CERAMIC MATERIAL

Abstract: Ceramic materials are in the form of a glass or a polycrystalline material, or a polycrystalline material additionally containing a glass phase, and are based on oxides of phosphorous and silicon and optionally one or more additional oxides selected from oxides of zirconium, aluminium, and alkali metals. The present invention provides a useful process for the production of such ceramic materials based on raw materials prepared by reaction of phosphoric acid with lower alkoxides of the other components or for the alkali metals, optionally alkali salts. Among the materials disclosed, an aluminium-modified Nasicon of increased stability towards molten sodium relative to known stoichiometric as well as Zr-deficient Nasicon ceramics, is of special interest.

Description: CERAMIC MATERIAL The present invention relates to ceramic materials and processes for preparing ceramic materials, and to the manufacturing of shaped articles from such materials. The ceramic materials are in the form of a glass, or a polycrystalline material, or a polycrystalline material additionally containing a glass phase, and are based on oxides of phosphorous and silicon and optio¬ nally one or more additional oxides selected from oxides of zirconium, aluminium, and alkali metals. In an important aspect, the invention relates to a process for preparing polycrystalline ceramic materials based on oxides of phosphorous, silicon, zirconium, aluminium and an alkali metal such as sodium, to shaped articles made from such cera¬ mic materials and to the use of such articles as ion conductors or electrolytes in electrochemical devices such as, e. g. , sodium-sulphur batteries. Polycrystalline ceramics which are promising for such uses and which have the general formula Na., Zr-P Si O.- -, ' π which 0 ≤ x ≤ 3 (Nasicon) , are known from e.g. German Published Specification No. 2.634.289, Materials Research Bulletin 11 , 1976, pp . 173-182, Euro- pean Published Specification No. 46932, and Solid State Ionics 3/4, 1981 , pp. 215-218. According to these publications, the ceramics are prepared by conventional methods comprising mixing oxides and salts in the appropriate ratios, followed by shaping and sintering. The preparation of pure dense ceramics of such materials from con- ventional oxide mixtures has, however, been hampered by residual ZrO 2 and volatilization of Na 2 O and P2®__- Alternatively, such ceramic materials may be prepared from raw materials by a process based on co recipitation and gelling in aqueous solutions, cf . Mat. res. bul. 14, 1979, pp. 1469-77. It has been shown, however, that gels based on polymerization and hydrolysis of alkoxides are the most useful raw materials for a low temperature preparation of ceramics in the form of powders, thin films, fibres or preshaped monoliths, this being due to the very high surface area and completely amorphous character of such gels . The properties of this type of gels and their usefulness as precursors in the preparation of dense ceramics depends strongly on the manner in which the gels are prepared. Thus, e. g. , it is known to reflux a dry solution of alkoxides at tem¬ peratures around 80°C for a few hours to prepare soluble mixed coordination complexes, which, by hydrolysis leads to clear gels (J. Mat. Sci. 15, 1980, pp. 1765-1771 ) . The gels prepared according to this ' reference are gels of the system Na-O-ZrOj-SiO- which were prepared in the form of fibres containing up to 7% by weight of Na«O and 33% by weight of ZrO 2 - The present invention provides an especially useful process for the production of ceramic materials by preparing a raw material in the form of a phosphorous-containing gel of a very high surface area and converting the gel into a ceramic material by sintering. The process of the invention is particularly suitable for preparing materials of the above-mentioned Nasicon type or closely related materials, but may also advantageously be used for preparing other ceramic materials containing phosphorus and silicon, e. g. , silicon-phoshorus glasses . I n its generel concept, the invention relates to a process for prepa¬ ring a ceramic material in the form of a glass, or a polycrystalline material, or a polycrystalline material additionally containing a glass phase, and based on oxides of phosphorus and silicon and optionally one or more additional oxides selected from oxides of zirconium, aluminium, and alkali metals, comprising preparing a gel by controlled mixing, in an organic medium, of anhydrous phosphoric acid with a lower alkoxide of silicon and optionally with one or more alkoxides selected from lower alkoxides of zirconium, aluminium, and the alkali metal, the optional alkali metal component alternatively being an alkali salt soluble in the organic medium, followed by gelling by addition of water, removing water and volatile components from the gel, and converting the water- and volatlles-free gel to a ceramic material. The main characterizing feature of the process of the invention is the use of phosphoric acid as the source of phosphorus for the prepara¬ tion of such ceramic materials . The use of phosphoric acid provides a number of important advantages: a One advantage is that phosphoric acid is more economic to use than other potential sou rces of phosphorus such as esthers of phosphoric acid with lower alcohols . Also, by using phosphoric acid instead of phophoric acid esthers, problems associated with residual organic materials in the gels are minimized. Furthermore, the acidic nature of phosphoric acid facilitates the reaction thereof with the alkoxides used, in particular with silicon lower alkoxide tetraethylorthosilicate which is the slowest reacting among the alkoxides . In the present specification, the term "lower" is used in connection with alcohols and alkoxides, designates that the alcohols or the alco¬ hol moieties of the alkoxides contain 1 -4 carbon atoms. Examples of such alcohols are methanol, ethanol, methanoi, propanol, isopropanol, n-butanol, sec. butanol, isobutanol and tert. butanol . The gelling may be performed by adding water in various manners . Thus, according to one embodiment, the water may be added by allowing the mixture of alkoxide to be exposed to a humid atmosphere. According to another method water dissolved in a lower alcohol such as n-propanol, methanol, isopropanol, etc. is added to the mixtu re. According to a third embodiment, the mixture may be spray-dried in a humid atmosphere. Important ceramic materials, which may advan¬ tageously be prepared by the method of the invention, contain al kali metal ions such as sodium ions. When preparing clear hydroxy gels from mixed solution of metal alkoxides and other chemicals, e.g . , in the system Na-O-ZrO-n-SiO.- ) - ^2^5"^'2^3 '* ' s ° P a rt ' cu ' ar importance to avoid selective precipita ¬ tion both when the initial components are mixed and when these solutions are gelled . It is thus necessary to ensure the formation of soluble copolymers composed of all the components before gelling . This may be achieved in various ways. In one embodiment (A) of the process of the invention, anhydrous phosphoric acid dissolved in a lower alcohol is reacted with a lower alkoxide of silicon, the resulting polymer is reacted with one or more alkoxides selected from lower alkoxides of zirconium, aluminium, and the alkali metal dissolved in a lower alcohol, and the resulting mixture is gelled by addition of water. This embodiment is preferably per¬ formed by reacting the polymer resulting from the reaction between the anhydrous phosphoric acid and the lower alkoxide of silicon with either a zirconium lower alkoxide or an aluminium lower alkoxide or a mixture of a zirconium alkoxide and an aluminium alkoxide diluted with a lower alcohol followed by addition of an equimolar amount, relative to zirconium and/or aluminium, of water diluted with an alcohol, after which water is added to gel the mixture, alkali metal alkoxided op- tionally being added to the mixture prior to gelling. This embodiment is particularly useful for preparing gels with a silicon/phosphorous ratio in the range of 1 .8-2.6, such as gels for the manufacture of Nasicon ceramics or Nasicon-iike ceramics . One advantage of this embodiment is that stable stock solutions containing both zirconium, silicium, phosphorus, sodium and optionally aluminium may be pre¬ pared which makes it unnecessary to prepared the mixtures for immediate subsequent gelling. Another advantage of the use of such stock solutions is that