JP2008179532A - Piezoelectric ceramic material and piezoelectric element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a non-lead piezoelectric ceramic material where the amount of generated electric charges per pressure of 1 N when a positive piezoelectric effect is utilized can be increased and wherein the temperature stability of the electric charge generated in the range of -40 to 200°C relative to that at 25°C is excellent, and to provide a piezoelectric element. <P>SOLUTION: The piezoelectric ceramic material containing Mn of 0.05-1.5 pts.mass in terms of MnO<SB>2</SB>to 100 pts.mass of a main component in a bismuth layered compound whose compositional formula is denoted as Bi<SB>4</SB>Ti<SB>3</SB>O<SB>12</SB>-xä(Sr<SB>1-a</SB>Ca<SB>a</SB>)<SB>(1-b)</SB>Ba<SB>b</SB>TiO<SB>3</SB>} (wherein, 1.30≤x≤1.75, 0.40≤a≤0.60, 0≤b≤0.20) is used. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a piezoelectric ceramic and a piezoelectric element, and is suitable for, for example, a resonator, an ultrasonic vibrator, an ultrasonic motor, or a piezoelectric sensor such as an acceleration sensor, a knocking sensor, and an AE sensor. Piezoelectric ceramics suitably used as pressure sensor elements that utilize high-frequency resonators and positive piezoelectric effects (effects in which electrical polarization occurs when stress is applied to an object and charges are generated on the object surface) And a piezoelectric element.

  Conventionally, products using a piezoelectric ceramic include, for example, a filter, a piezoelectric resonator, an ultrasonic vibrator (hereinafter referred to as a concept including an oscillator), an ultrasonic motor, a piezoelectric sensor, and the like.

  In recent years, piezoelectric elements are incorporated in parts such as automobile engines and suspensions, and the pressure applied to the piezoelectric elements is sensed using the positive piezoelectric effect and used for engine combustion control and vehicle body attitude control. In particular, as a piezoelectric element used for engine control, there is an anti-knock sensor among lean burn type engines that are widely used for the purpose of cleaning exhaust gas and improving fuel consumption. In addition, piezo-electric elements are being used for the measurement of combustion pressure for the purpose of stable combustion of dilute gas among HCCL (Homogenous Charge Compression Ignition) engines that do not use combustion plugs, which are being considered as next-generation engines. Has been.

  Since these piezoelectric elements are mounted in an engine room, element materials having high heat resistance are required. Furthermore, in order to improve fuel efficiency, it is necessary to precisely measure the pressure in the engine cylinder and perform fine lean burn control, so output signal characteristics (pressure characteristics of generated charge) with respect to sensor pressure and temperature changes. Further, an element material having a small change in temperature characteristics) is required.

  Conventionally, a resonator or a pressure sensor element has a high piezoelectricity, for example, a PT (lead titanate) -based material or a PZT (lead zirconate titanate) -based material capable of obtaining a large generated charge with respect to a large P / V or pressure. It was used. However, since PZT and PT-based materials contain about 60% by mass of lead, the elution of lead is caused by acid rain, and there is a risk of causing environmental pollution. Therefore, high expectations are placed on piezoelectric materials that do not contain lead.

  In addition, since PZT and PT-based materials have a Curie temperature Tc of about 200 to 300 ° C, the piezoelectric constant deteriorates when used at a high temperature of about 200 ° C. There was a great limitation in the use because of the large change in the piezoelectric constant. For example, when used as a pressure sensor, if the piezoelectric constant deteriorates with time, the output voltage changes even at the same pressure. If the change in the piezoelectric constant at 200 ° C. relative to the piezoelectric constant at room temperature is large, the pressure and output voltage Is not linear, it is difficult to calculate an accurate pressure from the output voltage.

  Therefore, a material mainly composed of a bismuth layered compound has been proposed as a piezoelectric ceramic composition not containing lead (for example, Patent Document 1).

  Many materials mainly composed of bismuth layered compounds have a Curie temperature of 400 ° C or higher, and such materials have high heat resistance and are used in environments exposed to high temperatures such as in an engine room. There is a possibility of application as an element.

