Zinc Oxide Varistor Gas Sensors: I, Effect of Bi2O3 Content on the H2-Sensing Properties

Current ( I )-voltage ( V ) characteristics of porous ZnO varistors with different Bi₂O₃ content have been investigated in air as well as in H₂-air mixtures in the temperature range room temperature (RT)-600°C. The I-V characteristics measured at RT remained unchanged in the presence of H₂, but the breakdown voltage clearly shifted to a lower electric field in the temperature range 400–600°C. The breakdown voltage decreased with increasing H₂ concentration in air. The optimum amount of Bi₂O₃ for the largest decrease was found to be 1.0 mol%. Thus, ZnO varistors can be used as a new type of H₂ sensor. The results presented in this study also suggest the important role of excess oxygen ions existing at the ZnO-ZnO grain boundaries in developing the Schottky barrier as well as in the H₂-sensing mechanism of the varistors.     

Deviation from Stoichiometry in Cr2O3 at High Oxygen Partial Pressures

The deviation from stoichiometry, δ, in Cr₂−δO₃ was measured by a tensivolumetric method in the high pO₂ range of ≊10⁴ to 10⁴ Pa at 1100°C. The value of δ, or chromium vacancy concentration, was≊9×10⁻⁵ mol/mol Cr₂O₃ in air for Cr₂O₃ with 99.999% purity. The chemical diffusion coefficient, DT, determined from equilibration data was ≊4.6× cm²·s⁻¹ at 1100°C for pO₂= 2.2 ×10¹ Pa. The self-diffusion coefficient of Cr ions was calculated from and δ and found to be≊1.6×10-17 cm²-s⁻¹, in good agreement with recently measured values.     

Ion-Probe Measurement of Oxygen Self-Diffusion in Single-Crystal Al2O3

Anion self-diffusion coefficients normal to (1102) were obtained for single-crystal Al₂O₃ in a 1.3 × 10 ³ N/m² (10⁻⁵ torr) vacuum at 1585° to 1840°C. Tracer was supplied from an initial 650 to 1300 A Al₂¹⁸O₃ layer produced by the oxidation of vapor-deposited Al metal films in an ¹⁸O₂ atmosphere at 520°C. Concentration gradients extended over depths of 3000 to 5000 A and were measured by mass spectrometry of material sputtered from the samples with a beam of Ar⁺ ions. Crystals which had not been preannealed to remove surface damage displayed enhanced diffusion. Diffusion coefficients from preannealed crystals may be described by D₀ =6.4×10⁵cm²/s, with an activation energy of 188 ± 7 kcal/mol. The diffusion is interpreted as an extrinsic vacancy mechanism.     

Characterization of the Thermally Contracting Tungstates Ta22W4O67, Ta2WO8, and Ta16W18O94

An investigation of the properties of high-purity (>99 wt%) tantalum tungstates (Ta₂₂W₄O₆₇, Ta, WO₈, and Ta₁₆W₁₈O₉₄) included determination of density (bulk and theoretical), refined lattice constants, maximum use temperatures, micro-hardness, heat capacity, thermal expansion (contraction) and diffusivity, calculated thermal conductivity, and electrical resistivity. Usable to ∼ 1700 K in air or inert atmospheres, these tantalum tungstates have theoretical densities of 7.3 to 8.5 g/cm³, are relatively soft (120 to 655 kg/mm² hardnesses), and are electrical insulators (6× 10³ to 2× 10⁸Ω^.cm resistivities). The distinguishing properties of the materials are their thermal expansion (average CTE values from + 0.6×10⁻⁸/K to −5.1× 10⁻⁶/K at 293 to 1273 K), thermal expansion hysteresis with minimal observable microcracking, and thermal diffusivity     

High-Temperature Conductivity and Creep of Polycrystalline AI2O3 Doped with Fe and/or Ti

