【作者】 罗明；

【导师】 李亚伟；

【作者基本信息】 武汉科技大学 ， 材料学， 2013， 博士

【摘要】 碳复合耐火材料具有优异的热震稳定性和抗渣侵蚀性能而被广泛用作转炉、电炉、钢包等炼钢和连铸系统的炉衬材料，而着眼于当前世界各国―低碳经济‖的外部环境，以及进一步满足冶炼洁净钢的要求，传统碳复合耐火材料必然向低碳、超低碳方向发展。但单纯降低传统碳复合耐火材料中的鳞片石墨含量，会导致材料的韧性降低、热震稳定性能变差。碳纳米管（carbonnanotubes, CNTs）作为一种新型纳米碳源具有非常优异的力学性能，将其部分或全部取代鳞片石墨引入到低碳碳复合耐火材料中，有望解决材料韧性低、热震稳定性差的问题。从目前来看，限制碳纳米管在碳复合耐火材料中应用的主要原因是其成本高、在材料中易发生团聚导致分散困难以及高温复杂环境下易发生结构蚀变等。针对上述存在的问题，本论文首先探讨了多壁碳纳米管（multi-walled carbon nanotubes,MWCNTs）在高温复杂环境下的结构演变规律，系统研究了MWCNTs表面修饰聚碳硅烷（Polycarbosilane, PCS）以及原位裂解形成SiCxOy陶瓷涂层的工艺条件，以解决MWCNTs在碳复合耐火材料中的结构蚀变问题。另一方面，系统研究了催化剂Fe、Co、Ni的硝酸盐掺杂酚醛树脂的裂解碳结构及原位催化形成碳纳米管的生成机理，旨在为解决碳纳米管的使用成本和分散问题提供另一条途径。在上述研究工作的基础上，研究了碳纳米管复合铝碳耐火材料的显微结构与力学性能之间的关系。本论文可以得到如下结论：（1）高温复杂环境下，MWCNTs的结构演变主要与不同硅源作用下体系中Si（g）和SiO（g）的分压密切相关。以Si为硅源时，体系中的Si（g）分压最高，其不断在MWCNTs表面反应和沉积，使得较低温度下MWCNTs表面生成了SiC反应层，而较高温度下MWCNTs演变成SiC纳米线；以Si+SiO₂作为硅源时，体系中的SiO（g）分压最高，其与CO（g）反应并不断沉积，使得MWCNTs表面形成了无定形SiO₂-SiC的反应层结构，且SiO₂反应层的厚度随着处理温度的上升而不断增大；而以Al+SiO₂作为硅源时，体系中Si（g）和SiO（g）的分压均最低，MWCNTs即使经高温（1500）处理后表面也只能生成很薄的反应层。MWCNTs的氧化温度和氧化活化能因其表面形成上述反应层而大幅度提高，主要由反应层的厚度决定的。（2）在MWCNTs表面功能修饰PCS，高温作用下原位裂解形成SiCxOy陶瓷涂层，为阻止MWCNTs的结构蚀变提供一条新的途径。MWCNTs的抗氧化性能也因其表面形成的陶瓷涂层而大幅度提高，主要由涂层的厚度决定，与处理过程中PCS的浓度和裂解温度密切相关。（3）在铝碳耐火材料中引入MWCNTs会对其显微结构和力学性能产生影响。MWCNTs的引入，提高了不同温度处理后材料的抗折强度、弹性模量和形变位移量等力学性能。当处理温度低于1000时，MWCNTs自身对材料进行增强增韧作用；高于1000时，MWCNTs和原位形成的陶瓷晶须对材料进行协同增强增韧作用。但随着MWCNTs含量的增加，其发生团聚降低了材料的力学性能。经PCS修饰后的MWCNTs在铝碳耐火材料中的分散性大幅度改善，同时材料热处理过程中PCS裂解在MWCNTs表面原位生成SiCxOy陶瓷涂层，阻止了MWCNTs的结构蚀变并提高了MWCNTs与基体之间的界面结合，进一步提高了材料的力学性能和抗氧化性。（4）采用过渡金属元素的硝酸盐对酚醛树脂进行掺杂处理，经高温裂解后原位催化形成CNTs，为解决CNTs在碳复合耐火材料中使用成本和分散问题提供一条新的途径。随着处理温度的升高，掺杂树脂裂解碳中形成的碳纳米管等纳米石墨碳结构含量增加，石墨化度不断增加。其中，硝酸镍在掺杂树脂裂解过程中更容易以金属单质形式存在，相对于硝酸铁和硝酸钴来说具有更好的催化性能。同时，上述催化剂还能够促进铝碳耐火材料基质内部AlN、Al4C3和SiC等陶瓷晶须的形成。（5）基于上述研究工作，将硝酸镍掺杂酚醛树脂引入到铝碳耐火材料中，在材料内部能够原位催化形成MWCNTs，同时在较高的温度（高于1000）下催化形成更多的陶瓷晶须。原位形成的MWCNTs以及其与陶瓷晶须协同增强增韧作用分别赋予Al₂O₃-C耐火材料在较低（低于1000）和较高温度（高于1000）下更加优异的力学性能。更多还原

