GB2468953A

GB2468953A - Freeze-cast ceramic components

Info

Publication number: GB2468953A
Application number: GB1004683A
Authority: GB
Inventors: Ronald William Jones
Original assignee: HORIZON COMPOSITES Ltd
Current assignee: HORIZON COMPOSITES Ltd
Priority date: 2009-03-20
Filing date: 2010-03-22
Publication date: 2010-09-29
Anticipated expiration: 2030-03-22
Also published as: GB201004683D0; GB0904783D0; GB201407073D0; GB2468953B

Abstract

Apparatus and methods of making components of an insulating or fire protective structure are disclosed, comprising making a plurality of freeze-cast ceramic elements, each element consisting of a shell 28 formed from a freeze-castable ceramic slurry having an internal void 29 which is filled with a fibrous and / or foam and / or particulate insulation or may be left empty. The casting process taught uses cryogenic freezing and a highly thermally conducting mould 20 so that flash freezing occurs to produce a mechanically strong ceramic shell, with an internal void which may be filled with an insulating material. This material may be a further freeze cast ceramic or loose insulation. Preferably the method involves pre-cooling the slurry and pre-cooling the mould in a cryogenic environment before use. The mould may be made from metal sheets which are flexible to promote the release of the article after casting. Further, making a tunnel kiln from interlocking components is disclosed.

Description

FREEZE -CAST COMPONENTS

This specification relates to freeze-cast components in general and particularly to engineering items, where dimensional accuracy and superior mechanical and I or thermal properties are required.

The properties of ceramic components are well known and ceramic techniques have been developed over thousands of years, initially to produce items for the storage of foodstuffs, etc. and for the preparation and consumption of food, etc., and more recently to make components for industrial and engineering usage.

This specification is concerned with engineering components, particularly those where dimensional accuracy is required combined with either / both of high mechanical properties, such as high compressive, tensile and bending strengths and loading-carrying capabilities, etc., and I or thermal properties, such as resistance to fire, good thermal insulation and thermal cycling capabilities, etc. Examples of typical components range from fire bricks and furnace linings to blades for gas turbine engines. The teaching of this specification is written with respect to kiln or furnace linings but is applicable to many other products, as indicated above.

The use of ceramics for insulation and fire resistance is well known. Different forms range from the heavy, mechanically strong fire bricks / blocks, providing extreme fire resistance in furnaces, to insulating boards, providing a degree of mechanical strength and limited fire resistance for use in homes and public buildings. Fibrous bodies, such as mats or blankets, can be formed as insulation around items, such as hot water tanks, and loose insulation, such as fibres, powder, flakes or granules, may be used as infill, e.g. around fire bricks in furnaces, etc. Of the above forms of insulation, only fire bricks could have provided any resistance to the blast damage caused by the incidents of 1 1th September 2001 in New York. However, such bricks are far too heavy to be used in high rise buildings. Ughtweight ceramics have been developed, e.g. as disclosed in the May 2002 issue of Ceramic lndustry in an article entitled: uUltraLightweight Kiln Cars, which described how low thermal mass kiln cars may be used in tunnel furnaces to carry items being fired. This teaching could be developed to provide a hollow structural ceramic shell item, with a fibrous or foam insulation filling (see below).

I

A range of conventional casting processes is known. In one, a mixture of powders and binders, optionally with water and I or lubricants, are pressed into a mould to form a component. The green' pressing is removed from the mould, dried and fired at high temperature. Due to internal friction between the particles and with the sides of the mould, pressing causes a variable compaction density within the mould and, hence, unpredictable shrinkage and I or distortion in the final dimensions of the fired component. This can be rectified by casting an oversize component, firing and machining to final size but this is costly, especially as ceramic powders are very abrasive and cause considerable wear both to the mould during pressing and to the cutting tools during machining. The variable compaction density can also lead to unpredictable mechanical properties, e.g. compressive strength.

Slip casting uses a water-based slurry, poured into a dry, absorbent mould. Water in the slurry layer adjacent to the mould surface is drawn into the mould by capillary forces so that, after a short period of time, the bulk of the slip slurry may be poured out, leaving a fining in the mould.

After a further period of water extraction, the lining is removed as a green' component, dried and fired. Because the ceramic slip contains a great deal of water, considerable shrinkage takes place during drying and firing but this shrinkage is more predictable.

Both these processes, and variations of them, have drawbacks, particularly in that they cannot produce components with the repeatable dimensional accuracy required for engineering applications. Pressing tends to be used for items such as fire bricks and slip casting for cups and bowls, etc. However, a need has been identified for engineering components with accurate external dimensions and having both exceptional strength and thermal insulating properties. Neither of the above methods could produce items anywhere near these requirements.

The process of freeze-casting can produce components with precise dimensions on a repeatable basis. The process uses a particulate mixture dispersed in an aqueous sol, to form an essentially homogeneous slurry. The particles are in the nanometer' size range (i.e. 1 0 m) and, when the water is removed from the slurry by freezing, the particles come essentially into direct contact with each other.

Due to the high speed of the freezing process, numerous tiny ice crystals form, drawing water molecules from the microscopic local slurry volume and displacing the particles in this volume.

Thus, the process leaves shells' of particles around each ice crystal. The particles in these shells are in such intimate contact with each other that they bond tightly together via the Van der Waals' attractive forces, which overcome any electrostatic repulsive forces. Similarly, as adjacent shells are in direct contact, they also bond together via the same forces. The result is a 3-dimensional, geodetic-type ceramic matrix, bonded at the nano-particulate level, forming a potentially very strong structure.

