Pore-size dependence of the thermal conductivity of porous silicon: A phonon hydrodynamic approach (original) (raw)
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A Phenomenological Study of Pore-Size Dependent Thermal Conductivity of Porous Silicon
Acta Applicandae Mathematicae, 2012
A phonon-hydrodynamics approach is used to analyze the influence of porosity and of pore size on the reduction of the thermal conductivity in porous silicon. Different geometrical arrangements of the pores have been considered. For any given value of the porosity, the theoretical results show that for increasing Knudsen number (i.e., decreasing pore size) the effective thermal conductivity decreases whatever the geometrical arrangement of the pores is.
Thermal conductivity across nanostructured porous silicon films
Journal of Physics: Conference Series, 2007
This paper models the effect of pore size, pore arrangement and porosity on room temperature thermal conductivity across low porosity meso-porous silicon (meso-PS) films known as n+ and p+ type PS. Whereas the meso-PS films are simulated using a threedimensional (3D) pore network generator, the heat conduction is modeled using the Monte Carlo method (MCM) recently developed for simulating steady-state phonon transport in submicron structures. The simulations show that (i) the thermal conductivity across mesoporous films decreases when the pore size decreases or the porosity increases, in accordance with past studies. (ii) The thermal conductivity of n+ type pore network is always greater than that of p+ type due to the difference in pore morphology. Finally, the simulations corresponding to p+ type pore network are shown in good agreement with existing experimental data for low porosity p+ type PS. Such comparison confirms the validity of the current 3D modeling.
Thermal conductivity of silicon nanomeshes: Effects of porosity and roughness
Journal of Applied Physics, 2014
We theoretically investigate thermal conductivity in silicon nanomeshes using Monte Carlo simulations of phonon transport. Silicon membranes of 100nm thickness with randomly located pores of 50nm diameter are considered. The effects of material porosity and pore surface roughness are examined. Nanomesh porosity is found to have a strong detrimental effect on thermal conductivity. At room temperature, a porosity of 50% results in ~80% reduction in thermal conductivity. Boundary roughness scattering further degrades thermal conductivity, but its effect is weaker. Thermal transport can additionally be affected by the specific arrangement of the pores along the transport direction.
Nanoscale nature and low thermal conductivity of porous silicon layers
Applied Surface Science
Recently discovered phenomenon of low thermal conductivity of porous silicon (PS) layers is discussed in detail. A proposed theoretical model explains the considerable decrease of the thermal conductivity of nanoscale PS in comparison with meso-PS and bulk silicon. The thermal conductance dependence of Si/porous Si structures on the formation conditions of PS layers has been studied. By varying the values of anodisation current density and anodisation time it is possible to optimize the thermal conductance values of the Si/porous Si structures. In this way, an efficient thermal isolation can be obtained by forming thick PS layers.
We present a rigorous analysis of the thermal conductivity of bulk silicon ͑Si͒ and Si nanowires ͑Si NWs͒ which takes into account the exact physical nature of the various acoustic and optical phonon mechanisms. Following the Callaway solution for the Boltzmann equation, where resistive and nonresistive phonon mechanisms are discriminated, we derived formalism for the lattice thermal conductivity that takes into account the phonon incidence angles. The phonon scattering processes are represented by frequency-dependent relaxation time. In addition to the commonly considered acoustic three-phonon processes, a detailed analysis of the role of the optical phonon decay into acoustic phonons is performed. This optical phonon decay mechanism is considered to act as acoustic phonon generation rate partially counteracting the acoustic phonon scattering rates. We have derived the analytical expression describing this physical mechanism which should be included in the general formalism as a correction to the resistive phonon-point-defects and phonon-boundary scattering expressions. The phonon-boundary scattering mechanism is taken as a function of the phonon frequency, incidence angles, and surface roughness. The importance of all the mechanisms we have involved in the model is demonstrated clearly with reference to reported data regarding the isotopic composition effect in bulk Si and Si NW samples. Namely, our model accounts for previously unexplained experimental results regarding ͑i͒ the isotope composition effect on the thermal conductivity of bulk silicon reported by Ruf et al. ͓Solid State Commun. 115, 243 ͑2000͔͒, ͑ii͒ the size effect on ͑T͒ of individual Si NWs reported by Li et al. ͓Appl. Phys. Lett. 83, 2934 ͑2003͔͒, and ͑iii͒ the dramatic decrease in the thermal conductivity for rough Si NWs reported by Hochbaum et al. ͓Nature ͑London͒ 451, 163 ͑2008͔͒.
