Study On The Preparation And Properties Of ABWs/Mullite Ceramics

Mullite ceramics have been considered as an ideal high-temperature structural ceramics due to the excellent high-temperature mechanical properties,chemical stability and low thermal conductivity.Thus,the mullite ceramics have good prospects for application in metallurgy,refractory,thermal insulation,aerospace,catalysis and other fields.However,the poor room-temperature mechanical properties of mullite ceramics greatly limits their practical application.In addition,the high sintering temperature and high thermal conductivity of mullite ceramics need to be solved,otherwise mullite ceramics can not meet the application of room temperature insulation field.Therefore,in this paper,the solid solution strengthening and whisker reinforcement were proposed to solve these problems.It introduced sintering additives Mn O₂ and aluminum borate whiskers（ABWs）as reinforcing and toughening materials for mullite ceramics.And the interlocking network structure of whiskers and the addition of porogenic agents were took into account their mechanical and thermal insulation properties.Based on the above background,the mullite ceramic materials were prepared by solid-phase sintering,and the optimal synthesis process of mullite ceramics was obtained by studying the raw material size,sintering temperature and holding time.On this basis,the strengthening mechanism of Mn O₂ as a sintering additive was further explored.Subsequently,based on the research idea of"multiphase strengthening and reaction self combination",the ABWs were generated in situ by adjusting the amount of H₃BO₃ addition,and the evolution of microstructure and phase transformation law of aluminum borate whisker reinforced mullite based（ABWs/Mullite）ceramic materials were systematically studied.Finally,the Response Surface Methodology（RSM）was used to introduce an appropriate amount of composite pore forming agent to optimize the parameters of ABWs/Mullite ceramics,and the matching law of thermal mechanical properties of ABWs/Mullite insulation ceramics was explored.The main research work was as follows:（1）Mullite ceramic materials were prepared by solid-phase reaction using Al₂O₃ and SiO₂ as raw materials,and the effects of raw material particle size,sintering temperature and holding time on the sintering behavior and reaction mechanism of mullite ceramics were investigated.It was found that in Al₂O₃-SiO₂ ceramic materials,the surface area of particle contact increases as the particle size of the raw material decreases,while the raw material activity is relatively large due to the existence of defects in the raw material,and mullite starts to be generated at a sintering temperature less than 1350°C when the average particle size of Al₂O₃ was d₅₀=10.17μm;With the extension of the holding time,the physical phase composition of the samples basically does not change,the bulk density gradually increased,the apparent porosity gradually decreased,and the change was not obvious after the holding time of 120 min;The flexural strength gradually increased with the increase of the holding time,and reached the maximum value of 19.1 MPa when the holding time was 180min.However,the mullite phase content reached 93.5 mol%when the holding time was 120 min,and the phase content change was not obvious when the holding time continued to be extended.Due to the small amount of impurities in the raw material and the mechanism of high-temperature decomposition of mullite phase,which led to the incomplete generation of mullite,the maximum value of 95.2 mol%was reached at 240 min of holding time.（2）Mullite based ceramic materials were prepared by using Al₂O₃(d₅₀=10.17μm)and SiO₂ as raw materials and Mn O₂ as sintering aid at 1300°C for 120 min to explore the effect of Mn O₂ on the properties and strengthening mechanism of mullite-based ceramic materials.The results showed that the addition of Mn O₂ effectively reduced the mullite phase generation temperature by nearly 50°C compared with that of the specimen without Mn O₂ addition.When the addition of Mn O₂ is 6 wt.%,the mechanical properties of mullite based ceramic materials were improved by nearly 6 times compared to the unadded samples,reaching 60.1 MPa.In addition,the thermal conductivity of the sample decreases and then increases with the increase of Mn O₂ addition,and the thermal conductivity of the sample reaches a peak of 2.91 W m^-1·°C^-1 at the addition amount of 4 wt.%.Research has also found that under the combined action of Al₂O₃-SiO₂-Mn O₂ ternary eutectic and solid solution strengthening,Mn ions first replace Al³⁺in the corundum lattice in the form of+3 valence,and ultimately exist in the corundum and mullite crystals in the form of Mn²⁺.The lattice distortion generated makes the reaction particles have higher activity.In order to balance the negative charge of Mn’_Al,the concentration of oxygen vacancies in the lattice increases,which greatly improves the diffusion efficiency of ions and effectively facilitates the reaction process.（3）Using Al₂O₃(d₅₀=10.17μm)and SiO₂ as raw materials,the influence of H₃BO₃,Mn O₂ and H₃BO₃ additions on the structure evolution mechanism and comprehensive properties of ABWs/Mullite ceramics was studied.The results show that aluminum borate is synthesized from Al₂O₃ by solid-phase sintering with 15 wt.%H₃BO₃ as raw material at a temperature between 1150°C and1200°C.Al₂O₃ and SiO₂ as raw material,aluminum borate was produced at1100°C.With the increase of temperature,aluminum borate crystal has obvious axial growth trend.With Al₂O₃ and SiO₂ as raw materials,6 wt.%Mn O₂ and H₃BO₃ larger than 25 wt.%were added together,and mullite-aluminium borate composite whisker was formed at 1200°C.As Mn O₂ has a great influence on the anisotropic growth rate of aluminium borate whisker,the formation of a large number of interlaced ABWs can significantly reduce the volume density of mullite-based ceramic materials and increase the porosity,thus reducing the thermal conductivity.The overall change trend of thermal conductivity of A-Hx and AS-Hx series samples decreases first and then increases with the increase of H₃BO₃ content at 1200°C sintering temperature,while that of A-M6Hx and AS-M6Hx series ceramics decreases continuously.It shows that adding Mn O₂and H₃BO₃ together at a certain temperature is more helpful to reduce the thermal conductivity of test samples.（4）The starch（S）and carbon black（BC）were as pore-making agents（S-BC）.The content of S-BC,the ratio of S/S-BC,the molding pressure were controlled to optimize the mechanical and thermal insulation properties of ABWs/Mullite ceramics by RSM method.Meanwhile,the matching rules of thermal-mechanical properties were explored.The results showed that the proposed models were effective and accurate.Under the defined constraints,the optimal process parameters were obtained as follows:the content of S-BC,the ratio of S/S-BC and molding pressure were 28.72 wt.%,40.45%and 20.32 MPa,respectively.The porosity,compressive strength,thermal shock resistance and thermal conductivity of optimized results were 63.72%,26.81 MPa,88.35%0.85 W m^-1·°C^-1,respectively.In addition,the matching law of thermal and mechanical properties of ABWs/Mullite thermal insulation ceramics could be predicted byσ=（1-P）^1.72e^5.02 and k_e=3.65795-5.60285P+1.93405P²,which provided a good guidance for practical application.