Process for making LaMnO3-coated ceramics

Details Process for making LaMnO3-coated ceramics Process for making LaMnO3-coated ceramics

1995-06-05

2001-12-25

Copenheaver, Blaine (Department: 1771)

Plastic and nonmetallic article shaping or treating: processes

Outside of mold sintering or vitrifying of shaped inorganic...

Utilizing sol or gel

C427S595000, C427S597000

Reexamination Certificate

active

06333000

ABSTRACT:

TECHNICAL FIELD
This invention relates generally to a process for making ceramics coated with LaMnO
3
or a related ceramic, to ceramics rigified by LaMnO
3
-family sol-gels, or to ceramics containing LaMnO
3
-family ceramics.
BACKGROUND OF THE INVENTION
Superconducting ceramic oxides (i.e., the IBM 1-2-3 superconductors) are perovskite ceramics and are a recent technological breakthrough with promising applications in a wide range of areas. Superconductors are materials which transmit electricity without significant resistive losses and can sustain high magnetic fields when cooled below their superconducting transition temperature, T
C
. The new class of superconducting ceramic oxides (sometimes also called superconducting mixed metal oxides or superconducting metal oxides) typically exhibit superconductivity at ambient pressure above 77° K. (−321° F.), the temperature of liquid nitrogen, which signifies the ability to prepare and maintain superconductive materials now in virtually any laboratory. Being superconductors at “high” temperatures is a tremendous advantage since the previously known superconductors only exhibited this property if they were cooled with liquid helium, which is expensive and difficult to handle.
Using superconducting ceramic oxides, it is often difficult to produce large, mechanically stable, complex shapes, or any stable shapes for that matter. Hence, commercial adoption of these superconductor ceramic oxides has been slower than the promise and potential foreseen when they were first discovered. The superconducting ceramic oxides are brittle, difficult to handle without damaging the ceramic (the materials lack glass formers and have low tensile strengths), and particularly difficult to form into a wire or fiber, the most desired form for high current applications. For small scale applications (such as for microcomponents for electronic devices) low current carrying superconducting ceramic oxides may be made in the form of single crystals. The technology for making large single crystals suitable for high current industrial, however, uses is not yet practical.
U.S. patent application Ser. No. 07/381,498 (abandoned), describes a method of manufacturing superconductive fiberformed ceramic composites which exhibit superconductivity at liquid nitrogen temperatures, which do not require high temperature consolidation of the superconducting metal oxide powder, and which can be produced in large complex shapes at relatively low cost. The present application describes related perovskite fiberform ceramics or coated ceramics that substitute sols from the LaMnO
3
-family for the superconductive sols earlier used. The family of LaMnO
3
sols includes pure LaMnO
3
sols and those sols doped with varying amounts of strontium or chromium or both. The dopants provide charge carriers to produce sols and corresponding ceramic oxides having tailorable electrical conductivities as reported by R. Koc in his Master's Thesis: “Structural, Sintering and Electrical Conductivity Studies of the LaCrO
3
-LaMnO
3
System, May 1986, University of Missouri-Rolla, incorporated by reference.
SUMMARY OF THE INVENTION
The present invention provides a process for forming a novel class of ceramics and the resulting products. The process involves, in one preferred embodiment, forming a slurry of ceramic particles in a suitable carrier; dispersing the slurry over a form-defining porous surface; felting out of the slurry a mat of the ceramic particles; drying the mat; contacting the dried mat with a LaMnO
3
-family sol; and drying and curing the sol and mat to form the fiberform or microform ceramic. The ceramic particles may be in fiber or particle form, including microballoons, and can be mixtures of fibers and particles.
Another embodiment of the method includes forming the slurry of ceramic particles; forming a mat (i.e., a felted fiberform or microform ceramic) from the slurry; drying the mat; contacting the mat with a sol-gel ceramic precursor binder; drying and curing the mat to convert the precursor to a ceramic; and finally contacting the ceramic composite with a LaMnO
3
-family sol which is subsequently cured and converted to the coating on the fiberform or microform.
Another embodiment involves making a fiberform or microform ceramic or a syntactic foam using some or all coated ceramic particles. The coated particles are made by contacting the particles with a LaMnO
3
-family sol to form the coating, drying the coating, and, optionally, sintering the coating. The coated particles are then either dispersed in a slurry for felting a fiberform/microform ceramic or mixed with a suitable organic resin to form a syntactic foam. If felted, the mat is densified or made rigid with a suitable sol-gel binder. If mixed with the resin, the resin is cured to complete the syntactic foam. Coated particles comprise all or part of the particules in either product.
A microwave sintering method for obtaining the correct crystal structure of the ceramic is also described. The process is effective because the perovskite structure is a strong absorber of microwave energy so rapid heating occurs once the conversion begins. Therefore, the process is easier to use than conventional resistive heating to the 800-2400° F. (427-1316° C.) temperatures needed for the sintering.


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Proceedings of the 4thAnnual Conference on fossil energy materials published Aug. 1990, Janney et al “Microwave sintering of fuel cell materials”, compiled by Oak Ridge National Laboratory.*
Eror, et al.,Polymeric Precursor Synthesis of Ceramic Materials, Proc. Mat'ls Res. Soc.,Better Ceramics Through Chemistry(Brinker et al. [ed.]), 1986.

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