Thermoelectric Properties for a Suspended Microribbon of Quasi-One-Dimensional TiS<Subscript>3</Subscript>

Transition-metal trichalcogenides MX₃ (M = Ti, Zr, Nb, Ta; X = S, Se) are well-known inorganic quasi-one-dimensional conductors. Among them, we have investigated the thermoelectric properties of titanium trisulfide TiS₃ microribbon. The electrical resistivity ρ, thermal conductivity κ, and thermoelectric power S were measured using 3ω method. The weight mean values were found to be ρ = 5 mω m and κ = 10 W K^?1 m^?1 along the one-dimensional direction (b-axis) of the TiS₃ microribbon. Combined with the thermoelectric power S = ?530 μV K^?1, the figure of merit was calculated as ZT = 0.0023. This efficiency is the same as that of randomly oriented bulk TiS₃. We also estimated the anisotropy of σ and κ using the present results and those for randomly oriented bulk material. The obtained weak anisotropy for TiS₃ is attributable to strong coupling between triangular columns consisting of TiS₃ units. These experimental results are consistent with theoretical results obtained using density functional theory (DFT) calculations.     

Beneficial Effect of S-Filling on Thermoelectric Properties of S<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript>4</Subscript>Sb<Subscript>11.2</Subscript>Te<Subscript>0.8</Subscript> Skutterudite

In this work, Te-doped and S-filled S _x Co₄Sb_11.2Te_0.8 (x = 0.1, 0.15, 0.2, 0.25, 0.3, 0.4) skutterudite compounds have been prepared using solid state reaction and spark plasma sintering. Thermoelectric measurements of the consolidated samples were examined in a temperature range of 300–850 K, and the influences of S-addition on the thermoelectric properties of S _x Co₄Sb_11.2Te_0.8 skutterudites are systematically investigated. The results indicate that the addition of sulfur and tellurium is effective in reducing lattice thermal conductivity due to the point-defect scattering caused by tellurium substitutions and the cluster vibration brought by S-filling. The solubility of tellurium in skutterudites is enhanced with sulfur addition via charge compensation. The thermal conductivity decreases with increasing sulfur content. The highest figure of merit, ZT = 1.5, was obtained at 850 K for S_0.3Co₄Sb_11.2Te_0.8 sample, because of the low lattice thermal conductivity.     

Optical Properties of K<Subscript>2</Subscript>O-Li<Subscript>2</Subscript>O-WO<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> Glasses: Evidence of Mixed Alkali Effect

Glass with compositions xK₂O-(30 ? x)Li₂O-10WO₃-60B₂O₃ for 0 ≤ x ≤ 30 mol.% have been prepared using the normal melt quenching technique. The optical reflection and absorption spectra were recorded at room temperature in the wavelength range 300–800 nm. From the absorption edge studies, the values of the optical band gap (E _opt) and Urbach energy (ΔE) have been evaluated. The values of E _opt and ΔE vary non-linearly with composition parameter, showing the mixed alkali effect. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple Di-Domenico model.     

Effects of Nb Content on the Ferroelectric and Dielectric Properties of Nb/Nd-Co-doped Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> Thin Films

Nd/Nb-co-substituted Bi_3.15Nd_0.85Ti_3?x Nb _x O₁₂ (BNTN _x , x = 0.01, 0.03, 0.05 and 0.07) thin films were grown on Pt/Ti/SiO₂/Si (100) substrates by chemical solution deposition. The effects of Nb content on the micro-structural, dielectric, ferroelectric, leakage current and capacitive properties of the BNTN _x thin films were investigated. A low-concentration substitution with Nb ions in BNTN _x can greatly enhance its remanent polarization (2P _r) and reduce the coercive field (2E _c) compared with those of Bi₄Ti₃O₁₂ (BIT) thin film. The highest 2P _r (71.4 μC/cm²) was observed in the BNTN_0.03 thin film when the 2E _c was 202 kV/cm. Leakage currents of all the films were on the order of 10^?6 to 10^?5 A/cm², and the BNTN_0.03 thin film has a minimum leakage current (2.1 × 10^?6 A/cm²) under the high electric field (267 kV/cm). Besides, the C–V curve of the BNTN_0.03 thin film is the most symmetrical, and the maximum tunability (21.0%) was also observed in this film. The BNTN_0.03 thin film shows the largest dielectric constant and the lowest dielectric loss and its maximum Curie temperature is 410 ± 5°C.     

