JPH0769646A - Production of palladium powder - Google Patents

Abstract

PURPOSE:To produce palladium powder as starting material for palladium paste used for forming an internal electrode of a laminated ceramic capacitor especially as powder less liable to expansion and shrinkage at the time of firing. CONSTITUTION:An aq. soln. of a palladium-ammonium complex salt adjusted to <=pH3 is mixed with an aq. soln. of a hydrazine compd. adjusted to <=pH3 at <=50 deg.C and the resulting mixture is heated to >=50 deg.C to deposit palladium powder.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a conductive paste using palladium powder, particularly a palladium powder which is a constituent of a palladium paste used for forming internal electrodes of a laminated ceramic capacitor (hereinafter abbreviated as MLCC). .

[0002]

2. Description of the Related Art As electronic parts become lighter, thinner, shorter and smaller, MLCCs, which are chip parts, are required to be smaller and have higher capacities. The most effective means for downsizing and increasing the capacity of the MLCC is to reduce the thickness of the internal electrodes and the dielectric layer to achieve a multilayer structure. MLCC is barium titanate (B
aTiO ₃ ), etc., and a dielectric green sheet made of an organic binder such as polyvinyl butyral, etc., with a palladium paste printed, dried, and laminated so that internal palladium electrodes are alternately laminated and thermocompression bonded. After that, it is cut and fired at a temperature of about 1300 ° C. to sinter the binder and the internal electrode and the dielectric, and then form an external electrode such as silver to manufacture. Palladium paste, palladium powder as an electrode forming component,
It is made of a cellulose resin, an acrylic resin, an organic binder component such as trimethylbenzene or terbineol as a solvent, and is kneaded and mixed and dispersed by a three-roll mill.

Various methods are known for producing palladium powder. Particularly, the wet reduction method in which a reducing agent such as hydrazine is caused to act on an aqueous solution of a palladium salt to precipitate the palladium powder is costly and has a simple reactor. This is a widely used method. For example, JP-A-3-277706 discloses a combination of an aqueous solution of tetraammine palladium salt and an aqueous solution of a hydrazine compound, and JP-A-1-22570.
No. 8 discloses a combination of a palladium nitrate aqueous solution and an L-ascorbate aqueous solution. When the thermogravimetric measurement (TG-DTA) is performed from room temperature to 1000 ° C. at a constant temperature increase using the palladium powder produced by these methods, PdO starts to be generated at about 300 ° C. or higher during the firing process, and about 700 ° C. It can be confirmed that at 100 ° C., almost 100% of the palladium undergoes a phase change to PdO and returns to the metallic palladium phase again at about 820 ° C. or higher. Further, according to the result of TG measurement of the change in the oxidation rate per hour under the constant temperature of 500 ° C. in air, it can be confirmed that the oxidation of the palladium powder rapidly progresses with time. PdO is a thermodynamically stable phase of PdO at about 820 ° C. or lower in the atmosphere.
This is because metallic palladium is a stable phase at 20 ° C or higher. However, since the conventional palladium powder is sensitive to oxidation, the palladium paste theoretically causes a volume expansion and contraction of 15% due to the oxidation and reduction of the palladium powder as a constituent component during the MLCC production firing process. Such a rapid volume change due to the oxidation of palladium and a rapid heat generation due to the combustion of the polymer resin, which is the binder component of the palladium paste, occur at the interface between the internal palladium electrode and the dielectric layer, causing thermal expansion. It is considered that the stress caused by the difference causes peeling, swelling, and cracking at the interface. This defect is called delamination. In particular, delamination has become a big problem in realizing multi-layered MLCCs. When conventional palladium powder which is sensitive to oxidation is used as a palladium paste for MLCC, delamination occurs in the MLCC manufacturing process. It becomes easy, and it becomes difficult to make the internal electrodes multi-layered and thin. Therefore, palladium powder which is sensitive to oxidation is required as a raw material for palladium paste.

[0004]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and is characterized in that the oxidative expansion and reduction shrinkage that accompany firing are small.
The present invention provides a method for producing palladium powder that can be used as a raw material for palladium paste.

[0005]

The present invention provides a method for producing the above-mentioned palladium powder, which comprises adding an inorganic acid to an aqueous solution of a palladium ammonium complex salt to adjust the pH to 3 or less and to prepare an acidic aqueous solution and a hydrazine compound aqueous solution. Add an inorganic acid to adjust the pH to 3 or less and use an acidic aqueous solution at 50 ° C.
It is characterized in that they are mixed below, and then the mixed solution is heated to 50 ° C. or higher to precipitate palladium powder.

Further, the present invention is characterized in that the mixed solution in the above method is a solution containing a palladium salt in an amount of 0.01 to 0.2 mol / l.

