Zirconia ceramic article

Abstract: A ZrO 2 ceramic article comprises a cylindrical member and a flexible member, at least a part of the surface of the flexible member being made of a ZrO 2 ceramic material having a relative density not lower than 75% and containing a cubic crystal phase in an amount not lower than 75%, thereby exhibiting high erosion resistance and high strength. It is preferable that the rest of the ZrO 2 ceramic article is composed of a tetragonal crystal phase or a mixture of cubic and tetragonal or of a mixture of cubic, tetragonal and monoclinic.

Description: Background of the Invention and Related Art Statement The present invention relates to ZrO 2 ceramic sensor elements for use in contact with a liquid, and to a fluid sensor including such a sensor element. A ZrO 2 porcelain is widely used as a material comprising an vibrating sheet which is used for disposing a piezoelectric/electrostrictive element in a fluid sensor. The fluid sensor usually comprises a sensor unit which consists of a plate like member 2 having a piezoelectric/electrostrictive element 10 mounted thereon and a support member 1 which supports the sensor unit, for example, as shown in Fig. 5. The fluid sensor is configured to allow a sample fluid to be introduced into the member 1 and used to measure, for example, a degree of deterioration of a battery by detecting a viscosity, density of the electrolyte or a specific compound in the electrolyte. Both the plate like member 2 and the member 1 are made of ZrO 2 . Ordinarily used as such a ZrO 2 is a partially stabilized ZrO 2 which contains 3 to 4 mol% of yttria. However, the ZrO 2 which contains 3 to 4 mol% of yttria poses a problem that it is low in long-term durability and reliability though it is sufficient in its strength. Speaking concretely, it poses problems described below when it is kept in contact with a liquid or a gas which contains a liquid vapor for a long term, for example, in a condition used as a member of a fluid sensor. When the fluid sensor is used to evaluate deterioration of a battery, for example, it evaluates a degree of deterioration by utilizing a definite relationship of a viscosity, density of a deteriorated electrolyte of the battery versus a concentration of sulfuric acid and a fact that a concentration of sulfuric acid changes in an electrolyte as the battery is deteriorated. In this case, an acid solution used as the electrolyte is kept in direct contact with the vibrating sheet and the support member. The vibrating sheet and the support member may be eroded when they are kept in contact with the acid solution continuously for a very long term, for example, several to some tens of years though no problem is posed in use for a relatively short term. The inventor et al. conducted accelerated tests by keeping vibrating sheet of ZrO 2 porcelains in continuous contact with 40% sulfuric acid at 80°C and found a problem that a porcelain containing yttria at 3 mol allowed the sulfuric acid to leak to an opposite surface of the surface where sulfuric acid contact in 5 to 30 days and a porcelain containing yttria at 4 mol allowed the sulfuric acid to leak to a side opposite to an vibrating sheet in 10 to 40 days. It is considered that the leakage is traced to gradual transformation of a tetragonal phase, out of the ZrO 2 crystalline structures, into a monoclinic phase which is caused when ZrO 2 is left standing in a liquid or a gas containing a liquid vapor. Though it can be considered to change the crystalline structure of ZrO 2 into a cubic crystal which is stable in a liquid or a gas containing a liquid vapor in order to avoid such inconvenience, the cubic crystal has low toughness and is fragile, thereby posing a problem that cracks 11 are formed as shown in Fig. 6 at portions of an integral structure consisting of an vibrating sheet and a support member on which stresses are concentrated. Furthermore, there is available a method which makes a ZrO 2 porcelain stable in a liquid or a gas containing a liquid vapor by reducing a particle size of a raw material powder or using an additive such as magnesia or alumina. However, this method poses a problem that it makes a stability of the ZrO 2 porcelain insufficient and requires high temperature to sinter a piezoelectric/electrostrictive element which is disposed on a vibrating sheet, thereby enlarging the particle size and lowering the stability. EP-A-671 772 describes a ceramic diaphragm structure, useful as a sensor having a flexible zirconia diaphragm which may be a high theoretical density cubic zirconia. Summary of the Invention The present invention has as an object to provide a ZrO 2 porcelain in a sensor element which is not only stable but also tough, can hardly be eroded and has sufficient strength in a liquid or a gas containing liquid vapor. The present invention provides zirconia ceramic sensor elements as set out in claims 1 and 12. It is preferable that the surface layer covers at least one surface of the second layer and that a thickness of the surface layer is 0.2 to 2 times of that of a thinnest portion of the second layer. The ZrO 2 sensor element may consist of a support member made of zirconia porcelain and a ZrO 2 porcelain flexible member which is composed of the surface layer and the second layer, and have an integral structure obtained by integrating the ZrO 2 porcelain flexible member with the support member so that an end surface of the support member is brought into contact with the surface layer and the surface layer closes an opening of the support member. Furthermore, the ZrO 2 sensor element may consist of a support member made of ZrO 2 porcelain and a ZrO 2 porcelain flexible member which is composed of the surface layer and the second layer, with a connecting layer which is formed on at least one end surface of the support member and has a configuration which is the same as that of the surface layer, and have an integral structure which is obtained by integrating the ZrO 2 porcelain flexible member with the support member so that the connecting