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Electrical contacts to YBCO using metal spray techniques
Electrical contacts to YBCO using metal spray techniques S.P. Ashworth, C. Beduz, K. Harrison*, R.G. Scurlock and Y. Yang Institute of Cryogenics, University of Southampton, Southampton SO9 5NH, UK *Metco Limited, Chobham, Surrey, UK
Received 6 April 1989 A method for making large scale electrical contacts to yttrium based high temperature superconductors is described. This involves flame spraying of copper wire onto an abrasion cleaned superconductor surface. Using this technique contact resistivities of the order of 500/.tf~ cm 2 can be easily achieved. This is a process which can be carried out without the need for large scale vacuum equipment or clean facilities, and as such represents a step towards the production of cryostabilized high Tc superconductors outside the laboratory environment. Keywords: high Tc s u p e r c o n d u c t i v i t y ; superconductors; electrical contacts
A major obstacle to the testing and application of the YBa2Cu307_x high temperature superconductors is the high contact resistivity between a normal metal conductor and the superconductor. This contact resistivity usually exhibits a semiconductor type behaviour, in that the resistance of any given contact increases as the temperature is decreased. The best types of metal-superconductor contact to date are those produced by thin film methods. This usually involves: 1 no exposure of the superconductor to air or moisture; 2 sputter etching of the superconductor surface immediately before applying the contacts; and 3 deposition of noble metal films by sputtering or other vacuum deposition techniques. Ekin et al. have reported contact resistivities of the order of 10-lo ttt2 cm 2 using these methods 1. Zero resistance contacts have been produced but only at helium temperatures 2. The thin film methods are useful laboratory techniques, and may prove to be suitable for applications where only small contact areas are required and clean, high vacuum techniques are already in use, for example, in electronic device manufacture. Such methods are not suited to large scale applications such as cryostabilization of magnets and high current devices. Techniques have also been described in which silver or silver oxide is added to the YBa2 Cu3 O7-x before the final sinter 3. These techniques produce a sample which appears to be 'metallic', i.e has a positive temperature coefficient of resistance and is quoted as having a very low contact resistivity. Investigations at Southampton indicate that care must be taken with data of this type. Above the critical temperature the 'metallic' behaviour is due to the current being carried by the silver matrix and not by the superconductor; below the critical temperature the current is transferred to the superconductor but the contact area is distributed 0011-2275/89/121124-04 $03.00 ~ 1989 Butterworth& Co (Publishers) Ltd 1124 Cryogenics 1989 Vol 29 December
throughout the volume of the sample and resistivity should no longer be calculated using the superficial area only. There is no clear evidence that these impregnation methods produce a 'metallic' contact rather than simply producing a large area of'semiconductor' type contact. Although such techniques are useful in that they produce a low resistance contact (necessary for injecting large currents into samples) they do not reduce the area resistivity of the contact, which is desirable for cryostabilization. Cryostabilization of a superconductor is a very severe t e s t o f superconductor to normal metal contacts. If a normal zone is produced in a current carrying superconductor the current must be carried by a parallel, low resistivity normal metal. The contact resistivity must be very low to minimize the heat generated by high currents and there must be a very low thermal resistivity at the boundary to allow any heat generated to be removed by the coolant, and thus avoid the production and propagation of normal zones in the superconductor4. These low resistivity contacts must be present throughout the length of a current carrying superconductor and so it is desirable to develop methods of producing low resistivity contacts over areas of square metres rather than the areas of square centimetres with thin film techniques. This paper is a preliminary report on an extensive project to develop such contact methods, and concentrates on metal spray techniques. Sample preparation
Samples of YBa2Cu3OT_x were prepared by a direct sintering method, starting with powders of Yz 0 3 (99.9% purity), BaCO3 (99.9%) and CuO (99.999%). These were mixed and sintered at 960°C for 24 h, reground and cold pressed at 600 MPa to form discs of 25 mm diameter and 2 mm thick. The samples were then annealed in flowing
Electrical contacts to YBCO." S. P. Ashworth et al.
oxygen for a total of 24 h. The superconductor discs were then cut into 20 x 5 x l mm bars and stored under dry air at room temperature before application of the contact films. The normal metal films were then cut to produce four contact pads to which copper wire was bonded by silver paint to perform the measurements. Contact films applied by plasma spray and flame spray will be described in this paper.
