Instability-induced wrinkling in thermoelectric thin film/substrate structures for thermal protection systems in supersonic space shuttle applications

ABSTRACT

This paper examines the behavior of wrinkling instability of a thermoelectric thin film bonded to substrate. The critical temperature differences for wrinkling occurrence and buckling initiation are obtained. Damage growth following wrinkling is also determined. These critical temperatures can provide guidelines for the design of thermoelectric thin film devices. Numerical results show that the stability of thermoelectric thin film is affected by the electric current. The critical temperature differences become smaller when the electric current density in thermoelectric thin film is higher. Effect of the wavelength of wrinkling on the critical temperature differences of wrinkling occurrence is also identified.     

Analysis of thermally induced delamination of thermoelectric thin film/substrate system

Thermoelectric thin film/substrate structures have many practical applications such as in heat recovery systems. The general problem of thermally induced delamination between a thermoelectric thin film and a substrate is investigated. The temperature varies along the length direction but is constant along the thickness direction of the film. Analytical solutions of the temperature field in the film and the stress intensity factors (SIFs) at the delamination crack tips are obtained. The combined heat convection and heat radiation between the film surfaces and the surrounding medium (i.e., the air) are taken into account. Numerical results show that the SIFs sharply increase as the tips of the delamination crack approach the ends of the film. The combined heat convection and heat radiation can increase or decrease the SIFs. The mechanism for the delamination propagation in the thermoelectric film/substrate system is examined. The critical (permissible) temperature difference across the film governing the delamination propagation is identified.     

Fracture toughness measurement of thin films on compliant substrate using controlled buckling test

Thin films and multilayered structures are increasingly used in the industry. One of the important mechanical properties of these thin layers is the fracture toughness, which may be quite different from the known value of the bulk sample due to microstructural difference. In the design towards device flexibility and scratch resistance, for example, fracture toughness is an important parameter of consideration. This work presents a testing scheme using controlled buckling experiment to determine the fracture toughness of brittle thin films prepared on compliant substrates. When the film is under tension, steady-state channelling cracks form in parallel to each other. Critical fracture strain can be calculated by the measuring the displacement of the buckled plate. The fracture toughness can then be obtained with the help of finite element calculation. When the substrate experiences plastic deformation, the energy release rate is increased by the degree of plasticity. Fracture toughness measurement of two types of thin film Cu-Sn intermetallic compounds has been given to illustrate the merits of such a test scheme.     

Buckling behaviour of laminated beam structures using a higher-order discrete model

A higher-order shear- deformation theory, assuming a non-linear variation for the displacement field, is used to develop a finite-element model to predict the linear buckling behaviour of anisotropic multilaminated or sandwich thick and thin beams. The model is based on a single-layer Lagrangean four-node straight-beam element. It considers stretching and bending in two orthogonal planes. The most common cross-sections and symmetric and asymmetric lay-ups are studied. The good performance of the present element is evident on the prediction of the buckling of several test cases of thin and thick isotropic or anisotropic beam structures. Comparisons show that the model is accurate and versatile.     

Growth and thermoelectric properties of multilayer thin film of bismuth telluride and indium selenide via rf magnetron sputtering

A bismuth telluride (BT)/indium selenide (IS) multilayer film was deposited at room temperature by rf magnetron sputtering on a sapphire substrate in order to investigate how the multilayered structure affects the microstructure and thermoelectric properties. The effect of annealing at different temperatures was also studied. The results were compared with those from a BT film with the same thickness. A TEM study showed that the interface between the BT and IS layers became vague as the annealing temperature increased, and the BT layer crystallized while the IS layer did not. The presence of thin IS layers can help to limit the evaporation of Te from the BT/IS multilayer film, thus increasing the amount of Bi2Te3 phase in the multilayer film as compared with that of the BT film. An abrupt increase in the Seebeck coefficient of the multilayer film was observed when annealed at 300 degrees C, and the resistivity of the annealed multilayer film was high compared to that of the BT film. This result can also be explained by the proposed role of the IS layer, which limits the evaporation of Te at high temperature. The highest power factor of -3.9 x 10(-6) W/K2 cm was obtained at room temperature from the multilayer film annealed at 300 degrees C.     

