Professor Erich Kisi

The tungsten bronzes (MxWO3) are non-stoichiometric metal oxides which have attracted interest for their potential applications in plasmonics. Although the cubic structures of the sodium tungsten bronzes (NaxWO3) have been extensively studied, reports into the tetragonal-II structures (TII-) of any tungsten bronze are comparatively rare. In this work, TII-NaxWO3 and TII-KxWO3 were prepared by a furnace-assisted method, and characterised by X-ray and neutron powder diffraction, selected-area electron diffraction and electron energy-loss spectroscopy (EELS). A structural determination of TII-NaxWO3 was performed in the space group I4/m, in a 2×2×2 supercell of the P4/mbm TII-KxWO3 structure. Density functional theory is then used to calculate the valence electronic structure and optical properties to support the EELS measurements. Similar to the cubic structures, the conduction band of TII- is composed of hybridised O 2p and W 5d states, which is filled by electrons donated from the inserted Na or K. The O sites which are in-plane with the W sites are found to have highly localised bands, resulting in low-energy interband transitions in the a/b direction of the unit cell. In contrast, purely free-electron behaviour is observed in the c-direction below 2 eV. High-quality plasmon resonances are thus only observed in the c-direction, with energy and quality similar to those of the cubic structures. These anisotropic optical properties make TII-NaxWO3 and TII-KxWO3 interesting materials for further study as potential plasmonic materials.

Rare-earth hexaborides, especially lanthanum hexaboride (LaB6) are popular materials as thermionic cathodes. It is a great challenge to synthesise these hexaborides at a low temperature without any post-synthesis purification treatments. In this research work, we demonstrate a simple synthesis technique, namely borothermal method for preparing pure lanthanum hexaboride by reacting lanthanum oxide (La2O3) with boron (B). In this investigation, different pressed pellets were made using the starting powders mixed either by milling mildly for 30 min or high-energy milling (HEM) for 8/16 h. Later, the prepared pellets were put into a low-vacuum furnace at different temperatures. The synthesised products were characterised using X-ray diffraction (XRD), scanning electron microscope (SEM), electron dispersive spectroscopy (EDS) and a Schottky device for their structural, chemical, morphological and thermionic properties. Both synthesis methods are found to produce LaB6 with a purity above 99% at a relatively low temperature of 1250 °C. High-energy ball milling does not result in reducing synthesis temperatures, rather introduces some iron impurities. From XRD and SEM investigations, the average crystallite size of pure LaB6 prepared by borothermal methods is observed to be in the range of 225¿250 nm. From thermionic emission data, partially sintered LaB6 pellets prepared from powders via the simple borothermal and the borothermal method with HEM exhibit work functions of 2.86 ± 0.05 eV and 2.64 ± 0.08 eV, respectively. The results suggest that nanostructured LaB6 with a low work function can be easily prepared at lower temperatures for applications in thermionic energy converters, high-power electron devices and photonics.

Results from an experiment involving the measurement of individual particle stresses in a two-dimensional mono-disperse assembly of 579 ball bearings are presented. Using a combination of neutron radiography and strain scanning techniques, the full bi-axial stress state was obtained for each particle from which the full contact force network could be established. The evolution of this network was examined over a series of five monotonically increasing loads. Significant levels of inhomogeneity were observed in the form of prominent force chains that showed complex interaction with regions of order and disorder within the assembly. A reduction in the level of inhomogeneity with increasing load was also observed. Graphical Abstract: [Figure not available: see fulltext.].

New miscibility gap alloys with a ceramic matrix have been explored in the ZrO2¿Al, AlN¿Al, AlN-(Al¿Si), Al2O3¿Al and MgO¿Al systems with a view to creating oxidation-resistant macroscopically solid, phase change-enhanced, thermal energy storage materials. Materials were manufactured by mixing the components, pressing and firing at 700¿°C under argon. Oxidation tests at 700¿°C in air for up to 72¿h showed a mix of responses. Despite favourable thermodynamic properties of some published Ellingham diagrams, samples in the ZrO2¿Al system underwent a strong exchange reaction, which did not result in a useful material. In the AlN¿Al, AlN¿(Al¿Si) systems, viable MGA-type materials with encapsulated Al particles providing latent heat storage within a solid matrix were formed. There was no degradation of the materials after 24¿h of oxidation testing; however, severe oxidation of the Al or Al¿Si was observed after 72¿h. The Al2O3¿Al and MgO¿Al systems not only formed a viable ceramic matrix material after firing, but also showed no signs of Al oxidation after 72¿h in air at 700¿°C. These results open up a promising new series of thermal energy storage materials, some of which appear to have very good oxidation resistance under the test conditions.

