JP2000252541A - Laminated piezoelectric transformer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laminated piezoelectric transformer formed of piezoelectric ceramic layers which are each 200 μm or below in thickness, large in number of laminated layers, sintered at a temperature of about 1100 deg. or below, possessed of fine and uniform crystalline structure, and excellent in piezoelectric characteristics and mechanical strength. SOLUTION: A laminated piezoelectric transformer is composed of a drive part where piezoelectric ceramic layers 1 that are each polarized in a thick-wise direction and inner electric electrodes 2 whose main component is Ag are laminated and a generating part, where the piezoelectric ceramic layers of the drive part are each 200 μm or below in thickness t, the inner electrode 2 is 2 mg/cm2 or above in surface density x that is represented by weight per unit area, and the ratio of the thickness t (μm) of the piezoelectric ceramic layer to the surface density x of the inner electrode or the ratio t/x is 12 or above.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laminated piezoelectric transformer, which is an inverter circuit component for a cold-cathode tube lighting circuit and a backlight for a small liquid crystal display.

[0002]

2. Description of the Related Art In general, in a liquid crystal display, a liquid crystal itself does not emit light, and thus a backlight system in which a discharge tube such as a cold cathode tube is disposed on the back and side surfaces of a liquid crystal display is mainly used. In order to drive this discharge tube, an alternating high voltage of several hundred volts or more is usually required, depending on the length and diameter of the discharge tube used. An inverter using a piezoelectric transformer is disclosed in Japanese Patent Application Laid-Open No. 5-114492 as a method for generating this high AC voltage.

A piezoelectric transformer has a very simple structure because no winding is required, and can be reduced in size, thickness, and cost. The structure and characteristics of this piezoelectric transformer are described, for example, in the technical magazine “Electro-Ceramics” 197
This is shown in the July, 2001 issue of "Characteristics of Piezoelectric Transformers and Their Applications".

The structure and operation of the most common piezoelectric transformer will be described below with reference to FIG. The one shown in FIG.
In 6 years, the US C.I. A. FIG. 2 is a schematic diagram for explaining a Rosen-type piezoelectric transformer announced by Rosen. The hatched portion particularly indicates an electrode portion.

[0005] In the figure, 1 is, for example, PbTiO ₃ -PbZr
O ₃ (PZT) piezoelectric ceramics. A pair of input voltages 3 and 4 provided by, for example, silver printing are formed on the upper and lower surfaces of the left half in the figure of the piezoelectric transformer, and an output electrode 5 is formed on the right end surface in the same manner. Then, the left half driving section of the piezoelectric transformer performs polarization processing in the thickness direction, and the right half power generation section performs the polarization processing in the length direction as indicated by arrows.

When an AC voltage having substantially the same frequency as the longitudinal resonance frequency is applied between the input electrodes 3 and 4 of the piezoelectric transformer, the piezoelectric ceramic 1 generates strong mechanical vibration in the longitudinal direction. Accordingly, electric charges are generated by the piezoelectric effect in the right half of the power generation unit, and the output voltage V is applied between the output electrode 5 and one of the input electrodes, for example, the input electrode 4.
o occurs.

The step-up ratio (Vo / Vi) (where Vi is an input voltage) obtained by the piezoelectric transformer having the above configuration is as follows:
It is expressed as in equation (1). (Vo / Vi) = A · k ₃₁ · k ₃₃ · Q _M · L / T (1) where k _{31 is the} electromechanical coupling coefficient of the lateral effect, and k _{33 is} :
Electromechanical coupling coefficient of longitudinal effect, Q _M : mechanical quality coefficient,
L: length of piezoelectric transformer, T: thickness of piezoelectric transformer,
A: It is a constant. k ₃₁ , k ₃₃ , and Q _M are material constants determined by the piezoelectric material, and L and T are determined by the dimensions and shape of the element.

The piezoelectric transformer used for the above-described backlight requires a high AC high voltage of several hundred volts or more, so that a high step-up ratio is required. for that purpose,
As can be seen from equation (1), it is effective to reduce the thickness T of the piezoelectric transformer shape or increase the length L.
From the viewpoints of mounting and element strength, there were naturally limits to possible values.

As a method of solving such a problem, a laminated piezoelectric transformer in which thin piezoelectric ceramics are laminated and the drive unit side is connected in parallel is disclosed in, for example, JP-A-07-3.
No. 02938. FIG. 2 is a schematic diagram for explaining a laminated piezoelectric transformer in which such a driving unit is laminated.
Shown in Note that the hatched portions particularly indicate electrode portions. Here, the driving section has a structure in which the piezoelectric ceramics 1 and the internal electrodes 2 are alternately stacked and connected in parallel.

