Detection of Surface Impurity Phases in High T.sub.C Superconductors Using Thermally Stimulated Luminescence

The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiment of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:

FIG. 1 shows thermally stimulated luminescence emission from samples of Y.sub.2 O.sub.3, Ba.sub.3 CuO.sub.4, BaCuO.sub.2, and BaCO.sub.3.

FIG. 2 shows thermally stimulated luminescence emission from samples of Y.sub.2 BaCuO.sub.5, YBa.sub.2 Cu.sub.3 O.sub.6.2, and YBa.sub.2 Cu.sub.3 O.sub.x (I).

FIG. 3 shows thermally stimulated luminescence emission from single-crystal YBa.sub.2 Cu.sub.3 O.sub.x, and two sintered samples of YBa.sub.2 Cu.sub.3 O.sub.x.

FIG. 4 shows absolute thermally stimulated luminescence emission intensities from the samples with thermally stimulated luminescence emission described in FIGS. 1-3 hereof.

FIG. 5 shows the relationship of measured integrated thermally stimulated luminescence and measured surface resistance for several samples.

Investigation of the thermally stimulated luminescence from a sample of superconducting material is a rapid, sensitive, and inexpensive technique for determining the presence of insulating, impurity phases therein. The estimated probe depth of this procedure is about 1 .mu.m, thereby rendering it valuable for examining superconductors having applications deriving from surface properties. In its simplest form, an apparatus for measuring thermally stimulated luminescence includes a sample holder, a heater with thermocouple output, and a photomultiplier tube all mounted in a light-tight enclosure. Current from the photomultiplier is amplified and recorded as a function of sample temperature with the resulting curve being referred to as a thermally stimulated luminescence glow curve. Exposure of the samples to ionizing radiation induces charge trapping. The resulting metastable electron-hole pairs in insulating substances recombine upon heating to yield luminescence. No radiation effect is expected to occur in the superconducting materials since the exposure is small.

It would be obvious to an artisan having ordinary skill in the field of thermally stimulated luminescence, after reading the subject disclosure, that in order to locate the source of the luminescence, that is, the insulating material, a single light pipe, or a series of light pipes could be employed to gather light from specific portions of the sample during the heating process. If a single light pipe is used, a mechanical table would be provided to sweep the surface of the sample under one end of the pipe so that the luminescence of the surface as a function of both temperature and location could be generated. In either situation where light pipes are employed, each having a detector associated therewith, or where a single detector views all of the emitted luminescence, a spectrophotometer, or several spectrophotometers might be employed to resolve the specific emitted wavelengths in order to identify the composition of the insulating material source.

Samples for investigation according to the method of the present invention were exposed to 1.53.times.10.sup.5 R of .sup.60 Co radiation at room temperature. It would also be obvious to one of ordinary skill in the art of thermally stimulated luminescence, after reading the present disclosure, that x-rays or high-energy electrons could be employed as ionizing radiation in the same manner as have gamma rays according to the teachings of the subject invention. Following exposure to gamma radiation, the samples were heated to approximately 400.degree. C. in a commercial luminescence reader at a constant rate of about 4.4.degree. C. sec.sup.-1 and the thermally stimulated luminescence signal recorded. The area under the recorded luminescence plotted as a function of temperature is proportional to the total light emitted by the sample, which in turn, is proportional to the amount of insulating material present in the sample under investigation.

The presence of insulating surface impurity phases in high-T.sub.c superconducting materials is deleterious to rf surface resistance. Normal conducting electrons of the insulating materials dominate the rf losses, thereby making R.sub.s higher than expected if only superconducting electrons were present. TSL represents a measure of the presence of these normal-state electrons and is therefore an indirect measure of R.sub.s. If the integrated thermally stimulated luminescence is determined for a series of superconducting samples having similar compositions and the results normalized for mass and surface area of the emitting material involved, and the surface resistance is measured, a simple relationship is found to exist for these two quantities. Superconducting samples with unknown resistivity may be evaluated by comparing their TSL with the known samples according to the relationship.

Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. FIGS. 1, 2, and 3 hereof, show the resulting thermally stimulated luminescence emission for samples of Y.sub.2 O.sub.3, Ba.sub.3 CuO.sub.4, BaCuO.sub.2, BaCO.sub.3, Y.sub.2 BaCuO.sub.5, YBa.sub.2 Cu.sub.3 O.sub.6.2, and various forms of YBa.sub.2 Cu.sub.3 O.sub.x. FIG. 3 shows the results for two high-quality sintered superconducting samples having x approximately equal to 6.9, and a superconducting crystal. All curves were recorded by heating the samples at 4.4 C/sec, and all have arbitrary ordinates. Absolute intensities are shown by the bar charts in FIG. 4 hereof. The intensities were determined by normalizing the area under each curve to the mass and to the surface area of the sample. Since all of the samples investigated are opaque, the absorption coefficient for the emitted light is large, which implies that the TSL signals emanate from within v m of the surface. Therefore, the values are normalized relative to the surface area as well as to the mass of the sample under investigation.

