Low Thermal Conductivity Materials Research Articles

(Invited) Template Assisted Electrochemical Deposition and High Power Factor Materials for Integrated Thermoelectric Devices

Next generation integrated micro-scale thermoelectric devices are envisioned for spot cooling of active electronic parts or as independent power supply units to exploit small hot spots within electronic circuits. In the first case, a thermoelectric cooler is added to manage the temperature stability of sensitive electronic or photonic devices, hence to remove high density heat fluxes from these critical parts. In the second case, a thermoelectric generator is intended to recover a proportion of waste heat of a hot spot, i.e. a small, but otherwise unlimited heat source, in the electronic circuits. Both cases are technologically challenging since they request for a fabrication route that is compatible with a CMOS back-end processing. But especially the latter application provides also novel requirements to the thermoelectric material. The typical paradigm of thermoelectric material's development aims towards low thermal conductivity. This is not necessarily helpful for the given example of application where the heat has to be efficiently removed from the active electronic part, utilizing a proportion of this heat for thermoelectric power generation. Hence, this paradigm has to shift towards the development of high power factor materials, rather than low thermal conductivity materials. A processing that, at least in principle, can be fully compatible with CMOS back-end technology is given by the combination of lithographic structuring and the electrochemical deposition of the functional thick films into pre-structured cavities. A material class that provides both, high power factors and sufficiently high thermal conductivity for the removal of waste heat, is given by metallic alloys. This paper will therefore provide experimental details of the processing technology of integrated micro-scale thermoelectric devices and a discussion of high power factor materials for thermoelectric devices.

Leidenfrost drops cooling surfaces: theory and interferometric measurement

When a liquid drop is placed on a highly superheated surface, it can be levitated by its own vapour. This remarkable phenomenon is referred to as the Leidenfrost effect. The thermally insulating vapour film results in a severe reduction of the heat transfer rate compared to experiments at lower surface temperatures, where the drop is in direct contact with the solid surface. A commonly made assumption is that this solid surface is isothermal, which is at least questionable for materials of low thermal conductivity, resulting in an overestimation of the surface temperature and heat transfer for such systems. Here we aim to obtain more quantitative insight into how surface cooling affects the Leidenfrost effect. We develop a technique based on Mach–Zehnder interferometry to investigate the surface cooling of a quartz plate by a Leidenfrost drop. The three-dimensional plate temperature field is reconstructed from interferometric data by an Abel inversion method using a basis function expansion of the underlying temperature field. By this method we are able to quantitatively measure the local cooling inside the plate, which can be as strong as 80 K. We develop a numerical model which shows good agreement with experiments and enables extending the analysis beyond the experimental parameter space. Based on the numerical and experimental results we quantify the effect of surface cooling on the Leidenfrost phenomenon. By focusing on the role of the solid surface we provide new insights into the Leidenfrost effect and demonstrate how to adjust current models to account for non-isothermal solids and use previously obtained isothermal scaling laws for the neck thickness and evaporation rate.

Evidence for hard and soft substructures in thermoelectric SnSe

SnSe is a topical thermoelectric material with a low thermal conductivity which is linked to its unique crystal structure. We use low-temperature heat capacity measurements to demonstrate the presence of two characteristic vibrational energy scales in SnSe with Debye temperatures θD1 = 345(9) K and θD2 = 154(2) K. These hard and soft substructures are quantitatively linked to the strong and weak Sn-Se bonds in the crystal structure. The heat capacity model predicts the temperature evolution of the unit cell volume, confirming that this two-substructure model captures the basic thermal properties. Comparison with phonon calculations reveals that the soft substructure is associated with the low energy phonon modes that are responsible for the thermal transport. This suggests that searching for materials containing highly divergent bond distances should be a fruitful route for discovering low thermal conductivity materials.

Reduction of material swelling and recovery of titanium alloys in diamond cutting by magnetic field assistance

Ultra precision machining (UPM) is extensively used to fabricate high accuracy products. However, the problematic material swelling/recovery effect due to the elastic recovery of materials in UPM remains unresolved. It causes a ragged surface and extra engineering tolerances which are unadoptable in extremely precise components. In particular to high elastic recovery rate with low thermal conductivity materials like titanium alloys, the swelling effect is intensified during machining processes. In this study, a magnetic field was superimposed on titanium alloys during the single point diamond cutting which aimed to minimize the material swelling effect on the machined surface using the magnetic field influence. In the experiments, titanium alloys were located at the center of two permanent magnets with intensity 0.02T and undergone a diamond groove cutting. The experimental results showed the material swelling/recovery on the machined surface was significantly reduced in presence of magnetic field in comparison to that of diamond cutting without a magnetic assistance; the accuracy of depth of cut, width and radius of cutting groove in a magnetic field reached satisfactorily over 98%. The proposed machining technology solves the problem of material swelling/spingback of low thermal conductivity materials by a cost-efficient way which is needless of complicated equipment.

