JP4147657B2 - Nickel powder for internal electrode paste of multilayer ceramic capacitor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to nickel powder used as a paste for internal electrode material of a multilayer ceramic capacitor.
[0002]
[Prior art]
At present, electronic components have been rapidly miniaturized as electronic devices have been miniaturized. In such a situation, multilayer ceramic capacitors (MLCCs) are used in large quantities as small-sized and high-capacitance capacitors. Conventionally, noble metals such as palladium and platinum have been mainly used as internal electrode materials for such multilayer ceramic capacitors.
However, the number of laminated layers has increased due to the increase in the capacity of capacitors, and there is a problem that the use of the above-mentioned precious metal powder increases the cost. Recently, nickel powder is frequently used as an internal electrode material to reduce the cost. Has been.
[0003]
Nickel powder used as an internal electrode material is dispersed in a binder to form a paste. This paste is printed and applied onto a substrate, stacked and pressure-bonded, and fired at about 1300 ° C. in a reducing atmosphere to form an electrode. , To exhibit the characteristics as a capacitor.
Normally, the thickness of the internal electrode is 2 to 3 μm after firing, but in recent years, it has become necessary to form a thinner electrode with the progress of higher capacity and smaller size of the capacitor.
However, the packing density of the nickel powder in the nickel coating is much lower than the packing density of the compact in powder metallurgy, and the amount of shrinkage associated with the sintering of the ceramic green sheet used as the substrate is smaller than that of the nickel electrode film. Therefore, there was a problem that the nickel film was cut into islands as the sintering progressed. If the nickel electrode breaks in this way, it will not function as a capacitor. Therefore, in order to reduce the size and increase the capacity of the capacitor, it is possible to form a dense and thin electrode while minimizing the shrinkage during sintering. It is essential.
[0004]
Another problem when the electrode is thinned is a decrease in reliability due to dielectric breakdown. This is often caused by a decrease in the interlayer distance between the protrusions of the nickel electrode, and the thinner the layer, the thinner the dielectric layer, and the more easily affected by the protrusions on the nickel electrode surface. is there.
[0005]
[Problems to be solved by the invention]
In the manufacturing process of the multilayer ceramic capacitor, the present invention prevents a function stop as a capacitor due to electrode interruption that occurs when the nickel electrode thickness is reduced, and a decrease in life and reliability due to the formation of protrusions on the surface of the nickel electrode. An object of the present invention is to provide nickel powder for internal electrode paste of a multilayer ceramic capacitor which is effective for the above.
[0006]
[Means for Solving the Problems]
In order to solve the above problems, the present invention is a nickel powder containing at least one of Ti and Si at least 0.02 to 1% by mass on the surface thereof,
1) The nickel powder has an oxidation weight increase rate of less than 15% when held in air at 400 ° C. for 2 hours,
2) The nickel powder has a crystallite size determined by an XRD method of 300 angstroms or more,
3) The true specific gravity of the nickel powder powder is 8.5 or more, and
4) In the SEM photograph observation of the nickel powder, particles having a particle size of twice or more of the average particle size are 1/100 or less of the total,
It is characterized by being.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Next, the present invention will be specifically described.
The present invention has the following configuration.
(1) Oxidized weight increase rate when kept in air for 2 hours is less than 15%;
Nickel powder is mixed with a resin serving as a binder to form a paste, which is coated on a dielectric sheet and dried. After this, the sheets are laminated and enter the firing step, but a step of removing the binder component is incorporated in the initial stage of firing. In most cases, the debinding method is such that the resin is decomposed and volatilized by heating at 400 ° C. or lower in an oxidizing atmosphere, but even the nickel powder is oxidized at this time.
The oxidized nickel powder surface is reduced again in the subsequent sintering step under a reducing atmosphere, and at that time, extremely fine nickel powder of 0.1 μm or less is produced. Such fine nickel powder has the function of lowering the starting temperature of the sintering, and as a result, a large time difference occurs between the contraction of the electrode and the dielectric layer during sintering, resulting in cracks and other structures Defects are likely to occur.
[0008]
The inventors used the rate of increase in oxidized weight when kept at 400 ° C. in air for 2 hours as an index of oxidation resistance of nickel powder. In the binder removal step in an oxidizing atmosphere in which this value is 15% or more, cracks occur. On the other hand, the occurrence of cracks is extremely reduced in an oxidizing atmosphere of less than 15%, preferably less than 13%, and the rate of increase in oxidized weight is determined by the X-ray diffraction (XRD) method of nickel powder. It was closely related to the child size, and it was found that the oxidation weight increase rate was less than 15% when the size was 300 angstroms or more.
