JPH098376A - Piezoelectric transformer and production thereof - Google Patents

Abstract

PURPOSE: To obtain a piezoelectric transformer of single plate structure provided, on the surface of a PZT based piezoelectric ceramic, with driving and power generating electrodes in which the silver migration resistance is enhanced between the electrodes while sustaining the efficiency and boost rate thereof. CONSTITUTION: A conductive paste containing silver/palladium alloy powder is employed for forming an electrode 22 on the surface of a PZT based piezoelectric ceramic. Significant effect can be achieved especially when it is fired at a temperature in the range of 680-725 deg.C. Furthermore, an electrode having appropriate interface resistance and excellent in solderability can be obtained when the rate between silver and palladium is set in the range of silver 70wt.% : palladium 30wt.% - silver 80wt.% : palladium 20wt.%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric transformer and a method for manufacturing the same, and more particularly to a piezoelectric transformer having a single plate structure in which a driving electrode and a power generating electrode are provided on the surface of a piezoelectric body made of PZT type piezoelectric ceramic. The manufacturing method is related.

[0002]

2. Description of the Related Art In manufacturing a piezoelectric transformer of this type, a step of forming a drive electrode and a power generation electrode on the surface of a ceramic piezoelectric body is indispensable. Conventionally, when forming the electrode, it is generally performed to form an electrode pattern on the surface of the piezoelectric body by using a conductive paste containing 100% silver and then to form the electrode by firing. .

By the way, not only the piezoelectric transformer but also other electronic parts made of ceramic such as a piezoelectric actuator, of course, need to form the electrodes as described above, but in these other electronic parts, the conductive paste is used. As the conductive paste containing 100% silver, a conductive paste containing an alloy of silver and another metal is used. For example, in Japanese Unexamined Patent Publication (Kokai) No. 62-199001, a conductive paste having a silver and palladium ratio of 40 to 90 Wt% and palladium of 60 to 10 Wt% is used for a pair of electrodes provided on a PTC semiconductor. ing.
In this porcelain semiconductor, after performing a plating treatment on both surfaces of a barium titanate-based ring-shaped positive-characteristic porcelain semiconductor element,
An electrode is formed by screen-printing a paste containing silver having an average particle size of 1 μm or less and palladium having an average particle size of 800 Å and baking at 600 ° C. for 15 minutes.

In Japanese Patent Laid-Open No. 4-96385, silver 70 Wt is used as an internal electrode of a laminated ceramic piezoelectric element.
%, Palladium 30 Wt%, a piezoelectric element using a mixed electrode is disclosed.

Further, in Japanese Patent Laid-Open No. 2-114601, in a temperature sensor composed of an alumina substrate and a platinum resistance film deposited on the substrate, the extraction electrode is composed of silver, palladium and lead borosilicate glass. Is 83: 10: 7
Formed by baking the paste of 620 ℃,
An electrode with little solder erosion and good solderability is obtained.

As described above, the conductive paste containing the silver / palladium alloy containing palladium is used instead of the conductive paste containing 100% silver. This is because when a potential difference is applied between the electrodes facing each other, a so-called silver migration phenomenon occurs in which silver in the electrodes moves from the positive electrode to the negative electrode of the pair of electrodes along the element surface. This phenomenon occurs remarkably in an atmosphere of high temperature and high humidity.

It is also recognized that the inclusion of palladium is effective in preventing "solder erosion" when using solder.

Generally, when using a paste containing a silver-palladium alloy, it is necessary to perform firing at a higher temperature range than when using a paste containing 100% silver.

[0009]

As described above, the PZ
Conventionally, a conductive paste containing 100% silver has been used for forming electrodes of a T-based piezoelectric transformer. When a conductive paste containing a silver-palladium alloy is used instead of this conductive paste, the firing temperature of the conductive paste must be higher than in the past.

