Studies on polycrystalline layered ceramic oxide: LiFeVO4

Abstract

A new polycrystalline layered ceramic oxide, LiFeVO₄, has been prepared by a standard solid state reaction technique. The preparation conditions were optimised using thermogravimmetric analysis (TGA) technique. Material formation under the reported conditions was confirmed by X-ray diffraction studies. A preliminary structural analysis indicated that the crystal structure was orthorhombic with lattice parameters: a=4·3368 Å, b=13·1119 Å and c=16·3426 Å. The phase morphology and surface property were studied by scanning electron microscopy. Complex impedance analysis of the sample indicated bulk contribution to electrical properties at T≤125°C, grain boundary effects at the temperatures ≥125°C, negative temperature coefficient of resistance (NTCR) effect and evidence of temperature dependent electrical relaxation phenomena in the sample. The dc conductivity σ_dc shows typical Arrhenius behaviour when observed as a function of temperature. The activation energy value was estimated to be 0·24 eV. The value of σ_dc, evaluated from complex impedance spectrum, shows a jump of nearly two orders of magnitude at higher temperature (～1·24 × 10^?5 S cm^?1 at 350°C) when compared with that of σ_dc (1·14 × 10^?6 S cm^?1 at 50°C). Alternating current conductivity spectrum obeys Jonscher's universal power law. The results of σ_ac v. temperature are also discussed.     

Elastic stresses in an adhesively-bonded functionally-graded tubular single-lap joint in tension

In this study, the elastic stress analysis of an adhesively-bonded tubular lap joint with functionally-graded Ni-Al₂O₃ adherends in tension was carried out using a 3D 8-node isoparametric multilayered finite element with 3 degrees-of-freedom at each node. Stress concentrations were observed along the edges of both outer and inner tubes in the overlap region. Thus, the outer tube region near the free edge of the inner tube and the inner tube region near the free edge of the outer tube experienced considerable stress concentrations. Normal σ_zz and shear σ_rz stresses were dominant among the stress components. In addition, both edges of the adhesive layer experience stress concentrations, and the von Mises σ _eqv stress decreases uniformly across the adhesive thickness at the free edge of the outer tube, whereas it increases at the free edge of the inner tube. However, different compositional gradients had only a small effect on the through-the-thickness normal and shear stress profiles of both outer and inner tubes, and the peak von Mises σ _eqv stresses occurred inside the tube walls. As the ceramic phase in the material composition of the outer and inner tubes was increased, peak von Mises σ _eqv stress appeared in the ceramic layer. However, its magnitude was increased 1.75-fold in both tubes. In addition, the peak adhesive stresses appeared at the edge of the outer tube–adhesive interface near the free edge of the inner tube and at the edge of the inner tube–adhesive interface near the free edge of the outer tube. Increasing the ceramic phase in the material composition caused 1.22–1.67-times higher von Mises stresses along the free edges of the adhesivetube interfaces. In addition, with increasing number of layers across the inner and outer tubes the profiles of the normal σ_zz , shear σ_r and von Mises σ _eqv stresses across the tube walls and adhesive layer become similar. Increasing the ceramic phase in the material composition of the tubes causes also evident increases in the normal σ_zz and von Mises stresses while it does not affect their through-the-thickness profiles. However, it affects only shear σ_r and von Mises stresses across the adhesive layer. Finally, the layer number and the compositional gradient do not affect considerably through-the-thickness normal and shear stress profiles but levels in a functionally graded plate subjected to structural loads.     

Use of heating microscopy to assess sintering anisotropy in layered glass metal powder compacts

Abstract

The objective of the present study was to demonstrate that heating microscopy can be used to investigate the deformation of layered materials during sintering. Three composite systems with layered microstructure were prepared using borosilicate glass matrix and vanadium particles as the inclusion phase. The sintering shrinkage of cylindrical compacts was recorded in axial and radial directions. As expected, sintering was impaired with increasing concentration of vanadium particles. A shrinkage anisotropy factor was determined based on experimental measurements and its evolution during sintering for each sample was discussed. In samples containing three layers with different volume fractions of vanadium inclusions, similar densification was observed in the bottom layer with maximum concentration of inclusions (30 vol.-%) and in the top layer with minimum concentration (2 vol.-%). This indicates that sintering anisotropy of the samples is dependent not only on the composition, but also on the position arrangement of the layers in the sintering part. The results show that heating microscopy is a simple technique which can be used to support the design and fabrication of layered (and by extension functionally graded) materials. Smart choice of composition, dimension and position of the sample in the furnace during sintering should lead to adequate control or prediction of the final sintered shape.     