they make it easy to prepare particular new compositions by mixture of two or more stock solutions, such as e.g . , mixing of a stock solution corresponding to a stoichiometric Nasicon composition and a solution efficient in zirconium. In the initial mixing it is preferable to mix a solution of phosphoric acid (1 -3 M) in a solvent such as i-propanol with the silicon lower alkoxide, which, as mentioned above, is the least reactive of the esterified metal oxides. The size of the resulting polymers and the number of unreacted protons in the phosphate groups is only the function of the composition of the mixture and does not change in time. In the next step, a solution of a zirconium lower alkoxide, or an aluminium lower alkoxide or a mixture of a zirconium lower alkoxide and an aluminium alkoxide (e.g. a zirconium propoxide or an alumini¬ um sec. butoxide and in a concentration between 1 .1 and 1 .5 M such as 0.5 M in the case of the zirconium in an alcohol such as methanol, ethanol or propanol) is added. The zirconium compound and/or the aluminium compound react with the remaining acid groups on the sili- con/phosphorus polymers . Premature gelling may be observed if the silicon/phosphorus ratio is less than 2, but the fraction of the solu¬ tion forming a gel increases with decreasing silicon/phosphorus ratio . When employing a solvent such as propanol a clear and apparently stable solution may be obtained for silicon/phosphorus ratios above 2. I n order to ensure complete copolymerization between the zirconium alkoxide and/or the aluminium alkoxide and the silicon/phosphorus polymers, a controlled amount of water diluted in an alcohol such as propanol in a water/alcohol ratio in the range from 1 :5 to 1 : 15, in particular 1 : 10, may be added. I n order to keep the size of the resulting polymers down and to avoid gelling, the amount of water added should be kept as small as possible. If insufficient water is added at this stage, howerver, precipitation of e. g. or similar double alkoxide may be observed shortly after the optional final addition of an alkali metal alcoholate such as sodium methoxide dissolved in e. g . methanol . However, the precipitate will usually slowly dissolve again if more water is added . Solutions of compositons as mentioned above are strongly basic. Neutral solutions may be obtained if the alkali metal alkoxide is re¬ placed with an alkali metal salt such as, e. g . sodium acetate, sodium nitrate, or sodium nitrite. Furthermore, gelling in an acid environ¬ ment can be obtained by additionally adding an acid, preferably an acid corresponding to the alkali metal salt, such as acetic acid or nitrite acid. pH of the solution when gelled influences the specific surface area and the porosity of the resulting gel . In another embodiment of the method of the invention (B) of the process of the invention, anhydrous phosphoric acid and a lower alkoxide of silicon are polymerized together, and the polymer is subsequently added to a refluxed mixture of silicon lower alkoxide and zirconium lower oxide or aluminium lower alkoxide or a mixture of zirconium lower alkoxide and aluminium lower alkoxide, optionally containing an alkali metal lower alkoxide. In this embodiment, it is the silicon/phosphorus-containing which polymer is reacted with the zirconium-(and/or aluminium) -silicon copolymerisate complex (optionally zirconium(and/or aluminium) -silicon-alkali metal complex) . When using silicon/phosphorus ratios lower than 2, the rate of polymerization of OMPI V/ϋ-O *> the silicon/phosphorus polymer may lead to a polyfunctional polymer with more than one HO-group . I n order to avoid the formation of precipitates during the addition of the refluxed alkoxide mixture, it may be necessary to use a dilution of the silicon/phosphorus polymer solution . It is preferable that the maximum concentration of phospho¬ rus in the silicon/phosphorus polymer solution is in the range be¬ tween 0.35 and 1 .4 mol P/l, in particular around 0.35 mol P/l . This is particularly pertinent if isopropanol is used for dilution whereas, when methanol is used as the diluent, the phosphorus concentration may be reduced to 0.2 mol P/l . This may be due to the fact that methanol as a result of the process of alcohol exchange between the alkoxide and the solvent gives rise to a less sterically hindered and consequently more reactive polymer. Also when working according to this embodiment of the invention, it is possible to add the optinal alkali metal component as an alkali metal salt and to add an acid corresponding to the salt, in order to provide neutral or acidic conditions. According to a third embodiment of the invention, a mixture compri¬ sing a lower alkoxide of silicon and one or more alkoxides selected from lower alkoxides of zirconium, aluminium and the alkali metal is refluxed, after which anhydrous phosphoric acid dissolved in a lower alcohol is added to the refluxed mixture, and the resulting mixture is gelled by additional water. By the additional water to this mixture, clear gels will normally be obtained. This is in contrast to the behavior of an unrefluxed mixture since the refluxing leads to a partial cόpolymerization in the mixture which will inhibit selective precipitation of any of the metal oxides. The phosphoric acid is preferably in dilute form when introduced into the refluxed mixture in order to avoid the premature formation of a gel . An appropriate maximum concentration of phosphoric acid in this embodiment is substantially around 0.2 mol/! . Furthermore, the phos¬ phoric acid is preferably diluted with a lower alcohol, such as metha¬ nol or propanol, in particular methanol. The apparent superiority of methanol may be due to the fact that the use of methanol provides a OMPI greater difference in viscosity between the refluxed mixtu re and the diluted phosphoric acid leading to a sch I ieren -effect promoting a quicker mixing of the phosphoric acid into the refluxed metal oxide ester system. Also in this third embodiment of the invention, the alkali metal com¬ ponent may be added as a salt, and an acid corresponding to the salt may be added, in order to provide neutral or acidic conditions. It is known that it is possible to prepare phosphorus-free gels through activation of the silicon lower oxide through partial hydroly- sis before the addition of the zirconium lower alkoxide. This proce¬ dure may also be employed in the case of compositions with very high silicon/phosphorus ratios . I n a preferred embodiment of the method of the invention, the content of zirconium relative to the content of silicon, phosphorus and sodium in the final ceramic material is less than or equal to the content of zirconium in ZrSiO ., ZrP 2 O.-, Na 2 ZrO 4 and Na. . Zr 2 P, Si O., 2 , in which 0 < x < 3, and the content of sodium relative to the content of silicon, zirconium and phosphorus in the final ceramic material is less than or equal to the content of sodium in Na 2 ZrO ., Na .SiO . and a 3 O 4 . I n a particularly preferred embodiment of the invention, the starting materials are mixed in such proportions relative to each other that the ceramic material produced has the general formula : Na 1 r 2-z P 3-x Si x°12-z + y/2 herei n 0 ≤ x ≤ 3, 0 < z < x/3, 0 ≤ a ≤ (3-x)/2 -0.3 < y ≤ 0.3. when a is 0, i . e. , no aluminium is present, such materials are either part of a Nasicon series or may be termed zirconium-deficient Nasicon materials (which consist of Nasicon crystals and a glass) . When alumi- C: Π A W il ' O i. nium is present, materials may be considered as aluminium-modified Nasicon materials . The inclusion of aluminium in Nasicon type materi¬ als is believed to result in an increased stability towards molten sodium and a decrease of the solubility of the glass phase in zirconi- u -deficient Nasicons in water. In the above generel formula, it is especially preferred that 1 .8 ≤ x ≤ 2.6, 0 < z < x/3, -0.3 < y < 0.3, in particular that 0 < a < 0.4. In a subset of the generel formula given above is the generel formula of Nasicon materials or zirconium-deficient Nasicon materials which do not contain aluminium and which are represented by the generel formula a 1 +χ+y Zr 2 _ z P 3 _ χ Si χ O l 2 . 