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The modulation principle of vacuum on the superconductivity of YBCO
PHYSICA
Physica C 203 (1992) 45-50 North-Holland
The modulation principle of vacuum on the superconductivity of YBCO Z h i s h u a n g Gao, Baoshi Du, Yiya Peng, Hui Gao, Zhongxing Wang, C h a n g y u Z h u a n d Jianxun Kang Henan Fundamental and Applied Science Research Institute, Zhengzhou University, Henan 450052, China
Jizhou Li, Jilian Yang, Baisheng Zhang, Yongfan Ding and Jian Kang China Institute ofAtomic Energy, Beijing 102413, China
Wannian Wang Institute of Semiconductors, Beijing 100083, China
Received 31 July 1992
By neutron diffractionand other experiments, we have found that oxygenions in YBCOcan diffuse out of the samplein vacuo at room and low temperature, while the T, decreasesgreatly.We have also found that if the vacuum-deoxidationprocesslasts for several days there will be a damping oscillation of T~with time (t), and highervacuum correspondsto a greater amplitude and a shorter period. We tentatively think that T¢should satisfy the followingfunction: T¢0ocTce-~cos (tot+~); it may be due to the diffusion of oxygenand the saturation of the valence state.
1. Introduction Many high-temperature superconductors belong to mixed oxides of perovskite-type structure. Under vacuum conditions and at room temperature or low temperature, reports about relations between oxygen content, vacuum oxygen diffusion and To(t) have seldom been found. Although high-To superconductors are made at high temperature, they are preserved at room temperature and applied at low temperature. Particularly, many film devices are used under vacuum conditions. Therefore, the fluctuation of superconductivity caused by oxygen diffusion and the age of superconductors will remain significant problems before they go into practical uses. Recently, we performed the following experiments at practical temperature ranges to study these problems. First, we treated YBCO samples under vacuum conditions for a certain period of time, then we #r This project was supported by the National Center for Research and Developmenton Superconductivityof China.
used neutron and X-ray diffraction and To-measurement to study the samples. It was proved that structural oxygen in YBCO samples can actually diffuse out of the samples. The results show that at practical temperature ranges oxygen diffusion affects the stability of the superconductivity of oxide superconductors and the effect on film samples is more serious than that on bulk samples. It has also been found that, under a certain vacuum condition and at room temperature or low temperature, the dynamic equilibrium between the movable ionic oxygen in YBCO samples and the molecular oxygen outside samples determines T~. We call the law "The Principle of Oscillation and Dynamic Equilibrium of Tj'.
2. Experimental YBCO (or YBa2 Cu307_ 6 123-phase) samples were synthesized by solid state sintering. Too> 91.6 K. X-ray diffraction and neutron diffraction showed that our samples were single phase, and the occu-
0921-4534/92/$05.00 © 1992 ElsevierSciencePublishers B.V. All rights reserved.
46
Z. Gao et al. / The modulation principle o f vacuum on the superconductivity o f YBCO
Table 1 Structural parameters of normal state YBCO determined by neutron diffraction Element
Coordinate
Occupation Number
Lattice constant
(k)
Atom
Site
x
y
z
N
Y Ba Cu~ Cu2 O1 02 03 04 O3
lh 2t la 2q le 2s 2r 2g lb
0.5 0.5 0 0 0 0.5 0 0 0.5
0.5 0.5 0 0 0.5 0 0.5 0 0
0.5 0.1812 0 0.3544 0 0'.3795 0.3753 0.1618 0
1.0 2.0 1.0 2.0 0.980 2.0 1.888 2.0 0.0
pation number of every element in the lattice was also determined, which indicated that the samples were 123 phase (see table 1 ). The quantity of oxygen shortage was ~= 0.132.
2. I. Vacuum-deoxidation at room temperature The pump can reach a vacuum of 10- 6 Torr, while during deoxidation it can keep up to 10-2-10 =5 TorT. The vacuum measurement is performed by a Fzh-1 model vacuum meter, Three groups of YBCO vacuum-deoxidized samples were prepared for X-ray diffraction, neutron diffraction and measuring T : p t curves, respectively (p is vacuum degree and t is time). All these experiments are used to verify the existence of oxygen diffusion at room temperature and low temperature and then to find the law of Tc affected by the oxygen diffusion under vacuum conditions.
