416
Chem. Mater. 1990,2, 416-420
In Situ Determination of the Effects of C 0 2 and Other Volatile Impurities on YBa2C~307-x C. Michael Greenlief,*y+Joseph F. Bringley, Bruce A. Scott, Stephen M. Gates, Steven S. Trail, and Christopher D'Emic I.B.M. Research Division, Thomas J. W a t s o n Research Center, Yorktown Heights, N e w York 10598 Received February 13, 1990 Heating YBa2Cu307, to temperatures of up to 925 K causes evolution of H20, CO, and C 0 2 as the major volatile impurities contained in this material, which are observed here mass spectrometrically by using temperature-programmed desorption. The level of these impurities can be reduced significantly by annealing of the samples in oxygen (100 Torr, 673 K), cooling to 400 K at a rate of 5 K/min, heating to 925 K in vacuum, and repeating the oxygen anneal. In situ resistivity measurements show that the superconducting transition sharpens as the impurity levels are decreased; however, no change in T, is observed. The out-diffusion of O2 was also followed by TPD, and these results agree well with previous studies. Both oxidized (YBa2C~306,9) and reduced (YBa2C~306,5) forms of the material were exposed to cozat mild temperatures (325-635 K). Reduced samples were found to adsorb significantly more C02than the oxidized material. The presence of adsorbed C02significantly inhibits reoxidation of the reduced YBa2CU306.5 to YBa2Cu306,*Indirect evidence by TPD suggests that C 0 2may insert into oxygen vacancies in the partially reduced material. Introduction The discovery of high-temperature superconductivity has near 90 K in the YBaCuO and related generated tremendous amounts of new experimental and theoretical research on these materials. Although these oxides have high superconducting critical temperatures (T, = 92-97 K), their superconductivity is often plagued by broad T , transitions, low Meissner fractions, and low critical current densities, especially in polycrystalline or sintered samples. These features are largely a result of grain boundary effects, bulk or adsorbed impurities, and poor oxygen h ~ m o g e n e i t y . ~ Impurities -~ such as carbonates and hydroxides present in and a t the grain boundaries of polycrystalline YBa2Cu307-, are believed, in part, responsible for their poor performance."1° These volatile impurities result from the reaction of YBa2Cu307with C 0 2 , H20, and other volatile species, which are often present in the processing atmosphere. It has been shown that YBa2Cu307-,will absorb C 0 2 from air even at room temperature." In addition, the unbiquitous carbon can be incorporated into YBa2Cu307and is reported to adversely affect its superconducting properties.'* Thus, the specific processing conditions are crucial i n determining t h e superconducting properties of ceramic YBa2Cu307and related systems. We have developed a procedure for reducing volatile impurity levels in polycrystalline YBa2Cu307-, and measuring in situ the effects of these impurities on the superconducting properties of YBa2Cu307-,. This approach allows the direct determination of the effects of different processing atmospheres, without the risk of intermediate exposure to volatile impurities. The experiments were conducted in a two-chamber ultrahigh-vacuum system capable of processing a sample of YBa2Cu307-,at pressures of up to 1 atm. The chamber can be evacuated and the sample evaluated with either temperature-programmed desorption (TPD) mass spectrometry or fourpoint resistivity measurements. The volatile impurity levels of YBa2C~307-x were then followed as a function of different processing procedures. 'Permanent address: Department of Chemistry, University of Missouri-Columbia, Columbia, MO 65211.
0897-4756/90/2802-0416$02.50/0
Experimental Section Experiments were performed in a two-section turbomolecular pumped (150 L s-l) ultrahigh-vacuum (UHV) system with a base pressure of 2 X Torr. A schematic of the UHV chamber is shown in Figure 1. The reaction section of the chamber was isolated with a gate valve and could be pressurized up to 1 atm. A halogen-filled infrared lamp was used to heat the sample. Heating rates of 0.5-1.0 K sW1were typically used for the TPD measurements. A small hole (0.012-in.0.d.) was drilled through the sample, and the thermocouple (chromel-alumel) junction was centered in the middle of the hole. A tight fit of the junction in the hole was required for accurate temperature measurement. The thermocouple also served as the sample support. A four-point resistivity probe and Dewar for cooling the sample with liquid nitrogen were also mounted in the sample section of the chamber. The four-point probe was mounted on a linear feed-through so that the probe could be retracted while heating the sample. The sample was then pressed against the Dewar with the probe when resistivity measurements were made. The base temperature attainable with this configuration was 82 K. A second section of the chamber contained the ion gauge and a Dycor quadrupole mass spectrometer. The O2 (Matheson, research purity) and COz (Matheson, research purity) were used throughout the study, and the purity of the gases was verified by mass spectrometry. The water content (1) Bednorz, J. G.; Mueller, K. A. 2.Phys. 1986, B64, 189.
