Dual-phase Composites Research Articles

Oxygen permeation through the LSCO-80/CeO 2 asymmetric tubular membrane reactor

Mixed conductive perovskite materials, e.g., La 1− x Sr x O 3− δ (LSCO), have been widely investigated to understand the leverages of doping extent and composition on the oxygen permeability with the aim of developing an oxygen-transport solid electrolyte membrane. However at the present stage fabrication of a dense thin layer of perovskite oxide on a porous tubular support possessing mechanically and chemically stability at high temperatures is still a technological challenge to the endeavor. This is because the asymmetric configuration is a desired model of the commercial oxygen-permeable ceramic membrane reactor. The present work develops a new approach that allows the formation of a complete gas-tight oxygen-permeable thin membrane on the outer surface of a porous CeO 2 tube by the means of slurry coating. The oxygen-permeable membrane is a dual-phase composite containing equal volume fractions of CeO 2 and LSCO-80 ( x = 0.8). In the membrane CeO 2 particles are uniformly embedded in the continuous LSCO phase, and this highly dispersed semi-continuous structure could successfully buffer the mechanical stress generated in the LSCO phase due to mismatch of coefficient of thermal expansion (CTE) between the membrane and the support. The oxygen permeation flux tests showed a low activation energy barrier (∼30 kJ/mol) of the whole electrochemical reaction in the temperature range from 400 to 900 °C. The surface de-sorption (or the anodic) process of the oxygen has been simulated using the extended Hückel theory (EHT). The activation energy obtained from the EHT simulation is found very close to the experiment data. In addition, according to the computer simulation, surface oxygen de-sorption activation energy relies on the surface oxygen vacancy density and thus the oxygen partial pressure.

Effect of internal stress disturbance on the stress-induced transformation toughening of an alumina/zirconia dual-phase composite

The toughening and strengthening of a dual-phase composite, consisting of alpha-alumina (α-Al2O3) and tetragonal-zirconia (t-ZrO2), were investigated. The toughness of the composite was evaluated through the precise measurement of work-of-fracture (WOF), which is a measure of total fracture resistance involving the rising R-curve effect. It was found that both the WOF and flexural strength of the composite were maximized at a t-ZrO2 volume fraction f Z of about 0.7. The thermal degradation of the mechanical properties was also observed. The effect of the internal stresses arising from the thermoelastic mismatch between α-Al2O3 and t-ZrO2 on the critical stresses of the reversible phase transformation of t-ZrO2 was numerically examined to describe the f Z - and temperature-dependencies of WOF quantitatively.

Electrical conductivity and oxygen transfer in gadolinia-doped ceria (CGO)–Co 3O 4− δ composites

Dense, mixed-conducting ceramic membranes can be used for separation of oxygen from air. Dual-phase composites are an interesting way to obtain mixed conductivity because it is easy to tailor the electrical and transport properties by varying the fractions of the ionic and electronic components. In this study, mixed-conducting Ce 0.8Gd 0.2O 2− δ (CGO)–Co 3O 4− δ composites were fabricated with density about 95% theoretical values. The solubility of Co 3O 4− δ in CGO was found to be lower than 1 mol% (or 1.6 vol.%). The total conductivity of the CGO–Co 3O 4− δ composites increased with increasing Co 3O 4− δ content. A microstructure with two interconnected phases was obtained for a Co 3O 4− δ content above 10 mol% (or 15 vol.%). Isotopic exchange depth profiling (IEPD)/secondary ion mass spectrometry (SIMS) method was used to investigate the oxygen diffusion and surface exchange reaction in the composites. The diffusivities of the composites were almost constant within the experimental error with significantly enhanced surface exchange kinetics by adding Co 3O 4− δ to CGO.

