Beta-alumina solid electrolyte and process for production thereof

Abstract: A beta-alumina solid electrolyte for use in sodium-sulfur battery is composed of the beta-alumina crystals having a degree of orientation toward C axis, of 0.2-0.4 and an aspect ratio of 4.0 or less. The beta-alumina solid electrolyte is composed of the beta-alumina crystals having a degree of orientation toward C axis, of 0.2-0.4 and has such a particle diameter distribution that the average particle diameter is 3 µm or less, the proportion of the particles having a particle diameter of 5 µm or less is 90% or more, and the maximum particle diameter is 300 µm or less. A process for producing a beta-alumina solid electrolyte using an alumina source material, a magnesium source material and a sodium source material has using a magnesium-aluminum spinel as the magnesium source material and subjecting all materials to mixing, granulation, molding and firing to obtain a beta-alumina solid electrolyte without subjecting the materials to calcination. The beta-alumina solid electrolyte has a low electrical resistance and a large strength to internal water pressure and exhibits excellent properties when used as a diaphragm for sodium-sulfur battery.

Description: BACKGROUND OF THE INVENTION (1) Field of the Invention The present invention relates to a beta-alumina solid electrolyte having excellent properties such as low electrical resistance, large strength and the like, as well as to a process for production thereof. More particularly, the present invention relates to a process for production of beta-alumina solid electrolyte, which can produce a beta-alumina solid electrolyte of excellent properties without subjecting the materials to calcination (calcination is conducted in conventional processes). (2) Prior Art Beta-Alumina solid electrolytes have a very high sodium ion conductivity, i.e. a low electrical resistance. Hence, attention is being paid to their use as a diaphragm for sodium-sulfur battery, etc. Of the beta-alumina solid electrolytes, a MgO-stabilized beta-alumina solid electrolyte has conventionally been produced as follows. That is, the materials, i.e. MgO, a sodium compound and α-Al 2 O 3 are mixed at an appropriate ratio; the mixture is calcinated to obtain beta-alumina; then, grinding is conducted; the ground material is then granulated, molded into a desired shape and fired to obtain a beta-alumina solid electrolyte. The reason why in the above conventional process, the materials are calcinated beforehand to obtain beta-alumina, is that when it is tried to produce beta-alumina by direct firing without conducting calcination, there occurs severe volume expansion in the phase transition from α-Al 2 O 3 to beta-alumina, making it difficult to obtain a beta-alumina solid electrolyte of uniform quality and high strength. In the conventional process, however, since the materials are calcinated beforehand to obtain beta-alumina, the steps become complex and invite a high cost. Hence, it is strongly desired in production of sodium-sulfur battery, etc. to shorten the total production steps and produce a beta-alumina solid electrolyte more efficiently. Further, since in the above-mentioned conventional process there are conducted preliminary calcination (to obtain beta-alumina), grinding, granulation, molding and firing, the crystals constituting the resulting beta-alumina solid electrolyte have high orientation and, moreover, contain large crystals because the crystal growth takes place while the beta-alumina formed by calcination is acting as a nucleus for crystal growth. In addition, many of the large crystals have a shape of large aspect ratio, i.e. a thin and long shape. DE-A-2917590 discloses an alumina part containing alpha- and beta-alumina in which beta-alumina crystallites are arranged parallel to each other and perpendicular to the plane of the alpha-alumina platelets in which they are contained. GB-A-2175582 describes a process for forming beta-alumina in which alumina or an alumina precursor, sodium oxide or a sodium oxide precursor, and a spinel-forming oxide are mixed and spray dried. Examples are given in which lithia is used as the spinel-forming oxide. OBJECT AND SUMMARY OF THE INVENTION An object of the present invention is to produce a beta-alumina solid electrolyte efficiently without the calcination of the materials. Another object of the present invention is to provide a beta-alumina solid