O. Abunumah, P. Ogunlude, E. Gobina, A. Giwa, E. Ogoun, R. Prabhu
The Robert Gordon University,
Keywords: Nano Insulating Materials, creep coefficient, mobility, young modulus, gas, nanotechnology
Summary:Nano Insulating Materials (NIM) are useful materials for thermal control in systems, such as buildings and endothermic reactions processes. NIM possess nanopores that could contain rarefied gases. The coupling of quantities, such as the matrix’s and gas’s thermal conductivities, flow driven by thermal (creep coefficient) and pressure gradients (young modulus), present design and manufacturing opportunities and challenges for NIM and instrumentations. Therefore, understanding the vacuum or rarefaction gas dynamics in the NIM matrix would facilitate the optimal design and use of these materials in domestic and industrial applications. Some authors have analytically investigated the thermal conductivity of NIM using rarefied gas flowrate, but none has investigated the opportunities of using gas mobility to characterize the insulating quality of NIM vis-à-vis low pressure and low-high thermal gradients. Hence the motivation for this study. Gas mobility is a combinatorial quantity that couples rock (permeability) and fluid (viscosity) properties, therefore making it an elite engineering quantity to potentially characterise NIM thermal quality. Thus, this work has provided a novel method for characterizing NIM systems using gas mobility. Methodology and Materials: An empirical approach involving gas experiments in nanoscale porous media was adopted for this study. Low pressures (0.20 – 1.00 atm), high temperature (up to 673K) were set as the working conditions. 5 analogous NIM core samples of varying structural parameters (pore size- 15nm, 200nm, and 6000nm; porosity- 3%, 4%, 13%, 14%, and 20%; and aspect ratios- 4.50E-05, 1.55E-03, 1.91E-02, and 3.36E-02) and 4 common industry gases (CH4, N2, Air and CO2) were the major materials used. A total of 1960 experimental runs and 8000 datasets were collected and used for various analyses. Experimental Results: The research was able to characterize NIM systems using gas mobility, industry criteria, such as Knudsen and Reynold’s numbers, and gas laws, such as Boyles and Charles laws. Unlike flow rates, mobility responses to pressure and thermal gradients were very low. It was observed that gas thermal interactions and creep coefficient were elevated as pressure increases from 0.20 to 1 atm, this is further validated by the improved correlation values (R2). CH4 was found to be the most responsive to pressure and thermal boundary disturbances in NIM, while CO¬2 is the least. Therefore suggesting that a NIM filled with CO2 gas offers better insulating quality than the other gases. Contribution to Practice and Knowledge: The multivariable (conditions, properties and parameters) settings to optimize mobility and the consequent optimal NIM system has been presented in detail. The merits and demerits of mobility-driven NIM analysis were highlighted. The outcome would find direct utility and practical application in NIM and instrumentation device manufacturing, and infill gas selection.