JP3969594B2 - Piezoelectric element and manufacturing method thereof - Google Patents

Description

  The present invention relates to a piezoelectric ceramic composition suitable for use in piezoelectric layers of various piezoelectric elements such as actuators, piezoelectric buzzers, sounding bodies and sensors, and further relates to a piezoelectric element using the same.

  For example, an actuator that uses displacement generated by the piezoelectric effect as a mechanical drive source has advantages such as low power consumption and heat generation, good response, and miniaturization and weight reduction. However, it has come to be used in a wide range of fields.

Piezoelectric ceramic compositions used for this type of actuator are required to have a large piezoelectric characteristic, particularly a piezoelectric strain constant. Examples of piezoelectric ceramic compositions that satisfy this requirement include lead titanate (PbTiO ₃ ) and lead zirconate. (PbZrO ₃ ) and zinc / lead niobate [Pb (Zn _1/3 Nb _2/3 ) O ₃ ], a ternary piezoelectric ceramic composition, and the ternary piezoelectric ceramic composition Piezoelectric ceramic compositions in which a part of Pb, which is a constituent element, is substituted with Sr, Ba, Ca, or the like have been developed.

  However, these conventional piezoelectric ceramic compositions need to be fired at a relatively high temperature, and since firing is performed in an oxidizing atmosphere, for example, in a laminated actuator that simultaneously fires internal electrodes, a high melting point is obtained. It is necessary to use a noble metal (for example, Pt, Pd, etc.) that does not oxidize even when fired in an oxidizing atmosphere. As a result, the cost is increased and the cost of the manufactured piezoelectric element is hindered.

  From such a situation, the applicant of the present application includes, in the ternary piezoelectric ceramic composition, a first subcomponent including at least one selected from Fe, Co, Ni, and Cu, and Sb, Nb, and Ta. It has been proposed that low temperature firing is possible by adding a second subcomponent including at least one selected, and that an inexpensive material such as an Ag—Pd alloy can be used for the internal electrode (see Patent Document 1). ).

  The invention described in Patent Document 1 includes the ternary piezoelectric ceramic composition and the piezoelectric ceramic composition in which part of Pb is replaced with Sr, Ba, Ca, etc. in the ternary piezoelectric ceramic composition. By adding a first subcomponent including at least one selected from Co, Ni, and Cu and a second subcomponent including at least one selected from Sb, Nb, and Ta, a high piezoelectric strain constant is obtained. Provided is a piezoelectric ceramic composition that is densified without impairing various piezoelectric characteristics even when fired at a low temperature and has increased mechanical strength, and has a piezoelectric layer composed of the piezoelectric ceramic composition. That's it.

  As described above, according to the invention described in Patent Document 1, a certain degree of low-temperature firing is possible, and an Ag—Pd alloy or the like can be used as an electrode material. However, a cheaper metal (Cu) is used as an electrode material. Is not necessarily sufficient when considered to be used. For example, when Cu is used as an electrode material, there is a concern that even if baked at a low temperature, the electrode material is segregated in the piezoelectric layer and adversely affects the characteristics.

For example, Patent Document 2 attempts to improve the segregation of Cu. In Patent Document 2, in a laminated dielectric element in which dielectric ceramic layers and electrode layers are alternately laminated, the electrode layer is mainly free of standard Gibbs of metal oxide at the firing temperature than the ceramic material constituting the dielectric ceramic layer. A laminated dielectric element made of a conductive base metal material having a large energy and having no segregation of a material containing a conductive base metal material in a portion of the dielectric ceramic layer sandwiched between adjacent positive and negative electrode layers. It is disclosed. According to Patent Document 2, the characteristics of the dielectric ceramic layer can be sufficiently exhibited because there is no segregation of a material containing a conductive base metal material in a portion sandwiched between adjacent positive and negative electrode layers. .
JP 2004-137106 A JP 2002-260951 A

  However, according to the subsequent studies by the present inventors, the quality of the piezoelectric element cannot be judged only by the presence or absence of granular segregation as discussed in Patent Document 2, for example, the granularity It has been found that the absence of segregation does not necessarily exhibit good characteristics (acceleration voltage load life, etc.).

In the invention described in Patent Document 2, basically, CuO is used as an electrode material, which is reduced to Cu in a so-called metallization process, and then fired in a reducing atmosphere having an oxygen partial pressure of about 10 ⁻⁴ atm. . When a base metal (for example, Cu) is used as an electrode material, the firing in an oxidizing atmosphere (for example, in air) may cause a disadvantage that the electrode material is oxidized and the conductivity is impaired even if the firing is performed at a low temperature. . On the other hand, when fired in a reducing atmosphere, the obtained fired body contains more oxygen vacancies than a sintered body fired in air, and therefore has an insulation resistance particularly at high temperatures (100 ° C. or higher). There is a risk of inconvenience that it causes a decrease and a decrease in the high temperature load life (insulation life) of the product. The temperature range of 100 ° C. to 200 ° C. is often the standard operating temperature of a product, and a decrease in insulation resistance in this temperature range is a serious problem because it significantly impairs the reliability of the product. From these facts, in the invention described in Patent Document 2, firing is performed in an atmosphere having a relatively high oxygen partial pressure, and it is accepted that a part of Cu as an electrode material is oxidized. The electrode characteristics are sacrificed.

