JPH0443873B2 - - Google Patents

Description

[Detailed description of the invention] [Technical field to which the invention pertains] The present invention relates to piezoelectric ceramics, and in particular, they have high electromechanical energy conversion efficiency and high mechanical strength, and are therefore suitable for use as high-output piezoelectric elements and high-frequency piezoelectric elements. related to piezoelectric ceramics.

[Background technology]

Lead zirconate titanate is one of the piezoelectric ceramic materials with high electrical/mechanical energy conversion efficiency. This lead zirconate titanate (hereinafter _abbreviated as PZT) is represented by _the composition formula Pb( _Zr It is known as a material that exhibits high electrical/mechanical energy conversion efficiency near the morphotropic phase boundary (hereinafter abbreviated as MPB) where x=0.52 to 0.53.

In the field of piezoelectric ceramics, as its applications expand to high power and high frequency regions, there is a demand for piezoelectric ceramic materials that have high energy conversion efficiency, high strength, and high toughness. In this case, from the point of view of energy conversion efficiency, it is desirable to use PZT with a composition near MPB, but there is a problem that mechanical strength decreases near MPB, and in addition,
PZT itself has a property of lower mechanical strength than barium titanate, which is a piezoelectric material with the same perovskite crystal structure, so PZT with a composition near MPB can be used for the above applications. However, there were some drawbacks in terms of mechanical strength.

For this reason, various attempts have been made to improve the mechanical strength of PZT, one of which is adding additives such as Sr, Mn or Ca to PZT. However, when such additives are added, the electrical properties of PZT are changed by the additives, and the desired properties cannot be obtained, so that satisfactory results have not been obtained.

In order to solve this problem, the present inventors have made various considerations and conducted numerous experiments, and as a result, have proposed the present invention.

[Purpose of the invention]

An object of the present invention is to provide piezoelectric ceramics that have both good piezoelectric properties and mechanical strength.

[Summary of the Invention] The present invention is a piezoelectric ceramic made by molding and firing a mixture of a raw material obtained by calcining and pulverizing a PZT-based oxide and 0.05 to 1.0% by weight of fibrous silicon carbide. be. PZT-based oxides are PZT having a composition near MPB, and are composed of two groups of compounds described below: (1) Group 1: Nb ₂ O ₅ , MnO ₂ , Ta ₂ O ₅ , Cr ₂ O ₃ , Fe _{2O3 ,
NiO} , _Co2O3 , _{PbNb2O6 , PbTa2O6} , _{BaTiO3 ,}
CaTiO ₃ , PbSrO ₃ and SrTiO ₃ (2) Group 2: Pb (Mg _1/3 Nb _2/3 ) O ₃ , Pb (Mn _1/3 Nb _2/3 ) O ₃ , Pb
(Cd _1/2 W _1/2 ) O ₃ , Pb (Sb _1/2 Sn _1/2 ) O ₃ , Pb (Y _1/2
Nb _1/2 ) O ₃ , Pb (Co _1/3 Nb _2/3 ) O ₃ , Pb (Ni _1/3 Nb _2/3 )
O ₃ , Pb (Zn _1/3 Nb _2/3 ) O ₃ , Pb (Ni _1/2 W _1/2 ) O ₃ , Pb
In addition to those using at least one compound selected from (Co _1/2 W _1/2 ) O ₃ and Sr (Li _2/5 W _3/5 ) O ₃ as an additive, lead titanate and zircon One compound selected from the compounds of Group 2 may be added to lead acid. Furthermore, at least one selected from the above-mentioned additives (excluding one compound selected from the above group) may be added to these three oxides.

The silicon carbide SiC is preferably a whisker, and in the examples described later, whiskers with a diameter of 0.2 to 0.5 μm were used, but depending on the particle size of the ceramic material to be fired, SiC with a diameter in the range of 0.05 to 1.5 μm may be used. Whiskers can be used. In addition, the length of the whisker is preferably longer than the particle size of the ceramic material serving as the matrix in order to increase the mechanical strength, and in the examples described later, the length is 30 to 300 μm.
SiC whiskers of approximately 0.1 to 500 μm can be used.

Further, the amount of SiC whiskers added that is effective for increasing the mechanical strength of piezoelectric ceramics is within the range of 0.05 to 1.0% by weight. The relationship between the amount of SiC whiskers added and mechanical strength differs slightly depending on the type and amount of additives in the PZT-based oxide that is the raw material, but piezoelectric ceramics with an appropriate amount of SiC whiskers added, It was confirmed that the mechanical strength increased by 35 to 44% compared to the case where it was not added.