2z+y/2 in which 0 ≤ x ≤ 3, -0.3 ≤ y ≤ 0.3 0 < z < 1 . I n this generel formula, particularly interesting materials are also obtained when -0.1 ≤ y ≤ 0.1 . For Nasicon compositions or zirconium-deficient Nasicon compositions with x between 2 and 3, optionally aluminium-modified as discussed above, the method A outlined above leads to clear solutions which may be stored for months without any apparent change. The solutions may optionally be concentrated, e. g. by fractional distillation. It is thus possible to repair a solution with a concentration as high as up to 50%, typically as high as up to 36%, by weight of oxide (where x = 2) . Stock solutions of this type constitute an aspect of the invention, which, in its broadest sense, is characterised as a solution in an organic medium containing 1 -50% by weight of predominantly oxygen- bridged polymers formed by reaction of phosphoric acid with a lower alkoxide or silicon and optionally with one or more alkoxides selected from lower alkoxides or zirconium, aluminium, and alkali metal (the optional alkali metal reactant alternatively being an alkali salt soluble in the organic medium) . Preferably, the composition of the polymer in such solutions is one which by hydrolysis results in a gel of a compo¬ sition corresponding to any of the generel formulae stated above. One interesting type of such solution is one in which the polymer contents substantially consists of a polymer formed by reaction of a lower alkoxide of silicon such a tetraethylorthosilicate, and phosphoric acid. The organic medium in these solutions is preferably a lower alcohol . In an especially preferred embodiment of the invention, x in the above generel formulae represents a value in the range of 1 .8 ≤ x ≤ 2.6 such as 1 .8 < x ≤ 2.4. In the process of the invention, the removal of water and volatile components from the gel may be performed by drying at temperatures of up to 110°C followed by calcination at temperatures above 110°C. In a preferred embodiment, the calcination temperature is a tempera- ture in the range between 110°C and 800°C. In another preferred embodiment the drying may be performed by such methods as spray drying, vacuum drying, evaporation, evaporation by destination, or fractional destination . The physical properties of the water- and volatiles-free gels are influenced by the concentration of metal alkoxides and/or polymeri- sates thereof in the solutions gelled and by the particular way in which the final hydrolysis and removal of water and volatiles is per¬ formed. Different treatments may give rise to varying characteristics in terms of such factors as surface area, density and residual organic material (determined as CO 2 ) . Thus, spray drying of gels will gene¬ rally give rise to hollow, partly collapsed spheres of a very compact gel with a comparatively low surface area and high amounts of resi¬ dual organics . For the preparation of ceramic powders useful as ion conductors, both gelling at room temperature of dilute solutions with a large excess of water and drying by fractional destination are of particular interest. The conversion of the dried or calcinated gel into a ceramic material may be performed by sintering the dried or calcinated gel. Such sintering is preferably performed at a temperature below the incon- gruent melting temperature of the crystalline phases with proportions corresponding to the dried or calcinated gel, but the sintering is in particular performed at a temperature above 700°C, often at a tempe- rature between 800°C and 1200°C. When Nasicon or Zr-deficient Nasicon gels are dried by calcination, they are at first converted to glasses . In these glasses, tetragonal zirconium oxide crystallizes out by heating at a rate of about 1 °C/mi- nute or slower at intermediate temperatures. The zirconium oxide is completely resor-Sed at higher temperatures concurrently with the formation of Nasicon . Depending on the content of zirconium oxide in the composition of the gel, the crystallizing out of tetragonal zirco¬ nium oxide may occur at temperatures between 400 and 600°C. Under the preferred heating conditions employed in the process of the invention (about 1°C/minute from 20 to 710°C and then about 0.4°C/- minute) , the crystallization of Nasicon occurs at temperatures of about 850°C. Without wishing to be limited to any theory, it is believed that under these conditions the tetragonal zirconium oxide formed functions as a nucleus for Nasicon crystallization, the zirconium oxide crystals having been formed due to the relatively slow heating. All the raw powders calcinated at the chosen standard temperature of about 710°C consist of glass containing small crystals of tetragonal zirconium oxide. For the Nasicon-type gels prepared according to the process of the invention, the first phase to nucleate under the preferred heating conditions (1°C/minute) is tetragonal zirconium oxide. Depending on the composition, crystal growth begins between 400 and 600°C, the higher the zirconium oxide content the lower the temperature. Without wishing to be limited to any particular theory, it is believed that the method used for preparation of the gel also affects the nucleation and growth of zirconium oxide. For gels with a composition like the Nasi¬ con series and zirconium-deficient Nasicon, the material crystallizes under heating, and the zirconium oxide is completely resorbed at temperatures between 800 and 900°C. Gel powders based on alkoxide-derived gels as raw material for the manufacturing of dense ceramics have the advantage of such factors I I as high specific surface area, homogeneity on a molecular scale and an amorphous character of the material . The invention further relates to a method for producing a shaped article of a ceramic material prepared by a process substantially as described above comprising shaping the dried or calcinated gel into a desired shape followed by sintering at a temperature below the incon- gruent melting temperature of the material, such as a temperature of above 700° C. I n a preferred embodiment, the dried or calcinated gel is comminuted prior to shaping . The comminution may be performed by any suitable method such as grinding or treatment in a ball mill . Preforms for the desired shaped article may be produced by dry pressing, isostatic pressing, wet pressing, hot pressing, extrusion or slip casting . I n a preferred embodiment, the preforms are prepared by cold pressing in a steel die. Depending on the desired characteristics of the final shaped article, the gel powders may be pressed at a pressure in the range of 50-1000 MPa . If the final shaped articles are intended to be used as ion conductors, a preferred pressure range for calcinated powders is a range of 75-300 MPa, preferably about 100-200 MPa . When pressed at a pressure about 100 MPa, gels dried at 110°C, spray dried gels and gels which, are calcinated at 710°C show a poro¬ sity of 40-50% in the preformed items. Preforms prepared from ground, dried gels are the most porous . The specific gravity of the calcinated gels is much higher than that of the dried gels so that, by using calcinated powders, the least possible shrinkage during sinter¬ ing is obtained . The strength of preforms pressed from spray dried powder is substantially smaller than that of preforms prepared from ground gels at the same compression pressure. For such applications as ion conductive ceramics, it is particularly useful to sinter the pressed preforms at temperatures above 700°C, since the specific density of the preforms tend to increase rapidly above this temperature, in particular at temperatu res in the interval between 800 and 1500°C, for most compositions in the interval between 800 and 1300°C. At any rate, the temperature used for a particular composition should not be so high that the composition melts . In a ceramic material of one composition of the Nasicon series pre¬ pared by the process of the invention, the major proportion of the sintering occurs in the temperature interval between about 725°C and 950°C. The sintering at this temperature is presumably due to the viscous flow in the glass phase formed . The amount of glass phase decreases during the final 100°C of this temperature interval concur¬ rently with the formation of Nasicon and the resorption of tetragonal ZrO 2 . The sintering is substantially finished at about 1000°C. It has, however, been found that the crystallinity of the material and an improved ion conductivity may be obtained by further heat treatment to about 1200°C. I n another material prepared by the process of the invention and similar to the above, the total sintering in the temperature interval of up to about 1000°C is less than with the material described above. This is presumably due to the fact that this composition contains less glass in this temperature interval due to a faster