Powder mixtures

Liquid-Ph se Sintering. Sintering ia the Hquid state refers to the sintering of a powder mixture of two or more components, of which at least one has a melting temperature lower than the others. The sintering temperature is then selected ia such a manner that a Hquid phase is formed ia which the soHd powder particles of the other components rearrange. A high density powder compact is the result. 

Processing. Tungsten carbide is made by heating a mixture of lampblack with tungsten powder in such proportions that a compound with a combined carbon of 6.25 wt % is obtained. The ratio of free-to-combined carbon is of extreme importance. Tantalum and titanium carbides are made by heating a mixture of carbon with the metal oxide. Multicarbide powders, such as M02C—WC, TaC—NbC, and TiC—TaC—WC, are made by a variety of methods, the most important of which is carburization of powder mixtures. 

Manufacturing, analytical, and quaUty control procedures are thus estabhshed. Specifications for taw and in-process materials, as well as for final products per USP/NF and in-house standards are also determined. Process and formula vaUdation assures that each technological procedure in manufacture accomplishes its purpose most efficiently, eg, blending times for powdered mixtures in tableting, and that each formula ingredient is present in optimal concentrations (12). Thus, it serves to ensure process control (qv), reproducibiUty, and content uniformity. 

Oxidizing elements such as oxygen, sulfur, and halides react with zinc at room temperature in the presence of moisture, but do not in its absence. At higher temperature, the reactions can be vigorous even when dry. Eor instance, a powdered mixture of zinc and sulfur explodes if warmed and zinc reacts... 

Metal powder mixtures with carbon black may also undergo carburization. 

The preferred method for synthesis of complex carbides is the powder metallurgy technique. Hot-pressed powder mixtures must be subjected to prolonged annealing treatments. If low melting or volatile components are present, autoclaves are used. 

Other metal chlorides and salts can also be used lea ding to formation of numerous other metal dichloroisocyanurates (34). Other routes to SDCC iuclude reaction of a moistened powdered mixture of TCCA, CA, and NaHCO (35), reaction of cmde CA with NaOCl (36), and reaction of NaH2 with HOCl (37) or CI2O (38). 

The finely powdered mixture of potassium salicylate and carbonate is placed in a 500-cc. round-bottomed flask which is immersed in an oil bath so that only a small portion of the neck protrudes from the bath (Note 2). The bath is heated to 240° (Note 3) and maintained at this temperature for one and one-half hours. During this time the solid in the flask is stirred occasionally with a curved glass rod flattened at the end. 

Fig. 6.2. The energy localization problem in shock-loaded porous powder mixtures involves a balance between the rate at which energy is applied locally and the rate at which it is removed from the site.

Horie and his coworkers [90K01] have developed a simplified mathematical model that is useful for study of the heterogeneous nature of powder mixtures. The model considers a heterogeneous mixture of voids, inert species, and reactant species in pressure equilibrium, but not in thermal equilibrium. The concept of the Horie VIR model is shown in Fig. 6.3. As shown in the figure, the temperatures in the inert and reactive species are permitted to be different and heat flow can occur from the reactive (usually hot) species to the inert species. When chemical reaction occurs the inert species acts to ther-... 

The synthesis effort was initiated by the Horie group on mechanically blended powder mixtures of 3 parts nickel with 1 part aluminum in molar proportions and a similar sample composed of a composite particle of nickel plated on aluminum in similar proportions. The powders were a 44 74 m nickel powder and a 5-15- m micron aluminum powder, a coarse fine mixture. The powder mixtures were shock loaded to peak pressures of 7.5 and 22 GPa with starting powder densities of 60% of solid density. 

Fig. 8A A cross section of a shock-treated, Ni-Al powder mixture compact shows a characteristic reaction in the hotter, outer portion of the compact. There is also a localized reaction in the axially focused region [85H01],...

Fig. 8.6. Elemental distribution maps of a shoek modified but unreaeted powder mixture in the work of Hammetter et al. [88H01] show that there is eonsiderable mixing of niekel into the aluminum.

