Thermal Conductivity Of Films Research Articles

Exploring the Effect of Porous Structure on Thermal Conductivity in Templated Mesoporous Silica Films

This work elucidates the effect of porous structure on thermal conductivity of mesoporous amorphous silica. Sol–gel and nanoparticle-based mesoporous amorphous silica thin films were synthesized by...

Thermoelectric properties of low-cost transparent single wall carbon nanotube thin films obtained by vacuum filtration

Abstract The dispersions of semiconducting (sc-) and metallic (m-) SWCNTs with purity more than 98 and 86%, correspondingly, were obtained by using the aqueous two-phase extraction method. The unseparated (un-) SWCNTs contained ~3/4 of semiconducting and ~1/4 of metallic nanotubes. Thin films based on unseparated, semiconducting and metallic SWCNTs were prepared by vacuum filtration method. An Atomic Force Microscopy (AFM) and a Transmission Electronic Microscopy (TEM) were used to investigate the thin film microstructure. The thin SWCNT film transmittance was measured in the wavelength range of 300–1500 nm. Thermoelectric properties were carried out in the temperature range up to 200 °C. The largest Seebeck coefficient was observed for thin films based on semiconducting SWCNTs. The maximum value was 98 μV/K under the temperature of 170 °C. The lowest resistivity was 7.5·10−4·Ohm·cm at room temperature for thin un-SWCNT films. The power factor for m-SWCNT and un-SWCNT films was 47 and 213 μW m−1 K−2, correspondingly, at room temperature and 74 and 54 μW m−1 K−2 at 200 °C, respectively. For a thin sc-SWCNT film the maximum power factor was 2.8 μW m−1 K−2 at 160 °C. The un-SWCNT film thermal conductivity coefficient was 5.63 and 3.64 W m−1 K−1 and a thermoelectric figure of merit was 0.011 and 0.016 at temperatures of 23 and 50 °C, respectively.

A perspective on effective medium models of thermal conductivity in (ultra) nanocrystalline diamond films

Thermal conductivity of nanocrystalline and ultra-nanocrystalline films is analyzed with effective medium theory (EMT) models. The existing EMT models use the spherical inclusion approximation. Although this approximation works quite well it is inconsistent, mostly with respect to the maximal packing of 74{\%}, which may be unrealistic for polycrystalline films. To check the consistency of these models we devise an EMT model with arbitrarily shaped inclusions. We pick the EMT model with cubic inclusions and we compare its results with the results of the EMT model with spherical inclusions. It is found a very good agreement between both calculations. This agreement is explained by general geometrical arguments. We further employ these models to analyze thermal conductivity of nanocrystalline and ultra-nanocrystalline diamond films. It is noticed that the effective conductivity is strongly affected not only by the boundary Kapitza resistance but also by intra-grain scattering for grain sizes below 100 nm. Generally, both intra-grain conductivity and Kapitza resistance increase with grain size. However, the effect of Kapitza resistance increase is negligible due to the geometrical factor accompanying Kapitza resistance contribution to the effective conductivity.

Thermal conductivity in nanoscale polysilicon structures with applications in sensors

Thermal conductivity of polysilicon nanowires and thin films has been experimentally studied. Thermal conductivity has been measured with a microstructure fabricated alongside the polysilicon nanowires and thin films and a robust fabrication process has been developed. Results suggest that the in-plane and out-of-plane thermal conductivity are limited by phonon grain boundary and film boundary scattering respectively. The nanowires are fabricated by patterning a thin layer of low-pressure chemical vapor deposited polysilicon using e-beam lithography, consequently circumventing the prevalent process of pick-and-place process, in which metallic contacts are deposited to insure significant heat flow to the nanowire. By avoiding the need for metallic contacts a significant source of error in the calculation of the thermal conductivity is eliminated. Thermal conductivity of nanowires with a cross section of ˜60 nm × 100 nm and thin film with a cross-section of 3μm × 108 nm were measured to be both ˜3.5 W/m.K, suggeting that the thermal conductiviy is dominated by the out-of-plane phonon scattering in both cases. This is an almost 8× reduction from the thermal conductivity of bulk polysilicon at ˜30 W/m.K. The effect of grain size and doping concentration on the thermal conductivity of ˜100 nm polysilicon films is also experimentally studied.