layer is brought into contact with the surface layer and the surface layer closes an opening of the support member. In the ZrO 2 sensor element according to the present invention, it is preferable to dispose a connecting layer which has a relative density (bulk density/theoretical density) not lower than 95% and contains cubic crystals at a ratio not lower than 75%, on inside walls of a space formed by the ZrO 2 flexible member and the support member. In the ZrO 2 sensor element according to the present invention, the support member may be composed of a substrate which consists of a tetragonal crystal or a mixture phase consisting of a cubic crystal and a tetragonal crystal or a cubic crystal, a tetragonal crystal, and a monoclinic crystal, the support member may be composed of a contact part which has a relative density (bulk density/theoretical density) not lower than 95% and contains a cubic crystal at a ratio not lower than 75% or the support member may be composed of an intermediate layer which contains a cubic crystal at a ratio higher than the substrate and lower than the contact part. Furthermore, the ZrO 2 sensor element according to the present invention may have an integral structure of a cover member and a support member wherein the cover member is attached to the other end surface of the support member so as to close an opening of the support member and a hole formed in the cover member communicates with an internal space of the support member. In this case, the cover member may be composed of a substrate which consists of a tetragonal crystal or a mixture phase consisting of a cubic crystal and a tetragonal crystal or a cubic crystal, a tetragonal crystal and a monoclinic crystal. In the ZrO 2 sensor element according to the present invention, an intermediate ZrO 2 layer which contains a cubic crystal at a ratio higher than the second layer or the support member and lower than the surface layer or the connecting layer may be disposed at least at a location between the substrate or the support member and the surface layer or the connecting layer, and it is preferable that the intermediate ZrO 2 layer contains cubic crystals at a ratio not lower than 60% and not higher than 95%. The ZrO 2 sensor element according to the present invention may comprise a piezoelectric/electrostrictive element which is disposed on a surface of the zirconia flexible member. Furthermore, the ZrO 2 sensor element according to the present invention may contain lead. Moreover, the present invention provides a fluid sensor which uses the ZrO 2 sensor element described above. Furthermore, the present invention provides such a fluid sensor comprising: a piezoelectric element having a piezoelectric film and at least a pair of electrodes disposed in contact with said piezoelectric film, on the opposite surface, relative to the surface where said support member is located, of the ZrO 2 flexible member, electrode terminals disposed on that opposite surface of the ZrO 2 flexible member and electrically connected to the pair of electrodes; a second supporting member fixed relative to said ZrO 2 flexible member; and a separating member maintaining said piezoelectric element and said second supporting member in contactless conditions; wherein said separating member is disposed on the opposite surface, with respect to the surface where said support member is located, of said ZrO 2 porcelain flexible member so as to surround said piezoelectric element, and said second supporting member is in contact with said ZrO 2 flexible member and fixed relative to it by way of said separating member, characterized in that a glass layer is formed on the opposite surface, with recpect to the surface where said support member is located, of said ZrO 2 porcelain flexible member which is kept in contact with said separating member. Thus the present invention can provide a waterproof ZrO 2 member to be used in an atmosphere containing water or water steam. The atmosphere containing water or water steam may be an acidic atmosphere. Brief description of the Drawings Figs. 1(a) and 1(b) are schematic sectional views showing an embodiment and another embodiment of the ZrO 2 porcelain according to the present invention. Figs. 2(a) and 2(b) are schematic sectional views showing still another embodiment of the ZrO 2 porcelain according to the present invention. Fig. 3 is a schematic sectional view showing still another embodiment of the ZrO 2 porcelain according to the present invention. Fig. 4 is a schematic sectional view showing still another embodiment of the ZrO 2 porcelain according to the present invention. Fig. 5 is a schematic sectional view showing a configuration of a fluid sensor. Fig. 6 is a schematic sectional view exemplifying a conventional ZrO 2 porcelain. Fig. 7 is a schematic sectional view showing further another embodiment of the ZrO 2 porcelain according to the present invention. Figs. 8(a) and 8(b) are schematic sectional views showing further another embodiment of the ZrO 2 porcelain according to the present invention. Fig. 9(a) is a schematic perspective view showing further another embodiment of the ZrO 2 porcelain according to the present invention. Fig. 9(b) is a schematic sectional view of Fig. 9(a). Detailed Description of Preferred Embodiment In the ZrO 2 sensor element according to the present invention, a part thereof which is to be brought into contact with a liquid or a gas containing a liquid vapor, that is, the surface layer has a relative density (bulk density/theoretical density) not lower than 95% and contains cubic crystals at a ratio not lower than 75%. Since the cubic crystal has a low toughness though it has a crystalline structure which is stable in the liquid or the gas containing the liquid vapor, it is possible to provide a ZrO 2 sensor element which can hardly be eroded and has high strength by composing only the surface layer of cubic crystals. Speaking concretely, the surface layer contains contains yttria (Y 2 O 3 ) at 6 to 15 mol%, preferably 8 to 10 mol% in order to compose the