Journal of Applied Physics, 2011
This study establishes that the effective thermal conductivity k eff of crystalline nanoporous silicon is strongly affected not only by the porosity f v and the system's length L z but also by the pore interfacial area concentration A i . The thermal conductivity of crystalline nanoporous silicon was predicted using non-equilibrium molecular dynamics simulations. The Stillinger-Weber potential for silicon was used to simulate the interatomic interactions. Spherical pores organized in a simple cubic lattice were introduced in a crystalline silicon matrix by removing atoms within selected regions of the simulation cell. Effects of the (i) system length ranging from 13 to 130 nm, (ii) pore diameter varying between 1.74 and 5.86 nm, and (iii) porosity ranging from 8% to 38%, on thermal conductivity were investigated. A physics-based model was also developed by combining kinetic theory and the coherent potential approximation. The effective thermal conductivity was proportional to (1 À 1.5f v ) and inversely proportional to the sum (A i =4 þ 1=L z ). This model was in excellent agreement with the thermal conductivity of nanoporous silicon predicted by molecular dynamics simulations for spherical pores (present study) as well as for cylindrical pores and vacancy defects reported in the literature. These results will be useful in designing nanostructured materials with desired thermal conductivity by tuning their morphology.
Modeling of thermal conductivity of Si in the range from the normal to near-critical conditions
Mathematica Montisnigri, 2019
In a wide temperature range, including the semiconductor-metal phase transition region and the near-critical region, the results of modeling the silicon phonon thermal conductivity are presented. Since the transfer of thermal energy is carried out by phonons and free charge carriers, it is necessary to take into account both the contribution of phonons and electrons in the total thermal conductivity. In contrast to metals, heat transfer in silicon in the solid state is determined by phonon thermal conductivity. Although the contribution of the electronic component to the total thermal conductivity increases with increasing temperature, the inclusion of phonon thermal conductivity is of particular importance in liquid silicon. At higher temperatures, phonon thermal conductivity plays an important role in the modeling of the mechanisms of interaction of pulsed laser radiation with silicon in the framework of the two-temperature continuum model. Obtaining the temperature dependence of phonon thermal conductivity in such a wide temperature range from experiment is problematic. In this work, phonon thermal conductivity was obtained in the range 300 ≤ T ≤ 6500 K from molecular dynamics simulation using the KIHS potential.
Journal of New Technology and Materials, 2013
Porous silicon (PS) was prepared by electrochemical etching method. Mirage effect in transverse photothermal deflection PTD ( skimming configuration) was used to determine thermal conductivity the experimental results of PS thermal conductivity was compared with theoretical results they were almost the same. Optical extinction coefficient and absorption coefficient were calculated from transmittance T and reflectance R curve which measured with UV-Vis-NIR Spectrophotometer, and they were used to calculate the optical conductivity and electric conductivity from the Shankar and Joseph equations, and optical conductivity and electric conductivity were studied with porosity in porous silicon.
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A Phenomenological Study of Pore-Size Dependent Thermal Conductivity of Porous Silicon
Acta Applicandae Mathematicae, 2012
A phonon-hydrodynamics approach is used to analyze the influence of porosity and of pore size on the reduction of the thermal conductivity in porous silicon. Different geometrical arrangements of the pores have been considered. For any given value of the porosity, the theoretical results show that for increasing Knudsen number (i.e., decreasing pore size) the effective thermal conductivity decreases whatever the geometrical arrangement of the pores is.
Applied Physics Letters, 2012
Surface passivation of nanoporous crystalline silicon can reduce its thermal conductivity. This was established using equilibrium molecular dynamics simulations. The porosity varied from 8% to 38% while the pore diameter ranged from 1.74 to 2.93 nm. Hydrogen and oxygen passivation reduced thermal conductivity by 11% to 17% and 37% to 51% depending on porosity at 500 K, respectively. The hydrogen passivation effect decreased with increasing temperature. Vibrational spectra of oxygen overlapped with those of silicon at low frequencies. Therefore, oxygen passivation enhanced phonon scattering at solid matrix boundaries, resulting in stronger thermal conductivity reduction than that caused by hydrogen passivation.