  Furthermore, in the invention described in Patent Document 2, in order to prevent the diffusion and segregation of Cu, a melting suppressing substance, a melting point increasing substance, a diffusion suppressing substance or the like is added to the electrode paste material. This may adversely affect the characteristics of the dielectric ceramic composition constituting the piezoelectric layer, and is also disadvantageous when considering the manufacturing cost, manufacturing man-hours, and the like.

  The present invention has been proposed in view of such a conventional situation. For example, when Cu is used for the internal electrode layer, the optimum structure is achieved, and the insulation property of the grain boundary is enhanced. An object of the present invention is to provide a piezoelectric ceramic composition and a piezoelectric element which are excellent in acceleration voltage load characteristics and the like without changing the composition in the grains important for the characteristics.

  In order to achieve the above object, the present inventors have made various studies over a long period of time. As a result, it is not preferable that Cu segregates in a granular form as described above, but the uneven distribution of Cu in the grain boundary contributes to the improvement of the acceleration voltage load characteristics by enhancing the insulation of the grain boundary, Since the uneven distribution hardly changes the composition in the grains, it has been found that the piezoelectric strain characteristics are not impaired.

The present invention has been completed based on such findings. That is, the piezoelectric element of the present invention includes a plurality of piezoelectric layers including a composite oxide containing Pb, Ti, and Zr as constituent elements, and an internal electrode layer containing Cu formed between these piezoelectric layers, The piezoelectric layer has a structure in which Cu is segregated only within a range of 5 nm from the grain boundary without segregating in a granular manner at the grain boundary .

  For example, when the piezoelectric ceramic composition contains Cu, such as when Cu is used as an electrode material, it is known that if Cu is segregated in a granular form, it adversely affects the characteristics. The detailed structure has not been studied. Even if Cu is not segregated in a granular form, the insulation at the grain boundary may be insufficient and the acceleration voltage load characteristic may be insufficient. However, no improvement has been attempted so far.

  In the present invention, for example, by controlling the firing conditions, etc., the piezoelectric ceramic composition has a structure in which Cu is unevenly distributed at the grain boundaries, thereby increasing the insulation at the grain boundaries and greatly improving the acceleration voltage load characteristics. The In addition, when Cu is unevenly distributed in the grain boundary as described above, the composition of crystal grains that are the main component of the piezoelectric ceramic composition is not affected, so that the original piezoelectric characteristics are not impaired.

  As described above, the piezoelectric ceramic composition and the piezoelectric element of the present invention improve the acceleration voltage load characteristics and the like with a structural feature that Cu is unevenly distributed at grain boundaries. It is not necessary to add such a diffusion inhibitor. Therefore, the present invention can be said to be a completely different technique from the invention described in Patent Document 2 in which only the presence or absence of granular segregation is a problem.

  According to the present invention, it is possible to provide a piezoelectric ceramic composition and a piezoelectric element that have excellent piezoelectric strain characteristics and excellent acceleration voltage load characteristics by adopting a structure in which Cu is unevenly distributed in grain boundaries. . For example, even when Cu is used as the electrode material of the internal electrode layer, it is possible to provide a piezoelectric element that has little decrease in electrical resistance at high temperatures and is excellent in electromechanical coupling coefficient kr. Further, since the addition of a diffusion suppressing substance is unnecessary, it is advantageous in reducing the manufacturing cost and the manufacturing man-hour. Therefore, according to the present invention, it is possible to provide a piezoelectric ceramic composition and a piezoelectric element that are inexpensive and have excellent insulation characteristics and high reliability.

  Hereinafter, a piezoelectric ceramic composition and a piezoelectric element to which the present invention is applied will be described in detail with reference to the drawings.

First, the piezoelectric ceramic composition of the present invention is mainly composed of a complex oxide containing Pb, Ti, and Zr as constituent elements. Here, the composite oxide is composed of, for example, lead titanate (PbTiO ₃ ), lead zirconate (PbZrO ₃ ), and zinc / lead niobate [Pb (Zn _1/3 Nb _2/3 ) O ₃ ]. Or a composite oxide obtained by substituting a part of Pb with Sr, Ba, Ca, or the like in the ternary composite oxide.

  Specific examples of the composition include composite oxides represented by the following formula (1) or (2). In these formulas (1) and (2), the oxygen composition is obtained stoichiometrically, and deviation from the stoichiometric composition is allowed in the actual composition. .

Pb _a [(Zn _1/3 Nb _2/3 ) _x Ti _y Zr _z ] O ₃ (1)
(However, 0.96 ≦ a ≦ 1.03, 0.05 ≦ x ≦ 0.15, 0.25 ≦ y ≦ 0.5, 0.35 ≦ z ≦ 0.6, and x + y + z = 1.)

