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synonyms for ball milling technique

high energy ball milling process for nanomaterial synthesis

high energy ball milling process for nanomaterial synthesis

It is a ball milling process where a powder mixture placed in the ball mill is subjected to high-energy collision from the balls. This process was developed by Benjamin and his coworkers at the International Nickel Company in the late of 1960. It was found that this method, termed mechanical alloying, could successfully produce fine, uniform dispersions of oxide particles (Al2O3, Y2O3, ThO2) in nickel-base superalloys that could not be made by more conventional powder metallurgy methods. Their innovation has changed the traditional method in which production of materials is carried out by high temperature synthesis. Besides materials synthesis, high-energy ball milling is a way of modifying the conditions in which chemical reactions usually take place either by changing the reactivity of as-milled solids (mechanical activation increasing reaction rates, lowering reaction temperature of the ground powders)or by inducing chemical reactions during milling (mechanochemistry). It is, furthermore, a way of inducing phase transformations in starting powders whose particles have all the same chemical composition: amorphization or polymorphic transformations of compounds, disordering of ordered alloys, etc.

The alloying process can be carried out using different apparatus, namely, attritor, planetary mill or a horizontal ball mill. However, the principles of these operations are same for all the techniques. Since the powders are cold welded and fractured during mechanical alloying, it is critical to establish a balance between the two processes in order to alloy successfully. Planetary ball mill is a most frequently used system for mechanical alloying since only a very small amount of powder is required. Therefore, the system is particularly suitable for research purpose in the laboratory. The ball mill system consists of one turn disc (turn table) and two or four bowls. The turn disc rotates in one direction while the bowls rotate in the opposite direction. The centrifugal forces, created by the rotation of the bowl around its own axis together with the rotation of the turn disc, are applied to the powder mixture and milling balls in the bowl. The powder mixture is fractured and cold welded under high energy impact.

The figure below shows the motions of the balls and the powder. Since the rotation directions of the bowl and turn disc are opposite, the centrifugal forces are alternately synchronized. Thus friction resulted from the hardened milling balls and the powder mixture being ground alternately rolling on the inner wall of the bowl and striking the opposite wall. The impact energy of the milling balls in the normal direction attains a value of up to 40 times higher than that due to gravitational acceleration. Hence, the planetary ball mill can be used for high-speed milling.

During the high-energy ball milling process, the powder particles are subjected to high energetic impact. Microstructurally, the mechanical alloying process can be divided into four stages: (a) initial stage, (b) intermediate stage, (c) final stage, and (d) completion stage.

(a) At the initial stage of ball milling, the powder particles are flattened by the compressive forces due to the collision of the balls. Micro-forging leads to changes in the shapes of individual particles, or cluster of particles being impacted repeatedly by the milling balls with high kinetic energy. However, such deformation of the powders shows no net change in mass.

(b) At the intermediate stage of the mechanical alloying process, significant changes occur in comparison with those in the initial stage. Cold welding is now significant. The intimate mixture of the powder constituents decreases the diffusion distance to the micrometer range. Fracturing and cold welding are the dominant milling processes at this stage. Although some dissolution may take place, the chemical composition of the alloyed powder is still not homogeneous.

(c) At the final stage of the mechanical alloying process, considerable refinement and reduction in particle size is evident. The microstructure of the particle also appears to be more homogenous in microscopic scale than those at the initial and intermediate stages. True alloys may have already been formed.

(d) At the completion stage of the mechanical alloying process, the powder particles possess an extremely deformed metastable structure. At this stage, the lamellae are no longer resolvable by optical microscopy. Further mechanical alloying beyond this stage cannot physically improve the dispersoid distribution. Real alloy with composition similar to the starting constituents is thus formed.

Theoretical considerations and explorations of planetary milling process have been broadly studied in order to better understand and inteprate the concept. Joisels work is the first report to study the shock kinematics of a satellite milling machine. This work focused on the determination of the milling parameters that were optimized for shock energy. The various parameters were determined geometrically and the theoretical predictions were examined experimentally using a specifically designed planetary mill. Schilz et al. reported that from a macroscopical point of view, the geometry of the mill and the ratio of angular velocities of the planetary to the system wheel played crucial roles in the milling performance. For a particular ductile-brittle MgSi system, the milling efficiency of the planetary ball was found to be heavily influenced by the ratio of the angular velocity of the planetary wheel to that of the system wheel as well as the amount of sample load. Mio et al. studied the effect of rotational direction and rotation-to-revolution speed ratio in planetary ball milling. Some more theoretical issues and kinematic modeling of the planetary ball mill were reported later in related references. Because mechanical alloying of materials are complex processes which depend on many factors, for instance on physical and chemical parameters such as the precise dynamical conditions, temperature, nature of the grinding atmosphere, chemical composition of the powder mixtures, chemical nature of the grinding tools, etc., some theoretical problems, like predicting nonequilibrium phase transitions under milling, are still in debate.

For all nanocrystalline materials prepared by high-energy ball milling synthesis route, surface and interface contamination is a major concern. In particular, mechanical attributed contamination by the milling tools (Fe or WC) as well as ambient gas (trace impurities such as O2, N2 in rare gases) can be problems for high-energy ball milling. However, using optimized milling speed and milling time may effectively reduce the contamination. Moreover, ductile materials can form a thin coating layer on the milling tools that reduces contamination tremendously. Atmospheric contamination can be minimized or eliminated by sealing the vial with a flexible O ring after the powder has been loaded in an inert gas glove box. Small experimental ball mills can also be completely enclosed in an inert gas glove box. As a consequence, the contamination with Fe-based wear debris can be reduced to less than 12 at.% and oxygen and nitrogen contamination to less than 300 ppm. Besides the contamination, long processing time, no control on particle morphology, agglomerates, and residual strain in the crystallized phase are the other disadvantages of high-energy ball milling process.

Notwithstanding the drawbacks, high-energy ball milling process has attracted much attention and inspired numerous research interests because of its promising results, various applications and potential scientific values. The synthesis of nanostructured metal oxides for gas detection is one of the most promising applications of high-energy ball milling. Some significant works have been reported in recent years. Jiang et al. prepared metastable a-Fe2O3MO2 (M: Ti and Sn) solid solutions by high-energy milling for C2H5OH detection. The 85 mol% a-Fe2O3SnO2 sample milled for 110 hours showed the highest sensitivity among all the samples studied. The best sensitivity to 1000 ppm C2H5OH in air at an operating temperature of 250 C was about 20. Zhang et al. synthesized FeSbO4 for LPG detection. They found that there were two-step solid-state reactions occurring in the raw powders during the ball milling:

The response and recovery times of their sensor were less than one second. The sensitivity to 1000 ppm C2H5OH at an operating temperature of 375 C was about 45. Diguez et al. employed precipitation method to prepare nanocrystalline SnO2 and planetary milling to grind the obtained powder for NO2 detection. They found that the grinding procedure of the precursor and/or of the oxide had critical effect on the resistance in air. As a result, the gas sensing properties to NO2 had been considerably improved. Cukrov et al. and Kersen et al. synthesized SnO2 powders by mechanochemical processing for O2 and H2S sensing applications, respectively. The O2 sensor exhibited stable, repeatable and reproducible electrical response to O2. More recently, Yamazoes group reported the sensing properties of SnO2Co3O4 composites to CO and H2. A series of SnO2Co3O4 thick films containing 0100% Co3O4 in mass were prepared from the component oxides through mixing by ball-milling for 24 h, screen-printing and sintering at 700 C for 3 h. The composite films were found to exhibit n- or p-type response to CO and H2 depending on the Co3O4 contents in the composites. The n-type response was exhibited at 200 C or above by SnO2-rich composites (Co3O4 content up to 5 mass%). The sensor response to both CO and H2 was significantly enhanced by the addition of small amounts of Co3O4 to SnO2, and the response at 250 C achieved a sharp maximum at 1 mass% Co3O4. The p-type response was obtained at 200 C or below by the composites containing 25100 mass% Co3O4. The sensitivity as well as selectivity to CO over H2 could thus be increased by the addition of SnO2 to Co3O4.

