The effects of space radiation on thin films of YBa2Cu3O(sub 7-x)

This investigation had two objectives: (1) to determine the effects of space radiation on superconductor parameters that are most important in space applications; and (2) to determine whether this effect can be simulated with Co-60 gamma rays, the standard test method for space materials. Thin films of yttrium barium copper oxide (YBCO) were formed by coevaporation of Y, BaF2, and Cu and post-annealing in wet oxygen at 850 C for 3.5 h. The substrate used was (100) silicon with an evaporated zirconia buffer layer. The samples were characterized by four point probe electrical measurements as a function of temperature. The parameters measured were the zero resistance transition temperature (T sub c) and the room temperature resistance. The samples were then exposed to Co-60 gamma-rays in air and in pure nitrogen, and to 780 keV electrons, in air. The parameters were then remeasured. The results are summarized. The results indicate little or no degradation in the parameters measured for samples exposed up to 10 Mrads of gamma-rays in nitrogen. However, complete degradation of samples exposed to 10-Mrad in air was observed. This degradation is preliminarily attributed to the high level of ozone generated in the chamber by the gamma-ray interaction with air. It can be concluded that: (1) the electron component of space radiation does not degrade the critical temperature of the YBCO films described, at least for energies around 800 keV and doses similar to those received by surface materials on spacecraft in typical remote sensing missions; and (2) for qualifying this and other superconducting materials against the space-radiation threat the standard test method in the aerospace industry, namely, exposure to Co-60 gamma-rays in air, may require some further investigation. As a minimum, the sample must be either in vacuum or in positive nitrogen pressure

The effects of space radiation on thin films of YBa2Cu3O(7-x)

This investigation had two objectives: (1) to determine the effects of space radiation on superconductor parameters that are most important in space applications; and (2) to determine whether this effect can be simulated with Co-60 gamma rays, the standard test method for space materials. Thin films of yttrium barium copper oxide (YBCO) were formed by coevaporation of Y, BaF2, and Cu and post-annealing in wet oxygen at 850 C for 3.5 h. The substrate used was (100) silicon with an evaporated zirconia buffer layer. The samples were characterized by four point probe electrical measurements as a function of temperature. The parameters measured were the zero resistance transition temperature T(sub c) and the room temperature resistance. The samples were then exposed to Co-60 gamma-rays in air and in pure nitrogen, and to 780 keV electrons, in air. The parameters were then remeasured. The results are summarized. The results indicate little or no degradation in the parameters measured for samples exposed up to 10 Mrads of gamma-rays in nitrogen. However, complete degradation is preliminarily attributed to the high level of ozone generated in the chamber by the gamma-ray interaction with air. It can be concluded that: (1) the electron component of space radiation does not degrade the critical temperature of the YBCO films described, at least for energies around 800 keV and doses similar to those received by surface materials on spacecraft in typical remote sensing missions; and (2) for qualifying this and other superconducting materials against the space-radiation threat the standard test method used in the aerospace industry, namely, exposure to Co-60 gamma-rays in air, may require some further investigation. As a minimum, the sample must be either in vacuum or in positive nitrogen pressure

The effect of temperature cycling typical of low earth orbit satellites on thin films of YBa2Cu3O(7-x)

The refrigeration of superconductors in space poses a challenging problem. The problem could be less severe if superconducting materials would not have to be cooled when not in use. Thin films of the YBa2Cu3O(7-x) (YBCO) superconductor were subjected to thermal cycling, which was carried out to simulate a large number of eclipses of a low earth orbit satellite. Electrical measurements were performed to find the effect of the temperature cycling. Thin films of YBCO were formed by coevaporation of Y, BaF2, and Cu and postannealing in wet oxygen at 850 C for 3.5 h. The substrates used were (100) SrTiO3, polycrystalline alumina, and oxidized silicon; the last two have an evaporated zirconia layer. Processing and microstructure studies of these types of films have been published. THe zero resistance transition temperatures of the samples used in this study were 91, 82, and 86 K, respectively. The samples were characterized by four point probe electrical measurements as a function of temperature. The parameters measured were: the zero resistance transition temperature, the 10 to 90 percent transition width, and the room temperature resistance, normalized to that measured before temperature cycling. The results for two samples are presented. Each sample had a cumulative exposure. Cycling in atmospheric pressure nitrogen was performed at a rate of about 60 cycles per day, whereas in vacuum the rate was only about 10 cycles per day. The results indicate only little or no changes in the parameters measured. Degradation of superconducting thin films of YBCO has been reported due to storage in nitrogen. It is believed that the relatively good performance of films after temperature cycling is related to the fact that BaF2 was used as an evaporation source. The latest result on extended temperature cycling indicates significant degradation. Further tests of extended cycling will be carried out to provide additional data and to clarify this preliminary finding

