JP6683461B2 - Conductive adhesive for piezoelectric element and piezoelectric element - Google Patents

Description

  The present invention relates to a conductive adhesive for piezoelectric elements and a piezoelectric element.

  A piezoelectric element is an element that mutually converts mechanical displacement and electrical displacement by utilizing the property of expanding when a voltage is applied. For example, a piezoelectric drive element (piezoelectric element for a fuel injection device of an internal combustion engine of an automobile) It is preferably used as an actuator). Further, in recent years, for the purpose of ensuring a larger displacement while miniaturizing the piezoelectric element, a laminated piezoelectric element in which a plurality of piezoelectric units each including a piezoelectric body (for example, piezoelectric ceramics) and electrodes are laminated has been used. ing. This type of piezoelectric element has a basic structure in which an electrode is provided on one surface of a piezoelectric body, and an external conductor is joined to the electrode. In order to bond the electrodes and the outer conductor while maintaining the conductivity, a conductive adhesive material in which conductive powder (for example, silver powder) is dispersed in a thermosetting resin such as epoxy resin has been conventionally used. It is used. Patent documents 1 and 2 are mentioned as documents which disclose this kind of prior art.

JP-A-11-185526 JP, 2005-166322, A

  However, the conductive adhesive made of the epoxy resin tends to have a high elastic modulus and a high breaking strength after curing. Therefore, there is a problem that the conductive adhesive after curing hinders the extension of the piezoelectric body (piezoelectric element sintered body) that is the base, and cracks occur in the piezoelectric body. Generally, the breaking strength of the piezoelectric element is 30 to 100 MPa, so that the breaking strength of the conductive adhesive bonded to the electrode is preferably 20 MPa or less. That is, the conductive adhesive is required not to apply a load to the piezoelectric element (and thus suppress the occurrence of cracks) by making it easy to break after curing. Further, there is a demand for a conductive adhesive having such breaking strength while maintaining adhesiveness and good conductivity.

  The present invention has been made in view of such a point, and an object thereof is to suppress the occurrence of cracks in the piezoelectric element, and to maintain the adhesiveness and to form a bonding portion having good conductivity, and to provide a conductive adhesive for a piezoelectric element. To provide the agent. Another object is to provide a piezoelectric element including a joint portion formed by using such a conductive adhesive.

  The conductive adhesive for a piezoelectric element provided by the present invention includes silver powder, a flexible epoxy resin, and a curing agent. At least 40% by mass or more of the silver powder is flaky silver powder having an average aspect ratio of 1.4 or more. By using the flake-shaped silver powder having an average aspect ratio of 1.4 or more in combination with the flexible epoxy resin, it is possible to suppress cracking of the piezoelectric element, and to maintain the adhesiveness and the conductive portion having good conductivity. It is possible to realize a conductive adhesive that can form

  In a preferable aspect of the conductive adhesive disclosed herein, the average aspect ratio of the flaky silver powder is 1.8 or more. Such flake silver powder can effectively contribute to the improvement of conductivity.

  In a preferable aspect of the conductive adhesive disclosed herein, the average major axis of the flake silver powder is 0.5 μm to 10 μm. With such flake-shaped silver powder, a lower resistance joint can be realized.

  In a preferable embodiment of the conductive adhesive disclosed herein, the flexible epoxy resin is an alkyleneoxylated bisphenol type epoxy resin. Such a bisphenol type epoxy resin can effectively contribute to the reduction in breaking strength.

  In a preferable aspect of the conductive adhesive disclosed herein, the content of the flexible epoxy resin is 5 to 30 parts by mass with respect to 100 parts by mass of the silver powder. When the content of the epoxy resin is within such a range, the effect of reducing the breaking strength can be more suitably exhibited.

  In a preferable aspect of the conductive adhesive disclosed herein, the curing agent is an imidazole curing agent. Thereby, the effects of the present invention can be stably exhibited at a higher level. Further, the imidazole-based curing agent has a long pot life (pot life) and is therefore suitable from the viewpoint of workability.

  In a preferable aspect of the conductive adhesive disclosed herein, the content of the curing agent is 15 to 30 parts by mass with respect to 100 parts by mass of the flexible epoxy resin. Thereby, the effects of the present invention can be stably exhibited at a higher level.

  Further, according to the present invention, there is provided a piezoelectric element including a single or a plurality of piezoelectric units including a piezoelectric body and an electrode provided on one surface of the piezoelectric body. In this piezoelectric element, the joint between the electrode of the piezoelectric unit and a predetermined outer conductor or the joint between the piezoelectric units is made of a cured product of any of the piezoelectric element conductive adhesives disclosed herein. ing. The volume resistivity of the joint is 100 μΩ · cm or less, and the breaking strength of the joint is 20 MPa or less. Such a piezoelectric element can effectively suppress cracking of the piezoelectric element due to breakage of the bonding portion while maintaining good adhesiveness and conductivity of the bonding portion. Therefore, it is possible to realize a high-performance piezoelectric element that could not be obtained conventionally.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than matters particularly referred to in the present specification (for example, the composition of the conductive adhesive) and matters necessary for carrying out the present invention (for example, the method for preparing the conductive adhesive and the joint portion). The forming method, etc.) can be grasped as a design matter of those skilled in the art based on the conventional technique in the field. The present invention can be carried out based on the contents disclosed in this specification and the common general technical knowledge in the field.