Quasi-local states of Sn in Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> according to the studies of galvanomagnetic effects in classical and quantizing magnetic fields

The magnetic-field dependences of the Hall coefficient and transverse magnetoresistance are studied experimentally and theoretically in p-Bi₂Te₃ crystals doped heavily with Sn and grown by the Czochralski method in the case of both classical and quantizing magnetic fields as high as 12 T with the magnetic-field orientation along the C ₃ trigonal axis. The Shubnikov-de Haas effect and quantum oscillations of the Hall coefficient were measured at temperatures of 4.2 and 11 K. The six-ellipsoid Drabble-Wolfe model of the energy spectrum and the magnetic-field dependence of the Hall coefficient are used as the basis for the method for determining the Hall factor and Hall mobility. New evidence is obtained in support of the existence of the narrow band of impurity Sn states occupied partially with electrons against the background of the light-hole band spectrum. The parameters of impurity states are estimated including their energy (E _Sn ≈ 15 meV), the broadening (Γ « kT), and the radius of localization of the impurity state (R ≈ 30 Å).     

Effect of ZnO Nanoparticles on the Sintering Behavior and Physical Properties of Bi<Subscript>0.5</Subscript>(Na<Subscript>0.8</Subscript>K<Subscript>0.2</Subscript>)<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript> Lead-Free Ceramics

Sintered Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ + x wt.% ZnO nanoparticle (BNKT–xZnO_n) ceramics have been fabricated by conventional annealing with the aid of ultrasound waves for preliminary milling. Because of the presence of the liquid Bi₂O₃–ZnO phase at the eutectic point of 738°C, the sintering temperature decreased from 1150°C to 1000°C, and the morphology phase boundary of BNKT–xZnO_n ceramics can be clarified by two separated peaks at (002)_T and (200)_T of 2θ in the x-ray diffraction (XRD) patterns. The improvement of ferroelectric properties has been obtained for BNZT–0.2 wt.% ZnO_n ceramics by the increase of remanent polarization up to 20.4 μC/cm² and a decrease of electric coercive field down to 14.2 kV/cm. The piezoelectric parameters of the ceramic included a piezoelectric charge constant of d ₃₁ = 78 pC/N; electromechanical coupling factors k _p = 0.31 and k _t = 0.34, larger than the values of 42 pC/N, 0.12 and 0.13, respectively, were obtained for the BNKT ceramics.     

Influence of Sn<Superscript>4+</Superscript> on Structural and DC Electrical Resistivity of Ni-Zn Ferrite Thick Films

Among the soft ferrites, Ni-Zn ferrite is one of the most versatile ceramic materials because of their important electrical and magnetic properties. These properties were improved by substituting Sn⁴⁺ in Ni-Zn ferrites with chemical composition of Ni _x Zn_1+y?x Fe_2?2y Sn _y O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0; y = 0.1, 0.2). To achieve homogenous ferrite powder at lower sintering temperature and smaller duration in nano-size form, the oxalate co-precipitation method was preferred as compared to other physical and chemical methods. Using this powder, ferrite thick films (FTFs) were prepared by the screen printing technique because of its low cost and easy use. To study structural behavior, the FTFs were characterized by different techniques. The x-ray diffraction and thermo-gravimetric and differential thermal analysis studies show the formation of cubic spinel structure and ferrite phase formation, respectively. There is no remarkable trend observed in lattice constants for the Sn⁴⁺ (y = 0.1)- and Sn⁴⁺ (y = 0.2)-substituted Ni-Zn ferrites. The bond lengths as well as ionic radii on the A-site of Ni-Zn-Sn ferrites were found to decrease with increasing nickel content. The bond length and ionic radii on the B-sites remained almost constant for Sn⁴⁺ (y = 0.1, 0.2)-substituted Ni-Zn ferrites. The energy dispersive x-ray analysis confirms the elemental analysis of FTFs. The Fourier transform infrared spectra show two major absorption bands near 400 cm^?1 and 600 cm^?1 corresponding to octahedral and tetrahedral sites, respectively, which also confirms the formation of the ferrites. The field emission scanning electron microscopy images shows that the particles are highly porous in nature and located in loosely packed agglomerates. The average particle size of the FTFs lies in the range 20–60 nm. Direct current (DC) resistivity of Ni-Zn-Sn FTFs shows the semiconductor nature. The DC resistivity of Ni-Zn-Sn_0.2FTFs is lower than Ni-Zn-Sn_0.1 FTFs. The DC resistivity is found to decrease with the increase in Ni²⁺ content up to x = 0.6. It increases thereafter for a further increase in Ni²⁺ content up to x = 1.0, and a similar trend is observed for the variations of activation energy with Ni²⁺ content.     

