8 Accelerator Engineering and Technology: Accelerator Technology
391
Fig. 8.32 Thermal
conductivity of selected
materials at low temperature
Electrical resistivity at low temperatures is a most interesting property both for
engineering applications, assessment of material purity and microstructure, and
understanding of its solid-state physics. Low temperature resistivity spans across
several orders of magnitude, depending upon the type of material, degree of alloying
and purity (Fig. 8.33). In all cases, the decrease of electrical resistivity upon
cooldown shows a plateau, corresponding to a residual value ρ 0 . Matthiessen’s rule
expresses total resistivity ρ(T) as the sum of two terms,
ρ(T ) = ρ 0 + ρ i (T ),
(8.23)
where ρ 0 is the residual resistivity due to electron scattering by impurities and
lattice imperfections, and ρ i (T) is the intrinsic resistivity due to electron scattering
by phonons, strongly temperature dependent.
A convenient way to characterize a pure metal or dilute alloy is to measure its
residual resistivity ratio RRR:
RRR = ρ (273 K) /ρ (4 K) .
(8.24)