Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Particular or specific manner of positioning – arranging – or...
Reexamination Certificate
1999-12-28
2002-09-10
Derrington, James (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Particular or specific manner of positioning, arranging, or...
C264S607000, C264S671000, C264S672000, C264S673000, C428S325000
Reexamination Certificate
active
06447712
ABSTRACT:
BACKGROUND OF THE INVENTION
Cracking, warping, and surface deterioration often occur during firing of ceramic tapes. In addition, for tapes containing volatile components out-diffusion of materials can be a significant problem. A variety of approaches have been applied in attempts to solve these problems. However, no solution has been completely satisfactory. In a conventional approach a flat refractory cover plate (or setter) is placed on top of the unfired (“green”) ceramic tapes in order to prevent severe warping or wrinkling of the tapes during bum-out of organic binders in the tapes and sintering of the tapes at high temperatures (M. Asakura et al. (1982) Annual Report of the Engineering Research Institute Faculty of Engineering, University of Tokyo, Vol. 41, pp. 1 85-190). The cover plates used must be heavy enough to press the green ceramic tape flat during the sintering process, but the weight of the cover plate generates shear forces on the faces of the relatively fragile tape as it contracts (Kaga et al. U.S. Pat. No. 5,620,637 and Sawada et al. U.S. Pat. No. 5,527,501). The forces imposed on the tapes by the use of the cover plate makes them very susceptible to fracturing and destruction, particularly if the composition contains a volatile or liquid phase, such as PbO. In addition, the cover plates, if not completely chemically inert and compatible with the tapes at high temperatures, can bond or stick to the ceramic tapes during firing.
Many commercially important ceramic tapes have a volatile phase, such as PbO in piezoceramic-based tapes, e.g., PZT or PLZT ceramic tapes. At high temperatures, PbO will readily diffuse into the cover plate from these tape, resulting in a lead-deficient PZT or PLZT tape with very poor functional properties after firing. Loss of PbO from these materials adversely affects electromechanical coupling factors and dielectric constants (Webster A. H. et al. (1967) J. Am. Ceram. Soc. 50(9):490).
One way to prevent or decrease cracking and sticking is by coating the green ceramic tape with a suitable dry refractory ceramic powder (setter powder) by sprinkling, sifting, rubbing and/or brushing the ceramic powder onto and between green tapes or other shaped green ceramic articles (e.g., Fries, R. and Moulson, A. J. (1994) J. Mater. Sci.: Mater. in Electronics 5:238). One major problem with this method is non-uniformities in powder dispersion which lead to surface deformations and defects in the thin ceramic plates that cannot be tolerated in many applications. It is not practical to disperse powders uniformly over the delicate, thin, unfired tape and keep the powders uniformly dispersed as the tape is covered with the cover plate, transported and then fired. The use of loose powders in tape processing is additionally disadvantageous because it is inherently a slow and tedious process and can be wasteful with respect to the powders employed. Further, the use of loose powders, which can become airborne, in a manufacturing process can represent a respiratory hazard.
A second approach to improving firing of ceramic tapes or other shaped articles is the use of a solvent to disperse a thin layer of setter powder on tapes by dipping or wet spraying the tapes prior to firing. This method may be used to achieve acceptable uniformity. The solvent used must, however, be carefully selected. The solvent can exhibit poor wetting characteristics that result in particle agglomeration or it can dissolve binders and additives in the green tape destroying the integrity of the tape.