<Conductive adhesive for piezoelectric element>
The conductive adhesive disclosed herein is a conductive adhesive for piezoelectric elements, and contains silver (Ag) powder, a flexible epoxy resin, and a curing agent. And, at least 40% by mass or more of the silver powder is flaky silver powder having an average aspect ratio of 1.4 or more. By using the conductive adhesive containing the flaky silver powder having such a specific average aspect ratio and the flexible epoxy resin, the breaking strength of the joint is lowered, and the adhesiveness is maintained and the conductivity is further improved. Improve well. The reason why such an effect is obtained is not particularly limited, but it can be considered as follows, for example. That is, the flexible epoxy resin contributes to lowering the breaking strength of the joint and lowering the viscosity of the conductive adhesive. When the flaky silver powder having a large average aspect ratio is contained in such a conductive adhesive in a specific ratio, the surfaces of the flaky silver powder easily overlap with each other, and the contact area between the silver powders increases. This is considered to contribute to improvement of conductivity and reduction of breaking strength. Hereinafter, the components of the conductive adhesive disclosed herein will be described in more detail.

<Silver dust>
The electrically conductive adhesive disclosed here contains the flake-shaped 1st silver powder. The flake shape as used herein typically means a flaky shape, and includes a shape that spreads mainly in two dimensions, such as a flat shape, a plate shape, and a scale shape. The average aspect ratio As of the first silver powder is approximately 1.4 or more. From the viewpoint of improving conductivity, silver powder having an average aspect ratio As of 1.5 or more can be preferably used as the first silver powder in the conductive adhesive of the above embodiment. For example, a first silver powder having an average aspect ratio As of 1.6 or more is preferable, one having 1.7 or more is more preferable, and one having 1.8 or more is particularly preferable. The upper limit of the average aspect ratio As of the first silver powder is not particularly limited, but may be 3 or less, for example. The average aspect ratio As is preferably 2.5 or less, more preferably 2.2 or less. For example, a first silver powder having an average aspect ratio As of 1.4 or more and 2 or less can be preferably adopted.

  In this specification, the “average aspect ratio” for one type of silver powder refers to the average value of the major axis / minor axis ratio of a plurality of particles contained in the one type of silver powder. That is, this average aspect ratio is a value indicating the average particle shape of the one type of silver powder. The average aspect ratio can be obtained by, for example, observing with an electron microscope. As a specific procedure, for example, a scanning electron microscope (SEM) is used to observe a predetermined number (for example, 100) of particles contained in the silver powder to be measured, and the area circumscribed to each particle image is minimum. Draw a rectangle. Then, with respect to the rectangle drawn for each particle image, a value obtained by dividing the length of the long side (value of the long diameter) by the length of the short side (value of the short diameter) is the ratio of the long diameter / short diameter of each particle. It is calculated as (aspect ratio). Then, the average aspect ratio can be obtained by arithmetically averaging the aspect ratios of the predetermined number of particles. The aspect ratio and average aspect ratio of each particle can be obtained by using general image analysis software.

  The average major axis of the first silver powder (hereinafter sometimes simply referred to as “L1”) is not particularly limited as long as the average aspect ratio As of the first silver powder satisfies the above range (that is, 1.4 μm ≦ As). It is typically 0.5 μm or more, preferably 0.7 μm or more, more preferably 1 μm or more, further preferably 3 μm or more, particularly preferably 5 μm or more, from the viewpoint of improving conductivity. With the first silver powder having an average major axis L1 of 0.5 μm or more, a lower resistance joint portion can be realized. The average major axis L1 of the first silver powder may be, for example, 10 μm or less. The average major axis L1 is preferably 9 μm or less, more preferably 8 μm or less. For example, a first silver powder having an average major axis D1 of 0.7 μm or more and 7 μm or less can be preferably used.

  In this specification, the “average major axis” for one type of silver powder refers to the average value of the major axes of a plurality (eg, 100) of particles contained in the one type of silver powder. The major axis of each particle can be obtained as an arithmetic mean value of the lengths of the long sides of a rectangle that circumscribes each particle image obtained by SEM and has the smallest area, similarly to the above-described aspect ratio. When the average aspect ratio is 1.00, the average major axis value and the average minor axis value match.

  The average minor axis of the first silver powder (hereinafter, may be simply referred to as “L2”) is not particularly limited as long as the average aspect ratio As of the first silver powder satisfies the above range, for example, 0.3 μm or more, It can typically be 0.5 μm or more (eg 1 μm or more). The average minor axis L2 of the first silver powder may be, for example, 8 μm or less, typically 5 μm or less.

