In this paper, we study the ceramics of cobaltite [Tb0.5Nd0.5CoO3]1−yCy with equiatomic contents of terbium and neodymium and with the addition of a carbon material flakes (СMF) with the content of carbon in the range of weight fraction 0 < y < 0.01. The ceramics were obtained by the method of solid-phase reactions according to the standard ceramic technology in air. Using X-ray diffraction and energy-dispersive analysis, it was established that the initial (undoped) solid solution is single-phase with perovskite structure. It was shown that Co3+ ions in undoped ceramics under study are in a non-magnetic low-spin state over the entire temperature range under consideration (2–320 K), and the entire contribution to the magnetization is due to Tb3+ and Nd3+ ions. An analysis of the temperature dependences of the electrical resistance R(T) shows that the initial solid solution behaves like a dielectric with a reduced activation energy of about 0.3–0.35 eV in the temperature range of 280–320 K, due to hopping conduction with a constant activation energy (constant range hopping) by defects in Tb0.5Nd0.5CoO3 matrix. At lower temperatures (200 – 275 K) in the initial (undoped) Tb0.5Nd0.5CoO3 samples, the Mott-type hopping conductivity mechanism with a variable range hopping and high values of the characteristic temperature To ∼ 109 K was observed. Simultaneously, at 300 K, we observed hopping conductivity on alternating current according to the law σ(ω) ∼ ω-α(ω), where the exponent α, which determines a hopping probability, depends on the frequency. In [Tb0.5Nd0.5CoO3]1−yCy samples, heavily doped with СMF with y = 0.01, carrier transport in the entire studied temperature range of 2–320 K mainly occurs along highly conductive carbon-based channels, formed inside of Tb0.5Nd0.5CoO3 matrix by СMF. At temperatures below 15–20 K, these samples show hopping behavior of R(T) with a variable range hopping over localized states, described by the Mott–Kirkpatrick law. In the temperature range 20–100 K electrical resistance versus temperature R(T) along carbon-based channels obey the law R(T) ∼ Ln T. In this case, the relative magnetoresistance MR(T, B) is characterized by a negative sign at temperatures below 10 K in magnetic fields less than 1 T. This behavior of the R(T, B) dependences were described on the basis of the theory of quantum corrections to the Drude conductivity for two-dimensional samples under conditions of weak localization, which is consistent with the behavior of layered carbon-based materials known from the literature. At T > 150 K curves R(B) follow to Lorentz-like positive magnetoresistive effect in Tb0.5Nd0.5CoO3 matrix in the whole magnetic field range.

The thermal, electrical and thermoelectric properties of ZnO– Me x O y ceramics with 1 ≤ x , y ≤ 3, where Me = Al, Co, Fe, Ni, Ti, have been studied. The specimens have been synthesized using the ceramic sintering technology from two or more oxides in an open atmosphere with annealing temperature and time variation. The structural and phase data on the ceramics have shown that post-synthesis addition of Me x O y doping powders to wurtzite-structured ZnO powder causes Zn x ( Mе ) y O 4 spinel-like second phase precipitation and a 4-fold growth of ceramics porosity. Room temperature heat conductivity studies have testified to predominant lattice contribution. A decrease in the heat conductivity upon doping proves to be caused by phonon scattering intensification due to the following factors: size factor upon zinc ion substitution in the ZnO lattice (wurtzite) by Me x O y doping oxide metal ions; defect formation, i.e., point defects, grain boundaries (microstructure refinement); porosity growth (density decline); secondary phase particle nucleation (Zn x ( Mе ) y O 4 spinel-like ones). The above listed factors entailed by zinc ion substitution for metal ions (Co, Al, Ti, Ni, Fe) increase the figure-of-merit ZT by four orders of magnitude (due to a decrease in the electrical resistivity and heat conductivity coupled with a moderate thermo-emf decline). The decrease in the electrical resistivity originates from a more homogeneous distribution of doping metal ions in the wurtzite lattice upon longer annealing which increases the number of donor centers.