crystallization of Nasicon on the basis of more nuclei of ZrO.,. The best results for this material are obtained by heating to about 1250°C at a rate of about 3°C/minute followed by sintering at a temperature of about 1300°C in order to obtain a lower porosity. The method of preparing ceramic material based on the gelling of alkoxides further provides the possibility of a method for producing a shaped article of a ceramic material wherein the mixture of esterified metal oxides and anhydrous phosphoric acid is gelled in a mould followed by removal of water and volatile components from the result¬ ing shaped gel and finally sintering the water- and volatiles-free and shaped gel . By this method it is possible to prepare shaped articles having a shape which is difficult to obtain by pressing of powders . The invention also relates to a shaped article of a ceramic material prepared by the process of the invention and which is adapted to be used as an electrolyte or ion conductor in an electrochemical storage cell or other electrochemical devices such as electrochemical sensors and electrochromic displays, sodium heat engines, or sodium winning devices. For the purpose of use in batteries, a specific resistivity of ι3 less than 15Ω/cm is generally regarded as desirable, and shaped articles prepared by the method of the invention may attain a resisti¬ vity which is as low as about 3 Ω/cm. Articles of such ceramic mate¬ rials may form part of advanced battery types such as sodium/sulphu r batteries, which are characterized by a high ratio between total energy content and weight. Such batteries may therefore be useful in electrically powered vehicles, storage systems for alternative power generating systems such as wind mills or solar cells, in emergency power systems in vital institutions such as hospitals, and in power buffers for conventional power grids for the accumulation of surplus power to be used in periods of peak load . The ceramic materials prepared by the process of the invention may, depending on their composition, be wholly amorphous or wholly cry¬ stalline or any combination thereof . It is not unusual to observe ceramic material of the Nasicon type which , upon sintering, shows- an amorphous or glass phase in the interstices between the crystals of stoichiometric Nasicon . The prepared ceramic materials sintered to at least 1100°C and pre¬ ferably 1200°C show an excellent durability when tested in sodium/- sodium cells at 300°C, and corrosion tests in molten sodium (400- 700°C) indicates that at 400, 350, 300°C it takes at least 100 hou rs, 7 weeks and 3 years, respectively, to dissolve a layer of the ceramic material of a thickness of 1 ym. OMPI_ sfy-m. WIxO , t In the following, the invention is further illustrated with reference to the drawings in which Fig. 1 shows an electron microscope picture of a surface in a sintered plate of a zirconium-deficient Nasicon material prepared according to the prior art; Fig. 2 shows an electron microscope picture of a surface in a sintered ppllaattee ooff aa ZZrr<O 2 ~deficient Nasicon material produced by the process of the invention; Fig . 3 shows a phase diagram for the 4-component system ZrO 2 , SiO 2 , Na 2 O, P 2 O 5 ; Fig. 4 shows an excerpt of the phase diagram corresponding to the Nasicon mixing series; Fig. 5 shows the variation in the number of unreacted HO-groups in mixtures of phosphoric acid and tetraethylorthosilicate; Fig. 6 shows the results of thermogravimetric analysis and differential thermal analysis on the NZPS-82-1 BC gel; Fig. 7 shows X-ray diffraction diagrams of the NZPS-82-1 BC gel heated to various temperatures; Fig. 8 shows the sintering behaviour of plates pressed from crushed gel of NZPS-82-1 BC; Fig. 9 shows the density of the gel NZPS-82-1 BC subjected to diffe¬ rent treatments and the bulk density of plates prepared from the thus treated gel material; Fig. 10 shows the bulk density of the corresponding sintered plates as a function of the pressing pressure and the sintering conditions; Fig. 11 is a Weibul-plot, which for a sample shows the fracture pro ¬ b baabbiilliittyy PP f , ((%%)) aass a function of the loading in a three-point bend/ten sile strength test; Fig. 12 shows the resistivity of stoichiometric Nasicon and Z -defi- cient Nasicon (including NZPS-82-1 BC) at 300° C. DETAI LED DESCRI PTION OF THE DRAWI NGS Fig. 1 shows an electron microscope picture of a sectioned surface in a plate of synthetic zirconium-deficient Nasicon ceramic prepared according to the prior art (Materials Research Bulletin 11, 1976, p. 173) by sintering to 1226°C for 6 hours . The picture width corres¬ ponds to 20 μm. The photograph shows white areas and black areas on a grey background . The white areas are free crystalline ZrO 2 (Baddeleyite) and the black areas are zirconium-deficient glass, while the light grey mass is crystalline Nasicon with grain sizes in i the range of 2-10 μm and a stoichiometric composition corresponding to x = 1 .89 as determined by X-ray diffraction . Fig. 2 shows an electron microscope picture of a sectioned surface in a plate of the synthetic ZrO 2 ~deficient Nasicon ceramic designated NZPS-82-1 BC prepared according to the gel process of the invention by sinterint to 1100°C for 10 hours . The picture width is again 20 μm. The material is seen to be a polycrystalline ceramic with a grain size less than 1 μm in which no inclusions of crystalline ZrO 2 are to be seen . An additional glass phase are seen between the Nasicon crystals . The Nasicon crystals have a stoichiometric composition corresponding to x = 2.10 as determined by X-ray diffraction . Fig. 3 shows a phase diagram for the 4-component system ZrO 2 , SiO 2 , Na 2 O, PoCς * " " " he composition (on a molar basis) of known crystalline compounds within the system is marked with squares . A complete mixing series between NaZr 2 P 3 O, 2 and Na .Zr 2 Si 3 O 12 (Nasicon SS) is marked with a hatched line and may be designated Na 1 +χ Zr 2 P 3 _ χ Si χ O 12 , where 0 < x < 3. lb Fig. 4 shows an excerpt of the phase diagram corresponding to the Nasicon mixing series . The diagram shows that Nasicon with a com¬ position corresponding to x = 2, in the vicinity of which the Nasicon compounds interesting for battery applications and with a high ion conductivity are situated, melts incongruently at a temperature close to 1276°C under the formation of a melt (L) + ZrO 2 + ZrSiO 4 + a more silicon-rich Nasicon . At 1300°C this combination consists of a melt (L) + crystalline ZrO 2 + Nasicon with a composition corresponding to x = 2.18. At 1306°C the combination consists exclusively of a melt (L) + crystalline ZrO 2 . Fig. 5 shows the variation in the number of unreacted HO-groups in mixtures of H 3 PO . and Si(OEt) . as a function of composition . The determination of the number of HO-groups was performed by pH-ti- tration using a glass electrode (Radiometer G202B) , a calomel refe- rence electrode with a salt bridge of LiCI dissolved in isopropanol (Radiometer K701 ) and sodium methoxide dissolved in methanol as base. The reaction of the mixture took place at room temperature. In + the figure is shown the number of primary hydrogen ions (PH ) , the + number of secondary hydrogen ions (SH ) and the sum of hydrogen + ions (ΣH ) . In the Figure, the dotted line designates the lower limit + of ∑H after complete reaction between phosphoric acid and tetra- ethylorthosilicate (LI∑H ) . Fig. 6 shows results of thermogravimetric analysis and differential thermo analysis on the gel NZPS-82-1 BC which had been dried at 110°C. Fig. 7A shows the total weight loss in percent as a function of the temperature. The rate of increase of the temperature was 8°C/mi- nute. Fig . 6B shows a differential thermal analysis curve obtained with a rate of increase of the temperature of 8°C/minute. The curve shows an endotherm maximum around 180°C corresponding to a maxi- mum in the release of H 2 O (this is observed in all the gels) . The well-defined exotherm peak at 1040°C reflexes crystallization of the major part of the glass . Fig . 6C shows qualitatively the differential release of water as a function of temperature at a rate of increase of the temperature of 10°C/minute. Fig . 