Ni alloy powder

Abstract

To provide an excellent Ni alloy powder for an excellent conductive paste composed of an alloy mainly composed of Ni, in particular, to provide a Ni alloy powder for a conductive powder having excellent oxidation resistance by raising the sintering starting temperature.SOLUTION: There is provided a Ni alloy powder which comprises 85.0 to 90.0 mass% of Ni and the balance Al with inevitable impurities, wherein the average particle diameter is 0.04 to 1.00 μm. In addition, it is preferable that the Ni alloy power is mainly composed of an Ni3Al phase of an intermetallic compound and the average crystallite diameter of the Ni3Al phase is more than 0.3 times the average particle diameter of the Ni alloy powder. Thereby, a Ni alloy powder for a conductive paste having a high sintering starting temperature and excellent oxidation resistance can be easily obtained.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a Ni alloy powder for conductive paste used for forming internal electrodes of multilayer ceramic capacitors. [Background technology]

[0002] Multilayer ceramic capacitors are conventionally manufactured by printing or spraying a metal powder paste onto ceramic dielectric sheets, stacking multiple dielectric sheets so that they form an electrode structure, compressing them together, sintering the resulting assembly, and baking in external lead electrodes. Such multilayer ceramic capacitors have properties such as a thin effective dielectric thickness, a high capacitance-to-volume ratio, low internal inductance, and use at high frequencies, for example, up to the GHz range.

[0003] Because these multilayer ceramic capacitors involve simultaneously firing the ceramic dielectric and internal electrodes, the electrode material must have a melting point higher than the sintering temperature of the ceramic and must not react with the ceramic. For this reason, precious metals such as Pt and Pd were previously used, but they had the drawback of being expensive. To overcome this drawback, the sintering temperature of the dielectric ceramic has been lowered to 900-1100°C, and electrodes made of Ag-Pd alloys or inexpensive metals such as Ni have become commercially available.

[0004] However, because Ni has a lower melting point than the dielectric, there is a difference in the sintering temperatures between the Ni electrode and the dielectric. Therefore, the difference in the sintering shrinkage curves (temperature dependence of sintering shrinkage) between Ni and the dielectric can lead to problems such as cracking or peeling of the electrode during the sintering process, or poor sintering of the dielectric.

[0005] To avoid this problem, a dielectric is added to the Ni paste, shifting the Ni's firing shrinkage curve toward higher temperatures and bringing it closer to the dielectric's firing shrinkage curve. However, while the more dielectric is added, the closer the firing shrinkage curves become, the lower the Ni content in the electrode layer becomes. This reduces the electrode coverage—that is, the ratio of the area that actually functions as an electrode to the area that should be covered by the electrode. This reduction in coverage reduces the capacitor's capacitance, which in turn reduces its performance. Therefore, it is desirable to use a Ni alloy powder that begins sintering at a high temperature, without adding a dielectric, and that has a firing shrinkage curve close to that of the dielectric.

[0006] However, Ni is easily oxidized by oxygen in the atmosphere during the binder removal and sintering processes. As a result, there are problems such as poor sintering of the electrode due to the inclusion of oxides and reduced conductivity. Conventionally, Ni oxidation has been prevented by using a reducing atmosphere, but to fully utilize the performance of ceramic dielectrics, it is desirable to sinter in an oxidizing atmosphere, and it is also desirable for electrode materials to be able to be sintered in an oxidizing atmosphere.

