Patent Application
20060028097
1. Field of the Invention
The present invention relates to a piezoelectric loudspeaker, and more specifically, to improvements of power loss, the decrease in sound pressure, and the reproducibility of sounds in a thin piezoelectric loudspeaker.
2. Description of the Related Art
For example, in a known ultra-thin piezoelectric loudspeaker having a thickness of 1 mm or less, at least one piezoelectric element (piezoelectric sheet) that is polarized in the thickness direction of the sheet is bonded on at least one surface of a metal diaphragm. In order to achieve a sufficient sound pressure, the piezoelectric element must be a layered product formed by laminating piezoelectric layers composed of a piezoelectric material and electrode layers. In such a layered product, since the electrode must be sintered at the same time, a silver-palladium alloy is used as the electrode material that can withstand the sintering process. For example, as disclosed in Japanese Unexamined Patent Application Publication No. 2003-078995, a metal foil having a conductive adhesive thereon and having a thickness of 0.1 mm or less is used as a conductive path that applies signals to the electrode disposed on the surface of the piezoelectric material. This structure provides a thin loudspeaker overall. In addition, decreasing the palladium ratio in the electrode is effective in order to decrease the cost. Accordingly, a piezoelectric material that is sintered at a relatively low temperature is used and, for example, a material wherein the ratio of silver to palladium is about 9:1 is used as the electrode. However, the shrinkage of such an electrode including a small amount of palladium proceeds at a temperature lower than the shrinkage temperature of the piezoelectric material, i.e., a base material. As a result, a stress generated by mismatched shrinkages during sintering may deform or break the piezoelectric element. In order to solve this problem, the same piezoelectric material as the base material is added to the electrode so as to match the shrinkage ratio of the base material and that of the electrode.
However, the above piezoelectric material added to the electrode is eliminated from the metals during sintering. Consequently, particles of the piezoelectric material are precipitated on the surface of an outer electrode of the piezoelectric element. Since the piezoelectric material is a nonconductor, the precipitated particles increase the contact resistance. As a result, the electric power consumed at the contact part is increased. This phenomenon decreases energy that is effectively converted to sounds, thereby decreasing the conversion efficiency. In order to prevent this problem, the area of the conductive path (i.e., metal foil) is increased so as to decrease the contact resistance. However, according to this method, the conductive path impedes the vibration of the diaphragm. In such a case, the resonant frequency becomes high and the sound pressure is decreased. Furthermore, when an electrical contact is provided using the conductive adhesive, the piezoelectric material itself, which is a nonconductor, becomes a barrier. Consequently, the contact between the conductive adhesive and the electrode becomes unstable. In such a case, the contact resistance is changed with the vibration and the reproducibility of sounds is significantly impaired.
In view of the above situation, it is an object of the present invention to provide a thin piezoelectric loudspeaker in which the contact resistance is decreased without impairing the sound quality and the power loss can be decreased. It is another object of the present invention to provide a thin piezoelectric loudspeaker in which a stable electrical connection is formed so as to provide an excellent sound reproducibility.
In order to achieve the above objects, according to an aspect of the present invention, a piezoelectric loudspeaker includes a piezoelectric element including a piezoelectric material and an electrode layer that is provided on at least one principal surface of the piezoelectric element; a diaphragm applied on the other principal surface of the piezoelectric element; a conductive path composed of a strip-shaped metal foil conductively connecting electrode layers of the piezoelectric element to each other or connecting one of the electrode layers of the piezoelectric element to an external circuit, the conductive path being bonded on the electrode layer by an adhesive layer provided on the reverse face thereof; and a conductive layer formed by applying conductive paste, the conductive layer being provided over the surface of the electrode layer and the top face of the conductive path, and the conductive layer having a Young's modulus of 100 MPa or less and a volume resistivity of 6×10−3 Ωcm or less.
According to the piezoelectric loudspeaker, each of the contact area of the surface of the electrode layer with the conductive layer and the contact area of the conductive path with the conductive layer is preferably at least 0.8 mm2 and the thickness of the conductive layer is preferably at least 0.01 mm. The contact area of the surface of the electrode layer with the conductive layer is preferably 20 mm2 or less. The diaphragm preferably has a diameter of 10 to 50 mm.
Furthermore, the piezoelectric element is preferably applied on at least one principal surface of the diaphragm. The piezoelectric element preferably has a layered structure formed by alternately laminating a plurality of piezoelectric layers and electrode layers. The piezoelectric element preferably includes at least three piezoelectric layers.
