Low voltage varistor ceramics based on SnO<Subscript>2</Subscript>

The nonlinear current (I)-voltage (V) characteristics of tin dioxide doped with either Nb₂O₅ and CoO or Sb₂O₃ and CoO show promising values of nonlinear coefficient (α) values (∼11) with low breakdown voltages (E _B, ∼40 V mm⁻¹). The pentavalent antimony or niobium acts as donor and increases the electronic conductivity. The crucial parameter for obtaining low breakdown voltage is the grain size, which depends upon sintering duration and temperature of these oxide ceramics.     

Vicker’s microhardness of Bi1?xSbx (x?=?0.05–0.30) crystals

The solid–liquid interface of Bi_1−x Sb _x crystal growth is very favorable for investigations of electron–phonon phenomena. Bismuth is a semimetal with high electron and hole mobilities. When Bi is doped with Sb in the range of 7–22 atomic percentage, it undergoes semimetal–semiconductor transition. Interest in Bi–Sb materials system has recently been stimulated due to promise of a new generation of thermoelectric materials based on these alloys. The starting materials used in this study, Bi and Sb, were both of 99.999% purity. The authors have studied microhardness of these crystals with the above said composition range. The crystals were grown using Zone melting method with 0.35 cm/h growth speed and 25 zone passes. The indentation method is the most widely used method for measurement of hardness of the crystals either of metallic or nonmetallic nature. This method does not require large specimens and even on a small specimen a number of measurements can be carried out. Microhardness indentation tests were carried out on the cleavage planes (111) of the Bi_1−x Sb _x (x = 0.5 to 0.30) crystals, using the Vickers diamond pyramidal indenter. The results of Vickers microhardness studies on Bi_1−x Sb _x (x = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30) are presented in this paper.     

Study on electrical properties of Ni-doped SrTiO3 ceramics using impedance spectroscopy

The ceramics, SrTiO₃ (ST) and 0.4, 0.8 atom % Ni doped SrTiO₃, were prepared by solid state reaction route. The average grain size of undoped and doped samples was measured and found to be 1.2, 1.9 and 3.7 μm, respectively. The impedance measurements were conducted at 400–600°C to separate grain and grain boundary contributions. The grain and grain boundaries relaxation frequencies were shifted to higher frequency with temperature. Bulk resistance of doped and undoped ST ceramics was more or less the same. Single grain boundary resistance of doped sample was higher than that of undoped one, indicating that GB resistance increases with acceptor doping. Activation energies were calculated to confirm the same.     

Dielectric properties of Y2O3 donor-doped Ba0.8Sr0.2TiO3 ceramics

Ba_0.8Sr_0.2TiO₃ ceramics doped with Y₂O₃ from 0 to 0.10 mol% exhibit normal ferroelectric phase transition, while the ceramics doped with Y₂O₃ from 0.20 to 0.30 mol% show a giant dielectric constant behavior with loss less than 0.15 at 1 kHz from −40 °C to 140 °C, which is suggested due to semiconductive grain and the Maxwell–Wagner effect by structure disordering in grain boundary. The analyses of unipolar charge for the semiconductive grain indicate three kinds of dielectric processes: thermally stimulated process of unipolar hopping, dispersion process of dielectric constant with frequency, and phase transition process accompanied with disappearance of giant dielectric constant in cubic phase. The XPS results confirm that some of the barium ions are in low energy state to form e-Ba²⁺ and to provide hopping sites for electrons. The ceramics doped with Y₂O₃ from 0.50 to 0.75 mol% recover the normal ferroelectricity. The possible mechanics are relevant to binding effect of cation vacancies on electrons.     

