CHINESE JOURNAL OF PHYSICS
VOL. 31, NO. 6-11
DECEMBER 1993
The Improvement and Kinetics of SRS Process for Synthesis
of Y1Ba2Cu307 and Related Superconductors+
Shu-en Hsu, Chih-ming Li, Keun-long Wang, and Yu-then Chang
Materials Research and Development Center,
Chung Shun Institute of Science and Technology,
Lungtan, Taiwan 325, R.O.C.
(Received August 27, 1993)
The Spontaneous Reaction Synthesis (SRS) of YiBazCusOr single phase superconductor, the quickest process ever known, from metals and peroxides with Y:Ba:Cu=1:2:3
chemical stoichiometric ratio has been reported in our previous paper [l].
By extending the application of SRS process, a Y1237 superconductor dispersed
with z (at %) Y2115 pinning centers can be synthesized by adjusting the stoichiometric
ratio of Y:Ba:Cu=(l + 21):(2 + 2):(3 + 2). Fo11owing the texture alignment by PowerMelt-Growth (PMG) technique, the synthesized YBaCuO bulk superconductor exhibits
an extra-ordinary levitation force, at least five-fold stronger than that of conventionally
prepared ones.
It is found that oxygen pressure in the reaction chamber during SRS process plays
a dominant factor to control the rate of reaction and the completeness of phase transformation. Kinetics of SRS reaction for YiBasCusOr superconductors will be proposed
in this report.
I. INTRODUCTION
Spontaneous Reaction Synthesis (SRS), is a novel material process for synthesizing
a single phase YrBa2Cus07 superconductor. By further extending the application of SRS
process, it is found that either Y1237 or Y2115 system of HTSC can be synthesized separately or directly by preparing the initial stoichiometric composition. In this report, the
SRS process is utilized to prepare a Y1237 superconductor dispersed with z (at %) Y2115 as
flux pinning centers by adjusting the stoichiometric ratio of Y:Ba:Cu=(lt2z):(2ts):(3+2).
Following the texture alignment by Powder-Melt-Growth (PMG) technique, the synthesized
bulk YBa2CusOr_&zY2BaCuOs superconductor exhibits a strong levitation force. It was
found that Y1237ta: (at %) Y2115 superconductor seems to be independent of the amount
of additive of z (at %) Y2115, but strongly dependent on the grain size of Y1237 when
t Refereed version of the invited paper presented at the International Conference on Superconductors, August 27-30, 1993, Sun Moon Lake, Nan-tou, Taiwan, R.O.C.
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@ 1993 THE PHYSICAL SOCIETY
OF THE REPUBLIC OF CHINA
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THE IMPROVEMENT AND KINETICS OF SRS PROCESS FOR ...
VOL. 31
the bulk material is treated by PMG process. The dependence of grain size on levitation
force has been reported by M. Murakami et al. [a]. That the independence of the amount
of additive Y2115 flux pinning centers on levitation force can be verified in detail by SRS
process in this paper.
The objectives of this report have three folds: (1) To further extend the application of
SRS process to prepare Y1237 superconductors containing a designed amount of flux pinning
centers of Y2115, (2) To investigate the influence of the addition of Y2115 dispersoid on
the levitation force for Y1237 superconductors, and (3) To propose kinetics of SRS process
to explain the effect of prime parameters on the reaction rate for synthesis of Y1237 and
related superconductors.
II. EXPERIMENT PROCEDURE
The principle in SRS process involves the use of an exothermic reaction between
the reactant powders. YzOs, barium peroxide and Cu powders with Y:Ba:Cu=1:2:3 were
used as starting materials for YrBazCusOr superconductors which were treated as fine as
possible. In the case of preparing Y1237 with the addition of z (at %) Y2115 flux pinning
centers, the stoichiometric ratio should be adjusted to Y:Ba:Cu=(l + 2x):(2 + 5):(3 + z),
where z is designed fraction, z = O,O.l, 0.2,0.3 respectively in our case. The mixed powder
was subsequently consolidated at 15 kg/ cm2 by a steel die, forming a pellet 10 mm in diameter and 30 mm height. The green compact was settled on ceramics (ceramic YzBaCuOs
in our case) in a vertical furnace and then ignited from upper end with a 30 V DC-heating
element. Environmental preheating temperature (To) was adjustable, setting at 380 “C
for Y1237+z % Y2115, at 880 “C for Y2115+liq.phase ceramics respectively. The energy
for propagation of the reaction front was obtained from the exothermic heat of formation
of the synthesized ceramics. The velocity of wave-front propagation and temperature of
adiabatic reaction were measured from 3 thermal couples, 10 mm apart. Data are taken by
an automatic recorder with a frequency of 9 times per second.
