Refractory metal carbides have been widely proven to be an effective strategy to enhance the oxidation resistance of ceramic matrix composites. The modification effect of ZrC-HfC-TaC system on cyclic oxidation and ablation behavior has rarely been explored. Herein, ZrC-HfC-TaC modified C/SiC composites were fabricated by polymer infiltration and pyrolysis and chemical vapor deposition processes. The mechanical strength, chemical composition and microstructure after cyclic static oxidation test under 1600 degrees C/5 h were investigated, and the corresponding oxidation mechanism was proposed according to the characterization results. The cyclic oxyacetylene torch tests under 1700 degrees C/4000 s were also employed to verify the effectiveness of ZrC-HfC-TaC modification on C/SiC composites. Results suggest that ZrC-HfC-TaC modified C/SiC composites possess outstanding cyclic oxidation and ablation behavior.

超高温陶瓷基复合材料是以连续碳纤维为增强体、超高温陶瓷为基体的一类复合材料,具有密度低、韧性好、耐高温、抗氧化及耐烧蚀等优异性能,在新型高速飞行器热结构应用方面有着不可替代的作用.碳纤维增强体和陶瓷基体是超高温陶瓷基复合材料的两个重要组成部分,对复合材料使役性能起着决定性作用,但是,碳纤维与陶瓷基体的理化性质差异大,如何将碳纤维与陶瓷基体进行有效复合,以便充分发挥碳纤维轻质、高强韧特性与陶瓷基体抗氧化、耐烧蚀特性,是超高温陶瓷基复合材料基础研究和工程应用需要解决的主要问题.本文论述了有机无机转化法制备超高温陶瓷基复合材料技术的发展思路,介绍了超高温有机陶瓷前驱体的设计与合成、C/ZrC-SiC和C/HfTaC-ZrC-SiC复合材料的研究结果,探讨了解决新型高速飞行器高温气动/燃气环境氧化烧蚀问题的材料技术方案,为连续纤维增强超高温陶瓷基复合材料的技术发展和工程应用提供借鉴.

为了提高超高温陶瓷基复合材料的力学性能和耐烧蚀性能,本文采用前驱体浸渍裂解(PIP)工艺制备了C/ZrC-SiC复合材料,研究了锆硅一体化陶瓷前驱体(ZS)的固化-裂解工艺对C/ZrC-SiC复合材料性能的影响.结果 表明:前驱体的裂解温度对复合材料的力学性能影响较大.较高的裂解温度会损坏碳纤维,导致力学性能降低;较低的裂解温度会使碳热还原反应不充分,基体氧含量较高,结构疏松,导致力学性能下降;制备的C/ZrC-SiC复合材料通过了2 850 K的电弧风洞试验考核后线烧蚀率为8.75× 10-4mm/s,呈现出优异的耐烧蚀性能.

A simple and environmentally friendly route was developed to synthesize graphene hydrogel (GH) from ascorbic acid with mesoporous structure. The symmetric supercapacitors were packaged using GH as electrodes (GH\\GH). GH\\GH supercapacitor in the neutral aqueous solution electrolytes of 1M Li2SO4 could enhance the operation voltage up to 1.8V, and exhibited a maximum energy density of 18.5Wh/kg which was 3times higher than that in 1M H2SO4. GH//GH in 1M Li2SO4 could maintain an ideal electrical double layer capacitive behavior even at a high scan rate of 500mV/s. Over 92.3% of capacitance was also maintained after cycling 2000times at 50A/ g.

研究了EP/PI和BMI/PES两种TS/TP共混体系的反应诱导相分离过程及形貌结构。采用相差显微镜原位研究了反应诱导相分离的过程,发现分相初期形成了均匀的相结构;随着相分离的发展,一定浓度区域样品中的双连续结构经过演化发展,分相后期样品内部与边缘的形貌不一致。通过对固化后样品断面的SEM观察,发现在TP浓度很低时形成海岛结构,当TP浓度稍高,样品形成了核壳结构,在样品边缘和与基板接触的上下等外侧形成了TS的富集区,只有极少量的TP分散颗粒存在;在样品的中间,TS和TP形成双连续结构,其中TP富集相具有细丝状的网络特征。分析认为,EP和BMI为热固性树脂,初始样品为小分子,在反应开始时表现为流体,为快组分;PI和PES为典型的热塑性聚合物,它们的黏弹性特征随着相分离的发展越来越显著,即Tg以下为玻璃态,Tg以上表现出弹性、黏弹性特征,为慢组分。在反应分相过程中,由于TP富集相缠结网络的松弛慢于相分离的速度,因此TP富集相网络的整体收缩不可避免,在TP与TS动力学极不对称的作用之下,初始均匀的双连续结构最终发展为核壳结构。

通过对Cf/SiC复合材料基体进行改性制备了碳纤维增韧的超高温陶瓷基复合材料,并研究了其结构形式及组分比例对高温抗氧化耐烧蚀性能的影响.电弧风洞的测试结果表明:经过超高温陶瓷改性的C f/SiC复合材料的抗氧化耐烧蚀性能明显提高,其中陶瓷基体中ZrC含量约为40wt％时,高温抗氧化耐烧蚀性能提高尤为显著,其在600 s来流条件为2 400 K/Ma0.6/0.5 MPa的电孤风洞考核试验条件下,质量烧蚀率仅为7.37×10-5 g/(cm2·s),有望满足超燃冲压发动机燃烧室的使用要求.