2.2. X-ray diffraction spectrum Our experiments were performed by a D / M A X 3B-type X-ray diffractometer. The samples were divided into four subgroups. One was the original superconductive sample, the others were divided according to the period during which they had been vacuum-deoxidized ( 10-4 Torr): for 6 days, 15 days and 21 days. Figure 1 shows that the two peaks of (014) and (005), and (113 ), change their locations gradually when the vacuum-deoxidation time increases; at the same time, the relative intensity of the two peaks of (200) and (006), and (020), decreases
a=3.8189 b=3.8820
Oxygen shortage
R-factor R (%)
0.132
0.93
c=11.6597
by degrees and the two peaks of (200) and (006), which are originally overlapping, also gradually decrease. All this indicates that more oxygen is lost from the C u - O chains and the CuO2 planes when the samples are vacuum-deoxidated for longer. The experiments show that the samples tend to change into the tetragonal phase [ 1 ]. But the change is not great, which indicates that vacuum-deoxidation at room temperature cannot easily change YBCO samples (orthogonal phase ) into tetragonal phase, and this is also accurately proved by neutron diffraction.
2.3. Neutron diffraction The neutron diffraction experiments were carried out on the powder neutron diffractometer at the Heavy Water Research Reactor of the China Institute of Atomic Energy. The YBCO sample(originaUy T¢0=91.6 K) of 10 g was ground into 325-mesh fine powder and placed in a vanadium cylindrical cell 5 m m in diameter, then put on the sample table and centred on the diffractometer. A 20-scan ranging from 3 ° to 92 ° with a step width of 0.2 ° was carded out with a neutron beam of 1.184 A wavelength from the (002) reflection of a pyrolytic graphite at take-off-angle 10 ° 10'. The neutron data were processed by a full-matrix least-square program and the structural parameters from the data fit are listed in table 1. In order to prove the change of oxygen content in the YBCO sample after vacuumdeoxidation, the neutron diffraction measurement on the sample deoxidized in a vacuum of 10 -4 Tort for 5 days was carded out, and its structural param-
47
Z. Gao et al. / The modulation principle of vacuum on the superconductivity of YBCO
10
20
48
6d
78
2 T H E F A (DEGRE-'b~) Fig. 1. X-ray diffraction spectra of YBCO samples vacuum-deoxidated at room temperature for different periods. Table 2 Structural parameters of 325-mesh powder sample of YBCO after vacuum deoxidation for five days Element
Coordinate
Atom
Site
x
y
z
Occupation number N
Y Ba Cul Cu2 O1 02 03 04 05
lh 2t la 2q le 2s 2r 2g lb
0.5 0.5 0 0 0 0.5 0 0 0.5
0.5 0.5 0 0 0.5 0 0.5 0 0
0.5 0.1764 0 0.3574 0 0.3751 0.3780 0.1610 0
1.0 2.0 1.0 2.0 0.931 2.0 1.867 2.0 0.0
eters are listed in table 2. F r o m the neutron diffraction results presented in tables 1 a n d 2, it can be seen that the value o f oxygen shortage J for the sample having undergone d e o x i d i z a t i o n in 10 - 4 T o r t vacu u m for 5 days increased to 0.202 from the original value o f J = 0.132, a total decrease in oxygen content o f 0.070 o f which 0.049 comes f r o m the C u - O chain a n d 0.021 from the CuO2 plane. It seems that the oxygen in the C u - O chain can diffuse out o f the sample m o r e easily.
