WO2008126946A1

WO2008126946A1 - 3-dimensional curved surface type piezoelectric transformer and manufacturing method thereof

Info

Publication number: WO2008126946A1
Application number: PCT/KR2007/001762
Authority: WO
Inventors: Tae-Shik Yoon; Man-Sun Yun
Original assignee: Tae-Shik Yoon; Man-Sun Yun
Priority date: 2007-04-11
Filing date: 2007-04-11
Publication date: 2008-10-23

Abstract

Disclosed is a 3-dimensional curved surface type piezoelectric transformer, including a piezoelectric body formed of piezoelectric material and electrode parts formed on a first upper curved surface and a second upper curved surface of the piezoelectric body, respectively, to simultaneously generate normal strain and shear strain. The piezoelectric transformer is manufactured through powder injection molding, including a mixture preparation step of preparing a pellet for injection molding using a binder at a predetermined ratio (1st step); an injection molding step of subjecting the pellet to injection molding using a mold to have a predetermined shape, thus producing an injection molded body (2nd step); a binder removal step of removing the binder from the injection molded body, thus obtaining a degreased body (3rd step); and a sintering step of sintering the degreased body at a high temperature, thus manufacturing a final product (4th step).

Description

3-DIMENSIONAL CURVED SURFACE TYPE PIEZOELECTRIC TRANSFORMER AND MANUFACTURING METHOD THEREOF

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates, generally, to a high-power piezoelectric transformer having a 3-dimensional (3D) curved surface shape and a method of manufacturing the same, and more particularly, to a 3D curved surface type piezoelectric transformer, which includes a piezoelectric body, formed of piezoelectric material, and electrode parts formed on a first upper curved surface and a second lower curved surface of the piezoelectric body, respectively, to thus enable the simultaneous generation of normal strain and shear strain.

2. Description of the Related Art

Practical research into piezoelectric transformers, functioning as devices for converting mechanical energy into electrical energy, was first conducted by C. E. Rosen et. al. of the GE Company, USA, in 1957. As the piezoelectric material used at that time, there was barium titanate, which has a voltage step-up range of only about 50-60 times under no load conditions, and thus had limitations in the practical applicability thereof. However, novel piezoelectric material, composed mainly of Pb (Zr, Ti) O₃, has been discovered, making it possible to realize the step-up of higher voltages, therefore leading to full-scale research for practical applications . Such a piezoelectric transformer has the following advantages, compared to electromagnetic wound-type transformers. That is, the piezoelectric transformer may be manufactured to be smaller, slimmer and lighter because a winding process is not needed. Further, productivity may be increased upon mass production, and, upon high-frequency operation, it is possible to achieve high efficiency because magnetic loss, such as eddy current loss or hysteresis loss, which has occurred in the wound-type transformer, is not caused. Furthermore, in the course of the change of energy using the piezoelectric transformer, the conversion into magnetic energy, as in the wound-type transformer, does not occur, desirably minimizing electromagnetic induction interference. Although the wound- type transformer may cause fires when it shorts and fails, the piezoelectric transformer is in an open state when it fails, and thus the failure does not exacerbate the situation.

The voltage step-up ratio of the wound-type transformer is determined by the winding factor, but the piezoelectric transformer has a voltage step-up ratio varying depending on the material properties, electrode structures, dimensions, and load properties. From the point of view of output power for changing the secondary side so that it has high voltage and low current, as the wound-type transformer should increase the winding factor, a leakage component is increased. However, the piezoelectric transformer adopts electrical-mechanical (primary side) -mechanical-electrical (secondary side) combination, and thus efficiency of 90% or more may be realized. In the case where this transformer is applied as a stabilizer, energy may be greatly saved compared to electronic stabilizers used in conventional discharge lamps. When the transformer, capable of attaining high voltage and low current at the secondary side, has a high impedance load, efficient impedance matching, and thus good load properties, are realized, resulting in increased energy conversion efficiency. Therefore, the development and application of piezoelectric transformers having high voltage and low current properties has been relatively vigorous to date, and piezoelectric inverters for backlights of notebook computers are commercially available. Recently, thorough research into high output of primary Rosen type or tertiary Rosen type transformers is being conducted, along with research into layering methods for parallel operation. In the case of a multilayered type, there is a certain limitation in ensuring the output of high current, in addition to poor durability and high manufacturing costs. That is, the Rosen type transformer, having a planar structure, has a driving part and an output part, which are polarized in directions which are perpendicular to each other, and the stress convergence at the boundary thereof is very intense. Moreover, since the output part is polarized through the application of a high electric field (3 kV/rran) , such polarization is difficult to realize, and high current is also difficult to obtain due to the small electrode area of the output terminal. Hence, this type is unsuitable for lighting a high-current lamp, such as a fluorescent lamp. To overcome these problems, although methods of manufacturing separate multiple transformers and driving them in parallel have been proposed, there are difficulties in controlling the exact dimensions of the transformers, and thus it is impossible to equalize resonant frequencies, and furthermore, the output properties are inevitably deteriorated, attributable to the problems of the polarization process in an unsolved state.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and an object of the present invention is to provide a piezoelectric transformer, which is of a single-plate type but enables the step-up of voltage in the same level as a multilayer type, to increase the low output current and output voltage, which are disadvantages of a low-power piezoelectric transformer.

