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Notes: Infrared (IR) reflection spectra of sintered SiC thermoelectric semiconductors are studied as a function of doping concentration as well as of sintering temperature. Attention is focused on the changes in the reststrahlen band. The peak and dip frequencies remain constant, implying we are probing stable optical phonon mode in the crystalline grains through the interaction with IR photons. In contrast, variation of spectral shape, including the peak and background reflectance seems to give valuable information regarding the quality of the sintered samples and dopant distribution within the crystal grains and grain boundaries. Measured spectra are analyzed by the use of a four-component effective medium model. In this model we assume the presence of: (1) pores, (2) intergranular materials, (3) crystalline grains, and (4) metallic inclusions (at higher doping levels of metallic impurities), with volume fractions fi(i=1–4), respectively. For frequency dependent complex dielectric functions we employed: (a) a single oscillator model appropriate to the single crystal for the crystalline grains, (b) a Drude model for the metallic inclusions, and assumed (c) another single oscillator model for the intergranular material. The parameters of the latter are determined to give a best fit to the spectrum of a low doping level sample. This model gives excellent overall fit to measured spectra with the ratio f2/f3 and the damping coefficients for the oscillators as adjustable parameters when the experimental estimates for the fraction of pores f1 and metallic inclusions f4 are used. This is remarkable in view of the situation that similar data could not, so far, be explained in terms of the simple Maxwell–Garnet model. The results suggest that careful analyses of spectral shape of IR reflection in the optical phonon resonance region can be used to get information on the intergranular to grain ratio or on the damping coefficients, which are related to the quality of sintered samples or to the distribution of doped impurities among grains and grain boundaries. The background reflectance also give information on the presence of metallic inclusions although the effects of scattering may affect the interpretation. These results combined with the possibility of local probing seems to imply that IR reflectance can offer a versatile means to check the quality of the sintered samples. © 1999 American Institute of Physics.

Notes: Abstract It is shown that the Peltier heat, previously regarded as a purely contact characteristic, must be included in the equation describing the heat-flux balance in the volume of a sample (simplified Ioffe-Stil’bans equation). Analytical expressions for the temperature distribution in a loaded thermocouple, which describe the main deviations from the linear distribution, are derived by a self-consistent calculation allowing for a temperature dependence of the Peltier heat. Equations which extend the Ioffe relations to the case of large temperature differentials and large heat fluxes are obtained for the efficiency of a thermocouple.

Notes: Abstract A multicomponent infrared study of boron nitride (BN) pyrocrystals was performed in polarized light. A series of textured polycrystals, which included samples with different dergees of ordering, with various ratios of the concentrations of the hexagonal and rhombohedral phase, was prepared by chemical vapor deposition. This study revealed the essential influence of the microstructure of the pyrocrystal on the reflection and transmission spectra. It is shown that to identify the normal oscillations of the hexagonal phase, crystal-optical effects and effects associated with phonon scattering must be taken into account. The normal oscillations of the rhombohedral phase of BN were identified directly from experimental spectra of pyrocrystals which are similar in their properties to a single crystal. On the basis of the results obtained by us it is shown that ionic-covalent bonds exist between some of the atoms of neighboring layers in anisotropic modifications of BN. A refined model of the lattice structure eliminates an entire host of contradictions between the experimental data and shows that in addition to sp 2 hybridized electrons, sp hybridized electrons participate in the formation of interatomic bonds in anisotropic BN.