As-milled powders

As mentioned above, borides can be synthesized by the carbothermic reduction of B2O3 with carbon and a metal or a metal oxide [337], e.g. according to Eq. (40). Since free carbon is present in the as-milled powder mixture and also develops during Reaction (43), the following process must also be considered a potential source of Fe2 B ... 

Most solid-state mechanochemical reactions have been displacement reactions, with two or more product phases being formed. The microstructure of the as-milled powders consists of a nanocomposite mixture of the product phases, with 5-20 nm crystallites of the reduced metallic phase being uniformly intermixed with an amorphous or nanocrystalline oxidized by-product phase. The separation or removal of unwanted... 

Heat treatment of the as-milled powders has generally been shown to result in the formation of equilibrium phases. For example, with Nd2Fei4B formed from the reaction of Nd203 with Ca, heat treatment at temperatures above 873 K is required to form the Nd2Fei4B phase (Liu et al. 1994c). However, some differences have been observed in the structures synthesized via different reactions. In particular, the stability of the SmCos phase appears to depend on the reductant and on whether an oxide or a halide is being reduced (Richmond et al. 1997). 

Small, complex-shaped glass articles such as thread guides for the textile industry and television gun mounts for the electronics industry are made by the multiform process. The dry-milled powder is mixed with an inorganic binder and a fluid vehicle, and then atomi2ed by a spray dryer into small, dried agglomerates of glass powder and binder with good flow characteristics. They are subsequently pressed to the desired shape and fired. 

Silicon Nitride. SiUcon nitride is manufactured either as a powder as a precursor for the production of hot-pressed parts or as self-bonded, reaction-sintered, siUcon nitride parts. a-SiUcon nitride, used in the manufacture of Si N intended for hot pressing, can be obtained by nitriding Si powder in an atmosphere of H2, N2, and NH. Reaction conditions, eg, temperature, time, and atmosphere, have to be controlled closely. Special additions, such as Fe202 to the precursor material, act as catalysts for the formation of predorninately a-Si N. SiUcon nitride is ball-milled to a very fine powder and is purified by acid leaching. SiUcon nitride can be hot pressed to full density by adding 1—5% MgO. 

Soaps Soaps in a finely divided form may be classified as soap powder, powdered soap, and chips or flakes. The term soap powder is applied to a granular product. No. 12 to No. 16 sieve size with a certain amount of fines, which is produced in hammer mills with perforated or slotted screens. 

When BlkPdr is used as a pressed grain, surface normal burning rates can be defined. These studies continue as an important tool in examining the complexities of the burning mechanism. These and other properties of various BlkPdrs have been compared (Ref 72), as has the temp dependence of the burning rates. The enormous volume of published data deals, however, only with conventional, mill process, powder, while the corresponding data on the jet mill powder await the completion of ongoing studies... 

Hydrogenation is sensitive to the surface modifications. For example, the ball-milled powders require less activation compared to the conventional powders. For nanocrystalline hydrides, the grain boundary does not dramatically affect the PCI, which describes the thermodynamic aspects of hydride formation. However, the pressure for hydrogen desorption of the unmilled MgH2 is lower than that of the milled one as seen in Figure 11.7 [66]. 

Two important morphological parameters characterizing ball-milled powders are the particle and grain size of constituent phases within the powders. In our laboratory, the size measurement of the powder particles is carried out by attaching loose powder to sticky carbon tape and taking pictures under secondary electron (SE) mode in the SEM. The images are then analyzed by an image analysis software. The size of the powders is calculated as the particle equivalent circle diameter, ECD = AA/nf, where A represents the projected particle area. Usually from -300 to 700 particles are analyzed for each batch. 

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