JPH08699B2 - Synthesis of oxide fine particles by spray drying - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for synthesizing a fine particle raw material of a bismuth-based superconductor.

"Prior art and its problems" Bismuth-based superconductors have a high critical temperature Tc of about 110 K compared to yttrium-based superconductors, and can take a large temperature difference when liquid nitrogen is used as a cryogen, and yttrium-based liquid It has the advantage that it is easily decomposed in an atmosphere containing it, but it has high chemical stability, and is being researched for practical use.

However, in the currently known bismuth superconducting phase, there is a low Tc phase (80K in the same) other than the so-called high Tc phase showing Tc = about 110K that is expected to be put into practical use, and in the normal synthesis process It is said that a low Tc phase is generated first, and then a high Tc phase is generated after this is decomposed, and once the crystal structure of the low Tc phase is made strong, it will take several tens in the atmosphere until the high Tc phase is synthesized. ~ It takes several hundred hours, and even under low oxygen partial pressure (PO ₂ = 1/5 to 1/13), where the high Tc phase generation temperature range spreads, 10 to several 10
It took time. In addition, compositional non-uniformity is likely to occur due to phase separation due to liquid phase formation in the middle of the reaction, while high Tc
The formation of the phase was accompanied by a diffusion reaction involving a liquid phase component, the crystal grain growth was remarkable, and only coarse crystal grains having a particle diameter of several tens to several μm were obtained. For this reason, it is difficult to densify the sintered body at the time of preparation, and it is unavoidable to increase the sintered body density by a method such as pressure sintering.

In addition, according to widely used methods such as the solid phase method and the oxalate coprecipitation method, carbonates and the like tend to be generated or remain in the stage before the formation of the low Tc phase. This has been a cause of the formation of a phase and the formation of a hardly decomposable compound and grain growth due to the nonuniform composition described above.

For these reasons, it was impossible to synthesize fine particles having a high Tc phase and having a particle diameter of several μm or less. For this reason, it is not possible to obtain the easily sinterable homogeneous fine particle raw material required in the ordinary ceramics manufacturing process, and it is difficult to improve the bulk density and homogenize by the atmospheric pressure sintering method, which is a major obstacle to the improvement of the critical current density. It was.

[Summary of the Invention] The object of the present invention is to overcome this problem and to synthesize a bismuth-based high Tc phase and a fine particulate raw material that is easily sinterable.

The present inventor, in the process of synthesizing homogeneous fine particles of a bismuth-based superconductor by the spray-drying method, the generated precursor powder is calcined by gradually increasing the temperature from a low temperature to about 800 ° C. It is easy to split into complex oxides and carbonates,
This causes a liquid phase to be generated when the temperature is further raised, which causes rapid particle growth and composition non-uniformity, which impairs the characteristics of the homogeneous fine particle dry powder prepared by the spray drying method.
In the temperature range of ~ 835 ℃ (in the air), the crystallinity of the low Tc phase becomes high, and it is difficult to generate the high Tc phase, and it passes through the temperature range where these phenomena occur in an extremely short time, High Tc
It was found that high Tc phase can be synthesized without causing phase separation or grain growth by conducting thermal decomposition to crystallization in the phase stable region. Further, the present invention has been reached by further studying that starting from a precursor having high homogeneity facilitates thermal decomposition-crystallization reaction due to solid phase diffusion in the course of the process.

That is, the present invention is to prepare precursor fine particles with high homogeneity by a spray drying method of a nitrate solution, and to directly charge the precursor fine particles into a reaction furnace kept at a predetermined temperature.
Sample temperature of 835-845 ℃ (in air) for 10 seconds to 1 minute
By setting the temperature range to, the high Tc phase is promptly crystal-grown to prevent the composition from becoming nonuniform and the grain growth to be generated, and the bismuth-based superconductor high Tc phase homogeneous fine particles are generated. The present invention will be described in detail below.

First, a nitrate solution is prepared in which the composition ratio of each element of bismuth, lead, strontium, calcium, and copper is adjusted within the above composition range in which the high Tc phase is formed as a single phase in the bismuth system.

This is sprayed by compressed air into high temperature air of about 200 ° C. using a two-fluid nozzle or the like, and is instantaneously dried to prepare micron-order dry fine particles in which the homogeneity of each component element in the solution is maintained.

Place this in a container such as a platinum crucible and immediately
It is installed in a reaction furnace set at 45 ° C, or is set in the above temperature range at a heating rate of at least about 15 ° C / sec or more by using a method such as image heating. By firing as it is for 1 to 2 hours, a powder composed of a high Tc phase having almost no peaks of a low Tc phase or an insulator phase as shown in FIG. 1 can be obtained. The oxygen partial pressure (PO ₂ ) is lower than in the atmosphere (PO ₂ = 1 /
In the case of 5 to 1/13), the firing temperature is lowered by 5 to 15 ° C and the optimum temperature range is slightly expanded. When the solution concentration is 0.01 mol / l, the particles have an average particle size of about 1 to 3 μm as shown in FIG.
It is also possible to obtain particles of 1 μm or less by further reducing the solution concentration.

 The main points in each process will be described below.

