Showing papers on "Thermal diffusivity published in 2010"

Oxygen ion diffusivity in strained yttria stabilized zirconia: where is the fastest strain?

Abstract: We present the mechanism and the extent of increase in the oxygen anion diffusivity in Y2O3 stabilized ZrO2 (YSZ) under biaxial lattice strain. The oxygen vacancy migration paths and barriers in YSZ as a function of lattice strain was assessed computationally using density functional theory (DFT) and nudged elastic band (NEB) method. Two competing and non-linear processes acting in parallel were identified to alter the migration barrier upon applied strain: (1) the change in the space, or electronic density, along the migration path, and (2) the change in the strength of the interatomic bonds between the migrating oxygen and the nearest neighbor cations that keep the oxygen from migrating. The increase of the migration space and the weakening of the local oxygen–cation bonds correspond to a decrease of the migration barrier, and vice versa. The contribution of the bond strength to the changes in the migration barrier is more significant than that of the opening of migration space in strained YSZ. A database of migration barrier energies as a function of lattice strain for a set of representative defect distributions in the vicinity of the migration path in YSZ was constructed. This database was used in kinetic Monte Carlo (KMC) simulations to estimate the effective oxygen diffusivity in strained YSZ. The oxygen diffusivity exhibits an exponential increase up to a critical value of tensile strain, or the fastest strain. This increase is more significant at the lower temperatures. At the strain states higher than the critical strain, the diffusivity decreases. This is attributed to the local relaxations at large strain states beyond a limit of elastic bond strain, resulting in the strengthening of the local oxygen–cation bonds that increases the migration barrier. The highest enhancement of diffusivity in 9%-YSZ compared to its unstrained state is 6.8 × 103 times at 4% strain and at 400 K. The results indicate that inducing an optimal strain state by direct mechanical load or by creating a coherent hetero-interface with lattice mismatch can enable desirably high ionic conductivity in YSZ at reduced temperatures. The insights gained here particularly on the nonlinear and competing consequences of lattice strain on the local bonding structure and charge transport process are of importance for tuning the ionic transport properties in a variety of solid-state conducting material applications, including but not limited to fuel cells.

Mapping Heat Origin in Plasmonic Structures

Abstract: We investigate the physics of photoinduced heat generation in plasmonic structures by using a novel thermal microscopy technique based on molecular fluorescence polarization anisotropy. This technique enables us to image the heat source distribution in light-absorbing systems such as plasmonic nanostructures. While the temperature distribution in plasmonic nanostructures is always fairly uniform because of the fast thermal diffusion in metals, we show that the heat source density is much more contrasted. Unexpectedly the heat origin (thermal hot spots) usually does not correspond to the optical hot spots of the plasmon mode. Numerical simulations based on the Green dyadic method confirm our observations and enable us to derive the general physical rules governing heat generation in plasmonic structures.

Thermal conductivity and dynamic heat capacity across the metal-insulator transition in thin film VO2

Abstract: The thermal properties of VO2 thin films, 90–440 nm thick, are measured by time-domain thermoreflectance (TDTR) across the metal-insulator transition (MIT). The thermal conductivity increases by as much as 60% in the metallic phase; this increase in conductivity is in good agreement with the expected electronic contribution to the thermal conductivity. For relatively thick layers, TDTR data are sensitive to the dynamic heat capacity and show a pronounced peak near the MIT temperature created by a contribution to the enthalpy from periodic transformations at the 10 MHz frequency of the thermal waves used in the experiment. The dynamic heat capacity increases as the amplitude ΔT of the thermal waves becomes comparable to the width of the MIT and reaches ≈30% of the bulk latent heat for ΔT≈1.6 K.

Thermophysical Comparison of Five Commercial Paraffin Waxes as Latent Heat Storage Materials

Abstract: Thermophysical properties of phase change materials (PCM) are of utmost importance in latent heat thermal energy storage (LHTES) applications. Therefore, an experimental study is conducted in order to determine thermophysical properties of five technical grade paraffin waxes produced by major Croatian oil company, INA d.d. Rijeka. The temperatures and enthalpies of melting and solidification (latent heat capacity) and specific heat capacities of solid and liquid paraffin waxes were measured by differential scanning calorimetry (DSC). The thermal diffusivity of paraffin waxes was determined utilizing transient method. The importance of eliminating phase transformation interferences to thermophysical properties determination is addressed. The densities and the coefficient of thermal expansion were measured using Archimedes methods. A self-adopted simple and inexpensive laboratory procedure for the determination of liquid density as a temperature function is presented. Finally, the thermal conductivities have been calculated from measured densities, heat capacities and diffusivities. Based on results obtained, the investigated paraffin waxes were evaluated in regard to their applicability as PCM for LHTES.

