GB2267454A - Forming hollow articles by using ice cores - Google Patents

Abstract

The invention provides a method of making a sintered artifact from a sinterable particulate material. An ice core 20 is located in a mass of the particulate material 18, and the particulate material is compressed and consolidated around the core to form a green artifact in which the core is embedded. The core is then removed, preferably by sublimation, from the artifact to leave a cavity therein; and the green artifact is then sintered to produce a sintered unitary artifact having at least one cavity left therein by the core removal. Sublimation may take place through the walls of the hollow artifact or through an opening provided in a wall of the artifact. <IMAGE>

Description

2267454 Method of Making Artifact THIS INVENTION relates to a method of

making an artifact from particulate material. More particularly it relates to a method of making an artifact in the form of a holder, eg of ceramic material suitable for holding electrode material in a high temperature rechargeable electrochemical cell.

According to the invention there is provided a method of making a sintered artifact from sinterable particulate material, the method comprising the steps of:

locating at least one core at least partially formed from ice in a mass of the particulate material; compressing and consolidating the particulate material around each core to form a green artifact wherein each core is at least partially embedded; removing each core from the green artifact to leave a cavity therein; and sintering the green artifact to produce a sintered unitary artifact having at least one---.

cavity left therein by the core removal.

While the particulate material may be metallic, it is preferably a ceramic material is or a precursor thereof, so that the sintered artifact produced is of ceramic material, the particulate material having a particle size of 10 - 200,um.

By 'precursors' with reference to the particulate ceramic material are meant particulate substances or mixtures which, when heated during the sintering step, are transformed or converted to the sintered ceramic material of the sintered artifact. The particulate ceramic materials or precursors thereof preferably have an average particle size of 20-50 pm when isostatic pressing is used and an average particle size of 50-100pm when uniaxial- or die pressing is used.

i 2 In particular, the particulate material may be a solid electrolyte ceramic material of precursor thereof, the artifact being in the form of a holder for holding active electrode material in a high temperature electrochemical power storage cell.

As each core will comprise ice, the core removal step may comprise sublimation of at least part of the ice of each core, at a temperature and pressure at which the ice evaporates without melting. In other words, the core removal step may thus involve, at least in part, removal of at least part of each core by freeze-drying, without the formation of any liquid water.

Typically, each core will be made by casting it of a suitable shape at atmospheric pressure by freezing it in a suitable mould or in the form of a slab. The water used for such casting may be gas-free, containing no dissolved gas and, in particular, preferably no carbon dioxide, so that no carbon dioxide gas is produced during the sublimation, which carbon dioxide can adversely affect solid electrolyte materials or their precursors. Gas removal from the water can easily be effected by boiling it prior to freezing it, and is deionized water is preferred. Accordingly, each core may be formed by casting it at atmospheric pressure by freezing, the water used therefor being gas-free.

While each core may naturally be formed entirely from ice, in a particular embodiment of the invention each core may be formed by casting a mixture of ice and of the sinterable particulate material, so that each cavity in the sintered artifact contains, in its interior, a filling formed from said particulate material, in porous, liquid-permeable form, the filling acting to enhance the strength of the sintered artifact. Instead, each core may be formed to be of a composite nature, comprising an inner portion, eg in the form of a mandrel, which is reusable, of steel or the like, th.e inner portion having a surface layer comprising ice frozen thereon.

The locating of each core and the mass of particulate material, and the subsequent compressing and consolidating of the particulate material, may take place at a c temperature and pressure such that each core remains solid during these steps. Thus, the locatina of each core in the particulate material may take place at atmospheric ambient c 3 pressure and at a temperature of <OT the compressing and consolidating taking place at a temperature of <4T and core removal taking place, after the compressing and consolidating have been effected, by reducing the ambient pressure on the green artifact to a value at which the ice of the core sublimes. More particularly, core removal may take place by exposing the green artifact to a temperature between -30T and OT, and to an ambient pressure of 10 - 500 Pa, preferably 50 - 500 Pa, a particular example being exposing the green artifact to a temperature between -150C and -100C, and to an ambient C C1 pressure of 100 Pa.