Partial ionic and electronic dc conductivities and compressional creep rate were measured for hot-pressed poly crystalline AI₂O₃ made from AI-isopropoxide (AI₂O₃(II)). The undoped material was found to contain 1.5×10¹⁸ cm⁻³ fixed valency acceptors (Mg). Properties of undoped material and material doped with Fe or Ti were investigated as a function of grain size, dopant concentration, oxygen pressure, and temperature. No fast ionic conduction along grain boundaries is found in either acceptor- or donor-dominated material. Absolute values of self-diffusion coefficients calculated from conductivity and creep indicate that both effects are limited by migration of AI, involving V _AI"in donor-, AI," in acceptor-dominated material. In creep, oxygen is transported along grain boundaries in a neutral form (O_i^p). The p_O2 dependence of σ _t and σ _h are as expected on the basis of a defect model. That of creep is weaker for reasons that are not entirely clear. An ionic conductivity with low activation energy, observed at low temperature, is attributed to the presence of AI-silicate second phase.     

Thermal Expansion of the Low-Temperature Form of BaB2O4

Thermal expansion of the low-temperature form of BaB₂O₄ (β-BaB₂O₄) crystal has been measured along the principal crystallographic directions over a temperature range of 9° to 874°C by means of high-temperature X-ray powder diffraction. This crystal belongs to the trigonal system and exhibits strongly anisotropic thermal expansions. The expansion along the c axis is from 12.720 to 13.214 Å (1.2720 to 1.3214 nm), whereas it is from 12.531 to 12.578 Å (1.2531 to 1.2578 nm) along the a axis. The expansions are nonlinear. The coefficients A, B , and C in the expansion formula L _t = L ₀(1 + At + Bt ²+ Ct ³) are given as follows: a axis, A = 1.535 × 10⁻⁷, B = 6.047 × 10⁻⁹, C = -1.261 × 10⁻¹²; c axis, A = 3.256 × 10⁻⁵, B = 1.341 × 10⁻⁸, C = -1.954 × 10⁻¹²; and cell volume V, A = 3.107 × 10⁻⁵, B = 3.406 × 10⁻⁸, C = -1.197 × 10⁻¹¹. Based on α _t = (d L _t /d t )/ L ₀, the thermal expansion coefficients are also given as a function of temperature for the crystallographic axes a , c , and cell volume V.     

Crystallization and Properties of Fe2O3—CaO—SiO2 Glasses

Nucleation and crystal growth rates and properties were studied in a two-stage heat treatment process for Fe₂O₃-CaO-SiO₂ glasses. Glass transition (T_g) and crystallization temperatures (T _c ) for the glasses lay between about 612.0° and 710.0°C, and 858.5° and 905.0°C, respectively, and magnetite was the main crystal phase. For a glass of 40Fe₂O₃. 20CaO·40SiO₂ (in wt%) the maximum nucleation rate was (68.6 ± 7) × 10⁶/mm³·s at 700°C, and the maximum crystal growth rate was 9.0 nm/min^1/2 at 1000°C. The mean crystal size of the magnetite increased from 30 to 140 nm with variation of nucleation and crystal growth conditions. The glass showed the maxima in saturation magnetization and coercive force, 212.1 × Wb/m² and 30.8 × 10³ A/m, when heat-treated for 4 h at 1000°C and 1050°C, respectively. The variation of the saturation magnetization could be quantitatively interpreted well in terms of the volume fraction of the magnetite, whereas that of the coercive forces could be explained only qualitatively in terms of the particle size of the magnetite. Hysteresis losses showed the maximum value of 1493 W/m³ when heat-treated at 1000°C for 4 h prenucleated at 700°C for 60 min, and increased linearly with increasing heat treatment time under a magnetic field up to 800 × 10³ A/m.     

Grain Growth in Sintered ZnO and ZnO-Bi2O3 Ceramics

Grain growth in a high-purity ZnO and for the same ZnO with Bi₂O₃ additions from 0.5 to 4 wt% was studied for sintering from 900° to 1400°C in air. The results are discussed and compared with previous studies in terms of the phenomenological kinetic grain growth expression: G ⁿ— G ⁿ₀= K ₀ t exp(— Q/RT ). For the pure ZnO, the grain growth exponent or n value was observed to be 3 while the apparent activation energy was 224 ± 16 kJ/mol. These parameters substantiate the Gupta and Coble conclusion of a Zn²⁺ lattice diffusion mechanism. Additions of Bi₂O₃ to promote liquidphase sintering increased the ZnO grain size and the grain growth exponent to about 5, but reduced the apparent activation energy to about 150 kJ/mol, independent of Bi₂O₃ content. The preexponential term K ₀ was also independent of Bi₂O₃ content. It is concluded that the grain growth of ZnO in liquid-phase-sintered ZnO-Bi₂O₃ ceramics is controlled by the phase boundary reaction of the solid ZnO grains and the Bi₂O₃-rich liquid phase.     

