Progress in material selection for solid oxide fuel cell technology: A review

Electrical properties of the grain boundaries of oxygen ion conductors: Acceptor-doped zirconia and ceria

Some thoughts on the mechanisms of in-reactor corrosion of zirconium alloys

The grain boundary electrical properties of high purity ZrO 2 ceramic materials doped with 2, 3, and 8 mol % Y 2 O 3 , and 8 mol % Y 2 O 3 co-doped with 0.4 mol % Al 2 O 3 were studied in the temperature range of 200 to 500°C by electrochemical techniques and were theoretically analyzed. Although the presence of a siliceous phase is shown to be a major cause for the grain boundary blocking effect, the grain boundary properties appear to he significantly influenced by space charges, particularly in materials of high purity. The oxygen vacancy distribution and the grain boundary resistivity were calculated for 8 mol % Y 2 O 3 doped ZrO 2 by assuming double Schottky barriers, and the results were compared with the experiment. It is shown that reasonable space charge potentials lead to grain boundary effects which are consistent with the experimental features. In contrary to the bulk in which defect associates prevail (at temperatures <560°C), in the boundary regions, association effects can be assumed to be much less pronounced due to the vacancy depletion.

Grain Boundary Blocking Effect in Zirconia: A Schottky Barrier Analysis

Titanium diboride (TiB2) based materials have received wide attention because of their high hardness and elastic modulus, good abrasion resistance, and superior thermal and electrical conductivity. Potential applications include high temperature structural materials, cutting tools, armour, electrodes in metal smelting and wear parts. Despite its useful properties, the application of monolithic TiB2 is limited by poor sinterability, exaggerated grain growth at high temperature and poor oxidation resistance above 1000°C. Pure TiB2 can be densified only at high temperatures (∼2000°C), with an applied pressure generally being necessary during sintering. However, these high sintering temperatures cause abnormal grain growth and microcracks, which are detrimental to the mechanical properties. Various sinter additives are commonly added to obtain dense TiB2 with optimised mechanical properties at lower sintering temperature. The present review surveys the current state of knowledge on development of bulk...

Processing and properties of monolithic TiB2 based materials

In this paper, a titanium diboride ceramic was produced by the hot pressing sintering method. The effects of hot pressing parameters on the TiB 2 ceramic microstructure and mechanical properties were studied. The bending strength and fracture toughness were measured by three point bending testing and single edge notched bending tests (SENB), respectively. The microstructure features of the TiB 2 sintered material were revealed by means of SEM and TEM. The results show that the TiB 2 grain size increases quickly with the increasing temperature and time during hot pressing sintering. The density and the TiB 2 grain size have a great influence on the mechanical properties. The bending strength decreases with increasing TiB 2 grain size, whilst the fracture toughness increases.

Influence of hot pressing sintering temperature and time on microstructure and mechanical properties of TiB2 ceramics

The grain boundary resistance of 9 mol% Y2O3-stabilized ZrO2 with Al2O3 addition was measured by the complex impedance approach. The effect of Al2O3 addition on the grain boundary resistance was analyzed from the aspects of microstructure and crystal defects by means of SEM, EPMA, TEM and the positron annihilation technique. The Al2O3 addition up to 5 mol% was found to reduce the grain boundary resistance per unit surface area of the grain boundaries and the activation energy for the grain boundary resistance. This phenomenon was explained by the scavenging of the grain boundary impurities, e.g. SiO2, by the Al2O3 and the formation of the crystal grain boundary phases with very high defect concentration. By the analyses with a simplified theoretical model, the Al2O3 addition was considered to have no detrimental effect on the emf of the ZrO2 oxygen sensor.

