GB2153813A - Radiation-reflective ceramic material - Google Patents

Abstract

Radiation-reflective and particularly heat-reflective materials comprise a porous ceramic matrix having a porosity in the range 20 to 60% by volume, the porosity/ceramic interfaces predominantly having a selected range of radii of curvature of a similar order of magnitude to the range of wavelengths of a given radiation to be reflected so as to scatter the radiation efficiently. Methods of making such materials are described involving the sintering of particles of ceramic of wavelength dimensions at such a temperature as to sinter them without substantially increasing the resultant grain size.

Description

SPECIFICATION Ceramic materials This invention relates to ceramic radiation-reflective materials, for example for use as heat-shielding materials.

Co-pending Application No. 84 02614 describes a particular application of the invention. The present application describes the theory of the invention and further applications of the invention.

Radiation may be "reflected" from a material in several ways, including specular and diffuse reflection from the surface of the material; absorption and re-radiation from the material; and radiation scattering within the material. Most radiation-reflective materials are designed to use primarily the specular and diffuse reflection from the surface of the material, for example metallised surfaces are often provided to increase the specular reflection from an article.

The applicants have taken the course of improving the scattering properties of a radiation reflector and have accordingly provided new materials for radiation reflection.

Scattering in a material occurs at interfaces in the material between regions of differing refractive index.

To increase the proportion of incident radiation scattered by a material the following criteria have to be met: (i) a low absorption coefficient for the radiation; (ii) a high volume proportion of scattering interfaces; (iii) the scattering interfaces must have radii of curvature of a similar order of magnitude to the wavelengths of the incident radiation; and (iv) the difference in refractive index across the interface must be large.

Accordingly the present invention provides a ceramic radiation-reflective material comprising a porous ceramic matrix having a porosity in the range of 20 to 60 per cent by volume, the porosity/ceramic interfaces predominantly having a selected range of radii of curvature of a similar order of magnitude to the range of wavelengths of a given radiation to be reflected so as to scatter the radiation efficiently.

Preferably the selected range of radii of curvature is from 0.2 to 1.5 times, and ideally 0.5 times the wavelengths of the radiation to be reflected.

For a black body radiation source approximately 90% of the radiation emitted will have wavelengths in the range 0.5 to 4 times the peak wavelength of the radiation. Therefore a suitable range of radii of curvature for a material to reflect such radiation is from 0.1 to 6 times the peak wavelength of the radiation, a more preferable range being 0.25 to 2 times the peak wavelength of the radiation.

The present invention further provides a method of preparing a material so described by the sintering together of a powder comprising particles of ceramic selected to have overall dimensions of a similar order of magnitude to the wavelength to be reflected. The sintering is done at such a temperature as is high enough to sinterthe powder to form a very porous material, but not so high as to lead to substantial grain growth of the particles in sintering. In sintering the particles form a ceramic matrix with a large amount of porosity, the dimensions of the pores necessarily having a similar size to the sintered particles.

The porosity range selected is chosen because at a porosity of less than 20% by volume the scattering efficiency is much reduced due to the low amount of porosity/matrix interface; and at a porosity of greater than 60% by volume the material is too weak for practical use.

The material of the present invention made by this method may be used alone by normal ceramic fabrication techniques such as pressing, extruding, casting, moulding etc. or as a surface coating on a suitable substrate. A coating thickness of 0.1 millimetre is adequate for this purpose.

Such a substrate must be capable of withstanding firing temperatures up to that of the coating without degradation or extensive reaction with the coating. However strength and adhesion of coating to substrate may be assisted by the presence of a glass phase at the coating/substrate interface. To this end inorganic materials such as certain naturally occurring clays and/or synthetic silicates may be used; at the coating/substrate interface (i.e. as a glaze); or as an additive in the coating to react with the substrate at the coating/substrate interface.

In choosing a substrate the following features are desirable properties: (i) Minimum transmission/retention of heat, i.e. lowthermal conductivity, low heat capacity, minimum volume consistent with strength; (ii) Porous surface for retention/keying of the coating; (iii) Rigid and non-distorting in the coating process (which includes the sintering of the ceramic particles); and (iv) Low cost and ease of production.

Suitable substrate materials include: (a) 'biscuit' pottery materials which are porous and have been once-fired, - such as are conventionally prepared in the tableware business priorto glazing and decorating.

(b) other partially-fired, porous ceramic articles of well-known formulations (steatites, porcelains, cordierites, sintered oxides, nitrides, sillimanites, carbides etc) such as are usual in technical ceramics and/or refractories manufacture.

