Structural and dielectric properties of Ba3V2O8 ceramics

Polycrystalline sample of Ba₃V₂O₈ was prepared by a high-temperature solid-state reaction technique. Preliminary X-ray diffraction (XRD) analysis confirms the formation of single-phase compound of hexagonal (rhombohedral) crystal structure at room temperature. Microstructural analysis by scanning electron microscope (SEM) shows that the compound has well defined grains, which are distributed uniformly throughout the surface of the sample. The dielectric properties of the compound studied in a wide frequency range (10²–10⁶ Hz) at different temperatures (25–400 °C), exhibits that they are temperature dependent. Detailed analysis of impedance spectra showed that the electric properties of the material are strongly dependent on frequency and temperature. The activation energy, calculated from the temperature dependence of ac conductivity (dielectric data), was found to be 0.23 eV at 50 kHz in the higher temperature region.     

Experimental and theoretical study of AC electrical conduction mechanisms of Organic–inorganic hybrid compound Bis (4-acetylanilinium) tetrachlorocadmiate (II)

A new organic–inorganic bis (4-acetylaniline) tetrachlorocadmate [C₈H₁₀NO]₂[CdCl₄] can be obtained by slow evaporation at room temperature and characterized by X-ray powder diffraction. It crystallized in an orthorhombic system (Cmca space group). The material electrical properties were characterized by impedance spectroscopy technique in the frequency range from 209 Hz–5 MHz and temperature 413 to 460 K. Besides, the impedance plots show semicircle arcs at different temperatures and an electrical equivalent circuit has been proposed to interpret the impedance results. The circuits consist of the parallel combination of a resistance (R), capacitance (C) and fractal capacitance (CPE). The variation of the exponent s as a function of temperature suggested that the conduction mechanism in Bis (4-acetylanilinium) tetrachlorocadmiate compound is governed by two processes which can be ascribed to a hopping transport mechanism: correlated barrier hopping (CBH) model below 443 K and the small polaron tunneling (SPT) model above 443 K.     

Correlation between sintering temperature and properties of ZnO ceramic varistors

The effect of sintering temperature on ZnO varistor properties is investigated in the range of 700–1400 °C. The increase of sintering temperature does not influence the well-known peaks related to hexagonal wurtzite structure of ZnO ceramics, whereas the average grain size is increased from (1.08 to 2.1 μm). With increasing sintering temperature up to 1200 °C, the nonlinear region is clearly observed in the I–V characteristics, whereas this region is completely absent only for the sample sintered at 1400 °C. As the sintering temperature increased, the breakdown field decreased over a wide range from 2838.7 to 6.41 V/cm, while the nonlinear coefficient is increased in the range of (23.86–47.76). Furthermore, the barrier height decreased from 1.76 to 0.974 eV, whereas electrical conductivity is improved. On the other hand, the optical band gap is gradually decreased in the range of 3.08–2.70 eV with increasing sintering temperature. These results showed a strong correlation between sintering temperature and the properties of ZnO ceramic varistor.     

Characterization and organic electric-double-layer-capacitor application of KOH activated coal-tar-pitch-based carbons: Effect of carbonization temperature

The present study reports the influence of pre-carbonization on the properties of KOH-activated coal tar pitch (CTP). The change of crystallinity and pore structure of pre-carbonized CTPs as well as their activated carbons (ACs) as function of pre-carbonization temperature are investigated. The crystallinity of pre-carbonized CTPs increases with increasing the carbonization temperature up to 600 °C, but a disorder occurs during the carbonization around 700 °C and an order happens gradually with increasing the carbonization temperatures in range of 800–1000 °C. The CTPs pre-carbonized at high temperatures are more difficult to be activated with KOH than those pre-carbonized at low temperatures due to the increase of micro-crystalline size and the decrease of surface functional groups. The micro-pores and meso-pores are well developed at around 1.0 nm and 2.4 nm, respectively, as the ACs are pre-carbonized at temperatures of 500–600 °C, exhibiting high specific capacitances as electrode materials for electric double layer capacitor (EDLC). Although the specific surface area (SSA) and pore volume of ACs pre-carbonized at temperatures of 900–1000 °C are extraordinary low (non-porous) as compared to those of AC pre-carbonized at 600 °C, their specific capacitances are comparable to each other. The large specific capacitances with low SSA ACs can be attributed to the structural change resulting from the electrochemical activation during the 1st charge above 2.0 V.     