According to another aspect of the present invention, a piezoelectric loudspeaker includes a piezoelectric element including a piezoelectric material and an electrode layer that is provided on at least one principal surface of the piezoelectric element; a diaphragm applied on the other principal surface of the piezoelectric element; a conductive path composed of a strip-shaped metal foil conductively bonded on the electrode layer in order to connect electrode layers of the piezoelectric element to each other or connect one of the electrode layers of the piezoelectric element to an external circuit conductively; a conductive adhesive layer provided on the reverse face of the conductive path; and a conductive layer formed by applying conductive paste, the conductive layer being provided over the surface of the electrode layer and the top face of the conductive path. In the piezoelectric loudspeaker, the conductive layer is provided so as to be connected to the electrode layer at a plurality of points along the edge of the conductive path. The conductive layer is preferably provided on both sides of the conductive path or across the conductive path. The conductive layer preferably has a Young's modulus of 100 MPa or less and a volume resistivity of 6×10−3 Ωcm or less.
According to the piezoelectric loudspeaker, each of the contact area of the surface of the electrode layer with the conductive layer and the contact area of the conductive path with the conductive layer is preferably at least 0.8 mm2 and the thickness of the conductive layer is preferably at least 0.01 mm. The contact area of the surface of the electrode layer with the conductive layer is preferably 20 mm2 or less. The diaphragm preferably has a diameter of 10 to 50 mm.
Furthermore, the piezoelectric element is preferably applied on at least one principal surface of the diaphragm. The piezoelectric element preferably has a layered structure formed by alternately laminating a plurality of piezoelectric layers and electrode layers. The piezoelectric element preferably includes at least three piezoelectric layers.
The above and other objects, features and advantages of the present invention will become apparent from the following detailed description taken with the accompanying drawings.
The present invention can provide numerous physical embodiments, depending upon the environment and requirements of use. Substantial numbers of the embodiments shown and described herein have been made, tested, and used, and all have performed in a highly satisfactory manner.
The best embodiment for carrying out the present invention will now be described in detail with reference to Examples.
Referring to
Examples of the material of electrode layers 16 disposed between the piezoelectric layers and on the top and reverse faces of the piezoelectric element 14 include an alloy of silver and palladium (silver/palladium=9/1 to 10/0 (molar ratio)) and silver. The electrode layers 16 are formed as follows. An alloy (or silver) powder, a piezoelectric material powder, and a binder are added to an appropriate solvent to prepare paste. The paste is then applied on green sheets composed of a piezoelectric material by, for example, screen printing. Subsequently, a strip-shaped metal foil having a thickness of 0.1 mm or less and having an adhesive layer 22 on the reverse face thereof is prepared. The metal foil is applied on the electrode layer 16 disposed on the surface of the piezoelectric element 14. The metal foil forms a conductive path 20 connecting the electrode layers 16 to each other or connecting the outer electrode layer 16 to an external circuit. In the present embodiment, the adhesive layer 22 is electrically conductive. Therefore, in order to prevent short-circuiting between the conductive adhesive layer 22 and the diaphragm 12, an appropriate measure for insulation must be taken. For example, as shown in
Subsequently, conductive paste having a low rigidity and a low volume resistivity is applied so as to be disposed over the surfaces of the conductive path 20 and the electrode layer 16. Thus, a conductive layer is formed. Regarding the application shape of the conductive paste, the conductive paste may be applied so as to be connected to the electrode layer 16 at a plurality of points along the edge of the conductive path 20. Preferably, the conductive paste is applied on both sides of the conductive path 20 or applied across the conductive path 20. For example, as shown in the piezoelectric loudspeaker 10 in
Conductive paste having a low rigidity and a low volume resistivity is used as the conductive layer 24, 26, or 28. More specifically, conductive paste having a Young's modulus of 100 MPa or less and a volume resistivity of 6×10−3 Ωcm or less is preferably used. When the Young's modulus exceeds the above value, the conductive paste is broken because the paste cannot withstand the stress caused by the deformation of the diaphragm 12. Furthermore, in such a case, since the conductive paste acts as a resistance against the displacement of the diaphragm 12, the sound quality is impaired. Table 1 shows an example of the relationship between the Young's modulus of the conductive paste used as the conductive layer and the occurrence of breaking caused by driving the piezoelectric loudspeaker 10. Paste A is polyester-based paste, Paste B is silicone-based paste, Paste C is epoxy-based paste, and Paste D is polyimide-based paste. All types of the paste include silver as conductive filler.
As is apparent from Table 1, although Paste B to Paste D satisfy the condition of the volume resistivity of 6×10−3 Ωcm or less, the rigidity is excessively high. As a result, the paste is broken by driving the piezoelectric loudspeaker.
