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MICROSTRUCTURE EVOL UTION IN THE PREPARATION OF CARBON AEROGELS BY SMALL ANGLE X-RAY SCATTERING

注意:本論文已在《新型碳材料》2002,17(3):31–36發表
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ZHANG Rui 1 ,  XU Yao 1 ,  LU Yonggen 1 ,  LI Zhihong 1 ,  MENG Qinghan 1 ,  LI Kaixi 1 , WU Dong 1 ,  LING Licheng 1 ,  WANG Jun 2,  ZHAO Hui 2 ,  RONG Lixiang 2 ,  DONG Baozhong 2
(1. State Key Laboratory of Coal Conversion , Institute of Coal Chemistry , Chinese Academy of Sciences , Taiyuan  030001 , China ;
2. Institute of High Energy Physics , Chinese Academy of Sciences , Beijing  100039 , China)

Abstract:Solgel polymerization of phenolic resole and methylolated melamine in basic aqueous solution at 85 ℃for 5 days led to the formation of hydrogels , which , after water in the hydrogels was exchanged with acetone , were dried under supercritical carbon dioxide to produce organic aerogels . Carbon aerogels were obtained by pyrolyzing these organic aerogels at 850 ℃for 3 h in nitrogen. Microstructure evolution during the preparation was followed by small angle X-ray scattering. It is found that hydrogel is monodispersed , swelled by water , and has a sharp peak in the scattering intensity profile , indicating a spatial periodical nanostructure formed by nanoscale phase separation via spinodal decomposition. After solvent exchanging , the acetone gel thus obtained is also monodispersed , but is partly deswelled , and has a shoulder2like scattering peak. This is a result of a substantial increase in scattering intensity near the lower limit of the scattering vector , q caused by domain growth of the scattering entity. After supercritical carbon dioxide drying , the organic aerogels produced are polydispersed and deswelled completely by removal of the solvent. The peak in the scattering intensity profile disappears completely by further growth of domains as a result of the structural relaxation from an unstable to a more stable state , induced by an increase in instability of the gel formed by supercritical drying. Moreover , a diffuse interface layer characterized by negative deviation from the Porod law is found in organic aerogels . After pyrolysis , the polydispersion is retained in the carbon aerogel , however , the diffuse interface layer disappears due to migration of the layer to particles to reduce the interface energy. Also in this process , joint actions of further growth in domain size and volume reduction by mass loss and densification slightly broaden the distribution of the radius of gyration of scattering entities , and shift the most probable peak radius of gyration to the small size direction.
Keywords: Carbon aerogel ; Pyrolysis ; Small angle X-ray scattering ; Microstructure

小角 X 射線散射研究制備炭氣凝膠過程中凝膠的微結構演化

張  睿1 ,  徐  耀1 ,  呂永根1 ,  李志宏1 ,  孟慶函1 ,  李開喜1 ,  吳  東1 ,  凌立成1 ,  王  俊2 ,  趙  輝2 ,  榮利霞2 ,  董寶中2
(1. 中國科學院 山西煤炭化學研究所 , 山西 太原  030001 ;  2. 中國科學院 高能物理研究所 , 北京  100039)


摘  要:  酚醛樹脂和羥甲基化蜜胺在堿性水溶液中在 85 ℃水浴中反應 5 d 經溶膠 - 凝膠過程形成了水凝膠。水 凝膠經丙酮置換產生酮凝膠 ,而后經超臨界二氧化碳干燥生成有機氣凝膠。有機氣凝膠在氮氣氛中 800 ℃下裂解 3 h 形成了炭氣凝膠。采用小角 X - 射線散射技術和散射理論對上述四種凝膠的微結構進行了分析。結果發現:水 凝膠粒子是單分散的 ,被水溶脹并在 0. 15 nm 處有強的散射峰 ,表明具有納米尺度的空間周期結構 ,這種結構是通 過不穩態納米尺度相變 ———旋節微相分離產生的。由丙酮置換產生的酮凝膠也是單分散的 ,被丙酮稍微溶脹并在 0. 15 nm 具有肩峰 ,表明水凝膠在溶劑置換過程中發生輕微團聚產生的大粒子在較小角處發生強烈的散射致使強 峰演化成肩峰。有機氣凝膠和炭氣凝膠都是多分散的 ,在小角處沒有散射峰 ,表明在超臨界干燥過程中粒子的團 聚繼續發生導致多分散性并掩蓋了納米尺度的空間周期結構致使散射峰消失。有機氣凝膠中存在的擴散界面層 在其裂解過程中消失 ,這可能是由于在裂解過程中擴散界面層向粒子遷移以降低界面能所致。從小角 X - 射線散 射數據用 Shull2Roess 法得到的有機氣凝膠和炭氣凝膠的回轉半徑分布比較可知 ,在裂解過程中 ,粒子的團聚長大、 質量損失和致密化共同作用的結果使得炭氣凝膠的回轉半徑分布比有機氣凝膠稍寬 ,最可幾回轉半徑稍微減小。
關鍵詞:  炭氣凝膠;裂解;小角 X 射線散射;微觀結構

 

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