The composition range of the nitrate solution of is the above-mentioned composition which is said to be capable of obtaining a single phase of bismuth-based superconductor high Tc phase at present. However, due to changes in composition conditions, there are uncertain points such as changes from the feed composition ratio due to the behavior of volatile components such as lead. As the sample solution, in addition to nitrate solutions, organic acid salt solutions such as citrate and co-precipitation slurry solutions such as oxalate may be considered, but intermediate compounds such as carbonates are generated in the thermal decomposition process after spray drying. Since it becomes easier, the nitrate solution is limited here. In this process, it is necessary to heat the collection chamber with a heater during spray drying, considering that the dried particles of nitrate are very hygroscopic at room temperature.
The obtained dry particles need to be immediately processed in the next process or stored in a vacuum desiccator or the like. In this case, since it is necessary to keep the sample in the temperature range of 835 to 845 ° C (in the air) as quickly as possible, use an electric furnace with a large capacity, for example, a platinum crucible and a container with high thermal conductivity and small reactivity with the sample. Place the powder as it is, and quickly place it in the above temperature range. A predetermined temperature is reached in a few tens of seconds, so that pyrolysis-crystallization is performed as it is. Since this system has a melting point close to the crystallization temperature of the high Tc phase, the temperature control is required to be highly accurate in order to suppress grain growth due to liquid phase formation. The crystal phase shown in FIG. 1 is obtained by holding for 1 to 2 hours. Since the cooling process suppresses the decomposition of the high Tc phase, it is desirable that the cooling process be performed under a temperature lowering condition than the in-furnace cooling.

In the conventional method, for example, when the heating rate is about 200 ° C./hour, the reaction process goes through the route of precursor → intermediate phase → semiconductor phase → low Tc phase → high Tc phase, and therefore the decomposition-synthesis reaction is repeated. Therefore, a long reaction time is required because the composition becomes non-uniform in the process.

In the present invention, the decomposition reaction is facilitated by increasing the heating rate to prevent sufficient crystallization of the reaction phase generated in the course of forming the high Tc phase. As a result, the next step reaction easily occurs. For example, when the heating rate is set to about 1 ° C./sec, the formation of the intermediate reaction phase up to the semiconductor phase is suppressed, but the crystallization progresses sufficiently at the low Tc phase generation stage, so the low Tc phase -> Decomposition of high Tc phase will be inhibited.

When the heating rate of about 15 ° C./second is used as in the present invention, the low Tc phase generation reaction and (due to the decomposition reaction of the low Tc phase)
Since the production reaction of the high Tc phase is under the temperature condition in which the production reactions of the high Tc phase occur simultaneously in parallel, the production reaction of the high Tc phase efficiently progresses, and as a result, the effect as in the present invention is obtained.

However, as a precondition for producing such a reaction process, a high homogeneity of the precursor substance is required to promote the solid phase diffusion reaction, and as the raw material used in, the method described in and is used. Need that.

"Example" Next, an example is given and the present invention is explained.

A nitrate solution (solution concentration: 0.01 mol / l) prepared in a composition ratio of Bi: Pb: Sr: Ca: Cu = 1.84: 0.34: 1.91: 2.03: 3.06 was spray-dried under the following conditions. Drying chamber inlet temperature 195 ° C, outlet temperature 90 ° C, spraying pressure 1.2 kgf / cm ² , cyclone suction amount 0.
45m ³ / min, sample supply rate 10cm ³ / min. The dried powder thus obtained was dried in a vacuum for 24 hours and then enclosed in a platinum crucible,
It was installed in a large tubular furnace at ℃, heated at an average of 17 ℃ / sec, baked for 90 minutes and taken out of the furnace. The obtained powder showed an X-ray diffraction pattern of a high Tc phase single phase, and became plate-like particles having an average particle diameter of about 1.8 μm. A compact with a pressure of ² t / cm ² was formed using the raw material fine particles, and sintered at 845 ° C. for 12 hours (cold pressing again during the process) to obtain a sintered compact with Tc = 105K. . The density of the sintered body is 5.95 g / cm ^3, which is 90 of the theoretical density.
% Or more.

[Brief description of drawings]

Fig. 1: X-ray diffraction pattern of bismuth-based superconductor particles showing a high Tc phase obtained by the present invention (calcined at 842 ° C for 90 minutes in the atmosphere. "(0011)" in the figure is an index for high Tc phase. The surface index of each peak is shown). FIG. 2: Photograph of the particle structure of a bismuth-based superconductor high Tc phase fine particle obtained by the present invention by a scanning electron microscope image (solution concentration 0.01 mol / l, 842 ° C. 90 minutes baking).

Claims (1)

[Claims] 1. A bismuth-based superconductor composed of bismuth, lead, strontium, calcium, and copper, wherein (Bi, P
b): Sr: Ca: Cu: = around 2: 2: 2: 3 with a predetermined composition ratio, that is, a crystalline phase with a critical temperature Tc = about 110 K (so-called high Tc phase), that is, the general formula Bi _U Pb _U Sr _W Ca _X Cu _Y O _Z spray-dried a nitrate solution having a cation composition ratio of 1.4 ≤ u ≤ 1.9 0.1 ≤ v ≤ 0.8 1.8 ≤ w ≤ 2.0 1.95 ≤ x ≤ 2.3 3.0 ≤ y ≤ 3.3 Then, by rapidly heating the obtained dry powder at a temperature rising rate of 15 ° C / sec or more, the sample temperature is rapidly increased to 835 to 845 ° C in the atmosphere, and the temperature is further maintained for 1 to 2 hours. Almost Tc = 110K class superconductor (high Tc phase) by heat treatment within
A method for synthesizing bismuth-based high-temperature superconducting ceramic fine particles, which comprises powder having the crystal structure of 1 and having a particle diameter of several μm to 1 μm or less.