GB2281561A - Alumina artefacts;release agents - Google Patents

Description

1 2281561 Ionically Conductive Ceramics THIS INVENTION relates to

ionically conductive ceramics. More particularly, the invention relates to an unsintered ceramic artefact, to a method of making a ceramic slip, to a ceramic slip, to a method of forming a green ceramic tape, to a green ceramic tape, and to a release agent for use in forming green ceramic tapes.

According to a first aspect of the invention, there is provided an unsintered ceramic artefact, which comprises an alumina component selected from the group consisting in partially hydrolysed 5-alumina, partially hydrolysed 911-alumina, and mixtures thereof; at least one water-soluble binder; and at least one water- soluble plasticizer.

onically conductive ceramics such as 5-alumina cl- Z11 -alumina are used in, for example, high temperature rechargeable is electrochemical power storage cells. Such a cell typically comprises a sodium anode, which is molten at the cell operating temperature, and a cathode comprising either sulphur and/or 2 polysulfides, or a transition metal chloride selected from the group consisting of FeCl, or NiCl, dispersed in an electrolytepermeable matrix, which is electronically conductive, with the solid ionically conductive ceramic or sodium ion conductor separating the anode from the cathode.

Preferred shapes of the solid sodium ion conductor, also termed the solid electrolyte, are cylindrical tubes with one end closed off, or flat envelopes having two opposed panels sealed together along their peripheral edges and spaced apart to provide a space -0 for holding the sodium.

The cylindrical solid sodium ion conductor tubes are preferentially made by isostatically pressing a suitable 9-alumina or S11-alumina powder in a suitable mould. Flat envelopes are described, for example, in British Patent 2 231 567 A, and are typically of a ceramic solid electrolyte conductor of sodium ions such as S-alumina or S11-alumina.

Typically, in the manufacture of the flat envelopes, particulate Salumina or S11-alumina is compounded into a green slip formulation, optionally containing a binder, plasticizer and an 0 organic solvent, from which a tape or sheet is formed, for example by tape casting, with the panels being formed from the green tape or sheet.

3 Jay "green" tape is meant an unsintered ceramic tape, i.e. a tape released from the substrate on which it is formed after drying of the cast slip.

ordinarily, nonaqueous organic solvents are used for tape casting high-tech ceramics, since they evaporate rapidly, are chemically inert towards the ceramic powder, and are compatible with a wide choice of thermoplastic binders which give cohesion and flexibility to the green tape. Tape off-cuts and other tape residues remaining after further processing can be recycled easily by redispersion in fresh solvent or in fresh slip. However, many organic solvents or constituents of solvent mixes are potentially dangerous to human health and are flammable, and their removal may call for special air detoxification equipment. In many respects, water is a more desirable dispersing medium for tape-casting slips.

However, water will hydrolyse hydrolysable materials such as the beta alumina compounds and become strongly alkaline, forming sodium hydroxide solution by this process of hydrolysis. Therefore, it would appear that the use of water for preparing an aqueous beta alumina slip would be unadviuable because it would destroy the material which is to be dispersed and tapeformed. Moreover, the alkalinisation of the slip would induce sodium carbonate formation by pickup of carbon dioxide from the air, and preclude the utilization of organic binders which would react or flocculate with aqueous alkali.

a Surprisingly, the Applicant has found that it is possible to formulate aqueous beta alumina slips containing compatible fy the requirements outlined organic binders which will satis above and yield homogeneous beta alumina ceramics after drying S and firing, despite hydrolysis of beta alumina in the slip.

Evidently, the beta alumina crystal structure is maintained in the ceramic powder despite hydrolysis, so that after drying, in the course of subsequent thermal treatment leading up to sintering, reintegration of the alkali metal ions into the lattice restores the original composition.

Thus, the unsintered ceramic artefact of the first aspect of the invention can be in the form of a self-supporting green ceramic tape. For the production of the green tape, = 4 ne S-alumina or S" alumina powders are preferably used, since these S4 inter to a dense ceramic body upon firing. The particle S4 ize d5o of the alumina component may thus be in the range of ahout i to 5 gm.

