JPH0151072B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a lead titanate-based piezoelectric ceramic made of fine crystal grains and a surface acoustic wave (SAW) element using the ceramic. Piezoelectric ceramics have a large electromechanical coupling coefficient and are relatively easy to improve, so they have attracted attention as materials for surface acoustic waves (SAW), and many material compositions have been developed, including PZT. Among them, some of the Pb in piezoelectric ceramics whose main component is PbTiO ₃ is
Rare earth elements such as La, Pr or Nd, and Ti
Porcelain in which some of the elements are replaced with Mn, In, Ga, or Al elements not only have a zero SAW delay time temperature coefficient, but also have excellent characteristics such as a large electromechanical coupling coefficient and a small dielectric constant. This material is particularly promising as a high-frequency substrate material. However, the applicable frequency range of piezoelectric ceramics is generally up to about 100 MHz, and materials for high frequencies of 100 MHz or higher have conventionally been targeted at single crystals and thin films, and piezoelectric ceramics have not been included. The reason is that the propagation loss of piezoelectric ceramics increases rapidly as the frequency increases. The present invention solves the above-mentioned problems and provides a piezoelectric ceramic with small propagation loss that can be used even in a high frequency range, which was previously thought to be impossible, and a surface acoustic wave element using the ceramic. This is the purpose. Hereinafter, the present invention will be explained in detail with reference to Examples. Example 1 First, in order to study the crystal grain size dependence and frequency dependence of the SAW propagation loss of PbTiO ₃ -based ceramics, a number of samples with different crystal grain sizes were prepared, and their crystallinity, dielectric properties, piezoelectric properties, and SAW characteristics were measured. The composition used was 45.6 mol% PbO and _49.4 mol% TiO2.
Mol%, _2.9 mol% _Nd2O3 , 1.1 mol% _MnO2
and In ₂ O ₃ is 1.0 mol %. Ceramics with this composition have a SAW delay time temperature coefficient of
It is a material with extremely excellent properties, with a SAW propagation speed of approximately 2610 m/s, an electromechanical coupling coefficient of 2.2%, a dielectric constant of 220, and a Curie temperature of 320°C. The raw material powders having the above composition ratio were wet-mixed for about 1 hour using a pot mill, and after drying, they were pre-sintered at 850°C for 2 hours. This pre-sintered body was pulverized using a Raikai machine and mixed again using a pot mill. After drying, press molding at a pressure of 350Kg/cm ² and 1150℃
Sintered at a temperature range of ~1300°C for 10 minutes to 20 hours.
The grain size was controlled by sintering temperature and sintering time.
The pellet size after sintering is 16φ× ^2mmt .
This sintered body was then polished to a thickness of 1 mm, chromium-gold electrodes were deposited on both sides, and polarization treatment was performed. In the polarization treatment, an electric field of 5.5×10 ³ V/cm was applied at 150° C. for 10 minutes. Afterwards, we measured dielectric and piezoelectric properties. One side was mirror polished, and four sets of Al interdigital electrodes were formed at equal intervals by photolithography. An RF pulse was applied to the SAW excitation electrode at the end, and the propagation loss was determined from the amount of signal attenuation at each of the remaining receiving electrodes, which had different propagation distances. FIG. 1 shows the dependence of the density ρ, the axial ratio c/a of the crystal lattice, the dielectric constant ε ^T ₃₃ and the dielectric loss tan δ on the crystal grain size. The density and axial ratio gradually decrease as the crystal grain size decreases, but the dielectric constant increases, and sharply increases when the crystal grain size is 0.1 μm or less. Further, the dielectric loss remains constant and hardly changes even when the crystal grain size is reduced to 0.1 μm, and shows a tendency to increase when the crystal grain size is 0.1 μm or less. Figure 2 shows the electromechanical coupling coefficients K _t and K _p for thickness direction vibration and radial direction vibration, frequency constants N _t and N _p for thickness direction vibration and radial direction vibration, and mechanical quality factor Q _M for radial direction vibration. Shows grain size dependence. However, only K _p shows a maximum value when the crystal grain size is approximately 0.2 μm, and conversely decreases as the crystal grain size increases. Conventionally, in piezoelectric ceramics, the crystal grain size is generally
It has been said that when the thickness becomes less than 1 μm, ferroelectricity and piezoelectricity deteriorate or disappear. for example,
In BaTiO ₃ porcelain, which has the same crystal structure as PbTiO ₃ porcelain, when the crystal grain size becomes 1 μm or less, the dielectric constant increases by more than four times, and ferroelectricity and piezoelectricity disappear. The cause of this is thought to be that internal stress due to crystal anisotropy (in the case of BaTiO ₃ , c-a/a = 1%, a and c are lattice constants) acts on the crystal grains. . However, contrary to conventional wisdom, the piezoelectric ceramic of the present invention has a crystal grain size of 0.1 μm or less even if the crystal grain size is 1 μm or less.
Up to m, the dielectric constant does not become extremely large as shown in FIG. 1, and the piezoelectric properties do not particularly deteriorate as shown in FIG. Next, we will discuss the measurement results of SAW propagation loss. Figure 3 shows the measurement frequency as a parameter.
The dependence of SAW propagation loss on grain size is shown. In FIG. 3, the SAW propagation loss α shows a curved downward convex change with respect to a change in the crystal grain size D.
The region where the SAW propagation loss is relatively small is about 0.1 to 5 μm at low frequencies (for example, 60 MHz), and about 0.1 to 1 μm at high frequencies (for example, 630 MHz). When considered as a high frequency material, the optimum crystal grain size can be determined to be 0.1 to 1 μm. In Figure 4, with the grain size D as a parameter,
This shows the frequency dependence of SAW propagation loss. D=0.2,
The propagation loss when 0.9 μm is D>1 μm (for example, D=5 μm) and D<0.1 μm (for example, D
It can be seen that the propagation loss is clearly smaller than the propagation loss when the value is 0.07 μm). Furthermore, the characteristics indicated by the broken line in FIG. 4 are those of PZT ceramics, which is said to exhibit the lowest propagation loss among the piezoelectric ceramics reported so far. D = 0.2, this PbTiO ₃ with a grain size of 0.9 μm
The propagation loss of PZT-based ceramics is about one-fourth of that of conventional PZT-based ceramics (at the same frequency).
dB value comparison), it is obvious that it is clearly small. Now, the characteristic changes shown in FIGS. 3 and 4 described above will be explained in some detail. In Figure 3, as a result of examining the characteristics when D≧1μm using the least squares method, the propagation loss α (dB/cm) of this PbTiO ₃ ceramic is
(cm) and frequency f (MHz), it was found that the relationship is approximately D1×10 ⁻³ f ² +3×10 ³ D ³ f ⁴ (1). The first term in equation (1) is loss due to internal friction of ceramics, and the second term is scattering loss due to crystal grains. According to equation (1), in a region where the grain size D is small and the second term can be ignored with respect to the first term,
SAW propagation loss can be thought of as the internal friction of ceramics itself. Therefore, the third
The reason why the propagation loss increases as D decreases in the range of D < 1 μm, especially D < 0.1 μm, is as follows.
It is estimated that this is due to an increase in internal friction, which is
It is thought that this corresponds to the deterioration of the piezoelectric characteristics in the vicinity of 0.1 μm or less, as shown in FIG. 2 described above. On the other hand, in a region where the first term of equation (1) can be ignored compared to the second term, the SAW propagation loss is almost solely due to scattering loss due to the crystal grain size. In Figure 4, D
=5μm propagation loss compared to D<1μm propagation loss,
This is considered to be the reason why it becomes clearly large in the frequency region of f≧100 MHz or higher. Example 2 The piezoelectric ceramics shown in Example 1 are as follows:
We fabricated a prototype high-frequency filter using a sample whose SAW propagation loss was extremely small compared to that of the sample used, and evaluated its filter characteristics. As a sample, D0.5μm obtained in Example 1
PbTiO ₃ ceramics were used. The electrode pattern includes electrode width,
Regular electrodes with constant pitch and intersection width were used. There are four set center frequencies: 130, 260, 325, and 500MHz. The main dimensions of the electrode pattern are shown in Table 1.
In designing the electrode pattern, we first determined the electrode width and the number of pitch electrode pairs, and then determined the intersection width to minimize mismatch loss in the 50Ω measurement system. A typical example of the amplitude frequency characteristics of the obtained element is shown in the fifth section.
As shown in FIG. Figures 5 and 6 are
These characteristics correspond to elements designed to have center frequencies of 260 MHz and 500 MHz, respectively, and exhibit extremely good filter characteristics with an insertion loss of approximately 20 dB. A summary of the device characteristics is summarized in Table 2. Furthermore, the temperature change in the center frequency of these filters was approximately 5PPm/°C, which was slightly larger than zero. As mentioned above, as Examples 1 and 2, PbO, TiO ₂ ,
Although only the material properties and surface acoustic wave properties of the ceramic composition consisting of Nd ₂ O ₃ and In ₂ O ₃ are shown, Nd
Instead, almost similar characteristics could be obtained in samples in which at least one rare earth element such as Pr, Sm, or Gd, or In was replaced with Al or Ga. The composition of these materials may be those known as piezoelectric ceramics.