Structural,dielectric and impedance properties of NaCa2V5O15 ceramics

Polycrystalline sample of NaCa₂V₅O₁₅ (NCV) with tungsten bronze structure was prepared by a mixed oxide method at relatively low temperature (i.e. 630 °C). Preliminary structural analysis of the compound showed an orthorhombic crystal structure at room temperature. Microstructural study showed that the grains are uniformly and densely distributed over the surface of the sample. Detailed studies of dielectric properties showed that the compound has dielectric anomaly above the room temperature (i.e. 289 °C), and shows hysteresis in polarization study. The electrical parameters of the compound were studied using complex impedance spectroscopy technique in a wide temperature (23–500 °C) and frequency (10²–10⁶ Hz) ranges. The impedance plots showed only bulk (grain) contributions, and there is a non-Debye type of dielectric dispersion. Complex modulus spectrum confirms the grain contribution only in the compound as observed in the impedance spectrum. The activation energy, calculated from the ac conductivity of the compound, was found to be 0.20–0.30 eV. These values of activation energy suggest that the conduction process is of mixed type (i.e. ionic–polaronic).     

Properties of Bi2O3 thin films prepared via a modified Pechini route

Thin bismuth oxide films have been prepared by a modified Pechini route on glass substrate and annealed at temperatures ranging between 400 °C and 700 °C using bismuth nitrate as raw material. The thin films were then characterized for structural, surface morphological, optical and electrical properties by means of X-ray diffraction (XRD), Atomic force microscopy (AFM), scanning electron microscopy (SEM), optical absorption and d.c. two-probe, respectively. Structural investigations indicated that as-prepared bismuth oxide films were polycrystalline and multiphase, and annealing temperatures played a key role in the composition and optical properties of these films. AFM and SEM images revealed well defined particles which are highly influenced by annealing temperatures. The optical studies showed a direct band gap which varied with annealing temperatures between 3.63 eV and 3.74 eV. The electrical measurement showed that the electrical resistivity increased with annealing temperatures and the films were typical semiconductors. As catalyst, bismuth oxide films annealed at 550 °C had the best photocatalytic performance for photodegradation of methyl orange.     

Thermal and electrical conductivities of silver–indium–tin alloys

The variations of thermal conductivity with temperature for the Ag–[x] wt% Sn–20 wt% In alloys (x=8, 15, 35, 55 and 70) were measured using a radial heat flow apparatus. From the graphs of thermal conductivity versus temperature, the thermal conductivities of solid phases at their melting temperature for the Ag–[x] wt% Sn–20 wt% In alloys (x=8, 15, 35, 55 and 70) were found to be 46.9±3.3, 53.8±3.8, 61.2±4.3, 65.1±4.6 and 68.1±4.8 W/Km, respectively. The variations of electrical conductivity of solid phases versus temperature for the same alloys were determined from the Wiedemann–Franz equation using the measured values of thermal conductivity. From the graphs of electrical conductivity versus temperature, the electrical conductivities of the solid phases at their melting temperatures for the Ag–[x] wt% Sn–20 wt% In alloys (x=8, 15, 35, 55 and 70) alloys were obtained to be 0.036, 0.043, 0.045, 0.046 and 0.053 (×10⁸/Ωm), respectively. Dependencies of the thermal and electrical conductivities on the composition of Sn in the Ag–Sn–In alloys were also investigated. According to present experimental results, the thermal and electrical conductivities for the Ag–[x] wt% Sn–20 wt% In alloys linearly decrease with increasing the temperature and increase with increasing the composition of Sn.     

Growth-induced perpendicular magnetic anisotropy and clustering in NixPt1−x alloys

Polycrystalline and epitaxial (1 0 0), (1 1 0), and (1 1 1)-oriented Ni₃Pt, NiPt, and NiPt₃ films were deposited over a range of growth temperatures from 80°C to 700°C. Films grown at moderate temperatures (200–400°C) exhibit growth-induced properties similar to Co–Pt alloys: enhanced and broadened Curie temperature, perpendicular magnetic anisotropy and large coercivity. As in Co–Pt, the magnetic properties suggest a clustering of Ni into platelets on the growth surface, as the films are being grown. Unlike Co–Pt, however, NiPt films exhibit a strong orientational dependence of anisotropy and enhanced Curie temperature, possibly resulting from different types of surface reconstructions which affect the growth surface.     

