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Study on the oxygenation process during the heat treatment of TFA-MOD YBCO thin films by in situ resistance measurement
Physica C 494 (2013) 148–152
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Study on the oxygenation process during the heat treatment of TFA-MOD YBCO thin films by in situ resistance measurement Timing Qu a,b,⇑, Yunran Xue a, Feng Feng b,c, Rongxia Huang c, Wei Wu c, Kai Shi c, Zhenghe Han c a
Department of Mechanical Engineering, Tsinghua University, Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, Beijing 100084, China Division of Advanced Manufacturing, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China c Applied Superconductivity Research Center, Department of Physics, Tsinghua University, Beijing 100084, China b
a r t i c l e
i n f o
Article history: Received 18 January 2013 Received in revised form 24 April 2013 Accepted 26 April 2013 Available online 4 May 2013 Keywords: TFA-MOD YBCO thin film In situ resistance measurement Oxygenation process Oxygen diffusion
a b s t r a c t The oxygen content is one key factor to determine the properties of YBa2Cu3O6+y (YBCO) high temperature superconductors. In this study, YBCO thin films were produced by TFA-MOD method. The oxygenation process was carried out at 450 °C for 40 min, in various oxygen partial pressures from 0.01 to 1 atm. An in situ resistance measurement system was built up to record the resistance evolution during the whole heat treatment process. It was found that the resistance decreased exponentially and reached a saturate value in a few minutes during oxygen annealing. It was also found both the balanced resistance and the c-axis length of YBCO decreased with increasing oxygen partial pressure. A defect reaction was found to control the mechanism of the oxygenation process. A porosity assisted oxygen diffusion mechanism was proposed to explain the fast diffusion kinetics of oxygen in MOD YBCO thin films. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction YBa2Cu3O6+y (YBCO) coated conductor is one of the most promising high temperature superconducting (HTS) materials, for its high critical current density and excellent magnetic field properties. The metal–organic decomposition (MOD) process can produce high quality YBCO coated conductors with low cost. The heat treatment of the MOD method using trifluoroacetate (TFA) as the precursor includes the pyrolysis process to grow indefinite form of oxide and fluoride oxide, the firing process to form YBCO films at a high temperature and the oxygen annealing process to adjust the oxygen content in the YBCO lattice. The in situ resistance measurement (IRM) is quite convenient and effective to characterize the kinetics of phase evolution in YBCO thin films during the heat treatment. In 2000, Solovyov et al. [1] successfully estimated the growth rate of YBCO films during the heat treatment by using IRM, and found that the growth rate of YBCO films remained constant and was proportional to the square root of the H2O partial pressure in the processing atmo1=2 sphere. Chen et al. [2] obtained the relationships, R ¼ AF 1=4 P H2 O P2=3 t , of the growth rate (R) against the gas flow rate (F), the hydraulic
⇑ Corresponding author at: Department of Mechanical Engineering, Tsinghua University, Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, Beijing 100084, China. Tel.: +86 (0)10 6278 4261; fax: +86 (0)10 6278 5913. E-mail address: [email protected] (T. Qu). 0921-4534/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2013.04.078
pressure ðPH2O Þ and the total pressure (Pt) by using the similar method. The oxygen content is one of the key factors for the superconducting properties of HTS materials. For Bi-2223/Ag tapes, Ic and Tc could be improved by adjusting oxygen content [3,4]. For YBa2Cu3O6+y bulk materials, it was reported that Tc dropped when oxygen content decreased (y ranges from 0.3 to 1) [5,6], and that high oxygen pressure could shorten oxygenation time and improve Tc and Jc [7,8]. The oxygenation process induced micro- and macrocracks [9], which significantly shortened the oxygen diffusion distances [10]. The oxygen diffusion process was systematically studied in YBCO bulk materials by using IRM. LaGraff and Payne [11,12] measured the diffusion coefficient and found that polycrystalline YBCO revealed concentration-dependent diffusion kinetics. A one dimensional diffusion model was proposed by Zhang et al. [13] to explain the resistance variation during oxygen diffusion process in melt-texture grown (MTG) YBCO bulks. An optimization work for the oxygen annealing temperature in TFA-MOD YBCO thin films was also carried out by Chikumoto et al. [14]. Oxygen diffusion process was also studied on YBCO thin films fabricated by pulsed laser deposition (PLD) using different method including electrical conductivity measurement [15], optical diagnosis [16,17], secondary-ion mass spectrometry [18] and X-ray diffraction [19]. In this work, we built up an IRM system for YBCO thin films fabricated by TFA-MOD. The resistance variation curve during the heat treatment process was achieved. The oxygenation of YBa2Cu3O6+y was studied in a small range (y from 0.9 to 1) by changing the