Lattice constant (A)
Oxygen shortage
R-factor R(%)
0.202
2.01
a=3.8205 b= 3.8894 c= 11.6689
2.4. Tc m e a s u r e m e n t s
Three different kinds o f e q u i p m e n t with different vacua were used to d e t e r m i n e Tc o f Y B C O samples (see table 3 ). The experiments show that as the vacu u m increases, Tc o f the Y B C O samples decreases notably, a n d different kinds o f oxide superconductors have a similar trend. It is clear that oxygen in the structure o f all kinds o f oxide superconductors can escape through diffusion, which is why their sup e r c o n d u c t i v i t y can be decreased. But alloy superconductors do not have this peculiar property.
48
Z. Gao et al. / The modulation principle of vacuum on the superconductivity of YBCO
Table 3 Three different kinds of equipment with different vacuum were used to compare Tooof Bi; Y- and another system Samples
T,~ (K)
YBa2Cu307_6 (bulk) Bil.7Pbo.3Sr2Ca2Cu3Olo_a(bulk) YBCO (film) Nb3Sn
Liquid N2 thermostat ( ~ 1 atm)
GM-Mode110 K microrefrigerator ( 10-4-10 -~ Tort)
VM-Model 4.2 K microrefrigerator ( 10-6-10 -7 Tort)
91.6 100.0 78 17.9
83 73 47 17.8
71 63 35 18.1
ment, we put the bulk into 02 to restore its Too). We found that the diffusion o f oxygen from the Y B C O sample also vibrates with time, The higher the vacu u m is, the shorter the vibrational p e r i o d and the higher the a m p l i t u d e will be.
2.5. Oscillating curve o f T~(t) in v a c u u m
Through experiments measuring Tc in situ and using the curve fitting method, we obtained two similar patterns. ( 1 ) Curve fitting m e t h o d - the experiments were p e r f o r m e d in vacua o f 10 -2 a n d 10 -4 Torr with Y B C O single-phase samples which had Too o f 93.4 K a n d 92. l K. We o b t a i n e d the oscillating curve o f T,o with d e o x i d a t i o n t i m e (see fig. 2). ( 2 ) In situ m e t h o d - In o r d e r to e x p o u n d the problem accurately, we designed a T¢ measuring instrum e n t which has a variable v a c u u m (see fig. 3 ( a ) ) . Using the in situ method, we o b t a i n e d curves o f T¢o - t for the same samples in vacua o f 10 -2 a n d 10 -4 Torr (see fig. 3 ( b ) ) . (After the first measure-
3. Analysis and discussion M a n y works on oxygen content have shown a closely d e p e n d e n t relationship between Tc a n d oxygen content [ 2 - 7 ]. O u r experiments also showed that oxygen can get in and out o f the Y B C O structure, not only at high temperature, but also at r o o m a n d low t e m p e r a t u r e [8-11 ]. This p h e n o m e n o n has a great effect on the stability o f superconductors, so
96 ~~__~,~
-
90
3 X 10-~Torr
#,
5 X lO-2torr
~Z
(a)
3X 10-STorr
80
5)< I 0 -ztoer
70
25
(b)
50
75
100
t(hr) Fig. 2. Curve of Too-p-t, using curve fitting method. (a) Experiment with liquid nitrogen thermostat. (b) Experiment with OM-Model I 0 K microrefrigerator.