Optical and physical characteristics of In-doped ZnO nanorods

In this paper the effect of indium dopants on structure, optical, electrical and mechanical properties of ZnO nanorods are studied. The average surface potentials and the surface currents of ZnO:In nanorods were 0.25–0.84 mV and 2.2–200 MΩ-cm, respectively. The turn-on threshold field for the vertical ZnO nanorods was around 2–16 V μm⁻¹. Emission current densities of 3.3–911.4 mA cm⁻² were obtained for an electrical field of 60–160 V μm⁻¹. The photoluminescence (PL) spectrum measured at 15–300 K showed that the intensity of the peak at 2.06 eV increased with decreasing temperature, while the peak at 2.06 eV further red shifted and the peak at 3.39 eV blue shifted.     

Finite size effect and influence of temperature on electrical properties of nanocrystalline Ni–Cd ferrites

Electrical conductivity and dielectric measurements have been investigated for four different average grain sizes ranging from 3 to 7 nm of nanocrystalline Ni_0.2Cd_0.3Fe_2.5−xAl_xO₄ (0.0 ⩽ x ⩽ 0.5) ferrites. The impedance spectroscopy technique has been used to study the effect of grain and grain boundary on the electrical properties of the Al doped Ni–Cd ferrites. The analysis of data shows only one semi-circle corresponding to the grain boundary volume suggesting that the conduction mechanism takes place predominantly through grain boundary volume in the studied samples. The variation of impedance properties with temperature and composition has been studied in the frequency range of 120 Hz–5 MHz between the temperatures 300–473 K. The hopping of electrons between Fe³⁺ and Fe²⁺ as well as hole hopping between Ni³⁺ and Ni²⁺ ions at octahedral sites are found to be responsible for conduction mechanism. The dielectric constant and loss tangent (tan δ) are found to decrease with increasing frequency, whereas they increase with increasing temperature. The dielectric constant shows an anomalous behavior at selected frequencies, while the temperature increases, which is expected due to the generation of more electrons and holes as the temperature increases. The behavior has been explained in the light of Rezlescu model.     

Dielectric relaxation of ZnO nanostructure synthesized by soft chemical method

Thioglycerol capped nanoparticles of ZnO have been prepared in methanol through chemical technique. Nanostructures of the prepared ZnO particles have been confirmed through X-ray diffraction measurement. The Debye–Scherrer formula is used to obtain the particle size. The average size of the prepared ZnO nanoparticles is found to be 50 nm. The frequency-dependent dielectric dispersion of the sample is investigated in the temperature range from 293 to 383 K and in a frequency range from 100 Hz to 1 MHz by impedance spectroscopy. An analysis of the complex permittivity (ε′ and ε′′) and loss tangent (tan δ) with frequency is performed assuming a distribution of relaxation times. The frequency-dependent maxima of the imaginary part of impedance are found to obey Arrhenius law with activation energy ∼1 eV. The scaling behavior of dielectric loss spectra suggests that the relaxation describes the same mechanism at various temperatures. The frequency-dependent electrical data are analyzed in the framework of conductivity and modulus formalisms. The frequency-dependent conductivity spectra obey the power law.     

Nanocrystalline diamond thin films deposited by 35 kHz Ar-rich plasmas

We give evidence of nanometric size (5–15 nm) crystalline diamonds in carbon thin films obtained at low substrate temperature (15 °C) under the action of low pressure (0.1–0.3 Torr) 35 kHz excited CH₄/Ar (80–95%) plasmas. The decrease in Ar concentration was found to lead to higher film hardness while crystalline nanodiamonds are observed in a wide interval (1–3.5 h) of deposition times but only for very high Ar concentration (95%). The polycrystalline nanodiamond grains are found over 10–20% within an amorphous carbon matrix. It is suggested that the distribution of nanodiamond grains might be connected to the nonuniform ion energy distribution in the Ar-rich plasma generated at 35 kHz. Morphological and structural features of the deposited films were also investigated.     

