Reactive Thermal Spray

Coatings of refractory carbides and nitrides can be deposited reac-tively in a manner similar to reactive evaporation and sputtering by spraying the pure metal in an atmosphere of either a hydrocarbon or nitrogen (see Secs. 6.2 and 6.3). 

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Difficult-to-process composites can be readily produced by thermal spray forming, and plasma spray is the process of choice for the most reactive matrix materials. Particulate-, fiber-, and whisker-reinforced composites have all been produced and used in various applications. Particulate-reinforced wear-resistant coatings such as WC/Co, Cr3C2/NiCr, and TiC/NiCr are the most common applications. Figure 8 illustrates the diverse forms of composites that can be thermally spray formed. Whisker particulates can be incorporated into sprayed deposits using so-called engineered powders, mechanical blending, or by... 

Thermal Spray Technologies Reactive Plasma Spray Forming... 

TiN coatings are deposited by CVD, reactive evaporation, reactive sputtering, and ion-beam-assisted deposition. They can also be obtained by thermal spray. TiN powder is produced by the nitridation of Ti metal with nitrogen or ammonia at 1200°C (see Chs. 14 and 15). 

Reactive plasma spraying of silicon in controlled nitrogen atmosphere, in Thermal Spray Industrial Applications (eds C.C. Bemdt and S. Sampath), ASM International, Materials Park, OH,... 

Dead Seas Periclase Ltd., on the Dead Sea in Israel, uses yet another process to produce magnesium oxide. A concentrated magnesium chloride brine processed from the Dead Sea is sprayed into a reactor at about 1700°C (127,128). The brine is thermally decomposed into magnesium oxide and hydrochloric acid. To further process the magnesia, the product is slaked to form magnesium hydroxide which is then washed, filtered, and calcined under controlled conditions to produce a variety of MgO reactivity grades. A summary of MgO purities, for the various processes is given in Table 20. 

Each type of metallic coating process has some sort of hazard, whether it is thermal energy, the reactivity of molten salt or metal baths, particulates in the air from spray processes, poisonous gases from pack cementation and diffusion, or electrical hazards associated with arc spray or ion implantation. 

The general chemical behavior of chitosan, however, should be considered in order to avoid certain difficulties stemming from its insolubility at pH higher than 6.5 and its reactivity under the thermal conditions of the sprayer. For example, spray-drying of 1-2% chitosan in acid solution at 168 °C seems... 

Microspheres made from polymers or reactive oligomers (RO s) are manufactured by thermally treating sprayed solutions or emulsions. A solution of any film-forming polymer can be used. When an RO is used the solvent has to be evaporated, the sprayed monolithic particles are then heated to expand the gas or vapor within the particle, and the final microspheres are hardened 22,23). 

TD = thermal decomposition C = ceramic TB = Teflon-bonded RS = reactive sputtering E = electrochemical decomposition or formation SC = single crystal PJS = plasma jet spray. 

The control of the nuclear and chemical reactivity in case of accidents is insured by the emergency shutdown systems. The safety function devoted to the thermal power extraction from the HYPP is directly linked to the control of the chemical reactivity because the kinetics of chemical reactions increases with the temperature. The HYPP must be cooled by emergency systems, water streaming on equipments, spraying systems, and so on. 

The design of the precursor as well as the pyrolysis technique have to be optimized to obtain satisfactory selectivities and yields in the thermal conversions. The starting material has to be stable enough to be transferred into the gas phase without decomposition at temperatures up to 400 °C (depending on the vapor pressure and therefore on the molecular weight and structure) but also to possess an inherent reactivity which can be exploited to yield the desired products with high selectivities and yields under conditions as mild as possible. Thus, enthalpies of activation of 200-400 kJ mol 1 [31, 32] are representative for pyrolyses as synthetic procedures with moderate or high yields. Numerous special techniques have been developed to transfer thermally labile products into the gas phase (spray pyrolysis [33], solvent assisted sublimation techniques... 

During plasma spraying of hydroxyapatite onto titanium alloy substrate surfaces minor amounts of highly reactive CaO are being formed by thermal decomposition that react with the native titanium oxide layer to form calcium titanate, CaTiOs. This thin reaction layer has been implicated to be an important... 

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