  From the viewpoint of better exerting the effect of using the flaky silver powder, the average major axis L1 of the first silver powder is preferably 0.2 μm or more, and more preferably 1.5 μm or more, than the average minor axis L2. The value obtained by subtracting L2 from L1 (that is, L1-L2) is preferably 5 μm or less. For example, L1-L2 may be 3 μm or less.

  In this specification, the “average short diameter” for one type of silver powder refers to the average value of the short diameters of a plurality (for example, 100) of particles contained in the one type of silver powder. The minor axis of each particle can be obtained as an arithmetic mean value of the lengths of the short sides of a rectangle that circumscribes each particle image obtained by SEM and has the smallest area, as in the aspect ratio described above. When the average aspect ratio is 1.00, the average major axis value and the average minor axis value match.

  The average thickness of the first silver powder (hereinafter, may be simply referred to as “D”) is not particularly limited as long as the average aspect ratio As of the first silver powder satisfies the above range, but is, for example, 0.1 μm or more, typical. Specifically, it can be 0.2 μm or more (for example, 0.5 μm or more). Further, the average thickness D of the first silver powder may be, for example, 1.5 μm or less, typically 0.7 μm or less.

  From the viewpoint of better exerting the effect of using the flaky silver powder, it is preferable that the relationship between the average major axis L1 of the first silver powder and the average thickness D satisfies 3 ≦ (L1 / D) ≦ 20. By using the flake-shaped silver powder having such a specific average major axis L1 and average thickness D relationship, improvement in conductivity and the like can be realized at a higher level. In the technology disclosed herein, for example, the relationship between L1 and D is 3.5 ≦ (L1 / D) ≦ 15, more preferably 3.5 ≦ (L1 / D) ≦ 10, and further preferably 5 ≦. It can be preferably carried out in a mode in which (L1 / D) ≦ 10, particularly preferably 7 ≦ (L1 / D) ≦ 10.

  In this specification, the “average thickness” for one type of silver powder refers to the average value of the thicknesses of a plurality (for example, 100) of particles contained in the one type of silver powder. The average thickness of the silver powder can be determined by observation with an electron microscope (SEM). As a specific procedure, for example, a sample in which silver powder to be measured is hardened with a resin is prepared, and a cross section of the sample is observed with a scanning electron microscope. Then, the average thickness is obtained by measuring the thickness (transmission length in the thickest portion) of a predetermined number (for example, 100) of particles included in the cross section and arithmetically averaging the thickness of the predetermined number of particles. be able to.

The average particle size of the first silver powder may be, for example, 0.5 μm or more. From the viewpoint of obtaining a lower resistance, the average particle diameter is preferably 2 μm or more, more preferably 3 μm or more. In addition, the average particle diameter is usually 10 μm or less, and preferably 9 μm or less.
In this specification, the "average particle diameter" means a particle diameter D ₅₀ value (corresponding to cumulative 50% from the fine particle side having a small particle diameter in a volume-based particle size distribution measured based on a laser diffraction / light scattering method) ( It is also called the median diameter.).

  The conductive adhesive disclosed herein may contain silver powder other than the flake-shaped first silver powder (that is, the second and subsequent silver powders) as long as the effect of the present invention is not impaired. The shape (outer shape) of the second and subsequent silver powders is not particularly limited. For example, silver powder (typically spherical silver powder) having an average aspect ratio of less than 1.4 (preferably 1.3 or less, more preferably 1.1 or less, still more preferably 1.05 or less) can be suitably used. . When the second and subsequent silver powders are contained, it is suitable that the mass of the first silver powder is 40% by mass or more of the total mass of the silver powder contained in the conductive adhesive, and from the viewpoint of improving conductivity, it is preferable. It is 60% by mass or more, and more preferably 90% by mass or more. Among them, a conductive adhesive in which 100 mass% of the silver powder contained in the conductive adhesive is the first silver powder is preferable.

  The conductive adhesive disclosed herein may contain a conductive powder made of a material other than silver (hereinafter, also referred to as non-silver powder) as long as the effect of the present invention is not impaired. Examples of such non-silver powder are gold (Au), copper (Cu), platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), iridium (Ir), osmium (Os), nickel. Examples of the non-silver powder include a metal such as (Ni) and aluminum (Al), and alloys thereof.

  The content of the non-silver powder is appropriately 30% by mass or less, preferably 20% by mass or less, more preferably 10% by mass, of the total mass of the conductive powder contained in the conductive adhesive. It is the following. The technique disclosed herein can be preferably implemented in a mode in which the total proportion of silver powder is greater than 90% by mass based on the total mass of the conductive powder contained in the conductive adhesive. The proportion of the silver powder is more preferably 95% by mass or more, further preferably 98% by mass or more, and particularly preferably 99% by mass or more. Above all, a conductive adhesive in which 100 mass% of the conductive powder contained in the conductive adhesive is silver powder is preferable.