  In addition, a material mainly composed of a bismuth layered compound has a characteristic that the mechanical quality factor (Qm) is relatively high as compared with PZT and PT materials, and can be applied as a piezoelectric material for a resonator. . The piezoelectric element is used as a resonator in a reference signal oscillator of a computer, for example, incorporated in an oscillation circuit such as a Colpitts oscillation circuit. FIG. 1 shows a Pierce oscillation circuit in which the inductor portion is replaced with a piezoelectric resonator in a circuit configuration based on the Colpitts oscillation circuit. This Pierce oscillation circuit includes capacitors 111 and 112, a resistor 113, an inverter 114, and a resonator 115. In order to generate an oscillation signal in the Pierce oscillation circuit, it is necessary to satisfy the following oscillation conditions.

That is, the amplification factor in the amplifier circuit composed of the inverter 114 and the resistor 113 is α, the phase amount is θ ₁ , the feedback factor in the feedback circuit composed of the resonator 115 and the capacitors 111 and 112 is β, and the phase amount is θ ₂ . Then, it is necessary that the loop gain is α × β ≧ 1 and the phase amount is θ ₁ + θ ₂ = 360 ° × n (where n = 1, 2,...).

In general, an amplifier circuit including a resistor 113 and an inverter 114 is built in a computer, and in order to obtain stable oscillation without causing erroneous oscillation or non-oscillation, the loop gain must be increased. To increase the loop gain determines the gain of the feedback factor beta, P / V (peak valley value) of the resonator 115, i.e., is necessary to increase the difference in resonance impedance R ₀ and anti-resonance impedance R _a Become. P / V is defined as a value of 20 × log (R _a / R ₀ ).
JP 2002-167276 A

  However, in a conventional piezoelectric ceramic mainly composed of a bismuth layer compound containing no lead, when used in a pressure sensor, the generated charge is small and is 15 pC or less per unit pressure of 1 N, and the temperature change of the generated charge is 20% or more ( Therefore, there is a problem that it cannot be used as a pressure sensor element that requires high accuracy.

  Therefore, the present invention can increase the amount of charge generated per 1N of pressure due to the positive piezoelectric effect, and is excellent in temperature stability of generated charge with respect to 25 ° C. in the temperature range of −40 to 200 ° C. An object of the present invention is to provide a piezoelectric ceramic and a piezoelectric element.

The composition of the piezoelectric ceramic according to the present invention is expressed as Bi ₄ Ti ₃ O ₁₂ · x {(Sr _1−a Ca _a ) _1−b Ba _b TiO ₃ }, and 1.30 ≦ x ≦ 1. 75, 0.40 ≦ a ≦ 0.60, and 0 ≦ b ≦ 0.20, 100 parts by mass of the main component of the bismuth layered compound, and 0.05 to 1.5 parts by mass of Mn in terms of MnO ₂ It is characterized by containing.

In the piezoelectric ceramic of the present invention, the amount of spontaneous polarization is preferably 13 μC / cm ² or more, and the amount of change in the amount of spontaneous polarization at −40 ° C. to 200 ° C. with respect to the amount of spontaneous polarization at 25 ° C. is preferably 5% or less.

  The piezoelectric element of the present invention is characterized in that electrodes are respectively formed on the opposing pair of principal surfaces of the piezoelectric ceramic having a pair of opposing principal surfaces.

  The piezoelectric element of the present invention is a pair of electrodes opposed to each other, and is preferably operated by thickness longitudinal vibration.

According to the piezoelectric ceramic of the present invention, the composition formula based on the molar ratio is expressed as Bi ₄ Ti ₃ O ₁₂ · x {(Sr _1−a Ca _a ) _1−b Ba _b TiO ₃ }, and 1.30 ≦ x ≦ 1.75, 0.40 ≦ a ≦ 0.60, and 0 ≦ b ≦ 0.20 satisfying 100 parts by mass of the main component of the bismuth layered compound, Mn is 0.05 to 1.5 in terms of MnO _2. By containing a part by mass, the amount of spontaneous polarization is greatly increased.

  As for the bismuth layered compound, researches are mainly made for those in which x is an integer. When x is an integer, it is considered that when the value of x is large, the number of quasi-perovskite layers in the crystal structure of the bismuth layered compound increases, so that the piezoelectric characteristics increase. However, as the number of layers increases, the Curie temperature decreases and the temperature change of the generated charge due to the positive piezoelectric effect increases, and cannot be used for highly accurate sensor applications.

