JP3261356B2 - Piezoelectric ceramics - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric ceramic widely applicable to fields such as a resonator and a high-temperature pressure sensor.

[0002]

2. Description of the Related Art A piezoelectric material is a material having a piezoelectric effect in which electric polarization is changed by receiving an external stress and an inverse piezoelectric effect in which distortion is generated by applying an electric field. Piezoelectric materials are sensors for measuring pressure and deformation,
It is applied to resonators and actuators.

Most of the piezoelectric materials currently in practical use are tetragonal or rhombohedral PZT (PbZrO _3).
-PbTiO ₃ solid solution) or tetragonal PT (PbT
Ferroelectrics having a perovskite structure such as iO ₃ ) are generally used. By adding various sub-components to these, various requirements are met. For example, an actuator for position adjustment, which requires a large displacement amount in a DC manner, has a large piezoelectric constant (d ₃₃ ) instead of a small mechanical quality factor (Q _m ). In applications where alternating current is used, such as an ultrasonic wave generating element used for a piezoelectric element, an element having a large mechanical quality factor (Q _m ) instead of a small piezoelectric constant (d ₃₃ ) is used.

However, PZT and PT piezoelectric materials are
Many practical compositions have a Curie point of about 200 to 400 ° C. At higher temperatures, they become paraelectric and lose their piezoelectricity. Is not applicable. Further, since these lead-based piezoelectric materials contain a large amount (approximately 60 to 70% by weight) of lead oxide (PbO), which is extremely volatile even at a low temperature, it is also preferable from an ecological point of view and pollution prevention. Absent. Specifically, when producing these lead-based piezoelectric materials as ceramics or single crystals, heat treatment such as firing and melting is inevitable, and when considered on an industrial level, lead oxide, a volatile component, is contained in the atmosphere. The amount of volatilization and diffusion to the water becomes extremely large. In addition, lead oxide released during the manufacturing stage can be recovered, but most of the lead oxide contained in piezoelectric materials put on the market as industrial products cannot be recovered at present, and these are widely used in the environment. If released to
It is inevitable that it causes pollution.

As a piezoelectric material containing no lead, for example, BaTi having a perovskite structure belonging to a tetragonal system is used.
O ₃ is well known and has a Curie point of 120
It is not practical because of low temperature. Also, JP-A-9-10
No. 0156 discloses a (1-x) having a perovskite structure.
(Bi _1/2 Na _1/2 ) TiO ₃ -xNaNbO ₃ solid solution is described, but there is no description of a solid solution having a Curie point exceeding 370 ° C. It is impossible to apply to an element for ultra-high temperature.

As a piezoelectric substance whose Curie point can be raised to 500 ° C. or higher, for example, a bismuth layer compound is known.
However, bismuth layered compound contains no lead, there is a problem that Q _m is small which is important when applied to a resonator. The Q _m becomes small, presumably because the coercive field can not be large enough polarization. JP-A-6-305817 discloses a bismuth layered compound containing Pb _x Bi _3-x Ti _1-x Nb _{1 + x} O ₉ (0.3
≦ x ≦ 0.75). In this composition, the Curie temperature is 500 ° C. or higher. However, according to the experiments of the present inventors, a sufficiently large Q _m cannot be obtained, and there is a problem that lead is contained. In addition, Japanese Patent Publication No. 54-32957 discloses (Pb _{1 -X} Sr _X ) Bi ₄ Ti ₄ O ₁₅ (where 0
The bismuth layer compound represented by <X <1) is described, but this composition also contains lead.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a piezoelectric ceramic which does not contain lead, has a high Curie point, and has excellent piezoelectric properties, particularly a large mechanical quality factor (Q _m ). is there.

[0008]

The above object is achieved by the following (1).
This is achieved by the present invention. (1) SrBi ₄ Ti ₄ O is a bismuth layered compound containing Sr, Bi, Ti and Ln (lanthanoid).
_A piezoelectric ceramic containing a _15- type crystal and having an atomic ratio Ln / (Sr + Ln) of 0 <Ln / (Sr + Ln) <0.5.

In addition, "Trend Survey Report on Piezoelectric Ceramic Materials" issued by the Electronic Materials Industry Association in March 1979
Table 5 on page 18 shows that SrBi ₄
A Ti ₄ O ₁₅ material is described. However, the addition of lanthanoids is not described.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION
It is a composite oxide containing r, Bi, Ti and Ln (lanthanoid). In the piezoelectric ceramic of the present invention, the atomic ratio Ln / (Sr + Ln) is 0 <Ln / (Sr + Ln) <0.5. If Ln is not included, it is impossible to increase the Q _m. On the other hand, Ln / (Sr + Ln) is Q _m too large is lowered. A preferred range of Ln / (Sr + Ln) is 0.03 ≦ Ln / (Sr + Ln) ≦ 0.3.