(Pb _a-b Me _b ) [(Zn _1/3 Nb _2/3 ) _x Ti _y Zr _z ] O ₃ (2)
(However, 0.96 ≦ a ≦ 1.03, 0 <b ≦ 0.1, 0.05 ≦ x ≦ 0.15, 0.25 ≦ y ≦ 0.5, 0.35 ≦ z ≦ 0.6 X + y + z = 1, and Me in the formula represents at least one selected from Sr, Ca, and Ba.)

  The composite oxide has a so-called perovskite structure, and Pb and the substitution element Me in the formula (2) are located at a so-called A site of the perovskite structure. Zn, Nb, Ti, and Zr are located at the so-called B site of the perovskite structure.

  In the composite oxide represented by the formula (1) or formula (2), the ratio a of the A site element is preferably 0.96 ≦ a ≦ 1.03. If the A-site element ratio a is less than 0.96, firing at low temperatures may be difficult. On the contrary, when the ratio a of the A site element exceeds 1.03, the density of the obtained piezoelectric ceramic is lowered, and as a result, sufficient piezoelectric characteristics may not be obtained, and the mechanical strength may also be lowered. is there.

  In the composite oxide represented by the above formula (2), a part of Pb is substituted with the substitution element Me (Sr, Ca, Ba), but this can increase the piezoelectric strain constant. However, if the substitution amount b of the substitution element Me is too large, the sinterability is lowered, and as a result, the piezoelectric strain constant is reduced and the mechanical strength is also lowered. Also, the Curie temperature tends to decrease as the substitution amount b increases. Therefore, the substitution amount b of the substitution element Me is preferably 0.1 or less.

  On the other hand, among the B site elements, the ratio x between Zn and Nb is preferably 0.05 ≦ x ≦ 0.15. The ratio x affects the firing temperature. If this value is less than 0.05, the effect of lowering the firing temperature may be insufficient. On the other hand, if it exceeds 0.15, the sinterability is affected, and as a result, the piezoelectric strain constant may be reduced and the mechanical strength may be reduced.

  Of the B site elements, the Ti ratio y and the Zr ratio z are preferably set in terms of piezoelectric characteristics. Specifically, the Ti ratio y is preferably 0.25 ≦ y ≦ 0.5, and the Zr ratio z is preferably 0.35 ≦ z ≦ 0.6. By setting it within the above range, a large piezoelectric strain constant can be obtained in the vicinity of the morphotropic phase boundary (MPB).

The piezoelectric ceramic composition of the present invention contains the composite oxide as a main component, but may contain subcomponents in addition to this. In this case, the subcomponent is at least one selected from Ta, Sb, Nb, and W. By adding the subcomponent, piezoelectric characteristics and mechanical strength can be improved. However, the content of these subcomponents is preferably 1.0% by mass or less in terms of oxide. For example, in the case of Ta, 1.0% by mass or less in terms of Ta ₂ O ₅ , in the case of Sb, 1.0% by mass or less in terms of Sb ₂ O ₃ , and in the case of Nb, 1.0% by mass in terms of Nb ₂ O ₅ Hereinafter, in the case of W, it is 1.0 mass% or less in terms of WO ₃ . If the content of the subcomponent exceeds 1.0% by mass in terms of the oxide, the sinterability may be reduced and the piezoelectric characteristics may be deteriorated.

  As described above, the piezoelectric ceramic composition of the present invention is mainly composed of a composite oxide containing Pb, Ti, and Zr as constituent elements, and has a structure in which Cu is unevenly distributed at the grain boundaries. Is a big feature. The piezoelectric ceramic composition containing the composite oxide as a main component is configured as an aggregate of crystal grains A of the composite oxide, for example, as schematically shown in FIG. Here, at the boundary portion of each crystal grain A, that is, at the grain boundary, a so-called grain boundary layer B exists as a very thin layer. In the piezoelectric ceramic composition of the present invention, Cu is present in the grain boundary layer B. It is unevenly distributed.

  Such uneven distribution of Cu in the grain boundary layer B cannot be analyzed by, for example, EPMA, and a technique such as FE-TEM (field emission transmission electron microscope) is required for the analysis. When the piezoelectric ceramic composition of the present invention is analyzed by FE-TEM, a Cu peak is seen at the grain boundary or triple point.

  Ideally, the Cu is unevenly distributed only at the grain boundaries, but Cu may partially enter the portion of the crystal grain A that contacts the grain boundary layer B. However, it is not preferable that Cu enters the inside so much that Cu is preferably within 10 nm from the interface of the crystal grains 1. In addition to Cu existing in the grain boundary layer B, the Cu may be segregated in a granular form, for example. However, it is preferable that the granular segregation is as small as possible, and it is more preferable that there is no granular segregation at all.

