Influence of forming conditions on characteristics of alumina–PSZ composites

Abstract

The purpose of the work reported in the present paper was to establish the correlation between the physical, mechanical, and microstructural properties of alumina matrix composites reinforced with (CeO₂, Nd₂ O₃, Y₂O₃ )–PSZ (partially stabilised zirconia) depending on the processing and thermal treatment conditions. The composites obtained from fine powder mixtures were formed by hydraulic pressing, ceramic injection moulding, and hot pressing under various temperature and pressure conditions. The samples were fired at 1550–1770°C in an oxidising atmosphere and in vacuum depending on the forming conditions. Comparative microstructure investigations were made by TEM on sample surfaces. The XRD results were in accordance with the determined properties of the investigated compositions. The results highlighted that the best physical and mechanical properties and homogenous microstructure for the ZTA composites were obtained by firing in vacuum.     

Measurement and Correlation of Electrical Conductivity of β-Cyclodextrin in Water Solution

The electrical conductivity of 4% β-cyclodextrin in water solution had been measured by electrode method from（323.65~353.65） K at atmospheric pressure. The expe...     

Phase interaction and oxygen transport in oxide composite materials

Abstract

The oxygen permeability of oxide composite membranes containing similar volume fractions of the components, including (La_0.9 Sr_0.1)_0.98 Ga_0.8 Mg_0.2 O_3-δ(LSGM)–La_0.8 Sr_0.2Fe_0.8Co_0.2O_3-δ (LSFC), LSGM–La₂Ni_0.8Cu_0.2O_4+δ (LNC), SrCoO_3-δ–Sr₂Fe₃O_{6.5 ±δ}, Ce_0.8Gd_0.2O_2-δ (CGO)–LSFC and CGO–La_0.7Sr_0.3MnO_3-δ (LSM), was studied at 973–1223 K. In most cases, oxygen transport is substantially affected by component interaction, decreasing ionic conductivity due to cation interdiffusion, and formation of intermediate phases and/or blocking layers at grain boundaries. This interaction is maximised in systems where the phase components have similar structure and thus may form continuous solid solutions, for example LSGM–LSFC, or intermediate compounds such as Roddlesden–Popper phases in LSGM–LNC composites. The results show that, in addition to knowledge of the transport properties and volume fractions of percolating phases, analysis of ionic conduction in oxide composite materials requires assessment of phase interaction and grain boundary processes.     

Oxide ionic conductivity and crystallographic properties of tetragonal type Bi2O3-based solid electrolyte doped with Ho2O3

Abstract

In the present work, the authors have investigated the binary system of (Bi₂O₃)_1–x(Ho₂O₃)_x. For the stabilisation of the tetragonal type solid solution, small amounts of Ho₂O₃ were doped into the monoclinic Bi₂O₃ via solid state reactions in the stoichiometric range 0·01≤x≤0·1. The crystal formula of the formed solid solution was determined as Bi(III)_4–4xHo(II)_4xO_6–2xVo_(2+2x) (where Vo is the oxide ion vacancy) according to the XRD and SEM microprobe results. In the crystal formula, stoichiometric values of x were 0·04≤x≤0·08, 0·03≤x≤0·09, 0·02≤x≤0·09 and 0·04≤x≤0·09 for annealing temperatures at 750, 800, 805 (quench) and 760°C (quench) respectively. The four probe electrical conductivity measurements showed that the studied system had an oxide ionic type electrical conductivity behaviour, which is increased with increasing dopant concentration and temperature. The obtained solid electrolyte system has an oxygen non-stoichiometry characteristic, and it contains O^2– vacancies, which have disordered arrangements in its tetragonal crystal structure. The increase in the amount of Ho₂O₃ doping and temperature causes an increasing degree of the disordering of oxygen vacancies and a decrease in the activation energy E_a.     

Effect of porosity and metallic insertions on electrical resistivity of A2011 aluminium alloy

Abstract

A rotational, contactless inductive measurement technique has been used to determine the effect of pores and metallic insertions on the electrical resistivity of A2011 aluminium alloy at different temperatures. It is shown that the electrical resistivity increases with the total volume of pores, and is also dependent on the location and orientation of pores. Additional energy losses were found on the contact surfaces between sample and insertions.     

Modelling the 'universal' dielectric response in heterogeneous materials using microstructural electrical networks

Abstract

The frequency dependent conductivity and permittivity of a ceramic composite are modelled using electrical networks consisting of randomly positioned resistors and capacitors. The electrical network represents a heterogeneous microstructure that contains both insulating (the capacitor) and conductive regions (the resistor). To validate model results, a model ceramic conductor–insulator composite was designed consisting of a porous lead zirconate titanate impregnated with different concentrations of water. Excellent agreement between experimental and model data was achieved with a strong correlation with many other ceramics, glasses and composites. It is proposed that the 'universal' dielectric response of many materials is a consequence of microstructural heterogeneity. The modelling approach could be used as a simple and effective method for microstructural design of ceramics and other materials with tailored dielectric properties.     

Relationship between microstructure and mechanical properties of fired Ecuadorean clay

Abstract

Hardness and fracture toughness measurements are reported for individual phases and interfacial composites present in fired specimens of an Ecuadorian clay mineral. Each investigation used a half disc from strength tests previously done and reported elsewhere. The aim was to rationalise trends in the macroscopic strength and density to quantify contributions from the important components of the microstructure. This was achieved through application of an etching and gold coating procedure prior to microindentation testing. The coating technique improved the clarity of the indents and made the diagonal and radial crack measurements more accurate. Two peaks in the strength versus firing temperature curve are shown to arise from opposite trends in the toughness of the quartz grains and the matrix phase, together with a no-linear build up of the volume of interfacial composite material which confines any cracks initiated in the residual quartz crystals at high firing temperatures.     

Research of the Thermal and Strength Properties for Materials Obtained with Sunflower Oil

The use of pulverized fly ash (PFA) obtained from thermal power plants and epoxidized sunflower oil (ESFO) as recycled material in the construction industry was investigated. Epoxidized sunflower oil, clay (C) and pulverized fly ash were mixed at various proportions and were fired at 160, 180 and 200°C. The thermal conductivity and strength (compressive strength, tensile strength and abrasion loss) of the samples were determinated. The lowest value of thermal conductivity, 0.25 W/mK, was measured for the sample with a 70% FA/30% C ratio and 50% ESFO processed at 200°C. This sample had the lowest compressivetensile strength (3.28 MPa–0.633 MPa) and the highest abrasion loss (4.39%). The highest compressivetensile strengths and the lowest value of abrasion loss observed, were 7.21 MPa–0.939 MPa and 1.15%, respectively.     

Effects of transition from lamellar to compacted graphite on thermal conductivity of cast iron

Abstract

Different levels of magnesium were added to a standard grey iron alloy in order to obtain a range of graphite morphologies from lamellar to compacted graphite. The thermal conductivity/diffusivity of samples, solidified at different cooling rates, was investigated by means of the laser flash technique. There is a significant decrease in the thermal conductivity as the morphology transits from lamellar to compacted graphite. The thermal conductivity of grey iron decreases considerably at elevated temperatures, whereas the thermal conductivity of compacted graphite iron is less sensitive to changes in temperature. At increased nodularities, compacted graphite irons exhibit a maximum thermal conductivity at ～400°C. The influence from the cooling conditions on the thermal conductivity decreases as the morphology alters from lamellar graphite to compacted graphite. The effective thermal conductivity of cast iron is modelled by means of existing models for composites.