ION-SENSITIVE MEASURING ELECTRODE DEVICE

Abstract: An ion-sensitive measuring electrode device with an ion-sensitive element based on ion-conducting crystalline ceramic material. The electrode device shows a selectivity to the Na<+> ion versus the H<+> ion corresponding to the selectivity of the known sodium-sensitive glass-based solid membrane electrodes. Ion-sensitive measuring electrode devices showing good selectivity properties are provided on the basis of ion-conducting crystalline material, the crystal structure of which comprises a three-dimensionally extending interstitial space containing positions for the ion, especially material in which the interstitial space has bottlenecks which just permits passage of the ion. A method for preparing polycrystalline ceramic material based on oxides of zirconium, phosphor and silicon is also provided.

Description: ION-SENSITIVE MEASURI NG ELECTRODE DEVI CE The present invention relates to an ion-sensitive measuring electrode device comprising an ion-sensitive solid element showing selectivity for a particular ion species and reference system. The ion-sensitive measuring electrode device according to the inven¬ tion is characterized in that the ion-sensitive element is constituted by or comprises an ion-conducting crystalline material, the crystal structure of which comprises a three-dimensional interstitial space comprising positions for the ion . Such ion-sensitive elements are novel . Up to now, solid membrane electrodes have primarily been based on single crystals , the ion conductivity of which is based on impurities in the crystal lattice or on glasses . Among the materials showing a crystal structu re of the above-men- tioned type are materials which are suitable for being shaped as polycrystalline bodies (tablets) by ceramic working, and such tablets are suitable for use in connection with the preparation of measuring electrodes . Especially, it has been found that in the case where the three-dimen- sional interstitial space has bottleneck passages, which just permit passage of the ion, it is possible to prepare ion-sensitive electro¬ chemical measuring electrode devices with ion-selective properties; thus, sodium-sensitive measuring electrodes with good selectivity properties to Na + ions versus H* ions have been manufactured. Representatives of the above-mentioned materials with three-dimensio¬ nal interstitial space are found among ceramic materials based on oxides of zirconium and phosphorus and/or silicon . Polycrystalline ceramic materials and materials with three-dimensional interstitial space are known from e. g. German published specification No. 2, 634,289 (ref. 1 ) , from Materials Research Bulletin 77 , 1976, p . 173, Cref. 2) , European published patent specification No. 46.932 (ref. 3) , and Solid State Ionics 3/4, (1981 ) , pp. 215-218 (ref. 4) . A number of materials, including the Nasicon mixture series Na. Zr-Si P., O,- . in which 0< x < 3, methods for preparing the materials and their use in electrochemical batteries are known from these references . However, the fact that among the materials descri¬ bed there are also materials, which in addition to ion -conductivity also show selectivity to the ion species in question, is a surprising recognition, which opens new possibilities for making ion-selective measuring electrodes in which the ion-selective element has a low impedance and is therefore less demanding with respect to the attached measuring equipment and shielding against noise than the presently used corresponding glass-based ion-selective measuring electrode devices. I n addition, the electrode response will be fast because of the high ion conductivity. A measuring electrode device with an ion-sensitive element in the form of a polycrystalline ceramic material of the composition Na- q^Zr., Λ Q~ Si- 2^0 pp Λ Q ~ a ' so designated ZrO^-deficient Nasicon or NZPS-82- 1 BC has been found useful for determining the Na + ion with satisfac- tory selectivity characteristics versus H + ions . It is also contemplated that ion-sensitive measuring electrode devices with selectivity towards also other ions than the sodium ion and based on polycrystalline ceramic materials with crystal structure of the type described in the above-mentioned literature references can be made. Interesting possibilities are to produce ion-selective measuring elec¬ trodes with selectivity to other alkali metal ions by replacing the Na + ion in the crystal structure with other alkali metal ions. Another interesting possibility would be to