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Suggested Citation:"Appendix D: Improving Powder Production." National Research Council. 2011. Opportunities in Protection Materials Science and Technology for Future Army Applications. Washington, DC: The National Academies Press. doi: 10.17226/13157.
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Appendix D

Improving Powder Production

For commercial-scale operations, SiC and B4C powders are produced by the carbothermic reduction of a silicon oxide or boric oxide in contact with a carbon source. The resultant powder has large grains and must be comminuted to produce the micron- to submicron-sized particles required for ceramic processing. As a consequence, process-related impurities are introduced or process-induced changes occur within the particles, requiring extraordinary cleaning processes to remove impurities and a greater understanding of the changes that take place during processing.1

Aluminum nitride powder is primarily produced by carbothermal nitridation of alumina (Al2O3) in contact with carbon in a nitrogen atmosphere. Oxygen content can dramatically affect the structure of AlN, so large-scale Acheson-type furnaces cannot be employed. Typically, pusher-type furnaces are employed to provide improved control in the moving-bed furnace. Impurities condense near cold zones, which can lead to variable chemistry powders. Also, like SiC, AlN must be comminuted to achieve micron-sized powders, leading to process-related impurities that must be cleaned.2,3

Alumina is by far the most widely used ceramic powder, being a precursor to aluminum smelting. As a result, worldwide availability for commodity-grade Al2O3 has changed with the economic conditions in recent years. Across-the-board production cuts and future uncertainty have been prevalent. This has dramatically reduced the availability of low-soda, high-purity (>99.99 percent) Al2O3. Economic challenges brought many U.S. producers to the brink of bankruptcy. The impact for armor is that as production levels return, there may not be sufficient U.S. supplies to meet armor needs.4

Spinel and aluminum oxynitride (AlON) are specialty materials typically produced in very small volumes for transparent crystalline ceramics. AlON powder is not commercially available but is typically prepared by a vertically integrated ceramic producer. Common methods for forming AlON are either direct reaction of Al2O3 + AlN or reduction nitridation of Al2O3 + C + (Al or H2) in nitrogen or ammonia. The latter process is the most widely utilized, although with this process it tends to be difficult to remove all residual carbon. As with AlN and SiC, this process results in powders that must be reduced in size by comminution. Consequently, these powders must be carefully milled to avoid particulate contaminations.5

Spinel powder is produced by direct reaction of magnesium and aluminum salts that are subsequently calcined to produce the powders. Spray pyrolysis has also been used for very high purity powders. There is one source, Baikowski International Corp. (France), of commodity spinel worldwide. As a result, the cost of spinel powder is high. Variability in chemistry, particle size, and degree of aggregation has led to challenges in producing transparent ceramics.6 The current cost of spinel, at$60/kg to $80/kg, is much too high to expect widespread use for transparent armor. There is a need 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, U.K.: Chapman and Hall.

2Dunn, D., M. Paquette, H. Easter, and R. Pihlaja. Continuous carbothermal reactor. U.S. Patent 4,983,553, filed December 7, 1989, and issued January 8, 1991, to the Dow Chemical Company, Midland, Mich.

3Henley, J., G. Cochran, D. Dunn, G. Eisman, and A. Weimer. Moving bed process for carbothermally synthesizing nonoxide ceramic powders. U.S. Patent 5,370,854, filed January 8, 1993, and issued December 6, 1994, to the Dow Chemical Company, Midland, Mich.

4Moores, S. 2009. Economy crashes, alumina burns. Industrial Minerals 497: 30-37.

5Zheng, J., and B. Forslund. 1995. Carbothermal synthesis of aluminum oxynitride (AlON) powder: Influence of starting materials. Journal of the European Ceramic Society 15(11): 1087-1100.

6Bickmore, C., K. Waldner, D. Treadwell, and R. Laine. 1996. Ultrafine spinel powders by flame spray pyrolysis of a magnesium aluminum double alkoxid. Journal of the American Ceramic Society 79(5): 1419-1423.

