V. K. Zaitsev

Effects of Ge Doping on Micromorphology of MnSi in MnSi∼1.7 and on Their Thermoelectric Transport Properties

MnSi layers in Ge-doped MnSi~1.7 increased with increasing Ge content up to x=0.00133, began to break at x=0.00265 and finally disappeared at x=0.00530. An experimental equation for the growth of MnSi was proposed for the interval between the MnSi layers and amount of doped Ge content. The crystallinity of Ge-doped MnSi~1.7 increased initially with increasing doped Ge content and saturated at high Ge content. Thermoelectric transport properties along the c-axis of Ge-doped MnSi~1.7 were measured as a function of Ge content at room temperature. Electrical conductivity and thermoelectric power of Ge-doped MnSi~1.7 were compared to those of Al-doped MnSi~1.7 in our previous work. A maximum in the electrical conductivity and a minimum in the thermoelectric power of Ge-doped MnSi~1.7 were observed at x=0.00133, reflecting a change in hole density which was influenced by the volume ratio of MnSi. Hole mobility depended on the existence of MnSi layers and/or of interfaces between MnSi~1.7 and MnSi and on the crystallinity of MnSi~1.7. The thermal conductivity of Ge-doped MnSi~1.7 had a maximum at x=0.00053. The increase in thermal conductivity at low Ge doping can be explained by the increase in the amount of MnSi segregated in doped MnSi~1.7, while the decrease at high Ge content was caused by the increase in phonon scattering of Ge. A maximum figure of merit of Ge-doped MnSi~1.7 was obtained at x=0.00974, reflecting a maximum power factor.

High Effective Thermoelectrics Based on the Mg2Si-Mg2Sn Solid Solution

The complex study of the thermoelectric properties in solid solutions between compounds Mg2X (X=Si, Ge, Sn) was accomplished. Analysis of the features of band structure, thermal conductivity and electrical properties in the wide range of temperature and carrier concentration has shown that the most effective thermoelectric can be achieved in the Mg2Si-Mg2Sn solid solution. Energy spectrum and carrier concentration optimization, and, also, lattice thermal conductivity minimization allowed to establish the most effective compounds for the thermoelectrics of n- and p- type. Thermoelectrics with the maximum dimensionless figure of merit of more than 1.2 and average ZT0.9 (in the temperature range 300-800K) were obtained with developed synthesis and doping techniques. These materials are cheap, wide spread and environment friendly, have non-toxic initial components. It is very favorable for practical application.

Thermoelectric elements based on compounds of silicon and transition metals

Semiconducting compounds based on silicides of 3d transition metals (MnSi1.75 and CoSi) are promising thermoelectric materials for mass-produced thermoelectric generators. Their practical use has been delayed so far by the lack of reliable switching. Electrochemical surface treatment technology and metal deposition were used to obtain thermal-cycling-resistant, switching and antidiffusion metal coatings (Ni, Co, Fe, Cr) with an electrical resistivity up to 10−6 Ω·cm2 in the metal-semiconductor region. This technology was applied to develop a series of thermoelements and thermoelectric generators having wide-ranging applications.

Physicochemical interaction at the contact of higher manganese silicide with chromium

Diffusion processes taking place at the contact of higher manganese silicide MnSi1.71−1.75 with chromium at elevated temperatures are considered. The microstructures of cast and annealed samples show a diffusion region that evolves at the HMS/Cr interface during the HMS melt crystallization on chromium flakes. As the temperature and time of annealing of HMS/Cr samples increase, the diffusion region grows thicker and new phases appear. It is shown that the diffusion is of reaction character. The diffusion coefficients of magnesium, chromium, and silicon in the solid solution based on manganese and chromium monosilicides and in the solid solution based on the Cr3Si compound are calculated. It is also shown that, when the temperature of the HMS used as a thermoelement positive leg is optimal, the depletion time of a 10-µm-thick chromium layer exceeds 15000 h. Intermetallic phases forming in the diffusion region between the HMS and Cr are of metallic conduction and exhibit a high thermal conductivity, which minimizes energy losses in the contact layers of thermoelements.

Universal thermoelectric unit

The problems of energy supply of low power electric devices very often can be solved with thermoelectric generator even with low coefficient of performance, when other electric energy sources are not convenient. The problems of thermoelectric and construction choice for such generators are discussed in the paper. A series of domestic thermoelectric generators was designed by the authors. The work is based on designing an universal thermoelectric unit—a battery which consist of ten thermoelements. The coefficient of performance of the unit is about 4%. Any thermoelectric generator can be made as a combination of these units. Principal opportunity of production such thermoelectric generators on industrial scale was proved.

Influence of heat treatment on the structure and thermoelectric properties of CrSi2

Textured CrSi2 crystals obtained by heating finely dispersed constituents (Si, Cr) are studied. The use of a mixture of Cr and Si powders makes it possible to lower the CrSi2 synthesis temperature by 100 K. Crystallization conditions and post-crystallization annealing are found to influence the thermoelectric properties and composition of samples.

Kinetic properties of p-type Mg/sub 2/Ge/sub 0.4/Sn/sub 0.6/ solid solutions

In this paper the results of the study of kinetic properties of the Mg/sub 2/Si/sub 0.4/Sn/sub 0.6/ solid solution are presented. It is shown that it is possible to produce the Mg/sub 2/Si/sub 0.4/Sn/sub 0.6/ solid solution with of p-type with various hole concentration (up to 4 /spl middot/ 10/sup 19/cm/sup -3/). Seebeck and Hall coefficients and electrical conductivity were measured in the temperature range 100 - 800 K. The hole mobility in these solid solutions is lower than that of electrons. Some parameters of valence band are determined.

Homogeneity domain and thermoelectric properties of CrSi2

The phase composition, structure, and thermoelectric properties of CrSi2 obtained by low-temperature synthesis are investigated. The results indicate the strong effect of the silicon sublattice on the thermoelectric properties of the material and the possibility of solid-phase low-temperature transformations in a CrSi2 crystal lattice.

Structure and thermoelectric properties of CrSi2 crystallized from a tin solution—melt

The thermoelectric properties of CrSi2 single crystals grown from a tin solution—melt are studied. A correlation is found between the unit cell parameters of the CrSi2 crystals, their thermoelectric properties, and solution—melt cooling conditions.

Thermoelectrics of n-type with ZT > 1 based on Mg/sub 2/Si-Mg/sub 2/Sn solid solutions

The paper presents the results of extensive experimental and theoretical study of the solid solutions in the system Mg/sub 2/Si-Mg/sub 2/Sn. It is shown that the system is a favorable base for the creation of excellent thermoelectrics with band type of conductivity. The reproducible results are obtained for Seebeck and Hall coefficients, electrical and thermal conductivity in the wide temperature range 77-850 K for the samples of various solid solution composition and various electron concentration (up to 5/spl middot/10/sup 20/ cm/sup -3/). At optimum solid solution composition and electron concentration the thermoelectric of ZT/sub max/=1.1 is produced reliably. Average ZT value in the temperature range 350-830 K is about 0.8 for this material. It is shown that the high value of ZT is achieved due to some features of band structure of these materials. It is of great interest that these thermoelectrically remarkable alloys have simple crystallographic structure, made by a traditional technology and are formed from very cheap and ecologically friendly components.