Homogeneous coprecipitation method for preparing YBaCuO superconducting powder

An oxalate route coprecipitation process for preparing a superconducting YBaCuO system precursor powder comprises preparing an aqueous solution of nitrates of yttrium, barium and copper, which is then mixed with an organic solution containing a dibasic oxalate ester, an organic base and acetone. An ultrasound vibration is applied to the mixture to coprecipitate oxalates of yttrium, barium and copper. The decomposition of the oxalate ester is enhanced to an optimum rate as the result of the homogeneous phase contributed by acetone which is miscible with both water and the oxalate ester. The ultrasound vibration segregates the precipitate particles thereby preventing the coagulation of the particles and assuring fine and uniform particle size.

BACKGROUND OF THE INVENTION 
This invention relates to a process for preparing a YBaCuO system 
superconducting precursor powder and particularly, to a process for 
preparing a fine and homogeneous YBaCuO system superconducting precursor 
powder by an oxalate route coprecipitation method using a dibasic oxalate 
ester as a precipitate forming agent. 
The coprecipitation process for preparing a YBaCuO system superconducting 
precursor powder via an oxalate route is known in the art. U.S. Pat. No. 
4,895,832 discloses a process in which nitrates of Y, Ba and Cu are 
dissolved in water and oxalates of Y, Ba and Cu are coprecipitated by 
adding oxalic acid to the aqueous solution of the nitrates. In an article, 
"Solid State communications, vol. 64, No. 6, pp.881-883, 1987 ", Wang et 
al disclose a process for preparing a superconducting precursor powder 
also via the oxalate route, wherein oxalates of Y, Cu and Ba are 
coprecipitated by adding an aqueous solution of oxalic acid to a mixture 
containing an aqueous solution of copper acetate plus yttrium nitrate and 
a solution of barium hydroxide plus acetic acid. Oxalic acid permits 
oxalates of Y, Cu and Ba to precipitate rapidly. It is, however, 
ineffective to achieve a fine and uniform superconducting powder. 
Japanese Patent Application No. 63-285116 discloses an oxalate route 
coprecipitation process in which oxalates of Y, Ba and Cu are precipitated 
by adding an organic dibasic ester such as dimethyl or diethyl oxalate to 
an aqueous solution containing ions of Y, Ba and Cu. It is described in 
this patent that the process produces fine and uniform coprecipitates 
because ethyl oxalate gradually decomposes to deliver oxalate ions used to 
form oxalate precipitates. However, this process is found to be not so 
efficient as excepted because dibasic esters such as ethyl oxalate are not 
readily soluble in water. It is difficult to achieve a homogeneous phase 
when mixing ethyl oxalate and the aqueous solution so that a long period 
is required for the decomposition of ethyl oxalate to an efficient state. 
The decomposition rate of ethyl oxalate is so undesirably slow that 
oxalate particles formed priorly coagulate to large particles, resulting 
in disuniform precipitate particles. This process does not use a base to 
adjust the pH value of the solution and thus desired components in the 
solution are liable to be lost thereby adversely affecting the desired 
stochiometry of the final product. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide an improved oxalate route 
coprecipitating process for producing a YBaCuO system superconducting 
precursor powder, which process still employs a dibasic oxalate ester as a 
precipitate forming agent, but is efficient to produce a fine and uniform 
superconducting powder, The particle size of the superconducting precursor 
powder obtained by this process is less than 0.5 micron. 
Another object of the invention is to provide a process of coprecipitating 
oxalates of Y, Ba and Cu by a dibasic oxalate ester, which process is free 
of contamination and efficient to achieve a stochiometrically desirable 
product. 
According to the present invention, a homogeneous coprecipitation process 
for preparing a superconducting YBaCuO system precursor powder comprises: 
(a) preparing an aqueous solution containing nitrates of yttrium, barium 
and copper; (b) preparing an organic solution by dissolving a dibasic 
ester in acetone, said dibasic ester being selected from the group 
consisting of dimethyl oxalate and diethyl oxalate; (c) adding an organic 
base in the organic solution for the adjustment of pH value; (d) mixing 
the aqueous solution and the organic solution; and (e) applying an 
ultrasound by an ultrasonic generator with a titanium tip to the mixture 
of the aqueous solution and the organic solution to coprecipitate oxalates 
of yttrium, barium and copper. 
In the process of the present invention, a coprecipitation solution with a 
homogeneous phase is obtained by employing acetone which is miscible with 
both water and the dibasic esters. The dibasic oxalate ester can therefore 
dissociate at an optimum rate. The fine and uniform particle size is 
achieved by the application of ultrasonic wave vibration which prevents 
coagulation of precipitates by segregating precipitate particles. The 
ultrasonic wave vibration also improves the dissociation rate of the 
dibasic oxalate ester. 
The organic base adjusts the pH value of the coprecipitation solution so as 
to assure effective coprecipitation of desired oxalates and prevent losses 
of desired components. The organic base may be diethyl or triethylamine. 
Unlike the inorganic base which is susceptible to contamination of 
coprecipitates, the organic base used in the present invention can be 
removed easily together with the liquid phase from the coprecipitates 
during the filtration process. Even when a small amount of the organic 
base is retained in the coprecipitates, it can escape from the 
coprecipitate when the coprecipitate is subjected to heat treatments such 
as calcining and sintering, to form a YBaCuO system superconductor. 
Therefore, this process will not create any contamination problem. 
According to the invention, the weight of the dibasic ester used may be 1.2 
times the total equivalent weight of ions of yttrium, barium and copper. 
The molar ratio of the dibasic ester and the organic base may be 1:1.8 to 
1:2.2. The volumetric ratio of water and acetone may be 1:1.5-1:2. 
The preferred examples of the invention will be described in detail with 
reference to the accompanying drawings. in which:

EXAMPLE 1 
Solution (A) was prepared by dissolving 1.375 gm of yttrium nitrate, 2.614 
gm of barium nitrate and 3.624 gm of copper nitrate in 80 ml of deionized 
water. Solution B was prepared by dissolving 5.3 ml of ethyl oxalate and 
1.8 ml of triethylamine in 50 ml of acetone. Solution A and B were 
agitated to form a blue solution which was then treated by an ultrasound 
vibration to obtain a light blue coprecipitate. After 2 hours, the 
precipitate which reaches a pH value of about 1.8 was filtered. The 
collected coprecipitate was a fine blue powder. The coprecipitate was 
heated at 120 deg C. for 4 hours in 1 atm oxygen atmosphere for 
decomposition followed by calcining at 900 deg C. for 10 hours also in an 
oxygen atmosphere. The calcined substance was ground and formed into 
pellets by a pressing process. The pellets were then sintered at 950 deg 
C. for 30 hours, cooled slowly to 550 deg C. and annealed for 20 hours. 
FIGS. 1a and 1b show scanning electron microscopic (SEM) pictures of the 
coprecipitate from solutions A and B, which reflect that a large portion 
of the coprecipitate is constituted of particles of far below 0.5 um and a 
small portion thereof is constituted of particles of over 0.5 um. Greater 
magnification reveals that the coprecipitate is constituted of particles 
of 0.1-0.2 um (FIG. 1c). An electron diffraction pattern of the 
coprecipitate shows that the coprecipitate is in an amorphous state (FIG. 
1d). 
FIG. 2a is a SEM picture of the calcined powder, which shows that the 
powder becomes a coherent substance and the particle size is increased by 
a few microms. However, the particle size is still smaller than those 
obtained conventionally. The electron diffraction pattern of FIG. 2b 
reflects that the calcined powder is in a crystalline state. 
EPMA pictures of FIGS. 3a, 3b and 3c show that the distributions of Y, Ba 
and Cu on the surface of the final sintered pellet are uniform. The SEM 
picture of FIG. 3d shows the good performance of the sintering process 
involving a partial-melting state which enables some crystals to grow to 
several microns. 
FIG. 4 is a pattern resulting from a X-ray diffraction, which reflects that 
the sintered substance has a YBa2Cu3Oy orthorohmbic structure almost 
devoid of impurities. 
FIG. 5 shows a resistance vs. temperature curve which indicates that the 
critical superconducting temperature is 92 deg K. 
FIG. 6 is a SQUID picture which shows the magnetic properties of the 
superconducting pellet, which picture proves that pellet is in a single 
phase at 90 deg K. 
COMATIVE EXAMPLE 
1.6625 gm of yittrium nitrate, 2.61135 gm of barium nitrate and 3.624 gm 
copper nitrate are dissolved in 80 ml of water. 5.3 ml of diethyl oxalate 
and 10.8 ml of triethyl amine was added to and mixed with the solution. 
Since diethyl oxalate is not immiscible with water, the mixture obtained 
has two liquid phases, one being colorless (diethyl oxalate) and the other 
being deep blue (aqueous solution). The mixture was emulsified when 
agitated vigorously. When the mixture was heated to 85 deg C. for four 
hours, the oxalate phase did not disappear completely. The whole reaction 
took about 5 hours. After reaction, the remaining liquid had a volume of 
only 20 ml which was viscous and unable to produce precipitates. 
With the invention thus explained, it is apparent that various 
modifications and variations can be made without departing from the scope 
of the invention. It is therefore intended that the invention be limited 
only as indicated in the appended claims.