The paper studies the thermal, electrical and thermoelectric properties of ZnO–MexOy ceramics with 1 ≤ x, y ≤ 3, where Me = Al, Co, Fe, Ni, Ti. The samples were made on the basis of ceramic sintering technology of powder mixtures of two or more oxides in an open atmosphere with variations in temperature and duration of annealing. Structural and phase studies of ceramics indicate that the addition of powders of MexOy alloying agents to ZnO powder with a wurtzite structure after the synthesis process leads to the release of secondary phases such as Znx(Me)yO4 spinels and a 4-fold increase in the porosity of the resulting ceramics. Studies of thermal conductivity at room temperature indicate the predominance of the lattice contribution. The decrease in thermal conductivity during doping is due to an increase in phonon scattering due to the influence of the following factors: (1) the size factor when replacing zinc ions in the ZnO (wurtzite) crystal lattice with metal ions from the added MexOy oxides; (2) the formation of defects – point, grain boundaries (microstructure grinding); (3) increase in porosity (decrease in density); and (4) formation of additional phase particles (such as spinels Znx(Mе)yO4). The effect of these factors in the substitution of zinc ions with metals (Co, Al, Ti, Ni, Fe) leads to an increase in the thermoelectric Q-factor of ZT by 4 orders of magnitude (due to a decrease in electrical resistivity and thermal conductivity with a relatively small decrease in the coefficient of thermal EMF). The reason for the decrease in electrical resistance is the more uniform redistribution of alloying metal ions in the wurtzite lattice, resulting in an increase in the number of donor centers, formed with an increase in the duration of annealing.

We have studied the interconnection between structure (grain sizes, chemical and phase composition, porosity) and some electric properties (resistivity, Hall and Seebeck coefficients, as well as power factor, concentration and mobilities of carriers) in composite ceramics (ZnO)(z)[(TM)(x)O-y](1-z) (TM = Fe and Co - transition metals, 0 <= x <= 3; 1 <= y <= 4, 0.5 <= z <= 10 wt%), prepared by one-step and/or two-step annealing on air of powder mixtures of ZnO and TM oxides. The structure of ceramic samples was studied by X-ray diffraction (XRD), Mossbauer (MS) and Raman (RS) spectroscopies, scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis and magnetometry. It was proved that phase composition of (ZnO)(z) [(TM)(x)O-y](1-z) ceramics depends on the type of doping agents. Addition of CoO to ZnO results in the formation of solid solutions with wurtzite structure after two-step annealing independently on Co concentration z <= 10 wt%. Addition of FexOy to ZnO results in the formation of the samples with three phase components after synthesis: submicron grains of wurtzute-like structure with large grains of ZnFe2O4 ferrite with spinel structure and residual oxides FexOy used as dopant in powder mixture. SEM measurements evidence that size of the wurtzite phase grains decreases from several tens of micrometers when using the one-step synthesis to a submicron level for the case of the two-step technology. Temperature dependences of electrical resistivity p(T), as well as Hall and Seebeck coefficients in the undoped ZnO in th range of 6-500 K have shown a competition of the Mott and Shklovsky-Efros variable range hopping conductance (due to disordering), percolative one (due to formation of large-scaled potential relief) and a standard electron transport by C-band. It was found that C-band contribution to the carrier transport in ceramic samples (ZnO)(z)[(TM)(x)O-y](1-z) above 100 K was provided by both low and deep donor centers which have been formed in the wurtzite phase Zn1-8(TM)(8)O. For example, in the TM doped samples we observed intrinsic shallow levels with ionization energies of about Delta E-1 = (0.04-0.05) eV as well as extrinsic deep levels with Delta E-2 = (0.24-0.37) eV. The iron doping of ZnO-based ceramic samples increased the room temperature Seebeck coefficient S(300 K) in average. In so doing, the S(300 K) dependence on electron concentration n of the studied samples (ZnO)(z)(F(e)xO(y))(1-z) approached maximal values (up to 1000 mu V/K) at n = 1021 m(-3). At the same time ZnO doping with Co resulted in more weak increase (up to 1.3-1.5 times) of S(300 K) values. Practically threefold growth of S(300 K) doe to Fe doping was attributed by us to the influence of particles of ferrite) formed in wurtzite matrix. The power factor P = (S-2/p), estimated for the studied samples, evidenced that for both dopants it's the highest values (= 10(-5) W/Kmiddotm) were approached in the (ZnO)(z)[(TM)(x)O-y](1-z) ceramics with the lowest resistivity values. (C) 2021 The Author(s). Published by Elsevier B.V. CC_BY_4.0