Further, the present invention is characterized in that the molar ratio of the palladium ammonium complex salt to the hydrazine compound is 1 to 10 in the above method.

[0008]

As a result of various experimental studies, the present inventor added an inorganic acid to an aqueous solution of a palladium ammonium complex salt and added p
An acidic aqueous solution in which H is adjusted to 3 or less and an acidic aqueous solution in which an inorganic acid is added to the hydrazine compound aqueous solution to adjust the pH to 3 or less are mixed at 50 ° C. or less to uniformly mix the reducing agent and the to-be-reduced reagent from the beginning. Then, it was found that palladium was uniformly reduced and precipitated by heating the mixed solution to 50 ° C. or higher, and the reaction temperature at the time of precipitation was relatively high. The inventors have found that particles having good crystallinity and little oxidation can be obtained. This method also has the effect of compensating for the drawbacks such as a local concentration gradient or a non-uniform reaction progressing in the conventional wet reduction method, that is, a method of adding a reducing agent to a palladium salt (or vice versa). Play.

In the present invention, a hydrazine compound is used as a reducing agent. Examples of the hydrazine compound include hydrazine monohydrate, hydrazine hydrochloride and hydrazine sulfate. Considering the ease of handling and the cost of the reducing agent, it is preferable to use hydrazine hydrate. Pure water is added to these reducing agents to form an aqueous solution, and an inorganic acid is added to this to prepare an acidic reducing agent aqueous solution. If hydrazine hydrochloride or hydrazine sulfate is used, it is not necessary to add an inorganic acid as long as it has a predetermined pH value. This solution is called solution A. The reducing agents may be used alone or in combination.

The palladium salt is Pd (NH ₃ ) ₂ C
l ₂ , Pd (NH ₃ ) ₂ Br ₂ , Pd (NH ₃ )
₂ Ammonium complex salts such as ₂ I ₂ and Pd (NH ₃ ) ₂ (NO ₃ ) ₂ are used. One or several kinds of these salts are dissolved in inorganic acid water to prepare an acidic aqueous solution of palladium salt. This is designated as solution B. As the inorganic acid, hydrochloric acid, sulfuric acid, nitric acid or the like may be used alone or in combination.

The pH of solution A and solution B is adjusted to 3 or less, and both solutions are mixed at room temperature. The reason for this is that after adjusting the pH values of solution A and solution B at room temperature with an inorganic acid and then examining the pH range in which no reduction reaction occurs when both solutions are mixed, the pH of both solutions is set to 3 or less. It is possible because of that. If the pH value is higher than the above value, the reduction reaction is caused by mixing, which is unsuitable, and the pH value is set as low as possible, preferably 1 to 2, in order to prepare a stable mixed solution. The solution A and the solution B are mixed at a temperature at which a reaction does not occur even if they are mixed, which is 50 ° C. or less, preferably 30 ° C. or less as a result of the experiment. Fifty
At temperatures above ℃, reduction reaction occurs when mixed,
The reducing agent and the reagent to be reduced cannot be uniformly mixed from the beginning.

Next, this mixed solution is held in a constant temperature bath or a constant temperature bath set at 90 ° C. to 100 ° C. with stirring using a stirrer or the like. The number of revolutions of the stirrer is not particularly limited, but as the number of revolutions increases, the particle size tends to become smaller and the particle size distribution tends to broaden, so that it is usually preferable to perform the rotation at about 100 rpm. It is visually confirmed that the liquid temperature of this mixed solution gradually rises, and when the liquid temperature reaches 50 ° C. or higher, the reducing action of the reducing agent gradually occurs and palladium is slowly deposited.

Here, the concentration of the palladium ammonium complex salt is adjusted so that the palladium concentration in the entire reaction solution (A + B) is 0.01 to 0.2 mol / l. This is because if the palladium concentration is lower than 0.01 mol / l, the particles become too small and the yield becomes low in operation, and if it is higher than 0.2 mol / l, the particles become non-uniform palladium particles. . The range of 0.05 to 0.1 mol / l is preferable.

The molar ratio N ₂ H ₄ / Pd of the reaction between the hydrazine compound and the palladium ammonium complex is set in the range of 1 to 10. Even if the molar ratio is larger than 10, the effect of the oxidizing property of the obtained palladium powder does not change and only the reducing agent cost is required. Further, when the molar ratio is 1 or less, the reduction reaction is slow and the powder tends to be aggregated.