The water-soluble binder may be a der 4 I_vative of starch and/or cellulose. When a starch derivative is used, it may be a starch ether, such as hydroxypropyl starch. When a cellulose derivative is used, it may be a cellu-7--se ether, such as hyd--oxyethyl cellulose.

The water-soluble plasticizer may be selected from the group consisting in glycerol, sorbitol, and mixtures thereof.

The resins, ie starch and cellulose, and plasticizers used for the preparation of green tapes in amounts disclosed herein provide a flexible, green tape, which is self-supporting, and which has a good green strength. Tapes of good quality were obtained by using the resin hydroxypropyl starch, which is sold under the tradename Emsol K 55 by Emslandstdrke, Emlichheim/Germany, and/or the resin hydroxyethyl cellulose, which is sold under the tradename Tylose H 10 by Hoechst AG, Frankfurt (Main) /Germany, together with the plasticizers glycerol io and sorbitol (both obtained from Merck, Darmstadt /Germany).

Sorbitol serves as a plasticizer as well as a weak deflocculating agent. Both resins are low molecular weight polymers, which are more soluble in water than the high molecular weight types. Good solubility of the polymeric binder is necessary for achieving high concentrations of S-alumina or 911-alumina powder in the aqueous slip, and, simultaneously, for adjusting the ratio ceramic powder/resin in these slips appropriately in order to achieve the above described properties of the green tape. The binders or resins hydroxypropyl starch and hydroxyethyl cellulose not only provide a tape as described above but also provide 2V formulations which are not prone to cracking during dryIng of the cast slip, so that substantially crack-free green tapes, essential for making flawless ceramic artefacts from green tape, are obtained.

It is possible to use as binder for the green tape either hydroxyproiDyl starch or hydroxyethyl cellulose. The quality of tape in terms of flexibility, green strength and absence of 6 cracks is improved by using both resins, with the ratio of hydroxypropyl starch / hydroxyethyl cellulose then preferentially ranging from S:1 to 14:1 in parts by weight.

According to a second aspect of the invention, there is provided amethod of making a ceramic slip, which comprises homogenizing an alumina component selected from the group consisting in particulate 9-alumina, particulate 911-alumina or mixtures thereof; at least one water soluble binder; at least one water-soluble plasticizer; and water.

The homogenizing may comprise grinding together the components. The grinding together may be effected by ball milling, utilizing spherical grinding media, the alumina component and an aqueous component containing the water, the binder, the plasticizer and, optionally, a deflocculating agent to form an initial slip. The ball milling may be effected for about two hours. After separating the grinding media from the initial slip, it may be de-aired to produce the castable ceramic slip. The slip thus has a viscosity rendering it suitable for the production of a green tape using the doctor blade technique.

For slips which are to be extruded the consistency in too high for ball milling. Other known methods of homogenization. such as kneading, will then be used. These methods are well known to persons skilled in the art. If no milling taXes place the particle size d5o of the alumina components should rather be at the lower end of the range of about I to S Am.

7 The invention thus also extends to a ceramic slip when produced by the method hereinbefore described.

The slip, when intended for casting into a green tape, may comprise 3" -A1203 - 20-25 vol %, hydroxypropyl starch 5-7 Vol t hydroxyethyl cellulose 0,4-1,5 Vol % glycerol 10-15 Vol % sorbitol 0,4-1,0 vol t water 50-60 vol % The slip, when intended for extruding into a green tape, may comprise :a"-A'203 - 40-50 Vol methyl cellulose - I - 3 Vol % is sorbitol - 2 5 val k water - 45 55 Vol With slips intended for extruding, a suitable plastic visoosity and a suitable yield value may easily be determined by a f ew preliminary experiments. The total binder content may, eg when using methyl cellulose, be reduced to almost I vol t, and the amount of glycerol may be lowered to half of that of a slip intended for casting, or even omitted. when extruding, the suitable plastic viscosity and yield value also depend on the type of extruder used.

8 The slip is a concentrated dispersion belonging to the class of plastic substances. The flow properties of the slip are fully described by the yield value and the plastic viscosity. The slip Iz is solid below the yield value. For a cast slip, the plastic viscosity typically lies in the range from 200 to 2000 mPa, and the yield value in the range f ram 5 to 40 Pa:.