Journal of Applied Physics, 2011
This study establishes that the effective thermal conductivity k eff of crystalline nanoporous silicon is strongly affected not only by the porosity f v and the system's length L z but also by the pore interfacial area concentration A i . The thermal conductivity of crystalline nanoporous silicon was predicted using non-equilibrium molecular dynamics simulations. The Stillinger-Weber potential for silicon was used to simulate the interatomic interactions. Spherical pores organized in a simple cubic lattice were introduced in a crystalline silicon matrix by removing atoms within selected regions of the simulation cell. Effects of the (i) system length ranging from 13 to 130 nm, (ii) pore diameter varying between 1.74 and 5.86 nm, and (iii) porosity ranging from 8% to 38%, on thermal conductivity were investigated. A physics-based model was also developed by combining kinetic theory and the coherent potential approximation. The effective thermal conductivity was proportional to (1 À 1.5f v ) and inversely proportional to the sum (A i =4 þ 1=L z ). This model was in excellent agreement with the thermal conductivity of nanoporous silicon predicted by molecular dynamics simulations for spherical pores (present study) as well as for cylindrical pores and vacancy defects reported in the literature. These results will be useful in designing nanostructured materials with desired thermal conductivity by tuning their morphology.
Contribution of the normal component to the thermal resistance of turbulent liquid helium
Zeitschrift für angewandte Mathematik und Physik, 2015
Previous results for the velocity profile of the normal component of helium II in counterflow are used to evaluate the viscous contribution to the effective thermal resistance. It turns out that such contribution becomes considerably higher than the usual Landau estimate, because in the presence of vortices the velocity profile significatively separates from the Poiseuille parabolic profile. Thus, a marked increase of the contribution of the normal component to the thermal resistance with respect to the viscous Landau estimate does not necessarily imply that the normal component is turbulent. Furthermore, we examine the influence of a possible slip flow along the walls when the radius of the tube becomes comparable to the phonon mean-free path; this implies a reduction of the thermal resistance with respect to that obtained for non-slip boundary conditions.
Thermal conductivity measurement of porous silicon by the pulsed-photothermal method
Journal of Physics D: Applied Physics, 2011
Thermal properties of two types of porous silicon are studied using the pulsed-photothermal method (PPT). This method is based on a pulsed-laser source in the nanosecond regime. A 1D analytical model is coupled with the PPT technique in order to determine thermal properties of the studied samples (thermal conductivity and volumetric heat capacity).
On calculation of thermal conductivity from Einstein relation in equilibrium molecular dynamics
The Journal of Chemical Physics, 2012
In systems evolving under classical dynamics, using Einstein relation is one method to calculate lattice thermal conductivity. This method, in theory, is equivalent to Green-Kubo approach and it does not require a derivation of an analytical form for the heat current. However, in application of Einstein relation to molecular dynamics (MD), a discrepancy exists regarding the calculation of the energy moment (integrated heat current) R. The classical definition for the energy moment for a single particle is the total energy of the particle multiplied by its unwrapped coordinate in simulation domain. The total energy moment of the system is then calculated by summation over all particles. With this formulation of R, Einstein relation gives incorrect thermal conductivity (i.e. zero) for non-diffusive solid systems in MD under periodic boundary conditions. In this paper, we propose a new formulation for R that produces correct thermal conductivity and overcomes some of the difficulties encountered when calculating J . We apply it to solid argon and silicon defined by twoand n-body interactions. For the silicon, we also investigated the effect of porosity in the lattice. In accordance with the experimental studies, we determined substantial reduction in thermal conductivity as a consequence of porosity, internal surface area and the existence of surface surface rattlers.
Nanometre-scale 3D defects in Cr2AlC thin films
Scientific reports, 2017
MAX-phase Cr2AlC containing thin films were synthesized by magnetron sputtering in an industrial system. Nanometre-scale 3D defects are observed near the boundary between regions of Cr2AlC and of the disordered solid solution (CrAl)xCy. Shrinkage of the Cr-Cr interplanar distance and elongation of the Cr-Al distance in the vicinity of the defects are detected using transmission electron microscopy. The here observed deformation surrounding the defects was described using density functional theory by comparing the DOS of bulk Cr2AlC with the DOS of a strained and unstrained Cr2AlC(0001) surface. From the partial density of states analysis, it can be learned that Cr-C bonds are stronger than Cr-Al bonds in bulk Cr2AlC. Upon Cr2AlC(0001) surface formation, both bonds are weakened. While the Cr-C bonds recover their bulk strength as Cr2AlC(0001) is strained, the Cr-Al bonds experience only a partial recovery, still being weaker than their bulk counterparts. Hence, the strain induced bon...