Low dielectric loss and enhanced tunable properties of Cr-doped barium strontium titanate solid solution

The dielectric response and tunable properties of Cr-doped Ba_0.6Sr_0.4TiO₃ solid solution were investigated. The XRD patterns revealed that all of the doped specimens are cubic perovskite phase, and the SEM micrographs showed that the grain size increases with the increase of Cr content below 0.6 mol%. The dielectric frequency spectra of the doped BST solid solution showed that an additional relaxation of the doped specimens is observed at frequencies above 10 MHz. Both of the tunability and dissipation factor were improved by doping with Cr concentration lower than 1.0 mol% compared with undoped material. The 0.6 mol% Cr-doped specimen reveals a maximum tunability and figure of merit of 23.3% and 518.3, respectively. Extremely low dissipation factor in order of 5 × 10⁻⁴ is found for 0.4–0.6 mol% Cr-doped BST solid solution. The dramatically low dissipation factor of the specimen is explained by the reducing of Cr³⁺ to Cr²⁺ and the acceptor action of Cr³⁺ and Cr²⁺ which neutralized the donor action of oxygen vacancies.     

Electrical properties and stability against DC accelerated aging stress of lanthania doped praseodymia-based zinc oxide varistor ceramics

The microstructure, electrical properties, and stability against DC accelerated aging stress of the varistor ceramics, which are composed of ZnO–Pr₆O₁₁–CoO–Cr₂O₃–La₂O₃-based ceramics, were investigated for various La₂O₃ contents. The increase of La₂O₃ content led to more densified ceramics, whereas abruptly decreased the nonlinear properties by incorporating beyond 1.0 mol%. The highest nonlinearity was obtained from 0.5 mol% La₂O₃, in which the nonlinear exponent is 81.6 and the leakage current is 0.1 μA. As the La₂O₃ content increased, the donor concentration increased in from 0.64 × 10¹⁸ to 16.89 × 10¹⁸/cm³ and the barrier height greatly decreased with increasing La₂O₃ content, reaching a maximum (1.47 eV) in 0.5 mol% La₂O₃. The varistors doped with 0.5 mol% La₂O₃ exhibited high stability, in which the variation rates of varistor voltage, nonlinear exponent, and leakage current were −1.14%, −3.7%, and +100%, respectively, for stressing state of 0.95 V _{1 mA}/150 °C/24 h.     

Influence of antimony doping on structure and conductivity of tin oxide whiskers

Tin dioxide whiskers doped with different concentrations of antimony (0-0.25 at.%) have been grown from SnO and Sb₂O₃ mixture in a tube furnace in a flowing mixture of argon and oxygen at a constant source temperature. The whiskers possess high structural perfection. Influence of Sb on crystal structure, morphology and conductivity of SnO₂ whiskers is investigated. Antimony doping allows a decrease in the resistance of SnO₂ whiskers up to 10⁶ times.     

Microwave dielectric ceramics in the BaO–TiO2–ZnO system doped with Nb2O5

The microwave dielectric properties of BaO–TiO₂–ZnO (BTZ) system ceramics were studied as a function of the amount of Nb₂O₅ dopant. With the addition of 0–0.025 mol% Nb₂O₅, the substitution of Ti⁴⁺ ions with Nb⁵⁺ ions decreased the sintering temperature and increased the dielectric constant ε_r and quality factor Q of the system due to the similar atomic radius of Nb⁵⁺ and Ti⁴⁺ ions. When the amount of Nb⁵⁺ increased further (>0.025 mol%), Q was decreased by increasing the titanium vacancies. When the system doped with 0.025 mol% Nb₂O₅ was sintered at 1160 °C for 6 h, the ε_r, Qf₀, and the temperature coefficient of resonant frequency (τ_f) were 36.7, 41,000 GHz, and −5.0 ppm/°C, respectively, at 5 GHz.     

Electrical properties of SnO<Subscript>2</Subscript> based varistor ceramics with CuO addition

Current–voltage characteristics, temperature dependences of the low-field dc conductivity and the relative dielectric permittivity of tin dioxide based SnO₂–Co₃O₄–Nb₂O₅–Cr₂O₃ ceramics with CuO addition in the range 0–8 mol% are studied. These materials exhibit non-Ohmic conduction (the varistor effect). If 0.5 mol% CuO is added, the nonlinearity coefficient is increased from 54 to 75 and the electric field at fixed current density is decreased from 628 to 390 kV m⁻¹. Higher amounts of CuO substantially diminish non-Ohmic behaviour. Thickness dependence of the nonlinearity coefficient and the electric field at fixed current density in SnO₂ varistor ceramics is observed. Electrical properties are discussed in the frames of grain-boundary controlled conduction mechanism.     