According to a third aspect of the invention, there is provided a method of forming a green ceramic tape, which includes shaping a ceramic slip as hereinbefore described by casting the ceramic slip on a substrate, or extruding the ceramic slip, optionally onto a substrate, drying the cast or extruded slip to f arm a green ceramic tape; and, where applicable, removing the resultant green tape from the substrate.

The substrate may, in particular, be a non-adhesive substrate.

is Non-adhesive substrates may be hydrophobic polymer substrates, usually sold as foils, such as silicones or fluorinated polymers such as PTFE, FBP or PFA. FEP is a copolymer of tetrafluoroethylene and hexafluoropropylene; PFA is a copolymer of tatrafluoroothylene and perfluorovinylether.

slip casting for the examples described below was done on a substrate in the form of a FEP-covered aluminium plate. The supplier of the 0,127 mm thick FEP film used on the plate was Norton Pampus (Will ich/Cermany). The slip container was fitted with two doctor blades having a spacing of 6,3 cm. and whose clearances were controlled by dial gauges. The slip container 9 was stationary while the plate was moved automatically. The internal width of the slip container was 20 cm.

The same substrate as used for casting may also be used for extruding the slip.

The antiadhesive properties of the substrate, eg the PEP-film, may not be sufficient for a crackfree release of the green tape.

In addition, green tapes tend to adhere even to hydrophobic, antiadhesive substrates on drying, whereas smooth and easy removal of the green tape from the substrate is important to prevent faults such as cracks and fissures in the finished product.

While there is less tendency for adhering with extruded tapes than with cast tapes, adherence nevertheless has to he considered in the case of extruded tapes.

Therefore, prior to the casting or extruding on the substrate, a release agent may be applied to the substrate to promote clean, easy and crackfree detaching of the green tape therefrom. A release agent is a material which is applied to the surface of the substrate prior to casting/doctor-blading or extruding the slip onto said substrate, to create a non-stick interface between the substrate and green tape, so that the green tape created by drying the cast or extruded slip can be lifted and separated from the substrate.

Usually, the release agent is a fluid, but it can also be in the form of a paste or a solid. For example, a release agent may be applied as a molten wax and cooled to form a solid wax coat.

The release agent should form a continuous interface layer. Also, it should not contain inorganic matter which would react with beta alumina and degrade its properties. Therefore, silicones are not suitable.

The release agent may comprise a combination of a f irst component comprising a paraf fin and/or a glycerol ester and a second component comprising a fatty acid and/or a salt of a fatty acid.

The drying of the cast or extruded slip may be effected by lowering its water content to about 10 wt V by blowing filtered hot air, typically having a maximum temperature of 750C, over the cast or extruded slip, which may be located in a drying tunnel is fitted to equipment for tape casting or extrusion. Further drying of the tape may then be effected. This further drying may comprise drying the tape to a final water content of about 4 wt on a taut nylon net in said tunnel by blowing hot filtered air, typically at a temperature of about 650C, over both sides of the green tape. Thus, the tape drying may comprise two drying steps in a drying tunnel. in the case that the tape is cast or extruded on a substrate, the further drying will thus be effected after the tape has been removed f rom the substrate, and inverted - 11 Naturally, during extruding hot air may also be blown against one or both faces of the tape, which results in a preliminary drying of the tape between its exiting the extruder die and its laying down on the substrate. With concentrated slips having a Ell-alumina content close to 50 vOl t and having a relatively low water content close-to the lower limit of about 45 vol t, this drying may be sufficient, depending on the distance between the die and the substrate, to lay the tape down directly on a taut nylon net or another air permeable substrate, and then to dry it from both sides to a final water content of about 4% by weight.

The invention extends further to a green ceramic tape, when formed in accordance with the method as hereinbefore described.

The green ceramic tape, and also the unsintered ceramic artef act hereinbefore described, may comprise is partially hydrolysed 5- or SIT-alumina 50-60t by volume; hydroxypropyl starch 11-18k by volume; hydroxyethyl cellulose 0,5-5 % by volume; glycerol. 23-31% by volume; sarbitol 0,5-3t by volume.

more particularly, it may comprise partially hydrolyzed 9- or Ell-alumina - 52-50 by volume, hydroxypropyl starch - 13-10 by Volumef hyd=xyethyl cellulose - 1-3V by volume, glycerol - 25-29k by volume, 12 sorbitol 1-2k by volume.

