Patent Application
20100139621
The present invention relates to a stacked piezoelectric device equipped with a ceramic laminate made up of a plurality of piezoelectric ceramic layers and a plurality of inner electrode layers which are laminated alternately, a pair of side electrodes formed on side surfaces of the ceramic layer laminate, and stress absorbing portions formed in slit-like areas depressed inwardly into the sides of the ceramic laminate.
Conventionally, stacked piezoelectric devices are used as drive source of fuel injectors. The stacked piezoelectric device is made up of for example, a ceramic laminate formed by stacking inner electrodes and piezoelectric ceramics alternately and a pair of outer electrodes connected to the inner electrode alternately.
The stacked piezoelectric device is used in severe environmental conditions over a long duration, especially when employed in fuel injectors. Therefore, in order to improve the electric insulation of the side surfaces, a ceramic laminate having inner electrode-unformed areas where a portion of an end of an inner electrode layer is recessed inwardly is adapted widely.
However, the formation of the inner electrode-unformed areas in order to improve the insulation may cause portions which are susceptible and insusceptible to deformation to appear in the ceramic laminate upon application of voltage thereto, resulting in concentration of stress at interfaces therebetween and cracks in the device.
In order to avoid the cracks arising from the concentration of stress, stacked piezoelectric devices are being developed which have grooves (stress absorbing portions) formed at a given interval away from each other in a laminating direction in the side surface of the ceramic laminate (see patent document 1).
However, even when the stress absorbing portions are formed, the application of the voltage to the stress absorbing portions also may result in cracks extending from the top end of the stress absorbing portions. In order to avoid this, it is necessary to increase the depth of the stress absorbing portion in a direction perpendicular to the laminating direction more than the distance of the inner electrode-unformed areas. Such a structure, however, causes great electric discharge to occur at the stress absorbing portions (grooves) upon application of great voltage thereto, so that they may be short-circuited. This gives rise to the problem of insufficient electric insulation, which results in a decrease in service life of the stacked piezoelectric devices.
Stacked piezoelectric devices are being developed in which the inner electrodes interleaving the stress absorbing portion therebetween are made to have the same polarity in order to avoid the formation of cracks (see patent document 2). In such conventional stacked piezoelectric devices, it is possible to make the inner electrodes interleaving the stress absorbing portion therebetween to have the same polarity to make the piezoelectric ceramic layer interleaved between them as voltage inactive layers, thereby concentrating the stress at the voltage inactive layers when the stacked piezoelectric device expands. This causes cracks to occur in the stress absorbing portions selectively or preferentially, thereby avoiding the crack in voltage active layers of the laminate to improve the durability.
When two of the inner electrode layers interleaving the stress absorbing portion, as described above, are designed to have the same polarity, it will cause cracks to occur in the stress absorbing portions selectively or preferentially. It is, therefore, possible to avoid the occurrence of cracks in the piezoelectric active layers of the stacked piezoelectric device and improve the durability.
However, in fact, even when no cracks occur in the stress absorbing portions, it is still difficult to ensure sufficient electric insulation, which gives rise to the problem of a drop in electric insulation, thus resulting in an electric short.
Patent Document 1: Japanese patent first publication No 62-271478
Patent Document 2: Japanese patent first publication No. 2006-216850
The present invention was made in view of the above problem and is to provide a stacked piezoelectric device designed to avoid a drop in insulation resistance surely to show an excellent durability.
The invention lies at a stacked piezoelectric device including a ceramic laminate formed by laminating a plurality of piezoelectric ceramic layers and a plurality of inner electrode layers alternately and a pair of side electrodes formed on side surfaces of the ceramic laminate, characterized in that said inner electrode layers are connected electrically to either of the side electrodes, said ceramic laminate has stress absorbing portions formed in slit-like areas recessed inwardly from the side surfaces thereof, the stress absorbing portions being easier to deform than said piezoelectric ceramic layers, and adjacent two of said inner electrode layers interleaving the stress absorbing portion therebetween are both connected electrically to a positive side of the side electrodes (claim 1).
The most notable point of the invention is that adjacent two of said inner electrode layers interleaving the stress absorbing portion therebetween are both connected electrically to the positive side of the side electrodes.
