C. C. Sorrell

Photo-electrochemical hydrogen generation from water using solar energy. Materials-related aspects

Abstract The present work considers hydrogen generation from water using solar energy. The work is focused on the materials-related issues in the development of high-efficiency photo-electrochemical cells (PECs). The property requirements for photo-electrodes, in terms of semiconducting and electrochemical properties and their impact on the performance of PECs, are outlined. Different types of PECs are overviewed and the impact of the PEC structure and materials selection on the conversion efficiency of solar energy are considered. Trends in research in the development of high-efficiency PECs are discussed. It is argued that very sophisticated materials engineering must be used for processing the materials that will satisfy the specific requirements for photo-electrodes. An important issue in the processing of these materials is the bulk vs. interface properties at the solid/solid interfaces (e.g., grain boundaries) and solid/liquid interfaces (e.g., electrode/electrolyte interface). Consequently, the development of PECs with the efficiency required for commercialization requires the application of up-to-date materials processing technology. The performance of PECs is considered in terms of: • excitation of electron–hole pair in photo-electrodes; • charge separation in photo-electrodes; • electrode processes and related charge transfer within PECs; • generation of the PEC voltage required for water decomposition. This work also gives empirical data on the performance of PECs of different structures and materials selection. It is argued that PEC technology is the most promising technology for hydrogen production owing to several reasons: • PEC technology is based on solar energy, which is a perpetual source of energy, and water, which is a renewable resource; • PEC technology is environmentally safe, with no undesirable byproducts; • PEC technology may be used on both large and small scales; • PEC technology is relatively uncomplicated. According to current predictions, the production of hydrogen will skyrocket by 2010 (Morgan and Sissine, Congressional Research Service, Report for Congress. The Committee for the National Institute for the Environment, Washington, DC, 20006-1401, 28 April 1995). Consequently, seed funding already has been allocated to several national research programs aiming at the development of hydrogen technology. The countries having access to this PEC technology are likely to form the OPEC of the future.

Sintering effects on the strength of hydroxyapatite.

Mechanisms underlying temperature-strength interrelations for dense (> 95% dense, pores closed) hydroxyapatite (HAp) were investigated by comparative assessment of temperature effects on tensile strength, Weibull modulus, apparent density, decomposition (HAp:tricalcium phosphate ratio), dehydroxylation and microstructure. Significant dehydroxylation occurred above approximately 800 degrees C. Strength peaked at approximately 80 MPa just before the attainment of closed porosity (approximately 95% dense). For higher temperatures (closed porosity), the strength dropped sharply to approximately 60 MPa due to the closure of dehydroxylation pathways, and then stabilized at approximately 60 MPa. At very high temperatures (> 1350 degrees C), the strength dropped catastrophically to approximately 10 MPa corresponding to the decomposition of HAp to tricalcium phosphate and the associated sudden release of the remaining bonded water.

Electrophoretic deposition of hydroxyapatite coatings on metal substrates : A nanoparticulate dual-coating approach

Hydroxyapatite coatings can be readily deposited on metal substrates by electrophoretic deposition. However, subsequent sintering is highly problematic owing to the fact that temperatures in excess of 1100°C are required for commercial hydroxyapatite powders to achieve high density. Such temperatures damage the metal and induce metal-catalysed decomposition of the hydroxyapatite. Furthermore, the firing shrinkage of the hydroxyapatite coating on a constraining metal substrate leads to severe cracking. The present study has overcome these problems using a novel approach: the use of aged nanoparticulate hydroxyapatite sols (lower sintering temperature) and a dual coating strategy that overcomes the cracking problem. Dual layers of uncalcined hydroxyapatite (HAp) powder were electrophoretically coated on Ti, Ti6Al4V and 316L stainless steel metal substrates, sintered at 875–1000°C, and characterised by SEM and XRD, and interfacial shear strength measurement. Dual coatings on stainless steel had an average high bond strength (about 23 MPa), and dual coatings on titanium and titanium alloy had moderate strengths (about 14 and 11 MPa, respectively), in comparison with the measured shear strength of bone (35 MPa). SEM and XRD demonstrated that the second layer blended seamlessly with the first and filled the cracks in the first. The superior result on stainless steel is attributed to a more appropriate thermal expansion match with hydroxyapatite, the thinner oxide layer, or a combination of these factors.

Interfacial bond strength of electrophoretically deposited hydroxyapatite coatings on metals.

Hydroxyapatite (HAp) coatings were deposited onto substrates of metal biomaterials (Ti, Ti6Al4V, and 316L stainless steel) by electrophoretic deposition (EPD). Only ultra-high surface area HAp powder, prepared by the metathesis method 10Ca(NO3)2 + 6(NH4)2HPO4 + 8NH4OH), could produce dense coatings when sintered at 875–1000°C. Single EPD coatings cracked during sintering owing to the 15–18% sintering shrinkage, but the HAp did not decompose. The use of dual coatings (coat, sinter, coat, sinter) resolved the cracking problem. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) inspection revealed that the second coating filled in the “valleys” in the cracks of the first coating. The interfacial shear strength of the dual coatings was found, by ASTM F1044-87, to be ∼12 MPa on a titanium substrate and ∼22 MPa on 316L stainless steel, comparing quite favorably with the 34 MPa benchmark (the shear strength of bovine cortical bone was found to be 34 MPa). Stainless steel gave the better result since α-316L (20.5 μm mK-1) > α-HAp (∼14 μm mK-1), resulting in residual compressive stresses in the coating, whereas α-titanium (∼10.3 μm mK-1) < α-HAp, resulting in residual tensile stresses in the coating.

