Calculation and measurement of the magnetostriction of multilayered composites

A new theoretical model that predicts the magnetostriction of multilayered composites has been developed. The model takes into account the shear stress between the composite layers and consequently predicts a non-uniform strain along their thickness. The model has been experimentally validated by producing composites formed from three materials with different magnetostrains and mechanical properties, and controlled layer thicknesses in the order of micrometers. Deformations of several ppm, up to 7.5% of the saturation magnetostrain were measured between the edge and the centre of such composites. © 2006 Elsevier B.V. All rights reserved.

Electrical transport and percolation in magnetoresistive manganite / insulating oxide composites: case of La0.7Ca0.3MnO3 / Mn3O4

We report the results of electrical resistivity measurements carried out on well-sintered La0.7Ca0.3MnO3 / Mn3O4 composite samples with almost constant composition of the magnetoresistive manganite phase (La0.7Ca0.3MnO3). A percolation threshold (fc) occurs when the La0.7Ca0.3MnO3 volume fraction is ~ 0.19. The dependence of the electrical resistivity as a function of La0.7Ca0.3MnO3 volume fraction (fLCMO) can be described by percolation-like phenomenological equations. Fitting the conducting regime (fLCMO > fc) by the percolation power law returns a critical exponent t value of 2.0 +/- 0.2 at room temperature and 2.6 +/-0.2 at 5 K. The increase of t is ascribed to the influence of the grain boundaries on the electrical conduction process at low temperature.

La 0.7Ca 0.3MnO 3 / Mn 3O 4 composites: Does an insulating secondary phase always enhance the low field magnetoresistance of manganites

Composites of magnetoresistive La 0.7Ca 0.3MnO 3 (LCMO) with insulating Mn 3O 4 are useful as a model system because no foreign cation is introduced in the LCMO phase by interdiffusion during the heat treatment. Here we report the magnetotransport properties as a function of sintering temperature T sinter for a fixed LCMO/Mn 3O 4 ratio. Decreasing T sinter from 1250 °C to 800 °C causes an increase in low field magnetoresistance (LFMR) that correlates with the decrease in crystallite size (CS) of the LCMO phase. When plotting LFMR at (77 K, 0.5 T) versus 1/CS, we find that the data for the LCMO/Mn 3O 4 composites sintered between 800 °C and 1250 °C follow the same trend line as data from the literature for pure LCMO samples with crystallite size >∼25 nm. This differs from the LFMR enhancement observed by many authors in the usual manganite composites, i.e., composites where the insulating phase contains cations other than La, Ca or Mn. This difference suggests that diffusion of foreign cations into the grain boundary region is a necessary ingredient for the enhanced LFMR. © 2012 American Institute of Physics.

Electrical transport and magnetic properties of Mn3O4-La0.7Ca0.3MnO3 ceramic composites prepared by a one-step spray-drying technique

La0.7Ca0.3MnO3/Mn3O4 composites can be synthesized in one step by thermal treatment of a spray-dried precursor, instead of mixing pre-synthesized powders. Another advantage of this composite system is that a long sintering step can be used without leading to significant modification of the manganite composition. The percolation threshold is reached at ∼20 vol% of manganite phase. The 77 K low field magnetoresistance is enhanced to ∼11% at 0.15 T when the composition is close to the percolation threshold. © 2007 Elsevier Ltd. All rights reserved.

Analytical model and experimental validation of friction laws for composites under low loads

In order to account for interfacial friction of composite materials, an analytical model based on contact geometry and local friction is proposed. A contact area includes several types of microcontacts depending on reinforcement materials and their shape. A proportion between these areas is defined by in-plane contact geometry. The model applied to a fibre-reinforced composite results in the dependence of friction on surface fibre fraction and local friction coefficients. To validate this analytical model, an experimental study on carbon fibrereinforced epoxy composites under low normal pressure was performed. The effects of fibre volume fraction and fibre orientation were studied, discussed and compared with analytical model results. © Springer Science+Business Media, LLC 2012.

Influence factors on wear resistance of two alumina matrix composites

We measured the wear resistances of alumina, alumina/silicon carbide composite and alumina/mullite composite by abrasive wear. And we studied the influence of fracture mode and worn surface pullout on wear resistance. The results are as follows: the main wear mechanisms of alumina and alumina/silicon carbide were fracture wear and plastic wear respectively, and for alumina/mullite composite, fracture wear and plastic wear mechanisms worked together. The wear resistance of the alumina/silicon carbide composite and the alumina/mullite composite was better by a factor of 1 similar to 3 than that of the monolithic alumina. There were two main reasons for the better wear resistance, i.e., the improved mechanical properties and the more smooth worn surfaces. However, The primary reason was the reduction of area fraction of pullout on the worn surfaces induced by fracture mode transition. (C) 2007 Published by Elsevier B.V.

Wear resistance of reaction sintered alumina/mullite composites

Alumina and alumina/mullite composites with mullite content of 0.96-8.72 vol.% were subjected to an abrasive wear test under loads of 0.1-2.0 N with a ball-on-disc apparatus. The wear rate and area fraction of pullout f(po) on the worn surfaces were measured. The wear resistances of the alumina/mullite composites were better by a factor of 1-2 than that of pure alumina. The main wear mechanism of alumina is fracture wear, and for alumina/mullite composites, fracture wear and plastic wear mechanisms work together. The influence of mechanical properties on wear resistance was estimated by Evans' method. It was found that the wear rate depends on f(po), and the primary reason for the better wear resistance of alumina/mullite composites is the reduction off, induced by fracture mode transition. (c) 2007 Elsevier B.V. All rights reserved.

Two-dimensional photonic lattices in polymer-dispersed liquid crystal composites

The relationship between liquid crystal orientational ordering and optical diffraction properties is investigated for a two-dimensional square photonic lattice fabricated in a polymer-dispersed liquid crystal (PDLC) composite. Modifications of the nematic director field in the liquid crystal domains were induced by an external applied voltage and by heating over the nematic-isotropic (N-I) phase transition. They were studied by optical polarization microscopy and by analysing far-field optical diffraction patterns. The intensities of various diffraction orders (from the zeroth up to the eighth diffraction order) were monitored with a CCD camera, and their variations were correlated with the modifications of the director field.

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF TI-BASED METALLIC GLASS MATRIX COMPOSITES

Ti40Cu40Ni10Zr10-xScx (x = 0.5 and 1, at%) alloys were prepared by copper mould casting method. Microstructures of the phi 3 mm rod alloys were investigated by XRD and SEM. The results showed that the phi 3 mm rods were glassy matrix with TiCu crystalline phase. Mechanical properties were studied by compressive test. Ti40Cu40Ni10Zr9Sc1 alloy exhibited good compressive strength over 2200 MPa and superior compressive deformation is about 7.9%.