Oscillatory thermocapillary instability of a film heated by a thick substrate

In this work we consider a new class of oscillatory instabilities that pertain to thermocapillary destabilization of a liquid film heated by a solid substrate. We assume the substrate thickness and substrate–film thermal conductivity ratio are large so that the effect of substrate thermal diffusion is retained at leading order in the long-wave approximation. As a result, the system dynamics is described by a nonlinear partial differential equation for the film thickness that is non-locally coupled to the full substrate heat equation. Perturbing about a steady quiescent state, we find that its stability is described by a non-self-adjoint eigenvalue problem. We show that, under appropriate model parameters, the linearized eigenvalue problem admits complex eigenvalues that physically correspond to oscillatory (in time) instabilities of the thin-film height. As the principal results of our work, we provide a complete picture of the susceptibility to oscillatory instabilities for different model parameters. Using this description, we conclude that oscillatory instabilities are more relevant experimentally for films heated by insulating substrates. Furthermore, we show that oscillatory instability where the fastest-growing (most unstable) wavenumber is complex, arises only for systems with sufficiently large substrate thicknesses. Finally, we discuss adaptation of our model to a practical setting and make predictions of conditions at which the reported instabilities can be observed.

Simultaneous Mapping of Thermal Conductivity, Thermal Diffusivity, and Volumetric Heat Capacity of Halide Perovskite Thin Films: A Novel Nanoscopic Thermal Measurement Technique.

Local thermal conductivity, thermal diffusivity, and volumetric heat capacity of all-inorganic halide perovskite thin films are mapped simultaneously and with highest spatial resolution for the first time. These various thermal properties are detected by a scanning near-field thermal microscope operated at two different frequencies simultaneously. We apply this technique to analyze the thermal properties of halide perovskites on the nanoscale. In addition to an ultralow thermal conductivity of 0.43 ± 0.03 and 0.33 ± 0.02 W/(m·K), a low thermal diffusivity of 0.3 ± 0.1 mm2/s and a small heat capacity of 0.29 ± 0.9 and 0.18 ± 0.6 J/(g·K) are obtained for CsPbBr3 and CsPb2Br5 films, respectively. The findings of our thermal microscopy are of great general importance for the thermal design of thin-film devices based on halide perovskites, while the measurement technique itself is generally applicable for other thin-film optoelectronic materials.

Temperature fields generated by a circular heat source (CHS) in an infinite isotropic medium: Treatment of contact resistances with application to thin films

The temperature fields and heat fluxes created by a circular heat source (CHS) are derived for the case of a CHS embedded between two identical and isotropic semi-infinite media, when different thermal contact resistances (CRs) are present on each surface of the CHS. Three different temperature fields are derived: on the CHS surface and in each one of the media surrounding the CHS. The derivation of the three-dimensional heat flow solution uses first principles with no prior assumptions, and employs the Hankel and Laplace transforms. The analytical solution presented here is exact with no approximations, but is given in integral form, which requires a facile numerical evaluation. Finite element simulations performed with COMSOL Multiphysics are provided and validate the analytical solution. The application of the solution to thin film thermal conductivity virtual measurements is demonstrated. Regressions of the numerical results yield thin film resistances in excellent agreement with the actual values (all within 1.5% error), which can readily be translated into thin film thermal conductivities.

Dicing Tape Performance in a Plasma Dicing Environment

Plasma dicing, as a means of isolating individual integrated circuits from within a fully processed semiconductor substrate, is still an emerging technology but is now considered the latest step in the evolution in device singulation. With the trend towards smaller, thinner more robust devices, many chip manufacturers are considering, or already switching, to a plasma dicing approach.[1, 2] The high aspect ratio, deep reactive ion etching of silicon using a Bosch process, leverages some distinct advantages over the more physical methods of device singulation. [3] Although the more established methods of diamond blade saw dicing, various laser based approaches or laser/mechanical hybrid dicing techniques, all introduce an element of heat and in some cases water for cooling purposes, they do not expose dicing tape to the unique conditions within high vacuum plasma. This work investigates how the properties of both dicing tape and adhesive are affected when exposed to the environment of a plasma dicing process used in the semiconductor industry. A preliminary fitness test utilising an aggressive exothermic etching regime was used to establish the compatibility of a range of standard Lintec Adwill dicing tapes. In essence, a measure of base film thermal conductivity and how quickly some of the lower molecular weight volatile components of base film and adhesive materials are boiled off or ‘out-gassed’. Polyvinyl Chloride (PVC) tapes performed less well than Polyethylene terephthalate (PET) tapes and some of the Polyolefin (PO) tapes exhibited the greatest resilience. A design of experiments to measure changes in tensile strength, elongation and adhesive properties were carried out to assess the performance of plasma dicing on PO low adhesion tape. Figure 2 shows that the samples exposed to an SF6 plasma impacted the tape elongation and tensile strength properties. Typically the stretch required for PnP is in the range 1–10 % showing that the plasma dicing does not impact the overall performance of the tape. Further tests employing a die pick-up force measurement system to compare saw diced die with that of plasma diced die, proved the feasibility of this technology. Figure 3 shows that pick-up force measured on plasma diced dies is comparable with saw diced dies.

