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 60.0 to 80.0 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 NiAl phase of an intermetallic compound and the average crystallite diameter of the NiAl 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-Al alloys, which are high-melting point intermetallic compounds with relatively good electrical conductivity. There are five types of Ni-Al binary alloys: NiAl3, Ni2Al3, NiAl, Ni5Al3, and Ni3Al. Among these, the Ni:Al=1:1 compound has a melting point of 1638°C, which is higher than the melting points of Ni and Al alone. It is known for its excellent high-temperature strength and creep resistance, as seen in its use in aircraft turbines and other applications.

[0011] In addition, the conductivity is 20.0% IACS (8.62 x 10 -8 It has sufficient conductivity as a conductive material, with a resistivity of Ω·m, and it has been discovered that by using the intermetallic compound NiAl phase as a single phase or an alloy mainly consisting of this phase, it is possible to obtain alloy powder for conductive paste with a high sintering temperature, i.e., excellent heat 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.

[0012] 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 60.0 to 80.0 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] The Ni alloy powder according to [1], characterized in that the Ni alloy powder is mainly composed of an NiAl phase of an intermetallic compound. [3] The Ni alloy powder according to [2], characterized in that the average crystallite size of the NiAl phase is 0.3 times or more the average particle size 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]

[0013] 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]

[0014] [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

[0015] Hereinafter, embodiments of the present invention will be described in detail.

[0016] [Alloy powder composition] The alloy powder of the present invention is a Ni-Al binary alloy containing 60.0 to 80.0 mass% Ni, with 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 NiAl phase, an intermetallic compound, and the average crystallite size of the NiAl phase is preferably 0.3 times or more the average particle size of the Ni alloy powder. Hereinafter, "%" in the composition means "mass%."

[0017] Next, the reasons for limiting the composition of the alloy powder will be explained.

[0018] [Ni: 60.0~80.0%] The Ni content of the alloy powder of the present invention is limited to the range of 60.0 to 80.0%. If the Ni content is less than 60.0%, a heterogeneous phase other than the NiAl phase of the intermetallic compound is formed. However, since this heterogeneous phase has a melting point 500°C or more lower than the NiAl phase, the sintering start temperature drops significantly. On the other hand, if the Ni content exceeds 80.0%, a heterogeneous phase other than the NiAl phase of the intermetallic compound is formed. However, this heterogeneous phase undergoes a phase transformation around 700°C. Therefore, if an alloy powder with this composition is used to manufacture capacitor electrodes, phase decomposition and changes in crystalline structure occur during the cooling process after firing of the multilayer ceramic capacitor, making the capacitor prone to defects such as breakage and cracks. Therefore, the Ni content is set to 60.0 to 80.0%, preferably 65.0 to 75.0%.

[0019] 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.

[0020] [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 also 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 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.

[0021] Here, the average particle size of 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.

[0022] [Crystal structure of alloy powder] As described above, the alloy powder of the present invention is a Ni-Al binary alloy containing 60.0 to 80.0 mass% Ni, with the remainder being Al and unavoidable impurities. Furthermore, the crystalline structure of the alloy powder preferably consists primarily of the NiAl phase, an intermetallic compound. This crystalline structure was identified by X-ray diffraction, as described below. In Ni-Al binary alloys, other phases than the NiAl phase, such as NiAl3 phase, Ni2Al3 phase, Ni5Al3 phase, and Ni3Al phase, exist. In the present invention, the crystalline structure preferably consists primarily of the NiAl phase.

[0023] Here, "mainly" refers to a case where the NiAl phase is a single phase, and a case where the NiAl phase contains a phase other than the NiAl phase as described above, 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 NiAl phase is larger than the diffraction peak area of ​​any of the other phases.

[0024] 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.

[0025] 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 (110) plane of the NiAl 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 (221) plane of the Ni5Al3 phase, and the area of ​​the diffraction peak of the (111) plane of the Ni3Al phase.

[0026] Furthermore, it is more preferable that the area of ​​the largest diffraction peak among the diffraction peaks 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 (110) 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.

[0027] [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."

[0028] The present invention achieves a high sintering temperature by utilizing the presence of a NiAl phase, which has excellent high-temperature strength and creep resistance. Furthermore, even with the same NiAl phase, a larger crystallite size, i.e., a more developed crystalline structure of the NiAl phase, increases the effect of increasing the sintering temperature. Therefore, the average crystallite size is preferably 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 larger average crystallite size does not cause any problems, there is no upper limit.

[0029] 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.

[0030] [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 resulting 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 1 mole of Ni, an alloy powder of the NiAl 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.

[0031] 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 0.2 mol / min, AlCl3 supply rate of 0.2 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.

[0032] 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.

[0033] [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).

[0034] [Characteristics of alloy powder] Next, the characteristics listed as the effects of the present invention will be described.

[0035] [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 the sintering start temperature 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.

[0036] Here, an experiment was conducted to investigate the state of the sintering start temperature. A compact of 65% Ni and 35% 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 starting position, which is the height of the compact before shrinkage, and the ending position, which is the height of the compact after shrinkage, normalized to compare the change in the amount of shrinkage with temperature.

[0037] 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 65%-Al 35%) of the present invention, shrinkage begins at around 900°C, indicating that the sintering start temperature is high and close to that of the dielectric.

[0038] [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.

[0039] 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%.

[0040] An experiment was conducted to investigate the temperature condition at which this mass increase reaches 0.5%. A compact of 65% Ni and 35% 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 conducted 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.

[0041] In curve 21 of the Ni alloy powder (Ni 65%-Al 35%) of the present invention, the mass gradually increased upon heating, reaching a mass increase of 0.5% at around 400°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]

[0042] The present invention will be further described below based on examples, although the present invention is not limited to the examples described below.

[0043] 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.

[0044] The sintering start temperature was determined by applying pressure to the alloy powder to produce a green compact, then measuring the relationship between the green compact's height (sample height) and temperature as the green compact's temperature was increased. The temperature at which the green compact's height began to decrease was determined as the sintering start temperature. The temperature at which the mass increase reached 0.5% 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.

[0045] [Table 1]

[0046] 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.

[0047] 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]

[0048] 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

[Claim 1] 1. A Ni alloy powder comprising 60.0 to 80.0 mass% Ni, the balance being Al and unavoidable impurities, wherein the Ni alloy powder is observed with a scanning electron microscope (SEM), images are taken, and an average particle size (D50) of 0.04 to 1.00 μm is determined by SEM image analysis of 1,000 to 2,000 measured particles at a magnification of 20,000. The Ni alloy powder has an intermetallic compound NiAl phase, which is either a single phase or contains a phase other than the NiAl phase in addition to the NiAl phase, and when the diffraction peak areas of the individual phases are compared by X-ray diffraction, the diffraction peak area of ​​the NiAl phase is larger than the diffraction peak areas of any of the other phases, and the average crystallite size of the NiAl 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 use in a conductive paste.

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