Surface and grain-boundary energies as well as surface mass transport in polycrystalline CeO2

The multiphase equilibration technique for the determination of the equilibrium angles that develop at the interphase boundaries of a solid–liquid–vapor system, has been used to calculate the surface and interfacial energies in polycrystalline CeO₂ and CeO₂/Cu system in argon atmosphere at the temperature range 1473–1773 K. Linear temperature functions were obtained by extrapolation, for the surface energy γ_sv (J/m²) = 2.465–0.563 × 10⁻³ T and the grain-boundary energy γ_ss (J/m²) = 1.687–0.391 × 10⁻³ T of the ceramic, as well as for the interfacial energy γ_sl (J/m²) = 2.623–1.389 × 10⁻³(T −1356 K) of the CeO₂/Cu system. Grain-boundary grooving studied on polished surfaces of CeO₂ annealed in argon atmosphere at the same temperature range has shown that surface diffusion was the dominant mechanism for the mass transport. The surface diffusion coefficient can be expressed according to the equation D_s (m²/s) = 3.82 × 10⁻⁴ exp(−308,250/RT).     

Surface and grain-boundary energies of cubic zirconia

Using the multiphase equilibration technique for the measurement of contact angles, the surface and grain-boundary energies of polycrystalline cubic ZrO₂ in the temperature range of 1173 to 1523 K were determined. The temperature coefficients of the linear temperature function obtained, are expressed as $$\frac{{{\text{d}}\gamma }}{{{\text{d}}T}}({\text{ZrO}}_{\text{2}} ){\text{ }} = {\text{ }} - 0.431{\text{ }} \times {\text{ }}10^{ - 3} {\text{ }} \pm {\text{ }}0.004{\text{ }} \times {\text{ }}10^{ - 3} {\text{ Jm}}^{ - {\text{2}}} {\text{ K}}^{ - {\text{1}}} $$ and $$\frac{{{\text{d}}\gamma }}{{{\text{d}}T}}({\text{ZrO}}_{\text{2}} - {\text{ZrO}}_{\text{2}} ){\text{ }} = {\text{ }} - 0.392{\text{ }} \times {\text{ }}10^{ - 3} {\text{ }} \pm {\text{ }}0.126{\text{ }} \times {\text{ }}10^{ - 3} {\text{ Jm}}^{ - {\text{2}}} {\text{ K}}^{ - {\text{1}}} $$ respectively. The surface fracture energy obtained with a Vickers microhardness indenter at room temperature is found to be γ _F=3.1 J m^?2.     

Surface and grain-boundary energies in yttria-stabilized zirconia (YSZ-8 mol%)

The multiphase equilibration technique has been used to measure the equilibrium angles that develop at the interphase boundaries of a solid-liquid-vapour system after annealing and also the surface (γ_sv)and the grain-boundary, (γ_ss) energies of polycrystalline yttria-stabilized zirconia (8 mol% Y₂O₃). The data was recorded in the temperature range 1573–1873 K. Linear temperature functions were obtained for the surface energy $$\gamma _{SV} (Jm^{ - 2} ) = 1.927 - 0.428x10^{ - 3} T$$ and for the grain-boundary energy $$\gamma _{SS} (Jm^{ - 2} ) = 1.215 - 0.358x10^{ - 3} T$$      

Dielectric properties of polycrystalline CaTiO3 doped with yttrium oxide

Y³⁺ was substituted for Ca²⁺ in polycrystalline CaTiO₃ in amounts up to 15 at %. Sintering conditions (1450° C, 15 h) were such that grain sizes were > 25 m. Stoichiometry was adjusted on the assumption that the excess charge of the dopant was compensated by the creation of calcium vacancies. This assumption was supported by measurements of the Ca/Ti ratio in the grains by electron-probe microanalysis. Unlike yttrium-doped SrTiO₃, material sintered in air was light-coloured with no evidence of semiconductivity. On the other hand, when sintering was done in nitrogen, dielectric relaxation characteristic of boundary layers was observed for dopant levels > 1 mol %. The experimental data support the view that dielectric relaxation in SrTiO₃ and CaTiO₃ results from semiconducting grains with resistive surface layers and that the semiconductivity arises because oxygen loss from the grains during sintering is increased by donor doping.     

Semiconducting and transport properties of mono- and polycrystalline nickel oxide

On the basis of high-temperature studies of electrical conductivity of poly- and monocrystalline nickel oxide and making use of the results of studies on chemical diffusion coefficients obtained by several authors and in the present work, the structure of point defects in nickel oxide has been considered. It has been shown that in the temperature range 900 to 1300° C and at the oxygen pressure from 10^–4 to 1 atm there occur in nickel oxide singly- and doubly-ionized cationic vacancies in comparable quantities.Assuming such to be the model of defect structure in Ni_1–yO, the equilibrium concentration of cationic vacancies as a function of temperature has been calculated for the oxygen pressure of 1 atm. It has been shown that the results obtained are in good agreement with the results of direct determinations of concentration of cationic vacancies in NiO.     