The Mn+1AXn phases Cr2GeC and Cr2GaC were synthesized with high phase-purity directly from Cr2O3; Ge or Ga; and C using a single-step carbothermal reduction. X-ray diffraction and scanning electron microscope analysis of the materials shows them to be >99 mol% Cr2GeC and 92 mol% Cr2GaC, respectively. Extension to non-Cr systems is briefly demonstrated by applying the method to the synthesis of V2GeC.

Neutron diffraction techniques are used to measure a series of bulk stress distributions in granular materials undergoing high stress die compaction. Two different granular materials are studied: a ductile material in the form of iron powder and a brittle material in the form of quartz sand. The behaviour of these two materials is examined in terms of the resulting stress distribution within two dies of different geometry: an axisymmetric converging die and a stepped cylindrical die. These measured distributions are examined and discussed to highlight both similarities and differences in the mechanical response of these contrasting materials at high consolidating loads. The potential for neutron diffraction techniques to provide full-field information on the mechanical response and stress within granular samples is demonstrated.

The potential of novel tungsten-barium titanate composites as thermionic emitters is explored. Composites ranging from 20% to 80% tungsten by mass were prepared by sintering in an Ar-H2 atmosphere. XRD and SEM studies indicate four major micro-constituents; W, BaTiO3, Ba4(Ti,Fe)12O27 and BaWO4. Richardson work functions (fR) and Richardson constants (AR) were determined using a Schottky diode arrangement at temperatures ranging from 1223 to 1473 K. Work functions ranged from 2.67 eV to 3.32 eV with a shallow minimum at 40% by mass W and were relatively constant (~2.7-2.8 eV) in the range 30-70% by mass W. The decrease in work function was accompanied by a strong decrease in AR from 39.3 A cm-2 K-2 to 0.02 A cm-2 K-2 over the range 20-70% by mass W. The reduction in both fR and AR was associated with the major conversion of the surface to BaWO4 and Ba4Ti12O27 during the activation treatment before emission testing.

The color of Cu-Zn brasses range from the red of copper through bright yellow to grey-silver as the Zn content increases. Here we examine the mechanism by which these color changes occur. The optical properties of this set of alloys has been calculated using density functional theory (DFT) and compared to experimental spectroscopy measurements. The optical response of the low Zn content a-brasses is shown to have a distinctly different origin to that in the higher content ß', ¿ and e-brasses. The response of ß'-brass is unique in that it is strongly influenced by an overdamped plasmon excitation and this alloy will also have a strong surface plasmon response.

The effective thermal conductivity of Cu-Fe and Sn-Al miscibility gap alloys over a range of temperatures and volume fractions was determined using the Lattice Monte Carlo method. The Cu-Fe system was found to have an effective conductivity predictable by the Maxwell-Eucken model. The Sn-Al system was not consistent with any empirical model analysed. The microstructures of physical samples were approximated using a random growth algorithm calibrated to electron or optical microscope images. Charts of effective conductivity against temperature for a number of volume fractions are presented for the two alloys. It was determined that the Cu-Fe alloy would benefit from an interstice type microstructure and the Sn-Al would be more efficient with a hard spheres type microstructure. More general conclusions are drawn about the efficiency of the two observed microstructures. © 2014 Elsevier Ltd. All rights reserved.