  Although the method of unevenly distributing the Cu at the grain boundaries is arbitrary, for example, the piezoelectric ceramic composition may be fired under appropriate firing conditions in a state where Cu is in contact. Thereby, Cu diffuses into the piezoelectric ceramic composition, and the structure is obtained. Therefore, in the piezoelectric element, when Cu is used for the internal electrode layer and fired under appropriate conditions, the structure is realized in each piezoelectric layer. Therefore, the piezoelectric element will be described below.

  FIG. 2 shows an example of a laminated piezoelectric element. As shown in FIG. 2, the multilayer piezoelectric element 1 includes a multilayer body 4 in which an internal electrode layer 3 is inserted between a plurality of piezoelectric layers 2, and the multilayer body 4 is displaced as an active portion. Contribute. Although the thickness per layer of the piezoelectric layer 2 can be set arbitrarily, it is usually set to about 1 μm to 100 μm, for example. On both sides of the multilayer body 4, there are piezoelectric layer regions in which the internal electrode layer 3 is not formed as inactive regions. The thickness of the piezoelectric layer in this portion is the thickness of the piezoelectric layer 2 between the internal electrode layers 3. In some cases, the thickness is set to be thicker.

  The internal electrode layers 3 are alternately extended in opposite directions, for example, and terminal electrodes 5 and 6 electrically connected to the internal electrode layers 3 are provided at end portions in the extension directions. The terminal electrodes 5 and 6 may be formed, for example, by sputtering a metal such as Au, or may be formed by baking an electrode paste. The thicknesses of the terminal electrodes 5 and 6 are appropriately set depending on the application, the size of the multilayer piezoelectric element 1 and the like, but are usually about 10 μm to 50 μm.

  In the piezoelectric element, the internal electrode layer 3 has a function as an electrode for applying a voltage to each piezoelectric layer 2 and is naturally formed of a conductive material. In this case, a noble metal such as Ag, Au, Pt, or Pd can be used as the conductive material, but in the present invention, an electrode material containing Cu is used for the purpose of uneven distribution at the grain boundaries. Specifically, the internal electrode layer 3 is formed by applying a Cu paste. By using Cu as an electrode material, the manufacturing cost of the multilayer piezoelectric element 1 can be reduced. On the other hand, a piezoelectric ceramic composition is used for the piezoelectric layer 2. The piezoelectric ceramic composition to be used is a piezoelectric ceramic mainly composed of a composite oxide containing Pb, Ti, and Zr as described above. It is a composition.

  The basic configuration of the multilayer piezoelectric element 1 has been described above. The characteristic of the piezoelectric element 1 according to the present invention is that, as described above, Cu is unevenly distributed at the grain boundaries of the composite oxide as a main component. Having a structure. Since the piezoelectric layer 2 has a structure in which Cu is unevenly distributed at the grain boundaries, the insulating properties of the grain boundaries are enhanced, and for example, acceleration voltage load characteristics are improved. Further, since the composition of crystal grains of the composite oxide which is the main body of the piezoelectric layer 2 is not changed by the uneven distribution of Cu, the piezoelectric strain characteristic is also maintained.

In the piezoelectric layer 2 of the piezoelectric element, in order to obtain a structure in which Cu is unevenly distributed at grain boundaries, it is necessary to appropriately control the firing conditions when firing the piezoelectric element. For example, first, the piezoelectric element of the present invention is preferably fired under reducing firing conditions. When the piezoelectric element is manufactured, if it is fired in an oxidizing atmosphere, for example, a noble metal needs to be used as the electrode material of the internal electrode layer 3. On the other hand, if firing is performed under reducing firing conditions, inexpensive Cu can be used for the internal electrode layer 3. Here, the reducing firing conditions are, for example, a firing temperature of 800 ° C. to 1200 ° C. and an oxygen partial pressure of 1 × 10 ⁻¹⁰ to 1 × 10 ⁻⁶ atm.

  When firing under the reduction firing condition, a decrease in electrical resistance at a high temperature becomes a problem. However, in the case of the piezoelectric element of the present invention, the piezoelectric layer 2 has a structure in which Cu is unevenly distributed at grain boundaries as described above. This is possible to avoid. That is, since the multilayer piezoelectric element of the present invention is fired under reducing firing conditions, it is possible to use Cu for the internal electrode layer 3 and to eliminate a decrease in electrical resistance at high temperatures. is there.

  Hereinafter, an example of a manufacturing method suitable for manufacturing the piezoelectric element 1 of the present invention will be described. By firing under the conditions described below, a structure in which the Cu is unevenly distributed at the grain boundaries can be created.

In order to manufacture the piezoelectric element 1, first, a lamination process is performed. In this lamination process, a raw material for the piezoelectric layer 2 is prepared, weighed according to the target composition, and then added with a binder or the like to be a ceramic raw material. Mix. The raw material of the piezoelectric layer 2 includes oxides, carbonates, oxalates, hydroxides, etc. of the elements constituting the piezoelectric layer 2. For example, the piezoelectric layer 2 is made of the above lead zirconate titanate. In some cases, lead oxide (PbO), titanium oxide (TiO ₂ ), and zirconium oxide (ZrO ₂ ) are used as raw materials. Next, this ceramic raw material mixture is formed into a sheet shape to form a ceramic precursor layer.

