57 research outputs found
Effective thermal conductivity of polycrystalline materials with randomly oriented superlattice grains
A model has been established for the effective thermal conductivity of a bulk polycrystal made of randomly oriented superlattice grains with anisotropic thermal conductivity. The in-plane and cross-plane thermal conductivities of each superlattice grain are combined using an analytical averaging rule that is verified using finite element methods. The superlattice conductivities are calculated using frequency dependent solutions of the Boltzmann transport equation, which capture greater thermal conductivity reductions as compared to the simpler gray medium approximation. The model is applied to a PbTe/Sb_2Te_3 nanobulk material to investigate the effects of period, specularity, and temperature. The calculations show that the effective thermal conductivity of the polycrystal is most sensitive to the in-plane conductivity of each superlattice grain, which is generally four to five times larger than the cross-plane conductivity of a grain. The model is compared to experimental measurements of the same system for periods ranging from 287 to 1590 nm and temperatures from 300 to 500 K. The comparison suggests that the effective specularity increases with increasing annealing temperature and shows that these samples are in a mixed regime where both Umklapp and boundary scattering are important
Analysis and Improvement of the Hot Disk Transient Plane Source Method for Low Thermal Conductivity Materials
The hot disk transient plane source (TPS) method is a widely used standard
technique (ISO 22007-2) for the characterization of thermal properties of
materials, especially the thermal conductivity, k. Despite its well-established
reliability for a wide variety of common materials, the hot disk TPS method is
also known to suffer from a substantial systematic errors when applied to low-k
thermal insulation materials. Here, we present a combined numerical and
experimental study on the influence of the geometry of hot disk sensor on
measured value of low-k materials. We demonstrate that the error is strongly
affected by the finite thickness and thermal mass of the sensor's insulation
layer was well as the corresponding increase of the effective heater size
beyond the radius of the embedded metal heater itself. We also numerically
investigate the dependence of the error on the sample thermal properties,
confirming that the errors are worse in low-k samples. A simple correction
function is also provided, which converts the apparent (erroneous) result from
a standard hot disk TPS measurement to a more accurate value. A standard
polyimide sensor was also optimized using both wet and dry etching to provide
more accurate measurement directly. Experimentally corrected value of k for
Airloy x56 aerogel and a commercial silica aerogel using the numerical
correction factor derived based on the standard TPS sensor is in excellent
agreement with the directly measured value from the TPS sensor using the
optimized polyimide sensor. Both of these methods can reduce the errors to less
than 4% as compared to around 40% error of overestimation from raw values
measured with the pristine sensor. Such results show that both the numerical
correction to a pristine senor or an optimized sensor are capable of providing
highly accurate value of thermal conductivity for such materials.Comment: 76 pages, 11 figure
Large enhancement of near-field radiative heat transfer in the dual nanoscale regime enabled by electromagnetic corner and edge modes
It is well established that near-field radiative heat transfer (NFRHT) can
exceed Planck's blackbody limit1 by orders of magnitude owing to the tunneling
of evanescent electromagnetic frustrated and surface modes2-4, as has been
demonstrated experimentally for NFRHT between two large parallel surfaces5-7
and between two subwavelength membranes8,9. However, while nanostructures can
also sustain a much richer variety of localized electromagnetic modes at their
corners and edges,10,11 the contributions of such additional modes to further
enhancing NFRHT remain unexplored. Here, for the first time, we demonstrate
both theoretically and experimentally a new physical mechanism of NFRHT
mediated by these corner and edge modes, and show it can dominate the NFRHT in
the "dual nanoscale regime" in which both the thickness of the emitter and
receiver, and their gap spacing, are much smaller than the thermal photon
wavelengths. For two coplanar 20 nm thick SiC membranes separated by a 100 nm
vacuum gap, the NFRHT coefficient at room temperature is both predicted and
measured to be 830 W/m2K, which is 5.5 times larger than that for two infinite
SiC surfaces separated by the same gap, and 1400 times larger than the
corresponding blackbody limit accounting for the geometric view factor between
the emitter and receiver. This enhancement is dominated by the electromagnetic
corner and edge modes which account for 81% of the NFRHT between these SiC
membranes. These findings are important for future NFRHT applications in
thermal management and energy conversion.Comment: 58 pages, 20 figures, 1 tabl
Thermal Boundary Conductance: A Materials Science Perspective
The thermal boundary conductance (TBC) of materials pairs in atomically intimate contact is reviewed as a practical guide for materials scientists. First, analytical and computational models of TBC are reviewed. Five measurement methods are then compared in terms of their sensitivity to TBC: the 3 omega method, frequency- and time-domain thermoreflectance, the cut-bar method, and a composite effective thermal conductivity method. The heart of the review surveys 30 years of TBC measurements around room temperature, highlighting the materials science factors experimentally proven to influence TBC. These factors include the bulk dispersion relations, acoustic contrast, and interfacial chemistry and bonding. The measured TBCs are compared across a wide range of materials systems by using the maximum transmission limit, which with an attenuated transmission coefficient proves to be a good guideline for most clean, strongly bonded interfaces. Finally, opportunities for future research are discussed
Cathodoluminescence-based nanoscopic thermometry in a lanthanide-doped phosphor
Crucial to analyze phenomena as varied as plasmonic hot spots and the spread
of cancer in living tissue, nanoscale thermometry is challenging: probes are
usually larger than the sample under study, and contact techniques may alter
the sample temperature itself. Many photostable nanomaterials whose
luminescence is temperature-dependent, such as lanthanide-doped phosphors, have
been shown to be good non-contact thermometric sensors when optically excited.