SOME IMPORTANT FACTORS IN THE WET PRECIPITATION PROCESS OF HYDROXYAPATITE

The precipitation process considered here, involved the addition of orthophosphoric acid solution to a calcium hydroxide solution. Several parameters such as the effects of atmosphere controlling, pH, adding extra acid solution, the amount of stirring speed, and post-chemical treatment were studied. In order to study the thermal stability of synthesised powders, they were sintered for 2 h at 1200 °C and the as-precipitated and sintered powder samples were examined using SEM, XRD, FTIR and ICP methods. The results clearly revealed that the final precipitated powder is hydroxyapatite (HA) with good purity, stoichiometry and successful thermal stability. SEM high magnification micrographs showed that the precipitated HA consists of small rod-like particles.

The wet precipitation process of hydroxyapatite

Stoichiometric hydroxyapatite (HA) powder was synthesized by wet method using orthophosphoric acid and calcium hydroxide, as raw materials. In this regard, the time-depending changes of the precipitated particles and effects of heating on them were investigated. The as-precipitated and heated powder samples were examined using field emission electron microscopy (FESEM), XRD and Raman spectroscopy methods. The results revealed that the morphology and size of the precipitated particles change in each stage of the process. After drying, these nanoparticles tend to form small agglomerates.

Photo-electrochemical properties of the TiO2-Pt system in aqueous solutions

Abstract Electrochemical characteristics of photo-electrochemical cell (involving photo-anode made of TiO2 and Pt as cathode) were investigated during the light-on and the light-off regimes using the sunlight. Maximum photo-voltage of the open cell involving undoped TiO2 as photo-anode is 0.7 and 1.6 V without and with chemical bias (Δ pH=14.6), respectively. Application of ceramic Cr-doped TiO2 (involving 1 wt % of Cr2O3) as photo-anode results in a substantial decrease of the cell voltage (to 0.8 V ) despite the application of chemical bias.

Defect chemistry and semiconducting properties of titanium dioxide: I. Intrinsic electronic equilibrium☆

Abstract The present work describes defect chemistry and semiconducting properties of TiO 2 within the n–p transition regime. Quantitative considerations on the relationships between the concentration of ionic and electronic defects at the minimum of electrical conductivity vs. oxygen partial pressure resulted in the derivation of a theoretical model. The model is based on the empirical data of electrical conductivity for Cr-doped TiO 2 (exhibiting n–p transition). This model was then applied for the determination of the intrinsic electronic equilibrium constant which is the following function of temperature: (1) K i =3.74×10 −2 exp − 3.039±0.053 eV kT A good agreement between this equilibrium constant and the experimental data of electrical conductivity for TiO 2 doped with donors (Cr) was revealed. It was observed that the concentration of oxygen vacancies determined using the derived model is only slightly dependent on the value of the electronic intrinsic equilibrium constant.

High-Temperature Superconducting Linear Synchronous Motors Integrated With HTS Magnetic Levitation Components

High-temperature superconductors (HTSs) including HTS bulks and tapes have potential applications in linear motion drive and magnetic levitation/suspension systems generating substantial advantages over conventional ones. When an HTS linear motor is integrated with an HTS magnetic suspension subsystem, it can inherit both merits of HTS linear motion drive and HTS magnetic suspension simultaneously and can be applied into various fields, such as the maglev and electromagnetic aircraft launch systems (EMALSs). Based on different HTS aspects and arrangements, three modes of HTS linear synchronous motor (HTSLSM) integrated with HTS magnetic suspension subsystems have been proposed in this paper. To verify the modes for the design of a practical HTSLSM, the structural features of these systems are described, the magnetization characteristics to obtain HTS bulk magnets, the trapped-field attenuation characteristics of the HTS bulk magnet exposed to external traveling field, and the magnetic field distribution characteristics for different permanent-magnet guideways have been studied with experimental verification. Based on the study, a demonstration prototype of a single-sided HTSLSM integrated with HTS magnetic suspension subsystems has been developed. Its performance and thrust characteristics have been obtained by experimental measurements and compared with theoretical results. With regard to practical applications, two modes of double-sided HTSLSM integrated with HTS magnetic suspension subsystems have been designed for the maglev and EMALS, respectively, and then, the 2-D finite-element-analysis models for the HTSLSMs were built to analyze their performance characteristics. The comprehensive simulations and experimental results constitute a framework for the structural and electromagnetic design of the HTSLSM integrated with HTS magnetic suspension for practical applications.

Defect chemistry and semiconducting properties of titanium dioxide: III. Mobility of electronic charge carriers☆ ☆

Abstract The present work performs quantitative analysis of the electrical conductivity data reported in the literature in terms of defect chemistry models. The analysis results in the determination of the mobility terms for electrons and electron holes leading to the following respective forms: μ n =0.106 ( cm 2 V −1 s −1 ) μ p =(1.05±0.89) 10 6 T exp − 0.853±0.073( eV ) kT The mobility data determined in this work were then used for verification of defect chemistry disorder models and a good agreement was revealed.