  Similarly, for example, metallic copper as a raw material of the internal electrode layer 3 is prepared, and a binder or the like is added to obtain an internal electrode raw material mixture. As a raw material for the internal electrode layer 3, the metal copper may be used alone or in combination with other materials. In this case, examples of the other material include a copper oxide or an organic copper compound that becomes metal copper after firing, a metal other than metal copper, a metal oxide, an organic metal compound, and the like. Moreover, you may add additives, such as a dispersing agent, a plasticizer, a dielectric material, an insulator material, to an internal electrode raw material mixture as needed.

  The internal electrode precursor layer is formed by, for example, screen printing the internal electrode raw material mixture on the ceramic precursor layer. As described above, a plurality of ceramic precursor layers on which the internal electrode precursor layers are formed are stacked to obtain a stacked body in which the ceramic precursor layers and the internal electrode precursor layers are alternately stacked.

  After the above-mentioned lamination process, a degreasing process is performed with respect to the obtained laminated body in a degreasing process. The degreasing step is a step of decomposing and removing the ceramic precursor layer and the binder contained in the internal electrode precursor layer constituting the laminate by heating.

This degreasing step is usually performed in an atmosphere containing oxygen (for example, in the air), and in the production method of the present invention, it may be performed in the atmosphere containing oxygen, but oxidation of metallic copper in the degreasing step In order to suppress this, it is preferable to perform the degreasing step in a reducing atmosphere. In addition, an inert gas such as nitrogen (N ₂ ) or argon (Ar), water vapor, and, if necessary, an atmosphere gas containing hydrogen is introduced, and the degreasing step is performed in an oxygen partial pressure atmosphere represented by Formula (3). It is also a preferred form to do.
p (O ₂ ) ≦ (25331 × Kp) ^2/3 (3)
[Wherein Kp represents the dissociation equilibrium constant of water. The unit of oxygen partial pressure p (O ₂ ) is Pa. ]

As described above, when an atmosphere gas containing an inert gas and water vapor is introduced and the degreasing step is performed in the oxygen partial pressure atmosphere shown in the formula (3), the oxygen partial pressure is within the range shown in the formula (4). It is more preferable that
Kp ² × 10 ⁶ ≦ p (O ₂ ) ≦ (25331 × Kp) ^2/3 (4)
[Wherein Kp represents the dissociation equilibrium constant of water. The unit of oxygen partial pressure p (O ₂ ) is Pa. ]

  When the oxygen partial pressure is below the above range, lead contained in the ceramic precursor layer is reduced and metal lead is easily generated, resulting in deterioration of the characteristics of the ceramic material or metal lead in the internal electrode precursor layer. There is a possibility that problems such as elution by reacting with metallic copper contained may occur. If metallic lead reacts with metallic copper and is eluted, internal electrode layer 3 formed by firing the internal electrode precursor layer may cause disconnection or the like.

As described above, when the degreasing treatment is performed using an atmosphere gas containing an inert gas and water vapor, the water vapor reacts with hydrocarbon or carbon which is a residual carbon component to promote decomposition and removal of the residual carbon. Demonstrate. Therefore, the amount of water vapor introduced is preferably set so as to be the oxygen partial pressure, specifically 7 mol% or more. If the amount of water vapor introduced is less than this, decomposition and removal of the binder will be insufficient, resulting in a large amount of residual carbon.In particular, the number of laminated ceramic precursor layers will increase, and the size of each ceramic precursor layer will increase. When it becomes large, there is a possibility that the amount of residual carbon inside increases. Further, if the amount of introduced water vapor is too large, the oxygen partial pressure becomes too large, and the metallic copper contained in the internal electrode precursor layer is likely to change into cuprous oxide (Cu ₂ O). The amount introduced is preferably 50 mol% or less. Cuprous oxide diffuses into the ceramic layer (piezoelectric layer 2) at, for example, 680 ° C., and causes deterioration in characteristics.

  Water vapor also generates oxygen by dissociation equilibrium and has an action of suppressing changes in oxygen partial pressure even when the hydrogen concentration increases. Here, by adjusting the oxygen partial pressure at the time of degreasing using the dissociation equilibrium of water vapor, it becomes possible to adjust to an extremely low oxygen partial pressure. If the oxygen partial pressure during the degreasing process is high, the copper metal contained in the internal electrode precursor layer may oxidize and expand to cause a failure such as a crack. Since the oxygen partial pressure can be controlled to a low value using the water vapor, it is possible to suppress the occurrence of cracks due to oxidation of the metallic copper. In addition, as the atmosphere control during the degreasing process, for example, oxygen may be introduced as an atmosphere gas. Also in this respect, the control of the oxygen partial pressure by water vapor is advantageous.