Microstructural and electrical properties of nanocomposite PZT/Pt thin films made by pulsed laser deposition

Dual phase composites made by dispersing a metallic phase into a ferroelectric matrix are of great interest due to significant enhancement in the dielectric constant ( ε). In this study thin films of nano sized platinum (Pt) particles were embedded in a crystalline lead-zirconate-titantate (PZT) matrix, using pulsed laser deposition. The microstructure of these films was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The electrical properties were investigated by capacitance versus voltage ( C– V) and leakage current versus voltage ( I– V) measurements. The influence of the Pt dispersoids on the crystallinity of the PZT matrix and the dielectric properties of the composites are discussed. Pt addition caused significant changes in the electrical properties, which showed the difference from what is observed for bulk PZT/Pt composites.

Quantitative Analysis of Microstructural Homogeneity and Capacitance Correlations in Palladium/Yttria‐Stabilized Zirconia Composites

Voronoi tessellation was applied to quantify the microstructural homogeneity of palladium particles in random Pd/cubic yttria‐stabilized zirconia (YSZ) composites with the Pd concentration ranging from 0 to 30 vol%. Room‐temperature impedance measurements were used to determine the capacitance of the composites. A fourfold enhancement in capacity near the percolation threshold of Pd was obtained. The percolation threshold was estimated at ∼30 vol% Pd using the normalized percolation theory (NPT). Data obtained from Voronoi diagrams resulted in a quantitative measure for the homogeneity of the dual‐phase composite. The homogeneity was found to decrease with increasing Pd concentration in the composites. Near the theoretical value of the percolation threshold, the Pd phase appeared to be distributed too inhomogeneously for preparing insulating composites. No large increase in capacity as predicted by NPT could therefore be experimentally verified.

Sintering and electrical properties of PZT/Pt dual-phase composites

Lead zirconate titanate–platinum dual phase composites, which have been shown to exhibit a very interesting electric field dependence of the dielectric constant, were prepared. The sintering properties and microstructure were analyzed. No chemical reaction between PZT and Pt was found in the composite. Neither did Pt atoms dissolve into the PZT lattice. The behavior of the effective electrical resistivity and the effective dielectric permittivity of the composites were investigated experimentally and theoretically in terms of the metal concentration. The dielectric constant of the composites increases with increasing metal concentration and approaches a maximum near the percolation threshold while the electrical conductivity and the dielectric losses remain small. It is indicative that the composites show a conspicuous effect of the tortuous, near-percolative metal phase.

Mechanical strength, and oxygen and electronic transport properties of SrCo 0.4Fe 0.6O 3−δ-YSZ membranes

Three membranes including SrCo 0.4Fe 0.6O 3− δ (SCF), 9 wt.% yttria-stabilized zirconia (YSZ) doped SrCo 0.4Fe 0.6O 3− δ (9%YSZ-SCF), 40 wt.% YSZ doped SrCo 0.4Fe 0.6O 3− δ (40%YSZ-SCF) were prepared. The dual-phase composite, 9%YSZ-SCF, consists of mainly perovskite type and SrZrO 3. The electronic conductivity of the three composites increases in the order of 40%YSZ-SCF<9%YSZ-SCF<SCF. The magnitude of mechanical strength increases in the opposite order. In 950–1050 K, oxygen vacancy concentration variation of the 9%YSZ-SCF membrane is higher than that of the SCF membrane due to the special effect of the doping YSZ. The oxygen permeation flux increases with temperature, and decreases with thickness. Doping the membrane with 9 wt.% YSZ does not influence the oxygen permeability significantly because both the ionic and the electronic conductivities remain high. In situ high-temperature X-ray diffraction (HTXRD) analysis indicates the 9%YSZ-SCF membrane is more stable than the SCF membrane at high temperatures under a low oxygen partial pressure.