[0009]
Conventionally, it has been known that a nickel powder having a large particle diameter obtained by a vapor phase method is suitable as a nickel powder having high oxidation resistance, and the nickel powder obtained by the vapor phase method is obtained by an XRD method. The crystallite size is 1000 angstroms or more. The nickel powder having a large crystallite size obtained by this method was said to have high oxidation resistance because it has few crystal grain boundaries and the progress of internal oxidation due to oxygen grain boundary diffusion is slow. Since the surface area is large, the rate of increase in oxidized weight when kept in air at 400 ° C. for 2 hours is 15% or more, and the problem that cracks are likely to occur depending on the oxygen partial pressure at the time of debinding remains as it is. .
[0010]
On the other hand, although the crystallite size of the nickel powder obtained by the usual wet reduction method of nickel hydroxide varies depending on the average particle diameter, the size obtained by the XRD method is as small as 100 to 500 angstroms, and is originally used as the nickel powder used in the present invention. It was considered unsuitable. However, when the crystallite size of the nickel powder obtained by the wet reduction method is less than 300 angstroms, the crystallite size can be reduced by carrying out crystal growth by heating and holding, preferably by the method described in Japanese Patent Application No. 10-268160. Can be made 300 angstroms or more, and it has been found that the effect of improving the oxidation resistance can be realized more reliably.
Crystal growth can also be realized by heating and holding at 400 to 600 ° C. in a non-oxidizing atmosphere after the addition treatment of Ti and / or Si.
[0011]
(2) containing 0.02% by mass to 1% by mass of one or both of Ti and Si;
If the crystallite size is 300 angstroms or more, the larger the crystallite size, the better. However, even with a large crystallite size nickel powder, the above problem cannot be overcome unless the oxidation resistance of the particle surface is increased. The present inventors have found that the oxidation resistance of the nickel powder particle surface is improved by allowing 0.02% by mass to 1% by mass of either or both of Ti and Si on the nickel particle surface. .
The effect of improving the oxidation resistance by adding Ti and / or Si is that a nickel oxide layer containing Ti and Si is formed on the particle surface due to the presence of these elements in the vicinity of the particle surface. It is estimated that progress can be suppressed.
These elements are added by suspending nickel powder in an aqueous solution, adding an organic compound containing Si or an organic compound containing Ti, and admixing the element to the surface of the nickel powder, followed by filtration and drying. Can be obtained.
[0012]
If the addition amount of Ti or Si is singly or in total less than 0.02% by mass, the above effect cannot be obtained. On the other hand, if it exceeds 1% by mass, nickel sintering may be inhibited, which is not preferable. . In particular, when the amount of Ti is large, there is a risk of changing the composition of the dielectric material of barium titanate as a counterpart, and the designed dielectric constant cannot be obtained.
[0013]
(3) The true specific gravity of the powder is not less than 8.2 ;
It was found that when the true specific gravity of the nickel powder exceeded 8.2, the shrinkage during sintering was reduced and breakage did not easily occur even with a 1 μm electrode. The higher the true density of nickel powder, the more advantageous .
(4) The ratio of particles having a particle size of at least twice the average particle size in SEM photo observation is 1/100 or less;
[0014]
As nickel powder with high true density, nickel powder having a large crystallite size by a vapor phase method is known. However, this nickel powder cannot avoid mixing particles having a size more than twice the average particle size due to the process characteristics.
When the electrode is thinned, these coarse particles cause the projection of the electrode, and the protruding portion of the electrode directly shortens the interlayer distance, so that dielectric breakdown is likely to occur, reducing the life and reliability of the capacitor. Therefore, it is not preferable. Further, the thinner the layer is, the thinner the dielectric thickness becomes, so that it becomes more sensitive to electrode protrusions. In general, the size of the coarse particles in question should not exceed the thickness of the designed electrode, but in practice, even if the size is smaller than that, the difference from the average particle size When many large particles are present, they often cause protrusions. As a result of the study, the present inventors have found that many particles having a particle size more than twice the average particle size among the particles contained in the nickel powder are likely to cause electrode protrusions, in other words, the average particle size It has been found that dielectric breakdown in the electrode can be prevented when the particles having a particle size of 2 times or more are less than 1/100 of the total.
[0015]
Practically, since the thickness of the electrode is 2 to 3 μm after firing, the average particle diameter by SEM photograph observation is 1 μm or less, and the amount of particles having a particle diameter of 2 μm or more, which is twice that, is minimized. Specifically, it has been found that this problem can be solved by setting it to 1/100 or less.
[0016]
【Example】
[Example 1]
Nickel powder (Sumitomo Metal Mining Co., Ltd.) having a crystallite size of 800 angstroms is suspended in an aqueous solution and a titanate coupling agent (KR-ET manufactured by Ajinomoto Co., Inc.) is added, pH 7 The nickel powder having a Ti content of 500 ppm was obtained by stirring and holding at room temperature to adsorb Ti.
This nickel powder has a true specific gravity of 8.5, an average particle size of 0.56 μm by SEM photographic diagonal method, no presence of particles of 1 μm or more, and an oxidized weight when kept at 400 ° C. in air for 2 hours. The increase rate was 11%.