The unique and unusual state of matter represented by granular materials has historically made it very difficult to develop models of stress distributions and was previously not able to be explored experimentally in the required detail. This paper reports the application of the neutron diffraction strain scanning method, originally developed for residual stress measurements within engineering components, to the problem of the stress distribution in granular Fe under a consolidating pressure. Strains were measured in axial, radial, circumferential and an oblique direction using the neutron strain scanning diffractometer KOWARI at ANSTO (Sydney). The full stress tensor as a function of position was able to be extracted for both straight walled, converging and stepped dies. © (2014) Trans Tech Publications, Switzerland.

The full triaxial stress state within individual particles in a monodisperse spherical granular assembly has been measured. This was made possible by neutron imaging and computed tomography combined with neutron diffraction strain measurement techniques and associated stress reconstruction. The assembly in question consists of 549 precision steel ball bearings under an applied axial load of 85 MPa in a cylindrical die. Clear evidence of force chains was observed in terms of both the shape of the probability distribution function for normal stresses and the network formed by highly loaded particles. An extensive analysis of the source and magnitude of uncertainty in these measurements is also presented.

Model validation remains a serious problem within the field of computational granular materials research. In all cases the rigor of the validation process is entirely dependent on the quality and depth of the experimental data that forms the point of comparison. Neutron and X-ray diffraction methods offer the only quantitative non-contact method for determining the spatially resolved triaxial stress field within granular materials under load. Measurements such as this can provide an unprecedented level of detail that will be invaluable in validating many models. In this paper the theoretical foundation underpinning diffraction-based strain measurements, their conversion to local stress in the particles and ultimately into the bulk stress field is developed. Effects such as elastic anisotropy within the particles of the granular material, particle plasticity and locally inhomogeneous stress distribution are shown to not offer any obstacles to the method and a detailed treatment of the calculation of the bulk stresses from the particle stresses is given. © 2013 Springer-Verlag Berlin Heidelberg.

A persistent problem in the synthesis of complex materials from three or more constituents is the retention of very stable intermediate phases with properties grossly different from the target phase. In our recently developed two-stage synthesis methodology, a binary precursor is prepared and then directly converted into the product phase, thereby avoiding intermediate phase formation. Crystallographic similarities between the precursor and product phases are manipulated such that the major structural elements remain chemically intact. Ordered structural vacancies are used to facilitate rapid and highly selective diffusion of the third element into the precursor. Since no crystallographic reconstruction is required, no extraneous phases can form. The method has been tested on the solid state synthesis of Ti3AlC 2, Ti3SiC2 and Ti3AlC2, Ti3SiC2, producing synthesis temperature reductions of 400 - 600°C. It is postulated that similar crystal structural relationships in layered oxides, nitrides and intermetallic compounds will allow an extension to many other systems. © Institute of Materials Engineering Australasia Ltd.

Thermionic energy conversion is an ideal way of converting concentrated solar energy into electricity. Developing suitable emitter/collector materials, with optimum work functions whilst being able to withstand elevated temperatures under high vacuum conditions, is a major challenge. Rare-earth hexaborides have great potential as thermionic emitters due to their superior chemical stability and low work function values, lanthanum hexaboride (f ~2.60eV) and cerium hexaboride (f ~2.60eV). This study forms part of a series of investigations aimed at developing rare-earth hexaboride cathode materials. Methods used to synthesise, characterise and fabricate these materials are discussed together with electron emission results. © 2014 Owned by the authors, published by EDP Sciences.

The current paper addresses the thermal characterisation of Miscibility Gap Alloys (MGAs). These novel materials combine two immiscible metallic phases with different melting temperatures. The fusible phase (i.e. the phase with a lower melting temperature) acts as a phase change material that stores latent heat (in addition to sensible heat) thus optimising energy storage capacity. The second phase forms an enclosure and prevents the leakage of liquid material. Due to the high inherent thermal conductivity of metals, MGAs exhibit excellent thermal conduction in comparison to traditional phase change materials such as hydrate salts or paraffin. The combination of high energy storage and fast heat transfer makes MGA uniquely suited for use as thermal capacitors in applications like space heating, concentrated power generation or temperature stabilisation of sensitive equipment. The current paper determines the thermal properties of MGAs using Lattice Monte Carlo analysis combined with micro-computed tomography imaging.