6D shows qualitatively the differential carbon dioxide release as a function of temperature at a rate of increase of the temperature of 10°C/minute. C-: TI vA Vv to " Fig. 7 shows X-ray diffraction diagram of NZPS-82-1 BC (vacuum- dried at nθ°C) heated to the indicated temperatu res for the indicated time spans . The temperatures were reached according to the following program: 20°C-710°C l °C/minute, 710°C-930°C 0.4°C/minute, and above 930°C 1°C/minute. The broad weak peaks in the XRD-diagram for gel dried at 110°C are typical for the gels . Heating to 550°C for 161 hours does not result in any marked changes in the XRD-dia¬ gram. Heating to 600°C for 140 hours results in beginning crystalliza¬ tion as shown by the two narrow peaks. The crystalline phase formed is a tetragonal zirconiumdioxide. Heating to 710°C for 5 hours also results in a partial crystallization of the gel . The phase formed is tetragonalzirconium dioxide. Heating to 885°C or above results in the formation of crystalline Nasicon . Fig. 8 shows the sintering course for plates pressed from the material NZPS-82-1 BC produced by the process of the invention and pressed at 95 MPa from gel dried at 110°C. The changes in specific density (SD) and bulk density (BD) are shown . The plates were heated according to the following program: 20-710°C 1 °C/minute, 710-930°C 0.4°C/minute, and above 930°C l°C/minute. It may be seen from the bulk density curve that the substantial sintering occurs at temperatu¬ res in the range of 800-900°C and that the bulk density at 1110°C constitutes some 97% of the theoretical density (TD) . In the figure are furthermore given the theoretical densities of Nasicon (Na 3 ιZr 2 Pg gSL ,0,-) and Vlasovite (Na 2 ZrSi .O, ,) , (the theoretical densities being 3.254 and 3.006, respectively) , which are the equili¬ brium phases for the composition in question at 1100°C. Fig. 9 shows (for the material NZPS-82-1 BC) the specific density of gel dried at 110°C (2.61 g/cm 3 , marked I 710°C) and of gel calcinated at 710°C (3.1 g/cm 3 , marked I I 110°C) as wejl as the green bulk density as a function of pressing pressure for crushed gel dried at 110°C (marked A 110°C) or calcinated at 710°C (marked B 710°C) , respectively, or calcinated at 710°C and thereafter crushed (marked C 710°C) . Fig. 10 shows the bulk density and the porosity of the corresponding sintered plates as a function of the pressure when pressing the plates (before the sintering) . The sintering was performed at 1100 ± 20°C . The top half of Fig . 10 shows the open and the total porosity for the samples I-I I I and IV-VI , respectively, as a function of the pressing pressure. Samples I and IV were prepared from crushed gel dried at 110°C, samples I I and V from crushed gel calcined at 710°C, and samples I I I and VI from gel calcined at 710°C and then crushed. The open porosity corresponds to surface irregularities on the plate and is quite low, while the total porosity is 3-5% and increases with increas¬ ing pressing pressure. The bottom half of Fig. 10 shows the bulk density of the sintered plates . Sample A was prepared from crushed gel dried at 110°C, sample B from crushed gel calcined at 710°C, and sample C from gel calcined at 710°C and then crushed. Samples A, B, and C were sintered in air. Sample D was prepared from gel calcined at 710°C and then crushed, and the sintering was performed in vacuum. Fig. 11 shows a Weibul-plot of the fracture probability P, (%) as a function of the loading (σ) of the sample (sintered plates from NZPS-82-1 BC) by a three-point bending method. The sample marked A 1100°C was sintered at 1100°C, while the sample marked B 1204°C was sintered at 1204°C. Both samples were sintered in air and both 3 samples had a specific volume of 0.07 cm . The data points for the two samples were subjected to regression analysis resulting in the two full-drawn lines passing through the data points. The sample marked C 1134°C was sintered in vacuum at 1134°C and had a specific volume of 0.09 cm. Regression analysis of the data points resulted in the dotted regression line. For comparison purposes, the shaded area outlines strength test data for β"-alumina specimens with a specific volume of 0.05 cm (cfr. Trans. Brit. Ceram. Soc. 79, 1980, pp. 120-127) . The relatively low fracture strengths of the samples are due to the presence of closed pores up to 0.1 mm in size. These pores can probably be removed by optimizing the processing conditions. Fig. 12 shows the resistivity at 300°C of three materials prepared by the process of the invention and marked E, F and G, respectively. ιa For comparison , data from the literature (A, B, C, D and H) are in¬ cluded. Samples A, B, C, D and E are of the general formula Na, + Zr 2 P 3 _ Si O- 2 . The data marked A are according to Mat. Res . Bull. 11, 1976, pp. 203-220, the data B according to Mat. Res . Bull. 14, 1979, pp. 1469-1477, the data C according to "Ion Transport in Solids, Electrode and Electrol tes" , Elsevier 1979, pp. 419-422, and the data D according to Solid State ionics 3/4, 1981 , pp. 243-248. Samples F, G and H are of the general formula Na. . Zr P,, Si O,~ . The data marked H are according to Solid l + y 2-z 3-x x 12-a 9 State Ionics 7, 1982, pp. 345-348. The abscissa axis shows the factor x from the general formulae. The invention is further illustrated by the following examples which, however, are not to be construed as limiting . EXAMPLE 1 Preparation of a Metal Alkoxide Gel (Method A) Anhydrous phosphoric acid in an amount corresponding to 0.1 mole was dissolved in 30 ml of ethanol . To this solution was added 0.2 moles of tetraethylorthosilicate diluted in 30 ml of ethanol at room temperature. The phosphoric acid and the tetraethylorthosilicate reacted immediately, and a copolymer was formed (cf. Fig. 5 for x = 2) . The resulting solution is stable for at least one year if it is kept isolated from the atmosphere and other sources of moisture. A portion of the solution was gelled as a monolith by addition of 0.5 moles of water dissolved in 10 ml ethanol . A clear gel was obtained. From a portion of the solution, a thin film of gel was prepared on a substrate of pyrex glass by pouring a small amount of the solution out on the glass and leaving it exposed to the atmosphere for a few hours. The resulting gel had an oxide composition corresponding to Si .P 2 O g . EXAMPLE 2 Preparation of a Metal Alkoxide Gel (Method A) Anhydrous phosphoric acid in an amount corresponding to 0.1078 mole was dissolved in 30 ml of ehtanol containing 0.2961 mole of water. To this solution was added 0.2961 mole of tetraethylorthosilicate contain¬ ing 30 ml of ethanol at room temperature. The reaction mixture was left to react for 16 hours at room temperature followed by addition of 0.2006 mole of zirconium-tetra-n-propoxide from a 95% stock solution OMPI in propanol and 0.3955 mole of sodium ethoxide from a 1 .5 M stock solution in ethanol . The mixture tu rned opaque, but no precipitation occu rred . The mixture was then left to react for 16 hou rs at room temperature. Then , the solution was gelled by addition of 2 moles of water dissolved in 36 ml ethanol . The gelling was performed at room temperature, after which the temperature was raised to 80°C for 24 hours . The resulting gel (NZPS-82-1 BC; cf. Table 1 ) was dried at 110°C in a vacuum drying cupboard for 24 hours and was then crushed. The resulting product was by means of X-ray diffraction shown to be 2 amorphous and had a large specific surface area of 94 m /g as deter¬ mined by the BET-method . The amount of alkoxide remaining in the gel (converted to CO- by burning) corresponded to 4% by weight of co 2 . The gels NZPS-81 -1 , NZPS-81 -3, NZPS-81 -4, NZPS-81 -5, NZPS-81-1 A and NZPS-82-2 (cf . Table 1 ) were also prepared according to ^this method, consideration being had to their different cemical composition . EXAMPLE 3 Preparation of a Metal Alkoxide Gel (Method A) Anhydrous phosphoric acid in an amount corresponding to 0.1 mole was dissolved in 60 ml of i-propanol . 