Another object of the present invention is to provide a piezoelectric transformer, which has various electrode structures and polarization direction arrangements using a piezoelectric element manufactured to have a new shape through power injection molding.

A further object of the present invention is to provide a high-efficiency piezoelectric inverter using the above piezoelectric transformer, which may stably light a general fluorescent lamp or three-wavelength lamp of high-voltage and high-current, which has been unable to drive using a conventional piezoelectric transformer, at a domestic input voltage (220 V/110 V) , even without the use of a wound-type transformer in the driving circuit.

In order to accomplish the above objects, the present invention provides a 3D curved surface type high-power piezoelectric transformer, which includes a piezoelectric body, formed of a piezoelectric material, and electrode parts formed on a first upper curved surface and a second lower curved surface of the piezoelectric body, respectively, to thus simultaneously generate normal strain and shear strain.

Further, the piezoelectric transformer of the present invention includes a piezoelectric body formed through powder injection molding, an upper electrode part having an input electrode and an output electrode separately formed on the upper curved surface of the body, and a lower electrode part formed on the entire lower curved surface of the body, wherein, when the piezoelectric transformer is oscillated through the application of voltage, the displacement occurring upon contraction and expansion of the surfaces of the body is maximized at the peripheral region and the central region of the body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are perspective views illustrating the piezoelectric transformer according to the present invention;

FIG. 3 is a cross-sectional perspective view illustrating the hemispherical piezoelectric transformer of the present invention;

FIG. 4 is a top plan view illustrating the transformer, in which predetermined electrodes are formed, according to the present invention; FIG. 5 is a bottom plan view illustrating the transformer in which a predetermined electrode is formed, according to the present invention;

FIGS. 6 to 9 are cross-sectional views illustrating the mold for use in manufacturing the piezoelectric transformer of the present invention; FIG. 10 illustrates the polarization direction formed in the cross-section of the 3D curved surface type piezoelectric transformer according to the present invention;

FIG. 11 illustrates the resonant frequency determined from the piezoelectric transformer, polarized as in FIG. 6;

FIGS. 12 and 13 illustrate the vibration displacement of each cross-section upon contraction vibration and expansion vibration;

FIGS. 14 and 15 illustrate the results of 3D display of the vibration displacement on the surface of the 3D hemispherical piezoelectric transformer;

FIGS. 16 and 17 illustrate the results of quantitative analysis for the maximum strain of the output side of each of the piezoelectric transformer of the present invention and a conventional Rosen type piezoelectric transformer;

FIGS. 18 and 19 illustrate the actual upper and lower shapes of an injection molded body, produced according to the present invention;

FIGS. 20 and 21 illustrate the electrode structure formed after a sintering process, according to the present invention;

FIG. 22 illustrates a scanning electron micrograph showing the fine texture of an internal fractured surface of the sintered sample;

FIG. 23 illustrates the results of measurement of the resonance/anti-resonance impedance curve of a primary side electrode (oscillation part) ;

FIGS. 24 and 25 illustrate the construction of measurement apparatuses for comparing the voltage step-up ratios; FIG. 26 illustrates the results of input and output voltages, measured using the apparatus of FIG. 24;

FIG. 27 illustrates the output waveform, measured at standard resistance R_Ξ using the apparatus of FIG. 25;

FIG. 28 illustrates the final results of measurement of the voltage step-up ratio;

FIGS. 29 and 30 illustrate the electrode configuration according to one embodiment of the present invention;

FIGS. 31 and 32 illustrate the electrode configuration according to another embodiment of the present invention; FIG. 33 illustrates the polarization direction arrangement; and

FIGS. 34 and 35 illustrate the electrode configuration according to a further embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a detailed description will be given of a 3D curved surface type high-power piezoelectric transformer and a manufacturing method thereof, according to preferred embodiments of the present invention, with reference to the accompanying drawings .

