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OCR for page 121
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.