Besides the above mentioned researches, significant efforts on the synthesis of nanostructured metal oxide with high-energy ball milling method for gas sensing have been actively pursued by the authors of this chapter. In our research, we use the high-energy ball milling technique to synthesize various nanometer powders with an average particle size down to several nm, including nano-sized a-Fe2O3 based solid solutions mixed with varied mole percentages of SnO2, ZrO2 and TiO2 separately for ethanol gas sensing application, stabilized ZrO2 based and TiO2 based solid solutions mixed with different mole percentages of a-Fe2O3 and synthesized SrTiO3 for oxygen gas sensing. The synthesized powders were characterized with XRD, TEM, SEM, XPS, and DTA. Their sensing properties were systematically investigated and sensing mechanisms were explored and discussed as well.

ball milling the role of media and bead mills - byk

ball milling the role of media and bead mills - byk

Ball milling is a grinding technique that uses media to effectively break down pigment agglomerates and aggregates to their primary particles. Using a rotor or disc impeller to create collisions of the grinding media, the impact and force created by the bead mills collisions break down the pigment agglomerates. The media can consist of either stainless steel, glass, or ceramic materials. The higher the bead hardness or density, the greater the collision force. The ball-milling process uses a higher concentration of grinding media to mill base in which the chambers are designed to maximize the energy transfer.

When a particle size has to be reduced below 10 microns, bead milling is the technique to use. However, if the material has a very low viscosity, ball milling is a better dispersing process than using a high shear mixing (vertical) system.

Currently, the VMA-Getzmann company offers three product lines for bead milling. They can be dedicated stand-alone systems or accessories that can be added to the high-speed vertical disperser models. Depending upon the model, sample quantities can be as low as 20 ml or up to 20,000 ml.

Our Dispermat SL model line is the current horizontal bead mill system. Milling chamber sizes can start at 50 ml to save on raw material costs. The beads are separated from the mill base by a dynamic gap system. The standard gap uses 1.0 mm diameter grinding media; an optional gap is available to use beads down to 0.3 mm diameter. The Dispermat SL can be selected to run as a single pass or as a recirculation configuration.

One of the unique features is an independent pumping system to feed the mill base into the milling chamber. Instead of the speed of the milling rotor controlling the sample volume the operator can control the volume, through the mixing system pump that fits on top of the milling chamber. Separating the rotor speed from the sample feed system provides more control over the milling process.

Basket bead milling is a relatively new design for ball milling applications. The grinding media is contained in a cylinder (basket), and the mill base is circulated through the basket. The VMA-Getzmann basket mill consists of a stainless-steel cylinder with an opening at the top and a sieve filter on the bottom. The standard diameter size of the grinding media is 1.0 mm. however, it can be ordered to use 0.3 mm bead size.

Since the Getzmann basket mill is attached to aHigh-Speed Dispersermodel, those with an adapter allow the user to switch between the basket mill system and a motor shaft for high-shear dispersing easily.

Attached to the bottom of the basket is a cowles blade that rotates at high speed. The purpose of the cowles blade is to circulate the mill base to ensure all materials enter the basket mill. When you have created the desired particle size, the basket mill is then raised out of the sample container, while the grinding media stays in the basket.

The third system for ball milling applications is the APS (air pressure system). The APS is attached to a high shear disperser. It consists of a sample containerwith a sieve filter at the bottom, a stand to elevate the sample container, along with a sealing system around the motor shaft, and a container lid. The mill base and grinding media is mixed 50/50 in the container. Adisk impeller or pearl mill impelleris immersed into the mixture and rotated from 500 to 5000 RPMs depending on the desired particle size. After the dispersion is completed the stop cock that covers the sieve filter is removed, the lid is clamped tight over the vessel. The lid has an air connection; the air is applied to force the sample through the sieve filter separating the mill base from the grinding media. Aside from the ability to produce small quantities of less than 25 milliliters, another advantage of the APS system is their ease of cleaning.

overview of milling techniques for improving the solubility of poorly water-soluble drugs - sciencedirect

overview of milling techniques for improving the solubility of poorly water-soluble drugs - sciencedirect

Milling involves the application of mechanical energy to physically break down coarse particles to finer ones and is regarded as a topdown approach in the production of fine particles. Fine drug particulates are especially desired in formulations designed for parenteral, respiratory and transdermal use. Most drugs after crystallization may have to be comminuted and this physical transformation is required to various extents, often to enhance processability or solubility especially for drugs with limited aqueous solubility. The mechanisms by which milling enhances drug dissolution and solubility include alterations in the size, specific surface area and shape of the drug particles as well as milling-induced amorphization and/or structural disordering of the drug crystal (mechanochemical activation). Technology advancements in milling now enable the production of drug micro- and nano-particles on a commercial scale with relative ease. This review will provide a background on milling followed by the introduction of common milling techniques employed for the micronization and nanonization of drugs. Salient information contained in the cited examples are further extracted and summarized for ease of reference by researchers keen on employing these techniques for drug solubility and bioavailability enhancement.

ball milling - an overview | sciencedirect topics

ball milling - an overview | sciencedirect topics

Ball milling is often used not only for grinding powders but also for oxides or nanocomposite synthesis and/or structure/phase composition optimization [14,41]. Mechanical activation by ball milling is known to increase the material reactivity and uniformity of spatial distribution of elements [63]. Thus, postsynthesis processing of the materials by ball milling can help with the problem of minor admixture forming during cooling under air after high-temperature sintering due to phase instability.

Ball milling technique, using mechanical alloying and mechanical milling approaches were proposed to the word wide in the 8th decade of the last century for preparing a wide spectrum of powder materials and their alloys. In fact, ball milling process is not new and dates back to more than 150 years. It has been used in size comminutions of ore, mineral dressing, preparing talc powders and many other applications. It might be interesting for us to have a look at the history and development of ball milling and the corresponding products. The photo shows the STEM-BF image of a Cu-based alloy nanoparticle prepared by mechanical alloying (After El-Eskandarany, unpublished work, 2014).

Ball milling, a shear-force dominant process where the particle size goes on reducing by impact and attrition mainly consists of metallic balls (generally Zirconia (ZrO2) or steel balls), acting as grinding media and rotating shell to create centrifugal force. In this process, graphite (precursor) was breakdown by randomly striking with grinding media in the rotating shell to create shear and compression force which helps to overcome the weak Vander Waal's interaction between the graphite layers and results in their splintering. Fig. 4A schematic illustrates ball milling process for graphene preparation. Initially, because of large size of graphite, compressive force dominates and as the graphite gets fragmented, shear force cleaves graphite to produce graphene. However, excessive compression force may damage the crystalline properties of graphene and hence needs to be minimized by controlling the milling parameters e.g. milling duration, milling revolution per minute (rpm), ball-to-graphite/powder ratio (B/P), initial graphite weight, ball diameter. High quality graphene can be achieved under low milling speed; though it will increase the processing time which is highly undesirable for large scale production.

Fig. 4. (A) Schematic illustration of graphene preparation via ball milling. SEM images of bulk graphite (B), GSs/E-H (C) GSs/K (D); (E) and (F) are the respective TEM images; (G) Raman spectra of bulk graphite versus GSs exfoliated via wet milling in E-H and K.

Milling of graphite layers can be instigated in two states: (i) dry ball milling (DBM) and (ii) wet ball milling (WBM). WBM process requires surfactant/solvent such as N,N Dimethylformamide (DMF) [22], N-methylpyrrolidone (NMP) [26], deionized (DI) water [27], potassium acetate [28], 2-ethylhexanol (E-H) [29] and kerosene (K) [29] etc. and is comparatively simpler as compared with DBM. Fig. 4BD show the scanning electron microscopy (SEM) images of bulk graphite, graphene sheets (GSs) prepared in E-H (GSs/E-H) and K (GSs/K), respectively; the corresponding transmission electron microscopy (TEM) images and the Raman spectra are shown in Fig. 4EG, respectively [29].

Compared to this, DBM requires several milling agents e.g. sodium chloride (NaCl) [30], Melamine (Na2SO4) [31,32] etc., along with the metal balls to reduce the stress induced in graphite microstructures, and hence require additional purification for exfoliant's removal. Na2SO4 can be easily washed away by hot water [19] while ammonia-borane (NH3BH3), another exfoliant used to weaken the Vander Waal's bonding between graphite layers can be using ethanol [33]. Table 1 list few ball milling processes carried out using various milling agent (in case of DBM) and solvents (WBM) under different milling conditions.

