Conductive paste

Abstract

This conductive paste includes nickel powder, ceramic powder, an organic binder, and an organic solvent, and the surfaces of nickel particles (10) constituting the nickel powder are coated with sulfur (11) and nickel oxide (12) at a proportion of 79-100%, on average. Preferably, the coverage rate of the sulfur (11) on the surface of each of the nickel particles (10) averages 1-89%, and the coverage rate of the nickel oxide (12) on the surface of each of the nickel particles (10) averages 9-99%. More preferably, the organic binder is included in an amount of 3-7 parts by mass with respect to 100 parts by mass of the nickel powder, and the ceramic powder is included in an amount of 2-18 parts by mass with respect to 100 parts by mass of the nickel powder.

Description

conductive paste

[0001] The present invention relates to a conductive paste for forming internal electrodes in multilayer ceramic electronic components, and more particularly to a conductive paste containing nickel powder as a conductive component.

[0002] One example of a multilayer ceramic electronic component is a multilayer ceramic capacitor. As a technology of interest to this invention, for example, Japanese Patent Laid-Open Publication No. 2017-25400 (Patent Document 1) describes a conductive paste that serves as an internal electrode material for a multilayer ceramic capacitor. This conductive paste contains nickel powder as a conductive metal powder, an organic binder, and an organic solvent, and, if necessary, also contains ceramic powder as a co-material for the dielectric ceramic layers in the multilayer ceramic capacitor.

[0003] Thinning the internal electrodes is one of the key factors for achieving high capacitance and miniaturization of multilayer ceramic capacitors. To achieve this, there is a trend toward using finer nickel powder. However, when attempting to manufacture multilayer ceramic capacitors with thinned internal electrodes using a conductive paste containing finer nickel powder, the firing process during manufacturing can result in structural defects such as cracks and delamination in the laminates included in the multilayer ceramic capacitor. In particular, when the particle size of the nickel particles constituting the nickel powder is, for example, 300 nm or less, it is predicted that the likelihood of structural defects occurring increases.

[0004] The invention described in Patent Document 1 provides a technology for suppressing structural defects that may occur during the binder removal process and firing process in the manufacture of multilayer ceramic capacitors, and is characterized by having sulfur present on the surface of nickel powder in a conductive paste. Patent Document 1 states that by adopting this configuration, the rate of occurrence of structural defects due to the firing process can be reduced.

[0005] JP 2017-25400 A

[0006] However, it has been found that simply having sulfur present on the surface of nickel powder may not be enough to sufficiently suppress structural defects in laminates provided in multilayer ceramic electronic components such as multilayer ceramic capacitors.

[0007] Therefore, an object of the present invention is to provide a conductive paste that can sufficiently suppress the occurrence of structural defects in the laminate of a multilayer ceramic electronic component during firing, even if the nickel powder is atomized.

[0008] The present invention is directed to a conductive paste for forming internal electrodes in a multilayer ceramic electronic component.

[0009] The conductive paste according to the present invention contains nickel powder, ceramic powder, an organic binder, and an organic solvent, and is characterized in that an average of 79% or more and 100% or less of the surfaces of the nickel particles constituting the nickel powder are coated with sulfur and nickel oxide.

[0010] According to the present invention, even if the nickel powder in the conductive paste that serves as the internal electrode material is atomized, the occurrence of structural defects in the laminate of the multilayer ceramic electronic component during firing can be sufficiently suppressed.

[0011] 1 is a cross-sectional view schematically showing a multilayer ceramic capacitor 1 as an example of a chip-type ceramic electronic component having internal electrodes 4 and 5 formed from the conductive paste according to the present invention. 2 is a cross-sectional view schematically showing, in enlarged form, nickel particles 10 according to Examples 2, 5, and 6 and Comparative Examples 1 and 3, which were produced in experimental examples. 3 is a diagram showing the distribution of the coverage of sulfur (S) and nickel oxide (NiO) for Examples 1 to 6 and Comparative Examples 2 to 4 shown in Table 1.

[0012] Referring to FIG. 1, the structure of a multilayer ceramic capacitor 1 will be described as an example of a chip-type ceramic electronic component to which the conductive paste according to the present invention can be applied.

