JP2910392B2 - Manufacturing method of thickness vibration pressure ceramic transformer - 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 transformer operable in a high-frequency band, and more particularly to a method for manufacturing an on-board piezoelectric transformer which requires a reduction in size and noise.

[0002]

2. Description of the Related Art In recent years, an electromagnetic transformer has been used as a switching power supply in order to reduce the size of a power supply circuit of an electronic device. To reduce the size of the switching power supply, it is desirable to increase the switching frequency. However, when the switching frequency is increased, the hysteresis loss, the eddy current loss, and the loss due to the skin effect of the conductive wire of the magnetic material used in the electromagnetic transformer increase sharply, and the efficiency of the transformer becomes extremely low. For this reason, the upper limit of the practical frequency band of the electromagnetic transformer is at most 500 kHz. On the other hand, the laminated piezoelectric transformer is used in a resonance state, and compared to a general electromagnetic transformer, (1) high energy density at the same frequency enables downsizing, (2) non-combustibility, 3) It has many advantages such as no noise due to electromagnetic induction. This multilayer piezoelectric transformer is manufactured by a polarization process described below. As shown in FIG. 6A, the piezoelectric ceramic layer 1
4 is 5 kV / m between external electrodes 15 (e) and 15 (f) provided on the upper and lower surfaces of the laminate by a low impedance portion 11 composed of five layers and a high impedance portion 12 composed of one layer.
A high DC voltage of m is applied so that the polarization direction of each layer of the laminate is one direction in the thickness direction. Next, as shown in FIG. 6B, 5 kV / V is applied between the external electrodes 15 (a) and 15 (b) in which the internal electrode layers are alternately connected to the low impedance portion 11.
A high DC voltage of 1 mm is applied to reverse the polarization direction every other layer.

[0003]

As shown in the above conventional example, the laminated piezoelectric ceramic transformer performs two polarization operations in order to reverse the polarization direction every other low impedance part. I have. At this time, in the layer in which the polarization direction is opposite to the first polarization direction at the time of the second polarization, the polarization direction once aligned in one direction is reversed, so that internal stress occurs at the time of the reversal, and cracks and piezoelectric ceramics are generated. There is a disadvantage that peeling between the layer and the internal electrode layer occurs. The present invention has been made to overcome the above drawbacks.

[0004]

SUMMARY OF THE INVENTION The present invention comprises a step of alternately laminating a plurality of internal electrode layers and piezoelectric ceramic layers, and a method of forming an internal electrode layer on each of a low impedance portion and a high impedance portion of the piezoelectric ceramic layer. Providing a step of providing external electrodes connected every other, and a step of polarizing adjacent piezoelectric ceramic layers through the internal electrode layers in opposite directions, comprising: A method of manufacturing a thickness vibrating piezoelectric transformer, wherein a DC voltage having the same electric field strength is simultaneously applied to an external electrode of a high impedance portion at a temperature equal to or higher than the Curie point, and the temperature is gradually lowered to cause polarization.

[0005]

According to the present invention, there is provided a piezoelectric transformer having low loss and sufficient functions at a high frequency of 1 MHz or more. As shown in FIG. 1, it is a feature of the present invention that the low impedance section and the high impedance section have a polarization step of simultaneously applying a DC voltage with the same electric field strength. At the same time, when a DC voltage is applied, polarization is performed by applying the DC voltage at a temperature equal to or higher than the Curie point of the piezoelectric ceramic material in a thermostat and gradually lowering the temperature.

As shown in FIGS. 1 and 2, a piezoelectric ceramic transformer according to the present invention comprises a low-impedance portion 11 in which a large number of piezoelectric ceramic layers 14 polarized in a thickness direction are stacked, and a single-layer or at most a few layers. And a high impedance portion 12 composed of a piezoelectric ceramic layer 14. Here, the piezoelectric ceramic layers of the low impedance section 11 are arranged so that the polarization directions of the adjacent layers are reversed. In addition, the internal electrode layer 13
Is exposed on the side end face of the piezoelectric transformer and is connected to predetermined external electrodes 15 (a), (b), (c), (d),
It is possible to apply an electric field to the piezoelectric ceramic layer in the thickness direction.