49
Z. Gao et al. / The modulation principle of vacuum on the superconductivity of YBCO
95, ~"
90
25
50
75
t(hr)
Fig. 3. (a) Tc measuring instrument with variable vacuum. (b) Curve of T~o-P--t, using in situ method. we have put forward a rule for controlling the quantity of oxygen to steady To. The principle can be applied to many practical problems, such as the sintering of superconductors at high temperature, their preservation at room temperature and their application at low temperature. We call the law "The Principle of Oscillation and Dynamic Equilibrium of Tj'. The diffusion of oxygen ions and the conductivity of YBCO are easily understood by analysing the structure of YBCO. Because they are a kind of oxygen ion conductor [12-14] and they belong to mixed oxides of perovskite-type structure, there are many oxygen vacancies. So oxygen ions can diffuse very fast, even at room and low temperatures. Free electrons and vacancies also take part in the equilibrium between the crystal lattice and the gas phase. O27~i~ ~ ½02 + V 2 + 2e - , where Olattie 2- e is a mobile oxygen ion in the lattice, 02 represents an oxygen molecule, V 2 represents an oxygen vacancy with two suspending valence bonds and e - a free electron [ 12 ]. The equation and the relationship between T¢ and oxygen content [ 2-7 ] are the basis of our theory. At room temperature and under vacuum, because the concentration difference between ions and molecules of oxygen is very great, oxygen ions must diffuse out of the structure [15-17]. The higher the vacuum is, the greater the concentration difference and the faster oxygen diffusion will be. Therefore, superconductivity changes greatly. On the other hand,
in order to maintain electric neutrality when oxygen ions diffuse and the valences of Yio~s3+and Ba 2+ do not change [ 12], some Cu 3+ ions must turn into Cu 2+. Inversely, on decreasing the degree of vacuum and increasing the partial pressure of oxygen outside the sample, Cu E+ is oxidized and turned into Cu 3+ until the valence state in the structure and the partial pressure of oxygen reach a saturation state corresponding to dynamic equilibrium. Diffusion in a bulk sample first takes place at its surface and then gradually moves inside. This makes the concentration of Cu 3+ ions inside greater than that on the surface, negative ions increase and the potential energy barrier gradually becomes higher, which is to prevent oxygen ions from returning back to the surface. In the meantime, the peaks of the Cu 3+ waves tend towards the surface with diffusion and cause an increase in strength of the positive electric field which strongly attracts the oxygen molecules, and then enter into the lattice. Thus, the peaks of the Cu 3+ waves turn back to the inside of the sample again. Such a process will be repeated again and again. So, the competition and growth-decline between oxygen diffusion and the inversely preventive effect causes the function of oxygen content with time to generate a damping oscillation. This makes the oxygen content vibrate, having polar values, and so To0 oscillates. Because the vacuum is formed by continual pumping and the oxygen content of YBCO has a consumption feature, so the oscillation is attenuated. The higher the vacuum, the faster the oscillation at-
50
z. Gao et aL / The modulation principle of vacuum on the superconductivity o f YBCO
tenuates. We tentatively think that Tc should satisfy the following function:
Wang Rongyao for their help and very nice advice.
Tco oc T c e - a t c o s ( o9t + (a) .
References
To0 is the temperature when the resistance o f YBCO just becomes zero, Tc is the mid-point temperature o f the superconductive transition, t is the time o f pumping the air. fl and o9 are quantities related to the saturation o f the valence state and oxygen diffusion, and (p is a constant related to the lack o f oxygen. Using different methods (in situ method and curve fitting m e t h o d ) in different vacua, we obtained the same oscillating curve with m a n y samples. The error o f T,o is smaller than 0.5 K, and so we surely believe the principle is correct. Similar phenomena in other systems of high-To superconductors can be attributed to their structural similarity. The experiments o f this paper support the oxygen ions pairing theory which is closely related to the structural Cu3+ion in oxide superconductors [ 18 ].
Acknowledgement It is a pleasure to thank S. Tanaka, Zhang Yuheng, Liu Fusui, Zhao Zusen, Wu Youlin, Fan Xiqing and
[ 1] Zhang Yuheng et al., Proc. Chinese Academy Conference on Superconductivity (Institute of Rare Metals, Baoji, China, 1988) p. 125. [2] R.J. Cava et al.. Phys. Rev. B 36 (1987) 5719. [3] W.K. Kwok et al., Phys. Rev. B 37 (1987) 106. [4] K. Kishio et al., Jpn. J. Appl. Phys. 26 (1987) L1228. [ 5 ] J.D. Jorgensen et al., Phys. Rev. B 36 (1987) 5731. [6] S.B. Goodenough et al., Progress in High Temperature Superconductivity, 5 (1988) 18. [7]R. Beyers--et al., Progress in High Temperature Superconductivity 5 ( 1988) 38. [ 8 ] Gao Zhishuang et al., Physica C 167 (1990) 49. [ 9 ] P.P. Freitas and T.S. Plastett, Phys. Rev. B 36 (1987) 5723. [ 10] Gao Zhishuang et al., J. Acta Phys. Sinica 39 (1990) 1320. [ 11 ] H. Shaked et al., Phys. Rev. B 39 (1988) 7363. [ 12 ] Shi Meilun, "Solid Dielectric" (Publishin$ House of Science and TechnologyLiterature, Chong Qing, 1982). [ 13 ] Cui Guowen, "Defect, Diffusion and Sintering" (Qinghua University Publishing House, Beijing 1990). [ 14] Fang Junxin, "Physics of Dielectrics" (Scientific Publishing House, Beijing 1982). [ 15 ] Zhu Shining, et al., Chinese J. Low Temp. Phys. 12 (1990) 219. [ 16] K.N. Tu et al., Phys. Rev. B 39 (1989) 304. [ 17] G. Ottaviani et al., Phys. Rev. B 39 (1989) 9069. [ 18] Liu Fusui, Beijing Int. Conf. on High-T~Superconductivity, (BHTSC'92) China.