make a hydrogen ion-sensitive measuring electrode by using Keldyshite, which is the H + form of Parakeldyshite, cf. Fig. 3, of the ion-sensitive element. It should be noted that the electrochemical measuring electrode devi¬ ces according to the invention may be in the form of both traditional measuring electrodes with liquid or gelled electrolyte between the ion-sensitive element and the reference electrode and electrodes with OMFI solid state contact to the ion-sensitive element. Furthermore, both single rod electrodes and combination electrodes combined with an exterior reference electrode system may contain the ion-sensitive element. The ion-sensitive element in the form of a compressed and sintered polycrystalline tablet of ceramic material may easily be mounted at the end of a tubular stock tube, e. g. of plastic material, using epoxy or the like. The ceramic material of the ion-sensitive electrode device according to the invention may, e. g . , be prepared by preparing a gel by con¬ trolled mixing, in an organic medium, of anhydrous phosphoric acid with lower alkoxides of zi rconium, silicon and an alkali metal and optionally a lower alkoxide of aluminium, the al kali metal component alternatively being an al kali salt soluble in the organic medium, fol- lowed 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 . The method makes it possible to prepare a very finely grained poly¬ crystalline material, cf . Fig. 2, with a structure which secures good material properties, e. g. workability to thin plates with thicknesses down to 0.6 mm. Under suitable pressure and sintering conditions, the material may be prepared in a very dense form with closed poro¬ sity of less than about 5%. Furthermore, the method of the invention has made it possible to provide Nasicon- or ZrO~- deficient Nasicon, which is sintered completely at temperatures between 1000°C and 1100°C, i . e. temperatures considerably below the temperature where incongruent melting takes place. Therefore, the sintered product will not show inclusions of crystalline ZrO . i nteresting polycrystalline materials for use as ion-sensitive elements in the electrode device of the invention are stated in claims 5-9. The drawings of Nasicon crystal structures given in Materials Research Bulletin 11, 1976, pp. 173-182 shows the three-dimensional interstitial space containing the Na + ion . There will be seen various positions between which the Na + ion may migrate through the material, since each Na (1 ) position is connected three-dimensionally with two Na(2) positions and four Na(3) positions, and that each Na(2) position and Na(3) position is connected with two Na(1 ) positions. For x = 2 are seen the so-called bottlenecks in Nasicon crystals . These are situated in the interstitial space and are passed by the Na ion during position changes . I n the bottlenecks there is, in the shown crystal structure, a distance between the centres of two O-ions of 4.95 A which is very close to the sum of the diameter of the Na ion and the diameter of the O ion, which sum is 4.8 A. As will be seen from the experimental data, the polycrystalline material tested not only exhibits ion conductivity for sodium ions, but also a certain selectivity towards these as opposed to protons and other alkali metal ions . It is regarded as probable that it is the presence of the three-dimensional interstitial space with the Na + ions and bottlenecks contained therein which ensures not only the well-known fast sodium transport, but also the so far unknown selectivity which makes it possible to produce electrochemical measuring electrodes with selectivity characteristics comparable to what has previously been found for known ion-selective glass electrodes for the determination of sodium. In the following, the invention is further illustrated with reference to the drawing, in which Fig. 1 shows a prototype of an ion-selective measuring electrode device according to the invention; Figs. 2a and 2b show an electron microscope picture of a surface in a sintered plate of a zirconium-deficient Nasicon material prepared according to the prior art (2a) and prepared according to a preferred embodiment of the invention (2b) ; Fig. 3 shows a phase diagram for the 4-component system ZrO-. 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¬ bability P, (%) as a function of the loading in a th ree-point bend/ten- sile strength test; Fig . 