The method may involve forming one or more closed cavities in the green artifact, in which case core removal may be entirely by sublimation/freeze drying, water vapour passing by diffusion through the consolidated particulate material until the core or cores have gone and only water vapour remains in each cavity. Instead, if one or more openings are formed in the green artifact, which place one or more cavities in the artifact in communication with the exterior of the artifact, then, depending on the shape of each core, it may be preferred to sublime off no more than a surface layer on the core, eg an ice layer on a mandrel as mentioned above, after which the core can be physically removed from the artifact, eg by unscrewing the mandrel from the artifact, if the mandrel is screw-shaped.

When the core or cores are in the form of mandrels, each mandrel may be screw shaped, as described in the Applicant's Published British Patent Application 2 250 467A or, instead, each mandrel may be of conventional shape, having a smooth cylindrical outer surface and a domed end. In each case the pressing may be in the form of isostatic pressing, using a sheath such as a latex sheath over the ice-coated mandrel for this purpose, a layer of the particulate material being provided around the end of the mandrel and the adjacent part of its length, with the sheath enclosing the particulate layer. If the mandrel is screw shaped it can, after sublimation of the surface layer, be unscrewed axially from the green artifact as mentioned above, or if it is of smooth cylindrical shape, it can simply be withdrawn axially therefrom.

4 Instead, the method may be used to make a holder in the form of a laterally compressed and flattened envelope, in which case the envelope can be shaped and pressed as described in the Applicant's published British Patent Application 2 255 309A,and in which case uniaxial or die pressing is conveniently employed, each core being shaped and the particulate material being arranged such that at least part of each core is in the form of a thin slab or layer sandwiched between a pair of layers of the particulate material so that, after the sintering, at least part of each cavity is in the form of a thin gap between opposed plates of sintered ceramic material, each slab or layer having at least one opening therethrough which is filled with the particulate material, the particulate material in each opening, after consolidation thereof in the gap by the compressing and after the sintering, forming a bridge across the gap between the associated plates and sintered thereto, the bridge being capable of acting as a strut or tie between the associated plates, for reinforcing the holder. This approach is suitable when the holder is intended to hold solid electrolyte material in a high temperature electrochemical cell, so that, as indicated above, the holder is pressed to have a compressed or flattened shape, so that it is, for example, a laterally flattened envelope with a pair of oppositely outwardly facing major faces, at least one of the cavities being close to at least one of said major faces of the holder.

In this case, when the cavity or cavities are closed as described above, each core-- is entirely surrounded by the particulate material so that, after the compressing, it is fully embedded in the consolidated particulate material, and so that the sintering results in an artifact having a closed cavity therein, the method including forming a loading opening C) 0 D into the cavity from the exterior of the holder after the sintering.

In this case the cavity in the artifact can be kept closed, until it is to be charged with active electrode material. Accordingly, a suitable charging or loading opening for active electrode material can, if required, be machined into the cavity immediately before loading with active electrode material. This keeps the surface of the artifact exposed to the cavity in a clean, pure state, and leads to enhanced shelf-life of the artifact. This can be important when the active electrode material is a molten alkali metal such as sodium.

Naturall, instead, the core can have a projectiori which projects throii"h the particulate y 11.

material and which, when the core is removed, leaves a feedthrough or loading opening, and in this case, during removal of the core by heating, sublimed core material can issue from this opening, instead of having to permeate through the particulate material, which is does when the cavity is kept closed.

In general, compressing and consolidating the particulate material may be by isostatic pressing or uniaxial (die) pressing, or uniaxial pressing followed by isostatic pressing, after locating the core in the mass of particulate material in the interior of a mould. The consolidation leads to the production of a green artifact surrounding the core, which should be of sufficient strength to remain intact during the subsequent core removal and sintering. The pressing can be carried out at pressures of 30 - 150 MPa, preferably - 100 MPa. To obtain good green densities and green strengths in the green artifact, the method may include admixing a suitable binder into the particulate material before the core is located therein. This binder can act as a lubricant to lubricate the pressing, and suitable binders include, for example, polymers or waxes which may be soluble in aqueous or organic solvents, such polymers or waxes including polyvinyl butyrate, polyvinyl acetate, polyvinyl alcohol, polyethylene glycol, polyethylene oxide, and other polymers, waxes and binders known in the art. These binders may form from 0,5-30% by mass of the mixture of binder and said electrolyte/precursor, conveniently 0,5 - 15%.

