Heat Flow Control Research Articles

Magnetic Control of Flexible Thermoelectric Devices Based on Macroscopic 3D Interconnected Nanowire Networks

AbstractSpin‐related effects in thermoelectricity can be used to design more efficient refrigerators and offer promising applications for the harvesting of thermal energy. The key challenge is to design structural and compositional magnetic material systems with sufficiently high efficiency and power output for transforming thermal energy into electric energy and vice versa. The fabrication of large‐area 3D interconnected Co/Cu nanowire networks is demonstrated, thereby enabling the controlled Peltier cooling of macroscopic electronic components with an external magnetic field. The flexible, macroscopic devices overcome the inherent limitations of nanoscale magnetic structures that are caused by insufficient power generation capability limiting the heat management applications. From properly designed experiments, large spin‐dependent Seebeck and Peltier coefficients of −9.4 µV K−1 and −2.8 mV at room temperature, respectively, are found. The resulting power factor of Co/Cu nanowire networks at room temperature (≈ 7.5 mW K−2 m−1) is larger than those of state‐of‐the‐art thermoelectric materials, such as BiTe alloys, and the magnetopower factor ratio reaches about 100% over a wide temperature range. Validation of magnetic control of heat flow achieved by taking advantage of the spin‐dependent thermoelectric properties of flexible macroscopic nanowire networks lays the groundwork to design shapeable thermoelectric coolers exploiting the spin degree of freedom.

A study on heat storage sizing and flow control for a domestic scale solar-powered organic Rankine cycle-vapour compression refrigeration system

This paper presents the off-design modelling of a domestic scale solar organic Rankine cycle (ORC) and vapour compression cycle (VCC) in a coupled operation in different operating modes by using evacuated flat plate (EFP) collectors. Thermodynamic and parametric studies of such coupled system in literature usually assume that the isentropic efficiencies of expander and compressor and the heat exchanger pinch temperature differences are constant. Moreover, studies for directly coupling the ORC-VCC system with solar collectors are somewhat rare. Transient performance of the solar ORC-VCC considering the off-design behaviour of the system components needs to be investigated. A simulation for a period of 24 hrs is conducted by considering the electricity and cooling demand of a 60 m2 office building during a typical day in July for Istanbul. The effect of heat storage capacity on meeting the demand is investigated for a given area of EFP collectors. Moreover, the water flow rate to the boiler is controlled periodically to provide the demanded electricity and cooling. The required EFP collector area and heat storage unit volume have been determined to be 80 m2 and 9.4 m3, respectively. For these design parameters, 25.6 kWh cooling and 18.76 kWh electricity can be generated during a typical day in July. Finally, a simulation is given for such design on a sunny winter day in February, 5.5 kWh electricity and 104 kWh heat can be produced.

Ion Write Microthermotics: Programing Thermal Metamaterials at the Microscale.

Considerable advances in manipulating heat flow in solids have been made through the innovation of artificial thermal structures such as thermal diodes, camouflages, and cloaks. Such thermal devices can be readily constructed only at the macroscale by mechanically assembling different materials with distinct values of thermal conductivity. Here, we extend these concepts to the microscale by demonstrating a monolithic material structure on which nearly arbitrary microscale thermal metamaterial patterns can be written and programmed. It is based on a single, suspended silicon membrane whose thermal conductivity is locally, continuously, and reversibly engineered over a wide range (between 2 and 65 W/m·K) and with fine spatial resolution (10-100 nm) by focused ion irradiation. Our thermal cloak demonstration shows how ion-write microthermotics can be used as a lithography-free platform to create thermal metamaterials that control heat flow at the microscale.

Thermal transport in layer-by-layer assembled polycrystalline graphene films

New technologies are emerging which allow us to manipulate and assemble 2-dimensional (2D) building blocks, such as graphene, into synthetic van der Waals (vdW) solids. Assembly of such vdW solids has enabled novel electronic devices and could lead to control over anisotropic thermal properties through tuning of inter-layer coupling and phonon scattering. Here we report the systematic control of heat flow in graphene-based vdW solids assembled in a layer-by-layer (LBL) fashion. In-plane thermal measurements (between 100 K and 400 K) reveal substrate and grain boundary scattering limit thermal transport in vdW solids composed of one to four transferred layers of graphene grown by chemical vapor deposition (CVD). Such films have room temperature in-plane thermal conductivity of ~400 Wm−1 K−1. Cross-plane thermal conductance approaches 15 MWm−2 K−1 for graphene-based vdW solids composed of seven layers of graphene films grown by CVD, likely limited by rotational mismatch between layers and trapped particulates remnant from graphene transfer processes. Our results provide fundamental insight into the in-plane and cross-plane heat carrying properties of substrate-supported synthetic vdW solids, with important implications for emerging devices made from artificially stacked 2D materials.

