Low Thermal Conductivity Materials Research Articles

Parameter-free model to estimate thermal conductivity in nanostructured materials

Achieving low thermal conductivity and good electrical properties is a crucial condition for thermal energy harvesting materials. Nanostructuring offers a very powerful tool to address both requirements: in nanostructured materials, boundaries preferentially scatter phonons compared to electrons. The computational screening for low-thermal-conductivity nanostructures is typically limited to materials with simple crystal structures, such as silicon, because of the complexity arising from modeling branch- and wave-vector-dependent nanoscale heat transport. The phonon mean-free-path (MFP) dependent Boltzmann transport equation (MFP-BTE) approach is a model that overcomes this limitation. To illustrate this, we analyze thermal transport in 75 nanoporous half-Heusler compounds for different pore sizes. Our calculations demonstrate that, in most cases, the optimization of thermal transport in nanostructures should take into account both bulk thermal properties and geometry-dependent size effects, two aspects that are typically engineered separately. To enable efficient calculations within this paradigm we derive a model, based on the ``gray'' formulation of the BTE, that can decouple the influence of the geometry and the material on the effective thermal conductivity with relatively little loss in accuracy compared to the MFP-BTE. Our study motivates the need for a holistic approach to engineering thermal transport and provides a method for high-throughput low-thermal conductivity materials discovery.

Reduced-scale hot box method for thermal characterization of window insulation materials

Highly transparent and low thermal conductivity materials have attracted great interest for the applications in window insulation in recent years. Accurate characterization of the thermal properties including thermal transmittance (U-value) and thermal conductivity of window insulation materials is very important for developing next-generation materials. The conventional hot box method, which is commonly used to measure the U-values of building materials, requires sample sizes > 1.0 m2 to minimize the influence of parasitic heat loss on the measurement accuracy. This characterization challenge hinders the development of novel window materials which are not yet available for large-scale deployment. To address this issue, a reduced-scale hot box system (RHS) was designed to measure both the U-value and the thermal conductivity of specimens that can be more readily made, with sizes < 0.2 m × 0.2 m. The developed reduced-scale hot box system has a very simple testing system and can avoid the challenging thermal insulation requirement of the conventional hot box. The fast turnaround of the reduced-scale hot box system can help facilitate the development of novel insulating materials for energy-efficient windows.

Coupling the High-Throughput Property Map to Machine Learning for Predicting Lattice Thermal Conductivity

Low thermal conductivity materials are crucial for applications such as thermoelectric conversion of waste heat to useful energy and thermal barrier coatings. On the other hand, high thermal conduc...

An Assessment of LEWICE Roughness and Convection Enhancement Models

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;During aircraft design and certification, in-flight ice accretions are simulated using ice prediction codes. LEWICE, the ice accretion prediction code developed by NASA, employs a time-stepping procedure coupled with a thermodynamic model to calculate the location, size and shape of an ice accretion. LEWICE has been extensively validated for a wide range of icing conditions. However, continuing improvements to LEWICE predictive capabilities require better understandings of 1) the fundamental physics of turbulent flow generated by ice accretion roughness during an icing event and 2) the mechanisms responsible for convective enhancement of real ice accretion roughness. Recent experiments in the Icing Research Tunnel (IRT) at NASA Glenn Research Center have enabled significant insights into the nature of ice accretion roughness spatial and temporal variations. Other recent investigations have employed scans from the IRT to generate scaled test surfaces to investigate convection enhancement and skin friction interactions of flow over the surfaces with real ice roughness. For this investigation, the measurements of ice roughness characteristics, skin friction, and convection heat transfer enhancement are compared directly to predictions calculated using the LEWICE models. Issues associated with the LEWICE model for predicting laminar-to-turbulent transition are also addressed. The comparisons show that while the LEWICE models significantly overestimate equivalent sand-grain roughness heights and the local skin friction, the resulting LEWICE predictions of convection heat transfer generally agree within 25% of the experimental values for surfaces constructed with low thermal conductivity materials. At each step of the LEWICE model, methods to improve the predictions are provided.&lt;/div&gt;&lt;/div&gt;

Diffused Lattice Vibration and Ultralow Thermal Conductivity in the Binary Ln-Nb-O Oxide System.

