Hierarchically porous lanthanum zirconate foams with low thermal conductivity from particle-stabilized foams

Lanthanum zirconate (LZO) ceramic foams with hierarchical pore structure were fabricated by particle-stabilized foaming method for the first time, and the as-prepared ceramics have high porosity of 90.7%-94.9%, low thermal conductivity, and relatively high compressive strength. The LZO powder was synthesized by solid-state method. The porosity of the ceramic foams was tailored by suspensions with different solid loadings (20-40 wt%). The sample with porosity of 94.9% has thermal conductivity of 0.073 W/(m·K) and compressive strength of 1.19 MPa, which exhibits outstanding property of thermal insulation and mechanical performance, indicating that LZO ceramic foam is a promising thermal insulation material in high temperature applications.     

Long‐term thermal conductivity of cyclopentane–water blown rigid polyurethane foams reinforced with different types of fillers

An understanding of the long‐term thermal conductivity of rigid polyurethane (RPU) foams presents great interest in the building field considering the conservation of energy efficiency. In this study, the effect of different types of particles (talc, diatomaceous earth and non‐porous silica) on the thermal conductivity of RPU foams blown with cyclopentane and water as blowing agents was investigated during 3 years of aging. The characterization of the cellular structure shows how the addition of particles causes a cell size reduction of the foams, and consequently an enhancement of the thermal properties just after production. However, this initial reduction is not maintained, because each foam shows a different thermal conductivity evolution with time. We have found, for the first time, a relationship between the slope of the thermal conductivity versus time at the first measurements and the internal temperature reached during the foaming process. The evolution of the RPU foams in which higher internal temperatures were reached is more pronounced than in those RPU foams where lower foaming temperatures were observed. This effect is related to the kinetics of the diffusion of the gas occluded inside the cells and imposes a new criterion for the selection of particles to reduce the thermal conductivity of RPU foams; these additives should ideally decrease the temperature reached during the foaming process. Moreover, the effect of aging on the thermal conductivity is explained by using theoretical models. © 2019 Society of Chemical Industry     

纳米流体有效热导率预测

纳米流体热导率实验表明了纳米流体具有不同于常规流体的导热特性。从颗粒小尺寸效应、布朗运动引起的微对流、固液吸附微界面和颗粒聚集结构上分析了纳米流体导热机理,并推导了适合纳米流体的热导率预测公式。与实验结果对比分析,该预测公式与实测值较为接近,对预测纳米流体的热导率有一定的参考价值。     

Microcellular polymeric foams based on 1‐vinyl‐2‐pyrrolidone and butyl‐acrylate with tuned thermal conductivity

Microcellular polymers have been produced by ScCO₂ foaming, based on 1‐vinyl‐2‐pyrrolidone (VP) and butyl‐acrylate (BA). Three different copolymers were prepared, varying the compositions of VP and BA, following a simple radical polymerization process using an UV initiator. The samples a good foaming behavior and also excellent flexibility and handle ability, with expansion ratios between 1.53 and 1.72, and cell sizes in the microcellular range (below 5 µm). However, it was observed that the gas distribution and, consequently, the cellular structure inside the polymer foams was highly dependent on the VP and BA proportions, leading to very different thermal conductivity values, even for similar volume gas fraction values. These results were related to the copolymer nanostructuration, which seems to have an influence in the final pore structure, thus opening the possibility of designing microcellular foams with similar macroscopic characteristics but different thermal conductivity values. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45872.     

一种计算泡沫金属等效热导率的新模型

提出一种针对泡沫金属等效热导率预测的新模型,该模型基于泡沫金属Kelvin十四面体元胞结构建模,以凹面三棱柱近似金属韧带,并考虑韧带交汇处的结点特点,通过热阻分析得到计算等效热导率的表达式。研究表明：该模型对充填不同介质的不同材质泡沫金属等效热导率均有较高的预测精度（平均偏差均小于10%）,与文献其他半经验模型相比能更好兼顾预测精确性和通用性。     

Novel ZrP2O7 ceramic foams with controllable structures and ultra-low thermal conductivity