Thermal Propagation Control Using a Thermal Diffusion Equation

Controlling heat flow is necessary in many fields, such as the communications, health care, and manufacturing industries. In the communication field, the techniques for the reproduction of thermal sensations have been researched for many years. In order to extend the area of controlling the heat, the thermal diffusion in time and space needs to be considered. However, attaining precise control based on conventional methods is difficult because it requires several sensors and devices to monitor and control the temperature distribution. In this study, the thermal system consists of a metal wire and a Peltier device attached to the end of the wire, and a one-dimensional thermal propagation system is constructed. Modeling the thermal diffusion equation, it was found that the thermal system has characteristics of damping and propagation delay. Therefore, a control method using these system characteristics is proposed. The thermal system is treated as a time delay system with damping, which is compensated by a time delay and damping compensator. The thermal diffusion through the system can be considered by the proposed method. Experimental results show the viability of the proposed control system.

An electrochemical thermal transistor

The ability to actively regulate heat flow at the nanoscale could be a game changer for applications in thermal management and energy harvesting. Such a breakthrough could also enable the control of heat flow using thermal circuits, in a manner analogous to electronic circuits. Here we demonstrate switchable thermal transistors with an order of magnitude thermal on/off ratio, based on reversible electrochemical lithium intercalation in MoS2 thin films. We use spatially-resolved time-domain thermoreflectance to map the lithium ion distribution during device operation, and atomic force microscopy to show that the lithiated state correlates with increased thickness and surface roughness. First principles calculations reveal that the thermal conductance modulation is due to phonon scattering by lithium rattler modes, c-axis strain, and stacking disorder. This study lays the foundation for electrochemically-driven nanoscale thermal regulators, and establishes thermal metrology as a useful probe of spatio-temporal intercalant dynamics in nanomaterials.

Spectrally Resolved Specular Reflections of Thermal Phonons from Atomically Rough Surfaces

New experiments provide evidence that terahertz thermal phonons undergo specular surface reflections at room temperature and are sensitive to surface imperfections of just a few atoms, a key insight that is needed for novel methods of controlling heat flow.

Novel aspects for thermal stability studies and shelf life assessment of modified double-base propellants

Abstract Modified DB propellants, based on energetic nitramine (RDX) were manufactured by solventless extrusion process. Thermal stability and shelf life assessment of modified DB propellant were investigated. Shelf life assessment was evaluated using Van't Hoff's formula and artificial aging at 70 °C up to 120 days. Quantification of total heat released and heat flow with aging time was conducted using differential scanning calorimetry (DSC) and thermal activity monitoring (TAMIII) respectively. Modified DB formulation based on 20 wt % RDX demonstrated enhanced thermal stability in terms of controlled heat flow, and slow decomposition reactions at elevated temperature. This formulation demonstrated extended service life up to 56 years compared with reference formulation. These novel finding was ascribed to the high thermal stability of RDX and its compatibility with DB constituents. This manuscript shaded the light on novel and effective approach for thermal stability via monitoring thermal activity with aging.

Modulating thermal conduction via phonon spectral coupling

We report an approach to modulate thermal conduction that utilizes phonon coupling in layered nanostructures. While phonon coupling has been used previously to enhance thermal transport of an embedded layer in a tri-layer structure, the impact of coupling on cladding layers has remained unclear. Here, we develop a methodology to quantitatively evaluate the impact of phonon coupling on each layer in a tri-layer structure. We uncover that the underlying phonon-injection mechanism behind thermal conductivity enhancement can also be leveraged to reduce the thermal conductivity of an embedded silicon thin-film below its free-standing value. We evaluate the dependence of resultant thermal conductivity modulations on structural parameters and find that they are critically dependent on layer spacings and interface properties. We also extend the tri-layer transport analysis to bi-layer structures and report how phonon coupling leads to analogous thermal conductivity modulations. The results of this work open new avenues within the rational thermal design by elucidating a new method that can be used to both increase and reduce thermal conductivities and advance the basic understanding of nanoscale thermal transport by incorporating the role of phonon spectral coupling. The prospects of being able to modulate the thermal conductivity can radically change how we control heat flow in electronic, optoelectronic, and thermoelectric 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.