12 shows the resistivity of stoichiometric Nasicon and ZrO--defi- cient Nasicon (including NZPS-82-1 BC) at 300° C. DETAI LED DESCRI PTION OF THE DRAWI NGS Fig. 1 shows an experimental measuring arrangement comprising an ion-sensitive measuring electrode device according to the invention, generally designated 10. The measuring electrode device according to the invention has a polycrystalline ceramic ion-sensitive membrane 2 in the form of a disc melted into a glass tube 1 . The measuring electrode device has an internal reference system consisting of an internal electrolyte 3 and an internal reference electrode 4. Together with a separate reference electrode 5, the measuring electrode device is immersed into a sample solution 6, and on the basis of the potential difference measured by means of the mV-meter 7 over the internal reference electrode and the reference electrode, the content of a particular ion species in the sample solution can be determined . A measuring arrangement of this type wherein the ceramic membrane shows Na + -selectivity is described in greater detail in connection with Example 5 below. Fig . 2a 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~ (Baddeleyite) and the black areas are zirconium-deficient glass, while the light grey mass is crystalline Nasicon with grain sizes in the range of 2-10 μm and a stoichiometric composition corresponding to x = 1 .89 as determined by X-ray diffraction . Fig . 2b shows an electron microscope picture of a sectioned surface in a plate of the synthetic ZrO j -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- 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, P2°5« The composition (on a molar basis) of known crystalline compounds within the system is marked with squares. A complete mixing series between NaZ^P-O^ and Na .Zr j Si O.. ,, (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. 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., + ZrSiO , + a more silicon-rich Nasicon . At 1300°C this combination consists of a melt (L) + crystalline ZrO- + Nasicon with a composition corresponding to x = 2. 18. At 1306°C the combination consists exclusively of a melt (L) + crystalline ZrO- . Fig. 5 shows the .variation in the number of unreacted HO-groups in mixtures of H-PO . and Si (OEt) 4 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 (∑H ) . I n the Figure, the dotted line designates the lower limit +• of ΣH 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 Z- 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. Fig. 7 shows X-ray diffraction diagram of NZPS-82-1 BC (vacuum- dried at 110°C) heated to the indicated temperatures for the indicated time spans . The temperatures were reached according to the following program: 20°C-710°C 1 °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 crystailiza- 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 1 °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 -| r 2 P 0 g Si 2 .0^ and Vlasovite (Na^rS^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 well as the green bul k 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 l - l l l and 1V-VI , respectively, as a function of the pressing pressure. Samples I and IV were prepared from crushed gel dried at πθ°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 pressu re. The bottom half of Fig . 10 shows the bul k 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 \o outlines strength test data for (."-alumina specimens with a specific 3 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. 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-Pg SI O,-. 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 Electrolytes" , 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,-_ a - " The data marked H are according to Solid State Ionics 7 , 1982, pp. 345-348. The abscissa axis shows the factor x from the general formulae. As mentioned above, the raw material for the polycrystalline material is prepared by controlled mixing and reaction between the al koxides of Na, Zr, Si and optionally Al and with dry phosphoric acid in an alcohol with addition of water. The alkoxides employed comprise methylates, ethylates, propanolates and butylates . The alcohols em¬ ployed comprise ethanol and propanol . The problems associated with the preparation of the al koxid based gels are as follows: 1 ) To limit the reactivity of the phosphoric acid towards the Zr-al- koxide. The problem is greater when using Zr(OEt) . than when using Z OBut) .. 2) To avoid precipitation of heavily soluble phosphates of Zr and Na in alcohol solutions . 3) To avoid precipitation of heavily soluble Zr-al koxide-Na-alkoxide compounds . This is especially a problem in the use of ethanol and ethanolates . 