  In the case of introducing an atmosphere gas containing the inert gas and water vapor and performing the degreasing step in the oxygen partial pressure atmosphere shown in the formula (3), hydrogen can be introduced together with water vapor as the atmospheric gas. This is because hydrogen also has an action of removing residual carbon. However, if the amount of hydrogen introduced is large, the oxygen partial pressure is lowered and the residual carbon is increased, or lead contained in the ceramic precursor layer is likely to be reduced, so the hydrogen concentration in the atmospheric gas is 10 molppm or less. It is preferable that

  In the degreasing step, the temperature during the degreasing treatment is preferably 600 ° C. or lower. This is because if the degreasing temperature exceeds 600 ° C., the lead-based ceramic material starts to sinter, so that the densification may occur and the ventilation holes may be blocked, and decomposition and volatilization of the binder may be hindered.

  After the degreasing step described above, the laminate is fired in the firing step. In manufacturing the piezoelectric element 1 of the present invention, it is important to control the atmosphere during firing. Hereinafter, atmosphere control in the firing process will be described.

FIG. 3 shows an example of a temperature profile during firing of the laminate. In firing, the laminated body is fired through a temperature rising period UT in which the temperature gradually increases, a firing stable period AT in which the temperature is maintained at a constant temperature, and a temperature lowering period DT in which the fired laminated body is cooled. Here, the firing stable period AT, so that substantial sintering is performed by maintaining the so-called firing temperature reached T _1. For example, in the case of the lead zirconate titanate ceramic material, the firing temperature T ₁ is set to 900 ° C. to 1000 ° C.

At the time of firing, an atmospheric gas is introduced into the furnace and the furnace atmosphere is set to a predetermined atmosphere. In the present invention, the predetermined atmospheric gas is introduced when the temperature in the furnace exceeds 100 ° C. In this case, the gas to be introduced is, for example, an atmosphere gas containing an inert gas (such as nitrogen or Ar), hydrogen, and water vapor, and these component gases have a partial pressure of oxygen within the range represented by the following formula (5). It was adjusted to enter.
10 ⁵ × Kp ² ≦ P (O ₂ ) ≦ 10 ⁹ × Kp ² (5)

  As described above, when the atmospheric gas containing an inert gas, hydrogen, and water vapor and adjusted to have a predetermined oxygen partial pressure is introduced into the furnace, the temperature in the furnace is less than 100 ° C. There is a risk that water vapor will condense. Here, the condensation of the water vapor is a major obstacle in adjusting the oxygen partial pressure of the atmospheric gas, so the atmospheric gas is introduced after the temperature in the furnace exceeds 100 ° C. The atmosphere in the furnace before the introduction of the atmospheric gas is arbitrary, and may be, for example, an inert gas atmosphere or air.

After the furnace temperature exceeds 100 ° C., when the atmosphere gas containing the inert gas, hydrogen, and water vapor is introduced into the furnace, the dissociation of water vapor progresses as the temperature rises, and the oxygen partial pressure gradually increases. FIG. 4 shows how the oxygen partial pressure increases as the temperature rises. Line a shows the state of change in oxygen partial pressure when P (O ₂ ) = 10 ⁵ × Kp ² , and line b Indicates the state of change in oxygen partial pressure when P (O ₂ ) = 10 ⁹ × Kp ² is set.

On the other hand, in FIG. 4, the line c shows the oxygen dissociation pressure of copper, and the line d shows the oxygen dissociation pressure of lead (Pb). In the case of copper, when the oxygen partial pressure falls below the line c, the state of metallic copper is maintained, and when the oxygen partial pressure exceeds the line c, it is oxidized to cuprous oxide (Cu ₂ O). In the case of lead (Pb), metallization occurs when the oxygen partial pressure falls below the line d. On the other hand, when the oxygen partial pressure exceeds the line d, the state of lead oxide (PbO) is maintained.

When the lines c and d indicating the oxygen dissociation pressure of copper and lead are compared with the oxygen partial pressure (lines a and b) of the atmospheric gas introduced in the present invention, the lines a and b are the lines c and c in all temperature ranges. Although not in the region sandwiched by d, at the firing attainment temperature T ₁ (900 ° C. to 1000 ° C.) set in the firing stable period AT, it is in the vicinity of the oxygen dissociation pressure (that is, in the vicinity of the lines d and c). . In the region where the temperature is lower than this, the lines a and b are below the line c.

As a result of repeated studies by the present inventors, in the atmosphere control during the firing, the oxygen partial pressure (region sandwiched between lines c and d) in which metallic copper and lead oxide can coexist at all temperatures is not necessarily required. It is not necessary to control the atmosphere so as to be within the range, and the purpose can be achieved if the oxygen partial pressure of the atmospheric gas is set near the oxygen partial pressure where metallic copper and lead oxide can coexist at the firing temperature T ₁ . I understood.