However, in the piezoelectric transformer using the PZT type piezoelectric ceramics, when the firing for forming the electrodes is performed in a high temperature range, the performance as the transformer is lower than the performance determined from the structure such as the ceramic material and dimensions. In particular, the efficiency and step-up ratio of the transformer deteriorate.

[0011]

In the piezoelectric transformer of the present invention, a long plate-shaped piezoelectric body made of a PZT-based piezoelectric ceramic is divided into two regions, a drive unit and a power generation unit, and each region is divided into two regions. Piezoelectric structure with a structure in which a drive electrode for inputting an external voltage to the drive unit is provided on the surface of the piezoelectric body, and a power generation electrode for taking out the transformed voltage to the outside is provided to the power generation unit. In the transformer, the drive electrode and the power generation electrode are formed of a conductive film containing an alloy of silver and palladium.

In the alloy of silver and palladium, the weight ratio of silver to palladium is 70 Wt of silver.
%: Palladium 30 Wt% to silver 80 Wt%: Palladium 20 Wt%.

In the above piezoelectric transformer, a conductive coating material containing an alloy powder of silver and palladium is used on the surface of the piezoelectric body to form a pattern of the driving electrode and the power generating electrode, which is then fired to form the driving electrode. In a method for manufacturing a piezoelectric transformer including a step of forming an electrode for power generation and an electrode for power generation,
The conductive coating is fired in a temperature range of 680 ° C. or higher and 725 ° C. or lower.

[0014]

When the piezoelectric transformer according to the present invention is manufactured, a conductive paste containing silver-palladium alloy powder as an electrode material is fired at 680 to 725 ° C. In general, a conductive paste containing palladium undergoes a reduction reaction of palladium in the vicinity of 450 to 680 ° C., and is therefore fired at about 850 ° C. in the high temperature range. However, the electrical characteristics of the piezoelectric transformer to which the present invention is applied tend to depend on the electrode firing temperature, and a high firing temperature causes deterioration of the electrical characteristics. If the firing temperature is 725 ° C or higher, the target performances of the transformer efficiency of 94% or more and the boosting ratio of 15 times or more cannot be satisfied. On the other hand, the migration property also tends to depend on the electrode firing temperature, and the lower the firing temperature, the lower the migration resistance. At 680 ° C. or lower, the migration resistance is comparable to that of silver electrodes.
Further, when the reduction reaction temperature of palladium is 680 ° C. or lower, palladium oxide is mixed in the electrode, so that the solder wettability is poor. Therefore, due to the electrical characteristics, migration resistance, and solderability of the piezoelectric transformer, firing is performed in the temperature range of 680 to 725 ° C. to form the electrodes of the PZT-based piezoelectric transformer using the conductive paste containing silver-palladium alloy. Is the most suitable.

Further, the content of palladium in the alloy is 20
It is known that although the migration resistance is improved when Wt% or more, the interfacial resistance gradually increases when the amount of palladium exceeds about 40 Wt%. Also, the solder wettability deteriorates due to the increase in the palladium content. Therefore, in the present invention, a conductive paste in which the weight mixing ratio of silver and palladium is 70 to 80 wt% silver and 30 to 20 wt% palladium is used.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, preferred embodiments of the present invention will be described with reference to the drawings. First, as a first embodiment, PZ
A piezoelectric transformer having a structure shown in FIG. 2 was manufactured using the T-based piezoelectric ceramic by the following manufacturing process.

P containing manganese and antimony as the third component
ZT-based piezoelectric ceramic plate 21 has an inorganic content of 77 W
A conductive paste containing silver / palladium alloy containing t% (of which the weight mixing ratio of silver and palladium is 73 Wt% of silver and 27 Wt% of palladium) was printed by a thick film screen printing method using ethyl cellulose as a vehicle. . Then, the electrode 2 baked at 600 to 850 ° C.
A piezoelectric transformer having No. 2 formed was manufactured. The electrode shape is as shown in FIG. The electrode thickness is 15 μm.