Buckling of the CCFF orthotropic rectangular plates under in-plane pure bending

Solution of the buckling problem for the CCFF orthotropic plate subjected to in-plane pure bending is presented. The two parallel clamped edges of the plate are loaded by linearly distributed in-plane loads statically equivalent to the in-plane bending moments. The problem is solved using method of lines for partial differential equations and Galerkin’s method. The buckling problems are solved for isotropic, orthotropic and multilayered CFRP composite plates with various aspect ratios. Results of calculations of critical loads are compared with those based on finite-element modelling and analyses. The comparisons demonstrate efficiency of the proposed approach to the buckling analysis of composite CCFF plates with various dimensional and stiffness parameters.     

Elastic buckling of multiwall carbon nanotubes under high pressure

This paper studies elastic buckling of individual multiwall carbon nanotubes under radial pressure. The analysis is based on a multiple-elastic-shell model in which each of the concentric tubes of a multiwall carbon nanotube is described as an individual elastic shell. According to their radius-to-thickness ratios, the multiwall carbon nanotubes discussed here are classified into three types: thin, thick, and (almost) solid. The critical pressure for elastic buckling is calculated for examples of all three types. It is found that a thin N-wall nanotube (defined by a radius-to-thickness ratio larger than 4) is approximately equivalent to a single-layer elastic shell whose effective bending stiffness and thickness are N times the effective bending stiffness and thickness of single-wall carbon nanotubes. Based on this result, an approximate method is suggested for replacing the problematic multiwall nanotube of many layers with a multilayer elastic shell of fewer layers. In particular, the critical pressure predicted by the present model is in good agreement with known experimental results.     

Microstructure and electrochemical properties of magnetron-sputtered LiCoO2/LiNiO2 multi-layer thin film electrode

LiCoO₂ single-layer and LiCoO₂/LiNiO₂ multi-layer thin film electrodes were successfully fabricated by magnetron sputtering. Their microstructure and electrochemical properties were investigated. Once annealed, both films had the (0 0 3) preferred orientation to minimize the surface energy. The initial discharge capacity of the multi-layer thin film was approximately 53.1 μAh/cm² μm, which was higher than that of the LiCoO₂ single-layer thin film having similar thickness. The capacity retention of the multi-layer thin film was superior to that of the single-layer thin film. These findings indicate that the multi-layer thin film is a promising cathode material for the fabrication of high-performance thin film batteries.     

The effect of physical vapor deposition parameters on the thermoelectric power of thin film molybdenum-nickel junctions

The effect of various physical vapor deposition parameters on the thermoelectric power of thin film (10 000 Å) Mo-Ni junctions deposited by electron beam evaporation was investigated in this study. The deposition parameters of interest were the substrate temperature, the deposition rate and the vapor source purity. The thermoelectric power of the thin film Mo-Ni junctions was dependent on the structure of the Mo thermoelectric element of the couple, the characteristics of which were significantly altered by varying the deposition parameters. Varying the deposition parameters caused a change in lattice imperfections in the metal, which changed the mean free path of the conduction electrons and the thermoelectric power of the couple. The parameters having the greatest effect on the thermoelectric power were the deposition rate and the substrate temperature. Couples deposited at a high deposition rate (10 000 Å min^-1) and a high substrate temperature (? 300 °C) demonstrated e.m.f. characteristics closest to bulk values.     

退火条件对磁控溅射MgO-Ag3Sb-Sb2O4柔性薄膜热电性能的影响

MgAgSb是一种具有潜力且元素储量相对丰富的室温热电材料, 有望用于构建高性能可穿戴温差电池。本研究尝试在聚酰亚胺(PI)基底上磁控溅射制备MgAgSb薄膜, 并系统研究退火条件对其热电性能的影响。结果表明样品未形成纯相的MgAgSb柔性热电薄膜, 而是形成了由Ag₃Sb、MgO及Sb₂O₄多相组成的柔性薄膜, 其中Ag₃Sb起主要热电功能。不同气氛退火可以显著提升MgO-Ag₃Sb-Sb₂O₄ (Mg-Ag-Sb)柔性薄膜的热电性能, 其中真空处理性能最佳。在真空条件下, 随着退火温度升高, 柔性薄膜的热电性能呈现先增加后减少的趋势, 当退火温度为573 K时热电性能最佳, 室温附近功率因子达到74.16 μW∙m^-1∙K^-2。并且, 薄膜表现出较好的柔性, 弯曲900次后, 电导率仅变化了14%。本研究为MgAgSb柔性热电薄膜的制备及可穿戴应用提供了参考。     

Thickness-dependent thermoelectric power factor of polymer-functionalized semiconducting carbon nanotube thin films

