JP6964306B2 - Piezoelectric material, its manufacturing method, piezoelectric element and combustion pressure sensor - Google Patents

Description

The present invention relates to a piezoelectric material, a method for producing the same, a piezoelectric element, and a combustion pressure sensor. It relates to the piezoelectric element and the combustion pressure sensor.

In order to improve fuel efficiency and _{reduce CO 2} / NO _x , research and development of automobiles with internal combustion engines have been carried out. In recent years, a combustion pressure sensor mounted on an internal combustion engine has attracted attention for this purpose. Since the combustion pressure sensor directly detects torque fluctuations, _{it is possible to achieve both low torque fluctuations and low CO 2} / NO _x. Therefore, improvement in fuel efficiency and _{reduction in CO 2} / NO _x are expected.

Such a combustion pressure sensor is made of a piezoelectric material, and as the piezoelectric material, La ₃ Ga ₅ SiO ₁₄ (langasite), a langasite type crystal having the same crystal structure as the crystal structure of langasite, and the like are known. Has been done.

Since langasite and langasite-type crystals do not undergo a phase transition up to the melting point and do not have pyroelectricity, there is little deterioration in characteristics at high temperatures, and electromotive force is generated in response to temperature changes derived from pyroelectricity. It has the feature that there is no risk of dielectric breakdown due to electrical signal disturbance or pyroelectric voltage. However, even in these Langasite and Langasite type crystals, the electrical resistivity at high temperature is not sufficient for use in an in-vehicle combustion pressure sensor.

Recently, among Langasite-type crystals, Ca ₃ TaAl ₃ Si ₂ O ₁₄ (CTAS) has been attracting attention as a preferable material for high-temperature sensing applications, and CTAS, which exhibits high electrical resistivity and good piezoelectric properties at high temperatures, has been reported. (See, for example, Non-Patent Document 1).

On the other hand, it is expected that the piezoelectric characteristics of CTAS will be further improved by substituting Ca of CTAS with Sr. _{For example, in Non-Patent Document 2, about Sr 3} TaAl ₃ Si ₂ O ₁₄ (STAS) having a composition in which Ca of CTAS is replaced with 100% Sr, the crystal structure and the piezoelectric characteristics such as the piezoelectric _{constant d 11 are simulated.} Results have been reported.

However, there have been no reports of actually producing STAS and confirming its piezoelectric characteristics. Further, Non-Patent Document 2 only describes STAS, and does not describe the relationship between the ratio of CTAS Ca replaced by Sr and the piezoelectric characteristics, and there is room for further research.

X. W. Fu et al., Cryst. Growth Des. , 16, 2151 (2016) Yan-qing ZHENG et al., "Advances in design, growth and application of piezoelectric crystals with langasite strucure", IEEE 2012

An object of the present invention is to provide a piezoelectric material for a combustion pressure sensor, a method for producing the same, and a combustion pressure sensor using the same.

The piezoelectric material according to the present invention is a piezoelectric material composed of crystals containing Sr, Ca, Ta, Al, Si and O, and the crystals have a crystal structure of langasite represented by _{La 3} Ga ₅ SiO _14. The crystal has the same crystal structure as that of Sr _x Ca _3-x TaAl ₃ Si ₂ O ₁₄ , and the parameter x is 0 <x <3, thereby solving the above-mentioned problem.
The crystal may be a single crystal.
The parameter x may be 0 <x ≦ 1.5.
The electrical resistivity of the crystal at 400 ° C. may be in the range of ^{1.0 × 10 10} Ω · cm or more and 9.0 × 10 ^{10 Ω · cm or less.}
The electrical resistivity of the crystal at 400 ° C. may be in the range of ^{3.0 × 10 10} Ω · cm or more and 9.0 × 10 ^{10 Ω · cm or less.}

