2414 IEEE TRANSACTIONS ON MAGNETICS, VOL. 25, NO. 2, MARCH 1989

ULTRASONIC MEASUREMENTS IN SINTER-FORGED HIGH Tc SUPERCONDUCTOR YBa2Cu307-&

M-F. Xu, A. Schenstrom, Y. Hong, D. Bein, Bimal K. Sarma and M. Levy

Physics Department, University of Wisconsin-Milwaukee, Milwaukee, WI 53201

2. Zhao, S . Adenwalla, A . Moreau, Q. Robinson, D. L. Johnson, S . J. Hwu, K. R. Poeppelmeier and J. B. Ketterson

Department of Physics & Astronomy. Northwestern Universitv. Evanston CL' 60208

Abstract

Both ultrasonic attenuation and velocity as a function of temperature from 4.2 K to room temperature were measured on sinter-forged samples of YBa Cu 0 which have more than 80% alignment of the c-a$i~~o$-~ the grains parallel to the forging axis. Strong anisotropy was found both in the attenuation and in the velocity when propagating sound waves in different directions relative to the forging axis. Hysteresis was observed in the velocity changes.

Introduction

Since large single crystals of high Tc YBa2Cu30,-6 for ultrasound studies are not easy to obtain, most of the investigations that have been reported[l-51 were done on isotropic polycrystalline samples. Measurements on small single crystal (0.5 X 2.5 X 2.5 mm3) have been reported[6] with different results from all the others[l-51. We report here ultrasonic attenuation and velocity measurements on sinter-forged samples in which the c-axes of the crystallites are preferentially aligned parallel to the forging axis so that they are different from ordinary sintered samples. sinter-forged samples result in anisotropy in both velocity and attenuation measurements when sound waves propagate along different directions of the samples. Hysteresis was observed in the velocity measurements as a function of temperature. Attenuation measurements of longitudinal waves travelling perpendicular to the forging axis show three maxima as a function of temperature, while both longitudinal and transverse waves propagating parallel to the forging axis exhibit only the middle of these three maxima.

The highly textured

Experiments

The sinter-forged samples were uniquely made by repeatedly calcining and grinding the powder, pressing it into pellets and sinter-forging the pellets in a resistance furnace. The detailed description of the process of making the samples has been reported elsewere[7].

Rectangular shaped samples were cut from pellets, ground and polished to have opposite faces parallel and smooth. The samples are typically 4.5 X 4.0 X 1.8 mm3 and have a density of 97% of that of the theoretical value for YBa Cu 0 superconducting transition temperature was determined as the mid point of the resistance drop, which was 91 K. mode quartz transducers were bonded to the samples with Epon epoxy. Longitudinal waves of 12 MHz were propagated perpendicular to the forging axis and 12 MHz and 15 MHz waves were propagated parallel to the axis. 14.5 MHz and the higher harmonic 41 MHz transverse waves were also measured and were sent parallel to the forging axis.

system was used to measure both attenuation and velocity. At the same time, another phase sensitive

2 3 7-6' The

12 MHz longitudinal mode and 14.5 MHz transverse

A Matec ESA-8000A phase coherent ultrasonic

Manuscript received August 22, 1988.

interferometer was also used to measure the velocity changes and a Matec 6600 ultrasonic system was used to measure the attenuation on the same samples. The velocity and attenuation measurement results obtained from these different instruments came out to be identical within experimental errors.

velocity change measurements were performed from room temperature to 4.2K, with typical cooling and warming rates of 1-2 K/min. However, some of the measurements were concentrated on effects near Tc and hence much slower cooling and warming rates were used in the vicinity of the transition temperature.

The temperature dependent attenuation and

Results

In the temperature dependent attenuation curves, peaks and a shoulder were found above Tc. One peak in the longitudinal mode, when propagating waves perpendicular to the forging axis, was found below Tc. In the velocity change measurements, large softening and hysteretic effects were observed. propagating longitudinal waves perpendicular to the forging axis, the hysteretic effect was small.

When

9 , I I 1

n

01 u W

c, c, a

n

w \ E

> W

a -1oL I I 1

0 100 200 300 T e m p e r a t u r e ( K )

Fig. 1. (a) The attenuation (one and a half round trips) and (b) the velocity changes (V m/sec) of 12 MHz longitudinal ultrason$Eo!aves propagating perpendicular to the forging axis.

=4.4Ox1O3

0018-9464/89/0300-2414$01 .0001989 IEEE

I

I

2415

r I I 1 Fig. 1 shows the longitudinal attenuation and

velocity changes as a function of temperature at 12 MHz when propagating waves perpendicular to the forging axis. Two broad peaks in attenuation (Fig. la) around 250 K and 180 K were observed above Tc, and one small peak at about 70 K was observed below Tc. Velocity data (Fig. lb) showed hardening when initially cooled from room temperature, and then softened below 150 K until about 30 K where it stopped softening. up, with a velocity maximum at around 180 K.

