Composite powders

Perhaps the most definitive result to come from the early nickel-aluminia synthesis work was the thermal analysis investigation of Hammetter [88HO 88W01], which showed explicit data on substantial changes in the shockec-but-unreacted mixtures. Differential thermal analysis was carried out on th -starting powder compacts of both the mechanically mixed and composite powders. Shocked and unreacted powders were compared to provide direc evidence for substantial changes introduced by the shock process. 

The shock-modified composite nickel-aluminide particles showed behavior in the DTA experiment qualitatively different from that of the mixed-powder system. The composite particles showed essentially the same behavior as the starting mixture. As shown in Fig. 8.5 no preinitiation event was observed, and temperatures for endothermic and exothermic events corresponded with the unshocked powder. The observations of a preinitiation event in the shock-modified mixed powders, the lack of such an event in the composite powders, and EDX (electron dispersive x-ray analysis) observations of substantial mixing of shock-modified powders as shown in Fig. 8.6 clearly show the first-order influence of mixing in shock-induced solid state chemistry. 

Han, J.K., Choi, S.M. and Lee, G.H. (2007) Synthesis and photocatalytic activity of nanocrystalline Ti02-SrO composite powders under visible lightirradiation. Materials Letters, 61, 3798-3801. 

FIGURE 6.3 (a) Morphology of ball-milled NiO and SDC powders produced separately and (b) NiO-SDC composite powders coproduced by spray pyrolysis, showing a possible mechanism by which the composite powders retain a higher surface area resulting in a higher performance [19]. Reprinted from [19] with permission from Elsevier. 

Misono T, Murat, K, Fukui T, Chaichanawong J, Sato K, Abe H et al. Ni-SDC cermet anode fabricated from NiO-SDC composite powder for intermediate temperature SOFC. J. Power Sources 2006 157 754—757. 

Sobolev et al)5111 conducted a series of analytical studies on droplet flattening, and solidification on a surface in thermal spray processes, and recently extended the analytical formulas for the flattening of homogeneous (single-phase) droplets to composite powder particles. Under the condition Re 1, the flattening ratios on smooth and rough surfaces are formulated as ... 

G.J.C. Carpenter, Z.S. Wronski, NanocrystaUine NiO and NiO-Ni(OH) composite powders prepared by thermal and mechanical dehydroxylation of nickel hydroxide, Nanostructured Mater. 11(1) (1999) 67-80. 

First, the decomposition temperature of NUjAlH overlaps with the decomposition temperature of MgH. Second, the particle size of the composite powders is still coarse (Fig. 3.37e, f). 

The nebulization was also employed to generate composite powders for specific applications, such as in ceramics, by hydrolyzing with water vapor droplets containing Al(5ec-OBu) and silicon methoxide in the atomic ratio Al/Si = 3. This ratio of alkoxides was chosen in order to produce mullite, which was achieved by calcination of the resulting amorphous particles at rather high temperatures (up to I400 C) (52). In another approach a mixed Al-Mg-Si ethoxide was first synthesized, and then nebulized and hydrolyzed as usual (77). Depending on the experimental conditions, the powders calcined at 500 C exhibited structures of pure cordierite, or mixed with forsterite. In all of these described cases the nebulization yielded spherical but polydisperse particles. 

In the field of metallic powder applications, a method of plasma spray coating suitable for biomedical materials has been developed using titanium and calcium phosphate composite powder. By means of the mechanical shock process, the appropriate composite powder was prepared, and plasma sprayed on Ti substrate under a low-pressure argon atmosphere. A porous Ti coating layer was obtained in which the surface and the inside of the pores were covered thinly with hydroxyapatite. This surface coating is expected to show excellent bone ingrowth and fixation with bone (21). 

In addition to single-component powders, gas-phase synthesis can be used to produce composite powders, such as SiC and BN. An example of an apparatus used for this particular synthesis, which is representative of the apparatuses used for most gas-phase synthesis of particles, is shown in Figure 7.38. 

Safety Compound". A composite powder, consisting of K chlorate and gambler (a yellowish... 

Beside the production of pure Si3N4, the gas phase processes are used for the preparation of Si3N4/SiC composite powders used for nanocomposite materials [210, 222, 223]. For the production of such complex powders different kinds of evaporable Si organic compounds are used, e.g., hexamethyldisilazane. 

Itatani, K., Akiyama, K., Aizawa, M., Scott Howell, F., Okada, I. Sinterability of apatite-zirconia composite powder prepared by ultrasonic spray freeze-drying technique. J. Soc. Inorg. Mater. Jpn. 286,194-202, 2000... 

Bayberry bark is one of the most widely used agents in the herbal practice. It figures in many of the compound powders and is the base of the celebrated composition powder, a prescription of which will be found in the "Herbal Formulae" section of this volume. In cases of coldness of the extremities, chills and influenza, an infusion of 1 ounce of the powdered bark to 1 pint of water is taken warm. This assists circulation and promotes perspiration, especially when combined with Cayenne as in... 

Capsicum is a constituent of many of the herbal compounds, including the well-known composition powders, Thomson s formula for which will be found in the appropriate section of this book. The dose of the powdered fruit is 5-20 grains. 

The possibility to obtain a uniformly dispersed composite powder was shown for the a-Fe-Al203 system where metal particles with an average size of 55 nm were formed in an amorphous/nano alumina matrix.18 Other studies attempting to obtain dense bulk composites based on the sol-gel route using conventional pressure-assisted sintering ( 1400°C and an applied force of 10 MPa) resulted in a coarse microstructure.16 However, if reaching theoretical density is not a necessary requirement, a porous ceramic microstructure containing nanometer-sized metal particles can be used as a catalytic material.19 Certain combinations of composite materials demand... 

The third way to prepare CNT-ceramic composite powders is via the synthesis of CNT by a CCVD process, in situ in the ceramic powder. A ceramic powder which contains catalytic metal particles at a nanometric size, appropriate to the formation of CNTs, is treated at a high temperature (600-1100°C), in an atmosphere containing a hydrocarbon or CO. In the method reported in 1997 by the present authors,27 iron nanoparticles are generated in the reactor itself, at a high temperature (>800°C), by the selective reduction in H2/CH4 (18% CH4) of an a-Al203 based oxide solid solution ... 

Flahaut, E., Peigney, A., Laurent, Ch. and Rousset, A., Synthesis of single-walled carbon nanotube-Co-MgO composite powders and extraction of the nanotubes , J. Mater. Chem., 2000, 10, 249-252. 

G. J. C. Carpenter, Z. S. Wronski, Nanocrystalline NiO and NiO-Ni(OH)2 Composite Powders prepared by Thermal and Mechanical Dehydroxylation of Nickel Hydroxide, Nanostructured Mater., 1999,... 

Bilgrien CJ et al. (1992) Storage-stable flowable organosiloxane composition powders and their preparation. US Patent 5 153238... 

V. M. Gun ko, V. M. Bogatyrev, V. V. Turov, R. Leboda, J. Skubiszewska-Ziqba, L. V. Petrus, G. R. Yurchenko, O. I. Oranska, and V. A. Pokrovsky, Composite powders with titania grafted onto modified fumed silica, Powder Technology, submitted for publication. 

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