Photothermal Effects and Heat Conduction in Nanogranular Silicon Films
Nanomaterials
We present results on the photothermal (PT) and heat conductive properties of nanogranular silicon (Si) films synthesized by evaporation of colloidal droplets (drop-casting) of 100 ± 50 nm-sized crystalline Si nanoparticles (NP) deposited on glass substrates. Simulations of the absorbed light intensity and photo-induced temperature distribution across the Si NP films were carried out by using the Finite difference time domain (FDTD) and finite element mesh (FEM) modeling and the obtained data were compared with the local temperatures measured by micro-Raman spectroscopy and then was used for determining the heat conductivities k in the films of various thicknesses. The cubic-to-hexagonal phase transition in Si NP films caused by laser-induced heating was found to be heavily influenced by the film thickness and heat-conductive properties of glass substrate, on which the films were deposited. The k values in drop-casted Si nanogranular films were found to be in the range of lowest k o...
Impact of pore anisotropy on the thermal conductivity of porous Si nanowires
Scientific Reports
Porous materials display enhanced scattering mechanisms that greatly influence their transport properties. Metal-assisted chemical etching (MACE) enables fabrication of porous silicon nanowires starting from a doped Si wafer by using a metal template that catalyzes the etching process. Here, we report on the low thermal conductivity (κ) of individual porous Si nanowires (NWs) prepared from MACE, with values as low as 0.87 W•m −1 •K −1 for 90 nm diameter wires with 35-40% porosity. Despite the strong suppression of long mean free path phonons in porous materials, we find a linear correlation of κ with the NW diameter. We ascribe this dependence to the anisotropic porous structure that arises during chemical etching and modifies the phonon percolation pathway in the center and outer regions of the nanowire. The inner microstructure of the NWs is visualized by means of electron tomography. In addition, we have used molecular dynamics simulations to provide guidance for how a porosity gradient influences phonon transport along the axis of the NW. Our findings are important towards the rational design of porous materials with tailored thermal and electronic properties for improved thermoelectric devices.
Polymer Engineering & Science, 2022
Materials able to store thermal energy can be a useful strategy in order to reduce energy consumption of buildings and to decrease greenhouse gases emissions. In this work, for the first time, the technique of salt leaching has been used for the production of novel polyethylene foams containing different amounts of a microencapsulated phase change material (PCM) with a melting point of 24 °C, to be potentially applied in building insulation. The microstructural, thermal and mechanical properties of the produced foams have been comprehensively investigated. The prepared foams were characterized by high values of open porosity (about 60 %) and by density values around 0.4 g/cm 3. Differential scanning calorimetry tests revealed that the adopted production process caused a partial loss of PCM, resulting in an effective PCM content of around 33 % for the sample with the highest PCM loading (56 wt%). Infrared thermography analysis demonstrated that the time required from the samples to reach a set temperature, thanks to the presence of PCM, was up to two times higher with respect to the reference foam. Shore-A measurements evidenced that the addition of PCM generally led to a softening of the foams. Tensile mechanical tests confirmed the softening effect provided by the addition of the microcapsules, with a decrease of the stiffness and of the strength of the material. Interestingly, strain at break values were considerably increased upon PCM introduction.
Photoreflectance in Monolayer Mesoporous Silicon Structures
Journal of Russian Laser Research, 2020
For the first time, we obtained the photoreflectance spectra in single-layer samples of porous silicon with 13 μm thick in the 550-1000 nm spectral region. To describe the observed reflection and photoreflectance spectra, we use a theory of multiple-beam interference, taking into account the strong absorption of Si in this spectral region. We show that, under the action of a laser radiation with a wavelength 532 nm of 30 mW power and pulse duration 3 ms, the change in the refractive index δn reaches values of the order of 10 −5 , and this takes place due to the thermal nonlinearity of the refractive index. We show that photoreflectance spectroscopy can be used to measure the thermo-optic coefficients of porous silicon.
Entropy
In this paper, we ask ourselves how non-local effects affect the description of thermodynamic systems with internal variables. Usually, one assumes that the internal variables are local, but that their evolution equations are non-local, i.e., for instance, that their evolution equations contain non-local differential terms (gradients, Laplacians) or integral terms with memory kernels. In contrast to this typical situation, which has led to substantial progress in several fields, we ask ourselves whether in some cases it would be convenient to start from non-local internal variables with non-local evolution equations. We examine this point by considering three main lengths: the observation scale R defining the elementary volumes used in the description of the system, the mean free path l of the microscopic elements of the fluid (particles, phonons, photons, and molecules), and the overall characteristic size L of the global system. We illustrate these ideas by considering three-dimen...
