V. V. Kaminskii

Emf induced by a change in the samarium ion valence as a result of a phase transition in SmS single crystals

The mechanism of the emf appearing in a semiconductor under heating in the absence of external temperature gradients, an effect revealed by the present authors, is considered. The experiments were performed on samarium sulfide (SmS) single crystals. It is shown that the emf is generated by an abrupt change in the samarium ion valence, which results from the ion screening by the electrons activated into the conduction band. We succeeded in obtaining emf pulses 1.3 s long with an amplitude of up to 2.5 V at T ∼ 460 K, as well as CW emf generation within the 375-to 405-K temperature interval with a maximum value of ∼50 mV.

Defect Samarium Ions and Electromotive-Force Generation in SmS

A model explaining electromotive-force generation in SmS under heating in the absence of external temperature gradients is considered. An analysis of data on the density of SmS single crystals with compositional deviations from stoichiometry in the homogeneity range suggests that excess samarium ions occupy vacancies on the sulfur sublattice. Possible concentrations of defect samarium ions are determined (up to 2.8×1021 cm−3). The temperature interval within which electromotive force appears in various SmS samples (440–640 K) and the critical conduction-electron concentrations at which the generation sets in [(6.0–8.5)×1019 cm−3] are calculated. An expression permitting estimation of the magnitude of the observed effect is proosed.

Electrical conductivity and band structure of thin polycrystalline EuS films

A study of the electrical resistance of thin polycrystalline EuS films (0.4–0.8 μm thick) in the temperature range 120–480 K has provided the basis for a model of the band structure of this substance. It has been shown that the main impurity levels in thin polycrystalline EuS films are those related with localized states near the conduction band bottom, as well as the Ei donor levels of Eu ions outside regular lattice sites. The “tail” of the localized states extends in energy up to at least −0.45 eV.

The structure of a metallic-phase film produced by mechanical polishing of polycrystalline SmS

X-ray diffraction is used to study the structure of a metallic-phase film that forms during controlled polishing of homogeneous polycrystalline semiconducting Sm1+xS samples. Structural changes that appear in the semiconducting phase under these conditions were studied. The x dependence of the thickness of the metallic layer forming on the sample surface is analyzed to explain the effect of excess samarium ions on the transformation parameters. The cause of the stabilization of the metallic modification of SmS after polishing is terminated is explained using estimates based on the measured sizes of coherent domains in samples of different compositions. The appearance and stabilization of the metallic phase are related to a decrease in and subsequent conservation of the coherent-domain size, respectively.

Thermovoltaic effect in samarium sulfide-based Sm1 − xEuxS heterostructures

The thermovoltaic effect in samarium sulfide-based Sm1 − xEuxS bulk heterostructures in the temperature interval 300–520 K is considered. It is shown that this effect is due to an artificially produced samarium ion concentration gradient, rather than to an external temperature gradient.

Temperature dependence of the SmS lattice parameter

The behavior of the lattice parameter of single-crystal SmS with temperature was studied by x-ray diffractometry in the range 100–700 K. The observed features are assigned to a temperature-induced variation in the filling of the Sm2+f-shell ground-state multiplet levels. The temperature dependence of the thermal expansion coefficient of SmS was measured. It is shown that the lattice constant behavior in samples that exhibit a pronounced emf generation effect under heating is influenced by the transition of defect samarium ions from the divalent to trivalent state and that the effect itself derives from phase transitions in SmS.

Electromotive force generation in homogeneously heated semiconducting samarium monosulfide

The emf generated in samarium monosulfide (SmS) single crystals heated in the absence of external temperature gradients was measured for the first time in a broad temperature range (from room temperature to 712 K). The phenomena of self-heating (up to T=866 K) and the emf generation were observed in the SmS samples after termination of the external heating. A high level of the emf (0.4–0.6 V) generated for 12 min (including 10 min without external heating) suggests that these effects can be used for the conversion of thermal energy into electricity.

The Mechanism of the Appearance of an Electromotive Force on Heating of SmS Single Crystals

We analyze the experimental variation of the concentration of conduction electrons in semiconducting SmS single crystals with increasing temperature within a shallow-impurity model. It is shown that the appearance of an electromotive force is due to accumulation of the critical concentration of free electrons, which results in screening of the Coulomb potential of Sm2+ impurity ions that are responsible for the creation of donor levels with an activation energy of 0.045 eV in the band gap of SmS single crystals.

Dynamics of the thermovoltaic effect in SmS

The thermovoltaic effect in the semiconductor rare-earth compound samarium sulfide (SmS) is briefly described. The qualitative dynamic model of the effect for explaining the possibility of the long-term electric voltage generation on the basis of the thermovoltaic effect is proposed. The model uses a combination of thermal and electronic processes.

Efficiency of Transformation of Heat Energy into Electric Energy Due to the Thermovoltaic Effect

Direct measurements of the efficiency of the transformation of heat energy to electric energy on the basis of the thermovoltaic effect are carried out for the first time. Experiments are performed using samarium sulfide specimens. The values of efficiency vary from 7.5 to 28%.