The Surfaces of Titanate Minerals, Ceramics and Silicate Glasses: Surface Analytical and Electron Microscope Studies

The review evaluates evidence of aqueous surface attack on glasses, titanate minerals and synthetic rock material (Synroc C) available from:- leach rates; surface analysis (XPS, SAM, SIMS); IR spectroscopy; and electron microscopy. Direct observations are described showing the formation of:- ion-exchanged, cation deficient layers; altered glass network and crystal line lattice layers and recrystallised, reprecipitated and surface-segregated layers. The titanate minerals react in the order perovskites \u3e hollandite \u3e fluorite structures (zirconolite, zirkelite, pyrochlore, polymigllyte) \u3e rutile. The formation of amorphous Ti-O films, recrystallising to TiO2 (anatase and brookite), is observed on perovskite and hollandite surfaces. The surface reactivity of the titanate minerals is essentially the same in the ceramic assemblage Synroc C but additional microstructure (intergranular films, pores, triple points, minor phases) complicates the interpretation of first-day leach rates and depth profiles of leached discs. Reinterpretation of apparently congruent and incongruent dissolution data, using this evidence, is given for the glasses and titanate minerals. It is shown that solution analyses do not adequately describe the processes occurring in leaching and dissolution. The three major mechanisms of surface attack - ion-exchange, base catalysed hydrolysis of the network or lattice, and recrystallisation - are compared. Cation-exchange is very fast but limited to a layer no more than 20nm ahead of the reacted layer. Reaction of the titanate lattice is relatively slow and, in ceramics, appears to be limited by the grain size of the perovskite phase. In situ recrystallisation of amorphous TiOx films to TiO2 is fast above 100°C giving a polycrystalline discontinuous layer of varying depth (up to the grain size)

Resistance to amorphisation in Ca1-xLa2x/3TiO3 perovskites – a bulk ion-irradiation study

The changes induced from 1 MeV Kr+ and 5 MeV Au+ ion irradiation at room temperature have been utilised to determine the impact of cation vacancies on the radiation damage response of bulk Ca1-xLa2x/3TiO3 perovskite structured ceramics. Perovskite systems have long been considered as candidate waste forms for the disposition of actinide wastes, and doping with multi-valent elements such as Pu may lead to cation deficiency. Based on GAXRD and TEM analysis, two regions of resistance/susceptibility to amorphisation have been confirmed with reference to CaTiO3. Increased resistance to amorphisation has been observed for 0.1 ≤ x ≤ 0.4, with an increased susceptibility to amorphisation for x ≥ 0.5. It is proposed that these processes are induced by enhanced recovery from radiation damage for 0.1 ≤ x ≤ 0.4, and reduced tolerance for disorder/the increasingly covalent nature of the A-O bond for x ≥ 0.5. Lattice parameter analysis of the x = 0 and 0.5 samples showed a saturation in radiation damage induced volume swelling at 4.7 ± 0.1% and 1.8 ± 0.1%, respectively, while the saturation limit for the b parameter was lower than the respective a and c orthorhombic parameters. In the x = 0.2 and 0.4 samples, amorphisation was not observed, however the b parameter was found to swell to a lesser extent than the a and c parameters. Swelling was not observed for the ion irradiated x ≥ 0.6 samples

Evidence of an atomic displacement process in electron irradiated α-tin

The d.c. resistance of semiconducting α-tin is found to be strongly dependent on the dose and energy of electron irradiation. A threshold displacement energy of about 12 eV is obtained. The annealing spectrum shows three distinct low-temperature stages

Galvanomagnetic measurements on n- and p-type α-tin electron-irradiated at 5 K

Galvanomagnetic measurements were obtained for pure and doped n- and p-type α-tin filaments irradiated by 1 MeV electrons at ∼ 5 K. Variations in the Hall coefficient for highly degenerate samples were in qualitative agreement with theory. Carrier removal rates are reported; changes in these as a function of the initial position of the Fermi level indicate the presence of donor defect sites near E = 0 and acceptor defect sites deeper in the valence band. Isochronal annealing data have been obtained

Electron-irradiation-induced damage production effects in lightly doped n - and p -type α-tin

Irradiations at 4.5 K using 0.675-MeV electrons have been performed on a number of n - and p -type α-tin filaments, with impurity concentrations from 4×10 to 2×10 cm. Large and reproducible changes with radiation dose have been observed in the electrical resistivity and the thermoelectric power, resulting from changes in carrier concentrations and carrier scattering. It is shown that a model, which requires that the induced defects behave as acceptor sites, accounts satisfactorily for the experimental results. A carrier removal rate of 0.6±0.1 cm is deduced and is compared to the theoretical damage production rate of 1.6±0.3 cm; this indicates that only a fraction of the induced defects are active as carrier removal centers. Mobilities (μ and μ) have been calculated as functions of impurity and defect concentrations. It is found that the observed changes in the polarity of the thermoelectric power with dose can be accounted for qualitatively

Tip-induced nano-writing/machining of Si and DLC surfaces – "anodic" versus thermal oxidation?

Tip-induced oxidative manipulation of conducting surfaces, e.g., Si and some metals, has conventionally been described by a field-induced anodic mechanism. Likewise, in the case of electrically conducting graphitic and diamond-like carbon (DLC) films, tip-induced conversion of carbon to CO2 was initially thought to be due to an ionisation process. There is now mounting evidence for thermal activation playing an important role. The state of the tip is a critical, but largely disregarded, factor in such experiments. The present project has been prepared and characterized by I–V analysis, tips with different initial characteristics (e.g., H-termination , Au-coating, native oxide). Likewise, several surfaces have been prepared (e.g., Si plus termination by either native or thermal oxide, or plus H-termination, DLC and Au), and also subjected to I–V analysis. The resultant point-contact characteristics were found to range from ohmic to non-ohmic (the latter due to either direct or Fowler–Nordheim tunnelling). The various combinations were tested with respect to oxidative yield and tip durability. It was found that the presence of a tunnelling barrier at the point of contact is essential for enhancing yield. Tip durability, on the other hand, is promoted by the barrier being located in the surface thus localizing thermal deposition in the surface rather than in the tip