Furthermore, in the piezoelectric ceramic according to the present invention, addition of fibrous SiC not only increases its mechanical strength, but also shows favorable changes in its electrical properties depending on the purpose of use. In other words, in a disc-shaped resonator that uses thickness vibration, the electromechanical coupling coefficient K _p in the radial direction decreases, but the electromechanical coupling coefficient K _t in the thickness direction hardly changes.
It was confirmed that the vibration mode in the thickness direction was emphasized.

The ratio of the electromechanical coupling coefficient K _t in the thickness direction to the electromechanical coupling coefficient K _p in the radial direction, K _t /K _s , was 0.78 when SiC whiskers were not added, but when SiC
When 0.25% by weight of whiskers is added, K _t /K _p is
1.0, and it was confirmed that it increased to 1.47 when 0.70% SiC whiskers were added.

[Explanation based on examples]

Example 1 Pb(Zr _0.53 Ti _0.47 )O ₃ with a composition near MPB
0.5 mol% Nb ₂ to improve piezoelectric properties
A material to which _O5 was added was used.

The starting materials used were oxides with a purity of 99.5% or higher, and taking into account the evaporation of lead during firing,
Since PbO was added in excess of 1 mol %, the formulation composition was expressed by the following compositional formula.

Pb (Zr _0.53 Ti _0.47 ) O ₃ + 0.5 mol% Nb ₂ O ₅ + 1 mol% PbO A mixture of the above composition was placed in a sealed hard alumina case and heated in an electric furnace to a temperature of 800 to 850°C. Calcining was performed for 2 hours.

Next, after crushing this calcined material in an alumina mortar, commercially available SiC whiskers are added from 0.05% to 10% by weight.
Wet mixing was performed using a ball mill for 1 to 10 hours. The SiC whiskers used in this example had a diameter of 0.2
~0.5 μm and lengths ranging from 30 to 300 μm.

Next, after over-drying, 8% PVA as a binder.
After adding 5% by weight of the solution and granulating and sizing, 1000
It was pressure-molded into a disk shape with a diameter of 25 mm and a thickness of 1.5 mm using a pressure of Kg/cm ² .

Next, this molded body was housed in a case, and main firing was performed at a temperature of 1100°C to 1280°C in an electric furnace. Firing time depends on temperature, 6 hours at 1100℃, 6 hours at 1150℃
3 hours at ℃, 2 hours at 1180℃, 1 hour at 1280℃
Required baking time. The case for housing the molded body had a lid to seal the lead atmosphere, and was made of dense alumina or magnesia.

Next, this fired body was processed into a disk shape with a diameter of 20 mm and a thickness of 0.5 mm, which was further polarized by baking silver electrodes on both sides, and was used as a sample for measuring electrical characteristics. Further, this fired body was processed into a rectangular parallelepiped with dimensions of 2.5 x 1 x 10 mm and used as a sample for measuring mechanical strength. Mechanical strength was measured by measuring transverse rupture strength using a three-point bending method.
The transverse rupture strength was measured for each sample in which the amount of SiC whiskers added was changed stepwise from 0% to 10% by weight.

FIG. 1 shows a plot of the average flexural strength determined from a Weibull plot of the flexural strength of piezoelectric ceramics obtained by firing at a temperature of 1180° C. for 2 hours against the amount of SiC whisker added.

According to Figure 1, the amount of SiC whiskers added is 0.7
It has been shown that the transverse rupture strength increases approximately proportionally to the amount of SiC whiskers added up to % by weight.
When no SiC whiskers are added, the average bending strength is
It is 890Kg/ ^cm2 . By adding 0.05% by weight of SiC whiskers, the average bending strength increases to 920Kg/cm ² , and when adding 0.25% by weight, it increases to 1060Kg/cm ² .
When adding 0.5% by weight, the mechanical strength is 1210Kg/ ^cm2 , which is a 36% increase in mechanical strength compared to the case without SiC whiskers, and when adding 0.7% by weight, the average flexural strength is 1270Kg/cm2.
Kg/cm ² , an improvement of approximately 43%. However, when the amount added exceeds 0.7% by weight, the bending strength begins to decrease, and when the amount added is 1.0% by weight, it decreases to 1030 Kg/ ^cm2 , and when the amount added is 1.2% by weight, the strength is lower than that without the addition. was confirmed.