This study demonstrated the synthesis of novel zirconium pyrophosphate (ZrP₂O₇) ceramic foams via a two-step method using a foam casting technique. The synthesised foams functioned as thermal insulators with a highly controllable performance. We investigated the effects of the addition of foaming and thickening agents as well as the solid content of the slurries on the slurry, mechanical properties, thermal conductivities, and microstructure of ZrP₂O₇ ceramic foams. The ZrP₂O₇ ceramic foams synthesised at 1473 K exhibited a porosity, compressive strength, and thermal conductivity of 75.2–89.1 %, 1.95–0.02 MPa, and 0.144–0.057 W/(m K) (298–573 K), respectively. The increase in the porosity to >60 % will facilitate applications based on the low thermal conductivities of the foams.     

Analysis and modeling of the pore size effect on the thermal conductivity of alumina foams for high temperature applications

Analytical and finite element analyses were carried out to investigate the influence of the pore sizes on the effective thermal conductivity, which is the main physical property related to the ceramic microstructure insulating capacity at high temperatures. Thermal conductivity was estimated by analytical models using Litovsky's and Rosseland's approaches for a monodisperse pore distribution, whereas via finite element analysis a high porosity microstructure with three different pore sizes was investigated. Based on this, an ideal pore size range (0.5–3.0 µm) was found that optimizes the reduction of thermal energy transmission in the 1000–1700 °C range. Furthermore, the ideal pore size range seems to be independent of the ceramic foam material. When considering a pore size distribution, the ideal range is narrowed due to less effective thermal radiation scattering by sub-micron and large pores. The results obtained showed that nanopores (< 0.1 µm) are not the best option to reduce thermal conductivity at high temperatures. This statement is supported by experimental data on nanopore aerogels, which show a significant thermal conductivity increase at the high temperature range.     

Production,cellular structure and thermal conductivity of microcellular (methyl methacrylate)–(butyl acrylate)–(methyl methacrylate) triblock copolymers

Microcellular foaming of a (methyl methacrylate)–(butyl acrylate)–(methyl methacrylate) triblock copolymer was carried out by means of supercritical CO₂ in a single‐step process. The experiments were performed at 40 °C using a pressure of 300 bar (30 MPa) during 24 h. The depressurization times were modified from 2 to 30 min, leading to cell sizes from 10 to 100 µm, and relative densities from 0.11 to 0.17. It was found that the key parameter to control cell size and density was depressurization time: longer depressurization times generated larger cell sizes and lower densities. The thermal conductivity of these materials was measured using the transient plane source technique, and it was found that this decreased as the density was reduced. Various models for the prediction of thermal conductivity by conduction were tested. It was found that all the models underestimated the experimental results due to a significant contribution of radiation heat flow for these materials. Copyright © 2010 Society of Chemical Industry     

Determination of heat conductivity of waste glass feed and its applicability for modeling the batch‐to‐glass conversion

The effective heat conductivity (λ) of reacting melter feed affects the heat transfer and conversion process in the cold cap, a layer of reacting feed floating on molten glass. A heat conductivity meter was used to measure λ of samples of a cold cap retrieved from a laboratory‐scale melter, loose dry powder feed samples, and samples cut from fast‐dried slurry blocks. These blocks were formed to simulate the feed conditions in the cold‐cap by rapidly evaporating water from feed slurry poured onto a 200°C surface. Our study indicates that the effective heat conductivity of the feed in the cold cap is significantly higher than that of loose dry powder feed, which is a result of the feed solidification during the water evaporation from the feed slurry. To assess the heat transfer at higher temperatures when feed turns into foam, we developed a theoretical model that predicts the foam heat conductivity based on morphology data from in‐situ X‐ray computed tomography. The implications for the mathematical modeling of the cold cap are discussed.     

