. '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 

. ___~ 

( NASA- J M- 2 13 3) WM&-UY I F Lr?B TRAY S T T fbht ~~~-2at151 

CONDUCTTVITY @F SUPFRCUWDUCTING CEAAMIC 

(NASA- Lewis Research Center) :? p CSLL 24cf Uncl as 

GJ/3R 0225962 

*- . 
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 

on the superconducting properties of SmBa2Cu30,. 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. 

INTRODUCTION 

There is an extensive international research effort underway to 

understand the origin of high critical temperature (T,) superconductivity. A considerable amount of this research is directed toward increasing 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 dependent on the bulk porosity [6]. However, the effect of porosity variations, 

existing within the bulk of these ceramics, on the superconducting properties 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, 

2.54 cm diameter, 3.0 mm thick pellet sintered for 8 hr at 930 "C and 

annealed for 5 hr at 440 OC 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 se of eddy currents for the detection 3f surface flaws is well 

known. In brief, a coil excited by a continuous high frequency 2lectrical 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 &Wz. 

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 [lo]. 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 surface 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 completed in 10 min. The temperature is continuously measured by a thermocouple placed next to the sample. 

2 
CURRENT 

DEFLECTOMETER 

rPROBE SUPPORT ARM 

,-INSULATING POSITION 

TABLES ,' COVER 

TEMPERATURE 

GUAGE 

COMPUTER , SAMPLE 

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 evaporates 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 [Ill 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, R2 

measured as a function of temperature. 

Six 

The 

were simultaneously 

, 

' SURFACE CRACK 

FIGURE 2. - RADIOGRAPHIC IMAGE OF SAWLE SHOVING THE AREA SCANNED, ELECTRICAL CONTACT POINTS. AND SURFACE CRACK. 

3 

0RIGINfl.L PAGE 

BLACK Allla WMiTE PHOTOGRAPH 
RESULTS 

The eddy current response images and the resistance versus temperature data are shown in figure 3. The resistance data are normalized, for 

NORMALIZED RESISTANCE i::K- 0.6 EDGE 

(k) ::: 

0.0 

a5 ai a9 91 95 97 

SURFACE 

CRACK-, 

\ 

\ 

TEMPERATURE (0 

T=WK 

1.0 (SrnRCONDUCTING) 

T=BBK 

as mm 

PROBE 

RfSPGUSE 

(ARBITRARY 

UNITS) 

0 0 (nORMAU 

T=a9K 

FIGURE 3. - EDDY CURRENT RESPONSE INAGES. AND RESISTANCE 

VERSES TEWERATURE DATA AT THE CENTER AkD EDGE OF THE 

SWLE . 

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 conductivity of the sample. The superconducting-to-normal transition begins at 

about the center of the image as indicated by the darkest area. The 

resistance, R1, 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 center 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 or porosity distributions. The porosity fractions at the center 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 LINE) AND THE EDGE 

(SOLID LINE) 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 temperature 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 superconducting 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 localized 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 

on the superconducting properties of SmBa2Cu30x. 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. 

REFERENCES 

1. Wu, N.K., et al.: Superconductivity at 98 K in the Y-Ba-Cu-0 Compound 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., vol. 58, no. 25, 

June 22, 1987, pp. 2684-2686. 

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 for 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. 46, 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 to 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 

YB~~CU~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. 57, 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