Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2000-04-10
2002-12-03
Copenheaver, Blaine (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S332000, C428S357000, C428S402000, C428S403000, C029S527500, C264S301000, C264S636000, C264S637000, C264S651000, C264S669000, C264S670000
Reexamination Certificate
active
06489017
ABSTRACT:
The invention relates to a method for producing a solid molded article, in particular a ceramic and/or metallic article made of pulverized particles. The invention also relates to stable dispersions of pulverized particles in an aqueous fluid medium, solid molded articles made of pulverized particles, and sintered ceramic and/or metallic molded articles.
High-tech ceramics exhibit a combination of exceptional qualities, which are not attained in this form by any other class of materials. Two of the main reasons why there has not been a breakthrough in the manufacturing and use of such ceramics so far have been the lack of cost-efficient manufacturing methods and the fact that mass-produced components have proved unreliable (Aldinger, Technische Keramik- eine Herausforderung, Keramik-zeitschrift 40 (1988), 312). The failure probability of ceramic components depends on the defect population in the material (Evans and Wiederhorn, Crack Propagation and Failure Prediction in Si
3
N
4
at Elevated Temperatures, J. Mat. Sci. 9 (1974), 270-278). Most defects are introduced into the component during processing and during manufacture of the component (Lange, “Powder Processing Science and Technology of Increased Reliability”, J. Am. Ceram. Soc. 72 (1989), 3-15).
In most of the standard processes used in industrial-scale ceramics manufacturing, the introduction of large defects is intrinsic and thus unavoidable. For example, the use of spray drying for the production of pourable granulated powder for dry press molding results in the formation of agglomerate relicts, which constitute the basis for very large defects in the resulting ceramic components. At the same time, the variables in the production process lead to differing defect populations and thus to varying statistical characteristics.
In order to avoid these defects, one can resort to a technique referred to as “colloidal processing” (cf. Lange (1989), supra). This colloidal processing technique involves the following steps: production of a liquid dispersion of ceramic particles in a medium, filtration to remove impurities, addition of a precipitant in order to obtain a solid product, and subsequent molding.
Specific examples of such processes are the Direct Coagulation Casting (DCC) process, the HAS process and the Vibraforming process. The basic idea behind all of these processes is that, in order to achieve solidification in an aqueous system, the surface charge of powder particles is altered by means of a chemical reaction, or else salt or other compounds have to be added to the system.
In the DCC process (cf. eg, Graule et al., DKG 71 (1994), 317-322; Graule et al., Chemtech 25 (1995), 31-37; Graule et al., Industrial Ceramics 16 (1996), 31-34 and Baader et al., Industrial Ceramics 16 (1996), 36-40), the surface charge of powder particles is increased strongly by addition of citric acid, so that a dispersion with a high volume content of powder particles can be produced. The dispersion also contains urea or modified urea, which is broken down to NH
3
and CO
2
by addition of the enzyme urease. This leads to a rise in pH of the dispersion, and initiates the solidification process. A disadvantage of this process, however, is that the solidification is not reversible. If the dispersion is not poured into the mold quickly enough, the entire slurry has to be disposed of. The same applies to the green body should it become damaged. An added disadvantage is that the green body is very brittle. The DCC process has, moreover, so far been limited to Al
2
O
3
ceramics, being poorly suited, in particular, for nitridic ceramics or mixtures of ceramics.
In the HAS process (see patent application S195 000 73), aluminum nitride is used as additive, and is converted during solidification into aluminum oxide. The process can therefore only be used for ceramics which contain aluminum oxide. With the HAS process, too, fragile molded articles of low mechanical strength are obtained, which cannot be redispersed.
The Vibraforming process (Lange, supra) involves adjusting the forces acting between the powder particles by means of adding salts and other substances, so that the molding compound is almost solid at room temperature and only liquefies—on account of shear forces—when subjected to vibration. The compound is then introduced under pressure into a mold. Disadvantages here are the high salt content and the fact that the molding compound is relatively difficult to handle. The high salt content, eg, NH
4
Cl, causes the release of toxic gases such as HCl and ammonia during burnout, or the presence of undesirable compounds such as metal chlorides in the grain boundary phase of the resulting ceramic. In addition, to achieve complete solidification, the pH of the suspension is altered by means of breaking down urea. Here too, fragile molded articles of low mechanical strength are obtained, which cannot be redispersed.
In another coagulation process, that of Bergstrom (U.S. Pat. No. 5,340,532), organic solvents are used and it is necessary to lower the temperature in order to consolidate the ceramic.
The WO84/02519 describes a method of producing silicon dioxide glass, in which colloidal silicon dioxide is added to a hydrolyzed metal oxide sol, the sol containing the colloidal silicon dioxide gels and is dried to a dry gel, and the dry gel is sintered to form silicon dioxide glass. The solidification of a dispersion of pulverized particles by means of a temperature increase is not disclosed.
The DE-A41 31 335 describes a method of producing ceramic articles, in which a dispersion of pulverized ceramic material is prepared and cast in shape. The raw material is dispersed in a curable organic compound, in particular a synthetic resin, to form a relatively thin dispersion. The dispersion is poured into the mold and allowed to cure prior to demolding. The demolded green body is annealed in an inert gas atmosphere, and the organic compound broken down by way of cracking into volatile components and maybe residual components. The solidification of an aqueous dispersion by means of a temperature increase is not disclosed.
The EP-A-O 260 577 describes a water-soluble polymer for the dispersion of a powder. The solidification of a dispersion of pulverized particles by means of a temperature increase is not disclosed.
The DE-A43 36 694 discloses a method of producing sintered metallic and ceramic articles and layers, in which nanocrystalline ceramic or metallic powder is dispersed—in the presence of at least one low-molecular organic compound with at least one functional group which can react and/or interact with groups present at the surface of the powder particles—in water and/or a polar organic solvent serving as dispersing medium, the dispersing medium is removed and the surface-modified ceramic or metallic powder, having been processed—either before or after removal of the dispersing medium—eg, by means of freeze drying or freeze spray drying, to green bodies or layers, is sintered. Solidification of the dispersion by means of heating is not disclosed.
The DE-A-44 17 405 describes a method of producing textured organic layers on substrates, in which a composition obtained by means of hydrolysis and polycondensation of at least one hydrolyzable silane, at least one organosilane and maybe one or more compounds of glass- or ceramic-forming elements, is applied to a substrate, the applied composition is textured and the textured coating is densified thermally to a textured layer. The production of molded articles by means of increasing the temperature and thus causing a dispersion of pulverized particles to solidify is not disclosed.
The DE-A-195 40 623 describes a method of producing composite materials with a high interfacial fraction, in which a nanoscale filler is dispersed in a polymeric matrix. The filler particles, which may be surface-modified and which have an affinity for the matrix phase and a particle size of not more than 200 nm, are incorporated in the matrix phase in such a way that they are dispersed in an essentially agglomerate-free stat
Aldinger Fritz
Sigmund Wolfgang
Yanez Joseph
Arent Fox Kinter Plotkin & Kahn PLLC
Copenheaver Blaine
Max-Planck-Gesellschaft zur Fulderung der Wissenschaften E.V.
Paulraj Christopher
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