In the pursuit of low thermal conductivity materials for thermal management, one always tries to increase the material entropy by increasing the number of components in the materials to scatter heat-carrying phonons. However, it also drastically increases the technological complexity to synthesize materials with the target complex composition. Here, a material family is presented with simple composition Ln3 NbO7 , which only contains binary oxides of Ln2 O3 (Ln = Dy, Er, Y, Yb) and Nb2 O5 . The thermal conductivities approach the theoretical minimum limit, where the large chemical inhomogeneity due to the charge disorder and fluctuation of bonding length in Ln3 NbO7 plays a major role. Despite the simple composition, Ln3 NbO7 demonstrates an unprecedentedly high scattering rate of vibration states, as confirmed by the highest elastic constant/thermal conductivity ratio, as well as the diffused wavevector-frequency dispersion. In contrast to the conventional wisdom that low thermal conductivity materials should be explored in the pool of "complex" multiple-component materials, this work points out an avenue to look into materials with simple composition but large internal chemical inhomogeneity, which would be of both scientific and technological significance in the fields of thermal barrier coating, thermoelectric materials, etc.

First-principles study of elastic, thermal and optical properties of a metal-shrouded two-dimensional semiconductor Tl2O

In recent years, two-dimensional (2D) metal-clad materials have been attracted much attention due to their unique properties. 2D-Tl2O is a newly discovered two-dimensional cells material with semiconducting properties, both 1T- Tl2O and 2H- Tl2O. In this work, we study the elastic, thermal and optical properties of single-layer 2D-Tl2O based on first-principles calculations. The elastic properties of 1T-Tl2O and 2H-Tl2O are found to be isotropic and have lower in-plane Young's modulus 28.03 N/m and 33.46 N/m respectively, suggesting the flexible nature. By solving the phonon Boltzmann equation, we note that 1T-Tl2O and 2H-Tl2O are the low thermal conductivity materials, and their lattice thermal conductivities at room temperature are 0.49 W/mK and 1.13 W/mK, respectively. It is found that the thermal conductivity is mainly contributed by the acoustic support, but the influence of optical support can not be ignored. The optical properties are obtained using the DFT + RPA method. Its optical absorption and reflection regions are mainly located in the ultraviolet region and have multiple reflection or absorption peaks. In contrast, 2H-Tl2O has the largest energy loss than 1T-Tl2O. Our results will provide a reference to subsequent theoretical and experimental research.

Sustainable and efficient cork - inorganic polymer composites: An innovative and eco-friendly approach to produce ultra-lightweight and low thermal conductivity materials

In this investigation cork was used as a low density aggregate in the production of ultra-lightweight and low thermal conductivity inorganic polymer (geopolymer) composites. This novel and highly sustainable material, synthesised at room temperature (23 °C), may decrease the energy losses inside buildings, thus contributing to the United Nations development goals regarding energy and climate change. The ultra-low density (260 kg/m3) and low thermal conductivity (72 mW/m K) shown by the cork-composites are the second lowest ever reported for inorganic polymer composites, only being surpassed by that of polystyrene-inorganic polymer composites. However, cork is a fully renewable and sustainable resource, while polystyrene is manufactured from non-renewable fossil fuels, and for that reason our strategy has an additional sustainability advantage. Moreover, the cork-inorganic polymer composites do not release any toxic fume when under fire conditions, which is a major advantage over polymeric-based foams.