[0010]

The above-described laminated piezoelectric transformer in which thin piezoelectric ceramics 1 and internal electrodes 2 are alternately laminated is generally manufactured by the same manufacturing method as a multilayer ceramic capacitor. That is, a calcined raw material powder composed of a composition containing a metal oxide such as lead, zirconium, or titanium as a main component is formed in a sheet shape, and a noble metal paste to be an internal electrode 2 is printed on a surface thereof, for example, by screen printing. Some of them are laminated, pressed and integrated, and then sintered.

The internal electrode 2 of such a laminated piezoelectric transformer
The noble metal used for the piezoelectric ceramic must not be oxidized and melted at the sintering temperature of the piezoelectric ceramic, and it is desirable that the metal be industrially inexpensive.・
In general, a Pd alloy is used as an internal electrode material. Since the melting point of Pd is high at 1554 ° C., the Ag / Pd alloy withstands sintering at a high temperature as the ratio of Pd is high. On the other hand, Pd tends to change its volume due to oxidation and reduction during sintering. If the ratio is too high, there is a problem that peeling (delamination) from the ceramic element tends to occur during sintering. On the other hand, if the ratio of Pd is suppressed, the melting point decreases, so that the sintering temperature of the piezoelectric ceramic must necessarily be lowered. In a multilayer ceramic capacitor, Ag: P is mainly used as an internal electrode material in a weight% ratio.
An alloy with d = 70: 30 is used. However, in order to use this in a laminated piezoelectric transformer, the piezoelectric ceramic must be sintered at a temperature of about 1100 ° C. However, conventional piezoelectric ceramics have a sintering temperature of about 1250 ° C., and there has been no piezoelectric ceramic that can be sintered at a temperature of about 1100 ° C. in the past.

A laminate made of a conventional piezoelectric ceramic and having an internal electrode made of an Ag / Pd alloy was used in air at 1100 mm.
When sintering at ℃, the sintering promoting action of the internal electrode is 100
It has been reported that a laminated piezoelectric ceramics resonator that can be sintered at a low temperature of ℃ and has excellent piezoelectric properties has been obtained (Jpn.J. App.
l.Phys.Vol34 (1995) p5270-5272). However, the resulting piezoelectric ceramic of the laminated piezoelectric ceramic vibrator has a very uneven crystal structure in which the crystals are coarse near the internal electrode and insufficiently densified and the crystal grain size is small near the internal electrode. Met.

As a piezoelectric vibrator for solving the above-mentioned problems, the present inventors disclosed in Japanese Patent Application No. 10-073744 a sintered composition mainly composed of an oxide of a metal such as lead, zirconium or titanium. The piezoelectric ceramic contains Fe and Ag, and the average crystal grain size of the apparent crystal structure in the cross-sectional observation image of the piezoelectric ceramic is 1 μm or less, and the area of 80% or more of the apparent crystal structure is 1 μm or less. Using piezoelectric ceramics composed of crystal grains, a piezoelectric ceramic having a sintering temperature of about 1100 ° C. or less and excellent in piezoelectric characteristics and mechanical strength, and a laminated piezoelectric ceramic vibrator using the same have been proposed. Furthermore, by the reaction between lead in the piezoelectric ceramic and Ag in the internal electrode, Ag diffuses into the piezoelectric ceramic to promote sintering, and by controlling the atmosphere during sintering, the reactivity is increased. It discloses that control can be performed, whereby a laminated piezoelectric vibrator having a dense and uniform structure at a low temperature of 1100 ° C. or lower can be obtained.

In the above invention, it is necessary to precisely control the atmosphere during sintering. However, in the case of industrially producing a large amount, the sintering density tends to vary between production lots. For a piezoelectric transformer with a piezoelectric ceramic thickness as thin as 200 μm or less and a large number of laminated layers, A
There is a problem that diffusion of g is remarkable and polarization is difficult due to a decrease in insulation resistance. In a piezoelectric transformer used for a backlight inverter, a high output voltage is required when the cold cathode fluorescent lamp is turned on. Therefore, in order to increase the step-up ratio, the number of stacked drive units increases, and the piezoelectric ceramics The plate thickness tends to be thin.