<Flexible epoxy resin>
The conductive adhesive disclosed herein contains a flexible epoxy resin in addition to the silver powder described above. The "flexible epoxy resin" as used herein is a thermosetting epoxy resin which is liquid at 25 ° C., and 1 part by mass of imidazole (eg, epoxy-imidazole adduct) is added to 5 parts by mass of the epoxy resin and heated. An epoxy resin that, when cured, has a dynamic elastic modulus of 900 MPa or less measured by a tensile method (temperature: 25 ° C., frequency: 1.0 Hz) of the cured product. In a preferred embodiment, the cured product may have a dynamic elastic modulus of 890 MPa or less. As an apparatus suitable for performing such dynamic elastic modulus measurement, a viscoelasticity measuring apparatus DMA7100 manufactured by Hitachi High-Tech Science Co., Ltd. can be exemplified. The flexible epoxy resin is a component for imparting good adhesion, flex resistance (flexibility), and durability to the joint.

The material of the flexible epoxy resin is not particularly limited as long as the dynamic elastic modulus after heat curing satisfies the above range. For example, an alkyleneoxylated bisphenol type epoxy resin is preferably used. The term "bisphenol type epoxy resin" as used herein means an epoxy resin obtained by reacting bisphenols with epichlorohydrin or the like. Further, the “alkyleneoxylated bisphenol type epoxy resin” refers to a compound in which a hydrocarbon group containing one or more ether bonds is bonded to an oxygen atom directly bonded to an aromatic ring constituting the bisphenol type epoxy resin. In the above aromatic ring, the hydrogen atoms bonded to the atoms constituting the aromatic ring are each independently an alkyl group (linear or branched having 1 to 10 carbon atoms), a halogen atom (for example, F, Cl, Br). ) And the like may be substituted. A compound in which a hydrocarbon group containing two or more ether bonds is bonded to the oxygen atom directly bonded to the aromatic ring can be preferably used. Examples of the hydrocarbon group containing an ether bond include a group having a structure of (-ROR'-) and a hydrocarbon group having a repeating unit of an alkyleneoxy group (-RO-), for example, an alkylene oxide group (-(RO) _n. -R ') etc. are mentioned. The hydrocarbon group containing an ether bond may be directly bonded to the oxygen atom directly bonded to the aromatic ring, or may be bonded via an acetal bond or the like.

ここで上記一般式（１）中、Ｒ^１〜Ｒ^４は、それぞれ独立に、水素原子またはメチル基であり得る。Ｒ^１〜Ｒ^４は同じであってもよく異なっていてもよい。Ｒ^１〜Ｒ^４のすべてがメチル基であるものが好ましい。Ｘは、アルキレンオキサイド基、炭素原子数２〜２０（好ましくは２〜１５）のアルキレン基、炭素原子数３以上（好ましくは４以上）のアルキル基からなる群から選択される基である。アルキレンオキサイド基としては、エチレンオキシエチル基、ジ（エチレンオキシ）エチル基、トリ（エチレンオキシ）エチル基、テトラ（エチレンオキシ）エチル基、プロピレンオキシプロピル基、ジ（プロピレンオキシ）プロピル基、トリ（プロピレンオキシ）プロピル基、テトラ（プロピレンオキシ）プロピル基、ブチレンオキシブチル基、ジ（ブチレンオキシ）ブチル基、トリ（ブチレンオキシ）ブチル基、テトラ（ブチレンオキシ）ブチル基、エチレンオキドとプロピレンオキドとの付加重合により得られる基；等が挙げられる。式中のｎは自然数であり、ｎ＝１〜２０、好ましくはｎ＝２〜５であり得る。 Specific examples of the flexible epoxy resin preferable for the technology disclosed herein include polyalkyleneoxylated bisphenol epoxy resins represented by the following general formula (1).

Here, in the general formula (1), R ^{1 to} R ⁴ may be each independently a hydrogen atom or a methyl group. R ^{1 to} R ⁴ may be the same or different. It is preferable that all of R ^{1 to} R ⁴ are methyl groups. X is a group selected from the group consisting of an alkylene oxide group, an alkylene group having 2 to 20 carbon atoms (preferably 2 to 15) and an alkyl group having 3 or more carbon atoms (preferably 4 or more). As the alkylene oxide group, ethyleneoxyethyl group, di (ethyleneoxy) ethyl group, tri (ethyleneoxy) ethyl group, tetra (ethyleneoxy) ethyl group, propyleneoxypropyl group, di (propyleneoxy) propyl group, tri (ethyleneoxy) ethyl group Of propyleneoxy) propyl group, tetra (propyleneoxy) propyl group, butyleneoxybutyl group, di (butyleneoxy) butyl group, tri (butyleneoxy) butyl group, tetra (butyleneoxy) butyl group, ethylene oxide and propylene oxide Groups obtained by addition polymerization; and the like. In the formula, n is a natural number and may be n = 1 to 20, preferably n = 2 to 5.

  The epoxy equivalent of the flexible epoxy resin (that is, the mass of the epoxy resin containing one equivalent of epoxy group) is not particularly limited, but can be typically 100 g / equivalent to 600 g / equivalent. From the viewpoint of improving flexibility, the amount is preferably 300 g / equivalent to 500 g / equivalent, and more preferably 400 g / equivalent to 450 g / equivalent.