Determination of modified figure of merit validity for thermoelectric thin films with heat transfer model: Case of CuCrO2:Mg deposited on fused silica

Thermoelectric performance of a material is determined using a figure of merit (FOM) determined as ZT (ZT = σS2T/κ where σ is the electrical conductivity, S is the Seebeck coefficient, κ is the thermal conductivity, and T is the temperature). In the case of a thin film, it is normal in the first approach to consider calculating the FOM by using the thermal conductivity of the film. However, both the thermal influence of the substrate and the emissivity of the film must also be taken into account. In the present work, the heat transfer model is used in order to study the influence of the thermal conductivity, the thickness, and the emissivity of the film on the thermal gradient of the stack (substrate + thin film). The limits of these three parameters are determined in order to have the temperature variation due to the presence of the film compared to the substrate alone that remains less than 1%. Under these limits, the thermal conductivity of the substrate can be taken into account instead of the thermal conductivity of the thin film, and a modified FOM (Z’T) can be calculated. The present study leads to the determination of the validity of modified ZT. In the case of CuCrO2:Mg thin films, the model shows that the use of Z’T is valid. The calculated value of Z’T with the measured Seebeck coefficient and the electrical conductivity as a function of the temperature for 100 nm thick films and the temperature dependent thermal conductivity taken from the literature reached 0.02 at 210 °C. A thermoelectric module made with this material showed 10.6 nW when 220 °C is applied at the hot side.

Enhanced thermal conductivity and mechanical property of flexible poly (vinylidene fluoride)/boron nitride/graphite nanoplatelets insulation films with high breakdown strength and reliability

Owing to the integration of power electronic devices and the development of flexible devices, demands for high quality thermal conductive composites have substantially increased. Thus highly flexible poly (vinylidene fluoride)/boron nitride (i.e. PVDF/BN-x) composite films were prepared in this work, and a low quantity of graphite nanoplatelets (GNP) was added to simultaneously improve the thermal and mechanical properties of the film (i.e. PVDF/BN-30/GNP-y). The synergistic effect of the two fillers has been discussed, and results show that the thermal conductivity of PVDF/BN-30/GNP-2.5 reaches up to 0.720 Wm−1K−1 compared with 0.634 Wm−1K−1 of PVDF/BN-30. And the tensile strength is also increased from 12.43 MPa to 20.21 MPa, which has been improved by 63%. In addition, the experiment results validate that the films possess high electrical insulation and satisfactory breakdown strength, which proves its applicability to high-voltage environment. Besides, the thermal conductivity of films can be preserved after 300 thermal shocks with extreme temperature changes. This study provides a kind of flexible thermal interface material with excellent comprehensive properties for application in field of power electronics.

An Analysis of the Effect of Protective Films on the Inside Surface of Headers and Steam Pipelines in Combined-Cycle Power Plants on Their Thermally Stressed State

The outlet header of the high-pressure superheater is among the critical components affecting the service life of heat recovery steam generators (HRSG) of combined-cycle power plants (CCPP). Headers, heat-transfer tubes, and steam pipelines are protected against corrosion by running proper water chemistry to form thin protective films on the surfaces. These films formed on the inside surface of heat-transfer tubes protect against corrosion and also reduce thermal stresses in the tube walls during transients. They are formed on the inside of tube systems of HRSGs in CCPPs at temperatures above 230°С and with good deaeration. Results are presented of a numerical analysis of the influence of thin protective films on the state of thermal stresses in headers and steam pipelines of HRSGs used in CCPPs during various transients induced, for example, by startups from different initial thermal state or thermal shocks. Predictions were obtained for different film thicknesses assuming that the film thermal conductivity was less than thermal conductivity of the pipeline metal. The numerical results enabled the determination of the heating-up modes for headers and steam pipelines under which the effect of protective films is the greatest. It is demonstrated that the protective films have a considerable effect on stresses and damage accumulation in the pipeline wall induced basically by large temperature disturbances, such as thermal shocks. The effect of protective films on the thermally stressed state of unheated pipes was assessed by reducing the resourse consumption and accumulation of equivalent operating hours during transients. It is demonstrated that a 50 μm thick protective film alone can increase the service life of an HP steam superheater’s outlet header with a standard size of 426 × 34 mm by 6%.