Specifically, the inventors of this invention have studied the disadvantages arising from the formation of the stress absorbing portions such as grooves in the stacked piezoelectric device and found that the piezoelectric ceramic layers interleaved between a negative electrode layer next to the stress absorbing portion and a positive electrode layer next to the negative electrode layer will drop in insulation resistance earliest.
First, a drop in insulation resistance of typical stacked piezoelectric devices will be discussed below for explaining the details of the above.
Generally, when high electric field continues to be applied to the stacked piezoelectric device at a high temperature, the phenomenon that a lower resistance area spreads from the negative electrode side will appear. For example, the cause is that when the stacked piezoelectric device is made integrally by the firing, conductive metallic ions, as spreading to the piezoelectric ceramic layers during the firing, are metalized by electrons emitted from the negative electrode. The above phenomenon results in a variation in distribution of electric field intensity oriented in the laminating direction between the positive electrode layer and the negative electrode layer. In other words, the electric field intensity drops in the low resistance area, thereby resulting in a rise in electric field intensity in areas other than the low resistance area. The rise in electric field intensity accelerates the deterioration of the insulation resistance. The spreading of the low resistance area is usually accelerated by the existence of water.
Specifically, the phenomenon occurs that Ag+ ions, as spreading from an inner electrode-formed areas made with an AgPd electrode to piezoelectric ceramic layers made of PZT when the piezoelectric device is being fired as a whole are metalized by electrons emitted from the negative electrode layers during driving of the piezoelectric device, thereby causing the low resistance area to be formed which, in turn, expands to the positive electrode layer (Ag++e−→Ag metal).
Particularly, in the case where the stacked piezoelectric device with the stress absorbing portions, the stress absorbing portions will usually be a path leading to the outside where water exists. The phenomenon that the low resistance area expands in the negative electrode layer closest to the stress absorbing portion, therefore, becomes pronounced.
Accordingly, the piezoelectric ceramic layer interleaved between the negative electrode layer next to the stress absorbing portion and the positive electrode layer next to the negative electrode layer drops in insulation resistance earliest. The drop in insulation resistance tends to occur in the case where at least one of adjacent two of the inner electrode layers interleaving the stress absorbing portion therebetween is at the negative polarity. The drop in insulation resistance is usually taken place between the inner electrode layer of the negative polarity and the adjacent inner electrode layer of the positive polarity, which may result in an electric short.
Specifically, the drop in insulation resistance tends to occur in the case where at least one of adjacent two of the inner electrode layers interleaving the stress absorbing portion therebetween is at the negative polarity. The drop in insulation resistance is usually taken place between the inner electrode layer of the negative polarity and the adjacent inner electrode layer of the positive polarity, which may result in an electric short.
When adjacent two of the inner electrode layers interleaving the stress absorbing portion therebetween are, like in the invention, both at the positive polarity, it will result in no inner electrode layers interleaving the stress absorbing portions therebetween which contribute to the drop in insulation resistance, thus avoiding the drop in insulation resistance and improving the durability of the stacked piezoelectric device.
The positive electrode layers and the negative electrode layers, as referred to above, are the inner electrode layers connected electrically to the positive and negative sides of the side electrodes, respectively.
Next, a preferred embodiment of the invention will be described.
The stacked piezoelectric device of the invention is equipped with the ceramic laminate and a pair of side electrodes formed on the side surfaces of the ceramic laminate.
The ceramic laminate is made by stacking the piezoelectric ceramic layers and the inner electric layers alternately. The ceramic laminate has the stress absorbing portion in the slit-like areas recessed inwardly from the side surfaces of the ceramic laminate.
The stress absorbing portions are portions of the ceramic laminate where crystalline particles making up the piezoelectric ceramic are separated in the laminating direction and which are easier to deform in shape than the piezoelectric ceramic layers.
The stress absorbing portions work to absorb the stress accumulated in the laminating direction of the ceramic laminate. When the stacked number is small, it will result in a decrease in ability of the stress absorbing portions to absorb the stress. It is, therefore, preferable that the stacked piezoelectric device has the twenty or more inner electrode layers. For the same reasons, the interval between the stress absorbing portions in the laminating direction is preferably greater than or equal to the ten inner electrode layers and smaller than or equal to the fifty inner electrode layers. When the interval between the stress absorbing portions is less than the ten inner electrode layers or greater than the fifty inner electrode layers, it may result in a lack in stress absorbing ability of the stress absorbing portions.