Note that when the firing temperature exceeded 1300°C, no increase in flexural strength was observed due to the addition of SiC whiskers. In other words, it was found that at temperatures exceeding 1,300°C, the reaction between the ceramic material and SiC occurs significantly, causing the strength of the SiC whiskers to deteriorate. Therefore, especially for lead zirconate titanate-based materials that have a high lead content, it is necessary to select a firing temperature and firing time that minimize reaction with SiC.

On the other hand, the measurement results of the dielectric and piezoelectric properties of the sample for measuring the electrical properties obtained under the same firing conditions as the sample used for measuring the mechanical strength, that is, firing at 1180°C for 2 hours, are as follows. That's right.

The dielectric constant was decreased by the addition of SiC whiskers. The dielectric loss tan δ is 1.75% without the addition of SiC whiskers, and although it increases slightly with the addition of SiC whiskers, it decreases from 1.9 to 1.9% up to the addition amount of 0.7% by weight.
It is in the range of 2.1%.

The frequency constant fr・d is approximately 2000 Hzm when no SiC whiskers are added, but it increases with the addition of SiC whiskers, and becomes 2200 Hzm with the addition of 0.5% by weight of SiC whiskers, and increases with the addition of 0.7% by weight. As a result, an increase of 10 to 12% was observed at 2250 Hzm.

Q _n (mechanical Q) is 53 when no additives are added, and increases slightly with the addition of SiC whiskers, but increases slightly to 55-62 when the amount added is up to 0.5% by weight. When the amount added is 0.7% by weight, it decreases slightly to 48, but the degree of decrease is within 10%.

Regarding the electromechanical coupling coefficient, the radial electromechanical coupling coefficient K _p of the disc-shaped element is 0.64 when no SiC whiskers are added. SiC whiskers
When 0.05% by weight is added, K _p becomes 0.59, which is 0.1% by weight.
When adding , it becomes 0.55, and when adding 0.35% by weight, it becomes 0.46,
When adding 0.50% by weight, 0.40, when adding 0.7% by weight
Addition of 0.34 and 1.0% by weight decreases the conversion efficiency to 0.29, but it maintains a conversion efficiency sufficient for practical use as a piezoelectric material.

On the other hand, the electromechanical coupling coefficient K _t in the thickness direction decreases at a smaller rate due to the addition of SiC whiskers than the electromechanical coupling coefficient K _p in the radial direction. Therefore, the ratio K _t /K _p increases as the amount of SiC whiskers added increases, as shown in FIG. As is clear from FIG. 2, K _t /K _p was 0.78 when no SiC whiskers were added, but when the amount of SiC whiskers added was 0.05% by weight, 0.35% by weight, 0.50% by weight,
As it increases from 0.70 wt% to 1.0 wt%,
K _t /K _p are 0.84, 1.13, 1.20, 1.47 and respectively
The value increases to 1.72, and the vibration mode in the thickness direction becomes emphasized. This property is similar to the piezoelectric property of titanate-based piezoelectric ceramics, for example, and can be used to convert longitudinal mode sound waves and mechanical vibration energy into electrical energy, and vice versa, such as in sound wave transceiver elements. It is suitable as a transducer material when the purpose is to convert electrical energy into mechanical vibration energy having a longitudinal mode and to reduce the influence of vibrations in the transverse mode.

The above characteristics are obtained by adding 0.5 mol% of Nb ₂ O ₅ to PZT.
This was obtained by adding SiC whiskers to piezoelectric ceramics by firing the added composition at a temperature of 1180°C for 2 hours, but if the firing conditions were changed, for example by changing the firing temperature between 1150°C and 1280°C. Depending on this, the baking time can be changed from 6 hours to 1 hour.
Similar results were obtained regarding mechanical, dielectric and piezoelectric properties when varying the time.

These results are similar to piezoelectric ceramic materials.
It is obtained by the microstructure of ceramics formed by composite with SiC.

In this example, SiC whiskers are added to a piezoelectric ceramic material in which Nb ₂ O ₅ is added to PZT, but a PZT-based piezoelectric ceramic material having a composition in which other composite oxides are added to PZT A similar effect can be achieved with a composite material in which SiC whiskers are added to. An example is shown in Example 2.

Example 2 A material having a composition in which Pb (Mg _1/3 Nb _2/3 ) O ₃ was added to PZT was composited with SiC whiskers. The composition of the piezoelectric material used is 0.275PbZrO ₃ −
_{0.350PbTiO3−0.375Pb} (Mg1 _/ 3Nb2 _/ 3) _O3 ,
After calcining this raw material mixture at a temperature of 850℃ for 2 hours and pulverizing the calcined product, SiC whiskers are
0.05% to 3% by weight was added, and wet mixing was performed for 2 to 6 hours using a stirring device.

