Yu. N. Parkhomenko

The Influence of Anisotropy and Nanoparticle Size Distribution on the Lattice Thermal Conductivity and the Thermoelectric Figure of Merit of Nanostructured (Bi,Sb)2Te3

Two factors that are important for proper estimation of the thermoelectric figure of merit of bulk nanostructured materials based on bismuth telluride and its solid solutions have been investigated. First, the anisotropy of the thermoelectric properties of nanostructured (Bi,Sb)2Te3 fabricated by the spark plasma sintering (SPS) method was studied experimentally as a function of sintering temperature and pressure. Two measuring methods were used: (a) the Harman method and (b) separate measurements of electrical conductivity, Seebeck coefficient, and thermal conductivity (laser flash method). Anisotropy and transport property values obtained by these methods are compared. Secondly, the influence of the nanoparticle size distribution on the lattice thermal conductivity was taken into account theoretically for scattering of phonons both on nanoprecipitates with different compositions and orientations and on grain boundaries. The results of estimations based on different theoretical approaches (relaxation-time approximation, Monte Carlo simulations, and effective medium method) are compared using typical size distribution parameters from available experimental data.

Energy filtration of charge carriers in a nanostructured material based on bismuth telluride

The dependences of the electrical conductivity and thermopower on the size of grains in a nanocrystalline material based on Bi2Te3-Sb2Te3 solid solutions of the p type have been investigated theoretically and experimentally. The relaxation time in the case of hole scattering by nanograin boundaries in an isotropic polycrystal has been calculated taking into account the energy dependence of the probability of tunneling of charge carriers and the dependence of the scattering intensity on the nanograin size Ln. A decrease in the probability of boundary scattering with an increase in the energy of charge carriers leads to an increase in the thermopower. The dependences of the thermopower and electrical conductivity on the nanograin size, which have been obtained taking into account the boundary scattering and scattering by acoustic phonons, are in good agreement with experimental data. For the material under consideration, the thermopower coefficient increases by 10–20% compared to the initial solid solution at Ln = 20–30 nm. This can lead to an increase in the thermoelectric figure of merit by 20–40%, provided that the decrease in the electrical conductivity and the decrease in the lattice thermal conductivity compensate each other. Despite the absence of a complete compensation, it has been possible to increase the thermoelectric figure of merit for the samples under investigation to ZT = 1.10–1.12.

Effect of annealing on the structure and phase composition of thin electro-optical lithium niobate films

We have studied the formation of thin textured LiNbO3 films in originally amorphous samples produced by rf magnetron sputtering of a single-crystal target on silicon substrates containing a native oxide layer. The results demonstrate that postgrowth annealing leads to the formation of two phases, LiNbO3 and LiNb3O8, and that the percentage of the nonferroelectric phase LiNb3O8 is minimal after annealing at a temperature of 700°C. Annealing at 700°C is optimal because it ensures the lowest surface roughness of the film, the highest degree of structuring of the ferroelectric phase, and the maximum contrasts corresponding to the vertical and lateral components of the ferroelectric polarization in piezoresponse force microscopy.

Formation of bidomain structure in lithium niobate plates by the stationary external heating method

The method of development of the bidomain structure in single crystalline lithium niobate plates based on the creation of a given temperature gradient distribution through a sample thickness by stationary heating is considered. Heating the LiNbO3 plate, which is placed between two silicon plates, is implemented by light energy emitted by lamps of the photonic annealing setup, which is absorbed by silicon. The scheme of the technological cell provides the formation and control over heat fluxes penetrating a ferroelectric plate and forming temperature gradients required for the controlled formation of two domains with the opposite polarization vectors (a head-to-head domain structure). The efficiency of light absorption for the formation of heat sources, which can be used for symmetric and asymmetric heating, which determines the position of the conditional surface with a zero temperature gradient and, consequently, a domain boundary position, is confirmed experimentally. In the LiNbO3 plate with a thickness of 1.6 mm and length 60 mm, a symmetric bidomain structure with oppositely directed polarization vectors is formed. The dependence of the bending strain of a console-clipped sample on electric voltage is studied in the temperature range −300 to +300 V; the strain amplitude is more than 35 μm. The high linearity and repeatability of the electric voltage-bending strain characteristic is shown.