Table 2 shows the change in sound pressure when the Young's modulus of conductive paste is changes, In this example, the conductive layer 26 having a shape shown in
As is apparent from the results in Table 2, in order to suppress the deterioration of sound pressure within, for example, 0.1 dB, the upper limit of the Young's modulus of the conductive paste is 100 MPa. Accordingly, in order to prevent the breaking by driving and to minimize the deterioration of sound quality, the Young's modulus of the conductive paste is preferably 100 MPa or less.
When the volume resistivity exceeds 6×1031 3 Ωcm, the contact resistance cannot be described sufficiently. As described above, the application shape of the conductive paste may be any shape such as a circular shape or a rectangular shape so long as the conductive paste is connected to the electrode layer 16 at a plurality of points along the edge of the conductive path 20. However, each of the area of the conductive paste overlapping with the conductive path 20 and the area of the conducting paste overlapping with the electrode layer 16 has an area of at least 0.8 mm2. When the area is smaller than 0.8 mm2, the resistance at the conductive paste portion (conductive layer 24, 26, or 28) is not sufficiently decreased and a stable contact state cannot be achieved. Table 3 shows the deterioration of sound pressure when the application area of the conductive paste (conductive layer 24, 26, or 28) on the electrode layer 16 is changed using conductive paste having a Young's modulus of 60 MPa. As shown in Table 3, when the area exceeds 20 mm2, the deterioration of sound pressure exceeds 0.1 dB. Accordingly, the application area on the electrode layer 16 is preferably 20 mm2 or less. In a relatively small piezoelectric loudspeaker including the diaphragm 12 having a diameter of about 10 to about 50 mm, the application area of the conductive paste significantly affects the sound quality. The reason for this is as follows: In such a relatively small piezoelectric loudspeaker, the diaphragm 12 has a high rigidity. Therefore, the loudspeaker is less affected by the conductive paste.
The conductive paste may be paste may be applied by a known method such as printing or spraying. The thickness of the conductive layer 24 (26 or 28 ) is, for example, at least 0.01 mm (10 μm). When the thickness is smaller than 0.01 mm, the resistance becomes excessively high and a stable contact state cannot be achieved. After the application, the conductive paste is cured by a predetermined method, for example, by irradiating ultraviolet rays or by heating. Thus, the piezoelectric loudspeaker 10 wherein the contact state is stable can be produced.
Examples and Comparative examples of the present invention will now be described.
Firstly, Example 1 will now be described. In a piezoelectric loudspeaker 30 of Example 1, piezoelectric elements 34 and 40 having a layered structure were bonded on both faces of a diaphragm 32 to form a bimorph type. Electrode layers 38A and 44A were provided on the surfaces of the piezoelectric elements 34 and 40, respectively. Conductive paths 46A and 46B composed of strip-shaped metal foils were provided on the electrode layers 38A and 44A, respectively. The diaphragm 32 was composed of an iron-nickel alloy and had a diameter of 23 mm and a thickness of 0.03 mm. The piezoelectric element 34 was a layered product formed by alternately laminating three piezoelectric layers 36A to 36C and four electrode layers 38A to 38D. Each of the piezoelectric layers 36A to 36C was composed of lead zirconate titanate and had a diameter of 19 mm and a thickness of 0.018 mm (18 μm). Each of the electrode layers 38A to 38D was composed of a silver-palladium alloy and had a diameter of 18.5 mm and a thickness of 0.001 mm. The electrode layers 38A to 38D were connected to each other by a through-hole. The other piezoelectric element 40 had the same structure as that of the piezoelectric element 34. The piezoelectric element 40 also had a layered structure formed by alternately laminating three piezoelectric layers 42A to 42C and four electrode layers 44A to 44D.