Physica C 203 (1992) 45-50 North-Holland
The modulation principle of vacuum on the superconductivity of YBCO Z h i s h u a n g Gao, Baoshi Du, Yiya Peng, Hui Gao, Zhongxing Wang, C h a n g y u Z h u a n d Jianxun Kang Henan Fundamental and Applied Science Research Institute, Zhengzhou University, Henan 450052, China
Jizhou Li, Jilian Yang, Baisheng Zhang, Yongfan Ding and Jian Kang China Institute ofAtomic Energy, Beijing 102413, China
Wannian Wang Institute of Semiconductors, Beijing 100083, China
Received 31 July 1992
By neutron diffractionand other experiments, we have found that oxygenions in YBCOcan diffuse out of the samplein vacuo at room and low temperature, while the T, decreasesgreatly.We have also found that if the vacuum-deoxidationprocesslasts for several days there will be a damping oscillation of T~with time (t), and highervacuum correspondsto a greater amplitude and a shorter period. We tentatively think that T¢should satisfy the followingfunction: T¢0ocTce-~cos (tot+~); it may be due to the diffusion of oxygenand the saturation of the valence state.
1. Introduction Many high-temperature superconductors belong to mixed oxides of perovskite-type structure. Under vacuum conditions and at room temperature or low temperature, reports about relations between oxygen content, vacuum oxygen diffusion and To(t) have seldom been found. Although high-To superconductors are made at high temperature, they are preserved at room temperature and applied at low temperature. Particularly, many film devices are used under vacuum conditions. Therefore, the fluctuation of superconductivity caused by oxygen diffusion and the age of superconductors will remain significant problems before they go into practical uses. Recently, we performed the following experiments at practical temperature ranges to study these problems. First, we treated YBCO samples under vacuum conditions for a certain period of time, then we #r This project was supported by the National Center for Research and Developmenton Superconductivityof China.
used neutron and X-ray diffraction and To-measurement to study the samples. It was proved that structural oxygen in YBCO samples can actually diffuse out of the samples. The results show that at practical temperature ranges oxygen diffusion affects the stability of the superconductivity of oxide superconductors and the effect on film samples is more serious than that on bulk samples. It has also been found that, under a certain vacuum condition and at room temperature or low temperature, the dynamic equilibrium between the movable ionic oxygen in YBCO samples and the molecular oxygen outside samples determines T~. We call the law "The Principle of Oscillation and Dynamic Equilibrium of Tj'.
2. Experimental YBCO (or YBa2 Cu307_ 6 123-phase) samples were synthesized by solid state sintering. Too> 91.6 K. X-ray diffraction and neutron diffraction showed that our samples were single phase, and the occu-
0921-4534/92/$05.00 © 1992 ElsevierSciencePublishers B.V. All rights reserved.