Thermal rectification in Y-junction carbon nanotube bundle

Active control of heat flow is one of the important concepts in phononic devices, among which thermal diode is a fundamental building block. The long-standing bottleneck is the relevant experiments lagging far behind theoretical results. In this paper, we experimentally demonstrated considerable thermal rectification in the Y-junction carbon nanotube (CNT) bundle with suspended thermal bridge method. The thermal rectification ratio is up to ∼8.3% ± 0.5% with a relatively low temperature difference (ΔT = 4K). Molecular dynamics simulation results show that asymmetric phonon transmission in different (forward and backward) directions is responsible for the thermal rectification observed in the asymmetric CNT structure.

Thermal Rectification via Heterojunctions of Solid-State Phase-Change Materials

Nonlinear thermal transport could provide an avenue to controlling heat flow, and a basis for innovative thermal devices. This study offers a theoretical prescription for realizing thermal rectification via heterojunctions of phase-change materials, a long-sought result in thermal physics. This general theory handles the complex heat flow arising at such heterojunctions, and indicates that a diode comprised of known phase-change materials should exhibit a thermal rectification ratio surpassing that of any individual material by a factor of 10.

Concept and method to create a distinct low-temperature region in a Si melt for growth of a Si single ingot with a large diameter ratio using the noncontact crucible method

The noncontact crucible (NOC) method is defined as a growth method to intentionally establish a distinct low-temperature region inside a Si melt. For the NOC growth of a uniform large Si single ingot without contact with the crucible wall, a large and deep low-temperature region must be created in the upper central part of a Si melt to allow natural crystal growth inside it. The novel method to create the low-temperature region has not been disclosed until now. In this paper, the concept and method are clearly shown on the basis of controlling the heat flow from the bottom heater into the Si melt using an insulating plate locally set under the crucible bottom. The effectiveness of such heat-flow control is proved by the growth of several Si single ingots with a large diameter ratio using the NOC method. The effects of the size and thickness of the insulating plate is also shown by the growth of Si single ingots.

Dynamic optical control of near-field radiative transfer.

Dynamic control of radiative heat transfer is of fundamental interest as well as for applications in thermal management and energy conversion. However, realizing high contrast control of heat flow without moving parts and with high temporal frequencies remains a challenge. Here, we propose a thermal modulation scheme based on optical pumping of semiconductors in near-field radiative contact. External photo-excitation of the semiconductor emitters leads to increases in the free carrier concentration that in turn alters the plasma frequency, resulting in modulation of near-field thermal radiation. The temporal frequency of the modulation can reach hundreds of kHz limited only by the recombination lifetime, greatly exceeding the bandwidth of methods based on temperature modulcation. Calculations based on fluctuational electrodynamics show that the heat transfer coefficient between two silicon films can be tuned from near zero to 600 Wm-2K-1 with a gap distance of 100 nm at room temperature.

Thermal Transport across a Semiconductor/Semiconductor Interface: A First-Principles-Based Approach

Controlling heat flow across material heterojunctions is important for thermoelectrics and thermal management of electronic devices. A clear physical understanding of what transport mechanism dominates near an interface can help benefit technology. We present a theoretical investigation of phonon transport across a semiconductor/semiconductor interface, specifically Si/Ge, and demonstrate how inelastic scattering and non-equilibrium effects play a key role. We treat phonon transport with the McKelvey-Shockley flux method, which is efficient, captures ballistic and non-equilibrium effects, inelastic scattering, and has shown excellent agreement with the more computationally-demanding Boltzmann equation. The Si and Ge phonon dispersions and 3-phonon scattering rates, serving as input for the transport modeling, are calculated from first-principles. The results show that, while the maximum phonon frequency in Si is nearly double that of Ge, significant heat currents are carried by the high-frequency Si phonons above the Ge cutoff. When approaching the interface, inelastic scattering redistributes energy to the phonon frequencies that can transfer elastically across the Si/Ge junction. We explain how this collective reorganization of phonons is driven by non-equilibrium effects near the interface. We also include a model for the contact resistance of an ideal interface, that depends on both phonon dispersions, which provides a lower limit for a given material combination. These results help provide clear physical insights into what controls phonon transport at semiconductor/semiconductor interfaces.

Transformation thermotics: Thermal metamaterials and their applications

In this paper, we review some recent achievements in thermal metamaterials, including novel thermal devices, simplified experimental method, macroscopic thermal diode based on temperature-dependent transformation thermotics, and the important role that soft matters play in the experimental confirmations of thermal metamaterials. These works pave the developments in transformation mapping theory and can surely inspire more designs of thermal metamaterials. What is more, some approaches provide more flexibility in controlling heat flow, and they may also be useful in other fields that are closely related to temperature gradient, such as the Seebeck effect and many other domains where transformation theory is valid.