The freezing phase may be used either to create a shell', after which the excess slurry is poured out of the mould (as for slip casting), or the whole slurry volume in the mould may be frozen, to give a solid component. The freezing process results in an irreversible physical / chemical change, giving the green component enough mechanical strength to be removed from the mould for drying (at ambient temperatures and / or in an oven) and firing (at elevated temperatures). As the dimensions are effectively fixed by the 3-dimensional ceramic matrix, the ice crystals either melt and vaponse, or sublime, and the water vapour diffuses out of the matrix without damaging its integrity. Thus, a dimensionally accurate, mechanically strong ceramic component results.

Examples of the process are DE 4037258, US 4428895, US 4552800, US 4569920, US 5647432, US 5716559, US 5954121, US 6024259, US 6199836, US 6322729, US 2001 000633-A and U5 2001 042929-A. The principle of freeze casting is applicable to metals as well as ceramics, e.g. maraging steels.

The normal process of freeze-casting involves filling a mould with the slurry and applying cryogenic freezing until sufficient / all of the slurry has frozen. The moulds are usually much heavier than the component being cast -typically ten or more times as massive. This leads to a number of practical, production problems, such as slow heat transfer rates, potentially higher mould costs, handling problems, etc. Applicant has found that light, thin, flexible moulds are much more suited to this type of process, having low thermal inertia and facilitate high rates of heat transfer. Such moulds can be made of high quality sheet metal. After casting, demoulding is easily achieved by flexing the sides away from the green component. Preferably, demoulding is undertaken with the mould still at sub-zero temperatures (ideally at about -20°C).

Such moulds may be reused immediately, so saving energy for cooling. Moulds may be used and re-used thousands of times. Thus, moulds, such as described, greatly reduce production costs in terms of numbers of moulds needed, energy input and rate of output.

Production costs can be further reduced if the component is designed as a shell, rather than a solid item. After a predetemiined period of cryo-freezing, during which a shell will have formed on the inside of the mould, the remaining liquid of the slurry is decanted and the void remaining either left empty or filled with a second slurry before returning to the cryo-freezer. Thus, it is possible to design composite ceramic components with specific properties for a wide range of engineering requirements, as will be explained hereinafter.

According to a first embodiment of the invention, there is provided an insulating or fire protective structure comprising a plurality of freeze-cast ceramic elements, each element consisting of a shell formed from a freeze-castable ceramic slurry having an internal void which is filled with a fibrous and I or foam and I or particulate insulation or may be left empty.

According to a first variation of the first embodiment of the apparatus of the invention, the elements have interengageable parts so that the interengageable part of one element is interengegeable with the complementary part of another element to form a structure or part of a structure.

According to a second variation of the first embodiment of the apparatus of the invention, the interengageable parts are of the form of male and female parts.

According to a third variation of the first embodiment of the apparatus of the invention, the male I female interengagement is in the horizontal plane or in the vertical plane or in both planes together or in a plane(s) at an angle to the horizontal.

According to a fourth variation of the first embodiment of the apparatus of the invention, the elements between which the male and female parts interengage form part of a curve or an angle.

According to a fifth variation of the first embodiment of the apparatus of the invention, the male I female interengagement is such as to prevent any line of sight from one side of the structure, through I between the element(s) of the structure to the other side of the structure.

According to a sixth variation of the first embodiment of the apparatus of the invention, the interengaging parts of two adjacent interengaged elements form a mechanically strong connection between said two elements restricting relative movement therebetween and that said two interengaged elements either form a structure or form the basis for incorporating further interengaging elements therewith so that a larger structure may be built from said interengaging elements.

According to a seventh variation of the first embodiment of the apparatus of the invention, the interengagement is such that the acute angle between two adjacent interengaging elements is 600, or less, so that the interengagement forms a positive location and a mechanically strong connection therebetween.

According to an eighth variation of the first embodiment of the apparatus of the invention, resilient padding is provided between the interengaging parts of the interengaging elements to accommodate and cushion minor relative movements between adjacent interengaging elements.

According to a ninth variation of the first embodiment of the apparatus of the invention, the elements are cast in a mould having a low thermal capacity and a high thermal conductivity through the sides thereof.

According to a tenth variation of the first embodiment of the apparatus of the invention, the mould is made of a metal sheet(s).

According to an eleventh variation of the first embodiment of the apparatus of the invention, the mould is made of a single metal sheet or a plurality of metal sheets which can be assembled into the complete mould and disassembled after casting to release the green component from the mould.

According to a twelfth variation of the first embodiment of the apparatus of the invention, quick assembly I disassembly means are provided to connect / disconnect the metal sheet(s) or part(s) of a metal sheet(s) together to form the mould and I or open I close said mould According to a thirteenth variation of the first embodiment of the apparatus of the invention, the quick assembly I disassembly means are operable when the mould is at the temperatures associated with cryogenic freeze casting.

According to a fourteenth variation of the first embodiment of the apparatus of the invention, the metal sheets forming the mould are flexible to promote the release of the green component from the mould after casting.

According to a fifteenth variation of the first embodiment of the apparatus of the invention, the metal sheet(s) forming the mould are designed so that they may be flexed progressively away from the point, where the quick assembly / disassembly means is operated, along the sides of the component, so that the whole of the component is successively and I or progressively separated from the metal sheet(s).

According to a sixteenth variation of the first embodiment of the apparatus of the invention, a release agent is applied to the faces of the mould which will be in contact with the freeze-casting slurry to promote the release of the green component after casting.

According to a seventeenth variation of the first embodiment of the apparatus of the invention, the metal sheet(s) of which the mould is formed are stainless steel or an equivalent high quality alloy.

According to an eighteenth variation of the first embodiment of the apparatus of the invention, the mould is provided with a base / support means on which to stand stably during the filling and freeze-casting process.

According to a nineteenth variation of the first embodiment of the apparatus of the invention, the base / support means is adapted to permit all parts of the external surfaces of the mould to have direct contact with the cryogenic cooling medium.