FIG. 1 is a perspective view of the piezoelectric transformer of the present invention, and FIG. 3 is a cross- sectional view of the piezoelectric transformer of FIG. 1. FIGS. 3 and 4 are a top plan view and a bottom plan view, respectively, of the piezoelectric transformer having electrodes.

As shown in FIGS. 1 and 2, the piezoelectric transformer of the present invention includes a 3D curved surface type piezoelectric body 100 formed of a piezoelectric material, and electrode parts 200, 300 formed on a first upper curved surface and a second lower curved surface of the piezoelectric body 100, respectively. Although the 3D curved surface type is preferably provided in the form of a hemispherical shape, any shape may be used, as long as it is a 3D curved surface enabling the presence of a shear strain component mentioned below. On the upper curved surface of the piezoelectric body 100, an input electrode 200, corresponding to a primary side electrode as an oscillation part, and an output electrode 300, corresponding to a secondary side electrode, are separately formed, and a lower electrode part is formed over the entire lower curved surface of the piezoelectric body. Thus, the piezoelectric transformer of the present invention is classified into two types, depending on the polarization direction of the input and output sides. As mentioned above, in the case where the piezoelectric transformer is in the form of the electrode structure, in which the input and output sides are polarized in the upper and lower directions, the lower electrode part, printed on the entire lower curved surface of the body, is used as a common ground of the input and output parts, one of the divided upper electrode regions is set as an input side, and the other electrode thereof serves as the output side. Alternatively, the electrode structure may be formed into various configurations in a manner such that the input side electrode is polarized in the upper and lower directions and the output side electrode is set to be in a circumferential direction after the shapes of upper and lower electrode parts are set to be the same, thus making it possible to provide a piezoelectric transformer, which may undergo optional designs suitable for corresponding uses, for example, various designs for electrode shapes, electrode structures, and polarization directions, if needed. Thereby, when the piezoelectric transformer thus manufactured is oscillated through the application of voltage to the primary side electrode thereof, vibration displacement is generated at the cross-section thereof. The displacement, occurring upon contraction and expansion, is maximized at the peripheral region (oscillation part) and the central region (output part) of the piezoelectric transformer. From the point of view of displacement vector, considering the directional properties of this displacement, in the case of the hemispherical piezoelectric transformer of FIGS. 1 and 2, the displacement vector present on the entire surface, except for the center point, creates normal strain and shear strain at the same time, thus enabling the simultaneous use of a normal piezoelectric mode d₃₃ and a shear piezoelectric mode di₅. Consequently, the piezoelectric transformer of the present invention may have a relatively higher voltage step-up ratio than conventional Rosen type planar piezoelectric transformers.

In order to manufacture such a 3D curved surface type piezoelectric transformer, powder injection molding (PIM) is used in the present invention. Powder injection molding is a combined technique of plastic injection molding, which can be used to accurately manufacture products having 3D complicated shapes in large quantities, and powder metallurgy, and includes a mixture preparation step of mixing powder with a binder at a predetermined ratio to thus prepare a pellet for injection molding (1^st step) , an injection molding step of subjecting the pellet to injection molding using a mold to have a predetermined shape to thus produce an injection molded body (2^nd step) , a binder removal step of removing the binder from the injection molded body to thus obtain a degreased body (3^rd step) , and a sintering step of sintering the degreased body at a high temperature, thus manufacturing a final product (4^th step) . In the case where metal powder is used, a uniform texture, which is the advantage of powder metallurgy, may be obtained, thus resulting in mechanical properties equal to or better than those of cast products. Further, hard-to-work products, such as ceramics or carbides, may be economically manufactured without post treatment.