When the ceramic artifact has a flattened shape and each core is a slab, the compressing and consolidating of the particulate material may be by uniaxial pressing or die pressing in a metal die. However, if a more complex ceramic artifact is required, isostatic pressing by means of a flexible bag or sheath is preferably employed for the compressing and consolidating.

Generally, the binder will remain in place after the ice part of the core has been sublimed. This can be desirable as it permits, after initial removal of the core material, - the binder to remain in place to strengthen the green artifact prior to sintering. The binder will typically be driven off during the initial part of the heating which is carried out to sinter the artifact.

6 When the holder is intended in use for connection to a reservoir of active electrode material, eg a reservoir of molten sodium, a single cavity can be formed therein which is thin and of low volume, ie a thin gap as described above, close to at least one surface of the holder. However, when the holder is not intended for connection to a reservoir of anode material, it may, in addition, have such reservoir formed therein by a further cavity, which can be of more substantial volume.

Accordingly, the method may employ eg two or usually three cores embedded in the particulate material, to provide two or three c avities in the sintered artifact, one of which will be a thicker core intended to provide the reservoir, the other core or cores being thinner and intended to provide an electrode space near the surface of the artifact for enhanced ion conduction. Two cores will be used when the artifact is intended to contain a reservoir of active electrode material and is intended for use in an electrochemical cell wherein the artifact is located on one side of the other active electrode material of the cell. When the artifact is intended to contain a reservoir of said active electrode material and is intended for use in an electrochemical cell sandwiched between two electrode portions of the other active electrode material of the cell, three cores will be employed. When three cores are employed, a thicker core will be a central core, and two smaller thinner cores will be arranged on opposite sides thereof.

As indicated above, an important application of the sintered ceramic artifacts made by the method is expected to be as electrode holders in high temperature rechargeable electrochemical cells, usually molten alkali metal anode holders. In this case the solid electrolyte material used, or its precursor, will be selected to provide a ceramic artifact which is a conductor of ions of the alkali metal in question. For cells of the sodium/sulphur type or those with molten sodium anodes and cathodes which comprise transition metal halide active cathode materials dispersed in a matrix of electronically conductive material which is porous and permeable and impregnated with alkali metal haloaluminate molten salt electrolyte, the said electrolyte material of the artifact may thus be nasicon, P-alumina or preferably P"-alurnina.

7 Suitable ceramic solid electrolytes can also include analogues of P- or P"-alurnina wherein the sodium ions of P- or P"-alumina are at least partially substituted by other metal ions, so that such ceramics are conductors of such other metal ions (for cells wherein the anodes are such other metals).

When the holder is of solid electrolyte ceramic material, it will typically be used in a high temperature rechargeable electrochemical power storage cell comprising a pair of electrodes, namely an anode and a cathode, and the holder, one of said electrodes being held in the holder, and the wall or walls of the holder acting as a solid electrolyte separator between the anode and the cathode, said solid electrolyte separator being a conductor of ions of the active anode material of the cell.

Furthermore, such solid electrolyte holder, when it holds said electrode material of a cell, can provide an electrode structure for a cell, eg an anode structure.

Conveniently the electrode held by the holder is the anode, the active anode material typically being a metal, such as an alkali metal, for example sodium (when the ceramic solid electrolyte material is nasicon, Palurnina or P"-alumina).

The invention extends to a sintered ceramic artifact whenever made by a method as described above.

The invention will now be described, by way of example, with reference to the followina illustrative Example and diagrammatic drawings, in which:

Figure 1 shows a schematic sectional side elevation of a green holder being made according to the method of the present invention,during uniaxial pressing thereof into a green artifact by means of a die while located in a mould; Figure 2 shows a schematic three-dimensional view of a core for use with the mould of Figure 1; Figure 3 shows a view similar to Figure 2 of another core for use with the mould of Figure 1; 0 8 Figure 4 shows a schematic sectional side elevation of a further core for use in the mould shown in Figure 1; Figure 5 shows a schematic sectional side elevation of a further green holder being made in accordance with the method of the present invention, during isostatic pressing thereof around a mandrel while located in a mould; Figure 6 shows a sectional side elevation of the mandrel shown in Figure 5; Figure 7 shows a schematic flow diagram of apparatus used for sublimation in accordance with the present invention; Figure 8 shows a plot of the melting point of ice against pressure; and Figure 9 shows a phase diagram of water wherein the phases are shown on a plot of pressure against temperature.