surface layer of cubic crystals. The relative density (bulk density/theoretical density) is specified at the level not lower than 95% since sufficient strength and airtightness cannot be obtained at a relative density lower than 95%. In addition, it is more preferable that the relative density (bulk density/theoretical density) is 98% or higher. On the other hand, the content of the cubic crystals is defined at the level not lower than 75% since it is impossible to impart sufficient erosion resistance at a cubic crystal content lower than 75%. In addition, it is preferable that the cubic crystals are present at a ratio not lower than 90% from a viewpoint of the erosion resistance. However, the content of the cubic crystals is to be adequately determined dependently on a desired degree of erosion resistance and a fundamental configuration of a substrate described later since the content of a cubic crystal not lower than 90% makes strength slightly lower than that at a ratio which is not lower than 75% and lower than 90%. From the X-ray diffractometry and Raman spectroscopy which are generally used for identifications of various kinds of crystalline phases contained in crystalline systems and calculations of their ratios, the inventor adopts the X-ray diffractometry to determine ratios of existing crystalline phases on the basis of intensity ratios among typical diffracted rays of various kinds of crystalline phases. Components of zirconium oxide in ceramic substrates are judged using an X-ray diffractometer for thin films in procedures which are described below. Since a monoclinic crystal and a cubic crystal have crystal lattice surface spacings which are largely different from each other due to a difference in symmetry of their crystal lattices in the crystalline phase of zirconium oxide, the monoclinic crystal and the cubic crystal can be quantitatively determined from a ratio between intensities of main diffracted X-rays. On the other hand, a tetragonal crystal and a cubic crystal which have lattice surface spacings close to each other cannot be separated and quantitatively determined directly and accurately from the main diffracted X-rays. Accordingly, existence of a tetragonal crystal in a sample is confirmed by comparing diffracted X-ray images of the cubic crystal (C): C(111), C(200), C(220), C(311) and C(222) with diffracted X-ray images of the tetragonal crystal (T) : T(111), T ( 002) , T(200), T(202), T(220), T(113), T(131) and T(222). The ratio of the cubic crystals present is defined as the ratio of the cubic crystals relative to the cubic crystals + the tetragonal crystals and the content of the cubic crystal is determined as a ratio between intensities of the main diffracted ray from the crystals. "Contains a cubic crystal at a ratio not lower than 75%" is defined as satisfaction of a condition expressed by [Equation 1]. [Equation 1] I·C(111) I·T(111) + I·C(111) ≥ 0.75 When it is difficult to separate main diffracted rays from the tetragonal crystal and the cubic crystal which are close to each other, diffracted rays of a higher order may be used in place of the main diffracted rays. In such a case, however, it is necessary to standardize measured intensities of the diffracted rays of the higher order with intensities of the main diffracted rays using values of intensities of the main diffracted rays and those of the diffracted rays of the higher order which are known from the JCPDS card or the like. To determine an existing ratio from intensities of diffracted rays of the cubic crystal (200) and the tetragonal crystal (002) + (200), for example. [Equation 2] 100/25 × I·C(200) 100/25 × I·C(200) + 100/43 × [I·T(002) + I·T(200)] ≥ 0.75 [Equation 2] is used in place of the condition mentioned above since the JCPDS card indicates 25 and 43 (a sum of (002) and (200) ) as intensities of the diffracted rays of the crystals for a (111) intensity 100 of the main diffracted ray. In addition, symbols used in the numeric formulae are as defined below: I.T (111): (111) diffraction intensity of tetragonal crystal I.C (111): (111) diffraction intensity of cubic crystal I.T (002): (002) diffraction intensity of tetragonal crystal I.C (200) : (200) diffraction intensity of cubic crystal I.T (200): (200) diffraction intensity of tetragonal crystal The expression "a gas containing a liquid vapor" used herein means a gas which contains a space in the vicinity of a liquid surface or a mist like liquid such as a gas contained in a space in a battery. Furthermore, the ZrO 2 porcelain flexible member is a member which is made of a ZrO 2 porcelain and partially or entirely bent in use such as an vibrating sheet or a diaphragm of a fluid sensor, actuator or the like. In the present invention, it is preferable that the ZrO 2 sensor element has a surface layer and a second layer adjacent to the surface layer and that the second layer is composed of a tetragonal crystal or a mixture phase consisting of a cubic crystal and a tetragonal crystal or a cubic crystal, a tetragonal crystal and a monoclinic crystal. This composition is preferable to impart a sufficient toughness to the ZrO 2 flexible member. Speaking concretely, yttria, for example, is contained in the second layer at 1.5 to 6 mol%, preferably 2.5 to 4.5 mol% , to compose the substrate of a tetragonal crystal, or a mixture phase consisting of a cubic crystal and a tetragonal crystal or a cubic crystal, a tetragonal crystal and a monoclinic crystal. Furthermore, it is preferable that the surface layer covers at least one surface of the second layer and has a thickness which is 0.2 to 2 times of that of a thinnest portion of the second layer. This thickness is preferable to impart sufficient erosion resistance to the ZrO 2 porcelain. The ZrO 2 sensor element according to the present invention may consist of a cylindrical support member 1 made of the ZrO 2 porcelain and a ZrO 2 porcelain flexible member 4 which is composed by disposing a surface layer hereinafter called contact part 3 on at least one surface of a plate like member 2 constituting the