Low-temperature instabilities of the electrical properties of Cd<Subscript>0.96</Subscript>Zn<Subscript>0.04</Subscript>Te:Cl semi-insulating crystals

The electrical properties in the temperature range 295–430 K and low-temperature (4.2 K) photoluminescence of Cd_1?xZn_xTe:Cl semi-insulating crystals grown from melts with a variable impurity content (C _Cl ⁰ = 5 × 10¹⁷–1 × 10¹⁹ cm^?3) are investigated. Nonequilibrium processes leading to a decrease in carrier concentration are observed in all the samples at low temperatures (T = 330–385 K). These changes are reversible. The activation energy of these processes E _a is found to be 0.88 eV. As with semi-insulating CdTe:Cl, the observed phenomena can be explained by a change in the charge state of background copper atoms: Cu_Cd ? Cu_i. The introduction of Zn changes the ratio of the concentrations of shallow-level donors Cu_i and Cl_Te from their levels in the initial material.     

First-Principles Study of Structural,Electronic, Optical,and Thermal Properties of BeSiSb<Subscript>2</Subscript> and MgSiSb<Subscript>2</Subscript>

Structural, electronic, optical, and thermal properties of ternary II–IV–V₂ (BeSiSb₂ and MgSiSb₂) chalcopyrite semiconductors have been calculated using the full-potential linearized augmented plane wave scheme?in the generalized gradient approximation. The optimized equilibrium structural parameters (a, c, and u) are in good agreement with theoretical results obtained using other methods. The band structure and density of states reveal that BeSiSb₂ has an indirect (Γ–Z) bandgap of about 0.61 eV, whereas MgSiSb₂ has a direct (Γ–Γ) bandgap of 0.80 eV. The dielectric function, refractive index, and extinction coefficient were calculated to investigate the optical properties, revealing that BeSiSb₂ and MgSiSb₂ present very weak birefringence. The temperature dependence of the volume, bulk modulus, Debye temperature, and heat capacities (C _v and C _p) was predicted using the quasiharmonic Debye model at different pressures. Significant differences in properties are observed at high pressure and high temperature. We predict that, at 300 K and 0 GPa, the heat capacity at constant volume C _v, heat capacity at constant pressure C _P, Debye temperature θ _D, and Grüneisen parameter γ will be about 94.91 J/mol K, 98.52 J/mol K, 301.30 K, and 2.11 for BeSiSb₂ and about 96.08 J/mol K, 100.47 J/mol K, 261.38 K, and 2.20 for MgSiSb₂, respectively.     

Dielectric Properties of (Bi<Subscript>0.5</Subscript>K<Subscript>0.5</Subscript>)ZrO<Subscript>3</Subscript> Modified (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript> Ceramics as High-Temperature Ceramic Capacitors

(1???x)K_0.5Na_0.5NbO₃-x(Bi_0.5K_0.5)ZrO₃ [abbreviated as (1???x)KNN-xBKZ, 0?≤?x?≤?0.08] lead-free ceramics have been fabricated by a solid-state processing route. Based on the x-ray diffraction data and temperature-dependent dielectric characteristics, an orthorhombic phase for x?≤?0.03 and single rhombohedral one for x?≥?0.05 at room temperature were determined. The cell volume firstly increases, then decreases and finally increases with increasing BKZ, depending on ionic size and crystallographic structure. For the sample of x?=?0.05, a temperature-stable high permittivity (~?1736?±?15%) along with low dielectric loss tangent (≤?5%) is recorded from 158°C to 407°C. In addition, the activation energies of dielectric relaxation and dc conductivity at high temperatures were characterized by impedance spectroscopy. A combined effect of lattice distortion and oxygen vacancies on the magnitude of activation energies was discussed.     

Study of Critical Behavior in Amorphous Fe<Subscript>85</Subscript>Sn<Subscript>5</Subscript>Zr<Subscript>10</Subscript> Alloy Ribbon

We have investigated the critical behavior in amorphous Fe₈₅Sn₅Zr₁₀ alloy ribbon prepared using a single-roller melt-spinning method. This alloy shows a second-order magnetic transition from paramagnetic to ferromagnetic (FM) state at the Curie temperature T _C (～306 K). To obtain more information on the features of the magnetic transition, a detailed critical exponent study was carried out using isothermal magnetization M (H, T) data in the vicinity of the T _C. Modified Arrott plot, Kouvel–Fisher plot, Widom’s scaling relation and critical isotherm analysis techniques were used to investigate the critical behavior of this alloy system around its phase transition point. The values of critical exponents determined using the above methods are self-consistent. The estimated critical exponents are fairly close to the theoretical prediction of the three-dimensional (3D) Heisenberg model, implying that short-range FM interactions dominate the critical behavior in amorphous Fe₈₅Sn₅Zr₁₀ alloy ribbon.     