Interdiffusion and Association Phenomena in the System NiO-Al2O3

The rate of formation of NiAl₂O₄ by reaction between single crystals of NiO and Al₂O₃ can be described by k = 1.1 × 10⁴ exp (−108,000 ± 5,000/ RT ) cm²/s. In NiO the behavior of D as a function of concentration supports the Lidiard theory of diffusion by impurity-vacancy pairs. A good fit of the theory to the experimental results was obtained by assuming that Al³⁺ ions diffuse as [Al_Ni· V_Ni]'pairs. The diffusion coefficient of pairs, D_p , obeys the equation 6.6 × 10⁻² exp (−54,000 ± 3,000/ RT ) cm²/s. The free energy of association for pairs was calculated to range from 6.5 kcal/mol at 1789°C to 9.0 kcal/mol at 1540°C. The interdiffusion coefficients in the spinel showed a constant small increase with increasing concentration of Al³⁺ dissolved in the spinel.     

Synthesis of TiB2 by High-Dose Implantation of Boron Ions in Titanium Films

Boron ions were implanted at room temperature in Ti films at a high dose (7.1 × 10^I7 and 2.3 × 10¹⁸ ions/cm²), The formation of TiB₂ films was confirmed by X-ray diffraction. Boron concentration profiles in implanted films were studied by secondary-ion mass spectrometry.     

Bi2O3 Vaporization in Microwave-Sintered ZnO Varistors

Vaporization of Bi₂O₃ in microwave-sintered ZnO varistors is discussed in this study. The Bi₂O₃ vaporization of ZnO varistors sintered by a conventional electric furnace is also studied for comparison. The results show that the Bi₂O₃ vaporization in microwave-sintered ZnO varistors is more homogenous from the surface to the inside of the sample, which results from the special thermal gradient inside the microwave-sintered samples, and we also find out that the Bi₂O₃ vaporization directly affects the electrical properties of ZnO varistors. Microwave-sintered samples exhibit more excellent electrical properties than the conventional ones because the homogenous Bi₂O₃ vaporization leads to more uniform microstructures.     

Thermal Diffusivity of Be4B, Be2B, and BeB6

The thermal diffusivities of polycrystalline Be₄B, Be₂B, and BeB₆ were measured by the flash method. Generally, the thermal diffusivities at a given temperature decrease with increasing boron content. The thermal diffusivities of Be₄B, Be₂B, and BeB₆ varied from 0.13 to 0.072 to 0.031 cm²/s, respectively, at 200°C and from 0.068 to 0.038 to 0.007 cm²/s at 1000°C. Heat transport in BeB₆ is expected to occur almost entirely by phonon conduction, whereas electronic conduction probably plays a major role in Be₄B and Be₂B. Analytical expressions for the thermal diffusivities (α) of Be₄B and Be₂B at 200° to 1000°C and of BeB₆ at 25° to 1500°C are: α(Be₄B)=1/(5.83+9.05×10 3 T ), α(Be₂B)=1/(10.92+1.40×10 2 T ), and α(BeB₆)=5.60×10 4+5.72/ T +77.3/T²-4.09×10⁴/T³ cm²/s.     

Transition from Ionic to Electronic Conduction in Pure ThO2 Under Reducing Conditions

