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The possibility of tuning the vibrational properties and the thermal conductivity of layered van der Waals materials either chemically or mechanically paves the way to significant advances in nanoscale heat management. Using first-principles calculations we investigate the modulation of heat transport in MoS2 by lithium intercalation and cross-plane strain. We find that both the in-plane and cross-plane thermal conductivity (, ) of MoS2 are extremely sensitive to both strain and electrochemical intercalation. Combining lithium intercalation and strain, the in-plane and cross-plane thermal conductivity can be tuned over one and two orders of magnitude, respectively. Furthermore, since and respond in different ways to intercalation and strain, the thermal conductivity anisotropy can be modulated by two orders of magnitude. The underlying mechanisms for such large tunability of the anisotropic thermal conductivity of MoS2 are explored by computing and analyzing the dispersion relations, group velocities, relaxation times and mean free paths of phonons. Since both intercalation and strain can be applied reversibly, their stark effect on thermal conductivity can be exploited to design novel phononic devices, as well as for thermal management in MoS2-based electronic and optoelectronic systems.

Step junctions are often present in layered materials, i.e. where single-layer regions meet multilayer regions, yet their effect on thermal transport is not understood to date. Here, we measure heat flow across graphene junctions (GJs) from monolayer-to-bilayer graphene, as well as bilayer to four-layer graphene for the first time, in both heat flow directions. The thermal conductance of the monolayer-bilayer GJ ranges from ∼0.5 to 9.1 × 108 W m-2 K-1 between 50 K to 300 K. Atomistic simulations of such a GJ device reveal that graphene layers are relatively decoupled, and the low thermal conductance of the device is determined by the resistance between the two distinct graphene layers. In these conditions the junction plays a negligible effect. To prove that the decoupling between layers controls thermal transport in the junction, the heat flow in both directions was measured, showing no evidence of thermal asymmetry or rectification, within experimental error bars. For large-area graphene applications, this signifies that small bilayer (or multilayer) islands have little or no contribution to overall thermal transport.

Gradual switching between multiple resistance levels is desirable for analog in-memory computing using resistive random-access memory (RRAM). However, the filamentary switching of HfOx-based conventional RRAM often yields only two stable memory states instead of gradual switching between multiple resistance states. Here, we demonstrate that a thermal barrier of Ge2Sb2Te5 (GST) between HfOx and the bottom electrode (TiN) enables wider and weaker filaments, by promoting heat spreading laterally inside the HfOx. Scanning thermal microscopy suggests that HfOx + GST devices have a wider heating region than control devices with only HfOx, indicating the formation of a wider filament. Such wider filaments can have multiple stable conduction paths, resulting in a memory device with more gradual and linear switching. The thermally enhanced HfOx + GST devices also have higher on/off ratio (>103) than control devices (<102) and a median set voltage lower by approximately 1 V (∼35%), with a corresponding reduction of the switching power. Our HfOx + GST RRAM shows 2× gradual switching range using fast (∼ns) identical pulse trains with amplitude less than 2 V. [ABSTRACT FROM AUTHOR]

A new model for determining non-equilibrium phonon distribution in semiconductor devices, beginning with phonon Boltzmann transport equation is presented. It is found that the emitted phonos relax completely at current switching speeds and will accumulate if switching period is reduced by half.

Room temperature thermal properties of vertically aligned metalized single walled nanotubes (SWNT) are studied using thermoreflectance technique. The results showed that by increasing the number of carbon nanotube-substrate contacts it is possible to reduce the total thermal resistance of a SWNT array.

Small capacity wind turbine performance is influenced in a large extent by the blade profiles aerodynamics. By this point of view, the paper presents the most important aspects of aerodynamic profiles, and aspects concerning their optimization in order to increase performance. The study of blade profiles polar led to the identification of a reference profile, NACA4418, for which the best aerodynamic lift drag ratio is obtained on a big range of attack angles. A flow analysis on the reference profile is performed, using a CFD model of numerical simulations in order to obtain performance for different wind speeds. [ABSTRACT FROM AUTHOR]