Specifically, the stress absorbing portions are, for example, slit-like chambers (grooves) and may be of a structure wherein the slit-like chamber is filed with resin material which is lower in Young's modulus than the piezoelectric ceramic layer, slit-like fragile layers formed by making the same material as the piezoelectric ceramic layer to be porous, slit-like fragile layers made by material such as titanate different from that of the piezoelectric ceramic layer, or crack-like slits made intentionally by the polarization or actuation.
The stress absorbing portions are preferably slit-like grooves recessed inwardly from the side surface of the ceramic laminate (claim 2).
This facilitates the formation of the stress absorbing portions.
The stress absorbing portions are formed in the side surfaces of the ceramic laminate. The stress absorbing portions may be partially formed in the side surfaces on which the side electrodes are disposed. In this case, it is preferable that a pair of the stress absorbing portions are formed which interleave the side surfaces of the ceramic laminate therebetween. The stress absorbing portions may also be formed so as to extend in the entire peripheral surface in a circumferential direction.
The stacked piezoelectric device is preferably made by firing the plurality of piezoelectric ceramic layers and the plurality of inner electrode layers integrally (claim 3).
In this case, as compared with when a stacked piezoelectric device made by bonding laminates, as described later, by adhesive, it is possible to improve the amount of displacement and to make the stacked piezoelectric device more easily.
The stacked piezoelectric device is preferably made by bonding a plurality of the ceramic laminates through adhesive in a laminating direction (claim 4).
In this case, as illustrated 1
The stress absorbing portions are preferably formed by providing non-bonding portions to which no adhesive is applied near an outer periphery of the ceramic laminates when the ceramic laminates are joined together through the adhesive (claim 5).
This facilitates the formation of the stress absorbing portions.
Specifically, as illustrated in
The stress absorbing portions are preferably made using burn-off material which will be burnt off in the firing process (claim 6).
As the burn-off material, powder-like carbon particles, resinous particles, or carbonized organic particles made by carbonizing organic powders may be used.
Particularly, when the carbon particles are used as the burn-off material, the stress absorbing portions are shaped accurately because the carbon particles are insusceptible to thermal deformation.
Particularly, when the carbonized organic particles are used as burn-off material, it will result in a decrease in production cost of the stress absorbing portions.
The use of the carbonized organic particles as the burn-off material will result in a decrease in production cost required to form the stress absorbing portions.
As the organic particles, there are particles made by grinding soya beans, Indian corns, resinous material.
The carbonized organic particles, as referred to herein, are fine or minute particles made by removing water contained in organic particles partially to carbonize them to the extent that the flowability and dispersibility are good.
The stress absorbing portions are preferably made by forming the slit-like areas by material which causes cracks to occur when the stacked piezoelectric device is polarized or actuated and cracking the slit-like areas when the stacked piezoelectric device is polarized or actuated (claim 7).
This also facilitates the formation of the stress absorbing portions.
Two of the inner electrode layers which are located most outward of the stacked piezoelectric device in a laminating direction are preferably both connected to a positive side of the side electrodes (claim 8).
This improves the durability of the stacked piezoelectric device further.
In the case where the stacked piezoelectric device has the integrally formed signal ceramic laminate, two of the inner electrode layers which are located most outwardly of the ceramic laminate are preferably connected to the positive side of the side electrodes. In the case where the stacked piezoelectric device is made by bonding the plurality of ceramic laminates, two of the inner electrode layers located most outward of the bonded ceramic laminate are preferably connected to the positive side of the side electrodes.
The stacked piezoelectric device is preferably used in a fuel injector (claim 9).
In this case, the stability of operation of the stacked piezoelectric device in heavy environmental conditions is ensured for an increased time.