The present invention has been made in order to solve the above-mentioned problems, and in particular, has a piezoelectric ceramic having a thickness of 200 μm.
m and a large number of laminated piezoelectric transformers, the sintering temperature is around 1100 ° C or less, the crystal structure of the piezoelectric ceramics is fine and uniform, and the piezoelectric characteristics and mechanical strength are excellent. The purpose is to provide.

[0016]

Means for Solving the Problems As a result of earnest studies to solve the above-mentioned problems, the inventors have conceived of a laminated piezoelectric transformer having a remarkably improved configuration. That is, the present invention is a piezoelectric transformer having a drive unit formed by alternately stacking piezoelectric ceramics polarized in the thickness direction and an internal electrode mainly composed of Ag and a power generation unit polarized in the length direction, The thickness t of the piezoelectric ceramic per one layer of the driving unit is 200 μm or less, the surface density x expressed by the weight of the internal electrode formed per unit area is 2 mg / cm ² or more, and the thickness of the piezoelectric ceramic is The ratio t / x between t (μm) and the area density x (mg / cm ² ) of the internal electrode is 1
Two or more laminated piezoelectric transformers.

The internal electrode is made of an alloy of Ag and Pd, and the compounding weight ratio of Ag and Pd (Ag: Pd) is (60: 4).
0) to (80:20). Also, as piezoelectric ceramics, lead, titanium, zirconium,
A piezoelectric transformer having particularly good characteristics can be obtained from ceramics containing strontium and iron metal oxides as main components. Further, it is desirable that the length L of the power generation unit in the piezoelectric transformer is 30 mm or less.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present application have conducted intensive studies on the effect of Ag / Pd alloy on sintering of piezoelectric ceramics. As a result, it was found that Ag in Ag / Pd alloy contributes to sintering of piezoelectric ceramics. The degree of Ag diffusion from the internal electrode to the piezoelectric ceramic is correlated with the surface density x (mg / cm ² ) of the internal electrode formed of the Ag · Pd alloy and the plate thickness t (μm) of the piezoelectric ceramic.
It has been found that by optimizing the values of t and t, a laminated piezoelectric transformer having a sintering temperature of about 1100 ° C. or less, a fine and uniform crystal structure of piezoelectric ceramics, and excellent piezoelectric characteristics and mechanical strength can be obtained. .

The sinterability of the piezoelectric ceramic in the piezoelectric transformer depends on the amount of Ag diffused from the internal electrodes.
Therefore, in order to obtain a piezoelectric ceramic having a dense and uniform structure, the amount of Ag in the Ag / Pd alloy as the internal electrode, the position where the internal electrode exists, the distance of the piezoelectric ceramic in which Ag diffuses, and the like must be determined. Must be strictly specified. The inventors of the present application have found through various studies that, in particular, by specifying the items of the present invention, it is possible to provide a piezoelectric transformer having good characteristics.

In the present invention, the Ag content of the internal electrode is (80:20) to (60: 4) in the (Ag: Pd) ratio.
0) is preferable. When the (Ag: Pd) ratio is (80:
If it is more than 20), the Ag content in the piezoelectric ceramics increases, and the insulation resistance is remarkably reduced.
If the ratio is less than (60:40), the diffusion of Ag from the internal electrode is insufficient and a dense sintered body cannot be obtained. Also,
It is desirable that the length L of the power generation unit is 30 mm or less.
This is because if the length of the power generation section exceeds 30 mm, the diffusion distance of Ag from the internal electrode in the drive section becomes too long, the amount of Ag reaching the end of the power generation section becomes insufficient, and the end of the power generation section burns. This is because the consolidation density decreases.

When the piezoelectric ceramic is composed of a piezoelectric material containing a metal oxide of lead, titanium, zirconium, strontium or iron as a main component, it is possible to provide an element suitable for a high power use such as a piezoelectric transformer. Become.
Hereinafter, the effects of the present invention will be specifically described with reference to examples.

[0022]

(Embodiment 1) First, a method of preparing a sample will be described. Lead oxide, titanium oxide, zirconium oxide, strontium carbonate, and piezoelectric ceramics (Pb
_0.95 Sr _0.05 ) (Zr _0.52 Ti _0.48 )
With a composition of O _3, lead oxide to the Pb amount is weighed so as to provide an excess or less 0.3 wt% 0.05 wt% or more, were mixed with a wet ball mill, which is dried, the solution The mixture was crushed and calcined at 850 ° C. for 2 hours to prepare a calcined powder. The crushed powder and Fe ₂ O ₃ were each weighed at 0.7% by weight and put into a ball mill, pulverized by a wet method, and then dried to prepare a calcined raw material powder. The particle size of the calcined raw material powder is appropriately adjusted by the grinding time in a ball mill so as to be 1 μm or less.