0.2 Mole of tetraethylorthosili¬ cate was diluted with 40 ml of i-propanol . The two solutions were mixed . After 5 minutes at room temperature, the first proton of the phosphoric acid had reacted completely with the tetraethylorthosili- cate. Then, 0.2 mole of zirconium-tetra-n-propoxide diluted with 260 ml iso-propanol were added. The mixture was left to react for 16 hours at room temperature followed by addition of 0.2 mole of water dissolved in 180 ml of iso-propanol . The mixture was left to react for 1 hour at room temperature followed by addition of 0.3 mole of sodium methoxide from a 1 .54 N solution in methanol. τι The resulting solution was clear and of low viscosity. When the solu¬ tion is kept isolated from the atmosphere and from other sources of humidity, it is stable for at least one year and does not gel . On standing for about 100 hours at room temperature exposed to the atmosphere, it gelled to a slightly yellow, but clear gel . When the solution was gelled as a monolith by addition of 1 .5 moles of water dissolved in 50 ml of i-propanol at room temperature, a clear gel was obtained (cf. NZPS-82-XA in Table 1 ) . The gel NZPS-83-2C was prepared in the same manner, consideration being had to its different chemical composition . * EXAMPLE 4 In the manner described in Example 3, a clear solution with an oxide composition corresponding to Na 3 Zr 2 PSi 2 O, 2 was prepared (NZPS-82-3) . The concentration of oxides in the solution was 10 per cent by weight. This solution was concentrated by distillation to an oxide concentration of 36 per cent by weight. The resulting concen¬ trated solution was viscous and was stable for at least 5 months when kept isolated from moisture. The solution was gelled as a monolith by addition of water dissolved in alcohol as described above. A portion of the original 10% by weight solution was spray-dried in a wet at¬ mosphere (relative humidity 100%) at an inlet temperature of 110°C and an outlet temperature of 90°C. The resulting powder consists of hollow spheres of a diametre of about 10 μm and a specific surface of 2.6 mVg. EXAMPLE 5 Preparation of a Metal Alkoxide Gel (Method β) 18 millimoles of tetraethylorthosilicate, 16.7 millimoles of zirconium- tetra-n-propoxide, 34 millimoles of sodium methoxide (1 .54 M in meth¬ anol) and 15 ml of n-propanol were mixed and heated at reflux for 3 hours . Upon cooling, a mixture of 4 millimoles of phosphoric acid and PI ' 8 millimoles of tetraethylorthosilicate dissolved in 0.75 mole of i- propanol was added . The mixture was heated at reflux for 2.5 hours . Upon cooling, 5 ml of water diluted with 30 ml methanol were added with stirring . The resulting clear solution gelled within 1/2 hour after the addition to a clear, slightly opalescent gel . The resulting gel had an oxide composition corresponding to Na 3 .Zr, -37S ^P 4O1Q 0 - EXAMPLE 6 Preparation of a Metal Alkoxide Gel (Method B) A mixture of 10 millimoles of tetraethylorthosilicate, 20 millimoles of zirconium-tetra-n-propoxide, 20 millimoles of sodium methoxide (1 .54 M in methanol) and 10 ml of n-propanol were heated at reflux for 3 hours . Upon cooling, a mixture of 10 millimoles of phosphoric acid and 10 millimoles of tetraethylorthosilicate dissolved in 0.3 moles of -iso¬ propanol was added dropwise. The resulting solution was clear. The solution was gelled by addition of water substantially as described in Example 3. The resulting gel had an oxide composition corresponding to Na 2 Zr 2 PSi 2 O 1 l ι 5 . 24 Table 1 Compositions after calcination of gels prepared according to the invention % by weight SiO, ZrO„ P 2°5 AI 2 O 3 Na 2 O NZPS-79-6.4, 2.1* 14.53 24.60 39.60 19.64 NZPS-81 -3 16.19 20.55 46.82 16.44 NZPS-81 -1 13.41 22.64 46.41 17.55 NZPS-82-2 7.91 26.78 45.76 19.56 NZPS-82-1A 12.24 28.47 39.59 19.70 NZPS-82-1 BC** 12.25 28.51 39.60 19.64 NZPS-81 -5 10.46 29.11 40.23 20.20 NZPS-82-XA 13.38 22.65 46.45 17.52 NZPS-81 -4 13.38 22.66 46.42 17.54 NZPS-82-3 13.36 22.66 46.41 17.57 NZPS-83-2C 5.83 31.33 41.52 21 .32 NZPSA-83-1 10.76 28.66 39.78 1 .12 19.69 * The material shown in Fig . 1 . Made from mixture of oxides accor¬ ding to the method described in Material Research Bulletin 11, 1976, p. 173. Not according to the invention . ** The material shown in Fig. 2. EXAMPLE 7 Preparation of a Metal Alkoxide Gel (Method C) A mixture of 10 millimoles of tetraethylorthosilicate, 20 millimoles of zirconium-tetra-n-propoxide, 20 millimoles of sodium methoxide (1 .54 M in methanol) and 10 ml n-propanol were heated at reflux for 3 hours. Upon cooling, 20 millimoles of phosphoric acid (0.2 M in methanol) were added dropwise with stirring . On standing for 24 hours, the clear solution gels to a clear gel rich in organic material . Water diluted with i-propanol was added to a portion of the clear solution . Hereby, the solution gelled more quickly, and a completely hydrolysed gel was obtained. This gel has an oxide composition corresponding to Na 2 Zr 2 P 2 SiO 12 . EXAMPLE 8 Preparation of a Metal Alkoxide Gel Using Sodium Acetate (Method D) 0.44 mol of tetraethylorthosilicate was reacted with 0.16 mol of an- hydrous phosphoric acid dissolved in 100 ml of n-propanol . The resulting mixture was added to 0.298 mole of zirconium-tetra-n-prop¬ oxide diluted with 400 ml of n-propanol . After reaction overnight, 0.298 mole of water diluted to 125 ml with methanol was added drop¬ wise. The resulting solution was clear. To the clear solution, a solu- tion of 0.588 mole of sodium acetate and 0.68 mole of acetic acid in 360 ml methanol was added . The resulting clear solution was gelled by addition of water in i-propanol . The resulting gel had an oxide com- position corresponding to Na 2 g4 Zr.. 4g Si 2 2 P Q gO 10 85 . EXAMPLE 9 Preparation of a Metal Alkoxide Gel Using Sodium Nitrite (Method D) 0.26 mol of tetraethylorthosilicate was reacted with 0.04 mol of an¬ hydrous phosphoric acid dissolved in 100 ml of n-propanol . The resulting mixture was added to 0.2 mol of zirconium-tetra-n-propoxide dissolved in 250 ml of n-propanol . After reaction overnight, 0.2 mol of water diluted to 60 ml with methanol was added. The resulting solution was clear. To this clear solution, a solution of 0.36 mol of sodium nitrite dissolved in 850 ml of methanol was added . This re¬ sulted in the formation of a white, fine-grained precipitate which dissolved after addition of 640 ml of methanol and standing for about CV;?I YVIrO 36 hours with stirring . The resulting clear solution was gelled by addition of water in methanol substantially as described in Example 2. The oxide composition of the resulting gel corresponds to: Na 3.6 Zr 2.0 Si 2.6 P 0.4°12- EXAMPLE 10 Preparation of a Metal Alkoxide Gel Using Sodium Nitrate (Method D To a clear solution made from the same amounts of tetraethylortho¬ silicate, zirconium-tetra-n-propoxide and anhydrous phosphoric acid as described in Example 9, 0.36 mol of sodium nitrate, 47 ml of water and 5.6 ml of concentrated nitric acid in 1800 ml of methanol was added. A milky solution containing a white precipitate was obtained. Upon addition of water, a slow gelling was observed . The gel has an oxide composition corresponding to Na 3 gZr~ QS -P 0 4 O 1 . EXAMPLE 11 Preparation of Aluminum-containing Nasicon Gel (Method A) 0.238 mol of tetraethylorthosilicate was reacted with 0.0758 mol of anhydrous phosphoric acid dissolved in 100 ml of n-propanol by mixing. 0.161 mol of zirconium-tetra-n-propoxide was reacted with 0.0110 mol of aluminum-sec. butoxide. A white precipitate was imme- diately formed. After addition of 200 ml of n-propanol and stirring, this precipitate dissolved again, and after 2.5-3 hours, the solution was clear again . Thereafter, the tetraethylorthosilicate-phosphoric acid mixture was added to the zirconium-tetra-n-propoxide-aluminum-sec. butoxide mix- ture, and the resulting solution, which was clear, was allowed to stand overnight with stirring . C ?I A 1 : 10 mixture of H 2 O and n-propanol was added dropwise. Thereby, a white precipitate was formed . Immediately upon formation of the precipitate, 0.317 mol of sodium methoxide dissolved in 200 ml of methanol was added . After about 3 hours of stirring, the solution was clear. Thereafter, the solution was gelled homogeneously by addition of water dissolved in n-propanol in the usual manner. Prior to the gelling, the total oxide concentration in the solution was 7.9% by weight. The solution was gelled by addition of 35 ml of water diluted in 300 ml of methanol . The resulting gel (NZPSA-83-1 ) was clear and had an oxide composition corresponding to: Na 2.93 Zr 1 .61 (A, 0.1 Sl 2.20 P 0.70 ) O 10.74- EXAMPLE 12 Preparation of Aluminium-containing Nasicon Ceramic by Powder Mix¬ ing 50 mg of the gel NZPS-82-1 BC prepared as described in Example 2 and 50 mg of AI(OH) 3 were mixed very carefully in a mortar. The resulting mixture was mixed with more NZPS-82-1 BC gel according to the following procedure: 100 mg of the mixture and 100 mg of NZPS-82-1 BC 200 mg of this mixture and 200 mg of NZPS-82-1 BC 400 mg of this mixture and 400 mg of NZPS-82-1 BC 800 mg of this mixture and 800 mg of NZPS-82-1 BC 1 .6 g of this mixture and 1 .45 g of NZPS-82-1 BC Thereby, 3.050 g of powder mixture was obtained . This powder mixture (NZPS-82-1 -A01 ) was pressed into tablets at a pressure of 100 MPa. The tablets were sintered to 1205°C for 10 hours . The bulk density of the resulting tablets was 2.908 g/cm 3 , and the open poro¬ sity was as low as 0.50 per cent. Specific resistivity data of this Nasocon ceramic are given in Table 3. EXAMPLE 13 Preparation of Aluminium-containing Nasicon Ceramic by Powder Mix¬ ing 100 mg of the gel NZPS-82-2 prepared as described in the Example 2, 50 mg of NaAIO 2 and 50 mg of NaNO 3 were mixed very carefully in a mortar. The resulting mixture was mixed with more NZPS-82-2 gel according to the following procedure: 200 mg of this mixture and 200 mg of NZPS-82-2BC 400 mg of this mixture and 400 mg of NZPS-82-2BC 800 mg of this mixture and 800 mg of NZPS-82-2BC 1 .6 g of this mixture and 1 .5 g of NZPS-82-2BC Thereby, 3.100 g of powder mixture was obtained . This powder mixture (NZPS-82-2-N 2 A01 ) was pressed into tablets and sintered in the same manner as described in Example 12. The resulting tablets had a bulk density of 3.058 and an open porosity of 1 . 