Effect of thallium doping on the mobility of electrons in Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> and holes in Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>

The Shubnikov–de Haas effect and the Hall effect in n-Bi_2–xTl_xSe₃ (x = 0, 0.01, 0.02, 0.04) and p-Sb_2–xTl_xTe₃ (x = 0, 0.005, 0.015, 0.05) single crystals are studied. The carrier mobilities and their changes upon Tl doping are calculated by the Fourier spectra of oscillations. It is found shown that Tl doping decreases the electron concentration in n-Bi_2–xTl_xSe₃ and increases the electron mobility. In p-Sb_2–xTl_xTe₃, both the hole concentration and mobility decrease upon Tl doping. The change in the crystal defect concentration, which leads to these effects, is discussed.     

Influence of Sedimentation of Atoms on Structural and Thermoelectric Properties of Bi-Sb Alloys

Functionally graded thermoelectric materials (FGTMs) have been prepared by sedimentation of atoms under a strong gravitational field. Starting samples of Bi _x Sb_1?x alloys with different composition x were synthesized by melting of metals and subsequent annealing of quenched samples. The thermoelectric properties (Seebeck coefficient, electrical conductivity) of the starting materials were characterized over the temperature range from 300 K to 525 K. Strong gravity experiments were performed in a unique ultracentrifuge apparatus under acceleration of over 0.5 × 10⁶ G at temperatures of 538 K and 623 K. Changes of the microstructure and chemical composition were analyzed using scanning electron microscopy with energy-dispersive x-ray spectroscopy analysis. The distribution of the Seebeck coefficient of the Bi-Sb alloys was characterized by scanning thermoelectric microprobe. As a result of sedimentation, large changes in chemical composition (x = 0.45 to 1) were obtained. It was found that the changes in chemical composition were correlated with alterations of the Seebeck coefficient. The obtained experimental data allowed the development of a semiempirical model for the selection of optimal processing parameters for preparation of Bi-Sb alloys with required thermoelectric properties.     

Superconducting properties of (Pb<Subscript>0.05</Subscript>Sn<Subscript>0.95</Subscript>)Te doped with indium under conditions of hydrostatic compression

The superconducting properties of Pb_0.05Sn_0.95Te semiconductor alloy doped with 5 at % of In are investigated at a hydrostatic pressure of P < 7 kbar. At increasing pressure P > 1.35 kbar and T ≥ 1.3 K, the resistivity step to ρ = 0 in the temperature and magnetic-field dependences ρ(T) and ρ(H), which corresponds to the transition to the superconducting state, disappears. The experimental results are indicative of a decrease in the density of states at the Fermi level with increasing pressure, which can be interpreted as an indium-level shift deep into the valence band. The obtained data refine and supplement the results of studies devoted to investigation of the baric dependences of the critical parameters of the superconducting transition in (Pb _z Sn_1–z )_0.95In_0.05Te with varying lead content z.     

Crystalline SiN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Ultrathin Films Grown on AlGaN/GaN Using <Emphasis Type="Italic">In Situ</Emphasis> Metalorganic Chemical Vapor Deposition

Surface passivation by SiN _x films is indispensable for high-power operation of AlGaN/GaN heterojunction field-effect transistors (HFETs) since it can effectively suppress collapse in the drain current. So far, the plasma-enhanced chemical vapor deposition technique has been used for the SiN _x deposition; however, possible damage induced by the plasma processing may affect direct-current performance or reliability. In this paper, we present subsequent deposition of SiN _x ultrathin films on AlGaN/GaN in the same metalorganic chemical vapor deposition reactor. It is experimentally found that this in situ SiN _x passivation doubles the sheet carrier density at the AlGaN/GaN interface from that of the unpassivated sample. High-resolution cross-sectional transmission electron microscopy reveals that in situ SiN _x is crystallized on the AlGaN layer as island-like structures via the Stranski-Krastanov growth mode. The lattice constants of in situ SiN _x are estimated to be a ≈ 3.2 Å and c ≈ 2.4 Å, which are quite different from those of well-known Si₃N₄ crystal structures. First-principles calculation predicts that the crystal structure of in situ SiN _x is the defect wurtzite structure, which well explains the experimental results. The passivation technique using crystalline SiN _x films would be promising for high-power and high-frequency applications of AlGaN/GaN HFETs.     