Although not particularly limited, the content of the flexible epoxy resin can be, for example, 30 parts by mass or less with respect to 100 parts by mass of the silver powder. From the viewpoint of breakability and conductivity, the content of the flexible epoxy resin is preferably 20 parts by mass or less, more preferably 15 parts by mass or less, based on 100 parts by mass of the silver powder. Further, from the viewpoint of achieving higher adhesiveness and durability, it is appropriate that the content of the flexible epoxy resin is 5 parts by mass or more based on 100 parts by mass of the silver powder, and may be, for example, 10 parts by mass or more. .
The content of the flexible epoxy resin in the whole conductive adhesive is, for example, 4% by mass or more, preferably 8% by mass or more, more preferably 10% by mass or more, and 25% by mass or less, preferably 20% by mass or less. It is good to set it as the mass% or less, more preferably 15 mass% or less. By satisfying the above range, it is possible to stably realize a bonded portion which is excellent in adhesiveness and durability, as well as excellent in conductivity and easiness of breakage.

<Curing agent>
The conductive adhesive disclosed herein contains a curing agent in addition to the above-mentioned silver powder and flexible epoxy resin. The curing agent may be a component that reacts with the flexible epoxy resin to generate a hydroxyl group when the conductive adhesive is heated and cured. For example, it may be a component that reacts with the epoxy group of the flexible epoxy resin to form a crosslinked structure and forms a hydroxyl group at the terminal of the crosslinked structure. As such a curing agent, a compound containing a functional group that reacts with an epoxy resin, for example, an imidazole-based curing agent and its derivative, an amine-based curing agent such as an aliphatic amine, a polyether amine, an aromatic amine, an acid anhydride-based curing agent. Agents, phenol-based curing agents, amide-based curing agents, isocyanate-based curing agents, organic phosphines and the like. These compounds may be used alone or in appropriate combination of two or more.

Examples of the imidazole-based curing agent include 2-methylimidazole, 2-ethyl-4-methylimidazole, 1-cyanoethyl-2-undecylimidazolium trimellitate, and epoxy-imidazole adduct.
Examples of the amine-based curing agent include linear aliphatic polyamines such as diethylenetriamine and triethylenetetraamine; cyclic aliphatic polyamines such as N-aminoethylpiperazine, mensendiamine, and isophoronediamine; and fats such as m-xylenediamine. Aromatic amines; aromatic amines such as methanephenylenediamine and diaminodiphenylmethane are exemplified.
Examples of the acid anhydride-based curing agent include phthalic anhydride, trimetic anhydride, pyrometic anhydride, maleic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, succinic anhydride and the like.
Among them, the imidazole-based curing agent and its derivative are preferable from the viewpoint of obtaining a joint having lower resistance and excellent breakability.

  Although not particularly limited, the content of the curing agent is, for example, 15 parts by mass or more with respect to 100 parts by mass of the flexible epoxy resin. By setting the content of the curing agent to 15 parts by mass or more, the curing reaction can be stably performed in a relatively short time. From this point, the content of the curing agent with respect to 100 parts by mass of the flexible epoxy resin is preferably 20 parts by mass or more. Further, the content of the curing agent with respect to 100 parts by mass of the flexible epoxy resin is preferably 30 parts by mass or less. By setting the content of the curing agent to 30 parts by mass or less, it becomes difficult for the unreacted curing agent to remain in the joint portion, and the volume resistivity can be further reduced. From the viewpoint of low resistance, the content of the curing agent with respect to 100 parts by mass of the flexible epoxy resin is preferably 25 parts by mass or less (for example, 22 parts by mass or less).

<Other ingredients>
The conductive adhesive disclosed herein may contain an organic dispersion medium (typically an organic solvent), if necessary. Thereby, the viscosity and thixotropy of the paste can be adjusted, and the workability and coatability can be improved. Examples of the organic dispersion medium include glycol solvents such as ethylene glycol, propylene glycol, diethylene glycol; ethylene glycol monomethyl ether (methyl cellosolve), ethylene glycol monoethyl ether (cellosolve), diethylene glycol monobutyl ether (butyl carbitol), diethylene glycol monobutyl ether. Glycol ether solvents such as acetate, dipropylene glycol methyl ether, dipropylene glycol methyl ether acetate, propylene glycol phenyl ether, and 3-methyl-3-methoxybutanol; 1,7,7-trimethyl-2-acetoxy-bicyclo- [ Esters such as 2,2,1] -heptane and 2,2,4-trimethyl-1,3-pentadiol monoisobutyrate Solvent; terpineol, dihydro terpineol, dihydro terpineol propionate, alcohol solvents such as benzyl alcohol; toluene, hydrocarbon solvents such as xylene; organic solvent having a high boiling point, such as other mineral spirits and the like.

