Physica C 190 ( 1992 ) 219-224 North-Holland
Sintering time effect on electrical properties of YBa2Cu3Ox A. K u l p a , A.C.D. C h a k l a d e r a n d G. R o e m e r Department of Metals and Materials Engineering, The University o.f British Columbia, l'ancouver, B.C, Canada Received 17 September ! 991
Pellets of ceramic YBa2Cu3Ox superconductors have been sintered at a fixed temperature of 950:C in an oxygen atmosphere for different periods of time in the range 8 to 200 h. Structural ( X-ray ), microstructural (SEM and EDX ) and electrical (R versus T) measurements were performed. All samples studied were found to be in the orthorhombic phase. No systematic change in lattice cell parameters (a, b and c) versus sintering time was found. With the SEM studies, an increase in grain sizes was noticed. Reduced resistance (R/R.,gs v,) was a linear function, decreasing with temperature for all the samples studied. Reduced "zero" resistance (Ro/R:,~5 K), temperature coefficients of resistance and the ratio of R29s r~/Rtoo ~ were found to change systematically with sintering time and after long sintering time ( > 100 h ) these values approached those in a single crystal for the a-b plane resistivity. The transition temperature from the normal to superconducting state was unaffected by the sintering time.
1. Introduction The solid-state sintering process is ve~' often applied to obtain YBa_,Cu30, ceramic materials. Solid state reactions between Y_-O3, BaCO3 or BaO and CuO, followed by a sintering process were widely studied to establish optimized experimental phase diagrams (see for example refs. [1,2]). Effects of sintering temperature, time and atmosphere on density, grain sizes and some mechanical properties were studied by Hojaji et al.  and other groups [ 4 - 6 ] . The influence of the sintering temperature on the normal state resistance was reported by Kuwabara  and Ota et al. . In this paper the effect of the sintering time on the normal state resistance is presented.
2. Experimental details Material was prepared by a standard metallurgical method using Y20~, BaO2 and CuO high purity powders. This orthorhombic material was pressed into pellets ( ~ 10 mm in diameter and ~ 3 mm thick) Present address. ,Alberta Research Council, Edmonton, Alberta, Canada.
and sintered at 950°C in flowing oxygen for different amounts of time. After the sintering processes, the pellets were furnace cooled in an oxygen atmosphere to room temperature. The pellets were marked as = 1, =2 ..... and = 13 for each sintering time. Only for two samples, =9 and =11, cooling processes were interrupted at 450 =C where annealing (in 100% O, ) for an additional 20 (=9) and 12 h ( = 1 I ) was carried out. X-ray powder diffraction patterns of the single orthorhombic phase were obtained at ambient temperature using Cu K, radiation. On the basis of these X-ray powder diffraction spectra the lattice cell parameters were determined using a computer program. The thermogravimetric ( T G A ) measurements in a hydrogen atmosphere at 1000 °C provided data on the total oxygen content ( x = 6 . 9 6 + / 0.02) of the samples studied, whereas TGA measurements in an oxygen atmosphere provided data about changes in stoichiometr3' during the thermal cycle, from rooni temperature to ar -, n7 . :, v,- ~.,"*--6. q~a,, ,,, + / - 0 . 0 0 5 ) . Details on these measurements have been published elsewhere . The SEM and EDX analyses were performed to obtain information on the microstructure. Electrical measurements were done on the pellets using a standard DC four-probe method. Contacts were prepared with a silver paint.
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A. Kulpa et al. / Sintering time effect on YBCO
All the samples were studied at 10 mA current. The electrical measurements were carried out in a nitrogen atmosphere in the temperature (AT= + / - 0 . 5 degree ) range between room temperature and liquid nitrogen tempe;ature.