T. Qu et al. / Physica C 494 (2013) 148–152
149
oxygen partial pressure during oxygen annealing. The relationship between the balanced resistance after oxygen annealing and the oxygen content inside the YBCO film was studied and the defect reaction equation from Mehta’s work [20] was discussed. A porosity assisted diffusion mechanism was proposed to explain the fast kinetics of the oxygen diffusion process in MOD YBCO thin films. 2. Experimental details The precursor TFA solution contains three trifluoroacetic acid salts ((CF3COO)3Y, (CF3COO)2Ba, (CF3COO)2Cu) with the mole ratio of 1:2:3 using methanol (CH3OH) as the solvent. 25% PEG2000 was added into the precursor solution to shorten the pyrolysis process to 15 min [21], where PEG means polyethylene glycol, 25% means PEG’s mass fraction relative to the three trifluoroacetic acid salts and 2000 refers to the molecular weight of PEG used. 5 mm 5 mm 1 mm LaAlO3 single crystals with single side polished were used as substrates. The precursor solution was spin coated under the speed of 8000 rpm. The thickness of the final YBCO film was about 150 nm after heat treatments. The pyrolysis process was applied at a heating rate of 25 °C/min. Then firing process was held for an hour at 800 °C. Finally, the oxygenation process was applied at three different temperature (To = 400 °C, 450 °C, 500 °C) for 40 min, which was particularly studied in different oxygen partial pressures (pO2) from 0.01 atm to 1 atm by adjusting the flow rates of nitrogen and oxygen. The actual pO2 of the mixture gas was measured by an oxygen analyzer, with the accuracy to 0.01%. The in situ measurement device (see Fig. 1) was employed to record the resistance evolution during the whole heat treatment process. Four silver wires were connected to the substrate by silver
Fig. 2. Resistance variation curve during the whole heat treatment process.
paint. Then the substrate with silver wires was heated at 700 °C for one hour to guarantee the strong connections between the substrate and the wires. After coating, the sample was put on the ceramic wafer of the specimen holder (see Fig. 1c), with the four attached leads connected to silver wires which went through the four-pass ceramic pipe and connected to a Keithley-2700 multimeter outside the furnace. The Keithley-2700 multi-meter was used to measure the resistance value between the two diagonal points directly. This multi-meter has a wide measurement range from 100 lX to 120 MX. The automatic range mode was used during the measurement. The resistance values of the two channels were recorded at a frequency of 1 Hz and the average value was used. X-ray diffraction (XRD) patterns were measured by a Rigaku Smartlab diffractometer (k = 1.5406 Å) in samples after different oxygen annealing process. The diffraction data were collected in the h–2h step-scan mode. The c-axis length was then extrapolated from the 2h values of the selected (0 0 l) diffraction peaks. The extrapolation function is cos2h/sinh + cos2h/h [22,23]. The surface morphology of selected samples was observed by using a LEO-1530 scanning electron microscope. Special attention was paid to the porosity on the surface. 3. Results
Fig. 1. Illustration for the IRM holder: (a) schematic figure of the IRM holder, (1) the flange, (2) the four-pass ceramic pipe, (3) Al2O3 ceramic plate; (b) overview for the IRM holder inside the quartz tube of the furnace; and (c) details for the specimen holder, (1) Al2O3 ceramic plate, (2) thermocouple, (3) YBCO sample, (4) silver paint, (5) silver wire, (6) connections between silver wires.
The resistance vs. time curve of the whole heat treatment process is shown in Fig. 2. The resistance value decreased from 108 X to 10 X in general, with different characteristics in different stages, such as pyrolysis, firing and oxygenation. For the oxygenation process, the resistance value was normalized by the resistance measured just before oxygen infusing (see the red1 circle in Fig. 2). The resistance curves in different pO2 for samples annealed at 450 °C are shown in Fig. 3. The normalized resistance RðtÞ of all samples decreased until a balance value (Rf) within a short time of less than one minute. The curves can be fitted by an exponential function RðtÞ ¼ A expðt=sÞ þ b (A and b are constants). Both Rf and the characteristic time (s) decreased with increasing pO2. s, which is an indication for the oxygen diffusion rate, decreases with the increasing To and pO2 (see Fig. 4). The X-ray diffraction (XRD) patterns for samples with the same annealing temperature of 450 °C and different pO2 are shown in Fig. 5. Different curves were shifted in order to distinguish them. The (0 0 l) diffraction peaks of YBCO are marked in the figure, whose positions can be used to calculate the c-axis length of 1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.
