The effects of microstructural variations on the superconducting properties of SmBa2Cu3Ox are investigated. A scanning eddy current probe revealed the onset and growth of a normal conducting region. Resistance versus temperature measurements taken at different regions of the sample support the concept of a physically mixed state system. Regional variations in porosity and grain size distributions affect the observed superconducting transition

Generazio, Edward R.

Hepp, Aloysius F.

Roth, Don J.

English

NASA Technical Reports Server

. ' 7 "  f . .  
NASA Technical Memorandum 102 133 
t 
Non-Uniform Transition Conductivity 
of Superconducting Ceramic 
Edward R. Generazio, Don J .  Roth, and Aloysius F. Hepp 
Lewis Research Center 
Cleveland, Ohio 
Prepared for the 
16th Annual Review of Progress in Quantitative Nondestructive Evaluation 
cosponsored by the DOE, AFWAL, ONR, and ASNT 
Brunswick, Maine, July 23-28, 1989 
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https://ntrs.nasa.gov/search.jsp?R=19890019480 2020-03-20T00:36:07+00:00Z
NONUNIFORM TRANSITION CONDUCTIVITY OF SUPERCONDUCTING CEFUMIC 
Edward R. Generazio, Don J. Roth, and Aloysius F. Hepp 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 44135 
SUMMARY 
In this work we investigate the effect of microstructural variations 
A scanning eddy current on the superconducting properties of SmBa2Cu30,. 
probe revealed the onset and growth of a normal conducting region. Resis- 
tance versus temperature measurements taken at different regions of the 
sample support the concept of a physically mixed state system. Regional 
variations in porosity and grain size distributions affect the observed 
superconducting transition. 
INTRODUCTION 
There is an extensive international research effort underway to 
understand the origin of high critical temperature (T,) superconductiv- 
ity. A considerable amount of this research is directed toward increas- 
ing both the critical current density (J,) and Tc [l-41. In order to 
increase both Tc and Jc further we must understand the mechanisms 
supporting the superconducting state and the mechanisms that degrade this 
state. 
An important difference between ceramic and metallic superconductors 
is that the microstructural variations throughout bulk superconducting 
metals are generally small when compared to variations in ceramics. For 
example, porosity variations exist to some degree in bulk superconducting 
ceramics [ 5 ] .  It has been found that the transition width, ATc is depend- 
ent on the bulk porosity [6]. However, the effect of porosity variations, 
existing within the bulk of these ceramics, on the superconducting prop- 
erties is unknown. 
In this work we investigate the effect of porosity and grain size 
variations on the superconducting properties of SmBa2Cu30,. 
regional variations in porosity and grain size distributions affect the 
observed superconducting transition. 
We show that 
EXPERIMENTAL 
The SmBa2Cu30, ceramic sample was a uniaxially cold pressed, 
2 . 5 4  cm diameter, 3.0 mm thick pellet sintered for 8 hr at 930 "C and 
annealed for 5 hr at 440 O C  under 02 .  The resulting grain microstructure 
was orthorombic, quasi-isotropic (i.e., randomly textured), and single 
phase as determined from x-ray diffraction analysis. One quadrant of the 
pellet was used for this investigation. The quadrant was radiographed 
1 
and polished for optical examination after eddy current and resistance 
measurements were completed. X tone-pulse encoding technique [ 7 ]  was 
used to determine porosity and grain size distributions ac selecced 
regions of the sample. 
The s e  of eddy currents f o r  the detection 3 f  surface flaws is well 
known. In brief, a coil excited by a continuous high frequency 2lectri- 
cal signal yields an output signal that has a magnitude sensitive to 
changes in the inductance of :he coil. A change In the coil inductance 
is brought about by placing materials that have different conductivities 
(assuming that we are working with nonmagnetic materials) within the 
range of the alectromagnetic field produced by the :oil (this distance 
called lift-off is usually held constant for scanning systems). Eddy 
current surface probe evaluation does not yield :he actual value of the 
conductivity where the probe is positioned, but i.- does identify chanses 
in conductivity. For normai materials the depth of penetration of the 
