Pub Date : 2018-06-19DOI: 10.1080/15567265.2018.1551449
Runqing Yang, Shengying Yue, Bolin Liao
ABSTRACT In this paper, we examine the application of an ideal phonon-hydrodynamic material as the heat transfer medium between two diffuse-gray boundaries with a finite temperature difference. We use the integral-equation approach to solve a modified phonon Boltzmann transport equation with the displaced Bose–Einstein distribution as the equilibrium distribution between two boundaries perpendicular to the heat transfer direction. When the distance between the boundaries is smaller than the phonon normal scattering mean free path, our solution converges to the ballistic limit as expected. In the other limit, we find that, although the local thermal conductivity in the bulk of the hydrodynamic material approaches infinity, the thermal boundary resistance at the interfaces becomes dominant. Our study provides insights into both the steady-state thermal characterization of phonon-hydrodynamic materials and the practical application of phonon-hydrodynamic materials for thermal management.
{"title":"Hydrodynamic Phonon Transport Perpendicular to Diffuse-Gray Boundaries","authors":"Runqing Yang, Shengying Yue, Bolin Liao","doi":"10.1080/15567265.2018.1551449","DOIUrl":"https://doi.org/10.1080/15567265.2018.1551449","url":null,"abstract":"ABSTRACT In this paper, we examine the application of an ideal phonon-hydrodynamic material as the heat transfer medium between two diffuse-gray boundaries with a finite temperature difference. We use the integral-equation approach to solve a modified phonon Boltzmann transport equation with the displaced Bose–Einstein distribution as the equilibrium distribution between two boundaries perpendicular to the heat transfer direction. When the distance between the boundaries is smaller than the phonon normal scattering mean free path, our solution converges to the ballistic limit as expected. In the other limit, we find that, although the local thermal conductivity in the bulk of the hydrodynamic material approaches infinity, the thermal boundary resistance at the interfaces becomes dominant. Our study provides insights into both the steady-state thermal characterization of phonon-hydrodynamic materials and the practical application of phonon-hydrodynamic materials for thermal management.","PeriodicalId":49784,"journal":{"name":"Nanoscale and Microscale Thermophysical Engineering","volume":"23 1","pages":"25 - 35"},"PeriodicalIF":4.1,"publicationDate":"2018-06-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1080/15567265.2018.1551449","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42490316","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2018-05-24DOI: 10.1080/15567265.2018.1476634
K. Rykaczewski, A. Phadnis
ABSTRACT This work uses elementary theoretical arguments to estimate whether softening of the surface could be used, along with surface texture and chemistry, to control superheat required for onset of nucleate boiling. For an ideal, smooth surface a mild decrease of the required superheat is predicted. In turn, an approximate closed-form model of vapor trapping and bubble seeding from soft surface with conical cavities shows linear dependence between the required superheat and the substrate’s shear modulus. Based on these results, considerations involved in implementing soft coatings for boiling applications and relevant outstanding fundamental questions are also briefly discussed.
{"title":"Could Use of Soft Surfaces Augment Onset of Nucleate Boiling?","authors":"K. Rykaczewski, A. Phadnis","doi":"10.1080/15567265.2018.1476634","DOIUrl":"https://doi.org/10.1080/15567265.2018.1476634","url":null,"abstract":"ABSTRACT This work uses elementary theoretical arguments to estimate whether softening of the surface could be used, along with surface texture and chemistry, to control superheat required for onset of nucleate boiling. For an ideal, smooth surface a mild decrease of the required superheat is predicted. In turn, an approximate closed-form model of vapor trapping and bubble seeding from soft surface with conical cavities shows linear dependence between the required superheat and the substrate’s shear modulus. Based on these results, considerations involved in implementing soft coatings for boiling applications and relevant outstanding fundamental questions are also briefly discussed.","PeriodicalId":49784,"journal":{"name":"Nanoscale and Microscale Thermophysical Engineering","volume":"22 1","pages":"230 - 238"},"PeriodicalIF":4.1,"publicationDate":"2018-05-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1080/15567265.2018.1476634","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43176055","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2018-05-24DOI: 10.1080/15567265.2018.1476633
S. Misyura
ABSTRACT Heat transfer of a droplet and layer during evaporation of aqueous solutions of salts has been studied. The behavior of salt solutions on a smooth and microstructured surface is compared here. Evaporation rate of aqueous salt solutions is greater for a microstructured surface than for a smooth wall. The behavior of heat transfer coefficient α can be described by two time regimes: quasi-constant values of α and significant increase in heat transfer at a multiple decrease in the liquid layer height. Measurements made with application of the particle image velocimetry showed that the structured surface increases liquid speed inside the sessile drop. The largest value of the heat transfer coefficient α on the structured surface corresponds to water for the final stage of evaporation. For salt solutions, the heat transfer coefficient is lower than that for water in the entire period of evaporation on the structured surface. The maximal excess (20–30%) of α of the structured wall above the smooth surface corresponds to the maximal height of the liquid layer at the beginning of evaporation. With increasing time, the excess is reduced. A drop of heat transfer intensification with a decrease in the layer height relates to suppression of free convection (a multiple decrease in the average velocity in the drop).