Data of the samples studied are presented in tables I and 2 and in figs. 1-5. Table 1 contains the lattice cell parameters. Table 2 shows the electrical resistance data. Figure 1 shows the microstructure of samples #1 and #8. Figure 2 presents the reduced resistance (R/R2~s K) as a function of temperature for two samples, #1 and #1 3; these are the first and last ones in the experiment. Curves were replotted from the original graphs using a digitizer. Figure 3 shows reduced "zero" resistance (Ro/R29s K) for all samples versus sintering time. Changes in the temperature coefficient of resistance (TCR*) with sintering time are plotted in fig. 4. Figure 5 presents R~,s K/R~oo K ratios versus sintering time.
From X-::ay diffraction patterns of the samples studied it is confirmed that all specimens are single phase orthorhombic YBa,Cu~O, material, because
all the spectra consisted only of the characteristic peaks of YBa2Cu30, orthorhombic phase. However, it slhould be noted that for X-ray diffraction, the lower limit of detection for the crystalline phase is between one to five weight percent and it is higher for the amorphous phase. The lattice cell parameters of the orthorhombic YBa2Cu30,. structure of the samples studied are presented in table 1. An orthorhombic distortion ( b - a) is ~ 7 × 10- 3 A,. Differences (P) between the b lattice parameter and ~ of c are small, ranging from 0.01 to 0.3%. The orthorhombic distortion and P values are typical for YBa2Cu30,. material with a high oxygen content [ 7 ]. As can be seen in table 1, the structural data obtained show no systematic variation with sintering time. The EDX and SEM analyses were done for all the samples. The EDX studies showed that in general the composition of the specimens is YBa2Cu30,.. With the SEM studies it was found that the samples consisted of randomly oriented grains. The grain size varied within each sample. Nevertheless, an increase in grain sizes with sintering time was observed, from approximately 10-30 ~tm for the sample #1 to 2050 ~tm for the samples ~:7-#13 (fig. 1 ). Occasionally a small amount of amorphous or semiamorphous phase was encountered. A large amount ( ~ 15%) of voids was present in ~II 'he specimens. For the samples with higher numb~ rs, #10-~13, larger voids were observed. These may result from the coalescence of small voids during very long sin-
Table ! The orthorhombic unit cell parameters determined from X-ray spectra (A20= 0.05 ). Differences between the b lattice parameter and of c are represented by P= [ ( b - c/3 ) / b ] x 100%. Additionally annealed samples are marked with a star Sample
~1 =2 ~3 =4 =5 :=6 =7 =8 =9 * =10 ~!! * = 12 =13
3.827 3.818 3.824 3.825 3.825 3.825 3.821 3.822 3.824 3.823 3.826 3.821 3.821
3.902 3.895 3.895 3.900 3.897 3.899 3.892 3.898 3.900 3.892 3.898 3.902 3.894
11.69 11,68 1 !,70 ! 1,69 i 1.68 11.69 11.70 11.70 11.71 11.71 11.69 11.70 I 1.68
3.896 3.894 3.901 3.897 3.894 3.898 3.899 3.898 3.905 3.903 3.897 3.899 3.894
8.00 33.42 5 !.25 63.08 80.08 92.92 105.17 117.17 129.17 149.75 171.08 183.08 201.58
0.2 0.03 0.2 0.07 0.07 0.03 0.2 0.01 0.1 0.3 0.02 0.08 0,01
A. Kulpa et al. / Sintering time effect on YBCO
Table 2 Resistances at room temperature ( R29s K ) and resistances estimated at 0 K ( Ro); temperature coefficients of resistance, TCR* = oq / (Ro/ R295 ~ ), and reduced "zero" resistances ( Ro/R,.95 K ). Additionally annealed samples are marked with a star Sample
R295 ( m f l )
Ro ( m ~ )
TCR* ( I / K )
#1 #2 ~3 #4 #5 ~6 #7 ~8 ~9" # !0 #11" #12 ~! 3
3.23 2.58 2.17 2,3 ! 2.38 2.82 2. ! 7 1.85 !.7 ! 2.13 1.61 !.79 1.73
0.95 0.75 0.40 0.49 0,35 0.35 0.25 0.20 0.05 0.25 0.14 0.24 0.22
0.0081 0.0083 0.015 0.0 ! 3 0.020 0.024 0.026 0.028 0.11 0.025 0.036 0.022 0.023
8.00 33.42 51.25 63.08 80.08 92.92 ! 05.17 I ! 7,17 129, ! 7 149.75 171,08 183.08 201.58
0.29 0.29 0,18 0.2 ! 0.15 0,12 0,12 0. I I 0.029 0.12 0.087 0.13 0.13
Fig. I. (a), (b) SEM photographs of the sample ~1 and :~8, respectively; (700×)
