R.St.C. Smart

Initial dissolution Kinetics of Ionic Oxides

It is shown th at factors previously recognized, but not regarded as critical, can dominate dissolution kinetics of ionic oxides. The use of the nearly perfect {100} MgO surfaces of smoke cubes to obtain very precise values of dissolution rates per unit surface area, in dilute HC1, HC1O4 and HNO3, has shown th at rates extrapolated to zero dissolution are almost independent of pH in the range 2.0- 3.5. Dissolution rates were measured by monitoring solution pH as a function of time. This revealed increasing rates with increasing pH up to about 5 % total dissolution, followed ultimately by a return to the linear relation between Ig(rate) and pH (slope ca. — 0.5) normally expected. The initial increase in rate is due to increasing [Mg2+] in solution and is observed with [Mg2+] as low as 1 % of the [H+]. A linear relation between lg(rate) and [Mg2+] is found during the early stages of dissolution. Other cations (Al3+, Na+) also increase the initial rate, to a similar extent. Electron-microscope observations of the cubes show alteration of the surfaces to a castellated structure (of {100}-based projections and intrusions) on wetting before dissolution, and the development of facets having an average (110}-natureduring dissolution. The results are in conflict with current theoretical models, and a qualitative account of the mechanism of the establishment of a ‘ stable ’ solution double layer is given.

Growth twinning in magnesium oxide smoke crystals

Abstract Growth twins in magnesium oxide are shown to occur when magnesium is burnt in air in the presence of water vapour. The twins are coherent and the external morphology shows mirror symmetry across the {111} composition plane. The significance of the existence and form of these twins in such a highly ionic material is considered in terms of the structure of the boundary layer and the role of the water vapour.

The surface layer formed on zinc-containing glass during aqueous attack

Abstract The glass-water interaction is reviewed via an outline of the processes of bulk dissolution, selective leaching and surface enhancement. Special attention is paid to the nature and effects of surface films. Glasses of three compositions, each containing about one-fifth by weight Zn, are subjected to aqueous attack. Solution analysis confirms the roles of Na, Si and Zn in leaching, dissolution and surface enhancement, respectively. Proton-induced X-ray emission corroborates Zn enhancement in the glass surface and X-ray photoelectron spectroscopy fixes the Zn build-up to the first few nanometres. Powder X-ray diffraction examination identifies the specific structures of hemimorphite and Zn orthosilicate while transmission electron microscopy finds such phases to be distributed in a largely amorphous matrix. Infrared spectra are consistent with the presence of a modified silicate structure and additionally indicate that water is loosely bound in the reacted surface layer. Attack of a non-Zn glass by a Zn-containing solution shows the ion in solution produces a Zn surface layer while attack of a Zn glass by a Zn complexing solution shows a Zn layer forms preferentially to complexation. Hence Zn, in either the solid or solution, induces a thin, protective, solvated and partially crystalline silicate zone on a glass under aqueous attack.

X-ray photoelectron spectroscopy studies of valence states of cerium and uranium in Synroc C

Abstract Small quantities of lanthanides and actinides have been added to the SYNROC precursor slurry as part-simulation of high level nuclear waste (HLW) species. There are two main parameters which influence the crystallographic destination of the HLW species — the cation size and its valence state. Lanthanides and actinides are predicted to incorporate as a trivalent species into the zirconolite phase and/or as a trivalent or tetravalent species into perovskite. X-ray photoelectron spectroscopy (XPS) was used to investigate the valence states of cerium and uranium in SYNROC C. The analysis of the characteristic satellite structures, binding energies and peak shapes in the XPS spectra showed that cerium and uranium in SYNROC C adopt trivalent and tetravalent states respectively, in accord with crystallochemical prediction.

Correspondence: Size distribution of MgO smoke particles

Abstract It has been shown by a combination of scanning and transmission electron microscopy that the size distribution of MgO particles prepared by burning magnesium in air is extremely asymmetric. Less than 1% of the particles account for well over half the mass. This result explains an anomaly observed in the infra‐red spectroscopy of MgO smoke. The size distribution can be used in conjunction with sorptometer measurements of specific surface area to give a measure of surface roughness.

