Cu<Subscript>3</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript>: An Ultralow-Firing Microwave Dielectric Ceramic with Good Temperature Stability and Chemical Compatibility with Aluminum

An ultralow-firing microwave dielectric ceramic Cu₃Mo₂O₉ with orthorhombic structure has been fabricated via a solid-state reaction method. X-ray diffraction analysis, Rietveld refinement, Raman spectroscopy, energy-dispersive spectrometry, and scanning electron microscopy were employed to explore the phase purity, crystal structure, and microstructure. Pure and dense Cu₃Mo₂O₉ ceramics could be obtained in the sintering temperature range from 580°C to 680°C. The sample sintered at 660°C for 4 h exhibited the highest relative density (～ 97.2%) and best microwave dielectric properties with ε _r = 7.2, Q × f = 19,300 GHz, and τ _f = ? 7.8 ppm/°C. Chemical compatibility with aluminum electrodes was also confirmed. All the results suggest that Cu₃Mo₂O₉ ceramic is a promising candidate for use in ultralow-temperature cofired ceramic applications.     

Ultra-Low Loss Microwave Dielectric Ceramic Li<Subscript>2</Subscript>Mg<Subscript>2</Subscript>TiO<Subscript>5</Subscript> and Low-Temperature Firing Via B<Subscript>2</Subscript>O<Subscript>3</Subscript> Addition

Li₂Mg₂TiO₅, a rock-salt structured ceramic fabricated by a solid-state sintering technique, was characterized at the microwave frequency band. As a result, a microwave dielectric permittivity (ε_r) of 13.4, a quality factor of 95,000 GHz (at 11.3 GHz), and a temperature coefficient of resonance frequency (τ_f) of ? 32.5 ppm/°C have been obtained at 1320°C. Li₂Mg₂TiO₅ ceramics have low permittivity, a broad processing temperature region, and a low loss, making them potential applications in millimeter-wave devices. Furthermore, B₂O₃ addition efficiently lowered the sintering temperature of Li₂Mg₂TiO₅ to 900°C, which opens up their possible applications in low-temperature co-fired ceramics (LTCC) technology.     

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.     

Theoretical Study of Electronic Structure and Thermoelectric Properties of Doped CuAlO<Subscript>2</Subscript>

The doping level dependence of thermoelectric properties of delafossite CuAlO₂ has been investigated in the constant scattering time (τ) approximation, starting from the first principles of electronic structure. In particular, the lattice parameters and the energy band structure were calculated using the total energy plane-wave pseudopotential method. It was found that the lattice parameters of CuAlO₂ are a = 2.802 Å and c = 16.704 Å, and the internal parameter is u = 0.1097. CuAlO₂ has an indirect band gap of 2.17 eV and a direct gap of 3.31 eV. The calculated energy band structures were then used to calculate the electrical transport coefficients of CuAlO₂. By considering the effects of doping level and temperature, it was found that the Seebeck coefficient S(T) increases with increasing acceptor doping (A _d) level. The values of S(T) in our experiments correspond to an A _d level at 0.262 eV, which is identified as the Fermi level of CuAlO₂. Based on our experimental Seebeck coefficient and the electrical conductivity, the constant relaxation time is estimated to be 1 × 10^?16 s. The power factor is large for a low A _d level and increases with temperature. It is suggested that delafossite CuAlO₂ can be considered as a promising thermoelectric oxide material at high doping and high temperature.     

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.     

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.     

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.     

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.     

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.     

Interphase interactions and features of structural relaxation in TiB<Subscript>x</Subscript>-<Emphasis Type="Italic">n</Emphasis>-GaAs (InP,GaP, 6<Emphasis Type="Italic">H</Emphasis>-SiC) contacts subjected to active treatment

The results of experimental studies of interphase interactions in TiB_x-n-GaAs (GaP, InP, 6H-SiC) contacts stimulated by external effects are described. These effects are rapid thermal annealing at temperatures as high as 1000°C, microwave treatment at f=2.5 GHz, and ⁶⁰Co γ radiation in the range of doses 10⁵–10⁷ rad. Possible thermal and athermal relaxation mechanisms of internal stresses are considered. It is shown that thermally stable TiB_x-n-GaAs (GaP, InP, 6H-SiC) interfaces can be formed.     