Other methods for protecting tapes rely on application of a thin ceramic coating to the cover plate, usually in the form of a relatively inert layer of zirconia or other refractory ceramic or as a thin platinum coating or foil (Hind, D. and Knott, P. R. (1997) in
Electroceramics: Production, Properties and Microstructures,
W. E. Lee and A. Bell (Eds.), The Institute of Materials, London, UK, p. 107; and Stevenson, J. W. et al. (1994) J. Am. Ceram. Soc. 77(9):2481). Platinum foils, refractory ceramic plates or setter powder can also be placed between stacked ceramic tapes during firing. These methods tend to have high materials and manufacturing costs. Some manufacturers supply cover plates with textured surfaces to provide only “point contacts” with the green tapes. These methods attempt to eliminate the need for uniformly disperse powder, prevent out-diffusion and unwanted bonding of tapes to the cover plate. A major limitation of these methods, however, is that they are not effective for use with large area ceramic tapes with thicknesses of a hundred microns or less that are required in many important current applications. The inherent shear forces and increased friction produced by cover plates having permanent barrier coatings or textured surfaces are still too high for more delicate thin ceramic tapes and their use results in cracked and broken tapes.
In a related method, Busse et al. in U. S. Pat. No. 5,359,760 report the use of solid ceramic separator plates (setters) with high PbO content to prevent out-diffusion of volatile PbO from PLZT and PZT ceramic tapes. These plates or setters lose lead and must be replaced relatively frequently and cannot maintain their flatness. Further, these solid separator cover plates are not expected to be useful for large area very thin tapes because the friction and shear forces remain too high.
As noted above, loss of PbO from PLZT and PZT tapes is a significant problem during sintering. A common way of reducing PbO loss is to place the PLZT or PZT green tapes in a closed crucible surrounded with lead-containing atmosphere powders (Snow, G. S., (1973) J. Am. Ceram. Soc. 56(2):91; Snow, G. S., (1973) J. Am. Ceram. Soc. 56(9):479; Kingon, A. I. and Clark, J. B. (1983) J. Am. Cerm. Soc. 66(4):253). Other approaches have focused on decreasing sintering temperature by adding liquid phase agents (Wittmer, D. E. and Buchanan, R. C. (1981) J. Am. Cerm. Soc. 64(8):485; Cheng et al. (1986) J. Mater. Sci. 21:571; Zhilun et al. (1989) J. Am. Cerm. Soc. 72(3):486). The presence of a liquid phase promotes densification kinetics during sintering (James, A. D. and Messer, P. F. (1978) Trans. J. Br. Ceram. Soc. 77(5):152; and German, R. M. (1985)
Liquid Phase Sintering,
Plenum Press, New York, N.Y.). Excess PbO can act as a liquid phase agent and has been reported to affect grain size and density of piezoceramics (James, A. D. and Messer, P. F. (1978) Trans. J. Br. Ceram. Soc. 77(5):152; Chian, S.-S. et al. (1981) Am. Ceram. Soc. Bull. 60(4):484). It has also been reported that high amounts of excess PbO can only increase densification rates in the early stage, and will lower the final density of ceramic bodies due to evaporation of PbO at elevated temperatures (Kingon, A. I. and Clark, J. B. (1983) J. Am. Ceram. Soc. 66(4):256).
PLZT and PZT tapes are often co-fired in multiple layer stacks. In this case, the tapes may stick together and be difficult to separate without fracturing. This problem can be solved by placing platinum foils, solid PZT plates, or setter powder between tapes and between the tapes and the cover plates. The use of platinum foil and PZT plates is not economical for mass production of tapes and the disadvantages of the use of loose setter powder has been discussed above.
The present invention provides an improved method and low cost buffer sheets useful in the method for firing ceramic tapes or other shaped articles, particularly thin tapes and articles with thin walls, and particularly those combining large surface area with thin and more particularly for PLZT and PZT tapes.
SUMMARY OF THE INVENTION
The present invention provides an inexpensive, non-toxic, simple and reliable method for obtaining uniform and thin layers of particles or powders that are particularly useful an anti-sticking and anti-friction coatings during the fusion and densification (or sintering) of ceramic tapes and other shaped articles. The method is particularly useful for sintering of large area, very thin ceramic tapes. In application to very thin tapes and
Dogan Fatih
Feng Jian-Huei
Ferguson Lucian G.
Derrington James
Greenlee Winner and Sullivan P.C.
University of Washington
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