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.     

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.     

Grain Growth of Zinc Oxide During the Sintering of Zinc Oxide—Antimony Oxide Ceramics

Grain growth in a high-purity ZnO with systematic additions of Sb₂O₃ from 0.29 to 2.38 wt% was studied for sintering in air from 1106° to 1400°C. The results are discussed and compared with previous studies of pure ZnO and ZnO with Bi₂O₃ additions in terms of the kinetic grain growth expression: Gⁿ – Gⁿ ₀= K ₀ t exp(— Q/RT ). Additions of Sb₂O₃ inhibited the grain growth of ZnO and increased the grain growth exponent ( n -value) to 6 from 3 for pure ZnO and 5 for the ZnO—Bi₂O₃ ceramic. The apparent activation energy for the grain growth of ZnO also increased to about 600 kJ/mol from 220 kJ/mol for pure ZnO and 150 kJ/mol for the ZnO—Bi₂O₃ ceramics. Both the grain growth exponent and the activation energy were independent of the Sb₂O₃ content. Particles of the Zn₇Sb₂O₁₂ spinel were observed on the grain boundaries and at the grain triple point junctions. It was also observed that the Sb₂O₃ additions caused twin formation in each ZnO grain. It is concluded that both the Zn₇Sb₂O₁₂ particles and the twins are responsible for the ZnO grain growth inhibition by Sb₂O₃.     

Formation of Varistor Characteristics by the Grain-Boundary Penetration of ZnO-PrOx Liquid into ZnO Ceramics

Penetration of a liquid (ZnO-PrO _x ) into the grain boundaries of sintered, cobalt-doped ZnO pellets resulted in varistors with breakdown voltages per grain boundary in the 1-2 V range and nonlinearity coefficients of 22-37. The varistors were fabricated by spreading a thin layer of Pr₆O₁₁ powder paste on the surface of ZnO pellets and heating to various temperatures (1200°-1400°C) and times (0-60 min). Comparing the varistor properties per grain boundary (e.g., threshold voltage, donor concentration, and barrier height) of liquid penetration to those of conventional method indicated the individual grain boundaries were electrically activated when the samples were heat-treated above liquid-phase formation temperature.     

Origin of ZnO Varistor

The varistor properties were examined in porous ZnO. The sample used contained no dopants such as Bi₂O₃ or Pr₆O₁₁, which are usually considered to be useful in developing varistors. Comparison of I-V curves for the oxidized and unoxidized samples indicated that oxygen, which had chemisorbed and diffused into the grain boundary, played a role in developing varistor characteristics.     

Grain Growth of ZnO during Bi2O3 Liquid-Phase Sintering

Grain growth of ZnO during the liquid-phase sintering of binary ZnO–Bi₂O₃ ceramics has been studied for Bi₂O₃ contents from 3 to 12 wt% and sintering from 900° to 1400°C. The results are considered in combination with previously published studies of ZnO grain growth in the ZnO–Bi₂O₃ system. For the Bi₂O₃ contents of the present study, the rate of ZnO grain growth is found to decrease with increasing Bi₂O₃. Activation analysis, when combined with the results of similar analyses of the previous studies, reveals a change in the rate-controlling mechanism for ZnO grain growth. Following a low-Bi₂O₃-content region of nearly constant activation energy values of about 150 kJ/mol, further Bi₂O₃ additions cause an increase of the activation energy to about 270 kJ/mol. consistent with accepted models of liquid-phase sintering, it is concluded that the rate-controlling mechanism of ZnO grain growth during liquid-phase sintering in the presence of Bi₂O₃ changes from one of a phase-boundary reaction at low Bi₂O₃ levels to one of diffusion through the liquid phase at about the 5 to 6 wt% Bi₂O₃ level and above.     