tering processes, because the density was approximately the same for all the samples. A relative density of 84% + / - 5% was determined by weight and dimensional measurements. The error of 5% resuited from the volume determination, because the samples were not perfectly cylindrical after sintering. Also, Chen et al.  while studying the influence of the oxygen concentration on processing YBa2Cu30, found that samples sintered in 100% oxygen had densities which remained constant with an increase
of sintering time; however, they reported some density changes for samples prepared in an oxygen deficient atmosphere. Electrical measurements, resistance (R) versus temperature (T), showed that all the specimens were "metallic", that means resistance was decreasing with decreasing temperature. The transition from the normal to the superconducting state occurred at 90 or 89 (AT,-, 1 K) for all samples. To describe electrical properties independently on a sample shape, the re-
,4. Kulpa et al. / Sintering time effect on YBCO
Fig. 2. Normalized resistance (R/R:~s K) as a function o f temperature o f two samples: *1 and ~13, that are the first a n d the last samples in these sintering experiments.
Fig. 4. T e m p e r a t u r e coefficit:nts o f resistance (TCR*) as a function o f s i n t e r i n g time. The additionally annealed samples * 9 and :~i I are excluded.
® ® ---1--
Fig. 3. Normalized "zero" resistance (Ro/R295K) vs. sintering time. T h e additionally annealed samples # 9 and ~1 1 are m a r k e d with circles.
Fig. 5, The ratio o f room temperature resistance and resistance at 100 K (R29s K/Rmo K) VS. sintering time.
sistivity (p) is usually used.
The function describing the dependence of reduced resistance on the temperature is similarly described by
where: R is the sample resistance, A the area through which current flows and d the distance between voltage electrodes (DC four-probe technique). In our case, because the specimens were not perfectly cylindrical, which was men.ioned before, reduced resistance (R/R:~5 K) was analyzed instead of resistivity. Resistivity as a function of temperature is described by a simple equation:
R/R2,~5 K =fit +O~l T,
/R " R/R>5 K versus T down to T---O K for the temperature range from room temperature to T=2Tc; and ~t = A ( R / R > s K)/AT. Figure 2 shows normalized resistance versus temperature of the *:1 and *:13 samples, and those are the firs', and the last ones in this experiment. The same dependence of R/Rn,~s K on temperature was
A. Kulpa et al. / Sintering time effect on YBCO
observed for all the samples studied. A long sintering time, ~ 100 h and more, resulted in lowering Ro/ R295 K down to the almost constant value of ~0.1 (fig. 3). A long sintering time improved the "'metallic" behavior of the specimens. This is reflected in the TCR* values (TCR*=oll/Ro/R295 K), that are presented in fig. 4 as a function of sintering time. The samples # 9 and # 11 are not included in the plot, because these were additionally annealed. A small decrease in the TCR* values of the specimens # 10-# 13 with sintering time may follow from larger voids present in these specimens. Another possibility is that some microcracks may be present inside large grains, because YBa2Cu30,. materials are anisotropic and thermal expansion coefficients are 9 X 10-6, I 1 × 10 -6 and 14X 10 -6 K - I for the b-, a- and c-axis of the orthorhombic unit cell, respectively . However, no microcracks were visible on the surfaces of the specimens studied. The residual resistivity of materials, Pros, is generally accepted as resistivity at 4.2 K. The 19295K/fires rati~ ,'effects the purity of the metals, and for example, for very pure copper can be as high as 2000 but, on the other hand, for transition metals it is usually not higher than 10, because of gas impurities. In YBa2Cu3Ox materials Prc~ has not been measured. For this kind of measurements, superconductivity at 4.2 K has to be suppressed by a very strong magnetic field, H > H~2. Because of this, quality of YBazCu30, materials is referred to the/9_,95 r
