Cadmium is a metal and one of the major constituents of II-VI compound semiconductors. The most remarkable characteristics of cadmium are its great resistance to corrosion, its low melting-point and excellent electrical conduction. Cadmium compounds exhibit excellent resistance to chemicals and to high temperatures. Finally, cadmium pigments produce intense colourings such as yellow, orange and red, and are well known pigments in artists' colours, plastics, glasses, ceramics and enamels.
C-MET, Hyderabad laboratory has developed process technology for the preparation of 6N (99.9999 at.%) cadmium (Cd) starting from 3N (99.9 at.%) purity. This highpure (=6N purity) Cd is one of the major constitutes in cadmium telluride (CdTe), cadmium zinc telluride (CdZnTe), mercury cadmium cadmium (HgCdTe), cadmium sulfide (CdS), cadmium selenide (CdSe), etc.) which are used in various opto-electronic applications such as solar cells, IR detectors, imaging devices, electro-optic modulators, fluorescence, etc due to their sensitivity to x-rays, gamma-rays and photonic (UV-visible-IR) radiation. CdTe and CdZnTe substrates are used for growing epi-layers for Focal Plane Arrays (FPAs), whose major applications are in night vision cameras and thermal imaging devices in the kilometers range, predominantly used by army during night time operations. High pure Cd possesses a large capture cross section for thermal neutrons and is also used to fabricate moderator rods for nuclear reactors.
3N purity cadmium used as raw material for preparation of 6N cadmium. The purification are divided into two main process stages viz., (a) Vacuum distillation and (b) De-oxidation under H2 gas and ingot preparation.
The basis of separation of impurities from the base metal is by evaporation and condensation due to difference in the vapour pressure (Fricke Jr. et al 1978; Varavi et al 2003). The separation coefficient ‘a’ is defined as a = (Pi/Pm)* (Ni/Nm)*( ?i/?m) *(vMm/Mi), where P is the vapor pressure, N is the mole fraction , ? is the activity co-efficient and M is the molecular weight of impurity (i) and base metal (m). This technique was employed to prepare the high pure cadmium by distilling under vacuum.
De-oxidation of distilled Cd under hydrogen gas and preparation of Cd bar
The chunks of vacuum distilled cadmium (6N pure except oxygen, nitrogen and carbon) of nearly 1.0 kg weight is taken in a thoroughly cleaned and etched quartz (GE-214 grade) boat container of dimensions 34 x 38 x 560 mm (ID x OD x L) and melted at a temperature, Tm, nearly 13 to 15% more than the melting point of cadmium in a uniform temperature gradient furnace under a constant flow of hydrogen gas (IOLAR I grade). The temperature of the furnace is controlled by ‘Maxthermo’ make PID temperature controllers with an accuracy of +1 0C was used to purify cadmium in a clean room environment of class 10000. After completely melting, the H2 gas flow continued till the cadmium bar temperature is cooled down to room temperature to avoid any possible oxidation. After taking out the cadmium bar, a small portion of the cadmium material at one end is cut and analyzed to determine final purity.
S. NoElements in Cd matrixImpurity (in ppb)
S. NoElements in Cd matrixImpurity (in ppb)1B70