Open-circuit emf and ac conductivity studies were conducted on two batches of dense polycrystalline ThO₂. The open-circuit emf data were used to delineate the low- p o₂ ionic domain boundary for "pure" ThO₂, which is presented as a log P_θ line on a log Po₂-1/ T diagram. In addition the ionic conductivity, σ_ion, and the high-Po₂ log P_θ boundary were also determined, mainly from ac conductivity measurements, which also confirmed the Po₂^I/4 dependence of σ_p, the p-type electronic conductivity, shown by other investigators. The main results are, for the first batch, log Pθ= 12.7−220.2 × 10³/4.575T, log σ_ion= 1.9−44.3×10³/4.575T, and log P_θ=−1.0−31.4 × 10³/4.575T; for the second batch, log Pθ=11.2−219.7 × 10³/4.575T, log σ_ion= 1.7−41.6 × 10³/4.575T, and log Pθ=0.6−40.4 × 10³/4.575T. The oxygen permeability of ThO₂ tubes and the oxidation rate constant of Th were predicted from the conductivity and emf data and compared with direct measurements previously reported. The calculated and previously measured permeabilities agreed very well; however, the correlation between the predicted and previously measured oxidation kinetics was somewhat less satisfactory.     

Annealing of Irradiation-Induced Thermal Conductivity Changes in ThO2-1.3 wt% UO2

The thermal conductivities of sintered pellets of ThO₂-1.3 wt% U0₂ were measured at 60°C before and after irradiation. The irradiation temperature was below 156°C, and the exposures varied from 3.1 × 10¹⁴ to 4.7 × lo^L7 fissions/cm³. Each fission fragment damaged a region of 2.2 × 10^-16 cm³ with the reduction in conductivity saturating by about 10¹⁷ fissions/cm³. Samples having exposures from 10¹⁵ to 10¹⁶ fissions/cm³ were annealed isothermally at 651 °C or isochronally from 300° to 1200° C to study the annealing of damage. Most of the annealing occurred between 500° and 900°C. The width of this interval plus the slow isothermal annealing suggest that the damage is annealed by a number of single order processes with a spectrum of activation energies from 1.8 to 3.9 eV or, less probably, by a high order process with an activation energy of 3.55 ± 0.4 eV.     

Enhancement of Giant Dielectric Response in CaCu3Ti4O12 Ceramics by Zn Substitution

Zn-substituted CaCu₃Ti₄O₁₂ ceramics were synthesized by solid-state sintering. Their microstructures and dielectric properties were investigated. Ca(Cu_{1− x} Zn _x )₃Ti₄O₁₂ single-phase structures were obtained up to x =0.1, and the Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺ mixed-valent structure was enhanced with increasing Zn substitution. The giant dielectric response was significantly enhanced by Zn substitution. The dielectric constant increased with increasing x , and a giant dielectric constant plateau as high as ∼9 × 10⁴ was achieved for x =0.1 at 10 kHz, while that for x =0 was ∼3 × 10⁴. The enhanced giant dielectric response was profoundly concerned with the modified mixed-valent structure.     

Low Breakdown Voltage Varistors by Grain Boundary Diffusion of Molten Bi2O3 in ZnO

Diffusion of molten Bi₂O₃ into the grain boundaries of sintered, alumina-doped (0.23 and 0.7 mol%) ZnO pellets resulted in varistors with breakdown voltages in the 3–5 V range and nonlinearity coefficients of 10–24. The varistors were fabricated by spreading a thin layer of Bi₂O₃ powder on the surface of ZnO pellets and heating the combination to various temperatures (860–1155°C) and different times. The highest nonlinearity coefficients (20–24) and lowest breakdown voltages (3–5 V) were recorded in samples annealed at 860°C for 35 min. Longer annealing times and/or higher temperatures resulted in progressively higher breakdown voltages. Eventually the devices became insulating, which was attributed to the formation of an insulating Bi₂O₃ layer between the grains. Separate wetting experiments have shown that the penetration of Bi₂O₃ into ZnO grain boundaries was a strong function of alumina doping —the penetration rate was decreased by a factor of 5–7 as the ZnO was doped with as little as 0.2 mol% alumina. It is this slowing down of the penetration of the ZnO grain boundaries that is believed to be critical in the development of the low breakdown voltages observed.     