[0013] The multilayer ceramic capacitor 1 includes a laminate 2 serving as a component body. The laminate 2 includes a plurality of ceramic layers 3 made of dielectric material, stacked together, and a plurality of internal electrodes 4 and 5 arranged along the interfaces between the ceramic layers 3. The internal electrodes 4 and 5 are classified into a plurality of first internal electrodes 4 and a plurality of second internal electrodes 5, which are arranged alternately in the stacking direction of the laminate 2. A first external electrode 6 and a second external electrode 7 are provided on the surface of the laminate 2, more specifically, on each of the opposing end faces. The first external electrode 6 is electrically connected to the first internal electrode 4, and the second external electrode 7 is electrically connected to the second internal electrode 5.

[0014] The ceramic layer 3 is made of, for example, ABO ３ (A is at least one of Ba, Ca and Sr, and B is at least one of Ti and Zr) as a main component. ３ The alloy may contain at least one of Mn, Mg, Si, Y, Dy and Gd as a secondary component.

[0015] The multilayer ceramic capacitor 1 is manufactured, for example, through the following steps. First, a ceramic slurry is prepared containing ceramic raw material powder having the above-described composition of the ceramic layers 3. Next, a suitable sheet forming method is applied to the ceramic slurry to form ceramic green sheets. Next, a conductive paste to become each of the internal electrodes 4 and 5 is applied by printing or the like to predetermined ceramic green sheets among the plurality of ceramic green sheets. Next, the plurality of ceramic green sheets are stacked and then pressed together to obtain a green laminate. Next, the green laminate is fired. In this firing step, the ceramic green sheets become the ceramic layers 3. After that, external electrodes 6 and 7 are formed on the end faces of the laminate 2.

[0016] The conductive paste that is the material for the internal electrodes 4 and 5 contains nickel powder as a conductive component, ceramic powder as a co-material for the ceramic layer 3, an organic binder, and an organic solvent. Here, the nickel powder has the following characteristics.

[0017] 2(1) to 2(3), for example, an average of 79% to 100% of the surface of each nickel particle 10 constituting the nickel powder is covered with sulfur 11 and nickel oxide 12. As will be derived from the experimental results described later, it is preferable that the coverage rate of sulfur 11 is, on average, 1% to 89% of the surface of each nickel particle 10, and the coverage rate of nickel oxide 12 is, on average, 9% to 99% of the surface of each nickel particle 10.

[0018] Furthermore, the preferred contents of the organic binder and ceramic powder in the conductive paste are derived from the experimental results described below. That is, it is preferred that the organic binder be contained in an amount of 3 parts by mass or more and 7 parts by mass or less per 100 parts by mass of nickel powder, and that the ceramic powder be contained in an amount of 2 parts by mass or more and 18 parts by mass or less per 100 parts by mass of nickel powder.

[0019] The external electrodes 6 and 7 are formed as thick films by applying a conductive paste containing silver or copper as a main conductive component to the end faces of the laminate 2 and baking the paste. If necessary, Ni plating and Sn plating may be applied to the thick film formed by baking.

[0020] The conductive paste that is the material for the internal electrodes 4 and 5 is preferably prepared as follows.

[0021] First, nickel powder and ceramic powder to be contained in the conductive paste are prepared. For example, barium titanate ceramic powder is used as the ceramic powder. The method for producing nickel particles that make up the nickel powder, i.e., nickel particles coated with sulfur and nickel oxide, will be described in detail in the experimental examples below.

[0022] Next, the nickel powder and ceramic powder are each placed in a solution containing a non-aqueous, low-polarity solvent, such as acetone or ethanol, and a dispersant, and dispersed using a ball mill or the like to obtain a nickel powder dispersion and a ceramic powder dispersion, respectively. Anionic dispersants, such as carboxylic acid, sulfonic acid, sulfate, or phosphate ester dispersants, are particularly useful as dispersants. Anionic dispersants can exhibit crosslinking cohesive force. Note that a dispersant may not be added to the dispersion obtained in the above process.

[0023] Next, the nickel powder dispersion liquid and the ceramic powder dispersion liquid are mixed and stirred.