In Figure 1 of the drawings, reference numeral 10 generally designates a mould and die arrangement, during the uniaxial pressing of a holder in the form of a compressed, laterally flattened envelope in accordance with the method of the invention.

The arrangement 10 comprises a mould or die body 12, a movable die plunger 14 and a movable die plunger 16.

In the interior of the mould body is shown a mass of particulate P"alumina particles 18 of a particle size of 50 - 100,um, admixed with 15% by mass of water soluble wax, namely polyethylene glycol. Embedded in the mass of particles 18 is a core 20, which is of ice (see also Figure 2 in which this core is designated 20) made from finely crushed ice pressed in a suitable mould at -2T at a pressure of 20 MPa. Thus, finely crushed ice can be pressed at a suitable pressure in a suitably shaped uniaxial die and plunger set at about -2T, to form the core with openings 22 (described hereunder) therethrough. The cores can then be stored at eg -15T until use thereof.

Referring also to Figure 2, the core 20, which is in the form of a flat rectangular slab or plate, has a plurality of tubular openings 212 therethrough, evenly distributed in 0 spaced relationship, over its full area and interconnecting its major faces 24.

9 In accordance with the method of the invention, the arrangement 10 is set up with the plunger 14 retracted and the plunger 16 in place, as shown in Figure 1. A particulate mixture of said P-alumina particles 18 and wax, premixed as described hereunder, is charged into the mould interior, and a preformed ice core 20 is embedded in the mixture 18 as shown. This is done by charging a more or less flat layer comprising about half the mixture 18 into the mould interior, placing the core 20 on said layer, and charging the remainder of said mixture 18 into the interior on top of the core 20, as a second flat layer, which also fills the openings 22 and the peripheral spaces between the core 20 and the walls of the mould or die body 12. The plunger 14 is then urged uniaxially in the direction of arrow 26 towards the plunger 16 which acts as an anvil, to compress the mixture 18 around the core 20 and in the openings 22. The plunger 14 is then retracted in the opposite direction and the green artifact produced, containing the core 20, is removed from the mould 12.

This pressing is carried out to a pressure of eg 45-50 MPa with the core 20 at -15'C is and the mixture 18, mould 12 and plungers 14, 16 at a temperature of - 10'C, in an environment at -10'C. Instead, if the procedure can be carried out sufficiently quickly to prevent the ice from melting, the compressing of the mixture can be carried out in an environment at ambient temperature, but with the core, powder and arrangement 10 respectively pre-cooled in similar fashion to -15'C or -10'C, as the case may be. - The green artifact is first heated gently under vacuum to sublime the ice core as described hereunder. Subsequently the green artifact is further heated up to 40WC (optionally up to 500'C) in air to remove free water and polyethylene glycol associated with the particulate mixture. The green artifact is then heated further, to 1600'C in air to sinter the P"-alumina particles together, to form a continuous, unitary sintered polycrystalline P"-alurnina artifact.

This artifact is a hollow envelope of flattened shape and has a flattened continuous interior cavity in the form of a gap vacated by the core 20 between sintered plates formed from sintered material 18 formed by the major faces of the envelope. The P"-alLIMina material 18 in the openings 22 is sintered into pillars (ties or strUtS) integral with, and strengthening, reinforcing and spacing apart, the major faces 24 of the envelope which are provided by plates formed from the layers of mixture particles 18 on opposite sides of the core 20 in the mould 12. These major faces 24 are bound together at the periphery of the envelope by mixture 18 charged into the peripheral spaces between the edges of the core 20 and the mould 12.

In this regard it will be noted that the core 20 (Figure 2) can have an outwardly projection in the form of a tab or ear 28, midway along one of its side edges. The core is loaded into the mould 12 so that the car 28 touches the mould wall at 30 (Figure 1).

After the core removal and sintering, the ear 28 leaves a space which forms a feedthrough or loading opening from the exterior of the envelope through said side edge thereof, into the interior cavity of the envelope vacated by the core 20.

In contrast, in Figure 3, the ear 28 is omitted and is replaced by a pair of truncated cylindrical bosses 32, respectively in central positions on opposite sides of the core on its outwardly directed major faces and one of which is visible in Figure 3. The core of Figure 3 is located in the mould in a fashion such that there is no mixed material 18 between the bosses 32 and the plunger 14 and anvil 16 respectively. After the core removal and sintering, the spaces vacated by these bosses provide the envelope with a pair of central opposed openings through the major faces of the walls of the envelope.