Densification and dielectric properties of strontium bismuth tantalate ceramics

Abstract

The effects of B₂O₃ doping on the sintering processes and dielectric properties of SrBi₂Ta₂O₉ were investigated in this study. The sinterability of SrBi₂Ta₂O₉ was significantly enhanced by doping B₂O₃ due to the formation of liquid phase at elevated temperatures. The addition of B₂O₃ also led to inhibited thermal decomposition and enhanced grain growth of SrBi₂Ta₂O₉. The amount of doped B₂O₃ was found to substantially influence the dielectric constants of SrBi₂Ta₂O₉ ceramics. When a proper amount of B₂O₃ was added, the dielectric constants greatly increased. For facilitating the densification process and improving the dielectric properties of SrBi₂Ta₂O₉, it is important to control the doping amount of B₂O₃.     

An experimental investigation of the effect of interface adhesion on the fracture characteristics of a brittle-ductile layered material

The effect of interface adhesion on the failure characteristics of brittle-ductile layered material was experimentally investigated. Single-edge-notched fracture specimens were prepared by bonding two Homalite-100 layers to a thin aluminum layer using three different types of adhesives. The specimens were loaded under three-point bending and photoelasticity was used for full-field observation of the failure process. Fracture tests revealed two competing modes of failure: delamination along the Homalite-aluminum interface, and crack re-initiation in the Homalite layer across the reinforcing aluminum layer. The failure modes were directly influenced by the characteristics of the adhesive bond. Maximum load retention and energy dissipation capability during the fracture process was observed for a urethane based adhesive that formed an interfacial bond that was resistant to delamination, and additionally exhibited low modulus and large strain-to-failure, thereby suppressing crack re-initiation.     

Axisymmetric compression test and hot working properties of alloys

Abstract

The paper investigates the use of the hot axisymmetric compression test for the determination of the hot working properties of alloys. Qualitative analysis shows that stress determinations from the test are subject to systematic errors. These arise from frictional effects and deformation heating. The errors are affected by the conditions of the test and the effects of these parameters have been investigated using a fully coupled finite element procedure. Specimen geometry, specimen volume, friction, temperature, strain rate, and strain have been investigated. Procedures outlining methods of error calculation for general testing conditions are given. Methods of using the data to correct stress–strain curves, to validate such curves and to optimise testing conditions are discussed.     

ADAPTIVE LAYERED CARTESIAN CUT CELL METHOD FOR THE UNSTRUCTURED HEXAHEDRAL GRIDS GENERATION

Adaptive layered Cartesian cut cell method is presented to solve the difficulty of the unstructured hexahedral anisotropic Cartesian grids generation from the complex CAD model. Vertex merging algorithm based on relaxed AVL tree is investigated to construct topological structure for stereo lithography (STL) files,and a topology-based self-adaptive layered slicing algorithm with special features control strategy is brought forward. With the help of convex hull,a new points-in-polygon method is employed to improve the Cartesian cut cell method. By integrating the self-adaptive layered slicing algorithm and the improved Cartesian cut cell method,the adaptive layered Cartesian cut cell method gains the volume data of the complex CAD model in STL file and generates the unstructured hexahedral anisotropic Cartesian grids.     

Delamination growth during fatigue of advanced polymer matrix composites

Abstract

Advanced polymer matrix composites such as carbon fibre reinforced polymers (CFRP), offer many advantages over more traditional materials such as metals. Usually, CFRP have greater strength/weight and stiffness/weight ratios than traditional engineering materials, which makes them ideal for use in many weight sensitive applications, especially in the aerospace sector. To maximise the use of these materials there is a need to gain a better understanding of how CFRP, and more generally FRPs, behave under fatigue load conditions. This work investigates the fatigue response and damage mechanisms found in a CFRP. Previous work has highlighted that fatigue with a compressive element is more damaging than pure tensile fatigue and that delamination is the dominant damage mechanism in both cases. However, in the tensile fatigue tests the primary delamination was on a different interface from the primary delamination found in the compression fatigue tests. The cause of this trend to delaminate along a particular interface has been investigated using mixed mode bend tests. These tests have been used to investigate the response of the interface to both static and fatigue loads. Initial tests have been carried out on the 0°/45° interface. Delamination growth was monitored at three levels of mode mixity, ratios of MI/MII of 1:1, 1:3 and 3:1. PRC/1848     

Effect of friction on deformation during rolling as revealed by embedded pin technique

Abstract

The paper investigates the effects of friction on the heterogeneity of deformation during rolling through studies using plane strain (2D) and three-dimensional (3D) finite element models designed to simulate the deformation of the embedded pin inserts during rolling. Redundant work due to friction is defined as a path function along the arc of contact. Since deformation during rolling is profoundly influenced by the amount of redundant work, which depends on friction as a path function along the arc of contact, the study has focused especially on methods of representing these frictional effects. The friction studied has been in one of two classes: the coefficient of friction being constant or varying parabolically along the arc of contact. The results show that the values of shear stress and normal pressure along the arc of contact depend upon the friction profile. The magnitude of these frictional effects is revealed by the through thickness variation of the relative pin insert displacement. This displacement changes in the 3D model because transverse spread reduces the amount of displacement along the axial direction. Comparison of the simulations with experimental pin insert shapes shows close agreement between the predicted and experimental results, while revealing the direction of further work needed to provide suitable mechanics models to interpret experimental pin insert data.