{"title":"Non-isothermal Evaporation of Salt Solutions on a Microstructured Surface","authors":"S. Misyura","doi":"10.1080/15567265.2018.1476633","DOIUrl":"https://doi.org/10.1080/15567265.2018.1476633","url":null,"abstract":"ABSTRACT Heat transfer of a droplet and layer during evaporation of aqueous solutions of salts has been studied. The behavior of salt solutions on a smooth and microstructured surface is compared here. Evaporation rate of aqueous salt solutions is greater for a microstructured surface than for a smooth wall. The behavior of heat transfer coefficient α can be described by two time regimes: quasi-constant values of α and significant increase in heat transfer at a multiple decrease in the liquid layer height. Measurements made with application of the particle image velocimetry showed that the structured surface increases liquid speed inside the sessile drop. The largest value of the heat transfer coefficient α on the structured surface corresponds to water for the final stage of evaporation. For salt solutions, the heat transfer coefficient is lower than that for water in the entire period of evaporation on the structured surface. The maximal excess (20–30%) of α of the structured wall above the smooth surface corresponds to the maximal height of the liquid layer at the beginning of evaporation. With increasing time, the excess is reduced. A drop of heat transfer intensification with a decrease in the layer height relates to suppression of free convection (a multiple decrease in the average velocity in the drop).","PeriodicalId":49784,"journal":{"name":"Nanoscale and Microscale Thermophysical Engineering","volume":"22 1","pages":"213 - 229"},"PeriodicalIF":4.1,"publicationDate":"2018-05-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1080/15567265.2018.1476633","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49341428","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2018-05-21DOI: 10.1080/15567265.2018.1475525
A. Mohammadi, A. Koşar
ABSTRACT This article reviews recent studies on the hydrodynamic and thermal characteristics of micro pin fin heat sink (MPFHS). In the studies reviewed in this article, liquid coolants such as water, HFE-7000, HFE-7200, R-123 were tested under both single-phase and two-phase flow conditions. Analytical, computational and experimental research studies were covered with a focus on configurations with traditional arrangements of micro pin fins (MPF) as well as original designs such as oblique finned MPFs, variable density MPF, vortex generators and herringbone structures. Single-phase flow results highlighted pressure drop penalty with achieved heat transfer enhancement. Many studies revealed the inability of conventional correlations to predict the hydrodynamic and thermal characteristics and proposed new correlations for different operating conditions and geometrical specifications. Regarding the studies on two-phase flows the number of performed studies is less than the ones in single-phase flow regime although the diversity of utilized coolants is more. Under flow boiling conditions, the focus was on determining flow patterns among MPFs for different arrangements and under different operating conditions. Unlike the studies on single-phase flows, the data could be relatively well predicted using the earlier suggested model by Lockhart and Martinelli with appropriate coefficients for different arrangements of MPFs.
