Prospects for the practical use of the Kaminskii thermovoltaic effect

The Kaminskii thermovoltaic effect, which is one of the new principles of converting thermal energy into electrical energy, is considered in the article. Samples of the rare-earth semiconductor samarium monosulfide (SmS), located in a uniform temperature field without any temperature gradient, generated an electromotive force (emf). The nature of the effect is associated with the concentration gradient of Sm impurity atoms in the sample, a combination of electronic and thermal processes at temperature about 200 °C, and the appearance of a concentration gradient of free electrons, which leads to the generation of emf. The following electrical characteristics of the effect were achieved on bulk samples: a 2.5 V emf in a pulsed mode with a pulse duration of 1.3 s and a 0.05 V emf in a continuous mode. The parameters of thermoelements based on the Kaminskii effect and based on the classical Seebeck thermoelectric effect were compared. The maximum values of the efficiency were 36 % and 10 %, respectively. The effect was observed on other semiconductors, however, its maximum values occur in SmS, which is explained by fundamental reasons related to the position of rare-earth elements in the periodic table. The thermovoltaic effect can be used for direct conversion of associated heat and utilization of waste heat as power sources for various devices.


Introduction
A qualitative theoretical analysis of the remarkable properties of samarium monosulfide and other rare-earth semiconductors was carried out on the basis of consideration of their features arising from the position of rare-earth elements in the periodic table of elements [1]. The value of the ionization potential I of the impurity element should be considered the most important parameter in semiconductors for the transition of an electron to a free state (conduction band). This follows from the fact that a rough estimate of the depth of the impurity donor level in the semiconductor material is proportional to I. Consideration of the second and third ionization potentials of the elements of the periodic table showed that rare-earth elements have their smallest values. We can conclude that the rare-earth element introduced into the matrix from any compound will most easily give its electron in comparison with all other elements. In semiconductors, an electron will fall into the conduction band and lead to the appearance of effects associated with such a transition. The reason for the smallness of ionization potentials in comparison with other elements may be due to the fact that the ions of rareearth elements have the largest sizes of ionic radii. Large radii of atoms and ions of rare-earth elements are associated with the presence of electrons localized on 4f shells. These shells are located most closely to the nucleus of the element, and their electrons substantially screen the Coulomb potential of the nucleus, which leads to an increase in the ionic radius.
The described feature of the properties of rare-earth compounds allows us to determine the range of the most appropriate practical applications of rare-earth semiconductors. Since the rare-earth ion in these compounds most easily donate its electron under various external influences, these materials can be used as sensitive elements in the manufacture of sensors of various resistive type physical quantities (mechanical, thermal and gas sensors), as well as in converters of various types of energy into electrical energy. Samarium monosulfide is of greatest interest in this sense, since in this semiconductor the 4f levels of the samarium ion are localized in the band gap most closely to the bottom of the conduction band (~ 0.2 eV) among other rare-earth semiconductors. In particular, SmS has a record strain sensitivity among all semiconductors [2][3][4].

Thermovoltaic effect
The remarkable properties of samarium monosulfide revealed in a completely new effect, discovered by Kaminskii V. V. with colleagues in 1999 [5,6]. The thermovoltaic effect is one of the new principles for the conversion of thermal energy into electrical energy. SmS samples located in a uniform temperature field without any temperature gradient generated an electromotive force. The nature of the thermovoltaic effect is associated with the presence of a concentration gradient of Sm impurity atoms over the sample volume, a change in the valence of defective ions located in the vacancies of the sulfur sublattice Sm 2+ →Sm 3+ +e, electron transitions from 4f-levels to the conduction band, and the creation of large local carrier concentrations. Such electron transitions are collective. Electron transitions are accompanied by the appearance of pulses of electrical voltage and thermal processes synchronized with them. Generation was observed when the samples were heated up to about 200 °C. At higher temperatures, impurity levels with an activation energy of 0.04 eV are depleted, the concentration gradient of which over the volume of the sample causes the presence of a thermovoltaic effect. The following characteristics of the effect were achieved on SmS bulk samples: a 2.5 V emf in a pulsed mode with a pulse duration of 1.3 s and a 0.05 V emf in a continuous mode. The layout of the thermoelement [7] and the sample of a continuous signal generated for more than 5 hours are shown in Figures 1, 2.

Parameters of thermocouples and prospects for their application
A comparison was made of the values of the parameters of thermoelements based on the Kaminskii effect and based on the classical Seebeck thermoelectric effect (table 1). Maximum values of the efficiency were 36 % [8] and 10 %, respectively.
The thermovoltaic effect can be used for direct conversion of associated heat and utilization of waste heat as power sources for low-power electronic devices, autonomous sensors of physical quantities used for remote monitoring, in the automobile industry, when servicing pipelines and other facilities, complete with solar panels. The exceptional radiation resistance of SmS will allow using of generators in nuclear energy and spacecraft. The Kaminskii effect can be considered as an effective method of converting thermal energy into electrical energy, which reflects the current trend of the transition to environmentally friendly and resource-saving energy. The maximum effect is obtained on SmS. Such results cannot be achieved at the compounds of other elements for fundamental reasons related to the structure of the electronic shells of rare-earth elements, which are reflected in their location in the periodic table.
A further increase in the conversion characteristics based on the effect under consideration is associated with the working formula of the effect generation voltage [9] and can be achieved by increasing the maximum working temperature and developing effective technological methods for obtaining higher concentration gradients of impurity donor levels in the volume of the generating element.