[0007] Therefore, Patent Document 1 discloses a technique for increasing both the sintering start temperature and oxidation start temperature of Ni by adding V, Cr, Zr, Nb, Mo, Ta, W, etc. to Ni to make a powder that has a higher sintering start temperature than Ni and is not oxidized by the atmosphere during sintering. However, even with this technique, the upper limit of the sintering start temperature is about 700°C, and the upper limit of the oxidation start temperature is about 360°C, so it has not yet reached a level where adding a dielectric to Ni paste is no longer necessary. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-60877 Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention has been made in consideration of the above circumstances, and aims to provide an excellent Ni alloy powder for conductive paste, which is made of an alloy mainly containing Ni, and in particular to provide an Ni alloy powder for conductive paste which has an increased sintering start temperature and excellent oxidation resistance. [Means for solving the problem]

[0010] In order to increase the sintering temperature of Ni alloy powder, the inventors focused on Ni-based superalloys, which have excellent high-temperature strength, good creep resistance, and can withstand use as structural materials up to approximately 1000°C. The high-temperature strength of these alloys is achieved by dispersing more than 60% by volume of Ni3Al ordered alloy phase (γ') (hereinafter also referred to as "Ni3Al phase") in Ni solid solution (γ phase). Ni-Al binary alloys contain five intermetallic compounds: NiAl3, Ni2Al3, NiAl, Ni5Al3, and Ni3Al. The Ni3Al phase has a conductivity of 4.6% IACS (37.4 × 10 -8 It has sufficient conductivity as a conductive material, with a resistivity of Ω·m, and it was discovered that by using the Ni3Al phase as a single phase or an alloy containing this phase, it is possible to obtain an alloy powder for conductive paste with excellent oxidation resistance. %IACS is the resistivity of International Annealed Copper Standard (International Annealed Copper Standard, resistivity at 293K: 1.7241×10 -8 It is an index that expresses the conductivity of a conductor as a ratio, with the conductivity of Ω·m being 100% IACS.

[0011] The present invention was completed based on these findings and further investigations. That is, the gist of the present invention is as follows. [1] A Ni alloy powder containing 85.0 to 90.0 mass% Ni, the remainder being Al and unavoidable impurities, characterized in that the Ni alloy powder has an average particle size of 0.04 to 1.00 μm. [2] In [1], the Ni alloy powder is characterized in that the Ni alloy powder is mainly composed of an Ni3Al phase of an intermetallic compound. [3] In [2], the Ni alloy powder is characterized in that the average crystallite diameter of the Ni3Al phase is 0.3 times or more the average particle diameter of the Ni alloy powder. [4] The Ni alloy powder according to any one of [1] to [3], characterized in that the Ni alloy powder is an alloy powder for a conductive paste. [5] In any one of [1] to [4], the average particle size is D50 based on the number of particles obtained by observing the Ni alloy powder with a scanning electron microscope (SEM), taking an image, and analyzing the SEM image at a magnification of 20,000 times, with 1,000 to 2,000 particles measured. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a Ni alloy powder for conductive paste, which has a high sintering initiation temperature and excellent oxidation resistance, as a metal for conductive paste for multilayer ceramic capacitors, and this has a significant industrial effect. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 2 is a schematic diagram showing the relationship between the height and temperature of a compact of Ni alloy powder. [Figure 2] FIG. 2 is a schematic diagram showing the relationship between the mass and temperature of a compact of Ni alloy powder. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be described in detail.

[0015] [Alloy powder composition] The alloy powder of the present invention is a Ni-Al binary alloy containing 85.0 to 90.0 mass% Ni, the remainder being Al and unavoidable impurities, and has an average particle size of 0.04 to 1.00 μm. Furthermore, the Ni alloy powder preferably consists mainly of an intermetallic compound Ni3Al phase, and the average crystallite size of the Ni3Al phase is preferably at least 0.3 times the average particle size of the Ni alloy powder. Hereinafter, "%" in the composition means "mass%."

[0016] Next, the reasons for limiting the composition of the alloy powder will be explained.

