Erich H. Kisi

Ti3SiC2—A layered ceramic exhibiting ultra-low friction

Abstract Currently known solid lubricants will sometimes exhibit ultra-low coefficients of friction, (μ

Group-theoretical analysis of octahedral tilting in ferroelectric perovskites

Group-theoretical methods are used to analyze perovskite structures where both ferroelectric cation displacements and simple tilting of octahedral units are present. This results in a list of 40 different structures, each with a unique space-group symmetry. The list is compared with that of Aleksandrov & Bartolomé [Phase Transit. (2001), 74, 255-335] and a number of differences are found. The group-subgroup relationships between the structures are also determined, along with an indication of those phase transitions that must be first order by Landau theory.

The hydrogen activation of LaNi5

Abstract Changes to the hydridable intermetallic compound LaNi 5 and its hydriding behaviour during the first few hydrogen absorption—desorption cycles (activation) have been studied. A number of bulk (manometric hydrogen content measurements, optical microscopy), structural (X-ray and neutron diffraction) and microstructural (scanning and transmission electron microscopy) techniques provide strong evidence that the major part of the observed reduction in hysteresis after activation is due to the formation of extended two-dimensional defects in the alloy. Evidence is presented that these defects also contribute to the observed increase in the rate of hydride formation after activation.

Modelling Dislocation‐Induced Anisotropic Line Broadening in Rietveld Refinements Using a Voigt Function. II. Application to Neutron Powder Diffraction Data

In paper I [Wu et al. (1998). J. Appl. Cryst. 31, 356–362] an approach was developed to the problem of modelling dislocation-induced X-ray or neutron-diffraction-line broadening. This paper applies those findings to the Rietveld refinement of the neutron powder diffraction profiles of deuterium-cycled LaNi5 and β-PdD0.66. These interstitially modified materials exhibit, respectively, strong and weak anisotropic strain broadening. The broadening in LaNi5 is consistent with a dislocation slip system a/3 (\bar 2110) {0\bar 110}, in agreement with transmission electron microscopy studies. In PdD0.66 the model predicts a regular distribution of screw dislocations, which remains to be confirmed by other techniques.

Ultra-low friction for a layered carbide-derived ceramic, , investigated by lateral force microscopy (LFM)

The carbide-derived ceramic has a layered structure with double TiC-like blocks separated by planes of Si in six-fold coordination. It also has an unusual combination of electrical, thermal and mechanical properties. The polycrystalline ceramic exhibits a preferred plane of easy cleavage. Fracture/cleavage faces have been investigated by lateral and atomic force microscopy, scanning electron microscopy and other techniques. The results show that the ceramic is an exceptional solid lubricant. Freshly prepared (by fracture, abrasion or scraping) cleavage faces exhibit coefficients of friction at or below the limit of resolution, about , which are comparable to the lowest values measured for the best solid lubricants ( and graphite). Exposure to air leads to slight degradation, but values below were obtained after exposures of several months. Analysis of other faces leads to outcomes in the range 0.1-0.3, which are consistent with the outcome of macroscopic analysis. The favourable mechanical and electrical properties, in combination with ultra-low solid friction, may have important technical implications.

Formation of titanium hydride at room temperature by ball milling

Titanium hydride with a body centred tetragonal structure was rapidly synthesized at room temperature by ball milling titanium powders under a hydrogen atmosphere. The structure evolution was monitored by x-ray diffraction and Rietveld analysis. Unit cell volume dilation suggests the hydrogenation process begins with the formation of a titanium - hydrogen solid solution followed by the formation of titanium hydride. Dehydrogenation of the hydride produced by milling and titanium hydride prepared by standard techniques was investigated by thermogravimetric analysis (TGA). The hydride formed by the milling process has lower initial and final dehydrogenation temperatures and a slightly narrower dehydrogenation temperature range compared to hydride prepared by traditional methods. The extremely fine particle size, large surface area and lattice defects introduced by milling are believed to be responsible for modification of the hydrogen storage behaviour.

Applications of neutron powder diffraction

1. Introduction to neutron powder diffraction 2. Theory - the bare essentials 3. Basic instrumentation and experimental techniques 4. Elements of data analysis 5. Crystal structures 6. Ab initio structure solution 7. Magnetic structures 8. Quantitative phase analysis 9. Microstructural data from powder patterns 10. Diffuse scattering - thermal, short range order, gaseous, liquid and amorphous scattering 11. Stress and elastic constants 12. New Directions

Modelling dislocation-induced anisotropic line broadening in Rietveld refinements using a Voigt function. I. General principles

The theory of dislocation-induced X-ray or neutron diffraction line broadening developed by Krivoglaz et al. and Wilkens has been adapted for Rietveld refinement of the powder diffraction profile by fitting a Voigt function to each peak. Information on both the type of slip system and the density of dislocations in the crystallites may then be found by evaluating the shape parameter and the index-dependent breadth of the Voigt function.

A neutron diffraction investigation of the LaNi5D phase diagram

Phase boundaries in the LaNi5D system determined by in situ neutron diffraction experiments have shown good agreement with those predicted from the LaNi5D pressure—composition phase diagram by intersecting extrapolated pressure plateaux and pure phase isotherms. Lattice parameter measurements show strong dilational strain in the α phase and severe compressive strain in the β phase in the two-phase region. In addition, there are systematic changes in linewidths in the two-phase region. Taken together, these provide strong evidence that the partially hydrided powder is microscopically inhomogeneous, i.e. individual micron-sized particles contain coexisting nanoscale α and β phase domains. The alternative model, consisting of a mixture of particles which are wholly α and particles which are wholly β phase, is not supported by these data. We observed hysteresis in the lattice parameters of the β phase and the linewidths of the α phase in the two-phase region, implying a fundamental asymmetry in the D absorption and desorption mechanisms.

The giant piezoelectric effect: electric field induced monoclinic phase or piezoelectric distortion of the rhombohedral parent?

Lead zinc niobate–lead titanate (PZN–PT) single crystals show very large piezoelectric strains for electric fields applied along the unit cell edges e.g. [001]R. It has been widely reported that this effect is caused by an electric field induced phase transition from rhombohedral (R3m) to monoclinic (Cm or Pm) symmetry in an essentially continuous manner. Group theoretical analysis using the computer program ISOTROPY indicates phase transitions between R3m and Cm (or Pm) must be discontinuous under Landau theory. An analysis of the symmetry of a strained unit cell in R3m and a simple expansion of the piezoelectric strain equation indicate that the piezoelectric distortion due to an electric field along a cell edge in rhombohedral perovskite-based ferroelectrics is intrinsically monoclinic (Cm), even for infinitesimal electric fields. PZN–PT crystals have up to nine times the elastic compliance of other piezoelectric perovskites and it might be expected that the piezoelectric strains are also very large. A field induced phase transition is therefore indistinguishable from the piezoelectric distortion and is neither sufficient nor necessary to understand the large piezoelectric response of PZN–PT.