When the liquid temperature rises and the reaction starts, the reaction solution slowly becomes turbid in color and becomes cloudy, and foaming by nitrogen gas occurs. After that, the reaction solution is no longer released of gas, and the reaction is continued until the obtained palladium powder precipitates and the reaction solution becomes transparent. Next, solid-liquid separation is performed by a commonly used method such as gravity settling or suction filtration to recover the reduced and precipitated palladium powder, and the solid-liquid separated palladium powder is washed with water to remove the ions, reactions Remove the solution and dry to remove water. The drying atmosphere may be any of vacuum, inert atmosphere and air, and the drying temperature is from room temperature to about 90 ° C. The palladium powder thus obtained is characterized in that the rate of oxidation of palladium that accompanies calcination is slower than that produced by the conventional wet synthesis method, and the particle size range of the obtained particles is Since it is 0.5 to 1.5 μm, it can be used as a raw material for palladium paste.

[0016]

EXAMPLES The present invention will be specifically described with reference to the following examples. Examples 1 and 2 (Experiment No. 1 and 2) The palladium concentration in the reaction solution (A + B) was 0.1 mol.
The reaction solution was adjusted at room temperature by the following operations so that the molar ratio N ₂ H ₄ / Pd was 1,10 so that the ratio became 1 / l. First, 5.11 g each of hydrazine-hydrate, 51.
Hydrochloric acid water and pure water are added to 1 g to bring the total volume to 500 ml.
And the pH was set to 1 (solution A). Next, 1.3m
77% of this solution was used using an aqueous solution of Pd (NH ₃ ) ₄ Cl ₂ ammonia (NH ₃ 3.6 mol / l) at 1 / l.
Hydrochloric acid was added to 6 ml to bring the volume to 500 ml, and the pH was adjusted to 1
Was adjusted to (B solution). Then, from the B solution, fine Pd
The (NH ₃ ) ₂ Cl ₂ salt was precipitated. Mix solution A and solution B at room temperature and put in a constant temperature bath maintained at 90 ° C for 100
Stir at rpm. The liquid temperature becomes 50 in about 15 minutes after holding.
C., reaching 60 ° C. in Example 1, and then Example 2
Then, after passing 55 ° C., a change in the reaction solution gradually started to appear, and the reaction solution became black and Pd was precipitated in about 30 minutes. Stirring was continued for 30 minutes after precipitation.

The obtained palladium powder was recovered by gravity settling, washed with pure water three times, and dried at 80 ° C. in a hot air dryer. The obtained Pd powder was evaluated as follows. For the particle size, the specific surface area value was measured by a specific surface area meter according to the nitrogen gas adsorption method, and the average particle size (BET size) was calculated from the formula (1). Average particle size (μm) = 6 ÷ (12.03 (g / cm ³ ) × specific surface area (m ² / g)) (1)

The oxidation rate is evaluated by a thermal analyzer (TG-DT).
Using A), 20 mg of palladium powder was placed in an alumina cell and set in an apparatus.
n. Then, the temperature of the furnace is maintained at 500 ° C, and then the air is switched to air (200 ml / min.), The change in the oxidation weight is measured, and the air is switched to 200 mi.
n. The subsequent oxidation rate was calculated from equation (2) and compared. Oxidation rate (%) = (W−W ₀ ) ÷ (W ₀ × 0.15) × 100 (2) W _{0 in the} formula (2); initial weight (about 20 mg), W; 200
Weight after holding at 500 ° C. for minutes.

The delamination occurrence frequency test was conducted as follows. The Pd powder obtained in Example 2 was used, ethyl cellulose and terbineol were used as the organic binder, aromatic hydrocarbon was used as the diluent, and the palladium powder / organic binder / diluent (weight ratio) was 50/34.
A palladium paste was prepared as / 16. Using this palladium paste, 20 30-layer multilayer capacitors each having a size of 5 × 3 mm were manufactured. Then, polishing was performed until the internal electrodes could be observed, and the occurrence of delamination was observed with an optical microscope. The dielectric sheet used is composed of barium titanate-based ceramics, and the thickness of one layer is 30 μm. The above results are shown in Tables 1 and 2.

Table 1 Experiment Pd concentration N ₂ H ₄ / Pd pH Average particle size Remark No. mol / l Molar ratio (A + B) (μm) ──────────────────── ───────────────── 1 0.1 1 1.0 0.9 Actual 2 0.1 10 1.0 1.0 0.6 3 0.1 0.1 2.5 1.1 0.8 Application 4 0.2 2.5 1.1 1.1 1.4 5 0.05 2.5 1.0 0.5 Example 6 0.1 2.5 2.1 0.5 0.5 ─────── ───────────────────────────── 7 0.1 2.5 (10 to 10.5) 0.6 Comparative example

