Investigation Of Pore Structure Control And Performance Of Porous Alumina Insulating Refractories Fabricated By Pore Forming Agent Method

The heat insulation performance of the industrial furnace plays a key role in the energy conservation and environment protection. Alumina insulating refractories are widely used as the best heat insulation materials for the high-temperature furnace owing to its excellent thermomechanical properties, high working temperature and superior strength. Most of the commercial alumina insulating refractories which applied in the industries is porous alumina bubble bricks. During the preparation of such material, alumina hollow spheres was fabricated by the electric smelting method. Such method is complicated and leads to a large amount consumption of electricity. Meanwhile, the thermal conductivity and bulk density of such products can be quite high. To conquer these drawbacks, direct sintering technology exhibit lower energy consumption, which may be a candidate method for the new generation of alumina insulting materials. For such materials, the pore structure is a key parameter to affect their performance among all the factors. However, it is rarely reported that the reduction of thermal conductivity and enhancement of mechanical properties of alumina insulating refractories by the optimization of pore structure parameters(porosity, pore size, pore size distribution, pore shape).Aiming at the problems mentioned above, in this thesis, the effects of the pore-forming agents such as rice husk, sawdust, starch and carbon black on the thermal conductivity, mechanical properties and thermal shock resistance of alumina insulating refractories were firstly investigated, MIAPS software(Image method) was carried out to analyze the pore structure. Based on the results, grey relation theory and organizational theory were also applied to reveal the correlation of pore structure and thermal conductivity; fractal dimension was also used to investigate the effects of pore structure on the mechanical properties; the parameters of the thermal shock resistance of the alumina insulating refractories were calculated with the assistance of fracture mechanics. The results above provide a new method the develop the alumina insulating refractories with low thermal conductivity, high strength and excellent thermal shock resistance. Based on these results, the pre-fired alumina insulating refractories specimens were vacuum-immersed by the alumina sol in the and then fired at high temperature. According to the experimental results, the following conclusions can be obtained:1. The alumina insulating refractories were prepared using rice husk and sawdust as the pore-forming agent, respectively. MIAPS analysis software was also applied to characterize the pore structure of the specimens. It is found that MIAPS software can be an efficient approach to evaluate the pore structure parameter of the insulating refractories. With the increment of such pore-forming agents, the porosity and average pore size of the specimens increased whereas the thermal conductivity tended to decrease. However, the presence of the slit shaped pores after the burning-out of the pore-forming agents lead to the stress concentration, the impurities in the pore-forming agents such as Na2 O and K2 O were also left, thus resulting in the lower mechanical properties and refractoriness under load. To improve the mechanical properties, the pore shape was modified by immersing by alumina sol. It was found the slit shaped pores were partly transformed into quasi-spherical shape with smaller pore size. Accordingly, the mechanical properties were improved.2. In order to further improve the overall performance of the alumina insulating refractories, starch and carbon black were introduced as the pore-forming agents due to their spherical shape and high purity. The introduction of starch with micron-scale particle size can generate a large number of spherical pores in alumina insulating refractories and was beneficial to decrease the thermal conductivity. Unfortunately, the large pore size also resulted in an inferior mechanical properties and thermal shock resistance. Compared with other pore-forming agents, the introduction of carbon black with nano-scale particle size into the alumina insulating refractories exhibited the lowest average pore size and thermal conductivity, the best mechanical properties and thermal shock resistance.3. The correlation of pore structure and thermal conductivity, mechanical properties, thermal shock resistance was investigated using fractal theory, grey correlation theory and organizational theory. The results showed that the thermal conductivity was mainly effected by the <2?m pores. The mechanical properties was influenced by the porosity and pore shape factor. The decrease of porosity, the increasing number of spherical pores and the homogenized distribution of pores lead to the excellent mechanical properties; the thermal shock resistance was mainly effected by average pore size, fractal dimension and the intervals of the pores. The average pore size plays the most important role in the thermal shock resistance.4. Based on the above results, alumina insulating refractories were fabricated using carbon black as the pore-forming agent, the specimens were fired at 1400? and then immersed with alumina sol in a vacuum environment. After that, the specimens were fired at 1600?. Finally, the insulating refractories with the pores of spherical shape, smaller size and lower fractal dimension were obtained. Compared with porous alumina bubble bricks, the specimens immersed in the alumina sol with 5wt% solid content exhibited much lower thermal conductivity. The considerable mechanical properties and thermal shock resistance were also obtained.