Ultrafine metal powders

In 1988, Tanaka et al. first reported the use of ultrafine metal powder in protein analysis. Since then, many inorganic materials, including graphite particles, fine metal or metal oxide powder, silver thin-film substrates or particles,and silica gel, have been used in the MALDI-TOF analysis of low-mass molecules. 

One important class of particulate composites is dispersion-hardened alloys. These composites consist of a hard particle constituent in a softer metal matrix. The particle constituent seldom exceeds 3% by volume, and the particles are very small, below micrometer sizes. The characteristics of the particles largely control the property of the alloy, and a spacing of 0.2-0.3 tim between particles usually helps optimize properties. As particle size increases, less material is required to achieve the desired interparticle spacing. Refractory oxide particles are often used, although intermetallics such as AlFes also find use. Dispersion-hardened composites are formed in several ways, including surface oxidation of ultrafine metal powders, resulting in trapped metal oxide particles within the metal matrix. Metals of commercial interest for dispersion-hardened alloys include aluminum, nickel, and tungsten. 

So-called ultrafine metal powders below 1 i in diameter can be made by pyrogenic dissociation of the vapors of the carbonyls of iron, nickel, and cobalt. Aluminum of a particle size below the resolving power of the electron microscope has been formed by evaporation and condensation under vacuum in an inert atmosph e. Similarly, evaporated magnesium has been quenched by JP-4 fuel for directly making a slurry fuel. Also, long ball-milling fine magnesium powder in the presence of a surfactant can lead to particles below I n. Iron or nickel electrolytically deposited on a mercury cathode form very active, often pyrophoric, fine powdm that, however, can be stabilized in order to be handled in air. 

Ultrafine metal powder (e.g., Al, Mg) and a strong oxidizer (e.g. potassium chlorate or perchlorate)... 

FIGURE 6.15 Laser desorption mass spectrum of lysozyme using a slurry of glycerol and ultrafine metal powder as the matrix. (Reprinted with permission from reference 53). 

Homogeneous deposition of ultrafine metal particles on the surfaces of fine powder is not easy using PVD. A device for stirring the powder support in a vacuum chamber is needed to avoid heterogeneous deposition. Sputter deposition units equipped with stirring powder supports have already been adapted for the industrial production of Ti02 and carbon-supported gold catalysts by 3M [35]. 

Synthesis by means of volatile compounds A number of halides (especially chlorides) of the transition metals display a high volatility and in the gaseous state they are easy to mix. They can be synthesized from oxides, or scrap metals, and chlorine they are highly reactive and can be utilized for the preparation of various compounds, either as powder or a coherent solid or as coatings. Mixtures, for instance, of TiCLj + CH4 + H2 have been used to prepare ultrafine TiC powder, to deposit TiC on graphite (at 1200-1300°C), etc. 

Wu, Xiuhua (2002). A study on the preparation and application of ultrafine copper-silver double metal powder. ME thesis, Eastern China University of Technology, Shanghai, China (in Chinese). 

Cao J., Li Z. Mechanically activated synthesis and apphcation of ultrafine ZrSi04 powder. Trans. Nonferrous Metal. Soc. China 1998 8 259-62. 

As with other metals, it becomes more reactive as it is more finely divided. Ultrafine iron powder is pyrophoric and potentially explosive. Explosive or violent reaction with ammonium nitrate + heat, ammonium peroxodisulfate, chloric acid, chlorine trifluoride, chloroformamidinium nitrate, bromine pentafluoride + heat (with iron powder), air + oil (with iron dust), sodium... 

Chromium Doping. Additions of chromium compounds can be used to produce very fine metal powder grades (<0.5 pm). Chromium oxide is formed during reduction and acts as a grain refiner. Such powders are subsequently carburized to ultrafine WC. Special precautions have to be taken because of the high pyrophoricity of the metal powder. 

Direct Carburization [9.13-9.16]. An alternative for producing WC powders on an industrial scale is the direct carburization process, which was patented in Japan. It has been exclusively used there for several years to produce submicron and ultrafine WC powders of high quality. Other than the conventional process, where steering of the WC particle size is mainly conducted via the metal powder process, this is done through the quality of the oxide and carbon source as well as the process parameters. 

Traditionally, diemiites are prepared by mixing fine conqwnent powders, such as ferric oxide and aluminum. Mixing fine metal powders by conventional means can be an extreme fire hazard sol-gel mediods reduce that hazard while achievii ultrafine particle dispersions that are not possible with normal processing methods. In conventional mixing, domains rich in either fiiel or oxidizer exist, vdiich limit die mass transport and dierefore decrease the efficiency of die bum. However, sol-gel derived nanoconqiosites should be more uniformly mixed, thus reducing die magnitude of this problem. 

Two approaches were independently developed i) the admixture of ultrafine cobalt powder (particle size about 30 nm) to analyte solutions in glycerol [8,9], and ii) the cocrystallization of the analyte with an organic matrix [10-13]. When combined with a time-of-flight (TOP, Chap. 4.2) mass analyzer, both methods are capable of producing mass spectra of proteins of about 100,000 u molecular weight. Nonetheless, the application of the ultrafine-metal-plus-liquid-matrix method remained an exception because the versatility of an organic matrix and the sensitivity of matrix-assisted laser desorption/ionization (MALDI) [10,11,14,15] made it by far superior to the admixture of cobalt powder [16-19]. In its present status, MALDI represents a major analytical tool in the modem life sciences [15,20-22] and in polymer science [23,24]. 

Schur D.V., Dubovoi A.G., Anikina N. S., Zaginaichenko S. Yu., Dobrovol skij V.D., Pishuk V.K., Tarasov B.P., Shul ga Yu.M., Meleshevich K.A., Pomytkin A.P., and Zolotarenko A.D. (2001) The production of ultrafine powders of fullerites by the salting out method, Proceedings of VII International Conference Hydrogen Material Science and Chemistry of Metal Hydrides , Alushta-Cremia-Ukraine, September lb-22, pp. 478-484. 

A soft chemical route, known as the sol-gel method, has also been employed for the preparation of nano-oxides with uniform size and shape. This is a multistep process, usually consisting of hydrolysis of a metal alkoxide in an alcoholic solution to yield a metal hydroxide, followed by polymerization by elimination of water (gel-formation), drying off the solvent, and densification of the product to yield an ultrafine powder (Rao and Raveau, 1998 Khaleel and Richards, 2001). 

Qian Y.-T., Chen Q.-W., Chen Z.-Y, Fan C.-G., Zhou G.-E., Preparation of ultrafine powders of Ti02 by hydrothermal H2O2 oxidation starting from metallic Ti. J. Mater. Chem., 3 (1993) pp. 203-205. 

In preparing fine particles of inorganic metal oxides, the hydrothermal method consists of three types of processes hydrothermal synthesis, hydrothermal oxidation, and hydrothermal crystallization. Hydrothermal synthesis is used to synthesize mixed oxides from their component oxides or hydroxides. The particles obtained are small, uniform crystallites of 0.3-200 jim in size and dispersed each other. Pressures, temperatures, and mineralizer concentrations control the size and morphology of the particles. In the hydrothermal oxidation method, fme oxide particles can be prepared from metals, alloys, and intermciallic compounds by oxidation with high temperature and pressure solvent, that is, the starting metals are changed into fine oxide powders directly. For example, the solvothermal oxidation of cerium metal in 2-mcthoxycthanol at 473-523 K yields ultrafine ceria particles (ca 2 nm). 

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