  The conductive adhesive disclosed herein is a surfactant, a dispersant, a thickener, a conductive auxiliary agent, a defoaming agent, a plasticizer, a stabilizer, an antioxidant, within a range that does not impair the effects of the present invention. A known additive that can be used for a conductive adhesive (typically, a conductive adhesive for a piezoelectric element) such as a pigment may be further contained as necessary.

Examples of the additives, titanium diisopropoxy bis ethyl acetoacetate, tetraisopropyl titanate, tetra-normal butyl titanate, butyl titanate dimer, tetraoctyl titanate, titanium acetylacetonate, titanium tetraacetylacetonate, titanium ethylacetoacetate, Organic titanium compounds such as titanium phosphate compounds, titanium octylene glycolate, titanium lactate ammonium salt, titanium lactate, and titanium triethanolaminate; zirconium tetraacetylacetonate, ruconium tetraacetylacetonate, zirconium dibutoxybisethylacetoacetate. , Normal propyl zirconate, normal butyl zirconate, zirconium monoacetylacetonate, etc. Aluminum compounds such as aluminum isopropylate, aluminum ethylate, aluminum ethylacetoacetate diisopropylate, aluminum trisethylacetoacetate, aluminum trisacetylacetonate, aluminum bisethylacetoacetate, monoacetylacetonate, acetoalkoxyaluminum diisopropylate. Organoaluminum compound; Organotin compounds such as monobutyltin tris2-ethylhexanoate, tin octylate, monobutyltin oxide, dibutyltin oxide, dibutyltin laurate, dibutyltin diacetate, and monobutyltin hydroxyoxide.
When the silver powder is 100 parts by mass, the content of the additive is generally 10 parts by mass or less (for example, 0.5 parts by mass to 10 parts by mass), typically 5 parts by mass or less (for example, 1 part by mass to 1 part by mass). 5 parts by weight).
When the flexible epoxy resin is 100 parts by mass, the content of the additive is generally 5 parts by mass or more (for example, 5 parts by mass to 40 parts by mass), preferably 10 parts by mass or more (for example, 10 parts by mass). ˜35 parts by mass), more preferably 20 parts by mass or more (eg 20 parts by mass to 30 parts by mass). Thereby, the effect of the present invention can be exhibited at a higher level.

<Preparation of conductive adhesive>
Such a conductive adhesive can be prepared by weighing the above-mentioned materials so as to have a predetermined content rate (mass ratio) and uniformly stirring and mixing them. The materials can be stirred and mixed by using various conventionally known stirring and mixing devices such as a three-roll mill, a magnetic stirrer, a planetary mixer, and a disper.

<Join>
The conductive adhesive disclosed herein is a piezoelectric element composed of a single or a plurality of piezoelectric units provided with a piezoelectric body and an electrode provided on one surface of the piezoelectric body, and is composed of an electrode of the piezoelectric unit and a predetermined outer conductor. It is preferably used for joining or joining piezoelectric units. The joining method can be performed in the same manner as the conventional conductive adhesive of this type. For example, when the electrode of the piezoelectric unit and a predetermined outer conductor are joined, the electrode of the piezoelectric unit to be joined and the portion to be joined of the outer conductor are contacted and connected to each other, and a conductive adhesive is applied to the connected portion. Apply. Alternatively, a conductive adhesive is applied on the electrodes of the piezoelectric unit, and the joined portion of the outer conductor is contacted and connected thereon. Then, the coated article is kept at an appropriate temperature (typically 150 to 250 ° C., preferably 160 to 240 ° C., for example 180 to 220 ° C.) for a predetermined time (typically 1 to 60 minutes, for example 20 to 40). Min) Dry. As a result, the coating material can be cured to form a joint between the electrode of the piezoelectric unit and the external conductor.

  The joint thus formed has a low resistance because it is formed by using a conductive adhesive containing flake-shaped silver powder having an average aspect ratio As1.4 or more and a flexible epoxy resin. obtain. Typically, the volume resistivity of the junction is 100 μΩ · cm or less, preferably 80 μΩ · cm or less, more preferably 60 μΩ · cm or less, further preferably 50 μΩ · cm or less, and particularly preferably 30 μΩ · cm or less. obtain. Further, such a joint portion may exhibit a lower breaking strength than the conventional one. For example, the initial breaking strength immediately after heat curing may be 20 MPa or less, more preferably 18 MPa or less, further preferably 16 MPa or less, and particularly preferably 13 MPa or less. Furthermore, if the joint is left in a high temperature environment, the breaking strength may tend to further decrease. For example, the breaking strength after holding at 180 ° C. for 300 hours may be 10 MPa or less. In the present specification, the volume resistivity may be a value measured by a four-probe method using a general resistivity meter. Further, as the breaking strength, a value measured at room temperature at a pulling rate of 0.5 mm / min using a general tensile tester can be adopted.