second layer as shown in Fig. 1(a), and have an integral structure obtained by integrating the ZrO 2 porcelain flexible member 4 with the member 1 which is a supporting member for the flexible member so that an end surface of the member 1 is brought into contact with the contact part 3 disposed on the plate like member 2 and the contact part 3 closes an opening of the member 1. Furthermore, the ZrO 2 sensor element according to the present invention may consist of the member 1 made of a ZrO 2 porcelain and a ZrO 2 porcelain flexible member 4 which is a plate like member 2 composed of the contact part 3 as shown in Fig. 2 (a), and have an integral structure obtained by integrating the ZrO 2 porcelain flexible member 4 with the support member 1 which is the supporting member for the flexible member so that an end surface of the member 1 is brought into contact with the plate like member 2 and the contact part 3 closes an opening of the member 1. Though the ZrO 2 porcelain flexible member 4 itself has a low toughness and is fragile when the ZrO 2 porcelain flexible member 4 is composed of the contact part which contains a cubic crystal at a high ratio, a ZrO 2 porcelain 5 as a whole can maintain sufficient strength since it is supported by the cylindrical member 1. Furthermore, the ZrO 2 sensor element according to the present invention may comprise a connecting layer 6 which has the same composition as the contact part and is disposed between at least one end surface of the member 1 and the contact part 3 of the ZrO 2 porcelain flexible member 4 as shown in Figs. 1 (b) and 2 (b). When the ZrO 2 sensor element is used, for example, as a fluid sensor to evaluate deterioration of a battery and cracks are formed in the contact part 3 which has low toughness due to stresses produced by transformation of the member 1, the connecting layer 6 is capable of preventing cracks from being formed, and sulfuric acid from leaking to an opposite surface of the surface of vibrating sheet where sulfuric acid contact. In the ZrO 2 sensor element according to the present invention which has the integral structure described above, a layer 8 which has a relative density (bulk density/theoretical density) not lower than 95% and contains cubic crystals at a ratio not lower than 75% may further be disposed on inside walls of a space 7 formed by the ZrO 2 porcelain flexible member 4 and the member 1 which is the supporting member for the flexible member as shown in Fig. 3. This configuration makes it possible to impart sufficient erosion resistance to all contact parts, and strengthen a seam between the ZrO 2 porcelain flexible member 4 and the. member 1 which is liable to be cracked due to stresses. In the ZrO 2 sensor element according to the present invention, a layer which has a relative density (bulk density/theoretical density) not lower than 95% and contains cubic crystals at a ratio not lower than 75% may be disposed only on inside walls of a space 7 formed by closing the opening of the member 1 as shown in Fig. 4. When such a layer is formed on the entire surfaces of the inside walls, the contact parts preferably have sufficient erosion resistance. Strength of the ZrO 2 sensor element according to the present invention can be further enhanced as a whole by composing the support member of a tetragonal crystal or a mixture phase of a cubic crystal and a tetragonal crystal or a cubic crystal, a tetragonal crystal and a monoclinic crystal. The seam between the ZrO 2 porcelain flexible member and the supporting member which is liable to be cracked due to stresses can be further strengthened by forming an intermediate ZrO 2 layer 9 which contains a cubic crystal at a ratio higher than the support member and lower than the contact part or the connecting layer at least at a location between the support member and the contact part or the connecting layer as shown in Figs. 1 through 4. It is preferable that the intermediate ZrO 2 layer 9 contains cubic crystals at a ratio not lower than 60% and not higher than 95% on a premise that the intermediate ZrO 2 layer contains cubic crystals at a ratio higher than the support member and lower than the contact part or the connecting layer. Speaking concretely, such an intermediate ZrO 2 layer 9 can be obtained, for example, by allowing the intermediate ZrO 2 layer 9 to contain yttria at 5 to 7 mol%, preferably 5.5 to 6.5 mol%. In the ZrO 2 sensor element according to the present invention, the cubic crystal content of the support member may be higher than that of the substrate and lower than that of the contact part described above. Such a cubic crystal content of the cylindrical member provides a merit to make erosion resistance compatible with strength. Mentioned as such a ZrO 2 sensor element is, for example, an element consisting of a ZrO 2 porcelain flexible member 4 which comprises a substrate, a contact part 3 and an intermediate layer 9, and a member 1 as shown in Fig. 1(a), wherein the member 1 contains cubic crystals at a ratio not lower than 60% and not higher than 95%. Speaking of concrete means to obtain a cubic crystal content within the range specified above, yttria, for example, is contained in the member 1 at 5 to 7 mol%, preferably 5.5 to 6.5 mol%. In the ZrO 2 sensor element according to the present invention, the support member may have a relative density (bulk density/theoretical density) of 95% or higher and contain cubic crystals at 75% or higher. Such a support member can enhance erosion resistance of the ZrO 2 porcelain. In this case, the support member contains yttria at 6 to 15 mol%, preferably 8 to 10 mol%. In this case, however, toughness of the support member is reduced, whereby strength of the ZrO 2 porcelain may be insufficient. Accordingly, it is preferable to use the support member in combination with a cover member which is described later. Strength of the ZrO 2 sensor element according to the present invention as a whole can be enhanced by attaching a cover member 12 to an end surface of the support member 1 different from the end surface