magnetic field that induces eddy currents is given by [ 8 ] ,  
d = [ 3.14 popof 
where po, p, u ,  and f are the permeability of free space, relative 
permeability, conductivity and ac frequency, respectively. Smaa?Cu30x is 
a paramagnetic material. The relative permeability is approximately 1, 
therefore, in its normal state the eddy current response is expected to 
be due to only changes in conductivity. 
state the penetration depth is about 250 prn at G & W z .  
For SmBa2Cu30, in its normal 
For superconductors the de?th of penetration of  the magnetic field 
is given by the London relation [91 ,  
where do is the penetration depth of the magnetic field at T = 0 Kelvin, 
and do is estimated to be 0.1 p for superconducting ceramics [ l o ] .  As 
T + Tc then the magnetic field completely penetrates the material. The 
London relation is only dependent on temperature. However, it has been 
found experimentally [ 9 ]  that the London depth is dependent on frequency, 
geometry of sample, surface roughness and the applied magnetic field 
strength. 
The depth of penetration between Tc and Tc + ATc for ceramic 
superconductors is unknown. It is expected to be a relatively smooth 
minimum value of about 0.1 p in its superconducting. 
' function having a maximum value of about 250 pm in its normal state and a 
A commercially available eddy current deflectometer was used for 
this work. The probe was operated at 4 MHz and had a 1 mm diameter coil. 
The probe spatial resolution, determined by detecting and imaging a s u r -  
face crack in SmBazCu30, in its normal state, is about 1 mm. 
The experimental setup is shown in figure 1. The sample was placed 
on a leveled supporting table that is located at the bottom of a large 
liquid nitrogen dewar. An eddy current probe, attached to a support 
arm, is scanned over the sample. The probe scans a LO- by 10-mm array 
(fig. 2) in 1 nun increments. One complete scan of the area could be com- 
pleted in 10 min. The temperature is continuously measured by a thermo- 
couple placed next to the sample. 
2 
CURRENT 
DEFLECTOMETER 
rPROBE SUPPORT ARM 
,-INSULATING POSITION TABLES ,' COVER 
TEMPERATURE 
GUAGE 
, SAMPLE COMPUTER 
'. 1. 
'.-TABLE 
\ 
'LNz DEWAR 
THERMOCOUPLE' 
WIRE 
LN2J 
FIGURE 1. - DIAGRAM OF EXPERIMENTAL SETUP. 
During a run the dewar is filled with liquid nitrogen so that the 
sample is completely immersed. The probe continuously scans the same 
area of the sample in a repeated fashion while the liquid nitrogen evap- 
orates from the dewar. A square hole in the insulating cover allowed the 
probe to be moved while limiting turbulent exchange of room air with the 
nitrogen gas. 
the sample, the temperature of the sample uniformly increased at a rate 
of 1 K in 10 min. The temperature difference between the top and bottom 
surface and center to edge of the sample was less than 0.3 K. 
After the liquid nitrogen level drops below the base of 
Resistance versus temperature measurements were generated during a 
separate run by using a modified four point measurement arrangement. 
indium contacts [ I l l  were made at the locations shown in figure 2 .  The 
extreme two contacts were connected to a constant current source. No 
"resistive heating" effects were observed for currents below 35 mA. 
resistance at the center, R1, and at the edge, R 2  
measured as a function of temperature. 
Six 
The 
were simultaneously 
, 
' SURFACE CRACK 
FIGURE 2 .  - RADIOGRAPHIC IMAGE OF S A W L E  SHOVING THE AREA SCANNED, ELECTRICAL CON- 
TACT POINTS. AND SURFACE CRACK. 
3 
0RIGINfl.L PAGE 
BLACK Allla WMiTE PHOTOGRAPH 
RESULTS 
The eddy current response images and the resistance versus tempera- 
ture data are shown in figure 3 .  The resistance data are normalized, for 
NORMALIZED RESISTANCE i::K- 0.6 EDGE 
(k) ::: 
0.0 
a5 a i  a9 91 95 97 
SURFACE 
CRACK-, 
\ 
\ 
TEMPERATURE (0 
T = W K  
1.0 ( S r n R -  
CONDUCTING) 
T = B B K  
as mm 
PROBE 
RfSPGUSE 
(ARBITRARY 
UNITS) 
0 0 (nORMAU 
T = a 9 K  
FIGURE 3.  - EDDY CURRENT RESPONSE INAGES. AND RESISTANCE 
VERSES TEWERATURE DATA AT THE CENTER AkD EDGE OF THE 
S W L E  . 
comparison, by dividing the measured value by the value at 97 K. The 
resistance versus temperature data exhibit temperature fluctuations of 
about 20.25 K. These fluctuations are due to the actual temperature 
drifts of the exchange gas within the large dewar. At 85 K, the sample 
is completely superconducting. At 88 K there is change in the conductiv- 