electrolyte has a low electrical resistance and a large strength to internal water pressure. In a first aspect of the present invention, there is provided a beta-alumina solid electrolyte tube for use in a sodium-sulfur battery according to claim 1. In a second aspect of the present invention, there is also provided a beta-alumina solid electrolyte tube for use in sodium-sulfur battery according to claim 2. The beta-alumina solid electrolyte of the present invention preferably has an electrical resistance of 4.0 Ω.cm or less, a burst strength to internal pressure, of 150 MPa or more and a density of 3.20 g/cm 3 or more. In a third aspect of the present invention there is provided a process for producing a beta-alumina solid electrolyte according to claim 4. In the process of the present invention, it is preferable that the molar ratio of magnesia and alumina in the magnesium-aluminum spinel is 1 or more and further that the sodium source material is sodium compound(s) containing at least either of NaHCO 3 and sodium oxalate. In the process of the present invention, it is also preferable for property improvement in electrical resistance, strength, etc. that the firing is conducted according to a heat curve comprising: (1) a step of keeping the system temperature at a constant temperature in the range of 1,400-1,550°C for 1-5 hours, or a step of increasing the system temperature at a rate of 50°C/hr or less in said temperature range, (2) a step of keeping the system temperature at the maximum temperature of 1,580-1,650°C for 0.1-3.0 hours, and (3) a step of keeping the system temperature at a constant temperature in the range of 1,400-1,550°C for 1-5 hours, or a step of decreasing the system temperature at a rate of 50°C/hr or less in said temperature range. In the present invention, the beta-alumina is a generic name for β-Al 2 O 3 (Na 2 O.11Al 2 O 3 ), β''-Al 2 O 3 (Na 2 O.5Al 2 O 3 ), β'''-Al 2 O 3 , etc. and refers particularly to a beta-alumina containing β''-Al 2 O 3 in an amount of 95% or more. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view of a single crystal of the beta-alumina solid electrolyte of the present invention. Fig. 2 is a perspective view showing a test piece obtained by cutting the beta-alumina sintered material of the present invention in the lengthwise direction. Fig. 3 is a view showing an example of the constitution of the Na/Na conductivity tester for measurement of electrical resistance, used in the present invention. DETAILED DESCRIPTION OF THE INVENTION The present invention is hereinafter described in detail. The process of the present invention consists in that in the production of a beta-alumina solid electrolyte, magnesium-aluminum spinel is used as a magnesium source material which is a stabilizer for beta-alumina, in place of MgO, etc. used in the conventional process. All materials including magnesium-aluminum spinel are mixed; the mixture is granulated without being calcinated, followed by molding and firing; thereby, a beta-alumina having excellent properties when used as a solid electrolyte can be obtained. The present inventor made a study from various angles in order to shorten the conventional steps for beta-alumina production. As a result, the present inventor found out that when magnesium-aluminum spinel is used in place of conventional MgO, as a magnesium source material, the spinel is presumed to act as a seed crystal for beta-alumina crystal formation in the firing step of beta-alumina production and the calcination of materials for beta-alumina formation becomes unnecessary. The magnesium-aluminum spinel used as a magnesium source material has a magnesia (MgO)/alumina (Al 2 O 3 ) molar ratio of preferably 1 or more (MgO-rich), more preferably 1.0-1.5. The reason is not clear but, when the molar ratio is outside the above range, the resulting beta-alumina has slightly inferior properties in electrical resistance, strength, etc. Further, the magnesium-aluminum spinel is preferably fine and specifically has an average particle diameter of preferably 1 µm or less, more preferably 0.5 µm or less. Furthermore, the magnesium-aluminum spinel preferably has a 90% particle diameter of 10 µm or less, preferably 5 µm or less. Moreover, the magnesium-aluminum spinel preferably has a specific surface area of 5 m 2 /g or more. When the magnesium-aluminum spinel has the particle diameter and specific surface area in the above ranges, the resulting