MSU, East Lansing, USA
Michigan State University, East Lansing, USA
FRIB, East Lansing, Michigan, USA
(MSU), East Lansing, USA
The phonon peak in the thermal conductivity kpp of high purity niobium is an unknown function of heat treatment temperature Th and RRR, amongst other variables. The relationship between Th and kpp of large grain niobium is investigated using two sets of four specimens. The specimens of Set 1 were randomly cut from four ingot discs with different RRR. These specimens were subjected to different heat treatments. Specimens of Set 2 were cut from the same grain of an ingot disc, with the heat flow direction of each specimen along the same crystal orientation. Each of these specimens was subjected to one heat treatment, at a temperature ranging between 600 C and 1200 C, while maintaining a constant temperature for an interval of 2 hours for each specimen. Results from the specimens of Set 1 show that there is no change in kpp after heating at 140 C for 48 hours. Set 1 specimens also show that for a given heat treatment protocol, the maximum in kpp shows a monotonic dependence on RRR. Results from the specimens of Set 2 suggest that the phonon conduction response to heat treatments for 2 hours shows no increase for Th < 600 C and plateaus at Th > 1000 C.
Michigan State University, East Lansing, USA
JLAB, Newport News, Virginia, USA
FRIB, East Lansing, Michigan, USA
Niowave, Inc., Lansing, Michigan, USA
Large grain niobium is being examined for fabricating superconducting radiofrequency cavities as an alternative to using rolled sheet with fine grains. It is desirable to know the grain orientations of a niobium ingot slice before fabrication, as this allows heterogeneous strain and surface roughness effects arising from etching to be anticipated. Characterization of grain orientations has been done using orientation imaging microscopy (OIM), which requires destructive extraction of pieces from an ingot slice. Use of a Laue camera allows nondestructive characterization of grain orientations, a process useful for evaluating slices and deformation during the manufacturing process. Five ingot slices from CBMM, Ningxia, and Heraeus are compared. One set of slices was deformed into a half cell and the deformation processes that cause crystal rotations have been investigated and compared with analytical predictions. The five ingot slices are compared in terms of their grain orientations and grain boundary misorientations, indicating no obvious commonalities, which suggests that grain orientations develop randomly during solidification.