on which the ZrO 2 flexible member 4 is disposed, so that the cover member 12 covers the opening of the member 1, and integrating the cover member 12 with the member 1 so that a hole 13 formed in the cover member 12 communicates with the internal space of the member 1 as shown in Fig. 7. When the ZrO 2 porcelain is to be used as a fluid sensor, the cover member 12 hardly allows external noise to enter the vibrating member. In this case, it is preferable to compose the cover member 12 of tetragonal crystals or a mixture phase of cubic crystal and tetragonal crystal or cubic crystal, tetragonal crystal and monoclinic crystal for enhancing strength of the sensor furthermore. A ZrO 2 sensor element such as a vibrating sheet often contains lead for use of piezoelectric material for adjusting atmosphere and diffusion of substances composing a piezoelectric material when sintering the piezoelectric material. In such a case, the ZrO 2 is more liable to be transformed and it is effective to dispose the contact part described above. For the ZrO 2 porcelain according to the present invention, erosion resistance may be further enhanced by covering the contact part (surface layer) with an acid-resisting resin such as polyester resin or an epoxy acrylate resin, glass or the like. Dipping, spraying, coating and the like can be mentioned as covering methods. The ZrO 2 sensor element according to the present invention may comprise a piezoelectric/electrostrictive element disposed on a surface of the ZrO 2 porcelain flexible member so that it can be used as a fluid sensor, an actuator or the like. In such a case, the piezoelectric/electrostrictive element is disposed on the substrate (second layer) of the ZrO 2 porcelain flexible member. A fluid sensor which uses the ZrO 2 sensor element according to the present invention may comprise a separating member 15 and a supporting member 18 as shown in Fig. 8. In a fluid sensor 16 shown in Fig. 8, the separating member 15 and the supporting member 18 are disposed to maintain a piezoelectric/electrostrictive element 17 and electrode terminals disposed on a surface of a ZrO 2 porcelain flexible member 4 in a condition airtight from a fluid. Since the piezoelectric/electrostrictive element 17 vibrates the ZrO 2 porcelain flexible member 4, the separating member 15 which is adopted to prevent the supporting member 18 from being brought into contact with the piezoelectric/electrostrictive element 17 is disposed so as to surround the piezoelectric/electrostrictive element 17. Connected to the electrode terminals are lead wires 14 which are externally distributed through holes formed in the supporting member 18. The holes are maintained airtight with an adhesive agent or the like. Furthermore, a glass layer 19 is formed on the ZrO 2 porcelain flexible member 4 at a location which is in contact with the separating member 15. This glass layer 19 is formed to maintain the separating member 15 in an airtight condition for a long term on the ZrO 2 porcelain flexible member 4. That is, in a fluid sensor having such a configuration as described above, the whole sensor is usually used in a condition where the sensor is submerged in a fluid. Since all parts are kept in contact with the fluid in such a case, except for the parts which are kept airtight by the separating member 15 and the supporting member 18, the substrate is kept in contact with the fluid and eroded in long-term use on the surface of the ZrO 2 porcelain flexible member 4 which is outside the separating member 15, whereby the maintenance of airtightness by the separating part 15 is degraded after long-term use. The glass layer 19 is provided to prevent such a situation. The separating member 15 is not restricted in particular in its shape and may be a block member which has a square section. From a viewpoint to maintain airtightness, it is preferable to use a member which is like an O-ring having a circular section. Furthermore, it is preferable that the separating member 15 is made of a material which has not only erosion resistance against a fluid but also a cushioning property from a viewpoint of airtightness. When the separating member 15 is made of a cushioning material, it is effective to suppress unwanted vibrations of the ZrO 2 porcelain flexible member 4 and provides a merit to enhance an accuracy of the sensor. And the supporting member 18 is neither restricted in its shape nor material. The supporting member 18 is sufficiently effective so far as it cooperates with the separating member 15 to sustain the ZrO 2 porcelain flexible member 4 and the cylindrical member 1, and maintain the piezoelectric/electrostrictive element 17 and the electrode terminals airtight from a fluid. A fluid sensor which is shown in Figs. 9(a) and 9(b), for example, adopts a configuration wherein a sensor element which is composed of a ZrO 2 porcelain flexible member 4, a cylindrical member 1, a cover member 12, a piezoelectric/electrostrictive element 17 and electrode terminals is enclosed by a supporting member 18 made of vinyl chloride, a separating member 15 consisting of an O-ring made of a fluoro-rubber and a clamp member 20 made also of a vinyl chloride, and sustained by an elasticity of the separating member 15. These members are fixed by passing a shaft body 21 through holes formed in the supporting member 18 and the clamp member 20. From viewpoints of strength and erosion resistance, it is preferable that the shaft body 21 is made of a ceramic material or a glass material though it can also be made of vinyl chloride. In order to enhance erosion resistance of the fluid sensor shown in Fig. 8 or 9, it is preferable that the support member 1 has a relative density (bulk density/theoretical density) not lower than 95% and contains cubic crystals at a ratio not lower than 75%. Furthermore, it is preferable that the ZrO 2 porcelain flexible member 4 consists of a substrate, a contact part 3 and an intermediate layer 9 which is disposed between the substrate and the contact part 3 as shown in Fig. 8(b), and contains a