The transformer shown in FIG. 2 is one of Rosen type piezoelectric transformers. In FIG. 2, reference numeral 21 is a ceramic portion of the transformer. Reference numeral 22 denotes an electrode portion of the transformer, which was manufactured by the manufacturing method of this embodiment. This piezoelectric transformer has two low-impedance drive parts and a high-impedance power generation part, and is polarized in the direction indicated by the thick arrow in the figure. When an AC voltage having the resonance frequency of the piezoelectric transformer is applied to the drive unit, a high voltage boosted by the power generation unit is output by the piezoelectric horizontal effect 31 mode and the piezoelectric vertical effect 33 mode.

After the piezoelectric transformer of this example was polarized, it was connected to a load and the relationship between the electrode firing temperature and the electrical characteristics was measured. FIG. 3 is a wiring diagram of the measuring circuit. As the load, a pseudo load 32 composed of a resistor and a capacitor was used.

Further, a water drop test was carried out by dropping pure water on the counter electrode, and the relationship between the electrode firing temperature and the migration resistance was also investigated. A test piece was prepared using the same ceramic as above and a paste containing silver-palladium alloy. The firing temperature is also the same as above.

FIG. 1 shows the firing temperature dependency of the above-mentioned piezoelectric transformer and the firing temperature dependency of migration resistance. Referring to FIG. 1, the migration resistance of the silver-palladium alloy-containing paste depends on the firing temperature.
I understand. At a firing temperature of 680 ° C. or lower, the migration resistance is as good as silver. Therefore, the advantage of using the silver-palladium alloy-containing paste for the purpose of suppressing migration cannot be found. From this, when pursuing the advantage of the silver-palladium alloy-containing paste in migration resistance, the firing temperature of the silver-palladium alloy-containing paste must be 680 ° C. or higher. Incidentally, since the migration resistance is remarkably deteriorated at a firing temperature of 700 ° C. or lower, firing at 700 ° C. or higher is desirable.

Further, it can be seen from FIG. 1 that the efficiency and step-up ratio, which are the most important electrical characteristics of the piezoelectric transformer, deteriorate as the electrode firing temperature rises. The target performance of the piezoelectric transformer manufactured in this example has an efficiency of 94% or more,
Although the pressurization ratio is 15 times or more, if the firing is performed at 725 ° C. or more, the above two cannot be satisfied. From this, in order to obtain a transformer having a target performance or higher, the firing temperature of the silver-palladium alloy-containing paste must be 725 ° C or lower.

From the viewpoint of the above-mentioned migration resistance and electric characteristics as a piezoelectric transformer, the piezoelectric transformer is made of silver.
When forming an electrode using a palladium alloy-containing paste, the paste must be fired at 680 to 725 ° C.

In this embodiment, the firing temperature is 680 ° C.
The characteristics of the piezoelectric transformer manufactured in 1. were 94.8% in efficiency, 16 times the step-up ratio, and about 1.5 times the migration resistance of the paste containing 100% silver. Also, the firing temperature is 70
At 0 ° C., the efficiency was 94.5%, the pressurization ratio was 15.8 times, and the migration resistance was about 6 times that of silver. Furthermore, 72
The characteristics at 5 ° C are: efficiency 94.6%, boost ratio 15.5 times,
The migration resistance was about 6 times that of silver. These piezoelectric transformers were tested for high temperature and high humidity operation (temperature 85 ° C, humidity 9
When subjected to 5% RH), no dielectric breakdown occurred due to silver migration.

Next, a second embodiment of the present invention will be described. In this example, a piezoelectric transformer having the same geometric structure as that of the first example was manufactured by using a conductive paste in which the weight mixing ratio of silver / palladium alloy was changed. The ratio of silver to palladium is 70 Wt% silver: 30 palladium.
Wt% and the firing temperature is 725 ° C. When the electrical characteristics were measured according to the measurement circuit of FIG. 3, the transformer efficiency was 94.4% and the step-up ratio was 15.3 times.

