纳米隔热材料的孔隙结构特征与气体热传输特性
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摘要
为研究纳米隔热材料孔隙结构内部的气体热传输特性,采用溶胶—凝胶工艺结合超临界干燥技术,制备了一系列具有不同孔隙结构特征的样品,通过热导率、氮气吸-脱附和真密度测试,全面、准确获取了其孔隙结构信息,并专门、系统研究了孔隙结构特征与气体热传输特性之间的关系.研究结果表明:与气相贡献热导率相对应,材料具有双尺度孔隙结构特征,并且当大孔隙尺度不及小孔隙的10倍时,可进一步等效为单尺度孔隙.考虑气固耦合传热的本征气相贡献热导率随孔隙尺度的增大而升高,与气相热导率变化类似且成一定的比例关系,孔隙尺度小于200 nm和大于500 nm时的比例系数分别为2. 0和1. 5,200～500 nm时则为2. 0～1. 5.当大、小孔隙尺度的比值不超过10时,或者这一比值为100～1000且大孔隙含量低于10%时,气相贡献热导率随环境气压的降低依次呈现快速下降、缓慢下降和无变化三个阶段;当这一比值超过3000时,即使大孔隙含量很低(不超过10%),气相贡献热导率也会依次呈现快速下降、缓慢下降、快速下降和无变化四个阶段.
The thermal insulation properties of nano-porous thermal insulating materials largely depend on thermal transport via gas phase within their pores,and this process relies on their pore structures. Therefore,investigating pore structures and thermal transport via gas phase is important to understand the heat transfer mechanism. Current research mainly focuses on the theoretical calculation and analysis from the perspective of heat transfer,and special and systematic studies based on actual materials have not been reported yet.In addition,accurate analysis of pore structures using usual techniques is difficult due to the complex pore network and the poor mechanical properties of their solid skeleton. In this study,nano-porous thermal insulating materials with different pore structures were synthesized via a sol-gel process followed by supercritical drying. The materials were then characterized by thermal conductivity tester,nitrogen adsorption-desorption,and helium pycnometer. The pore structures of the resulting materials were obtained,and the relationship between pore structures and thermal transport via gas phase was studied. Results show that the bimodal distribution of pores in the resulting materials,corresponding to gas-contributed thermal conductivity. All pores within the resulting materials can be equivalent to pores with a single diameter when the equivalent size of large pores is 10 times less than that of small pores. Similar to the pure gaseous thermal conductivity,the intrinsic gas-contributed thermal conductivity including gas-solid coupling effects rises with increasing pore diameter of the materials. The ratio of intrinsic gas-contributed thermal conductivity to pure gaseous thermal conductivity is 2. 0,1. 5,and 2. 0-1. 5 for pores smaller than 200 nm,larger than 500 nm,and with size between 200 and 500 nm,respectively. When the equivalent size of large pores is 10 times less than that of small pores or when the equivalent size of large pores is 100-1000 times that of small pores and the contribution of large pores to the total porosity is less than 10%,the gas-contributed thermal conductivity reduction of the resulting material with decreasing gas pressure can be divided into three stages(steep decreasing stage,slow decreasing stage,and hardly changing stage) according to decreasing rate. When the equivalent size of large pores is 3000 times larger than that of small pores,the gas-contributed thermal conductivity reduction of the resulting material with decreasing gas pressure can be divided into four stages(steep decreasing stage,slow decreasing stage,steep decreasing stage,and hardly changing stage) even if the contribution of large pores to the total porosity is very low(less than 10%).
The thermal insulation properties of nano-porous thermal insulating materials largely depend on thermal transport via gas phase within their pores,and this process relies on their pore structures. Therefore,investigating pore structures and thermal transport via gas phase is important to understand the heat transfer mechanism. Current research mainly focuses on the theoretical calculation and analysis from the perspective of heat transfer,and special and systematic studies based on actual materials have not been reported yet.In addition,accurate analysis of pore structures using usual techniques is difficult due to the complex pore network and the poor mechanical properties of their solid skeleton. In this study,nano-porous thermal insulating materials with different pore structures were synthesized via a sol-gel process followed by supercritical drying. The materials were then characterized by thermal conductivity tester,nitrogen adsorption-desorption,and helium pycnometer. The pore structures of the resulting materials were obtained,and the relationship between pore structures and thermal transport via gas phase was studied. Results show that the bimodal distribution of pores in the resulting materials,corresponding to gas-contributed thermal conductivity. All pores within the resulting materials can be equivalent to pores with a single diameter when the equivalent size of large pores is 10 times less than that of small pores. Similar to the pure gaseous thermal conductivity,the intrinsic gas-contributed thermal conductivity including gas-solid coupling effects rises with increasing pore diameter of the materials. The ratio of intrinsic gas-contributed thermal conductivity to pure gaseous thermal conductivity is 2. 0,1. 5,and 2. 0-1. 5 for pores smaller than 200 nm,larger than 500 nm,and with size between 200 and 500 nm,respectively. When the equivalent size of large pores is 10 times less than that of small pores or when the equivalent size of large pores is 100-1000 times that of small pores and the contribution of large pores to the total porosity is less than 10%,the gas-contributed thermal conductivity reduction of the resulting material with decreasing gas pressure can be divided into three stages(steep decreasing stage,slow decreasing stage,and hardly changing stage) according to decreasing rate. When the equivalent size of large pores is 3000 times larger than that of small pores,the gas-contributed thermal conductivity reduction of the resulting material with decreasing gas pressure can be divided into four stages(steep decreasing stage,slow decreasing stage,steep decreasing stage,and hardly changing stage) even if the contribution of large pores to the total porosity is very low(less than 10%).