Study of the temperature dependent transport properties in nanocrystalline lithium lanthanum titanate for lithium ion batteries

The nano-crystalline Li_0.5La_0.5TiO₃ (LLTO) was prepared as an electrolyte material for lithium-ion batteries by the sol–gel method. The prepared LLTO material is characterized by structural, morphological and electrical characterizations. The LLTO shows the cubic perovskite structure with superlattice formation. The uniform distribution of LLTO particles has been analyzed by the SEM and TEM analysis of the sample. Impedance measurements at various temperatures were carried out and the temperature dependent conductivity of as prepared LLTO nanopowders at different temperatures from room temperature to 448 K has been analyzed. The transport mechanism has been analyzed using the dielectric and modulus analysis of the sample. Maximum grain conductivity of the order of 10⁻³ S cm⁻¹ has been obtained for the sample at higher temperatures.     

Effect of nanocrystallization on the electronic conductivity of vanadate–phosphate glasses

Electronically conducting glasses of the composition xV₂O₅·(100 − x)P₂O₅ for 60 < x < 90 were prepared. The glasses of the composition corresponding to x = 90 exhibited the highest electrical conductivity and they were studied in more detail. The effects of the annealing of the samples on their electrical conductivity, structure and other characteristics were studied by impedance spectroscopy, X-ray diffractometry, DSC and SEM microscopy. It was shown that, at temperatures close to the crystallization temperature T_c (determined from DSC), these glasses turned into nanomaterials consisting of crystalline grains of V₂O₅ (average size 25–35 nm) embedded in the glassy matrix. Their electrical conductivity was higher and the temperature stability was better than those of the starting glasses. It is postulated that the major role in this conductivity enhancement is played by the interfacial regions between crystalline and amorphous phases. The annealing at temperatures exceeding T_c led to massive crystallization and to a conductivity drop. The XRD and SEM observations have shown that the material under study undergoes structural changes: from amorphous at the beginning, to partly crystalline after the annealing at 340 °C and to polycrystalline after the annealing at 530 °C.The obtained results are in agreement with those of our earlier studies on mixed electronic–ionic conducting glasses of the ternary Li₂O–V₂O₅–P₂O₅ system.     

Increase of critical current density with doping carbon nano-tubes in YBa2Cu3O7−δ

The effects of carbon nano-tubes (CNTs) on the crystal structure and superconducting properties of YBa₂Cu₃O_7?δ (Y-123) compound were studied. Samples were synthesized using standard solid-state reaction technique by adding CNT up to 1 wt% and X-ray diffraction data confirm the single phase orthorhombic structure for all the samples. Current–voltage measurements in magnetic fields up to 9 T were used to study the pinning energy U_J and critical current density J_c as a function of magnetic field at fixed temperature. We find that while T_c does not change much with the CNT doping (91–92 K), both U_J and J_c increase systematically up to 0.7 wt% CNT doping in a broad magnetic field ranges between 0.1 and 9 T and J_c in the 0.7 wt% CNT doped sample is at least 10 times larger than that of the pure Y-123. The scanning electron microscope image shows that CNTs are forming an electrical-network between grains. These observations suggest that the CNT addition to the Y-123-compounds improve the electrical connection between superconducting grains to result in the J_c increase.     

Electrical and optical properties of the GaInAsSb-based heterojunctions for infrared photodiode and thermophotovoltaic cell application

The electrical end optical characteristics of a type II double heterojunction (DH) in the GaSb/GaInAsSb/GaAlAsSb system with staggered band alignment were studied. An analysis of the photodiodes performance through the investigation into electrical and optical characteristics was carried out. The dark current mechanisms in the heterostructures were investigated at several temperatures. The experimental results show that at the low temperature region, the tunneling mechanism of the current flow dominates in both forward and reverse biases. At high temperatures region and in the range of voltage from 0.1 V to 1 V, the reverse current was defined by generation of carriers in the depletion region. Have been estimated the temperature coefficient of the shift of the long-wavelength edge of the spectral sensitivity at half-maximum as Δλ/ΔT = 1.6 nm/K. Quantum efficiency of 0.6–0.7 for the investigated photodiodes was reached without any antireflection coating. For GaSb/GaInAsSb/GaAlAsSb TPV cells, the internal quantum efficiency of 90% was achieved at wavelengths between 1.2 and 1.6 μm.     