Study on thermal conductivities of Si thin films and porous Si structures based on a modified lattice Boltzmann method

In this work, a modified lattice Boltzmann method (LBM) is developed to predict the thermal conductivity of silicon thin films and porous silicon structures, in which a probability parameter is introduced to determine whether a phonon collision event happens at a specific lattice point. The thickness dependent silicon thin film thermal conductivity calculated using this method shows a good agreement with prior experimental and simulation results. We also use this approach to study the porosity and pore-size dependent thermal conductivity of porous silicon structures. Furthermore, the simulated results about thermal conductivity of porous silicon are found to agree well with the previously reported data. The modified algorithm offers substantial computational improvement as compared to current LBM models and enables us to a clearer understanding about the meaning of collision step and streaming step in the LBM model.

H‐BN huddled starch reinforced polyethylhexylacrylate\polyvinyl alcohol thin films for packaging applications

Herein, starch hybrid polyethylhexylacrylate\polyvinyl alcohol (PEHA/PVA‐s) thin films are synthesized by in situ polymerization of ethylhexylacrylate (EHA) with incorporation of hexagonal boron nitride (h‐BN). The occurrence of h‐BN nanoparticles in PEHA/PVA‐s thin films is characterized by Fourier transform infrared (FTIR) spectroscopy, X‐ray diffraction (XRD), and energy dispersive X‐ray (EDX) analysis. The field emission scanning electron microscopy (FESEM) images illustrates the embedment of h‐BN nanoparticles in the PEHA/PVA‐s films. It is found that, h‐BN nanoparticles have an average particle size of 100 nm observed with a lattice fringe of 0.35 nm are known from high resolution transmission electron microscopy (HRTEM) images. The surface and nano‐tribological features of thin films are studied by atomic force microscope (AFM). The thermal conductivity of films is enhanced with increase in h‐BN concentrations. From rheological study, it is observed that, the tan δ value varies from 0.3 to 0.5. The biodegradation, refractive index, and chemical resistance properties of the thin films are studied. The oxygen barrier properties of the thin films are substantially reduced by 18 folds due to incorporation of huddled h‐BN layers. The prepared PEHA/PVA‐s@h‐BN thin films with improved physicochemical behaviors and reduction in gas barrier properties can enable the material for packaging applications. POLYM. COMPOS., 40:1810–1818, 2019. © 2018 Society of Plastics Engineers

Thermal conductivity of ultrathin BaTiO3 films grown by plasma-assisted atomic layer deposition

We report the first measurements of the room temperature thermal conductivity of ultrathin (12 and 24 nm) barium titanate (BaTiO3) films prepared by plasma-enhanced atomic layer deposition (PEALD). The measured thermal conductivities of as-deposited films are relatively low as compared to values reported previously. We further investigate the effects of a post-deposition remote oxygen plasma treatment on the crystallinity and thermal conductivity of our PEALD BaTiO3 films. We find that the thermal conductivity slightly decreases with increasing duration of plasma treatment, which is most likely due to increased phonon scattering at interfaces between amorphous and crystalline phases within the films.

Influence of Focusing on Phonon Propagation and Thermal Conductivity in Single Crystal Films with Different Types of Anisotropy of Elastic Energy

The influence of the anisotropy of elastic energy on the phonon propagation and phonon transport in single crystal nanofilms with different types of anisotropy of elastic energy in a Knudsen flow of a phonon gas is studied. The angular distribution of phonon mean free paths in the planes of the films and in their cross section is analyzed. The physical reasons leading to the dependence of the thermal conductivity on the orientation of the film planes and the directions of the heat flux relative to the crystal axes are studied. An analysis of the effect of focusing on the phonon propagation made it possible to explain the qualitative difference between the anisotropy of phonon mean free paths in films of cubic nanocrystals of various types having different orientations of the planes.

Experimental study of a SINIS detector response time at 350 GHz signal frequency

Response time constant of a SINIS bolometer integrated in an annular ring antenna was measured at a bath temperature of 100 mK. Samples comprising superconducting aluminium electrodes and normal-metal Al/Fe strip connected to electrodes via tunnel junctions were fabricated on oxidized Si substrate using shadow evaporation. The bolometer was illuminated by a fast black-body radiation source through a band-pass filter centered at 350 GHz with a passband of 7 GHz. Radiation source is a thin NiCr film on sapphire substrate. For rectangular 10÷100 μs current pulse the radiation front edge was rather sharp due to low thermal capacitance of NiCr film and low thermal conductivity of substrate at temperatures in the range 1-4 K. The rise time of the response was ~1-10 μs. This time presumably is limited by technical reasons: high dynamic resistance of series array of bolometers and capacitance of a long twisted pair wiring from SINIS bolometer to a room-temperature amplifier.