It is an advantage of the aqueous slip of the invention that thick green tapes as well as thin green tapes can be produced. The thickness of green tape can thus be in the range from about 0,4 mm to about 1,2 mm. The rheological properties of the slip according to the invention are another advantage. No phase separation was observed during drying of the cast slip. The thickness of extruded tapes may even be greater, and may easily be, eg, 2 mm.

According to a fourth aspect of the invention, there is provided a release agent for use in forming green ceramic sheets or tapes from beta alumina based aqueous slips by tape-casting or extruding, which release agent comprises a combination of a f irst component comprising at least one paraffin and/or at least one glycerol eater, and a second component comprising at least one fatty acid and/or a metal or ammonium salt of a fatty acid.

The second component may make up less than So mass %- of the combination.

According to a fifth aspect of the invention, there is a release agent for use in forming green ceramic sheets or tapes from beta alumina based aqueous slips by tape-casting or extruding, which release agent comprises a combination of a first component comprising at least one paraffin and/or at least one glycerol ester and a second component comprising at least one fatty acid 13 and/or a salt of a fatty acid, with the secoild component making up less than 50 mass % of the combination.

The release agent of the fourth and the fifth aspects of the invention may be in paste form at room temperature, and the second component may make up at least 5 mass k of the combination, eg between 10 and 20 mass lk of the combination.

The first component may consist of low viscosity paraffins. By 'paraffins, is meant saturated aliphatic hydrocarbons or mixturesthereof which should be non-volatile, that is, boil at 3.0 temperatures above 2000C.

Instead, the first component may consist of glycerol esters. Glycerol esters are natural or synthetic fats, preferably glycerol triesters, eg triolein or synthetic fluid fate such as PCL-liquid (Roth, Karlsruhe).

By 'fatty acid' is meant an saturated or unsaturated aliphatic carbonic acid, preferably with at least twelve carbon atoms per molecule. if desired, a mixture of fatty acids can be used as the second component.

The second component may instead, or additionally, comprise a fatty acid salt with additional -OR functional groups and/or unsaturated carbon-carbon bonds. Thus, long-chain fatty acids which are liquid at ambient temperature, have one or more double 14 bonds and, optionally, further functional groups, particularly OH- groups, can be used.

By 'ammonium salt' is meant a salt with a simple ammonium cation as well as a salt with a quaternary ammonium cation in which one or all four ammonium hydrogens are substituted by aliphatic or aromatic organic groups.

It has been found that the combination of the f irst and the second components was superior to the components used singly as release agents. When both components are homogenized into a mixture, separation- stable pastes with good spreading properties were generated, whereas paraffins on their own tend to separate into droplets when used as release agent. The second component reduces the surface tension particularly towards the aqueous beta alumina slip, so that edge retraction of the tape in drying is minimized.

The release agent according to the invention can be formulated to be fully emulsible in water. Thus paraf f in/oleate mixtures with more than 10 wtt of oleate are dilutable with water to form stable emulsions. This property facilitates cleaning the tape- casting or extruding substrate and associated machinery; it is not, however, a prerequisite for a good release effect.

variation of the fatty acid/salt ratio and of the proportions of the first and second components allows tuning the wetting/spreading properties of the release agent to the binder composition of the water-based tape.

it is particularly advantageous to use liquid paraffins in combination with liquid fatty acids. Paraffins are inert towards the alkaline beta alumina slips, whereas a hydrolysis of the eater bond cannot be excluded when fats (glycerol esters) are used instead on their own.

Fluid components facilitate mixing and application of the release agents, since soft pastes, which can be applied efficiently, are normally formed.

Further optimization with respect to viscous flow and release properties is possible by selecting the starting viscosities of the components, ie the viscosities of paraffin and of the fatty acids. For instance, exchanging oleic acid against linolic acid in mixes with 85 wt paraffin will yield pastes of mucous consistency.

The release agent is mixed in simple manner to form a homogeneous liquid mass by using the free fatty acid. only after this, is optional salt formation induced by adding a suitable reagent, eg sodium hydroxide solution in the case of the sodium salt, by which step the liquid is normally transformed into a paste.