The nickel powder was used to investigate the shrinkage behavior at 1000 ° C. by thermomechanical analysis (TMA) and observe the electrode state after firing the coating film. The results are shown in Table 1 below.
[0017]
[Example 2]
Nickel powder (made by Sumitomo Metal Mining Co., Ltd.) with a crystallite size of 800 angstroms is suspended in an aqueous solution, and an ethanol solution of γ-glycidoxypropyltrimethoxysilane, which is a kind of silane coupling agent, is added. The mixture was stirred and held at pH 10 and 70 ° C., and Si was adsorbed to obtain nickel powder having a Si content of 500 ppm.
This nickel powder has a true specific gravity of 8.8, an average particle diameter of 0.56 μm by SEM photographic diagonal method, and no particles larger than 1 μm are observed, and an oxidized weight when kept at 400 ° C. in air for 2 hours. The increase rate was 8%.
Using this nickel powder, the shrinkage behavior at 1000 ° C. by TMA and the electrode state after firing the coating film were observed. The results are also shown in Table 1 below.
[0018]
[Example 3]
A nickel powder having a Ti content of 500 ppm was obtained in the same manner as in Example 1 except that a nickel powder having a crystallite size of 150 Å was used. This nickel powder was heated and held at 600 ° C. for 1 hour in an air stream of 2% hydrogen and 98% nitrogen, and the crystallite size was set to 900 Å. This nickel powder has a true specific gravity of 8.5, an average particle size of 0.58 μm by SEM photographic diagonal method, no presence of particles of 1 μm or more, and an oxidized weight when held at 400 ° C. in air for 2 hours. The increase rate was 13%.
Investigation of the shrinkage behavior of this nickel powder by TMA at 1000 ° C. and the electrode state after firing the coating film were observed. The results are also shown in Table 1 below.
[0019]
[Comparative Example 1]
When the crystallite size is 700 angstroms, the true specific gravity is 7.9, the average particle diameter is 0.56 μm by the SEM photograph diagonal line method, and the presence of particles of 1 μm or more is not observed, and when kept at 400 ° C. in air for 2 hours Investigation of shrinkage behavior at 1000 ° C by TMA and electrode state after coating film firing using nickel powder (Sumitomo Metal Mining Co., Ltd.) with an increase in oxidized weight of 20% and a Ti and Si content of less than 10 ppm Was observed. The results are also shown in Table 1 below.
[0020]
[Comparative Example 2]
The crystallite size is 800 angstroms, the true specific gravity is 8.5, the average particle size of primary particles by SEM photo diagonal method is 0.57 μm, contains many secondary aggregates of 1 μm or more, and kept in air at 400 ° C. for 2 hours. Of the shrinkage behavior at 1000 ° C. by TMA and the electrode after firing the coating film using nickel powder (manufactured by Sumitomo Metal Mining Co., Ltd.) having an increase in oxidized weight of 11% and a Ti content of 500 ppm. The condition was observed. The results are also shown in Table 1 below.
[0021]
[Comparative Example 3]
When the crystallite size is 700 angstroms, the true specific gravity is 8.5, the average particle size is 0.56 μm by SEM photograph diagonal line method, and the presence of particles of 1 μm or more is not observed, and when kept in air at 400 ° C. for 2 hours Investigation of shrinkage behavior at 1000 ° C. by TMA and electrode state after coating film firing using nickel powder (Sumitomo Metal Mining Co., Ltd.) with an increase in oxidized weight of 20% and a Ti and Si content of less than 10 ppm Was observed. The results are also shown in Table 1 below.
[0022]
[Table 1]
[0023]
As can be seen from Table 1, in the examples of the present invention, the shrinkage rate at 1000 ° C. was 15% or less, no crack was generated, and there was no breakage of the nickel electrode. On the other hand, in Comparative Examples 1 and 2, the shrinkage ratios were as high as 20% and 25%, and cracks occurred and the nickel electrodes were interrupted. In Comparative Example 3, the shrinkage ratio was 15%, but the nickel electrodes were interrupted. It has occurred.
[0024]
【The invention's effect】
As described above, according to the present invention, in the production process of a multilayer ceramic capacitor, it is possible to prevent a function stop as a capacitor due to electrode breakage and a decrease in lifetime and reliability due to dielectric breakdown, and an extremely effective multilayer ceramic capacitor. The nickel powder for internal electrode paste can be provided.

Claims (1)

A nickel powder containing at least one of Ti and Si at least 0.02 to 1% by mass on the surface thereof,
1) The nickel powder has an oxidation weight increase rate of less than 15% when held in air at 400 ° C. for 2 hours,
2) The nickel powder has a crystallite size determined by an XRD method of 300 angstroms or more,
3) The true specific gravity of the nickel powder powder is 8.5 or more, and
4) In the SEM photograph observation of the nickel powder, particles having a particle size of twice or more of the average particle size are 1/100 or less of the total,
Nickel powder for internal electrode paste of multilayer ceramic capacitor, characterized in that