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Fig. 3. Normalized resistance curves in different oxygen partial pressure during the oxygen annealing process at 450 °C.
Fig. 6. The relationship between the c-axis length and pO2.
4. Discussion 4.1. R–t characteristics for the oxygenation process
Fig. 4. The characteristic time (s) as a function of the oxygen partial pressure pO2 at different oxygenation temperature.
The whole heat treatment process of TFA-MOD YBCO can be divided into three stages, pyrolysis, firing and oxygenation, as shown in Fig. 2. During the firing process, the resistance dropped rapidly till an balanced value after reaching the sintering temperature of 800 °C. It conforms to the result of linear growing conductance in Solovyov et al. [1] and Chen’s et al. [2] work. In the oxygenation process, the resistance decreased exponentially with time at the beginning and reached a saturate value in about one minute. In YBa2Cu3O6+y materials (y is very close to 1), the resistance decreases with increasing oxygen content due to the increase of the hole concentration. This can be explained by the defect reaction shown in Eq. (1), which means that the oxygen (O2) fills into oxygen vacancies ðVO Þ, and releases holes [20,24]. The hole concentration [h] = y increases with raising p O2, which contributes to the decreasing resistance. We focus on this oxygen annealing process in this work.
1 VO þ O2 OO þ 2h 2
ð1Þ
4.2. Oxygenation reaction analysis For YBa2Cu3O6+y, the mass-action expression of Eq. (1) can be given according to Ref. [20]:
KðTÞ ¼ K 0 eDH=kT ¼
½OO p2 1=2 ½VO pO2
¼
ð6 þ yÞy2 1=2
ð1 yÞpO2
ð2Þ
;
where y can be estimated from Refs. [5,6] according to the measured c-axis length, p is the hole concentration which equals to y [20], DH = 0.83 eV is the enthalpy change of the reaction [20], and K(T) is the mass-action constant at certain temperature.
Table 1 Key parameters for the defect reaction shown in Eq. (1) at 450 °C. Fig. 5. X-ray diffraction patterns of samples produced in different oxygen partial pressure at 450 °C.
selected samples. The c-axis length vs. pO2 curves can be achieved, as shown in Fig. 6. It is seen that the c-axis length decreases with increasing pO2.
T (K)
pO2 (atm)
c-axis length (nm)
y [5,6]
calculated K(T)
K(T) from [20]
723
0.071 0.126 0.246 0.520 0.999
1.16817 1.16795 1.16766 1.16743 1.16729
0.9130 0.9277 0.9473 0.9626 0.9719
248.5 232.3 238.4 239.3 234.5
209.1
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Values of K(T) calculated from the experimental data are shown in Table 1. In this work, the range of the c-axis length variation was quite small compared to the studied range in Refs. [5,6], which would bring certain deviation for the estimated y values. However, it is interesting to find that the value of K(T) calculated from different pO2 in the present YBCO thin film were all very close to 240, about 15% higher than that in bulk YBCO [20] (both at 450 °C), which is reasonable for the case of isothermal reaction. It confirms that the defect reaction shown in Eq. (1) is still effective in YBCO thin film system. The discrepancy may come from the improved diffusion mechanism caused by the excellent grain connectivity in the thin film system, which could promote the formation of Oo in Eq. (1) and lead to a larger K(T) than that of the bulk materials. According to the result in Fig. 6, there is approximately a linear relationship between c-axis length and log (pO2). We can draw the relationship between the normalized balanced resistance (Rf) and pO2, as shown in Fig. 7. It is interesting that Rf also has a linear relationship with log (pO2). It is clear that Rf is determined by the carrier concentration, which is directly related to the oxygen content in the YBCO lattice. There should have certain connection between Rf and c-axis length which was affected by the oxygen content. The details are still under investigation.
