SRF2011 - List of Authors (Zhou, C.)

Paper

Title

Page

SIMS and TEM Analysis of Niobium Bicrystals

776

P. Maheshwari, A.D. Batchelor, D.P. Griffis, F.A. Stevie, C. Zhou
NCSU AIF, Raleigh, North Carolina, USA
G. Ciovati, R. Myneni, J.K. Spradlin
JLAB, Newport News, Virginia, USA
M. Rigsbee
Materials Science and Engineering, Raleigh, USA

The behaviour of interstitial impurities(C,O,N,H) on the Nb surface with respect to grain boundaries may affect cavity performance. Large grain Nb makes possible the selection of bicrystal samples with a well defined grain boundary. In this work, Dynamic SIMS was used to analyze two Nb bicrystal samples, one of them heat treated and the other non heat treated (control). H levels were found to be higher for the non heat treated sample and a difference in the H intensity and sputtering rate was also observed across the grain boundary for both the samples. TEM results showed that the bicrystal interface showed no discontinuity and the oxide layer was uniform across the grain boundary for both the samples. TOF-SIMS imaging was also performed to analyze the distribution of the impurities across the grain boundary in both the samples. C was observed to be segregated along the grain boundary for the control sample, while H and O showed a difference in signal intensity across the grain boundary. Crystal orientation appears to have an important role in the observed sputtering rate and impurity ion signal differences both across the grain boundary and between samples

THPO032

TOF-SIMS Analysis of Hydrogen in Niobium, From 160°K to 475°K

788

P. Maheshwari, A.D. Batchelor, D.P. Griffis, F.A. Stevie, C. Zhou
NCSU AIF, Raleigh, North Carolina, USA
G. Ciovati, R. Myneni
JLAB, Newport News, Virginia, USA
M. Rigsbee
Materials Science and Engineering, Raleigh, USA

Niobium (Nb) is the material of choice for superconducting radio frequency (SRF) cavities due to its high critical temperature and critical magnetic field. Interstitial impurity elements such as H directly influence the efficiency of these cavities. Quantification of H in Nb is difficult since H is extremely mobile in Nb with a very high diffusion coefficient even at room temperature. In the presented work, Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) was used to characterize H in Nb over a wide temperature range (160°K to 475°K) in situ to check for changes in mobility. Multiple experiments showed that as the specimen temperature is decreased below 300 °K, the H/Nb intensity changes by first increasing and then decreasing drastically at temperatures below 200°K. As specimen temperature is increased from 300°K to 450°K, the H/Nb intensity decreases. Remarkably, the H intensity with respect to Nb increases with time at 475°K (approximately 200oC). Correlation between this data and the H-Nb phase diagram appears to account for the H behaviour.

Poster THPO032 [0.125 MB]

Paper	Title	Page
THPO028	SIMS and TEM Analysis of Niobium Bicrystals	776
	P. Maheshwari, A.D. Batchelor, D.P. Griffis, F.A. Stevie, C. Zhou NCSU AIF, Raleigh, North Carolina, USA G. Ciovati, R. Myneni, J.K. Spradlin JLAB, Newport News, Virginia, USA M. Rigsbee Materials Science and Engineering, Raleigh, USA
	The behaviour of interstitial impurities(C,O,N,H) on the Nb surface with respect to grain boundaries may affect cavity performance. Large grain Nb makes possible the selection of bicrystal samples with a well defined grain boundary. In this work, Dynamic SIMS was used to analyze two Nb bicrystal samples, one of them heat treated and the other non heat treated (control). H levels were found to be higher for the non heat treated sample and a difference in the H intensity and sputtering rate was also observed across the grain boundary for both the samples. TEM results showed that the bicrystal interface showed no discontinuity and the oxide layer was uniform across the grain boundary for both the samples. TOF-SIMS imaging was also performed to analyze the distribution of the impurities across the grain boundary in both the samples. C was observed to be segregated along the grain boundary for the control sample, while H and O showed a difference in signal intensity across the grain boundary. Crystal orientation appears to have an important role in the observed sputtering rate and impurity ion signal differences both across the grain boundary and between samples

THPO032	TOF-SIMS Analysis of Hydrogen in Niobium, From 160°K to 475°K	788
	P. Maheshwari, A.D. Batchelor, D.P. Griffis, F.A. Stevie, C. Zhou NCSU AIF, Raleigh, North Carolina, USA G. Ciovati, R. Myneni JLAB, Newport News, Virginia, USA M. Rigsbee Materials Science and Engineering, Raleigh, USA
	Niobium (Nb) is the material of choice for superconducting radio frequency (SRF) cavities due to its high critical temperature and critical magnetic field. Interstitial impurity elements such as H directly influence the efficiency of these cavities. Quantification of H in Nb is difficult since H is extremely mobile in Nb with a very high diffusion coefficient even at room temperature. In the presented work, Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) was used to characterize H in Nb over a wide temperature range (160°K to 475°K) in situ to check for changes in mobility. Multiple experiments showed that as the specimen temperature is decreased below 300 °K, the H/Nb intensity changes by first increasing and then decreasing drastically at temperatures below 200°K. As specimen temperature is increased from 300°K to 450°K, the H/Nb intensity decreases. Remarkably, the H intensity with respect to Nb increases with time at 475°K (approximately 200oC). Correlation between this data and the H-Nb phase diagram appears to account for the H behaviour.
	Poster THPO032 [0.125 MB]