Isotope fractionation in silicate melts by thermal diffusion

Abstract: The phenomenon of thermal diffusion (mass diffusion driven by a temperature gradient, known as the Ludwig-Soret effect) has been investigated for over 150 years, but an understanding of its underlying physical basis remains elusive. A significant hurdle in studying thermal diffusion has been the difficulty of characterizing it. Extensive experiments over the past century have established that the Soret coefficient, S(T) (a single parameter that describes the steady-state result of thermal diffusion), is highly sensitive to many factors. This sensitivity makes it very difficult to obtain a robust characterization of thermal diffusion, even for a single material. Here we show that for thermal diffusion experiments that span a wide range in composition and temperature, the difference in S(T) between isotopes of diffusing elements that are network modifiers (iron, calcium and magnesium) is independent of the composition and temperature. On the basis of this finding, we propose an additive decomposition for the functional form of S(T) and argue that a theoretical approach based on local thermodynamic equilibrium holds promise for describing thermal diffusion in silicate melts and other complex solutions. Our results lead to a simple and robust framework for characterizing isotope fractionation by thermal diffusion in natural and synthetic systems.

Characterization of PTFE Using Advanced Thermal Analysis Techniques

Abstract: Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer used in numerous industrial applications. It is often referred to by its trademark name, Teflon. Thermal characterization of a PTFE material was carried out using various thermal analysis and thermophysical properties test techniques. The transformation energetics and specific heat were measured employing differential scanning calorimetry. The thermal expansion and the density changes were determined employing pushrod dilatometry. The viscoelastic properties (storage and loss modulus) were analyzed using dynamic mechanical analysis. The thermal diffusivity was measured using the laser flash technique. Combining thermal diffusivity data with specific heat and density allows calculation of the thermal conductivity of the polymer. Measurements were carried out from − 125 °C up to 150 °C. Additionally, measurements of the mechanical properties were carried out down to − 170 °C. The specific heat tests were conducted into the fully molten regions up to 370 °C.

Effective medium theory of a diffusion‐weighted signal

Abstract: Living tissues and other heterogeneous media generally consist of structural units with different diffusion coefficients and NMR properties. These blocks, such as cells or clusters of cells, can be much smaller than the imaging voxel, and are often comparable with the diffusion length. We have developed a general approach to quantify the medium heterogeneity when it is much finer than the sample size or the imaging resolution. The approach is based on the treatment of the medium statistically in terms of the correlation functions of the local parameters. The diffusion-weighted signal is explicity found for the case in which the local diffusivity varies in space, in the lowest order in the diffusivity variance. We demonstrate how the correlation length and the variance of the local diffusivity contribute to the time-dependent diffusion coefficient and the time-dependent kurtosis. Our results are corroborated by Monte Carlo simulations of diffusion in a two-dimensional heterogeneous medium. Copyright © 2010 John Wiley & Sons, Ltd.

Some Possible Approaches for Improving the Energy Density of Li-air Batteries

Abstract: A physics-based model is proposed for the simulation of Li-air batteries. The model is carefully calibrated against published data and is used to simulate standard Li-air batteries with nonaqueous (organic) electrolyte. It is shown that the specific capacity is mainly limited by the oxygen diffusion length which is a function of the oxygen diffusivity in the electrolyte and the discharge current density. Various approaches to increase the specific capacity of the cathode electrode and the energy density of Li-air batteries are discussed. It is shown that, in order to increase the specific capacity and energy density, it is more efficient to use a nonuniform catalyst that enhances the reaction rate only at the separator-cathode interface than a catalyst uniformly distributed. Using uniformly distributed catalysts will enhance the current and power density of the cell but will not increase significantly the specific capacity and energy density. It is also shown that the specific capacity and energy density can be increased by suppressing the reaction rate at the oxygen-entrance interface in order to delay the pinch-off of the conduction channel in this region. Other possibilities to enhance the energy density such as using solvents with high oxygen solubility and diffusivity, and partly wetted electrodes are discussed.

Constant-Pressure Technique for Gas Diffusivity and Solubility Measurements in Heavy Oil and Bitumen

Abstract: A modified pressure decay method has been designed and tested for more reliable measurements of molecular diffusion coefficients of gases into liquids. Unlike the conventional pressure decay method, the experimental setup has been designed such that the interface pressure and consequently the dissolved gas concentration at the interface are kept constant. This is accomplished by continuously injecting the required amount of gas into the gas cap from a secondary supply cell to maintain the pressure constant at the gas−liquid interface. The pressure decay is measured in the supply cell. The advantage of the new technique is that, assuming the diffusion coefficient to be constant, a simple analysis allows determination of the equilibrium concentration and diffusion coefficient.