An investigation of grain-boundary plane crystallography in polycrystalline nickel

An investigation has been carried out to measure and categorize the grain-boundary plane indices of boundaries in pure nickel. Coincidence site lattices (excluding = 3s) were found to be either asymmetrical tilt boundaries with high indices, or have irrational boundary planes. For the =3s, almost half were asymmetrical tilt boundaries displaced from the 111/111 symmetrical tilt boundary on the 110 zone. Such boundaries have low energies compared to other =3s. The 211/211 incoherent twin was not observed, which was explained on the basis of its higher energy compared to other boundaries on the 110 zone. The results are compared and contrasted with previous data, where boundaries abutted the specimen surface during annealing, which is not the case for the present data. Comments are made with respect to the relationship between macroscopic and atomic-level boundary geometry and implications of the results for grain-boundary properties.     

Surface mass transport of alumina

The kinetics of thermal grooving at the intersection of rhombohedral twin boundaries with the \((10\bar 10)\) plane in aluminium oxide were measured from 1773 to 2273 K. Analysis of the data using the model of Mullins showed that surface diffusion was the dominant mechanism for mass transport. The results were compared with other similar published work on alumina, and the following equation for surface diffusion was determined: $$D_s (cm^2 sec^{ - 1} ) = 4.05 x 10^5 exp - (452kJ mol^{ - 1} /RT).$$      

Synthesis of polycrystalline yttrium iron garnet and yttrium aluminium garnet from organic precursors

Polycrystalline Y₃Fe₅O₁₂ (YIG) and Y₃Al₅O₁₂ (YAG) garnets have been prepared by the organic solution technique using a novel organic precursor. The thermal decomposition of the precursor and subsequent formation of the garnet phases was studied by thermal analysis, X-ray diffraction and Fourier transform infra red spectroscopy (FTIR) including diffuse reflectance FTIR (DRIFT). The precursor of YIG decomposes to give the garnet phase as the main component at 800 °C. YFeO₃ and Fe₂O₃ are also present, and react to give YIG at higher temperatures. Single-phase YAG can be obtained from its precursor at 1100 °C. The reaction proceeds via a hexagonal YAlO₃ intermediate which is formed at 850 °C.     

Carrier transport in polycrystalline transparent conductive oxides: A comparative study of zinc oxide and indium oxide

Highly doped indium-tin oxide films exhibit resistivities ρ as low as  1.2 × 10^− 4 Ω cm, while for ZnO films resistivities in the range of 2 to 4 × 10^− 4 Ω cm are reported. This difference is unexpected, if ionized impurity scattering would be dominant for carrier concentrations above 10²⁰ cm^− 3. By comparing the dependences of the effective Hall mobility on the carrier concentration of ZnO and ITO it is found that grain barriers limit the carrier mobility in ZnO for carrier concentrations as high as 2 × 10²⁰ cm^− 3, independently, if the films were grown on amorphous or single crystalline substrates. Depending on the deposition method, grain barrier trap densities between 10¹² and 3 × 10¹³ cm^− 2 were estimated for ZnO layers. Also, crystallographic defects seem to reduce the mobility for highly doped ZnO films. On the other hand, for ITO films such an influence of the grain barriers was not observed down to carrier concentrations of about 10¹⁸ cm^− 3. Thus the grain barrier trap densities of ZnO and ITO are significantly different, which seems to be connected with the defect chemistry of the two oxides and especially with the piezoelectricity of zinc oxide.     

Synthesis of europium-doped yttrium hydroxide and yttrium oxide nanosheets

A new approach has been developed for the preparation of Y(OH)₃:Eu and Y₂O₃:Eu nanosheets using the sol–gel method and hydrothermal reactions. XRD patterns showed that the product was purely hexagonal-phase Y(OH)₃. TEM images revealed that the nanosheets are square shaped (1 × 1 μm²) with a thickness of several tens of nanometers. In addition, it was found that cubic-phase Y₂O₃ nanosheets can be obtained by calcination of Y(OH)₃ at 900 °C for 1 h. More importantly, the thus-prepared Y(OH)₃:Eu and Y₂O₃:Eu nanosheet phosphors were found to exhibit a relatively high photoluminescence (PL) intensity.     

Spin wave relaxation processes in polycrystalline yttrium iron garnets

The wavevector and temperature dependence of the spinwave linewidth in yttrium iron garnets has been studied for polycrystalline samples of average grain diameter 12·8μm and has been compared with the calculated relaxation times of basic three-magnon confluence and four-magnon scattering processes.     

An evaluation of the grain-boundary sliding contribution to creep deformation in polycrystalline alumina

The occurrence of grain-boundary sliding during creep in fine grained alumina was examined by inscribing marker lines on the tensile surfaces of specimens, prior to testing in four-point bending mode. There was considerable microstructural evidence for the occurrence of grainboundary sliding and grain rotation during creep deformation. Experimental measurements of the offsets in the marker lines at grain boundaries reveal that the grain-boundary sliding contribution to the total strain during creep deformation is 70 ± 6.2%. The extensive grain boundary sliding observed, together with the other mechanical properties, suggests that polycrystalline alumina exhibits superplastic characteristics. Several possible rate controlling mechanisms are examined critically in light of the present results and it is concluded that creep occurs either by an independent grain-boundary sliding mechanism or by an interface controlled diffusion mechanism.