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Appendix D Improving Powder Production challenges brought many U.S. producers to the brink of For commercial-scale operations, SiC and B4C powders bankruptcy. The impact for armor is that as production lev- are produced by the carbothermic reduction of a silicon oxide els return, there may not be sufficient U.S. supplies to meet or boric oxide in contact with a carbon source. The resultant armor needs.4 powder has large grains and must be comminuted to produce Spinel and aluminum oxynitride (AlON) are specialty the micron- to submicron-sized particles required for ceramic materials typically produced in very small volumes for processing. As a consequence, process-related impurities are transparent crystalline ceramics. AlON powder is not com- introduced or process-induced changes occur within the par- mercially available but is typically prepared by a vertically ticles, requiring extraordinary cleaning processes to remove integrated ceramic producer. Common methods for forming impurities and a greater understanding of the changes that take place during processing.1 AlON are either direct reaction of Al2O3 + AlN or reduction nitridation of Al2O3 + C + (Al or H2) in nitrogen or ammonia. Aluminum nitride powder is primarily produced by The latter process is the most widely utilized, although with carbothermal nitridation of alumina (Al2O3) in contact with this process it tends to be difficult to remove all residual carbon in a nitrogen atmosphere. Oxygen content can dra- carbon. As with AlN and SiC, this process results in powders matically affect the structure of AlN, so large-scale Acheson- that must be reduced in size by comminution. Consequently, type furnaces cannot be employed. Typically, pusher-type these powders must be carefully milled to avoid particulate furnaces are employed to provide improved control in the contaminations.5 moving-bed furnace. Impurities condense near cold zones, Spinel powder is produced by direct reaction of magne- which can lead to variable chemistry powders. Also, like SiC, sium and aluminum salts that are subsequently calcined to AlN must be comminuted to achieve micron-sized powders, leading to process-related impurities that must be cleaned.2,3 produce the powders. Spray pyrolysis has also been used for very high purity powders. There is one source, Baikowski In- Alumina is by far the most widely used ceramic powder, ternational Corp. (France), of commodity spinel worldwide. being a precursor to aluminum smelting. As a result, world- As a result, the cost of spinel powder is high. Variability in wide availability for commodity-grade Al2O3 has changed chemistry, particle size, and degree of aggregation has led with the economic conditions in recent years. Across-the- to challenges in producing transparent ceramics.6 The cur- board production cuts and future uncertainty have been rent cost of spinel, at $60/kg to $80/kg, is much too high to prevalent. This has dramatically reduced the availability of expect widespread use for transparent armor. There is a need low-soda, high-purity (>99.99 percent) Al2O3. Economic for research to be conducted to determine whether a more affordable, uniform, ceramic-grade powder can be produced. 1Guichelaar, P. 1977. Acheson process. Pp. 115-128 in Carbide, Nitride and Boride Materials Synthesis and Processing, A.W. Weimer, ed. London, 4Moores, S. 2009. Economy crashes, alumina burns. Industrial Minerals U.K.: Chapman and Hall. 2Dunn, D., M. Paquette, H. Easter, and R. Pihlaja. Continuous carbo- 497: 30-37. 5Zheng, J., and B. Forslund. 1995. Carbothermal synthesis of aluminum thermal reactor. U.S. Patent 4,983,553, filed December 7, 1989, and issued January 8, 1991, to the Dow Chemical Company, Midland, Mich. oxynitride (AlON) powder: Influence of starting materials. Journal of the 3Henley, J., G. Cochran, D. Dunn, G. Eisman, and A. Weimer. Moving European Ceramic Society 15(11): 1087-1100. 6Bickmore, C., K. Waldner, D. Treadwell, and R. Laine. 1996. Ultrafine bed process for carbothermally synthesizing nonoxide ceramic powders. U.S. Patent 5,370,854, filed January 8, 1993, and issued December 6, 1994, spinel powders by flame spray pyrolysis of a magnesium aluminum double to the Dow Chemical Company, Midland, Mich. alkoxid. Journal of the American Ceramic Society 79(5): 1419-1423. 121
