先驱体法制备含异质元素SiC陶瓷纤维的现状与进展

先驱体聚碳硅烷进行物理与化学改性，可以制备出各种含Ti,Zr,Al,B等异质元素的耐温型和吸波型SiC陶瓷纤维。含异质元素的SiC陶瓷纤维，已成为当今SiC陶瓷纤维的发展主流，着承介绍了Si-C、O，Si-Ti、C-O，Si-B-Ti-C，Si-Zr-C-O,Si-Al-C纤维的先驱体的制备及其纤维的性能。介绍了我国含异质元素SiC纤维的现状与进展，指出国内SiC纤维的发展要立足于创新的基础上开发各种高性能SiC纤维，力争做到高起点、高瞻远瞩地发展新型SiC纤维，建立拥有自主知识产权的制备工艺。     

含铝碳化硅纤维耐高温性能

通过合成陶瓷纤维先驱体聚铝碳硅烷,制备了具有耐高温性能的含铝碳化硅SiC(Al)纤维。SiC(Al)纤维的化学组成为SiC1.15O0.026·Al0.013,主要结构是平均晶粒为95nm的βSiC,O和游离C含量均大大低于nicalon纤维,同时含有微量的Al和少量的αSiC。SiC(Al)纤维的平均直径为13μm,平均抗拉强度为2.3GPa。1400℃氩气中处理1h后,抗拉强度是原始强度的95%以上;1800℃氩气中处理1h后,抗拉强度保留率为71%。纤维的高温稳定性高于nicalon,Hi nicalon等商品SiC纤维,但低于TyrannoSA商品SiC纤维,并且SiC(Al)纤维的高温抗蠕变性能明显高于nicalon纤维。SiC(Al)纤维的高温稳定性取决于其低氧含量、低富碳含量以及异元素Al的助烧结和在高温下抑制SiC晶粒长大的作用,良好的抗蠕变性能决定于其高结晶度和低含量的SiCxOy相。     

基于聚硅碳硅烷与仲丁醇铝制备含铝碳化硅纤维

采用聚硅碳硅烷和仲丁醇铝常压高温反应合成碳化硅纤维先驱体——聚铝碳硅烷。通过GPC、FT-IR、元素分析、X射线光电子能谱分析等表征发现:其组成以Si—C、Si—O、O—Al等化学键结构为主,重均分子质量为2 600,软化点在190～200℃之间。经熔融纺丝制得聚铝碳化硅纤维,其预氧化后陶瓷产率达到75%,高温处理后得到的含铝SiC纤维。随着温度的升高,该纤维组成逐渐由无序结构转化为晶体结构,其中的SiC_xO_y不稳定相在1 500℃开始分解,但晶粒尺寸基本维持在10 nm左右,无过度长大。含铝SiC纤维表现出良好的耐温性能,1500℃高温处理后纤维表面没有明显缺陷。     

锆英石碳热还原合成ZrB_2—SiC复合粉体

以锆英石、氧化硼、活性炭为原料,采用碳热还原合成工艺制备了ZrB2—SiC复合粉体,并对合成过程进行了热力学分析。考察了反应温度及原料配比对碳热还原合成ZrB2—SiC复合粉体的物相的组成、含量和显微结构的影响。结果表明:提高反应温度有利于ZrB2—SiC复合粉体的合成,适当过量氧化硼及活性炭有利于ZrB2—SiC复合粉体的合成。合成ZrB2—SiC复合粉体的最优参数为:当ZrSiO4、B2O3和C的摩尔比为1∶2∶12,在1 773K保温3h,可得到几乎纯相的ZrB2—SiC复合粉体。     

溶胶-凝胶法制备锆/有机硅复合薄膜

以氧氯化锆(ZrOCl_2·8H_2O)和γ-氨丙基三乙氧基硅烷(KH-550)为先驱体,采用溶胶-凝胶法通过共缩聚反应在聚碳酸酯(PC)板表面制备了锆/有机硅复合薄膜,采用TG/DTA、FTIR、UV-VIS、金相显微镜及铅笔硬度测试方法对复合薄膜的结构及性能进行表征。研究结果表明,通过共缩聚反应,PC板表面形成带有机基团的无机交联网络结构,基本骨架由Si—O—Si,Si—O—Zr,Zr—O—Zr组成;当KH-550与氧氯化锆的物质的量比不低于21时,涂膜PC板的透过率较高,表面较平整;去离子水对薄膜性能的影响较小,水与氧氯化锆的用量在801～201内均可以。PC板镀膜后硬度也由2B提高到H,膜层与基体的结合较好,不易脱落。     

