GB2106093A

GB2106093A - Porous ceramic material

Info

Publication number: GB2106093A
Application number: GB08224073A
Authority: GB
Inventors: David James Godfrey
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1981-09-03
Filing date: 1982-08-20
Publication date: 1983-04-07

Abstract

A ceramic porous material comprising a fired mixture of 40% to 60% by weight of clay and 60% to 40% by weight of cenospheres combined with a suitable binder. The mixture is fired at 1100 DEG C for 0.5 hours, the firing temperature being reached in 1.25 to 1.5 hours. The cenospheres are screened through a 100 mesh sieve prior to use to provide a powder density of 0.7 gm/cm<3>. The material exhibits a high degree of closed porosity with comparatively little open porosity, the latter being sealable by glazing. The material is suitable for thermal insulation in wet environments.

Description

SPECIFICATION Ceramic composite material This invention relates to a ceramic composite material.

Ceramic composite materials for thermal insuation applications are well-known in the prior art.

In particular, it is known to produce ceramic composite materials such as cement incorporating a filler comprising hollow glass spheres.

It is also known to produce thermal insulants from ceramic-filled plastics materials. Generally speaking, ceramic-filled plastics materials exhibit far lower open porosity than wholly ceramic composites, but necessarily ceramic composites are capable of use at much higher temperatures.

It is an object of the present invention to provide a ceramic composite thermal insulant having a comparatively low degree of open porosity.

The present invention provides a ceramic composite material comprising a fired mixture of clay and cenospheres, the mixture having 40% to 60% by weight of clay prior to firing. Those skilled in the art of ceramics will appreciate that censopheres are produced as spherical particulates in the fly ash combustion products of pulverised coal.

It has been found that ceramic/cenosphere composite materials in accordance with the invention have high total porosity of which comparatively little is open porosity. Such materials are accordingly particularly suitable for use as thermal insulants in applications requiring low fluid permeability, since most of the porosity is closed. The invention is particularly suitable for thermal insulation employed in wet environments. It has also been found that the material of the invention exhibits much lower shrinkage during firing than ceramics produced wholly from clay, typically 5% shrinkage as opposed to at least 10%. Furthermore, the material has been found to exhibit as compared to clay improved tolerance to accelerated firing procedures, so that the final firing temperature may be reached more quickly with consequent reduced cost and increased throughput.

The ceramic composite material of the invention may be glazed to seal its outer surface.

The invention will now be described with reference to the examples.

Tables 1 and 2 show density and porosity data for four comparison composite materials (a) to (d), and clay/cenosphere example materials (1) to (7) of the invention. Table 3 gives details of the clays employed. The materials (1) to (7) were made from approximately equal parts by weight (60/40, 50/50 or 40/60) of cenospheres and clay. In each case, the cenospheres were screened through a 100 mesh sieve to yield a powder density of 0.7gm/cm3, as indicated by the parameter 100/7. In each case, the cenosphere/clay mixture was mixed with water to give a plastic consistency and extruded to a moulded wet form. The examples were fired at 1 1 00do for + hour and the furnace was then switched off and allowed to cool.The firing temperature in each case was reached in la to 1 + hours without adverse effects on the examples. Extrusion of the examples (1) to (7) was carried out using a cylindrical die to form plugs of the material 25mm in diameter and 30mm in length. Comparison example (a) consisted of borosilicate glass spheres fired together at 750 C, (b) spheres similar to (a) mixed with an equal quantity by weight of aluminium phosphate and fired at 1 200do, and (c) cenospheres type 100/7 in an organic binder, shaped under pressure and fired at 1 200 C. Porosity of the materials of Tables 1 and 2 was measured by boiling each fired example in distilled water for two hours and weighing in water; each example material was then removed from the water, surface moisture was removed and the example rapidly reweighed to indicate internal adsorbed water. The proportion of closed to open porosity was then estimated from the theoretical 2.4 gm/cm3 density of non-porous glass/clay mix. It can be seen from Tables 1 and 2 that the total porosity of Examples (1) to (7) was about 70% in each case, but that only between 9 and 23% of this total was due to open porosity. Accordingly, the materials of the invention combine comparatively high total porosity in the upper part of the approximately 30-90% range exhibited by the comparison materials (a) to (d). However, the open porosity of Examples (1) to (7) averages about 16%, much lower than the 30-50% range of the comparison materials.

Ceramic composite materials of the invention are suitable for glazing to seal residual open porosity.

High porosity is consistent with low thermal conductivity and hence good thermal insulation characteristics, whereas low open porosity is consistent with low fluid permeability. Accordingly.

and unlike the comparison materials of the invention exhibit porosity characteristics consistent with good thermal insulation properties but low fluid permeability.