beta-alumina sintered material has small crystal orientation and a small crystal aspect ratio and thus has good properties. As the sodium compound as sodium source material, there can be used conventionally known compounds such as Na 2 CO 3 and the like. However, use of sodium hydrogencarbonate (NaHCO 3 ) or sodium oxalate is preferable because there occurs no segregation of sodium compound in granulation product and there can be achieved uniform dispersion. That is, NaHCO 3 or sodium oxalate, having lower solubility in the solvent used (e.g. water) than Na 2 CO 3 , precipitates rapidly during drying for granulation and can be dispersed uniformly in the granulation production. Use of NaHCO 3 or sodium oxalate is also preferable because, when materials including NaHCO 3 or sodium oxalate are mixed in a slurry state, the slurry can be controlled at a pH of about 10, making easy the selection of organic binder. NaHCO 3 or sodium oxalate may be used singly, or may be used as a mixture with Na 2 CO 3 . The NaHCO 3 or sodium oxalate used has an average particle diameter of preferably 5 µm or less. The alumina source material is preferably α-Al 2 O 3 . It is preferably fine and has an average particle diameter of 1 µm or less, particularly preferably 0.5 µm or less. The α-Al 2 O 3 also has a specific surface area of preferably 5 m 2 /g or more, more preferably 10 m 2 /g or more. When the α-Al 2 O 3 has a particle diameter in the above range, the resulting beta-alumina sintered material scarcely contains large crystals and tends to be dense and have a low resistance. The alumina source material, the magnesium source material and the sodium source material all having desired properties in particle diameter, etc. as mentioned above, are mixed in given proportions allowing for beta-alumina formation. The mixing is conducted by grinding the materials in water, whereby a slurry is prepared. Mixing as uniform as possible is preferable because it can improve sinterability in the firing step. The slurry is granulated using a spray dryer or the like. This granulation step is conducted to make the mixing of materials uniform and improve moldability in the subsequent molding step. In the granulation step, a granulation product is produced so as to preferably have an average particle diameter of 50-100 µm. The granulation product is then molded into a desired shape. The molding is made into a tubular shape in many cases because the beta-alumina solid.electrolyte of the present invention is suitably used mainly as a diaphragm for sodium-sulfur battery. The molding is conducted at a pressure of e.g. 1.5 ton/cm 2 or more, preferably 2.0 ton/cm 2 or more, whereby a molded material having a density of 1.9 g/cm 3 or more is produced. Then, the molded material is fired under given conditions, whereby growth of uniform crystals is accelerated and beta-alumina crystals are formed. In this case, it is preferable to set the firing conditions and firing heat curve as described below. In the present invention, since magnesium-aluminum spinel is used as a seed crystal for beta-alumina formation, the volume expansion associated with the phase transition from α-Al 2 O 3 to beta-alumina poses no big problem. However, it is preferable to appropriately control the heat curve for firing in order to obtain a beta-alumina solid electrolyte of uniform quality, large strength and low electrical resistance. It is preferable to set the maximum firing temperature at 1,580-1,650°C in order to obtain a beta-alumina containing a high proportion of β"-Al 2 O 3 having good properties. Specifically, the heat curve preferably comprises at least three steps, i.e. (1) a step of keeping the system temperature at a constant temperature in the range of 1,400-1,550°C (in this temperature range, β-Al 2 O 3 changes to β"-Al 2 O 3 ) for 1-5 hours, or a step of increasing the system temperature at a rate of 50°C/hr or less in said temperature range, (2) a step of keeping the system temperature at the maximum temperature of 1,580-1,650°C for 0.1-3.0 hours, and (3) a step of keeping the system temperature at a constant temperature in the range of 1,400-1,550°C for 1-5 hours, or a step of decreasing the system temperature at a rate of 50°C/hr or less in said temperature range. The above heat curve is effective because it can increase the β"-Al 2 O 3 content in beta-alumina to 95% or more. The beta-alumina solid electrolyte obtained by the process of the present