Structural and Electrical Properties of Ag/<Emphasis Type="Italic">n</Emphasis>-TiO<Subscript>2</Subscript>/<Emphasis Type="Italic">p</Emphasis>-Si/Al Heterostructure Fabricated by Pulsed Laser Deposition Technique

We have investigated the structural and electrical characteristics of the Ag/n-TiO₂/p-Si/Al heterostructure. Thin films of pure TiO₂ were deposited on p-type silicon (100) by optimized pulsed laser ablation with a KrF-excimer laser in an oxygen-controlled environment. X-ray diffraction analysis showed the formation of crystalline TiO₂ film having a tetragonal texture with a strong (210) plane as the preferred direction. High purity aluminium and silver metals were deposited to obtain ohmic contacts on p-Si and n-TiO₂, respectively. The current–voltage (I–V) characteristics of the fabricated heterostructure were studied by using thermionic emission diffusion mechanism over the temperature range of 80–300 K. Parameters such as barrier height and ideality factor were derived from the measured I–V data of the heterostructure. The detailed analysis of I–V measurements revealed good rectifying behavior in the inhomogeneous Ag/n-TiO₂/p-Si(100)/Al heterostructure. The variations of barrier height and ideality factor with temperature and the non-linearity of the activation energy plot confirmed that barrier heights at the interface follow Gaussian distributions. The value of Richardson’s constant was found to be 6.73 × 10⁵ Am^?2 K^?2, which is of the order of the theoretical value 3.2 × 10⁵ Am^?2 K^?2. The capacitance–voltage (C–V) measurements of the heterostructure were investigated as a function of temperature. The frequency dependence (Mott–Schottky plot) of the C–V characteristics was also studied. These measurements indicate the occurrence of a built-in barrier and impurity concentration in TiO₂ film. The optical studies were also performed using a UV–Vis spectrophotometer. The optical band gap energy of TiO₂ films was found to be 3.60 eV.     

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.     

Radiative recombination in Ge<Superscript>+</Superscript>-implanted SiO<Subscript>2</Subscript> films annealed under hydrostatic pressure

Photoluminescence (PL) spectra and PL excitation spectra were recorded at room temperature from SiO₂ films implanted with Ge⁺ ions and annealed at temperature T _a =450–1100°C under hydrostatic pressure P=12 kbar. The emergence of features in the violet and green bands of the PL and PL excitation spectra correlates with the formation of hydrostatically strained Ge nanocrystals. The shift of the PL bands to higher energies, which occurs as the annealing temperature is raised to T _a ≥800°C, can be attributed to a shift of the energy levels related to the radiative recombination centers, which is caused by the increasing deformation potential. The observed PL is accounted for by the enhanced probability of direct radiative transitions in Ge nanocrystals with an X-like conduction band.     

Special features of charge transport in PbGa<Subscript>2</Subscript>Se<Subscript>4</Subscript> crystals

The current-voltage (I-V) characteristics of PbGa₂Se₄ single crystals grown by the Bridgman-Stockbarger method with a resistivity of 10¹⁰–10¹² Ω cm were measured. The value of the majority carrier mobility μ=14 cm² V^?1 s^?1, calculated by the differential method of analysis of I-V characteristics, makes it possible to evaluate a number of parameters: the carrier concentration at the cathode (n_c0=2.48 cm^?3), the width of the contact barrier d_c=5.4×10^?8 cm, the cathode transparency D _c ^* =10^?5–10^?4 eV, and the quasi-Fermi level E_F=0.38 eV. It is found that a high electric field provides the charge transport through PbGa₂Se₄ crystals in accordance with the Pool-Frenkel effect. The value of the dielectric constant calculated from the Frenkel factor is found to be equal to 8.4.     

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.     