The above-mentioned method for producing a piezoelectric material according to the present invention includes a step of melting a raw material containing Sr, Ca, Ta, Al, Si and O, and a seed crystal in the melt of the raw material obtained in the melting step. The step of melting and the step of pulling up are carried out under an inert gas, and the raw material is related to Sr, Ca, Ta, Al and Si in the raw material. It is prepared so as to satisfy Sr: Ca: Ta: Al: Si = x: (3-x): 1: 3: 2 (atomic number ratio), thereby solving the above-mentioned problems.
The oxygen content in the inert gas may satisfy the range of 0% by volume or more and 1.5% by volume or less.
The oxygen content in the inert gas may be 0.5% by volume or more.
The raw material is filled in an Ir crucible, and the melting step and the pulling step may be performed under an inert gas having an oxygen content of 0% by volume or more and 1.2% by volume or less.
It may also further include the step of heat treatment in an inert gas or air.
The heat treatment step may be performed in an inert gas in a temperature range of 1150 ° C. or higher and 1250 ° C. or lower for a time of 2 hours or more and 24 hours or less.

In the piezoelectric element provided with the piezoelectric material according to the present invention, the piezoelectric material is the above-mentioned piezoelectric material, thereby solving the above-mentioned problems.
In the combustion pressure sensor provided with the piezoelectric element according to the present invention, the piezoelectric element is the above-mentioned piezoelectric element, thereby solving the above-mentioned problems.

The piezoelectric material according to the present invention is composed of crystals containing Sr, Ca, Ta, Al, Si and O, and has the same crystal structure as the crystal structure of Langasite represented by _{La 3} Ga ₅ SiO _14. It is represented by the general formula Sr _x Ca _3-x TaAl ₃ Si ₂ O ₁₄ , and the parameter x is 0 <x <3. Therefore, there is little deterioration in characteristics at high temperatures, which is advantageous for piezoelectric elements used at high temperatures. Further, the piezoelectric material according to the present invention is a crystal having an ^{electrical resistivity of more than 1.0 × 10 10 Ω · cm at 400 ° C.} As a result, it is advantageous for a combustion pressure sensor equipped with a piezoelectric element in an internal combustion engine or the like used at a high temperature.

Schematic diagram of a single crystal growing apparatus used in the method for producing a piezoelectric material composed of a single crystal of the present invention. Schematic diagram showing the piezoelectric element of the present invention The figure which shows the observation result of the SCTAS single crystal of Example 1. The figure which shows the powder X-ray diffraction chart of the SCTAS single crystal of Example 1. The figure which shows the transmission spectrum of the SCTAS single crystal of Example 1. The figure which shows the temperature dependence of the electrical resistivity of the SCTAS single crystal of Example 1 and the CTAS single crystal of Comparative Example 1.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The same elements are given the same numbers, and the description thereof will be omitted.

First, the piezoelectric material of the present invention and a method for producing the same will be described.

The piezoelectric material of the present invention comprises crystals containing Sr, Ca, Ta, Al, Si and O. This crystal has the same crystal structure as the crystal structure of langasite represented by _{La 3} Ga ₅ SiO _14. Specifically, this crystal has a trigonal crystal structure and belongs to the point group 32 and the space group P321. As described above, since the crystal constituting the piezoelectric material of the present invention (simply referred to as the crystal of the present invention) has the same crystal structure as the crystal structure of langasite, the phase transition occurs up to the melting point as in the case of langasite. Since it does not have pyroelectricity, there is little deterioration in characteristics at high temperatures. Furthermore, since the crystal of the present invention has the same crystal structure as the crystal structure of Langasite, it has a high piezoelectric constant like Langasite. This makes the crystals of the present invention advantageous for piezoelectric elements used at high temperatures.