46
Z. Gao et al. / The modulation principle o f vacuum on the superconductivity o f YBCO
Table 1 Structural parameters of normal state YBCO determined by neutron diffraction Element
Coordinate
Occupation Number
Lattice constant
(k)
Atom
Site
x
y
z
N
Y Ba Cu~ Cu2 O1 02 03 04 O3
lh 2t la 2q le 2s 2r 2g lb
0.5 0.5 0 0 0 0.5 0 0 0.5
0.5 0.5 0 0 0.5 0 0.5 0 0
0.5 0.1812 0 0.3544 0 0'.3795 0.3753 0.1618 0
1.0 2.0 1.0 2.0 0.980 2.0 1.888 2.0 0.0
pation number of every element in the lattice was also determined, which indicated that the samples were 123 phase (see table 1 ). The quantity of oxygen shortage was ~= 0.132.
2. I. Vacuum-deoxidation at room temperature The pump can reach a vacuum of 10- 6 Torr, while during deoxidation it can keep up to 10-2-10 =5 TorT. The vacuum measurement is performed by a Fzh-1 model vacuum meter, Three groups of YBCO vacuum-deoxidized samples were prepared for X-ray diffraction, neutron diffraction and measuring T : p t curves, respectively (p is vacuum degree and t is time). All these experiments are used to verify the existence of oxygen diffusion at room temperature and low temperature and then to find the law of Tc affected by the oxygen diffusion under vacuum conditions.
2.2. X-ray diffraction spectrum Our experiments were performed by a D / M A X 3B-type X-ray diffractometer. The samples were divided into four subgroups. One was the original superconductive sample, the others were divided according to the period during which they had been vacuum-deoxidized ( 10-4 Torr): for 6 days, 15 days and 21 days. Figure 1 shows that the two peaks of (014) and (005), and (113 ), change their locations gradually when the vacuum-deoxidation time increases; at the same time, the relative intensity of the two peaks of (200) and (006), and (020), decreases
a=3.8189 b=3.8820
Oxygen shortage
R-factor R (%)
0.132
0.93
c=11.6597
by degrees and the two peaks of (200) and (006), which are originally overlapping, also gradually decrease. All this indicates that more oxygen is lost from the C u - O chains and the CuO2 planes when the samples are vacuum-deoxidated for longer. The experiments show that the samples tend to change into the tetragonal phase [ 1 ]. But the change is not great, which indicates that vacuum-deoxidation at room temperature cannot easily change YBCO samples (orthogonal phase ) into tetragonal phase, and this is also accurately proved by neutron diffraction.
2.3. Neutron diffraction The neutron diffraction experiments were carried out on the powder neutron diffractometer at the Heavy Water Research Reactor of the China Institute of Atomic Energy. The YBCO sample(originaUy T¢0=91.6 K) of 10 g was ground into 325-mesh fine powder and placed in a vanadium cylindrical cell 5 m m in diameter, then put on the sample table and centred on the diffractometer. A 20-scan ranging from 3 ° to 92 ° with a step width of 0.2 ° was carded out with a neutron beam of 1.184 A wavelength from the (002) reflection of a pyrolytic graphite at take-off-angle 10 ° 10'. The neutron data were processed by a full-matrix least-square program and the structural parameters from the data fit are listed in table 1. In order to prove the change of oxygen content in the YBCO sample after vacuumdeoxidation, the neutron diffraction measurement on the sample deoxidized in a vacuum of 10 -4 Tort for 5 days was carded out, and its structural param-
47
Z. Gao et al. / The modulation principle of vacuum on the superconductivity of YBCO
10
20
48
6d
78
2 T H E F A (DEGRE-'b~) Fig. 1. X-ray diffraction spectra of YBCO samples vacuum-deoxidated at room temperature for different periods. Table 2 Structural parameters of 325-mesh powder sample of YBCO after vacuum deoxidation for five days Element
Coordinate
Atom
Site
x
y
z
Occupation number N
Y Ba Cul Cu2 O1 02 03 04 05
lh 2t la 2q le 2s 2r 2g lb
0.5 0.5 0 0 0 0.5 0 0 0.5
0.5 0.5 0 0 0.5 0 0.5 0 0
0.5 0.1764 0 0.3574 0 0.3751 0.3780 0.1610 0
1.0 2.0 1.0 2.0 0.931 2.0 1.867 2.0 0.0
eters are listed in table 2. F r o m the neutron diffraction results presented in tables 1 a n d 2, it can be seen that the value o f oxygen shortage J for the sample having undergone d e o x i d i z a t i o n in 10 - 4 T o r t vacu u m for 5 days increased to 0.202 from the original value o f J = 0.132, a total decrease in oxygen content o f 0.070 o f which 0.049 comes f r o m the C u - O chain a n d 0.021 from the CuO2 plane. It seems that the oxygen in the C u - O chain can diffuse out o f the sample m o r e easily.