(2) Wu, M. K.; Asburn, J. R.; Torng, C. J.; Hor, P. H.; Meng, R. L.; Gao, L.; Huang, Z. J.; Wang, Y. Q.;Chu, C. W. Phys. Rev. Lett. 1987,58, 908. ( 3 ) Beyers, R.; Lin, G.; Engler, E. M.; Savoy, R. J.; Shaw, T. M.; Dinger, T. R.; Gallagher, W. J. Appl. Phys. Lett. 1987, 50, 1918. (4) Cava, R. J.; Batlogg, B.; van Dover, R. B.; Murphy, D. W.; Sunshine, s.;Siegrist, T.; Remeika, J. P.; Rietman, E. A.; Zahurak, s.;Espinosa, G. P. Phys. Reu. Lett. 1987, 58, 1676. (5) Clarke, D. R.; Shaw, T. M.; Dimos, D. J. A m . Ceram. SOC. 1989, 72, 1103. (6) Chaudhari, P.; Mannhart, J.; Dimos, C.; Tsuei, C. C.; Chi, J.; Oprysko, M. M.; Scheuermann, M. Phys. Reu. Lett. 1988, 60,1653. (7) Gallagher, P.K.; Grader, G. S.; O'Bryan, H. M. Mater. Res. Bull. 1988, 23, 1491. (8) Nakahara, S.;Fisanick, G. J.; Yan, M. F.; van Dover, R. B.; Boone, T.; Moore, R. J . Cryst. Growth 1987,85, 639. (9) Seibt, E. W.; Zalar, A. Mater. Lett. 1988, 7, 256.
(10) Fjellvag, H.; Karen, P.; Kjekshus, A.; Kofstad, P.; Norby, T. Acta Chem. Scand. 1988, A42, 178. (11) Keller, S. W.; Leary, K. J.; Stacy, A. M.; Michaels, J. N. Mater. Lett. 1987, 5, 357. (12) Shaw, T. M.; Dimos, D.; Baston, P. E.; Schrott, A. G.; Clarke, D. R.; Duncombe, P. R. J. Mater. Res., submitted.
0 1990 American Chemical Society
Effects of Impurities on YBazCu307-,
Reaction Side
Chem. Mater., Vol. 2, No. 4, 1990 417
Analysis Side
liquid nitrogen dewar
e .Z 0.6
.-3 In c &
c
1 0.4 c m
E
Ln
5
0.2
0.0 300
500 600 700 Temperature (K)
400
P: IE-8 10 IO00 Torr
Figure 1. Schematic diagram of the experimental system. The reaction and analysis parts of the system can be isolated by a gate valve so that the reaction side can be independently pressurized.
Results and Interpretation Cleaning of YBa2Cu306,9. Figure 2 shows the initial heating of a YBa2Cu306.9sample. This TPD represents the "worst case" sample under our conditions. This sample was inserted into the vacuum system after it was cut with the diamond wheel and not subjected to the final O2anneal (see the Experimental Section). The sample of Figure 2 was inserted into the vacuum system, annealed in vacuum at 400 K for 6 h, and cooled to room temperature before the start of the TPD experiment. Heating the sample to 925 K a t a rate of 0.5 K s-l resulted in the evolution of several gases. The most abundant desorption peak is H20, centered about 500 K; CO and COP closely follow each other up to about 600 K. The fixed ratio (mlz 44)/(m/z 28) indicates that C 0 2 is desorbing up to this point; however, above 600 K the ratio changes, indicating that both (13)Manthiram, A.; Swinnea, J. S.; Sui, Z. T.; Steinfink, H.; Goodenough, J. B. J. Am. Chem. SOC.1987, 109, 6667.
900
Figure 2. TPD spectrum of a YBa2C~306,9 sample heated at a rate of 0.5 K s-l prior to any annealing cycles. I
was