ATOMIC RESOLUTION OBSERVATIONS OF SOLUTE-ATOM SEGREGATION AND TWO-DIMENSIONAL PHASE TRANSITIONS AT INTERNAL INTERFACES

Solute-atom segregation effects to individual stacking faults (SFs) in Co-0.96 at.% Nb and Co-0.98 at.% Fe alloys have been studied employing the atom-probe field-ion microscope (APFIM) and transmission electron diffraction techniques. /1-7/ The mean composition of individual SFs was measured for bulk specimens which had been equilibrated in the range 450-575°C. In addition, the composition of the SFs was measured--employing the APFIM technique-- with a spatial resolution, within the plane of the SFs, of ≈ 0.1 nm and with a spatial resolution of < 0.4 nm perpendicular to the plane of the SFs. These measurements demonstrated the following : (a) The mean composition of the SFs increased with decreasing temperature according to an Arrhenius-like expression ; (b) The Nb or Fe concentrations fall off very quickly with distance-- within < 0.4 nm form the plane of the SF the bulk concentration is achieved ; (c) The SFs equilibrated above 450°C contained solute-atom fluctuations (≈ 0.5 to 2.0 nm diameter) which correspond to compositions with stoichiometries of ≈ Co2Nb or ≈ Co3Fe. The temperature dependence of these fluctuations suggests that in addition to solute-atom segregation we have observed a two-dimensional phase transition in the stacking fault. In the case of the Co(Nb) alloy a transmission electron diffraction pattern was obtained of a SF-- with the electron beam normal to the SF--which exhibited superlattice reflections with six-fold symmetry around the Bragg reflections from the hexagonal close-packed reflections. The latter result is consistent with a two-dimensional phase in the SF plane with the composition Co2Nb. Both the APFIM and the transmission electron diffraction studies are consistent with the existence of two-dimensional ordered phases within the SFs. The APFIM and transmission electron diffraction results demonstrate that it is possible to study both the chemistry and the spatial arrangement of solute atoms at a well-defined internal interface--the stacking fault. The APFIM measurements yield unique information about the chemistry of this interface--which, at present, is unobtainable by any other technique. Whereas the transmission electron diffraction patterns yield information about the ordering of solute and solvent atoms at the interface. The combination of these two techniques is a particularly powerful approach for the study of internal interfaces on an atomic scale

SOLUTE-ATOM SEGREGATION AND TWO-DIMENSIONAL PHASE TRANSITIONS IN STACKING FAULTS : AN ATOM-PROBE FIELD-ION MICROSCOPE STUDY

Nous avons étudié, à l'aide d'un microscope à émissions d'ions avec sonde atomique, des effets de ségrégation pour des fautes d'empilement individuelles dans les alliages Co-0.96 at.% Nb et Co-0.98 at.% Fe. La composition des fautes d'empilement a été mesurée dans ces alliages dans la gamme de température 450 à 575°C. La concentration moyenne de soluté dans les fautes d'empilement décroit avec l'augmentation de température. Des petites fluctuations (~5 to 20 A de diamètre) riches en soluté, dépendant de la température, dont la composition diffère de façon significative de la composition moyenne d'une faute, ont été observées dans le plan des fautes d'empilement. Le fait que ces fluctuations sont dépendantes de la température suggère qu'en plus de ségrégation solutée nous avons observé une phase de transition à deux dimensions dans les fautes d'empilement. Une ségrégation isotherme de type Fowler-Guggenheim a été dérivée ce qui explique de façon semiquantitative la dépendance de la température de la composition moyenne d'une faute.The atom-probe field-ion microscope has been employed to study solute-atom segregation effects to individual stacking faults in Co-0.96 at.% Nb and Co-0.98 at.% Fe alloys. The compositions of stacking faults have been measured in these alloys in the temperature range 450 to 575°C. The mean solute concentration in the stacking faults decreases with increasing temperature. Small temperature-dependent solute-rich fluctuations (~5 to 20 A diameter), whose compositions differ significantly from the average fault compositions, have been observed in the plane of the stacking fault. The temperature dependence of these fluctuations suggests that in addition to solute-atom segregation we have observed a two-dimensional phase transition in the stacking fault. A Fowler-Guggenheim type segregation isotherm has been derived, which semi-quantitatively explains the temperature dependence of the mean composition of a fault