Prediction of thermal conductivity of micro/nano porous dielectric materials: Theoretical model and impact factors

Abstract This study treats a new model of nonlocal fractal thermoelasticity beam theory for nanomaterials characterized by an apparent negative thermal conductivity which occurs in shaped graded materials. The model is based on the new concept of "product-like fractal measure" introduced by Li and Ostoja-Starzewski in their formulation of fractal continuum media with fractal dimension The basic setups of the theory are formulated, yet, we have restricted our analysis for the static case. The effects of fractal dimension on field variables, in particular the lateral deflection and the temperature increment of the resonator were analyzed analytically and graphically. It was observed that the physics of the thin elastic nanobeam is affected by the numerical value of the fractal dimension and the characteristic lengths of the theory. Comparable scenarios observed in complex material structures are obtained and discussed.

Fractal nonlocal thermoelasticity of thin elastic nanobeam with apparent negative thermal conductivity

Micro/nano-porous polymeric material is considered a unique industrial material due to its extremely low thermal conductivity, low density, and high surface area. Therefore, it is necessary to establish an accurate thermal conductivity prediction model suiting their applicable conditions and provide a theoretical basis for expanding their applications. In this work, the development of the calculation model of equivalent thermal conductivity of micro/nano-porous polymeric materials in recent years is summarized. Firstly, it reviews the process of establishing the overall equivalent thermal conductivity calculation model for micro/nanoporous polymers. Then, the predicted calculation models of thermal conductivity are introduced separately according to the conductive and radiative thermal conductivity models. In addition, the thermal conduction part is divided into the gaseous thermal conductivity model, solid thermal conductivity model and gas-solid coupling model. Finally, it is concluded that, compared with other porous materials, there are few studies on heat transfer of micro/nanoporous polymers, especially on the particular heat transfer mechanisms such as scale effects at the micro/nanoscale. In particular, the following aspects of porous polymers still need to be further studied: micro scaled thermal radiation, heat transfer characteristics of particular morphologies at the nanoscales, heat transfer mechanism and impact factors of micro/nanoporous polymers. Such studies would provide a more accurate prediction of thermal conductivity and a broader application in energy conversion and storage systems.

Thermal conductivity of micro/nano-porous polymers: Prediction models and applications

A detailed investigation of the physicochemical properties and thermal characteristics of argan fruit residues (AFRs) was carried out to identify their potential application as biofuels. This experimental study covered the four main by-products of argan fruit: argan pulp (AP), argan nut shell (ANS), argan oilcake (AOC) and argan deoiled cake (ADC). Physicochemical analysis was performed to characterize biomass-based materials. Thermogravimetric techniques (TG, DTG and DTA) were applied to assess structural decomposition, biomass reactivity and combustion parameters. Lastly, thermal conductivity and diffusivity between 25 and 100 °C were measured using the transient plane source technique (TPS). The study shows that heating values were in the range of 17–22.5 MJ kg−1, which is comparable to those of wood pellets and lignite coal. The maximum ash content was found in AP and the minimum in ANS. The residual oil of AOC was found to affect the drying process and calorific value. Thermal analysis showed that ANS has the highest ignition temperature and thermal conductivity values and therefore appears to be the most reactive material. Our data show that ANS has a good potential to produce heat through direct combustion. A pretreatment appears to be necessary to improve the characteristics of AP and AOC in order to make them suitable biofuels in the near future.

Physicochemical and thermal analysis of argan fruit residues (AFRs) as a new local biomass for bioenergy production

Nowadays, our society is facing problems related to energy availability. Owing to the energy savings that insulators provide, the search for effective insulating materials is a focus of interest. Since the current insulators do not meet the increasingly strict requirements, developing materials with a greater insulating capacity is needed. Until now, several nanoporous materials have been considered as superinsulators achieving thermal conductivities below that of the air 26 mW/(m K), like nanocellular PMMA/TPU, silica aerogels, and polyurethane aerogels reaching 24.8, 10, and 12 mW/(m K), respectively. In the search for the minimum thermal conductivity, still undiscovered, the first step is understanding heat transfer in nanoporous materials. The main features leading to superinsulation are low density, nanopores, and solid interruptions hindering the phonon transfer. The second crucial condition is obtaining reliable thermal conductivity measurement techniques. This review summarizes these techniques, and data in the literature regarding the structure and thermal conductivity of two nanoporous materials, nanocellular polymers and aerogels. The key conclusion of this analysis specifies that only steady-state methods provide a reliable value for thermal conductivity of superinsulators. Finally, a theoretical discussion is performed providing a detailed background to further explore the lower limit of superinsulation to develop more efficient materials.

/pdf/thermal-conductivity-of-nanoporous-materials-where-is-the-2nxq1bhi.pdf

Thermal Conductivity of Nanoporous Materials: Where Is the Limit?