Next, the stacked piezoelectric device according to embodiments of the invention will be described below using
As illustrated in
The ceramic laminate 15 has the stress absorbing portions 12 which are easier to deform in shape than the piezoelectric ceramic layers 11 in slit-like areas recessed inwardly from the side surfaces of the ceramic laminate 15. Adjacent two of the inner electrode layers 121 and 122 interleaving the stress absorbing portion 12 are both connected electrically to the positive side electrode 17. The remaining inner electrode layers 13 and 14 are connected electrically to the side electrodes 17 and 18 alternately.
The stress absorbing portions 12 of this embodiment are slit-like grooves (chambers) recessed inwardly from the side surface of the ceramic laminate 15. The stress absorbing portions 12 extend in the whole of the outer peripheral surface of the ceramic laminate 15 in a circumferential direction.
Next, a production method of the stacked piezoelectric device of this embodiment will be described below using
In this embodiment, the stacked piezoelectric device is made by a green sheet making process, an electrode printing process, an burn-out slit printing process, a pressure bonding process, a stack cutting process, and a firing process.
Next, each process of the production method will be described below.
<Green Sheet Making Process>
First, we prepared ceramic raw material powder such as lead zirconate titanate (PZT) which is a piezoelectric material. Specifically, we prepared Pb3O4, SrCO3, ZrO2, TiO2, Y2O3, and Nb2O5 as starting raw materials, weighted them at a stoichiometric proportion which was selected to produce a target composition PbZrO3—PbTiO3—Pb(Y1/2Nb1/2)O3, wet-blended, and calcined them at 850° C. for 5 hours. Next, we wet-ground the calcined powders using a pearl mill. We dried the calcined ground powders (Grain Size (D50): 0.7±0.05 μm) and blended with solvent, binder, plasticizer, and dispersant in a ball mill to make slurry. We agitated, vacuum-degassed, and adjusted the slurry in viscosity.
We applied the slurry on a carrier film using the doctor blade method to make elongated green sheet having a thickness of 80 μm. We cut the green sheet into a desired size to make wide green sheet 110, as illustrated in
The formation of the green sheet may alternatively be achieved by the extrusion molding or any other manners as well as the doctor blade method.
<Electrode Printing Process>
Next, as illustrated in
The formation of the electrode-printed sheets 31 and 32 will be described below in more detail.
The first electrode-printed sheet 31 was formed, as illustrated in
The second electrode-printed sheet 41 was, like the first electrode-printed sheet, formed by, as illustrated in
In the first and second electrode-printed sheets 31 and 32, the electrode materials 130 and 140 formed on the green sheets 110 are exposed to side surfaces different from each other.
In this embodiment, Ag/Pd alloy paste was used as the electrode materials 130 and 140, Ag, Pd, Cu, Ni, or Cu/Ni alloy may alternatively be used.
<Burn-Out Slit Printing Process>
In this embodiment, slits 12 (see
As illustrated in
In this embodiment, carbon powder material which is small in thermal deformation and will keep the shape of grooves to be formed by the firing process precisely was used as the burn-off material to make the burn-off slit layer 120. Carbonized organic particles may alternatively be used. The carbonized organic particles may be made by carbonizing powder-like organic particles or grinding carbonized organic substance. As the organic substance, cereal grains such as cones, soya beans, or flour may be used to save the production costs.
In the electrode printing and burn-off slit printing processes, as illustrated in
<Pressure Bonding Process>
Next, the first electrode-printed sheet 31 and the second electrode-printed sheet 32, and the burn-off slit-printed sheets 33 were, as illustrated in
The first electrode-printed sheets 31 and the second electrode-printed sheets 32 were stacked so that the electrode material 130 and the electrode material 140 were exposed alternately to the end surface which the printing areas face. As two of the electrode-printed sheets interleaving the burn-off slit-printed sheet 33, printed-sheets (i.e., the first electrode-printed sheets 31) which were identical in pattern formed by the electrode material with each other were used. Specifically, as illustrated in
The green sheet 110 not subjected to the printing process was disposed on an upper end of the sheets to be stacked.
The sheets stacked in this manner were heated at 100° C. and pressed at 50 MPa in the laminating direction to make a pre-stack 100. For the sake of convenience,
<Stack Cutting Process>
Next, as illustrated in
The pre-stack 100 may be cut in the unit of the intermediate stacks 10 or in the unit of two or more of them. In this embodiment, the pre-stack 100 was cut in the unit of each of the intermediate stacks 10 so that each of the electrode materials 130 and 140 and the burn-off slit layers 120 were exposed to the side surfaces of the intermediate stack 10.