A piezoelectric ceramic transformer having such internal electrodes can be manufactured by a multilayer ceramic technique (doctor blade method) used for a multilayer ceramic capacitor, a multilayer piezoelectric actuator, and the like. In the manufactured piezoelectric ceramic transformer, the layer interval is 20μ.
It is also possible to reduce the thickness to about m. Therefore, 2
1 / wavelength mode (fundamental mode with free ends) or 1
Even if a thickness longitudinal resonance vibration of a wavelength mode (a second-order mode free at both ends) is used, it is possible to use ceramic technology to reduce the thickness to 5 to 2 mm.
It is also possible to realize a piezoelectric ceramic transformer that operates in the ultra-high frequency range of the 0 MHz band.

In the high impedance section 12, the external electrodes 15 (c) and (d) are taken out from the internal electrode layers 13 located on the upper and lower surfaces.
External electrodes 1 in which internal electrode layers 13 sandwiching the
5 (a) and 5 (b) are taken out. In this connection, when a high voltage having a frequency equal to the resonance frequency of the thickness longitudinal vibration is applied between the electrode terminals 18 and 19 on the high impedance 12 side, the piezoelectric impedance transformer of the high impedance section 12 causes the entire piezoelectric ceramic transformer to be mechanically driven. The low impedance section 11 generates a voltage having the same frequency as the input voltage due to the positive piezoelectric effect and outputs the voltage to the electric terminals 16 and 17. At this time, the voltage between the output terminals 16 and 17 becomes lower than the voltage between the input terminals 18 and 19 due to a difference in impedance between the input side and the output side. Conversely, when converting a low voltage to a high voltage, a high voltage is output from between the electric terminals 18 and 19 by applying a low voltage between the electric terminals 16 and 17. The piezoelectric transformer according to the present invention can have a three-terminal configuration as well as a four-terminal configuration as shown in FIG.

The resonance frequency of the thickness longitudinal vibration is expressed by the following equation if the decrease in the resonance frequency due to the piezoelectric longitudinal effect is ignored. f _r = n · V _t / 2t (n = 1, 2, 3,...) where, f _r : resonance frequency of thickness longitudinal vibration n: mode order V _t : sound velocity of longitudinal wave in thickness direction t: Thickness

Further, in the rectangular parallelepiped vibrator, vibrations in the length direction and the width direction are also generated via k ₃₁ , and the resonance frequencies thereof are expressed as follows. f _ln = n · V _l / 21 (n = 1, 2, 3,...) f _wn = n · V _w / 2w (n = 1, 2, 3,...) where f _1n : length longitudinal vibration resonance frequency V _l: the length direction longitudinal wave acoustic velocity l: length f _wn: width longitudinal vibration resonance frequency V _w: the width direction of the longitudinal wave acoustic velocity W: width of these longitudinal, in the width direction The vibration is a spurious vibration with respect to the thickness longitudinal vibration. Therefore, in order to realize a wide-band piezoelectric transformer, it is necessary to prevent strong vibration in the length direction and the width direction near the driving frequency.

In the piezoelectric device utilizing a thickness longitudinal vibration as the invention, the material only electromechanical coupling coefficient kt is large to excite a thickness extensional vibration is a piezoelectric longitudinal effect is desirable, the piezoelectric transverse effect, such as k ₃₁ In a material having a large electromechanical coupling coefficient, spurious due to vibration in the length and width directions or contour vibration becomes a problem. PZ Therefore kt / k ₃₁ is small
PbTiO ₃ -based materials having a larger kt / k ₃₁ than T-based piezoelectric materials are more advantageous. However, the PbTiO ₃ -based material has a large crystal lattice anisotropy c / a, and if there is a region in the same device having a different polarization direction, a stress strain occurs there and the material becomes mechanically weak. Also, if there is a process of reversing the direction of polarization or a process of polarizing only a part of the piezoelectric transformer in the course of fabrication, it will be destroyed at the time of polarization due to peeling of the electrode due to the strain generated when the direction of polarization changes, or a mechanical quality factor. Qm may be reduced. On the other hand, according to the piezoelectric ceramic transformer of the present invention, the stress distortion generated in the piezoelectric ceramic transformer is reduced by simultaneously polarizing the low impedance part and the high impedance part, and the electric field strength is reduced by one digit or more by the electric field cooling method. Almost no stress distortion.

