Grain reduction

If tlie level(s) associated witli tlie defect are deep, tliey become electron-hole recombination centres. The result is a (sometimes dramatic) reduction in carrier lifetimes. Such an effect is often associated witli tlie presence of transition metal impurities or certain extended defects in tlie material. For example, substitutional Au is used to make fast switches in Si. Many point defects have deep levels in tlie gap, such as vacancies or transition metals. In addition, complexes, precipitates and extended defects are often associated witli recombination centres. The presence of grain boundaries, dislocation tangles and metallic precipitates in poly-Si photovoltaic devices are major factors which reduce tlieir efficiency. 

The raw ROM (run of mine) ore is reduced in size from boulders of up to 100 cm in diameter to about 0.5 cm using jaw cmshers as weU as cone, gyratory, or roU-type equipment. The cmshed product is further pulverized using rod mills and ball mills, bringing particle sizes to finer than about 65 mesh (230 p.m). These size reduction (qv) procedures are collectively known as comminution processes. Their primary objective is to generate mineral grains that are discrete and Hberated from one another (11). Liberation is essential for the exploitation of individual mineral properties in the separation process. At the same time, particles at such fine sizes can be more readily buoyed to the top of the flotation ceU by air bubbles that adhere to them. 

For negative films, the electrochemical reduction properties of the reducing agents must be properly positioned to provide rapid amplification of exposed grains and much slower spontaneous amplifica tion of unexposed grains. The abiUty to discriminate between exposed and unexposed grains is a... 

High purity 50% ferrosihcon containing <0.1% Al and C is used for production of stainless steel and corded wire for tires, where residual aluminum can cause harm fill alumina-type inclusions. These are also useflil in continuous cast heats, where control of aluminum is necessary. High purity grades of 50 and 75% ferrosihcon containing low levels of aluminum, calcium, and titanium are used for sihcon additions to grain-oriented electrical steels, where low residual aluminum content contributes to the attainment of desired electrical properties, eg, significant reduction of eddy currents. 

High density tungsten alloy machine chips are recovered by oxidation at about 850°C, foUowed by reduction in hydrogen at 700—900°C. Typically, the resultant powders are about 3-p.m grain size and resinter readily. There can be some pickup of refractory materials used in furnace constmction, which must be controUed. This process is important commercially. Eor materials that may be contaminated with other metals or impurities, the preferred recovery process is the wet chemical conversion process used for recovery of tungsten from ores and process wastes. Materials can always be considered for use as additions in alloy steel melting. 

Ahoys having 15 to 30 wt % Co and very coarse WC grain sizes (up to 20 -lm) have replaced steel roUs in the production of hot-roUed steel rods. Carbide roUs are also weU suited for the cold reduction and finishing of strip products in Sendzimir mills where rigidity and dimensional stabhity are particularly important. The compositions used in these apphcations have a 5.5 wt % Co and medium WC grain size (1—2 p.m). 

The characteristics of WC, especially grain size, are determined by purity, particle shape and grain size of the starting material, and the conditions employed for reduction and carburization. The course of the reaction WO3 — W — WC is dependent on temperature, gas flow rates, water-vapor concentration in the gas, and the depth of the powder bed. All these factors affect the coarsening of the grain. 

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