Morphology and thermal decomposition behavior of coprecipitated LaBaCu oxalate

278

Materials Chemistry and Physics, 37 (1994) 278-283

Morphology and thermal decomposition La-Ba-Cu oxalate Ma-Shine

Wu and Tsang-Tse

behavior of coprecipitated

Fang

Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101 (Taiwan, ROC) (Received

July 28, 1993; accepted

September

17, 1993)

Abstract The morphology and thermal decomposition behavior of coprecipitated La-Ba-Cu oxalate have been investigated. It was found that La-Ba-Cu coprecipitates first decompose into CuO, BaCO, and La,O,CO,; then La20,C0, reacts with some 010 to form La,CuO,. Thereafter, La,Cu04, BaCO, and residual CuO react together to form the solid-solution phase of LaBa,Cu,O,,, i.e., La 1+xBa2-xCu30y (0
Experimental

Introduction

High-T, ceramic superconductors have drawn considerable attention in the scientific community since the discovery of YBa,Cu,O,,, which has a superconductivity transition temperature (T,) above the boiling point of liquid nitrogen [l]. Among the 90 K superconductors, LaBa,Cu,O,, is particularly important because lanthanum is by far the most abundant of the lanthanides found to be useful in producing LnBa,Cu,O, superconductive materials (Ln = Y and lanthanide elements), and the large size of the LaBa,Cu,O, crystal lattice is more suitable for making epitaxial films [2, -1

31.

In view of the low melting point of these high-T, ceramic superconductors, a low sintering temperature is required for fabricating these materials. While uniformity of mixing and fine particle size could be obtained in some ceramic materials using the coprecipitation method [4-6], it was found that the calcination and sintering temperatures cannot be reduced for the highT, superconductor by using coprecipitated precursor powders. In this study, it was observed that the inhomogeneity of the composition and particle morphology of coprecipitated La-Ba-Cu oxalates might play an important role. The purpose of this study is to investigate the mechanism of formation and possible reasons for the development of inhomogeneity of composition and particle morphology of La-Ba-Cu oxalates.

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A solution with cation concentrations of 0.05, 0.10 and 0.15 M of La, Ba and Cu, respectively, was prepared. Oxalic acid, kept at 0.3 M, was used as a precipitating agent in 20% excess. The cation solutions were titrated into the oxalic acid solution at a rate of 10 ml min-‘. An ammonia solution was used to adjust the pH value. After (co)precipitation, the oxalates were washed using ethanol and then dried at 80 “C for 24 h. The dried powders were then calcined under different conditions of temperature, time, and atmosphere. X-ray diffraction (XRD) (Rigaku D/MAX 1II.V instrument), using nickel-filtered Cu KU radiation at a scanning rate of 4” min-I, was used to identify the compositions of (co)precipitates and the phases calcined at different temperatures, times and atmospheres. Scanning electron microscopy (SEM) (JEOL JSM-35 instrument) was used to observe the particle morphology, and wavelength dispersive X-ray spectroscopy (WDS) was used to analyze the composition of the particles. A Setaram TAG 24 thermal analyzer was used to examine the thermal behavior of the (co)precipitates at a heating rate of 10 “C min-’ in air. Results Thermal cation

and discussion behavior

of the oxalate of the individual

The composition of lanthanum oxalate prepared under the conditions of the present paper could be

279

but for temperatures above 950 “C, the reduction of CuO to Cu,O would reduce the total weight, because Cu,O is more stable than CuO at high temperature.

identified as La,(C,O,), . lOH,O. The thermal behavior (differential thermal analysis (DTA) and thermogravimetric analysis (TGA)) of La2(GOJ3. lOH,O is shown in Fig. 1. There are three weight losses, at about 180, 400 and 800 “C. The first weight loss is about 24% and is attributed to crystalline water, which is in agreement with the calculated value (24.94%). The second weight loss, about 22.75%, might be due to the decomposition of La2(cOJ3, which corresponds to the formation of La,O,CO,. Decomposition of La,O,CO, results in the final product, La,O,. The thermal behavior of barium oxalate (BaGO,. OSH,O) is consistent with that found in another report [7]. The thermal behavior of copper oxalate (CuGO,.O.lH,O) shown in Fig. 2 is similar to that reported by Kornienko [S]. The decomposition of copper oxalate leads to the formation of elemental copper, and the total weight loss is about 60%. The copper was gradually oxidized and became Cu,O and CuO in the temperature range 300-600 “C; complete formation of CuO occurred above 600 “C. The weight increases and the weight loss became 50% owing to the oxidation,

