POINT-DEFECTS AND THERMOELECTRIC PROPERTIES OF IRON DISILICIDE CERAMICS SINTERED WITH SIH4-PLASMA-PROCESSED MICROGRAINS

The point defects in beta-iron disilicide (beta-FeSi2) have been investigated by electron paramagnetic resonance (EPR) along with the thermoelectric properties. The samples used are FeSi(x) (1.9 less-than-or-equal-to x less-than-or-equal-to 2.8) ceramics sintered with FeSi(x) micrograins processed in SiH4-plasma and nonprocessed. EPR detects (1) several S=1/2 signals and (2) a multiplet signal. An S=1/2 signal with orthorhombic g factors (g1=2.061, g2=2.047, g3=2.024) is detected in all n-type specimens, and ascribed to unpaired electrons of donors associated with iron vacancies. The other S=1/2 signals are detected in both n -type and p -type specimens. The centers responsible for these signals are considered to exist in an oxidized intergrain region of the ceramics. The EPR intensity of the multiplet signal due to the closed-pair of a high spin Fe3+ ion (S=5/2) and an S=1/2 center is reduced by SiH4-plasma treatments. SiH4-plasma processing of FeSi(x) micrograins prior to sintering changes the Seebeck coefficient sign and magnitude, depending upon the condition of plasma processing. This Seebeck change is explained in terms of the introduction of silicon- and/or iron-vacancies. SiH4-plasma processing increases the electrical conductivity of nearly stoichiometric specimens and reduces the EPR intensity of the multiplet signal. A possible model for the increased conductivity is discussed using an energy band model for the ceramic grains and grain boundaries. We consider that the change of the thermoelectric properties by SiH4-plasma processing is mainly due to the modification of defects in and near the ceramic intergrain region.