JPH1045470A - Piezoelectric ceramics - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the deterioration of characteristics even under thermal shock from the outside and to prevent the considerable variation of piezoelectric characteristics from the initial characteristics by incorporating grains each having a specified angle between the polarization direction and each domain wall. SOLUTION: Piezoelectric porcelain represented by the formula (where x>0, y>0, 0<x+y<=0.12, 0.970<=a<=1.030, 0.100<=b<=0.700, 0.100<=c<=0.700, 0<=d<=0.170, 0.030<=e<=0.200, 0.480<=f<=0.60, A is at least one among Ca, Sr and Ba and B is at least one among Nd, Gd, La, Pr and Sm) is polarized by applying a DC electric field corresponding to <=75% of a saturation electric field for 0.2-1.0hr and it is aged by holding at a temp. corresponding to 80-97% of the Curie temp. (280-330 deg.C) for 0.5-3.0hr to obtain the objective piezoelectric ceramics contg. >=77% grains each having 40-140 deg. angle between the polarization direction A and each domain wall 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a ceramic filter, a ceramic resonator, an ultrasonic transducer,
Related to piezoelectric ceramics suitable for piezoelectric buzzers, piezoelectric ignition units, ultrasonic motors, piezoelectric fans, piezoelectric actuators and piezoelectric sensors such as acceleration sensors, knocking sensors, AE sensors, etc., and in particular to piezoelectric ceramics optimal as ceramic filters for cellular use. is there.

[0002]

2. Description of the Related Art Conventionally, as elements such as ceramic filters and ceramic resonators, PZT-based piezoelectric ceramics,
That, PbZrO ₃ -PbTiO ₃ and ceramics as a main component is utilized, this Nb ₂ O ₅ or metal oxides such as _{MnO 2, Pb (Nb 2/3 Mg} 1/3) O 3 and Pb ( Nb
By adding or replacing a composite perovskite oxide such as _2/3 Mn _1/3 ) O _{3, the} piezoelectric characteristics are improved.

Incidentally, PZT-based piezoelectric ceramics belong to ferroelectrics, and have spontaneous polarization at room temperature due to a shift in the center of gravity of charges in a crystal lattice. PZ
T-based piezoelectric ceramics are aggregates of sintered particles composed of such crystals. When produced by a solid-phase reaction, individual sintered particles are also formed from a plurality of domains in which spontaneous polarization has a certain direction. Be composed. Within one domain,
Although spontaneous polarization is aligned in a certain direction, the porcelain as a whole does not show piezoelectric characteristics because the direction of spontaneous polarization is isotropic. Therefore, in order to obtain piezoelectric characteristics, it is necessary to apply an electric field to the piezoelectric ceramics from the outside to perform a process of aligning the direction of spontaneous polarization of each domain, that is, a polarization process. In general, the piezoelectric ceramics immediately after the polarization processing are likely to deteriorate in the piezoelectric characteristics and cannot withstand actual use. Therefore, a process for forcibly deteriorating the piezoelectric characteristics by applying heat or the like from the outside is performed in advance. This aging process stabilizes the characteristics of the piezoelectric ceramic.

[0004] In recent years, surface mounting of components has been rapidly progressing along with miniaturization of electronic devices including communication devices. In this surface mounting, a component having no lead wire is temporarily mounted on a base, and the base is heated and soldered by a reflow furnace. You will be exposed to temperature. Due to the heating due to this, before and after the soldering reflow process, for example, for the piezoelectric element,
Problems such as the resonance frequency deviating from the initial frequency often occur.

Therefore, a piezoelectric element used for a piezoelectric filter or the like for a communication device, which requires high reliability and stability, has high stability even when a thermal shock is applied from the outside, and changes in piezoelectric characteristics. Must be very small.

[0006]

However, conventional piezoelectric ceramics are susceptible to external thermal shock, and have large changes in piezoelectric characteristics and resonance frequency before and after reflow soldering, which poses a practical problem. Further, for example, when the filter is used for a filter of a vehicle-mounted communication device or the like in which the environment changes drastically, there has been a problem that stable transmission and reception cannot be performed due to a change in element characteristics.