Sodium diffusion in cryolite at elevated temperatures studied by quasielastic neutron scattering

Quasielastic neutron scattering (QENS) has been applied to study the sodium mobility on nanosecond time scales in the perovskite fluoride cryolite, Na₃AlF₆, at high temperatures. Up to T = 1153 K the diffusion of Na ions is well described by a diffusion process of jumps between six and eight-fold coordinated sites. Above this temperature, where a step-like increase in the electrical conductivity occurs, the jump length increases, which indicates additional jumps over larger distances. The electrical conductivity derived from the self-diffusion coefficient via the Nernst–Einstein relation and the corresponding activation energy are in excellent agreement with the previous conductivity measurements. We conclude that the jump diffusion of sodium ions is the dominant mechanism for the electrical conductivity in cryolite at high temperatures up to T = 1153 K.     

Dielectric and ferroelectric properties of 0.8PZT–0.2PCN ceramics under sintering conditions variation

The influences of sintering conditions on electrical properties of the 0.8Pb(Zr_1/2Ti_1/2)O₃–0.2Pb(Co_1/3Nb_2/3)O₃ ceramics have been investigated with sintering temperatures of 1175, 1200, 1225, and 1250 °C and dwell times for 2, 6, and 10 h. The crystal structure of dense specimens showed coexistence between tetragonal, rhombohedral and pseudo cubic phases in all sintering temperatures, while tetragonal-rich phase appeared with increasing dwell times. A maximum dielectric constant was observed at sintering condition of 1200 °C for 2 h, while the transition temperature slightly increased with increasing dwell time. All ceramics also showed diffused phase transition behaviors with a minimum diffusivity at sintering condition of 1200 °C for 2 h. In addition, the polarization–electric field (P–E) hysteresis loops of the ceramic systems also changed significantly with sintering conditions. Interestingly, the ferroelectric parameters; remnant polarization (P_r) and loop squareness (R_sq) tended to increase with increasing sintering temperatures and dwell times.     

Effects of calcining temperature on structural and magnetic properties of nanosized Fe-doped NiO prepared by diol-based sol−gel process

Iron-doped nickel oxide (Fe_0.01Ni_0.99O, abbreviated as FNO) nanoparticles were prepared by sol–gel process using 1,3-propanediol as a solvent and also as a chelating agent, and calcined at the various temperatures (400–1000 °C) for 2 h. The phase composition and the microstructure of the calcined products were investigated by X-ray diffraction and scanning electron microscopy techniques, respectively. Magnetic properties were measured at room temperature using a vibrating sample magnetometer. All calcined samples showed the single phase of FNO cubic rock-salt structure without the presence of any impurity phases. The crystallite size from XRD and particle size from SEM increased as calcining temperature increased. The FNO powders calcined at 400?600 °C revealed the uniform and dense spherical particles in nanosize. The room-temperature ferromagnetism was observed for all samples. When the calcining temperature was increased, the saturation magnetization decreased whereas the coercivity increased, corresponding to the less dense and larger particles. The calcined sample at 400 °C had the best magnetic properties with the highest M_s of 5.34 emu/g (at 10 kOe) and the lowest H_c of 372 Oe.     

Performance and durability of Ni-coated YSZ anodes for intermediate temperature solid oxide fuel cells

NiO-coated YSZ composite powders were synthesized through the Pechini process in order to improve the performance and durability of SOFC anodes. Their microstructures and electrical properties have been investigated with thermal and redox cycling tests. The coverage of NiO crystals on the YSZ surface could be modulated by controlling the composition of the reaction mixture and the ratio of NiO and YSZ. Ni–YSZ electrodes were manufactured by sintering the die-pressed NiO–YSZ pellet at 1400 °C for 3 h, followed by reducing it to 800 °C under hydrogen atmosphere. The anode made from NiO/YSZ composite powder, which has a high homogeneity and plenty of contact sites between Ni and YSZ, has an excellent tolerance against thermal and redox cycling. The maximum power density of a single cell made from NiO/YSZ composite powder was 0.56 W cm^− 2 at 800 °C in reactive gases of humidified hydrogen and air. It can be concluded that the functional NiO/YSZ composite powder will suppress the degradation of anodes and enhance the long-term and redox stability of the unit cell at elevated temperatures.