cubic crystal at a ratio higher than the substrate and lower than the contact part. Furthermore, it is preferable that the fluid sensor 16 comprises a cover member 12 which is attached, so as to cover an opening of the member 1, to an end surface of the cylindrical member 1 different from the end surface to which the ZrO 2 porcelain flexible member 4 is attached and has a hole 13 communicating with an internal space of the member 1 as shown in Fig. 8(b). When the ZrO 2 sensor element according to the present invention is to be used a fluid sensor and the ZrO 2 porcelain flexible member has a width of 0.3 to 0.5 mm, it is preferable that the ZrO 2 porcelain flexible member has a thickness of 10 to 50 µm. In this case, it is preferable that the substrate is 5 to 30 µm thick and the contact part is 5 to 30 µm thick. When the intermediate layer is to be formed, it is preferable to configure it so as to have a thickness of 5 to 15 µm. If these parts have thicknesses exceeding the values specified above, the ZrO 2 porcelain flexible member will have a large thickness as a whole, thereby suppressing vibrations. If these parts have thicknesses smaller than the values specified above, ZrO 2 porcelain strength will be insufficient for the substrate, erosion resistance will be insufficient for the contact part, and maintenance of stress moderating function and continuity to the substrate and the contact part will be insufficient for the intermediate layer. Examples The present invention will be described more detailedly below with reference to its examples which are presented not in limitative sense. (Example 1) We manufactured a ZrO 2 porcelain for a fluid sensor consisting of a cylindrical support member and a vibrating sheet, wherein a contact part disposed on a vibrating sheet was in contact with a connecting layer disposed on a cylindrical member as shown in Fig. 1(b). First, we prepared a green sheet 10 µm thick which was to be used as an vibrating sheet and a green sheet 330 µm thick which was to be used as the support member. ZrO 2 containing 2 to 4 mol% of yttria was used for preparation of the green sheets. Then, an intermediate layer which was composed of a ZrO 2 containing 6 mol% of yttria was formed by screen printing or green sheet lamination-thermocompression bonding on a side of the green sheet for the vibrating sheet which was to be brought into contact with an acid. The intermediate layer was 6 µm thick. A contact part which was composed of ZrO 2 containing 8 mol% of yttria was formed by screen printing or green sheet lamination-thermocompression bonding on the intermediate layer which was formed on the green sheet for the vibrating sheet. The contact part was 9 µm thick. An intermediate layer which was composed of a ZrO 2 containing 6 mol% of yttria was formed by screen printing or green sheet lamination-thermocompression bonding on an end surface of the green sheet for the support member. The intermediate layer was 25 µm thick. A connecting layer which was composed of ZrO 2 containing 8 mol% of yttria was formed by screen printing or green sheet lamination-thermocompression bonding on the intermediate layer which was formed on the green sheet for the support member. The connecting layer was 25 µm thick. A support green sheet was prepared by forming a through hole in the green sheet for the support member. Finally, the green sheet for the vibrating sheet was laminated with the support green sheet so that the green sheet for the vibrating sheet closed an opening of the support green sheet and the green sheets were sintered at 1450°C for 2 hours, thereby obtaining a ZrO 2 porcelain. Using the ZrO 2 porcelain as a fluid sensor, we checked it for the number of days elapsed until 40% sulfuric acid at 80°C leaked to a surface of the vibrating sheet on a side opposite to the support member. (Example 2) We manufactured a ZrO 2 porcelain for a fluid sensor consisting of a cylindrical support member and an vibrating sheet, wherein a contact part disposed on the vibrating sheet was in contact with an end surface of the support member and a layer which had a relative density (bulk density/theoretical density) not lower than 95% and contained cubic crystals at a ratio not lower than 75% was formed on inside walls of the support member as shown in Fig. 3. The ZrO 2 porcelain was the same as that in Example 1, except for the intermediate layer and the connecting layer which were formed on the end surface of the green sheet for the support member in Example 1. However, a contact part was formed on the inside wall of the support member in procedures described below: First, an intermediate layer which was composed of ZrO 2 containing 6 mol% of yttria was formed by screen printing (through hole printing) or dipping. Formed on the intermediate layer was a connecting layer which was composed of ZrO 2 containing 8 mol% of yttria by the same way described above. The intermediate layer and the connecting layer were 6 µm and 13 µm thick respectively. Using the ZrO 2 porcelain described above as a fluid sensor, we checked it for the number of days elapsed until 40% sulfuric acid at 80°C leaked to a surface of the vibrating sheet on a side opposite to the support member. (Example 3) We manufactured a ZrO 2 porcelain which was to be used as a fluid sensor consisting of a cylindrical support member and an vibrating sheet, wherein the vibrating sheet was joined to an end surface of the support member so as to close an opening of the cylindrical member and a layer which had a relative density (bulk density/theoretical density) not lower than 95% and contained cubic crystals at a ratio not lower than 75% was formed on inside walls of a space formed by the cylindrical member and the vibrating sheet as shown in Fig. 4. First, we prepared a green sheet for an vibrating sheet and a green sheet for the support member. Thicknesses and yttria contents of the green sheets were the same as those in Example 1. A through hole was formed in the green sheet for the support member, the green sheet for the vibrating sheet was