Chemical reduction of hematite by sodium borohydride
Well-crystallized hematite was suspended in water and treated at room-temperature (RT) with sodium borohydride. The product of the reaction is a highly magnetic black powder, which is stable at RT. The NaBH4 treatment converts about half of the hematite to an amorphous Fe–B alloy and to a small fraction of sub-micron sized, amorphous metallic-Fe nodules. Heating at 400°C of this composite has resulted in the crystallization and/or oxidation of more than half of the amorphous Fe–B phase to α-Fe and Fe3O4 and B2O3, respectively. After treatment at 800°C, the metallic Fe and the amorphous Fe–B have completely vanished, and the resulting product consists of hematite and FeBO3 embedded in the matrix of α-Fe2O3.
Reduction of oxides by sodium borohydride solutions: A ‘chimie douce’ preparation of oxide bronzes
Abstract This work describes the use of sodium borohydride at room temperature as a reducing agent in aqueous medium in order to prepare oxide bronzes in which a cation is inserted. Two possibilities are offered depending on pH: a topotactic reaction leading to a final compound isostructural with the starting oxide, or a non-topotactic reaction leading to a bronze which may differ significantly from the parent phase. They are illustrated by two examples, respectively: the preparation of a hexagonal potassium tungsten bronze from a metastable hexagonal WO 3 (conservation of the tunnel structure), and that of the lamellar vanadyl vanadate (NH 4 ) 2 V 3 O 8 from the stable form of V 2 O 5 .
Microstructure of the solder-casting zone in bridges of dental gold alloys.
The composition and structure were studied in the cast and in the solder of bridges which had failed clinically. Both materials were gold alloys but with different contents of Pt, Ag and Zn. The metallographic investigation revealed defects mostly in the solder, situated in the subsurface layer. SEM studied of the fracture surface revealed large porosites and a structure of dense parallel lines--striations--, indicating that the material had failed from fatigue. The materials in the fractured bridges were identified by micro-probe measurements. It was stated that cast material and solder material used together, were of different composition.