16 it is important to note that the paste should be applied to the nonadhesive substrate with sufficient force to spread it evenly; brushing or rubbing is recommended.

The release agents according to the invention are inexpensive, inoffensive, environmetally friendly compositions offering problem-free handling.

The invention will now be described in more detail with reference to the following non-limiting examples.

463,9 g 911-A1203 comprising about 9 wt 4; Na20, about 0,7 wt V U20, with the balance being A".03, and S28,6 g aqueous solution containing 13-0 g/f Emao3 K 55, 20 g1f Tylose H 10, 191 S/I glycerol, and 10 g/I sorbitol were introduced together with 1,8 kg zirconia grinding media, into a 21 plastic bottle which was is rotated for 1, 5 h at 70 rpm. After separating the grinding media, the slip was de-aired in a spinning disc apparatus (type RRV, Koruma Company, Neuenburg/Germany) under a vacuum of about mbar for about 5 minutes. The olip was cast on a 1 m long and 24 cm, wide PEP-covered aluminium plate. Prior to casting a release agent in accordance with Example 2.2 was applied as a thin layer to the FEP-film (Norton Pampus Company, Willich/Germany) by hand, using a soft cloth. The clearance of the f irst doctor blade was set to 2,4 mm, and that of the second to 2,7 mm. Casting speed was 30 cm/min. The cast slip was dried in a drying tunnel which was fitted to the tape casting 17 equipment. Filtered air at a temperature of 7511C was blown over the slip. After 120 minutes the tape, with a water content of 11,2 wt which was determined by weighing the aluminium plate, was detached f rom the substrate. The released tape was dried upside down on a taut nylon net inside the drying tunnel by blowing hot air with a temperature of 6511C over the tape. Drying was terminated at a water content of 4,3 wt t. The thickness of the dried tape was 0,92 mm. The tape was flexible and flawless.

The green tape can be shaped into various configurations formed from one or two tapes or tape layers (envelopes, for example) or even more than two tapes (envelopes, for example). The manufacture of an envelope made from three tapes is described in TJK Patent No. 2 231 567 A. After debonding, ie burning out the organic constituents, the porous ceramic body is sintered to a dense ceramic artefact having the same properties in terms of sodium ion conductivity and density as artefacts made from the same 311-A-1203starting material by dry manufacturing methods, despite partial hydrolysis of the 9-alumina or Sn-alumina during slip preparation and tape drying. The hydrolysis of 311-alumina in aqueous slips containing 25 vol!k and 30 vol % 9"-alumina was determined by analyzing Na and Li in the supernatant liquid which had formed after a storage of 24 h. In both cases about 30 k of the Na20 dissolved from the 611-A'203, with the dissolution of lithium (about 0,02 of the Li-content) being negligible.

18 EMPLE 2:

FEP foil substrates were coated with less than 0,5 g/m of the t different release agents described in more detail below. The same aqueous sodium Gn-alumina slip containing starch ether and cellulose ether as binder and glycerol and a small amount of sorbitol as plastifier components, as described in Example 1, was cast onto the substrate with a tape-casting machine, and dried to 10% residual water content, using the procedure described in FIxample 1. This tape was in each instance removed from the substrate, with the weight of the tape normally being sufficient to separate it from the substrate.

2.3- The above procedure was performed without a release agent: the ceramic tape adhered so strongly to the substrate, that the force necessary for removal damaged the tape.

is 2.2 The experiment was performed with a release agent consisting of 85 wt;; low viscosity paraffin oil (type PC-02, Fauth, Mannheim) and 15 wt% sodium oleate, made from technical quality oleic acid (NO.471, Merck, Darmstadt): the tape parted from the substrate under its own weight.

2.3 The experiment was performed with a release agent made up from 85 wt%low viscosity paraffin oil as described above it and IS wt!k sodium linolate prepared from linolic acid (No.62240, Fluka, Switzerland)-. the tape parted from the substrate under its own weight.

19 2.4 The experiment was performed with a release agent made up from 85 wt!k low viscosity paraffin and 15 wt.% sodium ricinolate, prepared from ricinolic acid (No.8369, Roth, Karlsruhe). the tape parted from the substrate under its own weight.