invention has unique inherent characteristics in that the beta-alumina crystals constituting said solid electrolyte have small orientation and a small aspect ratio. Specifically, the beta-alumina crystals constituting said solid electrolyte have a degree of crystal orientation of the C-axis relative to the diameter direction of the tube of 0.2-0.4, preferably 0.25-0.35 and an aspect ratio of 4.0 or less, preferably 3.0 or less. Beta-Alumina crystals have a crystal form of hexagonal system and, in each single crystal as seen in Fig. 1 showing a schematic view of beta-alumina solid electrolyte single crystal, have a Na-ion-conductive plane in the plane formed by the A axis and the B axis, i.e. the A-B plane and shows no conductivity toward a direction perpendicular to the A-B plane, i.e. the C axis direction. Further, the crystals, having a cleaving property along the A-B plane, have low mechanical strengths against pulling in the C axis direction. Hence, the beta-alumina crystals preferably have low orientation. In the conventional process for production of beta-alumina crystals (said process includes a step of calcination of materials), however, it has been difficult to produce beta-alumina crystals of low orientation. Incidentally, the degree of crystal orientation toward C axis, of beta-alumina solid electrolyte which is a polycrystal, was measured as follows. The beta-alumina sintered material obtained by the given production process was cut to obtain a test piece of 1 mm in width and 40 mm in length, shown in Fig. 2. The test piece was polished at the outer surface to remove the unevennesses of said surface. The outer surface of the polished test piece was subjected to analysis by X-ray analyzer to obtain the diffraction pattern of said surface, and there were calculated the relative amounts of various crystal faces each directing toward the diameter direction of beta-alumina tube. In the specific measurement, an X-ray analyzer of goniometer type was used. Using characteristic X-rays, a tube voltage of 35 KV and a tube current of 20 mA, a diffraction pattern was recorded on a chart. For various crystal faces each having a given angle (0°, 33.3°, 60° or 90°) to the C axis of beta-alumina crystals, there were measured respective peak heights, and the proportion of each crystal face was expressed as a relative value. The relative values of beta-alumina crystal faces are shown in Table 1. Angle between C axis and crystal face(°) Crystal face Diffraction angle 2  (°) Peak height measured (mm) Orientation degree of crystal face (relative value) 0.0 006 15.0∼16.0 H 1 H 1 /(H 1 + H 2 + H 3 + H 4 ) 33.3 101 32.0∼32.5 H 2 H 2 /(H 1 + H 2 + H 3 + H 4 ) 60.0 104 20.0∼21.0 H 3 H 3 /(H 1 + H 2 + H 3 + H 4 ) 90.0 110 31.5∼32.0 H 4 H 4 /(H 1 + H 2 + H 3 + H 4 ) In the present invention, "degree of crystal orientation of the C axis relative to the diameter direction of the tube" refers to the relative value of the peak height (H 1 of Table 1) (when the angle between C axis and crystal face is 0°, that is, the crystal face agrees with the C axis, to total peak height (H 1 +H 2 +H 3 +H 4 ), i.e. H 1 /(H 1 +H 2 +H 3 +H 4 ). When the degree of crystal orientation toward C axis is high, the non-conductive face of beta-alumina crystals is directed to the Na-ion-conductive direction of beta-alumina tube and, as a result, the beta-alumina tube has a high resistance. In the present invention, the aspect ratio of beta-alumina crystals is measured as follows. The beta-alumina sintered material is cut to obtain a small test piece. The test piece is polished and etched at the section. The resulting section is observed using a scanning type electron microscope and photographed at a magnification allowing for the observation of individual particles. Using the photograph, the major axis and minor axis of each individual particle were measured and the average of their ratios was taken as the aspect ratio of the sintered material. When the degree of crystal orientation toward C axis, of beta-alumina crystals is smaller than 0.2, the crystals have a small electrical resistance but small mechanical strengths (e.g. small burst strength to internal pressure) and is unsuitable for use as a diaphragm for sodium-sulfur battery. When said degree of crystal orientation is larger than 0.4, the crystals have large mechanical strengths but too large an electrical resistance. When the aspect ratio of beta-alumina crystals deviates