\\ \ \

:

‘---------

-40

Figure 3(a) shows that the La-Ba coprecipitates are a mixture of La and Ba oxalates and a new precipitate, denoted by the number 4. The new precipitate and

1

1

--20

The thermal behavior of the coprecipitated oxalates of the binary and ternary cations

d’beq-

,_____---5

490

200

qo

Elp

Fig. 1. DT (-) and TG (- --) analyses of the oxalate at a heating rate of 10 “C min-’ in air.

25

35

45

55

Fig. 3. The X-ray diffraction patterns of (a) the La-Ba coprecipitated oxalates, (b) the precipitates in the supematant of La-Ba-Cu coprecipitation after aging for a long time, and (c) the precipitates of (b) calcined at 9.50 “C for 1 h. (1) Laz(C20&. lOH,O; (2) BaC204.0.5H20; (3) CuG04.0.1HZO; (4) Ba(HC,0&.2H,O; (6) BaCO,; (7) CuO; (8) BaCuO,.

%c

I

15

lop

lanthanum

TGl”bl

-0

-----------.

I !

4c

I

--20

I I

20 ‘-40

0 -60

-

2po

400

600

Fig. 2. DT (-) and TG (- - -) analyses at a heating rate of 10 “C min-’ in air.

600

of the copper

‘o,oo

oxalate

oc

Fig. 4. Comparison of the DT and TG analyses of the powders of the Ba(HG0&.2H20 [7] (- - - -) DTA, (---) TGA)) and the precipitates in the supematant of La-BaCu coprecipitation after aging for a long time ((-) DTA, (-) TGA)).

TGl%

HFlmV)

I

20.

=20 .---_________. ___----

0.

I

200

400

600

1000

600

DC

(a) HFI mV)

TG(%)

I

200

400

600

600

shows that the thermal behavior of the La-Ba coprecipitates is mainly that of the individual La and Ba oxalates, but because the decomposition temperature of BaC,O, shifted to 400 “C, this reaction would overlie the decomposition reaction of La2(&04)3. From the fact that no apparent Ba(HC204)2-2Hz0 peak appears in the DTA, it might be a minor phase and have an insignificant contribution to the total thermal behavior. Moreover, Ba(HGO,),.2H,O has the same thermal behavior as BaC,O, . O.SH,O above 300 “C, as Ba(H&O,), *2Hz0 also decomposed into BaC,O,. The compositions of the La-Cu and Ba-Cu coprecipitates are simply mixtures of their individual oxalates and hence their thermal behavior is similar to that of the individual components. Two extra peaks that appear at about 690 and 950 “C (Fig. 5(b) and (c)) are due to the formation of La,CuO, and BaCuO,, respectively. Figure 6(a) shows that the composition of the La-Ba-Cu coprecipitates is a mixture of the individual oxalates of La, Ba and Cu, in addition to the precipitate of Ba(HC,O,),-2H,O. As can be seen in Fig. 7, the thermal behavior of La-Ba-Cu coprecipitates at temperatures lower than 600 “C arises from the decom-

1

@I G(% I

HFf

mV)

5 6

(cl Fig. 5. DT (-) and TG (- - -) analyses of the coprecipitated oxalates of binary cations: (a) La-Ba, (b) La-Cu and (c) Ba-Cu oxalates.

