Hexagonal silicon carbides

Spitzer, W. G., D. Kleinman, and D. Walsh, 1959. Infrared properties of hexagonal silicon carbide, Phys. Rev., 113, 127-132. 

INTRODUCTION This data sheet contains information for single hexagonal silicon carbide crystals, type 6H. ... 

Table 69. Comparison of Si—C bond lengths in cyclic carbosilanes and in cubic and hexagonal silicon carbide (SiC) (S = chair form W = boat form TW = twisted boat form) ...

A Taylor, RM Jones. The crystal structure and thermal expansion of cubic and hexagonal silicon carbide. In Ref.l, p 147. 

Silicon carbide occurs in two slightly different crystal structures a single cubic form, (3SiC, and a large number of hexagonal... 

When Acheson found the hexagonal crystals in the voids, he sent some to B.W. Frazier, a professor at Lehigh University. Professor Frazier found that although the crystals were all silicon carbide, they differed in their crystalline structure. He had discovered the polytypism of SiC [18]. Polytypism will be explained in Section 1.3.2. 

Silicon carbide exhibits a two-dimensional polymorphism called polytypism. All polytypes have a hexagonal frame of SiC bilayers. The hexagonal frame should be viewed as sheets of spheres of the same radius and the radii touching, as illustrated in Figure 1.5. The sheets are the same for all lattice planes. However, the relative position of the plane directly above or below are shifted somewhat to fit in the valleys of the adjacent sheet in a close-packed arrangement. Hence, there are two inequivalent positions for the adjacent sheets. 

The structure of presolar silicon carbide grains can provide information about the conditions of formation. Crystalline silicon carbide is known to form about 100 different polytypes, including cubic, hexagonal, and rhombohedral structures. Presolar silicon carbide exists in only two of these, a cubic (fi-SiC) polytype and a hexagonal (a-SiC) polytype (Daulton et al.,... 

Crystal Structure. Silicon carbide may crystallize in the cubic, hexagonal, or rhombohedral structure. There is a broad temperature range where these structures may form. The hexagonal and rhombohedral structure designated as the a-form (noncubic) may crystallize in a large number of polytypes. 

HRTEM observations of three differently misoriented interphase boundaries between hexagonal boron nitride (h-BN) and 3C silicon carbide (3C SiC) grains showing an orientation dependence on equilibrium film thickness. In (a) and (b) the (0001) of the highly anisotropic b-BN are parallel to the interface, whereas in (c) they make an angle of 68° with the interphase boundary (reprinted from Ultramicroscopy, Knowles KM and Turan S, The dependence of equilibrium film thickness on grain orientation at interphase boundaries in ceramic-ceramic composites, 83(3/4) 245-259 (2000) with kind permission of Elsevier Science). 

Turan, S. and Knowles, K.M., (1997), Interphase boundaries between hexagonal boron nitride and beta silicon nitride in silicon nitride-silicon carbide particulate composites , J. Eur. Ceram. Soc., 17 (15/16), 1849-1854. 

Silicon carbide is covalently bonded with a structure similar to that of diamond. There are two basic structures. One is a cubic form, /i-SiC which transforms irreversibly at about 2000 °C to one of a large number of hexagonal polytypes, and the other is a rhombohedral form also with many polytypes. Both the hexagonal and rhombohedral forms are commonly referred to as a-SiC. 

The preparation, manufacture, and reactions of SiC have been discussed in detail in Gmelin, as have the electrical, mechanical, and other properties of both crystalline and amorphous of SiC. Silicon carbide results from the pyrolysis of a wide range of materials containing both silicon and carbon but it is manufactured on a large scale by the reduction of quartz in the presence of an excess of carbon (in the form of anthracite or coke), (Scheme 60), and more recently by the pyrolysis of polysilanes or polycarbosUanes (for a review, see Reference 291). Although it has a simple empirical formula, silicon carbide exists in at least 70 different crystalline forms based on either the hexagonal wurtzite (ZnS) structme a-SiC, or the cubic diamond (zinc blende) structme /3-SiC. The structmes differ in the way that the layers of atoms are stacked, with Si being fom-coordinate in all cases. 

Silicon carbide is remarkable for its unusually large variety of different morphologies, which differ in their stacking sequences of hexagonal and rhombohedral layers. All hexagonal and rhombohedral forms are often simply described as a-SiC. The commercially available SiC produced by the Acheson process is a-SiC. 

Properties For silicon carbide, crystalline form ranges from small to massive crystals in the hexagonal system, the crystals varying from transparent to opaque, with colors from pale green to deep blue or... 

Zhao[266] demonstrated the successful synthesis of highly ordered mesoporous silicon carbides with unusually high surface areas (430-720 m2/g), uniform pore sizes (<3.5 nm), and extremely high thermal stabilities (up to 1400 °C) replicated by mesoporous silica hard templates via a one-step nanocasting process. Highly ordered 2-D hexagonal (p6m) and bicontinuous cubic (Ia3d) SiC nanowire arrays have been cast from the hard templates, mesoporous silica SBA-15 and KIT-6, respectively. 

Microstructure of Chemical Vapour Deposition SiC Figure 6.8 shows the X-ray diffraction (XRD) patterns of CVD SiC deposited in a temperature range of 1000 to 1300°C. Detailed analyses of the X-ray results indicate that the deposits are pure silicon carbide mainly composed of //-SiC (cubic 3C crystal structure) with a small amount of er-SiC (hexagonal 4H crystal structure). It is clear from the figure that the diffraction angles of 35.6°, 41.3°, 60.1°, 72.1° and 75.5° correspond to //-SiC and the diffraction angle of 33.7° corresponds to er-SiC. As the deposition temperatures decrease, the deposits become poorly crystallised because the diffraction peaks become broader or its intensity shown in Y axis become lower. At the deposition temperature of 1000°C, the deposits are in a quasi-amorphous state. 

Refractive indices for several polytypes of silicon carbide have been measured [8-12]. The 6H polytype of SiC has been measured in the most detail [8]. For the hexagonal form the c axis is assumed to be perpendicular to the surface. Thus, a normal incidence wave can be used to measure transmission and/or reflection. This normal incidence wave is often called the ordinary ray. A prism with its sides perpendicular to the c-axis is used to determine both n0 and ne in the normal way (n0 = ordinary ray, ne = extraordinary ray). A short summary of the data is presented here for additional information please refer to [8,9]. 

Dean et al [7] measured the Zeeman splitting of a luminescence line involving the 2p donor state, obtaining the electron effective mass m t=(0.24 0.01)mo and m /m, =0.36 0.01 for n-type cubic crystals. Measurements of infrared Faraday rotation due to free carriers were made by Ellis and Moss [8] at room temperature in a number of n-type hexagonal specimens belonging to the 6H and 15R polytypes of silicon carbide. One component of the total density-of-states effective mass was explicitly determined by this method. A value for the... 

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