The effects of polymer structures on the thermoelectric properties of polymer-wrapped semiconducting carbon nanotubes have yet to be clarified for elucidating intrinsic transport properties. We systematically investigate thickness dependence of thermoelectric transport in thin films containing networks of conjugated polymer-wrapped semiconducting carbon nanotubes. Well-controlled doping experiments suggest that the doping homogeneity and then in-plane electrical conductivity significantly depend on film thickness and polymer species. This understanding leads to achieving thermoelectric power factors as high as 412 μW m^?1 K^?2 in thin carbon nanotube films. This work presents a standard platform for investigating the thermoelectric properties of nanotubes.     

Preparation and Properties of Amorphous NiFe/Cu/NiFe Thin Films

The amorphous of Permalloy on the copper subtract was studied using composite electroplating method. A portion of hydrogen brings the counteraction on the surface of cathode leading nickel-iron alloys to be anomalous in the process of co-depositing. The results of X-ray diffraction (XRD) show that the Ni-Fe alloys layer is amorphous. The Giant Magneto -Impedance (GMI) effect of Ni-Fe alloys was obtained under the optimal conditions, dependence on the soft magnetic property of Ni-Fe amorphous thin film. As a result, the ratios△ Z/Z of NiFe/Cu/NiFe amorphous thin film are 30% at 40 kHz which is in low frequency. Furthermore, the GMI value of NiFe/Cu/NiFe amorphous thin film with a sandwich structure is higher than that of single-layer ferromagnetic films of the same thickness.     

The resistivity, temperature coefficient of resistivity and thermoelectric power of thin continuous metal films II: A methodology for computer processing of thin film data to extract parameters with associated error estimates

A straightforward computer-based general methodology is presented which will enable parameter values and associated error estimates to be extracted from experimental thin film data points. The methodology operates on exact thin film relationships and overcomes problems in interpreting results, such as having to resort to the use of approximate thin film relationships.

The methodology is presented within the framework of the well-known Fuchs-Sondheimer model for conduction in thin continuous metal films. However, its general nature means that it is equally applicable to other theoretical thin film models. An illustration of the methodology's use is given by applying it to a set of thin film resistive, temperature coefficient of resistivity and thermoelectric power data obtained from measurements on thin continuous copper films.     

Thermoelectric properties of Bi2Te3-In2Se3 composite thin films prepared by co-sputtering

Bi2Te3-In2Se3 films were prepared by co-sputtering followed by annealing, and their structural and thermoelectric properties were investigated. The immiscible nature of the two alloys results in precipitation of the second phase, thus leading to structures with self-assembled dots that are a few nanometers in scale. HAADF-STEM and HRTEM were used to confirm that In2Se3 nanodots that were a few nanometers in size did indeed form in the Bi2Te3 thin film. It was found that the incorporation of these nanodots can reduce the thermal conductivity of the thin film.     

Sputter deposition of multilayered structures for use in sputter depth profile calibration

The development of standards and methods for calibration and comparison of depth scales, sputter removal rates and sputter depth profiling by surface analytical techniques (e.g. AES, SIMS, RBS and X-ray absorption) requires well controlled deposition of multilayered structures. The plasma beam-sputter deposition technique was used to produce combinations of multilayered structures, consisting of metal and oxide layers on Si(100) substrates. Ni, Cr, Ag, Ta, Au, Cr₂O₃ and Ta₂O₅ films were combined and produced in three structural designs of standard reference materials (SRM's), to be used for sputter depth profiling calibration namely, single thin film types, periodically modulated multilayered thin film structures and multilayered ‘marker’ structures. Tetrode sputtering equipment (SPUTRON II¹ of Balzers) was found to be an appropriate apparatus for the deposition of the chosen materials on the small production scale. Four in situ interchangeable targets were used to make highly reproducible layers, having the required quality and especially minimal layer and inter-layer contamination. Characterization of some of the multilayered structures developed showed, that periodically modulated system Ni/Cr/Ni … with well defined repetitive profiles and interface depth resolution is primarily suited for use in sputter depth profiling calibration. This multilayered structure is issued by NBS, Washington as Standard Reference Material No 2135.     

Evidence of vacuum between buckled films and their substrates

Closed-buckling structures have been opened by a focus ion beam process. Atomic force microscopy investigations prior to and after the opening have evidenced a significant increase of the maximum deflection of the buckling structures. These experimental results have been discussed in the framework of the Föppl-Von Karman theory of thin plates and are interpreted in terms of the existence of vacuum between the delaminated film and the substrate. The critical stress for buckling to occur is consequently modified taking into account this pressure mismatch.     