Thermal conductivity of Bi2(SexTe1−x)3 alloy films grown by molecular beam epitaxy

We studied the thermal conductivity of Bi2Se3, Bi2Te3, and their alloy Bi2(SexTe1−x)3 at room temperature using time-domain thermoreflectance measurements. The Bi2(SexTe1−x)3 films with various concentrations of Se and Te prepared by molecular beam epitaxy on GaAs substrates were investigated to study the dependence of thermal conductivity on film composition. We observed that the Bi2(SexTe1−x)3 ternary alloys can have much lower thermal conductivity values compared to those of Bi2Se3 and Bi2Te3. These results may provide useful information for developing and engineering low thermal conductivity materials for thermoelectric applications.

New approach for delaying the internal temperature rise of fire resistant mortar made with coated aggregate

Scientifically, resistant properties of steel structures decrease rapidly with an increase of the temperature. Therefore, steel structure is protected with low thermal conductivity materials from high temperatures. Thus, in this study, for developing fire resistance mortars on the surface of steel structure, the coated aggregate was developed with the zeolite aggregate, paraffin, and aluminium hydroxide. In case of paraffin and aluminium hydroxide, they were used as the coating materials on the surface of zeolite aggregates. First, aluminium hydroxide was coated on the surface of the zeolite aggregate with polymer as the bonding material. After that, paraffin was coated on the surface of coated aggregate with aluminium hydroxide. After the coated aggregate were developed, physical and chemical experiments were conducted. The results of the mechanical experiments presented that compressive strength of specimens with coated aggregates was decreased in comparison with the general specimen. In addition, the bonding strength test indicated that every specimen with or without coated aggregates showed similar results. However, fire resistance properties of specimens with coated aggregates presented that it has a lower thermal conductivity and internal temperature than the plain specimen. Especially, the specimen with coated aggregates was delayed at 17min compared with the plain specimen on the increase of the internal temperature around 200°C in the mortar. Therefore, it was confirmed that coated aggregates developed have an effect to decrease the internal temperature and delay time of the increase of the heat by reacting coating materials with heat.

Thermal contact theory for estimating the thermal conductivity of nanofluids and composite materials

Recently, extensive researches are going on the heat transfer enhancements of low thermal conductivity materials by adding a few percentage of high thermal conductivity materials, especially in the topic of the nanofluids and polymers. Also, metal meshes have been used in the thermal storage systems to enhancing the thermal conductivity of the systems. A few theories have been suggested on the heat transfer enhancement in nanofluids or composite materials. However, the discrepancies in the published experimental data are wide and results are not conclusive. In this work, the most popular models are discussed and critically analyzed. Also, a model is developed based on thermal resistance analysis and compared with those models. Furthermore, numerical analyses are performed to closely understand the effect of adding a few percentage of high thermal conductive material to a low thermal conductive material. The findings are very interesting, where more than 30% enhancements can be achieved if a small percentage of high thermal conductivity of material embedded in a low thermal conductivity materials, provided that the particles are long enough and align with the temperature gradient. However, if the particles align perpendicular to the temperature gradient, the heat transfer enhancement is insignificant. Hence, aspect ratio and orientation of the particles are the defining factors in heat transfer enhancement.

Thermoelectric functionality of Ca3Co4O9 epitaxial thin films on yttria-stabilized zirconia crystalline substrate

Abstract Layered cobaltite Ca3Co4O9 is one of the most interesting materials for energy conversion due to its excellent thermoelectric properties and stability at high temperatures. However, the best performance of this material is achieved for single crystals and epitaxial films. Within this framework, an important role is played by the substrate, which should promote epitaxial growth and that should have suitable thermal properties for its integration in devices. In this work, we report on the preparation of epitaxial thin films of Ca3Co4O9 by sputtering using low cost ceramic target on cubic yttria-stabilized zirconia (YSZ(100)) that is typically the standard for low thermal conductivity material and on hexagonal sapphire (Al2O3(0001)). The quality of the epitaxial films and its microstructure has been studied to explain the thermoelectric properties obtained for both substrates. YSZ(100) substrate provides Ca3Co4O9 films with enhanced Seebeck coefficient and Al2O3(0001) allows improved conductive Ca3Co4O9 films that finally, at 200 °C, results in a substrate-modified Z′T figure of merit one order of magnitude higher for Ca3Co4O9 on YSZ(100) than on Al2O3(0001) substrate.