After over-drying, add 6% by weight of 5% by weight methylcellulose solution as a binder, perform granulation and sizing, dry again, and form into discs with a diameter of 25mm and a thickness of 1.5mm under a pressure of 1000Kg/ ^cm2 . Pressure molded.

This molded body was fired at a temperature of 1150 to 1300°C for 6 to 1 hour to obtain a piezoelectric ceramic element, and its mechanical strength and electrical properties were measured in the same manner as in Example 1.

As an example, when looking at a sample fired at a temperature of 1280°C for 2 hours, an increase in the average bending strength was observed due to the addition of SiC whiskers. i.e. SiC
Whiskers 0.10 wt%, 0.25 wt% and 0.50
It was confirmed that the average bending strength increased by 8%, 19%, and 36% by adding % by weight, respectively. A similar increase in average flexural strength was also confirmed for composite materials fired at a temperature of 1150°C for 6 hours, 1250°C for 3 hours, and 1300°C for 1 or 2 hours.

However, if the amount of SiC whiskers added exceeds 0.6 to 0.8% by weight, the bending strength begins to decrease, so the amount of SiC whiskers added that can improve the mechanical strength of this composition is limited to 1.0% by weight. It was hot.

When the firing temperature is high, for example, when firing at 1330°C for 2 hours, no increase in strength is observed due to the addition of SiC whiskers, and the upper limit of the firing temperature in this example is
It was 1300℃.

Looking at the dielectric properties and piezoelectric properties, the dielectric constant decreased with the addition of SiC whiskers, as in Example 1. The dielectric loss remained constant up to 0.7% by weight of SiC whiskers, and increased when the amount exceeded 1.0% by weight.

The frequency constant and Q _n (mechanical Q) showed similar trends as in Example 1 as the amount of SiC whiskers added increased.

Also, the electromechanical coupling coefficient in the radial direction
K _p decreases as the amount of SiC whiskers added increases, but since K _t in the longitudinal direction is almost constant,
K _t /K _p increased as the amount of SiC whiskers added increased. In this way, also in the composition of this example, a material in which the longitudinal mode was emphasized was obtained by adding SiC whiskers. Note that if more than 1.0% by weight of SiC whiskers is added, K _p will be 0.25.
Or the value was smaller than this.

As described above, in Example 1, a piezoelectric ceramic material in which Nb ₂ O ₅ was added to PZT, and in Example 2,
In this case, SiC whiskers are added to PZT with Pb (Mg _1/3 Nb _2/3 ) O ₃ added, but not only that, but also the following two groups (1) and (2) A similar effect can be obtained by adding SiC whiskers to a composition in which one or more of the above compounds are selected and added to PZT as an additive.

(1) Group 1: Nb ₂ O ₅ , Ta ₂ O ₅ . _Cr2O3 , _{MnO2 , Fe2O3 ,}
NiO _{, Co2O3} , _{PbNb2O6 .} PbTa ₂ O ₆ , BaTiO ₃ ,
CaTiO ₃ , PbSrO ₃ and SrTiO ₃ , (2) Group 2: Pb (Mg _1/3 Nb _2/3 ) O ₃ , Pb (Mn _1/3 Nb _2/3 ) O ₃ , Pb
(Cd _1/2 W _1/2 ) O ₃ , Pb (Sb _1/2 Sn _1/2 ) O ₃ , Pb (Y _1/2
Nb _1/2 ) O ₃ , Pb (Co _1/3 Nb _2/3 ) O ₃ , Pb (Ni _1/3 Nb _2/3 )
O ₃ , Pb (Zn _1/3 Nb _2/3 ) O ₃ , Pb (Ni _1/2 W _1/2 ) O ₃ , Pb
(Co _1/2 W _1/2 ) O ₃ and Sr (Li _2/5 W _3/5 ) O ₃ In addition, lead titanate and lead zirconate contain one selected from the compounds of Group 2 above. A three-component system containing a compound may also be used. Furthermore, these three oxides have the above-mentioned groups 1 and 2.
(However, one compound selected from Group 2 is excluded) may be used as an additive.

In addition, in the above-mentioned composition, what is necessary is just to select an appropriate calcination temperature and main calcination temperature according to each material. However, since it is a composition containing lead,
It is necessary to select a firing temperature and time at which SiC and lead do not react or react less. For example, in Examples 1 and 2, no increase in mechanical strength was observed due to the addition of SiC whiskers when fired at a temperature exceeding 1300°C. Therefore SiC
In order to avoid the reaction between lead and lead, it is preferable to select a piezoelectric ceramic raw material that can be main fired at a relatively low temperature, for example, 1280°C or lower, preferably 1200°C or lower.