4.3. Kinetics for the oxygen diffusion process YBCO thin films have an excellent biaxial texture with the c-axis perpendicular and the ab plane parallel to the surface of the substrate. The HTS layer of our samples was as thin as 150 nm while its length and width were 5 mm. A diffusion model was built up to explain the kinetics of the oxygen diffusion process with the following assumptions: (1) The aspect ratio ? 1, so the oxygen diffusion process can be approximately regarded as a one dimensional diffusion process only along the thickness direction (c-axis
direction) (see Fig. 8). (2) According to Refs. [11–13], R / (1 y), when y is very close to 1, and R can be calculated from the average oxygen content of the whole thin film. Following the same arithmetic method of Refs. [11–13], the average normalized resistance RðtÞ can be described as:
t þ Rf ; RðtÞ ¼ ð1 Rf Þ exp
s
s¼
d
p
2
2D
;
ð3Þ
c;eq
where Rf is the normalized balanced resistance, s is the characteristic time, d is the thickness of the thin film, Dc,eq is the equivalent diffusion coefficient along the thickness direction. Eq. (3) can explain the exponential decrease of the resistance with the annealing time (see Fig. 3). It can be estimated from Fig. 4 that 2
Dc;eq ¼
d
ps 2
1011 cm2 =s;
ð4Þ
where d = 150 nm, s = 5–30 s according to Fig. 4. However, the measured Dc in bulk materials is about 1017– 14 10 cm2/s according to Ref. [25], which is far less than Dc,eq achieved in the present work. Actually Dc,eq is quite close to the diffusion coefficient along the ab plane (Dab). Such a fast diffusion kinetics is impossible to be achieved in the interior of pure YBCO materials, though the surface might be. For YBCO thin films fabricated by PLD method, Chen et al. [15] indicated that the grain boundaries acted as short circuits to fasten the oxygen transport. For this work, a porosity assisted diffusion mechanism was proposed to explain the oxygen diffusion process in MOD YBCO thin films (see Fig. 9). It is quite common that there were a lot of gas holes in MOD films (see Fig. 9b), which construct unimpeded ways for oxygen diffusion. The distance between neighboring holes is around 0.4–1 lm in the present sample according to the SEM photos. Oxygen can easily go through the porosity and then diffuse along the ab plane very fast since Dab 1046 Dc. In order to go further along the thickness direction, oxygen will try to meet other deeper porosities and then go through the ab plane in a deeper level. Thus the oxygen diffusion can reach a much higher rate under this porosity assisted diffusion mechanism. It should be noticed that the kinetics for the defect reaction of Eq. (1) is not considered here. Since O2 is one of the reactants in the defect reaction, higher pO2 during oxygen annealing
Fig. 7. Rf values (200 s after oxygenation began) in different oxygen partial pressures at 450 °C.
Fig. 8. Illustration for the one dimensional oxygen diffusion through the thickness direction of YBCO thin films.
Fig. 9. (a) The illustration for the porosity assisted oxygen diffusion mechanism in MOD YBCO thin films. (b) SEM photo for the surface morphology of a typical MOD YBCO thin film. The porosity positions are pointed by arrows.
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contributes to the increasing of the reactant concentration, which leads to a higher reaction rate and a shorter characteristic time s. Thus it is seen in Fig. 4 that s decreased with increasing pO2. This result also suggests that the evolution of the oxygen content in YBCO lattice (parameter y) might not be totally controlled by the diffusion mechanism. There seems to have two procedures, the diffusion of oxygen and the defect reaction shown in Eq. (1). The second procedure should be strongly influenced by the concentration of the oxygen atmosphere.
[5]
[6]
[7]
[8]
5. Conclusions [9]
In situ high temperature resistance as a function of time was measured for the whole heat treatment process of TFA-MOD YBa2Cu3O6+y thin films. The resistance variation could be related to the phase or structural evolution during the heat treatment. In particular, the oxygenation process of present YBCO thin films under different oxygen partial pressure (pO2) was proved to be controlled by the defect reaction shown in Eq. (1). The mass-action constant at 450 °C is about 15% higher than that in bulk YBCO. There is approximately a linear relationship between c-axis length and log (pO2), and also between the normalized balanced resistance (Rf) and log (pO2). There should have certain connection between Rf and c-axis length which was affected by the oxygen content. The oxygen diffusion rate increases with increasing oxygenation temperature (To) and pO2. The fast diffusion kinetics along the thickness direction in YBCO thin films can be explained by the porosity assisted diffusion mechanism.
[10]
[11] [12]
[13]
[14]
[15]
[16]
Acknowledgements
[17]
This work was supported by the Beijing Natural Science Foundation (2122026) and the Fundamental Research Program of Science and Technology Development Foundation of Shenzhen (JCYJ20120614193005764).