For the sake of convenience,
<Firing Process>
Next, binder resin contained in the green sheet 110 of the intermediate stacks 10 was removed thermally (degreased) by 90% or more. This was achieved by heating the binder resin gradually up to 500° C. for eighty hours and keeping it for five hours.
Next, the degreased intermediate stacks 10 were fired. The firing was achieved by heating the intermediate stacks 10 gradually up to 1050° C. for twelve hours, keeping them for two hours, and then cooling them gradually.
In this manner, the ceramic laminate 15 is, as illustrated in
After fired, the entire surface of the ceramic laminate 15 was polished to be 6 mm×6 mm square and 4.4 mm high. The side electrodes 17 and 18 were printed on the both side surfaces of the ceramic laminate 15. The inner electrodes 13 and 14 are connected electrically alternately to the side electrodes 17 and 18 respectively. Two of the inner electrode layers 121 and 122 interleaving the stress absorbing portion 12 therebetween are connected electrically to the side electrode 17. In this embodiment, the side electrode 17 to which the two inner electrode layers 121 and 123 interleaving the stress absorbing portion therebetween is a positive electrode.
In the above manner, the stacked piezoelectric device 1 was made which, as illustrated in
For the sake of convenience,
In this embodiment, the stacked piezoelectric device 1 (see
As a comparison with the sample E1, the stacked piezoelectric device 1 (see
As a comparison with the sample E1, the stacked piezoelectric device 1 (see
Additionally, in this embodiment, the stacked piezoelectric device 1 (see
As a comparison with the sample E2, the stacked piezoelectric device 1 (see
As a comparison with the sample E2, the stacked piezoelectric device 1 (see
Further, in this embodiment, the stacked piezoelectric device 1 (see
As a comparison with the sample E3, the stacked piezoelectric device 1 (see
As a comparison with the sample E3, the stacked piezoelectric device 1 (see
For the sake of convenience,
We performed the following durability tests on the stacked piezoelectric device (i.e., the samples E1 to E3, Ca1 to Ca3, and Cb1 to Cb3, as made in the above.
<Durability Test>
We applied an electric field of 3.1 kV/mm to the stacked piezoelectric device of each sample at 200° C. to drive it. We connected each sample to a resistor R whose resistance value was known in parallel thereto to develop a circuit. We read the voltage (leakage current value) applied to the resistor R through a digital meter while applying the electric field to each sample. We measured the time elapsed until the insulation resistance of the device (sample) drops below 10 MΩ and defines it as the service life of the device. The durability tests were performed on the five samples of each of the above types.
The results are shown in
It is found that especially, in the case where two of the inner electrode layers 13 which are located most outward in the laminating direction of the ceramic laminate are, like the sample E1, connected electrically to the positive side of the side electrodes (see
It is found that in the case where two of the inner electrode layers 13 which are located most outward in the laminating direction of the ceramic laminate are, like the sample E2, connected electrically to the negative side of the side electrodes (see
It is found that in the case where two of the inner electrode layers 13 which are located most outward in the laminating direction of the ceramic laminate, like the sample E3, are connected electrically to the different side electrodes, respectively (see
In contrast to the above, it is found that the stacked piezoelectric devices 1 (i.e., the samples Cb1 to Cb3), as illustrated in
As described above, the invention avoids the drop in insulation resistance surely and enables the stacked piezoelectric devices (i.e., the sample E1 to E3) which are excellent in the durability.
In this embodiment, the stress absorbing portions are formed using the burn-off material which will burn off in the firing process, but however, they alternatively be formed by material (crack material) which will be cracked when being polarized or actuated.
In this embodiment, the inner electrode layers 131 and 141, the recessed portions 135 and 145, and the slit layers 12 are formed in the combination pattern, as illustrated in
Possible combinations of the inner electrode portions 131 and 141 and the slit layers 12 are demonstrated in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-046071 | Feb 2007 | JP | national |
| 2008-042112 | Feb 2008 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2008/053228 | 2/26/2008 | WO | 00 | 2/2/2010 |