Dissolution Mechanisms of Oxides and Titanate Ceramics — Electron Microscope and Surface Analytical Studies

The mechanisms which control the dissolution of simple oxides in aqueous solution are not fully understood, despite substantial research; those for ceramics are even less certain, and may involve “leaching” (preferential release of some species from the solid into solution) rather than “dissolution” (all species released at equal rates). In this paper we consider the reactivity in aqueous solutions of two materials — MgO and CaTiO3 — to illustrate current attempts to establish the detailed mechanisms and rate-determining steps. The importance of using surface analytical and electron microscopy techniques to characterize aspects of the surfaces before and after attack is emphasized. In the case of MgO, there is an initial stage of dissolution related to the restructuring of the initial oxide surface; the dissolution process is controlled by the second protonation reaction. For the perovskite CaTiO3, ion—exchange of the Ca2+ from surface sites is accompanied by base—catalyzed hydrolysis of the titanate lattice: there is no significant leaching of the Ca2+ from the intact solid unless hydroxylation has occurred. However, crystal defects due to deformation and microstructural characteristics of the material can have major effects on the reactivity of polycrystalline perovskite.

XPS determination of band bending in defective semiconducting oxide surfaces

X-ray photoelectron spectroscopic studies of polycrystalline, p-type semiconducting nickel oxides and manganese oxides, with different defect levels, have revealed dependence of the surface potential, Vch, on X-ray power, bulk oxide defect concentrations, electron donor and acceptor adsorption and temperature. Mechanisms for the development of surface potential during X-irradiation are described and their dependence on these factors is examined. At low X-ray power (below 200 W) Vch is shown to be linearly dependent on X-ray tube current. At high X-ray power, Vch is given by: Vch = [(K1−K3)B(σ0)]exp(eVskT) where K1 and K3 are constants; B(σ0) represents a functional dependence on bulk conductivity of the sample and Vs is the band bending at the surface. From linear experimental plots of In Vch versus 1T, values of Vs between 0.24 and 0.63 V were determined for four oxide samples. More defective oxides, after evacuation at 400°C, have larger Vs values and the expected larger values of B(σ0). From related studies of these surfaces, the higher concentration of surface defects can be shown to produce higher concentrations of electron-accepting hydroxyl groups which, when partially removed (with some surface oxygen as CO) by high temperature evacuation, cause free hole annihilation and increased band bending.

XPS evidence for band bending at semiconducting oxide surfaces

Abstract The magnitude of the surface potential, directly influenced by defect properties and donor or acceptor chemisorption, has been studied using XPS for p-type nickel oxide surfaces. Above 200 W, the KE shifts are independent of X-ray intensity. A simple explanation of the dependence of the surface potential (Vch) on band bending (Vs) is proposed with preliminary experimental evidence allowing derivation of values of Vs.

Intergranular films and pore surfaces in Synroc C

High resolution electron microscopy and scanning electron microscopy were used to determine the distribution of intergranular films and microvoids in Synroc C. Diffraction contrast derived from these films, which were 1-3 nm thick, showed then to be ill-defined crystallographically, and they may be described as glassy. Pores were usually several micrometers in extent and occurred principally in rutile-rich areas. The chemical composition of these structural features was obtained using analytical transmission electron microscopy, secondary ion mass spectrometry, x-ray photoelectron spectroscopy and scanning Auger microscopy. Within intergranular films, elemental enhancement of cesium, sodium, potassium and aluminium, and possibly silicon and molybdenum was observed. Enhancement of cesium, sodium phosphorous, aluminium and silicon was found at triple points. Fracture faces preferentially expose grain boundaries, and dissolution of these surfaces proceeds rapidly at ambient temperatures. During the fabrication of Synroc C, microvoids trap cesium vapor, and after cooling this condenses onto pore surfaces. Recognition of the (simulated) waste species which segregate at grain boundaries and pores, permitted the reinterpretation of published leach data for monolithic and crushed Synroc C.

The Microstructure of Synroc

The Synroc concept was introduced by Ringwood in 19781 and, as is well known, the material now proposed for the disposal of hiah-level civilian nuclear waste consists primarily of hollandite (BaAl 2 Ti 6 0 16 ), perovskite (CaTiO 3 ) and zirconolite (CaZrTi 2 0 7 ).