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122 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS SILICON CARBIDE The exact kinetics of the reaction are highly dependent on carbon source, particle size, mixing uniformity, and pack- Silicon carbide (SiC) is not found in any appreciable ing of the silica and the carbon. During the heating of the quantities in nature but is one of the most widely used syn- graphite core, silica can react with carbon at temperatures thetic technical minerals. The market for SiC focuses on its as low as 1527°C to create b-SiC. At temperatures about hardness and refractoriness, but SiC is also used as a source 1900°C, the b-SiC converts to α-SiC. The various polytypes of silicon in the metallurgical processing of iron. SiC’s hard- formed are dependent not only on temperature but also on ness and high-temperature stability make it as widely used as the presence of impurities. For example, for α-SiC the 6H alumina as an abrasive grain. For higher-performance appli- polytype is most prevalent. However, in the presence of cations, the higher-purity (green) SiC powder is used, and for aluminum, either intentionally or as an impurity, the 4H lesser requirements the lower-purity (black) SiC powder is polytype becomes dominant. This change in polytype alters used. For advanced ceramic applications such as armor, only not only the shape of the resultant particles but also the mi- the high-purity green materials are used. Other applications crohardness, with the 4H being less hard.10 of high-purity SiC include space-based mirrors, semiconduc- Today’s Acheson furnaces are very large. The first tor processing equipment, wire-impregnated saws for silicon commercial furnace was 2 meters long and had a power wafer cutting, and automobile catalysts. These markets have input rate of 58 kW; today the largest furnace has a 240-ton driven the world supply of green SiC to more than 1 million capacity and a power input rate of nearly 6 MW! Aside from tons per year. Armor ceramics make up less than 1 percent SiC processing being a tremendous consumer of electricity, of the world market for high-purity SiC.7 for every pound of SiC produced, 1.4 pounds of CO are There are many methods for producing SiC, including emitted. Both electricity costs and environmental concerns carbothermic reduction of silica, chemical vapor-phase reac- shifted the manufacturing of powder offshore to the extent tions, and electrothermal techniques. The Acheson process, that today, the United States accounts for less than 5 percent which dates from 1893, places electrodes into a graphite core of the world’s production of SiC, whereas China accounts laid within a mixture of reactant carbon, salt, and sand. The for more than 60 percent. However, that 5 percent produced electric current resistively heats the graphite and in turn the in the United States supplies the abrasives and metallurgical surrounding reactants, resulting in the formation of a hollow markets, meaning that there was no supplier in 2010 provid- cylinder of SiC and the evolution of carbon monoxide (CO) ing SiC for advanced ceramics, including armor. gas.8 The chemical reaction that Acheson described for the Work by Choi et al.11 indicated that SiC sintered with manufacture of SiC from silica sand and carbon is as follows: AlN and oxide additives could have a marked effect on the mechanical properties of the resulting SiC. Zhou et al.12 SiO2 + 3C → SiC + 2CO showed the strong influence of rare-earth additions and resulting intergranular properties on the mechanical proper- Within the ceramic-grade zone, both green SiC (>99 ties of SiC. Thus a better understanding of the role of inter- percent SiC) and black SiC (95-98 percent SiC) can be granular phases could be used to engineer high-performance found, with metallurgical SiC (80-94 percent SiC) making armor materials. up the remainder of the reaction zone. The boundary between unreacted materials and the reaction zone is marked by a BORON CARBIDE layer of condensed impurities. This layer is discarded, but the unreacted precursors can be used again. Worldwide, 1,000 to 2,000 metric tons of boron carbide The formation of SiC is the result of four subreactions, are produced annually. The boron carbide market is driven by each of which provides vapor-phase mass transport:9 the use of boron carbide based on selected properties, such as its hardness—for example, as an abrasive grit or pow- C + SiO2 → SiO(g) + CO(g) der; its neutron absorption capacity (for use as control rods SiO2 + CO(g) → SiO + CO2(g) and shielding in pressurized water nuclear reactors, among C + CO2(g) → 2CO(g) other applications); and its specific hardness—as an armor 2C + SiO → SiC + CO(g) 10Poch, W., and A. Dietzel. 1962. Formation of silicon carbide from silica and carbon. Berichte der Deutschen Keramischen Gesellschaft 39(8): 413-426 (in German). 7Moores, 11Choi, H-J., Y-W. Kim, M. Mitomo, T. Nishimura, J-H. Lee, and D-Y. S. 2007. Energy prices prune SiC bloom. Industrial Minerals 475: 28-35. Kim. 2004. Intergranular glassy phase free SiC ceramics retain strength at 8Guichelaar, P. 1977. Acheson process. Pp. 115-128 in Carbide, Nitride 1500°C. Scripta Materialia 50(9): 1203-1207. 12Zhou, Y., K. Hirao, M. Toriyama, Y. Yamauchi, and S. Kanzaki. 2001. and Boride Materials Synthesis and Processing, A.W. Weimer, ed. London, U.K.: Chapman and Hall. Effects of intergranular phase chemistry on the microstructure and mechani- 9Weimer, A., K. Nilsen, G. Cochran, and R. Roach. 1993. Kinetics of cal properties of silicon carbide ceramics densified with rare-earth oxide carbothermal reduction synthesis of beta silicon carbide. AIChE Journal and alumina additions. Journal of the American Ceramic Society 84(7): 39(3): 493-503. 1642-1644.