12%. EXAMPLE 14 Shaping Articles of Ceramic Material For the preparation of green bodies for later conversion into cera¬ mics, the dried gel powders prepared by the process described in Examples 1 -11 may either be used directly as a powder for pressing and shaping by conventional pressing methods, or they may first be calcinated at temperatures up to 710°C. The calcination results in the removal of the remainder of water and organic material from the gel . The variation in the properties of gel powders prepared as described above is illustrated in Table 2. For NZPS-82-1 BC, the specific gravity of the dried powder and the calcinated powder is shown in Fig . 9 together with the bulk density of tablets (diameter of about 1 cm) produced from the gel, as a function of the pressure at which the tablets are shaped and of the preceding treatment of the powder. The clear gel monoliths prepared by the process described in Examp¬ les 3-7 and 11 may either be dried and powdered and used as de¬ scribed above, or they may be dried carefully, e. g. at room tempera¬ ture over a period of 7 days . I n this case, the gels retain their monolithic shape and sh rink without disintegrating into a powder. Thus, it is possible to cast mouldings directly by means of the solu¬ tions prepared in these examples by pouring them into a mould of the desired shape of the final article, but enlarged to compensate for the shrinkage, before gelling. EXAMPLE 15 Sintering Shaped Articles of Ceramic Material The course of sintering of the dried gel NZPS-82-1 BC as a function of temperature is shown in Fig. 8. The bulk density and porosity of the resulting articles when sintered to 1100 t 20°C is shown in Fig . 10 as a function of the pressure used to compress the articles. It was demonstrated that the strongest sintering of this gel occurs in a narrow range between 800°C and 900°C, and that the highest bulk density of the sintered plates surprisingly occurs at a low compres¬ sion pressure. The other types of gel prepared according to the method described in Examples 1 -11 show similar characteristics . Thus, all the compositions studied show enhanced sintering in this tempera¬ ture interval, but the absolute magnitude of the densification de¬ creases as the ZrO 2 content increases. The competing processes which control the sintering behaviour in this interval appear to be viscous flow in the glass phase and crystal growth . For stoichiometric Nasicon compositions, densities in excess of 93% of the theoretical were only obtained by sintering to temperatures close to the incongruent melting point (Fig . 4) . Table 2 Variation in properties of gels prepared by reaction polymerization specific organics surface determined specific area deter¬ as CO 2 density mined by Gel No. % by weight g/cm 3 BET m 2 /g Comments Vacuum dried 110° C NZPS- 82-3 - - 2.6 spray dried 82-2 5.1 - 109 original 1 .7 2.78 77 rehyd. 25-80°C 82-4 - - 127 82-1 A 4.9 2.59 50 82-1 BC 6.3 - 136 original 3.6 2.62 103 rehyd. 25°C 83-2C - - 231 dest. in steps* Calcined °C h 82-3 - 3.25 2.1 710 100 grey 82-2 0.7 3.26 15 710 160 grey 82-4 - 2.87. 5.6 710 140 white 82-1 BC - - 50 500 142 grey - - 40 550 161 grey <0.1 3.11 14 710 160 white H 2 O added to the partially dried gel . OMPI I n order to avoid cracking due to too rapid sintering, it is necessary to heat slowly. The gel NZPS-82-1 BC dried at 1 I0°C is suitably heated from 20°C to 710°C over a period of 6 hou rs, from 710°C to 1000°C over a period of 14 hou rs, from 1000°C to 1100°C over a period of 2 hours, and at 1100°C for 9 hours . The two final steps are not necessary for the sintering, but they are necessary in order to obtain the maximum crystallinity of the material . The crystalline equilibrium phases at 1150°C as determined by X-ray diffraction in the various compositions stated in the examples are shown in Table 3. The flexural and tensile strength of ceramic mate¬ rial prepared by the process described above and determined by the three-point method is shown in Fig . 11 . Table 3 Equilibrium Phases at 1150°C Determined by X-ray Diffraction Nasicon Other Phases Na 1 -x Zr 2 P 3-x Si x°12 NZPS-81 -3 1 .8 NZPS-81 -1 2.0 NZPS-82-2 2.4 NZPS-82-1 BC 2.14 Vlasovite (Na 2 ZrSi 4 O 4 ) NZPS-81 -5 2.25 Vlasovite (Na.,ZrSi 4 O 4 ) Parakeldyshite (Na 2 ZrSi 2 O 4 ) NZPS-82-XA 2.0 -^ fRE OMPI • SI The resistivity with respect to sodium ion conductivity obtained with polycrystalline ceramic articles of the composition stated above are shown in Table 4. It is possible to convert shaped articles cast from solutions which are prepared according to the method described in Examples 1 , 3-7 and 11 , to a compact glass by heating to 450-550°C; at higher tempera¬ tures, the materials containing more than 35% by weight of zirconium oxide begin to crystallize, whereas the SL PJJO Q gel prepared in Ex¬ ample 1 remains a glass even after heating temperatures above 900°C for prolonged periods. Cast plates of the gel NZPS-82-XA have been converted to a glass by heating over a period of 13 hours from 20°C to 491 °C. This glass is an ion conductor. EXAMPLE 16 Electrochemical tests The ion conductivity of the ceramic material based on NZPS-82-1 BC sintered to 1100°C for 9 hours was determined by the AC-method. In sodium/sodium ceils, the ceramic material was subjected to current 2 densities of up to 0.8 A/cm for brief periods . No decomposition of the ceramic material was observed. After a period of up to 1136 hours in sodium/sodium cells at 300°C, the ceramic material showed no signs of decomposition apart from a brownish staining of the surface. Static corrosion experiments to determine the durability of the ceramic material in molten sodium were conducted on the ceramic material based on NZPS-82-1 BC sintered to 1100°C for 9 hours. The results are shown in Table 5. Corrosion tests were also performed on ceramics of the same composi¬ tion, but sintered to 1200°C for 10 hours. This ceramic showed a higher resistance to sodium corrosion; thus, no corrosion was obser¬ ved at 400°C. Table 4 Specific resistivity in Nasicon ceramics measu red by the AC-method (blocking Au-electrodes) or by DC in Na/Na-cells Ω/cm bulk Comp. density 25°C 125°C 175°C 250°C 300°C 350° C 400° C no. °C h g/cm 3 AC AC DC AC DC AC DC AC DC DC NZPS- 82-2 1180 0.2 3.24 340 25 ■ - 11 - 8 - 6 81 -1 1010 1 0. 1 3.26 3290 177 ■ - 68 - 33 - 24 1250 1 0.1 3. 19 1128 41 - - 10 - 5 - 3.3 81 -3 1250 1 0. 1 - 1596 57 ■ ■ 26 - 16 - - 81 -5 1260 1 0.1 - 628 50 - - 22 - - - 79-6.4,2. 1 1226 6 3 - 5680 132 - . 1 _ 16 - 11 82-1 BC 1100 2 3 3.10 - - 162 - 49 - 17 - - - - 1100 9 3.06 3653 162 - 52 - 20 18 13 11 .3 7.7 4.9 1200 3" 3.01 2268 129 - 51 - 18 12 10.6 8.6 6.7 5.5 82-1 -A01 1205 10 2.908 1254 62 22 9.8 7.0 1 Hot-pressed 60 MPa, 2 Hot-pressed 30 MPa, 3 after 16h at 1201°C * after 9h at 1100°C. Table 5 Data on the corrosion of ceramic based on the ZrO 2 ~deficient Nasicon composition NZPS-82-1BC in liquid sodium M, = Observed weight loss, mg/cm 2 . M = Weight loss due to temperature independent fast reaction. o (M.-M )/(h x d); d=3.00 g/cm 3 . t o NZPS-82-1BC sintered to 1120°±20°C temp. time M t M.-M t o K Na °C h mg/cm 2 mg/cm 2 mg/cm 2 lOVcmh 400 6 1.42 _ _ 400 48 1.45 - - 400 96 (2.07) - - 400 144 1.45 - - 400* 36 0.33 - 0.33 0.031 450 96 4.24 1.44 2.90 0.097 500 6 0.95 - (0.95) (0.53) 500* 12 (3.28) - (3.28) (0.91) 500 72 14.04 1.44 12.60 0.58 550 7 9.09 1.44 7.65 3.6 600 6 21.33 1.44 19.89 11.1 650* 4 7.94 - 7.94 6.62 650 4 30.23 1.44 28.79 24.0 700 6 67.77 1.44 66.33 36.9 * surface exposed to the atmosphere removed immediately before experiment.

