High-thermal-conductivity SiC and applications

Abstract

Although it does not occur in nature and was first synthesized a little more than a century ago, silicon carbide is one of the most important industrial ceramic materials, with consumption greater than one million tons per year. Discovered by Pennsylvanian Edward Acheson in 1891 and patented shortly thereafter [1], silicon carbide justified a premium price (more than $800/lb) due to its unique abrasive character. The availability of inexpensive hydroelectric power in Niagara Falls led Acheson to set up his Carborundum facility in the vicinity. Slight modifications of the original process are used today to generate hexagonal forms of SiC from the high-temperature reaction of quartz sand and petroleum coke [SiO2 + 3C → SiC + 2CO]. Silicon carbide played a key role in the industrial revolution and is still widely used as an abrasive and as a steel additive, refractory, and structural ceramic. An excellent review is available in [2]. Silicon carbide crystallizes in a close-packed structure of covalently bonded silicon and carbon atoms. These atoms are arranged so that two primary coordination tetrahedra, SiC4 and CSi4, where four carbon or silicon atoms are bonded to a central Si or C atom, are formed [3]. These tetrahedra are linked together through their corners and stacked to form polar structures called polytypes, which are alike in the two dimensions of the closed packed plane but differ in the stacking sequence in the dimension perpendicular to these planes. The stacking sequence in SiC can be described by an ABC notation. In a cubic, 3C, or β-SiC, a sequence of three planes or the ABC stacking (⋯ABCABC⋯) is repeated to form a zinc-blend structure whereas in a simple hexagonal 2H-SiC, a sequence of two planes (⋯ABAB⋯) is repeated. In addition, more than 100 polytypes of SiC exist that contain more complex stacking arrangements derived from these two forms. All these noncubic forms of SiC are known as α-SiC. Silicon carbide (SiC) is a good material for high-heat-flux applications due to its many attractive properties, such as high thermal conductivity, which is exceeded only by diamond, low values of density and thermal expansion coefficient, and high values of hardness, elastic modules, flexural strength, and thermal shock resistance. Further, SiC is a wide-band-gap material (band gap = 2.2-2.86 eV) with good transmission in the wavelength region 0.5-6?m, so it can also be used as windows and domes for high-speed aircraft and missiles. The properties of SiC depend considerably on the specific method used to produce it. Four basic types of SiC are currently available: hot pressed or sintered, siliconized or reaction bonded, single crystal and chemical vapor deposited (CVD). In the hot-pressed process [4], [5], SiC powders are mixed with suitable sintering aids and grain growth inhibitors and are consolidated at high temperature and pressure to form near 100% dense SiC parts of relatively simple shapes. Use of hot isostatic pressing allows fabrication of small components of intricate shapes. Although this technique provides good mechanical properties, the resulting SiC does not provide high values of other properties such as thermal conductivity, optical transmission, or high surface quality. Further, reliable bonding techniques are required to fabricate large and complex-shaped parts. The reaction-bonded SiC (RB-SiC) is a two-phase material consisting of SiC and 10-40% of Si [4], [5[, [6]. First, a SiC porous body is formed by casting an alpha SiC slurry and then this body is infiltrated with Si to fill the pores and yield a near 100% dense material. Thus the properties of this material are primarily determined by the Si content. This method permits fabrication and in-process repair of parts such as lightweight structures for space mirrors. However, due to the presence of particles of different thermal conductivity, refractive index, and hardness, this material does not provide very high thermal conductivity, it is not useful for transmissive optics applications, and it cannot be polished to a high degree of surface figure and finish. The last drawback is usually overcome by overcoating this material with a layer of CVD-SiC, Si or any other suitable material. Single-crystal SiC (undoped) can provide good thermal, optical, and physical properties of interest for high-thermal-conductivity applications [7], [8]. However, single-crystal SiC is expensive, difficult to produce in large sizes, and susceptible to fracture along the cleavage planes. The hexagonal form of single-crystal SiC (α-SiC) has been produced in small sizes for semiconductor applications using sublimation and is currently available commercially. The single-crystal β-SiC, which is cubic and isotropic, is not readily available. Chemical-vapor-deposited SiC is a superior material for high-thermalconductivity applications [9], [10], [11], [12], [13], [14], [15], [16]. By varying the process parameters, the same CVD method can produce high-thermalconductivity SiC with other properties such as electrical resistivity or optical transmission optimized for a variety of applications. CVD-SiC is a theoretically dense, highly pure, polycrystalline material, which is free from voids and microcracks. The CVD process permits use of near-net shape and precision replication technologies to fabricate components, which require minimal Monolithic 0.5-m-diameter or 1.0-m-long and 25-mm-thick parts have been successfully produced. This process can be further scaled to yield multimetersize parts. Large-scale capability reduces cost and makes CVD-SiC components cost-effective in comparison to other competing materials. Finally, the CVD process is reproducible. This reproducibility has been demonstrated statistically by plotting important properties of CVD-SiC on statistical quality-control charts. A consequence of reproducibility and homogeneity of CVD-SiC is that a fabrication process can be developed to yield parts of consistent quality and finish from batch to batch. Table 6.1 shows a comparison of important properties of different forms of SiC. We see that CVD-SiC has properties superior to all other forms of SiC except the single-crystal SiC, which is not readily available in bulk form. Specifically, CVD-SiC has substantial advantage in terms of thermal conductivity and polishability in comparison to hot-pressed and RB-SiC. We present CVD β-SiC process and property data relevant for highthermal- conductivity applications. Discussion has been limited to bulk CVD-SiC, and no attempt has been made to include applications of CVD-SiC coatings. In Section 6.2, the process conditions used to produce high-thermalconductivity SiC are presented. Many applications may require not only high thermal conductivity but also optimum values of other properties such as flexural strength, optical transmission, electrical resistivity, and chemical purity. Consequently, important physical, thermal, mechanical, optical, and electrical properties of CVD-SiC are provided in Section 6.3. The high-thermalconductivity applications of CVD-SiC are discussed in Section 6.4. Finally, a summary and conclusions are presented in Section 6.5.