ity of the sample. The superconducting-to-normal transition begins at 
about the center of the image as indicated by the darkest area. The 
resistance, R 1 ,  at the center is slightly higher at the center than at 
the edge, R2, of the sample at this temperature. At 89 K an isolated 
region of high resistance is identified near the center of the image. 
At 89.5 K the resistance at the center exceeds that of the edge by about 
a factor of 2 .  The eddy current response at 90 K indicates that the cen- 
ter of the sample is almost completely normal and surrounded by a region 
having some residual superconductivity. 
A surface crack developed during the thermal cycling of the sample. 
The surface crack was located about 2 nun above the central normal zone 
(figs. 2 and 3) and was detectable only at temperatures above 91 K when 
the sample was everywhere in its normal state. 
The grain and porosity size distributions at the center and edge of 
the sample are shown in figure 4. No anisotropy was observed for either 
the grain o r  porosity distributions. The porosity fractions at the cen- 
ter and edge of the sample are 0.24 and 0.23, respectively. The mean 
pore sizes are 3421.5 and 3121.8 pm at the center and edge, respectively. 
The mean grain sizes are 7.320.35 and 6.220.32 p at the center and edge, 
4 
NUMBER 
(ARBITRARY 
UNITS) 
I 
GRAINS PORES 
,r CENTER 
-- . 
18 0 100 
LENGTH (/rm) LENGTH (pm) 
AT THE CENTER (DASHED L I N E )  AND THE EDGE 
(SOLID L I N E )  OF THE SAMPLE. 
FIGURE 4 .  - GRAIN AND POROSITY DISTRIBUTIONS 
respectively. The results indicate that the mean pore and grain sizes 
are about 9 and 18 percent, respectively, greater at the center of the 
sample than at the edge. 
DISCUSSION 
The eddy current response images (fig. 3) showed that as the sample 
temperature increases, the center of the sample begins to go normal before 
the outer edge. Between 89 and 90 K, both the eddy current response and 
the resistance measurements indicate that the superconducting-to-normal 
transition begins at and grows from the center of the sample. 
Previous work [6] indicated that the superconducting transition tem- 
perature decreases and the transition width increases with increasing 
bulk porosity fraction and grain size. Figure 3 indicates that the total 
transition width is about the same for both the center and edge, however, 
the data for the central region does exhibit a much sharper transition 
jump at 90 K. The increase in mean pore and grain size at the center of 
this sample is likely to be responsible for the different transition 
structure in this region. It should be noted that these microstructural 
variations may have affected the degree and uniformity of oxygenation 
throughout the bulk of the sample. 
The shape of the resistance versus temperature curves in figure 3 
resemble that of the superconducting bolometers used widely in superfluid 
physics research [12]. These bolometers are electronically biased into 
their transition region for the measurement of temperature variations in 
the microkelvin range. Wherever there is a nonzero resistance supporting 
a current there is joule heating. For superconducting metals, joule 
heating effects are minimized by their high thermal conductivity 
(1.0 W cm-lK-l) and by use of the appropriate thermal sinking methods. A 
typical metallic sample was, throughout the bulk, at the same temperature 
within a few millikelvin. In contrast, the low thermal conductivity 
(5.0~10-3 W cm-lK-l) of bulk high temperature ceramics [13] is expected 
to promote local joule heating. It has been speculated [6] that super- 
conducting ceramics may exhibit varying local superconducting properties 
or a physically mixed state system. Our results indicate that regional 
nonuniformity of porosity and grain size distributions encourage local- 
ized superconducting transitions to take place over different regions of 
a high critical temperature ceramic superconductor. We believe that 
local heating on a millikelvin level, at the porous or granular areas, 
is the mechanism responsible for affecting the observed superconducting 
transitions. 
5 
CONCLUSION 
In this work we investigate the effect of microstructural variations 
A scanning eddy current on the superconducting properties of SmBa2Cu30x. 
probe revealed the onset and growth of a normal conducting region. 