Electromechanical Properties of Pb(Yb1/2Nb1/2)O3-PbZrO3-PbTiO3 Ceramics

The electromechanical and electric-field-induced strain properties of x Pb(Yb_1/2Nb_1/2)O₃· y PbZrO₃·(1− x − y )PbTiO₃ ( x = 0.12, 0.25, 0.37; y = 0.10–0.40) ceramics have been studied systematically as a function of Pb(Yb_1/2Nb_1/2)O₃ (PYN) content and PbZrO₃/PbTiO₃ (PZ/PT) ratio. In addition, the effect of MnO₂ on the electromechanical properties of 0.12Pb(Yb_1/2Nb_1/2)O₃·0.40PbZrO₃·0.48PbTiO₃ was also investigated. The maximum transverse strain values of 1.6 × 10⁻³ for x = 0.12, 1.45 × 10⁻³ for x = 0.25, and 1.36 × 10⁻³ for x = 0.37 were obtained at the compositions which were regarded as the morphotropic phase boundary (MPB). The transverse strain was maximized at the MPB composition. The value of the maximum electromechanical coupling coefficient was 0.69 for y = 0.40 and x = 0.12 composition. In the 0.12Pb(Yb_1/2Nb_1/2)O₃·0.40PbZrO₃·0.48PbTiO₃ composition, the temperature of the maximum dielectric constant decreased and the grain size increased with an addition of MnO₂. The electromechanical coupling coefficient decreased while the mechanical quality factor rapidly increased with an addition of MnO₂. These resulted mainly from the acceptor effect of manganese ions that were produced by doping MnO₂ into the perovskite structure.     

Grain Growth of ZnO in ZnO-Bl2O3 Ceramics with Ai2O3 Additions

Grain growth of ZnO during liquid-phase sintering of a ZnO-6 wt% Bi₂O₃ ceramic was investigated for A1₂O₃ additions from 0.10 to 0.80 wt%. Sintering in air for 0.5 to 4 h at 900° to 1400°C was studied. The AI₂O₃ reacted with the ZnO to form ZnAl₂O₄ spinel, which reduced the rate of ZnO grain growth. The ZnO grain-growth exponent was determined to be 4 and the activation energy for ZnO grain growth was estimated to be 400 kJ/mol. These values were compared with the activation parameters for ZnO grain growth in other ceramic systems. It was confirmed that the reduced ZnO grain growth was a result of ZnAl₂O₄ spinel particles pinning the ZnO grain boundaries and reducing their mobility, which explained the grain-growth exponent of 4. It was concluded that the 400 kJ/mol activation energy was related to the transport of the ZnAl₂O₄ spinel particles, most probably controlled by the diffusion of O^2- in the ZnAl₂O₄ spinel structure.     

Phase Relations in the Na0.5Bi0.5TiO3–Li3xLa(2/3)−x□(1/3)−2xTiO3 System

The phase relations and the mechanism of solid-state synthesis for the Na_0.5Bi_0.5TiO₃–Li_{3 x} La_{(2/3)− x} □_{(1/3)−2 x} TiO₃ system were investigated using X-ray powder diffraction, scanning electron microscopy, and thermal analysis. The study revealed that the extent of the homogeneity range—which is related to the A-site substitution between (Na_0.5Bi_0.5)²⁺ and (Li_{3 x} La_{(2/3)− x} □_{(1/3)−2 x} )²⁺ pseudo cations of a perovskite structure—depends strongly on the ordering of the (Li_{3 x} La_{(2/3)− x} □_{(1/3)−2 x} )²⁺ species. The solid-state reaction of the compounds in the homogeneity range is completed only after multiple high-temperature firings. However, the system is also subjected to a slow thermal decomposition; this is particularly so for the compounds with a high × value and an increased Li_{3 x} La_{(2/3)− x} □_{(1/3)−2 x} TiO₃ concentration.     

Effect of Zn/Si Ratio on the Microstructural and Microwave Dielectric Properties of Zn2SiO4 Ceramics

Zn₂SiO₄ ceramics synthesized by the conventional solid-state method exhibited a low Q × f value, possibly due to the formation of a ZnO second phase. However, with a small ZnO reduction from the Zn₂SiO₄ ceramics, the ZnO second phase disappeared and grain growth occurred due to the formation of a Si-rich liquid phase. Specimens with a large grain size exhibited an improved Q × f value. In particular, the ceramics with nominal composition Zn_1.8SiO_3.8 sintered at 1300°C exhibited improved microwave dielectric properties of ɛ_r=6.6, Q × f =147 000 GHz, and τ_f=−22 ppm/°C.