Fine Defective Structure of Silicon Carbide Powders Obtained From Different Starting Materials

The fine defective structure of silicon carbide powders obtained from silicic acid— saccharose, aerosil — saccharose, aerosil —carbon black, and hydrated cellulose—silicic acid gel systems was investigated. The relation between IR absorption characteristics and the microstructure of SiC particles obtained from different starting materials was established. The numerical relationship between the lattice parameter а and the frequency νТО is presented.


Introduction
It is known that the microstructure of silicon carbide obtained from different mixtures depends to a great extent on starting components and conditions of preparation [1][2][3][4].It has been shown in [5] that in order to prepare dense materials, one should use monophase solid spherical silicon carbide particles.Hollow SiC particles are being used in the production of thermoelectric devices, since the materials produced from these powders exhibit a low thermal conductivity and high Seebeck coefficient.That is why control of the fine defective structure (FDS) of silicon carbide powders is of great importance.This is a problem of high priority when obtained powders are subjected to further treatment, for instance, sintering.
The theoretical approach to processing of IR spectra of silicon carbide powders was based on the theory of average dielectric constants (TADC), according to which the manifestation of phonon modes in the clearance between the longitudinal and the transverse optical components of the frequencies ν ТО and ν LO is characteristic of such particles [6].
In [6], it was established that for particles that satisfy the condition d≤λ and exhibit anisotropy, IR spectra are determined by the size, shape, and FDS of the particles, as well as by the nature of the matrix in which they are located.The relative intensity, location, and the width of IR absorption bands depend on the above-mentioned factors.
Electron-paramagnetic-resonance spectra were measured with a SE/X 2547 -Radiopan radiospectrometer, connected to a spectrum analyzer and a personal computer.

Experimental results and discussion
The mechanism of formation of silicon carbide particles, which was investigated in [1][2][3][4], is determined by the heterogeneous character of distribution of SiO 2 and С particles in mixtures and by their defective structure.It was noted that the state of the initial components of the SiO 2 -С system influences both the onset of carbide formation and the TDS of silicon carbide particles.
It has been shown by a number of investigators [1][2][3][4][6][7] that during synthesis of silicon carbide powders, both solid particles and particles with a core and pores can be obtained.The former ones are characterized by the presence of one absorption peak in the obtained IR spectra, and the latter are characterized by two peaks whose frequencies manifest themselves within ν ТО and ν LO .The difference between these frequencies ∆ν determines the core size or the pore size.
As shown in [8], for such particles, the frequency of absorption is associated with the change in the parameter a by the following relationship ( ) where ν = 797 cm TO 0 -1 is the frequency of absorption for solid particles in the form of fibers; ν TO 0 = 874 см -1 , is the frequency of absorption for solid spherical particles; a = 0.4359 nm is the lattice constant for solid particles; D = 3734±30 cm -1 .
An analysis of IR spectra obtained from the starting silicic acid -saccharose (1), aerosil -saccharose (2), aerosil -carbon black (3), hydrated cellulose -SiO 2 (4) systems performed using the theory of average dielectric constants has shown that, for these systems, manifestation of surface phonon modes that characterize the FDS of obtained particles is typical.The calculation of ν frequencies performed by the foregoing formula is presented in Tab.I.

TO
The calculated frequencies coincide with those obtained experimentally within the error connected with changes in the density (agglomeration) of powders.
For systems 1-4, two-hump IR spectra, that indicate the presence of the core in these particles, are characteristic.According to EPR data, this core is carbon.The size of the core changes with the change of frequencies ∆ν = (ν LO -ν TO ).The values of ∆ν are presented in Tab.I.The increase in the lattice parameter a in system 2 compared to that in system 1 agrees with the decrease in the frequency of absorption from the LO-side.Moreover, the decrease in the frequency range ∆ν in system 2 (see Tab. I) indicates that, in this system, the existing carbon core in silicon carbide particles is smaller that in systems 1 and 3.These conclusions agree with the character of the change (decrease) in the intensity of the absorption band from the LO-side b a The results presented in Tab.I point to the fact that, in different systems, the carbide formation process begins at different temperatures.In system 2, the presence of an intensive ----band characteristic of vibrations of Si-O-Si bonds in the frequency range ~1100 cm-1 after thermal treatment at 1873 K enables us to suggest that intensive carbide formation occur at Т treat ≥ 1873 К. (Tab.I, Fig. 1).
In system 3, the distinctive feature of carbide formation is that the band corresponding to Si-C bonds has an insignificant intensity and is the superposition of two wide bands with frequencies 950 and 850 cm -1 .This indicates that the size of the carbon core in silicon carbide particles increases in comparison with those in systems 2 and 3 and that coarsening and sintering of SiC particles occur, which is substantiated by electron microscopy data.The deformation of the absorption band of Si-С bonds from the LO-side (the increase in its intensity against that of the peak from the TO-side) also testifies to the growth of the carbon core.
For system 4, the increase in the lattice parameter correlates with the decrease in the frequency of IR absorption from the LO-side.The decrease in the intensities of the absorption bands with frequencies ν 1 (LO) indicates that the size of the carbon core decreases and that the microstructure of particles becomes closer to that of solid particles.This fact is substantiated by the decrease of the frequency range ∆ν.

Conclusions
The existence of a relation between IR characteristics of absorption and the microstructure of SiC particles prepared from different starting components was shown.The change in the lattice parameter a, caused by the change in the frequency of IR absorption ν for particles with sizes d ≥ λ, is a characteristic of the FDS of silicon carbide powders obtained from different starting materials.
TO 0 For all investigated systems, the frequency ∆ν is the indicator of the relative size of cores or pores in the structures.The use of IR spectroscopy is an effective method for the control of the FDS of silicon carbide powders.The obtained data are of great applied importance in further use of these powders.