The crystal of the present invention is represented by the general formula Sr _x Ca _3-x TaAl ₃ Si ₂ O ₁₄ , and the parameter x is 0 <x <3. In the present specification, the crystal of the present invention may be referred to as SCTAS crystal. The value of the parameter x can be appropriately set in consideration of desired characteristics according to the application of the SCTAS crystal, the viewpoint of ease of manufacturing the SCTAS crystal, and the like. For example, 0.001 ≦ x ≦ 2.999 The range of 0.001 ≦ x ≦ 2.0, the range of 0.01 ≦ x ≦ 1.5, the range of 0.01 ≦ x ≦ 1.0, and the like. For example, from the viewpoint of forming an SCTAS crystal having better piezoelectric properties (for example, _{showing a larger value of the piezoelectric constant d 11} under predetermined conditions), it is preferable that the value of x is closer to 3. On the other hand, from the viewpoint of producing SCTAS crystals with stable quality and safety, it is preferable to set the value of x to 1.5 or less. In this case, the value of x is, for example, x = 1.5, 1.0, 0.75, 0.5, 0.25, 0.1, 0.075, 0.05, 0.025, 0. 001, but not limited to these.

Further, the piezoelectric material of the present invention has an electrical resistivity at 400 ° C. ^{exceeding 1.0 × 10 10} Ω · cm. The electrical resistivity may be measured by, for example, the three-terminal method. This value clears the specifications of the combustion pressure sensor equipped with the piezoelectric element in an internal combustion engine or the like used at a high temperature.

The crystal of the present invention is preferably a single crystal. Thereby, the above-mentioned excellent piezoelectric characteristics and excellent electrical resistivity can be achieved.

Next, an exemplary method for producing the piezoelectric material composed of crystals of the present invention will be described, but it should be noted that this is only an example. Hereinafter, a case where the crystal of the present invention, particularly a single crystal, is produced by the Czochralski method (CZ method) will be described.

FIG. 1 is a schematic view of a single crystal growing apparatus used in the method for producing a piezoelectric material made of a single crystal of the present invention.

FIG. 1 shows a crystal pulling furnace 20 used for producing a single crystal. The crystal pulling furnace 20 includes an iridium (Ir) or platinum (Pt) crucible 21, a ceramic tubular container 22 for accommodating the crucible 21, and a high-frequency coil 23 wound around the tubular container 22. Mainly equipped with. The high-frequency coil 23 generates an induced current in the crucible 21 to heat the crucible 21. A single crystal to be the above-mentioned piezoelectric material is grown by the CZ method using the crystal pulling furnace 20. The manufacturing process will be described in detail.

Step S110: The raw material containing Sr, Ca, Ta, Al, Si and O is melted.

First, SrCO ₃ powder, CaCO ₃ powder, Ta ₂ O ₅ powder, Al ₂ O ₃ powder, and SiO ₂ powder are dry-mixed, the raw material powder is calcined in the air, and Sr, Ca, Ta, Al, and Si are fired. A raw material containing and O is prepared. Regarding the mixing ratio of the raw material powder, Sr, Ca, Ta, Al and Si satisfy the relationship Sr: Ca: Ta: Al: Si = x: (3-x): 1: 3: 2 (atomic number ratio). The calcined product also satisfies the same atomic number ratio. As a result, the finally obtained single crystal can have the same crystal structure as the crystal structure of Langasite. The type of raw material powder is not limited to this, but the above-mentioned oxides and carbonates are preferable because they are available and easy to handle.

Next, the crucible 21 is filled with the fired body. A high-frequency current is applied to the high-frequency coil 23 to heat the rutsubo 21, and the fired body in the rutsubo 21 is heated from room temperature to a temperature at which the raw material powder can be melted. As a result, the fired body is melted and the melt 24 is obtained.

Step S120: The seed crystal is brought into contact with the melt of the raw material obtained in step S110 and pulled up.

The seed crystal to be brought into contact with the melt 24 is, for example, a seed crystal 25 used as a rod-shaped crystal pulling shaft. Examples of the seed crystal 25 include langasite represented by _{La 3} Ga ₅ SiO ₁₄ and langasite type single crystal such as _{Ca 3} TaAl ₃ Si ₂ O ₁₄ (CTAS) and Ca _{3 TaGa 3} Si ₂ O _{14 (CTGS).} Crystals can be used.