As Ln, La, Nd, Sm, Eu,
At least one of Gd, Dy, and Ho is preferable;
at least one of a, Nd, Sm and Gd is more preferred, and at least La is more preferably used;
It is particularly preferred to use only La.

The piezoelectric ceramic of the present invention contains SrBi ₄ Ti ₄ O ₁₅ type crystal, which is a bismuth layered compound, and is preferably substantially composed of this crystal. Heterophases may be included.
In this piezoelectric ceramic, Ln is SrBi ₄ T
It is considered that the Sr site of the i ₄ O ₁₅ type crystal is mainly substituted, but a part may be substituted for another site or a part may be present at a crystal grain boundary. .

The overall composition of the piezoelectric ceramic of the present invention is as follows:
Generally it may be set as _{(Sr 1-x Ln x)} Bi 4 Ti 4 O 15 , but may be in the future offset. For example, the ratio of Sr + Ln to Ti or the ratio of Bi to Ti may deviate from the stoichiometric composition by about ± 5%. Further, the amount of oxygen can also change according to the valence of the metal element, oxygen vacancy, and the like.

The piezoelectric ceramic of the present invention may contain Ba, Ca, Pb or the like as an impurity or a small amount of an additive.
In terms of oxides such as O, CaO, PbO, etc.
It is preferably at most 5% by weight. If the content of these elements is too large, the effects of the present invention may be impaired.
It is most preferable that Pb is not contained in the piezoelectric ceramic of the present invention, but there is substantially no problem if the content is about the above.

The crystal grains of the piezoelectric ceramic of the present invention have a spindle shape or a needle shape. The average crystal grain size is not particularly limited, but is preferably 1 to 10 μm in the major axis direction.
m, more preferably 3 to 5 μm.

The Curie point of the piezoelectric ceramic of the present invention can be at least 450.degree.
It is easy to raise the temperature to not less than ° C. In addition, the Q _m, 4.5M
At least around 2000 in the vicinity of Hz and 300
It is also easy to set it to 0 or more.

The piezoelectric ceramic of the present invention is suitable for a resonator, a high temperature sensor and the like.

Next, an example of a method for producing the piezoelectric ceramic of the present invention will be described.

First, as a starting material, an oxide or
Compounds that can be converted to oxides by firing, for example, carbonates, hydroxides, oxalates, nitrates and the like, specifically Sr
Powders such as CO ₃ , Bi ₂ O ₃ , TiO ₂ , La ₂ O ₃ are prepared, and these are wet-mixed by a ball mill or the like.

Then, at about 800 to 1000 ° C., 1 to 3
Calcination is performed for about an hour, the obtained calcination product is converted into a slurry, and wet crushed using a ball mill or the like.

After the wet pulverization, the powder of the calcined product is dried, and a small amount of water (about 4 to 8% by weight) is added to the dried product, and 1 to 4 t / c
Press molding is performed under a pressure of about m ² to obtain a molded body. At this time, a binder such as polyvinyl alcohol may be added.

Next, the compact is fired to obtain a piezoelectric ceramic. The firing temperature is preferably 1200 to 1350
C., and the firing time is preferably about 1 to 5 hours. The baking may be performed in the air, or may be performed in an atmosphere having a lower or higher oxygen partial pressure than in the air, or in a pure oxygen atmosphere.

[0023]

EXAMPLE A piezoelectric ceramic sample shown in Table 1 was produced by the following procedure.

As starting materials, SrCO ₃ , Bi ₂ O ₃ ,
Powder _{TiO 2, La 2 O 3,} Nd 2 O 3, Sm 2 O 3, Gd 2 O 3, final composition is formulated so that the _{(Sr 1-x Ln x)} Bi 4 Ti 4 O 15, The mixture was wet-mixed in pure water by a ball mill using zirconia balls for 10 hours. Table 1 shows the value of x for each sample.

Next, the mixture was sufficiently dried and pressed, and then calcined at 800 to 900 ° C. for 2 hours. The obtained calcined product was pulverized by a ball mill, dried, and granulated by adding a binder (polyvinyl alcohol). The obtained granulated powder is 2,000 to 3,000 using a uniaxial press molding machine.
A load of kgf / cm ² was applied to form a thin plate having a plane size of 20 mm × 20 mm and a thickness of about 1.5 mm. After subjecting the obtained molded body to a heat treatment to volatilize the binder, 1200 to 135
Baking at 0 ° C. for 2 to 4 hours. The obtained sintered body is polished to a thickness of about 0.5 mm.
After cutting to 6mm, flat dimensions of 5mm x 5 on both sides
mm silver electrodes were formed. Next, an electric field of 5 to 15 MV / m was applied in a silicone oil bath at 200 to 250 ° C for 1 to 10 MV / m.
The sample was subjected to a polarization treatment by applying the voltage for minutes, thereby obtaining a sample for characteristic measurement.