(c) ceramic fibre artifacts made by felting and with possibly also some degree of compaction and containing fibres of alumino-silicates (e.g. Triton Kaowool (Trade Mark)) and!or refractory oxides (e.g. Saffil (Trade Mark)) and/or asbestos.

Suitable coating materials are high melting point metal oxides, with low absorption of radiation at the wavelengths concerned and preferably with high refractive index. Examples include, alumina, magnesia, ceria, titania, thoria and zirconia. Mixed metal oxides such as mullite (an alumino-silicate material) or zircon (a zirconia-silicate material) may also be used. The higher the refractive index the better the material will work as a scatterer of radiation, however cost and fabrication difficulties may lead to lower refractive index materials being chosen for a particular application.

The coating may be applied in the form of a suspension in a suitable liquid. Water is convenient, cheap, and suitable in most cases, though organic liquids may be used. The suspension may be applied by a variety of established methods such as spreading, painting, spraying, dipping, casting etc.

The powder used in performing the method of preparation of the invention must be in such a form as will allow sintering to form a porous material, without substantial grain growth. For some ceramics this may make advantageous the use of special 'reactive' forms of ceramic. For example alumina can be obtained as a reactive form in which the particles have a higher specific surface area and/or are surface treated so as to have a lowersintering temperature. Such reactive lauminas are used commercially for the preparation of highly dense alumina objects.

The following descriptions illustrate the invention by way of example only.

Example 1 Reactive alumina was prepared by impact fracturing 99.5% by weight pure alumina. A particle size of less than 2 microns (o.1-1 micron) was obtained. An extrusion mix in water was formed and to this 0.5% magnesium silicate was added in the form of LAPONITE (Trade mark) a synthetic hectorite material comprising 27% MgO, 62% SiO2, the balance being other materials. This magnesium silicate is in the form of very small plate-like sub-micron particles and acts as a lubricant in forming of the alumina, and as a grain growth inhibitor at normal sintering temperatures i.e. approximately 1610 C. The particles of magnesium silicate are small enough that they can coat 2 micron particles of alumina.The resulting mixture was extruded as a tube, cut into lengths as required, dried, and fired in a kiln at any temperature in the range 1020"C to 1400"C for 20-30 hours. It appears that the time of firing is relatively unimportant, the critical feature being the temperature of firing which must be high enough for sintering of the material but low enough that grain growth and subsequent densification does not occur. The usual sort of firing temperature to obtain dense aluminas is in the range 1500-1660"C.

The tubes produced had a length of 190 millimetres, diameter 2 millimetres and bore of 1.2 millimetres and were used as a heat shroud for a thermostat control rod. In this use the tubes out-performed tubes of similar dimensions made of gold-coated fully dense alumina.

Whereas fully dense alumina would have a density of 98-100% of theoretical density the material of the present invention would have a density only slightly higher than the green density; a typical value would lie between 55-70% of theoretical density.

Example 2 A reactive alumina (Alcoa A.16 (Trade mark) - a micronised and milled alumina sold as a reactive alumina) was blended with 1% by weight LAPONITE in a Z-blade mixer for 5 minutes. Liquid was then added in the proportions 4.5 parts by weight powder to 1 part by weight of liquid, the liquid comprising a 6% by volume solution of CELACOL M450 (Trade Mark- a methylcellulose derivative) in water. The mixing of liquid and powder continued for 15 minutes to form a friable mass capable of being squeezed by hand into cohesive lumps. This mass as extruded by ram pressure through a suitably formed nozzle, cut into lengths as required, and fired in air to a temperature in the range 1020into 1330"C for 20 to 30 hours.

The product had the same charactistics as the product of Example 1.

Example 3 Two coating slips were made to the following formulations: Weight % (i) Fine grain alumina (Alcoa A.16) 63.95 LAPONITE 1.9 Water 33.9 Dispersant (DISPEX A.40 (Trade Mark) .25 (ii) Fine grain alumina (Alcoa A.16) 72.9 Water 26.98 Dispersant (DISPEX A.40) .12 For each slip the materials wre put in a small ball mill and blended for about one hour (the ball mill was rubber lined and used alumina balls). A binder solution was added to the mill and blending continued for 20 minutes. The binder solution comprised a solution of 15.25 wt % dextrine and .75 wt % Celacol M450 in water. The amount of binder added was in the range 7.75-8.72 weight % binder to total weight of slip plus binder.