After the tip of the seed crystal 25 is brought into contact with the melt 24, the seed crystal 25 is pulled up at a predetermined pulling speed while rotating at a predetermined rotation speed. The rotation speed of the seed crystal 25 is preferably 3 to 50 rpm, more preferably 10 to 15 rpm. The pulling speed of the seed crystal 25 is preferably 0.1 to 10 mm / h, and more preferably 0.3 to 0.8 mm / h.

Steps S110 and S120 are carried out under the inert gas, and the oxygen content in the inert gas satisfies the range of 0% by volume or more and 1.5% by volume or less. When oxygen exceeding 1.5% by volume is contained, the amount of oxygen is too large and the oxidation of the rutsubo becomes large / oxygen defects cannot be controlled. Therefore, ^{electrical resistance exceeding 1.0 × 10 10} Ω · cm at 400 ° C. A single crystal having a resistivity cannot be obtained. Further, as the inert gas, N ₂ or a rare gas such as He, Ne, Ar can be used, but N ₂ is preferably advantageous from the viewpoint of controlling the amount of oxygen contained. The oxygen content of the inert gas is preferably 0.5% by volume or more. As a result, oxidation of the crucible is suppressed and oxygen defects are controlled to a predetermined amount, so that a high electrical resistivity can be achieved. The oxygen content is more preferably 1% by volume or more. The crucible 21 and the like of the crystal pulling furnace 20 may be sealed in a sealed housing to control the atmosphere.

Preferably, in the production method of the present invention, an Ir crucible is adopted as the crucible 21, and the steps S110 and S120 are under an inert gas satisfying the oxygen content in the range of 0% by volume or more and 1.2% by volume or less. Will be done. This makes it possible to achieve a particularly high electrical resistivity. By using an Ir crucible rather than a Pt crucible, the influence of impurities can be reduced and a good quality single crystal can be obtained.

By pulling up the seed crystal 25 in this way, a bulk-shaped grown crystal 26 can be obtained at the tip of the seed crystal 25. The grown crystal 26 is a piezoelectric material containing the above-mentioned Sr, Ca, Ta, Al, Si, and O, and is composed of a single crystal having the same crystal structure as the crystal structure of Langasite. By cutting out the obtained grown crystal 26 to a desired size, it can be used for a piezoelectric element described later.

Through the above steps, the grown crystal 26, which is a piezoelectric material made of the single crystal of the present invention, can be easily produced, and the grown crystal 26 can be easily increased in size.

The single crystal of the present invention is not limited to the CZ method described with reference to FIG. 1, and includes Edge Defined Film Fed Growth Method (EFG method), Bridgeman method, floating zone method (FZ method), Verneuil method, and the like. It can be obtained by the liquid phase growth method.

By heat-treating the obtained single crystal in an inert gas such as nitrogen or in the atmosphere, the electrical resistivity can be further improved. The heat treatment conditions are carried out in an inert gas or in the air at a temperature lower than the single crystal growth temperature in the range of 2 hours or more and 24 hours or less. This improves the electrical resistivity of the SCTAS single crystal. More preferably, the heat treatment conditions are carried out in nitrogen as an inert gas in a temperature range of 1150 ° C. or higher and 1250 ° C. or lower, and in a range of 2 hours or more and 24 hours or less. As a result, oxygen defects in the single crystal are controlled, and the electrical resistivity is surely improved.

Next, the piezoelectric element using the piezoelectric material of the present invention will be described.
FIG. 2 is a schematic view showing the piezoelectric element of the present invention.