from the above range, the beta-alumina crystals are too thin and long, making it difficult to obtain a beta-alumina solid electrolyte having excellent properties (e.g. low electrical resistance, high strength, etc.) in good balance. Preferably in the present invention, the beta-alumina crystals constituting the beta-alumina solid electrolyte have such a particle diameter distribution that the average particle diameter is 3 µm or less, the proportion of the particles having a particle diameter of 5 µm or less is 90% or more, and the maximum particle diameter is 300 µm or less, preferably 200 pm or less. In the beta-alumina crystals, the size of large crystals present therein is small and their amount is also small. As described above, the beta-alumina solid electrolyte of the present invention has a particular crystal structure and shows excellent properties when used as a diaphragm for sodium-sulfur battery. Specifically, the preferred beta-alumina solid electrolyte has an electrical resistance of 4.0 Ω.cm or less, preferably 3.5 Ω.cm or less, a burst strength to internal pressure, of 150 MPa or more, preferably 180 MPa or more, and a density of 3.20 g/cm 3 or more, preferably 3.22 g/cm 3 or more. The present invention is hereinafter described in more detail by way of Examples. However, the present invention is not restricted to these Examples. Example 1 and Comparative Example 1 There were weighed three raw materials, i.e. an α-Al 2 O 3 powder having an average particle diameter of 1.5 µm and a specific surface area of 3.5 m 2 /g as measured by BET adsorption method, Na 2 CO 3 having an average particle diameter of 1 µm and a magnesium-aluminum spinel having an average particle diameter, a 90% particle diameter and a MgO/Al 2 O 3 molar ratio all shown in Table 2, in proportions allowing for beta-alumina formation, i.e. proportions of 71% (Al 2 O 3 ), 14% (spinel) and 15% (Na 2 CO 3 ). The raw materials were uniformly mixed in water using a ball mill to prepare a slurry. The slurry was granulated into grains having an average particle diameter of 60 µm, using a spray dryer. The grains were molded into a tubular molded material having a 25 mm in diameter, 230 mm in length and 1.3 mm in thickness, at a pressure of 2 ton/cm 2 using an isostatic press. The molded material was placed in a MgO-made sagger and fired at the maximum temperature of 1,600°C for 30 minutes to obtain a beta-alumina sintered material. The crystal structure and properties of the beta-alumina sintered material are shown in Table 2. For comparison, beta-alumina sintered materials (sample Nos. 1-3 of Comparative Example 1) were obtained under the same conditions as in Example 1 except that there were added a step of calcinating the mixture of raw materials at 1,250°C for 120 minutes and a step of grinding the calcination product. Further, beta-alumina sintered materials (sample Nos. 4-6 of Comparative Example 1) were obtained under the same conditions as in Example 1 except that no magnesium-aluminum spinel was used. The crystal structures and properties of these beta-alumina sintered materials (sample Nos. 1-6 of Comparative Example 1) are shown in Table 2. In Table 2, electrical resistance and strength to internal water pressure were measured as follows. Test method for electrical resistance Electrical resistance was obtained as a value at 350°C using a Na/Na conductivity tester shown in Fig. 3. In Fig. 3, the Na/Na conductivity tester was constituted by a tubular beta-alumina sintered material (a beta-alumina tube) 1, insulating supports 2 and 3, a stainless steel electrode 4 and electrode terminals 5 and 6. By feeding a molten sodium 8 of 350°C into a container 7 and the beta-alumina tube 1 and passing a given electricity between the terminals 5 and 6, the electrical resistance of the beta-alumina tube to be measured was determined as a specific resistance. Test method for burst strength to internal pressure A water pressure was applied to the inner wall of a tubular beta-alumina sintered material (a beta-alumina tube) via rubber tube. From the water pressure when the beta-alumina tube broke and the dimension of said tube, the burst pressure to internal pressure, of said tube was determined. Example 2 In order to confirm the effect of addition of sodium compound, beta-alumina sintered materials were obtained under the same steps and conditions as in Example 1 except that there were used various sodium compounds as shown in