Pore structure of nano-porous thermal insulating materials and thermal transport via gas phase in their pores
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摘要: 為研究納米隔熱材料孔隙結構內部的氣體熱傳輸特性, 采用溶膠-凝膠工藝結合超臨界干燥技術, 制備了一系列具有不同孔隙結構特征的樣品, 通過熱導率、氮氣吸-脫附和真密度測試, 全面、準確獲取了其孔隙結構信息, 并專門、系統研究了孔隙結構特征與氣體熱傳輸特性之間的關系.研究結果表明: 與氣相貢獻熱導率相對應, 材料具有雙尺度孔隙結構特征, 并且當大孔隙尺度不及小孔隙的10倍時, 可進一步等效為單尺度孔隙.考慮氣固耦合傳熱的本征氣相貢獻熱導率隨孔隙尺度的增大而升高, 與氣相熱導率變化類似且成一定的比例關系, 孔隙尺度小于200 nm和大于500 nm時的比例系數分別為2.0和1.5, 200~500 nm時則為2.0~1.5.當大、小孔隙尺度的比值不超過10時, 或者這一比值為100~1000且大孔隙含量低于10%時, 氣相貢獻熱導率隨環境氣壓的降低依次呈現快速下降、緩慢下降和無變化三個階段; 當這一比值超過3000時, 即使大孔隙含量很低(不超過10%), 氣相貢獻熱導率也會依次呈現快速下降、緩慢下降、快速下降和無變化四個階段.Abstract: 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%).
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圖 1 納米隔熱材料的孔徑分布曲線
Figure 1. Pore diameter distribution of nano-porous thermal insulating materials
圖 2 納米隔熱材料氣相貢獻熱導率測試值與計算值. (a)?G1; (b)?G2; (c)?G3; (d)?G4; (e)?G5; (f)?G6; (g)?G7; (h)?G8; (i)?G9; (j)?G10
Figure 2. Measured and calculated gas-contributed thermal conductivity of nano-porous thermal insulating materials: (a)?G1; (b)?G2; (c)?G3; (d)?G4; (e)?G5; (f)?G6; (g)?G7; (h)?G8; (i)?G9; (j)?G10
圖 3 本征氣相貢獻熱導率與孔隙直徑之間的關系
Figure 3. Relationship between intrinsic gas-contributed thermal con-ductivity and pore diameter
圖 4 單一尺度孔隙結構內本征氣相貢獻熱導率和努森數隨環境氣壓的變化
Figure 4. Gas pressure dependence of intrinsic gas-contributed thermal conductivity and knudsen number of a porous material with a single pore diameter
圖 5 大尺度孔隙對納米隔熱材料本征氣相貢獻熱導率的影響. (a)?500 nm; (b)?5000 nm; (c)?50000 nm; (d)?150000 nm; (e)?250000 nm; (f)?350000 nm
Figure 5. Effect of large pores on intrinsic gas-contributed thermal conductivity of nano-porous thermal insulating materials: (a)?500 nm; (b)?5000 nm; (c)?50000 nm; (d)?150000 nm; (e)?250000 nm; (f)?350000 nm
表 1 納米隔熱材料的制備條件
Table 1. Synthesis condition of nano-porous thermal insulating materials
樣品 物質的量之比 催化劑濃度/(mmol·L-1) HCl AcH NH3·H2O G1 n [TMOS]: n [MeOH]: n [H2O] =1:8:4 — — 10.80 G2 n [TEOS]: n [EtOH]: n [H2O] =1:7:4 5.88 — 22.10 G3 n [TEOS]: n [EtOH]: n [H2O] =1:7:3 0.80 — 2.00 G4 n [TEOS]: n [EtOH]: n [H2O] =1:7:3 0.80 — 5.00 G5 n [TEOS]: n [EtOH]: n [H2O] =1:7:3 0.80 — 10.00 G6 n [TEOS]: n [EtOH]: n [H2O] =1:7:3 0.80 — 2.00 G7 n [TEOS]: n [EtOH]: n [H2O] =1:7:3 0.80 — 2.00 G8 n [TEOS]: n [EtOH]: n [H2O] =1:7:3 0.80 — 2.00 G9 n [TEOS]: n [EtOH]: n [H2O] =1:3.5:3 0.80 — 20.00 G10 n [TBOT]: n [EtOH] : n [H2O] =1:5:4 — 2.80 — 
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表 2 納米隔熱材料的物理性質
Table 2. Physical properties of nano-porous thermal insulating materials
樣品 表觀密度，ρ/(g·cm-3) 真密度，ρs/(g·cm-3) 孔隙率，?/% 理論孔體積，V/(cm3·g-1) 測試孔體積，VBJH/(cm3·g-1) 孔體積測得率，φ/% 外比表面積，Sext/(m2·g-1) 氣相貢獻熱導率，kg-c/(W·m-1·K-1) 理論平均孔隙直徑，Dc/nm 測試峰值孔徑，DBJH/nm 大尺度孔隙等效孔徑，Dg-c/nm R = Dg-c/DBJH G1 0.212 2.0759 89.79 4.24 3.87 91.38 798 0.00660 21 14 2000 143 G2 0.203 2.0830 90.25 4.45 4.25 95.59 913 0.00843 20 27 10000 370 G3 0.203 2.0041 89.87 4.43 3.48 78.61 341 0.01199 52 61 300 5 G4 0.193 1.9875 90.29 4.68 3.50 74.82 215 0.01389 88 59 300 5 G5 0.194 1.9773 90.19 4.65 0.65 13.98 87 0.01908 216 93 300 3 G6 0.269 2.0041 86.58 3.22 3.03 94.14 337 0.00955 39 58 300 5 G7 0.364 2.0041 81.84 2.25 2.20 97.85 330 0.00784 28 38 200000 5263 G8 0.419 2.0041 79.09 1.89 1.75 92.71 355 0.00729 22 27 200000 7407 G9 0.344 1.9280 82.16 2.39 0.95 39.78 48 0.02008 206 104 300 3 G10 0.369 3.5883 89.72 2.43 1.42 58.40 51 0.02345 193 106 700 7
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表 3 納米隔熱材料的氣相貢獻熱導率
Table 3. Gas-contributed thermal conductivity of nano-porous thermal insulating materials
樣品 kg-cB /(W·m-1·K-1) kg-cS /(W·m-1·K-1) kg-cI /(W·m-1·K-1) kg-cIB /(W·m-1·K-1) kg-cIS /(W·m-1·K-1) G1 0.00297 0.00363 — 0.03837 0.00442 G2 0.00148 0.00695 — 0.03719 0.00806 G3 0.00465 0.00734 0.01334 0.02419 0.01039 G4 0.00447 0.00942 0.01538 0.01966 0.01394 G5 0.01540 0.00368 0.02116 0.01985 0.02919 G6 0.00137 0.00818 0.01103 0.02700 0.01004 G7 0.00067 0.00717 — 0.03808 0.00895 G8 0.00211 0.00518 — 0.03660 0.00706 G9 0.01004 0.01004 0.02444 0.02029 0.03072 G10 0.01063 0.01282 0.02614 0.02848 0.02447
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