Advances in Applied Ceramics: Structural,Functional and Bioceramics

CSIRO has demonstrated a concept for utilising two of Australia's largest energy resources, solar energy and natural gas. Solar energy is used to reform natural gas to produce solar syngas (H₂/CO), then shifted to hydrogen. The resulting hydrogen contains 20% embodied solar energy. The concept allows the choice to use either the syngas directly, or hydrogen. Much of Australia's gas reserves exist in the high solar regions, and gas pipeline infrastructure is in place to transport the solar gas to the cities. Alternatively the solar gas can be used on site in large gas turbines, for distributed generation in gas engines in remote regions, or as a chemical feedstock. The most significant market penetration at present would be for syngas, however the project has demonstrated it is possible to produce hydrogen of a quality sufficient for a hydrogen fuel cell. An improved version is now under construction at Australia's National Solar Energy Centre.

The hydrogen fuel cell smart house (HFCSH) will incorporate many new materials and concepts demonstrating a high level of maturity in commercially available products associated with renewable energy systems. The aim is to put into practice, renewable energy technologies in a mode that will create support for an important change in future energy usage. The HFCSH cluster concept places emphasis on hydrogen and key fuel cell technologies. However, renewable technologies such as photovoltaics, solar heating and water treatment will also play a significant role in the HFCSH. Of critical importance are the software packages, which will control and display energy distribution, as well as demonstrating safety systems and efficiencies.

The main driving forces of the hydrogen economy are considered and its impact on the environment is discussed. The present work is focused on hydrogen generation from water using solar energy through photoelectrochemical energy conversion. Progress in this area, which is determined by the development of photosensitive materials and devices, is overviewed. The effect of materials selection and cell structures on the performance of photoelectrochemical cells is discussed. It is argued that TiO₂ and TiO₂ based oxide systems exhibit the most promising functional properties and, therefore, are the top candidates for photoelectrodes. The major challenges in the development of commercial photoelectrodes and photoelectrochemical energy conversion devices are discussed.

Templating techniques have been used to control the structure and hence properties of titanium dioxide materials, leading to enhanced performance in photocatalytic applications. In the present paper, titanium dioxide based structures synthesised by the author's group using preformed templates are reviewed. Variation in the morphology of the organic templates, including monolithic pellets, films, spherical particles and fibres, is discussed. The incorporation of the inorganic material within the porous structure was achieved by conducting either sol–gel chemistry in the pores of the template or infiltrating preformed nanoparticles through the template pores. In addition, the ability to alter the composition of the final inorganic material by mixing different precursor solutions, mixing solutions of preformed nanoparticles of two different metal oxides before conducting the templating or the controlled application of layers of one oxide after another onto the template is discussed. These techniques allow the fabrication of intricate titanium dioxide based porous structures with tailored properties.

More than three decades of research have not solved the problem of storing hydrogen on board for transport fuel. Hydrogen storage therefore constitutes a major technological barrier to the hydrogen economy. The present paper considers why the problem is so tough and assesses the prospects for the two most popular storage concepts, pressurised gas storage systems and condensed materials, exemplified by lithium based complexes. It is concluded that efforts to achieve ‘condensed matter’ hydrogen storage are showing promise and are still vitally important because pressurised gas storage for automobiles is very unlikely to meet the targets for energy density set by the US Department of Energy.

This paper presents an analysis of membrane reactor operation and design for enhanced hydrogen production. Silica derived membranes were used for gas permeation studies and a membrane reactor for the water gas shift reaction. A model of the equilibrium reaction is developed and analysed with respect to operational factors such as temperature and pressure analysed in consideration for production of a 99% pure H₂ stream. These factors influence the optimisation of the reaction and permeation rate as well as the equilibrium conversion. It was found that using H₂ permeation membranes, the H₂ equilibrium could be shifted towards the products. In turn, this provided better conversion at higher temperatures. The cost of H₂ production using membrane reactors is dependent upon several engineering process parameters such as reaction rates, permeation, selectivities, temperature and pressure. Silica membranes assembled in membrane reactors out performed conventional reactor systems. Silica membranes were synthesised showing permeations of 5 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ and H₂/CO selectivities >10. The silica membrane capital cost per kg H₂ produced ranged from US$0·25 to 3&middot00 for 10 to 80%H₂ separation respectively.