The details of PMG process are discussed elsewhere (31. In this study, the PMG
process is carried out with SRS process. The starting materials of Y203, BaO2 and Cu were
pre-mixed with stoichiometric ratio Y:Ba:Cu=( 1 + 2x):(2 + x):(3 + z) where z = 0,0.1,0.2
and 0.3 and then synthesized by SRS process by adjusting the preheating temperature
(To) in order to increase the adiabatic reaction temperature (Tad) above 1000 “C at which
Y203 will react with the molten liquid phase of BaO2 and CuO to produce Y2115 phase
and peritectic liquid enveloped. At this stage, the sample was rapidly cooled down to the
temperature at which the superconducting phase Y1237 starts to nucleate. Following slow
cooling, Y2115 phase continually reacts with liquid phase to produce Y1237 phase. The
final product of PMGtSRS process would be YrBa2CusOr_-6 with z (at %) YzBaCuOS
dispersions.
Phase analyses of SRS products and preferred orientation of SRS+PMG products
were conducted by X-ray diffraction. The levitation force was measured by moving a rareearth magnet toward and away from the SRS+PMG YBaCuO superconductor.
.
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SHU-EN HSU, CHIH-MING LI, KEUN-LONG WANG, AND . . .
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III. RESULTS
As reported in our previous paper [l], a YrBa&&Or__s single phase superconductor
can be synthesized very quickly by SRS process. Fig. 1 re-depicts the XRD spectra of
samples synthesized by SRS process in an oxygen environment of 1 atm, 5 atm and 10 atm
respectively. The background of impurities disappears with an increase of oxygen pressure.
In this paper, the SRS process was extended to prepare a Y1237 superconductor containing
2 (at %) Y-2115 dispersoids as flux pinning centers according to the following equation:
(l/2 + Z)YZOS
+
(2 + z)BaOz + (3 + z)Cu (1)
YBazCusOr-6 + zY2BaCuOs
Figure 2 depicts the result of phase analysis of the as-SRS-synthesized ceramics. It
is notable that there are only two phases, Y1237 and Y2115, presented. Fig. 3 shows the
SRS at 10 atm
.e
cdl P02= 1
dh
in 02
1: YBd2CU$+x
5
e
<
E
-
4
16
28
40
52
64
20
FIG. 1. XRD spectra ofY1237 IITSC synthesized by SRS in oxygen. (a) Po, = 1 atm (b) PO, = 5
atm (c) Po, = 10 atm (d) R vs T for (c). [re-depicted from [I]]
THE IMPROVEMEKT AND KINETICS OF SRS PROCESS FOR
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(1)
a-o.1
1
:YB&ba%=
2
:Y,BaCd
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FIG. 2. XRD pattern of Y1237+t (at %) Y2115 HTSC, where z = 0.1,0.3 and 0.5, respectively,
HTSC omposite prepared by SRS.
microstructure of the SRS-processed Y1237 matrix dispersed with Y2115 particles. Following the Powder-Melt-Growth (PMG) p recess as mentioned in experimental procedure, the
SRS+PMG product exhibits strong anisotropic property (as shown in Fig. 4) of which the
----
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SHU-EN HSU, CHIH-MING LI, KEUN-LONG WANG, AND . . .
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FIG. 3. SEM micrograph of Y1237+O.lY2115 HTSC synthesized by SRS.
basal planes of Y1237 phases are all parallel to the surface of the bulk superconductor.
Fig. 5 shows levitation force versus distance for the Y1237+10 at % Y2115 SRS+PMG
superconductor. The result of the normal sintered YBaCuO superconductor is shown (the
small loop) in the same figure for comparison.
In order to investigate the effect of the amount of addition of z (at %) Y2115 dispersed
on the levitation force, four SRS+PMG treated HTSC composites with z = 0,0.1,0.2 and
0.3 additions are tested. Fig. 6 illustrates levitation force as a function of z (at %) of
addition Y2115. It is interesting to note that the levitation force of Y2115 dispersed in
Y1237 superconductors is independent of the amount of addition of flux pinning centers
I,. I.
4
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28
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I
40
II
52
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64
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FIG. 4. XRD pattern of Y1237+O.lY2115 HTSC prepared by SRS+PMG, showing anisotropic
texture, all basal planes aligned along the surface.
2.:
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THE INPROVEMENT AKD KINETICS OF SRS PROCESS FOR ...