[0017] [Ni: 85.0-90.0%] The Ni content of the alloy powder of the present invention is limited to the range of 85.0 to 90.0%. If the Ni content is less than 85.0%, a different phase other than the Ni3Al phase will be generated. This different phase undergoes a phase transformation around 700°C. Therefore, if an alloy powder of this composition is used to manufacture capacitor electrodes, the Ni5Al3 phase will be generated during the cooling process after firing of the multilayer ceramic capacitor. This will cause phase decomposition and changes in the crystalline structure, making the capacitor prone to defects such as breakage and cracking. On the other hand, if the Ni content exceeds 90.0%, the alloy will enter the Ni solid solution region rather than the Ni3Al phase, approaching the behavior of Ni alone, and will not be expected to contribute to the aforementioned high-temperature strength. Therefore, the Ni content is limited to 85.0 to 90.0%, preferably 85.0 to 87.0%, and more preferably 85.5 to 86.5%.

[0018] The balance is substantially Al. Examples of unavoidable impurity elements include C, N, P, S, Mn, Cu, etc. These unavoidable impurity elements deteriorate the powder properties of the alloy powder, such as electrical conductivity, heat resistance, and oxidation resistance. However, if the total content is 1.0% or less, the powder properties are not practically deteriorated and therefore are acceptable.

[0019] [Average particle size of alloy powder: 0.04 to 1.00 μm] Generally, the thinner the electrodes of multilayer ceramic capacitors, the greater the capacitance of the capacitor; therefore, it is necessary to manufacture electrodes as thin as possible. To achieve this, the average particle size of the alloy powder in the electrode paste must be small. The surface roughness Ra (Ra is a surface roughness parameter defined in JIS B 0601) of films printed with a paste made using alloy powder with a small average particle size should be small. However, in actual paste production, the smaller the average particle size, the more likely the powder is to aggregate, forming large aggregates. Therefore, the surface roughness Ra of films printed with paste made with powders with excessively small average particle sizes actually increased. In the worst case scenario, large aggregates could penetrate the dielectric layer, causing a short circuit. Therefore, the lower limit of the average particle size was set at 0.04 μm. On the other hand, for the reasons mentioned above, a thin electrode is desirable, so powders with a large average particle size are undesirable. Therefore, based on the electrode thickness currently required in practical applications, the upper limit of the average particle size of the alloy powder was set at 1.00 μm. The thickness is preferably 0.04 to 0.80 μm, more preferably 0.04 to 0.60 μm, and even more preferably 0.04 to 0.40 μm.

[0020] Here, the average particle size of the Ni alloy powder is defined as D50 based on the number of particles, which is determined by observing the alloy powder particles with a scanning electron microscope (SEM), taking an image, and analyzing the SEM image of 1,000 to 2,000 particles measured at a magnification of 20,000 times.

[0021] [Crystal structure of alloy powder] As described above, the alloy powder of the present invention is a Ni-Al binary alloy containing 85.0 to 90.0 mass% Ni, with the remainder being Al and unavoidable impurities. Furthermore, the crystalline structure of the alloy powder preferably consists primarily of the intermetallic compound NiAl phase. This crystalline structure was identified by X-ray diffraction, as described below. In Ni-Al binary alloys, NiAl, NiAl, NiAl, and NiAl phases exist in addition to the NiAl phase. In the present invention, the crystalline structure preferably consists primarily of the NiAl phase.

[0022] Here, "mainly" refers to a case where the Ni3Al phase is a single phase, or a case where the Ni3Al phase and other phases other than the Ni3Al phase are included in addition to the Ni3Al phase, and when the diffraction peak areas of the individual phases are compared using the X-ray diffraction method described below, the diffraction peak area of ​​the Ni3Al phase is larger than the diffraction peak area of ​​any of the other phases.

[0023] The crystal structure can be identified by X-ray diffraction. This X-ray diffraction method measures diffraction peak intensities in accordance with JIS K 0131 "General Rules for X-ray Diffraction Analysis," and the crystal structure can be identified from the resulting diffraction pattern. To quantitatively analyze the crystal structure, the area (or integrated intensity ratio) of the diffraction peak of the identified crystal structure is calculated, and the quantitative ratio of the crystal structure can be determined by comparing the areas obtained.

