SISTEMA DE GESTIÓN DE MANTENIMIENTO DE CEMICON S.A.C

The resistance o f samples over the temperature range 4 - 300K was determined by the standard four terminal technique in an Oxford Instruments helium gas flow cryostat. A small bar (roughly 1mm2 x 5mm) was cut from the sintered pellet of each sample and was mounted on a specially designed printed circuit board (PCB) using shellac as an adhesive. Four electrical contacts were made to the sample from copper tracks on the PCB with conductive silver paint (see figure 2.2). The PCB was fixed to a copper block at the end o f the cryostat sample stick and four leads (a pair for both voltage and current) were soldered on. Dry helium (boil-off) gas, at reduced pressure (-30 kPa), was used as a heat exchange medium in the cryostat sample space.

Y iB a 2 C u j0 7

F ig u re 2.1 X-ray powder diffraction pattern o f Y123.

B

3 cm --- -

1/ / \

copper tracks / contacts

sample

F igu re 2.2 PCB used fo r resistance measurements showing sample

3 0

R esistivity measurements were controlled using an Apricot microcomputer via an IEEE bus. Current was supplied from a Keithley 244 constant current source, and the voltage dropped across the sample was read from a Keithley 184 nanovoltmeter. Sample temperature was m easured using a ca lib rated gold-iron :ch rom el therm ocouple monitored by a Phillips multimeter. Resistance was calculated from the average o f 30 voltmeter measurements (15 fo r each o f the two current directions) with a fixed current magnitude. The current was reversed so as to eliminate any thermal em f's (at the contacts or leads) from the calculation o f sample resistance.

The onset o f the superconducting transition was defined to be the point at which the resistance deviated from the extrapolated high tem perature behaviour (see figure 2.3). T c was taken to be the temperature at which the resistance had fallen to h alf the onset value. The width o f the transition. A T C> was defined to be the temperature difference between 10% and 90% o f the onset resistance.

Section 2^3 A.C. Magnetic Susceptibility

As explained in chapter 1, when a superconductor is cooled below its transition temperature it becomes diamagnetic. This can be exploited by susceptibility measurements not only for determining Tc but also for quantifying the fraction o f superconducting phase present.

R

es

is

ta

nc

e

F o r these measurements the same gas flow cryostat was employed as that for d.c. resistance (section 2.2). A separate probe was constructed with a system o f coils at the lower end as shown in figure 2.4. An a.c. voltage o f l v at 86.4 H z was supplied to the primary coil from an EG & G Princeton Applied Research two phase lock-in analyzer (PSD). The difference o f the induced voltages in the two secondary coils was measured on the PSD with the phase set to give x ' and X ° n separate channels. Samples were normally fine powders (100-200mg) although for quick checks o f a sample small lumps were often used. These were contained in standard 5mm (dia.) quartz NM R tubes screwed to the end of a thin walled stainless steel tube. This allowed samples to be changed without removing the coil assembly from the cryostat. Temperature at the sample position was monitored using a calibrated Southampton miniature diode (calibration: better than ±0.1K in the temperature range o f in terest) supplied with lOpA constant current. As in the resistance measurements data were collected on an Apricot microcomputer.

It was noticed that the measured signal was sensitive to any rapid changes in cooling/heating rate and that marked hysteresis occurred when these rates were too high. The former problem is believed to result from changes in the temperature gradient between the two secondary coils whereas the latter is assumed to be caused by the necessarily poor therm al contact between the coil system (where temperature was measured) and the sample. Acceptable results were obtained by heating at a constant rate of 0.2 Kmin_1 (or less) from well below T c. A t this rate any hysteresis was found to be negligible (< 0.5 K).

Prim ary Coil Upper Secondary C o «

Low er Secondary Coil

Figure 2.4 Schematic layo u t o f coils on the a.c. m agnetic susceptibility probe.

32

Recently Chen et al (1990) have investigated the problems involved in making quantitative measurements o f a.c. susceptibility in the High- T c Superconductors. Th ey compare samples from both cut sintered pellets and ground powders. The relevance o f weak link models to explain the low J c is discussed and they propose a "grain-cluster" model. In the present study the a.c. susceptibility measurements were used prim arily to determ ine Tc and to monitor sample quality — a "good" sample has a single sharp transition associated with a large change in susceptibility.

33

CHAPTER 3