Codoping of Ge with Al and Sb

Ge single crystals were doped with Al or Sb to various concentrations and codoped with Al and Sb in the ratios 3:1, 1:1, and 1 : 3. Using Hall effect measurements, the electron and hole concentrations in the doped crystals were determined. A relationship between the dopant-solubility limits and the maximum carrier concentration was found. The data obtained were shown to be consistent with the processes Al ↔ Al^- + h and Sb ↔ Sb⁺ + e in Ge doped with Al or Sb up to the solubility limit. In the material codoped with Al and Sb in the atomic ratio 3 : 1, the hole concentration is equal to the difference in the Al and Sb concentrations. At an Al : Sb ratio of 1 : 3, the electron concentration in Ge(Al, Sb) is higher than that in Ge<Sb> because of the increase in Sb solubility in the presence of Al. The crystals doped with equal concentrations of Al and Sb aren-type. The results are interpreted in terms of donor-acceptor interaction and compensation effect.     

The abnormal grain growth and dielectric properties of (Nb,Ba) doped TiO2 ceramics

The effect of consolidation pressure and crystallite size of powders crystal phases of TiO₂ on sintered microstructure of TiO₂ ceramics doped with 0.25 mol % Nb and 1.0 mol % Ba were investigated. Also, the development sequence of abnormal grain growth of (niobium, barium) doped TiO₂ ceramics was proposed. The second phases of as-sintered surface were determined. The dielectric properties of Ag-electroded samples were correlated with the resistivity of the bulk (Nb, Ba) doped TiO₂ ceramics. Abnormal grain growth lowered the resistivity of bulk material of (Nb, Ba) doped TiO₂ ceramics, and moved the relaxation frequency of fan δ to high frequency region over 10⁵ Hz. Controlling the sintered microstructures can obtain reasonably good dielectric properties.     

Electrical conductivity,relaxation, and scaling analysis studies of lithium alumino phosphate glasses and glass ceramics

Different compositions of lithium aluminum phosphate glasses were prepared by melt quenching technique. The best bulk conductivity achieved by the sample G₃, (28 mol% of lithium oxide). Further, the investigation extended by crystallizing the G₃ sample at different temperatures, 200 °C (GC₂₀₀), 300 °C(GC₃₀₀), 400 °C (GC₄₀₀), and 500 °C (GC₅₀₀). The electrical measurements for all the glasses and glass ceramics were carried out in the frequency range of 1–10⁵ Hz and at a temperature range of 393–513 K by the impedance spectroscopy. The variation of conductivity with frequency of the samples was explained in the light of different valency states of aluminum ions. AC conductivity data are fitted to a power law equation. Scaled spectra for ac conductivity and modulus data suggested that the present glass samples follow temperature independent conductivity distribution relaxation mechanism.     

Microstructure and thermoelectric properties of tungsten trioxide ceramics doped with a low amount of terbium dioxide

The effect of the nonstoichiometric compound terbium dioxide (Tb₄O₇) on the thermoelectric properties of tungsten trioxide (WO₃) ceramics was investigated. Among the sintered ceramics, the sample doped with 0.1 mol% Tb₄O₇ showed the maximum grain size and density. Doping with Tb₄O₇ also increased the electrical conductivity (σ) of the ceramics by about two orders of magnitude, and the sample doped with 0.1 mol% Tb₄O₇ showed the highest electrical conductivity. The absolute value of Seebeck coefficient (|S|) of the doped samples increased as well. Consequently, the power factor (σs ²) markedly increased. The sample doped with 2.0 mol% Tb₄O₇ demonstrated the maximum σs ² of 2.88 μW m^?1 K^?2 at 973 K, which was larger than the highest recorded σs ² for WO₃ ceramics (2.71 μW m^?1 K^?2 at 1,023 K). In addition, the low-doped sample (0.1 mol%) exhibited excellent thermoelectric properties.     