Grain boundary ionic conduction in zirconia-based solid electrolyte with alumina addition

Up to 5 mol% Al2O3 was added to 9 mol% Y2O3-stabilized ZrO2, and the roles of Al2O3 were systematically studied by means of the complex impedance approach, the positron annihilation technique, SEM, TEM, and electron probe microanalysis from the following aspects: (1) the existence of forms of Al2O3 in ZrO2, (2) the effects of Al2O3 on the microstructure of ZrO2, (3) the effects of Al2O3 on the resistance of ZrO2, (4) the microstructure and property changes of ZrO2 with Al2O3 addition during ageing at 940‡C. Two types of grain boundary phase, crystal and amorphous, were discovered. The Al2O3 segregation at grain boundaries can promote the mobility of the grain boundaries and thus results in a low density, because of entrapped pores. The Al2O3 addition decreases the grainboundary resistance in two ways: to scavenge the SiO2 and CaO located at grain boundaries, and to form crystal grain-boundary phases with very high crystal defect concentrations. Ordered microdomains of Zr3Y4O12 were precipitated from ZrO2 grains during ageing, and aluminium was found to facilitate the precipitation.

Roles of alumina in zirconia-based solid electrolyte

On the grain boundaries of ZrO2-based solid electrolyte

Metal/ceramic type functionally gradient materials [1 ] (FGMs) are new-generation composite materials for future use in aerospaee, nuclear engineering, etc. The key problem for their practical use is how to ensure no damage is caused by the thermal stresses in the preparation and working processes. In analysing the residual and working stresses of ceramic/metal FGMs, Kawasaki [2], Kato [3], and Yuan et al. [4] have done some interesting work. However, work which optimizes the composition and structure by comprehensively surveying the residual and working stresses has not been done until now. In fact, these studies are very important to the thermal stress relaxation design of FGM. I n this paper, a TiC/Ni3A1 FGM is studie& As a ceramic phase, TiC has a high electronic expansion coefficient and high-temperamre strength; as a metal phase, the intermetallic Ni3A1 has a fine creepresistance at elevated temperature, and an excellent interface bondability with TiC [5, 6]. The thermomechanical properties of TiC/Ni3A1 composites with different TiC-Ni3A1 ratios are first tested. The tested properties are then used as a basis of axisyrnmetric finite element calculation for the two different types of stresses. For a disc-like TiC/Ni3A1 FGM, by surveying the stresses, the stress sites, and the stress state at the pure ceramic side, the second optimization for the FGM compositional distribution and structure is obtained. TiC/Ni3A1 composites with Ni3A1 volume fraction of 0, 20%, 40%, 60%, 80% and 100% were sintered by the HP method for 2 h at 1300 °C and.300 MPa under Ar gas protection. These sintering conditions are the same as the conditions for sintering TiC/ Ni3A1 FGM. The sintered composites were cut and ground into 36 x 4 x 3 m m strip samples; the properties were then tested with a four-point bending test method in a materials test system. In the test, the top span of the samples was 3 0 m m and the underpart span 20 mm. Strain ganges were fixed at the centre of the samples. Young's modulus E was calculated through the tested load-defiection curve; t h e Poisson ratio /z was determined by the ratio of the tested cross-strain to the longitudinal strain; and the fracture strength of the samples was indicated as the four-point bending strength o-. The average values of E and /z were obtained from 30 samples and the average ofrom seven samples. In addition, the thermal expansion coefficient was tested by a nonloaded thermal dilatometer and the thermal diffusion coefficient ff and specific heat Cp were tested by a laser perturbation method (SHINKO RIKO TC-700). The samples were 10 mm long and 2 mm wide; they were tested at 25 °C, 300 °C, 600 °C, 800 °C and 1200 °C, respectively. The thermal conductivity A was calculated from A = ff x Cp x d, where d is the sample density. The tested properties are indicated in Table I. F ig . 1 is a finite element model for the thermal stress calculation of a disc-like TiC/Ni3A1 FGM. The material is 30 mm in diameter, 6 mm in thickness, and has 1 l gradient layers. The top side is the metal phase, the bottom is the ceramic phase, and between the two sides are the FGM interlayers. Due to the

Run-Zhang Yuan

Papers

Influence of hot pressing sintering temperature and time on microstructure and mechanical properties of TiB2 ceramics

Grain boundary ionic conduction in zirconia-based solid electrolyte with alumina addition

Roles of alumina in zirconia-based solid electrolyte

On the grain boundaries of ZrO2-based solid electrolyte

Residual and working stresses of a TiC/Ni3Al FGM and its structure optimization