mechanical grinding milling operation pdf

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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photo shows the STEM-BF image of a Cu-based alloy nanoparticle prepared by mechanical alloying (After El-Eskandarany, unpublished work, 2014).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.

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.

mechanical grinding - an overview | sciencedirect topics

Colmonoy 69 was selected as the starting alloy powder for subsequent compositional modifications. Single track and five-track coatings (33% track overlapping) of Zr-modified and Nb-modified Colmonoy 69 with a thickness of 0.9-1.1mm were deposited at a speed of 5mm/s on 50-mm-diameter S355 low carbon steel rods using a fiber laser beam. A Metco Twin 10C powder feeder was used to simultaneously inject Colmonoy 69 and industrial grade Zr (Zr+Hf>99wt.%) or Colmonoy 69 and commercially pure Nb powders through a side cladding powder injection nozzle using argon as powder carrier and melt pool shielding gas. In each case, the two powders were mixed in a cyclone before exiting the deposition nozzle. Zr or Nb powder feeding rates were adjusted to have up to 5wt.% Zr or up to 10wt.% Nb in the coatings. Deposits were made using a laser power of 800W on substrates preheated to 500C. Preheating of the substrates was done immediately before cladding using an electric tube furnace. Dilution from the substrate [36] evaluated from optical microscopy (OM) images of transversal cross-sections was kept lower than 10%.

Samples for further characterizations were cut and prepared by standard mechanical grinding with suspensions containing 9 and 3m diamond particles and polishing with colloidal Al2O3. A Philips XL30 Field Emission Gun Scanning Electron Microscope (SEM) was used for microstructural observations, mostly in BSE imaging mode. Mutual solubility of the phases was evaluated by quantitative energy dispersive spectroscopy (EDS). To ensure the accuracy and reliability of the EDS measurements, the following steps were taken:

The halographic peak deconvolution (HPD) function in the EDAX Genesis software was used to compare the model EDS spectra with the experimental ones to distinguish the overlap between B-K and Zr-M or B-K and Nb-M peaks.

Furthermore, a combination of EDS and electron backscatter diffraction (EBSD) was used for phase identification of the Cr-, Zr-, and Nb-rich precipitates. Full details on the application of EDS/EBSD for phase identification of boride and carbide phases in this alloy system could be found elsewhere [19]. As stated before, Thermo-Calc simulations were used to determine the sequence and temperature of phase formation reactions. The calculated phase formations were verified against the EDS/EBSD phase identification results. Simulations were repeated using a modified list of expected phases wherever necessary.

An unsuccessful attempt was made to measure the fracture toughness of Colmonoy 69 coatings, using the Palmqvist indentation technique. The procedure and the results are presented in [38]. Alternatively, the cracks in each clad layer were counted using a low-magnification (20) OM and their number per unit area of the clad layer was used as a measure of toughness. Hardness of the coatings was measured using Vickers indenter at a load of 4.9N.

Even in smaller scale with respect to the sawing operation, the mechanical grinding also leaves a certain subsurface damage (Zeng et al., 2007b). This damage can have a catastrophic effect on the detector properties in terms of charge collection and leakage current (Auricchio et al., 2008; Hossain et al., 2008). For these reasons, crystals are mechanically and chemo-mechanically polished to remove the damaged layer and improve surface quality. It is important to remember that after each step, a cleaning of few minutes in ultrasonic bath is necessary to remove any remaining abrasive particles and residual chemicals on the surface.

Mechanical polishing. The mechanical polishing involves the abrasion of material surface by using polishing clothes and slurries such as diamond paste alumina or magnesium oxide powders of size varying between 0.5 and 3 m suspended in DI water (Cui et al., 2005; Cui et al., 2007; Duff et al., 2008; Kargar et al., 2006; Li et al., 2006a; Marchini et al., 2008; Zhang et al., 2008b). Depending on the chemicals used as slurries, contamination of the surface of the sample may occur, for example, with carbon when diamond slurries are used (Dremlyuzhenko et al., 2004).

The process is carried out in a few steps by reducing the size of abrasive powder in order to decrease the surface roughness and to create a mirror-like surface without degrading the flatness. The choice of the polishing pads, slurries, pressure on the samples, pad rotational velocity, drip rate of solution, etc., are important factors to optimize the quality, uniformity, and reproducibility of polishing.