As a variation of the method described above, it should be noted that no special provision (such as the car 28 of Figures 1 and 2 or the bosses 32 of Figure 3) need be made for openings into the envelope, prior to sintering. In principle the evaporation or sublimation can take place without any opening into the interior of the envelope, as water vapour from the ice core 20 can diffuse out through the walls of the envelope before they densify on sintering. Any opening(s) into the interior of the envelope can then be made where desired after sintering, eg by machining.

A further variation of the method involves the use of profiled faces on at least one of the plungers 14, 16, c. as shown at 34 on the upper plunger 14 in Figure 1. The face 0 c) c 1) in question is recessed inwardly from a peripheral strip 36 by a shallow step at 38. This feature leads to enhanced densification along the periphery of the green envelope, and of the final envelope after sintering, the degree of densification increase depending on the compressibility of the core 20 and mixture 18.

A still further variation of the method involves the use of plungers whose pressing faces have been coated with a layer of flexible material, eg polyurethane. This assists with uniform pressure application across the entire face of the envelope.

In this regard it should be noted that, in use, the envelopes are intended to hold molten sodium anode material in high temperature electrochemical power storage cells having molten sodium anode material, and the openings provided by the car 28 or bosses 32 are intended as inlets/outlets for placing the interior cavity of the envelope in communication with reservoirs of molten sodium, and/or with other similar envelopes containing molten sodium.

As with the cores of Figures 2 and 3, the core 20 of Figure 4 is in the form of a flat slab or plate of rectangular outline, having a plurality of openings 22 therethrough which are evenly spaced from one another and are spread across its full extent. As in the case of Figures 2 and 3, each opening 22 of the core 20 interconnects the plates which provide the major faces 24 of the core 20 and is in the form of a passage of roughly----hourglass-shape when seen in sectional side elevation, as viewed in Figure 1, having walls which bulge convexly inwardly so that it has a narrow waist portion into which lead entrances at opposite ends of the passage. The entrances are countersunk and taper inwardly, being convexly curved in sectional side elevation. The peripheral edges 39 of the core 20 are rounded and convexly curved, being of similar profile in sectional side elevation to the walls of the passages 22.

With reaard to the hourglass-shape of the ties or struts arising from the shape of the passages 22, whereby they have rounded edges where they join said major faces, and with reeard to the rounded peripheral edges of the envelope (caused by the rounded edge W 0 0 39 of the core 20), it will be appreciated that these tend to resist cracking of the sintered artifact. Such crackin. can be caused by thermal stresses and by stresses arising from 12 pressure changes across the walls of the envelope. It should further be noted that in fact, as mentioned above, no opening into the green artifact is necessary to permit the subliming or evaporating ice to escape. Thus, no provision is made in the core of Figure 1 to provide such opening (in contrast to the car 28 and bosses 32 respectively of Figures 2 and 3). The water vapour can in fact diffuse through the walls of the green artifact, which are sufficiently porous for this purpose, although they become substantially hermetically airtight after sintering. The absence of such opening can be an advantage, as the interior of the holder is protected and kept in a pure state, for enhanced shelf-life.

If desired, an opening into the interior cavity of the artifact can be machined, eg by drilling, shortly before use.

In Figure 4 the core is generally designated 20 and the same reference numerals are used for the same parts thereof as in Figure 1. A further difference between the core of Figure 4 and those of Figures 2 and 3 is that the core 20 of Figure 4 has a surface layer 40 containing particles of wicking material, such as the particles 18 of P"-alumina is used for the artifact, mixed with polyethylene glycol as described above and with a suitable proportion of carbon balls of similar size. During the sintering the carbon is burnt off, to leave a porous sintered P"-alumina layer lining the cavity or gap of the sintered envelope. This porous lining is suitable for wicking molten sodium by capillary action during use as described hereunder, from the interior of the cavity into a layer of molten sodium coating the inner surface of the cavity.

As indicated above, in use, the holders produced by the arrangement shown in Figure 1 will typically be an anode holder (although it can naturally be a cathode holder), containing molten sodium, in a high temperature electrochemical power storage cell. In such cells the holder will be located sandwiched between two cathode portions in a cell housing. In this case the holder produced by the arrangement of Figure 1 can have, as indicated above, an opening machined therein for connection to an external reservoir of molten sodium.