Ball milling of graphite with appropriate stabilizers is another mode of exfoliation in liquid phase.21 Graphite is ground under high sheer rates with millimeter-sized metal balls causing exfoliation to graphene (Fig. 2.5), under wet or dry conditions. For instance, this method can be employed to produce nearly 50g of graphene in the absence of any oxidant.22 Graphite (50g) was ground in the ball mill with oxalic acid (20g) in this method for 20 hours, but, the separation of unexfoliated fraction was not discussed.22 Similarly, solvent-free graphite exfoliations were carried out under dry milling conditions using KOH,23 ammonia borane,24 and so on. The list of graphite exfoliations performed using ball milling is given in Table 2.2. However, the metallic impurities from the machinery used for ball milling are a major disadvantage of this method for certain applications.25

Reactive ball-milling (RBM) technique has been considered as a powerful tool for fabrication of metallic nitrides and hydrides via room temperature ball milling. The flowchart shows the mechanism of gas-solid reaction through RBM that was proposed by El-Eskandarany. In his model, the starting metallic powders are subjected to dramatic shear and impact forces that are generated by the ball-milling media. The powders are, therefore, disintegrated into smaller particles, and very clean or fresh oxygen-free active surfaces of the powders are created. The reactive milling atmosphere (nitrogen or hydrogen gases) was gettered and absorbed completely by the first atomically clean surfaces of the metallic ball-milled powders to react in a same manner as a gas-solid reaction owing to the mechanically induced reactive milling.

Ball milling is a grinding method that grinds nanotubes into extremely fine powders. During the ball milling process, the collision between the tiny rigid balls in a concealed container will generate localized high pressure. Usually, ceramic, flint pebbles and stainless steel are used.25 In order to further improve the quality of dispersion and introduce functional groups onto the nanotube surface, selected chemicals can be included in the container during the process. The factors that affect the quality of dispersion include the milling time, rotational speed, size of balls and balls/ nanotube amount ratio. Under certain processing conditions, the particles can be ground to as small as 100nm. This process has been employed to transform carbon nanotubes into smaller nanoparticles, to generate highly curved or closed shell carbon nanostructures from graphite, to enhance the saturation of lithium composition in SWCNTs, to modify the morphologies of cup-stacked carbon nanotubes and to generate different carbon nanoparticles from graphitic carbon for hydrogen storage application.25 Even though ball milling is easy to operate and suitable for powder polymers or monomers, process-induced damage on the nanotubes can occur.

Ball milling is a way to exfoliate graphite using lateral force, as opposed to the Scotch Tape or sonication that mainly use normal force. Ball mills, like the three roll machine, are a common occurrence in industry, for the production of fine particles. During the ball milling process, there are two factors that contribute to the exfoliation. The main factor contributing is the shear force applied by the balls. Using only shear force, one can produce large graphene flakes. The secondary factor is the collisions that occur during milling. Harsh collisions can break these large flakes and can potentially disrupt the crystal structure resulting in a more amorphous mass. So in order to create good-quality, high-area graphene, the collisions have to be minimized.

The ball-milling process is common in grinding machines as well as in reactors where various functional materials can be created by mechanochemical synthesis. A simple milling process reduces both CO2 generation and energy consumption during materials production. Herein a novel mechanochemical approach 1-3) to produce sophisticated carbon nanomaterials is reported. It is demonstrated that unique carbon nanostructures including carbon nanotubes and carbon onions are synthesized by high-speed ball-milling of steel balls. It is considered that the gas-phase reaction takes place around the surface of steel balls under local high temperatures induced by the collision-friction energy in ball-milling process, which results in phase separated unique carbon nanomaterials.

Conventional ball milling is a traditional powder-processing technique, which is mainly used for reducing particle sizes and for the mixing of different materials. The technique is widely used in mineral, pharmaceutical, and ceramic industries, as well as scientific laboratories. The HEBM technique discussed in this chapter is a new technique developed initially for producing new metastable materials, which cannot be produced using thermal equilibrium processes, and thus is very different from conventional ball milling technique. HEBM was first reported by Benjamin [38] in the 1960s. So far, a large range of new materials has been synthesized using HEBM. For example, oxide-dispersion-strengthened alloys are synthesized using a powerful high-energy ball mill (attritor) because conventional ball mills could not provide sufficient grinding energy [38]. Intensive research in the synthesis of new metastable materials by HEBM was stimulated by the pioneering work in the amorphization of the Ni-Nb alloys conducted by Kock et al. in 1983 [39]. Since then, a wide spectrum of metastable materials has been produced, including nanocrystalline [40], nanocomposite [41], nanoporous phases [42], supersaturated solid solutions [43], and amorphous alloys [44]. These new phase transformations induced by HEBM are generally referred as mechanical alloying (MA). At the same time, it was found that at room temperature, HEBM can activate chemical reactions which are normally only possible at high temperatures [45]. This is called reactive milling or mechano-chemistry. Reactive ball milling has produced a large range of nanosized oxides [46], nitrides [47], hydrides [48], and carbide [49] particles.

The major differences between conventional ball milling and the HEBM are listed in the Table 1. The impact energy of HEBM is typically 1000 times higher than the conventional ball milling energy. The dominant events in the conventional ball milling are particle fracturing and size reductions, which correspond to, actually, only the first stage of the HEBM. A longer milling time is therefore generally required for HEBM. In addition to milling energy, the controls of milling atmosphere and temperature are crucial in order to create the desired structural changes or chemical reactions. This table shows that HEBM can cover most work normally performed by conventional ball milling, however, conventional ball milling equipment cannot be used to conduct any HEBM work.

Different types of high-energy ball mills have been developed, including the Spex vibrating mill, planetary ball mill, high-energy rotating mill, and attritors [50]. In the nanotube synthesis, two types of HEBM mills have been used: a vibrating ball mill and a rotating ball mill. The vibrating-frame grinder (Pulverisette O, Fritsch) is shown in Fig. 1a. This mill uses only one large ball (diameter of 50 mm) and the media of the ball and vial can be stainless steel or ceramic tungsten carbide (WC). The milling chamber, as illustrated in Fig. 1b, is sealed with an O-ring so that the atmosphere can be changed via a valve. The pressure is monitored with an attached gauge during milling.

where Mb is the mass of the milling ball, Vmax the maximum velocity of the vial,/the impact frequency, and Mp the mass of powder. The milling intensity is a very important parameter to MA and reactive ball milling. For example, a full amorphization of a crystalline NiZr alloy can only be achieved with a milling intensity above an intensity threshold of 510 ms2 [52]. The amorphization process during ball milling can be seen from the images of transmission electron microscopy (TEM) in Fig. 2a, which were taken from samples milled for different lengths of time. The TEM images show that the size and number of NiZr crystals decrease with increasing milling time, and a full amorphization is achieved after milling for 165 h. The corresponding diffraction patterns in Fig. 2b confirm this gradual amorphization process. However, when milling below the intensity threshold, a mixture of nanocrystalline and amorphous phases is produced. This intensity threshold depends on milling temperature and alloy composition [52].

Figure 2. (a) Dark-field TEM image of Ni10Zr7 alloy milled for 0.5, 23, 73, and 165 h in the vibrating ball mill with a milling intensity of 940 ms2. (b) Corresponding electron diffraction patterns [52].

Fig. 3 shows a rotating steel mill and a schematic representation of milling action inside the milling chamber. The mill has a rotating horizontal cell loaded with several hardened steel balls. As the cell rotates, the balls drop onto the powder that is being ground. An external magnet is placed close to the cell to increase milling energy [53]. Different milling actions and intensities can be realized by adjusting the cell rotation rate and magnet position.

The atmosphere inside the chamber can be controlled, and adequate gas has to be selected for different milling experiments. For example, during the ball milling of pure Zr powder in the atmosphere of ammonia (NH3), a series of chemical reactions occur between Zr and NH3 [54,55]. The X-ray diffraction (XRD) patterns in Fig. 4 show the following reaction sequence as a function of milling time:

The mechanism of a HEBM process is quite complicated. During the HEBM, material particles are repeatedly flattened, fractured, and welded. Every time two steel balls collide or one ball hits the chamber wall, they trap some particles between their surfaces. Such high-energy impacts severely deform the particles and create atomically fresh, new surfaces, as well as a high density of dislocations and other structural defects [44]. A high defect density induced by HEBM can accelerate the diffusion process [56]. Alternatively, the deformation and fracturing of particles causes continuous size reduction and can lead to reduction in diffusion distances. This can at least reduce the reaction temperatures significantly, even if the reactions do not occur at room temperature [57,58]. Since newly created surfaces are most often very reactive and readily oxidize in air, the HEBM has to be conducted in an inert atmosphere. It is now recognized that the HEBM, along with other non-equilibrium techniques such as rapid quenching, irradiation/ion-implantation, plasma processing, and gas deposition, can produce a series of metastable and nanostructured materials, which are usually difficult to prepare using melting or conventional powder metallurgy methods [59,60]. In the next section, detailed structural and morphological changes of graphite during HEBM will be presented.