Using transient hot disk technique to measure the thermal conductivity of liquids, the sensor is traditionally placed in the vertical orientation. In this study, the horizontal orientation method was analyzed. Effects of convection were studied and compared when using both the vertical and the horizontal orientation transient hot disk methods to measure the thermal conductivities of liquids. The temperature field and the velocity field of the testing fluid in both methods were obtained when numerical simulations were utilized to get quantified results of convection effects. Moreover, experiments were executed to validate the numerical simulation settings. The velocity magnitude and the Rayleigh number distribution were monitored in the direction which was perpendicular to the sensor face. It was found that the horizontal method had a lower maximum velocity and convection intensity. Besides, the testing region which was affected by heating process was smaller in horizontal method. The temperature distribution in horizontal orientation method showed asymmetry around the sensor face, the heat transfer ability of up face was higher than the down face. Meanwhile, the flow intensity in the horizontal orientation method grew faster than the vertical orientation method with long testing time. Judging from the average temperature rise of the sensor and the flow intensity in the fluid region, it is proposed that the horizontal orientation method is a better way for thermal conductivity measurements of liquids when testing time is not too long. Experiments were performed to measure the thermal conductivities of water, ethanol and glycol in both vertical and horizontal methods. It was found that the thermal conductivities measured in horizontal method were more accurate than the vertical method for all three liquid samples compared with the values in the literature. For ethanol, the relative error in the horizontal method is 11.6 % lower than the results in the vertical method.

Influence of sensor orientations on the thermal conductivity measurements of liquids by transient hot disk technique

Macroporous metal foams are used as energy exchange materials and have been widely used in the industry. In this paper, the finite element method is used to calculate the equivalent thermal radiation conductivity and equivalent thermal conduction equivalent conductivity of the fractal-skeleton heat transfer model, and the heat transfer characteristics of the two components are analyzed. The results show that during thermal conduction progress, energy transfer mainly occurs at the junction of walls and holes perpendicular to the direction of heat flow. While during the thermal radiation process, the smaller the cell wall thickness and the larger the porosity, the larger the proportion of the heat radiation conductivity in the total heat conductivity. When the structural size is equal to the characteristic wavelength, the scattering coefficient is rapidly reduced, so that the radiation heat transfer effect is improved. Finally, the cause of this phenomenon is further explained by the micro-mechanism theory.

A fractal-skeleton model of high porosity macroporous aluminum and its heat transfer characterizes

The invention provides a thermal-insulation and rotary integrated loading device for use in measurement of high-temperature photo-thermal properties of a material and relates to the thermal-insulation and rotary integrated loading device. According to the thermal-insulation and rotary integrated loading device, the problem that measurement precision of the photo-thermal properties of the material is limited because an existing loading device cannot rotate in the measurement process of the high-temperature photo-thermal properties of the material and the thickness of a thermal insulation layer of the loading device is overlarge is solved. The thermal-insulation and rotary integrated loading device comprises a metal plate, a radiation reflection screen, a regulating ring, support pillars, a thermal insulation plate, a support seat, a spring, a rotating shaft, a magnetic sealing device and a servo motor. According to the thermal-insulation and rotary integrated loading device, the horizontal 360-degree controllable rotation of a to-be-measured sample can be realized. The invention acquires the thermal-insulation and rotary integrated loading device for use in measurement of high-temperature photo-thermal properties of the material.

Thermal-insulation and rotary integrated loading device for use in measurement of high-temperature photo-thermal properties of material

A dynamic region Monte Carlo method (DRMC) is proposed to simulate radiative heat transfer in participating medium. The basic principle and solution procedure of this method is described; radiative heat transfer in a two-dimensional rectangular region of absorbing, emitting, and/or scattering gray medium is analyzed. A comparison between DRMC and the traditional Monte Carlo method (TMC) is investigated by analyzing the simulated temperature distribution, the computing time, and the number of the sampling bundles. The investigation results show that, to compare with TMC, the DRMC can obviously reduce the computing time and storage capacity under the same solution precision for radiative transfer in optically thick medium; the DRMC allows bypassing the difficulties encountered by TMC in the limit of optically thick extinction.

/pdf/radiative-heat-transfer-in-participating-medium-and-dynamic-1qnsq5rf50.pdf

Xinlin Xia

Papers

Thermal conductivity of micro/nano-porous polymers: Prediction models and applications

Influence of sensor orientations on the thermal conductivity measurements of liquids by transient hot disk technique

A fractal-skeleton model of high porosity macroporous aluminum and its heat transfer characterizes

Thermal-insulation and rotary integrated loading device for use in measurement of high-temperature photo-thermal properties of material

Radiative Heat Transfer in Participating Medium and Dynamic Region Monte Carlo Method by Region Adaption