  The generation of electric charges due to the positive piezoelectric effect is caused by dipoles existing in the crystal, and is related to the spontaneous polarization generated by the dipoles. If the amount of spontaneous polarization is large, the amount of generated charges also increases. Further, the temperature dependence of the generated charge is caused by the temperature change of the spontaneous polarization as described above. In particular, when the number of pseudo-perovskite layers of the bismuth layered compound is an odd number, spontaneous polarization occurs in the c-axis direction. This spontaneous polarization in the c-axis direction tends to decrease as the temperature changes. By setting 1.30 ≦ x ≦ 1.75, in particular, the amount of generated charges can be increased because the amount of spontaneous polarization becomes larger than when x is an integer. Further, when x is 1.30 ≦ x ≦ 1.75, spontaneous polarization in the c-axis direction is added, and the spontaneous polarization becomes larger than when x is an integer, so that the temperature change of the spontaneous polarization of the entire crystal is controlled. It becomes easy. Therefore, the temperature change rate can be suppressed while increasing the amount of generated charge. Further, by setting a to 0.40 ≦ a ≦ 0.60, it is possible to improve the amount of generated charges while improving the Curie temperature, so that the amount of spontaneous polarization increases. Therefore, in the piezoelectric ceramic of the present invention, it is possible to increase the amount of generated charge per applied pressure 1N and decrease the rate of change in temperature of the generated charge amount. As a result, it exhibits useful characteristics as a piezoelectric ceramic for pressure sensors used in high temperature environments, such as a pressure sensor for directly detecting the pressure in a cylinder of a car engine, a shock sensor compatible with SMD, Since no deterioration in piezoelectricity is observed even at high temperatures, it can be used for nonvolatile ferroelectric memory elements that can be used at high temperatures.

According to the piezoelectric ceramic of the present invention, when the amount of spontaneous polarization is 13 μC / cm ² or more and the amount of change in the amount of spontaneous polarization at −40 ° C. to 200 ° C. with respect to the amount of spontaneous polarization at 25 ° C. is 5% or less, Stable piezoelectric characteristics can be obtained even at high temperatures.

  According to the piezoelectric element of the present invention, stable piezoelectric characteristics can be obtained even at high temperatures by forming electrodes on both main surfaces of the piezoelectric ceramic.

  As a result, when the piezoelectric element is used for a pressure sensor utilizing the positive piezoelectric effect, the generated charge amount per 1N of applied pressure can be increased, and the generated charge amount temperature in the temperature range of −40 ° C. to 200 ° C. Properties with excellent stability can be obtained.

  Furthermore, when the piezoelectric element of the present invention is used as a resonator, the P / V of the fundamental wave vibration in the thickness longitudinal slip and the thickness longitudinal vibration is increased and exceeds 10 ° between the resonance frequency and the anti-resonance frequency. The occurrence of phase distortion can be remarkably reduced, the temperature change rate of the resonance frequency is small, and the firing temperature range is widened, so that the generation of different phases due to firing variations is suppressed, and variations in sensitivity of the piezoelectric sensor and P / V characteristic variation can be suppressed, and high yield can be realized.

  According to the resonator of such a piezoelectric element, for example, in an oscillator that operates in the thickness slip and thickness longitudinal vibration modes, P / V increases, so that the oscillation margin increases, and between the resonance frequency and the anti-resonance frequency. Stable oscillation is obtained because phase distortion does not occur at the frequency, high-accuracy oscillation with excellent temperature stability of the oscillation frequency is obtained, and furthermore, the range of firing temperature is widened, so characteristic fluctuations due to firing variations It is possible to obtain a resonator that can be applied to a wide frequency range of 2 to 20 MHz, in which the above is suppressed significantly.

  When used as a piezoelectric resonator, it is possible to increase the P / V in the thickness-shear fundamental wave vibration, the thickness longitudinal fundamental wave, and the third-order overtone vibration.

  Furthermore, the piezoelectric element of the present invention is a pair of electrodes opposed to each other, and has excellent characteristics of temperature stability of oscillation frequency in a temperature range of −20 ° C. to + 80 ° C. when operated by thickness longitudinal vibration. can get.

