Compositions: ceramic – Ceramic compositions – Pore-forming
Patent
1996-11-04
1998-01-06
Marcheschi, Michael
Compositions: ceramic
Ceramic compositions
Pore-forming
501 84, 501 85, 252306, 252307, 252310, 252350, 264 42, 264 51, C04B 3800, C04B 3810
Patent
active
057054487
DESCRIPTION:
BRIEF SUMMARY
This application is pursuant to 35 U.S.C. .sctn. 371 entitled to the priority date of International Application No. PCT/GB95/01063, filed May 10, 1995.
The invention relates to the production of porous articles and is an improvement in or modification of the invention disclosed in our patent application PCT/GB92/01493 published under number WO 93/04103 on Mar. 4, 1993. All of the disclosure of that specification is hereby incorporated herein.
The method described and claimed in the earlier application comprises the step of forming a dispersion comprising particles of refractory material in a liquid carrier, introducing gas into the dispersion and removing the liquid carrier to provide a solid article having pores derived from the bubbles of gas, characterised by controlling the critical viscosity of the dispersion to be from about 5 mPa.s, (the level below which the entrapped gas bubbles will tend to escape), to about 1000 mPa.s, (the level above which the entrapped gas bubbles cannot be introduced). The critical viscosity is preferably from about 20 mPa.s to about 250 mPa.s. After the gas is introduced the liquid carrier is removed, typically water is driven off by heat.
One optional step is the inclusion of a polymerisable monomer to have an effect when the liquid carrier is removed. Soluble acrylates and acrylamides are given as suitable additives and they are polymerised by the use of catalysts and/or heat.
There may be a need in certain circumstances for a porous article in which the pores are highly interconnected. This invention is based on the appreciation that by carrying out foaming in a defined way such articles or articles having predetermined structure of pores can readily be formed.
According to the invention in one aspect there is provided a method of making a porous refractory article, the method comprising the steps of: polymerisable monomer component which on polymerisation generates an exotherm substance generates the gas before or during the initial stage of polymerisation of the monomer component whereby the gas forms bubbles which under the heat of the exotherm interconnect to form an interconnected open pore structure.
The thermally decomposable gas generating substance will, under the appropriate conditions, generate the gas in situ. The temperature at which the gas is generated depends on the substance in question, and the overall conditions, e.g. temperature and pressure. The substance is preferably one which after decomposition leaves no residues which in the context tend to contaminate, decomposes at a pH of between about 7.5 and 9, and does not generate free oxygen which would inhibit polymerisation. Preferred are ammonium carbonate, ammonium bicarbonate, ammonium carbamate; and the like. Metallic carbonates can also be used where the presence of the residual metal oxide will have no adverse effect.
It is an advantage of this invention that by controlling the temperature and pressure the decomposition rate of the gas generating substance is controlled. The foaming reaction could then be triggered by the increase in temperature associated with the onset of polymerisation. This triggering process requires control over the dispersion temperature and pressure prior to the onset of polymerisation and the use of a catalyst. Alternatively the dispersion could be stored at a predetermined pressure and temperature and microwave energy can be used to induce polymerisation and decompose the gas generating substance.
The particles can be derived from a wide variety of materials such as metals, metallic oxides, non-metallic ceramics and cermets. Examples, given for illustration and not limitation, include alumina, cordierite, magnesia, mullite, silica, silicon carbide, silicon nitride, tungsten nitride, zirconia; and the like.
In one optional step, foaming is carded out at atmospheric pressure using ammonium carbonate or the like as the gas generating substance by using an initiator which induces polymerisation at a temperature higher than that at which ammonium persulphate operates. Such i
REFERENCES:
patent: 5082607 (1992-01-01), Tange et al.
patent: 5563106 (1996-10-01), Binner et al.
Sambrook Rodney Martin
Smith Robert Terence
Dytech Corporation Limited
Marcheschi Michael
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