Oxidation-Resistant Fiber Coatings for Non-oxide Ceramic Composites

Oxidation of fiber/matrix interfaces can degrade the strength and toughness of ceramic composites containing non-oxide constituents. Oxidation can occur when the fiber ends are exposed or in the presence of cracks. Theoretical considerations indicate that, contrary to popular belief, oxide fiber coatings should be worse than non-oxide fiber coatings for protection against oxidation. Furthermore, oxidation of fiber/matrix interfaces is a composite system problem rather than that of the fiber coating.     

莫来石纤维含量对氧化铝基陶瓷复合材料性能的影响

本课题选用氧化铝粉和多晶莫来石纤维为主要原料,添加1wt%的TiO2和3wt%的CMS(CaO、MgO、SiO2混合物)助熔剂,用电磁振荡搅拌器混料与球磨机混料相结合的方式进行混料,采用单向加压方式成形,使用传统的无压烧结技术制备出了莫来石纤维增强增韧氧化铝陶瓷基复合材料,并对复合材料的性能进行测试.研究发现:复合材料的弯曲强度随纤维含量的增加先增大后降低,纤维含量为15wt%时,复合材料的弯曲强度最高,达504.52MPa,是普通氧化铝陶瓷的1.7倍;复合材料的断裂韧性随着纤维含量的增加先增加后降低,莫来石纤维含量为15wt%时,复合材料的断裂韧性最大达到4.46MPa·ml/2,是普通氧化铝陶瓷的1.6倍;复合材料的抗热震性能随纤维含量的增加而提高.当烧结温度为1450℃,纤维含量为15wt%时,MFTACC的综合性能较好.     

Fiber Coatings for SiC-Fiber-Reinforced BMAS Glass-Ceramic Composites

Interfaces in SK-fiber-reinforced glass-ceramic matrix composites were modified by applying different coatings to the fibers and varying the coating thickness. Coatings of SiC/BN, Si₃N₄/BN, and BN were applied to the fibers by CVD prior to composite fabrication. Interfacial microstructures were characterized using high-resolution and analytical transmission electron microscopy. Oxidation of the SiC fibers during composite fabrication was suppressed by the fiber coatings, provided that the coatings were sufficiently thick to prevent oxygen diffusion from the matrix. The SiC/RN and BN coatings were stable during high-temperature exposures, while the Si₃N₄/BN coating underwent chemical reactions.     

针刺碳纤维增强莫来石基复合材料的制备及性能

采用溶胶-凝胶法制备了针刺碳纤维增强莫来石基复合材料。借助于TG-DTA和XRD对合成凝胶的莫来石化过程进行研究,结果表明,在热处理过程中,凝胶在920°C左右形成铝硅尖晶石,1198°C左右形成莫来石。研究了烧结温度对复合材料性能的影响,结果表明,烧结温度为1500°C制备的复合材料弯曲强度最高,达到142.2 MPa,断裂韧性为8.77 MPa·m~(1/2)。借助于对复合材料微观结构的观察对复合材料力学性能的变化进行了解释。     

莫来石纤维增强SiO2气凝胶隔热材料的制备

以正硅酸乙酯（TEOS）为硅源,采用溶胶-凝胶及超临界干燥技术制备了莫来石纤维增强SiO2气凝胶隔热材料,并系统地研究了HF催化剂以及溶剂等制备因素对气凝胶品质的影响。结果表明：HF催化剂、H2O、乙醇和甲酰胺用量对TEOS的水解缩聚反应有较大影响,调节其配比可以制备具有不同密度的SiO2气凝胶隔热材料;添加甲酰胺可以调节凝胶内部网络结构,防止干燥时由于应力不均而开裂或破裂。     

碳结合碳纤维复合物用保护涂料

碳结合碳纤维复合物（CBCF）通过与熔融硅直接反应,在复合物表面得到一层碳化砖层并对其进行改进。随后,已渗透硅的CBCF材料用泥浆-浸渍工艺、用含钇和铝的硅质玻璃进行涂层。对玻璃热处理时就会产生一个玻璃-陶瓷层,因此就得到一个多层抗氧化和抗侵蚀保护系统。通过疆做技术和X-射线衍射测出了涂层的显微结构特征。受到控制的结晶过程中所产生的主要晶相有：玻璃状方英石、硅酸钇（Y2Si2O7,Keiviile,β形式）和莫来石。它们是具有抗氧化和抗侵蚀保护的SiC涂层的碳质复合物的优质陶瓷材料,这是因为它们的热膨胀系数（CTE）非常接近SiC的热膨胀系数。在1375℃热处理时,微裂纹有可能闭合,这是因为利用了玻璃一陶瓷中残余玻璃的粘性流,因此为了延长保护系统的使用寿命．我们对这些都进行了研究．     

