Tham khảo tài liệu 'materials handbook 15th ed - g. brady_ h. clauser_ j. vaccari (mcgraw-hill_ 2002) episode 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 800 RHODIUM molybdenite which contains to of the metal. But the usual source is from the flue dusts and from the sublimed rhenium oxide Re2O7 of stack gases in the smelting of copper and other ores. It is precipitated from the flue dust of molybdenum-bearing copper ores in the form of potassium perrhenate KReO4 or of ammonium perrhenate NH4ReO4. The Russians also obtain some rhenium from the Ural platinum ores. The stable rhenia used in alloying is rhenium trioxide ReO3 which comes as a red powder. The metal is obtained as a dense silvery powder which can be compacted sintered and cold-rolled with frequent annealing. It is marketed in the form of rod strip foil and wire. At the Technical University of Munich in Germany pure electrically conductive ReO3 ceramic films have been made by heating the polymer methyl-trioxorhenium Ch3ReO3 above 460 F 240 C causing it to lose its methyl groups and water. Rhenium has a higher electrical resistivity than tungsten has high arc resistance and does not become brittle after prolonged heating as tungsten does. Rhenium is used as an undercoat for tungsten coatings on graphite nozzles to prevent the formation of tungsten carbide and thus give the full heat resistance of the tungsten. Rhenium and rhenium-tungsten alloys used for electric contacts give very long service life. Rhenium-tungsten versus tungsten thermocouples are good for service to 5072 F 2800 C . Substantial amounts of rhenium are used in select tungsten- and molybdenum-base alloys. RHODIUM. A rare metal symbol Rh found in platinum ores such as the nickel-copper ores of Canada and pyroxenite of South Africa. It is very hard and is one of the most infusible of the metals. The melting point is 3565 F 1963 C . It is insoluble in most acids including aqua regia but is attacked by chlorine at elevated temperatures and by hot fuming sulfuric acid. Liquid rhodium dissolves oxygen and ingots are made by argon-arc melting. At temperatures above 2192 F 1200 C rhodium
