The thesis is introduced by a discussion of the technological importance of developing superconducting materials having low losses at power frequencies. The following two chapters are devoted to a presentation of the reversible magnetic properties of superconductors using the Ginzburg-Landau equations, and a discussion of the irreversible behaviour of real materials in terms of flux pinning and the critical state concept. Following this background discussion, previous work on a.c. losses is reviewed. Particular emphasis is given to the losses below Hc1 and it is shown that, while losses above Hcl can be interpreted using the critical state model, below Hc1 they are only partially understood. Modifications to an electronic wattmeter which enable accurate measurements to be made at 10- 4 WHz- 1 m-2 are discussed. An improved vibrating sample magnetometer, is described, together with the theory and design of an a.c , permeability apparatus used to obtain critical current values. The preparation of a range of niobium samples is described. The techniques employed include annealing, spark-machining, mechanical, chemical and electro-polishing, neutron irradiation and ion-implantation. The magnetization curves, surface profiles and other data for the treated specimens are presented. Measurements of the low frequency losses in these samples at 4.2 K are described. It is shown that in rough samples hysteretic losses arise below Hcl·from the penetration of surface asperities and that the results are in good agreement with an expression, similar to Buchhold’s (1963), which is derived, namely : L =4 μOHm2 f(Hm/HCL) KD where Hm is the peak field, HCL the lower critical field, HCl or HC, D the centre line average height of the surface profile and K a ‘hysteresis factor’. The losses in cold-worked and annealed niobium samples and lead, tin and indium specimens with values for D between 0.33 and 20 μm are found to fit (within a factor of two ) the expression : L = ½ μ0Hm2 (Hm/HCL)2D Different results are observed in smooth samples and explanations for these are suggested. The losses in samples previously penetrated by a large a.c. field are found to be increased by a factor of up to thirty. This is related to trapped flux and ppossible loss mechanisms arc discussed. The losses above Hcl are shown to be reduced in damaged samples. Neutron irradiation induces heavy bulk damage but has little effect on the surface. Mechanical treatment produces large surface currents and is most effective reducing the loss, the dissipation at 50 Hz being below 0.1 W m-2 at fields up to 85 kA(RMS)m- 1 (1500 0e - Peak) in a mechanically polished, annealed polycrystalline sample. Implantation of niobium ions to a depth of 10 nm does not alter the losses. Measurements of the critical currents and flux profiles in two annealed samples are presented and it is shown that a critical state exists within them. The losses above Hcl in both irreversible and annealed samples fit the Ullmaier (1966) expression L = ⅔ μO(Hm - ∆H/2)3/JC, but it is found that equating ∆H/2 with HCl gives critical currents an order of magnitude smaller than those obtained in other measurements. It is shown that ∆H/2 is not constant and that the shielding currents are negligible except near HCl.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:594897 |
| Date | January 1970 |
| Creators | Brankin, Paul Robert |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/73938/ |
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