Isotactic polypropylene (iPP)/organo-montmorillonite (OMMT) nanocomposite was modified by poly(ethylene-co-octene) (PEOc). PEOc-rich domains were well dispersed in the iPP matrix, with narrowly distributed size. OMMT layers were well dispersed, mainly intercalated and partially exfoliated. Compared with the case in the binary composites, many OMMT layers were preferential distributed inside and around the PEOc-rich domains in the ternary composite samples, which formed an enhanced OMMT filler network. The reason for the OMMT preferential distribution was considered to be dragged or wrapped by PEOc-rich domains during sample preparation and phase separation.

The mechanism and morphology of shear induced crystallization of isotactic polypropylene (iPP) in its blends or composites were analyzed, compared and discussed in this work. For pure iPP, crystalline structure of cylindrites can be observed easily under very low shear rate. But for iPP/poly (ethylene-co-octene) (PEOc) blends, cylindrites were observed after shear in the very late stage of phase. separation only when the shear rate was large enough. The morphologies of cylindrites were multi-formed and different from the pure iPP because PEOc is an elastermer and the PP/PEOc blend system is phase separated. For PP/OMMT (organo-montmorillonite, clay) composites the fillers could form mesoscale network in the polymer matrix. The effect of shear on the heterogeneous nucleation, cylindrites formation and-morphology of spherulites were discussed and compared with pure iPP and PP/PEOc blend. The clay content is also a very important parameter in the crystallization behavior. For all three systems the mechanism of shear induced cylindrites was initiated by the structure of the entangled polymer chain network which formed bunddled core nuclei.

Isothermal crystallization of isotactic polypropylene (iPP)/organic montmorillonite (OMMT) binary composite under shear field was investigated by in situ polarized optical microscopy, rheometry and transmission electron microscopy. When shear strain was small, shear flow could enhance the crystallization of iPP, and the crystallizing entity was spherulitic in iPP/OMMT composite in which the OMMT content was below the percolation threshold. With shear strain increasing, the orientation extent became stronger and cylindrites and strings of spherulites appeared in these samples. However, for iPP/OMMT composite with OMMT content higher than the percolation threshold, when the shear strain was not big enough to destroy the fillers network in the matrix, the crystallization of iPP was similar with that of the un-sheared sample. When shear strain was large enough, the fillers network was destroyed and clay layers were aligned along the flow direction. There formed oriented crystals including cylindrites and strings of spherulites, which were much smaller in size than those formed in the previous case, because the aligned clay layers acted as heterogeneous nucleation agents to promote crystallization of iPP.

Novel temperature and pH dual-responsive hydrogels were constructed by inclusion of poly(PEGMA)-co-poly(DMA) with a-cyclodextrin in aqueous solution. The temperature- or pH-induced sol/gel transition in the hydrogels was completely reversible. Studies on structure/property relationships show that chain uniformity, graft density and copolymer concentration affect the hydrogel behavior. A dual-responsive mechanism is proposed. The in vitro release of a model drug from this hydrogel was studied. It was found that the release kinetics were greatly accelerated at higher temperature and at acidic pH conditions, indicating potential applications in controlled drug delivery.

A fluorinated polyimide was employed to investigate reaction-induced phase separation of epoxy/polyimide blends. Three distinct morphological structures were obtained. Among these, an unusual three-layered structure was observed in a certain polyimide concentration range. The polyimide-rich phase was mainly localized in the middle layer and the outer layers were epoxy-rich. The thickness of the Outer layers (10-30 mu m) was Much larger than the characteristic length of the bicontinous domains in the middle layer. In the lower polyimide concentration range. polyimide-rich particles formed in the epoxy-rich matrix its aggregates without further coalescence. At higher polyimide concentrations, polyimide-rich phase formed continuous domains while epoxy-rich phase formed connected nonspherical particles. Evolution of the layered Structure was elucidated from scanning electron micrographs. No layer existed in the sample before phase separation began. The reaction conversion of epoxy was very low when layers started to form. During that period, the Outer layers thickened rapidly. The mechanism of this unusual layered structure formation Was discussed based on the characteristics of this epoxy/polyimide system. Also a refractive index cross-over was observed which was caused by the gradual increase of the refractive index of epoxy during the isothermal Curing process.