[0024] Next, the mixture is kneaded using a dispersion method such as a triple roll mill to uniformly disperse the nickel powder and ceramic powder, and then an organic binder and an organic solvent are added and stirred to obtain a conductive paste. Here, the organic binder is, for example, ethyl cellulose, and the organic solvent is, for example, dihydroterpinyl acetate.

[0025] Regarding the addition of the dispersant described above, it is preferable to add the dispersant to only one of the nickel powder dispersion and the ceramic powder dispersion before mixing, so that the bridging and cohesive force is exerted only in one of them, because the bridging and cohesive force of the dispersant is difficult to exert when mixed with other dispersants.

[0026] The dispersant may also be added after the nickel powder dispersion and the ceramic powder dispersion are mixed.

[0027] Next, an example of an experiment conducted to determine the scope of the present invention will be described.

[0028] [Production of Nickel Powder] Preparation of First Solution: 100 g of nickel sulfate hexahydrate and 40.3 g of trisodium citrate dihydrate (to give a molar ratio of trisodium citrate of 0.36 moles per mole of nickel) were dissolved in 270 g of water. Next, 0.18 g of a copper sulfate pentahydrate solution diluted to 0.4 mass % was added to give a copper ion concentration of 8.2 mass ppm per mole of nickel, to prepare a nickel salt solution containing copper ions. This solution was designated the first solution.

[0029] Preparation of Second Solution Next, 54 g of sodium hydroxide was dissolved in 250 g of water so that the molar number of sodium hydroxide was 3.55 moles per mole of nickel contained in the first solution to be mixed, to prepare a second solution.

[0030] Preparation of Third Solution Next, 66 g of 60% hydrazine hydrate was prepared so that the mole ratio of hydrazine was 2.03 moles per mole of nickel contained in the first solution to be mixed. This solution was designated as the third solution.

[0031] Preparation of fourth solution and precipitation of nickel powder The first, second, and third solutions were heated to 55°C, the second and third solutions were premixed, and the mixed solution was then added dropwise to the first solution over a period of 100 seconds to prepare a fourth solution. Nickel powder was precipitated in this fourth solution.

[0032] Adding sulfur and nickel oxide to nickel particles The fourth solution was aged for 30 minutes to obtain a reaction solution containing nickel precipitates. Next, after cooling the reaction solution, the supernatant was replaced twice with pure water, and then thiomalic acid (also known as mercaptosuccinic acid; HOOCCH(SH)CH) was added as a sulfur coating agent. ２ The nickel particles were washed and sulfurized by adding an aqueous solution of acetone (COOH; molecular weight: 150.15) and replacing the supernatant with pure water several times. After replacing the water with acetone, the nickel powder was dried in an oven set at 80°C and crushed in an airflow mill.

[0033] In the above process, the coverage of sulfur on the surface of the nickel particles was controlled by changing the amount of sulfur coating agent added, and the coverage of nickel oxide on the surface of the nickel particles was controlled by changing the drying time of the nickel powder in the oven.

[0034] In this manner, nickel powders were obtained consisting of nickel particles having the "S coverage," "NiO coverage," and "S coverage + NiO coverage" shown in Table 1. FIG. 2 shows an enlarged schematic view of nickel particles 10. In FIG. 2, (1) shows nickel particles 10 according to Example 2 (E2), (2) shows nickel particles 10 according to Example 5 (E5), (3) shows nickel particles 10 according to Example 6 (E6), (4) shows nickel particles 10 according to Comparative Example 1 (R1), and (5) shows nickel particles 10 according to Comparative Example 3 (R3). Sulfur 11 and nickel oxide 12 are shown schematically on the surface of the nickel particles 10.

[0035] The coverage of sulfur and nickel oxide on the surface of the nickel particles was determined by calculating the amounts of sulfur and nickel oxide on the surface of the nickel particles by mapping analysis using a TEM (transmission electron microscope).

[0036] The average particle size of the obtained nickel particles was measured by image analysis using a scanning electron microscope and was 200 nm, as shown in the "Particle Size" column in Table 1. As mentioned above, a particle size of 200 nm corresponds to a particle size of 300 nm or less, at which point the possibility of structural defects occurring increases.

[0037] [Production of Conductive Paste] A sample conductive paste containing the nickel powder, a barium titanate-based ceramic powder as the ceramic powder, ethyl cellulose as the organic binder, and dihydroterpinyl acetate as the organic solvent was produced through the above-mentioned processes of dispersion, mixing, stirring, etc.