Photochemistry is the basis of the life circle on earth and whereby we derive our energy from the sun. The dye sensitised solar cell attempts to mimic these systems utilising visible solar radiation in a cyclic manner to generate electricity. During this process, species are generated which possess highly oxidative properties which can be used to perform chemical reactions similar to oxidative sterilisation. The byproduct of such a process produces hydrogen and is shown to allow the visible light degradation of organic pollutants and the coupled production of hydrogen gas. A further extension is the use of the output as a sensor for both the concentration and species of organic pollutants present in a waste stream.

CSIRO has been involved in the research and development of fuel cell technology for a number of years. Initially the effort focused on solid oxide fuel cell technology, which led to a commercial activity, and now the polymer electrolyte membrane fuel cells including micro fuel cells for portable power applications are the focus of research and development. Other hydrogen and related technologies under development include solid state water electrolysis system for hydrogen generation and integration with sustainable energy sources, and hydrogen separation from byproducts of coal gasification or fossil fuel reformates. In this paper, a brief overview of these technology areas and progress made in CSIRO will be discussed.

The present paper presents a short overview of the TiO₂ coating technologies that have been applied for a variety of applications, including the formation of photocatalysts for the decomposition of water and the fabrication of antipollution building materials through functional coatings. Several coating processing procedures are outlined, including sol–gel technique, screen printing, roll on method and spraying procedure.

This work reports the reactivity of oxygen and water with the surface of TiO₂ from the perspective of surface science. The reactivity is considered in terms of surface properties studied using surface sensitive tools and their effect on adsorption of oxygen and water, and the related charge transfer. There is a general agreement that defected surfaces exhibit substantially enhanced reactivity over perfect surfaces. In other words, surface defects, such as oxygen vacancies and Ti interstitials are the local surface sites, which are responsible for reactivity with water and oxygen. However, the reported reactivity mechanisms on specific surface sites and the related charge transfer are conflicting. The proposed reactivity models are also not consistent with the present state of understanding of TiO₂ reactivity with water and the related charge transfer. The apparent discrepancy is due to the difference between the specific environment applied in surface studies, which often requires the imposition of high vacuum, and that related to their performance in electrochemical cells.

The present paper reports experimental data on the effect of processing of undoped polycrystalline TiO₂ on the microstructure. The processing included the precipitation of TiO₂ powder, obtained by adding water to Ti isopropoxide (99·999%)–ethanol mixture, drying, pressing and sintering. The applied procedures of pressing included both cold isostatic pressing and high temperature isostatic pressing. The obtained SEM micrographs are considered in terms of the effect of the applied processing procedure on microstructure and defect chemistry. The processing including sintering at 1688 K and subsequent high temperature isostatic pressing at low oxygen activity resulted in maximum density.

The present paper reports semiconducting properties and the related defect disorder for undoped TiO₂ single crystal using measurements of the electrical conductivity. Isothermal changes of the electrical conductivity as a function of oxygen activity were determined within oxygen activities ranging between 10 Pa and 75 kPa and temperatures between 1073 and 1323 K. The electrical conductivity data, involving both n and p type regimes, are considered in terms of defect disorder assuming that the predominant mobile defects are oxygen vacancies, which are compensated by immobile titanium vacancies. Application of this model led to the determination of the electrical conductivity components related to electrons, electron holes and ions. The determined empirical relationships between electrical conductivity and oxygen activity may be used for predicting the effect of experimental conditions on semiconducting properties of TiO₂. The activation energy for the standard conductivity component of electrons and holes is 226 and 102 kJ mol⁻¹ respectively.

