Dietrich Schultze

Carbothermal synthesis of vanadium and chromium carbides from solution-Derived precursors

Abstract Vanadium and chromium tartrate precursors prepared from aqueous solutions have been used as preceramic materials for carbothermal reactions with and without simultaneous nitridation. Their thermal behaviour has been investigated by TG/DTA, X-ray diffraction and measurement of surface areas. Under pyrolysis up to about 600°C, reactive composites consisting of intimately mixed carbon and amorphous M 2 O 3 oxides are formed by salt decomposition. The subsequent processes of crystallization of oxides and carbothermal reduction render both the composites and the reduction products porous. The carbothermal reactions leading either to V 8 C 7 , a V(C,N,O) solid solution, and Cr 3 C 2 , respectively, proceed at moderate temperatures between 800 and 1100°C. The final products result as assemblages of fine carbide or carbonitride particles, however, with extensive heat treatment the particles grow and the surface areas diminish. Air oxidation of the final products has been studied by simultaneous TG/DTA.

Formation of NbC and TaC from gel-derived precursors

Abstract Binary hydrogels, in which a niobium or tantalum oxide gel and a pyrolysable organic compound are combined, were prepared as preceramic materials. Carbonaceous gel or saccharose is used as the organic gel constituent, and alkoxides or peroxo acids are the starting materials for the transition metal component. Under pyrolysis at 600–700 °C, the gels are transformed into carbide precursors in which a reactive carbon and finely dispersed oxide particles are mixed intimately. The subsequent carbothermal reduction proceeds at lower temperatures compared to physical mixtures of oxides and carbon black. The high-temperature treatment was monitored by TG/DTA, X-ray diffraction and nitrogen adsorption. Microporosity is generated in the temperature range 700–800 °C, and mesoporosity is additionally formed at higher temperatures. The changes in porosity are controlled by the processes of crystallization, carbothermal reduction and sintering. The final products at 1400–1500 °C are assemblages of fine particles of the face-centred cubic carbides with grain sizes of 1 μm and less.

Preparation of NbC, TaC and Mo2C fibres and films from polymeric precursors

Polymeric precursors for carbothermal reactions were prepared from niobium alkoxide, tantalum alkoxide and molybdenum ethylene glycolate, respectively, by simultaneous reaction with a chelating reagent (acetylacetone) and organic compounds having two or more reactive OH groups, such as ethylene glycol, saccharose, tartaric acid or dihydroxybenzenes. The precursors exist in common polar and non-polar solvents mostly as linear polymers. The viscous solutions show rheological properties that allow for the preparation of polymer fibres and films. At temperatures up to 1500 °C, bulk precursors as well as fibres and films were thermally converted into carbide powders, fibres and coatings. The structural transformations of the polymeric materials into carbides were investigated using simultaneous thermogravimetric–differential thermal analyses (TGA–DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM).

Pore formation during the pyrolysis of C-Nb2O5 and C-Ta2O5 xerogels

Binary organic-inorganic gels have been prepared by mixing a carbonaceous hydrosol and a Nb2O5 or Ta2O5 sol derived by hydrolysis of the alkoxides. The gels are pyrolyzed under an inert atmosphere into precursors in which carbon and the metal oxides are mixed very intimately. High temperature treatment converts the precursors into the cubic face centered carbides. The precursors as well as the carbides have been shown to be micro- and mesoporous materials. Measurements of nitrogen adsorption reveal a characteristic change of the shapes of the isotherms (Type I → Type IV) and of the hysteresis loops (H4→H2→H1) during the thermal processes. Pore widening has been observed with rising temperature. The phenomena of crystallization, carbothermal reduction and sintering were found to control the pore shape and size. The results of the adsorption measurement correlate well with those of the thermoanalytical and X-ray diffraction studies.

Porosity of shapes from hydrothermally treated titania

Titania nanopovvders obtained by hydrolysis of titanium alkoxides and consecutive hydrothermal treatment were compacted and sintered. The influence of preparation conditions (alkyl, pH of hydrolysis, temperature of hydrothermal treatment, compacting pressure, sintering temperature) on the porosity of powders, compacts and sintered bodies was investigated by nitrogen sorption and mercury intrusion. Optimum conditions are derived to prepare high-density ceramics at relatively low sintering temperature.

Potassium Self-Diffusion in Potassium Bismuth/Rare Earth Molybdate Crystals

The anisotropy of self-diffusion of K+ ions, ionic conductivity, and static permittivity in stoichiometric disordered K5Bi1-xRx(MoO4)4 (R = Nd, Gd; x = 0, 0.92, 1) single crystals is studied. A high value of the 2-D diffusion and ionic conductivity by K+ ions is detected. The fast ionic transport is parallel to the double layers formed by MoO4 tetrahedrons.