Laminated piezoelectric element

Abstract: A laminated piezoelectric element comprising a laminate of piezoelectric ceramic plates and inner electrodes, the piezoelectric ceramic plates being laminated alternately together with the inner electrodes, the laminate having a peripheral surface, a positive outer electrode connected to alternate inner electrodes at the peripheral surface of the laminate, and a negative outer electrode connected to other alternate inner electrodes at the peripheral surface of the laminate, wherein fine ceramic particles are coated, or a ceramic layer is sputtered, on the peripheral surface of the piezoelectric ceramic plates so that an insulation between adjacent inner electrodes is improved.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laminated piezoelectric element particularly useful as an actuator. 2. Description of the Related Art Currently, in answer to the demand for a high speed printing, a laminated piezoelectric element capable of a very quick response when a voltage applied thereto is changed, is used as an actuator in a printing head. Nevertheless, in a laminated piezoelectric element, insulation breakdowns and current leaks occur due to recesses or pores present on the surface of piezoelectric ceramics, and accordingly, it is difficult to obtain a desired reliability. Therefore, there is a need for a highly reliable laminated piezoelectric element which is resistant to insulation breakdowns. A conventional laminated piezoelectric element is manufactured by preparing a plurality of green sheets made of piezoelectric ceramics, coating a metal paste film on one of the surfaces of each green sheet, to thus form an inner electrode, laminating and sintering the plurality of the green sheets, cutting the laminate to a predetermined size, and connecting positive and negative outer electrodes to the inner electrodes, which are exposed at both sides of the cut laminate, so that a voltage can be applied to each of the piezoelectric ceramics sandwiched between the inner electrodes. The thus formed laminated piezoelectric element is often further encapsulated by a resin, for an insulation thereof. Nevertheless, since piezoelectric ceramics contain many pores in the body thereof, which are formed during the fabrication thereof, and recesses are formed on the surface of the piezoelectric ceramics when cutting the piezoelectric ceramics, and water in the air is captured and adsorbed in the recesses if the laminate is used in a high temperature and high humidity atmosphere. Even if a resin coating is provided around the laminated piezoelectric element, since the resin coating may be slightly peeled from the surface of the piezoelectric ceramics by a repeated operation of the piezoelectric element, and thus water can penetrate the resin coating and reach the surface of the piezoelectric ceramics, in this case also water is captured and adsorbed in recesses on the surface of the piezoelectric ceramics. If water is present in the recesses on the surface of the piezoelectric ceramics, a current path can be formed through the water layer, whereby the insulation between the inner electrodes is broken and a current occurs, and accordingly, a voltage applied to the piezoelectric ceramic becomes inappropriate for the desired purpose, and thus the reliability of the laminated piezoelectric element is lowered. SUMMARY OF THE INVENTION Therefore, the object of the invention is to solve the above problem and prevent a current leakage caused by an insulation breakdown between the inner electrodes, and to provide a laminated piezoelectric element which is highly reliable even when used in a high temperature and high humidity atmosphere. The above object of the invention is attained by the features of claim 1. The presence of ceramic particles in the recesses on the peripheral surface of the piezoelectric ceramic plates ensures that, even if water is captured in those recesses, a current path between inner electrodes is not easily formed. Therefore, the size of the fine ceramic particles should be substantially smaller than the size of the recesses of the piezoelectric ceramic plate. Accordingly, as the size of recesses or pores in a piezoelectric ceramic is usually about 5 to 10 µm, the fine ceramic particles preferably have an average particle size of less than about 5 µm, more preferably less than 3 to 2 µm, most preferably less than 1 µm. The fine ceramic particles that may be used in the present invention are, for example, BN, TiN, Al₂O₃, BeO, Si₃N₄ and AlN, etc., but preferably BN and AlN are used. For example, the fine ceramic particles may be coated by spraying a mixture of fine ceramic particles with a volatile solvent. Alternatively, the fine ceramic particles may be coated on the peripheral surface of the piezoelectric ceramic plate, together with a resin as a binder. A preferred amount of a resin is less than 80 % by volume, more preferably about 10 to 30 % by volume. If the amount of the resin is too high, the proportion of fine ceramic particles present in the recesses on the peripheral surface of the piezoelectric ceramic plate is undesirably lowered. Examples of the resin used as a binder include epoxy, urethane, acryl, vinyl, etc. Further, after fine ceramic particles are first coated on the peripheral surface of the piezoelectric ceramic plate, on the fine ceramic particle layer may be covered by a resin layer, to fix the fine ceramic particles on the peripheral surface of the piezoelectric ceramic plate. Alternatively, a piezoelectric element may be coated with a ceramic layer by sputtering, whereby the recesses on a peripheral surface of the piezoelectric ceramic plates are filled by the ceramic layer. In this case, the sputtering should be performed at a low temperature, to prevent a fusing of an insulating glass provided on a peripheral surface of the inner electrodes. The piezoelectric ceramic plate may be made of any piezoelectric ceramics, for example, barium titanate, lead titanate, lead zirconate, etc., or a solid solution thereof. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a sectional view of a piezoelectric element; Fig. 2 is an enlarged sectional view of a portion of a piezoelectric element of the present invention; Fig. 3A shows a leak current of a piezoelectric element of the present invention in relation to an application of a voltage to the element; Fig. 3B shows a leak current of a conventional piezoelectric element in relation to an application of a voltage to the element; Fig. 4 is a partial perspective view of a printing head, particularly of an actuator for driving dot-impact wires or rods of a printer; and Fig. 5 is a front view of an actuator for driving a dot-impact wire or rod. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, a laminated piezoelectric element comprises a laminate of piezoelectric ceramic plates 1 and inner electrodes 2 arranged such that each of the piezoelectric ceramic plates 2 is sandwiched by the inner electrodes 2. The thickness of a piezoelectric ceramic plate 1 is about 110 µm and the number of the piezoelectric ceramic plates 1 is about 140, and thus a stroke of the laminated piezoelectric element becomes about 15 µm and it can be used as an actuator for a printer head, etc. An example of a printer head will be illustrated later with reference to Figs. 4 and 5. A commercially obtained laminate of piezoelectric ceramic plates and inner electrodes, is cut to a predetermined shape to form a laminated piezoelectric element, and the cut laminate has a peripheral surface 11 on which peripheral surface 21 of the inner electrodes 2 and peripheral surface of the piezoelectric ceramic plates 1 are exposed. The piezoelectric ceramic plates 1, made of a solid solution of Pb(Ni,Nb)O₃, PbTiO₃ and Pb 1-z Zr z O₃, for example, has recesses on the peripheral surface thereof and sizes of the recesses typically range from about 5µm to 10µm. Either of positive and negative electrodes 31 and 32 is connected to the alternate inner electrodes 2 at an end of the exposed peripheral surface 21 of the laminate, and the same alternate inner electrodes are insulated by covering same with glass 5 at the other end of the exposed peripheral surface 21 of the laminate. The inner electrodes 2 are alternately connected to the positive and negative electrodes 31 and 32, respectively. The peripheral surface 11 of the laminated piezoelectric element is coated with a layer 4 of fine ceramic particles, and thus the recesses 13 on the surface of the piezoelectric ceramic plates 1 derived from fine pores 12 formed during the fabrication thereof are filled with the fine ceramic particles. The layer 4 of fine ceramic particles may comprise a mixture of fine ceramic particles, e.g., boron nitride powders, with a resin, e.g., epoxy, as a binder. Such fine ceramic particles having a particle size smaller than that (5 to 10 µm) of the recesses on the surface of the piezoelectric ceramic plates 1 will fill the recesses, and thus the insulation between adjacent inner electrodes is improved, and as a result, breakdowns between adjacent inner electrodes due to water accumulated in the recesses in a high temperature and high humidity atmosphere are prevented. Also, since ceramic particles such as boron nitride have a high thermal conduction coefficient, a heating of the piezoelectric ceramic plates 1, and therefore a deterioration of the insulation due to the higher temperature, is prevented. The ceramic particle layer 41 may be formed, for example, by immersing a laminate of the piezoelectric element in a mixture of fine ceramic particles 42 with a fused resin 43, or by spraying a mixture of fine ceramic particles 42 dispersed in a volatile solvent onto a laminate; see Fig. 2. As an example, boron nitride powders 42 having the following distribution of particle size were coated on a piezoelectric element. particle size(µm) : 3 2 1.5 1 0.5 other ratio (wt%) : 15 33 27 11 9 5 As a comparative example, a piezoelectric element identical to the above was coated with an epoxy resin not containing a ceramic powder. A voltage of 120 V was continuously applied to the above conventional piezoelectric element and that of the present invention, in an atmosphere having a temperature of 40°C and a humidity of 90% and a leak current was measured in relation with time elapsed. The leak current was measured by passing a current between the positive and negative outer electrodes. The results are shown in Figs. 3A and 3B. Figure 3A shows the results of the present invention and Fig. 3B shows that of the conventional element. It is seen in Figs. 3A and 3B that the conventional piezoelectric element broke down after about 80 hours but the piezoelectric element of the present invention had not broken down after more than 150 hours. Figures 4 and 5 illustrate a printing head of a dot-impact printer in which a plurality of actuators comprising a piezoelectric element are used, wherein the printing head 110 comprises a cylindrical housing 120 and a plurality of actuators 130 arranged in the cylindrical housing 120. Each of the actuators 130 comprises an electro-distortion device 101, an impact printing wire 116, a frame 111, a movable member (or armature) 112, a first resilient member 113, a second resilient member 114, and a resilient connecting (or third resilient) member 115. The frame 111 is substantially L-shaped, having a base 111a and a side wall 111b extending substantially perpendicular to the base 111a. The electro-distortion device 101, such as a piezoelectric element, has a base portion 101a mounted on the frame base 111a, and therefore, the top free end of the electro-distortion device 101 is displaced upward