The composition of the piezoelectric ceramic according to the present invention is expressed as Bi ₄ Ti ₃ O ₁₂ · x {(Sr _1−a Ca _a ) _1−b Ba _b TiO ₃ }, and 1.30 ≦ x ≦ 1. 75, 0.40 ≦ a ≦ 0.60, and 0 ≦ b ≦ 0.20, 100 parts by mass of the main component of the bismuth layered compound, and 0.05 to 1.5 parts by mass of Mn in terms of MnO ₂ It contains.

  Here, the reason why the coefficient x is set in the above range will be described. By setting 1.30 ≦ x ≦ 1.75, in particular, the amount of generated charges can be increased because the amount of spontaneous polarization becomes larger than when x is an integer. Further, in the case of 1.30 ≦ x ≦ 1.75, spontaneous polarization in the c-axis direction is added, and the spontaneous polarization becomes larger than when x is an integer, so that the temperature change of the spontaneous polarization of the entire crystal can be controlled. It becomes easy.

  Next, the reason why the coefficient a is set in the above range will be described. By setting a to 0.40 ≦ a ≦ 0.60, it is possible to improve the amount of generated charges while improving the Curie temperature, so that the amount of spontaneous polarization increases. Therefore, in the piezoelectric ceramic of the present invention, it is possible to increase the amount of generated charge per applied pressure 1N and decrease the rate of change in temperature of the generated charge amount.

  Further, by replacing part of Sr with Ba, the sinterability is improved, and a dense sintered body can be obtained at a lower temperature. Further, as b increases, the amount of generated charges increases. However, as b increases, the temperature fluctuation of the spontaneous polarization amount increases, so b is preferably 0.20 or less. Further, as b increases, the Curie temperature also decreases. In order to reduce the temperature fluctuation of the spontaneous polarization amount and increase the Curie temperature, 0 ≦ b ≦ 0.10 is a more preferable range, and it is particularly preferable to set b = 0.

  Furthermore, when the additive content of Mn is 0.05 parts by mass or more with respect to the main component described above, this material system that is a plate-like crystal can be easily sintered, and a dense porcelain can be obtained and elastic. An increase in loss is suppressed, and the amount of charge generated when pressure is applied does not become lower than 18 pC / N. Moreover, by setting it as 1 mass part or less, it can suppress that a heterogeneous phase is produced | generated and the generated charge amount at the time of a pressure application does not become lower than 18 pC / N.

In the piezoelectric ceramic according to the present invention, the compositional formula is represented by Bi ₄ Ti ₃ O ₁₂ · x {(Sr _1−a Ca _a ) _1−b Ba _b TiO ₃ }, and the main crystal phase is a bismuth layered compound. It consists of That is, the piezoelectric ceramic of the present invention can be expressed as {(Sr _1-a Ca _a ) _1-b Ba _b } _x Bi ₄ Ti _{3 + x} O _{12 + 3x,} and (Bi ₂ O ₂ ) ²⁺ (α _m-1 β _m O _{3m + 1} ) In the general formula of the bismuth layered compound represented by ^2- , the structures of m = 4 and m = 5 are mixed with defects at the α site, β site and oxygen site, and Mn is partially fixed. It is thought that it is a dissolved bismuth layered compound. Mn may be dissolved in the main crystal phase and may partially precipitate as a crystal of the Mn compound at the grain boundary. As other crystal phases, there may be a pyrochlore phase, a perovskite phase, and a bismuth layered compound having a different structure.

Furthermore, since the spontaneous polarization generated by the dipole existing in the crystal is related to the charge due to the positive piezoelectric effect generated due to the dipole, the amount of generated charge increases when the amount of spontaneous polarization is large. The porcelain of the present invention has a spontaneous polarization amount of 13 to 22 μC / cm ² and a change amount of the spontaneous polarization amount at −40 ° C. to 200 ° C. with respect to the spontaneous polarization amount at 25 ° C. is 5% or less. The generated charge can be obtained with a high value of the positive piezoelectric effect.

In the piezoelectric ceramic according to the present invention, Zr or the like may be mixed from the ZrO ₂ ball at the time of pulverization.