Studies on semiconductive (Ba0.8Sr0.2) (Ti0.9Zr0.1)O3 ceramics

In order to produce semiconductive (Ba_0.8Sr_0.2) (Ti_0.9Zr0.1)O₃ ceramics (BSZT), providing low resistivity for boundary-layer capacitor applications, a controlled valency method and a controlled-atmosphere method were applied and studied. In the controlled-valency method, trivalent ions (La³⁺ Sb³⁺) and pentavalent ions (Nb⁵⁺, Sb⁵⁺, Ta⁵⁺) were doped into BSZT ceramics, while in the controlled-atmosphere method, samples were sintered in air and a reducing atmosphere. The doped BSZT ceramics sintered in the reducing atmosphere showed much lower resistivities and smaller temperature coefficient of resistivity (TCR) than those sintered in air, indicating that low partial pressure of oxygen will increase the solubility of the donor dopant and enhance the grain growth. In addition, a small negative TCR at low temperature, as well as a small positive TCR at higher temperature, are also observed for specimens fired in a reducing atmosphere. The former is attributed to the semiconductive grain and the latter to the small barrier layer formed at the grain boundary.     

Effect of aluminum doping on electrical and dielectric aging behavior against current impulse of ZPCCY-based varistors

The electrical and dielectric aging behavior against current impulse (5–1,200 A) in the Zn-Pr-Co-Cr-Y-based varistors was investigated with aluminum doping level (0–0.01 mol%). The varistors doped with 0–0.001 mol% Al were destroyed at higher current impulse beyond 900 A and the varistors doped with 0.005–0.01 mol% Al exhibited high stability against current impulse. The clamp ratio (K) at given current impulse ranges decreased with increasing Al doping level. The varistor doped with 0.01 mol% Al exhibited the lowest K value, with 1.65 at a current impulse of 10 A and 2.38 at a current impulse of 1,200 A. The best electrical and dielectric stability against current impulse of 1,200 A was obtained at 0.01 mol% Al, where %\Updelta E_{1  \textmA/cm²} = -3.4%\%\Updelta E_{1\;\text{mA/cm}^2} = -3.4\%, %Δα = 0%, %ΔJ _L = −26.3%, %Δε′_APP = +3.4%, and %Δtanδ = −7.7%. Conclusively, Al doping level was optimized at 0.01 mol% in terms of the surge withstand capability (SWC).     

Physicomechanical and thermophysical properties of SiC-based ceramics

Fine-grain SiC-based ceramics have been produced via infiltration of molten silicon into preforms fabricated from SiC and graphite powders, with a phenol-formaldehyde resin as a binder. The materials thus prepared have a density of 2.70–3.15 g/cm³, dynamic modulus of elasticity from 200 to 400 GPa, compressive strength from 800 to 1900 MPa, bending strength from 150 to 315 MPa, thermal expansion coefficient (KTE) of 4.1 × 10⁻⁶ K⁻¹, and thermal conductivity of 140–150 W/(m K). Their properties are compared to those of known silicon carbide materials fabricated by other processes. The results indicate that the density and physicomechanical properties of the silicon carbide ceramics depend little on the fabrication process and are determined primarily by the SiC content. Increasing the SiC content from 20 to 99.5 wt % increases the density of the ceramics from 2.2 to 3.15 g/cm³ and leads to an exponential rise in their physicomechanical parameters: an increase in modulus of elasticity from 95 to 430 GPa, in compressive strength from 120 to 4200 MPa, and in bending strength from 70 to 410 MPa. The thermal conductivity of the ceramics depends very little on the fabrication process, falling in the range 100–150 W/(m K) over the entire range of SiC concentrations. Their KTE decreases slightly, from 4.3 × 10⁻⁶ to 2.4 × 10⁻⁶ K⁻¹, as the SiC content increases to 99–100 wt %.     