[18]
[19] [20]
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Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Study on the oxygenation process during the heat treatment of TFA-MOD YBCO thin films by in situ resistance measurement Timing Qu a,b,⇑, Yunran Xue a, Feng Feng b,c, Rongxia Huang c, Wei Wu c, Kai Shi c, Zhenghe Han c a
Department of Mechanical Engineering, Tsinghua University, Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, Beijing 100084, China Division of Advanced Manufacturing, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China c Applied Superconductivity Research Center, Department of Physics, Tsinghua University, Beijing 100084, China b
a r t i c l e
i n f o
Article history: Received 18 January 2013 Received in revised form 24 April 2013 Accepted 26 April 2013 Available online 4 May 2013 Keywords: TFA-MOD YBCO thin film In situ resistance measurement Oxygenation process Oxygen diffusion
a b s t r a c t The oxygen content is one key factor to determine the properties of YBa2Cu3O6+y (YBCO) high temperature superconductors. In this study, YBCO thin films were produced by TFA-MOD method. The oxygenation process was carried out at 450 °C for 40 min, in various oxygen partial pressures from 0.01 to 1 atm. An in situ resistance measurement system was built up to record the resistance evolution during the whole heat treatment process. It was found that the resistance decreased exponentially and reached a saturate value in a few minutes during oxygen annealing. It was also found both the balanced resistance and the c-axis length of YBCO decreased with increasing oxygen partial pressure. A defect reaction was found to control the mechanism of the oxygenation process. A porosity assisted oxygen diffusion mechanism was proposed to explain the fast diffusion kinetics of oxygen in MOD YBCO thin films. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction YBa2Cu3O6+y (YBCO) coated conductor is one of the most promising high temperature superconducting (HTS) materials, for its high critical current density and excellent magnetic field properties. The metal–organic decomposition (MOD) process can produce high quality YBCO coated conductors with low cost. The heat treatment of the MOD method using trifluoroacetate (TFA) as the precursor includes the pyrolysis process to grow indefinite form of oxide and fluoride oxide, the firing process to form YBCO films at a high temperature and the oxygen annealing process to adjust the oxygen content in the YBCO lattice. The in situ resistance measurement (IRM) is quite convenient and effective to characterize the kinetics of phase evolution in YBCO thin films during the heat treatment. In 2000, Solovyov et al. [1] successfully estimated the growth rate of YBCO films during the heat treatment by using IRM, and found that the growth rate of YBCO films remained constant and was proportional to the square root of the H2O partial pressure in the processing atmo1=2 sphere. Chen et al. [2] obtained the relationships, R ¼ AF 1=4 P H2 O P2=3 t , of the growth rate (R) against the gas flow rate (F), the hydraulic
⇑ Corresponding author at: Department of Mechanical Engineering, Tsinghua University, Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, Beijing 100084, China. Tel.: +86 (0)10 6278 4261; fax: +86 (0)10 6278 5913. E-mail address: [email protected] (T. Qu). 0921-4534/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2013.04.078
pressure ðPH2O Þ and the total pressure (Pt) by using the similar method. The oxygen content is one of the key factors for the superconducting properties of HTS materials. For Bi-2223/Ag tapes, Ic and Tc could be improved by adjusting oxygen content [3,4]. For YBa2Cu3O6+y bulk materials, it was reported that Tc dropped when oxygen content decreased (y ranges from 0.3 to 1) [5,6], and that high oxygen pressure could shorten oxygenation time and improve Tc and Jc [7,8]. The oxygenation process induced micro- and macrocracks [9], which significantly shortened the oxygen diffusion distances [10]. The oxygen diffusion process was systematically studied in YBCO bulk materials by using IRM. LaGraff and Payne [11,12] measured the diffusion coefficient and found that polycrystalline YBCO revealed concentration-dependent diffusion kinetics. A one dimensional diffusion model was proposed by Zhang et al. [13] to explain the resistance variation during oxygen diffusion process in melt-texture grown (MTG) YBCO bulks. An optimization work for the oxygen annealing temperature in TFA-MOD YBCO thin films was also carried out by Chikumoto et al. [14]. Oxygen diffusion process was also studied on YBCO thin films fabricated by pulsed laser deposition (PLD) using different method including electrical conductivity measurement [15], optical diagnosis [16,17], secondary-ion mass spectrometry [18] and X-ray diffraction [19]. In this work, we built up an IRM system for YBCO thin films fabricated by TFA-MOD. The resistance variation curve during the heat treatment process was achieved. The oxygenation of YBa2Cu3O6+y was studied in a small range (y from 0.9 to 1) by changing the