[0024] That is, in the alloy powder of the present invention, in the X-ray diffraction pattern of the Ni alloy powder, the area of ​​the diffraction peak of the (111) plane of the Ni3Al phase is preferably larger than the area of ​​the diffraction peak of the phase of the other Ni-Al-based intermetallic compound. Here, the areas of the diffraction peaks of the phases of the other Ni-Al-based intermetallic compounds specifically include the area of ​​the diffraction peak of the (111) plane of the NiAl3 phase, the area of ​​the diffraction peak of the (012) plane of the Ni2Al3 phase, the area of ​​the diffraction peak of the (110) plane of the NiAl phase, and the area of ​​the diffraction peak of the (221) plane of the Ni5Al3 phase.

[0025] Furthermore, it is more preferable that the area of ​​the diffraction peak with the greatest intensity among the diffraction peak areas of the phases of the above-mentioned other Ni-Al based intermetallic compounds is 50% or less of the area of ​​the diffraction peak of the (111) plane of the NiAl phase. If it exceeds 50%, it becomes difficult to obtain a Ni alloy powder for conductive paste that has a high sintering initiation temperature, improved oxidation resistance, and excellent heat resistance. It is more preferable that it is 30% or less, and even more preferable that it is 10% or less.

[0026] [Average crystallite diameter] Next, the average crystallite size will be described. Typically, a single particle is composed of a complex of multiple crystals with different orientations. A crystallite refers to the individual crystals that make up the complex. The crystal orientation is consistent within each crystal, and each can be considered a single crystal. In X-ray diffraction of a single crystal, all crystal lattices simultaneously satisfy the Bragg diffraction condition at a specific incident angle, resulting in an extremely sharp diffraction peak. In contrast, as the crystallite size (also called "crystallite diameter") decreases, the number of crystals (crystallites) that make up the particle increases, and each crystal has a different crystal orientation, resulting in a broader diffraction peak. In X-ray diffraction, the crystallite diameter can be calculated using the Scherrer formula. In this invention, the average crystallite diameter is defined as the value calculated according to JIS H 7805, "Method for measuring crystallite diameter of metal catalysts by X-ray diffraction."

[0027] The present invention achieves high sintering temperatures through the presence of the Ni3Al phase, which has excellent high-temperature strength and creep resistance. Furthermore, even with the same Ni3Al phase, a larger crystallite size, i.e., a more developed crystalline structure of the Ni3Al phase, increases the effect of increasing the sintering temperature. Therefore, it is preferable that the average crystallite size be 0.3 times or more the average particle size of the alloy powder. More preferably, it is 0.5 times or more. However, since a large average crystallite size does not cause any problems, there is no upper limit.

[0028] Furthermore, the Ni alloy powder is preferably spherical in shape, as this provides excellent dispersibility and flowability when made into a paste. Such alloy powder can be easily produced by chemical vapor deposition (also known as "CVD") to form spherical alloy powder with uniform particle size.

[0029] [Method of manufacturing alloy powder] Here, a method for producing alloy powder by CVD will be described. For example, Ni and Al alloy powder can be produced by heating and evaporating Ni chloride and Al chloride, respectively, and then reducing the vapor with a reducing gas such as hydrogen gas. The composition and particle size of the alloy powder can be controlled by changing the reaction conditions. For example, if the reaction is carried out under conditions that produce 1 mole of Al for every 3 moles of Ni, an alloy powder of the Ni3Al phase, an intermetallic compound, is obtained. CVD allows different metal elements to mix at the atomic level, making it possible to produce alloy powder with a uniform composition.

