Preparation And Properties Of Environmentally Friendly Mullite Whiskers Reinforced Porous Ceramics

Construction waste,with its large quantity of emissions and low utilization rate,not only caused numbers of fertile land squeezed,water,soil and air polluted,but also caused serious secondary disasters.The main chemical components of the construction waste are SiO₂,Al₂O₃,CaO and so on,which are useful mineral resources.Mullite is a stable crystalline compound in the Al₂O₃-SiO₂ system under normal atmospheric pressure.Mullite porous ceramics,due to its excellent performance,such as low density,low thermal conductivity,good thermal shock resistance,low coefficient of thermal expansion,high temperature mechanical properties,corrosion resistance,etc.,are widely used in heat insulation,filtration,sound absorption,catalyst carrier and other suchlike fields.However,as porous material,it inevitably has the demerits that the porosity is high,but the mechanical strength is low,and traditionally,its preparation raw materials are kaolin or other suchlike non-renewable resources.So in this paper,the fine utilization of the construction waste,the preparation of the mullite porous ceramics,and the in situ whiskers reinforced porous ceramics were focused on,and the mullite whiskers reinforced porous ceramics were fabricated from the construction waste at low sintering temperature.While,the relationship between the technology,formula,structure and performance were detailedly studied,and the relevant mechanisms were detaildedly explored.Firstly,the physical and chemical properties of the construction waste and the fabrication and sintering mechanism of the mullite whiskers were studied.It was found that the main chemical constituents of the construction waste were SiO₂,Al₂O₃ and CaO,by means of infrared spectroscopy,thermogravimetric analysis,silicate rapid determination and crystal phase analysis,etal.Then,with construction waste as the raw material,Al₂O₃ powders as the supplementary aluminum source,the gas doping method and sintering aid doping method were comprehensively adopted,and the mullite whiskers reinforced porous ceramics were fabricated.Effects of the sintering temperature,holding time,heating rate,molding pressure and ball milling time on the open porosity,mechanical properties,pore size distribution,linear shrinkage,bulk density,microstructure,and phase composition of the sintered specimens were studied in detail.And the parameters in the ball milling process,molding process and the sintering process were observed.On the basis of better process parameters,effects of the whisker catalyst and sintering aid on the morphology of the mullite whiskers and the structure and properties of the porous ceramics were studied in detail.The experimental results showed that the morphology of the mullite whiskers and properties of the porous ceramics were greatly influenced by the fluoride?AlF₃,CaF₂?and metallic oxide?MoO₃,B₂O₃,V₂O₅,CeO₂?content and species,which were used as whisker catalyst and sintering aid,respectively.The introduction of a suitable amount of fluoride and sintering aid in the raw materials was beneficial for the mullitization between silicon dioxide and aluminum oxide,and enhanced the properties of the porous ceramics.The specimen obtained by co-adding 12 wt% AlF₃ and 3 wt% MoO₃,and sintering at 1350 °C for 1 h,exhibited excellent properties,including an open porosity of 67.4 ± 0.5% and biaxial flexural strength of 24.0 ± 0.8 MPa.The mullite whiskers were sintering aid uniformly distributed;the whiskers had a diameter of 0.05-0.5 ?m,length of 8-10 ?m,and aspect ratios?length to diameter ratio?of 20-30 on average.In addition,effects of different Al₂O₃ powder on the structure and properties of the porous ceramics were studied.The experimental results showed that the morphology of the mullite whiskers and properties of the porous ceramics were greatly influenced by alumina addition,particle size and crystal morphology.The raw materials with adding 50 wt% content of Al₂O₃ powder as the supplementary aluminum source was beneficial to the preparation of mullite whiskers with good morphology.The transitional alumina with different crystalline type prepared by precipitation method,due to its more surface defects and higher activity,promoted the fabrication of the mullite at low sintering temperature.Nano-alumina with smaller particle size and higher surface activation energy,also promoted the mullitization at low sintering temperature,but the reactant powders easily reunited,the whiskers was fine and unevenly distributed.Among them,?-Al₂O₃ powders,prepared from the precipitation method and calcined at 850?,when used as the supplementary aluminum source for the preparation of porous ceramics,the mullite whiskers was prepared at low sintering temperature of 1250?.And the specimens exhibited excellent properties,such as the opening porosity of 65.9 ± 0.8% and the bulk density of 1.28 g.cm^-3.Finally,effect of sintering aid on the preparation of mullite whiskers and whisker skeleton porous ceramics at low sintering temperature was studied.The results showed that,both the method of doping a separate of sintering aid and co-doping sintering aid effectively promoted the preparation of mullite whiskers at low sintering temperature and enhanced the properties of the whisker skeleton porous ceramics.For doping a separate of sintering aid method,the fluxing effect of B₂O₃ was obvious.The mullite whisker could be prepared at the sintering temperature of 1100?,aluminum borate whiskers were introduced,and the porous ceramics sintered at 1150? exhibited good performance,with an open porosity of 79.9±0.9%,the biaxial flexural strength of 15.6±0.7 MPa,the bulk density of 0.85 g.cm^-3,the thermal conductivity of 0.24 W.mK^-1,and the average sound absorption coefficient in 1000-2000 Hz of 0.80.For the co-doping sintering aid method,with doping 3 wt% MoO₃ and 3 wt% V₂O₅ and sintering at 1300?,the length of the mullite whiskers were long,the aspect ratio was high,the properties of the porous ceramics were excellent,including the open porosity of 73.4±0.7%,the flexural strength of 28.9±0.6 MPa,the bulk density of 0.93 g.cm^-3,the thermal conductivity of 0.36 W.mK^-1,and the average sound absorption coefficient in 1000-2000 Hz of 0.60.