{"title":"Review on Heat and Fluid Flow in Micro Pin Fin Heat Sinks under Single-phase and Two-phase Flow Conditions","authors":"A. Mohammadi, A. Koşar","doi":"10.1080/15567265.2018.1475525","DOIUrl":"https://doi.org/10.1080/15567265.2018.1475525","url":null,"abstract":"ABSTRACT This article reviews recent studies on the hydrodynamic and thermal characteristics of micro pin fin heat sink (MPFHS). In the studies reviewed in this article, liquid coolants such as water, HFE-7000, HFE-7200, R-123 were tested under both single-phase and two-phase flow conditions. Analytical, computational and experimental research studies were covered with a focus on configurations with traditional arrangements of micro pin fins (MPF) as well as original designs such as oblique finned MPFs, variable density MPF, vortex generators and herringbone structures. Single-phase flow results highlighted pressure drop penalty with achieved heat transfer enhancement. Many studies revealed the inability of conventional correlations to predict the hydrodynamic and thermal characteristics and proposed new correlations for different operating conditions and geometrical specifications. Regarding the studies on two-phase flows the number of performed studies is less than the ones in single-phase flow regime although the diversity of utilized coolants is more. Under flow boiling conditions, the focus was on determining flow patterns among MPFs for different arrangements and under different operating conditions. Unlike the studies on single-phase flows, the data could be relatively well predicted using the earlier suggested model by Lockhart and Martinelli with appropriate coefficients for different arrangements of MPFs.","PeriodicalId":49784,"journal":{"name":"Nanoscale and Microscale Thermophysical Engineering","volume":"22 1","pages":"153 - 197"},"PeriodicalIF":4.1,"publicationDate":"2018-05-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1080/15567265.2018.1475525","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46918441","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2018-05-03DOI: 10.1080/15567265.2018.1503382
Ke Chen, Xianghai Meng, Feng He, Yongjian Zhou, Jihoon Jeong, N. Sheehan, S. Bank, Yaguo Wang
ABSTRACT Optical grating technique, where optical gratings are generated via light inference, has been widely used to measure charge carrier and phonon transport in semiconductors. In this paper, compared are three types of transient optical grating techniques: transient grating diffraction, transient grating heterodyne, and grating imaging, by utilizing them to measure carrier diffusion coefficient in a GaAs/AlAs superlattice. Theoretical models are constructed for each technique to extract the carrier diffusion coefficient, and the results from all three techniques are consistent. Our main findings are: (1) the transient transmission change ∆T/T0 obtained from transient grating heterodyne and grating imaging techniques are identical, even these two techniques originate from different detection principles; and (2) by adopting detection of transmission change (heterodyne amplification) instead of pure diffraction, the grating imaging technique (transient grating heterodyne) has overwhelming advantage in signal intensity than the transient grating diffraction, with a signal intensity ratio of 315:1 (157:1).
{"title":"Comparison between Grating Imaging and Transient Grating Techniques on Measuring Carrier Diffusion in Semiconductor","authors":"Ke Chen, Xianghai Meng, Feng He, Yongjian Zhou, Jihoon Jeong, N. Sheehan, S. Bank, Yaguo Wang","doi":"10.1080/15567265.2018.1503382","DOIUrl":"https://doi.org/10.1080/15567265.2018.1503382","url":null,"abstract":"ABSTRACT Optical grating technique, where optical gratings are generated via light inference, has been widely used to measure charge carrier and phonon transport in semiconductors. In this paper, compared are three types of transient optical grating techniques: transient grating diffraction, transient grating heterodyne, and grating imaging, by utilizing them to measure carrier diffusion coefficient in a GaAs/AlAs superlattice. Theoretical models are constructed for each technique to extract the carrier diffusion coefficient, and the results from all three techniques are consistent. Our main findings are: (1) the transient transmission change ∆T/T0 obtained from transient grating heterodyne and grating imaging techniques are identical, even these two techniques originate from different detection principles; and (2) by adopting detection of transmission change (heterodyne amplification) instead of pure diffraction, the grating imaging technique (transient grating heterodyne) has overwhelming advantage in signal intensity than the transient grating diffraction, with a signal intensity ratio of 315:1 (157:1).","PeriodicalId":49784,"journal":{"name":"Nanoscale and Microscale Thermophysical Engineering","volume":"22 1","pages":"348 - 359"},"PeriodicalIF":4.1,"publicationDate":"2018-05-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1080/15567265.2018.1503382","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42301822","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2018-04-03DOI: 10.1080/15567265.2017.1416713
Jihoon Jeong, Ke Chen, E. S. Walker, N. Roy, Feng He, Philip Liu, C. Willson, M. Cullinan, S. Bank, Yaguo Wang
ABSTRACT We develop a nanosecond grating imaging (NGI) technique to measure in-plane thermal transport properties in bulk and thin-film samples. Based on nanosecond time-domain thermoreflectance (ns-TDTR), NGI incorporates a photomask with periodic metal strips patterned on a transparent dielectric substrate to generate grating images of pump and probe lasers on the sample surface, which induces heat conduction along both cross- and in-plane directions. Analytical and numerical models have been developed to extract thermal conductivities in both bulk and thin-film samples from NGI measurements. This newly developed technique is used to determine thickness-dependent in-plane thermal conductivities (κx) in Cu nano-films, which agree well with the electron thermal conductivity values converted from four-point electrical conductivity measurements using the Wiedemamn–Franz law, as well as previously reported experimental values. The κx measured with NGI in an 8 nm x 8 nm GaAs/AlAs superlattice (SL) is about 10.2 W/m⋅K, larger than the cross-plane thermal conductivity (8.8 W/m⋅K), indicating the anisotropic thermal transport in the SL structure. The uncertainty of the measured κx is about 25% in the Cu film and less than 5% in SL. Sensitivity analysis suggests that, with the careful selection of proper substrate and interface resistance, the uncertainty of κx in Cu nano-films can be as low as 5%, showing the potential of the NGI technique to determine κx in thin films with improved accuracy. By simply installing a photomask into ns-TDTR, NGI provides a convenient, fast, and cost-effective method to measure the in-plane thermal conductivities in a wide range of structures and materials.