Suggested Citation:"Appendix D: Improving Powder Production." National Research Council. 2011. Opportunities in Protection Materials Science and Technology for Future Army Applications. Washington, DC: The National Academies Press. doi: 10.17226/13157.
×

SILICON CARBIDE

Silicon carbide (SiC) is not found in any appreciable quantities in nature but is one of the most widely used synthetic technical minerals. The market for SiC focuses on its hardness and refractoriness, but SiC is also used as a source of silicon in the metallurgical processing of iron. SiC’s hardness and high-temperature stability make it as widely used as alumina as an abrasive grain. For higher-performance applications, the higher-purity (green) SiC powder is used, and for lesser requirements the lower-purity (black) SiC powder is used. For advanced ceramic applications such as armor, only the high-purity green materials are used. Other applications of high-purity SiC include space-based mirrors, semiconductor processing equipment, wire-impregnated saws for silicon wafer cutting, and automobile catalysts. These markets have driven the world supply of green SiC to more than 1 million tons per year. Armor ceramics make up less than 1 percent of the world market for high-purity SiC.7

There are many methods for producing SiC, including carbothermic reduction of silica, chemical vapor-phase reactions, and electrothermal techniques. The Acheson process, which dates from 1893, places electrodes into a graphite core laid within a mixture of reactant carbon, salt, and sand. The electric current resistively heats the graphite and in turn the surrounding reactants, resulting in the formation of a hollow cylinder of SiC and the evolution of carbon monoxide (CO) gas.8 The chemical reaction that Acheson described for the manufacture of SiC from silica sand and carbon is as follows:

SiO2 + 3C → SiC + 2CO

Within the ceramic-grade zone, both green SiC (>99 percent SiC) and black SiC (95-98 percent SiC) can be found, with metallurgical SiC (80-94 percent SiC) making up the remainder of the reaction zone. The boundary between unreacted materials and the reaction zone is marked by a layer of condensed impurities. This layer is discarded, but the unreacted precursors can be used again.

The formation of SiC is the result of four subreactions, each of which provides vapor-phase mass transport:9

C + SiO2 → SiO(g) + CO(g)
SiO2 + CO(g) → SiO + CO2(g)
C + CO2(g) → 2CO(g)
2C + SiO→ SiC + CO(g)

The exact kinetics of the reaction are highly dependent on carbon source, particle size, mixing uniformity, and packing of the silica and the carbon. During the heating of the graphite core, silica can react with carbon at temperatures as low as 1527°C to create b-SiC. At temperatures about 1900°C, the b-SiC converts to α-SiC. The various polytypes formed are dependent not only on temperature but also on the presence of impurities. For example, for α-SiC the 6H polytype is most prevalent. However, in the presence of aluminum, either intentionally or as an impurity, the 4H polytype becomes dominant. This change in polytype alters not only the shape of the resultant particles but also the microhardness, with the 4H being less hard.10

Today’s Acheson furnaces are very large. The first commercial furnace was 2 meters long and had a power input rate of 58 kW; today the largest furnace has a 240-ton capacity and a power input rate of nearly 6 MW! Aside from SiC processing being a tremendous consumer of electricity, for every pound of SiC produced, 1.4 pounds of CO are emitted. Both electricity costs and environmental concerns shifted the manufacturing of powder offshore to the extent that today, the United States accounts for less than 5 percent of the world’s production of SiC, whereas China accounts for more than 60 percent. However, that 5 percent produced in the United States supplies the abrasives and metallurgical markets, meaning that there was no supplier in 2010 providing SiC for advanced ceramics, including armor.

Work by Choi et al.11 indicated that SiC sintered with AlN and oxide additives could have a marked effect on the mechanical properties of the resulting SiC. Zhou et al.12showed the strong influence of rare-earth additions and resulting intergranular properties on the mechanical properties of SiC. Thus a better understanding of the role of intergranular phases could be used to engineer high-performance armor materials.

BORON CARBIDE

Worldwide, 1,000 to 2,000 metric tons of boron carbide are produced annually. The boron carbide market is driven by the use of boron carbide based on selected properties, such as its hardness—for example, as an abrasive grit or powder; its neutron absorption capacity (for use as control rods and shielding in pressurized water nuclear reactors, among other applications); and its specific hardness—as an armor

______________

7Moores, S. 2007. Energy prices prune SiC bloom. Industrial Minerals 475: 28-35.

8Guichelaar, P. 1977. Acheson process. Pp. 115-128 in Carbide, Nitride and Boride Materials Synthesis and Processing, A.W. Weimer, ed. London, U.K.: Chapman and Hall.

