Lithium oxide, sintering

Lithium oxide in its highly porous sintered form is used as an absorbent for carbon dioxide. 

Synthesis of niobates and tantalates of alkaline metals at high temperatures may result in the products with unintended stoichiometry because of high volatility of alkaline metal (especially lithium) oxides. Therefore, their synthesis in the form of efficiently sintering powders presents a serious problem. Films of alkaline niobates and tantalates can find wide range of applications in acousto- and optoelectronics and are usually prepared by rf-sputtering techniques, also giving rise to stoichiometry problems. 

Because of the potential importance for industrial-scale catalysis, we decided to check (i) whether an influence of a semiconductor support on a metal catalyst was present also if the metal is not spread as a thin layer on the semiconductor surface but rather exists in form of small particles mixed intimately with a powder of the semiconductor, and (ii) whether a doping effect was present even then. To this end the nitrates of nickel, zinc (zinc oxide is a well-characterized n-type semiconductor) and of the doping element gallium (for increased n-type doping) or lithium (for decreased n-type character) were dissolved in water, mixed, heated to dryness, and decomposed at 250°-300°C. The oxide mixtures were then pelleted and sintered 4 hr at 800° in order to establish the disorder equilibrium of the doped zinc oxide. The ratio Ni/ZnO was 1 8 and the eventual doping amounted to 0.2 at % (75). 

Other Semiconductors.—Kennedy et alf have continued to study FcjOj photoelectrodes, and their most recent work shows that high efficiencies are obtained with Si-doped sintered electrodes. Dare-Edwards et alf have characterized lithium-doped NiO in some detail but, as expected, the very low carrier mobility in this material makes it quite unsuitable for solar energy conversion. Gissler has investigated trigonal Se films, and Davidson and Willsher have given further details of the properties of HgS powder photoanodes. Derivatized tin-oxide electrodes have been prepared by Fox et al.f and Janzen et al. have successfully attached the photosynthetic reaction centre molecule isolated from Rhodopseudomones sphaeroides to tin oxide (see also Section 2). 

The working temperature of molten carbonate fuel cells is around 600-650°C. Mixed carbonate melts containing 62-70 mol% of lithium carbonate and 30-38 mol% of potassium carbonate, with compositions close to the eutectic point, are used in molten carbonate fuel cells as an electrolyte. Sometimes, sodium carbonate and other salts are added to the melts. This liquid melt is immobilized in the pores of a ceramic fine-pore matrix, made of sintered magnesium oxide or lithium aluminate powders. 

The reduced materials (category (f)) require special fusion conditions because dissolution produces an exothermic reaction that can destroy the platinum alloy fusion vessels. For example, the flux used for sihceous materials is reconstituted as lithium tetraborate and hthium carbonate. A SiC or other reduced sample is mixed with lithium carbonate and sintered on top of a protective layer of lithium tetraborate that has been fused and spread over the dish. The weight of the reduced sample therefore needs to be adjusted to maintain the flux to a (oxidized) sample ratio at 5 1. If samples lie in category (c), lithium tetraborate is replaced by boric oxide, which together with the lithium carbonate will ultimately give the appropriate lithium tetraborate/sample ratio (ignited basis). 

The concept of reactivity is a difficult one to define adequately. Comparison of the reactivity of iron oxides prepared from various starting compounds via several processes indicates that the reactivity depends upon the particular test applied. The relative order of activities determined by (1) solubility in acid at 81 °C, (2) extent of the reaction with lithium carbonate at 490 to 840 C, and (3) the ability to sinter to high density at 1200 °C is different for each. The reactivity of a powder is a transient phenomenon which dissipates with increasing temperature. To be useful, the test for reactivity has to occur in the specific temperature range associated with the particular activity of interest. 

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