Robert Freer

THE ROLES PLAYED BY AG AND AL DOPANTS IN CONTROLLING THE ELECTRICAL PROPERTIES OF ZNO VARISTORS

Four sets of ZnO‐based ceramic varistors (reference samples‐ZNR; doped with aluminium‐AL; doped with silver‐AG; doped with aluminium and silver‐AA) have been prepared by the conventional mixed oxide route. The current‐voltage (I‐V) characteristics, determined at current densities up to 1 mA/cm2, yielded nonlinear coefficients (α) of 38, 60, 22 and 56, respectively. Dc and ac degradation tests were performed at 115 °C. Ac impedance analysis was used to determine grain (rg) and grain boundary (Rb) resistances; capacitance‐voltage analysis enabled the donor density (Nd) and the barrier height (φ) to be determined. Doping the varistors with Al increased Nd, reduced rg, and improved the I‐V characteristics, but caused an increase in Zn interstitials which degraded the stability. In contrast, Ag acts as an amphoteric dopant in ZnO causing a decrease in Nd, an increase in rg and Rb, and a reduction in α ;  Ag located in the interstitial sites is able to block the formation and migration of new Zn interstitials, ...

DiC12: Magnesium titanate microwave dielectric ceramics

Abstract Magnesium titanate ceramics have been prepared by a chemical route (Pechini method) and by the conventional mixed oxide route. Dielectric properties of sintered samples were measured at 8 GHz by the Hakki and Coleman technique. Microstructures were examined by optical microscopy and scanning electron microscopy. The use of the chemical method enabled the sintering temperature to be lowered and good quality, single phase, dense ceramics were obtained at temperatures down to 1150°C. Dielectric constants were approximately 17.5 and Q values up to 21200 were obtained at 8 GHz. The effect of powder preparation processes, sintering temperature and cooling rate on the densification, microstructure and microwave dielectric properties are discussed.

The effect of Cr and La on MgTiO3 and MgTiO3–CaTiO3 microwave dielectric ceramics

Magnesium titanate and MgTiO 3 –CaTiO 3 ceramics were prepared by a chemical (Pechini) route and classical mixed oxide route. Selected specimens were doped with Cr or La. Specimens were sintered at 1350 °C and 1400 °C. Microstructures were examined by optical microscopy, scanning electron microscopy, and transmission electron microscopy. Dielectric properties were determined at 8 GHz by the Hakki and Coleman method. The highest Q values were obtained for undoped, chemically prepared MgTiO 3 (20800); any dopants caused the Q value to be degraded. Additions of small amounts of Cr (≤1 mol %) to mixed oxide magnesium titanate increased the density to 97.1% theoretical, and increased the Q value (from 7000) to 13,000. Additions of La led to the formation of La 2 Ti 2 O 7 second phase and reduction in the Q value for both materials. Both Cr and La acted as effective sintering aids, increasing density (to a maximum of 99% theoretical for 1% mol La in chemically prepared samples) and relative permittivity (to 18.1 for the same specimens). The relative permittivity of MgTiO 3 –CaTiO 3 ceramics increased with calcium content, but the corresponding Q values decreased (∈ r = 19.9, Q = 8500 for Mg:Ca = 94: 6). Small additions of La to Mg–Ca titanates enhanced the dielectric Q values but decreased the relative permittivities.

A Raman spectral characterization of ceramics in the system ZrO2-TiO2

Ceramics of zirconium titanate, ZrTiO4, were prepared by the mixed oxide route, sintered at 1400 °C and cooled at various rates: water quench to 1 °C h−1. Mixed oxide-prepared ceramics of Zr5Ti7O24 were sintered at 1400 °C and cooled at 6 °C h−1. TEM analysis of the ZrTiO4 ceramics showed the development of an incommensurate superstructure in the more slowly cooled specimens. The Zr5Ti7O24 ceramics exhibited a commensurate superstructure with a tripleda-axis. Raman spectra were obtained from polished ceramic specimens at room temperature. Major differences between spectra for ZrTiO4 ceramics are believed to reflect differences in the degree of cation ordering. The Raman spectrum for Zr5Ti7O24 is similar to that of slowly cooled ZrTiO4 but there are significant differences.

The mechanism of abnormal grain growth in Sr0.6Ba0.4Nb2O6 ceramics

Strontium barium niobate, Sr0.6Ba0.4Nb2O6 (SBN40) ceramics were prepared by the mixed oxide route; specimens sintered at 1300 °C–1450 °C for 2–4 h had densities of ∼95% theoretical. The use of “conventional’’ conditions (1300 °C; ∼200 °C below the melting temperature) gave rise to a duplex structure comprising a mixture of small grains and abnormally large grains (in excess of 100 μm). Transmission electron microscopy studies revealed a Nb-rich, Ba-poor phase at the grain boundaries; the low melting temperature of this phase caused localized liquid phase sintering, resulting in abnormal grain growth. Dual-stage sintering (sintering at 1250 °C for 4 h, followed by sintering at a temperature in the range 1350 °C–1450 °C) inhibited abnormal grain growth. The use of a fast heating rate (600 °C/h) and dual-stage sintering (1250 °C and 1450 °C) yielded samples with good transparency and preferential orientation along the c-direction of the tetragonal unit cell. The dielectric properties depended sensitively on ...

Thermoelectric Power Generation from Lanthanum Strontium Titanium Oxide at Room Temperature through the Addition of Graphene.