Waste-based geopolymeric mortars with very high moisture buffering capacity

In this study, lightweight waste-based geopolymeric mortars were evaluated for the first time regarding their potential to passively adjust indoor relative humidity (RH) levels. Geopolymer mortars were prepared using a mixture of fly ash (FA) and metakaolin (MK) as a binder, in a proportion of 75:25 wt% (FA:MK), construction and demolition waste as the fine aggregate and a pore forming agent in varying amounts. The results showed that the addition of a pore-forming agent to the compositions considerably increased the moisture buffer value (MBV) of the mortars, that is, from 0.80 (reference mortar) to 5.61 g/m2 Δ%RH (mortar with highest porosity). The moisture buffering capacity shown by these eco-friendly mortars is higher than values reported for other binder materials and can be classified as excellent (MBV > 2 g/m2 Δ%RH). The porous FA-based mortars also presented low thermal conductivity (as low as 0.19 W/m∙K), which suggests that these innovative binders could be simultaneously used for indoor moisture buffering and as low thermal conductivity materials.

Thermal Performance Optimization in Electric Vehicle Power Trains by Locally Orthotropic Surface Layer Design

In this paper, the application of orthotropic material orientation optimization for controlling heat flow in electric car power trains is presented. The design process is applied to a case model, which conducts heat while storing heat-sensitive electronic components. The core of the case is designed using a low thermal conductivity material on order to focus the heat flow into the surface layer, which is designed using a high thermal conductivity material. Material orthotropy is achieved in the surface layer of the case by removing the material at points determined by the optimization analysis. For this purpose, an orthotropic material orientation optimization method was extended to calculate optimal material distribution. This is achieved by transforming the initially obtained optimal orientation vector field into a scalar field through the use of coupled time-dependent nonisotropic Helmholtz equations. Multiple parameters allow the control of the scalar field and therefore the control over material distribution in accordance to the optimal orientation. This allows the material distribution pattern to be scaled depending on the desired manufacturing method. The analysis method is applied to divert heat flow from a specific section of the model while focusing the heat flow to another section. The results are shown for a model with a 0.1 mm thick surface layer of copper and are compared to those results from several other materials and layer thicknesses. Finally, the manufactured design is presented.

Analytical Multistage Thermal Model for FEOL Reliability Considering Self-and Mutual-Heating

Aggressively scaled devices with high power density wrapped around the low thermal conductivity material in narrow space are susceptible to self-heating effect, which includes serious mutual-heating due to proximity effect in multifinger/fin FETs. Obtaining local temperature instead of equivalent lumped temperature at device/circuit level quickly is crucial for designers to accurately predict front-end-of-line reliability (i.e., hot carrier injection, bias temperature instability, and time dependent dielectric breakdown) and achieve better thermal management. In this paper, we: 1) first propose an analytical model based on multistage thermal network to address effectively the self- and mutual-heating within the device; 2) validate the multistage network and proposed analytical model by COMSOL and SPICE simulation, respectively; and 3) utilize the model to investigate local temperature rise in multifinger and multifin FETs-based inverter and ring oscillator as well as provide thermal-aware IC design guideline for self-heating and reliability optimization. The results show that the analytical multistage thermal model can predict local temperature rise and quantify nonuniform temperature difference in full-frequency range, superior to the lumped thermal model. Moreover, the model can offer quick thermal analysis at the pre-Si stage for reliability prediction and thermal-aware design.

Image processing based quantitative damage evaluation in composites with long pulse thermography

Pulsed thermography is a contactless and rapid non-destructive evaluation (NDE) technique that is widely used for the inspection of fibre reinforced plastic composites. However, pulsed thermography uses expensive and specialist equipment such high-energy flash lamps to generate heat into the sample, so that alternative thermal stimulation sources are needed. Long pulse thermography was recently developed as a cost-effective solution to enhance the defect detectability in composites by generating step-pulse heat into the test sample with inexpensive quartz halogen lamps and measuring the thermal response during the material cooling down. This paper provides a quantitative comparison of long pulse thermography with traditional pulsed thermography and step heating thermography in carbon fibre and glass fibre composites with flat-bottomed holes located at various depths. The three thermographic methods are processed with advanced thermal image algorithms such as absolute thermal contrast, thermographic signal reconstruction, phase Fourier analysis and principal component analysis in order to reduce thermal image artefacts. Experimental tests have shown that principal component analysis applied to long pulse thermography provides accurate imaging results over traditional pulsed thermography and step heating thermography. Hence, this inspection technique can be considered as an efficient and cost-effective thermographic method for low thermal conductivity and low thermal response rate materials.

