The thermal conductivities of solid silicon thin films and silicon thin films with periodic pore arrays are predicted using a Monte Carlo technique to include phonon-boundary scattering and the Boltzmann transport equation. The bulk phonon properties required as input are obtained from harmonic and anharmonic lattice dynamics calculations. The force constants required for the lattice dynamics calculations are obtained from forces calculated using density functional theory. For both solid and porous films, the in-plane thermal conductivity predictions capture the magnitudes and trends of previous experimental measurements. Because the prediction methodology treats the phonons as particles with bulk properties, the results indicate that coherent phonon modes associated with the secondary periodicity of the pores do not contribute to thermal transport in porous films with feature sizes greater than 100 nm.

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The electrocaloric effect in thin films of a poly(vinylidene fluoride-trifluoroethylene chlorofluoroethylene) terpolymer (62.6/29.4/8 mol. %, 11-12 lm thick) is directly measured by infrared imaging at ambient conditions. The adiabatic temperature change is estimated to be 5.2 K for an applied electric field of 90 V/lm. The temperature change is independent of the operating frequency in the range of 0.03-0.3 Hz and is stable over a testing period of 30 min. Application of this terpolymer is promising for micro-scale refrigeration. V C 2014 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4890676] Micro-scale refrigeration systems are widely used for the cooling of integrated circuits, microelectromechanical sensors, and biomedical devices.1 Environment-friendly cooling technologies with a high efficiency are attractive due to growing energy demands and stringent environmental requirements. 2Although thermoelectric cooling is commonly applied and has been scaled to the micro-domain, 3-6 the low efficiency and challenges in material fabrication suggest that alternatives are needed. 6 While refrigeration based on the magnetocaloric effect 7 can be employed to achieve extremely low temperatures, miniaturization of devices is challenging while maintaining a high cooling performance due to the difficulty of realizing the large magnetic fields required. 2The electrocaloric (EC) effect is a phenomenon in which reversible, polarization-related temperature and entropy changes occur when an electric field is applied to certain materials. EC cooling, which operates on a refrigeration cycle analogous to magnetocaloric cooling, is an emerging technology. 8 The highest reported adiabatic temperature change in a bulk EC material is 2.5 K at an electric field of 3 V/lm and a temperature of 434 K 11 The P(VDF-TrFE-CFE) terpolymer demonstrates an adiabatic temperature change of 16 K at an electric field of 150 V/lm near room temperature 12 and is easily and economically fabricated, making it favorable for mass production.13 These findings point to the potential of applying EC cooling in micro-devices using polymer thin films.Direct and indirect techniques can be applied to measure the EC effect. In the indirect measurement, a differential scanning calorimeter is used to measure the heat flow under a high electric field and isothermal conditions. 14,15 This technique is best suited to bulk materials as the output heat flow signal for a thin film sample is very small. Jia and Ju 16 reported an approach for characterizing the EC effect in a thin film sitting on an insulating substrate. In this approach, the temperature response of a resistance thermometer deposited on the bottom of the EC film is monitored as an electric field is turned on and off. In the reported measurements, the temperature change is less than 10% of that expected because the heat loss from the EC film to the substrate is large. Lu et al.17 employed a specially designed calorimeter to measure the EC effect in a thin film. In this approach, the heat generated in the...

Energy barriers for flipping the transverse dipole moments in poly(vinylidene fluoride) (PVDF) and related copolymers and terpolymers are predicted using the nudged elastic band method. The dipole moments flip individually along the chain, with an order and energy barrier magnitudes (0.1-1.2 eV) that depend on the chain composition and environment. Trifluoroethylene (TrFE) and chlorofluoroethylene (CFE) monomers have larger energy barriers than VDF monomers, while a chain in an amorphous environment has a similar transition pathway as that of an isolated molecule. In a crystalline environment, TrFE and CFE monomers expand the lattice and lower the energy barriers for flipping VDF monomers. This finding is consistent with experimental observations of a large electrocaloric effect in P(VDF-TrFE-CFE) terpolymers.