〔Effect of the invention〕

As is clear from the above explanation, according to the present invention
It is possible to obtain piezoelectric ceramics with increased mechanical strength without impairing the excellent electrical properties originally possessed by PZT-based piezoelectric ceramics having a composition near MPB. Therefore, PZT-based piezoelectric ceramics can be applied to high-output devices, and because the frequency constant increases, it can also be used as high-frequency devices.

Furthermore, when the piezoelectric ceramic according to the present invention is applied to a disc-shaped resonator that utilizes thickness vibration, there is an advantage that the influence of radial vibration is small and the thickness direction mode is emphasized.

The above-mentioned effects can be obtained by structurally improving the piezoelectric ceramic material by adding fibrous SiC, so it is now possible to select the material of the piezoelectric material from a wide range, and the material can be adjusted depending on the purpose. Since the composition can be selected, the performance of various transducers can be improved.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the amount of SiC whiskers added and the average bending strength in piezoelectric ceramics according to the present invention, and FIG. 2 is a graph showing the relationship between the amount of SiC whiskers added and K _t /K _p .

Claims (1)

[Scope of Claims] 1. A piezoelectric product obtained by molding and firing a mixture of a raw material obtained by calcining and pulverizing a lead zirconate titanate oxide and 0.05 to 1.0% by weight of fibrous silicon carbide. Ceramics. 2. The piezoelectric ceramic according to claim 1, wherein the lead zirconate titanate-based oxide has Pb(Zr _0.53 Ti _0.47 )O ₃ as a main component. 3. In the piezoelectric ceramic according to claim 1, the fibrous silicon carbide is SiC.
The piezoelectric ceramic is made of whiskers. 4. In the piezoelectric ceramic described in claim 1, the lead zirconate titanate-based oxide contains lead titanate, lead zirconate, and the following group Pb (Mg _1/3 Nb _2/3 ) O ₃ , Pb (Mn _1/3 Nb _2/3 ) O ₃ , Pb
(Cd _1/2 W _1/2 ) O ₃ , Pb (Sb _1/2 Sn _1/2 ) O ₃ , Pb (Y _1/2
Nb _1/2 ) O ₃ , Pb (Co _1/3 Nb _2/3 ) O ₃ , Pb (Ni _1/3 Nb _2/3 )
O ₃ , Pb (Zn _1/3 Nb _2/3 ) O ₃ , Pb (Ni _1/2 W _1/2 ) O ₃ , Pb
The piezoelectric ceramic is a three-component system comprising one compound selected from (Co _1/2 W _1/2 )O ₃ and Sr(Li _2/5 W _3/5 )O ₃ . 5. In the piezoelectric ceramic described in claim 1, the lead zirconate titanate-based oxide includes lead zirconate titanate, Nb ₂ O ₅ , Ta ₂ O ₅ , Cr ₂ O ₃ , MnO ₂ , _{Fe2O3 ,
NiO} , _Co2O3 , _{PbNb2O6 , PbTa2O6} , _{BaTiO3 ,}
CaTiO ₃ , PbSrO ₃ , SrTiO ₃ , Pb (Mg _1/3 Nb _2/3 )
O ₃ , Pb (Mn _1/3 Nb _2/3 ) O ₃ , Pb (Cd _1/2 W _1/2 ) O ₃ ,
Pb (Sb _1/2 Sn _1/2 ) O ₃ , Pb (Y _1/2 Nb _1/2 ) O ₃ , Pb
(Co _1/3 Nb _2/3 ) O ₃ , Pb (Ni _1/3 Nb _2/3 ) O ₃ , Pb (Zn _1/3
Nb _2/3 ) O ₃ , Pb (Ni _1/2 W _1/2 ) O ₃ , Pb (Co _1/2 W _1/2 )
The piezoelectric ceramic is made of an additive containing at least one compound selected from O ₃ and Sr(Li _2/5 W _3/5 ) O ₃ . 6. The piezoelectric ceramic described in claim 4 further includes Nb ₂ O ₅ , Ta ₂ O ₅ ,
_{Cr2O3 , MnO2 , Fe2O3 ,} NiO, _Co2O3 , _PbNb2
O ₆ , PbTa ₂ O ₆ , BaTiO ₃ , CaTiO ₃ , PbSrO ₃ ,
The piezoelectric ceramic comprises, as an additive, at least one compound selected from the compounds of the group described above except for SrTiO ₃ and the one compound described above.