Ball milling and ultrasonication were used to reduce the particle size and distribution. During ball milling the weight (grams) ratio of balls-to-clay particles was 100:2.5 and the milling operation was run for 24 hours. The effect of different types of balls on particle size reduction and narrowing particle size distribution was studied. The milled particles were dispersed in xylene to disaggregate the clumps. Again, ultrasonication was done on milled samples in xylene. An investigation on the amplitude (80% and 90%), pulsation rate (5 s on and 5 s off, 8 s on and 4 s off) and time (15 min, 1 h and 4 h) of the ultrasonication process was done with respect to particle size distribution and the optimum conditions in our laboratory were determined. A particle size analyzer was used to characterize the nanoparticles based on the principles of laser diffraction and morphological studies.

mechanical milling - an overview | sciencedirect topics

mechanical milling - an overview | sciencedirect topics

Mechanical milling was used to prepare nanocrystalline AZ31 magnesium (Mg) alloy with titanium (Ti) addition.[26] During milling, the crystallite size of the AZ31 Mg matrix decreased steadily with the increase of milling time. After milling for 110h, the crystallite size of the Mg phase of the alloy with 18wt.% and 27wt.% Ti additions was refined to 86 and 66nm, respectively, leading to the formation of a nanocrystalline Mg matrix microstructure. Meanwhile, the size of Ti particles added in the alloy also decreased gradually with the increase of milling time, and the Ti dispersions presented a positive effect on the refinement of the Mg matrix grains. By milling for 110h, the average size of Ti particles was refined to less than 1m in the AZ31 Mg alloy with 18wt.% Ti addition. After mechanical milling, the as-milled powders were cold pressed into green compacts, and the mechanical properties were characterized by Vickers hardness and uniaxial compressive tests. It was found that the microhardness of mechanically milled AZ31 Mg alloy with 27wt.% Ti addition reached about 147Hv, which was almost 3 times larger than that of the as-received AZ31 Mg alloy. Meanwhile, the yield strength of mechanically milled Ti-added AZ31 Mg alloy was seen to be as high as 293MPa at room temperature and 60MPa at 300C, respectively.[26]

Mechanical milling is the simplest top-down manufacturing technique that can be performed with or without a solid state chemical reaction. In simple mechanical milling, the variables that determine the ENM product characteristics are the milling method (ball or attrition miller), the exerted power, the milling medium (e.g., tungsten carbide ball), the process control solution (e.g., toluene), speed (revolutions per minute), and the time (70). In the mechanochemical milling, a chemical reaction accompanies the milling process to synthesize the metal, oxide, or complex metal nanoparticles. The components of this process include the precursor materials, e.g., metal, alloy, or mixtures of powders (oxides, carbonates, sulfates, chlorides, fluorides, and hydroxides); appropriate reactants to aid or complete the solid state reaction to generate the desired composition of the nanomaterial; surface-modifying agents (e.g., carboxylic acid or other acids); and process control solutions such as stearic acid or toluene (70). The final products generally have broad size distribution and variable shape, contain impurities and defects, and thus are mostly used for nanograined bulk material or nanocomposites, but not for specific/precision application (38). Solid state reactions can also be carried out in the absence of milling in high temperature (e.g., 850 C) joule furnaces to synthesize nanoparticles or nanowires. As noted above, the components needed include the precursor material, solutions to aid the growth of nanomaterials, a surfactant such as nonylphenol ether, and a process control solution. The precursor materials need to be mixed, milled, or ground thoroughly to prevent inhomogeneity in the final product.

Mechanical milling is used to change the properties of silver powders, including disintegration of particle aggregates, particle shape, and particle surface characteristics. This process can be used for most types of silver powders with techniques such as ball milling, vibratory milling, or attrition milling.

A feature of these processes is that they are time consuming and require special precautions to prevent contamination from the mill and milling medium. Various types of milling media can be used, including zirconia balls (ZrO2), metallic balls, and glass balls. Continued milling produces two-dimensional silver flakes. The sizes of the milling media is between 10 and less than 1mm, depending on the sizes of the initial powder and the size of the resulting silver flake required. Generally, an organic surfactant is introduced during the milling process to prevent the agglomeration of particles with each other while grinding and to control the size and shape of the silver flake.

A new mechanical milling device with glow discharge has been developed for aiming of surface modification of nanoparticles without heating. The configuration of this device is based on that of Mechanofusion system (Hosokawa Micron Corp.), which consists of a rotating chamber and an arm fixed with a certain clearance against the inside wall of the chamber. By applying an electric power into the clearance under controlled ambient and rotating the chamber, any particles can be subjected to mechanical forces such as compression and shearing, in glow discharge. Using this device, TiO2 nanoparticles were processed under a gas pressure of 50Pa of Ar with 10% NH3. TEM-EELS spectrums of the particles indicated that the mechanical milling with glow discharge can promote effective solid-gas reaction without heating.

Ball milling technique, using mechanical alloying and mechanical milling approaches were proposed to the word wide in the 8th decade of the last century for preparing a wide spectrum of powder materials and their alloys. In fact, ball milling process is not new and dates back to more than 150 years. It has been used in size comminutions of ore, mineral dressing, preparing talc powders and many other applications. It might be interesting for us to have a look at the history and development of ball milling and the corresponding products. The photo shows the STEM-BF image of a Cu-based alloy nanoparticle prepared by mechanical alloying (After El-Eskandarany, unpublished work, 2014).

One advantage of MA, MD, and MM methods over any other methods is the possibility to monitor the progress of the solid-state reaction, solid-state mixing, phase transformations, and the degree of powder mechanical mixing that has taken place upon ball milling and the starting materials for different milling times. Within the last 30 years, almost all the published scientific research articles have focused on studying the effect of ball-milling time on the structural, morphological, thermal stability, chemical, physical, and mechanical properties of the ball-milled materials. It is worth mentioning here that the ball-milling time required to prepare the same material is dependent on the ball mill used. High-energy ball mills have high kinetic energy that allows the solid-state reaction, phase transformations, and particle size reduction to be achieved in shorter time, when compared with the low-energy ball mills that always require longer milling.

Several characterization techniques and devices, like XRD, transmission electron microscope (TEM), scanning electron microscope (SEM), differential scanning calorimeter (DSC), etc., have been employed to monitor the progress in the MISSR via ball-milling process. There is no more fast and easy way to get quick information on the milling progress than XRD that gives a full picture on the mean structural changes upon powders ball milling for different length of time (see, e.g., Figure 3.18). This technique can be also used to calculate the change on the lattice parameters, ao (Figure 3.22), crystallite size, and the lattice strains (Figure 3.23) upon powder ball milling for different ball-milling time. The development on the local structural changes of the powders milled for different ball-milling time can be monitored beyond the atomic level by TEM and high-resolution TEM (HRTEM) that provide the opportunity for local analysis with a regional diameter of less than 5nm (see, e.g., Figure 3.24).

Figure 3.24. HRTEM image (a and c) and the corresponding Nano Beam Diffraction Pattern (NBDP) (b and d) of mechanically alloyed Zr65Al7.5Ni10Cu12.5Pd5 powders after 259 and 432ks of ball milling, respectively [30].

In spite of the melt spinning technique that leads to the formation of amorphous and metallic glassy alloys through a single-stage step, MA offers an excellent opportunity to monitor the stages that the powders passed from its starting long-range-ordered to short-range-ordered structure. Such amorphization reaction that promoted in a linear fashion with increasing ball-milling time can be followed up by using DSC method, which gives several useful information, such as glass transition temperature (Tg), crystallization temperature (Tx), super cooled liquid region (Tx), and enthalpy change of crystallizations (Hx), as exemplified in Figure 3.25.

There is no more fundamental method to monitor the morphological changes, which are taken place in the ball-milled powders upon milling for different milling times, more than the SEM technique. The metallographic developments of the ball-milled powder particles can be followed up by light optical microscope and SEM techniques that can give primary judgment on the formed phase, as exemplified in Figures 3.26 and 3.27.