Thermoelectric Properties of Cu-doped Bi<Subscript>0.4</Subscript>Sb<Subscript>1.6</Subscript>Te<Subscript>3</Subscript> Prepared by Hot Extrusion

Cu_0.003Bi_0.4Sb_1.6Te₃ alloys were prepared by using encapsulated melting and hot extrusion (HE). The hot-extruded specimens had the relative average density of 98%. The (00l) planes were preferentially oriented parallel to the extrusion direction, but the specimens showed low crystallographic anisotropy with low orientation factors. The specimens were hot-extruded at 698 K, and they showed excellent mechanical properties with a Vickers hardness of 76 Hv and a bending strength of 59 MPa. However, as the HE temperature increased, the mechanical properties degraded due to grain growth. The hot-extruded specimens showed positive Seebeck coefficients, indicating that the specimens have p-type conduction. These specimens exhibited negative temperature dependences of electrical conductivity, and thus behaved as degenerate semiconductors. The Seebeck coefficient reached the maximum value at 373 K and then decreased with increasing temperature due to intrinsic conduction. Cu-doped specimens exhibited high power factors due to relatively higher electrical conductivities and Seebeck coefficients than those of undoped specimens. A thermal conductivity of 1.00 Wm^?1 K^?1 was obtained at 373 K for Cu_0.003Bi_0.4Sb_1.6Te₃ hot-extruded at 723 K. A maximum dimensionless figure of merit, ZT _max = 1.05, and an average dimensionless figure of merit, ZT _ave = 0.98, were achieved at 373 K.     

Temperature and Excitation Power-Density Dependences of the Photoluminescence of BaZrO<Subscript>2.9</Subscript> Compound

Temperature- and excitation density-dependent photoluminescence are presented and discussed in detail for the BaZrO_2.9 compound prepared from the non-luminescent BaZrO₃ sample. The thermal-quenching behavior of the photoluminescence peak intensity is described with two activation energies, E _a1 = 185 and E _a2 = 139 meV. The variation of the integrated photoluminescence intensity with the excitation density shows a linear behavior with the slope close to α = 1.06, indicating that the PL signal is attributed to the excitonic recombination.     

Transport Properties of CuInTe<Subscript>2</Subscript> Thin Films Obtained by the Electrochemical Route

CuInTe₂ (CIT) thin films were potentiostatically electrodeposited onto cadmium sulfide thin films coated on fluorine doped tin oxide (FTO) glass in an aqueous bath at 75°C by the standard three-electrode system at ??0.7 V and ??0.8 V, with respect to an Ag/AgCl reference electrode. The electrodeposited layers were heat treated at?～?80°C in air ambient for 60 min. X-ray diffraction pattern and Raman analysis confirmed the formation of chalcopyrite CIT thin films upon heat treatment. The optical band gap of heat treated CIT films was found to be?～?1.0 eV and 0.95 eV deposited at ??0.7 V and ??0.8 V, respectively. Compact and good adhesive growth of CIT layers onto CdS coated FTO substrates is confirmed by field emission scanning electron microscopy. The current density–voltage (J–V) and capacitance–voltage (C–V) measurement was studied to understand the electronic quality of material for development of CIT layers for solar cell applications. The current density was found to be increased by two orders of magnitude upon low-temperature heat treatment. The capacitance–voltage measurement showed sharp depletion and accumulation region. The built in potential was found to be ～?60 mV and 145 mV in the as-deposited samples, deposited at ??0.7 V and ??0.8 V, respectively, whereas upon heat treatment it shifted to 159 mV and 210 mV. The capacitance of the CIT films was found to be a function of applied bias and increased with increasing the bias voltage. The depletion width of the heat treated sample was found to be?～?20 nm and 200 nm for the sample deposited at ??0.7 V and ??0.8 V, respectively. Thus, the sample deposited at ??0.8 V shows optimum electronic properties and is found to be suitable for opto-electronic applications.     

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.     

Electrical properties of ZnSiAs<Subscript>2</Subscript> irradiated with protons

The electrical properties of p-ZnSiAs₂ irradiated with protons (energy E = 5 MeV, dose D ≤ 2 × 10¹⁷ cm^?2) are studied. Experimental data and results of calculations are used to estimate the limiting position of the Fermi level in the band gap of the irradiated material (at the midgap E _g/2). The thermal stability of radiation defects in the temperature range from 20 to 610°C was analyzed.