Finite element analysis of observed high strengthening in composites with regularly segregated microstructures

A distinct dual phase composite has been developed, comprising spherical reinforcement clusters and an unreinforced matrix, according to numerical simulation of crack initiation and propagation in discontinuously reinforced MMCs. The present work is aimed at interpretation of the high strengthening ratios which were actually measured in such dual phase composites. Elastic-plastic finite element modelling is utilised to analyse the strengthening ratio in a two-dimensional idealised microstructure with periodic clustering. As the degree of clustering increases, the strengthening ratio is predicted to increase. In composites with a networking cluster, much more strengthening is exhibited together with relatively uniform strain distribution. The primary mechanism leading to additional strengthening due to clustering derives from an optimum ratio in deformation resistance between a matrix and a reinforcing phase. In the proposed dual phase composites, each cluster can behave as a single reinforcement which can deform plastically and there is no distinct interface between the cluster and the softer phase.

強化材凝集部が導入された二相複合材料の高強度発現機構

強化材凝集部が導入された二相複合材料の高強度発現機構

Microstructural development, electrical properties and oxygen permeation of zirconia-palladium composites

Yttria-stabilized cubic zirconia (YSZ)-palladium dual phase composites have been investigated. The percolative composite containing 40 vol% Pd (ZYPd40) showed a much larger oxygen permeability than that of the non-percolative composite containing 30 vol% Pd (ZYPd30). For a 2.0 mm thick percolative composite, an oxygen flux of 4.3 × 10 −8 mol/cm 2/s was measured at 1100 °C with oxygen partial pressures at the feed and permeate sides being 0.209 and 0.014 atm, respectively. This value is two orders of magnitude larger than that observed for a 2.0 mm thick non-percolative composite at the same temperature with the oxygen partial pressures at the feed and permeate sides being 0.209 and 1.5 × 10 −4 atm, respectively. From the dependence of the oxygen permeation on the temperature and on the oxygen partial pressures, it was concluded that the transport of the oxygen ions through the YSZ phase in the percolative system was the rate limiting step.

Plastic flow of a tungsten-based composite under quasi-static compression

A micromechanics study is carried out on the plastic flow of a tungsten “heavy alloy” under uniaxial compression. The alloy-composed of a continuous tungsten phase in a tungsten-nickel-iron “matrix”—is modeled as a dual-phase composite, with the tungsten phase approximated by aligned, uniformly distributed and equal-sized particles. The stress-strain behavior of each phase is assumed to be power-law hardening of the Ramberg-Osgood type. Stress-strain curves of the composite under uniaxial compression are computed numerically using several cell models representing different tungsten particle shapes. It is found that the overall stress-strain behavior of the composite is essentially independent of the particle shape. However, the character of the local plastic deformation changes dramatically with the inclusion geometry. Two simple formulae are given that approximate the overall flow behavior of the composite in terms of the tungsten volume fraction and the behavior of the individual phases.

The tensile deformation of a martensitic dual-phase steel

A single 0.12 wt.% C alloy has been heat treated to produce dual-phase ferrite-martensite composites of a variety of volume fractions and grain sizes of both phases. Two separate models have been used to describe the 0.2% proof stress of these composites, one based on the topological model of Gurland and the other based on the yield propagation model of Petch. Using the topological model, the 0.2% proof stress in all dual-phase steels could be represented by σ 0.2% (MPa) = 280 + 17.4d I − 1 2 (1 − g′) + 576g′ where g′ is a parameter related to the volume fractions and contiguities of the two phases and d I is the size of type I grains. This model had some inconsistencies and in particular did not show good extrapolations to either 100% martensite or 100% ferrite. Using the yield propagation model, the 0.2% proof stress could be written as σ 0.2% (MPa) = 122 + {16.4 + 432 (V II d II ) 1 2 }d c − 1 2 where d c is an average between the ferrite grain size and the martensite islet size, d II is the size of type II grains and V II is the volume fraction of the secondary phase. This equation showed excellent extrapolation to 100% ferrite but some change in slope was observed towards 75% martensite. The reasons for the inadequacies of representation have been attributed mainly to autotempering in the martensite and to the shortcomings in the measurement of the ferrite and martensite continuities.