Below, the method of manufacturing a 3D curved surface type piezoelectric inverter using powder injection molding is stepwisely described. (1) 1^st Step: Preparation of Mixture (Material Powder and Binder)

In the present invention, material powder is composed mainly of Pb (Zr, Ti) O₃, and each element (Pb, Zr, Ti) powder is uniformly dispersed and ground through high-energy ball- milling to an average particle size of 2.0 μm or less. As a binder, polybutyl methacrylate (PBMA) and paraffin wax (PW) are added at a predetermined ratio within the range of 90:10-10:90. The amount of PBMA and PW, contained in the predetermined ratio, is set to be 60-95 wt% based on the total wt%, the balance being ethylenevinylacetate (EVA) , dissolved in a petroleum solvent. The material powder and the binder are weighed to be about 45-55% by volume, mixed at 150⁰C for about 1 hour using a pressure kneader having two banbury type blades for rotation, cooled, ground into pellets for injection molding, and granulated. (2) 2^nd Step: Injection Molding and Design of Mold Therefor

Using any mold of FIGS. 6 to 9 in a molding machine having the same structure as a general plastic injection molding machine, an injection molded body is produced. The mold for injection molding typically adopts a cold runner system or a hot runner system. In the two systems, the powder mixture is filled into a cavity via a sprue, a runner, and a gate, thus forming the injection molded body. FIG. 6 illustrates the mold, ^' in which the gate 14 is positioned at the top of the piezoelectric transformer. In the present invention, the reason why the gate is positioned at the top of the piezoelectric transformer is that the entire surface of the piezoelectric transformer, except for the top thereof, functions in a normal piezoelectric mode and a shear piezoelectric mode at the same time. If a gate mark or fine scratches exist after the injection molding, cracking may be caused. Thus, when the gate is disposed at the top of the piezoelectric transformer, where only the normal piezoelectric mode is present, it is possible to increase the lifetime of the product somewhat even when the gate mark is present at the top after molding. When the structure of the mold is more carefully observed with reference to FIG. 6, the mold 400 includes a fixed body 411 and a movable body 412. The product is molded in a manner such that the powder for molding, fed into a runner 413, is loaded into a cavity 415 via a gate 414, the movable body 412 is removed, and the product is obtained using an ejector pin 416. FIG. 7 illustrates another mold, which is an improvement of the structure of FIG. 6. The mold of FIG. 7 includes a protrusion 425 formed at the top of a cavity 426. When the powder for molding is loaded into a cavity 415 and a gate 424 is then removed, a piezoelectric transformer, comprising the cavity 426 and the protrusion 425, may be obtained. In this case, the gate mark is not formed on the surface of the injection molded body but is formed on the protrusion 425. Hence, after the production of the injection molded body or the sintered body, the protrusion is treated along the outer diameter of the hemisphere so that the surface of the hemisphere becomes smooth, thereby further lengthening the lifetime of the product.

When a cavity 435, as in FIG. 8, is manufactured, a runner 433 and a gate 434 are formed at the parting surface between a fixed body 431 and a movable body 432, such that a gate mark is formed on the side surface of a product. In this case, even though the gate mark remains after the molding, the electrode is not formed near the gate mark, and thus the generation of defects, including cracking, may be advantageously minimized upon the operation of the piezoelectric transformer. The mold of FIG. 9 has a structure similar to the mold of FIG. 8, with the exception that a gate 444 is formed at the straight portion 446 of the lower end of a product, therefore making it possible to form more electrode parts on the entire surface of a cavity 45. The injection molding is conducted at a pressure of 300 MPa or less. (3) 3^rd Step: Removal of Binder

The binder used in the powder injection molding functions to impart the powder with flexibility upon injection molding to thus enable it to be loaded into the mold and to retain the molded shape after a cooling process. When the binder remains after degreasing, it is mainly present in the form of a carbon component, which incurs poor piezoelectric properties and abnormal crystalline grain growth, resulting in greatly- worsened properties of piezoelectric ceramic products. Ultimately, the binder must be completely removed. The removal of the binder is a process of completely removing the binder, which is present in a large amount in the injection molded body, without generating defects, including distortion or cracking. This process is recognized to be the most important in the total process, and takes a long time. The removal of the binder is conducted using so-called pyrolysis or solvent extraction. The removal of the binder through pyrolysis includes a removal method using evaporation. According to this method, when the injection molded body is placed on a porous substrate in a furnace and then heated to a temperature not lower than the melting point of the binder, fast gas flow is caused around the injection molded body to thus form eddy currents at the binder evaporation interface of the surface of the injection molded body, thereby removing the binder using such eddy currents without the formation of a boundary layer.