[0038] As shown in the "Organic Binder" column of Table 1, the amount of ethyl cellulose added as the organic binder was varied within a range of 2 to 8 parts by mass relative to 100 parts by mass of nickel powder, and as shown in the "Ceramic" column, the amount of barium titanate-based ceramic powder added as the ceramic powder was varied within a range of 1 to 19 parts by mass relative to 100 parts by mass of nickel powder.

[0039] [Fabrication of Multilayer Ceramic Capacitor] A multilayer ceramic capacitor as a sample was fabricated by the method described above.

[0040] The resulting laminate of the multilayer ceramic capacitor had planar dimensions of 1.0 mm x 0.5 mm, with the ceramic layers each having a thickness of 2 μm and the internal electrodes each having a thickness of 1 μm.

[0041] [Evaluation] The laminates of the multilayer ceramic capacitor samples were visually inspected using a stereomicroscope to check for structural defects such as cracks and delamination. 200,000 samples were measured for each sample, and the structural defect occurrence rate (ppm) is shown in the "Structural Defects" section of Table 1.

[0042]

[0043] As shown in Table 1, in Examples 1 to 14, both the "S coverage" and the "NiO coverage" exceed 0%. In other words, the surfaces of the nickel particles are covered with sulfur and nickel oxide. Furthermore, the coverage with sulfur and nickel oxide, i.e., the "S coverage + NiO coverage," is 79% or more and 100% or less. In Examples 1 to 14, the "structural defects" are suppressed to 95 ppm or less.

[0044] In addition, Examples 1 to 14 satisfy the condition that the "S coverage" is 1% or more and 89% or less, and the "NiO coverage" is 9% or more and 99% or less.

[0045] Among Examples 1 to 14, Examples 9 and 10 had an "organic binder" content outside the range of 3 parts by mass or more and 7 parts by mass or less per 100 parts by mass of nickel powder. As a result, Examples 9 and 10 exhibited "structural defects" values ​​of 85 ppm and 95 ppm, respectively. In Example 9, the "organic binder" content was 2 parts by mass, which was lower than, for example, 5 parts by mass in Examples 1 to 6. This reduced the dispersibility of the conductive paste, thereby reducing the filling ability during printing and weakening the laminate structure, resulting in the occurrence of structural defects in some areas. In Example 10, the "organic binder" content was 8 parts by mass, which was higher than, for example, 5 parts by mass in Examples 1 to 6. This increased the amount of organic components released from the raw laminate during the firing process, resulting in the occurrence of structural defects in some areas.

[0046] Furthermore, among Examples 1 to 14, Examples 13 and 14 had a "ceramic" content outside the range of 2 parts by mass or more and 18 parts by mass or less per 100 parts by mass of nickel powder. As a result, Examples 13 and 14 had "structural defects" values ​​of 80 ppm and 85 ppm, respectively. In Example 13, the "ceramic" content was 1 part by mass, which was less than, for example, 10 parts by mass in Examples 1 to 6. This is thought to have reduced the filling ability of the paste coating film, weakened the laminate structure, and caused structural defects in some areas during the firing process. In Example 14, the "ceramic" content was 19 parts by mass, which was more than, for example, 10 parts by mass in Examples 1 to 6. This is thought to have increased the internal stress of the laminate, causing stress release during the firing process and causing structural defects in some areas.

[0047] In Examples 1 to 6, 7, 8, 11, and 12, excluding Examples 9, 10, 13, and 14, not only did the "S coverage" and "NiO coverage" both exceed 0%, and the "S coverage + NiO coverage" was 79% or more and 100% or less, but also the "S coverage" was 1% or more and 89% or less, the "NiO coverage" was 9% or more and 99% or less, and the "organic binder" was 3 parts by mass or more and 7 parts by mass or less, and the "ceramic" was 2 parts by mass or more and 18 parts by mass. As a result, the "structural defects" showed a value of 0 ppm.

[0048] On the other hand, in Comparative Example 1, although the "S coverage + NiO coverage" was 90%, which is 79% or more and 100% or less, the "NiO coverage" was 0%, so the "structural defects" were 405 ppm. In Comparative Example 1, as shown in Figure 2 (4), it is presumed that the nickel particles were coated only with sulfur, which made the reactivity of the nickel particles too high, resulting in the generation of structural defects during the firing process.