According to a twentieth variation of the first embodiment of the apparatus of the invention, a hole(s) is / are provided at a suitable position(s) in the mould so that, when standing stably on its base 1 support means, the hole(s) is I are at the highest point(s) of the mould so that it may be fi(led to the brim with a freeze-castable ceramic slurry.

According to a twenty first variation of the first embodiment of the apparatus of the invention, an attachment(s) is I are provided at a suitable point(s) on the mould so that, when standing stably on its base I support means, a freeze-castable slurry may be pumped into the mould so that it may be filled to the brim with said slurry.

According to a twenty second variation of the first embodiment of the apparatus of the invention, a vent hole(s) is I are provided in the mould so that, when standing stably on its base I support means, the mould may be completely filled with freeze-castable ceramic slurry without incorporating any air I gas bubbles therein.

According to a twenty third variation of the first embodiment of the apparatus of the invention, the freeze-castable ceramic slurry is pre-cooled to a temperature just above its freezing point before pouring I pumping into the mould.

1.5 According to a twenty fourth variation of the first embodiment of ihe apparatus.of.the invention, the. mould. is. pre-cooledto. a temperature below 0°C prior to. filling with. freeze-castable ceramic of sturry.

According to a twenty fifth variation of the first embodiment of the apparatus of the invention, a cryogenic medium is applied to the mould after the mould has been filled with freeze-castable ceramic slurry.

According to a twenty sixth variation of the first embodiment of the apparatus of the invention, the slurry-filled mould is placed in a cryogenic environment for a predetermined time.

According to a twenty seventh variation of the first embodiment of the apparatus of the invention, the cryogenic environment is provided with means of enhancing heat transfer from the slurry-filled mould.

According to a twenty eighth variation of the first embodiment of the apparatus of the invention, the means of enhancing heat transfer includes cryogenic sprays.

According to a twenty ninth variation of the first embodiment of the apparatus of the invention, the means of enhancing heat transfer includes immersion in a bath of cryogenic liquid.

According to a thirtieth variation of the first embodiment of the apparatus of the invention, the means of drying the green component is placing it on a structure in ambient air, with / without forced convection.

According to a thirty first variation of the first embodiment of the apparatus of the invention, the means of drying the green component is in an oven.

According to a thirty second variation of the first embodiment of the apparatus of the invention, the means of firing the green component is in a kiln or equivalent apparatus.

According to the first embodiment of the invention, there is provided a method of producing freeze-cast ceramic elements, consisting of a shell containing a fibrous, and / or foam and / or particulate insulation within the shell, for an insulating or fire protective structure comprising the steps of:-i) providing a mould; ii) placing the mould in a cryogenic environment for a predetermined period of time to pre-cool it prior to use; iii) removing the mould from the cryogenic environment and filling the mould with a pre-cooled, liquid, freeze-castable, ceramic slurry; iv) replacing the mould in a cryogenic environment for a predetermined period of time; v) removing the mould from the cryogenic environment and decanting the unfrozen, liquid slurry from the mould; vi) removing the green component from the mould; vii) drying the green component; viii) firing the green component to produce a freeze-cast ceramic shell element; ix) filling the void inside the shell element with a fibrous and I or particulate insulation; and x) providing the freeze-cast, ceramic, insulated element for use in an insulated or fire protective structure.

According to a first variation of the first embodiment of the method of the invention, the mould comprises a number of pieces which are assemblable prior to filling with the slurry and disassemblable to facilitate the removal of the green component after freeze-casting.

According to a second variation of the first embodiment of the method of the invention, a release agent is applied to the internal faces of the mould prior to filling with ceramic slurry.

According to a third variation of the first embodiment of the method of the invention, the cryogenic environment is created by placing the mould in a chamber and spraying a cryogenic liquid onto the external faces of the mould.

According to a fourth variation of the first embodiment of the method of the invention, the cryogenic environment is created by immersing the mould in a cryogenic liquid.

According to a fifth variation of the first embodiment of the method of the invention, the method of filling the mould with ceramic slurry is to pour the slurry into the mould through a hole provided at the topmost part of the mould and to fill the mould up to the rim of the filing hole.

According to a sixth variation of the first embodiment of the method of the invention, the method of filling the mould with ceramic slurry is to pump the slurry into the mould until it is brim full at the filling I air-exit hole(s) provided in the topmost part of the mould.

According to a seventh variation of the first embodiment of the method of the invention, the green component is removed from the mould while both mould and green component are at cryogenic temperatures.

According to an eighth variation of the first embodiment of the method of the invention, the method of removing the green component from the mould includes releasing the means locking the mould in the closed position and flexing the panels forming a part(s) of the mould to break adhesion(s) between the panels and the green component.

According to a ninth variation of the first embodiment of the method of the invention, the method of drying the green component includes drying in air at ambient temperatures.

According to a tenth variation of the first embodiment of the method of the invention, the method of drying the green component indudes drying in an oven.

According to an eleventh variation of the first embodiment of the method of the invention, the method of drying the green component includes forced convection.

According to a twelfth variation of the first embodiment of the method of the invention, the method of firing the dried, green component includes placing in a kiln.

According to a second embodiment of the invention, there is provided an insulating or fire protective structure comprising a plurality of freeze-cast ceramic elements, each element consisting of a shell formed from a first freeze-castable ceramic slurry having an internal void which is filled with a second freeze-castable ceramic slurry.

According to a first variation of the second embodiment of the invention, the shell formed from the first freeze-castable ceramic slurry has high mechanical properties and the material in the internal void formed from the second freeze-castable ceramic slurry has high thermal insulating properties.

