Metallic powder

Ammonium nitrate Acids, metal powders, flammable liquids, chlorates, nitrites, sulfur, flnely divided organic or combustible materials, perchlorates, urea... 

Chlorosulfonic acid Saturated and unsaturated acids, acid anhydrides, nitriles, acrolein, alcohols, ammonia, esters, HCl, HF, ketones, hydrogen peroxide, metal powders, nitric acid, organic materials, water... 

Metal passivation Metal pickling Metal powders Metal recovery... 

Electrically conductive mbber (13) can be achieved by incorporation of conductive fillers, eg, use of carbon or metal powders. These mbbers exhibit volume resistivities as low as lO " H-cm. Apphcations include use in dissipation of static charge and in conductive bridging between dissimilar electronic materials under harsh operating conditions. 

A. R. E. Singer and A. D. Roche, ia E. N. Aqua and C. I. Whitman, eds.. Modem Developments in Powder Metallurgy Metal Powder Industries... 

Reduction. Hafnium oxide can be reduced using calcium metal to yield a fine, pyrophoric metal powder (see Calciumand calciumalloys). This powder contains considerable oxygen contamination because of oxygen s high solubility in hot hafnium, and caimot be consoHdated into ductile metal. To obtain low oxygen ductile hafnium, the feed must be an oxygen-free halide compound such as hafnium tetrachloride or potassium hexafluorohafnate [16871-86-6]. 

Although a few simple hydrides were known before the twentieth century, the field of hydride chemistry did not become active until around the time of World War II. Commerce in hydrides began in 1937 when Metal Hydrides Inc. used calcium hydride [7789-78-8J, CaH2, to produce transition-metal powders. After World War II, lithium aluminum hydride [16853-85-3] LiAlH, and sodium borohydride [16940-66-2] NaBH, gained rapid acceptance in organic synthesis. Commercial appHcations of hydrides have continued to grow, such that hydrides have become important industrial chemicals manufactured and used on a large scale. 

Metallic Powders. These are usually either aluminum or bronze flakes and vary ia shades from silver to gold, depending on the composition of the metal used. The silver powders can also be toned with organic pigments to produce golds or copper shades usiag transparent yellow or red pigments. 

M. Jacobson, A. R. Cooper, andj. Nagy, Explosivity of Metal Powders, U.S. Bureau of Miaes, Report of Investigations 6516, Washiagton, D.C., 1964. 

Ferroalloys Association 1612 K Street, NW Washington, D.C. 20006 Metal Powder Industries Federation P.O. Box 2054 Princeton, NJ 08540 Gold Institute 1001 Connecticut Avenue, NW Washington, D.C. 20036 Silver Institute 1001 Connecticut Avenue, NW Washington, D.C. 20036... 

Miscellaneous Methods. Powdered metals such as aluminum, chromium, nickel, and copper, along with various aHoys, can be appHed to parts by electrostatic deposition. The metal strip containing the attached powdered metal must be further processed by cold rolling and sintering to compact and bond the metal powder. 

Mechanical Plating. Impact or peen plating is a mechanical process whereby the metal powder is compacted and welded to parts by mechanical energy. This process is limited to relatively small parts of no more than about one kilogram. The parts are placed ia a specially desigaed barrel... 

Metal powder—glass powder—binder mixtures are used to apply conductive (or resistive) coatings to ceramics or metals, especially for printed circuits and electronics parts on ceramic substrates, such as multichip modules. Multiple layers of aluminum nitride [24304-00-5] AIN, or aluminay ceramic are fused with copper sheet and other metals in powdered form. The mixtures are appHed as a paste, paint, or slurry, then fired to fuse the metal and glass to the surface while burning off the binder. Copper, palladium, gold, silver, and many alloys are commonly used. 

Miscellaneous Processes. Metal strip for cladding can be produced by cold pressing metal powder into alow density green strip, foUowed by sintering to compact the powder. AHoy powders can be made into strip, along with specialized strip with one powder bonded to a different powder on the opposite side. 

Shape. Metal powder particles are produced in a variety of shapes, as shown in Figure 4. The desked shape usually depends to a large extent on the method of fabrication. Shape can be expressed as a deviation from a sphere of identical volume, or as the ratio between length, width, and thickness of a particle, as weU as in terms of some shape factors. 

Fig. 4. Shapes of metal powder particles (a) spherical (b) rounded (c) angular (d) acicular (e) dendritic (f) kregular (g) porous and (h) fragmented. Density. The density of a metal powder particle is not necessarily identical to the density of the material from which it is produced because of...

Inasmuch as friction conditions determine the flow characteristics of a powder, coarser powder particles of spherical shape flow fastest and powder particles of identical diameter but irregular shape flow more slowly. Finer particles may start to flow, but stop after a short time. Tapping is needed in order to start the flow again. Very fine powders (fine powder particles to coarser ones may increase the apparent density, but usually decreases the flow quality. Metal powders having a thin oxide film may flow well. When the oxide film is removed and the friction between the particles therefore increases, these powders may flow poorly. 

The manufacture of metal in powder form is a complex and highly engineered operation. It is dominated by the variables of the powder, namely those that are closely connected with an individual powder particle, those that refer to the mass of particles which form the powder, and those that refer to the voids in the particles themselves. In a mass of loosely piled powder, >60% of the volume consists of voids. The primary methods for the manufacture of metal powders are atomization, the reduction of metal oxides, and electrolytic deposition (15,16). Typical metal powder particle shapes are shown in Figure 5. 

Fig. 5. Metal powder particle shapes (a) atomized copper (b) sponge iron and (c) atomized iron.

Mechanical comminution may be used to form metal powders. Relatively coarse particles are produced by machining, whereas ball mills, impact mills, gyratory cmshers, and eddy mills give fine powders of britde materials. 

Condensation of metal vapors followed by deposition on cooler surfaces yields metal powders as does decomposition of metal hydrides. Vacuum treatment of metal hydrides gives powders of fine particle size. Reaction of a metal haHde and molten magnesium, known as the KroU process, is used for titanium and zirconium. This results in a sponge-like product. 

Other methods of metal powder manufacture are also employed for specific metals. Selective corrosion of carbide-rich grain boundaries in stainless steel, a process called intergranular corrosion, also yields a powder. 

Consolidation. Metal powders are consoHdated by heat or by pressure followed by heat, or by heating during the appHcation of pressure (17). ConsoHdation produces a coherent mass of definitive size and shape for further working, heat treating, or use as is. 

---