J. Rabier

Densification and thermal conductivity of low-sintering-temperature AlN materials

Abstract The aim of the study was the sintering of aluminium nitride at rather low temperatures (≤1650°C). Various sintering aids were studied, in particular AlSi- and Ca-based oxides. CaCO3 and 3CaO-3SiO2-Al2O3 are the most efficient additives, because quantities as low as 0·5 wt% are enough to yield sintered AlN substrates with no open porosity. TEM observations showed that low concentrations of CaCO3 or CaSiO3 lead to location of the secondary phases at the triple points, whereas high concentrations lead to wetting of the grains. Thermal diffusivity of sintered materials was determined by a laser flash method. The highest conductivity (70 W m−1 K−1) is obtained with CaCO3 additions. These additions promote liquid formation, which cleans the surface of the AlN grains from oxygen and thereby prevents oxygen from entering the AlN lattice. The CaAl2O4 compound exhibits a lower thermal conductivity than the CaAl4O7 and Ca3Al10O18 compounds. The Ca2SiAl2O7 compound and the 27R AlN polytype have a strong negative influence on the thermal conductivity of sintered materials.

Microindentation of Al-Cu-Fe icosahedral quasicrystal

Reference CRPP-ARTICLE-1999-033doi:10.1016/S1359-6462(99)00242-0View record in Web of Science Record created on 2008-04-16, modified on 2017-05-12

Dislocation microstructures in Si plastically deformed at RT

Dislocation microstructures induced by plastic deformation at room temperature in Si have been investigated by TEM. Plastic deformation has been obtained by using two types of technique: deformation under a confining pressure of 5 GPa in an anisotropic multi-anvil apparatus and by surface scratching. The TEM observations show common features in the two deformation substructures which are characteristic of high stress-low temperature deformation. The deformation microstructures are built with dislocations with a/2110 Burgers vector in (111) planes which are undissociated. Such dislocations are mainly aligned along the screw orientation and 112 orientations at 30° from the Burgers vectors as well as along 132 orientations at 41° from the Burgers vector. The occurrence of those Peierls valleys confirms that different dislocation core configurations from those usually dealt with at higher temperatures have to be taken into account when dislocations are nucleated at very high stresses.

On a change in deformation mechanism in silicon at very high stress: new evidences

Abstract Dislocation configurations resulting from high stress low temperature deformation in silicon are found non-dissociated and elongated along different Peierls valleys as compared to usual results from the literature. Experiments on pre-deformed samples are reported and discussed in the context of a possible transition in the core structure at high stresses between dissociated glide and perfect shuffle configurations.

Chapter 93 Dislocations in Silicon at High Stress

Abstract This chapter is devoted to the structure and properties of dislocations obtained at high stress in silicon, which have been shown to have different core structures from the ones observed at low stress and high temperature. TEM observations of high-stress deformation microstructures revealing the existence of a specific type of dislocations, which exhibit a non-dissociated core and are expected to lie in the shuffle set, are reported together with relevant atomistic computations. The results of experiments and computations are confronted in order to obtain a comprehensive view of the high-stress deformation regime. The transition between the two deformation regimes is also discussed.

AIN plastic deformation between room temperature and 800°C. I. Dislocation substructure observations

Abstract Dislocations substructures in aluminium nitride (AIN) ceramics have been studied by transmission electron microscopy for samples plastically deformed under a high stress in the temperature range 20–800°C. The activation of basal slip ⅓ 〈1120〉(0001) as well as of the first-kind prismatic slip ⅓ 〈1120〉{1100} have been observed. In the basal plane, dislocations configurations attest to the role of the Peierls forces along the screw orientation. It is easier for a dislocation to cross-slip in the prismatic plane than to lie in the basal plane along an orientation other than the screw direction. In the basal plane as well as in the prismatic plane, no dissociation of dislocations has been resolved under weak-beam imaging conditions. Cross-slip is favoured by numerous pinning points on screw dislocations, giving rise to dipole dragging. From 20 to 500°C, AIN deformation proceeds by dislocation glide in the basal and the prismatic planes and frequent corss-slip between these two planes. At higher temper...

Indentation-induced twinning in LaAlO3 single crystals: An atomic force microscopy study

Abstract Micro-indentation tests performed from room temperature to 700 °C on LaAlO 3 single crystals show a plastic anisotropy in the rhombohedral phase, depending on the orientation of the Vickers indenter. The indentation-induced twins were investigated by atomic force microscopy. Their geometry appears to be different from those resulting usually from the cubic to rhombohedral phase transition.

High oxygen pressure generation of flux-pinning centers in melt-textured YBa2Cu3O7

A strong enhancement of the critical current density, Jcab(H,T), has been generated in melt-textured YBa2Cu3O7/Y2BaCuO5 composites by means of high oxygen pressure (≈100 bar, ∼400 °C). Enhancement factors as high as 180% have been found at low temperatures leading high sample performances at 77 K (Jcab≈1.2×105 A/cm2 at null field). Transmission electron microscopy studies show a high concentration of dendritic-like stacking faults generated as a consequence of the strong nonequilibrium conditions of the transformation from YBa2Cu3O7 to YBa2Cu4O8. The high concentration of 1/6〈031〉 partial dislocations within the (001) planes are proposed as point-defect pinning centers enhancing Jcab(H,T) for the H∥c configuration.

Atomistic calculations of point-defect interactions with an edge dislocation in NiO

Abstract The dislocation core structure of an (α/2) [110](1·0) edge dislocation has been computed in NiO using an atomistic description of the lattice. A shell model potential was used to describe the ionic interactions. The binding energies of the dislocation with vacancies of different charge states as well as with electronic holes have been calculated using the PDINT code. This code is based on a generalized Mott-Littleton procedure. It is found that doubly charged nickel vacancies have a lower binding energy with an edge dislocation than singly charged vacancies. These results are analysed in relation to possible differences between the stoichiometry of the dislocation core and the bulk crystal. Preliminary results on migration energies of vacancies within the dislocation core are also given.

Enhanced critical currents in melt textured YBa2Cu3O7 by cold isostatic pressing

An enhancement of the critical currents in the H‖c configuration, reaching 100% at 77 K, has been obtained in melt textured YBa2Cu3O7/Y2BaCuO5 composites after a cold isostatic pressing process (300 °C, PAr=200 MPa). A transmission electron microscopy analysis of the microstructure demonstrates that a high density of nanometric stacking fault loops, with anisotropic growth within the (001) plane, is generated. The improvement of critical currents is attributed to the flux pinning enhancement due to the increased length of the 1/6〈031〉 partial dislocations surrounding nanometric stacking faults. We stress that this technique has high potential for the large scale processing of high critical currents superconducting ceramic composites.