The Magnetic Thomson Effect for Heat Flow Control

This paper studies one of the magnetic analogs of well-known thermoelectric effects, the spin Thomson effect. This effect consists in the existence of a heat flux caused by the spin current and is described by a modified heat equation. When comparing the published experimental data and the theoretical model, it was possible to find the Thomson spin coefficient for iron-yttrium garnet. From a viewpoint of thermal conductivity, the presence of a magnetic field of 1750 Oe turns out to be equivalent to the motion of the medium at a rate of 6.5 mm/s and leads to the existence of a giant heat flux of 18.9 kW/m2 due to the transfer of heat by spins. The ability to control heat fluxes by means of a magnetic field allows the use of materials based on iron-yttrium garnet for practical applications as thermal switches for solid-state cooling.

Turbulence closure models for free electroconvection

Electroconvection has been simulated in a number of recent studies, given its application in heat transfer enhancement, electrostatic atomizers and flow control. In practical applications, such as in charge injection atomizers, the electric Reynolds number can be sufficiently high such that the well-described ordered large scale electrohydrodynamic (EHD) instabilities that normally appear in electroconvection can dissipate and form into a wider distribution of length-scales. This purely electrohydrodynamically driven chaotic flow has features that resemble turbulent natural convection, and can dominate the operational regime of practical devices. Despite its practical relevance, Reynolds averaged turbulence model closures for EHD are unavailable, which currently makes direct numerical simulation the main viable option for EHD flows. Closure of EHD turbulence in free electro-convection is examined here through implementation of Reynolds stress model (RSM) closures using EHD specific timescales for the unclosed terms appearing in the turbulent scalar flux and space-charge scalar variance equations. A new closure for the highly non-linear triple correlation (q′E′u′¯), a term which is specific to EHD, is also presented. The work demonstrates that Reynolds stress closures are a feasible modeling route for EHD flows, with errors approaching similar values as in thermal Rayleigh–Benard convection at analogous levels of turbulence.

A numerical study of adaptive building enclosure systems using solid–solid phase change materials with variable transparency

Buildings currently consume about 40% of all energy use in the US and therefore play an important role in mitigating global greenhouse gas emissions. Passive solar design strategies can be used to limit building heating and cooling demands. Current passive solar design strategies, however, require substantial design effort for each individual project, often require mechanical and electrical control systems, and the approach is also difficult to implement in building retrofit projects. Solid–solid phase change materials (SS-PCM) are currently emerging as alternative materials for thermal energy storage. Here we present an exploratory study on two innovative climate responsive building enclosure systems that employ the transparency change and latent heat storage capacity of SS-PCMs as mechanisms to passively control building temperature. The first system is based on a thin layer of SS-PCM that is placed on top of a highly reflective film to control solar heat gain. The second system uses a layer of SS-PCM foam to store thermal energy and control heat flow. The performance characteristics of the systems are evaluated using finite element modeling techniques. Simulation results shed light on the synergistic interactions between different components of the systems and indicate that both systems can reduce undesirable heat exchange between the building and its environment if designed properly. Recommendations are made regarding various material and system parameters, including the attenuation coefficient of the SS-PCM, system thickness, void ratio and distribution of the void ratio of the SS-PCM foam based system, and phase transition temperature.

Grain boundary thermal resistance and finite grain size effects for heat conduction through porous polycrystalline alumina

Design of high performance materials for thermal insulation or heat sinking requires understanding of the mechanisms controlling heat flow. The effect of grain size on the effective thermal conductivity of porous alumina ceramics has been investigated. After uniaxial pressing of a fine α-alumina powder, control of firing temperature and duration yielded ceramics with variations in grain size. Values of thermal conductivity corresponding to equivalent 100% dense ceramics were determined from laser flash measurements using Landauer’s relation to take porosity into account. These exhibited a strong decrease from 33 W m−1 K−1, for grain sizes larger than 2 μm, down to 8 W m−1 K−1 for an average grain size of 0.25 μm.A method to separate the contributions of localized thermal resistance (Kapitza resistance) at grain boundaries and grain conductivity for each sample is presented. The larger grain size samples (>0.5 μm) yield values for the effective grain boundary thermal resistance in the range 0.6–0.8 × 10−8 m2 K W−1 and a grain conductivity close to 35 W m−1 K−1. For smaller grain samples (<0.5 μm), the grain boundary thermal resistance increases to 2 × 10−8 m2 K W−1 while the grain conductivity reduces to 23 W m−1 K−1. Thus grain size controls both the number of grain boundaries in the heat path and the grain conductivity itself.