Compositions: ceramic – Ceramic compositions – Pore-forming
Reexamination Certificate
2002-05-15
2003-12-23
Derrington, James (Department: 1731)
Compositions: ceramic
Ceramic compositions
Pore-forming
C501S155000, C264S043000, C264S628000, C264S669000, C264S125000
Reexamination Certificate
active
06667261
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to heat-resistant, acid-resistant, low-density open-cell porous materials, which are made from hollow microspheres, permeable for gas and liquids. Microspheres are cellulated glass hollow or solid microballoons, also known as spherical glass crystalline particles. Cenospheres are a particular class of hollow microspheres which are a component of fly ash obtained from the combustion of coal. The open-cell glass crystalline porous material of the invention is made from cenospheres, and has properties useful as porous matrices for immobilization of liquid radioactive waste, heat-resistant traps and filters, supports for catalysts, adsorbents and ion-exchangers.
Open-cell porous ceramic materials have been prepared in the prior art by means of foaming melts with the use of gas evolving additives, impregnation of ceramics on to a plastic network skeleton, and consolidation of different primary units (granules, fibers etc.). The meaning of “open-cell” porous materials used herein is porous materials with accessible internal voids composed of both voids between microspheres and voids inside the microspheres. Formed porous ceramic bodies differ considerably in their properties including texture (cellular or granular), open porosity, size of openings, and hydro- and aerodynamic resistance. For example, while the open porosity of cellular ceramics is up to 96 vol. %, the open porosity of granular materials is limited to about 40 vol. %. In spite of this, the porous structure of granular ceramics can be controlled more precisely by the shape and size of the primary units, especially in the case of microspherical particles. Other advantages of porous ceramics formed by microspheres are high compression strength and deformation ability.
The prior methods of forming porous ceramic bodies from microspheres were aimed predominantly at the creation of structural and insulating materials of small open or completely closed porosity, such as disclosed in U.S. Pat. Nos. 3,458,332, Re. 25,564, 4,016,229, 4,035,545 and U.S. Statutory Invention Registration (SIR) No. H200.
Heat-resistant porous structural materials of a 30-35% porosity comprising 50-75 vol. % of microspheres, 10-200 micrometers in diameter, of high-melting point oxides such as ZrO
2
, Al
2
O
3
, Y
2
O
3
, are disclosed in U.S. Pat. No. 4,035,545. The microspheres are sintered directly to each other so that the diameter of their contact amounts to 0.2-0.5 of the microsphere diameter. Composition of the material can incorporate 20-50 vol. % of a filler as metal, metal alloy, intermetallic compound, phenol-formaldehyde resin, polyvinyl alcohol, glass etc. The process steps for making the heat-resistant porous structural material include the plasma processing of the powdered high-melting point oxides to form microspheres, molding and isothermal sintering in an oxygen-gas-fired furnace at 1850-2100° C. for 5-7 hours. The disadvantages of making such porous granular ceramic material are the high cost of initial components, high power consumption and complexity of the process.
U.S. Pat. No. 3,458,332 discloses the preparation of porous glass agglomerates of ⅛ to ½ inch (3,175-12,700 micrometers) in diameter by sintering a mass of hollow glass microspheres with diameters of 5-5,000 micrometers and an alkalinity in the range from 0.103 to 0.192 milliequivalent per gram. According to the method, agglomerates of glass microspheres are formed by fusing the microspheres to each other at their points of contact by subjecting them to a temperature of 900-1100° F. (482-593° C.). No information was given about the porosity.
Closed-cell porous insulating materials have been prepared from hollow glass or ceramic microspheres. “Closed-cell” is intended to mean that porous materials have internal voids with closed walls which are not permeable for gas and liquids Porous lightweight ceramic bodies are disclosed in U.S. Pat. No. 3,888,691. These porous bodies have a comparatively high strength per unit of weight, obtained by mixing hollow glass spheres with refractory components, including refractory particles (lithium-aluminum silicate) and a binder (calcium aluminate cement and/or colloidal silica). The mixture is formed into a solid body and heated at below the softening temperature of the refractory particles and above the melting temperature of the glass within the spheres, in order to cause the glass to be drawn into the composition. As a result, closed spherical pores are formed in the ceramic body.
The porous material disclosed in U.S. Pat. No. 4,016,229 is a closed-cell ceramic foam material which can be prepared by heating hollow glass crystalline microspheres, recovered from fly ash from coal combustion (cenospheres), in the presence of air at 1350-1650° C. for 0.25-1.5 hours. A coherent material having a bulk density of at least 0.50 g/cm
3
is formed. The cenospheres may be used directly after recovery from fly ash but it is preferred to pretreat them by a decrepitation and/or separation procedure. The cenospheres are decrepitated by heating at a temperature of from about 315-540° C. for 0.5-2 hours followed by separation in an organic liquid like heptane to obtain a fraction having a density of less than 0.35 g/cm
3
. To form the cenospheres into a predetermined shape a temporary organic binder such as gum arabic, or polyvinyl alcohol is used. Prior to firing, the decrepitated cenospheres can be admixed with 0.1-30 wt. % of an additive selected from the group consisting of transition metal and rare earth compounds, preferably transition metal and rare earth carbonates. The closed-pore ceramic foam may be used as a non-combustible insulation panel or structural member for a vide variety of applications.
A method of producing a structural insulating composite is disclosed in the U.S. Statutory Invention Registration H200. The method comprises (1) selecting hollow closed-cell ceramic beads having an outer diameter in the range of about 20-200 micrometers, a wall thickness of above about 2.0 micrometers, a softening temperature above about 800° C. and a bulk density of about 0.3-0.5 g/cm
3
, (2) forming a mixture of the ceramic beads with a compatible binder composition with the weight ratio of beads: binder of 1:1-2, (3) removing entrained gas bubbles in the mixture and compacting the mixture under sintering conditions and pressure to provide the structural insulating composition. The sintering conditions include a temperature of above about 700° C. but below the softening point of the microspheres. The final product obtained is characterized by a closely packed, bonded array of said beads with closed cells, useful as an insulating material at high temperatures.
Accordingly, an object of this invention is a method of producing an open-cell porous material, formed by cenospheres. Another object of the invention is a method of producing an open-cell glass crystalline porous material having open-cell porosity of up to 90 vol. %. A further object of the invention is a method of producing an open-cell glass crystalline porous material having a relatively low production cost. Another object of the invention is to produce an open-cell glass crystalline porous material having two types of openings, interglobular voids, i.e. voids between cenospheres, of 20-100 micrometers, and through-flow wall pores of 0.1-30 micrometers. An additional object of the invention is to produce an open-cell glass crystalline porous material having an open-cell porosity in the range of about 40 to about 90 vol. %, useful as a porous glass ceramic matrix for immobilization of liquid radioactive and other toxic waste, as a heat-resistant trap and filter, as a support for catalysts, an ion-exchanger and an adsorbent.
SUMMARY OF THE INVENTION
The material of high open-cell porosity which is characterized by two types of openings, interglobular voids and through-flow wall pores, is produced by separating cenospheres of fixed sizes and composition, molding the cenospheres and agglomerating the cenosphere arra
Aloy Albert S.
Anshits Alexander G.
Knecht Dieter A.
Lubtsev Rem I.
Macheret Yevgeny
Derrington James
Gottlieb Paul A.
Moody Julia Cook
The United States of America as represented by the United States
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