The Planetary Ball Mills are the most popular mills used in MM, MA, and MD scientific researches for synthesizing almost all of the materials presented in Fig.1.2. In this type of mills, the milling media have considerably high energy, because milling stock and balls come off the inner wall of the vial (milling bowl or vial) and the effective centrifugal force reaches up to 20times gravitational acceleration.

The centrifugal forces caused by the rotation of the supporting disc and autonomous turning of the vial act on the milling charge (balls and powders). Since the turning directions of the supporting disc and the vial are opposite, the centrifugal forces alternately are synchronized and opposite. Therefore, the milling media and the charged powders alternatively roll on the inner wall of the vial are lifted and thrown off across the bowl at high speed, as schematically presented in Fig.3.18.

However, there are some companies in the worldwide manufacture and sale numbers of planetary-type ball mills, Fritsch GmbH (www.fritsch-milling.com) and Retsch (http://www.retsch.com) are considered to be the oldest and principle companies in this area.

Fritsch produces different types of planetary ball mills with different capacities and rotation speed. Perhaps, Fritsch Pulverisette P5 (Fig.3.19A) and Fritsch Pulverisette P6 (Fig.3.19B) are the most popular models of Fritsch-planetary ball mills. A variety of vials and balls made of different materials with different capacities, starting from 80 up to 500mL, are available for the Fritsch Pulverisette planetary ball mills; these include tempered steel, stainless steel, tungsten carbide, agate, sintered corundum, silicon nitride, and zirconium oxide. Fig.3.20 presents 80mL-tempered steel vial (A) and 500mL-agate vials (B) together with their milling media that made of the same materials.

Figure 3.19. Photos of Fritsch planetary-type high-energy ball mill of (A) Pulverisette P5 and (B) Pulverisette P6. The equipment is housed in the Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR).

Figure 3.20. Photos of the vials used for Fritsch planetary ball mills with capacity of (A) 80mL and (B) 500mL. The vials and the balls shown in (A) and (B) are made of tempered steel agate materials, respectively (Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR)).

More recently and in Year 2011, Fritsch GmbH (http://www.fritsch-milling.com) introduced a new high-speed and versatile planetary ball milled called Planetary Micro Mill PULVERISETTE 7 (Fig.3.21). The company clams this new ball mill will be helpful to enable extreme high-energy ball milling at rotational speed reaches to 1100rpm. This allows the new mill to achieve sensational centrifugal accelerations up to 95 times Earth gravity. They also mentioned that the energy application resulted from this new machine is about 150% greater than the classic planetary mills. Accordingly, it is expected that this new milling machine will enable the researchers to get their milled powders in short ball milling time with fine powder particle sizes that can reach to be less than 1m in diameter. The vials available for this new type of mill have sizes as 20, 45, and 80mL. Both the vials and balls can be made of the same materials, which are used to manufacture of the large vials used for the classic Fritsch planetary ball mills that shown earlier of this text.

Retsch also produced a number of capable high-energy planetary ball mills with different capacities (http://www.retsch.com/products/milling/planetary-ball-mills/), namely Planetary Ball Mill PM 100 (Fig.3.22A), Planetary Ball Mill PM 100 CM, Planetary Ball Mill PM 200, and Planetary Ball Mill PM 400 (Fig.3.22B). Likewise Fritsch, Retsch offers high-quality ball milling vials with different capacities (12, 25, 50, 50, 125, 250, and 500mL) and balls of different diameters (540mm), as exemplified in Fig.3.23. These milling tools can be made of hardened steel as well as other different materials such as carbides, nitrides, and oxides.

Figure 3.22. Photos of Retsch planetary-type high-energy ball mill of (A) PM 100 and (B) PM 400. The equipment is housed in the Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR).

Figure 3.23. Photos of the vials used for Retsch planetary ball mills with capacity of (A) 80mL, (B) 250mL, and (C) 500mL. The vials and the balls shown are made of tempered steel (Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR)).

Both Fritsch and Retsch companies have offered special types of vials that allow monitoring and measure the gas pressure and temperature inside the vial during the high-energy planetary ball milling process. Moreover, these vials allow milling the powders under inert (e.g., argon or helium) or reactive gas (e.g., hydrogen or nitrogen) with a maximum gas pressure of 500kPa (5bar). It is worth to be mentioned here that such a development made on the vials design allows the users and researchers to monitor the progress tackled during the mechanical alloying and mechanical disordering processes by following up the phase transformations and heat realizing upon RBM, where the interaction of the gas used with the freshly created surfaces of the powders during milling (adsorption, absorption, desorption, and decomposition) can be monitored. Furthermore, the data of the temperature and pressure driven upon using this system is very helpful when the ball mills are used for the formation of stable (e.g., intermetallic compounds) and metastable (e.g., amorphous and nanocrystalline materials) phases. In addition, measuring the vial temperature during blank (without samples) high-energy ball mill can be used as an indication to realize the effects of friction, impact, and conversion processes.

More recently, evico-magnetics (www.evico-magnetics.de) has manufactured an extraordinary high-pressure milling vial with gas-temperature monitoring (GTM) system. Likewise, both system produced by Fritsch and Retsch, and the developed system produced by evico-magnetics allow RBM but at very high gas pressure that can reach to 15,000kPa (150bar). In addition, it allows in-situ monitoring of temperature and of pressure by incorporating GTM. The vial, which can be used with any planetary mills, is made of hardened steel with capacity up to 220mL. The manufacture offers also two-channel system for simultaneous use of two milling vials.

Different kind of mills are suitable for grinding, mechanical alloying and mechanical milling such as horizontal mills (tumbler ball mill), stirred mill (attritor, e.g. Szegvari attrition mill1), planetary ball mill, vibrating mill (tube vibrating mill, Sweco vibrating mill and shaker vibrating mill (e.g. Spex is a lab-scale mill3)). Their working principles and operating conditions are summarised in Fig. 12.1 and Table 12.1. The classification on a scale of increasing mill energy is: horizontal ball mill, attritor, planetary ball mill and vibrating ball mill. For example, a process that takes only a few minutes in the SPEX mill can take hours in an attritor and a few days in a commercial tumbler ball mill.

The choice of a milling technique is determined by many parameters. For example, attrition mills are more efficient than tumbler ball mills for mixing and blending WC-Co cutting tool powders because of short milling time, production of fine particle size (submicron sized) and enhanced smearing of Co onto carbide particles. However, as the product output is relatively low with attrition mills, tumbler ball mills are usually used for production runs of over 100kg/day. Moreover, powder contamination, which is an important criterion for many applications, can be due to the initial purity of the powder, the milling equipment (design), the milling operating conditions (mill speed, balls size and material, atmosphere) and/or the use of process control agent. It increases with milling time, milling intensity and with the reduction of the difference of strength/hardness between the powder and the milling balls.

In spite of the obvious benefits of using MA, MD, and MM for preparations of a wide spectrum of advanced materials, the contamination of the milled powders that usually happened upon high-energy ball milling of the reactant powders is considered to be the most serious problems of the ball milling process. The source of contaminations can be classified into the following categories:

Organic contaminations: It is usually introduced to the powders upon milling with liquid lubricants of some organic compounds known as process control agent (PCA). The PCA plays a very important role on milling of ductile materials that always subjected to severe plastic deformation and cold working that always lead to agglomerate the powders that tend to stuck on the surface of the milling tolls (balls and the internal surfaces of the vials) without breaking the powder particles and/or alloying. This phenomenon of powder agglomeration (Fig.4.3) is sometimes known as negative milling since the powders, instead to be reduced in sizes, tend to gain larger sizes due to the excessive cold working. These lubricants, which act as particle-agglomeration-inhibitor-surfactants, added to the powdersduring milling.Accordingly, the surface-tension of the powder particles, which their surfaces absorb the surfactant, tend to have lower values that permit perfect alloying between the reactant material powders without agglomerations. However, there is a long list of liquid organic PCA that can be used effectively (15wt.% of the powder charge) such as ethyl alcohol, ethanol, etc., but they can be also in the solid form such as oxalic acid, boric acid, and alumina. When using PCA, the as-milled powders of the end product should be subjected to drying and surface activation processes before any further process such as powder consolidations. In general, most of these lubricants used as PCA are not stable against temperature since they have low melting and boiling points. As a result, they so often decomposed during excessive high-energy ball milling and interact with the fresh surfaces of the powders to form undesired interstitial compounds, such as carbides, nitrides, etc., which change the expected properties of the end product. It is thought that the existence of such compound contaminations has no harmful effect on the properties since they are homogeneously dispersed in the matrix of the milled material powders. Hence, they can improve some of the mechanical properties of the end product, as it is pointed out by Frazier etal.[6]

Figure 4.3. Photo of Ni33Al67 powders after planetary dry ball milling for 25h at rotation speed of 250rpm in an argon gas atmosphere, using Cr-tempered steel milling tools and in the absence of PCA.