2.5 The experiment was performed with a release agent made up f rom 85 wtV PCL-liquid (synthetic triester of glycerol with branched fatty acids, No.5423, Roth, Karlsruhe) and is wt% sodium oleate; the tape parted from the substrate under its own weight.

2.6 The experiment was performed with a release agent made up from 85 wt% Triolein (triester of glycerol with oleic acid) and 15 wtt sodium oleate; the tape parted from the substrate under its own weight.

is ZXMP14E 3 460 g 911-A'203 as used in Example I were thoroughly homogenized with 360 g of the same aqueous solution as also uaed in Example 1, in a Werner and Pfleiderer type laboratory kneader. The particle size of the:911-A1203 was mostly at I to 3 Am. The slip 20 thus contained only 70% of the aqueous solution of Example 1, with a corresponding reduction of the amount of water, Emsol, Tylooe, glycerol and sorbitol. The slip was de-aired in the kneader and then extruded with a screw extruder having a die slot which was 150 mm wide and 2, 0 mm high, onto a FEP toil which was 25 located 10 cm below the extruder die as a taut slip running over two rolls and having a distance of 1 m with a speed of 30 cm/min for a total length of 1 m. The tape was laid down, in the same fashion as in Example 1, on an aluminium Plate and dried in the same fashion as in Example 1, but f or only 60 min. it then had a water content of about lit by weight. it was then detached from the substrate and, in the same fashion as in Example 1, dried upside down on a taut nylon net in the drying tunnel with hat air of 650C until the final water content wao 4,2%, by weight. The thickness of the dried tape was 1,2 mm. The 10 release agent on the FEP tape was, as in Example 1, a mixture of 85k by weight low viscosity paraffin oil and 15t by weight sodium oleate.

21

Claims (1)

1. CLAIMS -

   1. An unsintered ceramic artefact, which comprises an alumina component selected from the group consisting in partially hydrolysed 0-alumina, partially hydrolysed gn-alumina, and mixtures thereof; at least one water-soluble binder; and at least one water-soluble plasticizer.

   2. An artefact according to Claim 1, which is in the form of a self-supporting unsintered or green ceramic tape, and wherein the particle size clw of the alumina component is in the range of about 1 to 5 gm.

   3. An artefact according to Claim 1 or Claim 2, wherein the water-soluble binder is a derivative of starch andlor cellulose.

   4. An artefact according to Claim 3, wherein the is water-soluble binder is a starch ether.

   5. An artefact according to Claim 3, wherein the water-soluble binder is a cellulose ether.

   6. An artefact according to Claim 3, wherein the water-soluble binder is a combination of a starch ether and a cellulose ether, with the mass proportion of starch ether to cellulose ether being between 5:1 and 14:1.

   22 7. An artefact according to Claim 6, wherein the water soluble plasticizer is selected from the group consisting in glycerol, sorbitol. and mixtures thereof.

   An artefact according to Claim 7, which comprises partially hydrolysed Sor S"-alumina.50-60k by volume; hydroxypropyl starch 11-18k by volume; hydroxyethyl cellulose 0,5-5 k by volume; glycerol 23-31k by volume; sorbitol 0,5-3k by volume.

   9. A method of making a ceramic slip, which comprises homogenizing an alumina component selected from the group consisting in particulate S-alumina, particulate 91-alumina or mixtures thereof; at least one water soluble binder; at least one is water-soluble plasticizer; and water.

   10. A method according to Claim 9, wherein the homogenizing is effected by ball milling, utilizing spherical grinding media, the alumina component and an aqueous component containing the water, the binder, the plasticizer and, optionally, a deflocculating agent, to form a slip; separating the grinding media from the resultant slip; and de-airing the slip to produce a castable ceramic slip.

   11. A method according to Claim 9, wherein, when the slip is highly concentrated so that it has a consistency which makes it difficult to mill, the homogenizing comprises kneading.

   23 12. A ceramic slip when produced by the method of any one of Claims 9, 10 or 11.

   13. A ceramic slip according to Claim 12, which comprises 9"-A'203 20-25 Vol hydroxypropyl Starch 5-7 Vol % hydroxyethyl cellulose 0,4-1,S Vol V glycerol 10-15 Vol V sorbitol 0,4-1,0 Vol V water so-so Vol W and which is suitable for casting into a green tape.