[0012]

Embodiments of the present invention will be described below. In this embodiment, a piezoelectric ceramic transformer having the configuration shown in FIGS. 1 and 2 was manufactured by a green sheet method. First, a PbTiO ₃ -based piezoelectric ceramic powder as a piezoelectric material is dispersed in a solvent together with an organic binder to form a slurry. This is formed into a green sheet having a thickness of about 70 μm by a casting method using a doctor blade. Thereafter, the sheet is punched and cut by a punching machine into a shape necessary for lamination and pressure bonding by a hot press machine, that is, a size suitable for a press die. Next, a Pt paste including Pt, a binder, and an organic solvent is printed on the green sheet as an internal electrode layer by a screen printing method. Thereafter, a plurality of green sheets are laminated and pressed by a hot press machine to form an integrated green laminate. The green laminate is heat-treated in air at 500 ° C. for 10 hours to complete the binder removal. Then, it is baked at 1200 ° C. for 2 hours, and the external electrodes 15 (a),
15 (b), 15 (c), and 15 (d) are printed, and the electrode terminals 16, 17, 18, and 19 are connected.

Next, in a thermostat at a temperature equal to or higher than the Curie point of the ceramic material, the space between the electrode terminals 16 and 17
DC voltage (200
５００500 V / mm), and the temperature is lowered as it is to perform a polarization treatment. Finally, parallel plane polishing is performed to a thickness necessary for adjusting the resonance frequency. In this example, in order to drive the piezoelectric transformer at 2 MHz, parallel flat polishing was performed so that the thickness of the piezoelectric transformer was 2.2 mm.

FIGS. 4 and 5 show admittance characteristics of piezoelectric ceramic transformers obtained by the conventional method and the manufacturing method of the present invention, respectively. 4 and 5, it can be seen that the piezoelectric ceramic transformer (FIG. 5) obtained by the manufacturing method of the present invention has extremely small spurious and almost no cracks in the piezoelectric ceramic layer. The resonance frequency of the prototyped piezoelectric transformer was 2 MHz, the mechanical quality factor Qm was 850,
The maximum energy conversion efficiency was 97%. It is clear that the prototyped piezoelectric transformer realizes the step-down function of the transformer and high energy conversion efficiency. In addition,
It goes without saying that the input terminal can be used as a step-up transformer by taking out the output terminal from the low impedance part and the output terminal from the high impedance part.

[0015]

As described above, according to the manufacturing method of the present invention, since the stress generated in the piezoelectric ceramic transformer is greatly suppressed, cracks are generated inside the laminated body and the internal electrodes are separated from the piezoelectric ceramic layer. Is prevented.

[Brief description of the drawings]

FIG. 1 is a sectional view of a transformer for explaining a polarization step of a thickness vibration piezoelectric ceramic transformer according to a manufacturing method of the present invention.

FIG. 2 is a perspective view of an example of a piezoelectric ceramic transformer obtained by the manufacturing method of the present invention.

FIG. 3 is a cross-sectional view of an example of a piezoelectric ceramic transformer laminate obtained by the manufacturing method of the present invention.

FIG. 4 is a diagram showing admittance characteristics of a piezoelectric ceramic transformer element obtained by a conventional manufacturing method.

FIG. 5 is a diagram showing admittance characteristics of a piezoelectric ceramic transformer element obtained by the manufacturing method of the present invention.

FIG. 6 is a cross-sectional view of a piezoelectric ceramic transformer element for explaining a polarization step in a conventional manufacturing method.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 11 Low impedance part 12 High impedance part 13 Internal electrode layer 14 Piezoelectric ceramic layer 15 (a), 15 (b), 15 (c), 15 (d) External electrode 16,17,18,19 Electrode terminal

Claims (1)

(57) [Claims] 1. A step of alternately laminating a plurality of internal electrode layers and piezoelectric ceramic layers, and providing an external electrode in which a low impedance part and a high impedance part of the piezoelectric ceramic layer are connected to every other internal electrode layer. A method for manufacturing a thickness vibration piezoelectric ceramic transformer comprising the steps of: polarizing the piezoelectric ceramic layers adjacent to each other through the internal electrode layer in the opposite direction, wherein the external electrodes of the low impedance part and the high impedance part are: A method of manufacturing a thickness vibrating piezoelectric transformer, wherein a DC voltage having the same electric field strength is simultaneously applied at a temperature equal to or higher than the Curie point and the temperature is gradually lowered to cause polarization.