Influence of Processing Conditions on the Electrical Properties of CaCu3Ti4O12 Ceramics

The electrical properties of a series of CaCu₃Ti₄O₁₂ ceramics prepared by the mixed oxide route and sintered at 1115°C in air for 1–24 h to produce different ceramic microstructures have been studied by Impedance Spectroscopy. As-fired ceramics are electrically heterogeneous, consisting of semiconducting grains and insulating grain boundaries, and can be modelled to a first approximation on an equivalent circuit based on two parallel RC elements connected in series. The grain boundary resistance and capacitance values vary as a function of sintering time and correlate with the ceramic microstructure based on the brickwork layer model for electroceramics. The large range of apparent high permittivity values for CaCu₃Ti₄O₁₂ ceramics is therefore attributed to variations in ceramic microstructure. The grain-boundary resistance decreases by three to four orders of magnitude after heat treatment in N₂ at 800°–1000°C but can be recovered to the original value by heat treatment in O₂ at 1000°C. The bulk resistivity decreases from ∼80 to 30 Ω·cm with increasing sintering time but is independent of heat treatment in N₂ or O₂ at 800°–1000°C. The origin of the bulk semiconductivity is discussed and appears to be related to partial decomposition of CaCu₃Ti₄O₁₂ at the high sintering temperatures required to form dense ceramics, and not to oxygen loss.     

Pore–Boundary Separation Behavior during Sintering of Pure and Bi2O3-Doped ZnO Ceramics

Pore–boundary separation in ZnO and 99.95ZnO·0.05Bi₂O₃ (in mol%) specimens during sintering at 1200°C was investigated. In pure ZnO specimens, pores were attached to the grain boundaries and disappeared during the final stage of sintering. In the Bi₂O₃-doped specimens, on the other hand, many pores were separated from the boundaries and trapped inside the grains. Observation using transmission electron microscopy showed that a thin layer of Bi₂O₃-rich phase existed at the boundaries in the Bi₂O₃-doped specimens. The pore separation in 99.95ZnO·0.05Bi₂O₃ specimens was explained in terms of the dihedral angle change and the high mobility of a liquid film boundary.     

(Pr, Co, Nb)-Doped SnO2 Varistor Ceramics

The effect on microstructure and electrical properties of (Co, Nb)-doped SnO₂ varistors upon the addition of Pr₂O₃ was investigated by scanning electron microscopy and by determining I – V , ɛ– f , and R – f relations. The threshold electric field of the SnO₂-based varistors increased significantly from 850 to 2280 V/mm, and the relative dielectric constants of the SnO₂-based varistors decreased greatly from 784 to 280 as Pr₂O₃ concentration was increased up to 0.3 mol%. The significant decrease of the SnO₂ grain size, from 4.50 to 1.76 μm with increasing Pr₂O₃ concentration over the range of 0–0.3 mol%, is the origin for the increase in the threshold voltage and decrease of the dielectric constants. The grain size reduction is attributed to the segregation of Pr₂O₃ at grain boundaries hindering the SnO₂ grains from conglomerating into large particles. Varistors were found to have a superhigh threshold voltage and a comparatively large nonlinear coefficient α. For 0.15 mol% Pr₂O₃-doped sample, threshold electric field and nonlinear coefficient α were measured to be 1540 V/mm and 61, and for 0.3 mol% Pr₂O₃-doped sample, V and α were 2150 V/mm and 42, respectively. Superhigh threshold voltage and large nonlinear coefficient α qualify the Pr-doped SnO₂ varistor as an excellent candidate for a high voltage protection system.     

Microstructure and Crystal Phases of Praseodymium Oxides in Zinc Oxide Varistor Ceramics

The effect of sintering temperature on the microstructure and crystal phases of the intergranular praseodymium oxides in ZnO varistor ceramics was investigated using transmission electron microscopy and high-resolution electron microscopy. The ZnO grains were three-dimensionally separated from the intergranular praseodymium oxides. On the basis of microdiffraction analyses of the intergranular layer, the phase transformation from fcc-Pr₆O₁₁ into hcp-Pr₂O₃ was found when the sintering temperature increased from 1300° to 1350°C. The defect reaction equation and the decrease of donor concentration with increasing sintering temperature can verify the certainty of phase transition during the liquid-phase sintering observed by transmission elecron microscopy. Additionally, on the basis of the small variations of the breakdown voltage per grain boundary, the number of active grain boundaries is not a dominant factor for the donor concentration dependence on the sintering temperature.     

Effects of Samarium Oxide Addition on the Phase Composition, Microstructure, and Electrical Resistivity of Aluminum Nitride Ceramics

The phase composition, microstructure, and electrical resistivity of hot-pressed AlN ceramics with 0–4.8 wt% Sm₂O₃ additive were investigated. The phase composition was approximately consistent with that estimated from the Sm₂O₃–Al₂O₃ phase diagram using the amount of added Sm₂O₃ and oxygen content of the AlN raw material. When sintered at more than 1800°C, the AlN ceramics with 1.0–2.9 wt% Sm₂O₃ additive contained an Sm-β-alumina phase wetting the grain boundaries, and their electrical resistivity considerably decreased to 10¹⁰–10¹²Ω·cm. This resistivity decrease was caused by the continuity of the Sm-β-alumina phase with a resistivity lower than that of bulk AlN.     