Table 3. The crystal structures and properties of the beta-alumina sintered materials obtained are shown in Table 3. Example 3 In order to confirm the effect of alumina material, beta-alumina sintered materials were obtained under the same steps and conditions as in Example 1 except that the average particle diameter and BET specific surface area of α-Al 2 O 3 material were varied as shown in Table 4. The crystal structures and properties of the beta-alumina sintered materials obtained are shown in Table 4. Table 4 In order to confirm the effect of firing conditions, beta-alumina sintered materials were obtained under the same steps and conditions as in Example 1 except that the heat curve for firing was varied as shown in Table 5. The crystal structures and properties of the beta-alumina sintered materials obtained are shown in Table 5. As is clear from the above, the beta-alumina sintered materials produced in the scope of the present invention have good properties in electrical resistance, strength to internal water pressure, etc., and the beta-alumina sintered materials produced under more preferable conditions have better properties in electrical resistance, strength to internal water pressure, etc. As described above, in the present invention, since the magnesium source material is fed in the form of magnesium-aluminum spinel, a beta-alumina solid electrolyte can efficiently be produced without subjecting raw materials to calcination. Further, the beta-alumina solid electrolyte obtained by the present process, having a unique crystal structure in that the beta-alumina crystals constituting said solid electrolyte have small orientation and a small aspect ratio, has a low electrical resistance and a large strength to internal water pressure and is excellent as a diaphragm for sodium-sulfur battery.

Claims: A beta-alumina solid electrolyte tube for use in a sodium sulphur battery, characterized in that the beta-alumina crystals constituting said electrolyte have a degree of orientation of the C axis relative to the diameter direction of the tube in the range 0.2-0.4 and an aspect ratio of 4.0 or less. A beta-alumina solid electrolyte tube for use in a sodium sulphur battery, characterized in that the beta-alumina crystals constituting said electrolyte have a degree of orientation of the C axis relative to the diameter direction of the tube in the range 0.2-0.4 and have a particle diameter distribution such that the average particle diameter is 3 µm or less, the proportion of particles having a particle diameter of 5 µm or less is 90% or more, and the maximum particle diameter is 300 µm or less. A beta-alumina solid electrolyte tube according to claim 1 or 2, which has an electrical resistance of 4.0 Ω.cm or less, a burst strength to internal pressure of 150 MPa or more and a density of 3.20 g/cm 2 or more. A process for producing a beta-alumina solid electrolyte using an alumina source material, a magnesium source material and a sodium source material, which comprises subjecting all these materials to mixing, granulation, molding and firing to obtain a beta-alumina solid electrolyte without subjecting the materials to calcination after the mixing step, characterized in that a magnesium-aluminum spinel is used as the magnesium source material. A process according to claim 4, wherein the molar ratio of magnesia (MgO) and alumina (Al 2 O 3 ) in the magnesium-aluminum spinel is 1 or more. A process according to claim 4 or 5, wherein the sodium source material is at least one of NaHCO 3 and sodium oxalate. A process according to any one of claims 4 to 6, wherein the firing is conducted according to a heat curve comprising the steps of: (1) keeping the system temperature at a constant temperature in the range of 1,400-1,550°C for 1-5 hours, or increasing the system temperature at a rate of 50°C/hr or less in this temperature range, (2) after step (1), keeping the system temperature at the maximum temperature in the range of 1,580-1,650°C for 0.1-3.0 hours, and (3) after step (2), keeping the system temperature at a constant temperature in the range of 1,400-1,550°C for 1-5 hours, or decreasing the system temperature at a rate of 50°C/hr or less in this temperature range.

Inventors: KAJIHARA TAKEHIRO, KAJITA MASAHARU, TOTOKI TAKAO, KAJIHARA, TAKEHIRO, KAJITA, MASAHARU, TOTOKI, TAKAO
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1995
Main classification: H01M10
IPC codes: C04B35, H01M10