SYNTHESIS AND PROCESSING
2.04 Processing of Silicon Carbide-Based Ceramics
2.04.3 SiC Raw Materials Production
Although natural SiC with the mineral name Moissanite exists (Moissan, 1904), raw SiC is synthesized since there are no commercially recoverable deposits anywhere in the world. Some small- and large-scale synthesis concepts are described in the next section. Normally ceramics are consolidated through a sintering process. If highly densified materials are required, the corresponding raw materials batch should containdbesides special sintering aidsda fine-grained base powder. This especially applies to a nonreactive, covalent bonded com- pound like SiC. For special applications like refractories or grinding tools, the SiC particles may also be coarse grained. Thus SiC synthesis includes grinding and screening, which is also treated in the next section.
Generally SiC-based ceramics are extremely brittle if the SiC crystals within the ceramics are fairly equiaxial. However, the brittleness can be reduced if the grains are elongated or at least exhibit a large aspect ratio. Thus, the toughness of the ceramics might be improved through reinforcement using flattened or elongated particles like platelets, whiskers orfibers. The synthesis of SiC particles offering large aspect ratios is discussed below too.
2.04.3.1 SiC Synthesis, Grinding and Classification of SiC Powders 2.04.3.1.1 Acheson Process
To date large-scale production of SiC is based upon the historic Acheson process, which was applied for patent more than a century ago (Acheson, 1893). The inventor Edward Goodrich Acheson can be regarded as thefirst prominent pioneer of SiC-based ceramics. In his process, a mixture of pure quartz sand (>99.5% SiO2) and petrol coke poor in ashes (<0.2% ashes) is heated up to about 2500C in a batch resistance furnace to generate SiC according to the overall reaction,
SiO2þ3C 4 SiCþ2CO (1)
The grain sizes of the two starting components are comparatively coarse (up to 10 mm) with a narrow size distribution for a loose packing density in order to let CO leave the charge.
Because the process involves many partial reactions, the actual kinetics is much more complicated than the carbothermal reduction of silica shown inEqn (1). There are two different design versions of Acheson Figure 2 Possible positions of atoms in a single-layered close-pack lattice.
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Figure 3 Stacking sequence of Si–C bilayers forming different polytypes seen from½1120. Tairov, Y. M, Vodakov, Y. A. (1977). Group IV materials (mainly SiC). In J. I. Pankove (Ed.)Topics of applied physics, Vol. 17, Electroluminescence. Berlin, Heidelberg, New York:
Springer-Verlag, pp. 31–61, with permission from Springer ScienceþBusiness Media.
Processing of Silicon Carbide-Based Ceramics 93
furnace today. The ordinary design resembles the old Acheson patent application to a large extent, whereas the ESK-type construction has got some useful technical changes, and some of them are explained below.
The ordinary Acheson furnaces are lined up side-by-side in a long hall. A typical Acheson furnace has a rectangular cross-section with a length of about 25 m, a width of 4 m and a height of 4 m, with partly movable narrow sidewalls. The carbon electrodes for the current feed-through are installed in the center of refractory furnace heads. For efficient electrical installation, a single transformer is simultaneously connected to several furnaces in such a way that one furnace is in an operating state while the others are cooling down, dismounted or being newly fed.
SiC develops as a solid cylindrical ingot around the graphite resistance core, which is usually called the“roll”. In the center, where the maximum temperature (2500C) is reached, the crystal growth is quite considerable.
According to the temperature drop, the crystal sizes are scaled down with increasing distances from the core.
After the power is turned off, the furnace cools down for several days. Thereafter the sidewalls are removed and the cylinder is excavated from the nonconverted component mixture. The cylinder contains not only the primary resistance core but also an additional graphite internal ring originated from the dissociation of SiC.
After removing the graphite the inner part of the cylinder contains coarsea-SiC crystals whereas in the outer part finer SiC crystals of mainly b-type are dominant. The SiC crude material is carefully crushed, classified, sometimes ground again and optionally chemically treated in order to obtain the specific properties based on application.
The conventional design of the Acheson process has disadvantages. First is the fact that the reaction gases, which consist essentially of CO but also of H2S, cannot be captured completely and are lost for an energy gain.
The second drawback derives from the problem that these gases pollute the environment. These disadvantages can be reduced through the modified ESK version of the Acheson process (Figure 4) (Benecke, Korsten, Petersen,
Figure 4 Principle of SiC synthesis according to the ESK process: (a) longitudinal and (b) transverse sections. From ESK SiC GmbH, Frechen, Germany.
