Surface Composition Analysis of a Degraded Copper Plate and Scrapings from The Inner Surface of An SRF Coupler

Transcription

1 Surface Composition Analysis of a Degraded Copper Plate and Scrapings from The Inner Surface of An SRF Coupler Abstract: Andy T. Wu, K. Wilson, M. Wiseman, and I. Campisi Measurements have been undertaken to determine the composition of surface layer of a contaminated copper plate used in a superconducting radio frequency (SRF) coupler and chemical composition of scrapings from the inner surface of an SRF coupler employing a scanning electron microscope and an energy dispersive x-ray spectrometry. It is found that one surface of the copper plate is seriously degraded with typical composition of oxygen, sulfur, and copper presumably in the forms of CuO and CuSO 4. The other surface has typical composition of oxygen and copper in the form of CuO with small areas of almost pure copper covered by a surface layer of oxide with a thickness less than a few micrometers. For the scrapings, they can be mainly divided into two types. One is Cu with CuO surface cover. The other is presumably a mixture of stainless steel and Cu and CuO and CuSO 4. By using some suitable pickling technique, the degraded layer on the surfaces of the copper plate can be removed, at least on the less degraded side. It is suggested that the environment for processing SRF couples may need to be controlled in order to make good SRF couplers.

2 1. Introduction: A copper plate mounted on top of a superconducting radio frequency (SRF) coupler shows some degradation. Judging from the appearance, one surface (Fig.1) is more heavily degraded than the other (Fig.2). To study the surface composition, a scanning electron microscope (SEM) equipped with an energy dispersive x-ray spectrometry (EDX) was employed. Similar measurements have been performed on scrapings from the inner surface of an SRF coupler to determine their chemical composition. One surface of a double-sided carbon adhesive tab (25mm diameter) was used to hold the scrapings with the other surface in contact with the sample holder of the SEM system. 2. Experimental: 2.1 Copper plate The heavily degraded surface (named A Surface) was examined first. Fig.3 shows how it looks like under SEM. Basically speaking, A Surface can be classified into three different types of regions. The first type of regions (marked as 1 in Fig.3) looks brighter than the second type (marked as 2 in Fig.3) under SEM and occupies the majority of the surface. The third type (indicated by the white arrows) appears to be brightest among the three types and has a curved line appearance. EDX taken on the first type is shown in Fig.4a). The composition of this type is oxygen, sulfur, and copper with 2.47% of S and 97.53% of Cu. The suggested forms of compounds may be CuO and CuSO 4. Fig.4b) indicates that the second type of regions contains pure CuO. The third type consists of, in fact, two different kinds at larger magnifications as shown in Fig.5b). One kind has rectangular shape with corresponding EDX shown in Fig.4c). The composition of this kind is oxygen, silicon, sulfur, and copper with Si concentration of 4.74%, S of 27.48%, and Cu of 67.78%. The other kind looks like hills and contains only oxygen, sulfur, and copper with S concentration of 27.57% and Cu of 72.43% as shown in Fig.4d).

3 The other surface of the copper plate is less complicated. The general appearance under SEM is shown in Fig.6. The entire surface can be divided into to types. One is darker than the other and the major part of the surface is covered by the brighter type. The composition of the brighter part is oxygen and copper in the form of CuO presumably with typical EDX depicted in Fig.7b). The darker part is almost pure copper (Fig.7a)). 2.2 Scrapings from the inner surface of an SRF coupler A general SEM view of the scrapings is shown in Fig.8. Small flakes of different sizes are easily visible. Some small threads can also be seen as indicated by arrows in Fig.8, presumably due to contamination on the coupler surface or the paper that held the scrapings during transportation from manufacturer to our JLab. Generally speaking, the scrapings can be classified into two types. One is Cu with possibly CuO overlay. The other is mainly stainless steel with major composition of Cr, Fe, Cu, Ni, S, and O. EDX data of these two types are shown in Fig.9. The latter type may be a mixture of stainless steel and Cu and CuO and CuSO 4. A typical SEM image of this two types is depicted in Fig.10. Occasionally, small fiber-like threads are also seen. A typically example is also shown in Fig.10. EDX measurements on the surface of the thread (see Fig.11) indicate that the surface composition is Na, K, Cl, Ti, and Cu, presumably mainly due to the contamination from sweating. The other relatively frequent observed contamination is also indicated in Fig.10. This seems to be hydrocarbon mixed with Cl, S, O, and Cu as typically shown in Fig.12 from EDX measurements. The two types of contamination observed here are believed to be due to the processes related to sample collection and handling and the environment during sample collecting. Coupler test has suggested that the oxide layer on copper may reduce the secondary electron yield. It may therefore be a good idea to process or treat couple under a controlled environment, for instance, in an oxygen chamber. In order to get a reproducible performance, the temperature under which SRF couplers are

4 treated needs to be fixed. It would be ideal if SRF couplers can be processed or treated in oxygen environment at a fixed temperature of, for instance 45 o C, to increase the thickness of oxide layer on the surface of copper. 3. Conclusion: Present study indicates that one surface of the copper plate is seriously degraded with typical composition of oxygen, sulfur, and copper in the forms of presumably CuO and CuSO 4. Some small areas contain silicon in addition. The other surface has typical composition of oxygen and copper in the form of CuO with small areas of almost pure copper having a surface layer of oxide less than a few micrometers. By using some suitable pickling technique, the degraded layer on the surfaces of plate can be removed, at least on the less degraded side. SEM and EDX measurements have shown that the scrapings from the inner surface of a SRF coupler can be mainly divided into two types. One is Cu with CuO surface cover. The other is presumably a mixture of stainless steel and Cu and CuO and CuSO 4.

5 Fig.1: The heavily degraded side of the copper plate. Fig.2: The other side of the copper plate.

6 Fig.3: SEM image of the heavily degraded side of the copper plate. The surface can be divided into three types as marked by 1, 2, and arrows.

7 Fig.4a) Fig.4b)

8 Fig.4c) Fig.4d) Fig.4: EDX taken on various types of the regions on the heavily degraded side of the copper plate (see text for details).

9 Fig.5: Enlarged areas of the third type of the heavily degraded surface of the copper plate. b) is the enlarged part of a) under SEM.

10 Fig.6: SEM image of the less degraded side of the copper plate. This side of the plate can be divided into two regions. One marked by 1 with corresponding EDX shown in Fig.7a). The other marked by 2 with EDX of Fig.7b).

11 Fig.7a) Fig.7b) Fig.7: Typical EDXs taken on the less degraded side of the copper plate.

12 Fig.8: SEM image of the scrapings obtained from the inner surface of a SRF coupler held by a double-sided carbon tape. The arrows indicate fiber-like threads.

13 a) b) Fig.9: Typical examples EDX spectra observed on the scrapings, a) scrapings show composition of Cu and CuO, b) scrapings show composition of stainless steel and presumably and Cu and CuO and CuSO 4.

14 Fig.10: A typical SEM image of the scrapings showing all types of feature observed. 1 indicates the Cu with CuO overlay, 2 indicates the stainless steel mixed with Cu, CuO, and CuSO 4, 3 indicates the fiber-like threads with surface composition of Na, K, Cl, Ti, and Cu, 4 indicates the hydrocarbon mixed with Cl, S, O, and Cu.

15 Fig.11: A typical EDX spectrum taken on the fiber-like threads as shown in Fig.3.

16 Fig.12: A typical EDX spectrum taken on another contamination type typically shown in Fig.3 as 4.