9Weimer, A., K. Nilsen, G. Cochran, and R. Roach. 1993. Kinetics of carbothermal reduction synthesis of beta silicon carbide. AIChE Journal 39(3): 493-503.

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).

11Choi, H-J., Y-W. Kim, M. Mitomo, T. Nishimura, J-H. Lee, and D-Y. Kim. 2004. Intergranular glassy phase free SiC ceramics retain strength at 1500°C. Scripta Materialia 50(9):1203-1207.

12Zhou, Y., K. Hirao, M. Toriyama, Y. Yamauchi, and S. Kanzaki. 2001. Effects of intergranular phase chemistry on the microstructure and mechanical properties of silicon carbide ceramics densified with rare-earth oxide and alumina additions. Journal of the American Ceramic Society 84(7): 1642-1644.

Suggested Citation:"Appendix D: Improving Powder Production." National Research Council. 2011. Opportunities in Protection Materials Science and Technology for Future Army Applications. Washington, DC: The National Academies Press. doi: 10.17226/13157.
×

ceramic, for example.13,14,15 As mentioned in Chapter 5 of this report, boron carbide is a solid solution containing 10 percent to 20 percent carbon. The exact chemistry of boron carbide powders depends on the particular powder synthesis route. The carbothermic reduction processes provide the largest quantities of boron carbide powders produced.16Magnesiothermic reduction and vapor-phase reactions, while producing high-quality fine-grain powders, are very expensive (>$500/kg) and are not discussed here.

Carbothermic Reduction

Boron carbide, like silicon carbide, is most commonly produced by the reduction of boron oxide (or boric acid) with carbon. The reaction is commonly written as follows:

Boric oxide: 2B2O3 + 7C → B4C + 6CO

or

Boric acid: 4H3BO3 + 7C → B4C + 6CO + 6H2O

This process occurs in two stages:

B2O3 + 3CO → 2B + 3CO2
4B + C → B4C

Carbothermic reduction of boron carbide utilizes a Higgins or an electric arc furnace. Here, a water-cooled crucible is insulated with a packed wall of the mixed boric oxide and carbon precursors. An electric arc is used to generate temperatures between approximately 2500°C and 2800°C. Mixed precursor powders are added where they slowly melt, near the highest temperature areas. Because the melt is highly viscous and evolved CO2 must be allowed to escape, materials are gradually added and the electrode height is changed. When sufficient materials have been reacted, the electrodes are withdrawn and the melt is cooled. The result is an ingot that weighs between 25 kg and 1,000 kg. The outer edges of the ingot are covered with unreacted precursor powders, which must be manually removed and are typically recycled. The ingot then undergoes a series of crushing operations, and the powder grain is milled to size. Depending on the manufacturer, metallic impurities derived from the crushing and milling equipment can be eliminated through a series of acid leaching steps.17

The carbothermic method is a very high temperature operation having large temperature variations across the crucible, and the stoichiometry of the product boron carbide is typically rich in carbon, commonly B4-xC. A few percent of essentially pure carbon is typically found in the powder, resulting from unreacted graphite, graphite originating from the electrode, decomposed B4C, or vapor-phase condensates of CO/CO2.

Direct carbothermic reduction has been demonstrated on a pilot scale, where boric oxide and carbon are reacted in a vertical tube furnace at between 1973°C and 2073°C. Although this method produces a fine-grained (0.5-5 μ) and very controlled stoichiometric boron carbide, its yield is lower than that of the arc-melted grain method and at present it is not considered a viable option.18

ALUMINA

In 1887, Bayer discovered that aluminum hydroxide precipitated from alkaline solution was crystalline and could be more easily filtered and washed than that precipitated from acid medium. The process was a key to the development of modern metallurgy, since aluminum hydroxide is the raw material for the electrolytic aluminum process that was invented in 1886. The process that Bayer invented has remained essentially the same and produces nearly all of the world’s alumina as an intermediate in aluminum production. The Bayer process can be considered in three stages: (1) extraction, (2) precipitation, and (3) calcination.