In addition to the contamination by the PAC and in many cases, the milling tools used are not perfectly dried after washing and cleaning process. Accordingly, the powders milled with such humid surface of the milling tools lead the materials suffer from the existence of high moisture content that should be removed by drying in ovens at 120150C. It would be necessary to mention here that all the as-received and as-milled powders should be stored in vacuum-silica gel dissectors (Fig.4.4).

Gas contamination (e.g., oxygen, nitrogen): It is introduced to the milled powders due to the lack of sealing the milling vial that contains the powder charge. Milling of metallic powder on air atmosphere or using unsealed milling vials usually leads to the formation of significant volume fractions of high-temperature phases of metal oxide that cannot be decomposed simply upon power drying. In order to minimize the gas contamination content that usually happened upon exposure of the powders in the open air, the powders should be handled, mixed, balanced, and sealed into the vials together with the balls inside a glove box filled with an inert gas atmosphere of argon or helium (Fig.4.5). It is worth to be mentioned here that some materials like Ti, Zr, and Hf are very sensitive to nitrogen gas. Hence, significant mole fractions of metal nitride would be existed in the final product of the milled powders. Thus, nitrogen gas would not be recommended for using as an inert gas media.

Figure 4.5. Photos taken from KISR-Nanotechnology Lab, Kuwait, for the author while handling the ball-milled powders inside a glove box (mBRAUN Glove Box, Dieselstr. 31, D-85748 Garching-Germany; http://www.mbraun.com) under a helium gas atmosphere.

Solid contamination: powder contaminations with foreign solid materials may have one (or more) of the following reasons:Materials of the milling tools are introduced to the powders during the ball milling process as a result of frictions between the milling media (balls) or between the ball and the internal wall of the milling vial. It is well known that the magnitude of contamination depends on many parameters such as the milling medium, milling time, milling speed, and also the difference in strength and hardness between the powders and the milling tools. When using, for example, stainless, tool steel, Cr-tempered steel, and WC milling tools, fine chips of these materials are found to be existed in the milled powders. To minimize the effect of these foreign materials on changing the properties of the milled samples, it is recommended to consider the following issues:-When milling hard material powders like, for example, Nb, Ta, or W it is not suitable to use the soft stainless steel alloy of SUS304. The fine milled powder particles on the mill charge will act as abrasion media of the milling tools, thus a significant volume fractions of the SUS304 (Fe, Cr, Ni) would be peeled off from their surfaces and mixed and in many cases alloyed with the milled powders during the ball milling process. It was found that 33at.% of iron could be detected in the end product of pure W powders that were milled in steel milling tools for 50h, using SPEX Mixer/Mill.[7] In most cases, these metallic alloy contaminations with their constitution elements can react with most of the metallic system and form undesired phases with poor properties. Suryanarayana[7] pointed out that using steel as milling medium, a large amount of iron (20at.%) was found in WC/Co composite upon milling for 310h, using SPEX Mixer/Mill. Thus, using hard tool steel or WC always recommended to be used as milling tools in these cases.-Secondly, and more efficient is to use milling tools made of the same or similar materials of the milled powders. As an example, milling tools made of WC or WC/Co are always considered to be used when mixing of WC/Co powders to fabricate composite materials.-It is better to utilize specific milling tools for one system rather than using the same milling tools for milling different systems. If it is necessary to change from system to another, a perfect cleaning procedure should be tackled in order to insure the removal of all the powders of the previous system, especially in the dead zone of the vials at the corners and the internal walls. It is better in all cases to use new set of milling media for new powder system.-Existing of silica sand and dust in the as-milled sample powders are common problems that should be avoided by maintaining the MA lab in a proper clean conditions and to be isolated by simple closed doors from the outside. Moreover, the milling tools should be cleaned and kept inside vacuum-silica gel dissectors in order to protect them from any expected contamination by the dust. Such storing conditions offer prefect protection against corrosion for the steel-based milling tools.-The type of the ball mill used should be considered as an important factor. It is believed that roller ball milling leads to bring more contamination to the powders when compared with the roller rod mills.[8] This can be understood from the difference in the mechanisms between the two categories of mills, as previously discussed in Chapter3.

Materials of the milling tools are introduced to the powders during the ball milling process as a result of frictions between the milling media (balls) or between the ball and the internal wall of the milling vial. It is well known that the magnitude of contamination depends on many parameters such as the milling medium, milling time, milling speed, and also the difference in strength and hardness between the powders and the milling tools. When using, for example, stainless, tool steel, Cr-tempered steel, and WC milling tools, fine chips of these materials are found to be existed in the milled powders. To minimize the effect of these foreign materials on changing the properties of the milled samples, it is recommended to consider the following issues:-When milling hard material powders like, for example, Nb, Ta, or W it is not suitable to use the soft stainless steel alloy of SUS304. The fine milled powder particles on the mill charge will act as abrasion media of the milling tools, thus a significant volume fractions of the SUS304 (Fe, Cr, Ni) would be peeled off from their surfaces and mixed and in many cases alloyed with the milled powders during the ball milling process. It was found that 33at.% of iron could be detected in the end product of pure W powders that were milled in steel milling tools for 50h, using SPEX Mixer/Mill.[7] In most cases, these metallic alloy contaminations with their constitution elements can react with most of the metallic system and form undesired phases with poor properties. Suryanarayana[7] pointed out that using steel as milling medium, a large amount of iron (20at.%) was found in WC/Co composite upon milling for 310h, using SPEX Mixer/Mill. Thus, using hard tool steel or WC always recommended to be used as milling tools in these cases.-Secondly, and more efficient is to use milling tools made of the same or similar materials of the milled powders. As an example, milling tools made of WC or WC/Co are always considered to be used when mixing of WC/Co powders to fabricate composite materials.-It is better to utilize specific milling tools for one system rather than using the same milling tools for milling different systems. If it is necessary to change from system to another, a perfect cleaning procedure should be tackled in order to insure the removal of all the powders of the previous system, especially in the dead zone of the vials at the corners and the internal walls. It is better in all cases to use new set of milling media for new powder system.-Existing of silica sand and dust in the as-milled sample powders are common problems that should be avoided by maintaining the MA lab in a proper clean conditions and to be isolated by simple closed doors from the outside. Moreover, the milling tools should be cleaned and kept inside vacuum-silica gel dissectors in order to protect them from any expected contamination by the dust. Such storing conditions offer prefect protection against corrosion for the steel-based milling tools.-The type of the ball mill used should be considered as an important factor. It is believed that roller ball milling leads to bring more contamination to the powders when compared with the roller rod mills.[8] This can be understood from the difference in the mechanisms between the two categories of mills, as previously discussed in Chapter3.

When milling hard material powders like, for example, Nb, Ta, or W it is not suitable to use the soft stainless steel alloy of SUS304. The fine milled powder particles on the mill charge will act as abrasion media of the milling tools, thus a significant volume fractions of the SUS304 (Fe, Cr, Ni) would be peeled off from their surfaces and mixed and in many cases alloyed with the milled powders during the ball milling process. It was found that 33at.% of iron could be detected in the end product of pure W powders that were milled in steel milling tools for 50h, using SPEX Mixer/Mill.[7] In most cases, these metallic alloy contaminations with their constitution elements can react with most of the metallic system and form undesired phases with poor properties. Suryanarayana[7] pointed out that using steel as milling medium, a large amount of iron (20at.%) was found in WC/Co composite upon milling for 310h, using SPEX Mixer/Mill. Thus, using hard tool steel or WC always recommended to be used as milling tools in these cases.

Secondly, and more efficient is to use milling tools made of the same or similar materials of the milled powders. As an example, milling tools made of WC or WC/Co are always considered to be used when mixing of WC/Co powders to fabricate composite materials.