1
20 m
- - % - -\ Superconductor
Plasma spray
In this technique a metal powder (particle size < 100 #m) is injected into a plasma produced by a d.c. current flowing through argon gas. The high temperature of the plasma partially melts the metal and the semi-molten particles are accelerated towards the target by the high pressure argon gas. At the gun nozzle the plasma temperature can be of the order of 15 000°C and the particle velocity up to 600m s - l , slowing to 200m s -~ at a distance of 15 cm. On impact the particle melts, flattens and subsequently solidifies, producing a very strong bond with the target surface 5. In the present tests the intergrain bonding of the superconductor failed before the coppersuperconductor bond. This high bond strength is characteristic of plasma sprayed coatings. Flame spray
In this method of metal coating, coating material in wire form is fed into an oxygen-acetylene combustion flame. The metal wire melts and a high pressure air jet directs the molten metal particles towards the target. The flame spray produces metal particles with a lower velocity and larger size than the plasma spray. Both methods will produce particles of approximately the same temperature but the amount of energy tranferred to the target for a given coating thickness is much higher for the plasma spray technique. A Metco 9M plasma Spray system and a Metco 12E flame spray system were used with a spray head to target distance of 15cm. Both systems were used to deposit coatings to a thickness of 75/~m at a typical rate of 25/am per pass of the spray head. Feed metals of copper, nickel and tin/zinc (60/40) were used for the flame spray and copper only for the plasma spray. It must be emphasized that both these deposition techniques were used in air and, although areas of only 20 × 10 mm were coated in these tests, the same techniques are routinely used by a number of industries to produce even, well bonded coatings on complex shapes with areas of many square metres.
Figure 1 Schematic diagram of six wire arrangement for measuring sample and contact resistivities
across the bulk superconductor only, V2. Typical measuring current, i, was 10 mA, with contact pad sizes 5 x 3 mm. A silicon diode thermometer was mounted on the sample. The sample was then placed in a nitrogen flow cryostat and cooled to 77 K from room temperature over approximately 2 h and then warmed back to room temperature over a similar period. V1 and V2 were recorded as a function of temperature and time during both the cool-down and warm-up; no hysteresis was observed in I"1 or V2. The temperature difference between the upper surface of the sample and the cryostat base plate was less than 0.5 K at all times. The contact resistance is obtained from Rc=(VI-oW2)/i, where o~= AV1/AV2(Tc), as shown in Figure 2, and is assumed to be a geometrical factor. An equivalent electrical circuit for the system is shown as an inset on Figure 2. Figure 2 shows examples of V1 and V2 as a function of temperature for the tin/zinc flame sprayed contacts. The value of V2 shows the bulk superconductor undergoing a transition to zero resistance at 95 K, whilst 1,'1increases as the temperature is decreased. Figure 3 shows the value of ro extracted from such data for a number of types of contact. Curves 1, 2 and 3 (nickel flame spray, copper plasma spray and tin/zinc flame spray) all exhibit the semiconductor type behaviour characteristic of normal metal contacts on Y based high temperature superconductors; r~ is a very fast function of temperature. Note that the nickel sample shows contact resistivities an order of magnitude worse than the other samples. Curves 4 and 5 of Figure 3 are two examples of flame sprayed copper contacts. As well as showing a much lower
0.08
v~ 'E
O. 06
.~1
~ . ~ . o n
t act s ,
_V2k , ~ - ~ l ~ . _ . , . ~
t..