Typically, the mechanical polishing run leaves behind a roughness about 310 times the size of the abrasive particle used, with a stock removal of about 500 m for each side (Hossain et al., 2008; Kargar et al., 2006). While the mechanical polishing improves the apparent status of the surface, it can also induce subsurface damage and surface nonstoichiometry that enhance trapping effects and eventually lead to increase the surface leakage current. In addition, the final surface roughness is crucial for the leakage current (Hossain et al., 2008; Wenbin et al., 2004); therefore, the damaged layer after mechanical polishing has to be removed by a chemo-mechanical process as explained below (Tepper et al., 2001; Wenbin et al., 2004).

Chemo-mechanical polishing (CMP). Some authors (Miclaus et al., 2004) use a step of CMP after the mechanical polishing. In CMP, the mechanical component helps to maintain a flat surface through the abrasive particles while the surface roughness is strongly reduced by the chemical component. In general, to discuss the final surface preparation of optical quality surfaces for photonic applications, it is useful to first discuss the process inputs and process constraints. Singh et al. (2005) outlined some parameters for CdZnTe, and presented results obtained by polishing with Bromine using various slurry compositions (ethylene glycol, methyl cellulose, lactic acid, and methanol), applying different pressures (275gcm2), and evaluating the effects of noncontact polishing. Moravec et al. (2006) present the effects of load, slurry concentration, bromine concentration, and pad type on material removal rates. They concluded that lower bromine concentrations led to lower removal rates and improved surface morphologies. A more complete list of these variables is presented in Table 1, with optimal conditions listed for each author.

The physical parameters for polishing CdZnTe are similar for most of these authors. For rough polishing of CdZnTe without aggressive chemicals, the pads Logitechs Pellon Cloth and Buehlers Tex-met have proven to be suitable when jointly used with alumina-based slurries and moderate pressures. For chemical polishing using bromine-based slurries (or other aggressive slurries), more robust pads are required such as Logitechs Chemicloth, Buehlers Chemomet, or Rodel Haus DF200. Polishing plate rotation is set typically between 50 and 85 rpm (depending on slurry flow rate). Resultant surface roughness for these polished surfaces has been reported between 1.865 and 5.8nm.

Some authors have implemented CMP with SiO2 nanoparticles (Zhang et al., 2008b). Colloidal silica gel based on SiO2, H2O2, NaOH, and glycerine does not contaminate the surface layer (Dremlyuzhenko et al., 2004; Mandal et al., 2006). Other results have demonstrated that by using weaker oxidants based on iodine with lower redox potential value, compared with bromine, the surface is more stable with time, and with a surface roughness comparable to that obtained with bromine (Ivanitska et al., 2007).

Powders less than 1 m in size can be produced by wet grinding in attritors in the presence of substances preventing autoignition of the powder such as nitrobenzene, salts of oleic or stearic acid. The addition of surfactants to the dispersing liquid will aid in reducing the particle size further.

Cryogenic methods enable fine and ultrafine powders to be produced. To obtain ultrafine powder, the powder is suspended in an aqueous medium and subsequently frozen under ultrasonic impact. The water is evaporated in a vacuum; the resulting product is a powder with particles size less than 1 m.

Zinc powder can be produced as a by-product by treatment of zinc-containing wastes. The surface of sthe zinc-containing melt is blown with an inert gas. Zinc vapor is condensed as a fine powder on the surface of a rotating water-cooled drum (a rotational speed of 0.510rpm) located at a distance of 5003000 mm from the surface of the melt. The average size of the resulting particles is < 3 m.