<Piezoelectric element>
According to the technology disclosed herein, it is possible to provide a piezoelectric element including a single or a plurality of piezoelectric units each including a piezoelectric body and an electrode provided on one surface of the piezoelectric body. In this piezoelectric element, the joint between the electrode of the piezoelectric unit and a predetermined outer conductor or the joint between the piezoelectric units is made of a cured product of the above-mentioned conductive adhesive for piezoelectric element. The volume resistivity is 100 μΩ · cm or less, and the breaking strength is 20 MPa or less. Such a piezoelectric element may have higher performance (for example, a piezoelectric body is less likely to be cracked and has a low resistance) because the volume resistivity and the breaking strength of the bonded portion are lower than those of the related art.

  As a preferred example of the piezoelectric element disclosed herein, a volume resistivity of the joint is 100 μΩ · cm or less and a breaking strength is 20 MPa or less; the volume resistivity of the joint is 80 μΩ · cm or less, And the breaking strength is 18 MPa or less; the volume resistivity of the joint is 600 μΩ · cm or less and the fracture strength is 16 MPa or less; the volume resistivity of the joint is 40 μΩ · cm or less and the fracture is Those having a strength of 13 MPa or less; and the like. By having both the volume resistivity and the breaking strength of the joint portion within such a predetermined range, it is possible to realize a high-performance piezoelectric element that could not be obtained conventionally.

  Hereinafter, some embodiments of the present invention will be described, but the present invention is not intended to be limited to those shown in the embodiments.

The following materials were prepared as constituent components of the conductive adhesive.
<Silver powder (6 types)>
Flake silver powder 1 ("AgC-A" manufactured by Fukuda Metal Foil & Powder Co., Ltd .: average major axis 3.5 µm, average minor axis 2 µm, average thickness 0.6 µm, average aspect ratio As 1.8, average major axis / average thickness 5 .8)
Flake silver powder 2 (“FA-D-2” manufactured by DOWA Electronics Co., Ltd .: average major axis 5 μm, average minor axis 2.5 μm, average thickness 0.5 μm, average aspect ratio As2, average major axis / average thickness 8.3)
-Flake silver powder 3 ("FA-D-3" manufactured by DOWA Electronics Co., Ltd .: average major axis 7 µm, average minor axis 4 µm, average thickness 0.7 µm, average aspect ratio As 1.8, average major axis / average thickness 10)
-Flake silver powder 4 ("TC703S" manufactured by Tokuriki Kagaku Kenkyusho Co., Ltd .: average major axis 0.7 µm, average minor axis 0.5 µm, average thickness 0.2 µm, average aspect ratio As1.4, average major axis / average thickness 3. 5)
Flake silver powder 5 (“TC910” manufactured by Tokuriki Kagaku Kenkyusho Co., Ltd .: average major axis 0.9 μm, average minor axis 0.8 μm, average thickness 0.4 μm, average aspect ratio As1.1, average major axis / average thickness 2. 25)
・ Spherical silver powder (“AG2-8” manufactured by DOWA Electronics Co., Ltd.)
<Epoxy resin (4 types)>
-Flexible epoxy resin 1 ("EPICLON EXA4816" manufactured by DIC Corporation: polyalkyleneoxylated bisphenol A, epoxy equivalent 400)
The dynamic elastic modulus of the flexible epoxy resin 1 is 890 MPa.
-Flexible epoxy resin 2 ("EPICLON 4850-150" manufactured by DIC Corporation: polyalkyleneoxylated bisphenol A, epoxy equivalent 450)
-Non-flexible epoxy resin 1 ("HP4710" manufactured by DIC Corporation: epoxy equivalent 172)
-Non-flexible epoxy resin 2 ("HP7200" manufactured by DIC Corporation: epoxy equivalent 258)
<Curing agent>
・ "PN50" manufactured by Ajinomoto Fine-Techno Co., Ltd .: Epoxy-imidazole adduct <additive>
・ Matsumoto Fine Chemical Co., Ltd .: Zirconium tetraacetylacetonate <solvent>
・ Diethylene glycol monobutyl ether acetate

  The above materials were mixed so as to have the composition ratio (parts by mass) shown in Table 1 to prepare conductive adhesives (Examples 1 to 18).

<Measurement of volume resistivity>
The conductive adhesive of each example was printed on the surface of a glass substrate, and heat-dried at 200 ° C. for 30 minutes to form a heat-cured film of the conductive adhesive on the substrate. The volume resistivity of this heat-cured film was measured by a 4-probe method using a resistivity meter (model: Loresta GP MCP-T610) manufactured by Mitsubishi Chemical Analytech Co., Ltd. The results are shown in the "Volume resistivity" column of Table 1. The volume resistivity of the conductive adhesive for a piezoelectric element is preferably 100 μΩ · cm or less.