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& Wiebke, 1976; Wiebke, Korndörfer, Korsten, Benecke, & Petersen, 1976). Only the resistance core is still horizontally located. The current is supplied viafloor electrodes to the resistance core. The complete furnace unit is located outdoors. The bulk mixture of sand and coke is covered with a huge plastic sheet. The reaction gases are collected under the sheet, which will be blown up. CO will be exhausted partly through the porous bed of the floor directly underneath the sheet. The gases will be desulfurized and finally combusted in a power station to be transferred to electrical energy. The latter contributes to the electric power that is necessary for the synthesis of SiC.
The reaction time of the ESK process takes about a whole week. Another week is needed to cool the batch down through water spraying. The SiC roll is uncovered from the unreacted part of the batch. The length of the SiC roll is usually 60 m and the weight is about 300 tons. Because of space saving, the roll is generally U-shaped (Petersen & Korsten, 1979) as shown inFigure 5.
After cooling, the roll is broken into pieces and classified. Classification is necessary since only near the core pure largea-SiC crystals form. The outer area contains a lower grade (90–95% purity)fine-grainedb-SiC which is recycled again for the next run or applied as a dopent for the iron and steel metallurgy.
The subsequent preparation route depends upon the application. Targeted grain characteristics are defined:
color (green/black), shape (compact/splintery), mean-size and size distribution. In all cases, the route includes fracturing, ball milling, attrition milling, sieving, screening and elutriating. Grain sizes and their distributions for abrasives are defined in Europe according to the Fédération Européenne des Fabricants de Produits Abrasifs standard. Commercial SiC powders vary in terms of color between green and black. The purest commercial SiC grade (99.5% purity) has a light green color and the crystals are translucent. The usage of very pure quartz sand as a starting material is necessary to obtain such a pure grade. The sand for the ordinary nontransparent black grade with purity of only 98.5% contains some impurities, especially aluminum. The black color is due to the presence of elementary carbon. For some applications the SiC powders are further purified. Adherent impurities like elementary silicon, metals (in particular iron), metal compounds, graphite and silica can be removed chemically.
Even today, Acheson process variants represent the main source of SiC raw material for all applications; it should be noted that there are a few drawbacks concerning the synthesis of SiC using these routes. First, extremely high temperatures (above 2300C) are necessary for a complete reaction. Such high temperatures give rise to exaggerated grain growth. Furthermore, the large crystals are sintered together. Thus lumps of SiC have to be ground through elaborated milling and purification steps. Hence, roughly speaking the route via the Acheson process for the requirement offine SiC powders is usually involved with the input of excess energy in two respects,first the thermal excess energy for the formation of large crystals during carbothermal synthesis and second the mechanical excess energy for the pulverization of the large crystals after cooling down. Another Figure 5 U-shaped SiC roll. From ESK SiC GmbH, Frechen, Germany.
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drawback can be derived from the batch process itself that implies a distinct temperature gradient within the accumulation during thefiring cycle, resulting in a property gradient.
Hence, through the years many intensive attempts have been made to overcome these drawbacks while maintaining the advantageous carbothermal reduction of silica. Most approaches target lowering of the tem- perature level in order to reduce the grain size, which mostly leads tob-SiC. It should be noted that in some casesb-SiC powders as a starting material for some ceramics could cause some problems during the sintering process, as it will be shown below.
2.04.3.1.2 Sugar Solution Method
In a method developed by General Electric Company (Prener, 1963), first a silica gel in a sugar solution is generated through hydrolysis of SiCl4. Subsequently the gel is dehydrated at 300C in order to decompose the sugar and to achieve a thorough mixture of silica and amorphous carbon. Finally the mixture is heated in argon at 1800C to generateb-SiC powder. Thisfine powder wasfirst provided for the semiconductor and pigment industry. Later this method was modified for the production of submicronb-SiC powders (Prochazka, 1972;
Schwetz & Lipp, 1978) through the usage of combination of either tetraethyl silicate/sugar or pyrogenic silica/
sugar, which could be produced at lower temperatures.
2.04.3.1.3 Pyrolysis of Rice Hulls
Fineb-SiC powders with grain sizes around 0.1mm (without grinding) is possible if rice hulls are pyrolyzed between 1290 and 1600C (Lee & Cutler, 1975). Rice hulls contain both cellulose, which may provide carbon for the SiC synthesis, and amorphous silica. Hence the corresponding SiC production can be regarded as a carbothermal synthesis as well. The reaction temperature is extremely low since on the one hand, both components are homogeneously mixed and on the other hand, the uniformly distributed iron acts as catalyst.