Film quality and crystalline perfection of SOI layers obtained by direct bonding and mechanical grinding and polishing techniques are virtually identical to those of device grade bulk silicon wafers. If the polishing is performed to prime grade standards, the crystalline perfection of the initial bulk Si wafer can be retained. In thick-film bonded SOI wafers the device silicon wafer directly defines the crystal material of the SOI film. Accordingly, the crystal quality and defects are correlated to the control of the crystal pulling techniques. Because of the thick silicon layers, control of the crystal defects associated with precipitation of oxygen in the bulk of the device wafer needs consideration when deciding upon the silicon material. In the bonded SOI process the device silicon wafer is heat-treated at 11001200C to form a permanent chemical bond and reach the final bond strength; two high-temperature process steps are necessary when thermal oxide is grown on the device wafer to form the buried oxide. In such high-temperature annealing, oxygen incorporated into Czochralski-silicon precipitates, leading to formation of oxygen precipitates in the bulk of the device silicon wafer. In high-oxygen silicon, a denuded zone free of precipitates can form near the siliconoxide interface. If the SOI layer is thin (<10m) it is in the region of the denuded zone where precipitation of oxygen is greatly reduced, and low concentration of crystal defects in the silicon film can be achieved. In thick SOI layers oxygen precipitation in the bulk of the device silicon wafer actually defines the concentration of oxygen-related crystal defects in the SOI film. Stacking faults then grow on the oxygen precipitates during thermal oxidation of the SOI layer. For oxygen concentration of 14ppma or higher, the stacking fault density induced by thermal oxidation strongly depends on the actual oxygen concentration in the crystal and the silicon film thickness.

The choice of crystal material is critical to control oxygen precipitation and thereby, for example, the formation of oxidation-induced stacking faults (OISFs) in a thick device layer. Basically one can use float-zone silicon material with inherently low oxygen concentration, or epitaxial silicon wafers. However, it is also possible to use Czochralski (CZ) silicon with reduced oxygen concentration. Figure 7.20 shows the number of OISF as a function of the oxygen concentration in the crystal material of the device wafers. It summarizes data from different types of CZ crystals including weakly doped n- and p-type and highly doped p-type material. The device layer thickness in the wafers is from 8 to 50m. In SOI wafers in which the oxygen concentration in the device wafer is 11ppma or less there is very little or no oxygen precipitation that would lead to the for-mation of stacking faults. The OISF density is from 0 to 2cm2 independent of the type of crystal material or the SOI film thickness. Similar OISF densities are typically measured from prime quality polished silicon wafers indicating that the defect density is related to the overall crystal quality rather than the SOI fabrication process. At higher oxygen concentrations from 11 to 13ppma, OISF start to appear although there is quite a lot of variation according to the type of crystal, layer thickness, and formation of buried oxide, either on the device wafer or the handle silicon wafer. Finally, when the oxygen concentration is around 15ppma very high OISF concentrations of 1105cm2 or higher can be found.

Figure 7.20. Thermal oxidation induces stacking faults in thick-film SOI wafers. Thermal oxidation was carried out at 1100C for 2h (wet oxidation). Device wafer properties vary including n-type, p-type, and highly p-type silicon. SOI layer thickness varies from 8 to 50m. The buried oxide thickness varies from 400nm to 2m, and it was initially formed either on the device wafer or the handle silicon wafer by thermal oxidation.

For reference, IC requirements for the oxygen concentration in polished silicon wafers to control oxygen precipitation for internal gettering of metal impurities are most commonly in the range of 1216ppma. In thick-film SOI wafers designed for MEMS applications crystal material with lower oxygen concentration is advantageous.

Many methods of pretreatment have been suggested over the last few decades, and each has its advantages and disadvantages. They can be basically divided into the following categories: physical, chemical, biological, or a combination of these methods.

They are usually divided into two categories: mechanical and nonmechanical. In mechanical (grinding and milling), impact forces reduce particle size and crystallinity and increase specific surface and bulk density. In nonmechanical pretreatments, the cellulosic substrate is subjected to the action of external agents in order to cause changes in the structures of the original material. This pretreatment has the disadvantage of inefficiency in increasing biomass digestibility and the consumption of large amounts of energy in the process. Physical pretreatment combined with heating and adding chemicals can be an interesting option.

They are those in which chemical agents such as acids, bases, and organic solvents are used. They are intended to increase the surface of the substrate by swelling the fibers and the modification or removal of hemicelluloses and/or lignin in order to make the cellulose more accessible to the enzymes.