Microwave Dielectric Properties of BiCu<Subscript>2</Subscript>PO<Subscript>6</Subscript> Ceramics with Low Sintering Temperature

A BiCu₂PO₆ microwave dielectric ceramic was prepared using a solid-state reaction method. As the sintering temperature increased from 800°C to 880°C, the bulk density of BiCu₂PO₆ ceramic increased from 6.299 g/cm³ to 6.366 g/cm³; the optimal temperature was 860°C. The best microwave dielectric properties [permittivity (? _r ) = ～16, a quality factor (Q × f) = ～39,110 GHz and a temperature coefficient of resonant frequency (τ _f ) = ～?59 ppm/°C] were obtained in the ceramic sintered at 860°C for 2 h. Then, TiO₂ with a positive τ _f (～+400 ppm/°C) was added to compensate the τ _f value. The composite material was found to have a near-zero τ _f (+2.7 ppm/°C) and desirable microwave properties (? _r  = 19.9, Q × f = 24,885 GHz) when synthesized at a sintering temperature of 880°C. This system could potentially be used for low-temperature co-fired ceramics technology applications.     

Crystal Growth,Structure, and Stoichiometry of the Superconducting Pyrochlore Cd<Subscript>2</Subscript>Re<Subscript>2</Subscript>O<Subscript>7</Subscript>

We report single-crystal growth of the superconducting pyrochlore Cd₂Re₂O₇ using a vapor transport technique. Several parameters of the growth conditions, including hot-zone temperature and starting stoichiometry, were varied in order to control the formation of ReO₂ inclusions, as confirmed by the electron microscopy, resistivity, and magnetic susceptibility measurements. The Rietveld refinement of x-ray (neutron) powder diffraction was found to be consistent with a cubic structure Fd3m with lattice constant a = 10.2250 (10.2358) Å and reduced coordinate of O1 = 0.3184 (0.3177) at 293 K (250 K). We also studied the oxygen stoichiometry by means of redox reactions, electron microprobe analysis (EMPA), and x-ray/neutron diffractions. Particularly, the neutron powder diffraction on the ¹¹⁴Cd-enriched specimens yielded an oxygen deficiency δ = 0.14 ± 0.03 solely at the O2 site, which was consistent with the EMPA results. The EMPA indicated that the oxygen deficiency is homogeneous in the bulk and in a range of 0.01 ± 0.18–0.23 ± 0.19.     

Structural,Mechanical and Thermodynamic Properties of Cu<Subscript>2</Subscript>CoXS<Subscript>4</Subscript> (X = Si,Ge, Sn) Studied by Density Functional Theory

We have investigated the Structural, mechanical and thermodynamic properties of Cu₂CoXS₄ (X = Si, Ge, Sn) by using the density functional theory method. In this paper, we used GGA-PBE functional to find the equilibrium structural parameters and to calculate the elastic properties. The Mulliken population analysis indicates the bonds between S atoms and other three atoms in Cu₂CoXS₄ (X = Si, Ge, Sn) exhibit the feature of covalent bond. Furthermore, the calculated elastic constants prove the mechanical stability of Cu₂CoXS₄ (X = Si, Ge, Sn) in I\(\bar 4\)2m structure. The results are given for B/G and A ^U reveal Cu₂CoXS₄ (X = Si, Ge, Sn) can behave as a ductile and elastic material. Finally, the heat capacity, thermal expansion, entropy and Debye temperature are also reported at the different pressures (0~50 GPa) and temperatures (0~1000 K).     

Photoluminescence properties of thallium-containing GeSe<Subscript>2</Subscript> and GeSe<Subscript>3</Subscript> vitreous semiconductors

The photoluminescence properties of thallium-containing vitreous semiconductor systems with stoichiometric and nonstoichiometric compositions (GeSe₂)_1–x Tl _x and (GeSe₃)_1–x Tl _x (0 ? x ? 0.1) are studied at a temperature of T = 77 K. Intrinsic defects with negative correlation energy are responsible for the Gaussian shape of the photoluminescence spectra. It is established that an increase in x in the systems does not affect the shape of the spectrum, does not generate new emission bands, shifts the photoluminescence spectra to the region of low energies, reduces the intensity of radiation, and increases its half-width. Kinetics of the fatigue of photoluminescence is different for both systems and is characterized by one curve irrespective of Tl content in the systems.     

Local symmetry and electronic structure of tin atoms in (Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript>x</Subscript>)<Subscript>1−<Emphasis Type="Italic">z</Emphasis></Subscript>In<Subscript>z</Subscript>Te lattices

Mössbauer spectroscopy of the ¹¹⁹Sn isotope has shown that the superconducting transition in (Pb_1?xSn_x)_1?zIn_zTe solid solutions is not accompanied by a change in the local symmetry of tin atoms or in their electronic structure.