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Monte Carlo simulation of cross-plane thermal conductivity of nanostructured porous silicon films
Journal of Applied Physics, 2008
This paper presents a Monte Carlo ͑MC͒ modeling of heat conduction in heavily doped ͑p + and n + ͒ porous silicon ͑PS͒ films known as mesoporous silicon ͑meso-PS͒. A three-dimensional pore network generator is developed to better reproduce the structure of low porosity ͑f v Ͻ 50% ͒ meso-PS. The submicron scale heat conduction modeled by the Boltzman transport equation is simulated using the MC method in which the nonlinear phonon dispersion curves of bulk silicon and the phonon lifetime dependent on temperature, frequency, and polarization are taken into account. The proposed method has been applied to predict the effect of the porosity ͑10%-47%͒, pore sizes ͑10-20 nm͒, pore arrangement ͑p + -and n + -type͒, temperature ͑50-500 K͒, and film thickness ͑50 nm-1 m͒ on the cross-plane thermal conductivity of meso-PS films. Moreover, the simulation results enable to deduce the scattering mean free path ͑MFP͒ of phonons in the PS and the scattering MFP due to phonon-pore wall interaction. At room temperature, the thermal conductivity of meso-PS is shown one to two orders of magnitude smaller than that of bulk silicon. A drastic simplification of the phonon dispersion curves and phonon MFP, such as in the Debey approximation, results in an overestimation ͑by about three times͒ of the thermal conductivity of meso-PS. The thermal conductivity decreases when the pore size decreases or the porosity increases. For a given porosity and pore size, the thermal conductivity of doped p + -type PS is much smaller than that of doped n + -type PS. Finally, the simulations of thermal conductivity of doped p + -type PS are shown in good agreement with available experimental data which confirms the validity of the current modeling.
Thermal conductivity of highly porous Si in the temperature range 4.2 to 20 K
Nanoscale research letters, 2014
We report on experimental results of the thermal conductivity k of highly porous Si in the temperature range 4.2 to 20 K, obtained using the direct current (dc) method combined with thermal finite element simulations. The reported results are the first in the literature for this temperature range. It was found that porous Si thermal conductivity at these temperatures shows a plateau-like temperature dependence similar to that obtained in glasses, with a constant k value as low as 0.04 W/m.K. This behavior is attributed to the presence of a majority of non-propagating vibrational modes, resulting from the nanoscale fractal structure of the material. By examining the fractal geometry of porous Si and its fractal dimensionality, which was smaller than two for the specific porous Si material used, we propose that a band of fractons (the localized vibrational excitations of a fractal lattice) is responsible for the observed plateau. The above results complement previous results by the au...
Impact of pore anisotropy on the thermal conductivity of porous Si nanowires
Scientific Reports
Porous materials display enhanced scattering mechanisms that greatly influence their transport properties. Metal-assisted chemical etching (MACE) enables fabrication of porous silicon nanowires starting from a doped Si wafer by using a metal template that catalyzes the etching process. Here, we report on the low thermal conductivity (κ) of individual porous Si nanowires (NWs) prepared from MACE, with values as low as 0.87 W•m −1 •K −1 for 90 nm diameter wires with 35-40% porosity. Despite the strong suppression of long mean free path phonons in porous materials, we find a linear correlation of κ with the NW diameter. We ascribe this dependence to the anisotropic porous structure that arises during chemical etching and modifies the phonon percolation pathway in the center and outer regions of the nanowire. The inner microstructure of the NWs is visualized by means of electron tomography. In addition, we have used molecular dynamics simulations to provide guidance for how a porosity gradient influences phonon transport along the axis of the NW. Our findings are important towards the rational design of porous materials with tailored thermal and electronic properties for improved thermoelectric devices.
Microelectronics Journal, 1999
Recently measured low thermal conductivity of as-prepared and slightly oxidized meso-porous silicon (meso-PS) offers new possibility to apply this promising material for thermal isolation in microsensors and microsystems. We report here a theoretical model describing specific mechanisms of heat transport in as-prepared and oxidized meso-PS. The model is in good agreement with experimental data obtained earlier.
Study of Thermal Conductivity of Porous Silicon Using the Micro-Raman Method
Open Journal of Physical Chemistry, 2012
In this work, we are interesting in the measurement of thermal conductivity (on the surface and in-depth) of Porous silicon by the micro-Raman spectroscopy. This direct method (micro-Raman spectroscopy) enabled us to develop a systematic means of investigation of the morphology and the thermal conductivity of Porous silicon oxidized or no. The thermal conductivity is studied according to the parameters of anodization and fraction of silicon oxidized. Thermal transport in the porous silicon layers is limited by its porous nature and the blocking of transport in the silicon skeleton what supports its use in the thermal sensors.