Experimental details These measurements were performed using a six wire technique as shown in Figure 1. Current was passed through the outer pads and the voltage between the current carrying contacts measured using a second set of copper wires attached to the same contacts. The silver paint bonds of these two wires should be separate so as to measure only the voltage across the normal metal, resistive contact layer and bulk superconductor, Vt. A second set of metal contacts is used to measure the voltage
Bulk superconductor
J~ 0.04 t. to
f .,.., 0.02 >o
V2 contacts
o
80
120
160
i
l
200
240
280
320
Temperature (K)
Figure 2
Typical data for voltage v e r s u s temperature of a sample (I/2) and sample plus contacts (Vl)
Cryogenics 1989 Vol 29 December
1125
Electrical contacts to YBCO: S. P. Ashworth et al. 10
20
LJ
1G
a
>.12
"~
U
c.j o
80
I 120
I 160
I 200
J 2t10
1 280
~ 0
320
80
120
Temperature (K}
Figure 3 Contact resistivity versus temperature for a number of different samples. 1, Nickel flame spray; 2, copper plasma spray; 3, tin/zinc flame spray; 4, copper flame spray; 5, copper flame spray w i t h surface abrasion cleaning in air immediately before flame spraying
contact resistivity at 77 K than the previous discussed contact metals, the copper flame sprayed contacts show a very different dependence on temperature; a linear increase in r= rather than the more 'exponential' increase of the other samples. The best contacts obtained by vacuum deposition techniques rely on cleaning the superconductor surface by sputter etching immediately before deposition of the contact material. Cleaning the surface by abrasion (curve 5) in air immediately before flame spraying also produces a significantly lower re. A qualitative explanation of these results is as follows. The contact material produces a region of the YBaCuO material which is not superconducting above 77 K. This is in addition to an interface layer already degraded by exposure to atmosphere; removal of this layer by abrasion is helpful. The magnetic nickel obviously has more effect than the tin/zinc. The resistive contacts are apparently made worse by the transient heating of the surface produced by plasma spraying. The temperature achieved by this heating is, however, low (< 450 K) compared to the temperatures used during production of the superconductor (> 1300 K), indicating that the semiconductor contact is irreversibly damaged by quite low temperatures. This hypothesis was examined by exposing a plasma sprayed copper contact to 420 K for 300 s, before remeasuring the re(T) behaviour. This is shown in Figure 4. The room temperature contact resistivity increased by a factor of 1.5 but the r~(77 K) increased by a factor of 4. This sensitivity to moderate temperatures is also shown in Figure 5, where a sample was heated at 420 K for varying times and the contact resistivity measured at room temperature and at liquid nitrogen temperature. A large increase in rc(77 K) compared to the room temperature value is apparent. Measurements have been performed using flame sprayed silver contacts and these have produced contact resistivities which are below the sensitivity of the present apparatus, i.e. < 10-af~ cm 2. Further measurements on these samples will be reported at a later date s. There are two obvious mechanisms for the conversion of superconducting YBa 2 Cu3 O7-~ into a semiconductor by a metallic layer. These involve either the removal of oxygen by the metal or the diffusion of metal into the superconductor. Cava et al. 6 measured the resistivity of
1126
Cryogenics 1989 Vol 29 December
200
160
2q0
280
320
Temperature (K)
Figure 4 Degradation of contacts caused by heating to 420 K for 300s. Contacts produced by plasma sprayed copper, a, After heating; b, before heating
a bulk sample of YBa2Cu3OT_ x superconductor as a function of x and showed that semiconductor-like behaviour could be induced by an increase in x from 0.0 to 0.1. This was also accompanied by a reduction in the critical temperature of the bulk sample. The present results suggest that the superconductor-normal metal interface gives up oxygen very readily and that the best contacts will be produced when techniques are used to minimize this oxygen loss. It is also possible that the YBCO free surface is intrinsically non-metallic, as discussed by Egdell and Lavell7; this would also lead to high values of contact resistivity.