It is better to utilize specific milling tools for one system rather than using the same milling tools for milling different systems. If it is necessary to change from system to another, a perfect cleaning procedure should be tackled in order to insure the removal of all the powders of the previous system, especially in the dead zone of the vials at the corners and the internal walls. It is better in all cases to use new set of milling media for new powder system.

Existing of silica sand and dust in the as-milled sample powders are common problems that should be avoided by maintaining the MA lab in a proper clean conditions and to be isolated by simple closed doors from the outside. Moreover, the milling tools should be cleaned and kept inside vacuum-silica gel dissectors in order to protect them from any expected contamination by the dust. Such storing conditions offer prefect protection against corrosion for the steel-based milling tools.

The type of the ball mill used should be considered as an important factor. It is believed that roller ball milling leads to bring more contamination to the powders when compared with the roller rod mills.[8] This can be understood from the difference in the mechanisms between the two categories of mills, as previously discussed in Chapter3.

powder milling - an overview | sciencedirect topics

powder milling - an overview | sciencedirect topics

After ball milling, powders will be compacted in a die using a uniaxial press with adequate pressure. Based on the literature 300570 MPa pressure recommended for CNT based metal matrix composites (Esawi et al., 2010; Esawi et al., 2009; Al-Qutub et al., 2013). The relative density of the compacts mainly depends on the pressure applied and holding time (Ardestani et al., 2014). Holding time played a significant role for the cold welding effect during compaction. After the cold compaction microwave sintering was used by most of the researchers. The optimization of sintering parameters such as temperature, time, heating, and cooling rate are required for every composite (Uddin et al., 2010). The parameters such as holding time, ramp rate, pulse duration, pulse current, and voltage are used to control of sintering temperature.

Al-Qutub et al. (2013) reported that the microstructure of Al6061 with 1 wt% CNT sintered at the temperature of 400C for 20 min exhibited only very small pores were present, and the further increase of sintering temperature to 450C leads to the densification of the matrix. How the pores are decreasing is indicated in Fig. 6. Similarly, the hardness of the matrix increased from 57 to 66 HV with the further increase the sintering temperature of 50C. Hence, they concluded that higher sintering temperature increases the rate of diffusion and reduces the porosity. So the density and hardness of the composite increased significantly. Spark plasma sintering (SPS) is another sintering method which is suitable for the densification of unsinterable materials and creation of sound interface bonding (Kwon et al., 2010). The sintering temperature ranges from 500 to 650C suitable for AlCNT based metal matrix composites.

As was pointed out in the last two chapters, powder-milling process, using ball or rod mills, aims to produce a high-quality end product that can be composites and nanocomposites, and nanocrystalline powder particles of intermetallic compounds, amorphous, hydrides, nitrides, silicates, etc. As previously presented in Chapter3, powder-milling process has been continuously improving with introducing numerous innovated types of ball mills in order to improve the quality and homogeneity of the end products and to increase the productivity. This chapter discusses the factors affecting the mechanical alloying, mechanical disordering, and mechanical milling processes and their effects on the quality of the desired end products. Moreover, we will present some typical examples that show the effect of these factors on the physical and chemical properties of the milled powders.

There is a wide range of types of Al-based MMCs and many possibilities exist in terms of both the matrix and the reinforcement material. It should first be stressed that (unreinforced) aluminum is a material which is highly suited to recycling (Clyne, 1984; Peterson, 1995; Friesen et al., 1997; Legrand, 1997; Kirchner, 1998). This allows retention of the large energy content represented by the free energy of formation of aluminum from its ores (primarily bauxite), since the proportion of scrap aluminum which becomes converted to corrosion products during use is very small (as a consequence of the thin, coherent oxide layer which protects the surface). Furthermore, there are well-developed and straightforward techniques for the removal of contaminants from aluminum melts and this can be achieved without major expense or inconvenience. An obvious initial strategy for recycling of Al-based MMCs is to assume that this is likely to be worthwhile and to attempt to use the approaches developed for aluminum, with suitable modifications as necessary.

A broad overview of MMC recycling strategies is presented in Figure 1, which gives a flow chart showing routes for the handling of particulate MMCs during recycling and reclamation. This incorporates the concept of metallic foam production becoming a significant factor in the treatment of MMC scrap and also the usage of virgin or partially recycled MMC material for this purpose. While the production of metallic foam is still in the relatively early stages of development, it is certainly conceivable that its usage could soon become industrially quite significant. The material also has attractions in terms of its potential role in a recycling strategy (Degischer and Simancik, 1994). Metallic foam processing is examined in detail later in this chapter.

The matrix metal can be extracted for conventional secondary products. The reinforcement may be segregated to the dross and discarded, or recovered together with some matrix for subsequent milling and powder processing as above.

Schuster et al. (1993) suggested that castings and extrusion billets of particulate-reinforced MMCs could be recycled directly by remelting of primary material during MMC production. It was reported that there was no degradation of the mechanical properties, even after four remelting cycles. This indicates that the producer of primary particle-reinforced MMCs should be able to handle specified scrap particulate MMC. Alternatively, a secondary processing cycle can be established to collect assorted particulate MMC scrap for remelting. This is indicated in the MMC cycle shown in Figure 1, into which conventional foundry scrap can also be incorporated. During the production of particulate MMC castings, recycling within the foundry of processing scrap, such as gates and risers, in the same way as with conventional metal castings, is economically attractive.

The reaction between 6XXX alloys and alumina reinforcement is sensitive to the Mg content, since a common reaction product is MgAl2O4 spinel. Schuster et al. (1993) reported that repeated remelting of 6XXX-Al2O3 MMCs led to spinel contents which stabilized at just above 3 vol.%. At such levels, the mechanical properties of the material are apparently not significantly impaired. Wrought alloys reinforced with SiC particulate, produced by powder metallurgy routes, cannot be remelted without the danger of substantial interfacial reaction to form aluminum carbide, Al4C3. This is detrimental both because it raises the viscosity of the melt and, more seriously, it dramatically reduces the corrosion resistance (Bhat et al., 1991) of the castingparticularly in the presence of water, which reacts strongly with Al4C3. More details of this problem are given in Chapter 3.18 this volume. The reaction between molten Al and SiC to form Al4 C3 is inhibited by the presence of Si in the melt. This has been studied in some detail by Lloyd and co-workers (Lloyd, 1989,1994; Lloyd et al., 1989) and is also covered in Chapter 3.21 this volume. Provided the Al casting alloy contains at least about 8 wt.% Si, then the extent of the reaction is expected to be very limited, particularly if the melt temperature is kept below about 750Cwhich is recommended practice for primary MMC aluminum foundry technology (Duralcan, 1990). It has been shown (Lin et al., 1998) that, for the A380/SiCp system, repeated recycling, with no degradation of properties, can be achieved provided there is careful control over the silicon content of the melt during processing.

A detailed study of the recycling of AlSi casting alloys reinforced with SiC particulate is given by Provencher et al. (1992), who outline the quality criteria of the melt, as well as describing the recommended practices for remelting, melt holding, and recycling procedures. A summary of their main recommendations and observations is listed below.

The melt should be agitated mechanically in order to distribute the SiC particles homogeneously in the melt. Excessive stirring must be avoided, since it tends to result in vortex formation and consequent entrainment of bubbles, which tend to become stabilized by the adsorption of SiC particles.

It was reported (Provencher et al., 1992) that there was no drop in SiC particle content during recycling and the removal of porosity, oxide films, and hydrogen were found to be efficient. No degradation of mechanical properties was observed. The quantities of dross typically generated during the process are, however, relatively high ( 10% of the total weight). It is reported (Chamberlain and Bruski, 1998) that 2550% by weight of the scrap charge may be recycled in a production environment, although this is accompanied by a recommendation that, for quality products, the yield should not exceed 33%.

It may also be mentioned that, as indicated in Figure 1, nonreactive systems could in principle be used as melt feedstock for spray deposition (see Chapter 3.23 this volume), perhaps reducing or eliminating the need to co-inject ceramic particulate. In practice, however, the increase in melt viscosity induced by the presence of the ceramic would probably cause difficulties.

Particulate MMC containing more than about 50 vol.% of ceramic are difficult to fluidize at all, even when the matrix is fully molten. However, MMCs such as Al or Cu alloys containing 6075 vol.% SiC particles can be recycled by melting the matrix (to give a highly viscous charge) and reshaped by direct squeeze casting. This process is attractive in some respects, although there is a danger that the product may incorporate defects such as cracks and inhomogeneities in reinforcement distribution.