If it is within the above range, that is, if the oxygen partial pressure curve of the atmospheric gas is between the lines a and b, even if a base metal such as copper is used for the internal electrode layer, complicated atmosphere control is not performed. In addition, it is possible to manufacture a laminated piezoelectric element excellent in quality without oxidation or elution of the internal electrode layer. In this case, for example, the firing temperature reached _{T 1} is a 900 ° C. to 1000 ° C., oxygen partial pressure in the temperature will generally 1 × ¹⁰ -6 Pa~2 × ^{10 -11} atm.

Preferably, the atmospheric gas having an oxygen partial pressure p (O ₂ ) within the range represented by the following formula (6) is introduced when the temperature reaches 100 ° C. or higher, and more preferably, the firing is achieved. oxygen partial pressure is to introduce at the time of reaching the metallic copper and lead oxide can coexist oxygen partial becomes pressure such an atmosphere gas of more than 100 ° C. the temperature in the temperature T _1. Incidentally, shown in the preferred range of 4 lines _{e [p (O 2) =} 10 6 × Kp 2], and the line _{f [p (O 2) =} 10 8 × Kp 2]. In this case, the oxygen partial pressure at the firing temperature T ₁ (= 900 ° C. to 1000 ° C.) is approximately 1 × 10 ⁻⁷ Pa to 2 × 10 ⁻¹⁰ Pa.
10 ⁶ × Kp ² ≦ p (O ₂ ) ≦ 10 ⁸ × Kp ² (6)
[Wherein Kp represents the dissociation equilibrium constant of water. The unit of oxygen partial pressure p (O ₂ ) is Pa. ]

  After the introduction of the atmospheric gas, firing is performed according to the temperature profile shown in FIG. At this time, it is not necessary to change the atmosphere gas at all, and the firing is performed while maintaining the set atmosphere gas. Therefore, complicated atmosphere control becomes unnecessary, and productivity can be improved. Further, since the apparatus configuration is not complicated and the furnace atmosphere is not uniform at the time of temperature rise, it is possible to manufacture a product with uniform quality.

  Hereinafter, specific examples to which the present invention is applied will be described based on experimental results.

Production of laminated piezoelectric element A piezoelectric ceramic composition was produced as follows. First, PbO powder, SrCO ₃ powder, ZnO powder, Nb ₂ O ₅ powder, TiO ₂ powder, and ZrO ₂ powder were prepared as raw materials of the main component, and weighed so as to have the following main component composition. Next, these raw materials were wet-mixed for 16 hours using a ball mill, and calcined at 700 ° C. to 900 ° C. for 2 hours in the air.
Main component: (Pb _0.965 Sr _0.03 ) [(Zn _1/3 Nb _2/3 ) _0.1 Ti _0.43 Zr _0.47 ] O ₃

The obtained calcined product was finely pulverized and then wet pulverized for 16 hours using a ball mill. After drying, a vehicle was added and kneaded to prepare a piezoelectric layer paste. At the same time, Cu powder as a conductive material was kneaded with a vehicle to produce an internal electrode paste. Subsequently, using the piezoelectric layer paste and the internal electrode paste, a green chip, which is a precursor of the laminate, was produced by a printing method. Furthermore, binder removal processing was performed, and firing was performed under reducing firing conditions to obtain a laminate. As reducing firing conditions, an atmosphere gas to be introduced is set so that the oxygen partial pressure at the firing temperature T ₁ (= 950 ° C.) is in the vicinity of the oxygen partial pressure at which metallic copper and lead oxide can coexist. When the temperature reached 100 ° C., it was put into the furnace and stabilized for 1 hour, and then the temperature was raised.

Analysis by EPMA EPMA analysis was performed on the piezoelectric layer of the laminated piezoelectric element produced. FIG. 5 shows the analysis result by this EPMA. In the EPMA analysis, almost no segregation of Cu was observed, and there were about 2 to 3 granular segregations of Cu in the field of view of 900 μm × 900 μm.

Analysis by FE-TEM Next, a region in which Cu segregation was not observed in the EPMA analysis was analyzed by FE-TEM. FIG. 6 is a TEM photograph of the piezoelectric layer. The piezoelectric layer is formed as an aggregate of crystal grains, and a grain boundary extending in three directions is observed around a triple point indicated by a point D in the photograph. Composition analysis was performed by TEM-EDS for points D (triple point), E (grain boundary), F (grain boundary), G (grain boundary), and H (inside grain) in FIG. The results are shown in FIG. The presence of Cu was confirmed at point D (triple point), point E (grain boundary), point F (grain boundary), and point G (grain boundary). On the other hand, no Cu peak was observed at point H (intragranular).