引文
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[18]Pirard R,Rigacci A,Marechal J C,et al.Characterization of hyperporous polyurethane-based gels by non-intrusive mercury porosimetry.Polymer,2003,44(17):4881
[19]Pirard R,Alie C,Pirard J P.Characterization of porous texture of hyperporous materials by mercury porosimetry using densification equation.Powder Technol,2002,128(2-3):242
[20]Alie C,Pirard R,Pirard J P.Mercury porosimetry:applicability of the bucking-intrusion mechanism to low-density xerogels.JNon-Cryst Solids,2001,292(1-3):138
[21]Wiener M,Reichenauer G,Braxmeier S,et al.Carbon aerogelbased high-temperature thermal insulation.Int J Thermophys,2009,30(4):1372
[22]Bi C,Tang G H.Study of coupling heat transfer between solid and gas phases in nanoporous aerogel.J Eng Thermophys,2015,36(6):1315(毕成,唐桂华.多孔材料气凝胶气固耦合传热研究.工程热物理学报,2015,36(6):1315)
[23]Zhao J J,Yu H T,Duan Y Y,et al.Analysis of aerogel thermal conductivity based on the microstructure.J Eng Thermophys,2013,34(10):1926(赵俊杰,于海童,段远源,等.基于微观结构的气凝胶热导率分析.工程热物理学报,2013,34(10):1926)
[24]Zhao J J,Duan Y Y,Wang X D,et al.A 3-D numerical heat transfer model for silica aerogels based on the porous secondary nanoparticle aggregate structure.J Non-Cryst Solids,2012,358(10):1287
[25]Li Z Y,Zhu C Y,Zhao X P.A theoretical and numerical study on the gas-contributed thermal conductivity in aerogel.Int J Heat Mass Transfer,2017,108:1982
[26]Bi C,Tang G H,Hu Z J,et al.Coupling model for heat transfer between solid and gas phases in aerogel and experimental investigation.Int J Heat Mass Transfer,2014,79:126
[27]Lee O J,Lee K H,Yim T J,et al.Determination of mesopore size of aerogels from thermal conductivity measurements.J NonCryst Solids,2002,298(2-3):287
[28]Tamon H,Kitamura T,Okazaki M.Preparation of silica aerogel from TEOS.J Colloid Interface Sci,1998,197(2):353
[29]Swimm K,Vidi S,Reichenauer G,et al.Coupling of gaseous and solid thermal conduction in porous solids.J Non-Cryst Solids,2017,456:114
[2]Hu Z J,Li J N,Sun C C,et al.Recent developments of nano-superinsulating materials.Mater China,2012,31(8):25(胡子君,李俊宁,孙陈诚,等.纳米超级隔热材料及其最新研究进展.中国材料进展,2012,31(8):25)
[3]Koebel M,Rigacci A,Achard P.Aerogel-based thermal superinsulation:an overview.J Sol-Gel Sci Technol,2012,63(3):315
[4]Chen D P,Hou K Y,Wang L J,et al.Status and development of fire protection materials based on super thermal insulator and their application prospect in urban underground space.Chin J Eng,2017,39(6):811(陈德平,侯柯屹,王立佳,等.超级绝热型防火材料的研究进展及其在城市地下空间的应用展望.工程科学学报,2017,39(6):811)
[5]Hüsing N,Schubert U.Aerogels-airy materials:chemistry,structure,and properties.Angew Chem Int Ed,1998,37(1-2):22
[6]Qiao J H,Bolot R,Liao H L,et al.Knudsen effect on the estimation of the effective thermal conductivity of thermal barrier coatings.J Therm Spray Technol,2013,22(2-3):175
[7]Spagnol S,Lartigue B,Trombe A,et al.Experimental investigations on the thermal conductivity of silica aerogels by a guarded thin-film-heater method.J Heat Transfer,2009,131(7):074501-1
[8]Zhu C Y,Li Z Y,Zhao X P,et al.The DSMC study on gas heat conduction in nanoscale.J Eng Thermophys,2016,37(5):1027(朱传勇,李增耀,赵新朋,等.纳米尺度下气体导热的DSMC模拟.工程热物理学报,2016,37(5):1027)
[9]Swimm K,Reichenauer G,Vidi S,et al.Gas pressure dependence of the heat transport in porous solids with pores smaller than10μm.Int J Thermophys,2009,30(4):1329