Preparation of Zn(In)Se films from alloyed precursors

Single-phase cubic Zn(In)Se thin film growth by Se vapor selenization of Zn(In) alloy precursors films is described. Depositing the Zn(In) precursor film at higher substrate temperatures changes the In/Zn composition and also yields highly crystalline Zn(In)Se films. The In/Zn ratio in the selenized film is higher in comparison to that of the precursor due to differential selenization kinetics and the complex In-Se and Zn-Se reaction chemistry. The resistivity of the Zn(In)Se film depends on the In/Zn ratio. Initially, the resistivity increases with increased indium incorporation due to increased defect concentration and then decreases at higher In/Zn ratios because of lower grain-boundary effects and reduced trap density owing to improvement in film crystallinity. Treatment with vapor-phase Zn compensates for Zn vacancies in the film, reduces electrically inactive defects, and increases doping efficiency, thereby lowering the resistivities to ∼1 Ω cm. Hot-probe and thermoelectric power measurements show that all low resistive ZnSe films are n-type.     

Nonlocal plate model for nonlinear analysis of thin films on elastic foundations in thermal environments

Postbuckling, nonlinear bending and nonlinear vibration analyses are presented for a simply supported stiff thin film resting on a two-parameter elastic foundation in thermal environments. The stiff thin film is modeled as a nonlocal orthotropic plate which contains small scale effects. The elastomeric substrate with finite depth is modeled as a two-parameter elastic foundation. The thermal effects are included and the material properties of the substrate are assumed to be temperature-dependent. The governing equation that includes plate-foundation interaction is solved by a two-step perturbation technique. The numerical results reveal that the small scale parameter e₀a reduces the postbuckling equilibrium paths, the static large deflections and natural frequencies, but increases the nonlinear to linear frequency ratios of the thin film slightly. The results also reveal that the effect of the small scale parameter is significant for compressive buckling, but less pronounced for static bending and marginal for free vibration of the thin film resting on an elastic foundation.     

Structural and thermoelectric properties of HfNiSn half-Heusler thin films

The bulk thermoelectric properties of half-Heusler alloys have recently been extensively studied due to their potential as thermoelectric materials. However, only a few publications have been addressed on thin film systems. The present study investigated the structural and thermoelectric properties of HfNiSn half-Heusler alloy thin films grown at different substrate temperatures: 25 °C, 200 °C, and 400 °C. The crystalline phase and structural variation of the films were determined by X-ray diffraction and scanning electron microscopy. Polycrystalline thin films were obtained for utilizing lower substrate temperatures. The HfNiSn thin films exhibited preferred (111) orientation when substrate temperature was higher than 400 °C. The in-plane Seebeck coefficient and resistivity of HfNiSn thin films with preferred orientation were much lower than those of films without orientation. This implies the thermoelectric properties of HfNiSn alloy may exhibit anisotropic characteristics. The best Seebeck coefficient and power factor of HfNiSn thin films obtained in this work are −68 μV/K and 1.3 μW/K²cm, respectively, measured at room temperature. The effects of partial substitution of Sn by Sb on thermoelectric properties of HfNiSn thin films were also studied with a “pseudo-combinatorial” approach.     

Thermoelectric properties of zinc antimonide thin film deposited on flexible polyimide substrate by RF magnetron sputtering

Zinc-antimony binary system is one of the most promising P-type thermoelectric materials for low cost intermediate temperature thermoelectric application. In this work, zinc antimonide thin film was deposited on the flexible polyimide substrate using zinc antimonide alloy target. All the samples were annealed in argon atmosphere at different temperatures and the thermoelectric properties of all the samples were significantly boosted. X-ray diffraction results displayed that single ZnSb phase was obtained when the annealing temperature above 300 °C. The thin film annealed at 325 °C possessed the carrier concentration of 3.59 × 10¹⁹ cm^?3, which was the most optimum carrier concentration. The maximum Seebeck coefficient of 280 μV K^?1 and the maximum power factor of 2.35 × 10^?3 Wm^?1 K^?2 was obtained at 260 °C. The Seebeck coefficient and the power factor increase with the increasing of the testing temperature. The thermoelectric properties of thin film annealed at 325 °C were better than other samples.