According to the second embodiment of the invention, there is provided a method of producing an insulating or fire protective structure comprising a plurality of freeze-cast ceramic elements, each element consisting of a shell formed from a first freeze-castable ceramic slurry having an internal void which is filled with a second freeze-castable ceramic slurry comprising the steps of:-i) providing a mould; ii) placing the mould in a cryogenic environment for a predetermined period of time to pre-cool it prior to use; iii) removing the mould from the cryogenic environment and filling the mould with a first, pre-cooled, liquid, freeze-castable, ceramic slurry; iv) replacing the mould in a cryogenic environment for a predetermined period of time; v) removing the mould from the cryogenic environment and decanting the unfrozen, first, liquid slurry from the mould; vi) refilling the mould with a second, liquid, freeze-castable, ceramic slurry; vii) replacing the mould in the cryogenic environment for a further predetermined period of time until all the second freeze-castable slurry is frozen; viii) removing the mould from the cryogenic environment; ix) removing the green component from the mould; x) drying the green component; and xi) firing the green component to produce a freeze-cast ceramic element; and xii) providing the freeze-cast, ceramic element for use in an insulated or fire protective structure.

According to a first variation of the second embodiment of the method of the invention,, the mould containing the second, liquid, freeze-castable, ceramic slurry is returned to the cryogenic environment for a further predetermined period of time until a given quantity of the second slurry has frozen after which the mould is removed from the cryogenic environment, the remaining second, liquid slurry decanted, the mould refilled with a third, liquid, freeze-castable, ceramic slurry and returned to the cryogenic environment until all the third, liquid slurry has frozen.

According to a third embodiment of the invention, there is provided an insulating or fire protective structure comprising a plurality of freeze-cast ceramic elements, each element consisting of a shell formed from a first freeze-castable ceramic slurry having an empty internal void.

In a preferred application of the method of the invention, a mould consisting of a number of panels of sheet metal is put together using quick assembly means, e.g. spring clips, turn buckles, nuts and bolts, etc. The mould is pre-.cooled by spraying with a cryogenic liquid, e.g. liquid nitrogen, before filling to the brim with a first, cold, freeze-castable, ceramic slurry. The mould and its contents are returned to the cryogenic chamber and the spraying resumed.

The excessive temperature difference (T) between the cryogenic liquid and the slurry, combined with the high thermal conductivity of the metal sheets of the mould, results in extremely rapid removal of heat from the slurry, i.e. flash freezing, so that microscopically small water-ice crystals are formed, displacing the ceramic particles from these crystals. The displaced particles concentrate in the zones surrounding the ice crystals and become bonded together by Van der Waals forces to form an inherently-strong, 3-dimensional matrix around the crystals. After a predetermined period of time, when the freeze cast layer will have reached the required thickness, the mould is removed from the cryogenic chamber and the remaining liquid slurry is poured out.

At this point, three options re possible. In one (first embodiment), the green shell is removed from the mould, dried, fired and the void inside the shell filled with fibrous or particulate insulation. Here, a mechanically strong shell supports a lightweight insulation to give blocks that can be built up into thermally insulating structure.

In the second option (second embodiment), the mould is refilled with a second, cold, freeze-castable, ceramic slurry and returned to the cryogenic chamber and left there until all the slurry has frozen. After removal from the mould, drying and firing, the same shaped insulating block is provided but here, the core of the block, provided by the second ceramic is intimately bonded to the inside of the shell and, if, for example, the second ceramic is produced from a foamed slurry, the insulating properties are likely to be significantly superior to those of the loose filled first option blocks.

In the third option, the internal void is left empty. This may be desirable to produce a lighter block, or where the insulating requirements are not onerous.

The green, mechanical strength of the shell allows the component to be removed from the mould, e.g. by undoing the clips, etc. and flexing the panels away from the green component, while the mould is still at cryogenic temperatures. This allows the mould to be re-used while still cold, i.e. minimising the pre-cooling required.

It is a feature of the second method of the invention that the core is firmly bonded to the inside of the shell and is continuous and uniform, i.e. there are no voids, which would allow convective process to occur within the core or between the shell and core. Thus, a dimensionally accurate, mechanically strong component is produced with both exceptional strength and thermal insulating properties. By varying the thickness of the shell in relation to that of the core, the balance of mechanical and thermal properties can be adjusted for any particular requirement.

The composite structure of the elements (components) of the invention is shown in Fig. 5. The second method of the invention can be adapted to provide a three layered construction for very specialised applications.

For a clearer understanding of the invention and to show how it may be put into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:-Figure 1 is perspective view of a kiln built of blocks according to the invention, shown open, ready for loading; Figure 2 is a perspective view of the kiln of Fig. 1, shown closed, during a period of firing; Figure 3 is a sectional elevation through the kiln of Fig. 1, looking axially parallel to lines 6; Figure 4 is a section through block I 1A forming a typical part of the structure of the kiln shown in Fig. 1; Figure 5 is a sectional elevation of a sheet metal mould showing how a typical kiln block 11 C is manufactured inside the mould and how the finished green block is removed from the mould.

In the following description, the same reference numeral is used for the same component or for different components fulfilling an identical function.

Referring to Figs. I and 2, a kiln I is formed with an inverted U-shaped body with ends 3 and 4. One end 4 and the kiln floor 5 are movable along rails 6 to allow kiln I to be opened (Fig. 1) for loading before firing and emptying after firing and to be dosed (Fig. 2) during the firing cycle. Firing, with internal temperatures of up to 1350°C inside kiln I causes high stresses in the blocks 11 (Fig. 3) forming the sides 2, floor 5 and ends 3, 4 of kiln I due to the combination of absolute temperatures inside the kiln, the temperature gradient through the blocks and thermal cycling so that end pieces 8 and tensioning means 9 are provided to maintain the shape and integrity of body 2.

