| Recently, SiO2 nanoporous thermal insulating material has become a research hotspot in the field of high-performance thermal insulating material, because of its excellent thermal insulation performance. It mainly contains SiO2 aerogel based thermal insulating material and SiO2 nano-powder based thermal insulating material.Firstly, SiO2 xerogel was prepared by sol-gel process and azeotropic distillation drying process in this study. The influence of azeotropic distillation technology on the formation of gel porous network structure was studied, and the changes of xerogel phase and microscopic pore structure of SiO2 xerogel during calcining process were investigated. The feasibility of SiO2 xerogel as substitution of aerogel for the preparation of aerogel based thermal insulating material was also discussed, for the preparation of aerogel based thermal insulating material in the condition of ambient pressure drying.Then, for the research on SiO2 nano-powder based thermal insulating material, the thermal behavior and the changes of chemical bond and phase during calcining process of the silica fume powder, precipitated SiO2 powder and fumed silica powder were analysed, respectively. The thermal insulation performance of the powder based thermal insulating materials as well as the pore structure parameters and microscopic structure of the all three powder compacts were also investigated. The research finally confirmed that the optimal matrix material of SiO2 nano-powder based thermal insulating material was fumed silica.Last, the researches on fumed silica based thermal insulating material mainly focused on the following three parts, including of the microstructure evolution of fumed silica compacts during compression, thermal treatment and doping process, the effects and action mechanisms of fibers, opacifiers and sintering inhibitors on the thermal insulation performance, mechanical property and thermal stability of fumed silica based thermal insulating material. And the effects of fumed silica based thermal insulating material on the temperature of molten steel and heat transfer of tundish layers using finite element analysis method. The obtained results are as follows:(1) SiO2 xerogel was prepared by sol-gel process and azeotropic distillation drying process. The hydroxyl group on the particle surface had been replaced by Si-OC4H9, and the particle surface hydrophobic modification and solvent replacement could be successfully completed. With calcination temperature increasing, SiO2 xerogel remained amorphous phase up to 1000?, but the viscosity sintering could induce the decrease of the specific surface area and pore volume. After calcined at 300?, the specific surface area, pore volume and average pore size were 924.62m2·g-1,0.754cm3·g-1 and about 4nm, respectively. The pore size was mainly smaller than 10nm and its distribution also spreaded over 100-160nm. But, there was still glaring gap of SiO2 xerogel as substitution of aerogel for the preparation of aerogel based thermal insulating material, because of the non-uniform distributions of micropores, mesopores and macropores.(2) Compared with silica fume powder and precipitated SiO2 powder, fumed silica powder had more excellent thermal stability and remained amorphous phase up to 1000?. Simultaneously, the thermal conductivity of fumed silica based thermal insulating material was obviously lower than the others, because of the typical nanoporous adsorption characteristics and the nanopore microstructure in the fumed silica compact. So, the research finally confirmed that the optimal matrix material of SiO2 nano-powder based thermal insulating material was fumed silica.(3) During the compression process of fumed silica, the microscopic morphology of aggregates did not change significantly and nitrogen adsorption-desorption isotherm of compacted samples remained almost unchanged at low relative pressure range, especially below 0.16. With the rising forming pressure, the enhanced hysteresis loops at high relative pressure range were observed and the increased effective porosity measured by nitrogen adsorption was also analysed. The specific surface area maintained basically stable. The total pore volume of nitrogen adsorption first steadily increased, then decreased, last increased to an approximately equal value with empty volume of compacts. While the forming pressure of 80MPa, the adsorption saturation indication of the compact was observed.(4) During the thermal treatment process of fumed silica compact, thermal driving force directly acted on the surface of original particles to produce mass transfer phenomenon, resulting in a significant reduction of specific surface area and nitrogen adsorption total pore volume. With the thermal treatment degree intensifying, transitional pore rearrangement phenomenon would occur in the compacts, and pore size became smaller and its distribution gradually narrowed. Adjacent particles in aggregate consolidated to induce the formation of a new bigger aggregate unit.(5) Regional nanopore structure would be destroyed by introducing opacifiers and fibers, resulting in the weakening of adsorptive capacity. But, the adsorptive capacity of fibers doped sample enhanced at high relative pressure range, because of the formation of fumed silica coating layer on fiber. The capillary condensation of fumed Al2O3 doped sample appeared at high relative pressure range due to the increased effective porosity measured by nitrogen adsorption.(6) Fumed silica based thermal insulating material with lower thermal conductivity and better flexural strength was obtained under the forming pressure of 2MPa. When the hot surface temperature was 1000?, the thermal conductivity was only 0.023W/(m-K) by introducing 20 mass% SiC with D50 of 3.029?m as opacifier. Compared with the fumed silica based thermal insulating material obtained by traditional mixing method, the dense fumed silica coating layer with the thickness of 4?m resulted from mechanofusion effect could make its thermal conductivity reduction up to 50%. The thermal stability at high temperatures would be significantly improved by introducing 10 mass% fumed Al2O3, and the volume shrinkage under 900? and 1000? were only 0.3% and 1.1%, respectively.(7) When using fumed silica based thermal insulating material as heat insulating layer of tundish, the thermal insulation performance was effectively improved. When the running time up to 3600s, compared with the tundish without heat insulating layer and the tundish with common thermal insulating material as heat insulating layer, the temperatures of molten steel raised 5.2? and 3.9?, respectively, and the cold surface temperatures of steel shell dropped 353? and 262?, respectively. When the heat insulating layer thickness was 5mm, its hot surface temperature was only 1096?. If increasing heat insulating layer thickness or placing heat insulating layer in the place between permanent layer and working layer, the effects on improving thermal insulation performance of tundish were not obvious. |
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