基于前驱体转化制备孔隙梯度分布的碳化硅纤维

以SiC陶瓷前驱体——聚碳硅烷(PCS)的纤维为原料,经臭氧不熔化和热解至1 600℃获得多孔结构的碳化硅(SiC)纤维。采用扫描电镜、X射线衍射仪、元素分析仪和比表面积与孔径分析仪等研究了臭氧不熔化时间和热解温度对纤维形貌与组成的影响。结果表明:当热解温度从1 300℃提升至1 600℃,SiC纤维呈现出氧元素含量显著降低,比表面积显著增加,形态从无定形转化为结晶形态,以及SiC纤维壁形成多孔结构且孔径从外到内呈梯度缩小等变化;当臭氧不熔化时间为20 min时,所得的多孔结构的SiC纤维还具有中空结构,比表面积为10.45 m~2/g,纤维壁富含尺寸介于2～30 nm的介孔。     

整筒聚碳硅烷原丝直径分布和缺陷分析

先驱体聚碳硅烷的纺丝工艺是制备高性能连续SiC纤维过程的关键环节,纺丝工艺决定了原丝的性能优劣,而纺丝工艺对聚碳硅烷原丝的损伤程度也将决定最终的SiC纤维性能.为此,利用扫描电子显微镜对整桶纺出原丝的直径、表面和截面缺陷进行了观察,利用统计方法对直径和缺陷的分布进行了分析.研究发现:原丝平均直径从外层到内层大体呈减小的趋势,并且每层数值波动逐渐减小,靠近中间层的原丝直径与整体平均值最为接近.表面划痕由外层到内层划痕逐渐减少.鼓泡在各层之间呈均匀分布.断面气孔主要集中在最外几层原丝中.根据统计结果得到了结构特征分布模型,建立了评价整桶原丝性能优劣的有效方法.     

金属锆醇盐对钛合金表面溶胶-凝胶转化膜结构及漆层结合力的影响

采用有机硅烷和金属锆醇盐为主成分、无水乙醇为溶剂，在TC4钛合金表面制备了溶胶-凝胶转化膜。采用扫描电镜(SEM)、能谱(EDS)、辉光放电质谱(GDMS)、X射线光电子能谱(XPS)等技术分析了金属锆醇盐含量对转化膜表面微观形貌和成分，以及纵深方向元素分布和氧元素价态的影响。通过电化学阻抗谱(EIS)分析了转化膜的电化学特性和结构特点。最后通过长时间湿热试验研究了金属锆醇盐含量对后续有机漆层结合力的影响。结果表明，制备的溶胶-凝胶转化膜分成有机层和无机层两部分，其中无机层主要是由Ti─O─Zr/Si共价键结构组成，有机层主要由硅烷的有机碳链交联形成。当金属锆醇盐含量为2 000 mg/kg时，转化膜具有最佳的结构完整性，能够保证钛合金表面有机底漆优异的结合力和耐久性。     

用聚碳硅烷制备柔性疏水型碳化硅纤维毡(英文)

以聚合物先驱体转化法用陶瓷工艺与静电纺丝技术相结合制备了碳化硅(SiC)陶瓷纤维毡。以聚碳硅烷为先驱体,利用静电纺丝技术制备先驱体原纤维毡。原纤维毡经过低温交联和1 000℃以上热解,得到SiC纤维毡。当温度达到1 200℃时,纤维毡为多晶态,保持典型的三维网络结构,纤维直径约为1.1mm。电子探针分析表明,纤维毡化学组成为Si、C、以及少量O元素。SiC纤维毡具有良好的疏水性,疏水角大于130°。1 000℃热解制备的SiC纤维毡的拉伸强度为0.6MPa,断裂伸长率为45%,可应用于高温极端环境。     

连续Si-C-Fe-O功能陶瓷纤维的制备

为了改善碳化硅纤维的电磁性能,首次由聚二甲基硅烷和二茂铁合成了聚铁碳硅烷,当二茂铁的添加量(以质量计)为2%时,所制得的聚铁碳硅烷经熔融纺丝、预氧化,惰性气氛中烧成后,可制得低电阻率(10-2Ω·cm)、拉伸强度为2.0GPa、长度大于500m的连续SiC FeO纤维。探索了连续SiCFeO纤维的制备工艺。研究了铁对纤维电阻率和βSiC结晶的影响。结果表明:铁含量的增加有利于βSiC晶粒的增长和电阻率的降低。     

Direct observation of freeze-thaw instability of latex coatings via high pressure freezing and cryogenic SEM