laminated with the support green sheet so that the green sheet for the vibrating sheet closed an opening of the support green sheet and the green sheets were sintered at 1450°C for 2 hours. Finally, an intermediate layer and a contact part were formed by screen printing (through hole printing) or dipping on inside walls of a space formed by the support member and the vibrating sheet, and a ZrO 2 porcelain was obtained by sintering the members. Thicknesses of the intermediate layer and the contact part, yttria contents and sintering conditions were the same as those in Example 1. Using the ZrO 2 porcelain described above as a fluid sensor, we checked it for the number of days elapsed until 40% sulfuric acid at 80°C leaked to a surface of the vibrating sheet on a side opposite to the support member. (Example 4) We manufactured a ZrO 2 porcelain which is to be used as a fluid sensor consisting of a cylindrical support member and an vibrating sheet, wherein the vibrating sheet was composed of a contact part and the vibrating sheet was joined to an end surface of the support member so as to close an opening of the support member in a condition where a connecting layer formed on the support member was in contact with the vibrating sheet as shown in Fig. 2(b). First, we prepared a green sheet for an vibrating sheet and a green sheet for a the support member. A thickness and yttria content of the green sheet for the support member were the same as those in Example 1. The green sheet for the vibrating sheet was 20 µm thick. The green sheet for the vibrating sheet was composed of ZrO 2 containing 8 mol% of yttria. An intermediate layer and a connecting layer were formed on an end surface of the green sheet for the support member as in Example 1. The intermediate layer and the connecting layer were 25 µm and 25 µm thick respectively. A through hole was formed in the green sheet for the support member, thereby preparing a support green sheet. Finally, the green sheet for the vibrating sheet was laminated with the support green sheet so that the green sheet for the vibrating sheet closed an opening of the support green sheet and the green sheets were sintered at 1450°C for 2 hours, thereby obtaining a ZrO 2 porcelain. Using the ZrO 2 porcelain described above as a fluid sensor, we checked it for the number of days elapsed until 40% sulfuric acid at 80°C leaked to a surface of the vibrating sheet on a side opposite to the support member. (Example 5) We manufactured a ZrO 2 porcelain which was to be used as a fluid sensor consisting of a cylindrical support member, an vibrating sheet and a cover member as shown in Fig. 7. In a ZrO 2 porcelain 5 shown in Fig. 7, an vibrating sheet 4 had a substrate, a contact part 3 and an intermediate layer 9 which was formed between the substrate and the contact part 3, and contained cubic crystals at a ratio higher than the substrate and lower than the contact part. Furthermore, a cover member 12 was attached to an end surface of the support member 1 different from an end surface to which the vibrating sheet 4 was attached so that the cover member 12 closed an opening of the member 1 and has a hole 13 communicating with an internal space of the support member 1. Furthermore, the member 1 had a relative density (bulk density/theoretical density) not lower than 95% and contained cubic crystals at a ratio not lower than 75% like the contact part 3. As in Example 1, an intermediate layer which contained 6 mol% of yttria and a contact part which contained 8 mol% of yttria were formed on a green sheet for an vibrating sheet like that used in Example 1. A green sheet for a support member and a green sheet for a cover member were manufactured using a ZrO 2 containing 2 to 4 mol% of yttria. A through hole was formed in the green sheet for the support member to prepare a support green sheet and a hole was formed also in the green sheet for the cover member. The green sheet for the vibrating sheet was laminated with the support green sheet so that the green sheet for the vibrating sheet closed an opening of the support green sheet, the green sheet for the cover member was laminated so as to close the other opening of the support green sheet and these members were sintered at 1450°C for 2 hours, thereby obtaining a ZrO 2 porcelain. Using the ZrO 2 porcelain as a fluid sensor, we checked it for the number of days elapsed until it allowed 40% sulfuric acid at 80°C to leak to a surface of the vibrating sheet on a side opposite to the support member. (Comparative example 1) We manufactured a ZrO 2 porcelain for a fluid sensor using a cylindrical support member and an vibrating sheet both of which were made of a ZrO 2 containing 3 mol% of yttria, actually used it as a fluid sensor and checked it for the number of days elapsed until it allowed 40% sulfuric acid at 80°C to leak to a surface of the vibrating sheet on a side opposite to the support member. Though the ZrO 2 porcelain manufactured in Comparative example 1 allowed 40% sulfuric acid at 80°C to leak upon lapse of 5 to 30 days, the ZrO 2 porcelains manufactured in Examples did not allow 40% sulfuric acid at 80°C to leak upon lapse of 120 or more days. The ZrO 2 sensor element according to the present invention can hardly be eroded and has high strength owing to its crystalline structure wherein a portion to be brought into contact with a liquid or a gas containing the liquid vapor is made of cubic crystals. Accordingly, the ZrO 2 sensor element does not allow a liquid such as sulfuric acid to leak to a surface of the vibrating sheet on a side opposite to surface where sulfuric acid contact even when it is used as a fluid sensor or the like, thereby being capable of prolonging service lives of implements which use ZrO 2 porcelain flexible members. The ZrO 2 sensor element according to the present invention can have more sufficient toughness when a second layer is composed of tetragonal crystals or a mixture phase consisting of cubic crystals and tetragonal crystals or cubic crystals, tetragonal crystals and monoclinic crystals.