Decoupled trends for electrical and thermal conductivity in phase-confined CNT co-continuous blends

In the present work, the morphology and the electrical and thermal conduction properties of co-continuous poly(vinylidene fluoride) (PVDF), maleated polypropylene (PPgMA) and multiwall carbon nanotubes (CNT) nanostructured blends are investigated. CNT preferentially locates in the PPgMA phase and clearly causes a refinement in the co-continuous structure. Electrical conductivity experiments show that nanocomposites are well above the percolation threshold and evidence for one order of magnitude enhancement in conductivity for the co-continuous nanocomposites compared to the monophasic nanocomposites with the same CNT volume fraction. On the other hand, thermal diffusivity enhancement for the co-continuous blends is found lower than that for the monophasic nanocomposites at the same CNT volume fraction. An explanation is proposed in terms of large interfacial area, causing phonon scattering at the interface between immiscible PVDF and PPgMA domains. Results described in this paper open the way to the preparation of high electrical and low thermal conductivity materials with possible application as thermoelectrics.

Superhydrophobic fluorine-rich conjugated microporous polymers monolithic nanofoam with excellent heat insulation property

The exploitation of multifunctional materials with both low thermal conductivity and high flame retardant property is of great importance for construction of modern energy-effective buildings. Here, we report the design and synthesis of fluorine-rich conjugated microporous polymers (FCMPs) monolithic nanofoam through a Sonogashira–Hagihara coupling reaction. The FCMPs monolithic nanofoam exhibits a high specific surface area of up to 1194 m2 g−1 and interestingly superhydrophobic wettability with a water contact angle of 160°. The FCMPs monolithic nanofoam possesses a low thermal conductivity of 0.021 W m−1 K−1, which can compete with the most of low thermal conductivity materials such as silicon aerogel (0.02 W m−1 K−1), indicating excellent thermal insulation capability. Microcalorimetry (MCC) test results show that the FCMPs monolithic nanofoam possesses excellent flame retardant performance with a low peak heat release rate (pHRR) value of 10.4–11.2 W g−1, which is nearly one order magnitude lower than that of the flame retardant materials reported so far. Compared with those brittle inorganic thermal insulation materials as well as the CMPs usually are formed as unprocessable powder, the elastomeric FCMPs monolithic nanofoam is flexible and mechanical robust, which in turn could be engineered as different shapes for practical applications. Taking these advantages, the FCMPs monolithic nanofoam may have great potentials as anti-proofing, novel heat insulation materials with better flame retardant properties for modern building construction or other energy-efficient coatings.

Thermal bridges in building envelopes – An overview of impacts and solutions

Increasing building energy performance has become an obligatory objective in many countries. Thermal bridge is a major cause of poor energy performance, durability, and indoor air quality of buildings. This paper starts with a review of thermal bridges and their negative impacts on building energy efficiency. Based on published literatures, various types of building thermal bridges are discussed in this paper, including the most effective solutions to diminish their impacts. In addition, various numerical and experimental studies on the balcony thermal bridge are explored. Results show that among all types of thermal bridges, the exposed balcony slab produces the most challenging thermal bridging problem where an integrated thermal and structural design is required. Using low thermal conductivity materials in building construction could help in reducing the impact of thermal bridges. Finally, further investigations are needed to develop more innovative and effective solutions for the balcony thermal bridge.

