Gerhard Pfaff

Chemical synthesis of calcium stannates from peroxo precursors

Abstract A chemical route for the low-temperature synthesis of the calcium stannates CaSnO 3 and Ca 2 SnO 4 , potential materials for dielectric applications, is reported. Mixed cation precursor powders, derived using calcium chloride, tin tetrachloride and hydrogen peroxide, were calcined at relatively low temperatures of about 700–1200 °C to form homogeneous crystalline CaSnO 3 and Ca 2 SnO 4 . The structural evolution of the crystalline phases was studied using X-ray diffraction, differential thermal analysis, thermogravimetric analysis, scanning electron microscopy, Brunauer-Emmett-Teller measurements and chemical analysis. The thermal degradation of the X-ray amorphous peroxo precursors to the single-phase stannates occurs in two steps for the formation of CaSnO 3 and in three steps for the formation of Ca 2 SnO 4 . CaSnO 3 is one of the intermediate products formed during the thermal decomposition to Ca 2 SnO 4 . The ultrafine powders resulting after the thermal degradation of the precursors have a small impurity content. They show a high sinterability at temperatures above 1000 °C which increases from Ca 2 SnO 4 to CaSnO 3 . The highests densities reached for ceramic samples were about 97% of the theoretical value for CaSnO 3 and 95% for Ca 2 SnO 4 by sintering at 1400 °C for 4 h.

A novel reaction path to barium zirconates by the decomposition of peroxide precursors

Abstract Barium zirconate BaZrO 3 can be prepared at 900 °C with high purity using the peroxide method. Large specific surface area and small grain sizes of the powder lead to high sinterability. A peroxo-precursor is formed in a first step from aqueous solution. The thermal degradation of the X-ray amorphous precursor leads directly to BaZrO 3 , Ba 2 ZrO 4 and Ba 3 Zr 2 O 7 are formed as metastable phases after thermal decomposition of corresponding peroxo-precursors at temperatures of about 700 °C.

High radiant flux photovoltaic cells for solar proximity missions

Solar cells are widely used in all near-Earth space missions, with solar panels increasingly getting bigger to supply the growing power requirements of sophisticated spacecrafts. Most solar cells begin to lose their efficiency significantly when their bulk temperature rises above about 60 °C. This work describes gallium arsenide solar cells coated with infrared-reflecting pigments, contained in a silica film. Measurements indicate that such coated cells retain more of their efficiency at higher temperatures than similar uncoated cells with the former producing around 7.6% more power than the latter. The structure, fabrication and photovoltaic characteristics of pigment-coated single-junction gallium arsenide solar cells are described. The basic idea could be extended to triple-junction cells which are widely used for space missions.