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oxygen partial pressure during oxygen annealing. The relationship between the balanced resistance after oxygen annealing and the oxygen content inside the YBCO film was studied and the defect reaction equation from Mehta’s work [20] was discussed. A porosity assisted diffusion mechanism was proposed to explain the fast kinetics of the oxygen diffusion process in MOD YBCO thin films. 2. Experimental details The precursor TFA solution contains three trifluoroacetic acid salts ((CF3COO)3Y, (CF3COO)2Ba, (CF3COO)2Cu) with the mole ratio of 1:2:3 using methanol (CH3OH) as the solvent. 25% PEG2000 was added into the precursor solution to shorten the pyrolysis process to 15 min [21], where PEG means polyethylene glycol, 25% means PEG’s mass fraction relative to the three trifluoroacetic acid salts and 2000 refers to the molecular weight of PEG used. 5 mm 5 mm 1 mm LaAlO3 single crystals with single side polished were used as substrates. The precursor solution was spin coated under the speed of 8000 rpm. The thickness of the final YBCO film was about 150 nm after heat treatments. The pyrolysis process was applied at a heating rate of 25 °C/min. Then firing process was held for an hour at 800 °C. Finally, the oxygenation process was applied at three different temperature (To = 400 °C, 450 °C, 500 °C) for 40 min, which was particularly studied in different oxygen partial pressures (pO2) from 0.01 atm to 1 atm by adjusting the flow rates of nitrogen and oxygen. The actual pO2 of the mixture gas was measured by an oxygen analyzer, with the accuracy to 0.01%. The in situ measurement device (see Fig. 1) was employed to record the resistance evolution during the whole heat treatment process. Four silver wires were connected to the substrate by silver
Fig. 2. Resistance variation curve during the whole heat treatment process.
paint. Then the substrate with silver wires was heated at 700 °C for one hour to guarantee the strong connections between the substrate and the wires. After coating, the sample was put on the ceramic wafer of the specimen holder (see Fig. 1c), with the four attached leads connected to silver wires which went through the four-pass ceramic pipe and connected to a Keithley-2700 multimeter outside the furnace. The Keithley-2700 multi-meter was used to measure the resistance value between the two diagonal points directly. This multi-meter has a wide measurement range from 100 lX to 120 MX. The automatic range mode was used during the measurement. The resistance values of the two channels were recorded at a frequency of 1 Hz and the average value was used. X-ray diffraction (XRD) patterns were measured by a Rigaku Smartlab diffractometer (k = 1.5406 Å) in samples after different oxygen annealing process. The diffraction data were collected in the h–2h step-scan mode. The c-axis length was then extrapolated from the 2h values of the selected (0 0 l) diffraction peaks. The extrapolation function is cos2h/sinh + cos2h/h [22,23]. The surface morphology of selected samples was observed by using a LEO-1530 scanning electron microscope. Special attention was paid to the porosity on the surface. 3. Results
Fig. 1. Illustration for the IRM holder: (a) schematic figure of the IRM holder, (1) the flange, (2) the four-pass ceramic pipe, (3) Al2O3 ceramic plate; (b) overview for the IRM holder inside the quartz tube of the furnace; and (c) details for the specimen holder, (1) Al2O3 ceramic plate, (2) thermocouple, (3) YBCO sample, (4) silver paint, (5) silver wire, (6) connections between silver wires.
The resistance vs. time curve of the whole heat treatment process is shown in Fig. 2. The resistance value decreased from 108 X to 10 X in general, with different characteristics in different stages, such as pyrolysis, firing and oxygenation. For the oxygenation process, the resistance value was normalized by the resistance measured just before oxygen infusing (see the red1 circle in Fig. 2). The resistance curves in different pO2 for samples annealed at 450 °C are shown in Fig. 3. The normalized resistance RðtÞ of all samples decreased until a balance value (Rf) within a short time of less than one minute. The curves can be fitted by an exponential function RðtÞ ¼ A expðt=sÞ þ b (A and b are constants). Both Rf and the characteristic time (s) decreased with increasing pO2. s, which is an indication for the oxygen diffusion rate, decreases with the increasing To and pO2 (see Fig. 4). The X-ray diffraction (XRD) patterns for samples with the same annealing temperature of 450 °C and different pO2 are shown in Fig. 5. Different curves were shifted in order to distinguish them. The (0 0 l) diffraction peaks of YBCO are marked in the figure, whose positions can be used to calculate the c-axis length of 1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.