Revisiting PbTe to identify how thermal conductivity is really limited

Due to the long range interaction in lead telluride (PbTe), the transverse optical (TO) phonon becomes soft around the Brillouin zone center. Previous studies have postulated that this zone-center softening causes the low thermal conductivity of PbTe through either enlarged phonon scattering phase space and/or strengthened lattice anharmonicity. In this work, we reported an extensive sensitivity analysis of the PbTe thermal conductivity to various factors: range and magnitude of harmonic and anharmonic interatomic force constants, and phonon wavevectors in the three-phonon scattering processes. The analysis reveals that the softening by long range harmonic interaction itself does not reduce thermal conductivity and it is the large magnitude of the anharmonic (cubic) force constants that realizes low thermal conductivity, however, not through the TO phonons around the zone center but dominantly through the ones with larger wavevectors in the middle of Brillion zone. The work clarifies that local band softening cannot be a direct finger print for low thermal conductivity and the entire Brillion zone needs to be characterized on exploring low thermal conductivity materials.

Heat transfer experiment in a partially (internally) blocked 19-rod bundle with wire spacers cooled by LBE

Internal blockages in fuel rod bundles can be rather small and may have a negligible global effect on the flow rate in a fuel assembly, and yet they can still produce significant local hot spots potentially leading to clad failure. Within the European project MAXSIMA the thermal-hydraulic effects of internal flow blockages in a heated rod bundle cooled by lead-bismuth eutectic (LBE) are investigated in prototypical experiments. This work builds up on previous experiments in an unblocked 19-rod bundle with wire spacers within the project SEARCH.In all cases, operating conditions representative of the scenarios expected in the MYRRHA reactor, developed by SCK• CEN in Belgium, are imposed. Similarly the characteristics of the blockage scenarios are selected according to the worst-case postulated events, in a conservative approach. Only solid blockages are installed, while a certain porosity can be expected if they are formed by accumulation of particles. Moreover, they are constructed using a filling material of low thermal conductivity, instead of being metallic. All blockage elements have the same length (54.6 mm, i.e. one sixth of the wire pitch) but different position and size. A first scenario includes two small blockages covering one central and edge sub-channel each. In the second scenario, a large blockage covering six sub-channels surrounding the central rod is investigated.The experimental results are presented following two approaches. First, selected reference cases are described in detail, allowing for a direct comparison with numerical simulations. Second, the results for all cases are correlated in dimensionless terms. An empirical model named Karlsruhe Blockage Equation (KBE) is proposed for representing the heat transfer resistance at the hottest spots in the blocked region for each element. Extrapolating these results to full-power conditions, the temperature increase produced by solid blockages with low thermal conductivity can be acceptable if they are small and excessive if they are larger. Further results include the wall and fluid temperature in the wake region and differential pressure in blocked and unblocked sections of the bundle.

On the Steady-State Temperature Rise During Laser Heating of Multilayer Thin Films in Optical Pump–Probe Techniques

In this study, we calculate the steady-state temperature rise that results from laser heating of multilayer thin films using the heat diffusion equation. For time- and frequency-domain thermoreflectance (TDTR and FDTR) that rely on modulated laser sources, we decouple the modulated and steady-state temperature profiles to understand the conditions needed to achieve a single temperature approximation throughout the experimental volume, allowing for the estimation of spatially invariant thermal parameters within this volume. We consider low thermal conductivity materials, including amorphous silicon dioxide (a-SiO2), polymers, and disordered C60, to demonstrate that often-used analytical expressions fail to capture this temperature rise under realistic experimental conditions, such as when a thin-film metal transducer is used or when pump and probe spot sizes are significantly different. To validate these findings and demonstrate a practical approach to simultaneously calculate the steady-state temperature and extract thermal parameters in TDTR, we present an iterative algorithm for obtaining the steady-state temperature rise and measure the thermal conductivity and thermal boundary conductance of a-SiO2 with a 65-nm gold thin film transducer. Furthermore, we discuss methods of heat dissipation to include the use of conductive substrates as well as the use of bidirectional heat flow geometries. Finally, we quantify the influence of the optical penetration depth (OPD) on the steady-state temperature rise to reveal that only when the OPD approaches the characteristic length of the temperature decay does it alter the temperature profile relative to the surface heating condition.