Viscoelastic polymer blends of polybutadiene (PB)low vinyl content polyisoprene (LPI), with a lower critical solution temperature (LCST), show interesting theological behaviors in temperature ramp measurements. In this report, a systematic study has been carried out, and the underlying physics has been investigated for the storage modulus G' at various temperatures and shear frequencies as the system passes through the binodal and the spinodal phase boundary lines. We considered the nucleation mechanism, spinodal fluctuations, shear induced mixing, and theological models in the interpretation of these interesting phenomena. Shear induced mixing is varied in our system, and the frequency dependence is obvious. Competition between the kinetics of the nucleation process and the droplet growth process has a prominent effect on the storage modulus for samples of noncritical compositions, while for samples with near-critical compositions the morphological evolution is responsible for the viscoelastic changes. Time-dependent experiments provide important information about morphological evolution at different temperatures. The region where fluctuations play a dominant effect on G' can be discerned from our treatment of putting G' and {G'(omega)/[G ''(2)(omega)T](2/3) in the same reference frame. On the basis of the results from both heating and cooling processes, it seems that there also exist competition between fluctuations and interfacial gradient on the determination of the value of G'.

Isothermal crystallization of isotactic polypropylene (iPP)/organic montmorillonite (OMMT) binary nanocomposite and iPP/OMMT/poly(ethylene-co-octene) (PEOc) ternary nanocomposites has been investigated by polarized optical microscopy (POM), rheometry and scanning electron microscopy (SEM). At the stage of nucleation the heterogeneous nucleation effect of OMMT was much greater than the concentration fluctuation assisted nucleation effect in the ternary nanocomposite. Besides, PEOc played a role of inhibitor of OMMT nucleation agents at the nucleation stage because many of OMMT layers were distributed around PEOc-rich domains. At stage II of the crystal growth process, the entanglement effect of PEOc greatly affected the rheological response (storage modulus (G′) and its growth rate) due to the long side chains of PEOc component. In stage III of the growth process, OMMT layers and the entanglement of PEOc chains limited the motion of polypropylene chains. So the growth rate of G′ was slowed down. During the shrinkage and cooling process after isothermal crystallization, some fibril links between the spherulites, consisting of PEOc chains and iPP chains, were formed from the amorphous phases surrounding the spherulites.

Rheological and thermal properties of isotactic polypropylene (iPP)/organo-montmorillonite (OMMT)/poly(ethylene-co-octene) (PEOc) ternary nanocomposites and iPP/OMMT binary nanocomposites were studied by X-ray diffraction (XRD), rheometry, thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) in this paper. The clay layers were mainly intercalated and partially exfoliated and well dispersed in these nanocomposites with the help of maleic anhydride modified polypropylene (PPgMA). Clay layers were mainly localized close to/inside the PEOc-rich phase from the direct observation of morphological study. A compact and stable network structure was formed in ternary composites when clay content was 2phr (parts per hundred parts of iPP/PPgMA) or higher, which resulted in the lower stress relaxation rate and a pseudo-solid like behavior in low frequency region. Compared with iPP/OMMT composites, iPP/OMMT/PEOc composites had a much stronger ability to resist thermal decomposition. In another word, combining with the filler network, PEOc greatly improved the structural and thermal stabilities of iPP/OMMT nanocomposites.

In this article, the rheological properties of polypropylene (PP)/ethylene–propylene–diene terpolymer (EPDM)/silicon dioxide (SiO2) ternary composites were systematically investigated. Two kinds of nano-SiO2 particles (with hydrophobic (denoted as A-SiO2) or hydrophilic (denoted as B-SiO2)) as well as two processing methods (one-step or two-step) were first employed to prepare PP/EPDM/SiO2 ternary composites. Then the deep mixing and morphology evolution of polymer composite with mixing time were assessed by rheological method, on the focus of formation of filler-network, and compared with scanning electron microscopy (SEM) observations. Linear viscoelastic behavior was observed for PP/EPDM and PP/SiO2 binary system, showing no evidence of the formation of filler-network structure. However, a solid-like rheological behavior, which was attributed to the formation of the filler-network structure as confirmed by SEM observation, could be observed in some PP/EPDM/SiO2 ternary systems, depending on the SiO2 surface property, processing method and EPDM content. It seemed that SiO2 with hydrophilic surface was necessary for the formation of filler-network in PP/EPDM/SiO2 ternary system. Besides, two-step processing method made the solid-like behavior occurred at an earlier stage compared with that of a one-step processing method, also, the higher elastomer content facilitated the formation of the filler-network structure. The results were in good agreement with those reported in our previous publications [Yang H, Zhang Q, Guo M, Wang C, Du R, Fu Q. Polymer 2006;47:2106] [Yang H, Zhang X, Qu C, Li B, Zhang L, Zhang Q, et al. Polymer 2007;48:860].