In the case of the pyrolysis method, used in the present invention, the injection molded body is placed on an alumina substrate so that the gate faces upward, and then the temperature is slowly increased. The temperature is increased to 130⁰C, which is lower than 140⁰C, the softening temperature of the mixture, and should be maintained there for at least 20 hours. This is because the shape of the injection molded body may be distorted when maintained at a high temperature exceeding the above temperature. The molded body is heated at 13O⁰C for about 20 hours, after which the temperature of the furnace is increased to 500⁰C at a rate of 0.5~3 °C/min and then maintained for 1 hour or longer, thereby removing 99% or more of the added binder. In this case, the amount of carbon residue is reduced to 0.1% or less. Alternatively, when the temperature of the furnace is increased to 700⁰C and that temperature is maintained for 1 hour or longer, the carbon residue is reduced to 0.01% or less. In this case, the heating rate may be increased to 5 °C/min. On the other hand, the solvent extraction method is conducted by extracting only a specific material of the binder with a solvent and removing the remaining binder using heat.

(4) 4^th Step: Sintering

The 3D curved surface type piezoelectric body thus degreased is sintered at 1300⁰C for 1-2 hours in a closed alumina crucible in an atmosphere containing oxygen, thus producing a 3D curved surface type piezoelectric body having a dense structure.

To evaluate the performance of the 3D curved surface type piezoelectric body of the present invention, the 3D hemispherical piezoelectric body, manufactured through powder injection molding, and the disk type piezoelectric body, manufactured through powder pressing, are measured for piezoelectric properties according to the EMAS6001 standard. The results are shown in Table 1 below.

TABLE 1

The dimensions of the 3D hemispherical shape, manufactured through PIM, are shown in Table 2 below.

TABLE 2

Outer Inner Curvature Thick. Primary Electrode Secondary Electrode

To evaluate the resonant frequency and vibration mode of the piezoelectric transformer thus manufactured, computer simulation analysis is conducted using an ATILA computer program. As the results, the polarization direction, formed in the cross-section of the 3D hemispherical piezoelectric transformer, is shown in FIG. 10. The resonant frequency analyzed for the polarized piezoelectric transformer is shown in FIG. 11. The arrow direction shown in FIG. 10 corresponds to the polarization direction of the 3D hemispherical piezoelectric transformer of the present invention. In the case where the sign wave IV of the resonant frequency is applied to the primary side electrode for oscillation, the vibration displacement, occurring at the cross-section, includes contraction vibration and expansion vibration, which are illustrated in FIGS. 12 and 13, respectively. Here, the displacement generated upon contraction and expansion may be seen to be maximized at the peripheral region (oscillation part) and central region (output part) of the 3D hemispherical piezoelectric transformer. The results of 3D display of the vibration displacement at such a resonant frequency on the 3D hemispherical shape are illustrated in FIGS. 14 and 15. FIG. 14 illustrates the vibration displacement vector generated on the upper curved surface of the 3D hemispherical piezoelectric transformer, upon resonance expansion, and FIG. 15 illustrates the vibration displacement vector generated in the contraction direction on the lower curved surface thereof. When observing the direction of the displacement vector, indicated by the arrows, the displacement vector component of the entire surface except for the center point includes a shear strain component. That is, the displacement vector, which is present on the entire spherical surface, except for the center point, has the normal strain and shear strain generated at the same time, which means that d₃₃ and di₅ modes may be simultaneously used, thus resulting in higher voltage step-up ratios than general planar piezoelectric transformers. The displacement generated at the output side has a relationship proportional to the output. Hence, to compare the output of the piezoelectric transformer of the present invention with the output of a conventional Rosen type piezoelectric transformer, the maximum strain of the output side is quantitatively analyzed through simulation under the same electric field. The results are shown in FIG. 10. As seen in FIG. 16, in the longitudinal direction of the Rosen type piezoelectric transformer, the maximum total displacement is 2 x 5.385 x 10^~8 mm, the total length is 25 mm, and the thickness is 2.5 mm,

_ A/ and thus the longitudinal-direction strain ( ° ) is determined to be 4.3 x 10⁸. On the other hand, in the 3D hemispherical piezoelectric transformer of the present invention, the displacement under the same conditions is shown in FIG. 17. The maximum total displacement, generated at the center point of the output part, which is a maximum displacement point, is 2 x 1.695 x ICT⁷, and the central

Δ/ portion is 2.5 mm long, and the strain ( ° ) is thus determined to be 1.92 x 10^"7. The ratio of the maximum strain, generated at the output part is calculated using Equation 1 below. Consequently, the 3D hemispherical piezoelectric transformer of the present invention can be seen to have a strain at least 4.8 times the conventional Rosen type piezoelectric transformer. Equation 1 (Strain of 3D Hemispherical Piezoelectric Transformer) / (Strain of Rosen type Piezoelectric Transformer)

Accordingly, the piezoelectric transformer of the present invention may easily assure higher output than a conventional Rosen type piezoelectric transformer.