Acid pretreatment consists of placing the biomass in direct contact with inorganic, diluted, or concentrated acids to solubilize the hemicelluloses in order to obtain more accessible cellulose. The most common diluted acid pretreatment consists of immersing the material in an acid solution of approximately 4% (w/w) and then heating to temperatures in the range of 140C200C for a period of several minutes to 1hour. Different reactions occur during pretreatments with acids, and one of them is the hydrolysis of hemicelluloses, especially xylans and glycans. Depending on the pretreatment conditions, there may be formation of furfural and hydroxymethylfurfural by the degradation of pentoses and hexoses, respectively, which may negatively influence the fermentation step (Palmqvist et al., 2000). An advantage of this pretreatment is the solubilization of hemicelluloses, increasing the accessibility of the cellulose to the enzymes, and a higher yield compared to the basic hydrolysis. On the other hand, there is a risk of corrosion of the process equipment, as well as the formation of volatile carbon degradation products, which in many cases slow the conversion to ethanol. However, volatile products can be converted to methane. The condensation and precipitation of solubilized lignin components is also an undesired reaction as it decreases digestibility. Pretreatments with concentrated acid for the production of ethanol are not attractive processes, due to the great risk of production of inhibitory components by the degradation of carbohydrates. In a study of optimum pretreatment conditions that used dilute sulfuric acid (H2SO4), the best condition was found at 155C, using H2SO4 at 1.5% in 25minutes of reaction. This same study used these conditions showing an economic feasibility study of a theoretical ethanol production plant reaching production values of 187.5L of ethanol per tonne of raw biomass of sugarcane straw, one of the values found in the literature (Mesa et al., 2017).

In alkaline pretreatment, dilute alkaline solutions are generally used under mild conditions compared to acidic systems. The main effect of this pretreatment is the removal of lignin. Soaking the material in an alkaline solution, such as sodium, potassium, or ammonium hydroxide, followed by heating, leads to the decrease in the crystallinity degree of polymerization of cellulose. It also causes breakdown of lignincarbohydrate bonds, as well as disruption of the lignin structure. In some cases, it can be conducted at room temperature, but this requires high reaction times of the order of hours, days, or weeks. Unlike acid pretreatments, a limitation occurs because some bases can be converted into salts unrecoverable or incorporated as salts in the biomass through the pretreatment reactions (Chang and Holtzapple, 2000). It is also worth noting that this method is more effective in agricultural residues and grasses than in woody materials, as these materials generally contain less lignin. For woody materials, the alkali concentration must be increased considerably, so that the process is similar to kraft pulping (Galbe and Zacchi, 2010). A dilute alkaline pretreatment is normally used when it is known that there may be a large increase in the yield of ethanol production. Using an alkaline pretreatment with 3% NaOH at 60C for 9hours, it was possible to obtain the highest glucose recovery (20.6%) and a high hydrolysis efficiency (72%) using a high cellulose grass (42.7%) (Siripong, 2018).

Ionic liquids have also been recently studied as a pretreatment option for lignocellulosic biomass to increase saccharification yield and ethanol production. In general, ionic liquids are salts that, during their liquid state (usually at high temperatures and pressures), have the ability to break recalcitrant biomass bonds due to their strong ionelectrostatic interaction (Zhang et al., 2017). Although the use of ionic liquids in biomass has been reported since 2002, only in recent years there has been more interest in the technique due to the development of new ionic liquids and a better understanding of the pretreatment processes using this technique.

They involve the use of microorganisms (fungi and bacteria) or enzymes (laccases) to delignify and reduce the degree of polymerization of cellulose and hemicelluloses. The advantage of these pretreatments is the low energy requirement. Although effective, clean, and without production of undesirable metabolites, the process is rather slow to be applied industrially. Another disadvantage is that these microorganisms are relatively not selective in biomass; thus besides degrading hemicelluloses and lignin, they degrade cellulose (Sun and Cheng, 2002). Some microorganisms, mainly filamentous fungi, have been studied in solid-state fermentation as a pretreatment of lignocellulosic biomass and even as precursors for the production of enzymes such as cellulases for the enzymatic hydrolysis processes (Yu et al., 2016).

