Preparation,Structure And Properties Of Mullite Whiskers/Anorthite Porous Ceramics

The development of near-space vehicles involves national security and the peaceful use of space.It is one of the focuses of the current international competition for space technology and has tended to be a manifestation of comprehensive national strength.The harsh working condition puts forward higher requirements on the heat insulation materials used for hypersonic vehicles:ultra-light,high strength,and excellent thermal insulation performance.Referring to the application of porous ceramics as thermal insulation parts in space shuttle,a series of mullite whisker/anorthite porous ceramics with a designable structure have been fabricated through process adjustment and optimization in this thesis.At the same time,a systematic study on microstructure and properties has been carried out.The main research contents include:investigating the influence of important processing parameters?foaming agent concentration,solid content,mullite whisker content,etc.?on the structure and properties of the prepared materials;exploring the reaction mechanism of in-situ synthesis mullite whisker/anorthite porous ceramics;analyzing the heat-insulation mechanism and strengthening mechanism of the prepared composite materials.The main results are as follows.Using ?-Al₂O₃,Si O₂ and Ca CO₃ as raw materials,a series of porous anorthite ceramics with a porosity of 82.3?91.5%,a room temperature compressive strength of0.19-4.17 MPa,and a room temperature vacuum thermal conductivity of 0.025?0.09W/?m·K?have been fabricated by foam-gelcasting method through adjusting the foaming agent concentration?0.5?32 g/L?and solid content?25?55 vol.%?in a wide scale.The material with the bulk density of 0.33 g/cm³ and porosity of 88.2%has been prepared under the condition of a foaming agent concentration of 16 g/L and a solid content of 35 vol.%,and it shows excellent high temperature performance.The compressive strength at 1000°C is as high as 2.03 MPa,which is 1.47 times of that at the room temperature;the retention rate of residual strength after a thermal shock at1200?is 114.7%;the thermal conductivity from the laser flash method increases from0.085 W/?m·K?to 0.258 W/?m·K?as testing temperature raises from 25°C to 1200°C.The relationship between thermal conductivity and temperature can be expressed by formula of"l=0.0455ln?T?-0.0716".The refinement of the anorthite grains accounts for the excellent high-temperature mechanical properties.The combination of high porosity,small pore size,a large number of nano-domains in the anorthite grains and the glass phase in the grain boundary endow the material lower thermal conductivity.Using foam-gelcasting method,the extra mullite whiskers reinforced anorthite porous ceramic with a porosity>91%,a median pore size of about 50?m,and a room temperature thermal conductivity of less than 0.065 W/?m·K?was successfully prepared.The addition of mullite whiskers can not only effectively stabilize the foam to obtain fine pores,but also promote to form fine anorthite grains.When the amount of mullite whiskers is 20 mol.%,the porosity of the material is as high as 91.6%,the thermal conductivity is as low as 0.034 W/?m·K?.Its compressive strength at 1000 ^oC is 0.64MPa,three times to the pure anorthite porous ceramics with similar porosity.Small pore size and grains,more phase boundaries and grain boundaries are the main factors leading to lower thermal conductivity;the stable three-dimensional network skeleton composed of mullite whiskers and anorthite,small pore size and grains also contribute to the strength increment of the material.Using ?-Al₂O₃,Si O₂ and Ca CO₃ as the main raw materials and Al F₃·3H₂O as additive,porous green bodies were prepared by foam-gelcasting method.The effects of the crucible sealing and the calcinations times on the composition and structure of the prepared material were investigated in detail.The results show that the crucible always in a sealed state at the initial stage of calcination would result in a large amount of residual carbon dispersed in the sample.The presence of residual carbon will hinder the synthesis of anorthite and affect the densification of the skeleton.The synthesis and growth of mullite whiskers are mainly controlled by the gas-solid reaction mechanism,and its final morphology and aspect ratio are closely related to the state of crucible sealing.Using graphite paper to control the crucible sealing time during the sintering process,high-purity mullite whiskers/anorthite were in-situ synthesized by adjusting the ratio of alumina to silica in the raw material.The porosity,compressive strength and thermal conductivity of fabricated composites are 84.8?88.1%,1.2?2.3 MPa and0.05?0.12 W/?m·K?,respectively.Combined with foam-freeze casting,a novel mullite seed assisted in-situ synthesis technique was successfully adopted to prepare mullite whisker/anorthite porous ceramics with three-dimensional network structure.The addition of mullite seed not only effectively increases the number and aspect ratio of mullite whiskers,but also plays a role in reducing sintering shrinkage and pore size,inhibiting the growth of anorthite grains.Through this technique route,the prepared materials exhibited high purity,high porosity?87.7?90.2%?,low bulk density?0.29?0.36 g/cm³?,high compressive strength?0.65?3.31 MPa?and low thermal conductivity?0.067?0.112 W/?m·K??.The material containing a mullite content of 60 mol.%together with a seed content of 10%exhibits excellent high-temperature mechanical properties.The high-temperature compressive strength at 800 ^oC and 1000 ^oC are determined to be 2.99 MPa and 3.02 MPa,respectively,which is comparable to the value?3.03 MPa?at room temperature.The residual strength after one-cycle thermal shock at 1200?is 3.05 MPa.