In this work, we study the effect of adding iron oxides FeO and Fe2O3 in a ratio of 1 : 9 on the thermal and thermoelectric characteristics of ceramics based on zinc oxide ZnO. The samples themselves were made in two stages based on the ceramic technology of sintering powder mixtures in an open atmosphere. Thermal conductivity studies point to the dominant contribution of lattice thermal conductivity at room temperature. The decrease in thermal conductivity as a result of alloying is due to an increase in phonon scattering at point defects introduced into the ZnO lattice (due to the replacement of zinc ions by iron ions) and at grain boundaries (due to microstructure refinement), as well as an increase in porosity (a decrease in density) and the formation of particles of an additional ZnFe2O4 ferrite phase. Alloying wit iron and the accompanying change in the structure of ceramics (decrease in grain size, increase in porosity, precipitation of the ferrite phase) leads to an increase in the thermoelectric figure of merit ZT by 2 times (due to a decrease in electricalresistivity and thermal conductivity with a relatively small decrease in the thermoelectric coefficient). The results obtained can be used to fabricate ZnO-based ceramics with optimal thermoelectric characteristics.

Currently, special attention is paid to the search for new ceramic materials based on wide-gap oxides, as well as to the study of their structure and properties with a view to their application in various areas of electronic and optoelectronic industry. Conventional double-step ceramic technology has been used to obtain samples in this experiment. After compacting at the pressure of 6 GPa of ZnO and ZnO-FexOy powders in different weight relations, the samples were subjected to the procedure of synthesis at 1173 K for 2 h and then to the annealing at 1473 K for 3 h on air. The samples structure was investigated by the Scanning Electron Microscopy (SEM), Energy-dispersive X-ray Spectroscopy (EDX), X-ray Diffraction (XRD) and Raman spectroscopy methods. Temperature dependences of resistivity, magnetoresistance, Hall and Seebeck effects were experimentally studied in the range from 4 to 700 K. As the experiments have shown, the size of grains in the obtained ceramic samples after synthesis was in submicron range. An XRD study showed the saving of the wurtzite structure in Zn1-delta Fe delta O solid solutions where 0.66 < delta < 0.81 at. % regardless of the type of the doping agent. At the same time, the replacement of zinc by iron atoms led to the contraction of the ZnO lattice. All the samples studied (ZnO and Zn1-delta Fe delta O) demonstrate n-type conductivity. The temperature dependencies of resistivity have shown two specific features: the presence of energy level about 0.35 eV below the conduction band bottom for the doped ceramic samples (unknown in literature) and conductance with the changing activation energy at temperatures below 200 K for the undoped ZnO ceramic samples. Seebeck coefficient increased by100-150% with doping due to growth of electron concentration. Some model concepts about scattering mechanisms and reasons of Seebeck effect enhancement have been developed. (C) 2020 Elsevier B.V. All rights reserved.

The relationship between the chemical composition, phase structure, grain size, electrical resistance, and Seebeck coefficient in composite ceramics (ZnO)z[(TM)xOy]1 – z (TM = Fe, Co (transition metals); 0 ≤ x ≤ 3, 1 ≤ y ≤ 4, 0.5 ≤ z ≤ 50 wt %), obtained using one- and two-stage technology has been examined. Experimental data on the electrical conductivity, Hall effect, and thermoelectric parameters of doped and undoped ceramics are presented. In the wurtzite phase of the studied ceramics, two levels were found: a shallow level with ionization energy ΔE1 ≈ 0.04–0.05 eV and a deep level with ΔE2 ≈ 0.24–0.37 eV. The relationship between the phase composition, electrical conductivity, and thermoelectric power in the studied ceramics has been established. Doping of zinc oxide with iron is shown to be promising for increasing the Seebeck effect.

The study presents frequency dependences of real part of admittance Z' (f) and phase shift angle theta(T, f) in nanogranular films containing CoFeZr nanoparticles with "core-shell" structure embedded into SiO2 matrix. The 3 mu m thicknesses (Co41Fe49Zr10)(x)(SiO2)(100-x) films with 20 <= x <= 80 at.% were deposited in vacuum chamber evacuated either with pure Ar (Set 1 samples) or Ar + O-2 gas mixture (Set 2) using ion-beam sputtering technique. After characterization by X-Ray diffraction, Mossbauer spectroscopy, scanning electron microscopy and magnetization studies, the films both of Set 1 and Set 2 samples were subjected by admittance measurements at 300 K in the frequency range of 0.1-1000 kHz. Mossbauer spectroscopy have shown that oxidized CoFeZr nanoparticles in Set 2 samples contain semiconducting iron-based oxides with Fe3+ charge states of iron ions. The observed Z' (f) and theta(f) dependencies for the Set 2 films below the x(c) have shown dielectric regime of carrier transport. They also exhibited that at weak AC electric fields inductive-like contribution to reactive part of admittance (with positive theta values) prevails over the capacitive one f > 10 kHz. This effect of the so-called "negative capacitance" was explained by the delay of current, formed by electrons hopping between nanoparticles, relative to applied bias voltage. This delay is forced by formation of dipoles of charged FeCoZr nanoparticles with native Fe-based oxide "shells" around them that results in the increase of mean life time of hopping electrons on nanoparticles with "core-shell" structure.

