JP2004262693A - Piezoelectric ceramics - Google Patents

Abstract

[PROBLEMS] The effect of lead on the environment is a problem because it contains a large amount of lead. In addition, since it has a large relative dielectric constant and a small mechanical quality factor, it cannot be used for electronic components used in a high frequency band.
SOLUTION: The general formula (1-x) is represented by SrBi ₂ Nb ₂ O ₉ -xLnBi ₂ TiNbO ₉ (where Ln is a rare earth metal), and x is in a composition range of 0.05 <x <0.2. Main component is ceramics.
[Effect] The absolute value of the temperature coefficient of the resonance frequency can be reduced, the relative dielectric constant can be reduced, and the mechanical quality factor Qm can be increased. In addition, environmental problems caused by lead could be solved.

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a piezoelectric ceramic used for a ceramic resonator, a ceramic filter, a temperature sensor, and the like.
[0002]
[Prior art]
As a conventional piezoelectric ceramic, there is a PbZrO ₃ -PbTiO ₃ solid solution called PZT having a composition near the MPB (morphotropic phase boundary). Piezoelectric ceramics having such a composition are used for various electronic components because they are inexpensive and have good electric characteristics.
However, since this kind of piezoelectric ceramics contains a large amount of lead, lead is released into the atmosphere during heat treatment such as sintering and melting in the manufacturing process, or after being marketed in the form of electronic components, Even when these electronic components are discarded, lead is released into the soil and the effect of lead on the environment becomes a problem, and industrial use has been prohibited or restricted.
In recent years, the conversion of electric signals from analog to digital has been rapidly performed, and the use range of electronic components has been expanding to a high frequency range. However, the above-described piezoelectric ceramics cannot be used for electronic components used in a high frequency band because the relative dielectric constant is as large as 1000 or more and the mechanical quality factor Qm is as small as 2500 or less.
In such a situation, a bismuth layered ferroelectric has attracted attention as a material that does not contain lead, has a small relative dielectric constant, and has a large mechanical quality factor Qm. (See, for example, Patent Documents 1 and 2.)
[0003]
[Patent Document 1]
JP-A-2002-145669 [Patent Document 2]
JP 2001-294486 A
[Problems to be solved by the invention]
However, the bismuth layer structure ferroelectric has an absolute value of the temperature coefficient of the resonance frequency of more than 30 ppm / ° C., and has not been put to practical use in electronic parts.
[0005]
The present invention can reduce the absolute value of the temperature coefficient of the resonance frequency, reduce the relative dielectric constant, and increase the mechanical quality factor Qm, and eliminate the problem of environmental problems caused by lead. The purpose is to provide.
[0006]
[Means for Solving the Problems]
The piezoelectric ceramic of the present invention solves the above-mentioned problem by forming a solid solution of Sr-Bi-Nb-based ceramics and Ln-Bi-Ti-Nb-based ceramics, which are a kind of bismuth layer structure ferroelectric.
The piezoelectric ceramic of the present invention is represented by the general formula (1-x) SrBi ₂ Nb ₂ O ₉ -xLnBi ₂ TiNbO ₉ (where Ln is a rare earth metal), and x has a composition of 0.05 <x <0.2. The main component is ceramics in the range.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
SrBi-Nb-based ceramics (in _{particular, SrBi 2 Nb 2 O 9)} LnBi-Ti-Nb based ceramics (in _particular, LnBi 2 TiNbO ₉₎ is solid solution. As Ln, any one of La, Nd, Sm, and Gd, which is a kind of rare earth element, is used. Therefore, in the piezoelectric ceramic of the present invention, the absolute value of the temperature coefficient of the resonance frequency can be adjusted by the amount of solid solution of the Ln-Bi-Ti-Nb-based ceramic.
[0008]
【Example】
Hereinafter, examples of the present invention will be described.
First, a method for manufacturing a piezoelectric ceramic according to the present invention will be described. Raw material powders of SrCO ₃ , La ₂ O ₃ , Nd ₂ O ₃ , Sm ₂ O ₃ , Gd ₂ O ₃ , Bi ₂ O ₃ , Nb ₂ O ₅ , and TiO ₂ are weighed so as to have a predetermined composition, and a ball mill. And wet-mixed for 7 hours. These mixed powders were calcined at 800 to 1000 ° C., and the calcined product was wet-ground using a ball mill or the like for 7 hours. After the pulverized product was dried, a binder such as PVA was added to the pulverized product, granulated, molded, and fired to obtain a material according to the present invention.
[0009]
The characteristics of the piezoelectric ceramics according to the present invention are measured by adding a binder such as PVA to the dried pulverized material in an amount of 1 to 3% by weight and granulating the dried pulverized material into a disk having a diameter of 15 mm and a thickness of 1 to 2.5 mm. Press molding at a pressure of 100 MPa, and baking so that the maximum holding temperature is 1100 to 1250 ° C. and the holding time at the maximum holding temperature is 2 hours to obtain a disc-shaped porcelain. Is 0.2 to 2 mm, and silver electrodes are formed on both sides. Then, in an insulating oil, the temperature is 100 to 250 ° C., the electric field is 5 to 12 kV / mm, and the time is 1 to 30 minutes. An evaluation sample was obtained by performing a polarization treatment under the following conditions. For the measurement of this characteristic, the electromechanical coupling coefficient Kt and the mechanical quality coefficient Qm of the thickness longitudinal fundamental vibration were calculated using the resonance / antiresonance method. The temperature change of the resonance frequency was measured using a thermostat and an impedance analyzer, and the temperature coefficient of the resonance frequency (fr-TC) was calculated using the following equation.