Thermal conductivity of periclase (MgO) from first principles.

Abstract: We combine first-principles calculations of forces with the direct nonequilibrium molecular dynamics method to determine the lattice thermal conductivity k of periclase (MgO) up to conditions representative of the Earth's core-mantle boundary (136 GPa, 4100 K). We predict the logarithmic density derivative a = (partial derivative lnk/partial derivative ln rho)(Tau) = 4.6 +/- 1.2 and that k = 20 +/- 5 Wm(-1) K-1 at the core-mantle boundary, while also finding good agreement with extant experimental data at much lower pressures.

Scaling of phononic transport with connectivity in amorphous solids

Abstract: The effect of coordination on transport is investigated theoretically using random networks of springs as model systems. An effective medium approximation is made to compute the density of states of the vibrational modes, their energy diffusivity (a spectral measure of transport) and their spatial correlations as the network coordination z is varied. Critical behaviors are obtained as z→zc where these networks lose rigidity. A sharp crossover from a regime where modes are plane-wave–like toward a regime of extended but strongly scattered modes occurs at some frequency ω*~z-zc, which does not correspond to the Ioffe-Regel criterion. Above ω* both the density of states and the diffusivity are nearly constant. These results agree remarkably with recent numerical observations of repulsive particles near the jamming threshold (Xu N. et al., Phys. Rev. Lett., 102 (2009) 038001). The analysis further predicts that the length scale characterizing the correlation of displacements of the scattered modes decays as with frequency, whereas for ωω* Rayleigh scattering is found with a scattering length ls~(z-zc)3/ω4. It is argued that this description applies to silica glass where it compares well with thermal conductivity data, and to transverse ultrasound propagation in granular matter.

Molecular dynamics simulation of the sintering of metallic nanoparticles

Abstract: The sintering of two different-sized nickel nanoparticles is simulated by a molecular dynamics method in this work. The particles are partitioned into different regimes where tracing atoms are arranged to investigate the sintering kinetics. The detailed sintering process of two nanoparticles, 3.52 and 1.76 nm in diameter, respectively, is subsequently examined by the shrinkage ratio, gyration radius, mean square displacement, sintering diffusivity, and activation energy. A three-stage sintering scenario is established, and the layered structure shows a regime dependent behavior of diffusivity during the sintering process. Besides the surface diffusion, sintering of different-sized nanoparticles is found to be affected by a few other mechanisms.

Characterization of thin metal films via frequency-domain thermoreflectance

Abstract: Frequency-domain thermoreflectance is extended to the characterization of thin metals films on low thermal diffusivity substrates. We show how a single noncontact measurement can yield both the thickness and thermal conductivity of a thin metal film with high accuracy. Results are presented from measurements of gold and aluminum films 20–100 nm thick on fused silica substrate. The thickness measurements are verified independently with atomic force microscope cross sections, and the thermal conductivity measurements are verified through electrical conductivity measurements via the Wiedemann–Franz law. The thermoreflectance thermal conductivity values are in good agreement with the Wiedemann–Franz results for all the films at least 30 nm thick, indicating that our method can be used to estimate electrical conductivity along with thermal conductivity for sufficiently thick films.

Experimental investigation into the convective heat transfer and system-level effects of Al 2 O 3 -propanol nanofluid

Abstract: It has been speculated that the application of nanofluids in real systems could lead to smaller, more compact heat exchangers and reductions in material cost. However, few studies have been conducted which have carefully measured the thermo-physical properties and thermal performance of these fluids as well as examine the system-level effects of using these fluids in traditional cooling systems. In this study, dilute suspensions of 10 nm aluminum oxide nanoparticles in propanol (0.5, 1, and 3 wt%) were investigated. Changes in density, specific heat, and thermal conductivity with particle concentration were measured and found to be linear, whereas changes in viscosity were nonlinear and increased sharply with particle loading. Nanofluid heat transfer performance data were generally commensurate with that measured for the baseline. For the 1 wt% concentration, a small but significant enhancement in the heat transfer coefficient was recorded for 1800 < Re < 2800, which is attributed to an earlier transition to turbulent flow. In the case of high particle loading (i.e. 3 wt%), the thermal performance was observed to deteriorate with respect to the baseline case. Discoloration of the fluid was also observed after being cycled at high flow rates and increased temperature.