In the piezoelectric ceramic having the composition of the present invention, for example, various oxides composed of SrCO ₃ , CaCO ₃ , BaCO ₃ , Bi ₂ O ₃ , MnO ₂ , and TiO ₂ or salts thereof can be used as raw materials. A raw material is not limited to this, You may use metal salts, such as carbonate and nitrate which produce | generate an oxide by baking.

These raw materials are weighed so as to have the above-described composition, pulverized so that the average particle size distribution (D ₅₀ ) after mixing is in the range of 0.3 to 1 μm, and this mixture is calcined at 850 to 1000 ° C. Then, pulverization is performed so that the average particle size distribution (D ₅₀ ) after calcination is in the range of 0.3 to 1 μm, and a predetermined organic binder is added again, followed by wet mixing and granulation.

  The powder thus obtained is molded into a predetermined shape by a known press molding or the like, and calcined at a temperature range of 1000 to 1250 ° C. for 2 to 5 hours in an oxidizing atmosphere such as the atmosphere to obtain the composition of the present invention. The piezoelectric ceramic which has is obtained.

  The piezoelectric ceramic having the composition of the present invention is optimal for a piezoelectric ceramic of a pierce oscillation circuit as shown in FIG. 1, particularly for a high-frequency resonator using the fundamental wave vibration of thickness shear vibration. It can also be used for piezoelectric sensors such as piezoelectric resonators, ultrasonic vibrators, ultrasonic motors and acceleration sensors, knocking sensors, and AE sensors.

  FIG. 2 is a schematic perspective view of an 8 MHz piezoelectric resonator (piezoelectric oscillator) which is an example of an embodiment of the piezoelectric element of the present invention. This piezoelectric resonator is configured by forming electrodes 2 and 3 on both surfaces of a piezoelectric ceramic 1 having the above composition. In such a piezoelectric resonator, the P / V of the fundamental wave in thickness shear vibration and thickness longitudinal vibration can be increased, the oscillation margin is increased, and phase-shift distortion does not occur at a frequency between the resonance frequency and the anti-resonance frequency. Therefore, stable oscillation can be obtained. Furthermore, highly accurate oscillation excellent in temperature stability of the oscillation frequency can be obtained, and in particular, a piezoelectric resonator that can be adapted to a frequency of 2 to 20 MHz can be obtained.

  In addition, by using a piezoelectric ceramic made of a polycrystal, chipping due to processing can be greatly suppressed as compared to a single crystal, and furthermore, a piezoelectric can be produced by producing and firing a molded body so as to have a desired shape by a molding die. The element can be obtained, and phase-shifting distortion due to spurious vibration does not occur between the resonance frequency and anti-resonance frequency due to chipping (missing the ceramic edge for the resonator), and the phase-shift condition is not satisfied. There is no oscillation, and stable oscillation can be obtained.

  Therefore, the piezoelectric resonator does not cause phase shift distortion at a frequency between and near the resonance frequency and the antiresonance frequency, and can increase the P / V of the thickness shear vibration and the thickness longitudinal vibration. Thus, a lead-free piezoelectric ceramic having excellent temperature stability of the oscillation frequency in the temperature range of -20 ° C to + 80 ° C can be obtained.

  FIG. 3 shows a schematic perspective view of a pressure sensor element which is an example of an embodiment of the piezoelectric element of the present invention. This pressure sensor includes a pair of electrodes 12 and 13 which are formed on a pair of opposed main surfaces of a piezoelectric substrate 11 made of a piezoelectric ceramic having the above-described composition, respectively, and are opposed to each other. Polarization is performed in a direction perpendicular to the main surface. In such a pressure sensor, electric charges are generated on the main surfaces due to the pressure applied between the main surfaces. Therefore, the pressure applied between the main surfaces can be measured by measuring the electric charges.

First, SrCO ₃ powder having a purity of 99.9%, CaCO ₃ powder, BaCO ₃ powder, Bi ₂ O ₃ powder, TiO ₂ powder as a starting material, and a composition formula by molar ratio is represented by Bi ₄ Ti ₃ O ₁₂ · x {( Sr _1-a Ca _a ) _1-b Ba _b TiO ₃ }, x, a, b are the main components having the values shown in Table 1, and MnO ₂ powder is added to 100 parts by mass of the main components. The mixture was weighed and mixed so as to have the mass parts shown in Table 1.