2.04.3.1.4 Fluidized Bed Technique
A continuous production of mainlyb-SiC powder was developed by the Superia Graphite Company (Gold- berger & Reed, 1984, 1985; Goldberger, 1985). The synthesis of SiC occurs using an electrically heatedfluidized bed technique at temperatures between 1500 and 1600C. The discharged fractions have grain sizes smaller than 3 mm, which are polycrystalline and contain elementary carbon. The material can easily be ground and the elementary carbon removed.
2.04.3.1.5 Rotary Furnace Process
Fineb-SiC powder is continuously produced through carbothermal reduction of silica in a rotary furnace at temperatures between 1600 and 1700C (van Dijen & Metselaar, 1989; Wei, 1983). The rotary tube consists of graphite.
2.04.3.1.6 Elementary Synthesis
A quite different approach is the elementary synthesis of SiC according to the reaction:
Siðl;gÞþCðsÞ 4 SiCðsÞ (2) Prochazka (1972)generatedb-SiC with a specific surface area of 7 m2g1in the temperature range between 1500 and 1600C through this method. For his investigations he used extremely pure silicon and carbon as starting materials. He tried to apply this powder for sintering experiments, but the sintering activity of the generated powder proved to be only moderate. He attributed this low activity to the presence elementary silicon, which inhibits sintering.
2.04.3.1.7 Vapor-Phase Synthesis
The preparation of veryfine powders without using grinding techniques can be successfully achieved by the vapor-phase synthesis techniques: chemical vapor deposition (CVD) or physical vapor deposition (PVD). In principle, b-SiC powders with spherical uniform particles can be obtained using both routes. CVD uses chemical reactions of metals, metalloids or the corresponding compounds, whereas PVD uses evapo- ration–condensation of solids with similar compositions. The usage of CVD technique is more popular since its production rate as a rule is higher than PVD. Additionally, because in the former often the reactants have high purities or may be purified easily, the powders generated from the gas phase generally exhibit low- 96 Processing of Silicon Carbide-Based Ceramics
impurity concentrations. Sometimes it is even possible to integrate a little quantity of sintering aids into the batch (Chen, Goto, & Hirai, 1996) which are then homogeneously distributed. Another advantage of CVD is derived from short reaction times. On the other hand, it should be noted that there are a few severe problems concerning the use of vapor-phase synthesis techniques. For instance, it is difficult to complete the reaction among the starting components, thus the products often contain unreacted fractions. It can also be supposed that the transference from a laboratory scale fabrication to large-scale production takes a long development time. This may be the reason that large-scale production of vapor-phase synthesis techniques for SiC powders does not exist so far.
The CVD technique (Section 2.04.4.3) for the synthesis of SiC powders includes several variations depending on the source for chemical reactions: heated tube furnace (Suyama & Yamaguchi, 1991), d.c. arc plasma (Baumgartner & Rossing, 1989), high-frequency plasma (Stroke, 1981) or laser (Cannon, Danforth, Flint, Haggerty, & Marra, 1982; Suyama, Marra, Haggerty, & Bowen, 1985). Either one or two precursor gases, with hydrogen as the carrier one, are commonly used for the deposition, for instance:
CH3SiCl3ðgÞ 0H2 SiCðsÞþ3HClðgÞ (3)
SiCl4ðgÞþCH4ðgÞ 0H2 SiCðsÞþ4HClðgÞ (4) As a summary, all mentioned SiC synthesis versions are compiled inFigure 6. As noted above, both Acheson variants mainly producea-SiC, whereas all others generateb-SiC. Though presently there exist two continuously running SiC synthesis processes (fluidized bed technique and the rotary furnace method), large-scale produc- tion has only been realized through the two batched Acheson techniques.
2.04.3.2 Whiskers, Platelets and Fibers of SiC
The brittleness of ceramic materials is often a big problem in respect of their use in structural applications.
Monolithic SiC materials are inherently brittle ceramics. Hence in the last decades one of the most important domains in ceramic research programs has been toughening of ceramic materials, and this is particularly true for
Figure 6 SiC synthesis variants.
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high-strength SiC ceramics. One successful approach has been the ex situ incorporation of particles exhibiting large aspect ratios like whiskers, platelets and fibers into ceramic microstructures. The basic idea of this development has been the potentiality that mechanisms which absorb energy are generated through the reinforcement of those elongated orflattened particles into a matrix with isotropic microstructure. The increase of crack growth resistance as it extends is achieved through energy dissipation processes like deflection, branching and bridging of microcracks as well as delamination and pull-out effects. All these mechanisms have positive effects on crack propagation resistance and damage tolerance of the ceramic composites. The attributes of SiC-type nonisometric particles are both high-specific Young’s modulus (i.e. ratio between stiffness and density) and good high-temperature properties. It should be noted that these particles have been not only used as reinforcing components for SiC-based ceramics or other ceramics but also successfully incorporated into metals.
