Vol 48, No 8 (2019)

The results of investigating the crystal structure, ionic conductivity, and local structure of the (ZrO₂)_{1 –x}(Gd₂O₃)_x and (ZrO₂)_{1 –x}(Y₂O₃)_x (x = 0.04, 0.08, 0.10, 0.12, and 0.14) solid solutions are reported. The crystals are grown by directional crystallization of the melt in a cold container. The phase composition of the crystals is investigated by X-ray diffractometry and transmission electron microscopy. The transport characteristics are studied by impedance spectroscopy in the temperature range of 400 to 900°C. The local crystal structure is examined by optical spectroscopy. Eu³⁺ ions were used as a spectroscopic probe. The study of the local structure of the ZrO₂–Y₂O₃ and ZrO₂–Gd₂O₃ solid solutions revealed the features in the formation of optical centers, which reflect the character of localization of oxygen vacancies in the crystal lattice depending on the stabilizing oxide concentration. It is established that the local crystal environment of Eu³⁺ ions in the (ZrO₂)_{1 –x}(Y₂O₃)_x and (ZrO₂)_{1 –x}(Gd₂O₃)_x solid solutions is determined by the stabilizing oxide concentration and is practically independent of the stabilizing oxide type (Y₂O₃ or Gd₂O₃). The maximum conductivity at a temperature of 900°C is observed in the crystals with 10 mol % of Gd₂O₃ and 8 mol % of Y₂O₃. These compositions correspond to the t'' phase and are close to the interface between the cubic and tetragonal phase regions. It is found that in the ZrO₂–Y₂O₃ system the highly symmetric phase is stabilized at a lower stabilizing oxide concentration than in the ZrO₂–Gd₂O₃ system. The analysis of the data obtained makes it possible to conclude that, in this composition range, the concentration dependence of the ionic conductivity is mainly affected by the phase composition rather than the character of the localization of oxygen vacancies in the crystal lattice.

The short-circuit current flow in crystals with the low-temperature phase transitions including Rochelle salt NaKC₄H₄O₆ · 4H₂O and triglycine sulfate (CH₂ · NH₂ · COOH)₃ · H₂SO₄ is investigated. The experiments are conducted on polar cut samples without preliminary polarization with the symmetric indium conducting coatings. The short-circuit currents remaining for a fairly a long time and the current decay with time are observed at room temperature on all the samples. The temperature dependences of the short-circuit currents in the temperature ranges of 16 to 45°С for Rochelle salt and 16–110°С for triglycine sulfate are obtained. The short-circuit currents are observed in these crystals both in the ferroelectric and paraphase. It is shown that, upon heating in the ferroelectric phase, the total short-circuit current is determined by competing processes: the pyroelectric currents and electrochemical decomposition currents. In the paraphase, the short-circuit currents are the electrochemical self-decomposition currents. Based on the experimental results obtained, it is demonstrated that the short-circuit current flow through the polar cut samples of Rochelle salt and triglycine sulfate crystals is induced by the intrinsic emf caused by the electrochemical self-decomposition of the opposite surfaces of the sample polar cuts when in contact with the conducting coatings due to the anisotropy of these surfaces. A model of the electrochemical self-decomposition in such crystals is proposed.

We calculate the temperature regime in nanosized AlAs/GaAs binary heterostructures. When modeling the heat transfer in nanocomposites, it is important to take into account that heat dissipation in multilayer structures with the size of layers of the order of the mean free path of the energy carriers (phonons and electrons) occurs not in the lattice but at the boundaries (interfaces) of the layers. In this regard, the use of classical numerical models based on the Fourier law is limited, causing significant errors. To obtain more accurate results, we use a model in which the heat distribution is assumed to be constant inside the layer, and the temperature changes stepwise at the boundaries of the layers. A hybrid approach is used for these calculations: the finite-difference method with an implicit time approximation scheme and a mesh-free model based on a set of radial-basis functions for the spatial approximation. The parameters of the bases are calculated by solving the systems of linear algebraic equations. In this case, only the weights of neuroelements with fixed centers and widths are chosen. As approximators, the set of frequently used basis functions is considered. To increase the speed of the calculations, the algorithm is parallelized. The computation time is evaluated to assess the performance gain with the use of the parallel implementation of the method.

The distribution of the potential and the parameters of the potential barrier for electrons in semiconductor crystallite is numerically calculated. The calculations are made in a spherical crystallite with uniformly distributed surface states and uniformly distributed donors. It is considered in the calculations that the surface charge is screened both by the ionized donors and on free electrons; the contibution of free electrons should not be neglected in semiconductors with a high concentration of free electrons. It is demonstrated that the height of the potential barrier depends non-monotonically on the concentration of the donors in the crystallite. Moreover, in the curve of the height of the potential barrier as a function of the concentration of the donors, it is possible to highlight two segments corresponding to the cases of the complete and partial depletion of the crystallite. The height of the potential barrier increases with the concentration of the donors in the first segment and decreases in the second segment. It is established that the height of the potential barrier increases with the increase in the concentration of the surface states. The possibility of the existence of surface potential barriers in nano- and polycrystalline metal-oxide semiconductors, which are applied as a sensitive layer of gas sensors, is estimated. It is concluded that if the crystallite radius in metal-oxide semiconductors does not exceed 10 nm the sensor’s sensitivity to gas could hardly be attributed to the usual barrier model. It is demonstrated that shape of crystallite and the contribution of free electrons to screening of surface charge have to be taken into account to calculation of width of potential barrier.

The features and changes in the microstructure of the electrode material of the negative electrode of the lead–acid starter accumulator battery appearing on the addition of two different specimens of carbon carbon black and hybrid carbon, are investigated. The X-ray phase analysis and the scanning electron microscopy analysis are conducted. It is established that using carbon black or hybrid carbon as an additive to the material of the negative electrode influences its structure causing changes in the processes of its impregnation and formation. Based on the structural analysis, a qualitative description, according to which hybrid carbon increases the dispersity of the negative active mass and impedes the diffusion of sulfate-ions, is proposed. The standard tests were conducted by intensive cycling in the partial state of charge lead–acid starter batteries in the charge–discharge mode. The batteries were manufactured using negative plates with additives of technical or hybrid carbon. The influence of each type of carbon additive on the electrical characteristics of the starter batteries is determined. It is shown that the additive of hybrid carbon increases the service life of starter batteries under operation in the partial state of the charge. This additive increases the charge acceptance on average by 9% and the deep discharge stability of the battery. The capacity loss after deep discharge is less than 4.4% if hybrid carbon is used as an additive and 7.2% in the case of carbon black.