Specialized metallurgical processes – compositions for use therei – Processes – Producing solid particulate free metal directly from liquid...
Reexamination Certificate
2001-04-12
2002-09-03
Wyszomierski, George (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing solid particulate free metal directly from liquid...
C075S338000
Reexamination Certificate
active
06444009
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for producing ultra fine metallic alloy powders, which are environmentally stable. More particularly, it relates to a method for producing substantially nanometer-sized, environmentally stable reactive alloy particles at a high production rate.
BACKGROUND
The interest in ultra-fine particles, particularly nanometer-sized clusters (d≦200 nm), is due to the unique processing characteristics as well as performance properties exhibited by very small particles of metals, semiconductors and ceramics. Ultra-fine particles have enormous potential in metal and ceramic processing. For example, smaller particles can be sintered at much lower temperatures. Not only the structure, but also the mechanical, electronic, optical, magnetic and thermal properties of nano-crystalline materials are different from those exhibited by their bulk counterparts. Nano-phase metals and ceramics derived from nanometer-scaled particles are known to exhibit unique physical and mechanical properties. The novel properties of nano-crystalline materials are the result of their small residual pore sizes (small intrinsic defect sizes), limited grain size, phase or domain dimensions, and large fraction of atoms residing in interfaces. Specifically, ceramics fabricated from ultra-fine particles are known to possess high strength and toughness because of the ultra-small intrinsic defect sizes and the ability for grain boundaries to undergo a large plastic deformation. In a multi-phase material, limited phase dimensions could imply a limited crack propagation path if the brittle phase is surrounded by ductile phases so the cracks in a brittle phase would not easily reach a critical crack size. In addition, dislocation movement distances in a metal could be limited in ultra fine metallic domains, leading to unusually high strength and hardness. Even with only one constituent phase, nano-crystalline materials may be considered as two-phase materials, composed of distinct interface and crystalline phases. Further, the possibilities for reacting, coating, and mixing various types of nano materials create the potential for fabricating new composites with nanometer-sized phases and novel properties. For a review on nano-phase materials please refer to R. P. Andres, et al. “Research Opportunities on Clusters and Cluster-Assembled Materials,” in Journal of Materials Research, Vol. 4, 1989, pp.704-736 and A. N. Goldstein, “Handbook of Nanophase Materials,” Marcel Dekker, Inc., N.Y., 1997.
Ultra-fine metal particles, despite their usefulness, are sometimes difficult to work with due to their high surface-to-volume ratio which makes them more susceptible to environmental degradation (e.g., oxidation and corrosion) than their bulk counterparts. Some ultra-fine metal powders tend to be pyrophoric and present danger in their manufacture, transportation, handling, and storage.
The techniques for the generation of nanometer-sized particles may be divided into three broad categories: vacuum, gas-phase, and condensed-phase synthesis. Vacuum synthesis techniques include sputtering, laser ablation, and liquid-metal ion sources. Gas-phase synthesis includes inert gas condensation, oven sources (for direct evaporation into a gas to produce an aerosol or smoke of clusters), laser-induced vaporization, laser pyrolysis, and flame hydrolysis. Condensed-phase synthesis includes reduction of metal ions in an acidic aqueous solution, liquid phase precipitation of semiconductor clusters, and decomposition-precipitation of ionic materials for ceramic clusters. Other methods include high-energy milling, mix-alloy processing, chemical vapor deposition (CVD), and sol-gel techniques. All of these techniques have one or more of the following problems or shortcomings:
(1) Most of these prior-art techniques suffer from a severe drawback: extremely low production rates. It is not unusual to find a production rate of several grams a day. Vacuum sputtering, for instance, only produces small amounts of particles at a time. Laser ablation and laser-assisted chemical vapor deposition techniques are well-known to be excessively slow processes. The high-energy ball milling method, known to be a “quantity” process, is capable of producing only several kilograms of nano-scaled powders in approximately 100 hours. These low production rates, resulting in high product costs, have severely limited the utility value of nano-phase materials. There is, therefore, a clear need for a faster, more cost-effective method for preparing essentially nanometer-sized powder materials.
(2) Condensed-phase synthesis such as direct reaction of metallic silicon with nitrogen to produce silicon nitride powder requires pre-production of metallic silicon of high purity in finely powdered form. This reaction tends to produce a silicon nitride powder product which is constituted of a broad particle size distribution. Furthermore, this particular reaction does not yield a product powder finer than 100 nm (nanometers) except with great difficulty. Due to the limited availability of pure metallic silicon in finely powdered form, the use of an impure metallic powder necessarily leads to an impure ceramic product. These shortcomings are true of essentially all metallic elements, not just silicon.
(3) Most of the prior-art processes require heavy and/or expensive equipment (e.g., a high power laser source or a plasma generator), resulting in high production costs. In the precipitation of ultra fine particles from the vapor phase, when using thermal plasmas or laser beams as energy sources, the particle sizes and size distribution cannot be precisely controlled.
(4) The conventional mechanical attrition and grinding processes have the disadvantages that powders can only be produced up to a certain fineness and with relatively broad particle-size distribution. As a matter of fact, with the currently familiar large-scale process for manufacturing powders it is rarely possible, or only possible with considerable difficulty, to produce powders having average particle sizes of less than 0.5 &mgr;m (microns).
(5) Most of these prior-art processes do not allow for production of environmentally stable metal particles.
A relatively effective technique for producing fine metal particles is atomization. Atomization involves the breakup of a liquid into small droplets, usually in a high-speed jet. The preparation of high-quality powders, including aluminum, copper alloys, nickel alloys, cobalt alloys, zinc alloys and the like has been achieved by using the atomization technology. The breakup of a liquid stream by the impingement of high-pressure jets of water or gas is referred to as water or gas atomization, respectively. Other commonly used atomization techniques include centrifugal atomization, vacuum atomization, and ultrasonic atomization. By judiciously varying the parameters of the atomization process, the particle size, particle size distribution, particle shape, chemical composition and micro-structure of the particles can be varied to meet the requirements of a specific application.
The major components of a typical atomization system include a melting chamber (including a crucible, a heating device, and a melt-guiding pipe) in a vacuum or protective gas atmosphere, an atomizing nozzle and chamber, and powder-drying (for water atomization) or cooling equipment. The metal melt can be poured into first end of a guiding pipe having a second end with a discharging nozzle. The nozzle, normally located at the base of the pipe, controls the shape and size of the metal melt stream and directs it into an atomizing chamber in which the metal stream (normally a continuous stream) is disintegrated into fine droplets by the high-speed atomizing medium, either gas or water. Liquid droplets cool and solidify as they settle down to the bottom of the atomizing chamber. A subsequent collector system may be used to facilitate the separation (from the waste gas) and collection of powder particles.
Environmentally stable metal alloy particles have been
Jang Bor Zeng
Liu Junhai
Nanotek Instruments, Inc.
Wyszomierski George
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