Anion-deficient layered cobaltites Sr0.75Ln0.25CoO3–x (Ln is a lanthanide) have attracted the special attention of the scientists who study the nature of phase transformations in perovskite-like cobaltites, the anomalous behavior of the temperature magnetization of which is still the subject of scientific discussion. The purpose of this work is to investigate the regularity of changes in the elastic, magnetic, and electrical properties of layered cobaltites Sr1–уYуCoO3–x in the composition range 0.2 ≤ y ≤ 0.3 over a wide temperature range. The studied polycrystalline samples were obtained by the known ceramic technology in the air. Electron microscopic studies were performed on a LEO 1455 PV scanning electron microscope. The temperature dependence of the Young’s modulus was studied by the method of resonance vibrations in the frequency range 1000–6000 Hz and in the temperature range 100–450 K. X-ray phase analysis was performed on a DRON-3M diffractometer under Cu-Kα radiation. Magnetic measurements were performed using a physical property measurement system (Cryogenic Ltd.) in the temperature range 5–325 K. As a result of the studies, it was found that in the temperature range 25–300 K, Sr1–уYуCoO3–x solid solutions (0.2 ≤ y ≤ 0.3) are characterized by the semiconductor-like conductivity. No significant magnetoresistive effect was observed in this temperature range for the studied compositions. It was shown that the Sr1–уYуCoO3–x solid solution (у = 0.25) exhibits two magnetic phase transformations: low-temperature near 220 K and high-temperature at 350 K. The nearby compositions of the concentration range 0.2 ≤ y ≤ 0.3 exhibit magnetic phase transformations at temperatures above room temperature. No low-temperature phase transitions were detected in them. It has been established that magnetic phase transformations are accompanied by structural transitions at corresponding temperatures.

Neutron powder diffraction and magnetization measurements have been performed for La0.7Sr0.3Mn0.7Ti0.3-xAlxO3 (0 <= x <= 0.15) stoichiometric compounds. Increase of the Al3+ content enlarges the Mn4+ ions fraction from 0% (x = 0) up to around 20% (x = 0.15). The x = 0 composition around 150 K exhibits a structural transition from the rhombohedral phase to the orthorhombic one whereas the crystal structure of the compounds with x = 0.1 and 0.15 remains to be rhombohedral down to 2 K. The substitution of Ti4+ by Al3+ ions is accompanied by a gradual increase in the bond angle Mn-O-Mn and decrease in the Mn-O bond length which lead to enhancement of the covalent component of the chemical bond. All these compounds exhibit ferromagnetic components below 100 K. Magnetic moments estimated per manganese from the neutron powder diffraction data are found to be around 1.3 mu(B) (x = 0) and 1.7 mu(B) (x = 0.1 and 0.15) at 2 K. It is suggested that ferromagnetism is originated predominantly from the Mn3+-O-Mn3+ and Mn3+-O-Mn4+ superexchange interactions whereas bond angles fluctuation leads to magnetic frustrations. Enhancement of covalence slightly increases ferromagnetism.

In the present paper the investigations of the influence of swift heavy ion irradiation on the magnetotransport in the antimony (Sb) delta-layer in silicon are reported. Temperature and magnetic field dependences of the resistance R(T, B) and the Hall coefficient R-H(T, B) in the temperature range of 2 K < T < 300 K and B <= 8 T before and after the 167 MeV Xe+26 ion irradiation (ion fluence of 10(8) cm(-2)) were measured. At the temperatures below 50 K there is observed the transition from the Arrhenius log R(1/T) to a logarithmic R approximate to -log(T) dependence both before and after the swift heavy ion exposure which confirms the assumption that the carrier transport goes through the delta-layer mainly. Moreover, the transition from the positive to negative magnetoresistance was observed with the temperature decrease that is characteristic of the two-dimensional quantum corrections to the conductivity in the case of weak localization regime. The appropriate Thouless lengths L-Th(T) approximate to A x T-p (where p and A are dependent on the scattering mechanism) indicated their approximate to 25 - 30% decrease after the swift heavy ion exposure. It was shown that the exponent p values were close to the theoretical one of p = 1, confirming the realization of 2D weak localization regime in the carrier transport.

