Critical-roles of Pores in Thermal Insulation Materials

Significance 

Thermal insulation materials, especially those with lower thermal conductivity compared with that of stationary air, are prospective candidates for minimizing heat loss in numerous practical applications. Since gases generally have much lower thermal conductivity than solids and liquids, introducing pores in the material is one common way of reducing heat conduction. The pores not only reduce the solid conduction cross-sectional area but also induce torturous structures that prolong the heat conduction path, thereby reducing the overall heat loss through the material.

The pores can be broadly classified into two categories: open and closed pores. The former has complex interconnections, while the latter are separated by solid materials. Depending on the size of the pores, the gaseous heat transfer is mainly governed by heat convection and conduction. Suppressing the heat conduction by introducing the nm-scale pores in the material is deemed an effective strategy for realizing ultra-low thermal conductivity. Nevertheless, utilizing nm-scale pores to realize thermal conductivity below 0.01 W/(mK) robustly and cost-effectively remains a big challenge. This is mainly attributed to the trade-off relationships between gas and solid conduction.

Numerous building blocks have been explored for potential application in fabricating super-thermal insulators with different pore structures. However, most existing works rarely develop materials with specific structures suitable for achieving ultralow thermal conductivity. This can be attributed to poor understanding of heat transfer in highly porous structures, which is critical in their design and applications. Moreover, developing an effective and robust theoretical model to analyze the geometry and insulation performance relationship in porous structure is highly desirable.

Herein, Dr. He Liu from Zhengzhou University of Light Industry together with Dr. Xinpeng Zhao from the University of Colorado Boulder analyzed the thermal conductivity of highly porous structures in open and closed pores. The influence of thermophysical and geometric parameters on the thermal conductivity performance of the porous structures was analyzed using two analytical models. Porous structures with closed and open pores were represented by cubic array of hollow cubes and intersecting rods, respectively. Their work is currently published in the International Journal of Heat and Mass Transfer.

The research team showed that regardless of the pore type, the effective thermal conductivity of the porous structures approached the pore size-dependent gaseous thermal conductivity as the porosity increased. This phenomenon was characterized by an increase and decrease in the effective thermal conductivity when the solid conduction was less and more prominent than the gas conduction, respectively. However, for equal thermophysical and geometric properties, structures with open pores exhibited lower effective thermal conductivity than those with closed pores, except for the cases where the solid thermal conductivity was significantly lower than that of stationary air.

Effective strategies for reducing the thermal conductivity below that of stationery air include minimizing the gaseous thermal conductivity by either introducing nm-scale pores in the materials or decreasing the pressure in the pores. Besides, significantly reducing the solid thermal conductivity lower than that of the air within the pores could also minimize the thermal conductivity of porous structures with μm-scale pores. Furthermore, decreasing the gas pressure, pore size and solid thermal conductivity were found to substantially decrease the thermal conductivity regardless of the pore types.

In summary, He Liu and Xinpeng Zhao reported successfully the analysis of the thermal conductivity of the porous structures with closed and open pores represented by irregular structures. The proposed theoretical framework and model permitted the derivation of effective thermal conductivity of porous structures as a function of thermal accommodation coefficient, pores size, solid thermal conductivity, porosity and gas pressure. The study provided more insights into the heat transfer in porous structures with different pores. In a statement to Advances in Engineering, Dr. Xinpeng Zhao, the corresponding author explained that the findings would guide the design and production of super-thermal insulators for different applications such as buildings, aerospace, cryogenic engineering, and cold-chain transportation.

About the author

Dr. He Liu is currently an assistant professor in the School of Energy and Power Engineering at the Zhengzhou University of Light Industry, China. She received her Ph.D. in Engineering Thermophysics from Xi’an Jiaotong University in 2016. She joined the Zhengzhou University of Light Industry via the “ High-level Talents Introduction Project” in 2017. She currently focuses on the heat and mass transfer mechanism of porous materials with nano/micro-pores and their applications in energy-efficient buildings.

About the author

Dr. Xinpeng Zhao is currently a postdoc associate in Materials Science and Engineering at the University of Maryland College Park. He received his Ph.D. in Mechanical Engineering from the University of Colorado Boulder in 2020, his MS degree in Engineering Thermophysics from Xi’an Jiaotong University in 2015, and his BS degree in Flight Vehicle Propulsion Engineering from Northwestern Polytechnical University in 2012. His research focuses on the micro/nanoscale heat transfer, thermal and light management of buildings, and sustainable thermal insulation materials.

Reference

Liu, H., & Zhao, X. (2022). Thermal Conductivity Analysis of High Porosity Structures with Open and Closed PoresInternational Journal of Heat and Mass Transfer, 183, 122089.

Go To International Journal of Heat and Mass Transfer

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