【Abstract】 Carbon containing refractories have been widely used in steelmaking and continuous castingsystems like converter, electric furnace, steel ladles, due to their excellent thermal shock and slagresistance. With a view to current―low carbon economy‖environment all over the world and therequirement to develop clean steel, carbon containing refractories toward low or ultra-lowcarbonization is the direction of development in refractory field. However, decreasing the content ofgraphite flake in the traditional carbon containing refractories can decrease the tougthness anddeteriorate the thermal shock resistance of the refractories. Carbon nanotubes （CNTs） as a kind ofnew carbon sources, possess many excellent mechanical properties. Therefore, when they arepartially or toally used to replac graphite flake and incorporated into carbon containing refractories,the problems of low toughness and bad thermal shock resistance can be solved by this way. From thepresent point of view, the main challenges which limit the usage of CNTs in carbon containingrefractories are their high cost, difficulty in homogeneous dispersion in the matrix as well asstructural transformation and so on.Based on the problems above, the microstructural evolution of multi-walled carbon nanotubes（MWCNTs） in the high-temperature and complicated enviroment is firstly studied in this thesis.Secondly, in order to slove the problem of structural evolution, MWCNTs are modified withpolycarbosilane （PCS） which pyrolyzes into SiCxOyceramic coating in situ on the surface ofMWCNTs during the heating treatment. On the other hand, the structure of pyrolysis carbon andformation mechanism of CNTs from Fe, Co and Ni nitrate doped phenolic resin are studied atdifferent treated temperatures, in order to offer another way to solve the high cost and dispersionproblem of CNTs used in carbon containing refractories. On the basis of the work above, therelationship between microstructure and mechanical properties of Al₂O₃-C refractories is studied indetail and the conclusions can be drawn as follows:（1） In high-temperature and complicated enviroment, the microstructural evolution of MWCNTs isclosely associated with the partial pressures of Si（g） and SiO（g） in the systems using different siliconsources. When Si is used as the silicon source, Si（g） partial pressure in the system is the highest andSiC reaction layer forms on the surface of MWCNTs at low treated remperature due to the depositionand reaction of Si（g）. Most of MWCNTs transform into SiC nanowires with gradually increasing thethe coking temperature. Using Si+SiO₂as the silicon source, the partial pressure of SiO（g） is thehighest and amorphous SiO₂-SiC reaction layer forms on the surface of MWCNTs due to thedeposition and reaction between SiO（g） and CO（g）. Meanwhile, the thickness of SiO₂layer increaseswith increasing the treated temperature. Using Al+SiO₂as the silicon source, the partial pressures ofSi（g） and SiO（g） are both the lowest. Only a very thin reaction layer forms on the surface ofMWCNTs even after treated at1500oC. Compared with as-received MWCNTs, the oxidationtemperatures and oxidation activation energy of the treated MWCNTs improves greatly, which isdetermined by the thickness of the reaction layer. （2） The surface of MWCNTs can be functionally modified with PCS molecules, which pyrolyzesinto SiCxOyceramic coating on the surface of MWCNTs in situ during the heating process, whichoffers a new way to solve the problem of MWCNTs transformation. The oxidation resistance ofcoated MWCNTs improves greatly compared with as-received ones, which is closely related to thePCS concentration and the treated temperatures that determine the thickness of the coating.（3） Addition of MWCNTs has a big influence on the microstructure and mechanical properties ofAl₂O₃-C refractories. The mechanical properties such as cold modulus of rupture, modulus of elastics,and deformation displacement of refractories with MWCNTs improve greatly. When the treatedtemperature is lower than1000oC, MWCNTs can strengthen and toughen the materials bythemselves. In addition, when the treated temperature is higher than1000oC, the synergeticstrengthening and toughening mechanisms of MWCNTs and ceramic whiskers can endow theAl₂O₃-C refractories with better mechanical properties compared with that containing only graphiteflake. With increasing the amount of MWCNTs, the mechanical properties of Al₂O₃-C refractoriesdecrease due to the agglomeration of MWCNTs. By comparison with as-received MWCNTs, thedispersion of MWCNTs after functionally modified is greatly improved in the matrix. Meanwhile,SiCxOyceramic coating forms in situ on the surface of MWCNTs during the heating treatmentprocess, which protects the intact structure of MWCNTs and improves the interface bond betweenMWCNTs and the matrix, leading to the improvement on the mechanical properties and oxidationresistance of Al₂O₃-C refractories.（4） The CNTs can grow in situ by the catalytic pyrolysis of transition metals nitrate doped phenolicresin, which offers a new way to solve the problems of high cost and dispersion of CNTs in one stepwhen they are used in carbon containing refractories. The graphitization degree of pyrolysis carbonof doped phenolic resin increases greatly due to the catalytic formation of graphite structureincluding the crystalline graphite structure like nano carbon and CNTs with increasing the treatedtemperature. By comparison with Fe and Co nitrate, Ni nitrate has the best catalytic activity due tothe easy existence in the form of metal Ni during the heating treatment of doped phenolic resin. Aswell, except for the formation of MWCNTs, the catalyst can also promote the growth of ceramicwhiskers such as AlN, Al4C3and SiC simultaneously in situ in Al₂O₃-C matrix specimens.（5） Based on the work above, Ni nitrate doped phenolic resin replacing as-received one isintroduced into Al₂O₃-C refractories. On the one hand, MWCNTs can form in situ due to the catalyticpyrolysis of doped phenolic resin. On the other hand, the catalyst can promote the formation of AlN,Al4C3and SiC whiskers in the matrix at the temperature higher than1000oC. The in situ formedMWCNTs as well as the synergistic effect of MWCNTs and whiskers can endow the Al₂O₃-Crefractories with much better mechanical properties at different temperature stages.更多还原