fr-TC = (fr _{50 ° C.} −fr− _{10 ° C.} ) / (fr _{25 ° C.} × 60) × 10 ⁶
(However, fr _{50 ° C.} indicates a resonance frequency at 50 ° C., fr _{−10 ° C.} indicates a resonance frequency at −10 ° C., and fr _{25 ° C.} indicates a resonance frequency at 25 ° C.)
The Curie temperature Tc was calculated by measuring a change in relative permittivity with temperature.
[0010]
FIG. 1 shows (1-x) SrBi ₂ Nb ₂ O ₉ -xLnBi ₂ TiNbO ₉
(Where Ln is any one of La, Nd, Sm, and Gd), the electromechanical coupling coefficient Kt, mechanical quality coefficient Qm, and resonance frequency of the thickness longitudinal fundamental vibration when the value of x is changed. The temperature coefficient fr-TC, the Curie temperature Tc, and the firing temperature are summarized in a table. In addition, * mark of a sample No has shown that it is out of the range of the present invention.
When Sm was used as Ln, the temperature coefficient of the resonance frequency at which the value of x was 0 was −36 ppm / ° C., but when the value of x was 0.05, the temperature coefficient became −31 ppm / ° C. The absolute value of the temperature coefficient of the resonance frequency became −26 ppm / ° C. when the value of x was 0.1, and the absolute value of the temperature coefficient of the resonance frequency became the minimum when the value of x was 0.1. When the value of x exceeded 0.1, the absolute value of the temperature coefficient of the resonance frequency increased as the value of x increased. When La is used as Ln, the temperature coefficient of the resonance frequency at which the value of x is 0 is −36 ppm / ° C., but when the value of x is 0.1, the temperature coefficient becomes −28 ppm / ° C. The absolute value of the temperature coefficient of the resonance frequency became smaller, and when the value of x was set to 0.2, the absolute value of the temperature coefficient of the resonance frequency became larger than when the value of x was 0.1. Further, when Nd is used as Ln, the temperature coefficient of the resonance frequency where the value of x is 0 is −36 ppm / ° C., but when the value of x is 0.1, the temperature coefficient becomes −22 ppm / ° C. The absolute value of the temperature coefficient of the resonance frequency became smaller, and when the value of x was set to 0.2, the absolute value of the temperature coefficient of the resonance frequency became larger than when the value of x was 0.1.
[0011]
As described above, when the value of x is 0.05 or less, the absolute value of the temperature coefficient of the resonance frequency is large, and when the value of x is 0.2 or more, the absolute value of the temperature coefficient of the resonance frequency is 30 ppm / ° C. or more. Or the mechanical quality factor Qm is 3500 or less. Further, the relative dielectric constant was 200 or less when the value of x was in the range of 0.05 <x <0.2. When the value of x is increased, the Curie temperature Tc tends to decrease. However, the temperature is always 400 ° C. or more in the range of 0.05 <x <0.2. It is in a state sufficiently higher than the reflow temperature at the time of attachment, and is not depolarized by soldering.
[0012]
As described above, the piezoelectric ceramic of the present invention can obtain a good mechanical quality factor Qm when the value of x is in the range of 0.05 <x <0.2, and can reduce the absolute value of the temperature coefficient of the resonance frequency. Could be smaller. In particular, in the case where Nd is used as Ln and the value of x is set to 0.1, the absolute value of the temperature coefficient fr-TC of the resonance frequency is greatly improved as compared with the case where the value of x is 0.
[0013]
【The invention's effect】
The piezoelectric ceramic of the present invention is represented by the general formula (1-x) SrBi ₂ Nb ₂ O ₉ -xLnBi ₂ TiNbO ₉ (where Ln is a rare earth metal), and x has a composition of 0.05 <x <0.2. Since the main component is ceramics in the range, the absolute value of the temperature coefficient of the resonance frequency can be made smaller than 30 ppm / ° C. as compared with the conventional one, and the relative dielectric constant is reduced and the mechanical quality factor Qm is reduced. Can be larger. In addition, since the piezoelectric ceramic of the present invention does not contain lead, lead is not released during the manufacturing process or when electronic components are discarded, and there is no environmental problem.
[Brief description of the drawings]
FIG. 1 is a table of characteristics for explaining characteristics of a piezoelectric ceramic of the present invention.

Claims (3)

General formula (1-x) SrBi ₂ Nb ₂ O ₉ -xLnBi ₂ TiNbO ₉
(Where Ln is a rare earth metal), and x is in the composition range of 0.05 <x <0.2 as a main component. The piezoelectric ceramic according to claim 1, wherein Ln is a lanthanoid. The piezoelectric ceramic according to claim 1, wherein Ln is any of La, Nd, Sm, and Gd.

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