Sorption of uranyl ions on Si0 2 • xH 2 0 (silica gel) is investigated in absence and in presence of carbonate as function of pH. The curves obtained are very similar to those observed for sorption of uranyl ions on Ti0 2 • xH 2 0, indicating the dominating influence of the uranium species in solution. Between pH 2 and 5 the sorption ratio R s increases with hydrolysis of uranyl ions (formation of U0 2 0H + ), around pH 7 it is nearly independent of pH, and at higher pH it decreases again. The equilibrium constants are calculated for these ranges. In presence of carbonate R s decreases drastically above pH 6, due to the formation of carbonato complexes in solution. Sorption of uranyl ions on Si0 2 · XH 2 0, on Ti0 2 · xH 2 0, and on cryst. Si0 2 and A1 2 0 3 is compared. The problems of "surface complexation" modelling are discussed.
Group 4 element mononitrides (d-MN, M = Ti, Zr, Hf) with a NaCl type structure are well known as materials with extreme hardness, high melting points, metallic behavior, and remarkable chemical resistance.[1] They are industrially used for wear and chemical protection, as diffusion layers, decorative coatings, and in microelectronic devices. For example, d-TiN is broadly used as a monolayer or multilayer component for coating of cutting and milling tools because it considerably increases their service life. [2] In contrast to oxides of the group 4 elements, the existence of a stoichiometric nitride with cation oxidation state of 4+ was known, for a long time, only for zirconium. [3] In 2003 the novel cubic zirconium(IV) and hafnium(IV) nitrides with a Th 3 P 4 -type structure (c-M 3 N 4 , where M = Zr or Hf) were synthesized at high pressures and temperatures using a laser-heated diamond anvil cell (LH-DAC) technique.[4] They were obtained by the chemical reaction of elemental metals or their mononitrides with molecular nitrogen at 18 GPa and 2800 K for hafnium and at 15.6-18 GPa and 2500-3000 K for zirconium. These compounds are members of a rapidly growing group of new hard high-pressure nitrides exhibiting advanced properties.[5] Preliminary experiments on compression of c-Zr 3 N 4 and c-Hf 3 N 4 indicated high bulk moduli, B 0 , of about 250 GPa and 260 GPa (with assumed B 0 ′ = 4), respectively, which suggested a high hardness of these materials. In our recent accurate compressibility measurements the bulk modulus of c-Hf 3 N 4 was determined to be B 0 = 227(7) GPa (B 0 ′ = 5.3(6)) or B 0 = 241(2) GPa (with B 0
Hervorragende mechanische Eigenschaften zeichnen γ‐Si2AlON3 aus, eines der ersten Spinell‐Sialone und 4‐3‐Spinelle. Diese bei 130 000 bar/1800°C synthetisierte Phase übertrifft mit einer Härte von 27.5 GPa deutlich die der Niederdruckmodifikationen α‐ und β‐Sialon, die zur Metallbearbeitung und in Strukturkeramiken eingesetzt werden. Das Bild zeigt einige zur Bestimmung der Härte auf einer γ‐Si2AlON3‐Oberfläche eingebrachte Vickers‐Eindrücke und deutet die strukturelle Verwandtschaft zwischen β‐ und γ‐Sialon und den entsprechenden Si3N4‐Phasen an.
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