Infrared-Transparent Mullite Ceramic

Mullite ceramic, transparent in the infrared, was prepared by hot-pressing and hot-isostatically pressing starting materials derived from alkyloxides. A composition with 72.3 wt% Al₂O₃ yielded transparent, submicrometer grain size bodies at 1630°C, whereas higher temperatures produced glass-containing microstructures. A composition with 76 wt% A1₂O₃ formed precipitates of α-Al₂O₃ at the consolidation temperature, which could be removed by subsequent annealing between 1800° and 1850°C. Spectral transmittance and absorption coefficients of the bodies are reported. The formation of the second phases was linked to phase equilibria and grain growth that promoted compositional equilibration of the mullite phase. The results suggest adjustments to phase boundaries in the high-temperature segment of the SiO₂-Al₂O₃ phase diagram.     

Microstructural and Thermoelectric Characteristics of Zinc Oxide-Based Thermoelectric Materials Fabricated Using a Spark Plasma Sintering Process

M-doped zinc oxide (ZnO) (M=Al and/or Ni) thermoelectric materials were fully densified at a temperature lower than 1000°C using a spark plasma sintering technique and their microstructural evolution and thermoelectric characteristics were investigated. The addition of Al₂O₃ reduced the surface evaporation of pure ZnO and suppressed grain growth by the formation of a secondary phase. The addition of NiO promoted the formation of a solid solution with the ZnO crystal structure and caused severe grain growth. The co-addition of Al₂O₃ and NiO produced a homogeneous microstructure with a good grain boundary distribution. The microstructural characteristics induced by the co-addition of Al₂O₃ and NiO have a major role in increasing the electrical conductivity and decreasing the thermal conductivity, resulting from an increase in carrier concentration and the phonon scattering effect, respectively, and therefore improving the thermoelectric properties. The ZnO specimen, which was sintered at 1000°C with the co-addition of Al₂O₃ and NiO, exhibited a ZT value of 0.6 × 10⁻³ K⁻¹, electrical conductivity of 1.7 × 10⁻⁴Ω⁻¹·m⁻¹, the thermal conductivity of 5.16 W·(m·K)⁻¹, and Seebeck coefficient of −425.4 μV/K at 900°C. The ZT value obtained respects the 30% increase compared with the previously reported value, 0.4 × 10⁻³ K⁻¹, in the literature.     

Phase Diagram and Microstructure in the ZnO–Pr2O3 System

The phase diagram of the ZnO–Pr₂O₃ system with PrO_1.83 contents from 0.5 to 98 mol% was studied via differential thermal analysis (DTA). The system ZnOPr₂O₃ showed a simple eutectic type of phase diagram with no solid solutions and no other compounds. The eutectic temperature was determined to be 1382 ± 5°C. The liquidus line near the eutectic point was determined by a quenching method.     

Effects of Bismuth Sesquioxide on the Characteristics of ZnO Varistors

The nonlinearity of ZnO varistors is significantly influenced by the Bi₂O₃ and Sb₂O₃ contents, as well as by the phase composition of the Bi₂O₃. Degradation of the current-voltage characteristics due to the applied voltage is not always lowered by the β—γ transition of the Bi₂O₃ phase. Lattice parameter determinations and stress analyses suggest that the Bi₂O₃-rich phase in multigrain junctions causes mechanical strain at the grain boundary which may play an important role in the current-voltage characteristics of ZnO varistors.     

Ionically Conducting Composite Membranes from the Li2O–Al2O3–TiO2–P2O5 Glass–Ceramic

This paper reports processing of lithium ion-conducting, composite membranes comprised of 14Li₂O·9Al₂O₃·38 TiO₂·39P₂O₅ glass–ceramic and polyethylene. The processing involved tape casting of 14Li₂O·9Al₂O₃·38TiO₂·39P₂O₅ glass powder with organic additives into tapes, subjecting the green tape to binder burnout and thermal soaking in the temperature range of 950°–1100°C, and finally infiltrating the porous tape with polyethylene solution. The ionic conductivity and microstructure of 150–350 μm thick membranes were characterized and are discussed in this paper. The crystallites of the glass–ceramic show liquid-like conductivity at ambient temperature, whereas the grain boundary conductivity is lower by a factor of five. The lower grain boundary conductivity is explained on the basis of crystallographic mismatch and the existence of AlPO₄ at the grain boundary. The polyethylene infiltration in the porous membrane improved mechanical resilience with a minor adverse effect on conductivity.     