  The weighed raw material powder was put into a 500 ml polypot together with zirconia balls having a purity of 99.9% and ion-exchanged water, and mixed in a rotary mill for 16 hours.

The slurry after mixing was dried in the air, passed through # 40 mesh, and then calcined by holding at 950 ° C. for 3 hours in the air, and this synthetic powder was charged with 99.9% purity ZrO ₂ balls and ions. It put into a 500 ml polypot with exchange water, and grind | pulverized for 20 hours, and obtained evaluation powder.

  An appropriate amount of an organic binder is added to this powder, granulated, molded with a mold press at a pressure of 150 MPa, and finally fired in the air at 1050 ° C. to 1250 ° C. for 3 hours. A circle having a diameter of 7 mm and a thickness of 2.5 mm A columnar piezoelectric ceramic for evaluating a positive piezoelectric effect and a piezoelectric ceramic for evaluating a resonant frequency were obtained in a plate shape having a length of 25 mm, a width of 38 mm, and a thickness of 1 mm.

  After the piezoelectric ceramic for evaluating the positive piezoelectric effect was polished to a thickness of 2 mm, Ag electrodes were formed on both main surfaces (upper and lower surfaces of the cylinder), and polarization treatment was performed to obtain a piezoelectric element for evaluating the positive piezoelectric effect.

The positive piezoelectric effect was evaluated as follows. A pressure of 300 N was applied as a preload between both main surfaces of the evaluation piezoelectric element described above, and a pressure of 50 N was applied as a 10 Hz triangular wave pressure application waveform in addition to the preload. The output waveform from the piezoelectric ceramic was detected by a charge amplifier (Kistler 5011B), and the generated charge due to the positive piezoelectric effect with respect to the pressure application was measured. The amount of charge d (pC / N) generated when pressure is applied is
d = D / T
D: Generated charge (pC)
T: Applied stress (dynamic range, N)
Calculated as That is, it calculated with d = D / 50 on the above-mentioned application conditions.

  Subsequently, spontaneous polarization (Ps) was evaluated. The piezoelectric ceramic for evaluating the positive piezoelectric effect was lapped to a thickness of 200 μm, electrodes were formed on both main surfaces (upper and lower surfaces of the cylinder) by Au deposition, and then cut into 8 mm × 8 mm using a dicing saw to obtain an evaluation sample. . Spontaneous polarization was determined by applying a triangular wave having a frequency of 10 Hz to TF2000HS manufactured by aixACT and measuring PE hysteresis for this sample.

Moreover, volume specific resistance was evaluated based on JIS-C2141. These results are shown in Table 1.

  As is apparent from Table 1, sample nos. 2 to 6, 11, 12 and 15 to 22, 23 and 24, the generated charge amount when pressure is applied is as high as 18 pC / N, and the spontaneous polarization at −40 ° C. and 200 ° C. with respect to the spontaneous polarization at 25 ° C. The rate of change of polarization was within ± 10% and the temperature dependency was low.

  On the other hand, x is a sample No. outside the scope of the present invention. Sample Nos. 1, 7 and 8 having a small value of x No. 1 shows that the change rate of the spontaneous polarization at 200 ° C. with respect to the spontaneous polarization at 25 ° C. is larger than 10%, and sample No. 1 with a large value of x is obtained. In 7 and 8, the change rate of the spontaneous polarization at 200 ° C. with respect to the spontaneous polarization at 25 ° C. was larger than −10%, and the temperature dependency was high.

  In addition, a is a sample No. outside the scope of the present invention. In 9, 10, 13, and 27, the generated charge amount when applying pressure was as low as 17 pC / N or less.

  Also, b is a sample No. outside the scope of the present invention. In No. 26, the rate of change of spontaneous polarization was greater than 10%, and the temperature dependency was high.

Furthermore, sample No. ₂ with 0 addition of MnO ₂ was used. No. 14 was not sufficiently densified of the porcelain, and the amount of charge generated when pressure was applied was as low as 10 pC / N. Sample No. with MnO ₂ addition amount of 1.6 In No. 23, the porcelain was polarized, but could not be polarized.