VOL. 31
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FIG. 5. Levitation force vs. distance curve for ‘1’1237+O.lY2115 IITSC prepared by SRS+PMG;
a conventionally sintered YBazCusOT_a IITSC also shown for comparison.
FIG. 6. Maxium repulsion (levitation) force versus z (at %) Y2115 additives in the matrix ofY1237
IITSC prepared by SRS+PMG.
but strongly dependent on the grain size of PMG-treated Y1237 superconducting matrix.
IV. DISCUSSION
IV-l. Activation Energy
Spontaneous Reaction Synthesis (SRS) process involves many complicated mechanisms simultaneously, including combustion, chemical reaction and phase transformations
In our case of synthesizing a superconductor which involves more complicated parameters
not only because there are three starting raw materials participated but a gas-solid reaction is also engaged due to oxygen introduction. During SRS process, the energy equation
for transient heat conduction including the source term containing heat release due to the
exothermic reaction is given as:
L
_
._^
-
SHU-EN HSU, CHIH-MING LI, KEUN-LONG WANG, AND . . .
VOL. 31
av
pat=
where
aPcg)
_
4h(T d- To)
az
+ pQv(l- p)(l - q)exps
977
(1)
Q = enthalpy of exothermic reaction,
E* = activation energy,
To = initial temperature,
Tad = adiabatic reaction temperature,
Ii = heat conductivity,
p = density,
cp = enthalpy,
I = reaction coordinate,
h = surface heat transfer coefficient,
d = diameter of compact,
v = frequency factor,
p = molar fraction of diluent,
R = gas constant,
77 = the porosity of the green compact.
In the equation, left hand term indicates the energy required for heating the SRS
product from initial temperature to adiabatic temperature, T = Tad. The terms on the right
hand side are the conduction heat transformation, heat of surface radiation and heat release
due to exothermic reaction respectively. Some parameters are measurable from experiment
but some are uncertain, especially for the multiple reactant components. However, the
activation energy E* for SRS reaction can be obtained from the theory of “wave velocity
analysis”. According to Zeldovich’s theory of combustion [4-51, the velocity of propagation
of combustion wave front (u) can be expressed in the following equation:
G RT,2, w
V2 = f(n)KaQ
E*
where
( 1
-E'
RTad
f(n) = a function of the order of the reaction
cy = constant including frequency factor v
C, = the specific heat of the product
The activation energy for SRS reaction can be drawn by plotting In(&) versus (&)
by supposing the factors f(n), Ii, cy, and E’ are not functions of Tad. Fig. 7 illustrates
the result of activation energy for SRS reaction, E* = 137 KJ/mole, for YrBazCusOr_6
superconductor at ambient pressure. In this plot the velocity of wave propagation (v) and
the temperature of adiabatic reaction (Tad) are measured from three contact points of the
thermal couples, A, B and C, with 1 cm apart. The temperature profile and the rate of
change of reaction temperature are plotted as a function of time as shown in Fig. 8. It is
notable that the rate of change of temperature (5) at different points, A. B and C ,
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THE IMPROVEMENT AND KINETICS OF SRS PROCESS FOR . .
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FIG. 7. The activation energy for SFtS reaction (E’) at PO, = 1 atm and 5 atm for synthesizing
Y1237 HTSC was obtained by plotting In(&) vs. (&).
YBCO SRS
r-4uE I.v
I
FIG. 8. Temperature profile and temperature change rate (z) as functions of time during SFtS
for Y1237 HTSC; data taken from points A, B and C with 10 mm apart.
during SRS reaction reveals two distinct peaks indicating that SRS reaction is a nonequilibrium process which combines different mechanisms of synthesis and completeness of
phase transformation.
IV-2. Oxygen Pressure
For synthesizing high temperature superconducting ceramics by SRS process, oxygen
pressure plays a very important role, particularly for producing a superconductor phase.
Impurity phases disappear as oxygen pressure increases from 1 atm to 5 atm. Apparently,
the requirement of oxygen from starting oxide, YzOs and BaOz, is not enough for forming
superconducting phase of YrBazCusOr. Deficit of oxygen results in the co-existence of
non-superconductive phase. Increasing oxygen pressure during SRS reaction proceeding,
VOL. 31
SHU-EN HSU, CHIH-MING LI, KEUN-LONG WANG, AND ..+
979
oxygen atoms get more probability to migrate into the reaction zone to settle at the defect
sites. Fig. 7 also shows the activation energy for SRS reaction at 5 atmosphere, E* = 109
KJ/mole, in comparison with E* = 137 KJ/mole for SRS process at 1 atm. The lower
value of activation energy may result in an exponential increase in the propagation velocity
as well as reaction rate. Moreover, a decrease of activation energy may cause a more
stable propagation and more complete reaction for phase transformation from the nonsuperconductive to superconductive phase.