Claims: CLAIMS 1 . A process for preparing a ceramic material in the form of a glass, or a polycrystalline material , or a polycrystalline material additionally containing a glass phase, and based on oxides of phosphorus and silicon and optionally one or more additional oxides selected from oxides of zirconium, aluminium, and alkali metal, comprising preparing a gel by controlled mixing, in an organic medium, of anhydrous phos¬ phoric acid with a lower alkoxide of silicon and optionally with one or more alkoxides selected from lower alkoxides of zirconium, aluminium, and the alkali metal, the optional alkali metal component alternatively being an alkali salt soluble in the organic medium, followed by gelling by addition of water, removing water and volatile components from the gel, and converting the water- and volatiles-free gel to a ceramic material . 2. A process according to claim 1 , wherein the water for the gelling is provided by exposure of the mixture to a humid atmosphere or by adding a solution of water in a lower alcohol . 3. A process according to claim 1 or 2, wherein the gel is prepared by one of the following methods : A) anhydrous phosphoric acid dissolved in a lower alcohol is reacted with a lower alkoxide of silicon, the resulting polymer is reacted with one or more alkoxides selected from lower alkoxides of zirconium, aluminium, and the alkali metal dissolved in a lower alcohol, and the resulting mixture is gelled by addition of water. B) anhydrous phosphoric acid dissolved in a lower alcohol is reacted with a lower alkoxide of silicon, the resulting polymer is added to a refluxed mixture comprising a lower alkoxide of silicon and one or more alkoxides selected from lower alkoxides of zirconium, aluminium, and the alkali metal, and the resulting mixture is gelled by addition of water, OMPI C) a mixture comprising a lower alkoxide of silicon and one or more alkoxides selected from lower alkoxides of zirconium, aluminium and the alkali metal is refluxed, after which anhydrous phosphoric acid dissolved in a lower alcohol is added to the refluxed mixture, and the resulting mixture is gelled by addition of water, D) the procedure according to A) , B) , or C) is followed with the modification that the alkali metal component is an alkali salt soluble in the organic medium rather than an alkoxide, in which case an acid corresponding to the alkali salt is optionally added to acidify the solution before gelling . 4. A process according to any of the preceding claims, wherein the alkali metal is sodium. 5. A process according to any of the preceding claims, wherein the content of zirconium relative to the content of silicon, phosphorus and sodium in the final ceramic material is less than or equal to the con¬ tent of the zirconium corresponding to the surface in the four-compo¬ nent system (SiO 2 , P 2 O 5 , ZrO 2 , Na 2 O (Fig. 3) defined by ZrSiO 4 , ZrP 2 Oy, Na 2 ZrO 4 and Na. Zr 3 Si χ O« 2 , in which 0 < ≤ 3, and the content of sodium relative to the content of silicon, zirconium and phosphorus in the final ceramic material is less than or equal to the content of sodium corresponding to the plane in the same four-compo¬ nent system defined by Na 2 Z > 4 , Na 4 SiO 4 and Na 3 PO 4 . 6. A process according to any of the preceding claims, wherein the starting materials are mixed in such proportions relative to each other that the ceramic material produced has the general formula Na Tx*y*2a Zr 2-z P 3-x Sl x A, a°12-2z + y/2' wherei n 0 ≤ x < 3, 0 < z < x/3, 0 < a < (3-x)/2 -0.3 ≤ y ≤ 0.3. 7. A process according to claim 6 in which 1 .8 ≤ x ≤ 2.6, 0 < z < x/3, -0.3 < y < 0.3. 8. A process according to claim 7 in wich 0 < a < 0.4. 9. A process according to claim 8, wherein the starting materials are mixed in such proportions relative to each other that the polycrystal¬ line ceramic material produced has the general formula Na 1 + x + y Zr 2-z P 3-x S i x°12-2z + y/2 i n wh ich 0 < x < 3, -0.3 < y < 0.3 0 < z < 1 . 10. A process according to claim 9, wherein the starting materials are mixed in such proportions relative to each other that the polycrystal- line ceramic material produced has the general formula Na 1 + x*y Zr 2-z P 3-x Sl x°12-z*y/2 i π wh ich 0 < x < 3, -0.1 y 0.1 , 0 < z ≤ 1 . 11 . A process according to claim 9 or 10, wherein x represents a value which is greater than or equal to 1 .8 and smaller than or equal to 2.6. 12. A process according to any of the preceding claims, wherein the removal of water and volatile components from the gel is performed by drying at temperatures of up to 110°C or calcination at temperatures above 110°C. 13. A process according to claim 9, wherein the calcination tempera¬ ture is a temperature between 110°C and 800°C. 14. A process according to claim 9, wherein the drying is performed by spray drying, vacuum drying, evaporation, evaporation by distil- lation, or fractional distillation . 15. A process according to any of the preceding claims, wherein the conversion of the dried or calcinated gel into a ceramic material is performed by means of sintering the dried or calcinated gel . 16. A process according to claim 12, wherein the sintering is per- formed at a temperature below the incongruent melting temperature of the crystalline phases with compositions corresponding to the dried or calcinated gel . 17. A process according to claim 13, wherein the sintering is per¬ formed at a temperature of above 700°C, in particular a temperature between 800 and 1200°C. 18. A ceramic material when prepared by the process according to any of claims 1-17. 19. A method for producing a shaped article of a ceramic material prepared by a process according to any of claims 1 -17, comprising shaping the dried or calcinated gel prepared according to any of claims 1-14 into a desired shape followed by sintering at a temperatu¬ re below the incongruent melting temperature of the material, but above 700° C. 20. A process according to claim 16, wherein the dried or calcinated gel is comminuted prior to shaping. 21 . A process according to claims 16 or 17, wherein the shaping is performed by means of dry pressing, isostatic pressing, wet pres¬ sing, extrusion or slip casting. 22. A process for producing a shaped article of a ceramic material prepared by a process according to any of claims 1 -17, wherein the gelling is performed in a mould, water and volatile components are removed from the resulting shaped gel , and the dried, shaped gel is sintered . 23. A process for producing a thin layer of a ceramic material pre¬ pared by a ' process according to any of claims 1 -17 on a substrate, comprising performing the gelling of the mixture in a thin layer on a substrate, removing water and volatile components from the resulting thin gel layer, and sintering the dried thin gel layer. 24. A method according to claim 22 or 23, wherein the gel is trans¬ formed into glass by sintering at temperatures below 1250°C. 25. A shaped article as prepared by any of the methods claimed in any of claims 19-24. 26. A shaped article of a ceramic material prepared by a process according to any of claims 1 -17, which is adapted to be used as an electrolyte or ion conductor in an electrochemical storage cell or other electrochemical devices such as electrochemical sensors and electro- chromic displays, sodium heat engines, or sodium winning devices . 27. A solution in an organic medium containing 1 -50% by weight of predominantly oxygen-bridged polymers formed by reaction of phos¬ phoric acid with a lower alkoxide or silicon and optionally with one or more alkoxides selected from lower alkoxides or zirconium, aluminium, and alkali metal (the optional alkali metal reactant alternatively being an alkali salt soluble in the organic medium) . 28. A solution according to claim 27 in which the composition of the polymer is one which by hydrolysis results in a gel of a composition corresponding to any of the compositions stated in claims 5-11 . 29. A solution according to claim 27 in which the polymer contents substantially consists of a polymer formed by reaction of a lower alkoxide of silicon such as tetraethylorthosilicate, and phosphoric acid. 30. A solution according to any of claims 27-29 in which the organic medium is a lower alcohol . 31 . A solution according to any of claims 27-30 in which the concen¬ tration of polymer is in the range between 5 and 40% by weight. 30. A ceramic material composed of glass or a polycrystalline material, or a polycrystalline material additionally containing a glass phase, the composition of the material corresponding to the general formula Na 1 + x + y*2a Zr 2-z P 3-x Si x Al a°12-2z + y/2' wherei n 0 < x ≤ 3, 0 < z < x/3, 0 ≤ a < (3-x)/2 -0.3 < y ≤ 0.3. 31 . A ceramic material according to claim 30 in which 1 .8 ≤ x ≤ 2.6, 0 ≤ z < x/3, -0.3 < y < 0.3. 32. A ceramic material according to claim 31 in wich 0 ≤ a ≤ 0.4. 33. A ceramic material according to any of claims 30-32 which is in the form of a polycrystalline material or in the form of a polycrystal¬ line material additionally containing a glass phase. OMPI

Inventors: ENGELL JOHN EMIL, MORTENSEN SVEND, ENGELL, JOHN, EMIL, MORTENSEN, SVEND
Applicants: KONGELIGE PORCELAINSFAB, DEN KONGELIGE PORCELAINSFABRIK A/S
Application year: 1983
Main classification: C04B35
IPC codes: C04B35, G01N27, H01M6