In Figure 5 of the drawings reference numeral 42 generally designates apparatus C) C for use i n the method of the present invention. The apparatus 42 is in the form of a 13 pressing jig comprising a hollow cylindrical mild steel outer housing 44 which is split longitudinally, and has its portions held together by outer steel ring clamps 46. The jig 42 is shown in an upright operative condition with its upper end closed off by a cylindrical resiliently flexible latex plug 48. Instead of latex, flexible polyurethane may be used for tD the plug 48.

Located concentrically in the housing 44 below the plug 16 is a longitudinally split mould 50, having portions which are held together by the housing 44. The top of the mould 50 has a central opening 52 closed by a plug 54 of similar material to that of the plug 46. The mould 50, as described in more detail hereunder, has a hollow screw threaded interior into which the opening 52 leads, the mould 50 also having a mouth 56 at its lower end, leading into said interior.

A screw-threaded mandrel 58 is shown located concentrically in the hollow interior of the mould 50, spaced radially inwardly from and loosely screwingly engaged therewith.

The mandrel 58 has a root 60 received in the mouth 56, and a central shank or stem 62 which is provided with a helically extending screw thread 64, extending from the root 60 to the opposite end of the stem 62.

As will be noted from Figure 5 and as mentioned above, the interior cavity of the mould 50 has a screw-threaded shape, being provided with an internal helically extending screw thread 68.

The pitch of the thread 64 is the same as that of the thread 68, the mandrel 58 being shaped and sized so that it can be located in the mouldspaced radially inwardly from the walls of the cavity of the mould 50, and so that the flights of the thread 64 are spaced axially from the flights of the thread 68 to register complementary therewith. The mould 50 and mandrel 58 accordingly define, therebetween, an annular space 70 of internally and externally threaded screw-like shape. In this space 70 is provided a resiliently flexible hermetically continuous latex sheath 72. The sheath 72 is in abutment with and forms a lining for the screw threaded interior surface of the mould 50, and, at its upper end, is open, and projects Outwardly through the opening 52, having the C1 c 14 periphery of its open upper end clamped between the plug 48 and the top of the mould 50, the plug 54 closing off said upper end. The helically shaped space 70 accordingly exists between the sheath 72 and the outer surface of the mandrel 58.

In use, the jig 10 is set up as shown in Figure 5, its being appreciated that the clamps 46 hold the housing 44 tightly against the outer cylindrical surface of the mould 50, and tightly against the plug 48 and the root 60 of the mandrel 58. The plug 48 accordingly seals one end of the housing 44, and the root 60 of the mandrel 58, having the open end 74 of the sheath 72 sandwiched between it and the housing 44, seals the lower end of the housing.

To make an electrode holder in accordance with the method of the invention, the jig 42 is set up as shown in Figure 5, except that the plugs 48, 54 are left out, and the lowermost clamp 46, around the root 60 of the. mandrel 58, is tightened sufficiently only to prevent powder from running downwardly out of the space 70.

A suitable powder such as P- or P"-alurnina of a particle size of 20-50pm, and is mixed with 15% by mass polyethylene glycol, is then charged into the space 70 between the sheath 72 and the mandrel 58, via the opening 52, with suitable vibration to compact and consolidate said powder. When the space 70 is filled with powder, designated 76, the plug 54 is inserted into position, followed by the plug 48, and followed by final tightening of the clamps 46 to seal the housing 44 tightly against the plug 48 and root 60 of the mandrel 58. A small open space 78 is left above the powder 46, below the plug 54.

The mould 50 is provided with one or more suitable passages and a plurality of pinholes (not shown) which communicate with the exterior, eg via one or more suitable passages (also not shown) in the plug 48.

Water is then introduced to the exterior of the sheath 72 via the passages and the C) various pinholes in the mould 50 at a suitable pressure for isostatically pressing a green screw-shaped hollow electrode holder from the layer of powder, designated 76, filling the space 70. This pressure may, for example, be 35 -50 MPa.

1 After the pressing the pressure is relieved, the sheath 72 which is resiliently flexible and is shaped to conform automatically with the inner surfaces of the mould 50, springs back into contact with the mould 50, free of the green holder. The green holder and mandrel 58 can then, together, be unscrewed from the interior of the mould 50, in an axially downward direction, after the clamps 46 are released.