The present paper investigates the temperature/frequency dependences of admittance Z in the granular Cu-x(SiO2)(1-x) nanocomposite films around the percolation threshold x(C) in the temperature range of 4-300 K and frequencies of 20-10(6) Hz. The behavior of low-frequency ReZ(T) dependences displayed the predominance of electrons hopping between the closest Cu-based nanoparticles for the samples below the percolation threshold x(C) approximate to 0.59 and nearly metallic behaviour beyond the x(C). The high-frequency curves ReZ(f) at temperatures T > 10 K for the samples with x < x(C) exhibited behavior close to ReZ(f) approximate to f(-s) with s approximate to 1.0 which is very similar to the known Mott law for electron hopping mechanism. For the samples beyond the percolation threshold (x > x(C)), the frequency dependences of ReZ(f) displayed inductive-like (not capacitive) behaviour with positive values of the phase shift angles.

In the present paper we have studied the peculiarities of carrier transport properties of nano-heterostructures containing silicon substrate covered with porous silicon layer, where pores were either filled or non-filled with ferromagnetic Ni clusters. We have carried out DC conductivity experiments as a function of temperature (ranging from 2 to 300 K) and porosity of porous silicon layer (between 30% and 70%). Presence of a surface layer with high resistance on the porous silicon top and its role in nano-heterostructure formation was revealed. It was shown that specific electrochemical kinetics of Ni deposition into porous silicon significantly influences resultant nanostructure resistance and high temperature conductance activation energy. (C) 2015 Elsevier B.V. All rights reserved.

A detailed analysis of the experimental temperature dependences of the resistivity of silicon doped with arsenic with a concentration of 10 18 cm −3 is performed for the region 1.8 K < T < 25 K. It is shown that, as a result of cooling to a temperature lower than 4.5 K, a transition from the Mott mode with variable hopping length to the mode of hopping conduction via nearest neighbors is observed, while, at T < 2.5 K, a transition to the Shklovskii-Efros mechanism is possible. A model for such a temperature crossover is suggested; the model is based on simplified solution of the percolation problem with the use of an interpolation expression for the density of states. Performed estimates show that the model is in satisfactory agreement with experimental data when the minimum number of adjustable parameters are used.

We describe here structure and temperature dependences of conductivity sigma(T), the Seebeck coefficient alpha(T), thermal conductivity lambda(T) and figure-of-merit ZT(T) in Ca3Co4O9 ceramics, doped with Fe and Y, depending on compacting pressure (0.2 or 6 MPa) and temperature (300 < T < 700 K). It is shown that introduction of iron and yttrium to ceramics does not alter the crystalline structure of the material. Increasing the pressure in the compacting process before the additional diffusion annealing leads to a smaller-grained structure and increase sigma and lambda due to reducing of the synthesized samples porosity. The Seebeck coefficients of nanocomposite ceramics Ca3Co3.9Fe0.1O9 and (Ca2.9Y0.1)(Co3.9Fe0.1)O-9 have linear dependences on temperature is not changed after increase of compacting pressure. Electrical-to-heat conductivity ratio (sigma/lambda) for the samples compacted at high (6 GPa) pressure increases not more than 20-30% in comparison with ones compacted at low (0.2 GPa) pressure, whereby ZT is increased more than 50%. The main reason for this effect is samples porosity reduction with the compacting pressure increase.

Crystal structure formation of BaAl2Si2O8 known as polymorphic compound is investigated in present work depending on conditions of preparation. Characteristics of ceramics have been studied for different modifications of crystal structure. Additional technologic operations (grinding with following heat treatment) have been found to result in polymorphic transformation. Dielectric properties of BaAl2Si2O8 ceramics have been studied for hexagonal, monoclinic crystal structure modifications as well as for that based on phase mixture. It has been shown that the sintering of ceramic material based on the monoclinic crystal structure modification of BaAl2Si2O8 takes place in temperature diapason of 1300-1350 degrees C. Sintering of material with the hexagonal crystal structure modification occurs in temperature diapason of 1450-1500 degrees C. Ceramics materials based on compound BaAl2Si2O8 are found to have low porosity, high Q-factor and dielectric characteristics, allowing use of these ceramic materials for production of resonators and other microwave equipments.