Preparation and microstructure evolution of novel ultra-low thermal conductivity calcium silicate-based ceramic foams

In this study, municipal solid waste incineration fly ash (MSWI FA) was used as a new raw material for the ceramics industry and a novel ultra-low thermal conductivity calcium silicate-based foams (CSFs) was prepared by the direct foaming method. The effects of the addition of foam and borax on the sintering behavior and microstructural evolution of the CSFs were investigated. With the optimal amount of foam, the CSFs had an apparent porosity of 63.43%–67.49%, bulk density of 0.75–0.84 g/cm³, compressive strength of 1.83–3.21 MPa, and room-temperature thermal conductivity of 0.213–0.235 W/(m·K). Notably, the whisker morphology, pore structure, and sintering behavior of the samples can be controlled by changing the amount of borax. The prepared ceramic foams can be applied in the fields of thermal insulation, filtration, and catalyst carriers.     

Significant enhancement of thermal conductivity in graphite/polyester composite via interfacial π–π interaction

Interfacial thermal resistance between matrix and filler is one of the most serious factors hindering heat transfer in composites. Here, a type of liquid crystalline polyester (LCP) containing phenyl pendant groups was intended to blend with pristine graphite by interfacial interaction. The intensity at 26.6° of the wide angle X‐ray diffraction pattern which exceeded that of pristine graphite indicated the existence of a strong interfacial π–π interaction. Both DSC and XRD tests showed that the ordered structure of the LCP matrix is directly affected by the mass fraction of graphite, indicating the interfacial interaction between LCP and graphite. By increasing the content of graphite, the thermal diffusivity showed a sharp increment by 1004%. The maximum thermal conductivity of the composite reached 28.613 W m^?1 K^?1, which was seven times that of traditional thermoplastic blended with graphite. Compared with the data calculated using effective medium theory, interfacial interaction plays a significant role in enhancing the thermal conductivity of the composites. Furthermore, the maximum tensile strength of this series of composites reached 13.3 MPa and the maximum Young's modulus reached 1067 MPa, exhibiting a potential guideline for further applications in flexible electronics. © 2019 Society of Chemical Industry     

Intrinsically flame retarded foams based on melamine − formaldehyde condensates: thermal and mechanical properties

Two kinds of foam based on melamine ? formaldehyde (MF ) condensates (PVA /MF (PVA , polyvinyl alcohol), PVA /APTES /MF (APTES , 3‐triethoxysilylpropylamine)) were prepared by chemical modification. Pure MF foam has the serious disadvantage that it is very hard and brittle, breaks easily and crumbles when handled. After modification, PVA /MF and PVA /APTES /MF display excellent resilience. The structures of the foams were characterized by Fourier transform infrared (FTIR ), SEM and XRD . XRD data indicate that modifiers hinder the crystallization of MF , which might contribute to the improvement of resilience. Flame retardancy of the foams was characterized by limiting oxygen index testing, and the thermal degradation behavior was studied using TGA . The mode of flame retardant action is suggested by gaseous and solid phase analysis. TGA‐FTIR results demonstrate that the sublimation of the melamine of MF and foams based on MF occurs during thermal decomposition, which contributes to the high flame retardancy. © 2016 Society of Chemical Industry     

The effective thermal conductivity of three-dimensional reticulated foam materials

In this paper, the effective thermal conductivity k _eff of three-dimensional (3D) reticulated SiC foams were investigated through experimental and numerical methods. The results showed that the k _eff of SiC foams increases as the volume fraction f increases from 30% to 50%. However, there are no systematic changes detected in k _eff when the cell size of the foam varies at a fixed volume fraction. The k _eff of SiC foams as a function of f was obtained. Compared the experimental results with the calculated ones, it indicated that the outcome can be widely applied in estimating the effective thermal conductivity of other foam materials.     

Influence of pore distribution on the equivalent thermal conductivity of low porosity ceramic closed-cell foams

The microstructures of porous alumina materials with different porosities were established by introducing the departure factor of pore position and acentric factor of pore diameter to describe the distribution of pores in space and in size, respectively. The contribution of radiation and influence of pore distribution on the equivalent thermal conductivity were discussed based on numerical simulations by the finite volume method (FVM) considering both thermal conduction and radiation. When the pore diameter was less than 10?µm, the radiation component was less than 2%, and radiation could be neglected. Radiative heat transfer played a dominant role for materials with high porosity and large pore size at high temperatures. For micro pore materials (<?100?µm), broad pore size and non-uniform pore space distribution decreased the thermal conductivity across the entire temperature range. For materials with macro pores (>1?mm), broad pore distribution decreased the thermal conductivity at low temperatures and increased it at high temperatures. The basic prediction model of effective thermal conductivity for a two-component material, the Maxwell–Eucken model (ME1) and its modified model were corrected by introducing the pore structure factor. The results from experiments prove that the numerical values were satisfactory.     