[0030] Reaction conditions include the blending amounts (molar amounts) of raw materials, heating temperature (900-1100°C), flow rate of reducing gas such as hydrogen gas (10-100 NL / min), and gas temperature (900-1100°C). To obtain the Ni alloy powder of the present invention, for example, conditions of NiCl2 supply rate of 0.3 mol / min, AlCl3 supply rate of 0.1 mol / min, carrier N2 gas flow rate of 10-200 NL / min, H2 gas flow rate of 20-50 NL / min, and reaction temperature of 1100°C can be achieved. When the gas flow rate is high, the particle size of the obtained alloy powder becomes small, and when the gas flow rate is low, the particle size of the obtained alloy powder becomes large.

[0031] After the reduction reaction, the obtained alloy powder is further subjected to a dechlorination process. In the dechlorination process, the obtained alloy powder is washed with a solvent to adjust the chlorine concentration. The solvent used is preferably one that dissolves unreacted chlorides and by-products generated by the reduction reaction. Examples of such solvents include water and alcohol. When a predetermined chlorine concentration (e.g., 30 ppm) is reached, the dechlorination process is completed, and the target alloy powder is obtained.

[0032] [Conductive paste] The conductive paste can be produced by a conventional method. For example, the paste can be produced by mixing 100 parts by weight of Ni alloy powder with 1 to 5 parts by weight of a binder (e.g., ethyl cellulose) and 5 to 20 parts by weight of an organic solvent (e.g., terpineol).

[0033] [Characteristics of alloy powder] Next, the characteristics listed as the effects of the present invention will be described.

[0034] [Sintering start temperature] The present invention is characterized by a high sintering start temperature. This is an important indicator because the sintering shrinkage curve of the alloy powder is similar to that of the dielectric in both temperature range and shrinkage amount. If the sintering start temperature of the alloy powder is significantly lower than that of the dielectric, the electrode layers will begin to sinter and shrink before the dielectric layers shrink during the heating process of the multilayer ceramic capacitor. As a result, the electrode layers, constrained by the dielectric layers, will not be able to shrink as a single plate maintaining overall electrical continuity and will fracture in various places, resulting in loss of electrode continuity and resulting in defective capacitors. In contrast, if the sintering start temperature of the alloy powder is the same as that of the dielectric and their sintering shrinkage curves are similar, the dielectric layers and electrode layers will sinter and shrink simultaneously during the heating process of the multilayer ceramic capacitor. This allows the electrode layers to shrink as a single plate maintaining overall electrical continuity without being constrained by the dielectric layers and breaking, making it possible to manufacture capacitors with guaranteed electrode continuity.

[0035] Here, an experiment was conducted to investigate the state of the sintering start temperature. A compact of 87% Ni and 13% Al was used as the Ni alloy powder of the present invention, and the change in the amount of shrinkage due to heating was measured using a thermomechanical analyzer (TMA) TMA4000SE manufactured by NETZSCH. As a comparative example, a compact of 100% Ni metal powder was also used and similar measurements were carried out. The results are shown in Figure 1. The horizontal axis of Figure 1 represents the heating temperature, and the vertical axis represents the height of the compact before shrinkage at the start position and the height of the compact after completion of shrinkage at the end position, normalized to compare the change in the amount of shrinkage with temperature.

[0036] In curve 12 of the Ni metal powder (Ni 100%) of the comparative example, shrinkage begins at around 400°C, but in curve 11 of the Ni alloy powder (Ni 87%-Al 13%) of the present invention, shrinkage begins at around 800°C, indicating that the sintering start temperature is high and close to the sintering start temperature of the dielectric.

[0037] [Oxidation resistance] When firing multilayer ceramic capacitors, it is desirable to sinter alloy powder with a large specific surface area in a reducing atmosphere to prevent oxidation of the alloy powder, but firing in a reducing atmosphere would cause the dielectric layer to become a semiconductor, resulting in a defective capacitor, so firing is performed in an oxidizing atmosphere. In order to form electrodes without oxidizing the alloy powder, the alloy powder must be oxidation-resistant.

[0038] Oxidation resistance indicates the difficulty of oxidation, and is expressed by the change in mass of the produced green compact as the temperature rises. Specifically, the degree of oxidation resistance can be compared based on the temperature at which the mass increase due to oxidation reaches 0.5%.