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Fig. 3. Normalized resistance curves in different oxygen partial pressure during the oxygen annealing process at 450 °C.
Fig. 6. The relationship between the c-axis length and pO2.
4. Discussion 4.1. R–t characteristics for the oxygenation process
Fig. 4. The characteristic time (s) as a function of the oxygen partial pressure pO2 at different oxygenation temperature.
The whole heat treatment process of TFA-MOD YBCO can be divided into three stages, pyrolysis, firing and oxygenation, as shown in Fig. 2. During the firing process, the resistance dropped rapidly till an balanced value after reaching the sintering temperature of 800 °C. It conforms to the result of linear growing conductance in Solovyov et al. [1] and Chen’s et al. [2] work. In the oxygenation process, the resistance decreased exponentially with time at the beginning and reached a saturate value in about one minute. In YBa2Cu3O6+y materials (y is very close to 1), the resistance decreases with increasing oxygen content due to the increase of the hole concentration. This can be explained by the defect reaction shown in Eq. (1), which means that the oxygen (O2) fills into oxygen vacancies ðVO Þ, and releases holes [20,24]. The hole concentration [h] = y increases with raising p O2, which contributes to the decreasing resistance. We focus on this oxygen annealing process in this work.
1 VO þ O2 OO þ 2h 2
ð1Þ
4.2. Oxygenation reaction analysis For YBa2Cu3O6+y, the mass-action expression of Eq. (1) can be given according to Ref. [20]:
KðTÞ ¼ K 0 eDH=kT ¼
½OO p2 1=2 ½VO pO2
¼
ð6 þ yÞy2 1=2
ð1 yÞpO2
ð2Þ
;
where y can be estimated from Refs. [5,6] according to the measured c-axis length, p is the hole concentration which equals to y [20], DH = 0.83 eV is the enthalpy change of the reaction [20], and K(T) is the mass-action constant at certain temperature.
Table 1 Key parameters for the defect reaction shown in Eq. (1) at 450 °C. Fig. 5. X-ray diffraction patterns of samples produced in different oxygen partial pressure at 450 °C.
selected samples. The c-axis length vs. pO2 curves can be achieved, as shown in Fig. 6. It is seen that the c-axis length decreases with increasing pO2.
T (K)
pO2 (atm)
c-axis length (nm)
y [5,6]
calculated K(T)
K(T) from [20]
723
0.071 0.126 0.246 0.520 0.999
1.16817 1.16795 1.16766 1.16743 1.16729
0.9130 0.9277 0.9473 0.9626 0.9719
248.5 232.3 238.4 239.3 234.5
209.1
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Values of K(T) calculated from the experimental data are shown in Table 1. In this work, the range of the c-axis length variation was quite small compared to the studied range in Refs. [5,6], which would bring certain deviation for the estimated y values. However, it is interesting to find that the value of K(T) calculated from different pO2 in the present YBCO thin film were all very close to 240, about 15% higher than that in bulk YBCO [20] (both at 450 °C), which is reasonable for the case of isothermal reaction. It confirms that the defect reaction shown in Eq. (1) is still effective in YBCO thin film system. The discrepancy may come from the improved diffusion mechanism caused by the excellent grain connectivity in the thin film system, which could promote the formation of Oo in Eq. (1) and lead to a larger K(T) than that of the bulk materials. According to the result in Fig. 6, there is approximately a linear relationship between c-axis length and log (pO2). We can draw the relationship between the normalized balanced resistance (Rf) and pO2, as shown in Fig. 7. It is interesting that Rf also has a linear relationship with log (pO2). It is clear that Rf is determined by the carrier concentration, which is directly related to the oxygen content in the YBCO lattice. There should have certain connection between Rf and c-axis length which was affected by the oxygen content. The details are still under investigation.