Table 2 Experiment No. Oxidation rate% Delamination frequency Remark ─────────────────────────────────── ─ 1 20 − − Actual 2 23 1/20 (5%) 3 20 0/20 (0%) Application 4 16 − − 525 − − Example 6 27 −− ───────────── ─────────────────────── 7 45 9/20 (45%) Comparative example

Examples 3 to 5 (Experiment Nos. 3 to 5) The palladium concentration in the reaction solution (A + B) was 0.1, 0.
₂ , molar ratio N ₂ H ₄ /
The reaction solution was adjusted by the following operations so that Pd was 2.5. Same as Examples 1 and 2 Pd (NH ₃ ) ₄ Cl ₂
Aqueous ammonia solution 77.6, 155.3 and 38.8, respectively
Hydrochloric acid water was added to ml to make the volume 500 ml and the pH was adjusted to 1 (solution B). Also, hydrazine hydrate 12.
Hydrochloric acid water and pure water were added to 8 g to make the liquid volume 500 ml, and p
H was set to 1 (Solution A). The solution A and the solution B were mixed at room temperature and placed in a constant temperature bath maintained at 90 ° C. and stirred at 100 rpm. In Examples 3 and 4, after passing 60 ° C., in Example 5, after passing 70 ° C., gradually. A change began to appear in the reaction solution, the reaction solution gradually became black, and palladium powder was deposited. Stirring was continued for 30 minutes as it was after the deposition of the palladium powder, and the operations thereafter were performed in the same manner as in Example 1. The evaluation was performed in the same manner as in Example 1, and the delamination test was performed on the powder obtained in Example 3. The above results are shown in Tables 1 and 2.

Example 6 (Experiment No. 6) The palladium concentration in the reaction solution (A + B) was 0.1 mol.
/ L so that the molar ratio N ₂ H ₄ / Pd is 2.5, hydrochloric acid water is added to 77.6 ml of the same Pd (NH ₃ ) ₄ Cl ₂ solution as in Examples 1 and ₂ to obtain a liquid volume. Solution of which pH was adjusted to 2 with 500 ml and hydrazine hydrate 1
Hydrochloric acid water and pure water were added to 2.8 g to make the liquid volume 500 ml, and the A solution whose pH was set to 2 was mixed at room temperature to 90 ° C.
When the mixture was placed in a constant temperature bath maintained at 100 rpm and stirred at 100 rpm, the reaction solution gradually became black after 52 ° C. and palladium powder was deposited. Stirring was continued for 30 minutes from the precipitation of palladium powder, and the subsequent operation was performed in the same manner as in Example 1. The evaluation was performed in the same manner as in Example 1, and the delamination test was performed on the powder obtained in Example 3. The above results are shown in Tables 1 and 2.

Comparative Example 1 (Experiment No. 7) The palladium concentration in the reaction solution (A + B) was 0.1 mol.
/ L such that the molar ratio N ₂ H ₄ / Pd is 2.5, and 77.6 ml of the above Pd (NH ₃ ) ₄ Cl ₂ solution
Pure water was added to the solution B to make the volume 500 ml, and pure water was added to 12.8 g of hydrazine hydrate to make the volume 500.
A solution A made up in ml was prepared, and both were heated to 60 ° C. Here, the pH of the solutions A and B was in the range of 10 to 10.5 because no acid was added. When the B solution was directly added to the A solution stirred at a rotation speed of 100 rpm for about 5 seconds, violent foaming occurred immediately after the B solution was added and the reaction solution became black turbid. The stirring was continued for 30 minutes from the end of the addition, and the operations thereafter were performed in the same manner as in Example 1. The evaluation and the delamination test were performed in the same manner as in Example 1. The above results are shown in Tables 1 and 2.

[0025]

From the above results, the palladium powder obtained according to the present invention is characterized in that the rate of oxidation of palladium which accompanies calcination is slow. Therefore, this powder is used as a palladium paste for MLCC internal electrodes. In this case, it is effective in preventing delamination which is the largest defect of MLCC at the interface between the internal palladium electrode and the dielectric layer.

Claims (3)

[Claims] 1. An acidic aqueous solution in which an inorganic acid is added to an aqueous solution of a palladium ammonium complex salt to adjust the pH to 3 or less, and an acidic aqueous solution in which an inorganic acid is added to an hydrazine compound aqueous solution to adjust the pH to 3 or less is 50 ° C. or less. And then heating the mixed solution to 50 ° C. or higher to precipitate the palladium powder. 2. The mixed solution comprises 0.01 to about palladium salt.
The method for producing palladium powder according to claim 1, which is a solution containing 0.2 mol / l. 3. The method for producing a palladium powder according to claim 1, wherein the molar ratio of the palladium ammonium complex salt to the hydrazine compound is 1 to 10.