BN纤维织物增强陶瓷透波材料的制备及其力学性能初探

以全氢聚硅氮烷(PHPS)为先驱体,采用聚合物浸渍裂解工艺制备BN纤维织物增强陶瓷透波材料,研究了复合材料的致密化工艺和力学性能。结果表明:先驱体PHPS在1637℃裂解产物主晶相为α-Si3N4。以PIP工艺制备BNf/Si3N4复合材料,经过4个浸渍裂解周期密度达到1·5g/cm3,复合材料的室温弯曲强度达到39·6MPa。裂解过程中,PHPS与BN纤维发生了强界面反应,导致复合材料力学性能不高。     

裂解升温速率对C／C—SiC复合材料性能的影响研究

以聚碳硅烷(PCS)为先驱体,采用先驱体浸渍裂解(PIP)工艺制备了C/C-SiC复合材料,研究了先驱体转化过程中不同裂解升温速率对材料力学和抗氧化性能的影响。结果表明:以较低的裂解升温速率制备的C/C-SiC复合材料的力学和抗氧化性能较好。采用20℃/h裂解升温速率制得的C/C-SiC复合材料的弯曲强度达278 MPa,在1400℃氧化2 h后,失重率为3.1%。     

C/C-SiC-HfC复合材料的微观结构及力学性能研究北大核心

采用炭纤维和陶瓷粉末混编技术将HfC粉引入到炭纤维预制体中,制备了含有HfC超高温陶瓷粉末的炭纤维预制体;随后采用化学气相渗透(CVI)和先驱体浸渍裂解工艺(PIP)制备了密度高达1.94 g/cm^(3)的C/C-SiC-HfC复合材料,分析了复合材料的微观结构及力学性能。结果表明,制备的复合材料主要由C、SiC及HfC等物相组成,复合材料的平均弯曲强度达到了78.3 MPa,平均压缩强度为127.9 MPa;断裂过程中,断口处出现明显的裂纹扩展、纤维拔出及脱粘现象,从而使得材料呈现出一定的假塑性断裂特征。     

石英纤维织物增强氮化物陶瓷基复合材料的性能

采用先驱体转化法制备了2.5维石英纤维织物增强氮化物陶瓷基复合材料.对所制备材料的力学性能、热物理性能、烧蚀性能和断口显微形貌进行了研究.研究了裂解温度及纤维体积含量对复合材料抗弯强度的影响.结果表明:该材料具有较好的力学性能和优良的抗烧蚀性能,随着纤维体积含量的提高,复合材料的抗弯强度随之提高.当纤维体积分数为48%,裂解温度为800℃时,复合材料显示出最高径向抗弯强度(134.6MPa).烧蚀过程中无分层和剥离现象,表面平整,线烧蚀率为0.038mm/s.材料的增韧机制是基体与纤维界面的脱粘和纤维的拔出吸收了大量的能量.     

纳米SiO2对先驱体浸渍裂解法制备SiCf/SiC复合材料力学性能的影响

以纳米SiO2为填料,采用先驱体浸渍裂解法制备2.5D-SiCf/SiC（D为维数,SiCf为SiC纤维）复合材料,研究了前驱液中纳米SiO2含量对复合材料力学性能的影响。结果表明,纳米SiO2的添加能有效抑制先驱体裂解过程中的体积收缩,提高致密度,但过量引入易导致浸渍液黏度过高,浸渍效率降低。纳米SiO2含量对材料力学性能有较大影响,添加纳米SiO2后材料的抗弯强度和断裂韧性均高于没有添加的样品,材料抗弯强度随纳米SiO2含量的增加先增大后降低。当浸渍液中纳米SiO2含量为6%时,复合材料具有优异的力学性能,抗弯强度达到211.1MPa。     

碳化硅陶瓷先驱体聚甲基硅烷的研究进展

介绍了聚甲基硅烷的主要合成方法和性能,特别是其反应活性和高温热裂解性能.综述了聚甲基硅烷及其改性先驱体应用于制备碳化硅纤维、碳化硅基复合材料、多孔陶瓷材料等领域的研究进展.聚甲基硅烷作为碳化辞陶瓷先驱体,其制备简单、热解产物接近碳化硅的化学计量比,具有广阔的应用前景.未来该领域的研究重点是聚甲基硅烷的规模化合成,低成本改性聚甲基硅烷先驱体研究,聚甲基硅烷系列复合先驱体的制备等.     

纤维类型对Cf/SiC复合材料力学性能的影响

本工作以AIN和Y2O3为烧结助剂,采用先驱体转化－热压烧结的方法制备出了Cf/SiC复合材料,研究了纤维类型影响复合材料力学性能的本质原因,由于T300纤维的制备温度明显低于M40JB纤维的制备温度,因此,与M40JB纤维相比,T300纤维的石墨化程度较低且含有较多的杂质,从而导致T300纤维表面的活性强,而M40JB纤维表面的活性较弱,正是这种结构和成分的差别,使T300纤维与基体的结合较强,而M40JB纤维与基体的结合较弱,因此以T300纤维为增强的复合材料呈现脆性断裂,而以M40JB纤维为增强相的复合材料则呈现韧性断裂,谈复合材料具有较好的力学性能。     

陶瓷先驱体催化裂解研究进展

有机聚合物衍生陶瓷技术具有聚合物分子可设计性强、成型容易和制备温度低等优点,已经成为陶瓷及其复合材料的主要制备技术之一。裂解是陶瓷先驱体实现从有机到无机转化的关键步骤,对目标陶瓷的组成、结构和性能有着决定性的影响。在陶瓷先驱体中添加过渡金属进行催化裂解,可以改变其裂解行为,进而调控和拓展裂解产物的结构和性能。本文从不同过渡金属对陶瓷先驱体的催化裂解作用入手,总结了陶瓷先驱体催化裂解的研究现状,探讨了催化机理,并就后续深化研究与应用提出了发展建议。     

