Preparation And Properties Of Thermal Insulating/thermal Sealing Materials With Damage Tolerance And Low Thermal Conductivity

The extreme service condition of hypersonic vehicles,such as high heat flow,strong thermal shock and strong vibration is a considerable challenge for the thermal insulating materials and thermal sealing materials applied for thermal protection of hypersonic vehicles.However,there are still many shortcomings for the currently thermal insulating materials and thermal sealing materials applied in aerospace industry?like SiO₂,Al₂O₃,ZrO₂ and mullite?,such as poor damage tolerance,poor machinability and severe sintering at high temperature,which results that these materials are difficult to meet the performance requirements of thermal protection materials of hypersonic vehicles.Therefore,it is of great significance to develop new thermal insulating materials and thermal sealing materials with damage resistance and low thermal conductivity.Thus,this dissertation focused on the preparation and the properties investigation of thermal insulating materials and thermal sealing materials with damage tolerance,machinability and low thermal conductivity.In this dissertation,phase-pure ZrP₂O₇ and?-Zr₂O?PO₄?₂ powders were synthesized by a precipitation method and followed by calcination of precursors.To obtain dense ZrP₂O₇ and?-Zr₂O?PO₄?₂ ceramics,a 2 wt%MgO powder was added as sintering additive to promote the densification of ceramics.Based on the optimized sintering process parameters obtained by analyzing the dynamic curve of the sintering process,bulk ZrP₂O₇ and?-Zr₂O?PO₄?₂ with a relative density of 96%were prepared by pressureless sintering method.The mechanical properties and thermal properties of as-prepared bulk ZrP₂O₇ and?-Zr₂O?PO₄?₂ were investigated.The mechanical properties of ZrP₂O₇ and?-Zr₂O?PO₄?₂ demonstrate that these ceramics exhibit low shear deformation resistance,low hardness and high damage tolerance.Moreover,bulk ZrP₂O₇ and?-Zr₂O?PO₄?₂ also possess good machinability and can be machined readily by conventional cemented carbide tools.From the perspective of thermal properties,ZrP₂O₇ and?-Zr₂O?PO₄?₂ present low thermal expansion coefficients(3.2-3.3×10^-6/?),good thermal expansion recovery and low thermal conductivity,and their lowest thermal conductivity are 0.71 W·m^-1·K^-1?900??and 1.84W·m^-1·K^-1?1050??,respectively.In addition,?-Zr₂O?PO₄?₂ has good thermal shock resistance,and its critical thermal shock temperature difference?T_c is 700?.It is concluded that ZrP2O7 and?-Zr2O?PO4?2 ceramics are a new class of thermal insulating materials and thermal sealing materials with great application prospects.Good mechanical,thermal and sintering resistance properties of ZrP₂O₇ make it possess great prospects for thermal insulating application.Thus,a porous ZrP2O7ceramic with adjustable porosity was prepared by using starch as pore forming agent.The as-prepared porous ZrP₂O₇ ceramics present excellent structural stability at high temperature.After annealing at 1500?for 6 h,the volume shrinkage of porous ZrP₂O₇ ceramics is only 1.4%.Excellent structural stability also makes porous ZrP₂O₇ceramics have extremely low thermal conductivity at high temperature(0.14-0.18W·m^-1·K^-1).Overall,porous ZrP₂O₇ ceramic is considered as a competitive thermal insulating material due to its excellent sintering resistance.In order to improve the thermal stability of ZrP₂O₇ at high temperature,inspired by the high entropy effect,a new type of high-entropy ceramic?TiZrHf?P₂O₇ was designed and successfully prepared,which maintain phase stability at 1550?.Moreover,the thermal conductivity of?TiZrHf?P₂O₇ is 15%lower than that of ZrP₂O₇.This result also firstly proves the feasibility of improving the thermal stability of ceramics by forming high-entropy solid solution.For the lack of the recognition of the advantages of high-entropy ceramics in properties at present,a series of high entropy thermal insulating ceramics are also designed.It is found that high-entropy ceramics exhibit many superior properties due to the increase of the configuration entropy,such as improved phase stability,adjustable thermal expansion coefficients,sluggish grain growth rate and reduced thermal conductivity,which opens a new window for the development of novel thermal insulating materials.