Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Utilizing sol or gel
Reexamination Certificate
1999-11-24
2002-05-07
Derrington, James (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Utilizing sol or gel
C264S650000, C264S658000, C264S660000, C501S012000
Reexamination Certificate
active
06383443
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to alpha-alumina articles and, more particularly, to alpha-alumina articles made using a sol-gel process.
Porous ceramics having interconnected pores ranging in size from nanometers to millimeters have been used as filters, catalyst carriers, acoustic absorbers, membranes, and heat exchangers in various industrial applications. Alumina, silica, mullite, corderite, aluminosilicate, and zeolites are among the commonly used ceramic materials.
Porous alumina is an excellent candidate for many applications, because of its good mechanical strength, thermal stability, and chemical durability. Hollow alumina honeycombs are used in food and beverage processing and in biotechnology for purposes such as removing bacteria in breweries and filtering gases and fluids. The same material also can be used to remove sulfur and nitrogen from coal gasifiers. Alumina materials having small pore sizes also are used as molecular sieves to separate hydrogen from coal gasification gases. Other applications of porous alumina include filters for high temperature gas clean-up and catalyst support for removing NO
x
and SO
x
from flue gases. Recently, porous alumina has been used as a casting mold for slip casting processes.
In a particularly important application, porous alumina substrates are used as the diffusion rate-determining member in thin-film, limiting-current type oxygen sensors for both low and high oxygen concentrations. In such applications, the alumina substrate serves as a diffusion barrier for oxygen transport. When the admission of oxygen is restricted, such as by the porosity and pore size of the substrate, a saturated region is reached in which the sensor output current remains constant despite a voltage increase. This phenomenon occurs because of the rate-determining oxygen transport process from the outside environment, through the porous substrate, and onto the sensor electrode. Consequently, a porous (sintered) alumina substrate can be used effectively as the rate-determining member for the oxygen diffusion process.
Currently, such porous alumina substrates typically are made by tape casting of alumina slurries that incorporate alumina powders of several different particle sizes. This casting process generally leads to an inhomogeneous microstructure in the substrate, with low manufacturing yields, especially for sensor applications. Other disadvantages of the tape casting process include large pore sizes in the substrates and uneven pore size distribution. For the oxygen sensor applications described above, large pore sizes are detrimental because they are outside the Knudsen diffusion regime, causing: (1) loss of linearity between current and concentration at high oxygen concentrations, (2) limited low oxygen concentration detection capabilities (detection lower limit ~100 ppm at ~50 nm pore size, 50 ppm at 30 nm pore size), and (3) slow response time.
The sol-gel process is a well-known technique for making ceramic materials in varying forms such as thin film, bulk, fiber and powder. Boehmite (Al—O—O—H) and pseudoboehmite are good precursors for the fabrication of alpha-alumina-based ceramics. Sol-gel processing with boehmite provides better chemical homogeneity and improved microstructure control. When heated to high temperatures, boehmite transforms to several transition aluminas before the formation of the thermodynamically stable alpha phase, at about 1200° C.
Monolithic alumina also has been made by hydrolyzing aluminum alkoxides, such as aluminum sec-butoxide, at 90° C. Although this process has produced transparent monolithic boehmite gels having small pore sizes and a narrow pore size distribution, the densities of these gels have been unduly low after sintering, i.e., less than a 45% theoretical density, or greater than a 55% porosity.
Boehmite gels of high density originally were considered impossible to obtain at sintering temperatures below 1600° C. However, by seeding the boehmite with alpha-alumina powders, the seed particles can function as nucleation sites that increase the transformation kinetics and decrease the required transformation temperature. Seeded boehmite gel-derived alpha alumina substrates can be sintered to a density of about 99% at temperatures as low as 1150° C. However, monolithic alpha-alumina gels generally have not been obtained directly from gels in this manner, because the gels generally crack during drying. This cracking problem has restricted the development of alumina gels to small dimensions, such as thin-film coatings and abrasive grains.
Thus, although seeding has allowed boehmite gels to be sintered to a high density, the fabrication of alumina monoliths generally has required the cracked gel fragments first to be ground into powder and then pressed into pellets, for subsequent consolidation into dense compact forms before sintering. Therefore, a powder-dispersion-consolidation process still has been required to fabricate dense alumina monoliths.
It should therefore be appreciated that there is a need for a practical method for producing monolithic porous alumina articles having small, well-defined pore radii. The present invention fulfills this need.
SUMMARY OF THE INVENTION
The present invention resides in a high-density, crack-free monolithic alpha-alumina article, and a sol-gel process for making it, having small, well-defined pore diameters. The process includes: (1) casting in a mold a solution of an aluminum alkoxide (e.g., aluminum sec-butoxide), alpha-alumina powder, water, and a peptizing agent, (2) drying the cast solution in a controlled environment having a temperature in the range of about 25 to 40° C. and a humidity in the range of about 75 to 95%, to produce a dried gel, and (3) sintering the dried gel, to produce the high-density, crack-free monolithic alpha-alumina article.
In a separate feature of the invention, the process uses a mold formed of a material selected from the group consisting of PMP (polymethylpentene), Teflon PFA (perfluoroalkoxy resin), Teflon FEP (fluorinated ethylenepropylene copolymer), and Teflon PTFE (poly tetrafluoroethylene polymer). The entire mold, or alternatively merely an inner liner of the mold, can be formed of such material. Further, sintering occurs at a temperature in the range of about 1000° C. to about 1400° C., and preferably in the range of about 1000° C. to about 1100° C., for a time period in the range of about 3 to about 12 hours. In addition, casting can include preliminarily applying a mold release agent, preferably a silicone agent, to the mold. Casting and drying can occur using the same mold.
The resulting high-density, crack-free monolithic alpha-alumina article has a density in the range of about 83 to 98%, with pores having diameters in the range of about 8 to 120 nanometers. The article, thereby, is suitable for use as a substrate for such devices as a gas sensor device, e.g., an oxygen sensor device. The preferred article has a density greater than about 95%, with pores having an average radius of less than about 30 nanometers. In addition, the article preferably has the shape of a disk, with a diameter greater than about 25 millimeters.
Other features and advantages of the present invention should become apparent from the following detailed description of the invention, which discloses, by way of example, the principles of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention resides in a sol-gel process for making high-density porous alpha-alumina articles, of theoretical density in the range of about 83 to 98%. The gels are produced by casting a suitable aluminum alkoxide (e.g., aluminum sec-butoxide or aluminum iso-propoxide), alpha-alumina, water, and a suitable peptizing agent (e.g., nitric acid, hydrochloride acid, or ammonium hydroxide). Cracking of the gels during drying is prevented by controlling the temperature and humidity of the drying condition and by using a mold formed of PMP (polymethylpentene), Teflon PFA (perfluoroalkoxy resin), Teflon FEP (fluorinated ethylene pr
Chen Chorng-Jeou
Jeng De-Yin
Suzuki Takayuki
Brueggemann James R.
Derrington James
Sheppard Mullin Richter & Hampton LLP
Yazaki -Corporation
LandOfFree
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