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123 APPENDIX D ceramic, for example.13,14,15 As mentioned in Chapter 5 of and milling equipment can be eliminated through a series of acid leaching steps.17 this report, boron carbide is a solid solution containing 10 percent to 20 percent carbon. The exact chemistry of boron The carbothermic method is a very high temperature carbide powders depends on the particular powder synthesis operation having large temperature variations across the route. The carbothermic reduction processes provide the crucible, and the stoichiometry of the product boron carbide largest quantities of boron carbide powders produced. 16 is typically rich in carbon, commonly B4-xC. A few percent Magnesiothermic reduction and vapor-phase reactions, while of essentially pure carbon is typically found in the powder, producing high-quality fine-grain powders, are very expen- resulting from unreacted graphite, graphite originating from sive (>$500/kg) and are not discussed here. the electrode, decomposed B4C, or vapor-phase condensates of CO/CO2. Direct carbothermic reduction has been demonstrated Carbothermic Reduction on a pilot scale, where boric oxide and carbon are reacted Boron carbide, like silicon carbide, is most commonly in a vertical tube furnace at between 1973°C and 2073°C. produced by the reduction of boron oxide (or boric acid) Although this method produces a fine-grained (0.5-5 μ) and with carbon. The reaction is commonly written as follows: very controlled stoichiometric boron carbide, its yield is lower than that of the arc-melted grain method and at present Boric oxide: 2B2O3 + 7C → B4C + 6CO it is not considered a viable option.18 or Boric acid: 4H3BO3 + 7C → B4C + 6CO + 6H2O ALUMINA This process occurs in two stages: In 1887, Bayer discovered that aluminum hydroxide precipitated from alkaline solution was crystalline and could B2O3 + 3CO → 2B + 3CO2 be more easily filtered and washed than that precipitated 4B + C → B4C from acid medium. The process was a key to the develop- ment of modern metallurgy, since aluminum hydroxide is Carbothermic reduction of boron carbide utilizes a Hig- the raw material for the electrolytic aluminum process that gins or an electric arc furnace. Here, a water-cooled crucible was invented in 1886. The process that Bayer invented has is insulated with a packed wall of the mixed boric oxide and remained essentially the same and produces nearly all of the carbon precursors. An electric arc is used to generate temper- world’s alumina as an intermediate in aluminum production. atures between approximately 2500°C and 2800°C. Mixed The Bayer process can be considered in three stages: (1) precursor powders are added where they slowly melt, near extraction, (2) precipitation, and (3) calcination. the highest temperature areas. Because the melt is highly The aluminum-bearing minerals in bauxite are dissolved viscous and evolved CO2 must be allowed to escape, materi- in a solution of sodium hydroxide (caustic soda) to selec- als are gradually added and the electrode height is changed. tively extract them from the insoluble components (mostly When sufficient materials have been reacted, the electrodes oxides). Then the ore is milled to make the minerals more are withdrawn and the melt is cooled. The result is an ingot available for extraction and to reduce the particle size. It is that weighs between 25 kg and 1,000 kg. The outer edges then combined with the process liquor in a heated pressure of the ingot are covered with unreacted precursor powders, digester. Temperature and pressure within the digester re- which must be manually removed and are typically recycled. flect the type of ore. Temperatures vary between 140°C and The ingot then undergoes a series of crushing operations, 240°C and pressures vary up to 35 atm. After the aluminum- and the powder grain is milled to size. Depending on the containing components dissolve, the insoluble residue is manufacturer, metallic impurities derived from the crushing separated from the liquor by settling. Crystalline aluminium trihydroxide (ATH) is then pre- cipitated from the digestion liquor: Al(OH)4 + Na+ → Al(OH)3 + Na+ + OH 13Lipp, A.,Pacific Northwest Laboratory, U.S. Atomic Energy Commis- sion. 1970. Boron carbide: Production, properties, applications. Richland, The ATH crystals are then classified into size fractions Wash.: Battelle Northwest Laboratories. and fed into a rotary kiln at temperatures greater than 1050°C 14Thévenot, F. 1990. Boron carbide—A comprehensive review. Journal of the European Ceramic Society 6(4): 205-255. 15Schwetz, K. 2000. Boron-carbide, boron nitride, and metal borides. 17Scott, J. 1964. Arc furnace process for the production of boron carbide. Ullmann’s Encyclopedia of Industrial Chemistry. DOI: 10.1002/14356007. U.S. Patent 3,161,471, filed February 25, 1958, and issued December 15, a04_295. 1964, to Norton Company, Worcester, Mass. 16Suri, A., C. Subramanian, J. Sonber, and T. Murthy. 2010. Synthesis and 18Rafaniello, W., and W. Moore. 1989. Producing boron carbide. U.S. consolidation of boron carbide: A review. International Materials Review Patent 4,804,525, filed July 14, 1987, and issued February 14, 1989, to the 55(1): 4-40. Dow Chemical Company, Midland, Mich.
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124 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS If the ATH is to be used for ceramics, it can undergo for calcination. The ATH is calcined to form alumina, which multiple washing steps to reduce the ionic sodium to less can be directly used for aluminum processing or can be used than 0.01 percent. The particle size of the calcined powder for ceramic applications: is reduced in size, depending on specifications determined 2Al(OH)3 → Al2O3 + 3H2O by the end user.