The piezoelectric element 200 includes the piezoelectric material 210 of the present invention. Since the piezoelectric material 210 is a piezoelectric material made of the above-mentioned crystals, the description thereof will be omitted. In FIG. 2, it is assumed that the piezoelectric material is a single crystal. An electrode 220 is provided on one surface of the piezoelectric material 210, and an electrode 230 is provided on the opposite surface. The electrodes 220 and 230 are arranged on the y-plane of the single crystal constituting the piezoelectric material 210. When a stress is applied in the direction perpendicular to the surface of the piezoelectric material 210y of the piezoelectric element 200, an electric charge is generated and output via the electrodes 220 and 230. Since the piezoelectric element 200 of the present invention includes the above-mentioned piezoelectric material made of a single crystal, it has not only a high piezoelectric constant but also a high electrical resistivity even at a high temperature. Therefore, if such a piezoelectric element 200 is incorporated in an appropriate housing and installed in a combustion chamber having a high temperature such as an internal combustion engine, it functions as a combustion pressure sensor. That is, it can be attached to the cylinder of the engine and the combustion pressure of the engine can be detected as a minute change in electric charge. However, the design of the device is not necessarily limited to the y-plane.

Next, the present invention will be described in detail with reference to specific examples, but it should be noted that the present invention is not limited to these examples.

[Example 1]
In Example 1, a _{piezoelectric material made of SCTAS single crystal having a parameter x of the general formula Sr x} Ca _3-x TaAl ₃ Si ₂ O ₁₄ of 0.75 is produced by the CZ method described with reference to FIG. bottom.

First, a raw material containing Sr, Ca, Ta, Al, Si and O was prepared. Specifically, 99.99% pure strontium carbonate (SrCO ₃ ) raw material powder (44.583 g), 99.99% pure calcium carbonate (CaCO ₃ ) raw material powder (90.668 g), and 99. 99% tantalum oxide (Ta ₂ O ₅ ) raw material powder (88.955 g), 99.99% pure aluminum oxide (Al ₂ O ₃ ) raw material powder (61.576 g), and 99.99% pure dioxide Silicon (SiO ₂ ) raw material powder (48.377 g) was dry-mixed and placed in an aluminal pot to obtain a fired body.

Sr, Ca, Ta, Al and Si in the mixed raw material powder satisfied Sr: Ca: Ta: Al: Si = 0.75: 2.25: 1: 3: 2 (atomic ratio). The firing temperature profile was raised from 0 ° C. to 1260 ° C. over 8 hours, held at a temperature of 1260 ° C. for 25 hours, and then lowered to 0 ° C. over 8 hours. When the obtained calcined product was subjected to powder X-ray diffraction, it was confirmed that it had the same crystal structure as the crystal structure of Langasite and was obtained in a single phase. Further, Sr, Ca, Ta, Al and Si in the obtained fired body satisfy Sr: Ca: Ta: Al: Si = 0.75: 2.25: 1.25: 1: 3: 2 (atomic ratio). It was confirmed. The fired product thus obtained was used as a raw material containing Sr, Ca, Ta, Al, Si and O.

The fired body thus obtained was filled in an Ir crucible and melted (step S110). The shape of the crucible was cylindrical, with a diameter of about 50 mm and a height of about 50 mm.

Next, the tip of a CTGS square rod-shaped seed crystal (size: 2 mm × 2 mm × 40 mm, a-axis) represented by _{Ca 3} TaGa ₃ Si ₂ O _{14 is brought into contact with the melt, and the seed crystal is rotated at 12 rpm.} The seed crystal was pulled up at a speed of 0.5 mm per hour while rotating by a number, and a bulk single crystal was grown (step S120). After the growth was completed, it took 40 hours to cool to room temperature.

The growth of these crystals was carried out in an _{N 2} gas atmosphere containing oxygen as an inert gas. The oxygen content was 1.0% by volume. The appearance of the obtained single crystal was observed. The results are shown in FIG.

Next, powder X-ray diffraction (XRD) was performed on the obtained single crystal. The results are shown in FIG. Note that FIG. 4 shows side-by-side simulated diffraction charts of CTAS obtained from databases generally available in the art for comparison.