<Measurement of breaking strength>
A conductive adhesive of each example was formed into a film having a film thickness of 120 μm using an applicator, and heat-dried at 200 ° C. for 30 minutes to prepare a 10 mm × 30 mm test piece. Then, the test piece immediately after curing was pulled under the following conditions using a tensile tester (a tensile / compression tester RTC-1210 manufactured by Orientec Co., Ltd.), and the breaking strength (initial breaking strength) was measured. Further, the breaking strength after holding the test piece at 180 ° C. for 100 hours and the breaking strength after holding at 180 ° C. for 300 hours were measured by the same procedure. The results are shown in the columns of "Initial breaking strength", "100 hour breaking strength", and "300 hour breaking strength" in Table 1. The breaking strength of the conductive adhesive for a piezoelectric element is preferably 20 MPa or less.
Tensile speed: 0.5mm / min
Gauge distance (measurement distance): 20 mm
Measurement temperature: room temperature

<Adhesion test>
A silver film sintered body was formed on the surface of the alumina substrate. The conductive adhesive of each example was printed on the silver film, and heat-dried at 200 ° C. for 30 minutes to form a heat-cured film of the conductive adhesive on the silver film. Next, after holding at 180 ° C. for 300 hours, cellophane tape (registered trademark: CT-24 manufactured by Nichiban Co., Ltd.) was attached to the heat-cured film, and a load was applied to bring them into close contact. Then, the cellophane tape was peeled off, and the one in which the heat-cured film was not peeled from the silver film was evaluated as "◯", and the one in which the heat-cured film was peeled from the silver film was evaluated as "x". The results are shown in the column of "Adhesion after 300 hours" in Table 1.

As shown in Table 1, Example 5 using silver powder having an average aspect ratio As1.1 had a volume resistivity of 150 μΩ · cm or more, and lacked conductivity as compared with Examples 1 to 4. Further, in Example 9 in which the mass ratio of the flake-shaped silver powder and the spherical silver powder was 20:80, both the volume resistivity and the initial breaking strength tended to increase as compared with Examples 1 and 6 to 8. Furthermore, in Examples 17 and 18 using the flaky silver powder having an average aspect ratio As1.4 or more and the non-flexible epoxy resin, both the volume resistivity and the initial breaking strength tend to increase as compared with Examples 12 and 15. became.
On the other hand, Examples 1-4, 6-8, 10-16 which contain silver powder and a flexible epoxy resin, and made at least 40 mass% or more of the silver powder into flake silver powder with an average aspect ratio As1.4 or more, All the resistivities were 100 μΩ · cm or less, indicating good conductivity. In addition, the initial breaking strength was 20 MPa or less in all cases, indicating good easiness of breaking. Further, the breaking strength after 300 hours was 10 MPa or less, and when left in a high temperature environment, the breaking strength tended to further decrease. Also, the adhesiveness was good after 300 hours.
From the above, by using at least 40% by mass or more of silver powder as a flake-shaped silver powder having an average aspect ratio As1.4 or more, which contains silver powder, a flexible epoxy resin, and a curing agent, a breaking strength of 20 MPa or less can be achieved, and adhesion is achieved. It was confirmed that it is possible to realize a conductive adhesive for a piezoelectric element, which can maintain the property and form a joint having good conductivity.

The present invention has been described in detail above, but these are merely examples, and the present invention can be modified in various ways without departing from the gist thereof.

Claims (8)

A conductive adhesive used for a piezoelectric element,
Contains silver powder, a flexible epoxy resin, and a curing agent,
Wherein at least 40 wt% of silver powder, the average aspect ratio of Ri 1.4 or more flaky silver powder der,
The content of the flexible epoxy resin, the silver powder Ru 5-30 parts by der respect to 100 parts by mass, the conductive adhesive piezoelectric element.   The conductive adhesive for a piezoelectric element according to claim 1, wherein the flake-shaped silver powder has an average aspect ratio of 1.8 or more.   The conductive adhesive for a piezoelectric element according to claim 1, wherein the flake-shaped silver powder has an average major axis of 0.5 μm to 10 μm.   The conductive adhesive for a piezoelectric element according to claim 1, wherein the flexible epoxy resin is an alkylene-oxylated bisphenol type epoxy resin. The flexible epoxy resin has the following formula (1):

(In the formula (1), R ^{1 to} R ⁴ are each independently a hydrogen atom or a methyl group, and X is an alkylene oxide group, an alkylene group having 2 to 20 carbon atoms, or 3 or more carbon atoms. Is a group selected from the group consisting of alkyl groups, and n is 1 to 20.), which is a polyalkyleneoxylated bisphenol epoxy resin represented by the formula 1. The electrically conductive adhesive for a piezoelectric element according to item 1.   The conductive adhesive for a piezoelectric element according to claim 1, wherein the curing agent is an imidazole-based curing agent.   Content of the said hardening | curing agent is 15-30 mass parts with respect to 100 mass parts of said flexible epoxy resins, The electroconductive adhesive agent for piezoelectric elements as described in any one of Claims 1-6. A piezoelectric element comprising a single or a plurality of piezoelectric units having a piezoelectric body and an electrode provided on one surface of the piezoelectric body,
The joint between the electrode of the piezoelectric unit and a predetermined outer conductor or the joint between the piezoelectric units is made of a cured product of the conductive adhesive for a piezoelectric element according to any one of claims 1 to 7. And
A piezoelectric element in which the volume resistivity of the joint is 100 μΩ · cm or less and the breaking strength is 20 MPa or less.