【table】

【table】

[Table] As shown above, the crystal grain size of the present invention is 0.1 μm to 1 μm.
PbTiO ₃ ceramics in the range of SAW
It is a material that has lower propagation loss than conventional materials, has a low dielectric constant, and has excellent temperature stability.Surface acoustic wave elements using this material have a low insertion loss of about 20 dB even in the high frequency range, and have excellent performance. It is clear that the device exhibits temperature characteristics.

[Brief explanation of drawings]

Figure 1 shows the crystal grain size dependence of the crystallographic properties and dielectric properties of the piezoelectric ceramic of the present invention, and the second
The figure shows the dependence of piezoelectric properties on crystal grain size.
The grain size dependence of SAW propagation loss, Figure 4, is
The frequency dependence of the propagation loss of the ceramics of the present invention compared to the propagation loss of PZT-based ceramics,
FIGS. 5 and 6 are diagrams showing representative examples of the amplitude characteristics of the obtained high frequency filter.

Claims (1)

[Claims] 1. A piezoelectric ceramic composition containing PbTiO ₃ as a main component and comprising crystal grains with a grain size of 0.1 to 1.0 μm. 2 The main component is _PbTiO3 , and some of the Pb is Pr, Nd,
At least one element selected from the group consisting of Sm, and Gd is substituted, and a part of Ti is replaced with Mn, In, and Al.
and at least one selected from the group consisting of Ga
Claim 1 having a composition substituted with an element
The piezoelectric ceramic composition described in . 3. In a surface acoustic wave element in which at least one metal electrode having a function of exchanging electric signals and surface acoustic wave signals is attached on a piezoelectric substrate, as the piezoelectric substrate
A surface acoustic wave device using a piezoelectric ceramic composition containing PbTiO ₃ as a main component and having crystal grains with a grain size of 0.1 to 1.0 μm. 4 The piezoelectric substrate has PbTiO ₃ as a main component, and a part of Pb is replaced with at least one element selected from the group consisting of Pr, Nd, Sm and Gd, and
Claim 3 using a piezoelectric ceramic composition having a composition in which a part of Ti is replaced with at least one element selected from the group consisting of Mn, In, Al, and Ga.
The surface acoustic wave device described in .