Further, 4% by weight of PVB as a binder and 4% by weight of BPBG as a plasticizer were added to the calcined raw material powder, and kneaded in a ball mill for 24 hours using ethyl alcohol as a solvent. After kneading, defoaming and viscosity adjustment were performed, and a green sheet of 60 to 350 microns was produced by a doctor blade method. Thereafter, an internal electrode was printed on the green sheet using a paste of (Ag: Pd) = (70:30) by screen printing, laminated, pressed, and cut into a predetermined shape to obtain a molded body. The number of layers was adjusted by adjusting the number of layers according to the thickness of the green sheet so that the thickness after sintering was about 2 mm. The desired areal density of the internal electrodes was obtained by adjusting the thickness of the printed electrodes by adjusting the emulsion thickness of the screen. This compact was arranged in a firing jig made of alumina or magnesia, degreased, and fired at 1100 ° C. for 2 hours in the atmosphere.
Furthermore, I / O electrodes are printed and baked at 140 ° C, 2kV
/ Mm by applying a polarization electric field of 30 mm
A 6.5 mm × 2 mm Rosen-type laminated piezoelectric transformer was used.

The Ag content in the piezoelectric ceramic of the laminated piezoelectric transformer according to the present invention needs to be confirmed by quantitatively analyzing the Ag content in the sintered body. In this embodiment, as a method of the quantitative analysis, ICP emission spectrometry (Inductively Coupled Plasma Emission Spectroscop) is used.
y) was adopted, and the piezoelectric ceramics of the power generation section after sintering were cut out and analyzed. The analysis points are the central part and the end face in the longitudinal direction of the power generation area of the laminated piezoelectric transformer.

The efficiency of the piezoelectric transformer was measured under the following conditions. A 100 kΩ resistor as a load resistor was connected to the output side of the transformer, the driving frequency was adjusted by an oscillator, and the current was amplified by a power amplifier to supply 2.5 W of power to the input portion of the piezoelectric transformer. The output current was measured by an AC ammeter, and the ratio of the output power to the input power was evaluated as the efficiency of the piezoelectric transformer. The efficiency is desirably 90% or more, and this is used as a reference value for evaluation. The driving was performed in a one-wavelength mode.

The input side insulation resistance of the multilayer piezoelectric transformer was measured and evaluated at 100 V DC. The insulation resistance is 100
MΩ or more is desirable, and this was used as a reference value for evaluation.

The density of the piezoelectric transformer was measured with a gamma ray density measuring device. The spot diameter was 2φ, and the local density of the piezoelectric transformer was measured. Table 1 shows the results obtained as described above.

[0028]

[Table 1]

In Table 1, the sample No. Nos. 1 to 4 are examples according to the present invention. Sample numbers 5 to 7 are comparative examples, and sample numbers are given in parentheses to distinguish them from the examples. Hereinafter, an example and a comparative example will be described in comparison.
No. 1 in Table 1. Samples 1 to 4 are laminated piezoelectric transformers in which the thickness t of the piezoelectric ceramic and the surface density x of the internal electrodes were changed within the range according to the present invention. In each of the examples, Ag diffused almost evenly to the end of the power generation unit, and as a result, the sintered density was sufficiently dense in both the drive unit and the power generation unit. It has good characteristics as a transformer.
Since the diffusion amount of g is also diffused in an appropriate range of 200 to 350 ppm, the insulation resistance shows a sufficiently high value.

On the other hand, in No. 5 shown as a comparative example, since the thickness of the piezoelectric ceramics was as thick as 300 μm, the diffusion of Ag was insufficient, and the density was significantly reduced especially at the end of the power generation part. Is insufficient. In addition, since the density is low, the strength is low, and there is a possibility of breaking during driving. No. 6 is a case where the ratio of the surface density x between the piezoelectric ceramic t and the internal electrode is high. Since the amount of the internal electrodes is larger than the thickness of the piezoelectric ceramic, the diffusion amount of Ag exceeds 500 ppm. For this reason, the insulation resistance was remarkably reduced, polarization was not possible, and evaluation as a transformer was impossible. In No. 7, the area density x of the internal electrode was 1 mg / cm.
A case ² to be low similarly low diffusion insufficient density of Ag and No5, resulting, not obtained sufficient characteristics as transformer.