Conclusions Copper flame spraying provides a good method for producing large scale, moderately low resistance electrical contacts to YBa2Cua07_x high temperature superconductors. If the surface of the superconductor is cleaned immediately before deposition of the copper layer, surface contact resistivities of 500/~f~ cm 2 are easily achievable. This allows current densities of over 100A cm -2 to be passed through contacts without exceeding the critical heat flux for the onset of film boiling in liquid nitrogen. 20
E
16
>- 12
t=
20
tl0
60
80
' 100
Time (min)
Figure 5 Change in room temperature ( O ) and 77 K ( l l ) contact resistivity due t o heat treatment at 420 K. Plasma sprayed copper sample
Electrical contacts to YBCO: S. P. Ashworth et al. Exposure to moderate temperatures damages the contact layer in an, as yet, unknown way but evidence seems to indicate that the damage is due to migration of oxygen atoms to the normal metal rather than the migration of metal atoms into the superconductor.
Acknowledgements The authors would like to acknowledge the technical assistance of J. Mayne (Metco Limited) and financial assistance from BOC Limited. Y. Yang acknowledges financial support from the British Council and the Chinese Government.
References 1 Ekin, J.W., Larson, T.M. and Bergen, N.F. Appl Phys Lett (1988) 52(21) 1819 2 Weiek,A.D. Appl Phys Lett (1988) 53(13) 1216 3 Yokoynma, S., Yamada, T., gubo, Y., Epwa, K. and Simuzu, Y. Cryogenics (1988) 28 734 4 Ashworth,S.P., Beduz, C., Scurlock, R.G. and Yang, Y. Proc ICEC 12 Butterworths, Guildford, UK (1988) 5 Herman,H. Scientific American reviewarticle (September 1988)p 88 6 Cava, R.J., Batlog, B., Chen, C.H., Rietman, E.A., Zahurak, S.M. and Werber, D. Phys Rev B (1987) 36 5917 7 Egdell, R.G. and Lavell, W.R. Z Phys B (1989) 74 279 8 Ashworth,S.P., Bndt~ C., Harrison, K., Scurlock, R.G. and Yang, Y. Adv Cryog Eng (1989) 34
Cryogenics 1989 Vol 29 December 1127
Received 6 April 1989 A method for making large scale electrical contacts to yttrium based high temperature superconductors is described. This involves flame spraying of copper wire onto an abrasion cleaned superconductor surface. Using this technique contact resistivities of the order of 500/.tf~ cm 2 can be easily achieved. This is a process which can be carried out without the need for large scale vacuum equipment or clean facilities, and as such represents a step towards the production of cryostabilized high Tc superconductors outside the laboratory environment. Keywords: high Tc s u p e r c o n d u c t i v i t y ; superconductors; electrical contacts
A major obstacle to the testing and application of the YBa2Cu307_x high temperature superconductors is the high contact resistivity between a normal metal conductor and the superconductor. This contact resistivity usually exhibits a semiconductor type behaviour, in that the resistance of any given contact increases as the temperature is decreased. The best types of metal-superconductor contact to date are those produced by thin film methods. This usually involves: 1 no exposure of the superconductor to air or moisture; 2 sputter etching of the superconductor surface immediately before applying the contacts; and 3 deposition of noble metal films by sputtering or other vacuum deposition techniques. Ekin et al. have reported contact resistivities of the order of 10-lo ttt2 cm 2 using these methods 1. Zero resistance contacts have been produced but only at helium temperatures 2. The thin film methods are useful laboratory techniques, and may prove to be suitable for applications where only small contact areas are required and clean, high vacuum techniques are already in use, for example, in electronic device manufacture. Such methods are not suited to large scale applications such as cryostabilization of magnets and high current devices. Techniques have also been described in which silver or silver oxide is added to the YBa2 Cu3 O7-x before the final sinter 3. These techniques produce a sample which appears to be 'metallic', i.e has a positive temperature coefficient of resistance and is quoted as having a very low contact resistivity. Investigations at Southampton indicate that care must be taken with data of this type. Above the critical temperature the 'metallic' behaviour is due to the current being carried by the silver matrix and not by the superconductor; below the critical temperature the current is transferred to the superconductor but the contact area is distributed 0011-2275/89/121124-04 $03.00 ~ 1989 Butterworth& Co (Publishers) Ltd 1124 Cryogenics 1989 Vol 29 December