value [7,12-15]. Single crystals studied (see for instance refs. [ 11 ] and [ 16] ) suggest that the other crystallographic defects rather than oxygen vacancies affect only the normal resistance whereas T¢ values were high ( > 90 K) and remained almost constant. For the best quality single crystals for both directions, along a-b planes and c-axis, a linear dependence of resistivity on temperature is observed, however, the values of the resistivity for the c-axis [ 16] were higher. The nonlinear behavior of p, versus temperature, observed by others [ 11 ], was explained by crystallographic defects present inside a single crystal [ 17 ]. The transition temperatures of the specimens studied were high and almost the same (89 and 90 K). because the oxygen content was high and samples were not quenched but slowly cooled. Any influence of sintering time on Tc could not be noticed. Almost the same Tc for all samples suggests that grain interiors experienced no change (or little change) in ordering of oxygen atoms and oxygen vacancies. Sintering time improved only normal resistance, lowering intergrain and intragrain resistances. Intergrain resistance should be understood as resistance resulting from connections between grain boundaries and as resistance of the boundaries themselves. If only intergrain resistance is improved, reduced resistance versus temperature is almost the same for different specimens, as was shown [ 18 ] for YBa,Cu30, materials with different amounts of silver added. The R3oo~/RiooK ratio of those YBa2Cu30,+Ag samples equals 3, that is ,--17% higher than for well prepared YBa2Cu~O, materials without silver. In our case, changes in reduced resistance versus temperature were observed with sintering lime. This means that grain interior resistances were decreasing with sintering time as well as intergrain resistances. lntragrain resistance was lowered because crystallographic defects were eliminated and also because grains have grown larger during ~.he long sintering time. With the equation D=rF/v. frequencies (v) of electrical charge scattering on c~,staliographic defects inside grains can be estimated for average grah~ sizes (D). To avoid using the value of Fermi velocity, t'v, a relative frequency, v=t/v=7, for average grain sizes, was estimated to be 1.75. At the same time, the ratio of (Ro/R2,~5 K)=I/(Ro/R295 K)=7 iS 2.23. These
,4. Kuipa et al. / Sintering time effect on vRCO
rough estimations suggest that the difference ( 2 . 2 3 - 1.75 ), which is approximately 20% of the reduced "zero" resistance, resulted from improved grain boundaries and improved connections between them. Lower resistance and, at the same time, higher TCR* values which were obtained for the additionally annealed samples can result from better local ordering which means that these samples contained more full chains. If chains were a reservoir of electrical carriers [24,25], then the electrical carrier density could be higher in these additionally annealed samples and this would imply lower resistance. The supposition, that some grains were not saturated with Oz and additional annealing redistributed oxygen from grain surfaces into the grain interiors is not very probable because generally grains were small (in lam ranges) and oxygen content was high x = 6.96. For high values of x, differences in resistivity have been found to be negligible because of this effect [ 26 ].
5. Conclusions Specimens were sintered at a temperature of 950°C in an oxygen atmosphere for different periods of time. Influences of sintering time on microstructure and normal resistance were found. The reduced resistance was a linear function of temperature for all the samples studied. After a long sintering time ( > 100 h) reduced "zero" resistance dropped down to a constant value of ~0.1 and TCR* values approached ~0.02 K -~. These and also R2,~sK/ R~o K~ 2.5, are similar to those in a single crystal for the a-b plane resistivity. No systematic variation in the lattice cell parameters with sintering time was found. Also, the transition temperature, T~,=89 or 90 K, was not affected by sintering time.
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