4.3. Kinetics for the oxygen diffusion process YBCO thin films have an excellent biaxial texture with the c-axis perpendicular and the ab plane parallel to the surface of the substrate. The HTS layer of our samples was as thin as 150 nm while its length and width were 5 mm. A diffusion model was built up to explain the kinetics of the oxygen diffusion process with the following assumptions: (1) The aspect ratio ? 1, so the oxygen diffusion process can be approximately regarded as a one dimensional diffusion process only along the thickness direction (c-axis
direction) (see Fig. 8). (2) According to Refs. [11–13], R / (1 y), when y is very close to 1, and R can be calculated from the average oxygen content of the whole thin film. Following the same arithmetic method of Refs. [11–13], the average normalized resistance RðtÞ can be described as:
t þ Rf ; RðtÞ ¼ ð1 Rf Þ exp
s
s¼
d
p
2
2D
;
ð3Þ
c;eq
where Rf is the normalized balanced resistance, s is the characteristic time, d is the thickness of the thin film, Dc,eq is the equivalent diffusion coefficient along the thickness direction. Eq. (3) can explain the exponential decrease of the resistance with the annealing time (see Fig. 3). It can be estimated from Fig. 4 that 2
Dc;eq ¼
d
ps 2
1011 cm2 =s;
ð4Þ
where d = 150 nm, s = 5–30 s according to Fig. 4. However, the measured Dc in bulk materials is about 1017– 14 10 cm2/s according to Ref. [25], which is far less than Dc,eq achieved in the present work. Actually Dc,eq is quite close to the diffusion coefficient along the ab plane (Dab). Such a fast diffusion kinetics is impossible to be achieved in the interior of pure YBCO materials, though the surface might be. For YBCO thin films fabricated by PLD method, Chen et al. [15] indicated that the grain boundaries acted as short circuits to fasten the oxygen transport. For this work, a porosity assisted diffusion mechanism was proposed to explain the oxygen diffusion process in MOD YBCO thin films (see Fig. 9). It is quite common that there were a lot of gas holes in MOD films (see Fig. 9b), which construct unimpeded ways for oxygen diffusion. The distance between neighboring holes is around 0.4–1 lm in the present sample according to the SEM photos. Oxygen can easily go through the porosity and then diffuse along the ab plane very fast since Dab 1046 Dc. In order to go further along the thickness direction, oxygen will try to meet other deeper porosities and then go through the ab plane in a deeper level. Thus the oxygen diffusion can reach a much higher rate under this porosity assisted diffusion mechanism. It should be noticed that the kinetics for the defect reaction of Eq. (1) is not considered here. Since O2 is one of the reactants in the defect reaction, higher pO2 during oxygen annealing
Fig. 7. Rf values (200 s after oxygenation began) in different oxygen partial pressures at 450 °C.
Fig. 8. Illustration for the one dimensional oxygen diffusion through the thickness direction of YBCO thin films.
Fig. 9. (a) The illustration for the porosity assisted oxygen diffusion mechanism in MOD YBCO thin films. (b) SEM photo for the surface morphology of a typical MOD YBCO thin film. The porosity positions are pointed by arrows.
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contributes to the increasing of the reactant concentration, which leads to a higher reaction rate and a shorter characteristic time s. Thus it is seen in Fig. 4 that s decreased with increasing pO2. This result also suggests that the evolution of the oxygen content in YBCO lattice (parameter y) might not be totally controlled by the diffusion mechanism. There seems to have two procedures, the diffusion of oxygen and the defect reaction shown in Eq. (1). The second procedure should be strongly influenced by the concentration of the oxygen atmosphere.
[5]
[6]
[7]
[8]
5. Conclusions [9]
In situ high temperature resistance as a function of time was measured for the whole heat treatment process of TFA-MOD YBa2Cu3O6+y thin films. The resistance variation could be related to the phase or structural evolution during the heat treatment. In particular, the oxygenation process of present YBCO thin films under different oxygen partial pressure (pO2) was proved to be controlled by the defect reaction shown in Eq. (1). The mass-action constant at 450 °C is about 15% higher than that in bulk YBCO. There is approximately a linear relationship between c-axis length and log (pO2), and also between the normalized balanced resistance (Rf) and log (pO2). There should have certain connection between Rf and c-axis length which was affected by the oxygen content. The oxygen diffusion rate increases with increasing oxygenation temperature (To) and pO2. The fast diffusion kinetics along the thickness direction in YBCO thin films can be explained by the porosity assisted diffusion mechanism.
[10]
[11] [12]
[13]
[14]
[15]
[16]
Acknowledgements
[17]
This work was supported by the Beijing Natural Science Foundation (2122026) and the Fundamental Research Program of Science and Technology Development Foundation of Shenzhen (JCYJ20120614193005764).
[18]
[19] [20]
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