Conductive adhesive layers 48A and 48B were provided on the reverse faces of the conductive paths 46A and 46B, respectively. Each of the conductive paths 46A and 46B was composed of a copper foil and had a thickness of 0.07 mm, a length of 10 mm, and a width of 2 mm. Alternatively, the adhesive layers 48A and 48B may be nonconductive. Furthermore, in order to prevent short-circuiting at a peripheral part of the diaphragm 32 where the metal was exposed, insulating tapes 50A and 50B were applied inside of the conductive paths 46A and 46B, respectively. Polyester-based conductive paste (DW-250H-5 from Toyobo Co., Ltd., Young's modulus: 60 MPa, volume resistivity: 1×10−3 Ωcm) including silver as conductive filler was applied on the conductive paths 46A and 46B to form conductive layers 52A and 52B, respectively. Regarding the application shape of the conductive paste, as in the embodiment shown in
The resistance between the conductive paths 46A and 46B and the electrode layers 38A and 44A provided on the surfaces of the piezoelectric elements 34 and 40, respectively, of the resultant piezoelectric loudspeaker was measured by a four probe method. Table 4 shows the results. Furthermore, the piezoelectric loudspeaker was installed in a jig so as to fix the periphery thereof. Sine waves with a voltage of 3 Vrms and having a frequency of 1 kHz were then applied to the terminals. The generated sounds were corrected with a microphone and signals amplified with a pre-amplifier were checked with an oscilloscope to observe the presence of waveform distortion. Table 4 shows the results. In Table 4, when waveform distortion was observed, the piezoelectric loudspeaker was determined to be in an unstable contact state, and when such waveform distortion was not observed, the piezoelectric loudspeaker was determined to be in a stable contact state.
The same conductive paste as that in Example 1 was applied on the conductive paths 46A and 46B by printing so as to form rectangular conductive layers 52A and 52B having a dimension of 1.6×4 mm, respectively, as in the embodiment shown in
A piezoelectric loudspeaker was prepared as in Example 1 except the conductive paste, in other words, except that the conductive layers 52A and 52B were not formed. The measurement of resistance and the observation of the presence of waveform distortion were performed by the methods described above.
A piezoelectric loudspeaker was prepared as in Example 2 except that the conductive layers 52A and 52B had a Young's modulus of 1,000 MPa, a volume resistivity of 2×10−3 Ωcm, and a thickness of 0.02 mm. The measurement of resistance and the observation of the presence of waveform distortion were performed by the methods described above.
A piezoelectric loudspeaker was prepared as in Example 1 except that the conductive layers 52A and 52B had a Young's modulus of 40 MPa and a volume resistivity of 1×10−1 Ωcm. The measurement of resistance and the observation of the presence of waveform distortion were performed by the methods described above.
A piezoelectric loudspeaker was prepared as in Example 2 except that the conductive layers 52A and 52B had a thickness of 0.005 mm. The measurement of resistance and the observation of the presence of waveform distortion were performed by the methods described above.
Table 4 shows the physical properties, the application shapes, the application areas, and the thicknesses of the conductive layers 52A and 52B, and in addition, the measured values of the contact resistance between the conductive paths 46A and 46B and the electrode layers 38A and 44A provided on the surfaces of the piezoelectric elements 34 and 40 of the piezoelectric loudspeakers, respectively, and the presence of waveform distortion of the piezoelectric loudspeakers in Examples 1 and 2 and Comparative examples 1 to 4.
Examples 1 and 2 satisfy the following conditions specified in the present invention: The conductive layers 52A and 52B have a Young's modulus of 100 MPa or less and a volume resistivity of 6×10−3 Ωcm or less, each of the contact area of the electrode layer 38A (44A) with the conductive layer 52A (52B) and the contact area of the conductive path 46A (46B) with the conductive layer 52A (52B) is at least 0.8 mm2, and the thickness of the conductive layers 52A and 52B is at least 0.01 mm (10 μm). Referring to the results in Table 4, regardless of the application shape of the conductive paste, the piezoelectric loudspeakers in Examples 1 and 2 had contact resistances of 0.11 Ω and 0.08 Ω, respectively. In other words, the contact resistance could be maintained within 0.5 Ω and the power loss of signals could be suppressed. Accordingly, a thin piezoelectric loudspeaker having a high efficiency could be achieved. Furthermore, regardless of the application shape, waveform distortion was not observed in Examples 1 and 2. In other words, a stable contact state could be achieved.
In contrast, referring to the results of contact resistance in Comparative examples, the piezoelectric loudspeaker in Comparative example 1, which did not include conductive paste, had a very high contact resistance of 4.22 Ω. In Comparative example 3 for comparing with Example 1, although the Young's modulus satisfied the above condition, the volume resistivity was higher than the above condition. Consequently, the contact resistance in Comparative example 3 was 2.09 Ω. In Comparative example 2 for comparing with Example 2, although the thickness and the volume resistivity satisfied the above conditions, the Young's modulus was larger than the above condition. Consequently, the contact resistance in Comparative example 2 was 1.86 Ω. In Comparative example 4 for comparing with Example 2, although the Young's modulus and the volume resistivity satisfied the above conditions, the thickness was smaller than 0.01 mm. Consequently, the contact resistance in Comparative example 4 was 3.33 Ω. These results showed that piezoelectric loudspeakers prepared under conditions other than the above ranges had a contact resistance of at least 1 Ω, which increased the power loss in this area.