(Example 2) Using a piezoelectric ceramic having the same composition as in Example 1, a 120 μm green sheet was prepared, and a predetermined electrode was printed so that the internal electrode had a surface density of 4 mg / cm ² . Lamination was performed so that the thickness after sintering was about 2 mm, and a piezoelectric transformer was manufactured and evaluated in the same manner as in Example 1 below. Note that Ag / Pd was used as the internal electrode.
Samples were made using alloys with different ratios. Table 2 shows the evaluation results.

[0032]

[Table 2]

No. 2 in Table 2 Samples Nos. 1 to 5 are laminated piezoelectric transformers in which the internal electrodes are formed of Ag / Pd paste (Ag: Pd) of (50:50) to (90:10) and sintered in air. No. 2 to No. 4 are examples of the present invention, and No. 1 and No. 5 are comparative examples. In each of the examples, Ag diffused almost evenly to the end of the power generation part, and as a result, the sintered density was sufficiently dense in both the drive part and the power generation part, and showed good characteristics. On the other hand, in the comparative example, N where (Ag: Pd) = (50:50) is satisfied.
o. The sample No. 1 has an insufficient diffusion amount of Ag, so that good characteristics cannot be obtained. (Ag: Pd) = (9
0:10), the Ag content was 700 p.
pm, the insulation resistance was significantly deteriorated, and polarization was not possible.

Example 3 Using a piezoelectric ceramic having the same composition as in Example 1, a 120 μm green sheet was prepared, and a predetermined electrode was printed so that the surface density of the internal electrode was 4 mg / cm ² . Lamination was performed so that the thickness after sintering was about 2 mm. Thereafter, piezoelectric transformers having different element lengths were manufactured and evaluated in the same manner as in Example 1. Element length is 30-7
5 mm, and the length L of the power generation unit was の of the element length.
Table 3 shows the evaluation results.

[0035]

[Table 3]

Nos. 1 to 3 in Table 3 are examples of the present invention, and No. 4 is a comparative example. In each of the examples, Ag diffused almost evenly to the end of the power generation part, and as a result, the sintered density was sufficiently dense in both the drive part and the power generation part, and showed good characteristics. On the other hand, the comparative example No.
In No. 4, since the power generation unit was long, Ag could not be sufficiently diffused to its end, and as a result, the density near the power generation unit end decreased,
Sufficient characteristics cannot be obtained.

Although the Rosen-type multilayer piezoelectric transformer has been described in these embodiments, the present invention is not particularly limited to this, and the effect of the present invention is not limited to other multilayer piezoelectric transformers using a multilayer technology. Needless to say, there is nothing.

[0038]

As described above, according to the present invention, there is provided a laminated piezoelectric transformer having a sintering temperature of about 1100 ° C. or less, a fine and uniform crystal structure of piezoelectric ceramics, and excellent piezoelectric characteristics. Can be.

[Brief description of the drawings]

FIG. 1 is a perspective view illustrating the structure of a Rosen-type piezoelectric transformer.

FIG. 2 is a perspective view for explaining the structure of a laminated piezoelectric transformer.

[Explanation of symbols]

 1 Piezoelectric ceramics 2 Internal electrode 3, 4 Input electrode 5 Output electrode

Claims (4)

[Claims] 1. A piezoelectric transformer comprising: a drive unit formed by alternately stacking piezoelectric ceramics polarized in a thickness direction and internal electrodes mainly composed of Ag; and a power generation unit polarized in a length direction. The thickness t of the piezoelectric ceramic per one layer of the driving portion is 200 μm or less, and the surface density x expressed by the weight of the internal electrode formed per unit area is 2 mg / c.
m ² or more, and the thickness t of the piezoelectric ceramics
A ratio t / x between (μm) and the areal density x (mg / cm ² ) of the internal electrode is 12 or more. 2. An internal electrode comprising an alloy of Ag and Pd,
The compounding weight ratio (Ag: Pd) is (60:40) to (80:
20. The multilayer piezoelectric transformer according to claim 1, wherein: 3. The multilayer piezoelectric transformer according to claim 1, wherein the piezoelectric ceramic mainly contains a metal oxide of lead, titanium, zirconium, strontium, and iron. 4. The length L of the power generation section in the piezoelectric transformer is 30.
4. The laminated piezoelectric transformer according to claim 1, wherein the thickness is equal to or less than mm. 5.