throughout the volume of the sample and resistivity should no longer be calculated using the superficial area only. There is no clear evidence that these impregnation methods produce a 'metallic' contact rather than simply producing a large area of'semiconductor' type contact. Although such techniques are useful in that they produce a low resistance contact (necessary for injecting large currents into samples) they do not reduce the area resistivity of the contact, which is desirable for cryostabilization. Cryostabilization of a superconductor is a very severe t e s t o f superconductor to normal metal contacts. If a normal zone is produced in a current carrying superconductor the current must be carried by a parallel, low resistivity normal metal. The contact resistivity must be very low to minimize the heat generated by high currents and there must be a very low thermal resistivity at the boundary to allow any heat generated to be removed by the coolant, and thus avoid the production and propagation of normal zones in the superconductor4. These low resistivity contacts must be present throughout the length of a current carrying superconductor and so it is desirable to develop methods of producing low resistivity contacts over areas of square metres rather than the areas of square centimetres with thin film techniques. This paper is a preliminary report on an extensive project to develop such contact methods, and concentrates on metal spray techniques. Sample preparation
Samples of YBa2Cu3OT_x were prepared by a direct sintering method, starting with powders of Yz 0 3 (99.9% purity), BaCO3 (99.9%) and CuO (99.999%). These were mixed and sintered at 960°C for 24 h, reground and cold pressed at 600 MPa to form discs of 25 mm diameter and 2 mm thick. The samples were then annealed in flowing
Electrical contacts to YBCO." S. P. Ashworth et al.
oxygen for a total of 24 h. The superconductor discs were then cut into 20 x 5 x l mm bars and stored under dry air at room temperature before application of the contact films. The normal metal films were then cut to produce four contact pads to which copper wire was bonded by silver paint to perform the measurements. Contact films applied by plasma spray and flame spray will be described in this paper.
1
20 m
- - % - -\ Superconductor
Plasma spray
In this technique a metal powder (particle size < 100 #m) is injected into a plasma produced by a d.c. current flowing through argon gas. The high temperature of the plasma partially melts the metal and the semi-molten particles are accelerated towards the target by the high pressure argon gas. At the gun nozzle the plasma temperature can be of the order of 15 000°C and the particle velocity up to 600m s - l , slowing to 200m s -~ at a distance of 15 cm. On impact the particle melts, flattens and subsequently solidifies, producing a very strong bond with the target surface 5. In the present tests the intergrain bonding of the superconductor failed before the coppersuperconductor bond. This high bond strength is characteristic of plasma sprayed coatings. Flame spray
In this method of metal coating, coating material in wire form is fed into an oxygen-acetylene combustion flame. The metal wire melts and a high pressure air jet directs the molten metal particles towards the target. The flame spray produces metal particles with a lower velocity and larger size than the plasma spray. Both methods will produce particles of approximately the same temperature but the amount of energy tranferred to the target for a given coating thickness is much higher for the plasma spray technique. A Metco 9M plasma Spray system and a Metco 12E flame spray system were used with a spray head to target distance of 15cm. Both systems were used to deposit coatings to a thickness of 75/~m at a typical rate of 25/am per pass of the spray head. Feed metals of copper, nickel and tin/zinc (60/40) were used for the flame spray and copper only for the plasma spray. It must be emphasized that both these deposition techniques were used in air and, although areas of only 20 × 10 mm were coated in these tests, the same techniques are routinely used by a number of industries to produce even, well bonded coatings on complex shapes with areas of many square metres.