[0007]

The present inventors have conducted intensive studies on the above problem and found that when the distribution of the angle between the polarization direction and the domain wall was obtained in the range of 0 to 180 °, the distribution was 4 ° C.
By controlling the polarization state so that the proportion distributed in the range of 0 to 140 ° becomes 77% or more, a domain structure having high stability can be obtained, and even a thermal shock from the outside can be obtained. The present inventors have found that characteristic deterioration can be suppressed, and have reached the present invention.

That is, the piezoelectric ceramic of the present invention is a polycrystal of crystal grains having individual domain walls at an angle of 0 to 180 ° with respect to the polarization direction. The ratio of crystal grains having an angle of 40 to 140 ° is 77% or more.

[0009]

According to the piezoelectric ceramics of the present invention, even when a thermal shock is applied to the piezoelectric ceramics from the outside as in the reflow soldering process, the characteristic deterioration is small and the piezoelectric characteristics do not largely change from the initial characteristics.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The piezoelectric ceramic of the present invention is a polycrystal of crystal grains having individual domain walls at an angle of 0 to 180 ° with respect to the polarization direction. The ratio of crystal grains having an angle of 40 to 140 ° is 77% or more.

As described above, the angle between the polarization direction and the domain wall within the range of 40 to 140 ° is 77% of the whole.
The reason for the above is that when less than 77%, the filter is susceptible to thermal shock, has large changes in the piezoelectric characteristics and resonance frequency before and after reflow soldering, and is used for filters of communication devices mounted on vehicles, which are subject to drastic environmental changes. This is because when used, the characteristics of the element are greatly deteriorated. When the angle between the polarization direction and the domain wall is in the range of 40 to 140 °, 82
% Is desirable.

Here, the structure of the domain in the sintered particles of the piezoelectric ceramic is observed by cutting a sample of the sintered body to several tens of microns, performing ion etching, and then performing TEM (transmission electron microscopy) of the sample thin. (Microscope) Take a picture. When preparing a sample slice, it is necessary to minimize external heating so that the domain structure of the sample is not changed by this.

When a TEM photograph of a sample slice having a plane parallel to the direction of the polarization axis is taken, a clear streak structure due to a domain structure is observed inside each sintered particle. FIG. 1 shows a schematic diagram of this domain structure.
In FIG. 1, the boundary of the streak structure is referred to as the domain wall 1. When the distribution of the angle θ formed between the polarization direction A and the domain wall 1 is examined, the domain wall is randomly present in the sample before the polarization treatment. The angular distribution is isotropic. on the other hand,
With respect to the porcelain after the polarization treatment, the domain wall moves according to the electric field strength, and the orientation becomes higher. As compared with the case of the sample before the polarization treatment, the proportion distributed in the range of 40 to 140 ° becomes higher.

When the distribution of the angle between the direction of the polarization axis and the domain wall of the piezoelectric ceramic subjected to the polarization treatment and the aging treatment is examined, the range of 40 to 140 ° is 77 °.
%, The piezoelectric characteristics do not fluctuate significantly from the initial characteristics even when exposed to high temperatures due to soldering reflow processing, etc.
A highly stable piezoelectric ceramic can be provided.

[0015] The piezoelectric ceramic of the present invention, for example, first, the composition formula _{(Pb 1-xy A x B} y) a [{(Nb 1/2
Yb _1/2 ) _b (Nb _1/2 Cr _1/2 ) _c (Nb _2/3 Co
_{1/3) 1-bc} 1-} d Nb d] e (Ti f Zr 1-f) 1-e
When expressed as O ₃ , the x, y, a, b, c, d,
When the values of e and f are 0 <x, 0 <y, 0 <x + y ≦ 0.1
2, 0.970 ≦ a ≦ 1.030, 0.100 ≦ b ≦
0.700, 0.100 ≦ c ≦ 0.700, 0 ≦ d ≦
0.170, 0.030 ≦ e ≦ 0.200, 0.480
A piezoelectric ceramic satisfying ≦ f ≦ 0.60 is manufactured. Here, A is at least one of Ca, Sr and Ba, and B is at least one of Nd, Gd, La, Pr and Sm. The present invention is not limited to the piezoelectric ceramic having the above composition, but may be any PZT piezoelectric ceramic.