Structure, electrical and magnetic properties, and the origin of the room temperature ferromagnetism in Mn-implanted Si

The structure and the electrical and magnetic properties of Mn-implanted Si, which exhibits ferromagnetic ordering at room temperature, are studied. Single-crystal n- and p-type Si wafers with high and low electrical resistivities are implanted by manganese ions to a dose of 5 × 1016 cm−2. After implantation and subsequent vacuum annealing at 850°C, the implanted samples are examined by various methods. The Mn impurity that exhibits an electric activity and is incorporated into the Si lattice in interstitial sites is found to account for only a few percent of the total Mn content. The main part of Mn is fixed in Mn15Si26 nanoprecipitates in the Si matrix. The magnetization of implanted Si is found to be independent of the electrical resistivity and the conductivity type of silicon and the type of implanted impurity. The magnetization of implanted Si increases slightly upon short-term postimplantation annealing and disappears completely upon vacuum annealing at 1000°C for 5 h. The Mn impurity in Si is shown to have no significant magnetic moment at room temperature. These results indicate that the room temperature ferromagnetism in Mn-implanted Si is likely to be caused by implantation-induced defects in the silicon lattice rather than by a Mn impurity.

Interdomain region in single-crystal lithium niobate bimorph actuators produced by light annealing

The interdomain region of a bidomain strucrture formed in 127°-cut lithium niobate single crystals using light annealing has been studied by optical and scanning probe microscopies. A periodic subdomain structure on the 180° macrodomain wall is visualized by piezoresponse force microscopy. The piezoresponse signal (polarization) is shown to be a power-law function of the domain width with an exponent n = 0.53.

Propagation of polarization of ferroelectric grains in electrically isolated lithium niobate films

Piezoresponse force microscopy has been applied for the study of ferroelectric properties of lithium niobate thin films. The films have been obtained on a silicon surface {110}, coated with a thin layer of SiO2 using high-frequency magnetron spraying of lithium niobate. It has been demonstrated that after polarization of a sample surface segment with positive or negative potential using a conductive probe of an atomic force microscope the effect of propagation (spreading) of the region of grain polarized state with a decrease in the piezoresponse amplitude is observed. Moreover, the polarization occurs stepwise, that is, the Barkhausen effect takes place at the nanometer level.

Investigation of the possibilities for increasing the thermoelectric figure of merit of nanostructured materials based on Bi2Te3-Sb2Te3 solid solutions

The transport coefficients and thermoelectric figure of merit ZT for bulk nanostructured materials based on Bi2Te3-Sb2Te3 solid solutions have been investigated theoretically. Similar materials prepared by rapid quenching of the melt with the subsequent grinding and sintering contain amorphous and nanocrystalline regions with different sizes of particles. According to the performed estimations, the thermoelectric figure of merit of the amorphous phase can exceed the value of ZT for the initial solid solution by a factor of 2–3 primarily due to the significant decrease in the thermal conductivity. The effective transport coefficients of the medium as a whole have also been investigated as a function of the parameters of each phase, and the concentration range of the amorphous phase, which corresponds to the effective values ZT > 1, has been determined.

Study of LiNbO3 single crystals with a regular domain structure by piezoresponse force microscopy

Lithium niobate crystals with a regular domain structure have been studied by piezoresponse force microscopy. The period of regular domains and the domain-wall width (w = 45 nm) have been calculated for Z- and Y-cut crystals based on an analysis of two-dimensional images of the domain-structure piezoresponse. It is shown that for the Y-cut crystal, both positive and negative domain boundaries can be identified when recording the lateral component of piezoresponse.

Bidomain structures formed in lithium niobate and lithium tantalate single crystals by light annealing

The bidomain structures produced by light external heating in z-cut lithium niobate and lithium tantalate single crystals are formed and studied. Interdomain regions about 200 and 40 μm wide in, respectively, LiNbO3 and LiTaO3 bidomain crystals are visualized and studied by optical microscopy and piezoresponse force microscopy. Extended chains and lines of domains in the form of thin layers with a width less than 10 μm in volume, which penetrate the interdomain region and spread over distances of up to 1 mm, are found.