The composition of the single crystal (molar ratio of Sr, Ca, Ta, Al and Si) was confirmed by ICP emission spectroscopic analysis. The results are shown in Table 1. As a result, it was confirmed that an SCTAS single crystal having a composition almost matching the designed composition was obtained.

A 10 mm square, 1 mm thick y-cut plate was cut out from the single crystal, and both sides were mirror-polished. The transmission spectrum of this sample was measured. The results are shown in FIG.

Further, as described with reference to FIG. 2, a platinum electrode was formed on the y-plane of this sample by sputtering to manufacture a piezoelectric element. The temperature dependence of the electrical resistivity of this piezoelectric element was measured by the three-terminal method, and the piezoelectric characteristics (relative permittivity and piezoelectric constant d ₁₁ ) were measured by an impedance analyzer and an LCR meter. The results are shown in FIG. 6 and Table 2.

[Comparative Example 1]
In Comparative Example 1, a piezoelectric material composed of a CTAS single crystal was produced by the CZ method described with reference to FIG. 1 in the same manner as in Example 1.

First, a raw material containing Ca, Ta, Al, Si and O was prepared. Specifically, 99.99% pure calcium carbonate (CaCO ₃ ) raw material powder (120.143 g), 99.99% pure tantalum oxide (Ta ₂ O ₅ ) raw material powder (88.404 g), and purity. 99.99% aluminum oxide (Al ₂ O ₃ ) raw material powder (61.218 g) and 99.99% pure silicon dioxide (SiO ₂ ) raw material powder (48.082 g) are dry-mixed into an alumina pentoxide pot. Arranged to obtain a fired body.

Ca, Ta, Al and Si in the mixed raw material powder satisfied Ca: Ta: Al: Si = 3: 1: 3: 2 (atomic ratio). The firing temperature profile was raised from 0 ° C. to 1300 ° C. over 8 hours, held at a temperature of 1300 ° C. for 20 hours, and then lowered to 0 ° C. over 8 hours. When the obtained calcined product was subjected to powder X-ray diffraction, it was confirmed that it had the same crystal structure as the crystal structure of Langasite and was obtained in a single phase. Further, it was confirmed that Ca, Ta, Al and Si in the obtained fired body satisfy Ca: Ta: Al: Si = 3: 1: 3: 2 (atomic ratio). The fired body thus obtained was used as a raw material containing Ca, Ta, Al, Si and O.

The fired body thus obtained was filled in a Pt crucible and melted (step S110). The weight of the raw material powder filled in the crucible was 262 g. The shape of the crucible was cylindrical, with a diameter of about 50 mm and a height of about 50 mm.

Next, the tip of a CTAS square rod-shaped seed crystal (size: 2 mm × 2 mm × 40 mm, a-axis) represented by _{Ca 3} TaAl ₃ Si ₂ O _{14 is brought into contact with the melt, and the seed crystal is rotated at 12 rpm.} The seed crystal was pulled up at a speed of 0.5 mm per hour while rotating by a number, and a bulk single crystal was grown (step S120). After the growth was completed, it took 40 hours to cool to room temperature.

The growth of these crystals was carried out in an _{N 2} gas atmosphere containing oxygen as an inert gas. The oxygen content was 1.2% by volume. The appearance of the obtained single crystal was observed. Next, when the obtained single crystal was subjected to powder X-ray diffraction, it was confirmed that it had the same crystal structure as the crystal structure of Langasite and was obtained in a single phase.

The composition of the single crystal (molar ratio of Ca, Ta, Al and Si) was confirmed by XRF, and it was confirmed that a CTAS single crystal that substantially matched the designed composition was obtained.