Figure 1 Schematic diagram of six wire arrangement for measuring sample and contact resistivities
across the bulk superconductor only, V2. Typical measuring current, i, was 10 mA, with contact pad sizes 5 x 3 mm. A silicon diode thermometer was mounted on the sample. The sample was then placed in a nitrogen flow cryostat and cooled to 77 K from room temperature over approximately 2 h and then warmed back to room temperature over a similar period. V1 and V2 were recorded as a function of temperature and time during both the cool-down and warm-up; no hysteresis was observed in I"1 or V2. The temperature difference between the upper surface of the sample and the cryostat base plate was less than 0.5 K at all times. The contact resistance is obtained from Rc=(VI-oW2)/i, where o~= AV1/AV2(Tc), as shown in Figure 2, and is assumed to be a geometrical factor. An equivalent electrical circuit for the system is shown as an inset on Figure 2. Figure 2 shows examples of V1 and V2 as a function of temperature for the tin/zinc flame sprayed contacts. The value of V2 shows the bulk superconductor undergoing a transition to zero resistance at 95 K, whilst 1,'1increases as the temperature is decreased. Figure 3 shows the value of ro extracted from such data for a number of types of contact. Curves 1, 2 and 3 (nickel flame spray, copper plasma spray and tin/zinc flame spray) all exhibit the semiconductor type behaviour characteristic of normal metal contacts on Y based high temperature superconductors; r~ is a very fast function of temperature. Note that the nickel sample shows contact resistivities an order of magnitude worse than the other samples. Curves 4 and 5 of Figure 3 are two examples of flame sprayed copper contacts. As well as showing a much lower
0.08
v~ 'E
O. 06
.~1
~ . ~ . o n
t act s ,
_V2k , ~ - ~ l ~ . _ . , . ~
t..
Experimental details These measurements were performed using a six wire technique as shown in Figure 1. Current was passed through the outer pads and the voltage between the current carrying contacts measured using a second set of copper wires attached to the same contacts. The silver paint bonds of these two wires should be separate so as to measure only the voltage across the normal metal, resistive contact layer and bulk superconductor, Vt. A second set of metal contacts is used to measure the voltage
Bulk superconductor
J~ 0.04 t. to
f .,.., 0.02 >o
V2 contacts
o
80
120
160
i
l
200
240
280
320
Temperature (K)
Figure 2
Typical data for voltage v e r s u s temperature of a sample (I/2) and sample plus contacts (Vl)
Cryogenics 1989 Vol 29 December
1125
Electrical contacts to YBCO: S. P. Ashworth et al. 10
20
LJ
1G
a
>.12
"~
U
c.j o
80
I 120
I 160
I 200
J 2t10
1 280
~ 0
320
80
120
Temperature (K}
Figure 3 Contact resistivity versus temperature for a number of different samples. 1, Nickel flame spray; 2, copper plasma spray; 3, tin/zinc flame spray; 4, copper flame spray; 5, copper flame spray w i t h surface abrasion cleaning in air immediately before flame spraying
contact resistivity at 77 K than the previous discussed contact metals, the copper flame sprayed contacts show a very different dependence on temperature; a linear increase in r= rather than the more 'exponential' increase of the other samples. The best contacts obtained by vacuum deposition techniques rely on cleaning the superconductor surface by sputter etching immediately before deposition of the contact material. Cleaning the surface by abrasion (curve 5) in air immediately before flame spraying also produces a significantly lower re. A qualitative explanation of these results is as follows. The contact material produces a region of the YBaCuO material which is not superconducting above 77 K. This is in addition to an interface layer already degraded by exposure to atmosphere; removal of this layer by abrasion is helpful. The magnetic nickel obviously has more effect than the tin/zinc. The resistive contacts are apparently made worse by the transient heating of the surface produced by plasma spraying. The temperature achieved by this heating is, however, low (< 450 K) compared to the temperatures used during production of the superconductor (> 1300 K), indicating that the semiconductor contact is irreversibly damaged by quite low temperatures. This hypothesis was examined by exposing a plasma sprayed copper contact to 420 K for 300 s, before remeasuring the re(T) behaviour. This is shown in Figure 4. The room temperature contact resistivity increased by a factor of 1.5 but the r~(77 K) increased by a factor of 4. This sensitivity to moderate temperatures is also shown in Figure 5, where a sample was heated at 420 K for varying times and the contact resistivity measured at room temperature and at liquid nitrogen temperature. A large increase in rc(77 K) compared to the room temperature value is apparent. Measurements have been performed using flame sprayed silver contacts and these have produced contact resistivities which are below the sensitivity of the present apparatus, i.e. < 10-af~ cm 2. Further measurements on these samples will be reported at a later date s. There are two obvious mechanisms for the conversion of superconducting YBa 2 Cu3 O7-~ into a semiconductor by a metallic layer. These involve either the removal of oxygen by the metal or the diffusion of metal into the superconductor. Cava et al. 6 measured the resistivity of
1126
Cryogenics 1989 Vol 29 December
200
160
2q0
280
320
Temperature (K)
Figure 4 Degradation of contacts caused by heating to 420 K for 300s. Contacts produced by plasma sprayed copper, a, After heating; b, before heating
a bulk sample of YBa2Cu3OT_ x superconductor as a function of x and showed that semiconductor-like behaviour could be induced by an increase in x from 0.0 to 0.1. This was also accompanied by a reduction in the critical temperature of the bulk sample. The present results suggest that the superconductor-normal metal interface gives up oxygen very readily and that the best contacts will be produced when techniques are used to minimize this oxygen loss. It is also possible that the YBCO free surface is intrinsically non-metallic, as discussed by Egdell and Lavell7; this would also lead to high values of contact resistivity.
Conclusions Copper flame spraying provides a good method for producing large scale, moderately low resistance electrical contacts to YBa2Cua07_x high temperature superconductors. If the surface of the superconductor is cleaned immediately before deposition of the copper layer, surface contact resistivities of 500/~f~ cm 2 are easily achievable. This allows current densities of over 100A cm -2 to be passed through contacts without exceeding the critical heat flux for the onset of film boiling in liquid nitrogen. 20
E
16
>- 12
t=
20
tl0
60
80
' 100
Time (min)
Figure 5 Change in room temperature ( O ) and 77 K ( l l ) contact resistivity due t o heat treatment at 420 K. Plasma sprayed copper sample
Electrical contacts to YBCO: S. P. Ashworth et al. Exposure to moderate temperatures damages the contact layer in an, as yet, unknown way but evidence seems to indicate that the damage is due to migration of oxygen atoms to the normal metal rather than the migration of metal atoms into the superconductor.
Acknowledgements The authors would like to acknowledge the technical assistance of J. Mayne (Metco Limited) and financial assistance from BOC Limited. Y. Yang acknowledges financial support from the British Council and the Chinese Government.
References 1 Ekin, J.W., Larson, T.M. and Bergen, N.F. Appl Phys Lett (1988) 52(21) 1819 2 Weiek,A.D. Appl Phys Lett (1988) 53(13) 1216 3 Yokoynma, S., Yamada, T., gubo, Y., Epwa, K. and Simuzu, Y. Cryogenics (1988) 28 734 4 Ashworth,S.P., Beduz, C., Scurlock, R.G. and Yang, Y. Proc ICEC 12 Butterworths, Guildford, UK (1988) 5 Herman,H. Scientific American reviewarticle (September 1988)p 88 6 Cava, R.J., Batlog, B., Chen, C.H., Rietman, E.A., Zahurak, S.M. and Werber, D. Phys Rev B (1987) 36 5917 7 Egdell, R.G. and Lavell, W.R. Z Phys B (1989) 74 279 8 Ashworth,S.P., Bndt~ C., Harrison, K., Scurlock, R.G. and Yang, Y. Adv Cryog Eng (1989) 34
Cryogenics 1989 Vol 29 December 1127