Measurements of specific heat,thermal conductivity and thermal diffusivity of Utah tar sands

A constant applied heat flux method has been used to measure the specific heat and thermal conductivity of large samples of Utah (North-west Asphalt Ridge) tar sands as a function of temperature. Independent measurements of density allowed for the calculation of thermal diffusivity. Constituent analysis of the tar sand samples also permitted the calculation of bitumen and sand specific heats. Specific heat of the bitumen was found to increase with temperature from 1.85 to 3.9 kJ kg^?1 K^?1 for temperatures between 300 and 480 K. Specific heat of the sand matrix increased only slightly, from 0.85 to 1.0 kJ kg^?1 K^? for the same range of temperature. Corresponding thermal diffusivities for tar sand were found to decrease with temperature, and had a range of 5 · 10^?7–9 · 10^?7 m² s^?1 over the measured temperatures. It was concluded that the latent heat of both bitumen and water have a strong influence on the apparent overall specific heat of tar sand.     

Temperature dependence of effective thermal conductivity and thermal diffusivity of treated and untreated polymer composites

Thermal properties, such as thermal conductivity, thermal diffusivity, and specific heat, of treated and untreated oil palm fiber–reinforced PF composites were measured simultaneously at room temperature and normal pressure using the transient plane source (TPS) technique. An increase in thermal conductivity was observed in the fiber‐treated and resin‐treated composites. Surface modifications of fibers by prealkali, potassium permanganate, and peroxide treatments increased the fiber–matrix adhesion by increasing porosity and pore size of the fiber surfaces. The increase in crosslinking enhanced the thermal conductivity of a composite of resin treated with peroxide compared to other composites. Also an attempt was made to explain the temperature dependence of thermal conductivity and thermal diffusivity of amorphous polymer samples using the same technique. It was observed that at the glass‐transition peak of the polymer, thermal conductivity and diffusivity were maximum. Below and above this temperature their values decreased. This has been explained on the basis of predominant scattering processes. An empirical relationship was established for the theoretical prediction of thermal conductivity and diffusivity. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1708–1714, 2003     

Effect of selected boroorganic compounds on thermal and heat properties of rigid polyurethane–polyisocyanurate foams

The effect of selected borates [i.e., tri(hydroxypropyl) borate (BTHP), tri[(3‐chloro‐2‐hydroxy‐1‐propoxy)propyl] borate (BTClHPP), and tri[(3‐chloro‐2‐hydroxy‐1‐propoxy)‐1‐methylethyl] borate (BTClHPME)] on the heat and thermal properties of the rigid polyurethane–polyisocyanurate foams was investigated. Increasing amounts of BTHP and BTClHPP in the foam composition, from 0.1 to 0.4 of chemical equivalent, caused increases in the softening point, the temperature of the first decrement of foam mass, the extrapolated temperature of the main decrement of the foam mass, and temperature of the highest rate of the mass decrement. In the case when BTClHPME was added to the foam compositions, the softening point decreased but temperatures characterizing their thermal resistance were higher compared to that of standard foam. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 400–403, 2005     

热界面材料热导率和接触热阻的测试

热界面材料通常用于降低电子器件中固体界面的热阻。热界面材料的性质, 如热导率、界面材料与固体表面间的接触热阻, 对于电子器件的散热分析非常重要。然而, 这些参数通常难以获得。依据ASTM D-5470测试标准, 搭建了一个热界面测试系统。通过该系统测试了硅油和导热硅脂的热导率, 以及它们与固体基板间的接触热阻。经分析, 测试热导率和接触热阻的相对误差分别小于11.3%和41.3%。