A 10 mm square, 1 mm thick y-cut plate was cut out from the single crystal, and both sides were mirror-polished. The transmission spectrum of this sample was measured. Further, as described with reference to FIG. 2, a platinum electrode was formed on the y-plane of this sample by sputtering to manufacture a piezoelectric element. The temperature dependence of the electrical resistivity of this piezoelectric element was measured by the three-terminal method, and the piezoelectric characteristics (relative permittivity and piezoelectric constant d ₁₁ ) were measured by an impedance analyzer and an LCR meter. The results are shown in FIG. 6 and Table 2.

First, the appearance of the SCTAS single crystal of Example 1 will be described.

FIG. 3 is a diagram showing the observation results of the SCTAS single crystal of Example 1. According to FIG. 3, the SCTAS single crystal of Example 1 was a transparent single crystal having a length of 20 mm × 10 mm, a length of 45 mm, and a weight of 22 g. Although not shown, a transparent single crystal was also obtained in Comparative Example 1.

Next, the powder X-ray diffraction chart of the SCTAS single crystal of Example 1 will be described.

FIG. 4 is a diagram showing a powder X-ray diffraction chart of the SCTAS single crystal of Example 1.

According to FIG. 4, the lattice constants of the SCTAS single crystal of Example 1 are a = 8.07 Å and c = 4.94 Å, and the lattice constants of CTAS by simulation (a = 8.02 Å, c = 4.91 Å). ) Is almost the same. From this, it was confirmed that the SCTAS single crystal of Example 1 had the same crystal structure as the crystal structure of Langasite and was obtained in a single phase. Further, in the powder X-ray diffraction chart of the SCTAS single crystal of Example 1, compared with CTAS by simulation, 2θ = 18 °, 39 °, 45 °, 60.5 °, 62 °, 100. Characteristic peak intensities were confirmed in the vicinity of °, while the peak intensities in the vicinity of 2θ = 12.5 ° tended to be weaker than those of CTAS. These peak profiles for SCTAS single crystals are believed to be due to the partial substitution of Ca in CTAS with Sr.

Next, the transmission spectrum of the SCTAS single crystal of Example 1 will be described.

FIG. 5 is a diagram showing a transmission spectrum of the SCTAS single crystal of Example 1.

According to FIG. 5, in the SCTAS single crystal of Example 1, both the a-axis and the c-axis showed almost the same transmission spectrum, but the a-axis showed slight absorption at a wavelength of 250 to 400 nm.

Next, the electrical resistivity and the piezoelectric characteristics of the SCTAS single crystal of Example 1 and the CTAS single crystal of Comparative Example 1 will be described.

FIG. 6 is a diagram showing the temperature dependence of the electrical resistivity of the SCTAS single crystal of Example 1 and the CTAS single crystal of Comparative Example 1.

According to FIG. 6, the electrical resistivity of the SCTAS single crystal of Example 1 was slightly lower than that of the CTAS single crystal of Comparative Example 1 in the measured total temperature range (400 ° C. to 800 ° C.). , It was about the same. In particular, as the SCTAS single crystal of Example 1, a single crystal having an electrical resistivity in ^{the range of 3.0 × 10 10} Ω · cm or more and 9.0 × 10 ¹⁰ Ω · cm or less at 400 ° C. can be obtained, and a piezoelectric sensor can be obtained. It was confirmed that it would be a piezoelectric material for use. The activation energies Ea of the SCTAS single crystal of Example 1 and the CTAS single crystal of Comparative Example 1 were 1.34 eV and 1.44 eV, respectively.
Further, in FIG. 6, for comparison, the electrical resistivity of the LTGA single crystal, which is an example of the conventional Langasite-type crystal, is shown. From this, the electrical resistivity of the SCTAS single crystal of Example 1 was much higher than that of the LTGA single crystal, and the difference in electrical resistivity at 400 ° C. was particularly remarkable. The activation energy Ea of the LTGA single crystal was 0.82 eV.

Table 2 below, at room temperature (25 ° C.), the results of measuring the dielectric constant and piezoelectric constant _{d 11} of SCTAS monocrystalline and of Comparative Example 1 CTAS single crystal of Example 1.