The unreacted samples and starting powder mixtures were studied in DTA experiments to investigate the effects of morphology on the shock modifica-... 

The response of titanium-aluminum powder mixtures in a 3 1 molar ratio was investigated under the same shock-loading conditions as in the nickel aluminides. Such mixtures are especially interesting in that the shock impedances of the materials are approximately equal and both are relatively hard and difficult to deform. In addition to any chemical differences, such materials should prove to be difficult to mix with the shock conditions. 

Very high pressure and temperature experiments with the Sawaoka fixture on Nb-Si powder mixtures show that the silicon melted but the higher melt temperature niobium did not. Under these conditions, only chemical reaction... 

Figure 10. Hydrogen effect in compacted powder mixtures Ti + TiH2 (8 1). Compacted at 9 kbar - y, additionally annealed in vacuum at 735°C for 10 min - A, outgassed - Q-...

Compacted powder mixtures of titanium and titanium dihydride demonstrate the hydrogen-enhanced plasticity effects on deformation over 500 C, like titanium alloys hydrogenated from the gas phase. 

Komatsu [478] has put forward the hypothesis that reaction in many powder mixtures is initiated only at interparticle contact and that product formation occurs by diffusion through these contact zones. Here, one of the participating reactants is not covered with a coherent product layer. Quantitative consideration of this model leads to a modified Jander equation. 

In a DTA study [1193] of decomposition reactions in Ag2C03 + CaC03 mixtures, the presence of a response peak, absent on heating the silver salt alone, resulted in the identification of the double salt Ag2C03 2 CaC03, stable at <420 K. One important general consideration which arises from this observation is that the formation of a new phase, by direct interaction between the components of a powder mixture, could easily be overlooked and, in the absence of such information, serious errors could be introduced into attempts to formulate a reaction mechanism from observed kinetic characteristics. Due allowance for this possibility must be included in the interpretation of experimental data. 

Johnson and Gallagher [410] showed that, in finely divided powder mixtures, Li2C03 and Fe203 react significantly below the usual temperature of carbonate dissociation, so that C02 evolution can be used in kinetic studies of the solid state reaction... 

Kutty and Murthy [1159] have made a kinetic study of the solid—solid reaction between tricalcium phosphate and urea nitrate, a process of possible technological importance. A reduction in particle size, notably of Ca3(P04)2, increased the rate of reaction in powder mixtures and also changed the kinetic characteristics (318—338 K). Reaction in relatively coarse material (between —180 and +200 mesh) obeyed the parabolic... 

A laboratory preparation of pure Li uses a finely powdered mixture of Li,0 and a noble metal (Pt, Pd or Ir), which is heated > 1100°C in a stream of Hj. The pure intermetallic phase so produced is then heated under vacuum and the Li can be isolated by condensation from the vapor phase. ... 

Synthesis of ruthenocene from fission-product ruthenium isotopes was done by neutron irradiation ofU30g and FeCpj powder mixtures. It was shown that most of the ruthenocene found was actually produced by the decay of a precursor. Subsequent knowledge makes it apparent that the fission product recoils formed a rhodium dicyclopentadienide whose structure was preserved through the decay . The total yield of ruthenocene reached a value of 60% under some experimental conditions and was rarely less than 40%. 

Baumgartner and Reichold prepared carrier-free Mo(CO)g in high yield by neutron irradiation of powdered mixtures of UjOg and Cr(CO)g. As with their preparation of ° RuCp2, the Cr(CO)g acted only as a catcher for fission-product molybdenum (and for its precursors niobium and zirconium). The yield of 60% found for Mo(CO)6 is higher than the fractional chain yield of Mo in fission, so that the reaction must be partly thermal, starting with molecular fragments which survive j8 decay. 

The synthesis of organometallic compounds as a consequence of nuclear fission is also noteworthy In this way, for example, a powdered mixture of Fe(C5H5)2 and UjOg gives good yields of rutheno-cene and iodoferrocene on fission ... 

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