Specialized metallurgical processes – compositions for use therei – Processes – Producing solid particulate free metal directly from liquid...
Reexamination Certificate
1998-02-24
2001-08-21
Wyszomierski, George (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing solid particulate free metal directly from liquid...
C075S338000, C075S346000, C075S351000, C075S369000
Reexamination Certificate
active
06277169
ABSTRACT:
FIELD OF THE INVENTION
The present invention involves an aerosol method for producing silver-containing particulate materials, powders including silver-containing particles producible by the method, uses of the powders in the manufacture of electronic and other devices and the devices so manufactured.
BACKGROUND OF THE INVENTION
Silver particles are widely used in the manufacture of electrically conductive thick films for electronic devices and for other uses. Examples of thick film applications for silver particles include to make internal electrodes in multi-layer capacitors; interconnections in multi-chip components; conductive lines in auto defoggers/deicers, photovoltaic modules, resistors, inductors, antennas and membrane switches; electromagnetic shielding (such as in cellular telephones), thermally conductive films; light reflecting films; and conducting adhesives. A trend continues, however, to produce ever smaller particles for these and other applications. Generally, desirable features in such small particles include a small particle size; a narrow particle size distribution; a dense, spherical particle morphology; and a crystalline grain structure. Existing technology for manufacturing silver-containing particles could, however, be improved with respect to attaining all, or substantially all, of these desired features for particles used in thick film applications.
One method that has been used to make small particles is to precipitate the particles from a liquid medium. Such liquid precipitation techniques are often difficult to control to produce particles with the desired characteristics. It is especially difficult by the liquid precipitation route to obtain particles having dense, spherical particle morphology and with good crystallinity.
Aerosol methods have also been used to make small silver particles. One aerosol method for making small particles is spray pyrolysis, in which an aerosol spray is generated and then converted in a reactor to the desired particles. Spray pyrolysis systems involving silver have, however, been mostly experimental, and unsuitable for commercial particle production. Furthermore, control of particle size distribution is a concern with spray pyrolysis. Also, spray pyrolysis systems are often inefficient in the use of carrier gasses that suspend and carry liquid droplets of the aerosol. This inefficiency is a major consideration for commercial applications of spray pyrolysis systems.
There is a need for improved silver-containing powders, for improved methods of manufacturing silver-containing powders and for improved products incorporating or made using improved silver-containing powders.
SUMMARY OF THE INVENTION
The present invention provides high quality, micro-size silver-containing particles of a variety of compositions and particle morphologies, and with carefully controlled particle size and size distribution, and an aerosol method for producing the particles. The particles are useful for making a variety of products.
Through careful control of the aerosol generation, and particularly when used in combination with other features of the present invention, the present invention permits preparation of very high quality powders of silver-containing particles that preferably have a weight average size in a range of from about 0.1 micron to about 4 microns, and for many applications with a range of from about 0.5 micron to about 2 microns in size. The powders have a narrow size distribution such that typically at least about 90 weight percent of the particles are smaller than about twice the weight average size of the particles. Furthermore, typically less than about 10 weight percent, and preferably less than about 5 weight percent, of the particles are smaller than about one-half the weight average size.
The invention includes both single-phase and multi-phase, or composite, particles useful for a variety of product applications, including for use as catalysts and in the preparation of thick film paste formulations, such as are used for depositing silver-containing films during manufacture of various electronic and other products. Multi-phase materials may be in a variety of morphological forms, for example in an intimate mixture of two or more phases or with one phase forming a surface coating over a core including another phase.
One preferred class of multi-phase particles includes a metallic silver-containing phase and a nonmetallic phase, which frequently includes a ceramic material. The nonmetallic phase could be in the form of a coating around a core of the metallic phase, in the form of small domains dispersed in a matrix of the metallic phase, or in some other form. A variety of ceramic materials can be used to effect a variety of beneficial modifications to particle properties, such as a modification of densification/sintering properties for improved compatibility and bonding with ceramic layers used in electronic devices, and especially when the silver-containing powder is used to make internal electrodes in multi-layer capacitors. An important use of these multi-phase particles is to reduce or delay film shrinkage during firing in the manufacture of electronic devices. One preferred group of ceramic materials for use in the multi-phase particles are titanates, as are frequently used in dielectric layers of multi-layer capacitors. Another preferred group of ceramic materials for use in multi-phase particles includes silica, alumina, titania and zirconia.
The silver in the powder, whether in single phase or multi-phase particles, is typically present in a metallic phase. In one embodiment, the particles in the powder include a high quality silver alloy, with one preferred alloy being with palladium. It has been found that the quality of the alloy is highly dependent upon the manner in which the process is carried out and the specific processing conditions. When preparing particles including a silver/palladium alloy, the reactor temperatures should preferably be in the range of from about 900° C. to about 1200° C., with even narrower temperature ranges being more preferred.
The process of the present invention for making the silver-containing particles involves processing of a high quality aerosol including a silver-containing precursor. The aerosol includes droplets of controlled size suspended in and carried by a carrier gas. In a thermal reactor, typically a furnace reactor, the liquid of the droplets is vaporized, permitting formation of the desired particles in an aerosol state. According to one embodiment of the present invention, an aerosol at a high droplet loading and at a high volumetric flow rate is fed to a reactor, where particles are formed. In addition to the high droplet loading and high volumetric flow rate, the aerosol also includes a narrow size distribution of droplets such that the particles exiting the reactor also have a narrow size distribution, with preferably at least about 75 weight percent, and more preferably at least 90 weight percent, of the particles being smaller than about twice the weight average particle size.
With the process, and accompanying apparatus, of the present invention, it is possible to produce high quality silver-containing powders at a high production rate using spray pyrolysis. This represents a significant advancement relative to the small laboratory-scale processes currently used.
An important aspect of the method of the present invention is aerosol generation, in which a high quality aerosol is produced having a controlled droplet size and narrow droplet size distribution, but at a high volumetric flow rate and with high droplet loading. An ultrasonic generator design is provided for generation of the high quality aerosol at a high generation rate.
The aerosol generation is Particularly advantageous for producing aerosol droplets having a weight average size of from about 1 micron to about 5 microns, preferably with no greater than about 30 weight percent of the droplets being larger than about two times the average droplet size.
High quality aerosol production is accomp
Caruso James
Chandler Clive D.
Hampden-Smith Mark J.
Kodas Toivo T.
Powell Quint H.
Marsh & Fischmann & Breyfogle LLP
Superior Micropowders LLC
Wyszomierski George
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