Beams Document 4730-v1

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First principles study of impurity and vacancy structures in niobium

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Fernanda G Garcia
Updated by:
Fernanda G Garcia
Document Created:
16 Dec 2014, 14:35
Contents Revised:
16 Dec 2014, 14:35
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16 Dec 2014, 14:35
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The interstitial chemical impurities hydrogen, oxygen, nitrogen, and carbon are important to SRF niobium and the optimization of SRF technology. These atoms are present in the refined sheets at 10 to 50 ppm concentration, and they can be absorbed into niobium, either intentionally or unintentionally, during processing treatments applied to SRF cavities, resulting in changes to the residual resistance and the performance of the cavities. These impurities may reside in niobium lattice interstices or site vacancies, and concentrations up to 1% can significantly lower the superconducting transition temperature of the niobium. Higher concentrations can result in precipitates with properties that differ from niobium. More subtle interactions also occur, such as the formation of hydrides or the suppression of hydrides by trapping hydrogen next to the other atoms (O, N, or C). Some of these subtle effects can be beneficial for SRF cavities. A first principles approach is taken to study the properties of chemical impurities in niobium, with particular focus given to the most recent analyses of carbon and metal vacancy complexes.
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