251 research outputs found

    An electrical probe of the phonon mean-free path spectrum

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    Most studies of the mean-free path accumulation function (MFPAF) rely on optical techniques to probe heat transfer at length scales on the order of the phonon mean-free path. In this paper, we propose and implement a purely electrical probe of the MFPAF that relies on photo-lithographically defined heater-thermometer separation to set the length scale. An important advantage of the proposed technique is its insensitivity to the thermal interfacial impedance and its compatibility with a large array of temperature-controlled chambers that lack optical ports. Detailed analysis of the experimental data based on the enhanced Fourier law (EFL) demonstrates that heat-carrying phonons in gallium arsenide have a much wider mean-free path spectrum than originally thought

    Thermal property measurement with frequency domain thermoreflectance

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    Heat transfer at the nanoscale has been one of the primary concerns in the design of nanoelectronics and nanostructured materials for applications such as thermal management and thermoelectric energy conversion. This thesis examines the thermal transport in nanoscale thin films and two-dimensional (2D) materials using an optical pump-probe technique based on frequency domain thermoreflectance (FDTR). The design and implementation of a continuous-wave laser based FDTR system is described in detail. The system is extended to an imaging microscope capable of producing micrometer scale maps of several thermophysical properties simultaneously. An analytical formula, which accounts for experimental noise and uncertainty in the controlled model parameters, is derived to calculate the precision of thermoreflectance measurements. The FDTR system is used to study the anisotropic heat conduction in periodic nanoscale Mo/Si superlattices and a 2D material, graphene. The measured in-plane thermal conductivity values of the superlattices are in good agreement with calculations taking into account both electron and phonon thermal transport, using a phonon mean free path which depends on the Mo layer thickness. The measurement procedure of graphene is described in detail, including the sample preparation, sensitivity analysis, and parameter fitting. Various graphene flakes supported on SiO2 surfaces and atomically flat Muscovite mica surfaces are measured. The results show that the thermal conductivity of single-layer graphene can be improved by ~3 times by using a mica substrate compared to commonly used SiO2 substrates. In addition, comparison with the reported values of suspended graphene suggest that the out-of-plane flexural phonon modes may contribute at least 70% to the thermal conductivity of graphene. Finally, the thermal model is modified to include volumetric heating for the measurement of materials without a transducer layer. An amorphous silicon film deposited on fused silica and silicon substrates is measured to validate the model

    Parametric study of the frequency-domain thermoreflectance technique

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    Without requiring regression for parameter determination, one-dimensional (1D) analytical models are used by many research groups to extract the thermal properties in frequency-domain thermoreflectance measurements. Experimentally, this approach involves heating the sample with a pump laser and probing the temperature response with spatially coincident probe laser. Micron order lateral resolution can be obtained by tightly focusing the pump and probe lasers. However, small laser beam spot sizes necessarily bring into question the assumptions associated with 1D analytical models. In this study, we analyzed the applicability of 1D analytical models by comparing to 2D analytical and fully numerical models. Specifically, we considered a generic n-layer two-dimensional (2D), axisymmetric analytical model including effects of volumetric heat absorption, contact resistance, and anisotropic properties. In addition, a finite element numerical model was employed to consider nonlinear effects caused by temperature dependent thermal conductivity. Nonlinearity is of germane importance to frequency domain approaches because the experimental geometry is such that the probe is always sensing the maximum temperature fluctuation. To quantify the applicability of the 1D model, parametric studies were performed considering the effects of: film thickness, heating laser size, probe laser size, substrate-to-film effusivity ratio, interfacial thermal resistance between layers, volumetric heating, substrate thermal conductivity, nonlinear boundary conditions, and anisotropic and temperature dependent thermal conductivity

    Length Dependence thermal conductivity of Zinc-Selenide (ZnSe) and Zinc Telluride (ZnTe)- A combined first principles and Frequency Domain Thermoreflectance (FDTR) study

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    In this study, we report the length dependence of thermal conductivity (k) of zinc-blende Zinc-Selenide (ZnSe) and Zinc Telluride (ZnTe) for length scales between 10 nm and 10000 nm using first-principles computations based on density-functional theory. k value of ZnSe is computed to decrease significantly from 11.3 W/mK to 1.75 W/mK as the length scale is diminished from 10 nm to 10 nm. k value of ZnTe is also observed to decrease from 10 W/mK to 1.2 W/mK for the same decrease in length. We also measure the k of bulk ZnSe and ZnTe using Frequency Domain Thermoreflectance (FDTR) technique and observed a good agreement between FDTR measurements and first principles calculations for the bulk ZnSe and ZnTe. Understanding of thermal conductivity reduction at nanometer length scales provides an avenue to incorporate nanostructured ZnSe and ZnTe for thermoelectric applications.Comment: 19 pages, 13 figure

    In-plane thermal diffusivity determination using beam-offset frequency-domain thermoreflectance with a one-dimensional optical heat source

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    We present an innovative contactless method suitable to study in-plane thermal transport based on beam-offset frequency-domain thermoreflectance using a one-dimensional heat source with uniform power distribution. Using a one-dimensional heat source provides a number of advantages as compared to point-like heat sources, as typically used in time- and frequency-domain thermoreflectance experi- ments, just to name a few: (i) it leads to a slower spatial decay of the temperature field in the direction perpendicular to the line-shaped heat source, allowing to probe the temperature field at larger distances from the heater, hence, enhancing the sensitivity to in-plane thermal transport; (ii) the frequency range of interest is typically < 100 kHz. This rather low frequency range is convenient regarding the cost of the required excitation laser system but, most importantly, it allows the study of materials without the presence of a metallic transducer with almost no influence of the finite optical penetration depth of the pump and probe beams on the thermal phase lag, which arises from the large thermal penetration depth imposed by the used frequency range. We also show that for the case of a harmonic thermal excitation source, the phase lag between the thermal excitation and thermal response of the sample exhibits a lin- ear dependence with their spatial offset, where the slope is proportional to the inverse of the thermal diffusivity of the material. We demonstrate the applicability of this method to the cases of: (i) suspended thin films of Si and PDPP4T, (ii) Bi bulk samples, and (iii) Si, glass, and highly-oriented pyrollitic graphite (HOPG) bulk samples with a thin metallic transducer. Finally, we also show that it is possible to study in-plane heat transport on substrates with rather low thermal diffusivity, e.g., glass, even using a metallic transducer. We achieve this by an original approach based on patterning the transducer using focused ion beam, with the key purpose of limiting in-plane heat transport through the thin metallic transducer
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