.
IV-3. Pinning Centers
It is observed from Fig. 8 that the adiabatic reaction temperature on the surface of
Y1237 HTSC during SRS reaction is about 900 “C, which is below the peritectic reaction
temperature in the pseudobinary phase diagram of the YBaCuO system along the tie line
of Y211 and Y123 as depicted in Fig. 9. Since SRS process exhibits a unique advantage
for synthesizing a designed stoichiometric compound, the Y1237+z (at %) Y2115 HTSC
composites with x = 0,0.1,0.2 and 0.3 fraction, can be synthesized at points a, b, c a n d
d respectively on the phase diagram of Fig. 9. After cooling down to room temperature,
the products exist the exact stoichiometric composition as prepared from starting raw
materials. In the process of PMG, the composite superconductors should be reheated up to
the temperature region of 1000-1200 “C and rapidly cooled down to the temperature (about
980 “C) at which the superconducting phase starts to re-nucleate by slow cooling. During
the remelting, YzBaCuOs and melted liquid phase are presented, and during the final slow
cooling YzBaCuOs reacts with liquid phase to produce a textured YrBazCusOr-6 phase
plus the un-reacted x % YzBaCuOs remainder. So, even for the case of no-additive (z = 0),
the fine YzBaCuOs particles are still dispersed in the matrix of YBazCusOT-6 phase.
YIOI
+ )I
L
L
FIG. 9. Pseudobinary phase diagram of YBaCuO system along the tie line of Y211 and Y123.
980
THE IMPROVEMENT AKD KINETICS OF SRS PROCESS FOR
VOL. 31
Fine particles of these Y2115 phases contribute to the increase of critical current density (Jc) ,
as well as levitation force [2]. This is the reason to explain the independence of levitation
force on the amount of Y2115 pinning centers.
V. CONCLUSIONS
The conclusions are summarized as following:
.
(I) Spontaneous Reaction Synthesis (SRS) is further proved to be the most effective and
quickest process ever known for fabricating bulk YBazCusOr single phase and Y1237
related superconductors.
(2) SRS process is improved to fabricate Y1237 HTSC composites containing z % of
Y2115 which dispersed in the Y1237 matrix as flux pinning centers.
(3) Following the technique of Powder-Melt-Growth (PMG), the SRS-synthesized Y1237
HTSC composite exhibits an extra-ordinary strong levitation force.
(4 It is found that the levitation force of the Y1237/z % Y2115 HTSC composite after
SRS+PMG processing is independent of the amount of z % additives but is strongly
dependent on the grain size of Y1237 matrix.
(5) Kinetics of SRS reaction for YiBazCusOr is proposed to explain the rate of reaction,
the influence of oxygen pressure, the effect of diluent and dependence of additions
and grain size on levitation forces.
REFERENCES
111
PI
PI
141
PI
PI
PI
S. E. Hsu, J. Y. Wang, C. M. Li, T. W. Huang, and K. L. Wang, “A Quick Process
for Synthesis of YrBazCuzOr Single Phase Superconductors,” Physica C207, (1993)
p. 159.
M. Murakami, T. Oyama, H. Fujimoto, T. Tagnchi, S. Gotoh, Y. Shiohara, N.
Koshizaka, and S. Tanaka, “Large Levitation Force due to Flux Pinning in YBaCuO
Superconductors Fabricated by Melt-Powder-Melt-Growth Process”, Jpn. Jour. of
App. Phys. 29, 11 (1990) p. L1991.
H. Fujimoto, M. Murakami, S. Gotoh, T. Oyama, Y. Shiohara, N. Koshizuka, and S.
Tanaka: Advances in Superconductivity II (Springer-Verlog, Tokyo, 1990) p. 285.
Z. A. Munir, “The Mechanism of Synthesis of High Temperature Materials by Compulsion Process” in High Temperature Science 27, (1990) p. 279, Humana Press
Published.
E. I. Maksimov and Shkadinski, comb. Explos, Shockwaves 7, 392 (1971).
M. Murakami, M. Morita, K. Doi, K. Miyamoto, and H. Hamada, Jpn. Jour. App.
Phys. 28, (1989) p. L332.
F. C. Moon, P. Z. Chang, H. Hojzji, A. Barkatt, and A. N. Thorpe, “L evitation
Force, Relaxation and Magnetic Stiffness of Melt-Quenched YBa&u302”, Jpn. Jour.
of Appl. Phys. 29, 7 (1990) p. 1257.