Vol 46, No 8 (2017)

The features of using radioactive isotopes when creating off-line power supplies are considered. The analysis of the substances used in radioisotope thermoelectric generators (RTGs) is carried out. The prospects for manufacturing beta-voltaic generators are justified and they are compared with other electric power sources. The mechanism of β-decay and its place among other types of nuclear transformations is considered. The basic requirements for radiation safety and the used materials of the frame and converter are formulated. Some designs of radioisotope beta-voltaic sources proposed earlier are considered. A list of isotopes that can be used as a power source in a beta-voltaic generator is presented. The methods for obtaining the radioactive materials demonstrating β-decay and their basic properties and natural isotopes are considered. It is concluded that the choice of nickel-63 isotope is preferable for use in beta-voltaic generators due to the optimal combination of its half lifetime, average particle energy, and radiation intensity.

Copper selenide is a promising material for power generation in a medium-temperature range 600–1000 K. A number of features of the Cu–Se system, namely, the existence of phase transition in a Cu2Se compound, the high speed of the diffusion of Cu ions, and the high vapor pressure of Se at elevated temperatures, make it necessary to carry out a series of experimental investigations to develop and optimize the methodology for obtaining the bulk material based on copper selenide. The influence of mechanochemical synthesis regimes and subsequent compaction method on the thermoelectric properties and structure of copper selenide is studied. The source material is obtained by mechanochemical synthesis. The methods of hot pressing (HP) and spark plasma synthesis (SPS) are used to obtain the bulk samples. The investigation of the structure and phase composition is performed by the X-ray diffraction and scanning electron microscopy. It is shown that increasing the duration of the mechanochemical synthesis up to 5 h leads to the depletion of copper in powders and to the formation of nonstoichiometric β-phase Cu_1.83Se, which persists after SPS. A comparison of the structure and properties of the material obtained by SPS and HP showed that the material obtained by HP has a greater degree of grain defects. The highest thermoelectric efficiency ZT = 1.8 at a temperature of 600°C is achieved for the material obtained by SPS. It is shown that low thermal conductivity is the main factor affecting the value of the thermoelectric efficiency ZT of the studied materials. The difference in the values of thermal conductivity of the materials obtained by different methods is related to the electronic component of thermal conductivity.

Bidomain single crystals of lithium niobate (LiNbO₃) and lithium tantalate (LiTaO₃) are promising materials for use as actuators, mechanoelectrical transducers, and sensors capable of working in a wide temperature range. One need to take into account the anisotropy of the properties of the crystalline material when such devices are designed. In this study we investigated deformations of bidomain round shaped Y + 128°-cut wafers of lithium niobate in an external electric field. The dependences of the piezoelectric coefficients on the rotation angles were calculated for lithium niobate and lithium tantalate and plotted for the crystal cuts which are used for the formation of a bidomain ferroelectric structure. In the experiment, we utilized an external heating method and long-time annealing with the lithium out-diffusion method in order to create round bidomain lithium niobate wafers. Optical microscopy was used to obtain the dependences of the bidomain crystals’ movements on the rotation angle with central fastening and the application of an external electric field. We also modelled the shape of the deformed bidomain wafer with the suggestion that the edge movement depends on the radial distance to the fastening point quadratically. In conclusion, we revealed that the bidomain Y + 128°-cut lithium niobate wafer exhibits a saddle-like deformation when a DC electric field is applied.

The results of creating a system of models and algorithms for calculating the parameters of technological processes for obtaining materials of micro- and nanoelectronics and designing equipment are considered. It is shown that the distinctive feature of the teaching methods for special technological courses in electronic engineering materials is the construction of courses by analogy with the technological processes for obtaining materials for electronics: from a bulk single crystal to instrumental structures, whose dimensions do not currently exceed several tens of nanometers. A scientific model approach to the solution of technological problems was formed in the study of heat and mass transfer processes, which together with the processes of interaction in liquids and gas, taking heterogeneous reactions into account, are the theoretical basis of the technology of electronic engineering materials. The possibilities of physical and mathematical modeling are compared. Approaches to create mathematical models of the growth processes of single crystals of semiconductors, epitaxial layers, and heterostructures are considered and the possibilities of their practical use are determined. It is determined that the ideas proposed by V.V. Krapukhin at the initial stages of training specialists in the field of the technology of electronic material and developed by his students identified the possibility of training several generations of qualified specialists.