changes at 12 MHz with longitudinal waves propagating parallel to the forging axis. attenuation (Fig. 2a) was observed around 180 K, and a distinct thermal hysteresis in velocity was found between 6 5 K and 270 K (Fig. 2b). On the initial cooling, the velocity slowly increased up to a maximum around 220 K, and then decreased rapidly below 180 K. When warming up from 4 . 2 K, the velocity initially did not change much up to 65 K. A sudden drop at this temperature deviates the warming up velocity curve from the cooling down curve, forming a distinct hysteresis. Above 180 K, the velocity increased rapidly and slowed down at about 250 K, and finally merged with the cooling down curve before the temperature reached room temperature.

propagated parallel to the forging axis. The attenuation and velocity changes are shown in Figs. 3a and b. A shoulder in the attenuation curve appears at

correctly locate the maximum that may have produced this shoulder, the background attenuation was interpolated as indicated by the dashed line and then subtracted from the original curve. lower curve has a broad peak at around 180 K. In the velocity measurements, again a large hysteretic effect was observed. Note that both the attenuation and the

Small hysteresis was observed during warm

Fig. 2 displays the attenuation and velocity

A pronounced peak in

Transverse sound waves at 14.5 MHz were

' a temperature around 160 K-170 K. In order to

The resultant

2 . 5

- 1.5

n

PI U

iJ

CT +, 0.5

30

10

n

VI \ E v

> a

I I

0 100 200 300 -10'

T e m p e r a t u r e ( K ) Fig. 3. (a) The attenuation (half round trip) and (b) the velocity changes (Vroom=2.66x103 m/sec) of 14.5 MHz transverse waves propagating parallel to the forging axis.

velocity for this transverse mode have higher values at low temperatures than those at room temperature.

At a higher harmonic of the transverse sound, 41 MHz signals were obtained and measured, with the wave vector parallel to the forging axis. The attenuation curves are shown in Fig. 4 . By applying the same method as in Fig. 3a, the lower resultant curve is obtained, which displays a maximum at 195 K. The velocity curve measured at 41 MHz is essentially the same as in Fig. 3b.

0.8 I I

h ( a > 1 J

*. . , . -:\

f .:. . . . * - . ' 0.41

12

9 n

m U v

6

3 I- t- [r 0 I -200 0 100 200 300

T e m p e r a t u r e ( K ) -3

0 100 200 300 TEMPERATURE ( K )

Fig. 2. (a) The attenuation (half round trip) and (b) the velocity changes (Vroo -3.77~10~ m/sec) of 1 2 MHz longitudinal waves propagaFing parallel to the forging axis.

Fig. 4 . transverse waves travelling parallel to the forging axis.

The attenuation (half round trip) of 41 MHz

2416

2

1 8 . 7

s W

> \ > U

0

- 1

I - - - ' I

L O 100 200 300 T e m p e r a t u r e ( K )

Fig. 5. The velocity hysteresis of 15 MHz longitudinal sound waves travelling parallel to the forging axis.

In order to determine the details of the velocity hysteresis, multicycle velocity measurements in the range of 4.2-220 K were performed. successive cycle the upper temperature was reduced by 15 K. Shown in Fig. 5 are the resulting velocity hysteresis curves when propagating 15 MHz longitudinal sound waves parallel to the forging axis. The arrows represent warming up or cooling down in temperature. Hysteresis loops are obvious and shrink with decreasing upper limit temperatures.

simulteneously with the velocity hystersis measurements to see if there were any correlations between the velocity hysteresis and changes in the superconducting transition temperatures[8]. For the cyclical curves shown in Fig. 5 we observed shifts in the transition temperature as measured during decreasing temperature runs that were within 2 K of each other.

For each

Resistance measurements were also done

Discussion

It has been suggested that in the perovskite superconductor YBa Cu 0 important role. Ties2 :;%I Planes are perpendicular to the c-axis. When longitudinal sound waves travel perpendicular to the c-axis, distortions in the Cu-0 planes will occur. When longitudinal waves propagate parallel to the c-axis, only the separations between the planes are affected. In the case when transverse waves travel parallel to the c-axis, the planes undergo shear motion. In the two latter cases, no distortion of the Cu-0 planes occurs. These facts could explain why there is only one peak in attenuation at around 180 K in these two latter cases, while there are three peaks in attenuation when longitudinal waves travel perpendicular to c-axis which, in the case of sinter-forged samples, is parallel to the forging axis. The distortion of the Cu-0 planes may be responsible for the peaks at 250 K and 70 K. And the 180 K peak in the cases of waves travelling parallel to the c-axis may due to the fact that Cu-0 planes are moving relative to each other. The reason that the 180 K peak appears in longitudinal waves propagating perpendicular to the forging axis could be that the crystallites in the sample are not 100% in alignment with the forging axis or that the interaction that produces this effect could be isotropic.

on cooling when propagating longitudinal sound waves

Cu-0 Planes play an

In the velocity measurements, the large softening

perpendicular to the c-axis may be due to lattice instabilities, samilar to the phenomenon that is observed in V,Si[9], an A-15 superconductor. The hysteretic effects in velocity may come from strain arrested gradual first order transition between 40 K and 270 K.

Acknowledeements

This research was supported under AFOSR Grant No. AFOSR 84-350 at the University of Wisconsin-Milwaukee and under DMR-85-20280 and DMR-86-02857 at Northwestern University.

Weinstock for drawing our attension to Ref. 8 , and suggesting the measurements of correlating Tc with velocity hysteresis.

The authors at UWM want to thank Dr. Harold

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1