Particle production aluminum

Though the density of aluminized PBX is high, the measured detonation velocity is low when compared with nonaluminized PBX. Since HMX and RDX are stoichio-metrically balanced materials, no extra oxygen is available to oxidize the aluminum particles. The aluminum particles are oxidized by CO molecules in the combustion products of HMX or RDX. Furthermore, the oxidation of the aluminum particles takes a much longer time than that of the crystalline HMX or RDX particles. The aluminum particles do not react in the detonation wave but react downstream of the CJ point shown in Fig. 3-5. Thus, no increase in the detonation velocity occurs even if the density of the PBX is increased by the addition of aluminum particles. However, when an aluminized PBX is used in water, the high temperature aluminum particles react with water to produce hydrogen gas and thus produce bubbles in the water. The bubbles generate additional pressure and a shock wave in the water. 

The brownish-red cloud in the photograph is excess bromine. The reactions product, which is solid particles of aluminum bromide (AlBrs), settles on the bottom of the beaker. 

Magnalium substituted for the coarsest particles of aluminum produces outstanding effects. Blown or atomized magnalium is preferred to the more angular ground product of the same size because of the fluid flow properties. Because magnalium is more volatile and flammable, larger particles can be used. 

The pioneering investigation in the use of RESS for particle production was conducted by Krukonis (55). In 1984, Krukonis reported the preparation of small particles and fibers via RESS from several classes of materials, including aluminum isopropoxide, dodecanolactam, polypropylene, P-estradiol, ferrocene, navy blue dye, and soybean lecithin (55). Since that time, many investigations have been focused on RESS and its application to materials preparation and processing. Some of the more representative investigations and the associated key results are summarized here. 

The Beckstead-Derr-Price model (Fig. 1) considers both the gas-phase and condensed-phase reactions. It assumes heat release from the condensed phase, an oxidizer flame, a primary diffusion flame between the fuel and oxidizer decomposition products, and a final diffusion flame between the fuel decomposition products and the products of the oxidizer flame. Examination of the physical phenomena reveals an irregular surface on top of the unheated bulk of the propellant that consists of the binder undergoing pyrolysis, decomposing oxidizer particles, and an agglomeration of metallic particles. The oxidizer and fuel decomposition products mix and react exothermically in the three-dimensional zone above the surface for a distance that depends on the propellant composition, its microstmcture, and the ambient pressure and gas velocity. If aluminum is present, additional heat is subsequently produced at a comparatively large distance from the surface. Only small aluminum particles ignite and bum close enough to the surface to influence the propellant bum rate. The temperature of the surface is ca 500 to 1000°C compared to ca 300°C for double-base propellants. 

Aluminum-containing propellants deflver less than the calculated impulse because of two-phase flow losses in the nozzle caused by aluminum oxide particles. Combustion of the aluminum must occur in the residence time in the chamber to meet impulse expectations. As the residence time increases, the unbumed metal decreases, and the specific impulse increases. The soHd reaction products also show a velocity lag during nozzle expansion, and may fail to attain thermal equiUbrium with the gas exhaust. An overall efficiency loss of 5 to 8% from theoretical may result from these phenomena. However, these losses are more than offset by the increase in energy produced by metal oxidation (85—87). 

Aluminum hydroxide gel may be prepared by a number of methods. The products vary widely in viscosity, particle size, and rate of solution. Such factors as degree of supersaturation, pH during precipitation, temperature, and nature and concentration of by-products present affect the physical properties of the gel. 

An important appHcation of MMCs in the automotive area is in diesel piston crowns (53). This appHcation involves incorporation of short fibers of alumina or alumina—siHca in the crown of the piston. The conventional diesel engine piston has an Al—Si casting alloy with a crown made of a nickel cast iron. The replacement of the nickel cast iron by aluminum matrix composite results in a lighter, more abrasion resistant, and cheaper product. Another appHcation in the automotive sector involves the use of carbon fiber and alumina particles in an aluminum matrix for use as cylinder liners in the Prelude model of Honda Motor Co. 

Al-Pb. Both lead [7439-92-17, Pb, and bismuth [7440-69-9] Bi, which form similar systems (Fig. 17), are added to aluminum ahoys to promote machinahility by providing particles to act as chip breakers. The Al—Pb system has a monotectic reaction in which Al-rich Hquid free2es partiahy to soHd aluminum plus a Pb-rich Hquid. This Pb-rich Hquid does not free2e until the temperature has fahen to the eutectic temperature of 327°C. SoHd solubiHty of lead in aluminum is negligible the products contain small spherical particles of lead which melt if they are heated above 327°C. 

Specifications and Packaging. Aluminum chloride s catalytic activity depends on its purity and particle size. Moisture contamination is an important concern and exposure to humid air must be prevented to preserve product integrity. Moisture contamination can be deterrnined by a sample s nonvolatile material content. After subliming, the material remaining is principally nonvolatile aluminum oxide. Water contamination leads to a higher content of nonvolatile material. 

The large majority of activated alumina products are derived from activation of aluminum hydroxide, rehydrated alumina, or pseudoboehmite gel. Other commerical methods to produce specialty activated aluminas are roasting of aluminum chloride [7446-70-0], AIQ calcination of precursors such as ammonium alum [7784-25-0], AlH2NOgS2. Processing is tailored to optimize one or more of the product properties such as surface area, purity, pore size distribution, particle size, shape, or strength. 

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