Resistance versus temperature measurements taken at different regions of 
the sample support the concept of a physically mixed state system. 
Regional variations in porosity and grain size distributions affect the 
observed superconducting transition. 
REFERENCES 
1. Wu, N . K . ,  et al.: Superconductivity at 98 K in the Y-Ba-Cu-0 Com- 
pound System at Ambient Pressure. Phys. Rev. Lett., vol. 58, Yar. 2 ,  
1987, pp.  908-910. 
2. Maeda, H., et al.; A New High-T(,) Oxide Superconductor h'ithout a 
Rare Earth Element. Jpn. J. Appl. Phys. Lett., vol. 27, no. 2, Feb. 
1988, pp. L209-L210. 
3. Ekin, J: Transport Critical Current in Bulk Sintered Y1Ba2Cu30, and 
Possibilities for Its Enhancement. Adv. Ceram. ?later., vol. 2, 
no. 3B, 1987, pp. 586-592. 
4 .  Chaudhari, P., et al.: Critical-Current Yeasurements in Epitaxial 
Films of YBa2C~307-~ Compound. Phys. Rev. Lett., v o l .  58, no. 2 5 ,  
June 2 2 ,  1987, p p .  2 6 8 4 - 2 6 8 6 .  
5 .  Roth, D.J.; Generazio, E.R.; Stang, D.B.; and Hepp, A: A Study of  
Porosity Variation in the Bulk YBa2Cu307-, High Temperature 
Superconductor by Ultrasonic Imaging. NASA TN-102130, 1989, to be 
published. 
6. Cima, M.J.; and Rhine, W.E.: Powder Processing f o r  Nicrostructural 
Control in Ceramic Superconductors. Adv. Ceram. Mater., vol. 2, no. 
3B, 1987, pp. 329-336. 
7. Generazio, E.R.: Determination of Grain-Size Distribution Function 
Using Two-Dimensional Fourier Transforms of Tone-Pulse-Encodsd 
Images. Mater. Eval., vol. 4 6 ,  no. 4 ,  Yar. 1988, pp. 528-53h. 
8. Dover, W.D., Charlesworth, F.D.W., Taylor, K.A., Collins, R., and 
Michael, D.H.: The Use of A-C Field Pleasurements t o  Determine the 
Shape and Size of a Crack in a Netal. Eddy Current Characterization 
of Platerials and Structures, G. Birnbaum and G. Free, eds., ASTN PCN 
04-722000-22. 
9. Applied Superconductivity, V.L. Newhouse, ed., Academic Press, 
New York (1975), Chapter 2 ,  pp. 200-207. 
10. Cava, R.J., et al.: Bulk Superconductivity at 91 K in Single Phase 
Oxygen-Deficient Perovskite BazYCu309-6. Phys. Rev. Lett. s(16) 
pp. 1698-1700, 1987. 
11. Wieck, A.D.: Vanishing Contact Resistance on Polycrystalline 
Y B ~ ~ C U ~ O ~ - ~ ,  Applied Physics Letters, Vol. 52, No. 12, Mar. 21, 
1988, p. 1017. 
6 
12. Generazio, E.R.; and Reed, R.W.: The Attenuation and Phase Velocity 
of Ultrasonic Third Sound for Optically Smooth and Roughened 
Substrates. J. Low Temp. Phys., vol. 5 7 ,  no. 5 / 6 ,  Dec. 1984, 
pp. 501-527. 
13. Morelli, D.T.; Heremans, J.; and Swets, D.E.: Thermal Conductivity 
of Superconductive Y-Ba-Cu-0. Phys. Rev. B, vol. 36, no. 7, Sept. 
1987, pp. 3917-3919. 
7 
National Aeronautics and 
Space Administration 
1. Report No. 
NASA TM-102133 
Report Documentation Page 
2. Government Accession No. 
17. Key Words (Suggested by Author(s)) 
7. Author@) 
Edward R. Generazio, Don J. Roth, and Aloysius F. Hepp 
18. Distribution Statement 
9. Performing Organization Name and Address 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 44135-3191 
19. Security Classif. (of this report) 20. Security Classif. (of this page) 
Unclassified Unclassified 
2. Sponsoring Agency Name and Address 
National Aeronautics and Space Administration 
Washington, D.C. 20546-0001 
21. No of pages 22. Price' 
8 A03 
3. Recipient's Catalog No. 
5. Report Date 
6. Performing Organization Code 
8. Performing Organization Report No. 
E-4928 
10. Work Unit No. 
506-43- 1 1 
11. Contract or Grant No. 
13. Type of Report and Period Covered 
Technical Memorandum 
14. Sponsoring Agency Code 
__ 
5. Supplementary Notes 
Prepared for the 16th Annual Review of Progress in Quantitative Nondestructive Evaluation, cosponsored by DOE, 
AFWAL, ONR, and ASNT, Brunswick, Maine, July 23-28, 1989. 
6. Abstract 
In this work we investigate the effect of microstructural variations on the superconducting properties of 
SmBa2Cus0,. A scanning eddy current probe revealed the onset and growth of a normal conducting region. 
Resistance versus temperature measurements taken at different regions of the sample support the concept of a 
physically mixed state system. Regional variations in porosity and grain size distributions affect the observed 
superconducting transition. 
Superconductors, Ceramic superconductors, 
Eddy currents, Porosity, High temperature 
superconductors 
I Unclassified - Unlimited 


Non-uniform transition conductivity of superconducting ceramic

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