This slip was applied to several different substrates with success including biscuit pottery bowls and felted ceramic fibre articles. In every case the final product had a thickness of coating in the range 1-2 millimetres and was found to perform well.

The only perceptible difference between the articles made using the two slips was an improved adhesion of the coating containing LAPONITE.

DISPEX A.40 is a commercial dispersant and deflocculant It is the ammonium salt of a polycarboxylic acid.

Examples 2 and 3 used Alcoa A.16 alumina which is a 99.5% pure alumina. Alcoa A.17 alumina is a similar material but comprises micronised alumina is a similar material but comprises micronised alumina without any subsequent milling, as such it has jagged particles of alumina and on theoretical grounds should be more suitable for use in the invention than the A.16 alumina. The applicants have not tested this as yet. By micronised is meant reduced to particles of the order of a micron in size.

Example 4 Titania tubes were made using the following formulations: Weight % Pure fine grain tiO2 (of the order of 1 micron in size) 68 LAPONITE 1 VERSICOL EA9 (Trade Mark) 1 6% solution of CELACOL M450 30 The method of mixing used was that of Example 2; the powders (TiO2 and LAPONITE) being mixed first and the liquids (VERSICOL and CELACOL) being added afterwards. The processing and firing regime was the same as for Example 2, the firing temperature being 1 1 50"C.

Infrared radiation from an ideal black body source has a peak wavelength as follows: Temperature 'C Approximate peak wavelength (microns) 500 3.76 1000 2.28 1500 1.64 It can be seen that a material made as above, having a particle size of the order of 1 micron, would act as an efficient scatterer of heat radiation over the temperature range shown. By suitable selection of particle size a material according to the invention can be tailored to a given application.

Claims (15)

1. A ceramic radiation-reflective material comprising a porous ceramic matrix having a porosity in the range 20 to 60% by volume, the porosity/ceramic interfaces predominantly having a selected range of radii of curvature of a similar order of magnitude to the range of wavelengths of a given radiation to be reflected so as to scatter the radiation efficiently.

2. A ceramic radiation-reflective material as claimed in Claim 1 in which the selected range of radii of curvature is from a tenth to six times the peak wavelength of the radiation to be reflected.

3. A ceramic radiation-reflection material as claimed in Claim 2 in which the selected range of radii of curvature is from a quarter to twice the peak wavelength of the radiation to be reflected.

4. A ceramic heat-reflective material comprising a material as claimed in any of Claims 1 to 3 in which the selected range of radii of curvature is from 0.1 to 1.6 micron.

5. A method of preparing a ceramic radiation-reflective material, comprising the steps of: (i) providing ceramic as a powder of small particles selected to have overall dimensions of a similar order of magnitude to the wavelengths of the radiation to be reflected; (ii) shaping the ceramic by any convenient means; (iii) drying if necessary; and (iv) firing at such a temperature as to sinter the particles of ceramic without substantially increasing the resultant sintered grain size as compared with the original particle size and maintaining the porosity in the range 20 to 60% by volume.

6. A method as claimed in Claim 5 in which the particle overall dimensions lie between a fifth and twelve times the peak wavelength of the radiation to be reflected.

7. A method as claimed in Claim 6 in which the particle overall dimensions lie between a half and four times the peak wavelength of the radiation to be reflected.

8. A method of forming an article having a radiation-reflective coating comprising the steps of: (i) providing ceramic as a powder of small particles selected to have overall dimensions of a similar order of magnitude to the wavelengths of the radiation to be reflected; (ii) applying the ceramic to the surface of an article to be coated; (iii) drying if necessary; and (iv) firing at such a temperature as to sinter the particles of ceramic without substantially increasing the resultant grain size as compared with the original particle size and maintaining the porosity in the range 20 to 60% by volume.

9. A method as claimed in Claim 8 in which the article to be coated is an unfired pottery article which is fired on sintering of the ceramic.

10. A method as claimed in Claim 8 in which the article to be coated is a felted ceramic fibre body.

11. A method as claimed in any of Claims 5 to 10 in which the ceramic is a reactive alumina and the firing temperature is in the range 1020to 1400 C.

12. An article having a radiation-reflective surface comprising a porous matrix having a porosity in the range 20 to 60% by volume, the porosity/matrix interfaces predominantly having a selected range of radii of curvature of a similar order of magnitude to the range of wavelengths to be reflected so as to scatter the radiation efficiently.

13. A method of making a radiation-reflective material, substantially as described.

14. A radiation-reflective material substantially as described.

15. An article having a radiation-reflective surface, substantialy as described.