It will be appreciated that the pinholes will be distributed over the threaded inner surface of the mould 50, to provide a layer of water under said pressure between the mould 50 and the sheath 72, this water pressing the sheath inwardly to consolidate said powder 76 against the outer threaded surfaces of the mandrel 58.

As described above with reference to Figures 1 -3, the pressing can be carried out in an environment with the powder 76 and the jig 42 eg at -10'C, the water used for the isostatic pressing also being at -10T and having suitable anti-freeze agents dissolved therein.

In Figure 6, the mandrel is designated 58, and its root 60, stem 62 and thread 64 0 are shown. A particular feature of the mandrel 58 is that it is of stainless steel and has an ice surface layer 80 about 0,5 mm thick.

This Surface layer is formed by locating the steel part of the mandrel in a split mould (not shown), generally similar to the mould 50 shown in Figure 5 but having an inner surface which is threaded and is of a shape and size such as to provide the surface layer 80 on the steel part of the mandrel. The layer will, in the usual way, be formed by freezing deionized water on to the steel part of the mandrel, followed by removal of the mould. In this regard it will be appreciated that the surface of the steel part of the mandrel can be treated, eg by being roughened, and the inner surface of the mould can be treated, e. by providing it with a plastics lining, so that the ice layer 80 adheres more strongly to the steel part of the mandrel than to the mould, to facilitate mould removal.

b J The mandrel 58, with its ice layer can then be stored at -15'C.

16 Turning to Figure 7, a test apparatus is shown wherein the feasibility of the core removal step of the present invention has been demonstrated although, naturally, a commercially available freeze-drying apparatus can be used instead. The apparatus is generally designated 82 and comprises a vacuum desiccator vessel 84 having wide-bore tubing 86 leading therefrorn. to a vacuum pump (not shown), connected to the end of the tubing 86 remote from the vessel 84. The tubing 86 has a U-shaped portion 88 dipping into a cold-trap in the form of a vessel 90.

In use, the pressed green artifact at -15T (either the flattened green envelope around the core 20 as shown at 92 in Figure 7 or the green screw-shaped hollow holder around the mandrel - see 28 and the layer 76 in Figure 5) is placed in the vessel 84 with the vessel 84 at -15'C, a cold medium at < -60'C such as liquid nitrogen 94 is charged into the vessel 90 and a vacuum of about 100 Pa is drawn in the tubing 86 by means of the vacuum pump.

In tests conducted by the Applicant, both on green artifacts with openings leading to their cavities (see Figures 2, 3 and 5) or on completely closed green artifacts with no openings leading to their internal cavities (see Figure 4), it has been shown that the ice core 20 or ice layer 80, as the case may be, can easily be sublimed, without any production of liquid water. In these tests, mass loss of the vacuum desiccator arising from sublimation of ice was used to monitor ice core removal. It was found that ice cores of up to 30g in mass could be removed in about 12 hours, the desiccator being allowed to heat up slowly to ambient temperature over this period while sublimed vapour froze in the U-shaped tubing portion 88. Similar results were obtainable in commercial freeze drying apparatus.

With the envelopes produced as described above with reference to Figures 1-4, these were, after the sublimation/freeze drying, ready for sintering, the polyethylene glycol binder providing sufficient green strength for handling. When the method described above with reference to Fiaures 5 and 6 was followed, the steel part of the mandrel 50 had to be removed after sublimation/freeze drying. Sublimation of the layer 80 however permitted easy unscrewing of the steel part of the mandrel 50 from the green artifact 17 which, similarly, retained adequate green strength for this purpose and for subsequent sintering.

EXAMPLE

It is contemplated that, in a typical embodiment of the invention, the core 20 (see Figures 1-4) or mandrel 50 with layer 80 (see Figures 5 and 6), will be cast from deionized water as described above. Separately, a mixture will be made up of P"-alurnina powder of average particle size of 50 - 100 pm or 20-50im, as the case may be, and polyethylene glycol. The polyethylene glycol is admixed with the P"-alurnina as a solution of 30% by mass thereof in water, in a proportion amounting to 15% by mass on a dry basis of the mixture thereof with P"-alumina. This mixing is followed by spray drying to a drier outlet temperature of 130T to a moisture content of no more than 10% by mass.