Distribution of Cu in the vicinity of the grain boundary Furthermore, the vicinity of the grain boundary was enlarged, and the distribution of Cu was examined by FE-TEM. FIG. 8 is an enlarged TEM photograph. FIG. 9 shows the result of the composition analysis performed by TEM-EDS in the vicinity of the grain boundary. In FIG. 9, (a) is a composition analysis result in the grain (10 nm from the grain boundary), (b) is a composition analysis result in the grain (5 nm from the grain boundary), and (c) is a composition analysis in the grain boundary. It is a result. A Cu peak is clearly observed at the grain boundary, and a Cu peak is observed even at a position of 5 nm from the grain boundary, but a Cu peak is hardly seen at a position of 10 nm from the grain boundary.

For comparison of comparative examples, a piezoelectric ceramic composition was prepared by adding CuO. FIG. 10 is a TEM photograph of the obtained piezoelectric ceramic composition. As a result of the compositional analysis, the presence of Cu was not observed at point C (grain boundary) or point E (triple point). On the other hand, the crystal grains where the point F exists are PbO 3.7% by mass, ZrO 0.8% by mass, Ta ₂ O ₅ 1.9% by mass, and CuO 93.6% by mass, most of which is composed of CuO. I found out. Therefore, it was found that CuO segregates in a granular form in the piezoelectric ceramic composition prepared by adding CuO.

Accelerates the laminated piezoelectric element (with Cu unevenly distributed at the grain boundary) fabricated for comparison of characteristics and the laminated piezoelectric element (without Cu unevenly distributed at the grain boundary) that does not diffuse Cu into the piezoelectric layer under different conditions. The voltage load characteristic (HALT) was measured. As a result, in the multilayer piezoelectric element having no Cu uneven distribution at the grain boundary (corresponding to the comparative example), the acceleration voltage load characteristic was 0 seconds, whereas the multilayer piezoelectric element having the Cu uneven distribution at the grain boundary. In the element (corresponding to the example), the acceleration voltage load characteristic was significantly improved to 2.0 × 10 ⁴ seconds.

It is a schematic diagram which shows the grain boundary of a piezoelectric ceramic composition. It is a schematic sectional drawing which shows one structural example of a lamination type piezoelectric element. It is a figure which shows an example of the temperature profile at the time of baking. It is a figure which shows the oxygen partial pressure range of the atmospheric gas introduce | transduced at the time of firing with the oxygen partial pressure in which metallic copper and lead oxide can coexist. It is an EPMA image of the piezoelectric element produced in the Example. It is a FE-TEM image of the piezoelectric element produced in the Example. It is a composition analysis result by TEM-EDS in each point of the FE-TEM image shown in FIG. It is a FE-TEM image which expands and shows the piezoelectric element produced in the example further. It is a composition analysis result by TEM-EDS in the vicinity of a grain boundary. It is the TEM image of Cu which segregated granularly.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Piezoelectric element, 2 Piezoelectric layer, 3 Internal electrode, 4 Laminated body, 5, 6 Terminal electrode

Claims (5)

A plurality of piezoelectric layers including a composite oxide having Pb, Ti, and Zr as constituent elements, and an internal electrode layer formed between these piezoelectric layers and containing Cu,
The piezoelectric layer has a structure in which Cu is segregated only within a range of 5 nm from the grain boundary without segregating Cu into the grain boundary . The piezoelectric layer is a complex oxide, Pb _a [(Zn _1/3 Nb _2/3 ) _x Ti _y Zr _z ] O ₃ (where 0.96 ≦ a ≦ 1.03, 0.05 ≦ x ≦ 0.15, 0.25 ≦ y ≦ 0.5, 0.35 ≦ z ≦ 0.6, and x + y + z = 1.), And (Pb _a−b Me _b ) [ (Zn _1/3 Nb _2/3 ) _x Ti _y Zr _z ] O ₃ (where 0.96 ≦ a ≦ 1.03, 0 <b ≦ 0.1, 0.05 ≦ x ≦ 0.15, 0 .25 ≦ y ≦ 0.5, 0.35 ≦ z ≦ 0.6, x + y + z = 1, and Me in the formula represents at least one selected from Sr, Ca, and Ba. The piezoelectric element according to claim 1, comprising at least one selected from the complex oxides.   The piezoelectric layer contains at least one selected from Ta, Sb, Nb and W as subcomponents, and the content of the subcomponents is 1.0% by mass or less in terms of oxide. The piezoelectric element according to claim 1 or 2. A method for manufacturing a piezoelectric element comprising: a plurality of piezoelectric layers including a composite oxide having Pb, Ti, and Zr as constituent elements; and an internal electrode layer formed between these piezoelectric layers and containing Cu,
A method for manufacturing a piezoelectric element, characterized in that, by firing under reducing firing conditions, the piezoelectric layer has a structure in which Cu is segregated only within a range of 5 nm from the grain boundary without segregating in a granular manner at the grain boundary . 5. The method for manufacturing a piezoelectric element according to claim 4, wherein the reduction firing conditions are a firing temperature of 800 ° C. to 1200 ° C. and an oxygen partial pressure of 1 × 10 ⁻¹⁰ to 1 × 10 ⁻⁶ atm.

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