Synthesis and properties of a zinc cadmium sulfide based low-voltage cathodoluminescent phosphors

This paper describes the fabrication of a red-orange-emitting (Zn,Cd)S:(Ag,In,Sb) cathodoluminescent phosphor which offers enhanced brightness at a low excitation voltage. The effects of antimony and chlorine added to the starting mixture as coactivators are analyzed. The largest brightness gain, observed at C _Sb/C _Ag ≃ 1.3, is probably due to the formation of Ag⁺-Sb³⁺ associates. Increasing the C _Sb/C _Ag ratio to above ≃ 2.2 leads to quenching of luminescence.     

Spark-plasma-sintering temperature dependence of structural and piezoelectric properties of BNT–BT<Subscript>0.08</Subscript> nanostructured ceramics

(Bi_0.5Na_0.5)TiO₃ doped with 8 mol% BaTiO₃ powder prepared by sol–gel was compacted and sintered by spark-plasma-sintering method. The influence of spark-plasma-sintering temperature on the densification and piezoelectric properties of these ceramics was studied. Starting from BNT–BT_0.08 powder gel with a microstructure consisting of particles with average size of 50 nm, ceramics with grain size of 60–90 nm and density of about 98.9–99.6% of the theoretical density were obtained by spark-plasma-sintering at 800–900 °C. Increasing the sintering temperature by SPS from 800 to 900 °C lead to the increase of d ₃₃, k _p, ε₃₃^T and, decrease of Q _m. Typical d ₃₃ and k _p values of BNT–BT_0.08 ceramics sintered by spark-plasma-sintering at 900 °C were 8 and 0.029, respectively.     

Microstructure and electrical properties of Sm2O3 doped Bi2O3-based ZnO varistor ceramics

The dependence of the bulk density, microstructure and dc electrical properties of bismuth oxide (Bi₂O₃)-based zinc oxide (ZnO) varistor ceramics for various samarium oxide (Sm₂O₃) contents was investigated. The value of bulk density was found to 5.43-5.50 g cm⁻³ with Sm₂O₃ (mol%) content. The maximum value of bulk density is observed to be 5.50 for 0.30 mol% Sm₂O₃ containing varistor ceramics. The grain sizes for all the samples calculated from the scanning electron micrographs were found to decrease as Sm₂O₃ increases. The presence of ZnO phases, Bi-rich phases, spinel phases and Sm₂O₃ phases were observed in the samples by the energy dispersive X-ray analysis and X-ray diffraction analysis. As the Sm₂O₃ amount increased in the Bi₂O₃-based ZnO varistor ceramics, the nonlinear coefficient, α increased up to 0.10 mol%, reaching a maximum value of 58 and then decreased. The breakdown electric field, E_b, increased with the increase of Sm₂O₃ content with a maximum value of 3220 V cm⁻¹ for the 0.75 mol% Sm₂O₃ doped ZnO varistor ceramics. The leakage current, I_L, showed a minimum value of 1.10 μA for the composition of 0.30 mol% Sm₂O₃ doped Bi₂O₃-based ZnO varistor ceramics. The 0.30 mol% Sm₂O₃-doped Bi₂O₃-based ZnO varistor ceramics sintered at 1200 °C exhibited a good stability for dc accelerated aging stress of 0.90 V_1 mA/90 °C/12 h.     

The FTIR studies of SnO2:Sb(ATO) films deposited by spray pyrolysis

Thin films of SnO₂:Sb were prepared by ultrasonic spray pyrolysis method. The role of antimony in the SnO₂ is demonstrated by FTIR. It is found that at low Sb concentrations, the Sb ions prefer to substitute Sn⁴⁺ in form of Sb⁵⁺. While beyond a certain Sb concentration, Sb ions start to be reduced to Sb³⁺, which have a negative effect on carrier concentration that, in turn, affects the reflectivity of SnO₂:Sb films. The increased disorder of SnO₂ at high Sb concentration is also shown by FTIR. The discussion of carrier scattering suggests that the ionized impurity and/or neutral impurity scattering are the dominant scattering mechanisms.