High throughput combinatorial method for fast and robust prediction of lattice thermal conductivity

The lack of computationally inexpensive and accurate ab-initio based methodologies to predict lattice thermal conductivity, without computing the anharmonic force constants or time-consuming ab-initio molecular dynamics, is one of the obstacles preventing the accelerated discovery of new high or low thermal conductivity materials. The Slack equation is the best alternative to other more expensive methodologies but is highly dependent on two variables: the acoustic Debye temperature, θa, and the Grüneisen parameter, γ. Furthermore, different definitions can be used for these two quantities depending on the model or approximation. In this article, we present a combinatorial approach to elucidate which definitions of both variables produce the best predictions of the lattice thermal conductivity, κl. A set of 42 compounds was used to test the accuracy and robustness of all possible combinations. This approach is ideal for obtaining more accurate values than fast screening models based on the Debye model, while being significantly less expensive than methodologies that solve the Boltzmann transport equation.

Grinding Characteristics of Waspaloy with High-Speed Reciprocation Grinding

Grinding machines that employ a high-speed reciprocating worktable or wheel head are known as high-speed reciprocation grinding. Grinding heat generated in the high-speed reciprocation grinding process is low. Therefore, it is considered that the high-speed reciprocation grinding is suitable for grinding material of low thermal conductivity. In this study, the high-speed reciprocation grinding is applied to grind nickel-based super alloy “Waspaloy” which is known as difficult-to-cut material. And grinding characteristics of Waspaloy are investigated. As a result, it is found that the effect of grinding heat is smaller than the effect of grain depth of cut in grinding process of Waspaloy.

Thermal conductivity of silicon nanocrystals and polystyrene nanocomposite thin films

Silicon nanocrystals (SiNCs) are well known for their size-dependent optical and electronic properties; they also have the potential for low yet controllable thermal properties. As a silicon-based low-thermal conductivity material is required in microdevice applications, SiNCs can be utilized for thermal insulation. In this paper, SiNCs and polymer nanocomposites were produced, and their thermal conductivity, including the density and specific heat, was measured. Measurement results were compared with thermal conductivity models for composite materials, and the comparison shows a decreasing value of the thermal conductivity, indicating the effect of the size and presence of the nanostructure on the thermal conductivity. Moreover, employing silicon inks at room temperature during the fabrication process enables a low cost of fabrication and preserves the unique properties of SiNCs.

Defect engineering in development of low thermal conductivity materials: A review

Low thermal conductivity is the key property dominating the heat insulation ability of thermal barrier coatings (TBC). Reducing the intrinsic thermal conductivity is the major topic for developing advanced TBCs. Defect engineering has attracted much attention in seeking better TBC materials since lattice defects play a crucial role in phonon scattering and thermal conductivity reduction. Oxygen vacancies and substitutions are proven to be the most effective, while the accompanying lattice distortion is also of great importance. In this paper, recent advances of reducing the thermal conductivity of potential thermal barrier coating materials by defect engineering are comprehensively reviewed. Effects of the mass and size mismatch between the defects and the host lattice are quantitatively estimated and unconventional thermal conductivity reduction caused by the lattice distortions is also discussed. Finally, challenges and potential opportunities are briefly assessed to further minimize the thermal conductivity of TBC materials in the future.

A second generation of low thermal noise cryogenic silicon resonators

We have set up an improved vertically mounted silicon cavity operating at the zero-crossing temperature of the coefficient of thermal expansion (CTE) near 123 K with estimated thermal noise limited instability of 4 x 10-17 in the modified Allan deviation. Owing to the anisotropic elasticity of single-crystal silicon, the vertical acceleration sensitivity was minimized in situ by axially rotating the resonator with respect to the mounting frame. The control of the resonator temperature is greatly improved by using a combination of two thermal shields, monitoring with several temperature sensors, and employing low-thermal conductivity materials. The instability of the resonator stabilized laser was characterized by comparing with another low-noise system based on a 48 cm long room temperature cavity of PTB's strontium lattice clock, resulting in a modified Allan deviation of 7 x 10-17 at 100 s.

Protected steel columns vs partially encased columns: Fire resistance and economic considerations

As is commonly known, resistant properties of steel columns decrease quickly with an increase in temperature. Thus, steel columns are usually surrounded with low thermal conductivity materials that protect them at high temperatures. Another interesting alternative is the use of partially encased columns, a sort of steel-concrete composite column. Focused on fire performance and economic cost of a standard column, this paper presents a comparison study between partially encased composite columns and I-shaped steel columns with and without protection. A range of geometric cross-sections and material properties have been tested and the Pareto frontier has been used to show the cheapest columns with the best performance. The study is carried out assuming geometrical and material restrictions accepted by the European codes and imposing a constant axial load. Results show that most of studied protected columns resist around 120min before collapsing in fire conditions. The structural response of partially encased composite (PEC) columns under simulated fire conditions is good and it can be seen from this study that significant savings can be obtained with a good design of PEC column, savings around 50%. The relative costs of the three design options studied are quantified. Considerations about the geometry and materials criteria are provided. All steel sections described in this study are commercial ones, which includes European, British, American, Japanese and Russian standard steel sections.