Lattice constant (A)
Oxygen shortage
R-factor R(%)
0.202
2.01
a=3.8205 b= 3.8894 c= 11.6689
2.4. Tc m e a s u r e m e n t s
Three different kinds o f e q u i p m e n t with different vacua were used to d e t e r m i n e Tc o f Y B C O samples (see table 3 ). The experiments show that as the vacu u m increases, Tc o f the Y B C O samples decreases notably, a n d different kinds o f oxide superconductors have a similar trend. It is clear that oxygen in the structure o f all kinds o f oxide superconductors can escape through diffusion, which is why their sup e r c o n d u c t i v i t y can be decreased. But alloy superconductors do not have this peculiar property.
48
Z. Gao et al. / The modulation principle of vacuum on the superconductivity of YBCO
Table 3 Three different kinds of equipment with different vacuum were used to compare Tooof Bi; Y- and another system Samples
T,~ (K)
YBa2Cu307_6 (bulk) Bil.7Pbo.3Sr2Ca2Cu3Olo_a(bulk) YBCO (film) Nb3Sn
Liquid N2 thermostat ( ~ 1 atm)
GM-Mode110 K microrefrigerator ( 10-4-10 -~ Tort)
VM-Model 4.2 K microrefrigerator ( 10-6-10 -7 Tort)
91.6 100.0 78 17.9
83 73 47 17.8
71 63 35 18.1
ment, we put the bulk into 02 to restore its Too). We found that the diffusion o f oxygen from the Y B C O sample also vibrates with time, The higher the vacu u m is, the shorter the vibrational p e r i o d and the higher the a m p l i t u d e will be.
2.5. Oscillating curve o f T~(t) in v a c u u m
Through experiments measuring Tc in situ and using the curve fitting method, we obtained two similar patterns. ( 1 ) Curve fitting m e t h o d - the experiments were p e r f o r m e d in vacua o f 10 -2 a n d 10 -4 Torr with Y B C O single-phase samples which had Too o f 93.4 K a n d 92. l K. We o b t a i n e d the oscillating curve o f T,o with d e o x i d a t i o n t i m e (see fig. 2). ( 2 ) In situ m e t h o d - In o r d e r to e x p o u n d the problem accurately, we designed a T¢ measuring instrum e n t which has a variable v a c u u m (see fig. 3 ( a ) ) . Using the in situ method, we o b t a i n e d curves o f T¢o - t for the same samples in vacua o f 10 -2 a n d 10 -4 Torr (see fig. 3 ( b ) ) . (After the first measure-
3. Analysis and discussion M a n y works on oxygen content have shown a closely d e p e n d e n t relationship between Tc a n d oxygen content [ 2 - 7 ]. O u r experiments also showed that oxygen can get in and out o f the Y B C O structure, not only at high temperature, but also at r o o m a n d low t e m p e r a t u r e [8-11 ]. This p h e n o m e n o n has a great effect on the stability o f superconductors, so
96 ~~__~,~
-
90
3 X 10-~Torr
#,
5 X lO-2torr
~Z
(a)
3X 10-STorr
80
5)< I 0 -ztoer
70
25
(b)
50
75
100
t(hr) Fig. 2. Curve of Too-p-t, using curve fitting method. (a) Experiment with liquid nitrogen thermostat. (b) Experiment with OM-Model I 0 K microrefrigerator.