Referring to the results of the presence of waveform distortion in Comparative Examples, in the piezoelectric loudspeaker in Comparative example 1, which did not include conductive paste, waveform distortion was observed, and in addition, sounds themselves were not generated. In all the piezoelectric loudspeakers in Comparative examples 2 to 4 prepared under conditions other than the above ranges by changing the physical properties and the thickness of conductive paste, waveform distortion was observed. This result showed that a satisfactory sound reproducibility could not be achieved. In particular, in the piezoelectric loudspeaker in Comparative example 2 using conductive paste having a high Young's modulus, the conductive paste was broken by vibration and the sounds themselves were not generated.
As described above, in a piezoelectric loudspeaker including a diaphragm and a piezoelectric element including an electrode layer provided on at least one principal surface of the piezoelectric element, a conductive path composed of a strip-shaped metal foil having an adhesive layer provided on the reverse face thereof connects electrode layers to each other or connects the outer electrode layer to an external circuit. In addition, in the piezoelectric loudspeaker, a conductive layer is provided over the surface of the electrode layer and the top face of the conductive path using conductive paste having a low rigidity and a low volume resistivity. This structure keeps the contact resistance low (for example, 0.5 Q or less) without impairing the sound quality and prevents the power loss of signals. Accordingly, a thin piezoelectric loudspeaker having a high efficiency can be provided.
Furthermore, in a piezoelectric loudspeaker including a diaphragm and a piezoelectric element including an electrode layer provided on at least one principal surface of the piezoelectric element, a conductive path composed of a strip-shaped metal foil having an adhesive layer provided on the reverse face thereof connects electrode layers to each other or connects the outer electrode layer to an external circuit. In addition, in the piezoelectric loudspeaker, a conductive layer is provided over the surface of the electrode layer and the top face of the conductive path so as to be connected to the electrode layer at a plurality of points along the edge of the conductive path using conductive paste having a low rigidity and a low volume resistivity. This structure provides the conductive path having a low resistance. Accordingly, even when particles of a piezoelectric material, which become a barrier, are precipitated on the surface of the electrode layer by a simultaneous sintering process using an electrode material with a low cost, the contact resistance is not varied by vibration to provide a stable electrical connection state. Thus, a thin piezoelectric loudspeaker having a satisfactory sound reproducibility can be provided with a low cost.
The present invention includes a plurality of embodiments, which can be variously modified according to the above disclosure. For example, the embodiments include the following:
(1) The materials, the shapes, and the dimensions described in the Examples are examples and can be appropriately changed so as to provide the same operation. For example, the application shapes of the conductive layers 24, 26, 28, 52A, and 52B are examples. The application shape may be appropriately changed within the range of the above conditions (the area and the thickness) so long as the conductive layer is connected to the electrode layer 16 at a plurality of points along the edge of the conductive path. Furthermore, for example, when the piezoelectric loudspeaker is a bimorph type, the shape of a conductive layer formed on an electrode layer of one piezoelectric element may be different from the shape of another conductive layer formed on the other electrode layer of the other piezoelectric element.
(2) The number of the piezoelectric layers and the electrode layers may be changed according to need. In the Examples, three piezoelectric layers are laminated in order to achieve a sufficient driving force. The number of the layers may be further increased so long as the total thickness of the layered product does not exceed 0.1 mm. Also, for example, the connecting structure of the inner electrode layers may be appropriately changed according to need.
(3) Although the adhesive layers 48A and 48B in the Examples are composed of a conductive material, the adhesive layers 48A and 48B may be composed of a nonconductive material.
(4) Examples of the preferable application of the present invention include a loudspeaker of various electronic devices such as cellular phones (including PHS), personal digital assistances (PDA), voice recorders, and personal computers (PC). The present invention may be used for other various applications.
As described above, according to the present invention, the contact resistance can be kept low without impairing the sound quality and the loss of signals can be prevented to achieve a high efficiency. According to the present invention, a stable electrical connection state can be provided. Accordingly, the present invention can be applied to a thin piezoelectric loudspeaker, and in particular, to an ultra-thin piezoelectric loudspeaker having a thickness of 1 mm or less.
As many apparently widely different embodiments of the present invention may be made without departing from the spirit and scope thereof, it is to be understood that the present invention is not limited to the specific embodiments thereof except as defined in the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-181815 | Jun 2004 | JP | national |
| 2004-181816 | Jun 2004 | JP | national |