some copper oxalates were also found in the supernatant of La-Ba-Cu coprecipitation after aging for a long time, as shown in Fig. 3(b). After the above powders were calcined at about 950 “C (Fig. 3(c)), only barium carbonate, copper oxide and BaCuO, were found, with no lanthanum compound. This new precipitate might be some kind of barium oxalate. On the basis of Xray data (Fig. 3(b)) and thermal behavior (Fig. 4), this new barium oxalate might be Ba(HC,O&-2H,O, as suggested by Fang et al. [7, 9, lo]. Therefore, more barium could be detected in coprecipitation than in the individual precipitation in a solution of oxalic acid, which supports the previous work [ll]. Figure 5(a)

,

20

I 5

26

15

32

25

38

44

, 50

35

45

4 55

Fig. 6. X-ray diffraction patterns of the La-Ba-Cu coprecipitated oxalate calcined at various temperatures: (a) as-coprecipitated; (b)6OO”C; (c) 750°C; (d)850”C; (e) 920°C. (l)La~(C~0&~10H~O; (2) BaGO,. 0.5HrO; (3) CuCrO,. O.lHzO; (4) Ba(HGO& .2H@; (5) L.a,+,Baz_,Cu30,; (6) BaCO,; (7) CuO; (8) BaCtQ; (9) LazCuOl; (10) Laz02C03.

281 TABLE

1. Formation

mechanism

Chemical

of the coprecipitated

reactions

La-Ba-Cu

oxalate Phases

derived from DTAJTGA

observed

in X-ray patterns

&) . lOH,O + 1 SO2 + La202C03 + lOH,O + .5C02 BaGO,. OSHsO + 0.502 -+ BaC03 + 0.5H20 + CO? CUC~O~~~.~H~~+~.~~~-+C~O+O.~H~O+~CO, Ba(HC,O&. 2H,O + O2 + BaCOS + 3H,O + 3COs

La,O,CO, BaCO, cue

690

La,O,CO,

La,CuO, BaCO, cue

800

aLa,CuO,+bBaCO,+cCuO+La,+,Ba,_~Cu,OY+bCO, a =(l +x)/2, b=2-x, c=(5-x)/2, and y=(13+x)/2

<600

La,(C,O,)a

920

+ CuO + La,CuO, + COP

BaCO, + CuO + BaCuO,+

(O
CO2

La,CuO, BaCO, cue La, +,Bas J&O, La, +rBa2-rCu30y BaCuO,

2b

(a) 12

200

400

600

ap

1000

08

“C 8

Fig. 7. DT (-) and TG (-- -) analyses of the La-Ba-Cu coprecipitated oxalate at a heating rate of 10 “C min-’ in air.

position of the individual oxalates of the La, Ba and Cu cations. On the basis of the XRD (Fig. 6) and DTA/TGA (Fig. 7) studies, a formation mechanism of LaBa,Cu,O, superconductor powders derived from the coprecipitated La-Ba-Cu oxalates can be proposed (Table 1). The La-Ba-Cu coprecipitates first decompose into CuO, BaCO, and La,O,CO,. Then La,O,CO, reacts with some of the CuO to form La,CuO, at about 690 “C, which is similar to the behavior of the La-Cu coprecipitates. La,CuO,, BaCO, and residual CuO react together at about 800 “C to form the solid-solution phase, i.e., La, +rBaa -xCu,O,,. The formation of La,+,Ba,_,Cu,O,, is not complete at about 800 “C, because BaCO, does not decompose readily into BaO and cannot react with La,CuO, and CuO to form La,+xBaZ-rCusO,, at this temperature. It was found that the final product was multiphase (La, +xBaZ_-xCu30,, and BaCuO,) when the La-Ba-Cu coprecipitates were fired at about 920 “C for 4 h. Single-phase LaBa,Cu,O, could be synthesized, as proposed in our previous paper [12], by heating the

2 i 04

0 l-

o

(b)

160 TEMPERATURE

240

(lo

Fig. 8. X-ray diffraction pattern (a) and resistivity as a function of temperature (b) for the coprecipitated LaBa,Cu,O, superconductor heated at 950 “C in NZ for 12 h, then annealed at 300 “C in O2 for 24 h.

calcined powders at 880 “C for 6 h in N, atmosphere. The single-phase powders were pressed into pellets and sintered at 950 “C for 12 h in Nz atmosphere, then annealed at about 300 “C for 24 h in 0, atmosphere. Figure 8 shows a single-phase LaBa,Cu,O, with an orthorhombic structure; the transition to superconductivity begins at T,,,,,,t =90 K and is complete at Tc,oeet= 83 K.