Powder metallurgical processing of MMC (see Chapter 3.25 this volume) sharply reduces the danger of interfacial reaction occurring. Any MMC scrap (discontinuously or continuously reinforced, and irrespective of reinforcement content) can be milled to powder (or machined to chips). This may serve as raw material for secondary MMC-powder consolidation.

Ensure the bottle selected is of appropriate size for the amount of powder that requires milling:Dry: 25% powder, 50 vol% media, 25% free spaceWet: 40% slurry (2550 vol% powder loading), 50 vol% media, 10% free space

If using wet milling, the powder slurry should then be transferred to a drying oven. When using water as the fluid, a hard agglomerate may be formed. Softer agglomerates (and faster drying) are obtained when alcohols are used as milling fluid.

The production of nanoparticles by milling a combination of compounds to form a new product by a solid-state displacement reaction has received increasing attention over recent years. The process is referred to as Mechanochemical Processing (MCPTM). In this process two or more materials are simultaneously milled to produce, through an exchange reaction, a nanoscale composite that can be further processed into dispersed nanopowders by removing the matrix phase. For example, ZnO powder has been produced by simultaneously milling powders of ZnCl2 and Na2CO3 to form ZnCO3 and NaCl by the following reaction:

The nanostructured product mix is then heat treated (170380 C) to thermally decompose ZnCO3 to ZnO, washed (to separate the sodium chloride from ZnO) and dried. Particles produced by this method had an average particle size of 27 nm. Amounts of NaCl (excess to stoichiometry) were added to act as a diluent and assisted both in particle separation and size control [29]. A range of materials has been produced by this method commercially and in the laboratory. These include oxides, sulphides, carbonates doped metal oxides and metals. Table 1.4 presents some examples of these.

Precursors can be selected from oxides, carbonates, sulphates, chlorides, fluorides, hydroxides and reported products are not limited to those cited above but vary across a range of metals including, for example, Cu, Ni, Al, Cd, Pb and Se. Process control is dependent on a number of variables, which include milling time, the level of diluent and the choice of starting material and thermal treatment parameters [3037].

Fundamentally, the term milling may be referred to as the breaking down of relatively coarse materials to the ultimate fineness. Apart from the milling of ores, milling is also used for preparing materials for some industrial applications, such as milling of quartz to fine powder (under 70 m in diameter), milling of talc to produce body powder, milling of iron ore for preparation of pellets, and many others. Over the past three decades, ball milling has evolved from being a standard technique in mineral dressing and powder metallurgy, used primarily for particle size reduction, to its present status as an important method for the preparation of either materials with enhanced physical and mechanical properties or, indeed, new phases, or new engineering materials. Accordingly, the term mechanical alloying (MA)[1] is becoming increasingly common in the materials science and metallurgy literatures.[2]

So far, the MA process, using ball-milling[3] and/or rod-milling techniques,[4] has received much attention as a powerful tool for fabrication of several advanced materials (Fig. 1.1), including equilibrium, nonequilibrium (e.g., amorphous, quasicrystals, nanocrystalline, etc.), and composite materials.[5][7] In addition, it has been employed for reducing some metallic oxides by milling the oxide powders with metallic reducing agents at room temperature.[8][10] In fact, MA is a unique process in that a solid state reaction takes place between the fresh powder surfaces of the reactant materials at room temperature. Consequently, it can be used to produce alloys and compounds that are difficult or impossible to be obtained by conventional melting and casting techniques.[11]

Figure 1.1. Mechanical alloying is a pioneer process for fabrication of a wide variety of alloys and compounds at room temperature. Fabrication of high thermal stable amorphous alloys, nanocrystalline and nanocomposite materials at room temperature are some advantages of this process.

The delta phase on the Ti-H phase diagram has a wide range of hydrogen solubility, from 2.16 wt% to above 5 wt%. The friability of the hydride is a strong function of the hydrogen content. Below 3.5 wt% hydrogen, the best control of hydride sizing can be achieved, with minimization of dust (fines) generation. Residual toughness of lower hydrogen content powder can lead to several challenges; however, including higher crushing and grinding forces and a greater likelihood of contamination through abrasive wear of the tooling.

The propensity to create dust clouds while milling powders with hydrogen contents above 4 wt% must be minimized or avoided. Titanium hydride (sponge, CP-Ti, and Ti-6Al-4V) is classified as a flammable solid. While the hydride powder can be physically difficult to ignite in bulk containers, a dust cloud will readily generate a bright white flash without significant sound if uncontained. Deflagration forces generated by ignition of a dust cloud in a highly confined milling environment can be devastating, potentially leading to secondary explosions.

Any hazardous powder should be classified by an accredited laboratory to understand the maximum explosion pressure (Pmax), the maximum rate of pressure rise (dP/dt)max, and the explosion class (or Kst) before attempting these operations. These laboratory services can also offer guidance for safest practices.

Many factors must be taken into account when designing milling and screening circuits. Potential ignition sources can include sparks from metalmetal contact or static discharge, excessive heat in the powder from the grinding operation, or from hot surfaces or motors. Control over these sources must be employed at all steps, especially while milling and screening and during powder transfer operations.

Chemical vapor deposition. In this technique, the substrate is exposed to a volatile precursor, which then reacts or decomposes on the surface of the substrate and is then obtained as the desired deposit. Materials are collected in various forms such as monocrystalline, polycrystalline, amorphous, and epitaxial.

High-energy ball milling process. This method is used for the synthesis of nanoparticles of three types:Mechanical alloyingmixtures of powders are milled together and then material transfer takes place to obtain homogenous alloy;Mechanical millingpowders with uniform composition are milled together as no material transfer occurs;Mechanochemical synthesischemical reaction between the powders takes place during the milling process at low temperature, which is far from equilibrium conditions.

Microwave synthesis. Nonaqueous solgel technique is widely used for the synthesis of metallic nanoparticles. The size of the nanoparticles can be controlled by choosing the appropriate reaction conditions including temperature, reactant amount, and the reaction time.

Polymerized complex method. In this technique, metal ions are first chelated to form complexes and are then polymerized to form a gel. Among other chemical processes, this method is the most suitable due to homogenous dispersion of cations in the polymer network.

Solgel synthesis. This method involves the production of quite high ultrapure materials at atomic scale and offers the advantage of tailoring the composition. Solgel synthesis is the most viable method for the production of homogenous alloys and composites in an efficient and cost-effective manner.

Supercritical hydrothermal synthesis. This process is used mainly for the synthesis of metal oxide nanoparticles. During supercritical hydrothermal synthesis, water is used as a solvent due to its high dielectric constant.

The processing flaws discussed in this section are larger than the characteristic dimension of microstructures (generally the grain size). They can be attributed to process failures or fluctuations. For instance, in sintered ceramics, flaws form during the successive processing steps: milling, mixing, forming, sintering and cooling down from the sintering temperature. Some of the more common processing flaws are agglomerates, pores, voids, inclusions, large grains and cracks (Table 1.3). Figures 1.91.11 show the various fracture origin defects detected in broken ceramic test specimens: an agglomerate in a sintered silicon nitride ceramic (Figure 1.9), a metallic inclusion coming from the ball mill during the step of powder milling (Figure 1.10), voids located at the surface or beneath the surface in silicon carbide bending bars (Figure 1.11). It is worth noting some features of these flaws: the size as small as about 100m, the shape, the nature and the curvature radius which looks larger than the crack tip one. However, the surface of voids is uneven with some sharp parts at grain boundary junctions. Thus, the curvature radius can be locally smaller than the apparent one. Void severity will depend on local curvature radius and flaw orientation with respect to stress direction.

In ceramic, glass or carbon fibers, fractures are generally surface-located flaws which are created by scratching or shocks (microcontact flaws) during handling, or by processing flaws such as voids, grains, chemical heterogeneities and contamination surface flaws. Figure 1.12 shows a fracture-inducing pore in a SiC-based fiber (Nicalon grade). Fracture origin is indicated by the visible associated mirror-like zone. It is important to note the submicron size of the pore, which is commensurate with the nanostructure length scale of fiber made up of nanograins of silicon carbide. The pore is thus bigger than the characteristic dimension of nanostructure indicated by grain size (less than 100nm).

The nature of flaws depends on processing mode, as indicated by Table 1.3 which summarizes the types of flaws identified in ceramics made via hot pressing, sintering, chemical vapor deposition or from polymer precursors.

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