{"title":"In-plane Thermal Conductivity Measurement with Nanosecond Grating Imaging Technique","authors":"Jihoon Jeong, Ke Chen, E. S. Walker, N. Roy, Feng He, Philip Liu, C. Willson, M. Cullinan, S. Bank, Yaguo Wang","doi":"10.1080/15567265.2017.1416713","DOIUrl":"https://doi.org/10.1080/15567265.2017.1416713","url":null,"abstract":"ABSTRACT We develop a nanosecond grating imaging (NGI) technique to measure in-plane thermal transport properties in bulk and thin-film samples. Based on nanosecond time-domain thermoreflectance (ns-TDTR), NGI incorporates a photomask with periodic metal strips patterned on a transparent dielectric substrate to generate grating images of pump and probe lasers on the sample surface, which induces heat conduction along both cross- and in-plane directions. Analytical and numerical models have been developed to extract thermal conductivities in both bulk and thin-film samples from NGI measurements. This newly developed technique is used to determine thickness-dependent in-plane thermal conductivities (κx) in Cu nano-films, which agree well with the electron thermal conductivity values converted from four-point electrical conductivity measurements using the Wiedemamn–Franz law, as well as previously reported experimental values. The κx measured with NGI in an 8 nm x 8 nm GaAs/AlAs superlattice (SL) is about 10.2 W/m⋅K, larger than the cross-plane thermal conductivity (8.8 W/m⋅K), indicating the anisotropic thermal transport in the SL structure. The uncertainty of the measured κx is about 25% in the Cu film and less than 5% in SL. Sensitivity analysis suggests that, with the careful selection of proper substrate and interface resistance, the uncertainty of κx in Cu nano-films can be as low as 5%, showing the potential of the NGI technique to determine κx in thin films with improved accuracy. By simply installing a photomask into ns-TDTR, NGI provides a convenient, fast, and cost-effective method to measure the in-plane thermal conductivities in a wide range of structures and materials.","PeriodicalId":49784,"journal":{"name":"Nanoscale and Microscale Thermophysical Engineering","volume":"22 1","pages":"83 - 96"},"PeriodicalIF":4.1,"publicationDate":"2018-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1080/15567265.2017.1416713","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46161792","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2018-04-02DOI: 10.1080/15567265.2018.1456581
Alireza Najafi Amel, S. Kouravand, P. Zarafshan, A. Kermani, M. Khashehchi
ABSTRACT This paper experimentally investigates the performance of micro and nano metfoam regenerators in alpha-type Stirling engine conditions. The thermal efficiency of this engine depends on performance of regenerator. Therefore, increase the heat recovery of regenerator raises the total efficiency. Accordingly, two types of regenerators from porous media are designed and simulated with different materials. Three-dimensional regenerator CFD simulation shows that randomize porous open cell metfoam made of silver as high conductivity and high heat capacity material is the best structure to fabricate metfoam regenerator. The porosity and matrix element diameter are optimized. The nano coating methodology enhances the activated surface. The regenerators are fabricated using casting polymer mold layer deposition. The nano silver particles are coated on the metfoam by sol-gel coating method. Experimental results show the improvement in regenerator percentage of heat recovery by 3.40% and 5.93% for micro metfoam and nano metfoam, respectively. The maximum improvement is achieved up to 8.65% in case of using the nano metfoam regenerator at 543 K.