Despite its industrial importance, the subject of freeze-thaw (F/T) stability of latex coatings has not been studied extensively. There is also a lack of fundamental understanding about the process and the mechanisms through which a coating becomes destabilized. High pressure (2100 bar) freezing fixes the state of water-suspended particles of polymer binder and inorganic pigments without the growth of ice crystals during freezing that produce artifacts in direct imaging scanning electron microscopy (SEM) of fracture surfaces of frozen coatings. We show that by incorporating copolymerizable functional monomers, it is possible to achieve F/T stability in polymer latexes and in low-VOC paints, as judged by the microstructures revealed by the cryogenic SEM technique. Particle coalescence as well as pigment segregation in F/T unstable systems are visualized. In order to achieve F/T stability in paints, latex particles must not flocculate and should provide protection to inorganic pigment and extender particles. Because of the unique capabilities of the cryogenic SEM, we are able to separate the effects of freezing and thawing, and study the influence of the rate of freezing and thawing on F/T stability. Destabilization can be caused by either freezing or thawing. A slow freezing process is more detrimental to F/T stability than a fast freezing process; the latter actually preserves suspension stability during freezing. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27–29, 2004 in Chicago, IL. Tied for first place in The John A. Gordon Best Paper Competition.     

Ethanol and (−)-α-Pinene: Attractant Kairomones for Bark and Ambrosia Beetles in the Southeastern US

In 2002–2004, we examined the flight responses of 49 species of native and exotic bark and ambrosia beetles (Coleoptera: Scolytidae and Platypodidae) to traps baited with ethanol and/or (−)-α-pinene in the southeastern US. Eight field trials were conducted in mature pine stands in Alabama, Florida, Georgia, North Carolina, and South Carolina. Funnel traps baited with ethanol lures (release rate, about 0.6 g/day at 25–28°C) were attractive to ten species of ambrosia beetles (Ambrosiodmus tachygraphus, Anisandrus sayi, Dryoxylon onoharaensum, Monarthrum mali, Xyleborinus saxesenii, Xyleborus affinis, Xyleborus ferrugineus, Xylosandrus compactus, Xylosandrus crassiusculus, and Xylosandrus germanus) and two species of bark beetles (Cryptocarenus heveae and Hypothenemus sp.). Traps baited with (−)-α-pinene lures (release rate, 2–6 g/day at 25–28°C) were attractive to five bark beetle species (Dendroctonus terebrans, Hylastes porculus, Hylastes salebrosus, Hylastes tenuis, and Ips grandicollis) and one platypodid ambrosia beetle species (Myoplatypus flavicornis). Ethanol enhanced responses of some species (Xyleborus pubescens, H. porculus, H. salebrosus, H. tenuis, and Pityophthorus cariniceps) to traps baited with (−)-α-pinene in some locations. (−)-α-Pinene interrupted the response of some ambrosia beetle species to traps baited with ethanol, but only the response of D. onoharaensum was interrupted consistently at most locations. Of 23 species of ambrosia beetles captured in our field trials, nine were exotic and accounted for 70–97% of total catches of ambrosia beetles. Our results provide support for the continued use of separate traps baited with ethanol alone and ethanol with (−)-α-pinene to detect and monitor common bark and ambrosia beetles from the southeastern region of the US.     

Novel polyurethane coating technology through glycidyl carbamate chemistry

Glycidyl carbamate chemistry combines the excellent properties of polyurethanes with the crosslinking chemistry of epoxy resins. Glycidyl carbamate functional oligomers were synthesized by the reaction of polyfunctional isocyanate oligomers and glycidol. The oligomers were formulated into coatings with several amine functional crosslinkers at varying stoichiometric ratios and cured at different temperatures. Properties such as solvent resistance, hardness, and impact resistance were dependent on the composition and cure conditions. Most coatings had an excellent combination of properties. Studies were carried out to determine the kinetics of the curing reaction of the glycidyl carbamate functional oligomers with multifunctional and model amines. Detailed kinetic analysis of the curing reactions was also undertaken. The results indicated that the glycidyl carbamate functional group is more reactive than a glycidyl ether group. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, on October 27–29, 2004, in Chicago, IL.     

Highly moisture-proof polysilsesquioxane coating prepared via facile sol-gel process

A highly moisture-proof polysilsesquioxane coating was obtained from a new bis-silylated precursor, which was synthesized from 3-aminopropyltriethoxysilane (APTES) and m-xylylene diisocyanate (m-XDI) in tetrahydrofuran (THF) and verified by ¹H MAS NMR. For direct comparison purposes, an SiO₂ coating was also prepared by the Stöber method using tetraethoxysilane (TEOS) as the reactant. Interestingly, the coating obtained from the polysilsesquioxane sol exhibited a much higher moisture resistance capability than its counterpart, which was attributed to its more compact feature between nanoparticles as characterized by N₂ absorption experiment and transmission electron microscopy (TEM). Furthermore, its high transparency of about 92% showed potential for application in the protection of optical crystals.