Refractory Metals
RRR Grade Niobium
It is well-known that, among of all pure metals, niobium, with its high critical temperature and critical magnetic field, is the favorite material for the fabrication of superconducting RF cavities. Niobium is chemically inert (at room temperature the surface is covered by a protecting oxide layer), it can be machined, deep drawn, welded and it is available as bulk and sheet material in any size. For good performance of superconducting RF cavities a high thermal conductivity in the cavity wall is required to guide the dissipated radio frequency (RF) power to the liquid helium coolant. For bulk niobium cavities this demands niobium of high purity.
The residual resistivity ratio (RRR=300 for a high gradient cavity) is a common indicator of the level of purity. The main interstitially dissolved impurities, oxygen, nitrogen, hydrogen and carbon (O, N, H, C), act as scattering centers for unpaired electrons and reduce the RRR. Oxygen is dominant due to its high affinity to Nb. The influence of hydrogen on the RRR is not so significant, but the content of hydrogen should be kept small (less then 3-5 µg/g) to prevent hydride precipitation and degradation of the Q-value of the high RRR cavities under certain cool-down conditions.
Chemical requirements for RRR superconducting grade niobium (wt%, max.)
UNS
C
N
O
H
Zr
Ta
Fe
Si
W
Ni
Ti
Al
R04220
0.003
0.003
0.004
0.0005
0.01
0.1
0.005
0.005
0.007
0.003
0.005
0.005
Mechanical properties for RRR superconducting grade niobium
Grade
Condition
Tensile strength
Yield strength
Elongation in 1 inch
R04220
Annealed
95
50
30
(MPa, min.)
(MPa, min.)
(%, min.)
* Annealed condition - 90 % Minimum Recrystallized
Available forms
Ingot
Rod
Plate and sheet
Specifications
ASTM B393-09
Typical application
Superconducting RF cavities
