Solid electrolytes with oxygen ion conduction

Single phaseβ-sialon ceramics, Si6−z
Al
 z
 O
 z
 N8−z
, have been prepared from carefully balanced powder mixtures, also taking account of any excess oxygen in the starting materials. Sintering powder compacts in a nitrogen atmosphere (0.1 MPa) at 1800° C or higher transforms the starting mixture into aβ-sialon solid solution atz-values up to about 4.3, but the sintered material has an open porosity. Addition of 1 wt% Y2O3 to the starting mix improved the sintering behaviour somewhat and the density of the sintered compacts reached 95% of the theoretical value. By glass-encapsulated hot isostatic pressing at 1825° C, however, sintered materials of virtually theoretical density could be obtained, with or without the 1 wt% Y2O3 addition. These latter samples have been studied by X-ray diffraction and electron microscopy, and their hardness and indentation fracture toughness have been measured. It was found that the maximum extension of theβ-sialon phase composition at 1825° C and 200 MPa pressure is slightly below 4,z∼ 3.85 and about 4.1 at atmospheric pressure, and that the hexagonal unit cell parameters are linear functions of thez-value. The single-phaseβ-sialon ceramics had no residual glassy grain-boundary phase. The grain shape was equi-axed and the grain size increased from about 1μm at lowz-values to 5μm at highz-values. At lowz-values the hardness at a 98 N load was 1700 and the fracture toughness 3, whereas an increase inz above 1 caused both the hardness and fracture toughness to decrease significantly. Addition of 1 wt % Y2O3 to the starting mix prior to the HIP-sintering gave rise to a small amount of amorphous intergranular phase, changes in grain size and shape, a clear increase in fracture toughness and a moderate decrease in hardness.

Dense single-phase β-sialon ceramics by glass-encapsulated hot isostatic pressing

https://ris.utwente.nl/ws/files/6512956/ion92gellings.pdf

Ion and mixed conducting oxides as catalysts

Besides temperature at one atmosphere, the applied pressure is another important parameter for influencing and controlling reaction pathways and final reaction products. This is relevant not only for the genesis of natural minerals, but also for synthetic chemical products and technological materials. The present critical review (316 references) highlights recent developments that utilise high pressures and high-temperatures for the synthesis of new materials with unique properties, such as high hardness, or interesting magnetic or optoelectronic features. Novel metal nitrides, oxonitrides as well as the new class of nitride-diazenide compounds, all formed under high-pressure conditions, are highlighted. Pure oxides and carbides are not considered here. Moreover, syntheses under high-pressure conditions require special equipment and preparation techniques, completely different from those used for conventional synthetic approaches at ambient pressure. Therefore, we also summarize the high-pressure techniques used for the synthesis of new materials on a laboratory scale. In particular, our attention is focused on reactive gas pressure devices with pressures between 1.2 and 600 MPa, multi-anvil apparatus at P < 25 GPa and the diamond anvil cell, which allows work at pressures of 100 GPa and higher. For example, some of these techniques have been successfully upgraded to an industrial scale for the synthesis of diamond and cubic boron nitride.

High-pressure chemistry of nitride-based materials

The O-Ta (Oxygen-Tantalum) system

High-density sintered disks of the composition 0.13YO1.5·0.87ThO2 are shown to be mixed conductors at high oxygen pressures (>10−6 atm) by electrical conductivity and electrochemical cell measurements. The ac and dc conductivity measurements were made between 900° and 1600°C over a wide range of oxygen partial pressures. A blocking-electrode polarization technique for determining transference numbers was not applicable at high oxygen pressures but appeared to work at the lower pressures, indicating a transition to n-type behavior. The electrochemical cell measurements show essentially completely ionic behavior at low oxygen pressure but indicate at least 0.1% electronic contribution at 10−13 atm at 1000°C. The lower oxygen pressure limit for completely ionic behavior has not been determined but extends below the equilibrium pressures of an Mn-MnO2, Cr-Cr2O3 electrochemical cell at 1000°C.

Electrical Properties and Defect Structure of a (0.13YO1.5·0.87ThO2) Electrolyte

A product or quasi-equilibrium diagram for compounds formed in the system Si-Al-O-N at 1800°C in 1 atm N2 is presented, X-ray diffraction data for phases are given, and the reaction and sintering processes are discussed. Data for the lattice expansion of the β-Si3N4 structure accompanying the incorporation of Al-O is presented and used to calculate theoretical densities.

Compounds and Properties of the System Si‐Al‐O‐N

Electrical conductivity, thermoelectric power, and weight change were measured for polycrystalline Ta2O5 from 900° to 1400°C. The predominant ionic and electronic defects in this temperature range are oxygen vacancies and electrons. The oxygen-vacancy and electron mobilities are 8.1 × 103exp (−1.8 eV/kT) and ∼0.05 cm2/V-s, respectively. At O2 partial pressures near 1 atm, the ionic-defect concentration is essentially fixed by the presence of lower-valence cation impurities, and the total electrical conductivity is predominantly ionic, whereas at low Po2's the conductivity is electronic and proportional to PPo2−1/6.

Defect Structure of Ta2O5

The oxidation-reduction kinetics of MnO in CO/CO2 atmospheres at 1230°C were measured by gravimetric and electrical conductivity techniques. The dependence of the kinetics on time, inert gas dilution, total pressure, and specimen thickness indicates that the kinetics are controlled by a slow surface reaction step. Attempts to analyze the data quantitatively using equations for linear heat flow in a solid with a radiation boundary condition were unsuccessful.

Oxidation-Reduction Kinetics in MnO

Controlled surface flaws were produced in commercial reaction-densified SiC by Knoop microhardness indentation. The flaws themselves could not be observed easily, thus an etching technique was used to delineate their semielliptical shape, thereby enabling calculation of the critical stress- intensity factor KIC at room temperature. Room-temperature fracture was insensitive to annealing environment (air or vacuum), flaw “healing” being observed at ≫1000°C. The variation in fracture stress of indented specimens with temperature showed 3 distinct regions of behavior which were interpreted in terms of residual stress relief, flaw healing, and Si-SiC bond weakening.

J. M. Wimmer

Papers

Electrical Properties and Defect Structure of a (0.13YO1.5·0.87ThO2) Electrolyte

Compounds and Properties of the System Si‐Al‐O‐N

Defect Structure of Ta2O5

Oxidation-Reduction Kinetics in MnO

Controlled Surface Flaw-Initiated Fracture in Reaction-Densified Silicon Carbide