This category includes combinations of physical and chemical pretreatments, such as steam pretreatment with the addition of a catalyst (acid or alkaline). A typical pretreatment in this category is the ammonia fiber explosion (AFEX), which is also alkaline, increasing the reactivity of the cellulose fraction due to the swelling of it, combined with hydrolysis of hemicelluloses and fiber disintegration. The biomass is treated with liquid ammonia for a period of 1060minutes at moderate temperatures (100C) and high pressures (30bar). Up to 2kg of ammonia is used per kilogram of dry biomass. One of the disadvantages of this pretreatment is the low solubilization of hemicelluloses and lignin (Fuentes, 2009). Like other alkaline pretreatment methods, AFEX is more efficient in agricultural waste and has no efficiency with wood due to its higher lignin content. Therefore in this case, ammonia recycled percolation pretreatment is more suitable for short-fiber wood and agricultural residues, since it is less efficient for long-fiber wood (Galbe and Zacchi, 2010). Another example of combined treatment is the use of continuous double screws together with some solvent (hot water or glycerol), without the need for strong chemical reagents or the need for high temperatures and pressures. Also, this pretreatment was efficient in the use in continuous systems, good for large-scale use, and with the potential to reduce environmental costs and impacts (Moro et al., 2017). Also, recently there has been interest in the use of other technologies to aid in the disintegration of biomass such as microwaves (Bezerra and Ragauskas, 2016; Chen et al., 2011), ionic liquids (Bian et al., 2014), and various combinations thereof (Moghaddam et al., 2014; Bahrani et al, 2015) to increase the yield of the pretreatment with the intention of improving the final product (ethanol).

The association between one or more pretreatments and the structure and composition of the biomass will determine the success of the pretreatment chosen as a facilitator of the hydrolysis process. Table 11.2 presents the main characteristics that differentiate some pretreatments. The pretreatments with diluted acid, calcium hydroxide, biological, steam explosion, and AFEX basically remove hemicelluloses, with minimal changes in lignin structure. However, the organosolve pretreatment removes lignin and hemicelluloses.

Source: Adapted from Santos, F.A., Queiroz, J.H., Colodette, J.L., Fernandes, S.A., Guimares, V.M., Rezende, S.T., 2012. Potencial da palha de cana-de-acar para produo de etanol. Qumica Nova 35 (5), 10041010.

Knowing how to evaluate each pretreatment method for the advantages and disadvantages in the case of each biomass is essential for the viability of second-generation ethanol production. For this, economic evaluations are focused not on the determination of the cost of ethanol, but on the comparison between the technological options available.

For bagasse and cane straw, some methods have proven to be adequate, but all still need to be optimized to reduce high-cost expectations. In the agroindustry, sugarcane is subjected to a physical pretreatment at the time of its production, that is, after the sugarcane milling, and becomes a heterogeneous set of particles with sizes varying between 1 and 25mm, with an average of 20mm. Owing to the reduced granulometry, the bagasse does not require grinding prior to the physicalchemical pretreatment, which represents an advantage in terms of raw material preparation cost. However, low density and low bagasse compaction pose a problem in terms of reactor feed operation, as well as the difficulty in conducting pretreatment with solids loads greater than 50% (Bonomi, 2010).

Bagasse has a high absorption capacity of liquids as well as reduced hardness. In addition, the high humidity of the bagasse from the mills (45%50%) facilitates the impregnation of this biomass with acidic and alkaline solutions. This aspect is of fundamental importance with respect to the efficiency of acidhydrolytic pretreatment, which requires adequate concentrations of hydroxyl ions (H3O+), formed from water and dissociated acid. The lack of water in the biomass would result in lower formation of hydroxyl ion, as well as reduced availability of the transport fluid inside the biomass, and consequent loss of efficiency of the hydrolytic capacity. Therefore in this case, the challenge is to determine the optimum quantity of water in the bagasse, in order to guarantee effective impregnation of the biomass, while obtaining a solids load in the reactor, mainly in steam explosion processes. Autocatalytic steam pretreatments, or in the presence of catalysts, hot water and dilute sulfuric acid are more promising methods for sugarcane (Bonomi, 2010).

There are several pretreatment projects in pilot scale, in demonstration phase, and already in commercial use aiming at the production of second-generation ethanol. However, there is no best pretreatment, that is, the most efficient for all types of lignocellulosic biomass. Each type of biomass requires a particular pretreatment method to minimize substrate degradation, maximize sugar yield and, at the same time, reduce consumption of chemical inputs and energy.

This special type of grinding uses electrochemical material removal mechanism. This process is alike to mechanical grinding uses for generation of good surface finish. However, here metallic wheel is employed in place of abrasive bonded grinding wheel. In between metal wheel and workpiece, a small gap is maintained which is submerged in electrolyte solution. Unlike conventional grinding, there is no cutting force, heat generation, distortion or stress development on the workpiece because of no contact between tool and the workpiece.