The migration resistance was about 6.5 times that in the case of using the paste containing 100% of silver. When the piezoelectric transformer of this example was subjected to a high temperature and high humidity operation test,
There were no defects due to dielectric breakdown.

Further, a third embodiment of the present invention will be described. In this example, the electrical characteristics of a piezoelectric transformer in which an electrode was formed by firing a conductive paste in which the ratio of silver to palladium was 80 Wt% silver: 20 Wt% palladium was fired at 725 ° C. were measured. The efficiency of the transformer was 94.4% and the boost ratio was 15.8 times.

The migration resistance was about 1.5 times that of the paste containing 100% silver. This embodiment is the first
Compared with Examples 1 and 2, the migration resistance is inferior, but a comparison of the manufacturing costs shows that the paste is inexpensive because of the low palladium content. When the piezoelectric transformer of this example was subjected to a high-temperature and high-humidity operation test, it was confirmed that there were no defects due to dielectric breakdown, and that it could withstand practical use.

It should be noted that the Rosen type piezoelectric transformer used in the above embodiments is only an example, and it has been confirmed that the same effects as in the embodiments can be obtained even in transformers other than this type.

[0030]

As described above, in the present invention, the electrode of the piezoelectric transformer using the PZT system piezoelectric ceramic is formed by firing the conductive paste containing the silver-palladium alloy powder. Thus, according to the present invention,
It is possible to provide a piezoelectric transformer having excellent silver migration resistance and high electrical characteristics as a piezoelectric transformer, particularly high efficiency and step-up ratio. The effect is particularly remarkable when the conductive paste is fired in the temperature range of 680 to 725 ° C.

When the ratio of silver to palladium in the above-mentioned silver-palladium alloy is in the range of 70% silver: 30% palladium to 80% silver: 20% palladium, the solderability is good and the interfacial resistance is good. Also gives a suitable electrode.

[Brief description of drawings]

FIG. 1 is a diagram showing electrode firing temperature dependence of migration resistance, transformer efficiency, and boost ratio in an example of the present invention.

FIG. 2 is a perspective view of a piezoelectric transformer according to an exemplary embodiment of the present invention.

FIG. 3 is a circuit diagram of a circuit used for measuring electrical characteristics of a piezoelectric transformer according to an example of the present invention.

[Explanation of symbols]

 21 Piezoelectric Ceramic 22 Silver / Palladium Alloy Containing Electrode 31 Drive Power Supply 32 Pseudo Load

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location H02M 5/04 H01L 41/22 Z

Claims (3)

[Claims] 1. A long plate-shaped piezoelectric body made of PZT-based piezoelectric ceramic is divided into two regions, a drive unit and a power generation unit, and the piezoelectric body surface is provided for each region and the drive unit is provided for each region. A piezoelectric transformer having a structure in which a drive electrode for inputting a voltage from the outside is provided, and a power generation electrode for taking out a transformed voltage to the outside is provided for a power generation unit, wherein the drive electrode and the power generation A piezoelectric transformer, wherein the electrodes are made of a conductive film containing an alloy of silver and palladium. 2. The piezoelectric transformer according to claim 1, wherein
The weight ratio of silver to palladium in the alloy of silver and palladium is 70 Wt% silver: 30 Wt palladium.
% To 80 Wt% silver: 20 Wt% palladium, a piezoelectric transformer. 3. The method for manufacturing the piezoelectric transformer according to claim 1, wherein the driving electrode is formed by using a conductive paint containing an alloy powder of silver and palladium on the surface of the piezoelectric body. And a method of manufacturing a piezoelectric transformer including a step of forming a pattern of a power generation electrode and then baking the pattern to form the driving electrode and the power generation electrode, wherein the conductive paint is baked at a temperature of 680 ° C. or higher and 725 ° C. or lower. A method for manufacturing a piezoelectric transformer, which is carried out in a range.