Home Appliances Efficiency Improvements for Energy Conservation in Debre Berhan City; Ethiopia

This paper presents the analysis of efficiency improvements of home appliances such as Electrical Stove (Injera Mitad), Electrical Showers, Kerosene Stoves, Charcoal Stoves and Small Electrical Stoves for energy conservation. Most of the existing stoves have been manufactured by small facilities without any regard for energy efficiency standard and safety. It suffers from many shortfalls, such as, poor insulation, lack of temperature regulation, bulkiness and overall poor design that encourages wastage of heat. The energy efficiency of the ordinary electric stove is very low since much of the heat energy is lost through the bottom, its sides, on the top part of the oven and also due to the much delay in the existing Injera baking procedure. The overall efficiency of the electrical shower is affected by the reservoir efficiency and the characteristics of the pipe. Kerosene and charcoal stoves have high heat energy loss due to lack of insulation to their external cover. Therefore, the efficiency of those electrical home appliances can be improved by minimizing the losses through their covers and parts. This can be achieved by selecting appropriate low thermal conductivity materials and insulations.

Thermal conductivity anisotropy in holey silicon nanostructures and its impact on thermoelectric cooling

Artificial nanostructures have improved prospects of thermoelectric systems by enabling selective scattering of phonons and demonstrating significant thermal conductivity reductions. While the low thermal conductivity provides necessary temperature gradients for thermoelectric conversion, the heat generation is detrimental to electronic systems where high thermal conductivity are preferred. The contrasting needs of thermal conductivity are evident in thermoelectric cooling systems, which call for a fundamental breakthrough. Here we show a silicon nanostructure with vertically etched holes, or holey silicon, uniquely combines the low thermal conductivity in the in-plane direction and the high thermal conductivity in the cross-plane direction, and that the anisotropy is ideal for lateral thermoelectric cooling. The low in-plane thermal conductivity due to substantial phonon boundary scattering in small necks sustains large temperature gradients for lateral Peltier junctions. The high cross-plane thermal conductivity due to persistent long-wavelength phonons effectively dissipates heat from a hot spot to the on-chip cooling system. Our scaling analysis based on spectral phonon properties captures the anisotropic size effects in holey silicon and predicts the thermal conductivity anisotropy ratio up to 20. Our numerical simulations demonstrate the thermoelectric cooling effectiveness of holey silicon is at least 30% greater than that of high-thermal-conductivity bulk silicon and 400% greater than that of low-thermal-conductivity chalcogenides; these results contrast with the conventional perception preferring either high or low thermal conductivity materials. The thermal conductivity anisotropy is even more favorable in laterally confined systems and will provide effective thermal management solutions for advanced electronics.

Quantitative study of free convective heat losses from thermodynamic partitions using Thermal Imaging

The following paper presents a simple method of determining the presence, distribution and values of heat losses from external building walls as thermodynamic partitions using a Thermal Imaging Camera (TIC).According to Fourier's equation, the value of heat loss is proportional to the temperature gradient ∂t/∂y|y=0 in air in the y direction perpendicular to the heated surface. Unfortunately, air temperature cannot be measured with a TIC, as gases do not emit thermal radiation. It is therefore suggested that a grid placed vertically in the air and perpendicularly to the heated surface should be used as a detector of temperature field. Warmed by convective air flows, such a grid becomes a source of infrared radiation. The temperature distribution reproduced on the grid constructed from thin strands of a low thermal conductivity material is sufficiently distinct and sharp to be recorded with a TIC.The correctness of the TIC method has been tested in laboratory conditions on the example of a vertical heating plate, as well as in real-life conditions − on a single pane of glass serving as a building partition. When compared to the works of other authors, the result obtained as Nu = 0.571·Ra0.25 relation reveals a satisfactory correlation for the heating plate, while estimated temperature distributions and heat fluxes from the room to the surface of thermodynamic partition wall Qloss,conv,in and on the outside of the building Qloss,conv,ext successfully and quite precisely correlate (5–7% for both internal and external sides) with theoretical calculations.Therefore, the TIC method proposed in this paper can significantly extend the range of application of TICs in energy audits, especially for buildings