The present paper reports the electrical properties of undoped TiO₂ single crystal in terms of the conductivity components for electrons, holes and ions and the related transference numbers within the n–p transition regime. These data indicate that ionic conductivity in TiO₂ may assume a substantial value, which cannot be ignored. The minimum of the electrical conductivity at the n–p transition and its electronic component, were used for the determination of the band gap, which is 3·13 and 3·1 eV respectively.

The present work considers the effect of oxygen activity on the chemical diffusion coefficient in terms of defect disorder and the related semiconducting properties. The transport kinetics under a chemical potential gradient in amphoteric oxide semiconductors exhibits a complex dependence within the n–p transition. It is shown that the effect may be related to the concentration of electronic charge carriers. The derived analytical form explains the empirical relationship between the chemical diffusion coefficient and oxygen activity reported for TiO₂.

The present paper reports the effect of oxygen activity p(O₂) on Nb segregation in a polycrystalline specimen of Nb doped TiO₂ (2·91 at.-%). The determined segregation induced enrichment factor following annealing at 1773 K in p(O₂) = 10 Pa and p(O₂) = 21 kPa is 3·1 and 7·3 respectively. The data on the effect of p(O₂) on the segregation induced Nb enrichment factor are considered in terms of defect disorder of Nb doped TiO₂ and the related lattice charge neutrality requirements. The reported segregation data may be used for controlled modification of surface concentration gradients in TiO₂ through the imposition of well defined gas phase composition during processing procedures.

This paper provides an outline for the use of secondary ion mass spectrometry (SIMS) in the determination of diffusion data in metal oxides. The focus is on the determination of Nb bulk and grain boundary diffusion coefficients in TiO₂ and zirconia. Specifically, the diffusion of Nb in TiO₂ and yttria doped (10 mol.-%) ZrO₂ (10YSZ) has been assessed. The following bulk diffusion coefficients D_93Nb were obtained

D_93Nb =(1·03±0·051) × 10⁻¹⁸ m² s⁻¹ 10YSZ(1273K)

D_93Nb =(1·91±0·096) × 10⁻¹⁶ m² s⁻¹ TiO₂(1273K)

The grain boundary diffusion parameter for Nb grain boundary diffusion in 10YSZ was also determined

D_93Nbδα =(7·48 ± 0·37) × 10⁻²⁵ m² s⁻¹ 10YSZ(1273K)

The Nb grain boundary diffusion coefficient D′_93Nb was determined to be

D′_93Nb =(3·99 ± 0·20) × 10⁻¹⁶ m² s⁻¹ 10YSZ(1273K)

First principles pseudopotential density functional calculations have been performed to investigate the reactivity of O₂ on ideal and defected surface of rutile TiO₂ (110). All ionic positions are allowed to relax on slab geometry with periodic boundary conditions. The results reveal that both the molecular and dissociated adsorption of O₂ is most favourable on a defective surface in the presence of titanium vacancy and a substantial change in surface relaxation was observed. This remarkable surface ionic displacement and titanium vacancy induced acceptor-like environment favoured in stable adsorption of oxygen for both molecular and dissociated states. The least favourable adsorption or no adsorption of O₂ is observed on an ideal surface.

The effect of oxygen activity on electrical conductivity and thermoelectric power for CaTiO₃ is determined within the n–p transition. The oxygen activity related to the n–ip transition point, which is determined from the minimum of the electrical conductivity and the zero value of thermoelectric power, exhibits a discrepancy. The discrepancy is considered in terms of the different mobilities of electrons and electron holes. The mobility ratio determined in the present work in the temperature range 1173–1323 K varies between 1·1 and 2·7.

The present work provides a brief but wide ranging survey of the different methods that may be used to apply thick films of titanium dioxide for self-cleaning building materials. This survey is preceded by a general description of the photocatalytic applications of TiO₂, a tabulated list of some relevant properties of the three naturally occurring polymorphs of TiO₂ and a tabulated survey of world market shares and resources of TiO₂. The survey of coating technologies consists of succinct coverage of twenty potential periodic and continuous routes for the application of liquid borne suspensions of TiO₂ to building units. This coverage consists mainly of an illustration accompanied by relevant comments concerning the advantages and disadvantages of each method.