Effect of Metal Oxide Additions on the High-Temperature Electrical Conductivity of Alumina

Several metal oxide additions were made to typical 99 and 96% alumina compositions to study their effect on the electrical conductivity of alumina from 500° to 1400°C. The metal oxide additions investigated were CO₂O₃, Cr₂O₃, CuO, Fe₂O₃, MnO₂, NiO, and TiO₂. Using a guarded two-probe technique, dc resistivities were measured on nonporous ceramic specimens. Additions of 0.5 to 2 mole % Co₂O₃, 2 mole % CuO, 1 mole % Fe₂O₃, or 2 mole % NiO to either a 96 or a 99% alumina composition increased the electrical resistivity. The addition of 1 mole % Cr₂O₃ to either a 96 or a 99% alumina showed practically no change in the resistivity. All changes in resistivity seemed to be structure dependent.     

Effects of La2O3/ZnO Additives on Microstructure And Microwave Dielectric Properties of Zr0.8Sn0.2TiO4 Ceramics

Dielectric ceramics of Zr_0.8Sn_0.2TiO₄ containing La₂O₃ and ZnO as sintering aids were prepared and investigated for microstructure and microwave dielectric properties. Low-level doping with La₂O₃ and ZnO (up to 0.30 wt%) is good for densification and dielectric properties. These additives do not affect the dielectric constant and the temperature coefficient. Dielectric losses increase significantly at additive levels higher than 0.15 wt%. The combined additives La₂O₃ and ZnO act as grain growth enhancers. With 0.15 wt% additives, a ceramic having a dielectric constant, a quality factor, and a temperature coefficient of frequency at 4.2 GHz of 37.6, 12 800, and –2.9 ppm/°C, respectively, was obtained. The quality factor was considerably improved by prolonged sintering.     

Effect of Bismuth of Oxide Content on the Sintering of Zinc Oxide

Densification and microstructure de velopment in Bi₂O₃-doped ZnO have been studied with a special emphasis on the effect of the Bi₂O₃ content. A small amount of Bi₂O₃ in ZnO (0.1 mol%) retarded densification, but the addition of Bi₂O₃ to more than 0.5 mol% promoted densification by the formation of a liquid phase above the eutectic temperature (∼740°C). The liquid phase increased grain-boundary mobility, which was responsible for the formation of intragrain pores and the decrease in the sintered density. The increase in the Bi₂O₃ content increased the probability of the formation of skeleton structure, which reduced the grain growth rate and the sintered density.     

Synthesis of BaCu(B2O5) Ceramics and their Effect on the Sintering Temperature and Microwave Dielectric Properties of Ba(Zn1/3Nb2/3)O3 Ceramics

BaCu(B₂O₅) ceramics were synthesized and their microwave dielectric properties were investigated. BaCu(B₂O₅) phase was formed at 700°C and melted above 850°C. The BaCu(B₂O₅) ceramic sintered at 810°C had a dielectric constant (ɛ_r) of 7.4, a quality factor ( Q × f ) of 50 000 GHz and a temperature coefficient of resonance frequency (τ_f) of −32 ppm/°C. As the BaCu(B₂O₅) ceramic had a low melting temperature and good microwave dielectric properties, it can be used as a low-temperature sintering aid for microwave dielectric materials for low temperature co-fired ceramic application. When BaCu(B₂O₅) was added to the Ba(Zn_1/3Nb_2/3)O₃ (BZN) ceramic, BZN ceramics were well sintered even at 850°C. BaCu(B₂O₅) existed as a liquid phase during the sintering and assisted the densification of the BZN ceramic. Good microwave dielectric properties of Q × f =16 000 GHz, ɛ_r=35, and τ_f=22.1 ppm/°C were obtained for the BZN+6.0 mol% BaCu(B₂O₅) ceramic sintered at 875°C for 2 h.