Zirconia porcelain

The present invention relates to a ZrO 2 porcelain and a waterproof ZrO 2 member which are to be used as members of a fluid sensor or the like

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 cylindrical 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 cylindrical 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 cylindrical 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 cylindrical member. The vibrating sheet and the cylindrical 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 Five to thirty days and a porcelain containing yttria at 4 mol allowed the sulfuric acid to leak to a side opposite to an vibrating sheet in ten to Forty 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 cylindrical 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 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.

The present invention has been achieved in view of the circumstances described above and has an object to provide a ZrO 2 porcelain 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.

That is, the present invention provides a ZrO 2 porcelain comprising a ZrO 2 porcelain flexible member having at least a part which is brought into contact with a liquid or a gas containing a liquid vapor,.wherein the contact part has a relative density (bulk density/theoretical density) not lower than 95% and contains a cubic crystal at a ratio not lower than 75%.

In the ZrO 2 porcelain, it is preferable that the ZrO 2 flexible member consists of a contact part and a substrate adjacent to the contact part, and that the substrate 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. Furthermore, it is preferable that the contact part covers at least one surface of the substrate and that a thickness of the contact part is 0.2 to 2 times of that of a thinnest portion of the substrate.

The ZrO 2 porcelain may consist of a cylindrical member made of a ZrO 2 porcelain and a ZrO 2 porcelain flexible member which is composed by disposing a contact part at least on one surface of a plate like member consisting of the substrate, and having an integral structure obtained by integrating the ZrO 2 porcelain flexible member with the cylindrical member which is a supporting member for the flexible member so that an end surface of the cylindrical member is brought into contact with the contact part disposed on the plate like member and the contact part closes an opening of the cylindrical member.

Furthermore, the ZrO 2 porcelain may consist of a cylindrical member made of a ZrO 2 porcelain and a ZrO 2 porcelain flexible member which is composed by disposing a contact part at least on one surface of a plate like member consisting of the substrate, comprise a connecting layer which is formed on at least one end surface of the cylindrical member and has a configuration which is the same as that of the contact part, and having an integral structure which is obtained by integrating the ZrO 2 porcelain flexible member with the cylindrical member which is a supporting member for the flexible member so that the connecting layer is brought into contact with the contact part disposed on the plate like member and the contact part closes an opening of the cylindrical member.

Furthermore, the ZrO 2 porcelain may consist of a cylindrical member made of a ZrO 2 porcelain and a ZrO 2 porcelain flexible member which is a plate like member consisting of the contact part, and have an integral structure obtained by integrating the ZrO 2 porcelain flexible member with the cylindrical member which is a supporting member for the flexible member so that an end surface of the cylindrical member is brought into contact with the plate like member and the contact part closes an opening of the cylindrical member.

Furthermore, the ZrO 2 porcelain may consist of a cylindrical member made of a ZrO 2 porcelain and a ZrO 2 porcelain flexible member which is a plate like member consisting of the contact part, comprise a connecting layer which is formed on at least one end surface of the cylindrical member and has the same configuration as that of the contact part, and have an integral structure obtained by integrating the ZrO 2 porcelain flexible member with the cylindrical member which is a supporting member for the flexible member so that the connecting layer is brought into contact with the plate like member and the contact part closes an opening of the cylindrical member.

For the ZrO 2 porcelain according to the present invention, it is preferable to dispose 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%, on inside walls of a space formed by the ZrO 2 porcelain flexible member and the cylindrical member which is a supporting member for the flexible member.

The ZrO 2 porcelain according to the present invention may comprise a cylindrical member made of a ZrO 2 porcelain; and a plate like member composed of the substrate, wherein an end surface of the cylindrical member is in contact with the plate like member, 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% is disposed on inside walls of a space formed by closing an opening of said cylindrical member, and a ZrO 2 porcelain flexible member is integrated with the cylindrical member which is a supporting member for the flexible member.

In the ZrO 2 porcelain according to the present invention, the cylindrical 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 cylindrical 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 cylindrical 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 porcelain according to the present invention may have an integral structure of a cover member and a cylindrical member wherein the cover member is attached to the other end surface of the cylindrical member so as to close an opening of the cylindrical member and a hole formed in the cover member communicates with an internal space of the cylindrical 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 porcelain according to the present invention, an intermediate ZrO 2 layer which contains a cubic crystal at a ratio higher than the substrate or the cylindrical member and lower than the contact part or the connecting layer may be disposed at least at a location between the substrate or the cylindrical member and the contact part or the connecting layer, and it is preferable that the intermediate ZrO 2 layer contains a cubic crystal at a ratio not lower than 60% and not higher than 95%. The ZrO 2 porcelain according to the present invention may comprise a piezoelectric/electrostrictive element which is disposed on a surface of the ZrO 2 porcelain flexible member. Furthermore, the ZrO 2 porcelain according to the present invention may contain lead.

Moreover, the present invention provides a fluid sensor which uses the ZrO 2 porcelain described above. Furthermore, the present invention provides 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 of the surface where said cylindrical member is located, of said ZrO 2 porcelain flexible member composing the ZrO 2 porcelain described above, electrode terminals disposed on the opposite surface of the surface where said cylindrical member is located, of said ZrO 2 porcelain flexible member and electrically connected to said pair of electrodes; a supporting member fixing said ZrO 2 porcelain flexible member; and a separating member maintaining said piezoelectric element and said supporting member in contactless conditions; wherein said separating member is disposed on the opposite surface of the surface where said cylindrical member is located, of said ZrO 2 porcelain flexible member so as to surround said piezoelectric element, and said supporting member is in contact with said ZrO 2 porcelain flexible member and fix it by way of said separating member, characterized in that a glass layer is formed on the opposite surface of the surface where said cylindrical member is located, of said ZrO 2 porcelain flexible member which is kept in contact with said separating member.