(4 Fig. 9.

zoum

20pm

2oum

20 urn

(e)

10urn

(0

iopm

The morphologies of the particles of lanthanum, barium and copper oxalates precipitated in a 0.3 M oxalic acid solution at various pH values and various concentrations of the cation ion: (a) [La3+] =0.017 M, pH=1.5; (b) [La3+]=0.05 M, pHc5.0; (c) [Ba’+]=O.033 M, pH=1.5; (d) [Ba*‘]=O.lO M, pH=5.0; (e) [Cuzc]=0.050 M, pH=1.5; (f) [Cu’+]=O.15 M, pH~5.0.

Particle morphology of La-Ba-Cu coprecipitates The calcination temperature of the coprecipitated

powders is not effectively lowered compared with the solid state reaction method, although a nearly chemical stoichiometry is obtained (La:Ba:Cu = 1:1.95:2.94). Figure 9 shows that the morphologies of the individual precipitates of La, Ba and Cu cations in a solution of oxalic acid are platelike, cylindrical and disklike, respectively. All the particles became smaller with increasing pH or cation concentration, because at higher concentrations of [C204’-] or [M”+] more nuclei could be formed at the same time and hence smaller particles could be obtained. It was found (Fig. 10(a)) that many coarse particles exist in the La-BaCu coprecipitated oxalates, which have been identified to be particles of lanthanum oxalate

(4

iopm

@)

iopm

(cl

2w

(d)

lopm

Fig. 11. The particle morphologies of the coprecipitated oxalates of the binary cations: (a) La-Ba; (b) La-Cu; (c) Y-Ba; (d) Y-Cu.

(a) Fig. 10. Observed SEM particle morphology (a) and WDS (b) of the Laz(GO,),-based particles showing that Ba*+ ions are adsorbed on the particle surface.

on whose surfaces Ba ions were adsorbed (Fig. 10(b)). The inhomogeneity of the composition and particle morphology might explain why the calcination temperature of the coprecipitates could not be lowered. The inhomogeneity of the composition might be attributed to the similarity of the ionic radii (1.22 and

283

1.43 8, for La3+ and Ba’+, respectively) and structure (monoclinic structure for La and Ba oxalates), allowing Ba2+ to be occluded easily in the La,((;O,),-based particles. This was not observed in the Y-Ba-Cu system. The coarsening mechanism of La,(C,O,),-based particles could not be explained in the same manner (the inhomogeneity of the composition), because coarse particles were also observed in the coprecipitates of La3+ and Cu2+ (Fig. 11(a) and (b)). Moreover, coarse particles were not observed in the coprecipitates of Y3+ with Ba2’ (Fig. 11(c)) or Cu2+ (Fig. 11(d)). Comparing the particle morphologies of the individual precipitates of La3 + (platelike, shown in Fig. 9) and Y3+ (disklike), it is seen that anisotropic crystal growth seems to play an important role. It is suggested that owing to the low solubility product, La,(&.O,),-based particles might precipitate first during coprecipitation with Ba2+ or Cu2+. Anisotropic crystal growth might be enhanced by the adsorption of Ba2+ or Cu2+ on the particle surface. However, more work is needed to clarify this point.

Conclusions

(1) The thermal behavior of La-Ba-Cu coprecipitates was as follows: first they decompose into CuO, BaCO, and La,O,CO,; then La,02C03 reacts with some CuO to form La,CuO,. Thereafter, La,CuO,, BaCO, and residual CuO react together to form the solid-solution phase of LaBa,Cu,O,,, i.e., La1 +xBa2--xCu30y (0
(2) The high calcination temperature for the La-Ba-Cu coprecipitated oxalates is due to the inhomogeneity of the composition and particle morphology.

Acknowledgement

The authors are grateful to the National Science Council for financial support under Contract No. NSCSO-0405E006-06.

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