Porous biomass fly ash-based geopolymers with tailored thermal conductivity

In this work novel, porous biomass fly ash-based-geopolymers were produced by using hydrogen peroxide as pore forming agent. This innovative material can help to mitigate the energy losses inside buildings, while simultaneously reducing the volume of generated wastes, thus contributing towards sustainable construction. Results demonstrate that the content of hydrogen peroxide controls the geopolymer physical properties such as porosity, mechanical resistance and thermal conductivity. The latter allows the production of geopolymers displaying suitable properties for thermal insulation applications. Geopolymers exhibiting thermal conductivity as low as 0.107 W/m K and a density of 560 kg/m3 were produced, showing the potential of these materials in applications requiring lightweight and low thermal conductivity materials. Besides this material is produced using a simple and cost effective approach, promoting waste valorization by reuse of biomass fly ash, which further demonstrates its potential.

Rapid thermal conductivity measurement of porous thermal insulation material by laser flash method

Measuring the thermal conductivity of a high performance thermal insulation material within a short time is challenging. In the present study, a laser flash method is adopted to measure the thermal diffusivity of porous thermal insulation material. Small sample size and short measuring time are major advantages of this method. The specific heat capacity was measured by a differential scanning calorimeter (DSC). With known density, the thermal conductivity was obtained. Applying a calibration routine, the measured thermal conductivity of the porous material is only 0.039W/(mK) at room temperature. This value is in good agreement with the data obtained using conventional method. It demonstrates that the laser flash method is a reliable technique to determine the performance of a very low thermal conductivity material.

Effect of thermal boundary conditions and thermal conductivity on conjugate heat transfer performance in pin fin arrays

The present work experimentally investigates the conjugate heat transfer performance for pin fin arrays. The geometry of pin fin arrays is typical of x/D=y/D=2.5 and H/D=1. The effect of two main factors: material thermal conductivity and thermal boundary conditions, i.e. conjugate and convective boundary conditions, are quantified by comparing the nondimensional temperature and Nusselt number.For conjugate heat transfer, models are constructed with materials with thermal conductivity ranging from 0.23Wm−1K−1 to 16Wm−1K−1. Uniform heat flux is imposed along the external wall of pin fin arrays and highly resolved temperature distributions of internal wall is obtained with steady liquid crystal, meanwhile external wall temperature is measured through thermocouples. For convective heat transfer, model is constructed with low thermal conductivity material to ensure the usage of transient liquid crystal to obtain heat transfer coefficients of the internal wall on the same configuration.Experimental data of temperature and heat transfer coefficient distribution is used as boundary conditions to conduct FEM calculations of pin fin array. Internal and external wall non-dimensional temperature distributions, as well as iso-thermal line distributions of the whole domain, are compared. Results indicate that thermal conductivity can significantly impact the heat transfer capacity, i.e. Nusselt number; meanwhile, the necessity of taking conjugate heat transfer effect is demonstrated by comparison with purely convective results.

ハイレシプロ研削法による耐熱合金の研削加工

Grinding machines that employ a high-speed reciprocating worktable or wheel head are known as high-speed reciprocation grinding. Grinding heat generated in the high-speed reciprocation grinding process is low. Therefore, it is considered that the high-speed reciprocation grinding is suitable for grinding material of low thermal conductivity. In this study, the high-speed reciprocation grinding is applied to grind nickel-based super alloy “Waspaloy” which is known as difficult-to-cut material. And grinding characteristics of Waspaloy are investigated. As a result, it is found that the effect of grinding heat is smaller than the effect of grain depth of cut in grinding process of Waspaloy.

Thermal transport properties of antimonene: an ab initio study.

Searching for low thermal conductivity materials is crucial for thermoelectric devices. Here we report on the phonon transport properties of recently fabricated single layer antimony, antimonene [Ares, et al., Adv. Mater., 2016, 28, 6332]. Ab initio calculations in combination with the Boltzmann transport equation (BTE) for phonons show that antimonene has a low lattice thermal conductivity (15.1 W m-1 K-1 at 300 K), indicating its potential thermoelectric applications. The low lattice thermal conductivity is due to its small group velocity, low Debye temperature and large buckling height. We also investigate in detail the mode contributions to total thermal conductivity and find at low frequency that the longitudinal acoustic (LA) branch dominates the thermal conductivity. Moreover, we show that the lattice thermal conductivity of antimonene can further be reduced by minimizing the sample size. Our findings open the field for thermoelectric applications based on antimonene.

Approaching extremely low thermal conductivity by crystal structure engineering in Mg2Al4Si5O18

Abstract