Fig. 3 shows a sectional; elevation through kiln 1, looking towards end 3 and showing joints 12 between adjacent end blocks 3. End blocks 3, 4, body 2 and floor 5 are constructed of essentially parallel-sided blocks, e.g. as shown 11 (Fig. 3), with a number of standard shapes being used to simplify manufacture and assembly. The standard wall and floor blocks are central floor block 1 IA, base corner blocks II B, sides II C, roofs lID and roof-keys 11 E. All blocks 11 include at least one of a male engagement member 13 and / or a female engagement member 14.

Block I 1A is typical of all blocks and will be described as such (Fig. 4). It is manufactured according to the method of the invention, as wifl be described later, but here, for simplicity, the shells and cores of all blocks 11 are shown with a single hatching. Also shown are kiln support legs 7 and the wheels 10 which move floor 5 and end 4 along rails 6.

Refemng to Fig. 4, block I IA is a parallel-sided freeze-cast ceramic block. Blocks I 1A and liE have two female engagement members 14, blocks 11 C and lID have one male 13 and one female 14 engagement member and blocks 11 B have a single male 13 engagement member.

Fig. 4 shows the geometry of the male I female interengagement. The acute angle 15, between faces 14A and 14B is less than 60° and ideally about 58°, as shown. Thus, the complementary angle 16 is about 302°. Finite element analysis of the interengagement, when subject to real heat transfer, thermal conductivity, thermal expansion I contraction and thermal cycling, etc. under load-bearing conditions, shows that structures based on acute angles greater than 60° are inherently unstable and will collapse, particularly where regular thermal cycling occurs, as would be the case in a batch-operated kiln 1. Long, permanently heated, tunnel kilns are less subject to thermal cycling but the instability problem remains.

The acute angle is taught as about 58°. This is because, with the thin sheet metal moulds preferred for this application (Fig. 5), which are regularly opened, closed and flexed to demould castings, it is difficult to maintain an exact interengement angle and 58° � about %° is found to be a practicable and reliable value. All the faces 1 3A and I 3B of the male interengaging and faces 14A and 14B of the female interengaging faces are cast to the same 58° nominal angle so that all blocks 11 are interchangeable in their design positions. The symmetrical 58° angle means that the angle of points 34 are each 29°; this angle gives adequate strength with freeze-cast components according to the invention to stop tips 34 from being broken off during normal handling and operation. Symbol 18 indicates that the full length of block I 1A is not shown.

Fig. 3 shows the interengagement between adjacent blocks 11 in the vertical plane (sides) and horizontal plane (floor). It will be understood that identical interengagements between adjacent blocks 11 are provided in the longitudinal plane (i.e. parallel to lines 6) for U-shaped body 2 and floor 5, i.e. in the plane perpendicular to that of the paper. The problems with thermal cycling have been taught and to minimise fretting of adjacent faces 13A I 14A and 13B I 14B, strips of thermally resistant, resilient padding (not shown) are placed between all interengaging members.

The design of the interengaging membersl3 and 14 in both horizontal and vertical planes means that there are no lines of sight through walls 2, 3 or 4 or floor 5 of kiln 1 for radiation to shine' out. It will be understood that appropriate no-shine arrangements are provided between end blocks 11 and end faces 8 and that end waIls 3 and 4 mate neatly with end faces 8. In a typical kiln 1, the internal width 19 might be 1000 mm and the thickness 28 of blocks II would be 200 mm.

Figs. 1 and 2 show the blocks 11 of body 2 of kiln I arranged one vertically above the other.

This is acceptable for short kilns I provided with restraints 9 but for long tunnel kilns (not shown), staggering of blocks 11 in adjacent rows is preferred.

Fig. 5 shows the process, of casting a typical wall block, e.g. 1 IC. A stainless steel, sheet metal mould 20 consists of a parallel-sided body with a female engagement section at the lower left and a male engagement section at the upper right. Mould 20 is secured with a quick release means 22 -a nut and bolt are shown as indicative of a suitable means which is operable at cryogenic temperatures. The acute angle 15 (nominally 58° as taught previously) is shown defining the female engagement 14 and this is replicated at the upper right (male engagement 13) of the mould (though not shown to avoid confusing detail). A leg 35 is provided so that, when standing on a horizontal surface 36 on leg 35 and edge 37, filling hole is in the uppermost part of the mould body 20. Vent holes 24 are provided to release any air I gas trapped during the filling of the mould 20.

Mould 20 may be used cool, e.g. at about 4°C, or placed in the cryogenic chamber (not shown) and sprayed 27 with liquid nitrogen at -196°C to pre-cool to about -50°C. Then, mould 20 is filled 26 (brim full, as shown, Fig. 5) with a first freeze-castable ceramic slurry, which has been cooled to just above its freezing point. Slurry 29 may be poured or pumped in 26 through hole 25. The filling may be done in the cryogenic chamber, with the sprays 27 turned off, or outside it, as convenient. Mould 20 is immediately placed I returned to the cryogenic chamber and sprayed 27 for a predetermined period. As shown, sprays 27 are all round mould 20 so that every part is cooled and the rate of cooling is essentially uniform. After the predetermined period, an essentially-uniformly, thick, frozen layer 28 will have formed on the inside of mould 20. Now mould 20 is removed from the cryogenic chamber and the remaining liquid slurry 29 poured out 26 through hole 25.

According to the invention, there are now two options, viz:- 1. Removing the green component from mould 20, drying and firing to give a hollow shell (first embodiment); or 2. Refilling with a second slurry and re-freezing to give a solid component (second embodiment) (Shown in Fig. 5).