4) To control the copolymerisatiori of Si (OR) , and Zr(OR) , (where R is the "hydrocarbon moiety" of an alcohol) , so that there will not be formed inhomogenities because of the difference in rate of hydrolysis between the two components . »2- Apart from depending on the selection of the starting chemicals (me- thylate, ethylate, etc.), the properties of the resulting gels depend on the concentration of alkoxide in the alcohol used as solvent, on the ratio between alkoxide and added water which is to bring about the final hydrolysis and gelling, and on the temperature at which the gelling is performed. The reactions which take place in the mixed alkoxide solution are, iinntteerr aalliiaa ooff tthhee following types (M metal ion of the valency n, OR = alkoxide group) I M π+ (OR " ) + H-PO, ■ * M n+ (OR " ) - H-PO. - HOR n 3 4 *■ 'n-1 2 4 II M n + (OR " ) + H-O → M n + (OR " ) , OH + HOR n 2 n-1 ( " R 1 0) n1 - l M l π1+ -O-M 2 π2+ (OR 2 " ) n2 _ 1 + HOR IV 2 M n + (OR " ) - OH - ( ~ RO) ^ M π+ -Q__M π + (Rθ " ) , + H-O π- 1 n- 1 n- 1 A The following examples describe preferred embodiments of the method of the invention in greater detail: IS EXAMPLE 1 Preparation of a Metal Alkoxide Cel 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 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 occurred . The mixture was then left to react for 16 hours 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 hou rs . 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-82-1A and NZPS-82-2 (cf . Table 1 ) were also prepared according to this method, consideration being had to their different cemical composition . EXAMPLE 2 Preparation of a Metal Alkoxide Gel 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 . 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 3 Preparation of a Metal Alkoxide Gel 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 8 millimoles of tetraethylorthosilicate dissolved in 0.75 mole of i- propanoi 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., ,Zr. e S gP Q 4 O, 0 0 - IS EXAMPLE 4 Preparation of a Metal Alkoxide Gel A mixture of 10 millimoles of tetraethylorthos-ilicate, 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-Zr-PSi-O., , ... EXAMPLE 5 Preparation of a Metal Alkoxide Gel 0.44 mol of tetraethylorthosilicate was reacted with 0. 16 moi 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 mi 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 ac'id 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^Zr- , 4 gSi 2 - 2 0 . 8 O 10 > 85 . Table 1 Compositions after calcination of gels prepared according to the invention % by weight P 2°5 SiO. ZrO. AI 2 O 3 a 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-82-1 A 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 mixtu re 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 6 Preparation of a Metal Alkoxide Gel 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 .7 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 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 Sl 2.6 P 0.4°12' EXAMPLE 7 Preparation of a Metal Alkoxide Gel To a clear solution made from the same amounts of tetraethylortho¬ silicate, zirconium-tetra-n-propoxide and anhydrous phosphoric acid as described in Example 6, 0.36 mol of sodium nitrate, 47 ml of water and 5.6 ml of concentrated nitric acid in 1800 mi of methanol was added. A mil ky 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- e r- -Si- -P- 4 O, ~. EXAMPLE 8 Preparation of Aluminum- containing Nasicon Gel 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 . O...PI 12 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 . A 1 : 10 mixture of H-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 l .6l CAi 0. l Si 2.20 P 0.70 ) O 10.74' EXAMPLE 9 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 -8 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 2-4 and 8 may either be dried and powdered and used as de- 12 scribed above, or they may be dried carefully, e. g . at room tempera¬ tu re over a period of 7 days . I n this case, the gels retain their monolithic shape and shrink 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 10 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 ± 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 occu rs in a narrow range between 800°C and 900°C, and that the highest bulk density of the sintered plates su rprisingly occu rs at a low compres¬ sion pressu re. The other types of gel prepared according to the method described in Examples 1 -8 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- 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 temperatu res close to the incongruent melting point (Fig. 4) . In order to avoid cracking due to too rapid sintering, it is necessary to heat slowly . The gel NZPS-82-1 BC dried at 110°C is suitably heated from 20°C to 710°C over a period of 6 hours, from 710°C to 1000°C over a period of 14 hours, from 1000°C to 1 100°C over a period of 2 hours, and at 1100°C for 9 hours . -2.0 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 fiexural 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 . The resistivity with respect to sodium ion conductivity obtained with polycrystalline ceramic articles of the composition stated above are shown in Table 4. The temperature interval for maximum sintering of some gels appears from Table 5. Table 2 Variation in properties of gels prepared by reaction polymerization specific organics surface determin ed specific area deter¬ as CO 2 density mined by Gel No. % by wei ght 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-O added to the partially dried gel . Table 3 Equilibrium Phases at 1150°C Determined by X-ray Diffraction Nasicon Other Phases Na Tx Zr 2 P 3-x Si x 0 12 NZPS-81-3 1.8 NZPS-81-1 2.0 NZPS-82-2 2.4 NZPS-82-1BC 2.14 Vlasovite (Na ZrSi 4 O 4 NZPS-81-5 2.25 Vlasovite Na 2 ZrSi 4 O 4 ) Parakeldyshite (Na 2 ZrSi 2 O 4 ) NZPS-82-XA 2.0 Table 4 Specific resistivity in Nasicon ceramics measured 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 - 41 - 16 - 11 - 82-1BC 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, Hot- pressed 30 MPa, 3 after 16h at 1201 °C k after 9h at 1100°C. 2 Table 5 Temperature I nterval for Maximum Sintering of the Prepared Gels Dried at 110°C and Sintered in Atmospheric Air Bulk density Interval Maxi¬ Relative Sin¬ g/cm 3 mum Sintering tering in % at G reen Sintered T -T °C Temperature ' l ' 2 U < Tl τ τ 2 τ 2 < NZPS-81 -1 1 .55 2.94 860-970 38 59 3 NZPS-82-1 BC 1 .074 3.06 800-885 23 70 7 NZPS-81 -5 1 .58 2.88 845-990 42 68 0 EXAMPLE 11 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. I n sodium/sodium cells, 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 6. Table 6 Specific resistivity in Nasicon ceramics measured by the AC-method (blocking Au-electrodes) or by DC in Na/Na-cells ../cm bulk Comp. density 25°C 125°C 175°C 250°C 300°C 350°C 400°C no. h g/cm 3 AC AC DC AC DC AC DC AC DC DC NZPS- 82-2 1 180 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 - - 41 - 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. 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. EXAMPLE 11 The Use of NZPS-82-1BC for Sodium- Sensitive Measuring Electrodes Potentiometric membrane electrodes as shown in Fig. 1 were prepared from the above mentioned material by melting a polycrystalline slide thereof (ø5 mm x 2 mm) in a glass tube (1 ) with a coefficient of ex- pansion corresponding to the ceramic membrane (2) . An electrolyte so¬ lution (3) consisting of 0.2 M NaCI stabilized at pH 7 with citrate buffer is filled into the glass moiety, and a silver/silver chloride electrode as a so-called inner reference (4) is immersed in the soluti¬ on . As reference electrode (5) for the membrane electrodes, a con- ventional calomel electrode (S . C. E. ) is used, which by means of a salt bridge of saturated potassium chloride is in contact with the sample solution (6) , which is to be measured by the membrane elec¬ trodes . The measurements are performed using a pH-meter (7) , which functions as a high impedance mV-meter, but in view of the low resistivities of the electrodes (1 -10 kohm) it would also be possible to perform the measurements with relatively low impedance measuring instruments . ■ Jϊ y S j Tϊj- * The mV-signals at 25°C were recorded for 4 test electrodes in the following solutions: 0.1 M NaCI, 1 mM NaCI, 1 mM HCI and 0.1 M KCI. Measuring results: El. 1 El. 2 El. 3 El. 4 0.1 M NaCI -8 mV -11 mV -4 mV -4 mV 1 mM NaCI -80 mV -80 mV -120 mV -97 mV 1 mM HCI -36 mV -38 V -48 mV -49 mV 0.1 M KCI -62 mV -61 mV -96 mV -91 mV 15 14 15 10 K NaH 0.04 0.04 0.03 0.07 The selectivity factors are calculated from the Nicholsky-equation: E = E- ♦ S • log (a Na ♦ K Naχ • a χ ) Compared to conventional solid-state sodium-sensitive electrodes, the electrodes with NZPS-82-1BC especially show a surprisingly good se¬ lectivity versus hydrogen ions. Another advantage is the low mem¬ brane resistance of the electrodes, which makes it possible to prepare both robust and miniaturized sodium-sensitive electrodes.