After loading the P"-alurnina, pre-cooled to -10'C, in the arrangement 10 (Figure 1) or jig 42 (Figure 5), also pre-cooled to -10T, the pressing will be to a pressure of 48-50 MPa (Figure 1 and Figure 5) or 23)0-240 MPa (Figure 5), to reduce the wall thickness of the envelope or layer 76 (Figure 5) to about 40% of its original value, eg from 5mm to 2mm.

After sublimation freeze-drying as described above with reference to Figure 7, and removal of the steel part of the mandrel 50 (Figures 5 and 6), the green artifact is then heated according to the following heating regime, under atmospheric air, to remove the polyethylene glycol, to remove any residual water by evaporating it and debonding it from the P"-alurnina, and then to sinter the artifact:

Ambient - 400T at 25T/hr (in air) 400 - 16000C at 100T/hr (in air) 1600 1617C at WC/hr (under air) 1617 - 10000C at 240'C/lir (under air) 18 10OTC - ambient at 360T/hr (tinder air).

Figure 8 shows that, at the temperatures contemplated for the pressing, ie -10 to -15T, pressing pressures of 35 -100 MPa can easily be employed without any danger of melting of the core 20 (Figure 1) or surface layer 80 (Figure 6). Figure 9 in turn shows the phase diagram of water, using a plot of pressure against temperature. Figure 9 it is clear that the freeze/sublime cycle (shown by the arrowed line in Figure 9) employed by the method of the present invention is feasible. Liquid water is frozen at 1 atmosphere pressure (about 100 kPa) and cooled to -15T, followed by a pressure drop to 10' atmospheres (about 100 Pa), at which pressure a temperature increase leads to sublimation.

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Claims (12)

Claims:

1. A method of making a sintered artifact from sinterable particulate material, the method comprising the steps of:

locating at least one core at least partially formed from ice in a mass of the particulate material; compressing and consolidating the particulate material around each core to form a green artifact wherein each core is at least partially embedded; removing each core from the green artifact to leave a cavity therein; and sintering the green artifact to produce a sintered unitary artifact having at least one cavity left therein by the core removal.

2. A method as claimed in claim 1, in which the particulate material is a ceramic material or a precursor thereof, so that the sintered artifact produced is of sintered ceramic material, the particulate material having a particle size of 10 - 200pm.

3. A method as claimed in claim 2, in which the ceramic material is a solid electrolyte ceramic material or precursor thereof, the artifact being in the form of a holder for holding active electrode material in a high temperature electrochemical power storage cell.

4. A method as claimed in any one of claims 1 - 3 inclusive, in which the core removal step comprises sublimation of at least part of the ice of each core, at a temperature and pressure at which the ice evaporates Without meltina.

5. A method as claimed in any one of the preceding claims, in which each core is formed by casting at atmospheric pressure by freezing, the water used therefor being gas free.

6. A method as claimed in claim 5, in which each core is formed by casting a mixture of ice and of the sinterable particulate material, so that each cavity in the sintered artifact contains, in its interior, a filling formed from said particulate material in porous, c liquid-permeable form, the filling acting to enhance the strength of the sintered artifact.

7. A method as claimed in any one of the preceding claims, in which each core is formed to be of a composite nature, comprising an inner portion which is reusable, the 0 inner portion having a surface layer comprising ice frozen thereon.

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8. A method as claimed in any one of the preceding claims, in which the locating of each core in the mass of particulate material, and the subsequent compressing and 0 consolidating of the particulate material, take place at a temperature and pressure such that each core remains solid during these steps.

9. A method as claimed in claim 8, in which the locating of each core in the particulate material takes place at atmospheric ambient pressure and at a temperature of < O'C, the compressing and consolidating taking place at a temperature of < 4'C and core removal taking place after the compressing and consolidating have been effected, by reducing the ambient pressure on the green artifact to a value at which the ice of the core sublimes.

10. A method as claimed in claim 9, in which core removal takes place by exposing the green artifact to a temperature between -30T and OT, and to an ambient pressure of 10 - 500 Pa.

11. A method as claimed in claim 1, Substantially as described and as illustrated herein with reference to the accompanying drawings.

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12. A sintered ceramic material whenever made by a method as claimed in any one of the preceding claims.

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