Mullite/Alumina Mixtures for Use as Porous Matrices in Oxide Fiber Composites

Weakly bonded particle mixtures of mullite and alumina are assessed as candidate matrixes for use in porous matrix ceramic composites. Conditions for the deflection of a matrix crack at a fiber-matrix interface are used to identify the combinations of modulus and toughness of the fibers and the matrix for which damage-tolerant behavior is expected to occur in the composite. Accordingly, the present study focuses on the modulus and toughness of the particle mixtures, as well as the changes in these properties following aging at elevated temperature comparable to the targeted upper-use temperature for oxide composites. Models based on bonded particle aggregates are presented, assessed, and calibrated. The experimental and modeling results are combined to predict the critical aging times at which damage tolerance is lost because of sintering at the particle junctions and the associated changes in mechanical properties. For an aging temperature of 1200°C, the critical time exceeds 10 000 h for the mullite-rich mixtures.     

Preparation of Mullite Fiber

Transparent mullite fibers have been prepared using aluminum carboxylates (ACs) and tetraethyl orthosilicate (TEOS) as starting materials. The ACs are derived from the catalyzed dissolution of elemental aluminum in a mixture of formic acid and acetic acid. The solubility of aluminum in the acids is influenced by the concentrations of the acids, water, and additives and the preparation temperature. A 1:4:3:24 molar ratio of aluminum, formic acid, acetic acid, and water dissolves the aluminum completely to give a colorless, clear solution suitable for fiber synthesis. The mixture of the ACs and TEOS, in the presence of ethyl alcohol as a mutual solvent at 50°–60°C, is concentrated to give a spinnable dope, from which mullite precursor fibers can be drawn. Heat treatment of the precursor at 1250°C yields crystallized and transparent mullite fibers.     

PMMA/MWCNT复合材料的热降解动力学

采用溶液共混法制备了聚甲基丙烯酸甲酯/多壁碳纳米管（PMMA/MWCNT）复合材料,利用热重法研究了不同气氛下复合材料的热降解,并采用Flynn-Wall-Ozawa、Kissinger和Friedman等3种方法计算其动力学参数。结果表明,MWCNT的添加量为3 %时,在氮气和氧气中复合材料的初始降解温度较纯PMMA分别提高了54.62 ℃和70.4 ℃,最大热失重速率温度也有一定程度的提高,说明MWCNT能显著提高PMMA的低温热稳定性,尤其是在有氧环境中,而对高温热稳定改善不明显;采用Kissinger法、Flynn-Wall-Ozawa法和Friedman法计算得到的活化能（Ea）变化趋势一致,当MWCNT的添加量为3 %时,Ea较纯PMMA提高最多,在氮气中分别为45.99、95.10、72.46 kJ/mol,在氧气中分别增加53.42、120.63、110.41 kJ/mol;由Friedman法求解出复合材料的反应级数（n）在氮气中约为1.5,在氧气中约为0.9。     

Porous Yttrium Aluminum Garnet Fiber Coatings for Oxide Composites

A porous oxide fiber coating was investigated for Nextel^™ 610 fibers in an alumina matrix. Polymeric-solution-derived yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) with a fugitive carbon phase was used to develop the porous fiber coating. Ultimate tensile strengths of tows and minicomposites following heat treatments in argon and/or air were used to evaluate the effect of the porous fiber coating. The porous YAG fiber coatings did not reduce the strength of the tows when heated in argon, and they degraded tow strength by only ∼20% after heating in air at 1200°C for 100 h. Minicomposites containing porous YAG-coated fibers were nearly twice as strong as those containing uncoated fibers. However, after heating at 1200°C for 100 h, the porous YAG coatings densified to >90%, at which point they were ineffective at protecting the fibers, resulting in identical strengths for minicomposites with and without a fiber coating.     

