In-situ Synthesis Of CNTs And SiC Whisker Bonding Phase And Their Effect On The Properties Of Al₂O₃-SiC-C Castables

Al₂O₃-SiC-C castable refractory samples were prepared from the main raw materials of tabular alumina,silicon carbide,ball pitch,reactive alumina powder and silica powder.Aluminium-silicon alloy powder was used as additives and calcium aluminate was used as binder.By means of X-ray diffraction,scanning electron microscope,energy spectrum analyzer and refractory material mechanical performance testing instrument,the effect of heat treatment temperature and carbon content on the phase composition,microstructure characteristics and mechanical property of Al₂O₃-SiC-C castable has been investigated with the change of ball pitch content.Ni（NO₃）₂·6H₂O was added to Al₂O₃-SiC-C castable,the effect of in situ one-dimensional ceramic phase such as SiC whiskers and catalyst content on structure and performance of the castable was investigated.The main research results were given as follows:（1）With the improvement of ball pitch content,the carbon nano tube with high aspect ratio was formed in a coke bed after fired at 1100℃.When the temperature was1450℃,it is found that the ceramic bonding phase of SiC whisker were well developed in situ,both of the aspect ratio and content of SiC whisker were increased with the improvement of pitch content.（2）The cold modulus of rupture（CMOR）and cold crushing strength（CCS）of Al₂O₃-SiC-C castable were decreased under the same heat treatment temperature with the increased of ball pitch content.The hot modulus of rupture（HMOR）of castable was improved by increase the amount of pitch added,in this experiment,the HMOR of castable sample with 5%ball pitch reached a maximum of 2.6MPa.It is also found that after three thermal shock cycles,the descend range of residual strength ratio of samples was increased with the increment of ball pitch to join.As the heat-treated temperature rising,the mechanical properties of materials with the same content of ball pitch were increased.It is believed that the substantial enhancement in the thermo-mechanical properties of Al₂O₃–SiC–C castable refractory specimens under room and elevated temperatures could be attributed to the synergistic effect of in situ formed SiC whiskers and CNTs.（3）The experimental results show that the in situ formed SiC whiskers and CNTs were detected in castable samples which added a certain amount of Ni（NO₃）₂·6H₂O as a catalyst after fired at 1450℃.On the other hand,the specimens added catalyst with 5%and 7%ball pitch respectively after fired at 1450℃presented a better performance compared to ones without catalyst,the property test show that the value of CMOR was increased by 11%and 22.4%,respectively.It is also found that the HMOR of samples containing catalyst shows almost 20.5%and 68.4%increased compared to specimens without catalyst respectively.Furthermore,after three thermal shock cycles,the residual strength ratio of samples with 5%and 7%ball pitch and 0.6%Ni（NO₃）₂·6H₂O was increased by 8.1%and 21.5%over the ones without Ni（NO₃）₂·6H₂O respectively.In addition,the elastic modulus of specimen added catalyst was almost 40.6%and 25%increased respectively compare with the ones without catalyst,also,the load displacement was almost 54.2%and 6.4%increased respectively.Moreover,the flexural toughness of the refractory castable was improved by 49.5%and 15.3%,respectively.（4）It is found that under the experimental condition,the particles of metallic nickel derived from catalyst precursor of Ni（NO₃）₂·6H₂O encourage the formation of liquid phase,which can dissolve SiO_（g）and CO_（g）effectively.The SiC whiskers were generated via dissolution-diffusion-precipitation mechanism.It is indicated that the growth process of SiC whiskers was follow V-L-S growth mechanism.The ball pitch have a process of pyrolysis at high temperature,the carbon atoms was generated after the pyrolysis of hydrocarbon gas and participate in the vapor deposition reaction with metallic Ni particle together.Finally,the carbon atoms will be segregated in the case of supersaturation,thus,the CNTs were formed.The growth of CNTs conform to bottom growth model.（5）When the catalyst of Ni（NO₃）₂·6H₂O is added in Al₂O₃–SiC–C castable,the formation of in situ SiC whiskers was detected after heat treating at 1450℃.It is found that the yield and aspect ratio of SiC whiskers increased as the amount of catalyst was increased.（6）With the increase of catalyst Ni（NO₃）₂·6H₂O amount,the CMOR,CCS,HMOR,fracture displacement and residual strength of Al₂O₃–SiC–C castable all have different range to ascend after fired at 1450℃.The experiment result shows that the castable specimen with 0.6wt%Ni（NO₃）₂·6H₂O addition exhibits better comprehensive property.