Physical review letters, 2015
The electron-phonon interaction is well known to create major resistance to electron transport in metals and semiconductors, whereas fewer studies are directed to its effect on phonon transport, especially in semiconductors. We calculate the phonon lifetimes due to scattering with electrons (or holes), combine them with the intrinsic lifetimes due to the anharmonic phonon-phonon interaction, all from first principles, and evaluate the effect of the electron-phonon interaction on the lattice thermal conductivity of silicon. Unexpectedly, we find a significant reduction of the lattice thermal conductivity at room temperature as the carrier concentration goes above 10^{19} cm^{-3} (the reduction reaches up to 45% in p-type silicon at around 10^{21} cm^{-3}), a range of great technological relevance to thermoelectric materials.
Thermal conductivity of thick meso-porous silicon layers by micro-Raman scattering
Journal of Applied Physics, 1999
We report here a theoretical model describing specific mechanisms of heat transport in as-prepared and oxidized meso-porous silicon layers. The model is in good agreement with experimental measurements performed by micro-Raman scattering on the layers surface. For the first time, thermal conductivity inhomogeneity along the porous layer thickness of 100 m is studied. Direct correlation between the thermal conductivity and morphology variations along the layer thickness is brought to the fore. A new approach to estimate local porosity of the porous layers based on thermal conductivity and Si nanocrystallite size measurements is also proposed.
Journal of Applied Physics, 2020
Irradiating porous silicon is expected to reduce thermal conductivity without altering the porous structure and can be studied by optical techniques provided optical properties can be established reliably. Toward this end, meso-porous silicon (PSi), with a porosity of 56%, was prepared from p + Si wafer (0.01-0.02 .cm-1 resistivity) and was partially amorphized by irradiation in the electronic regime with 129 Xe ions at two different energies (29 MeV and 91 MeV) and five fluences ranging from 10 12 cm-2 to 3.10 13 cm-2. The PSi structure is monitored by scanning electron microscopy. High-resolution transmission electron microscopy shows that the amorphous phase is homogeneous in volume and that there is no formation of amorphous-crystalline core-shell structures. An agreement is found between the thermal conductivity results obtained with micro-Raman thermometry, which is an optical contactless technique heating the sample in the depth, and scanning thermal microscopy, which is an electrical technique heating the sample by contact at the sample surface. A linear relation is established between the effective thermal conductivity and the amorphous fraction, predicting the thermal conductivity of fully-amorphous porous Si below 1 W.m-1 .K-1. The obtained values are comparable to that of SiO2, reduced by a factor 6 in comparison to non-irradiated porous samples (~6.5 W.m-1 .K-1) and smaller than bulk silicon by more than two orders of magnitude.
An analytical model for the thermal conductivity of silicon nanostructures
Journal of Applied Physics, 2005
A simple model of thermal conductivity, based on the harmonic theory of solids, is used to study the heat transfer in nanostructures. The thermal conductivity is obtained by summing the contribution of all the vibration modes of the system. All the vibrational properties ͑dispersion curves and relaxation time͒ that are used in the model are obtained using the data for bulk samples. The size effect is taken into account through the sampling of the Brillouin zone and the distance that a wave vector can travel between two boundaries in the structure. The model is used to predict the thermal conductivity of silicon nanowires and nanofilms, and demonstrates a good agreement with experimental results. Finally, using this model, the quality of the silicon interatomic potential, used for molecular-dynamics simulations of heat transfer, is evaluated.
Study of Nano-Porous Silicon with Low Thermal Conductivity as Thermal Insulating Material
Journal of Porous Materials
Recently discovered phenomenon of extremely low thermal conductivity of nano-porous silicon (nano-PS) is discussed in detail. A theoretical model describing specific mechanisms of heat transport in as-prepared and oxidized nano-PS layers is described. The theoretical estimations are in a good agreement with experimental data obtained earlier. The low thermal conductivity values allow to use this promising material as thermal insulator in microsensors and microsystems. To ensure an efficient thermal isolation, a nano-PS layer has to be as thick as possible and mechanically stable. We describe here the procedures to form thick (up to 200 m) and stable nano-PS layers. Distribution of Si oxidized fraction along the layer thickness after thermal oxidation in dry O2 atmosphere at 300C during 1 h is studied.
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