Control of Interfacial Properties through Fiber Coatings: Monazite Coatings in Oxide–Oxide Composites

Fiber pushout tests were used to quantify the effects of fiber coating thickness on the mechanical properties of two model composite systems: a monazite-coated (LaPO₄-coated) alumina (Al₂O₃) fiber in an Al₂O₃ matrix and a LaPO₄-coated yttrium aluminum garnet (YAG) fiber in an Al₂O₃ matrix. Interface properties were quantified using the Liang and Hutchinson (LH) pushout model and mechanistically rationalized by considering the change in residual thermal stresses with changes in the coating thickness. Measures of the pure Mode II interfacial fracture energy, the coefficient of friction, and a radial clamping pressure are extracted by fitting the LH equations to the experimental results. Using the approach that has been developed herein, a methodology is available for measuring the interfacial properties, predicting the effect of coating thickness, and selecting the coating thickness to     

玻璃纤维／环氧树脂复合材料的热性能

本文采用E-玻璃纤维作为增强材料、双酚A环氧树脂和芳胺类固化剂作为基体制成复合材料试样,利用动态与静态热分析方法测定玻璃纤维／环氧树脂基复合材料的热性能,研究了玻璃纤维含量对复合材料动态热机械性能、玻璃化温度等热性能的影响。     

Reaction-Bonded Mullite/Zirconia Composites

The feasibility of fabricating dense, low-shrinkage, mullite/ ZrO₂ composites based on the reaction bonding of alumina (RBAO) process and the reaction sintering of zircon is examined. Compacts pressed from an attrition-milled powder mixture of Al, A1₂O₃ and zircon were heated in air according to a two-step heating cycle. The phase evolution and microstructural development during reaction bonding were traced by X-ray diffraction, nuclear magnetic resonance, and scanning electron microscopy on samples extracted from various points along the heating cycle. It is seen that, as in conventional RBAO, AI oxidizes to γ-Al₂O₃ which then transforms to α-AI₂O₃ between 1100° and 1200°C. The zircon dissociation commences at ∼1400°C and is practically complete by 1500°C. Mullite enriched in Al₂O₃ forms initially, but 3:2 stoichiometry is attained in the final product which consists of mullite, t - and m-ZrO₂, and residual α-AI₂O₃. The flexure strength of the composite is superior to that of pure mullite, and ∼80% of the strength is retained up to 1200°C. Although there was no toughness enhancement relative to mullite, this should be achievable by optimizing the fabrication procedure.     

Thermal Exposure Effects on the Strength and Microstructure of a Novel Mullite Fiber

The mechanical behavior of the novel fiber CeraFib75 after various thermal exposures is examined. This fully crystalline mullite fiber was developed to exceed the thermal stability of commercially available oxide fibers. Therefore, heat treatments at temperatures ranging from 1000°C to 1400°C for 25 h were performed and results compared to the well‐established Nextel^? 720 fibers. Mechanical characterization was realized with bundle tensile tests using acoustic emission sensors to determinate the fiber failure distributions. Investigations showed that the initial fiber microstructure of mullite grains with traces of alumina transforms starting at 1200°C. Changes include dissociation of the alumina‐rich mullite phase and grain growth. Thus, strength reduction is measured as a result of these microstructure transformations. Remarkably, at 1400°C, fibers become more fragile and Weibull statistics can no longer describe the failure distribution. A relation between the distribution shape and the load redistribution capability of fibers is suggested. This is more pronounced for Nextel^?720 fibers, which present much bigger grains and retain only 10% of their original strength. However, CeraFib75 fibers are more stable and exhibit a strength retention of 50% at the same conditions, which is attributed to the higher amount of mullite phase.     