Claims: IS CLAIMS 1 . An ion-sensitive measuring electrode device comprising an ion-sen¬ sitive solid element with selectivity to a particular ion and a reference system, characterized in that the ion-sensitive element is constituted by an ion-conductive crystalline material, the crystal structure of which comprises a three-dimensional interstitial space comprising positions for the ion . 2. An ion-sensitive measuring device according to claim 1 , characterized in that the ion-sensitive element is a polycrystalline material . 3. An ion-sensitive measuring electrode device according to claim 1 or 2, characterized in that the three-dimensional interstitial space has bottlenecks which just allow passage of the ion . 4. An ion-sensitive measuring electrode device according to any of claims 1 -3, characterized in that the ceramic material is based on oxides of zirco¬ nium and phosphorus and/or silicon and optionally aluminium. 5. An ion-sensitive measuring electrode device according to any of claims 1 -4, characterized in that the ion is an alkali metal ion and that the cera¬ mic material comprises M. + Zr-Si P- O, 2 , where 0 ≤ x ≤ 3, and M designates an alkali metal , or ZrO--deficient M- + Zr-Si P«_ O.- . 6. An ion-sensitive measuring electrode device according to claim 5 and adapted for the determination of Na ions, characterized in that the ion-sensitive element of ion-conducting cera¬ mic material comprises or consists of Nasicon, Na l 4 . Zr-Si P- O«-, 0 1 + x 2 x 3-x 12 ≤ x ≤ 3, or ZrO 2 -deficient Nasicon . 7. An ion-sensitive measuring electrode device according to claim 5, characterized in that the ion-sensitive element comprises or consists of ZrO--deficient Nasicon with the formula: Na 2.94 Zr 1 .49 s '2.2 P 0.8°10.85 * 8. An ion-sensitive measuring electrode device according to claim 5, characterized in that the ion-sensitive element comprises or consists of a ceramic material of the general formula Na 1 ÷x + y + 2a Zr 2-z P 3-x S i 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. 9. An ion-sensitive electrode device according to claim 8 in which 0 < a < 0.4 10. An ion-sensitive electrode device according to any of the preceding claims, characterized in that the polycrystalline, ion-conducting ceramic material has a dense structure with a bul k density of > 95 % of the specific gravity of the phases involved. 11. An ion-sensitive electrode device according to any of the prece¬ ding claims in which the ceramic material of the ion-sensitive element is prepared by preparing a gel by controlled mixing, in an organic medium, of anhydrous phosphoric acid with lower alkoxides of zir¬ conium, silicon and an alkali metal and optionally a lower alkoxide of aluminium, the 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 . 12. An ion-sensitive electrode device according to claim 11 in which the gel is prepared by reacting anhydrous phosphoric acid dissolved in a lower alcohol with a lower alkoxide of silicon , reacting the re¬ sulting polymer with a lower alkoxides of zirconium and of the alkali metal dissolved in a lower alcohol and optionally with a lower alkoxide of aluminium, and gelling the resulting mixture by addition of water, or reacting anhydrous phosphoric acid dissolved in a lower alcohol with a lower alkoxide of silicon, adding the resulting polymer to a refluxed mixture comprising a lower alkoxide of silicon, zirconium and the alkali metal and optionally a lower alkoxide of alumnium, and gelling the resulting mixture by addition of water. 13. An electrode according to claim 12 in which the alkali metal alco- holate is sodium alcoholate. 14. An electrode device accordigg to claim 13, characterized that such ratios between the starting materials have been used that polycrystal¬ line material of Nasicon- or ZrO--deficient Nasicon-type with 1 .8 < x < 2.4 is obtained .

Inventors: ENGELL JOHN EMIL, MORTENSEN SVEND, ENGELL, JOHN, EMIL, MORTENSEN, SVEND
Applicants: RADIOMETER AS, RADIOMETER A/S
Application year: 1983
Main classification: G01N27
IPC codes: C04B35, G01N27, H01M6