Abstract
We report a dramatic and irreversible reduction in the lattice thermal conductivity of bulk crystalline silicon when subjected to intense plastic strain under a pressure of 24 GPa using high-pressure torsion (HPT). Thermal conductivity of the HPT-processed samples were measured using picosecond time domain thermoreflectance. Thermal conductivity measurements show that the HPT-processed samples have a lattice thermal conductivity reduction by a factor of approximately 20 (from intrinsic single crystalline value of 142 Wm−1 K−1 to approximately 7.6 Wm−1 K−1). Thermal conductivity reduction in HPT-processed silicon is attributed to the formation of nanograin boundaries and metastable Si-III/XII phases which act as phonon scattering sites, and because of a large density of lattice defects introduced by HPT processing. Annealing the samples at 873 K increases the thermal conductivity due to the reduction in the density of secondary phases and lattice defects.
Highlights
Nanomaterials possess unique abilities to control thermal transport [1]
We show that bulk single crystalline silicon, when subjected to intense plastic strain through high-pressure torsion (HPT) processing, shows a dramatic reduction in room temperature thermal conductivity from its intrinsic single crystal value of 142 W m−1 K−1 to a low thermal conductivity of approximately 7.6 W m−1 K−1
The thermal conductivity of the HPT-processed silicon at 24 GPa was approximately 18 Wm−1 K−1 which is an order of magnitude less than the intrinsic literature value of 142 Wm−1 K−1 for single crystalline silicon
Summary
Engineering the thermal properties of nanostructured materials have a promising application in the field of thermoelectrics. The thermoelectric system performance is evaluated by the dimensionless figure of merit, ZT = S2σT/k, where S is the Seeback coefficient, σ is the electrical conductivity, T is the temperature, and k is the thermal conductivity [2]. To achieve higher ZT, lattice thermal conductivity of the thermoelectric material needs to be reduced without compromising the charge carrier mobility. Significant work has been done in recent years by using chemically distinct secondary phases either in the bulk form, or in the form of thin films, to reduce lattice thermal conductivity [3]. The introduction of nanostructured interfaces to scatter phonons efficiently and thereby reducing the thermal conductivity of the material has yielded high ZT in thermoelectric devices [4,5,6]
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