Considering the first option (embodiment), green component 28 is now removed from mould 20 by undoing securing means 22 and gently flexing 30 the two sides 21 away from centreline 23. (For simplicity, mould 20 is shown as a single line 20, 21, spaced slightly apart from the frozen edge of block 11 C.) With mould 20 still at about -20°C, gentle flexing 30 of sides 21 releases faces I 3A and I 3B. Further flexing 31 releases the parallel sides of block 11 C and still further flexing 32 frees faces 14A and 146, without breaking tips 34. It will be noted that the size and position of leg 35 does not impede gentle flexing 30, 31, 32 of mould 20.

The green component 28 is now allowed to warm up and dry, firstly in the ambient air and then in a oven at 40-100°C before firing at 1150-1350°C. The result is a mechanically very strong shell 28 with a hollow centre 29 and an access hole 26. For applications in a kiln 1, the internal void 29 is filled with insulation through hole 26, e.g. by packing with a fibrous or particulate insulating material. To retain the insulation in place, hole 26 may be filled with a plug (not shown).

Thus, by using the first embodiment of the invention, blocks 11 for a kiln I can be made. If the insulation is fibrous in nature, e.g. Rockwool, the blocks will be light and easy to handle. As can be seen, the combination of a mechanically strong shell with the option of a variety of fillings gives a wide range of potential applications all falling with in the scope of the invention.

If required, the internal void may be left empty.

Considering the second option (embodiment), mould 20 remains unopened and a second, cold, liquid, freeze-castable ceramic slurry is poured in 26 through hole 25 to fill void 29 right up to the brim (as shown, Fig. 5). Mould 20 is now returned to the cryogenic chamber (not shown) and spraying 27 resumed until all the second slurry has frozen.

Because second slurry 29 is directly in contact with the inner surface 28A of cryogenic shell 28, freezing occurs from this interface (28A) inwards, i.e. towards centreline 23. Thus, second freeze casting 29 will be rigidly bonded to first freeze casting 28 via interface 28A and will completely fill void 29. This bond is important as it means that there are no voids at I near interface 28A, which could act as paths for promoting heat transfer, e.g. via convection currents, in an otherwise contiguous insulating medium.

When all the second slurry 29 was frozen, mould 20 would be removed from the cryogenic chamber and the green casting demoulded, dried and fired as described above. It will be noted that it would be possible to decant the remaining unfrozen second slurry part way through the freezing process and either leave a small void around centreline 23 or refill with a third slurry and complete the freezing.

Thus, the second embodiment of the invention greatly increases the design options for precision-cast, engineering ceramics. Though components for kilns, etc. have been taught in the description, applications in many other areas of engineering are equally possible, for example, the second casting 29 could have particular dielectric properties for use in electrical insulation, etc. Other applications include making moulds from which panels for motor car can be manufactured, linings for components in the combustion chambers of boilers and engines, etc. Another major application could be as insulating linings for structural members in buildings, e.g. profiled members to fit round the H-shaped metal beams, or concrete columns, used in towers such as were destroyed on I 1th September 2001.

As examples of the slurries applicable to the invention, a typical sillimanite first slurry formulation is:-Ingredient Grade I Supplier Wt% Alumina (tabular) -325 mesh / Alcoa 9.4 Fused Silica -325 mesh I Minco Ltd. 1.9 Alumina CT3000 I Alcoa 4.2 Alumina CT9FG /Alcoa 11.0 Molochite 16/30 I ECC International Ltd. 22.0 Molochite 8/16 / ECC International Ltd. 22.0 Molochite -200 mesh I ECC International Ltd. 17.7 Nyacol Silica sol 15/30/ Nyacol Nano Technologies Inc. 11.8 A typical second, insulating slurry formulation based on fused silica is:-Fused Silica -325 mesh! Ceradyne Minco 20.6 Fused Silica 30/50/ Ceradyne Minco 19.8 Fused Silica 50/100/ Ceradyne Minco 11.9 Fused Silica 120 mesh/ Ceradyne Minco 23.8 Fumed Silica 983U Elkem 3.2 Momsol AS 20/40 Momsons Chemicals 15.9 Glycerol Reagent or food grade 4.8 The skilled person will appreciate that the key to the invention is achieving the optimum combination of mechanical and other properties of each of the two ceramics and the correct balance between the porportions of the two ceramics. An important factor in achieving this is to maximise the temperature difference driving the freeze casting process. In engineering parlance, this temperature difference is referred to as aT and the apparatus of the invention is designed to optimise T, for example:- * The freeze-castable slurry is pre-cooled (e.g. ideally to less than +1°C) prior to filling the mould (this minimises sensible heat transfer requirements).

* The mould is pre-cooled (to about -20°C) before filling (also minimising sensible heat transfer requirements).

* The mould is made of highly, thermally, conductive, thin, sheet metal to minimise thermal inertia and maximise heat transfer.

* Liquid nitrogen, at -196°C, is used as the cryogenic coolant.

* The liquid nitrogen is sprayed 27 directly onto the metal surface of the mould. This results in direct evaporative contact with the metal surface so that the latent heat of evaporation contributes directly to the cooling and the boiling of the liquid on the metal surface maximises heat transfer.

* Continuous spraying eliminates the creation of any boundary layer, which might otherwise reduce heat transfer.

Thus, the T driving the heat transfer is (-)196 + 1) = (-)197°C + the latent heat of evaporation of nitrogen.

The mechanics of freeze casting has been taught and, as a general rule, the smaller the ice crystals and, hence, the smaller the geodetic-type structures surrounding them, the stronger the ceramic structure will be. Thus, adjacent to the metal surface of mould 20, freezing will be extremely rapid under effectively the whole of the (-)197°C M. However, as the thickness 288 of the layer 28 increases so the rate of heat transfer will reduce and progressively larger ice crystals will be formed as layer 28 gets thicker, giving a slightly weaker structure. Thus, there is an optimum thickness 28B for shell 28 and this is achieved by the predetermined freezing time before first slurry is poured out. Second casting 29 Will also contribute to the overall strength of block 11 C but the greater strength resides in shell 28.

