Effect of Zn- and Ca-oxides on the structure and chemical durability of simulant alkali borosilicate glasses for immobilisation of UK high level wastes

Compositional modification of United Kingdom high level nuclear waste (HLW) glasses was investigated with the aim of understanding the impact of adopting a ZnO/CaO modified base glass on the vitrified product phase assemblage, glass structure, processing characteristics and dissolution kinetics. Crystalline spinel phases were identified in the vitrified products derived from the Na2O/Li2O and the ZnO/CaO modified base glass compositions; the volume fraction of the spinel crystallites increased with increasing waste loading from 15 to 20 wt%. The spinel composition was influenced by the base glass components; in the vitrified product obtained with the ZnO/CaO modified base glass, the spinel phase contained a greater proportion of Zn, with a nominal composition of (Zn0.60Ni0.20Mg0.20)(Cr1.37Fe0.63)O4. The addition of ZnO and CaO to the base glass was also found to significantly alter the glass structure, with changes identified in both borate and silicate glass networks using Raman spectroscopy. In particular, these glasses were characterised by a significantly higher Q3 species, which we attribute to Si–O–Zn linkages; addition of ZnO and CaO to the glass composition therefore enhanced glass network polymerisation. The increase in network polymerisation, and the presence of spinel crystallites, were found to increase the glass viscosity of the ZnO/CaO modified base glass; however, the viscosities were within the accepted range for nuclear waste glass processing. The ZnO/CaO modified glass compositions were observed to be significantly more durable than the Na2O/Li2O base glass up to 28 days, due to a combination of the enhanced network polymerisation and the formation of Ca/Si containing alteration layers

Electrochemical characterization of porous diaphragms in development for gas separation

The alkaline electrolyzer industry is facing the imminent necessity of replacing the asbestos diaphragms with a separator made of a new environmentally friendly material. The influence of a diaphragm's material and porosity on the ionic conductivity was studied for asbestos, zirconium dioxide, wollastonite and olivine diaphragms. Developed electrochemical impedance spectroscopy (EIS) set-up with four-electrode cell offers the possibility for an accurate measurement of diaphragms’ ionic conductivity and enables its correlation to the materials porosity and tortuosity. The ion conductivity of investigated diaphragms approaches zero value for total porosities below 20% and increases when increasing porosities in agreement with Archie's law

Effect of the surface oxidation of LiBH4 on the hydrogen desorption mechanism

The surface oxidation behavior of LiBH4 and NaBH4 was investigated in view of the formation and structure of the surface oxidation and its effect on the hydrogen desorption kinetics. The sample surfaces were intentionally modified by exposure to oxygen in the pressure range from 10 -10 mbar up to 200 mbar. The induced surface changes were systematically studied by means of X-ray photoelectron spectroscopy. NaBH 4 shows a low reactivity with oxygen, while LiBH4 oxidizes rapidly, accompanied by surface segregation of Li. The hydrogen desorption kinetics of LiBH4 were studied by thermal desorption spectroscopy with particular emphasis on the analysis of the desorbed gases, i.e. diborane and hydrogen. The surface oxidation induces the formation of a Li2O layer on LiBH4, significantly reduces the desorption of diborane, and enhances the rate of hydrogen desorption. © 2010 the Owner Societies

Description of the capacity degradation mechanism in LaNi5-based alloy electrodes

The mechanism of the capacity degradation of LaNi5-based alloy electrodes was investigated with a special focus on the influence of the alloy and surface composition, as well as the unique structure obtained by gas atomisation. The electrochemical properties, especially the cycle life curve (i.e. the capacity as a function of the cycle number of LaNi4.5Al0.5, LaNi2.5Co2.4Al0.1, (La + Mm) Ni3.5Co0.7Al0.35Mn0.4Zr0.05, and MmNi(4.3)Al(0.2)Mn(0.5) alloy electrodes), was analysed and modelled. The capacity degradation upon cycling is determined by the chemical state of the alloy elements and the solubility of their oxides. The cycle life curves for the alloy electrodes without Co exhibited a rapid activation (3-4 cycles to reach maximum capacity), as well as rapid degradation (130-180 cycles for 50% maximum discharge capacity). LaNi2.5Co2.4Al0.1 and (La + Mm) Ni3.5Co0.7Al0.35Mn0.4Zr0.05 alloy electrodes activated after 7-10 cycles and showed very stable discharge behaviour (more than 400 cycles). The Co-containing alloy electrodes primarily lose the cycle stability because of mechanical decrepitation, whereas the alloys without Co suffer from selective dissolution of the unstable elements in the potential window, which was shown by our model of alloy degradation and confirmed by means of SEM, WDX, and ICP-OES data. (C) 2014 Elsevier B.V. All rights reserved