A schematic diagram of EC Grinding is shown in Fig. 5.2.39. Here, metallic wheel acts as cathode and rotates over the anodic workpiece. Both are immersed in electrolytic chamber. This process is specially utilized for producing flat surface with very good surface finish hence it requires very small gap between job and the metallic wheel. Wheel is fed downwards by controlling Z drive of the setup. Servo feed system is utilized to maintain and control the small gap between metallic wheel and job. Job is given a horizontal motion under the metallic wheel by X-Y stage for which feed rate can be controlled utilizing servo system. Rotational speed of the metallic wheel force circulates the electrolyte solution into the machining gap.

This process is mainly utilized for good flat surface generation. However, abrasive bonded metallic wheel can be employed for faster metal removal and efficient machining. In this case, micro cutting by abrasive as well as electrochemical machining take place. This process is more popular for machining as well as grinding and discussed in details in Chapter 6 under hybrid machining.

Low-temperature grinding produces compressive residual stresses and improves fatigue life. This beneficial result is due to the mechanical grinding force applied to the surface which has a similar effect to surface peening. Conversely, high-temperature grinding of hardened components produces tensile residual stresses that shorten fatigue life. Other forms of damage can also be caused. This chapter concerns the nature of thermal damage in engineering materials. Damage may include any of the following:

The material of the specimen is selected from the Nanyang marble of Henan Province. The rock is a fine-grained crystal structure with crystal grains between 0.1 and 0.3mm. The main mineral components are calcite, dolomite, and wollastonite. The material density is 2.762g/cm3. The pine ratio is 0.26 and the modulus of elasticity is 69.04GPa while the longitudinal wave velocity cd=5681.1m/s, the transverse wave velocity cs=3235.3m/s, and the Rayleigh wave speed cR=2979.8m/s. On the RMT-150B rock mechanics test machine of Henan Polytechnic University, the average static fracture toughness of the marble was measured to be 1.57MPam1/2.

First, the homogeneous stone is made into a cylinder, and then four steps of disc cutting, end grinding, mechanical slitting (herringbone grooving), and artificial precracking (straight crack) are carried out.

Fix discs fixed by self-manufacturing of polymer nylon rod materials to ensure that the grooving surfaces formed by the two cutting faces coincide. The cutting machine with the RSG-200 grinder is used to cut the center of the disc to form a herringbone groove, and the circular cutter with a grooving diameter is 100mm.

Fig. 2.60 shows the geometry of the P-CCNBD specimen. The specimen was obtained by grinding a slotted tip into a straight crack on the basis of a cracked chevron notched Brazilian disc (CCNBD). B is the thickness of the disc, R is the radius of the disc, D is the diameter of the disc, and the slotted round tool has a diameter of 100mm. a is the initial grooving length, a1 is the maximum grooving length, and ap is the precrack length. The geometric parameters of the specimens are shown in Table 2.13.

The CaSiO3 used in the present study was prepared from a homogeneous mixture of SiO2 and CaO in a stoichiometric ratio by mechanical grinding in a wet state. The mixture obtained by mechanical grinding was calcined in air at 523K for ten hours. The characterization of prepared CaSiO3 was carried out by SEM, XRD, TG-DTS and N2 adsorption for measuring pore size distribution and BET surface area.

Pt supported catalysts were prepared by impregnation using H2PtCl6 solution. Ni supported catalysts also were prepared by impregnation using Ni(NO3)2 solution. Besides the impregnation, an ion exchange method was applied to prepare Ni supported CaSiO3. Moreover, in addition, CaSiO3, SiO2 (Aerosil 300), -Al2O3 (Aerosil C) and MgO (Konoshima HP-30) were used as supports for preparing reference catalysts.

Each of the prepared supported metal catalysts (40mg) was pressed into self-supporting pellet of 20mm in diameter, and was placed in an infrared cell, which was connected to an iso-volumetric system equipped with a vacuum line. The pressure of this system can be measured to the order of 1103 Torr by a capacitance manometer. The catalysts were pretreated with O2 at 723K for ten hours and then reduced under H2 at 723K for ten hours, followed by evacuation at the same temperature for one hour before use. Adsorption of CO was carried out at 298K.

Infrared spectra were recorded by an FT-IR equipped with an MCT detector. The spectra were obtained by 100 scans at 4cm1 resolution. The spectra of the adsorbed CO were obtained from the ration of the background spectra of the catalyst to those of adsorbed CO.

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Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

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