In the indirect band gap semiconductors, and in particular, in silicon, the lifetime of nonequilibrium charge carriers is determined by recombination through impurity centers and it is inversely proportional to their concentration. Therefore, this is the most important parameter for determining the quality of the material. The noncontact methods of its measurement, in particular, the noncontact measurements of the decay of the photoconductivity (PC) constant, are the most important ones. The surface recombination strongly influences the shape of the curve. The calculation of the lifetime in the bulk by the decay constant remains relevant since there is no single-valued analytic solution of the continuity equation for this case. In the samples of monocrystalline silicon with unpassivated surfaces, the relaxation of PC is analyzed by numerical methods. The applicability of the known formulas for estimating the contribution of surface recombination to the effective PC relaxation time is discussed. It is shown that the period of time for which the fast exponents disappear depends on the relative thickness of the sample to be measured. The effective decay time is determined by the maximum value of the surface component of the relaxation time and is described by wellknown formulas only in this piece of the relaxation curve. The effective relaxation time attains saturation by the time when the signal intensity reaches 45% of the maximum value (the origin of the effective decay time according to the recommendation of the SEMI MF 1535 standard) only for the samples with a thickness of less than 3–5 diffusion lengths. For larger thicknesses, the contribution of the fast exponents to the effective PC relaxation time is observed right up to 5% of the maximum signal (i.e., until reaching the noise level of the signal to be measured). In this case using the recommended by SEMI MF 1535 standard formulas leads to a sufficiently large systematic error (up to 20%) in evaluating the free charge carrier’s lifetime.

The literature concerning the features of creating ohmic contacts for GaAs/AlGaAs heterostructures with a two-dimensional (2D) electron gas with a high level of electron mobility is analyzed. The process of annealing the contacts based on the Ni/Au/Ge system is considered. The recommended published parameters of the layers to be sprayed and the regimes of their annealing are presented, which make it possible to obtain ohmic contacts with low resistance up to temperatures lower than 4 K. Several mechanisms are considered, which can lead to the experimentally observed dependence of the characteristics of the contact on its crystallographic orientation. A method for creating contacts using Au/Ge/Pd metallization, in which the contact is formed due to the mutual diffusion and interaction of metals and a semiconductor in the solid phase at temperatures lower than 200°C, is described. This ensures a higher degree of homogeneity of the contact in the composition and a smooth metal–semiconductor interface, and it can lead to decrease of the influence of orientation effects on the electric characteristics of the contact.

Based on the analysis of numerous experimental data, it is shown that the currently generally accepted glass transition temperature T_g is not correct. It was the result of borrowing from G. Tamman the symbol T_g, which corresponds to the temperature at which a liquid in a viscous flow is transformed into a solid brittle glassy state, and using it instead of the symbol Tw. The latter corresponded to the bending temperature (for Tamman) on the property–temperature curve of glass-forming substances lying above Tamman’s T_g. The disclosed physico-chemical nature of the temperature curve Tw (currently, generally taken to be T_g), which is the temperature of the reverse direction of the interconversion of nanofragment patterns (of polymorphs) of the high- and low-temperature polymorphs that coexist in a vitreous substance, is based on the application of polymer-polymorphing representations of the structure of glass-forming substances. In the past few decades, the discovery of the preendothermal effect located, just as Tamman’s T_g, below the standard T_g, confirms the correctness of Tamman’s T_g characterized by the increased specific heat of the heated glass.