Large Enhancement of Thermal Conductivity and Lorenz Number in Topological Insulator Thin Films.

Topological insulators (TI) have attracted extensive research effort due to their insulating bulk states but conducting surface states. However, investigation and understanding of thermal transport in topological insulators, particularly the effect of surface states, are lacking. In this work, we studied thickness-dependent in-plane thermal and electrical conductivity of Bi2Te2Se TI thin films. A large enhancement in both thermal and electrical conductivity was observed for films with thicknesses below 20 nm, which is attributed to the surface states and bulk-insulating nature of these films. Moreover, a surface Lorenz number much larger than the Sommerfeld value was found. Systematic transport measurements indicated that the Fermi surface is located near the charge neutrality point (CNP) when the film thickness is below 20 nm. Possible reasons for the large Lorenz number include electrical and thermal current decoupling in the surface state Dirac fluid, and bipolar diffusion transport. A simple computational model indicates that the surface states and bipolar diffusion indeed can lead to enhanced electrical and thermal transport and a large Lorenz number.

Measurements of thermal conductivity and the coefficient of thermal expansion for polysilicon thin films by using double-clamped beams

In this paper, a test structure for simultaneously determining thermal conductivity and the coefficient of thermal expansion (CTE) of polysilicon thin film is proposed. The test structure consists of two double-clamped beams with different lengths. A theoretical model for extracting thermal conductivity and CTE based on electrothermal analysis and resonance frequency approach is developed. Both flat and buckled beams are considered in the theoretical model. The model is confirmed by finite element software ANSYS. The test structures are fabricated by surface micromachined fabrication process. Experiments are carried out in our atmosphere. Thermal conductivity and CTE of polysilicon thin film are obtained to be (29.96 ± 0.92) W · m · K−1 and (2.65 ± 0.03) × 10−6 K−1, respectively, with temperature ranging from 300–400 K.

The Influence of a TiN Film on the Electronic Contribution to the Thermal Conductivity of a TiC Film in a TiN-TiC Layer System

TiC and TiN films were deposited by reactive magnetron sputtering on Si substrates. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterization of the microstructure and interface structure have been carried out and the stoichiometric composition of TiC is determined. Thermal conductivity and interface thermal conductance between different layers in the films are evaluated by the transient thermo reflectance (TTR) and three-omega (3-ω) methods. The results showed that the thermal conductivity of the TiC films increased with temperature. The thermal conductivity of TiC in the absence of TiN is dominated by phonon contribution. The electronic contribution to the thermal conductivity of TiC in the presence of TiN is found to be more significant. The interface thermal conductance of the TiC/TiN interface is much larger than that of interfaces at Au/TiC, TiC/Si, or TiN/Si. The interface thermal conductance between TiC and TiN is reduced by the layer formed as a result of interdiffusion.

Phonon-boundary scattering in nanoporous silicon films: Comparison of Monte Carlo techniques

The thermal conductivities of silicon thin films with periodic pore arrays (i.e., nanoporous films) and square silicon nanowires are predicted at a temperature of 300 K. The bulk phonon properties are obtained from lattice dynamics calculations driven by first-principles calculations. Phonon-boundary scattering is included by applying three Monte Carlo-based techniques that treat phonons as particles. The first is a path sampling technique that modifies the intrinsic bulk mean free paths without using the Matthiessen rule. The second uses ray-tracing under an isotropic assumption to calculate a single, mode-independent boundary scattering mean free path that is combined with the intrinsic bulk mean free paths using the Matthiessen rule. The third modifies the ray-tracing technique to calculate the boundary scattering mean free path on a modal basis. For the square nanowire modeled using isotropic ray-tracing, the maximum mean free path is comparable to the wire width, an unphysical result that is a consequence of the isotropic approximation. Free path sampling and modal ray-tracing produce physically meaningful mean free path distributions. The nanoporous film thermal conductivity predictions match a previously measured trend, suggesting that coherent effects are not relevant to thermal transport at room temperature. A line-of-sight for phonons in the nanoporous films is found to change how thermal conductivity scales with porosity.