The aluminum-bearing minerals in bauxite are dissolved in a solution of sodium hydroxide (caustic soda) to selectively extract them from the insoluble components (mostly oxides). Then the ore is milled to make the minerals more available for extraction and to reduce the particle size. It is then combined with the process liquor in a heated pressure digester. Temperature and pressure within the digester reflect the type of ore. Temperatures vary between 140°C and 240°C and pressures vary up to 35 atm. After the aluminum-containing components dissolve, the insoluble residue is separated from the liquor by settling.

Crystalline aluminium trihydroxide (ATH) is then precipitated from the digestion liquor:

Al(OH)4 + Na+ → Al(OH)3 + Na+ + OH

The ATH crystals are then classified into size fractions and fed into a rotary kiln at temperatures greater than 1050°C

______________

13Lipp, A., Pacific Northwest Laboratory, U.S. Atomic Energy Commission. 1970. Boron carbide: Production, properties, applications. Richland, Wash.: Battelle Northwest Laboratories.

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. Ullmann’s Encyclopedia of Industrial Chemistry. DOI: 10.1002/14356007. a04_295.

16Suri, A., C. Subramanian, J. Sonber, and T. Murthy. 2010. Synthesis and consolidation of boron carbide: A review. International Materials Review 55(1): 4-40.

17Scott, J. 1964. Arc furnace process for the production of boron carbide. U.S. Patent 3,161,471, filed February 25, 1958, and issued December 15, 1964, to Norton Company, Worcester, Mass.

18Rafaniello, W., and W. Moore. 1989. Producing boron carbide. U.S. Patent 4,804,525, filed July 14, 1987, and issued February 14, 1989, to the Dow Chemical Company, Midland, Mich.

Suggested Citation:"Appendix D: Improving Powder Production." National Research Council. 2011. Opportunities in Protection Materials Science and Technology for Future Army Applications. Washington, DC: The National Academies Press. doi: 10.17226/13157.
×

for calcination. The ATH is calcined to form alumina, which can be directly used for aluminum processing or can be used for ceramic applications:

2Al(OH)3 → Al2O3 + 3H2O

If the ATH is to be used for ceramics, it can undergo multiple washing steps to reduce the ionic sodium to less than 0.01 percent. The particle size of the calcined powder is reduced in size, depending on specifications determined by the end user.

Suggested Citation:"Appendix D: Improving Powder Production." National Research Council. 2011. Opportunities in Protection Materials Science and Technology for Future Army Applications. Washington, DC: The National Academies Press. doi: 10.17226/13157.
×
Page 121
Suggested Citation:"Appendix D: Improving Powder Production." National Research Council. 2011. Opportunities in Protection Materials Science and Technology for Future Army Applications. Washington, DC: The National Academies Press. doi: 10.17226/13157.
×
Page 122
Suggested Citation:"Appendix D: Improving Powder Production." National Research Council. 2011. Opportunities in Protection Materials Science and Technology for Future Army Applications. Washington, DC: The National Academies Press. doi: 10.17226/13157.
×
Page 123
Suggested Citation:"Appendix D: Improving Powder Production." National Research Council. 2011. Opportunities in Protection Materials Science and Technology for Future Army Applications. Washington, DC: The National Academies Press. doi: 10.17226/13157.
×
Page 124
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Armor plays a significant role in the protection of warriors. During the course of history, the introduction of new materials and improvements in the materials already used to construct armor has led to better protection and a reduction in the weight of the armor. But even with such advances in materials, the weight of the armor required to manage threats of ever-increasing destructive capability presents a huge challenge.

Opportunities in Protection Materials Science and Technology for Future Army Applications explores the current theoretical and experimental understanding of the key issues surrounding protection materials, identifies the major challenges and technical gaps for developing the future generation of lightweight protection materials, and recommends a path forward for their development. It examines multiscale shockwave energy transfer mechanisms and experimental approaches for their characterization over short timescales, as well as multiscale modeling techniques to predict mechanisms for dissipating energy. The report also considers exemplary threats and design philosophy for the three key applications of armor systems: (1) personnel protection, including body armor and helmets, (2) vehicle armor, and (3) transparent armor.

Opportunities in Protection Materials Science and Technology for Future Army Applications recommends that the Department of Defense (DoD) establish a defense initiative for protection materials by design (PMD), with associated funding lines for basic and applied research. The PMD initiative should include a combination of computational, experimental, and materials testing, characterization, and processing research conducted by government, industry, and academia.

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