JPH02229722A - Method for synthesizing oxide fine particle by spray drying - Google Patents

Abstract

PURPOSE:To obtain Bi-based high-temp. superconducting ceramic fine particles consisting of a crystal structure of a high-Tc phase by spray-drying a nitrate soln., directly charging the obtained Bi-based superconductor starting material into a reaction furnace and heat-treating the material under specified conditions. CONSTITUTION:A nitrate soln. having a cationic composition shown by the general formula, BiuPbvSrwCaxCuyOz, is prepared (where u=1.4-1.9, v=0.1-0.8, w=1.8-2, x=1.95-2.3 and y=3-3.3). The soln. is spray-dried, and the dried powder is charged directly into the reaction furnace. The powder is heated at the rate of >=15 deg.C/sec, and then heat-treated in the atmosphere at 835-845 deg.C for 1-2 hr. Consequently, the fine particles of the micron order in the ratio of about (Bi, Pb):Sr:Ca:Cu=2:2:2:3 consisting of a crystal structure of a high-Tc crystal phase having >=110K Tc is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a method for synthesizing fine particle raw materials for bismuth-based superconductors. "Prior art and its problems" Bismuth-based superconductors have a higher critical temperature Tc of about 110 K than yttrium-based superconductors, and for example, when liquid nitrogen is used as a cryogen, a large temperature difference can be maintained, and yttrium-based superconductors can absorb moisture. It has advantages such as high chemical stability, whereas it easily decomposes in atmospheres containing it, and research on practical application is progressing.

However, it is currently known that bismuth-based superconducting phases include, in addition to the so-called high Tc phase with a Tc of approximately 110K, which is expected to be put into practical use, there are also low Tc phases (80K), which cannot be achieved through normal synthesis processes. It is said that a low Tc phase is formed first, and then a high Tc phase is formed after this decomposes. It takes ~ several hundred hours under low oxygen partial pressure (P02
= 1/5 to 1/13), it took 10 to several 10 hours. In addition, the composition tends to become non-uniform due to phase separation accompanying liquid phase formation in the intermediate reaction process, while the formation of a high Tc phase is accompanied by a diffusion reaction involving liquid phase components, resulting in grain growth. was remarkable, and only coarse crystal grains with particle diameters of several tens to several micrometers were obtained. For this reason, it is difficult to make the sintered body dense when producing it, and it is necessary to increase the density of the sintered body by methods such as pressure sintering.

Furthermore, according to the solid-phase method and the oxalate coprecipitation method that have been widely used, carbonates are likely to be generated or remain at a stage before the formation of the low Tc phase, and this is likely to occur below the superconducting phase synthesis temperature. This produced a liquid phase, which caused the formation of refractory compounds and grain growth due to the non-uniform composition mentioned above. For these reasons, it has been impossible to synthesize fine particles consisting of a high Tc phase and having a particle size of several μm or less. For this reason, it is not possible to obtain homogeneous fine particle raw materials that are easy to sinter, which is required in the normal ceramic manufacturing process, and it is difficult to improve bulk density and homogenize by pressureless sintering, which is a major obstacle to improving critical current density. It became. ``Summary of the Invention'' The purpose of the present invention is to overcome this problem and synthesize a homogeneous fine particle raw material that is easily sinterable in a bismuth-based high Tc phase. In the process of conducting research on the synthesis of homogeneous fine particles of bismuth-based superconductors using a spray drying method, the present inventor discovered that the normal method of calcining the generated precursor powder by gradually increasing the temperature from a low temperature to around 800℃. It is easy to phase separate into double oxides and carbonates,
When the temperature is further increased, a liquid phase is generated, resulting in rapid particle growth and uneven silk formation, which impairs the properties of the homogeneous fine dry powder prepared by the spray drying method.
We focused on the fact that in the temperature range of 0 to 835°C (in the atmosphere), the crystallinity of the low Tc phase becomes high, making it difficult to form a high Tc phase, and we passed through the temperature range where these phenomena occur in an extremely short period of time. We also found that a high Tc phase can be synthesized without phase separation or grain growth by performing thermal decomposition and crystallization in the high Tc phase stability region. The present invention was also achieved by considering that by starting from a highly homogeneous precursor, thermal decomposition and crystallization reactions occur easily due to solid-phase diffusion during the process. That is, in the present invention, highly homogeneous dry fine particles of a precursor are prepared by spray drying a nitrate solution, and then the particles are directly introduced into a reactor kept at a predetermined temperature.
By setting the sample temperature in the temperature range of 835 to 845 °C (in the atmosphere) for about 0 seconds to 1 minute, the high Tc phase can be rapidly grown into crystals, preventing compositional non-uniformity and grain growth. ,
It is characterized by producing homogeneous fine particles of bismuth-based superconductor high Tc phase. The present invention will be explained in detail below.

(1) First, a nitrate solution is prepared in which the composition ratios of the elements bismuth, lead, strontium, calcium, and copper are adjusted to the above-mentioned composition range in which a bismuth-based high-Tc phase is generated as a single phase.

