Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Shaping against forming surface
Reexamination Certificate
1998-11-25
2001-04-17
Ortiz, Angela (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Shaping against forming surface
C264S302000, C205S352000
Reexamination Certificate
active
06217812
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method in accordance with the preamble of claim
1
.
Such a method is disclosed in the PCT application Wo 96/08050.
Various proposals exist in the prior art for producing, for example, a matrix plate in a fuel cell.
The oldest proposal comprises the so-called hot press technique. In a comparatively complicated manner this technique yields comparatively thick plates. The result of this, on the one hand, is an increase in production costs and, on the other hand, a reduction in the efficiency of the cell.
An alternative for producing matrix plates is a process which is similar to the papermaking process. This involves mixing pulp comprising a binder, water and the supporting materials of the matrix. The matrix material then adheres to the pulp, giving rise to floccules. Via the filtration process the floccules are separated from the water material, and the binder is removed by subsequent baking. This process is attractive since the suspension medium used is inexpensive, environment-friendly water. A drawback is its controllability and, in particular, the pore size and the pore size distribution. It did not prove possible to fabricate matrix plates reproducibly in this way on a commercially acceptable scale.
The third most commonly used technique is the so-called “tape casting”.
This involves a suspension in which a binder, fibres and the matrix material are present being cast onto a flat table and the suspension medium present therein being evaporated. A green matrix plate is then produced which can either be sintered before being placed into a fuel cell, after which the electrolyte can be introduced, or is placed directly into the fuel cell. In the latter case the electrolyte is introduced in some other way.
When the suspension was being prepared it was not uncommon to employ vacuum conditions in order thus to regulate the viscosity of the fluid.
Such a method is described, for example, in the article “Review of Carbonate Fuel Cell Matrix and Electrolyte” by H. C. Maru et al. in Proceedings of 2nd Symposium on MCFC Technology, Vol. 90-16, pp. 121-136. The binder described therein was dissolved in a mixture of xylenes and ethanol. When the suspension is heated there is the problem, in particular, of removing xylene. It is no longer permissible for a solvent such as xylene to be discharged into the environment, and recycling leads to a considerable increase in costs.
A similar consideration applies to the use of acrylate as a binder which is dissolved in a mixture of acetone, dichloromethane and petroleum naphtha.
In the case of the method described in the abovementioned PCT application 96/08050 it was found that in the process of forming that suspension, solid particles will agglomerate, resulting in a decrease in the strength of the tape. This is caused by uneven dispersion of the additions and a pore structure which is not well defined. Moreover the introduction of additions such as fibre strengtheners, dissolution retardants and carbonate proved particularly difficult. Upon introduction of such substances agglomeration has likewise been observed, which counteracts the strength, on the one hand, and the dispersal of these substances, on the other hand.
Moreover it was found that this production gives rise to rejects and waste which cannot be put to any further use.
SUMMARY OF THE INVENTION
The object of the present invention is to avoid these drawbacks.
This object is achieved by means of the characterizing measures of claim
1
.
By preparing two ancillary suspensions and then mixing these, it was found, surprisingly, that agglomeration of solids does not occur. Moreover, the above-described additions can readily be added to the suspension in question without the risk of agglomeration or inhomogeneities, as the case may be.
Should one of the two suspensions not meet the set requirements, it can be readily recycled.
The above-described method requires less aftertreatment (such as rolling) and makes it possible for production rejects or waste to be used directly a starting material. In the process, production waste after “tape casting” can be reused by the addition of water/alcohol mixture; the gelled binder present is water-soluble. In the case of other binder systems such as acrylates this is not possible. In this context it must be ensured that the properties of the plate-shaped component obtained are uniform and adjustable, in a simple manner, as a function of the requirements imposed thereon, such as pore size and thickness.
This object is achieved in the case of an above-described method by a first suspension being prepared, comprising an alcohol as the dispersion medium and the base material, by a second suspension being prepared, comprising the binder and an alcohol as the dispersion medium m and water as the gelling medium, and these two separately prepared suspensions being mixed with one another before being cast.
By using a different dispersant gelling agent/binder system, the binder suspension and the suspension which comprises the material of the component being fabricated it proved possible to dispense with organic solvents other than alcohols.
In contrast to the view held in the prior art (see for example the article “Physical Property Optimization of Lithium Aluminates for Fabrication of Molten Carbonate Fuel Cell Matrices” by Patrick M. Brown in Fuel Cell Seminar 1990, pp. 289-293) the addition of water needs not to have a negative effect on the results obtained with a suspension prepared in this manner which is then cast.
It was found that a component which, in the abovementioned manner, was fabricated from suspensions on a water/alcohol basis with the aid of the “tape casting” technique, can be held in intermediate storage for considerably longer, be produced more cheaply and results in less pollution of the environment than a plate which has been fabricated with the aid of an acrylate binder system.
It was found, moreover, that if this dispersant/gelling agent/binder is used, production rejects can be used directly as a starting material.
Employing a dispersing/gelling agent/binder system on the basis of water-alcohol mixtures makes it possible to use suspensions within a wide viscosity range of 500-3500 cPs, preferably 1000-20000 cPs of the various suspensions. As a result it is no longer necessary to employ a vacuum for precise regulation of the viscosity of the suspension.
The amount of water added to the gelling binder suspension is preferably at least 10-25 times the weight of the binder.
If a surfactant is added to prevent coagulation, this is preferably introduced into the first above-described suspension.
The first suspension may comprise any of the granular materials known from the prior art. Examples to be mentioned are: nickel, nickel oxide, nickel aluminum, both alloyed and intermetallic, lithium cobaltate, chromium, lithium aluminate and aluminium oxide, and also other metals or metal oxides in the form of powders or fibres. The solid may also consist of mixtures of the abovementioned substances.
The use in an MCFC cell and more in particular as a matrix plate involves, as generally known in the prior art, (&ggr;-)lithium aluminate. By employing the “tape casting” technique it is possible to cheaply fabricate comparatively thin plate-shaped components which after drying can be used directly in the cell. The low thickness can result in optimum capacity of e.g. a fuel cell.
It is also possible for the plate-shaped component fabricated by means of the “tape casting” technique then to be baked or sintered and then to add an electrolyte to the said component in some way known from the prior art, if the plate-shaped component is employed as an electrode or matrix in an electrochemical cell.
Such an addition may comprise impregnation with e.g. an electrolyte material. It is also possible for a layer of solid carbonate to be placed into a fuel cell between, for example, two plates and for the fuel cell to be heated subsequently, as a result of which the carbonate material, which then melts,
De Rooij Robert
Nammensma Pieter
Ortiz Angela
Stichting Energieonderzoek Centrum Nederland
Sughrue Mion Zinn Macpeak & Seas, PLLC
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