Using such nanomaterials, in this work we accomplished the key milestone of
enabling far-field thermometry with a spatial resolution that is not
diffraction-limited at readout.
We explore thermal effects on the cathodoluminescence of lanthanide-doped
NaYF nanoparticles. Whereas cathodoluminescence from such lanthanide-doped
nanomaterials has been previously observed, here we use quantitative features
of such emission for the first time towards an application beyond localization.
We demonstrate a thermometry scheme that is based on cathodoluminescence
lifetime changes as a function of temperature that achieves 30 mK
sensitivity in sub-m nanoparticle patches. The scheme is robust against
spurious effects related to electron beam radiation damage and optical
alignment fluctuations.
We foresee the potential of single nanoparticles, of sheets of nanoparticles,
and also of thin films of lanthanide-doped NaYF to yield temperature
information via cathodoluminescence changes when in the vicinity of a sample of
interest; the phosphor may even protect the sample from direct contact to
damaging electron beam radiation. Cathodoluminescence-based thermometry is thus
a valuable novel tool towards temperature monitoring at the nanoscale, with
broad applications including heat dissipation in miniaturized electronics and
biological diagnostics.Comment: Main text: 30 pages + 4 figures; supplementary information: 22 pages
+ 8 figure
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Ion Write Microthermotics: Programing Thermal Metamaterials at the Microscale.
Considerable advances in manipulating heat flow in solids have been made through the innovation of artificial thermal structures such as thermal diodes, camouflages, and cloaks. Such thermal devices can be readily constructed only at the macroscale by mechanically assembling different materials with distinct values of thermal conductivity. Here, we extend these concepts to the microscale by demonstrating a monolithic material structure on which nearly arbitrary microscale thermal metamaterial patterns can be written and programmed. It is based on a single, suspended silicon membrane whose thermal conductivity is locally, continuously, and reversibly engineered over a wide range (between 2 and 65 W/m·K) and with fine spatial resolution (10-100 nm) by focused ion irradiation. Our thermal cloak demonstration shows how ion-write microthermotics can be used as a lithography-free platform to create thermal metamaterials that control heat flow at the microscale
Direct Measurement of Pyroelectric and Electrocaloric Effects in Thin Films
An understanding of polarization-heat interactions in pyroelectric and electrocaloric thin-film materials requires that the electrothermal response is reliably characterized. While most work, particularly in electrocalorics, has relied on indirect measurement protocols, here we report a direct technique for measuring both pyroelectric and electrocaloric effects in epitaxial ferroelectric thin films. We demonstrate an electrothermal test platform where localized high-frequency (approximately 1 kHz) periodic heating and highly sensitive thin-film resistance thermometry allow the direct measurement of pyrocurrents (<10 pA) and electrocaloric temperature changes (<2 mK) using the â2-omegaâ and an adapted â3-omegaâ technique, respectively. Frequency-domain, phase-sensitive detection permits the extraction of the pyrocurrent from the total current, which is often convoluted by thermally-stimulated currents. The wide-frequency-range measurements employed in this study further show the effect of secondary contributions to pyroelectricity due to the mechanical constraints of the substrate. Similarly, measurement of the electrocaloric effect on the same device in the frequency domain (at approximately 100 kHz) allows for the decoupling of Joule heating from the electrocaloric effect. Using one-dimensional, analytical heat-transport models, the transient temperature profile of the heterostructure is characterized to extract pyroelectric and electrocaloric coefficients
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