The insulating phases, Y.sub.2 O.sub.3, Ba.sub.3 CuO.sub.4, BaCuO.sub.2, BaCuO.sub.2, BaCO.sub.3, and Y.sub.2 BaCuO.sub.5, all exhibit characteristic TSL glow curves with well-defined maxima. In addition, strong luminescence is observed from a sample which has been intentionally depleted of a large fraction of the oxygen content (see, e.g., the curve labeled YBa.sub.2 Cu.sub.3 O.sub.6.2 in FIG. 2). This sample was prepared from YBa.sub.2 Cu.sub.3 O.sub.x (III) shown in FIG. 3, which is characterized by very weak TSL. This result demonstrates that not only can TSL detect the impurity phases present, but that it can also detect the nonsuperconducting tetragonal phase, which results from the poor oxygenation of YBa.sub.2 Cu.sub.3 O.sub.x. No TSL can be observed from the superconducting orthorhombic phase having .times..gtoreq.6.5; as for all metals, there is no band gap of sufficient magnitude to trap the radiation-induced charges. Thus, no pure superconductor will exhibit TSL, and any observed signal must come from the insulating surface impurities. This result is especially important since high-temperature superconducting materials are formed by sintering, and consequently, are susceptible to inclusion of secondary phases.

Many applications of high-T.sub.c superconducting materials will depend on surface properties. To date, measured values of R.sub.s at 4 K for sintered pellets of high-T.sub.c superconductors are considerably higher than that for Nb at the same temperature. It may be that the relatively high values of R.sub.s for bulk materials are, in part, associated with insulating surface impurity phases present in the material.

The TSL glow curves for three superconducting materials are shown in FIGS. 2 and 3. YBa.sub.2 Cu.sub.3 O.sub.x (I) is a low-quality superconductor as evidenced by its measured low diamagnetic shielding value of 38% of -1/4.pi. and by the indication of second phase constituents from x-ray diffraction measurements. Accordingly, it is characterized by strong luminescence and a high value of R.sub.s of 9.7 m.OMEGA. as measured in a 3-GHz Nb cavity operating in the fundamental mode TM.sub.OIO at 4 K. Typical values or R.sub.s for high-T.sub.c materials at this frequency and temperature are 0.4-0.7 m.OMEGA.. By contrast, YBa.sub.2 Cu.sub.3 O.sub.x (II) and YBa.sub.2 Cu.sub.3 O.sub.x (III) are both high-quality samples exhibiting 100% diamagnetic shielding. As shown in FIG. 3, they exhibit weak luminescence--two orders of magnitude less than YBa.sub.2 Cu.sub.3 O.sub.x (I). See also FIG. 4 hereof. Their corresponding values of R.sub.s are 0.1-0.2 These results indicate that insulating surface impurities contribute to high values of R.sub.s, and that these values correlate with TSL emission.

FIG. 3 shows the TSL glow curves of several small (about 4 mm.sup.3) YBa.sub.2 Cu.sub.3 O.sub.x single crystals. The absolute TSL intensity of these crystals is equivalent to that found in Ba.sub.3 CuO.sub.4 as is shown in FIG. 4. Two factors contribute to the observed emission: 1. insulating flux left on the surface; and 2. poor oxygenation.

The composite glow curves of the superconducting samples are broad, making it difficult to correlate the insulating constituents with any particular impurity phase. Computer construction of the composite curve from individual constituent curves may be possible and may permit identification of the principal insulating phases and the fractional composition thereof. An improved identification scheme will be to allow a portion of the emitted light to pass through a monochromator, whereby its spectral content is determined. The resulting emission spectrum will be an excellent signature of the individual insulating impurities.

FIG. 5 shows the relationship between the integrated TSL of three superconducting samples prepared as described hereinabove with the rf surface resistance thereof. Surface resistance is measured in a microwave cavity as described above at 3 GHz and 4 K. By observing the integrated TSL of an unknown sample, one may evaluate its surface resistance by utilizing the relationship defined in FIG. 5. This indirect surface resistance measurement represents significant savings in time and a reduction in complexity when compared to direct rf cavity techniques.

The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.