  The piezoelectric ceramic for resonance frequency evaluation was processed to have a length of 6 mm and a width of 30 mm, and then an end face electrode for polarization in the length direction was formed and subjected to polarization treatment. Thereafter, the polarization electrode was removed and processed by a lapping machine to a thickness of about 0.17 mm. Thereafter, Cr—Ag was vapor-deposited on both sides of the main surface (surface consisting of a length of 6 mm and a width of 30 mm), and annealed at 250 ° C. for 12 hours in order to increase the adhesion strength between the electrode and the porcelain.

  Thereafter, the electrode corresponding to the non-electrode is removed by etching so that the electrode structure shown in FIG. 2 is obtained, and the length is 2.2 mm (L), the width is 0.9 mm (W), and the thickness is 0.17 mm (H ) The shape was processed using a dicing saw to obtain a resonator for fundamental wave vibration of a small thickness longitudinal vibration corresponding to 8 MHz oscillation. In FIG. 2, P indicates the polarization direction.

The characteristic of the resonator is that an impedance waveform is measured by an impedance analyzer, and P / V at a fundamental wave vibration of thickness-shear vibration is calculated by an expression of P / V = 20 × log (R _a / R ₀ ) (however, R _a : anti-resonance impedance, R ₀ : resonance impedance).

  Furthermore, the temperature change rate of the oscillation frequency was calculated by the following equation with respect to the oscillation frequency change at −20 ° C. or + 80 ° C. with reference to the oscillation frequency of 25 ° C.

F _osc change rate _{= {(F osc (drift)} -F osc (25)) / F osc (25)}, however, F _osc (drift) is the oscillation frequency at -20 ° C. or + 80 ° C., F _osc (25) is the oscillation frequency at 25 ° C. These results are shown in Table 2.

  From Table 2, in the sample within the range of the present invention, the P / V of the fundamental wave vibration in the thickness shear vibration can be increased to 45 dB or more, and the temperature change rate of the oscillation frequency is decreased to within ± 2100 ppm.

Sample No. of the present invention. When 4 was analyzed by X-ray diffraction, a bismuth layered compound of m = 5 was recognized as the main crystal phase. The bismuth layered compound has a crystal structure in which Bi ₂ O ₂ is inserted in a stack of perovskite structures. The number of units having a perovskite structure sandwiched between Bi ₂ O ₂ layers is m. From this, it can be considered that the perovskite compound is incorporated in a bismuth layered compound of m = 3 and has a crystal of m = 5, and the piezoelectric ceramic of the present invention has a structure of m = 5. And m = 4 are mixed. It is considered that the structure is a bismuth layered compound in which Mn is partly dissolved.

  In addition, the composition of the sample prepared in the example was analyzed with a fluorescent X-ray analyzer. As a result, the composition of the porcelain of each sample was the same as the prepared raw material composition.

It is the schematic which showed the Pierce oscillation circuit which made the Colpitts type oscillation circuit a prototype. It is a schematic perspective view of the resonator for 8 MHz which is an example of embodiment of the piezoelectric element of this invention. It is a schematic perspective view of the pressure sensor which is an example of embodiment of the piezoelectric element of this invention.

Explanation of symbols

l, 11 ... Piezoelectric ceramic 2, 3, 12, 13 ... Electrode P ... Polarization direction

Claims (4)

The composition formula is expressed as Bi ₄ Ti ₃ O ₁₂ × x {(Sr _1−a Ca _a ) _1−b Ba _b TiO ₃ }, and 1.30 ≦ x ≦ 1.75, 0.40 ≦ a ≦ 0. A piezoelectric ceramic comprising 0.05 to 1.5 parts by mass of Mn in terms of MnO ₂ with respect to 100 parts by mass of a main component of a bismuth layered compound satisfying 60, 0 ≦ b ≦ 0.20. ^2. The piezoelectric according to claim 1, wherein the amount of spontaneous polarization is 13 μC / cm ² or more, and the amount of change in the amount of spontaneous polarization at −40 ° C. to 200 ° C. with respect to the amount of spontaneous polarization at 25 ° C. is 5% or less. porcelain. 3. A piezoelectric element comprising a pair of opposing principal surfaces and electrodes formed on the opposing principal surfaces of the piezoelectric ceramic according to claim 1 or 2. The piezoelectric element according to claim 3, wherein the electrodes are a pair of electrodes facing each other and are operated by thickness longitudinal vibration.

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