Study On In-situ Mullite Reinforced Aluminum-chrome Phosphates High Temperature Wave-transparent Composites

High temperature wave-transparent materials are the protections of high-speed,ultra high-speed aircraft communication systems, and one of the key materials forhigh-speed precision-guided aircraft. With the escalation of flight speed and serviceenvironment, high temperature wave-transparent materials have been required formore excellent mechanical property, dielectric property and longer life. However,researchers have not found an ideal wave-transparent material with goodcomprehensive performance of heat resistance, transmission efficiency and bearingcapacity yet. Given that there are few varieties of present products, it is significant tocarry out a low-cost, high-performance aluminum-chrome phosphates compositesresearch.Aluminum-chrome phosphates have low dielectric constant, good chemicalstability and high melting point, but also have low mechanical property and highmoisture-absorb. Therefore researchers usually adopt combination way of siliceousfiber-reinforced and moisture-proof coatings to improve. However, the servicetemperature of siliceous fiber reinforced aluminum-chrome phosphates compositesare limited by low temperature resistance of siliceous fiber and the erosion after beingheated. Throughout the current state of phosphate-based composites research,utilizing particles and whiskers reinforced aluminum-chrome phosphates ceramicmatrix composites is one of the best ways to decrease moisture absorption rate and toimprove high temperature service capacity. But few ceramic particles and whiskerscan match the chemical and thermal compatibility of aluminum-chrome phosphates,and mullite is one of them, which also has a low dielectric constant and hightemperature performance. Thus, new aluminum-chrome phosphates composite systemand enhance process have been proposed namely in-situ mullite particles andwhiskers reinforced aluminum-chrome phosphates multiphase ceramics system, onbasis of the good chemical and thermal compatibility between aluminum-chromephosphates and mullite, and the insufficient heat resistant ability of siliceousfiber-reinforced composites. Meanwhile preliminary theoretical prediction of its hightemperature dielectric property has been taken in order to provide more reference forthe development of high-temperature wave-transparent materials.The indications of the study are as follows：1. Thermal properties study of aluminum-chrome phosphates matrix showed that,the matrix was amorphous polymeric structure in160~900, and then crystallizedand decomposed successively in900~1300, finally, constituted by final productsAlPO4and Cr2O3in1300~1600. Thermal transformation of matrix from160to1600lead to a fluctuant dielectric constant, but finally stabled after heatingabove1300, which moisture absorption rate <0.002wt%.2. Obtained an in-situ mullite particles reinforced aluminum-chrome phosphatesmatrix multiphase ceramics, which normal temperature flexural strength,1000flexural strength, fracture toughness, dielectric constant, loss tangent, Vickers hardness and critical thermal shock temperature difference are157.45MPa,143.78MPa,4.08MPa·m1/2,<3.8,<0.05(8~12GHz),550Hv and <200. Controlingraw materials crystal forms at mullitization temperature is the key step to prepare acracking-free composite. Thermodynamics analysis of this system shows that: bothstandard Gibbs free energy nonlinear reduced with temperature rise, whilealuminum-chrome phosphates has a lower, mullite reaction met the mullitizationkinetics model with maximum rate at1500, matrix densification controlled by freeSiO2. System is grain boundary enhanced multiphase ceramics and TEM displayed noobvious boundary between the two phases. Mullite had obviously pinning effect formatrix grain boundaries. Both particles were incorporated to improve the strength andtoughness by residual stress induced crack deflection, crack branching and grain’spull-out.3. Obtained an in-situ mullite whiskers reinforced aluminum-chrome phosphatesmatrix multiphase ceramics, which normal temperature flexural strength,1000flexural strength, fracture toughness, dielectric constant, loss tangent, Vickershardness and critical thermal shock temperature difference are135.60MPa,121.71MPa,4.52MPa·m1/2,<3.6,<0.05(8~12GHz),400Hv and300. Mullitewhiskers improved thermal shock and dielectric properties of particle reinforcedaluminum-chrome phosphates system. Internal adding whiskers trigger is moresuitable for this system. In-situ mullite whisker had a stepped surface increasing theinterface mechanical locking probability. Both whisker and matrix were incorporatedto improve the strength and toughness by whisker bridging, pull out and crackdeflection to achieve enhanced toughening purpose.4. In-situ mullite reinforced aluminum-chrome phosphates multiphase ceramicshas a better broadband wave transmittance and transmission efficiency more than60%in the range of8~12GHz. Transmittance is optimal when thickness is7mm. Hightemperature dielectric modeling results show that, aluminum phosphate dielectricconstant increased rapidly when temperature raise to1000K because of the relativelyhigh iconicity of "P-O" bond. Mullite dielectric constant and loss tangent increasedrapidly when temperature raise to800K, which is duo to the contribution of oxygenvacancies, thus control the number of mullite oxygen vacancies is one of the means toreduce its high temperature dielectric loss.Chemical bond theory of complex crystalsis feasible for multiple compounds dielectric simulation.