The skilled person will appreciate the immense value that the invention gives to designers with the wide range of options to create specific blocks 11, or other components, with unique properties for any particular application.

Claims

Claims:- 1. In a first embodiment of the invention, there is provided apparatus to make the components of an insulating or fire protective structure comprising a plurality of freeze-cast ceramic elements, each element consisting of a shell formed from a freeze-castable ceramic slurry having an internal void which is filled with a fibrous and I or foam and I or particulate insulation or may be left empty.
2. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in Claim 1, wherein the elements have interengageable parts so that the interengageable part of one element is interengegeable with the complementary part of another element to form a structure or part of a structure.
3. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in Claim 2, wherein the interengageable parts are of the form of male and female parts.
4. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in Claim 3, wherein the male I female interengagement is in the horizontal plane or in the vertical plane or in both planes together or in a plane(s) at an angle to the horizontal.
5. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as daimed in Claim 3, wherein the elements between which the male and female parts interengage form part of a curve or an angle.
6. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as daimed in Claims 4 or 5, wherein the male I female interengagement is such as to prevent any line of sight from one side of the structure, through I between the element(s) of the structure to the other side of the structure.
7. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in Claim 6, wherein the interengaging parts of two adjacent interengaged elements form a mechanically strong connection between said two elements restricting relative movement therebetween and that said two interengaged elements either form a structure or form the basis for incorporating further interengaging elements therewith so that a larger structure may be built from said interengaging elements.
8. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in Claim 7, wherein the interengagement is such that the acute angle between two adjacent interengaging elements is 600, or less, so that the interengagement forms a positive location and a mechanically strong connection therebetween.
9. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in any preceding Claim, wherein resilient padding is provided between the interengaging parts of the interengaging elements to accommodate and cushion minor relative movements between adjacent interengaging elements.
10. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as daimed in any preceding Claim, wherein the elements are cast in a mould having a low thermal capacity and a high thermal conductivity through the sides thereof.
11. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in Claim 10, wherein the mould is made of a metal sheet(s).
12. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in Claim 11, wherein the mould is made of a single metal sheet or a plurality of metal sheets which can be assembled into the complete mould and disassembled after casting to release the green component from the mould.
13. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as daimed in Claim 12, wherein quick assembly / disassembly means are provided to connect / disconnect the metal sheet(s) or part(s) of a metal sheet(s) together to form the mould and / or open / close said mould.
14. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in Claim 13, wherein the quick assembly I disassembly means are operable when the mould is at the temperatures associated with cryogenic freeze casting.
15. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in Claims 11 to 13, wherein the metal sheets forming the mould are flexible to promote the release of the green component from the mould after casting.
16. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in Claim 15, wherein the metal sheet(s) forming the mould are designed so that they may be flexed progressively away from the point, where the quick assembly I disassembly means is operated, along the sides of the component, so that the whole of the component is successively and I or progressively separated from the metal sheet(s).
17. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in any preceding Claim, wherein a release agent is applied to the faces of the mould which will be in contact with the freeze-casting slurry to promote the release of the green component after casting.
18. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in Claim 17, wherein the metal sheet(s) of which the mould is formed are stainless steel or an equivalent high quality alloy.
19. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in any preceding Claim, wherein the mould is provided with a base I support means on which to stand stably during the filling and freeze-casting process.
20. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in Claim 19, wherein the base / support means is adapted to permit all parts of the external surfaces of the mould to have direct contact with the cryogenic cooling medium.
21. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as damed in any preceding Claim, wherein a hole(s) is I are provided at a suitable position(s) in the mould so that, when standing stably on its base / support means, the hole(s) is I are at the highest point(s) of the mould so that it may be filled to the brim with a freeze-castable ceramic slurry.
22. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as daimed in any preceding Claim, wherein an attachment(s) is I are provided at a suitable point(s) on the mould so that, when standing stably on its base / support means, a freeze-castable slurry may be pumped into the mould so that it may be filled to the brim with said slurry.
23. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in any preceding Claim, wherein vent hole(s) is / are provided in the mould so that, when standing stably on its base I support means, the mould may be completely filled with freeze-castable ceramic slurry without incorporating any air I gas bubbles therein.
24. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in any preceding Claim, wherein the freeze-castable ceramic slurry is pre-cooled to a temperature just above its freezing point before pouring I pumping into the mould.
25. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in Claim 24, wherein the mould is pre-cooled to a temperature below 0°C prior to filling with freeze-castable ceramic of slurry.
26. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as daimed in any preceding Claim, wherein a cryogenic medium is applied to the mould after the mould has been filled with freeze-castable ceramic slurry.
27. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in Claim 26, wherein the slurry-filled mould is placed in a cryogenic environment for a predetermined time.
28. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in Claim 27, wherein the ayogenic environment is provided with means of enhancing heat transfer from the slurry-filled mould.
29. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as daimed in Claim 28, wherein the means of enhancing heat transfer includes cryogenic sprays.
30. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as daimed in Claim 28, wherein the means of enhancing heat transfer includes immersion in a bath of cryogenic liquid.
31. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as daimed in any preceding Claim, wherein the means of drying the green component is placing it on a structure in ambient air, with / without forced convection.
32. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in Claim 31, wherein the means of drying the green component is in an oven.
33. A first embodiment of an apparatus to make the components of an insulating or fire protective structure, as claimed in any preceding Claim, wherein the means of finng the green component is in a kiln or equivalent apparatus.
34. A first embodiment of a method of producing freeze-cast ceramic elements, consisting of a shell containing a fibrous, and I or foam and I or particulate insulation within the shell, for an insulating or fire protective structure comprising the steps of:-i) providing a mould; ii) placing the mould in a cryogenic environment for a predetermined period of time to pre-cool it prior to use; iii) removing the mould from the cryogenic environment and filling the mould with a pie-cooled, liquid, freeze-castable, ceramic slurry; iv) replacing the mould in a cryogenic environment for a predetermined period of time; v) removing the mould from the cryogenic environment and decanting the unfrozen, liquid slurry from the mould; vi) removing the green component from the mould; vii) drying the green component; viii) firing the green component to produce a freeze-cast ceramic shell element; ix) filling the void inside the shell element with a fibrous and / or particulate insulation; and x) providing the freeze-cast, ceramic, insulated element for use in an insulated or fire protective structure.
35. A first embodiment of a method of producing freeze-cast ceramic elements, as claimed in Claim 34, wherein the mould comprises a number of pieces which are assemblable prior to filling with the slurry and disassemblable to facilitate the removal of the green component after freeze-casting.
36. A first embodiment of a method of producing freeze-cast ceramic elements, as claimed in Claim 35, wherein a release agent is applied to the internal faces of the mould prior to filling with ceramic slurry.
37. A first embodiment of a method of producing freeze-cast ceramic elements, as claimed in Claim 34, wherein the cryogenic environment is created by placing the mould in a chamber and spraying a cryogenic liquid onto the external faces of the mould.
38. A first embodiment of a method of producing freeze-cast ceramic elements, as claimed in Claim 34, wherein the cryogenic environment is created by immersing the mould in a cryogenic liquid.
39. A first embodiment of a method of producing freeze-cast ceramic elements, as claimed in Claim 34, wherein the method of filling the mould with ceramic slurry is to pour the slurry into the mould through a hole provided at the topmost part of the mould and to fill the mould up to the rim of the filing hole.
40. A first embodiment of a method of producing freeze-cast ceramic elements, as claimed in Claim 34, wherein the method of filling the mould with ceramic slurry is to pump the slurry into the mould until it is brim full at the filling I air-exit hole(s) provided in the topmost part of the mould.
41. A first embodiment of a method of producing freeze-cast ceramic elements, as claimed in Claim 34, wherein the green component is removed from the mould while both mould and green component are at cryogenic temperatures.
42. A first embodiment of a method of producing freeze-cast ceramic elements, as claimed in Claim 41, wherein the method of removing the green component from the mould includes releasing the means locking the mould in the closed position and flexing the panels forming a part(s) of the mould to break adhesion(s) between the panels and the green component.
43. A first embodiment of a method of producing freeze-cast ceramic elements, as claimed in Claim 34, wherein the method of drying the green component includes drying in air at ambient temperatures.
44. A first embodiment of a method of producing freeze-cast ceramic elements, as claimed in Claim 34, wherein the method of drying the green component includes drying in an oven.
45. A first embodiment of a method of producing freeze-cast ceramic elements, as claimed in Claim 34, wherein the method of drying the green component includes forced convection.
46. A first embodiment of a method of producing freeze-cast ceramic elements, as claimed in Claim 34, wherein the method of firing the dried, green component includes placing in a kiln.
47. In a second embodiment of the invention, there is provided an insulating or fire protective structure comprising a plurality of freeze-cast ceramic elements, each element consisting of a shell formed from a first freeze-castable ceramic slurry having an internal void which is filled with a second freeze-castable ceramic slurry.
48. An insulating or fire protective structure, according to the second embodiment of the invention, as daimed in Claim 47, wherein the shell formed from the first freeze-castable ceramic slurry has high mechanical properties and the material in the internal void formed from the second freeze-castable ceramic slurry has high thermal insulating properties.
49. A method of producing an insulating or fire protective structure, according to the second embodiment of the invention, wherein the structure comprises a plurality of freeze-cast ceramic elements, each element consisting of a shell formed from a first freeze-castable ceramic slurry having an internal void which is filled with a second freeze-castable ceramic slurry comprising the steps of.-i) providing a mould; ii) placing the mould in a cryogenic environment for a predetermined period of time to pre-cool it prior to use; iii) removing the mould from the cryogenic environment and filling the mould with a first, pre-cooled, liquid, freeze-castable, ceramic slurry; iv) replacing the mould in a cryogenic environment for a predetermined period of time; v) removing the mould from the cryogenic environment and decanting the unfrozen, first, liquid slurry from the mould; vi) refilling the mould with a second, liquid, freeze-castable, ceramic slurry; vii) replacing the mould in the cryogenic environment for a further predetermined period of time until all the second freeze-castable slurry is frozen; viii) removing the mould from the cryogenic environment; ix) removing the green component from the mould; x) drying the green component; and xi) firing the green component to produce a freeze-cast ceramic element; and xii) providing the freeze-cast, ceramic element for use in an insulated or fire protective structure.
50. A method of producing an insulating or fire protective structure, according to the second embodiment of the invention, as claimed in Claim 49, wherein the mould containing the second, liquid, freeze-castable, ceramic slurry is returned to the cryogenic environment for a further predetermined period of time until a given quantity of the second slurry has frozen after which the mould is removed from the cryogenic environment, the remaining second, liquid slurry decanted, the mould refilled with a third, liquid, freeze-castable, ceramic slurry and returned to the cryogenic environment until all the third, liquid slurry has frozen.
51. A method of producing an insulating or fire protective structure, according to the second embodiment of the invention, as claimed in Claim 49, wherein there is provided an insulating or fire protective structure comprising a plurality of freeze-cast ceramic elements, each element consisting of a shell formed from a first freeze-castable ceramic slurry having an empty internal void.
52. Apparatus and methods for an insulating or fire protective structure as described in and by the above statement, with reference to the accompanying drawings.