49
Z. Gao et al. / The modulation principle of vacuum on the superconductivity of YBCO
95, ~"
90
25
50
75
t(hr)
Fig. 3. (a) Tc measuring instrument with variable vacuum. (b) Curve of T~o-P--t, using in situ method. we have put forward a rule for controlling the quantity of oxygen to steady To. The principle can be applied to many practical problems, such as the sintering of superconductors at high temperature, their preservation at room temperature and their application at low temperature. We call the law "The Principle of Oscillation and Dynamic Equilibrium of Tj'. The diffusion of oxygen ions and the conductivity of YBCO are easily understood by analysing the structure of YBCO. Because they are a kind of oxygen ion conductor [12-14] and they belong to mixed oxides of perovskite-type structure, there are many oxygen vacancies. So oxygen ions can diffuse very fast, even at room and low temperatures. Free electrons and vacancies also take part in the equilibrium between the crystal lattice and the gas phase. O27~i~ ~ ½02 + V 2 + 2e - , where Olattie 2- e is a mobile oxygen ion in the lattice, 02 represents an oxygen molecule, V 2 represents an oxygen vacancy with two suspending valence bonds and e - a free electron [ 12 ]. The equation and the relationship between T¢ and oxygen content [ 2-7 ] are the basis of our theory. At room temperature and under vacuum, because the concentration difference between ions and molecules of oxygen is very great, oxygen ions must diffuse out of the structure [15-17]. The higher the vacuum is, the greater the concentration difference and the faster oxygen diffusion will be. Therefore, superconductivity changes greatly. On the other hand,
in order to maintain electric neutrality when oxygen ions diffuse and the valences of Yio~s3+and Ba 2+ do not change [ 12], some Cu 3+ ions must turn into Cu 2+. Inversely, on decreasing the degree of vacuum and increasing the partial pressure of oxygen outside the sample, Cu E+ is oxidized and turned into Cu 3+ until the valence state in the structure and the partial pressure of oxygen reach a saturation state corresponding to dynamic equilibrium. Diffusion in a bulk sample first takes place at its surface and then gradually moves inside. This makes the concentration of Cu 3+ ions inside greater than that on the surface, negative ions increase and the potential energy barrier gradually becomes higher, which is to prevent oxygen ions from returning back to the surface. In the meantime, the peaks of the Cu 3+ waves tend towards the surface with diffusion and cause an increase in strength of the positive electric field which strongly attracts the oxygen molecules, and then enter into the lattice. Thus, the peaks of the Cu 3+ waves turn back to the inside of the sample again. Such a process will be repeated again and again. So, the competition and growth-decline between oxygen diffusion and the inversely preventive effect causes the function of oxygen content with time to generate a damping oscillation. This makes the oxygen content vibrate, having polar values, and so To0 oscillates. Because the vacuum is formed by continual pumping and the oxygen content of YBCO has a consumption feature, so the oscillation is attenuated. The higher the vacuum, the faster the oscillation at-
50
z. Gao et aL / The modulation principle of vacuum on the superconductivity o f YBCO
tenuates. We tentatively think that Tc should satisfy the following function:
Wang Rongyao for their help and very nice advice.
Tco oc T c e - a t c o s ( o9t + (a) .
References
To0 is the temperature when the resistance o f YBCO just becomes zero, Tc is the mid-point temperature o f the superconductive transition, t is the time o f pumping the air. fl and o9 are quantities related to the saturation o f the valence state and oxygen diffusion, and (p is a constant related to the lack o f oxygen. Using different methods (in situ method and curve fitting m e t h o d ) in different vacua, we obtained the same oscillating curve with m a n y samples. The error o f T,o is smaller than 0.5 K, and so we surely believe the principle is correct. Similar phenomena in other systems of high-To superconductors can be attributed to their structural similarity. The experiments o f this paper support the oxygen ions pairing theory which is closely related to the structural Cu3+ion in oxide superconductors [ 18 ].
Acknowledgement It is a pleasure to thank S. Tanaka, Zhang Yuheng, Liu Fusui, Zhao Zusen, Wu Youlin, Fan Xiqing and
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