Incidentally, for comparison, Japanese Patent Application Laid-Open No.
As an example of the bismuth layered compound described in Japanese Patent No. 5817, a sintered body of Pb _0.5 Bi _2.5 Ti _0.5 Nb _1.5 O ₉ was prepared, and a sample for measuring characteristics was also prepared.

The Curie point of each sample was measured using an LCR meter HP4394A manufactured by Hewlett-Packard Company and an electric furnace. In addition, the DE hysteresis was measured using RT-6000HVS manufactured by Radiant Technology Co., and the coercive electric field at 250 ° C. was obtained. Moreover, was measured by Hewlett Packard impedance analyzer HP4194A, determine the thickness longitudinal electromechanical coupling coefficient (k _t) and mechanical quality factor (Q _m) by using a resonance-antiresonance method. Note that k _t and Q
_m was calculated by the following equation.

[0028]

(Equation 1)

[0029]

[Table 1]

From Table 1, it can be seen that in SrBi ₄ Ti ₄ O ₁₅
A part of Sr by replacing Ln, it is understood that more than 2000 a sufficiently high Q _m are obtained. Also, Ln
, The Curie point is hardly reduced. In contrast, in the comparative samples using the described bismuth layer compound in JP-A Hei 6-305817, the Curie point is high although Q _m is low. Also, this comparative sample has a larger coercive electric field than the sample of the present invention,
It turns out that it is hard to polarize.

FIG. 1 shows a frequency-phase angle curve and a frequency-impedance curve of Sample No. 4. From the figure, it can be seen that spurious is not generated in this sample. Sample No. 4 was also used for other samples.
Similarly, the resonance frequency was around 4.5 MHz.

Sample No. 1 (Comparative Example) and Sample N
For o.4, scanning electron micrographs of the cross sections are shown in FIGS. 2 and 3, respectively. From these figures, it can be seen that pores are reduced by the addition of La, and the sintered body is densified. In these photographs, the pores are located at the center of the white spots.

Sample No. 1 and Sample No. 4
After etching the cross section, a scanning electron micrograph was taken at a higher magnification than in FIGS. 2 and 3. Sample No. 1 is shown in FIG. 4, and sample No. 4 is shown in FIG. These photographs show that the crystal grains are spindle-shaped or needle-shaped.

FIG. 6 shows (Sr _{1 -x} La _x ) Bi ₄ Ti ₄ O ₁₅
5 shows the analysis results of powder X-ray diffraction when x was changed in Example 1. The figure also shows the results of peak identification. FIG.
Therefore, the sintered body with x = 0 to 0.5 is SrBi ₄ Ti ₄ O
It turns out that it is a single phase of a _15- type crystal.

[0035]

According to the present invention, a piezoelectric ceramic containing no lead, having a high Curie point, a large mechanical quality factor, and a high density can be realized.

[Brief description of the drawings]

FIG. 1 is a graph showing a frequency-phase angle curve and a frequency-impedance curve in the piezoelectric ceramic of the present invention.

FIG. 2 is a drawing substitute photograph showing a particle structure, and is a scanning electron microscope photograph of a cross section of a comparative sample.

FIG. 3 is a drawing substitute photograph showing a particle structure, and is a scanning electron microscope photograph of a cross section of a sample of the present invention.

FIG. 4 is a drawing substitute photograph showing a particle structure, and is a scanning electron microscope photograph of a cross section of a comparative sample.

FIG. 5 is a drawing substitute photograph showing a particle structure, and is a scanning electron microscope photograph of a cross section of a sample of the present invention.

FIG. 6 is a chart showing an analysis result of powder X-ray diffraction when ( _x ) is changed in (Sr _1-x La _x ) Bi ₄ Ti ₄ O ₁₅ .

Continuation of front page (56) References JP 2000-143340 (JP, A) Shrout, HIGH DIELECTRIC CON STANT NPO DIELECTRIC IC MATERIALS BASED ON (100-x) La0.1Bi0.9) 2Ti2O7-xSrTiO3, C ARTS 97, 17th Capacitor or Registry, Vol. 56-61 (58) Fields investigated (Int. Cl. ⁷ , DB name) C04B 35/42-35/50 CA (STN) REGISTRY (STN)

Claims (1)

(57) [Claims] 1. A bismuth layered compound containing Sr, Bi, Ti and Ln (lanthanoid), wherein SrBi
Including ₄ Ti ₄ O ₁₅ type crystal, atomic ratio Ln / (Sr + Ln)
Is 0 <Ln / (Sr + Ln) <0.5.