[0039] An experiment was conducted to investigate the temperature condition at which this mass increase reaches 0.5%. A compact of 87% Ni and 13% Al was used as the Ni alloy powder of the present invention, and the change in mass with temperature was measured using a NETZSCH thermogravimetric analyzer STA 2500 Regulus at a heating rate of 20 K / min in an air atmosphere. As a comparative example, a similar measurement was also performed using a compact of 100% Ni metal powder. The results are shown in Figure 2. The horizontal axis of Figure 2 represents the heating temperature, and the vertical axis represents the mass increase rate (%), which is the ratio of the increase in mass from the initial compact.

[0040] In curve 21 of the Ni alloy powder (Ni 87%-Al 13%) of the present invention, the mass gradually increased upon heating, reaching a mass increase of 0.5% at around 380°C. However, in curve 22 of the Ni metal powder (Ni 100%) of the comparative example, the mass increase reached 0.5% at around 250°C. From the above, it can be seen that the Ni alloy powder of the present invention is a material that is difficult to oxidize and has excellent oxidation resistance. [Example]

[0041] The present invention will be further described below based on examples, although the present invention is not limited to the examples described below.

[0042] Ni alloy powders were produced by adjusting and varying the components and average particle size of the Ni alloy powders according to the present invention and the alloy powders of the comparative examples, and the sintering start temperature and the temperature at which the mass increase reaches 0.5% during heating were measured for these Ni alloy powders. The measurement results are shown in Table 1.

[0043] The sintering start temperature was determined by applying pressure to the alloy powder to produce a green compact, and then measuring the relationship between the green compact height (sample height) and temperature as the temperature of the green compact increased. The sintering start temperature was determined as the temperature at which the green compact height began to decrease. The mass gain was measured using a NETZSCH STA 2500 Regulus thermogravimetric analyzer at a heating rate of 20 K / min in air. The results are shown in Table 1.

[0044] [Table 1]

[0045] Furthermore, a multilayer ceramic capacitor was manufactured by laminating multiple dielectric sheets manufactured by the doctor blade method and conductive paste prepared by dispersing the Ni alloy powder of the present invention in an organic solvent, which was then printed onto the laminate, and firing the laminate. The resulting multilayer ceramic capacitor showed no defects such as cracks or peeling and exhibited good operating characteristics.

[0046] As described above, the Ni alloy powder of the present invention has a high sintering initiation temperature and excellent oxidation resistance, making it ideal for use in the internal electrodes of multilayer ceramic capacitors. [Explanation of symbols]

[0047] 11 Curve showing the change in the height of the green compact with heating in the present invention example 12. Curve showing the change in the height of the compact with heating in the comparative example 21 Curve showing the change in mass increase rate with heating in the present invention example 22 Curve showing the change in mass increase rate with heating in the comparative example

Claims

【Request Item 1】 A Ni alloy powder containing 85.0 to 90.0 mass% Ni, the balance being Al and unavoidable impurities, wherein the Ni alloy powder is observed with a scanning electron microscope (SEM), and an average particle size, D50 on a number basis, determined by SEM image analysis of 1,000 to 2,000 measured particles at a magnification of 20,000 times, is 0.04 to 1.00 μm, and the Ni alloy powder is an intermetallic compound Ni. ３ The Al phase is a single phase or the Ni ３ In addition to the Al phase, the Ni ３ The Ni phase is obtained by comparing the diffraction peak areas of the individual phases obtained by X-ray diffraction. ３ The Al phase exists as a phase in which the diffraction peak area is larger than the diffraction peak areas of any other phases, and the Ni ３ A Ni alloy powder characterized in that the average crystallite size of the Al phase is 0.5 times or more the average particle size of the Ni alloy powder, and the Ni alloy powder is an alloy powder for a conductive paste.

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