Composites made in accordance with the invention exhibit high tolerance to accelerated firing procedure as compared to conventional pottery. Pottery clay may require over 16 hours to reach 11 00'C (or 22 hours to 1 400 C) as compared to 141 to 1 + hours for cenosphere-clay material.

Firing tests have shown that subjecting ordinary pottery clay without additives to a rise from room temperature to 1 00 C in 11 hours may result in violent decrepitation of the clay, with pieces of clay being dispersed throughout the furnace interior Accordingly, a much higher throughput with lower energy costs is obtainable with cenosphere-clay material as compared to clay alone. In addition, clay normally shrinks by 10-12% when fired of 5-6% when dried, whereas the material of the invention has been found to shrink when fired by only 6%, or 1.5mum in an original 25mm diameter body. It is therefore believed that the material of the invention is capable of improved dimensional stability when fired as compared to conventional pottery.

TABLE 1 COMPARISON MATERIALS (a) to (d) AND EXAMPLES 1 to 4

CONSTITUENTS DENSITY OF % POROSITY FIRING CONDITIONS FIRED BODY TOTAL OPEN CLOSED 3M - borosilicate glass 0.238 89.7 54.8 34.9 (a) hollow spheres type 337/2000, heated to 750 C 3M-B37/2000/A1PO4 1.185 53.0 37.7 9.3 (b) (50/50 proportions), 1200 C 1.188 51.9 41.1 7.0 Cenospheres type 100/7, 1.532 31.6 31.6 0 (c) 3% methylcellulose soln (MCS) binder1 compaction at 200 lbf/inch pressing, 120000 (d) As (c) above, no compaction 0.350 84.7 41.5 43.2 1 60 w/o Cenospheres 100/7 0.538 77.6 31.2 46.4 40 w/o Clay type P P Water binder ) 21 hour at 110000 2 40 w/o Cenospheres 100/7 ) 0.816 66.0 19.4 46.6 60 w/o Clay type P Water binder ) 9 hour at 1100 C 3 50 w/o Cenospheres 100/7 ) 0.684 71.6 22.7 48.9 50 w/o Clay type P Water binder ) hour at 1100 C 4 50 w/o Cenospheres 100/7 } 0.661 72.521.650.9 50 w/o Clay type P Water binder l hour at 110000 TABLE 2 EXAMPLES 5 TO 7 CONSTITUENTS DENSITY OF % POROSITY EXAMPLE 50/50 proportions FIRED BODY TOTAL OPEN CLOSED 5 Cenospheres 100/7) 0.701 70.8 8.6 62.2 Clay R + H20 0.677 71.8 8.5 63.3 1100 C 0.674 71.9 11.6 60.3 *0.659 72.6 15.7 56.9 6 Cenospheres 100/7) 0.725 69.8 15.5 54.3 Clay E+ H20 0.719 70.0 15.1 54.9 1100 C 0.722 69.9 14.9 55.0 '0.697 71.0 18.0 53.0 7 Cenospheres 100/7) 0.734 69.4 9.0 60.4 Clay H + H20 0.646 73.1 21.8 51.3 1100 C 0.711 70.4 15.6 54.8 *0.662 72.4 17.0 55.4 *Larger cylindrical specimen, rest of data for discs.

TABLE 3 CLAYS CLAY TYPE DESCRIPTION P PK CLAY (PIKE Bros, Waveham, Dorset) Rational Analysis:%: (1) Felspar Basis: Clay Substance 84.0; Felspar 9.7; Quartz 3.3; (2) Mica Basis: Clay Substance 74.0; British Mica 11.8; Soda Mica 2.9; Quartz 7.5.

E Hymod Excelsior) R Hymod RN H Hymod HSM

Dorset Clays. Full chemical analyses given in "Ball clays for the Ceramic Industry", published by ECC International, No C60, March 1980.

Claims

1. A ceramic composite material comprising a fired mixture of clay and cenospheres, the mixture having prior to firing 40% to 60% by weight of clay and 60% to 40% by weight of clay combined in a suitable binder.

2. A ceramic composite material according to claim 1 wherein the mixture has prior to firing 50% to 60% by weight of clay.

3. A ceramic composite according to claim 1 to 2 wherein the cenospheres have a powder density of 0.7 gm/cm3 after screening through a 100 mesh sieve.

4. A ceramic composite material according to claim 1, 2 ot 3 incorporating clay type P, E, R or H as herein described.

5. A ceramic composite material according to any preceding claim produced by firing at a temperature of substantially 11 00'C.

6. A ceramic composite material according to claim 5 wherein the firing temperature is reached in 1.25 to 1.5 hours, and the firing time is substantially 3 hour.

7. A ceramic composite material in accordance with any one of Examples 1 to 7.