[10]Reichenauer G,Heinemann U,Ebert H P.Relationship between pore size and the gas pressure dependence of the gaseous thermal conductivity.Colloids Surf A,2007,300(1-2):204
[11]Zhang H,Li Z Y,Dan D,et al.The influence of gas pressure on the effective thermal conductivity of nano-porous material.J Eng Thermophys,2013,34(4):756(张虎,李增耀,丹聃,等.气氛压力对纳米多孔材料等效热导率的影响.工程热物理学报,2013,34(4):756)
[12]Zhao J J,Duan Y Y,Wang X D,et al.Effects of solid-gas coupling and pore and particle microstructures on the effective gaseous thermal conductivity in aerogels.J Nanopart Res,2012,14(8):1024
[13]He Y L,Xie T.A review of heat transfer models of nanoporous silica aerogel insulation material.Chin Sci Bull,2015,60(2):137(何雅玲,谢涛.气凝胶纳米多孔材料传热计算模型研究进展.科学通报,2015,60(2):137)
[14]Zhu C Y,Li Z Y.The numerical study on the gas-contributed thermal conductivity of aerogel.J Eng Thermophys,2017,38(8):1753(朱传勇,李增耀.气凝胶中气相贡献热导率的数值求解.工程热物理学报,2017,38(8):1753)
[15]Coquil T,Fang J,Pilon L.Molecular dynamics study of the thermal conductivity of amorphous nanoporous silica.Int J Heat Mass Transfer,2011,54(21-22):4540
[16]Raed K,Gross U.Modeling of influence of gas atmosphere and pore-size distribution on the effective thermal conductivity of knudsen and non-knudsen porous materials.Int J Thermophys,2009,30(4):1343
[17]Yang H L,Wang X T,Wang Q,et al.Study on mercury porosimetry and gas sorption for pore structure characterization of nano-porous super thermal insulating materials.Acta Mater Compos Sin,2013,30(Suppl):273(杨海龙,王晓婷,王钦,等.压汞和气体吸附在纳米超级隔热材料孔隙结构表征中的应用研究.复合材料学报,2013,30(增刊):273)
[18]Pirard R,Rigacci A,Marechal J C,et al.Characterization of hyperporous polyurethane-based gels by non-intrusive mercury porosimetry.Polymer,2003,44(17):4881
[19]Pirard R,Alie C,Pirard J P.Characterization of porous texture of hyperporous materials by mercury porosimetry using densification equation.Powder Technol,2002,128(2-3):242
[20]Alie C,Pirard R,Pirard J P.Mercury porosimetry:applicability of the bucking-intrusion mechanism to low-density xerogels.JNon-Cryst Solids,2001,292(1-3):138
[21]Wiener M,Reichenauer G,Braxmeier S,et al.Carbon aerogelbased high-temperature thermal insulation.Int J Thermophys,2009,30(4):1372
[22]Bi C,Tang G H.Study of coupling heat transfer between solid and gas phases in nanoporous aerogel.J Eng Thermophys,2015,36(6):1315(毕成,唐桂华.多孔材料气凝胶气固耦合传热研究.工程热物理学报,2015,36(6):1315)
[23]Zhao J J,Yu H T,Duan Y Y,et al.Analysis of aerogel thermal conductivity based on the microstructure.J Eng Thermophys,2013,34(10):1926(赵俊杰,于海童,段远源,等.基于微观结构的气凝胶热导率分析.工程热物理学报,2013,34(10):1926)
[24]Zhao J J,Duan Y Y,Wang X D,et al.A 3-D numerical heat transfer model for silica aerogels based on the porous secondary nanoparticle aggregate structure.J Non-Cryst Solids,2012,358(10):1287
[25]Li Z Y,Zhu C Y,Zhao X P.A theoretical and numerical study on the gas-contributed thermal conductivity in aerogel.Int J Heat Mass Transfer,2017,108:1982
[26]Bi C,Tang G H,Hu Z J,et al.Coupling model for heat transfer between solid and gas phases in aerogel and experimental investigation.Int J Heat Mass Transfer,2014,79:126
[27]Lee O J,Lee K H,Yim T J,et al.Determination of mesopore size of aerogels from thermal conductivity measurements.J NonCryst Solids,2002,298(2-3):287
[28]Tamon H,Kitamura T,Okazaki M.Preparation of silica aerogel from TEOS.J Colloid Interface Sci,1998,197(2):353
[29]Swimm K,Vidi S,Reichenauer G,et al.Coupling of gaseous and solid thermal conduction in porous solids.J Non-Cryst Solids,2017,456:114