{"title":"Study the Heat Recovery Performance of Micro and Nano Metfoam Regenerators in Alpha Type Stirling Engine Conditions","authors":"Alireza Najafi Amel, S. Kouravand, P. Zarafshan, A. Kermani, M. Khashehchi","doi":"10.1080/15567265.2018.1456581","DOIUrl":"https://doi.org/10.1080/15567265.2018.1456581","url":null,"abstract":"ABSTRACT This paper experimentally investigates the performance of micro and nano metfoam regenerators in alpha-type Stirling engine conditions. The thermal efficiency of this engine depends on performance of regenerator. Therefore, increase the heat recovery of regenerator raises the total efficiency. Accordingly, two types of regenerators from porous media are designed and simulated with different materials. Three-dimensional regenerator CFD simulation shows that randomize porous open cell metfoam made of silver as high conductivity and high heat capacity material is the best structure to fabricate metfoam regenerator. The porosity and matrix element diameter are optimized. The nano coating methodology enhances the activated surface. The regenerators are fabricated using casting polymer mold layer deposition. The nano silver particles are coated on the metfoam by sol-gel coating method. Experimental results show the improvement in regenerator percentage of heat recovery by 3.40% and 5.93% for micro metfoam and nano metfoam, respectively. The maximum improvement is achieved up to 8.65% in case of using the nano metfoam regenerator at 543 K.","PeriodicalId":49784,"journal":{"name":"Nanoscale and Microscale Thermophysical Engineering","volume":"22 1","pages":"137 - 151"},"PeriodicalIF":4.1,"publicationDate":"2018-04-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1080/15567265.2018.1456581","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47794545","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2018-03-27DOI: 10.1080/15567265.2018.1505987
E. Chávez‐Ángel, N. Reuter, P. Komar, S. Heinz, U. Kolb, H. Kleebe, G. Jakob
ABSTRACT The quest to improve the thermoelectric figure of merit has mainly followed the roadmap of lowering the thermal conductivity while keeping unaltered the power factor of the material. Ideally an electron-crystal phonon-glass system is desired. In this work, we report an extraordinary reduction of the cross-plane thermal conductivity in crystalline (TiNiSn):(HfNiSn) half-Heusler superlattices (SLs). We create SLs with thermal conductivities below the effective amorphous limit, which is kept in a large temperature range (120–300 K). We measured thermal conductivity at room temperature values as low as 0.75 W m−1 K−1, the lowest thermal conductivity value reported so far for half-Heusler compounds. By changing the deposition conditions, we also demonstrate that the thermal conductivity is highly impacted by the way the single segments of the SL grow. These findings show a huge potential for thermoelectric generators where an extraordinary reduction of the thermal conductivity is required but without losing the crystal quality of the system
摘要提高热电性能因数的主要途径是在保持材料功率因数不变的同时降低热导率。理想情况下，需要电子-晶体-声子玻璃系统。在这项工作中，我们报道了晶体（TiNiSn）：（HfNiSn的）半Heusler超晶格（SL）的跨平面热导率的显著降低。我们创造了热导率低于有效非晶极限的SL，该极限保持在大的温度范围（120–300 K）内。我们在室温下测量了低至0.75 W m−1 K−1的热导率，这是迄今为止报道的半Heusler化合物的最低热导率值。通过改变沉积条件，我们还证明了SL的单段生长方式对热导率的影响很大。这些发现表明，热电发电机具有巨大的潜力，需要显著降低热导率，但又不会损失系统的晶体质量
{"title":"Subamorphous Thermal Conductivity of Crystalline Half-Heusler Superlattices","authors":"E. Chávez‐Ángel, N. Reuter, P. Komar, S. Heinz, U. Kolb, H. Kleebe, G. Jakob","doi":"10.1080/15567265.2018.1505987","DOIUrl":"https://doi.org/10.1080/15567265.2018.1505987","url":null,"abstract":"ABSTRACT The quest to improve the thermoelectric figure of merit has mainly followed the roadmap of lowering the thermal conductivity while keeping unaltered the power factor of the material. Ideally an electron-crystal phonon-glass system is desired. In this work, we report an extraordinary reduction of the cross-plane thermal conductivity in crystalline (TiNiSn):(HfNiSn) half-Heusler superlattices (SLs). We create SLs with thermal conductivities below the effective amorphous limit, which is kept in a large temperature range (120–300 K). We measured thermal conductivity at room temperature values as low as 0.75 W m−1 K−1, the lowest thermal conductivity value reported so far for half-Heusler compounds. By changing the deposition conditions, we also demonstrate that the thermal conductivity is highly impacted by the way the single segments of the SL grow. These findings show a huge potential for thermoelectric generators where an extraordinary reduction of the thermal conductivity is required but without losing the crystal quality of the system","PeriodicalId":49784,"journal":{"name":"Nanoscale and Microscale Thermophysical Engineering","volume":"23 1","pages":"1 - 9"},"PeriodicalIF":4.1,"publicationDate":"2018-03-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1080/15567265.2018.1505987","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45873624","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2018-03-06DOI: 10.1080/15567265.2018.1446065
Anirudh Krishna, Jaeho Lee