when an electric power is supplied to the electro-distortion device 101. The impact printing wire 116 is fixed to an end of the movable member 112 at a position A thereof. A plurality of such printing wires 116 driven by the respective actuators 130 constitute a wire-dot matrix. The first resilient member 113 is fixedly connected at the lower end thereof to the top end of the electro-distortion device 101 and extends upward in the same direction as the displacement of the elector-distortion device 101 the first resilient member 113 is also fixedly connected at the upper end thereof to the movable member 112 at a position B thereof. The second resilient member 114 is arranged in parallel to the first resilient member 113 and fixedly connected at the lower end thereof to the side wall 111b of the frame 111 at a position E. The upper end of this second resilient member 114 is fixedly connected to the movable member 112 at a position C thereof. A distance from the first position A to the second position B is much larger than a distance from the second position B to the third position C, so that a displacement of the electro-distortion device 101 can be enlarged by the movable member 112 and transmitted to the impact printing wire 116. A resilient connecting member 115 made of a metal wire having a circular cross-section extends substantially perpendicular to the first and second resilient members 113 and 114, which comprise metal strips arranged in parallel to each other. The connecting wire 115 has one end fixedly connected to the movable member 112 at a position D thereof and the other end fixedly connected to the side wall 111b of the frame 111 at a position F thereof, and passes through respective openings of the resilient members 113 and 114. The position D is located nearer the frame base 111a with respect to a plane on which the positions B and C lie. Also, the position F is located opposite to the position D with respect to the second resilient member 114. The operation of the printing head is as shown below. When printing, an electric power is supplied via a drive circuit (not shown) to the electro-distortion device 101, for a predetermined time. In this case, the upper portion of the electro-distortion device 101 is displaced upward, and therefore, the movable member 112 is turned in the counterclockwise direction in Fig. 5. Accordingly, the displacement of the electro-distortion device 101 is enlarged by the movable member 112, which moves upward, as shown by an arrow P, to conduct a dot-printing. After the printing operation is finished, the movable member 112 and the first and second resilient members 113 and 114 are returned to their original positions.

Claims: A laminated piezoelectric element comprising: a laminate of piezoelectric ceramic plates (1), having recesses (12) at the peripheral surface thereof, and inner electrodes (2), the piezoelectric ceramic plates being alternately laminated together with the inner electrodes, the laminate having a peripheral surface (11), a positive outer electrode (31; 32) connected to alternate inner electrodes at the peripheral surface of the laminate, and a negative outer electrode (32; 31) connected to other alternate inner electrodes (2) at the peripheral surface of the laminate, wherein fine ceramic particles (42) are coated on the peripheral surface of the piezoelectric ceramic plates, wherein the fine ceramic particles (42) have an average particles size smaller than the size of the recesses. A laminated piezoelectric element according to any preceding claim, wherein the fine ceramic particles (42) have an average particle size of less than 3 µm. A laminated piezoelectric element according to any preceding claim, wherein the fine ceramic particles (42) have an average particle size of less than 1 µm. A laminated piezoelectric element according to any of claims 1 to 5, wherein a ceramic layer is coated on the peripheral surface of the piezoelectric ceramic plates. A laminated piezoelectric element according to any preceding claim, wherein the fine ceramic particles (42) are boron nitride. An actuator comprising a laminated piezoelectric element according to any of claims 1 to 5. A printing head comprising a plurality of actuators according to claim 6. Procedure for producing a laminated piezoelectric element comprising: laminating of piezoelectric ceramic plates (1), having recesses (12) at the peripheral surface thereof, and inner electrodes (2), the piezoelectric ceramic plates being alternately laminated together with the inner electrodes, the laminate having a peripheral surface (11), connecting a positive outer electrode (31; 32) to alternate inner electrodes at the peripheral surface of the laminate, and connecting a negative outer electrode (32; 31) to other alternate inner electrodes (2) at the peripheral surface of the laminate, wherein fine ceramic particles (42) are coated on the peripheral surface of the piezoelectric ceramic plates, wherein the fine ceramic particles (42) have an average particles size smaller than the size of the recesses. A laminated piezoelectric element according to claim 8, wherein the ceramic coating or layer (4, 4,) has been formed by sputtering. Procedure according to claim 8, wherein the fine ceramic particles (42) are coated on the peripheral surface of the piezoelectric ceramic plates by spraying with a volatile solvent. Procedure according to claim 10, wherein the fine ceramic particles (42) are coated on the peripheral surface of the piezoelectric ceramic plates (1) together with a resin as a binder (43). A laminated piezoelectric element according to claim 11, wherein the resin (43) is an epoxy resin.

Inventors: KIMURA DAISUKE, OSHIME YUJI, KIMURA, DAISUKE, OSHIME, YUJI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1991
Main classification: H01L41
IPC codes: H01L41