热压烧结制备SiO2f/SiO2复合材料的析晶行为及力学性质

热压烧结制备了单向连续石英纤维补强的石英复合材料.研究了热压压力和烧结温度对复合材料的析晶行为及力学性能的影响,借助扫描电镜和透射电镜等手段观察和分析了材料的显微结构并用X射线衍射法鉴别物相变化.结果表明:热压烧结温度和压力对复合材料中方石英的析出有着显著的促进作用;而热压中石英纤维比石英基体易于析晶是造成复合材料力学性能下降的主要因素.在1 250℃,12 MPa热压制备的样品抗弯强度能达到113.9 MPa.     

PVC/竹片木塑复合材料制备工艺

以竹片为原料,复合聚氯乙烯(PVC)薄膜,通过热压–冷压工艺制备了新型PVC木塑复合材料。通过改变热压时间、热压温度和PVC添加量,研究了不同工艺条件下所制备的PVC木塑复合材料的力学性能和界面性质。实验结果表明,制备厚度为1 cm,面积为10 cm2的木塑复合材料的最佳加工工艺为:在180℃的热压温度下热压时间750 s,PVC添加量为0.3 g。加入硅烷偶联剂KH550可以有效提高PVC木塑复合材料的力学性能和界面相容性,在最佳加工条件下加入KH550为1%时,材料的胶合强度为1.212 MPa。     

Many Drugs of Abuse May Be Acutely Transformed to Dopamine,Norepinephrine and Epinephrine In Vivo

It is well established that a wide range of drugs of abuse acutely boost the signaling of the sympathetic nervous system and the hypothalamic–pituitary–adrenal (HPA) axis, where norepinephrine and epinephrine are major output molecules. This stimulatory effect is accompanied by such symptoms as elevated heart rate and blood pressure, more rapid breathing, increased body temperature and sweating, and pupillary dilation, as well as the intoxicating or euphoric subjective properties of the drug. While many drugs of abuse are thought to achieve their intoxicating effects by modulating the monoaminergic neurotransmitter systems (i.e., serotonin, norepinephrine, dopamine) by binding to these receptors or otherwise affecting their synaptic signaling, this paper puts forth the hypothesis that many of these drugs are actually acutely converted to catecholamines (dopamine, norepinephrine, epinephrine) in vivo, in addition to transformation to their known metabolites. In this manner, a range of stimulants, opioids, and psychedelics (as well as alcohol) may partially achieve their intoxicating properties, as well as side effects, due to this putative transformation to catecholamines. If this hypothesis is correct, it would alter our understanding of the basic biosynthetic pathways for generating these important signaling molecules, while also modifying our view of the neural substrates underlying substance abuse and dependence, including psychological stress-induced relapse. Importantly, there is a direct way to test the overarching hypothesis: administer (either centrally or peripherally) stable isotope versions of these drugs to model organisms such as rodents (or even to humans) and then use liquid chromatography-mass spectrometry to determine if the labeled drug is converted to labeled catecholamines in brain, blood plasma, or urine samples.     

ICP-MS法测定地球化学样品中As、Cr、Ge、V等18种微量痕量元素的研究

建立了测定地球化学样品中包括As、Cr、Ge、V等18种微量、痕量元素的ICP-MS方法。地化试样用HF-HNO3混酸分解后,以1 1 HNO3溶解干渣。由于制样不使用盐酸,避免了Cl对As、Cr、Ge、V的质谱干扰。用国家一级地球化学标准物质GBW 07309制备溶液优化仪器工作参数,并用于校准。方法测定限(6s)为:0.007~6.4μg/g,精密度(RSD%,n=12)为:29%~9.4%,经过国家一级地球化学标准物质的分析验证,结果与标准值吻合。方法已应用于国土资源调查的试样分析。     

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.     

Insect antifeedant properties of anthranoids from the genusVismia

Vismiones and ferruginins, representatives of a new class of lypophilic anthranoids from the genusVismia were found to inhibit feeding in larvae of species ofSpodoptera, Heliothis, and inLocusta migratoria.     

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.     

基于ATRP法制备的触角状亲水性羟胺树脂的吸硼性能的研究

以大分子引发剂氯乙酰化聚苯乙烯微球(PS-acyl-Cl)经原子转移自由基聚合(ATRP)法引发丙烯酰胺(AM)和甲基丙烯酸缩水甘油酯(GMA)单体的共聚接枝,制得一种触角状亲水性环氧载体(PS-acyl-g-P(AM-co-GMA)),再经二乙醇胺(DEA)的环氧基开环胺化反应,得到一种含多个-NCH2CH2OH螯合配基的多齿-五元螯合环的触角状亲水性羟胺树脂(PS-acyl-g-P(AM-co-GMA)-DEA)。将此树脂用于硼吸附研究,结果表明,PS-acyl-g-P(AM-co-GMA)-DEA树脂对硼的吸附满足Langmuir方程,为单分子层吸附;饱和吸附量约为37.7 mg·g-1,且树脂5 min即可达到吸附平衡,与其它已报道的吸硼树脂相比,该树脂具有更高的吸附量和吸硼速率。吸附动力学研究表明,树脂吸附硼的过程主要由颗粒扩散过程控制。重复使用5次后该树脂的吸附量基本不变,解吸率均在90%以上,重复使用性能良好。