■This is sprayed with compressed air into high-temperature air at around 200°C using a two-fluid nozzle, etc., and dried instantly to produce dry fine particles on the order of microns that maintain the homogeneity of each component element in the solution. .

■Put this in a platinum crucible container and immediately add 835
The reactor is placed in a reactor set at ~845°C, or the temperature is set in the above temperature range at a heating rate of at least about 15°C/second using a method such as image heating. 1~
By firing for 2 hours, as shown in FIG. 1, a powder consisting of a high Tc phase with almost no peaks of the low Tc phase or the insulating phase can be obtained. Note that when the oxygen partial pressure (P02) is lower than that in the atmosphere (P02 = 1/5 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/I, the particles have an average particle diameter of about 1 to 3 μm, as shown in FIG. 2, and by further decreasing the solution concentration, it is also possible to obtain particles of 1 μm or less.

The main points in each process are explained below. The composition range of the nitrate solution in (2) is the above-mentioned composition that is currently believed to be able to obtain a single phase of bismuth-based superconductor high Tc phase.

However, due to changes in firing conditions, there are some unclear points such as changes in the composition ratio due to the behavior of volatile components such as lead.

In addition to nitrate solutions, sample solutions may include solutions of organic acid salts such as citrate, co-precipitation slurry solutions such as oxalate, etc., but intermediate compounds such as carbonates are generated during the thermal decomposition process after spray drying. Here, we limited ourselves to nitrate solutions because they are easier to use. In process (2), it is necessary to heat the collection chamber with a heater during spray drying, taking into account that dried nitrate particles are highly hygroscopic at room temperature. The obtained dry particles must be immediately processed in the next process or stored in a vacuum desiccator or the like.

In case ①, it is necessary to keep the sample in the temperature range of 835-845℃ (in the atmosphere) as quickly as possible, so for example, use a large-capacity electric furnace and use a platinum crucible, which has high thermal conductivity and is less reactive with the sample. Place the powder in a small container and place it in the above temperature range immediately. Since the predetermined temperature is reached in several tens of seconds, thermal decomposition and crystallization are carried out as is. Since the crystallization temperature and melting point of this system are close to the crystallization temperature of the high Tc phase, high precision temperature control is required to suppress grain growth due to liquid phase formation. By holding for 1 to 2 hours, the crystal phase shown in FIG. 1 is obtained. In order to suppress the decomposition of the high Tc phase, the cooling process is desirably carried out under conditions where the temperature is lowered more rapidly than in the furnace cooling.

"Example" Next, the present invention will be explained with reference to Examples.

Bi:Pb:Sr:Ca:Cu=184:0,34:
A nitrate solution prepared to have a composition ratio of 1.91:2.03:3.06 (solution concentration=0.01 mol/I) was spray-dried under the following conditions. Drying chamber temperature: 195°C, outlet temperature: 90°C, spray pressure: 1. 2kgf/cm', cyclone suction amount 0.45n+3/min, sample supply amount 1
0cm3/IIIin. The obtained dry powder was dried in vacuum for 24 hours and then sealed in a platinum crucible. 842℃
It was placed in a large tubular furnace, and after firing for 90 minutes, it was taken out of the furnace. The obtained powder showed an X-ray diffraction pattern of almost a single high Tc phase, and was plate-shaped particles with an average particle diameter of about 1.8 μm. A molded body was prepared using the raw material fine particles at a pressure of 2t/c++2, and sintered at 845°C for 12 hours (cold pressing was performed again in the middle) to obtain a sintered body with Tc=t05K. The density of the sintered body is 5.95g/cII13
The density was 90% or more of the theoretical density.

[Brief explanation of the drawing]

Figure 1: X-ray diffraction pattern of bismuth-based superconductor fine particles exhibiting a high Tc phase obtained by the present invention (84
Bake at 2℃ for 90 minutes. ``'' (0011) etc. in the figure indicate the surface index of each peak indexed in the high Tc phase). Figure 22 Bismuth-based superconductor high T obtained by the present invention
A photograph of the particle structure of EA fine particles obtained by scanning electron microscopy (solution concentration 0.01 mol/I, calcination at 842°C for 90 minutes). Figure 2

Claims (1)

[Claims] In a bismuth-based superconductor consisting of bismuth, lead, strontium, calcium, and copper, (Bi,
Pb):Sr:Ca:Cu:=2:2:2:3,
A predetermined composition ratio that is said to be able to obtain a crystal phase (so-called high Tc phase) with a critical temperature Tc = approximately 110K, that is, general formula B
In i_uPb_vSr_wCa_xCu_yO_z, positive 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 By spray-drying a nitrate solution with an ionic composition ratio and rapidly heating the resulting dry powder at a heating rate of approximately 15°C/second or more, the sample temperature is rapidly raised to 835-845°C in the atmosphere. Then, by further heat treatment at the same temperature for 1 to 2 hours, the crystal structure becomes almost Tc = 110K class superconductor (high Tc phase), and the crystal structure is reduced to several μm ~
A method for synthesizing bismuth-based high-temperature superconducting ceramic fine particles to obtain powder having a particle size of 1 μm or less.