[0049] In Comparative Examples 2 to 4, the "S coverage + NiO coverage" was 74%, 40%, and 73%, respectively, which was below the range of 79% or more and 100% or less, and therefore the "structural defects" were 420 ppm, 315 ppm, and 195 ppm, respectively. In Comparative Examples 2 to 4, the "S coverage + NiO coverage" was low, so the catalytic effect of the nickel particles could not be suppressed, organic gas was rapidly generated during degreasing, and it is presumed that structural defects occurred in the laminate when the organic gas escaped outside the laminate chip.

[0050] 3 shows the distribution of the coverage of sulfur (S) and nickel oxide (NiO) for Examples 1 to 6 and Comparative Examples 2 to 4 shown in Table 1. In FIG. 3, the coverage of "NiO" is plotted on the vertical axis, and the coverage of "S" is plotted on the horizontal axis. In FIG. 3, Examples 1 to 6 are indicated by black circles, Comparative Examples 2 to 4 are indicated by white circles, Examples 1 to 6 are indicated by E1 to E6, respectively, and Comparative Examples 2 to 4 are indicated by R2 to R4, respectively.

[0051] From the distribution of the S coverage and NiO coverage shown in FIG. 3, it can be seen that there is a distinguishable boundary between the Examples and the Comparative Examples.

[0052] The present invention has been described above in relation to a conductive paste for forming internal electrodes in a multilayer ceramic capacitor. However, the present invention can be widely applied to conductive pastes for forming internal electrodes in multilayer ceramic electronic components other than multilayer ceramic capacitors, as long as they include a laminate having a laminate structure with internal electrodes.

[0053] The embodiments of the present invention include the following:

[0054] <1> A conductive paste for forming internal electrodes in a multilayer ceramic electronic component, comprising nickel powder, ceramic powder, an organic binder, and an organic solvent, wherein an average of 79% or more and 100% or less of the surfaces of nickel particles constituting the nickel powder are coated with sulfur and nickel oxide.

[0055] <2> The conductive paste according to <1>, wherein the sulfur coverage is 1% or more and 89% or less on average of the surface of each of the nickel particles.

[0056] <3> The conductive paste according to <1> or <2>, wherein the nickel oxide has an average coverage of 9% or more and 99% or less of the surface of each of the nickel particles.

[0057] <4> The conductive paste according to any one of <1> to <3>, wherein the organic binder is contained in an amount of 3 parts by mass or more and 7 parts by mass or less with respect to 100 parts by mass of the nickel powder.

[0058] <5> The conductive paste according to any one of <1> to <4>, wherein the ceramic powder is contained in an amount of 2 parts by mass or more and 18 parts by mass or less with respect to 100 parts by mass of the nickel powder.

[0059] REFERENCE SIGNS LIST 1 Multilayer ceramic capacitor 2 Laminate 3 Ceramic layer 4, 5 Internal electrodes 6, 7 External electrodes 10 Nickel particles 11 Sulfur 12 Nickel oxide.

Claims

1. A conductive paste for forming internal electrodes in multilayer ceramic electronic components, It contains nickel powder, ceramic powder, an organic binder, and an organic solvent. On average, 79% or more and 100% or less of the surface of the nickel particles constituting the nickel powder is coated with sulfur and nickel oxide. The sulfur coating rate is 1% or more and 89% or less on average on the surface of each nickel particle. Conductive paste.

2. The conductive paste according to claim 1, wherein the coverage of the nickel oxide is 9% or more and 99% or less on average on the surface of each nickel particle.

3. The conductive paste according to claim 1, wherein the organic binder comprises 3 parts by mass or more and 7 parts by mass or less per 100 parts by mass of nickel powder.

4. The conductive paste according to claim 2, wherein the organic binder comprises 3 parts by mass or more and 7 parts by mass or less per 100 parts by mass of nickel powder.

5. The conductive paste according to any one of claims 1 to 4, wherein the ceramic powder comprises 2 parts by mass or more and 18 parts by mass or less per 100 parts by mass of the nickel powder.