The applications of strontium titanium oxide based thermoelectric materials are currently limited by their high operating temperatures of >700 °C. Herein, we show that the thermal operating window of lanthanum strontium titanium oxide (LSTO) can be reduced to room temperature by the addition of a small amount of graphene. This increase in operating performance will enable future applications such as generators in vehicles and other sectors. The LSTO composites incorporated one percent or less of graphene and were sintered under an argon/hydrogen atmosphere. The resultant materials were reduced and possessed a multiphase structure with nanosized grains. The thermal conductivity of the nanocomposites decreased upon the addition of graphene, whereas the electrical conductivity and power factor both increased significantly. These factors, together with a moderate Seebeck coefficient, meant that a high power factor of ∼2500 μWm(-1)K(-2) was reached at room temperature at a loading of 0.6 wt % graphene. The highest thermoelectric figure of merit (ZT) was achieved when 0.6 wt % graphene was added (ZT = 0.42 at room temperature and 0.36 at 750 °C), with >280% enhancement compared to that of pure LSTO. A preliminary 7-couple device was produced using bismuth strontium cobalt oxide/graphene-LSTO pucks. This device had a Seebeck coefficient of ∼1500 μV/K and an open voltage of 600 mV at a mean temperature of 219 °C.

The microstructure and microwave dielectric properties of zirconium titanate ceramics in the solid solution system ZrTiO4–Zr5Ti7O24

Zirconium titanate (ZT) ceramics having compositions in the range of ZrTiO4-Zr5Ti7O24 were prepared via the mixed oxide route, using ZnO and CuO as sintering aids and Y2O3 as stabilizer. Specimens were sintered at 1450°C for 4 h and then cooled at 6°C h-1, 120°C h-1 or air-quenched. All products exhibited densities exceeding 95% of the theoretical values. The amount of ZnO and CuO in the products decreased as the cooling rate decreased and as the content of TiO2 increased. Energy dispersive analytical spectroscopy studies suggested that a grain boundary phase, rich in ZnO and CuO, existed as a continuous layer. Both composition and cooling rate were found to have significant effects on the microstructure of the zirconium titanate ceramics. Transmission electron microscopy showed that as the TiO2 content increased, a superstructure with a tripled a-axis developed, but there was no obvious change in the lattice parameters. As the cooling rate decreased, extra peaks were observed in X-ray spectra and the lattice parameter in the b direction shortened dramatically; both are associated with cation ordering. A short-range commensurate superstructure with a ZTTZZTTZTTZZTT (or ZTTZZTTZTTZZTT) stacking sequence was observed in the ordered ZrTiO4 specimens. All the samples showed poor dielectric properties at microwave frequency (4 GHz). The low dielectric Q values (400–1000) were due to the presence of the structural stabilizer, Y2O3, within the grains. The Q value increased slightly with increasing TiO2 content. The air-quenched samples had the highest Q values (850–1000); slower cooling led to the formation of microcracks within the samples and the reduction of Q values. The relative permittivity was controlled by bulk composition, the presence of a grain boundary phase, microcracks, oxygen vacancies and cation ordering. The ordering of cations and the presence of microcracks reduced the relative permittivity; rapidly cooled samples with higher TiO2 content had higher relative permittivities (with a maximum of 44.3 for air-quenched Zr5Ti7O24).

The energetics and structure of the hydrogarnet defect in grossular: A computer simulation study

We have used computer simulation methods to model the structure and energetics of the hydrogarnet defect in grossular. The predicted structure is in good agreement with experimental data. The calculated energy for the reaction of water with grossular to form the hydrogarnet defect is 1.02 Ev (98 kJ mol-1). This low energy of reaction suggests that such defects will be common in garnets where they could play an important role in effecting processes such as atomic transport.

The relationship between the microstructure and microwave dielectric properties of zirconium titanate ceramics

Zirconium titanate (ZrTiO4) ceramics have been prepared by the mixed oxide route using small additions of ZnO, Y2O3 or CuO. Specimens were sintered mainly at 1400 °C and cooled at various rates: water-quench, air-quench, 300 °C h−1, 120 °C h−1, 6 °C h−1 and 1 °C h−1. Products prepared with additives exhibited densities of at least 93% of the theoretical value. As the cooling rate after sintering was decreased, the length of the lattice parameter in the b direction was reduced and transmission electron diffraction revealed superlattice reflections associated with cation ordering. For specimens cooled at 1 °C h−1, electron diffraction patterns exhibited features consistent with an incommensurate superstructure in the a direction. The dielectricQ value of rapidly cooled (air-quenched) ceramics was 2000 at 5 GHz. With an increase in the degree of cation ordering theQ value increased to a maximum of 4400 for specimens cooled at 6 °C h−1. For specimens cooled at the slowest rate (1 °C h−1) theQ value fell to 2000 due in part to the presence of microcracks and exsolved ZrO2. Diffusion of trivalent impurities (yttria) into the host ZrTiO4 grains also led to a lowering of theQ values.The microwave dielectric properties of zirconium titanate ceramics are sensitive to processing conditions and mircrostructural features. The highestQ values (lowest loss) should be achieved in homogeneous specimens, free of trivalent impurities and lattice defects, in which lowQ-value second phases, microcracks and pores are eliminated.

Heterojunction gas sensors for environmental NO2 and CO2 monitoring

Abstract The NO 2 and CO 2 sensing properties of a heterojunction gas sensor formed between n -type ZnO and a p -type composite based on a mixture of BaTiO 3 /CuO/La 2 O 3 have been evaluated and compared with the performance of its component p - and n -type materials. It was found that the individual ZnO and BaTiO 3 /CuO/La 2 O 3 sensors showed resistance increases when exposed to NO 2 . When exposed to CO 2 , the ZnO based sensor showed no response, while the BaTiO 3 /CuO/La 2 O 3 sensor showed a small resistance increase. The heterojunction sensor was found to be insensitive to 2 although at higher concentrations it showed a small decrease in resistance. The resistance also decreased when exposed to low levels of NO 2 , indicating that a different detection mechanism was operative at the heterocontact compared with the single-phase materials.