2.04.3.2.1 SiC Whiskers
Initially the research projects had been focused on the capabilities of reinforcement through SiC whiskers, which are monocrystalline trichoid short fibers with diameters of 0.1–5mm, lengths of 5–100mm; and thus, aspect ratios ranging between 20 and 100, respectively (Figure 7). The Young’s modulus and the tensile strength values reach high levels, up to 580 and 16 GPa, respectively. But the main reason for the early interest in those reinforcing components had been the idea of being able to handle them by using simple conventional powder preparation techniques. However, a severe health risk was later identified through medical investigations, especially if the diameter of the whiskers is smaller than 3mm. Thus both the preparation and the subsequent treatment of the whiskers require in most cases strict safety-related measures.
SiC whiskers can be fabricated using three methods (Shaffer, 1994):
l Vapor–solid (VS) reaction
l Evaporation–condensation or vapor condensation (VC) l Vapor–liquid–solid (VLS) reaction.
The most cost-efficient and therefore the prevalent method of producing SiC whiskers is based on the carbo- thermal reduction of silica. The maximum reaction temperature range for the synthesis of these VS-b-SiC whiskers varies between 1500 and 1700C. There are a few processing versions, which vary in the raw materials or in the kind of catalyst. It should be mentioned that in nearly any case, a catalyst must be used otherwise only globular particulates but no whiskers would be generated. The only known exception is the formation ofb-SiC whiskers through the pyrolysis of rice hulls under reduced pressure of N2(Raju & Verma, 1997). The porous cellular structure of the rice hulls seem to act favorably for whisker growth.
If loosely packed, SiC powders are evaporated at temperatures above 2200C at low gas pressures, a-SiC whiskers are generated during the cooling down period. The formation mechanism of this whisker generation,
Figure 7 b-SiC whiskers. This photo is kindly released by Advanced Composite Materials, SC, USA.
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which is referred to as VC method, seems to resemble the mechanisms of both the introductorily mentioned bulk SiC single crystals and the below treated recrystallized SiC (RSiC) which are also generated through an evaporation–condensation process.
VLS method is a mechanism for the growth of whiskers from CVD. Generally the growth rate of a crystal through direct adsorption of a gas phase on a solid surface is comparatively low. Through VLS this problem is avoided by placing a catalytic liquid alloy phase being able to adsorb vapor to supersaturation levels on the surface of a substrate. Thus crystal growth occurs from the nucleated seed at the liquid–solid interface. The VLS mechanism wasfirst proposed to explain the growth of silicon whiskers in the gas phase in the presence of a liquid gold droplet located upon a silicon substrate (Wagner & Ellis, 1964). For the generation of SiC whiskers, molten transition metals and iron alloys fulfill the requirement of catalyst (Milewski, Gac, Petrovic,
& Skaggs, 1985; Shyne & Milewski, 1971).Figure 8illustrates the VLS growth of b-SiC whiskers. First metal microparticulates (usually iron) are distributed on the carbon surface of the substrates. At about 1400C the solid particulate melts and forms liquid catalyst droplets. Both carbon and silicon from vapor feeds, CH4þH2 and SiO, are dissolved in the liquid catalyst, which soon will be supersaturated. Solid SiC crys- tallizes at the liquid–graphite interface. Further dissolution of the gas species in the liquid catalyst induces the whisker to grow. The droplet stays on top of the growing whisker surface during the whole process. It should be mentioned that surprisingly VLS technique has not reached large-scale production yet, though VLS whiskers are longer than VS whiskers and offer diameters (4–6mm) which are presently not regarded to be harmful to health.
2.04.3.2.2 SiC Platelets
SiC platelets aredlike whiskersdmonocrystals with improved but reverse aspect ratios, thus they areflat and compact (Figure 9). Because of the enhanced aspect ratios platelets offer similar possibilities concerning ex situ toughening of materials. However, they are less expensive and nontoxic. Their aspect ratio is less, only between 4 and 15, which means that generally their toughening effects are lower compared to those caused by the whiskers. The diameter of the platelets ranges from 5 to 100mm and their thickness from 1 to 5mm. The crystal structure of the SiC platelets is always noncubic.
SiC platelets are industrially fabricated at temperatures between 1600 and 2100C in inert gas using low-cost raw materials like silica, carbon orfineb-SiC powders (Boecker, Chwastiak, Frechette, & Lau, 1989). The crystal habit depends on some doping elements (Kristler-De Coppi & Richarz, 1986; Meier, Hamminger, & Nold, Figure 8 Schematic illustration of SiC whisker growth through the VLS mechanism.
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