Then, a polarization process and an aging process are performed on the piezoelectric ceramic. The polarization treatment is performed by applying a DC electric field of 75% or more to the electric field (saturation electric field) when the polarization of the porcelain is saturated, in particular, by applying a saturation electric field for 0.2 to 1.0 hour, thereby obtaining a PZT piezoelectric ceramic. The ratio at which the angle between the polarization direction and the domain wall is distributed within the range of 40 to 140 ° increases. In the piezoelectric ceramic of the above composition formula, the saturation electric field is 1.5 to 3 kV / mm under the condition of 80 ° C.

Thereafter, an aging process for stabilizing characteristics is performed. This aging treatment is performed at a Curie temperature of 8
At a temperature of 0-97%, especially 87-97%,
By holding for 3.0 hours, the distribution of the angle between the polarization axis direction of the PZT-based piezoelectric ceramic and the domain wall is reduced to 0 to 180.
When determined in the range of °, the distribution ratio in the range of 40 to 140 ° can be increased. In the piezoelectric ceramic of the above composition formula, the Curie temperature is 280 to 330 ° C., and the aging temperature is 220 ° C. to 320 ° C., particularly 2 ° C.
40-320 ° C. In addition, as an aging treatment method after performing the polarization treatment, it is possible to control the orientation of the domain walls of the piezoelectric ceramics by performing aging treatment by applying pressure or an electric field in addition to aging by heat. Aging is most desirable.

When the distribution of the angle between the polarization direction and the domain wall is determined in the range of 0 to 180 °, the ratio of the distribution in the range of 40 to 140 ° is 77% or more. It is obtained by the polarization treatment and the aging treatment under the above conditions.

[0019]

EXAMPLES As starting materials, PbO, ZrO ₂ , TiO
_{2, Nb 2 O 5, Yb} 2 O 3, Cr 2 O 3, CoO, S
Using a raw material of rCO ₃ and La ₂ O ₃ , the composition formula is (Pb
_{0. 96 Sr 0.03 La 0.01) 1.00} [{(Nb 1/2 Yb 1/2)
_0.263 (Nb _1/2 Cr _1/2 ) _0.263 (Nb _2/3 C
o _1/3 ) _0.474 ｝ _0.88 Nb _0.12 ] _0.06 (Ti _0.495 Zr
_0.505) were weighed so that _0.94 O _3, the mixture Z
The mixture was wet-mixed in a ball mill using an rO ₂ pole for 12 hours.

Next, after dehydrating and drying the mixture,
The calcined product was calcined at 1000 ° C. for 3 hours in the air, and the calcined product was pulverized again by a ball mill. Thereafter, an organic binder (PVA) was mixed with the pulverized product to granulate. The obtained powder was compressed under a pressure of 1.5 t / cm ^{2 to} a diameter of 23 mm and a thickness of 2 m.
It was press-formed into a disk having a size of m.

Further, these compacts were sealed in a container made of MgO or the like and fired at 1300 ° C. in the atmosphere for 2 hours to obtain samples A to E.

A silver electrode was baked on the obtained piezoelectric ceramics, and a sample A was 1.4k in silicon oil at 80 ° C.
V / mm (70% of the saturation electric field), sample B was 1.5 kV /
mm (75% of the saturation electric field), sample C is 1.7 kV / mm
(85% of the saturation electric field), 1.9 kV / mm (95% of the saturation electric field) for sample D, and 2.0 kV / mm (100% of the saturation electric field) DC electric field for sample E for 30 minutes to perform polarization treatment. did. The saturation electric field was 2.0 kV / mm.

The polarized piezoelectric ceramics were aged at a temperature of 250 to 295 ° C. (Curie temperature of 310 ° C.) for 1 hour to obtain samples A to E having the same degree of polarization. Sample A was 250 ° C. (80% of the Curie temperature), and Sample B was 270 ° C. (87% of the Curie temperature).
%), Sample C was 280 ° C. (90% of the Curie temperature), sample D was 292 ° C. (94% of the Curie temperature), and sample E was 2%.
Aged at 95 ° C. (95% of Curie temperature). Samples A to E have the same degree of polarization, and Kp = 45%. Then, the samples A to E are shaved to several tens of microns, and ion etching is performed. Then, a TEM (transmission electron microscope) photograph of the sample slice is taken, and the structure of the domain of each sample is observed in an area of 20 μm × 20 μm. I went in. in this case,
The area of a region having a domain wall at a fixed angle (for example, 40 to 50 °) with respect to the polarization axis was determined, the ratio to the entire area was determined, and the ratio having a domain wall in the range of 40 to 140 ° C. was determined. In some cases, domain walls having different angles are present in one crystal grain. In this case, the measurement was performed separately for each region. In some cases, the domain wall is not shown in the photograph, but this was not included in the total area.