According to Table 2, the relative permittivity of the SCTAS single crystal of Example 1 at 25 ° C. was about 3.2% lower than that of the CTAS single crystal of Comparative Example 1. _{Further, the piezoelectric constant d 11} of the SCTAS single crystal of Example 1 at 25 ° C. was about 7.6% higher than that of the CTAS single crystal of Comparative Example 1.

From these results, it was confirmed that the SCTAS single crystal of Example 1 of the present invention has a low relative permittivity and a high piezoelectric constant, and is a piezoelectric material. Further, it was found that the electrical resistivity of the SCTAS single crystal of Example 1 of the present invention at 400 ° C. ^{exceeded 1.0 × 10 10} Ω · cm and could withstand practical use as a piezoelectric material for a piezoelectric sensor.

The piezoelectric material composed of a single crystal containing Sr, Ca, Ta, Al, Si, and O of the present invention has a high electrical resistivity at a high temperature and is advantageous for use at a high temperature, and thus has a combustion pressure. It is suitable for a piezoelectric sensor mounted on a sensor.

20 Crystal pulling furnace 21 Rutsubo 22 Cylindrical container 23 High frequency coil 24 Melt 25 Seed crystal 26 Growing crystal 200 Piezoelectric element 210 Single crystal 220, 230 Electrode

Claims (12)

A piezoelectric material composed of crystals containing Sr, Ca, Ta, Al, Si, and O.
The crystal has the same crystal structure as the crystal structure of langasite represented by _{La 3} Ga ₅ SiO _14.
The crystals are represented by the general formula _{Sr x Ca 3-x TaAl 3} Si 2 O 14, parameter x Ri is 0 <x <3 der,
A piezoelectric material in which the electrical resistivity of the crystal at 400 ° C. is in the range of 1.0 × 10 ¹⁰ Ω · cm or more and 9.0 × 10 ¹⁰ Ω · cm or less. The piezoelectric material according to claim 1, wherein the crystal is a single crystal. The piezoelectric material according to claim 1, wherein the parameter x is 0 <x ≦ 1.5. The piezoelectric material according to claim 1 , wherein the electrical resistivity of the crystal at 400 ° C. is in the range of 3.0 × 10 ¹⁰ Ω · cm or more and 9.0 × 10 ¹⁰ Ω · cm or less. The method for producing a piezoelectric material according to any one of claims 1 to 4.
A step of melting a raw material containing Sr, Ca, Ta, Al, Si, and O,
Including the step of bringing the seed crystal into contact with the melt of the raw material obtained in the step of melting and pulling it up.
The melting step and the pulling step are performed under an inert gas.
In the raw material, Sr, Ca, Ta, Al, and Si in the raw material have a relationship of Sr: Ca: Ta: Al: Si = x: (3-x): 1: 3: 2 (atomic number ratio). A method that is prepared to meet. The method according to claim 5 , wherein the oxygen content in the inert gas satisfies the range of 0% by volume or more and 1.5% by volume or less. The method according to claim 6 , wherein the oxygen content in the inert gas is 0.5% by volume or more. The raw material is filled in an Ir crucible and
The method according to claim 5 , wherein the melting step and the pulling step are performed under an inert gas having an oxygen content of 0% by volume or more and 1.2% by volume or less. The method of claim 5 , further comprising the step of heat treating with an inert gas or air. The method according to claim 9 , wherein the heat treatment step is performed in an inert gas in a temperature range of 1150 ° C. or higher and 1250 ° C. or lower for a time of 2 hours or more and 24 hours or less. A piezoelectric element equipped with a piezoelectric material.
The piezoelectric element, which is the piezoelectric material according to any one of claims 1 to 4. A combustion pressure sensor equipped with a piezoelectric element
The piezoelectric element is a combustion pressure sensor according to claim 11.

Images