Below, the above simulation results and the actual test product results are compared and analyzed. A test product is manufactured using the above-mentioned powder injection molding process. The actual upper and lower shapes of the injection molded body thus produced are illustrated in FIGS. 18 and 19. After the sintering process, the electrode structure is as illustrated in FIGS. 20 and 21. The scanning electron micrograph of the fine texture of the internal fractured surface of the sintered sample is shown in FIG. 22, which confirms the formation of a uniform and dense sintered body of 1.5~2 μm. For comparison, the Rosen type piezoelectric transformer has standard dimensions of 25 mm width x 7 mm length x 2.5 mm thickness. The piezoelectric transformer thus manufactured is immersed in silicone oil at 150°C, and then a DC electric field of 2.5 kV/mm is applied thereto for 40 min, after which the transformer is removed from the oil, washed, and then subjected to aging at 150⁰C for 5 hours. The secondary electrode of the obtained sample is short-circuited with the ground, after which the resonance/anti-resonance impedance curve of the primary side electrode (oscillation part) is measured using an HP4194 impedance/gain phase analyzer. The results are shown in FIG. 23. As is apparent from these results, the primary resonant frequency is determined to be 93.9056 kHz, which is similar to the simulation results. To compare the voltage step-up ratio of the Rosen type, having the highest voltage step-up ratio of conventional single-plate type oscillators, with the voltage step-up ratio of the 3D hemispherical piezoelectric transformer of the present invention, the measurement apparatus of FIG. 24 or 25 is constructed, after which a spherical wave is generated from a waveform generator, amplified to 5Vp-p using a power amplifier, and then input into the primary side electrode of the piezoelectric transformer. Thereafter, the voltage occurring at the secondary side electrode is measured using an oscilloscope. At a load resistance of 10 kΩ, as in FIG. 24, a voltage probe is directly connected to the load resistor to thus measure the output voltage. At 100 kΩ or higher, to remove the error attributable to internal resistance of the oscilloscope, as seen in FIG. 25, a standard resistor R₃ of 10 kΩ is connected to a load resistor R_L in parallel, the voltage probe is connected to the standard resistor to thus measure the voltage, and then the output voltage depending on the load resistance is determined using Equation 2 below: Equation 2

V_s V^z~^δiΞ ^SRz wherein V_L is the output voltage when load resistance is R_L, and V₃ is the output voltage at a standard resistor R_s of 10 kΩ. FIG. 26 shows the results of input and output voltages measured using the apparatus of FIG. 24 when the load resistance is 10 kΩ, and FIG. 27 shows the output waveform measured at the standard resistor of R_s using the apparatus of FIG. 25 when the load resistance is 10 MΩ. The input and output waveforms thus measured are analyzed, and the output voltage is divided by the input voltage to thus determine the voltage step-up ratio. The final results are shown in FIG. 28. As shown in FIG. 28, the conventional Rosen type piezoelectric transformer has a maximum step-up ratio of 20 times at 10 MΩ, whereas the piezoelectric transformer of the present invention has a maximum step-up ratio of 290 times, which is increased by more than 14 times over the Rosen type piezoelectric transformer.