Next, FIG. 2 shows the relationship between the temperature of the aging treatment and the ratio of the domain wall distributed in the range of 40 to 140 ° with respect to the polarization direction. In addition, a heat resistance test was performed using a soldering reflow furnace. The heat resistance test is performed by placing the samples A to E in a plastic case on a substrate and passing the sample three times through a reflow furnace having a peak temperature of 240 ° C., and changing the aging temperature and the mechanical coupling coefficient Kp before and after the solder reflow process. About the relationship with the rate,
As shown in FIG.

According to FIG. 2, when the aging temperature is 270 ° C. or more, the ratio of the angle formed between the polarization axis direction and the domain wall within the range of 40 ° to 140 ° is 77% or more, and the aging temperature is higher. It can be seen that when the temperature is 280 ° C. or more, it becomes 82% or more. From FIG. 3, when the ratio between the polarization direction and the domain wall distributed within the range of 40 to 140 ° is 77% or more, the reduction rate of the mechanical coupling coefficient Kp is 2% or less, When the angle formed between the polarization direction and the domain wall in the range of 40 to 140 ° is 82% or more, the reduction rate of the mechanical coupling coefficient Kp is 1% or less, and the heat resistance is excellent and the characteristics are excellent. Changes are small.

In comparison with this, the ratio of the domain wall distributed in the angle range of 40 to 140 ° with respect to the polarization direction is 72%.
%, The rate of reduction of the mechanical coupling coefficient Kp is 4.5%, which indicates that the change in the mechanical coupling coefficient Kp before and after the reflow soldering process is large.

FIG. 4 and FIG. 5 show the angular distribution of the domain wall for the samples A and D. In the case of the sample D having a high degree of polarization before the aging treatment, compared with the sample A, 40 to
It can be seen that the ratio of distribution in the range of 140 ° increases, and that 86% of the entire domain wall is distributed in this angle range.

Therefore, the polarization treatment is performed at 70% of the saturation electric field, and the aging treatment temperature is set to 270 ° C. or more.
By controlling the polarization state so that the ratio of the domain wall distributed in the angle range of 40 to 140 ° with respect to the polarization direction is 77% or more, the characteristics are not deteriorated even when exposed to a high temperature in the reflow soldering process. It is small, and the piezoelectric characteristics do not greatly change from the initial characteristics. On the other hand, if the proportion of the domain wall distributed in the angle range of 40 to 140 ° with respect to the polarization direction is smaller than 77%, the deterioration of the characteristics becomes larger than 2%, which is not practical.

[0029]

According to the piezoelectric ceramics of the present invention, even when a thermal shock is applied from the outside such as a reflow soldering process, the piezoelectric characteristics do not largely fluctuate from the initial characteristics before and after the thermal shock, and the reliability is high. And stable piezoelectric ceramics can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic diagram for explaining an angle between a polarization direction and a domain wall.

FIG. 2 shows that the angle between the polarization direction and the domain wall is 40 to 1
It is the figure which showed the ratio distributed in the range of 40 degrees with respect to the temperature of an aging process.

FIG. 3 is a diagram showing characteristic deterioration of a piezoelectric ceramic in which the orientation of domain walls is controlled before and after a reflow test.

FIG. 4 is a diagram showing a distribution of an angle between a polarization direction and a domain wall of a sample A subjected to a polarization process and an aging process.

FIG. 5 is a diagram showing a distribution of an angle between a polarization direction and a domain wall of a sample D that has been subjected to a polarization process and an aging process.

[Explanation of symbols]

 1 ... domain wall A ... polarization direction θ ... angle between the polarization direction and domain wall 1

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location H04R 17/00 H01L 41/18 101B 330 41/22 B

Claims (1)

[Claims] 1. A polycrystal of crystal grains having individual domain walls at an angle of 0 to 180 ° with respect to the polarization direction,
The angle between the polarization direction and the domain wall is 40 to 1
A piezoelectric ceramic, wherein a ratio of crystal grains at 40 ° is 77% or more.