Claims: A zirconia ceramic sensor element for use in contact with a liquid, comprising a zirconia support member (1) and a zirconia flexible member (4), said flexible member (4) being supported by said support member and comprising a flexible sheet which in use contacts the liquid, wherein said flexible sheet has a multi-layer structure having at least a surface layer (3;8) which in use contacts the liquid and is composed of a first zirconia material which has a relative density (bulk density/theoretical density) of not less than 95% and contains cubic crystals in a ratio of not less than 75% relative to the content of cubic crystals and tetragonal crystals determined as a ratio between intensities of the main diffracted X-rays from the crystals, and a second layer which is composed of a second zirconia material which is composed of tetragonal crystals or of a mixed crystal phase consisting of cubic and tetragonal crystals or cubic, tetragonal and monoclinic crystals. A zirconia ceramic sensor element according to claim 1, wherein said multi-layer structure includes an intermediate zirconia layer (9) between said surface layer (3) and said second layer. A zirconia ceramic sensor element according to claim 1 or 2, wherein at least said surface layer (3) extends over said support member (1), and there is a connecting layer (6) formed of a zirconia material which has a relative density (bulk density/theoretical density) of not less than 95% and contains cubic crystals in a ratio of not less than 75%, the connecting layer (6) being united with said surface layer and being between said surface layer and said support member. A zirconia ceramic sensor element according to claim 3, wherein there is an intermediate zirconia layer (9) united with said connecting layer (6) and said support member and having a cubic crystal content higher than the material of said second layer and the material of said support member (1) and lower than the material of said surface layer (3) and the material of the connecting layer (6). A zirconia ceramic sensor element according to any one of claims 1 to 4 wherein said support member (1) has an opening through it which is closed at one end by said flexible sheet. A zirconia ceramic sensor element according to claim 5, wherein said surface layer (8) of said multi-layer structure faces said opening, and said support member (1) has a surface layer bounding said opening thereof composed of a zirconia material which has a relative density (bulk density/theoretical density) of not less than 95% and contains cubic crystals in a ratio of not less than 75% and an intermediate layer (9) between said support member (1) and said surface layer thereof composed of zirconia containing cubic crystals in a ratio higher than that of said support member (1) and lower than that of said surface layer thereof. A zirconia ceramic sensor element according to claim 5 or 6, wherein a cover member (12) is arranged at the opposite end of said opening of said support member and integrated therewith so as partially to close said opening, a hole (13) of said cover member communicating with said opening. A zirconia ceramic sensor element according to claim 7, wherein said cover member (12) is made of a material which is composed of tetragonal crystals or of a mixed crystal phase consisting of cubic and tetragonal crystals or cubic, tetragonal and monoclinic crystals. A zirconia ceramic sensor element according to claim 4, wherein said intermediate layer (9) contains cubic crystals in a ratio of not less than 60% and not more than 95%. A zirconia ceramic sensor element according to any one of the preceding claims wherein said support member comprises a body of zirconia material which is composed of tetragonal crystals or of a mixed crystal phase consisting of cubic and tetragonal crystals or cubic, tetragonal and monoclinic crystals. A zirconia ceramic sensor element according to claim 10, wherein said surface layer extends over a surface of said second layer and the thickness of said surfaces layer is in the range 0.2 to 2 times the thickness of the thinnest part of said second layer. A zirconia ceramic sensor element for use in contact with a liquid, comprising a zirconia support member (1) and a zirconia flexible member (4), said flexible member (4) being supported by said support member and comprising a flexible sheet which in use contacts the liquid, wherein said support member (1) is a cylindrical member which has an opening which is closed by said flexible sheet, wherein said flexible sheet has at least a surface layer which in use contacts the liquid and is composed of a first zirconia material which has a relative density (bulk density/theoretical density) of not less than 95% and contains cubic crystals in a ratio of not less than 75% relative to the content of cubic crystals and tetragonal crystals determined as a ratio between intensities of the main diffracted X-rays from the crystals, and wherein said support member is composed of a second zirconia material which is composed of tetragonal crystals or of a mixed crystal phase consisting of cubic and tetragonal crystals or cubic, tetragonal and monoclinic crystals. A zirconia ceramic sensor element according to claim 12, wherein said surface layer extends over said support member (1), and there is a connecting layer (6) formed of a zirconia material which has a relative density (bulk density/theoretical density) of not less than 95% and contains cubic crystals in a ratio of not less than 75%, the connecting layer (6) being united with said surface layer and being between said surface layer and said support member. A zirconia ceramic sensor element according to claim 13, wherein there is an intermediate zirconia layer (9) united with said connecting layer (6) and said support member and having a cubic crystal content higher than the material of said second layer and lower than the material of said surface layer (3) and the material of the connecting layer (6). A zirconia ceramic sensor element according to any one of the preceding claims, having a piezoelectric/electrostrictive element disposed on a surface of said flexible sheet. A zirconia ceramic sensor element according to any one of the preceding claims, wherein said flexible member contains lead. A fluid sensor device for sensing a property of a fluid, having a zirconia ceramic sensor element according to any one of the preceding claims. A fluid sensor device for sensing a property of a fluid, having a zirconia ceramic sensor element according to claim 15, wherein said piezoelectric/electrostrictive element has electrodes on both sides of a piezoelectric film and is disposed on the opposite surface of said zirconia flexible member (4) from said support member, electrode terminals are disposed on said zirconia flexible member (4) on said opposite surface thereof, a covering member (18) is maintained in position relative to said zirconia flexible member (4) without contact therewith by a separating member (15) disposed on said opposite surface of said flexible member (4) so as to surround said piezoelectric/electrostrictive element, and a glass layer formed on said opposite surface (19) of said zirconia flexible member (4) is contacted by said separating member (15). Use of a flexible zirconia ceramic member (4) in contact with a medium which is water or an atmosphere containing water or steam, wherein said member (4) has a multi-layer structure having a surface layer exposed to said medium and composed of a material which has a relative density (bulk density/theoretical density) of not less than 95% and contains cubic crystals in a ratio of not less than 75% relative to the content of cubic crystals and tetragonal crystals determined as a ratio between intensities of the main diffracted X-rays from the crystals, and a second layer which is composed of tetragonal crystals or of a mixed crystal phase consisting of cubic and tetragonal crystals or cubic, tetragonal and monoclinic crystals. Use according to claim 19, wherein said medium is acidic.

Inventors: HIROTA TOSHIKAZU, NAKAO SHIGEKI, SHIBATA KAZUYOSHI, TAKEUCHI YUKIHISA, HIROTA, TOSHIKAZU, NAKAO, SHIGEKI, SHIBATA, KAZUYOSHI, TAKEUCHI, YUKIHISA, Nakao, Shigeki, A-330 NGK Yagoto-ryo
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1999
Main classification: C04B35
IPC codes: C04B35, B32B18, G01N29, H01L41, G01N11