   14. A ceramic slip according,to Claim 12, which comprises 2"-A1203 - 40-50 Vol % methyl cellulose - I - 3 Vol % sorbitol - 2 - S Vol V is water - 55 vol W and which is suitable for extruding into a green tape.

   15. A method of forming a green ceramic tape, which includes shaping a ceramic slip in accordanc-- with any one of Claims 12, 13 or 14 by casting the ceramic slip on a substrate, or extruding the ceramic slip, optionally onto a substrate; drying the cast or extruded slip to form a green ceramic tape; and, where applicable, removing the resultant green tape from the substrate.

   24 16. A method according to Claim 15, which includes, prior to the casting or extruding on the substrate, applying a release S. agent to the substrate to promote crackfree detaching of the green tape therefrom.

   17. A method according to Claim 16, wherein the release agent is in the form of a paste and comprises a combination of a first component comprising a paraffin and/or a glycerol ester and a second component comprising a fatty acid and/or a salt of a fatty acid, and wherein the substrate comprises a fluorinated polymer carrier sheet.

   18. A method according to any one of Claims 15 to 17 inclusive, wherein the drying of the cast or extruded slip is is ef f ected by lowering its water content to. about io wt %- by blowing filtered hot air over the cast or extruded slip in a drying tunnel, with further drying of the tape to a water content of about 4 wt It being ef f ected on a taut nylon net in said tunnel by blowing hot filtered air over both sides of the green tape, Provided that, if the tape has been cast or extruded on a substrate, the further drying is effected after the tape has been removed from the substrate and inverted.

   19. A method according to Claim 18, wherein the slip is extruded and wherein hot air is blown onto the extruded tape between an extrusion die through which the tape is extruded and the substrate, with the air being blown onto one or both sides of the tape, and the tape then optionally being dried in a drying tunnel directly on a taut nylon net.

   20. A green ceramic tape, when formed in accordance with the method of any one of Claims 15 to 19 inclusive.

   21. A green ceramic tape according to Claim 20, which comprises partially hydrolysed 5- or 311-alumina 50-60V by volume; hydroxypropyl starch 11-10t by volume; hydroxyethyl cellulose 0,5-5 t by volume; glycerol 23-31% by volume; sorbitol 0,5-31; by volume.

   is 22. A release agent for use in forming green ceramic sheets or tapes from beta alumina based aqueous slips by tape-casting or extruding, which release agent comprises a combination of a first component comprising at least one paraffin and/or at least one glycerol ester, and a second component comprising at least one fatty acid and/or a metal or ammonium salt of a fatty acid.

   23. A release agent according to Claim 22, wherein the second component makes up less than 50 mass Ir of the combination.

   24. A release agent for use in forming green ceramic sheets or tapes from beta alumina based aqueous slips by tape-casting 26 or extruding, which release agent comprises a combination of a first component comprising at least one paraffin and/or at least - one glycerol ester and a second component comprising at least one C.

   f atty acid and/or a salt of a fatty acid, with the second component making up less than 50 mass of the combination.

   25. A release agent according to any one of Claims 22 to 24 inclusive, which is in paste form at room temperature, and wherein the second component makes up between 10 and 20 mass V of the combination.

   26. A release agent according to any one of Claims 22 to inclusive, wherein the f irst component consists of low Viscosity paraffins.

   27. A release agent according to any one of Claims 22 to inclusive, wherein the first component consists of glycerol is triesters.

   28. A release agent according to any one of Claims 22 to 27 inclusive, wherein the second component comprises a fatty acid salt with additional -OH functional groups and/or unsaturated carbon-carbon bonds.

   29. A novel unaintered ceramic artefact, substantially as - described and exemplified herein.!Z 27 30. A novel method of making a ceramic slip, substantially as described and exemplified herein.

   31. A novel slip, substantially as described and exemplified herein.

   32. A novel method of forming a green ceramic tape, substantially as described and exemplified herein.

   33. A novel green ceramic tape, rubstantially as described and exemplified herein.

   34. A novel release agent, substantially as described and exemplified herein.