膨胀型无卤阻燃聚丙烯的热分解动力学研究

采用四种不同的动力学分析方法,即Kissinger法、Ozawa法、Freeman-Carroll法和Coats-Redfem法对含N、P阻燃剂(IFR)的膨胀型无卤阻燃聚丙烯体系的热分解动力学进行了探讨。四种方法的计算结果不太一致,但是都得出阻燃聚丙烯的活化能比未阻燃的聚丙烯的活化能高,活化能顺序为:EB>EA>EPP[A:m(PP)∶m(IFR)=1∶0.3,B:m(PP)∶(IFR)=1∶0.4],说明了阻燃聚丙烯有更好的热稳定性。     

PP/MWCNTs复合材料的热降解及热寿命研究

利用热重法研究了聚丙烯/多壁碳纳米管（PP/MWCNTs）复合材料的热降解过程,采用Achar法、Coats-Redfern法研究其热降解机理,并用Kissinger法求得动力学参数及热寿命方程。结果显示：复合材料的TG曲线均表现为一个失重阶段,MWCNTs含量为1.5 %时起始失重温度和最大热失重速率温度达到最大,PP热稳定性提高。该复合材料的热降解机理为一级扩散反应。添加MWCNTs后,复合材料的热寿命显著提高,同一温度下,随着MWCNTs含量的增加,复合材料的热寿命先增大后减小,在含量为1.5 %达到最大,利用Kissinger法求解材料的热寿命方法可靠。     

Spark-Plasma-Sintering Consolidation of SiC-Whisker-Reinforced Mullite Composites

Spark plasma sintering was applied to consolidate a series of SiC-whisker-reinforced mullite-matrix composites containing up to 30 vol% of whiskers. Bodies with relative densities higher than 99% were obtained at 1400°3C, using a holding time of 3 min and a uniaxial pressure of 50 MPa. The analysis of densification kinetics revealed that a shrinkage rate of about 10^-3 s^-1 was obtained even with a high whisker load, showing that the spark plasma sintering process is a rapid technique for fabricating whisker composites. The very rapid densification that occurs during spark plasma sintering allows little time for growth of the matrix grains. This, in turn, leads to improved mechanical properties.     

Thermal Expansion Discontinuities of Mullite

Thermal expansion data of mullite were collected between 25° and 800°C with high accuracy by means of an X-ray high-temperature Guinier powder camera, and high-resolution single-crystal Bond techniques. 3/2-type mullites, synthesized from chemically coprecipitated precursors and 2/1-type fused-mullite single crystals, were used for the measurements. Mullite displays low and nonlinear thermal expansions along the crystallographic a, b , and c axes below about 200°C, and linear expansions between 200° and 400°C. Between about 400° and 500°C, expansion curves are discontinuous. Above 500°C, a, b , and c expansions are linear again. The temperature-induced expansion discontinuities are relatively strong parallel to a , but are much weaker parallel to b and c . The slopes of the a and volume expansion curves are higher above the expansion discontinuity than below, while there is no significant tendency for those of b and c . Thermal expansion coefficients and discontinuity effects are reversible without any hysteresis. The increase of the a and volume expansion curves above the expansion discontinuities may be due to possible deformations, rotations, and tiltings of the [AlO₆] octahedra. Another factor responsible for the higher a and volume expansion coefficients at high temperatures may be a geometrical deformation of the [Al^⋆O₄] tetrahedra. At present, no conclusive model exists to explain the discontinuous change of thermal expansion coefficients between 400° and 500°C.     

含硫磷三分子缩合物阻燃聚乙烯复合材料的热降解动力学研究

采用不同升温速率研究含硫磷三分子缩合物无卤阻燃剂(S-TRIMER)阻燃LLDPE体系在氮气氛围中的热降解动力学.并采用两种不同的动力学分析方法(即Kissinger方法和Flynn-Wall-Ozawa方法)对阻燃LLDPE体系的热降解动力学进行了理论分析.其中,Kissinger方法计算出的活化能顺序为LLDPE＜LLDPE/S-TRIMER＜LLDPE/IFR,而Flynn-Wall-Ozawa方法计算出的活化能在热降解的初始阶段阻燃LLDPE的活化能小于LLDPE,但是当转化率α≥0.3时阻燃LLDPE的活化能大于LLDPE,这说明对于该体系第一热解阶段(即0.05≤α≤0.2)主要是阻燃剂发生分解,分解的阻燃剂抑制了基体LLDPE的进一步降解,使体系的活化能提高.上述两种方法计算的结果接近,阻燃聚乙烯的活化能比未阻燃聚乙烯的高,说明了阻燃聚乙烯有着更好的热稳定性,即阻燃剂S-TRIMER能提高LLDPE基体的热降解性能.