In addition, the present invention provides a waterproof ZrO 2 member to be used in an atmosphere containing water or water steam, wherein a substrate is made of ZrO 2 which is 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, and a ZrO 2 layer 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% is disposed at a location to be exposed to an atmosphere containing water or water steam. The atmosphere containing water or water steam may be an acidic atmosphere.

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 ).

In the ZrO 2 porcelain 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, a contact part has a relative density (bulk density/theoretical density) not lower than 95% and contains a cubic crystal 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 impart to the ZrO 2 porcelain a nature that it can hardly be eroded and has high strength by composing only the contact part of a cubic crystal. Speaking concretely, the contact part contains yttria (Y 2 O 3 ) at 6 to 15 mol %, preferably 8 to 10 mol % in order to compose the contact part of a cubic crystal.

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 crystal 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 crystal is contained at a ratio not lower than 90% from a viewpoint of the erosion resistance. However, a content of the cubic crystal 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%.

Out of 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 the 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 or 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).

An existing ratio of the cubic crystal is defined as an existing ratio of the cubic crystal relative to the cubic crystal+the tetragonal crystal and a 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]. I · C  ( 111 ) I · T  ( 111 ) + I · C  ( 111 ) ≥ 0.75 [ Equation     1 ]

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. 100 / 25 × I · C  ( 200 ) 100 / 25 × I · C  ( 200 ) + 100 / 43 × [ I · T  ( 002 ) + I · T  ( 200 ) ] ≥ 0.75 [ Equation     2 ]

[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 vibrating sheet or a diaphragm of a fluid sensor, actuator, or the like.

In the present invention, it is preferable that the ZrO 2 porcelain is composed of a contact part and a substrate adjacent to the contact part, and that the substrate 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 porcelain. Speaking concretely, yttria, for example, is contained in the substrate 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 contact part covers at least one surface of the substrate and has a thickness which is 0.2 to 2 times of that of a thinnest portion of the substrate. This thickness is preferable to impart sufficient erosion resistance to the ZrO 2 porcelain.

The ZrO 2 porcelain according to the present invention may consist of a cylindrical member 1 made of the ZrO 2 porcelain and a ZrO 2 porcelain flexible member 4 which is composed by disposing a contact part 3 on at least one surface of a plate like member 2 consisting of the substrate as shown in FIG. 1 ( a ), and have an integral structure obtained by integrating the ZrO 2 porcelain flexible member 4 with the cylindrical member 1 which is a supporting member for the flexible member so that an end surface of the cylindrical 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 cylindrical member 1 .

Furthermore, the ZrO 2 porcelain according to the present invention may consist of the cylindrical 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 cylindrical member 1 which is the supporting member for the flexible member so that an end surface of the cylindrical member 1 is brought into contact with the plate like member 2 and the contact part 3 closes an opening of the cylindrical 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 porcelain 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 cylindrical 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 porcelain 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 cylindrical 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 porcelain according to the present invention which has an integral structure as described above, a layer 8 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% may further be disposed on the inside walls of a space 7 formed by he ZrO 2 porcelain flexible member 4 and the cylindrical 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 cylindrical member 1 which is liable to be cracked due to stresses.

In the ZrO 2 porcelain according to the present invention, a layer 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% may be disposed only on inside walls of a space 7 formed by closing the opening of the cylindrical 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 porcelain according to the present invention can be further enhanced as a whole by composing the cylindrical 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 substrate or the cylindrical member and lower than the contact part or the connecting layer at least at a location between the substrate or the cylindrical 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 the cubic crystal at a ratio not lower than 60% and not higher than 95% on a premise that the intermediate ZrO 2 layer contains the cubic crystal at a ratio higher than the substrate or the cylindrical 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 mole %, preferably 5.5 to 6.5 mol %.

In the ZrO 2 porcelain according to the present invention, a cubic crystal content of the cylindrical 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 porcelain is, for example, a ZrO 2 porcelain consisting of a ZrO 2 porcelain flexible member 4 which comprises a substrate, a contact part 3 and an intermediate layer 9 , and a cylindrical member 1 as shown in FIG. 1 ( a ), wherein the cylindrical member 1 contains a cubic crystal 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 cylindrical member 1 at 5 to 7 mol %, preferably 5.5 to 6.5 mol %.

In the ZrO 2 porcelain according to the present invention, the cylindrical member may have a relative density (bulk density/theoretical density) of 95% or higher and contain a cubic crystal at 75% or higher. Such a cylindrical member can enhance erosion resistance of the ZrO 2 porcelain. In this case, the cylindrical member contains yttria at 6 to 15 mol %, preferably 8 to 10 mol %. In this case, however, toughness of the cylindrical member is lowered, whereby strength of the ZrO 2 porcelain may be insufficient. Accordingly, it is preferable to use the cylindrical member in combination with a cover member which is described later.

Strength of the ZrO 2 porcelain according to the present invention as a whole can be enhanced by attaching a cover member 12 to an end surface of the cylindrical 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 cylindrical member 1 and integrating the cover member 12 with the cylindrical member 1 so that a hole 13 formed in the cover member 12 communicates with the internal space of the cylindrical 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 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 for enhancing strength of the sensor furthermore.

A ZrO 2 porcelain 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 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 porcelain 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 of the ZrO 2 porcelain flexible member.

A fluid sensor which uses the ZrO 2 porcelain 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 23 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 22 are 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 cylindrical member 1 has a relative density (bulk density/theoretical density) not lower than 95% and contains a cubic crystal 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 cylindrical 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 cylindrical member 1 as shown in FIG. 8 ( b ).

When the ZrO 2 porcelain 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.

The present invention will be described in more detail below with reference to its examples which are presented not in limitative sense.