Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1999-01-04
2001-01-09
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S294000, C429S047000, C429S047000, C429S047000
Reexamination Certificate
active
06171460
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to improved ceramic electrodes and more particularly to the chemical protection of ceramic electrodes by coating and infiltrating with polymers.
Background—Description of the Prior Art
A new class of composite materials is that which combines ceramics with polymers. Typically, the chemically disparate ceramic and polymer components are combined in powdered form prior to heating, implosive compaction, or other form of processing. U.S. Pat. No. 4,146,525 to Stradley discloses a composite material formed by low temperature fusing of finely divided ceramic powders and a powdered polymer such as epoxy. U.S. Pat. Nos. 4,726,099 and 4,933,230 to Card, et. al. discuss creation of fiber layers of piezoelectric ceramic that are subsequently immersed in polymer. U.S. Pat. No. 5,109,080 to Wang, et. al. discloses an optically transparent composite formed from a combination of refractive index-matched ceramic and polymer. The composite material is formed by sol-gel synthesis of a metal alkoxide and an alkoxysilane polymeric component. In contrast to these methods of forming ceramicpolymer composites, the subject of the present invention pertains to the polymer impregnation of the porous structure of a previously formed ceramic article. In the formation of ceramics that preclude the initial combination of ceramic and polymer materials, impregnation of bulk porous ceramic with the polymer must indeed be pursued. Recently, Vipulanandan, et. al.
1
have successfully fabricated a thermoplastic polymer (polymethylmethacrylate) impregnated ceramic. The uncured polymer had a viscosity less than that of water and was simply “poured into” the ceramic. The ceramic was a high temperature superconductor and impregnation was for the purpose of increasing the mechanical strength of the ceramic. Low and Lim
2
have impregnated a YBa
2
Cu
3
O
7−x
superconductor using an epoxy resin with curing agent. A thesis by Lim
3
discusses the protection afforded by the epoxy to aqueous phase decomposition of the ceramics in water, acid or alkali. The work of Vipulanandan, et. al. and Lim and Low does not address the impregnation of ceramics of small pore size. Their work does not make use of impregnation liquids having high viscosity. In many applications, the salient feature of the impregnation liquid may be resistance to chemical attack and low viscosity versions of these liquids may be unavailable.
U.S. Pat. No. 4,892,786 to Newkirk discloses a method of making a composite material by reacting a molten metal with an oxidant to form a porous non-sintered ceramic and then infiltrating the ceramic with a polymer. Newkirk teaches a polymer infiltration process used with this non-sintered ceramic having a mean pore size on the order of 25 microns. The ceramic was first surrounded with polymer liquid then iteratively exposed to one half atmosphere of air pressure in an effort to withdraw air entrapped within the ceramic and then one atmosphere of air pressure to cause infiltration of the polymer into the ceramic. There are a number of reasons why the method disclosed by Newkirk will in general not be applicable in the case of viscous polymer-forming liquids with ceramics having small (microns or smaller) or even moderate pore sizes (tens of microns). The reasons are both physical and chemical as described next.
The flow rate of a viscous liquid through a porous material is approximated by Poiseuille's equation:
Q/A=(&pgr;&Dgr;p/8 &mgr;&dgr;)&Sgr;d
i
4
where Q/A is the volumetric flow rate per unit area of material, &Dgr;p is the pressure drop across the slab of material, &mgr; is the liquid viscosity, &dgr; is the slab thickness, and d
i
represents the individual pore diameters within the area A. As is shown by the above equation, flow rate at a given pressure is proportional to the fourth power of pore diameter. Therefore, ceramics with small pore sizes require extreme overpressures to achieve impregnation in reasonable times. As an example, using a polymer-forming liquid with a relatively low viscosity of 200 centipoises for impregnation of a centimeter thickness of ceramic having a mean pore diameter of 1 micron will require close to 1500 psi of pressure to achieve impregnation within an hour's time. Hence, the viscosity of most polymer-forming liquids will require significant overpressure to achieve impregnation of moderate pore size ceramics in practical manufacturing timeframes. Additionally, for curing techniques that elevate the polymer precursor and ceramic temperatures, residual air that would remain in the ceramic in the process disclosed by Newkirk would ultimately cause oozing of the polymer from the porosity. This would be caused by expansion of entrapped air. Such residual air can also cause secondary porosity as air bubbles migrate to external ceramic surfaces and form channels within the polymer. Other contaminants are adsorbed into the surface of the ceramic and may require heating for removal. An example is that of water which is adsorbed by most metal oxide ceramics; if the adsorbed water is not removed it can interfere with the polymer activating agents. Finally, the overpressure should be provided by a “chemically inert” gas. This would not necessarily be a noble gas such as argon, but a gas that would not react with the polymer or the ceramic host—for example dry nitrogen may be acceptable for many situations. An example of the deleterious effects caused by a chemically active pressurizing gas is the improper curing of polymer precursors such as polyester formulations. Many of these use oxidizing activators; hence, premature curing will ensue as the polymer precursor is exposed to oxygen if the pressurizing gas is air.
Among the categories of ceramics that invite the use of polymer impregnation are electrically conductive (EC) ceramics used in electrode applications
4
. Electrically conductive ceramic electrodes are applicable to magnetohydrodynamic (MHD) systems, fuel cells, electro-remediation of soils and groundwater, batteries, cathodic protection systems, electrostatic discharge systems, and electrochemical cells. Electrochemical cells are useful in the chemical breakdown of compounds, electrolysis; the refining and production of metals, electrowinning; creation of compounds, electosynthesis; and the separation of chemical species in aqueous solution; electrodialysis. A prime motive for the selection of ceramic electrodes for use in electrochemical cells is the improvement in corrosion resistance offered by these materials over metals and carbon. An example of one such chemically resistant ceramic is variable stoichiometry titanium oxide. Given the relative chemical resistance of such sintered ceramics, porosity of these materials remains problematic for many electrode applications. Migration of corrosive agents into the porous voids can degrade the lifetime of a given ceramic electrode. When in operation, the ceramic admits the influx of corrosive chemical species that may be present in given electrolyte solutions. Non-ionic corrosives will diffuse into the electrode, however corrosive ionic species, by virtue of electomigration can achieve concentrations in the bulk of the electrode material that are well above corresponding concentrations in the electrolyte external to the electrode. The tendency for these ionic species to concentrate (due to electromigration) in the proximity of the metal conductor connection to the ceramic electrode can dramatically reduce the lifetime of the electrical connection. Furthermore, high concentrations of the ionic corrosive in the bulk of the ceramic electrode can degrade the chemical and structural integrity of the ceramic. It is a purpose of the present invention to provide chemical protection to porous ceramic electrodes by means of impregnating materials in order to significantly extend the operational lifetime of this class of electrodes.
Prior art methods for providing electrodes in general with improved chemical resistance include physical-chemical treatment of the electrod
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