To use various vibration modes, occurring from the 3D curved surface type piezoelectric transformer, some upper electrode part examples are illustrated in FIGS. 29 to 32. As in FIGS. 29 and 30, while the upper and lower electrode parts have the same shapes, the electrode having a larger area (the central region of the spherical shape) is determined as a primary side electrode, and the electrode having a smaller area (the peripheral region of the spherical shape) is determined as a secondary side electrode. Thereafter, as in FIG. 33, the primary side electrode is polarized in the upper and lower directions, so that the primary electrode and the secondary electrode are polarized in a direction of a tangential line to the curved surface. Thereby, the capacitance ratio of the secondary side and the primary side may be increased, and the voltage step-up ratio of a maximum of 450 times at 10 MΩ may be obtained. In addition, according to another embodiment, electrodes may be provided in the forms shown in FIGS. 31 and 32 and may be arranged in the polarization direction seen in FIG. 33. Compared to the Rosen type, the above-mentioned electrode configurations are more effective in preventing breakdown around the boundary of the primary electrode and the secondary electrode because the primary side and the secondary side are polarized in directions which are not perpendicular to each other, in making the polarization process feasible, and in assuring a high yield and a high voltage step-up ratio. Further, in the case of requiring low voltage and high current of the output side, as in an AC-DC transformer, as the electrode area ratio of the input and output sides is low, preferable results may be obtained. In this case, the upper electrode part and the lower electrode part may be provided in the forms shown in FIGS. 34 and 35, according to a further embodiment. Accordingly, a 3D hemispherical piezoelectric transformer, which has an electrode over its entire lower surface (FIG. 35) , as the lower electrode part, and is polarized in a thickness direction, as in FIG. 10, may be provided. As described hereinbefore, according to the present invention, the piezoelectric transformer manufactured to have a 3D curved surface shape using a powder injection molding technique may have various vibration modes and large vibration displacement, unlike conventional planar piezoelectric transformers, thus greatly improving the limited properties of conventional planar piezoelectric ceramics. Further, the method of the present invention is advantageous because mass productivity and dimensional accuracy are superior, and because large displacement and power may be attained without the complexity of manufacture of a layered structure, compared to other methods. Furthermore, the above method may be applied to material having no limitation in thickness and radius of curvature, is able to realize resonant operation, and also effectively enables mass production. Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims .

Claims

WHAT IS CLAIMED IS:

1. A piezoelectric transformer having a 3-dimensional curved surface, which comprises a piezoelectric body formed of a piezoelectric material, and electrode parts formed on a first upper curved surface and a second lower curved surface of the piezoelectric body, respectively, to thus simultaneously generate normal strain and shear strain.

2. The piezoelectric transformer according to claim 1, wherein the electrode parts comprises an input electrode, corresponding to a primary side electrode as an oscillation part, and an output electrode, corresponding to a secondary side electrode, which are separately formed, and a lower electrode part of the electrode parts is formed on the entire lower curved surface of the piezoelectric body.

3. The piezoelectric transformer according to claim 1, wherein the electrode parts comprises an input electrode, corresponding to a primary side electrode, and an output electrode, corresponding to a secondary side electrode, which are polarized in a direction of a tangential line to the curved surface.

4. The piezoelectric transformer according to claim 1, wherein the electrode parts comprises an input electrode, corresponding to a primary side electrode, and an output electrode, corresponding to a secondary side electrode, which are polarized in directions which are not perpendicular to each other.

5. The piezoelectric transformer according to any one of claims 1 to 4, wherein the curved surface has a hemispherical shape.

6. A piezoelectric transformer, comprising: a piezoelectric body produced through powder injection molding; an upper electrode part, including an input electrode and an output electrode separately formed on an upper curved surface of the piezoelectric body; and a lower electrode part, formed on an entire surface of a lower curved surface of the piezoelectric body; wherein, when the piezoelectric transformer is oscillated through application of voltage, displacement occurring upon contraction and expansion of surfaces of the piezoelectric body is maximized at a peripheral region and a central region of the piezoelectric transformer.

7. A method of manufacturing a piezoelectric transformer, comprising: a mixture preparation step of mixing piezoelectric ceramic powder with a binder at a predetermined ratio, thus preparing a pellet for injection molding (a first step) ; an injection molding step of subjecting the pellet to injection molding using a mold to impart a predetermined shape thereto, thus producing an injection molded body (a second step) ; a binder removal step of removing the binder from the injection molded body, thus obtaining a degreased body (a third step) ; and a sintering step of sintering the degreased body at a high temperature, thus manufacturing a final product (a fourth step) .

8. The method according to claim 7, wherein the mold of the second step has a gate disposed at a top of a cavity, where only a normal piezoelectric mode is present.

9. The method according to claim 8, wherein the top of the cavity includes a protrusion.

10. The method according to claim 7, wherein the mold of the second step has a runner and a gate formed at a parting surface between a fixed body and a movable body.

11. A piezoelectric transformer, manufactured using the method of any one of claims 7 to 10.