ABSTRACT Spectral emissivity of surface materials has a strong impact on thermal properties of systems that are exposed in the ambient environment. While the solar spectrum heating up the surface ranges from 200 to 2,500 nm, the atmospheric transmission spectrum allowed for infrared cooling ranges from 8 to 14 µm. However, conventional surface materials have emissivity values that are either high or low throughout the spectrum. For example, ceramic materials are typically emissive and metallic materials are typically reflective and not emissive. Here, we show that surface materials with artificial periodicities can have a selectively controlled emissivity and that the surface morphology can transform ceramic materials to be reflective or metallic materials to be emissive. As a model system, we use microscale tree-like structures, or briefly micro-trees, to demonstrate wide variations of morphology-driven emissivity spectra. Our computation based on the rigorous coupled-wave analysis shows that optimal designs of micro-trees can act as a nearly perfect reflector or a black body depending on the spectral range and offer radiative cooling or heating capabilities beyond the limits of conventional materials. For cooling, metallic micro-trees provide a surface temperature 10 K lower than that of bare metallic surfaces in a normal ambient condition, and for heating, ceramic micro-trees provide a surface temperature 8 K higher than that of bare ceramic materials. The morphology-driven emissivity of micro-trees can offer a net cooling power of 136 W/m2 or a net heating power of 12 W/m2 depending on the application without requiring any active devices, and these results guide optimal designs of artificial materials for thermal management.
{"title":"Morphology-Driven Emissivity of Microscale Tree-like Structures for Radiative Thermal Management","authors":"Anirudh Krishna, Jaeho Lee","doi":"10.1080/15567265.2018.1446065","DOIUrl":"https://doi.org/10.1080/15567265.2018.1446065","url":null,"abstract":"ABSTRACT Spectral emissivity of surface materials has a strong impact on thermal properties of systems that are exposed in the ambient environment. While the solar spectrum heating up the surface ranges from 200 to 2,500 nm, the atmospheric transmission spectrum allowed for infrared cooling ranges from 8 to 14 µm. However, conventional surface materials have emissivity values that are either high or low throughout the spectrum. For example, ceramic materials are typically emissive and metallic materials are typically reflective and not emissive. Here, we show that surface materials with artificial periodicities can have a selectively controlled emissivity and that the surface morphology can transform ceramic materials to be reflective or metallic materials to be emissive. As a model system, we use microscale tree-like structures, or briefly micro-trees, to demonstrate wide variations of morphology-driven emissivity spectra. Our computation based on the rigorous coupled-wave analysis shows that optimal designs of micro-trees can act as a nearly perfect reflector or a black body depending on the spectral range and offer radiative cooling or heating capabilities beyond the limits of conventional materials. For cooling, metallic micro-trees provide a surface temperature 10 K lower than that of bare metallic surfaces in a normal ambient condition, and for heating, ceramic micro-trees provide a surface temperature 8 K higher than that of bare ceramic materials. The morphology-driven emissivity of micro-trees can offer a net cooling power of 136 W/m2 or a net heating power of 12 W/m2 depending on the application without requiring any active devices, and these results guide optimal designs of artificial materials for thermal management.","PeriodicalId":49784,"journal":{"name":"Nanoscale and Microscale Thermophysical Engineering","volume":"22 1","pages":"124 - 136"},"PeriodicalIF":4.1,"publicationDate":"2018-03-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1080/15567265.2018.1446065","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47993568","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2018-02-06DOI: 10.1080/15567265.2018.1434844
Xiaohu Wu, C. Fu
ABSTRACT We propose a method to realize ultra-broadband perfect absorption by using multiple slabs of asymmetric hyperbolic metamaterial (AHM) made of doped silicon nanowire arrays. Our numerical results show that the absorptance of the structure is greater than 0.99 in the wavelength range from to for an incident transverse magnetic (TM) plane wave at an angle of incidence equal to . Moreover, the broadband absorptance can still be above 0.9 when the angle of incidence is in the range from to . The underlying mechanism is elucidated as due to the combination of matching of impedance at the interfaces and enhanced absorption in the AHM slabs of the structure. This work may provide in the design of metamaterial absorbers with some inspiring guidelines for obtaining highly enhanced absorption over an ultra-broadband and in a wide range of angle of incidence.
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