Synthese fluorhaltiger Polycarbosilane

Synthesis of Fluorinated Polycarbosilanes Fluorine containing polycarbosilanes could be obtained by the reaction of the completely halogenated compounds poly(dichlorocarbosilane) and poly(dibromocarbosilane) with fluorinating agents like ZnF₂, KHF₂, SbF₃, NH₄F, NH₄HF₂, HF and LiF. The most successfully applied fluorinating agent was LiF. The substitution reaction of LiF yielded a fluoropolycarbosilane with a fluorine content of more than 30 weight per cent. This fluorinated compound D-PFC with a high hydrolitic resistance was analytically and spectroscopically investigated.     

聚碳硅烷的结构和形态

通过GPC-VPO-[η]测得了聚碳硅烷（PCS）的分子量、分子量分布和分子的形态与尺寸;通过IR-NMR-元素分析联合,定量表征了组成PCS的结构基团及支化程度。结果表明,常压高温裂解法合成的PCS是一种分子量较低,分子量分布较窄的低聚物,分予内以Si—C键为主的六元稠环组成的扁球体状分子,分子的支化程度很高,分子结构很紧密,等效球体半径（Re）在甲苯中约为1nm左右。     

聚碳硅烷的催化合成

本文研究了在硼酸丁酯［Ｂ（ＯＢｕ）＿３］存在下由低分子聚硅烷（ＬＰＳ）合成聚碳硅烷（ＰＣ）的反应。结果表明，Ｂ（ＯＢｕ）＿３具有催化聚合的作用，可在较低温度下合成较高分子量的产物。本文研究了合成条件对产物特性的影响，并对催化作用原理进行了分析与讨论。     

聚碳硅烷的短程有序结构

应用广角X射线散射和径向分布函数方法对聚碳硅烷(PCS)的非晶态结构进行了分析.结果表明,PCS是一种非晶态聚合物,其骨架结构为Si-C键,含有Si与C交替连接的六元环;且PCS在r约为12×10-10 m范围内存在短程有序.PCS的最邻近原子的平均距离r3为1.86×10-10 m,最有可能对其做出贡献的是PCS中的Si-C键;另外,其中程有序周期r1约为12×10-10 m,近程有序畴rs为5.12×10-10 m,r1=2rs.由此可知,PCS主链原子的排列在两个重复单元后变得无序.     

Pyrolysis Behavior of Titanium-Containing Polycarbosilane in Air

A synthesis of titanium-containing polycarbosilane (Ti-PCS) and transformation to SiO₂/TiO₂ hybrid ceramics are investigated. The Ti-PCS is prepared by blending polycarbosilane (PCS) and tetrabutyl titanate in xylene. The structural evolution and chemical composition change during the pyrolysis of the Ti-PCS are characterized by chemical analysis, TG-DTA, XRD, XPS and TEM. The results reveal that the polymer-to-ceramic transition of the Ti-PCS involves three steps. The final ceramics obtained at 1200 °C contain amorphous silica and rutile-TiO₂ nanocrystallites of ~10 nm.     

聚碳硅烷熔融纺丝的动力学模拟

对聚碳硅烷(PCS)熔融纺丝进行动力学模拟，得到了其丝条参数随纺程的变化关系，找出其玻璃化温度，对其纺丝过程的控制有着重要的指导意义。     

Silicon Carbide from Laser Pyrolysis of Polycarbosilane

Laser pyrolysis offers a means of extending solid free-form fabrication to polymeric precursors. Laser pyrolysis of polycarbosilane (PCS) produces controlled β-SiC shapes with nanometer grain size although properties are currently limited by high porosity. By the addition of filler powders, either inert or reactive, ceramic-metal and ceramic-ceramic composite shapes are possible. The results of laser pyrolysis of PCS alone and in mixtures with 3-SiC, Al, Si, Ti, and Zr are presented showing that the technique has the potential to produce a range of materials and shapes customized to particular design requirements.     

Ceramic Precursor Aluminum-Containing Polycarbosilane: Preparation and Structure

The aluminum-containing polycarbosilane (Al-PCS) with various quantities of aluminum was obtained by adding aluminium acetylacetonate (Al(AcAc)₃) as an aluminum source to polysilacarbosilane (PSCS). Subsequently, thermal decomposition and condensation at various conditions were carried out. The molecular weight distribution, yield and softening point of the resulting Al-PCS differ with the Al(AcAc)₃:PSCS weight ratio, the thermolysis temperature and the reaction time. The larger the Al(AcAc)₃:PSCS weight ratio, the higher the thermolysis temperature and the longer the reaction time, the greater the yield and softening point of Al-PCS. In addition, the oxygen and aluminum contents of Al-PCS increase at higher Al(AcAc)₃:PSCS weight ratios. As the mixing weight ratio of Al(AcAc)₃:PSCS increased, additional oxygen was introduced into Al-PCS with the increased Al content. The structure of Al-PCS was characterized by the FT-IR. These results show that Al-PCS is very similar to polycarbosilane in structure.     

Silicon Carbide Films by Laser Pyrolysis of Polycarbosilane

Thin films of polycarbosilane were deposited on Si and SiO₂ substrates. Instead of conventional oven annealing (high temperatures, inert atmosphere), laser pyrolysis was used to achieve the polymer-to-ceramic conversion. In some conditions, especially when laser radiation absorption was enhanced by depositing a carbon layer on the surface of as-deposited films or by embedding graphite particles, this processing method yielded SiC ceramic coatings, without damaging the substrate. Processing in air or low vacuum did not result in oxidized coatings, contrary to what happens for oven pyrolysis. Laser-converted films were similar to oven-heated films processed at 1000° to 1200°C.     

Green State Joining of Silicon Carbide Using Polycarbosilane

Green state joining of SiC was investigated using a paste consisting of polycarbosilane polymer and SiC powder. The joining process and densification were described. Initial experiments resulted in the formation of symmetrical black bands and cracks on both sides of the joint. However, with modifications in processing conditions, the cracks were eliminated and the resulting joints were indistinguishable from the matrix. The flexural strength of joined samples was measured to be 234 MPa, which was comparable to that of the control sample with similar density. As the applied pressure during joining was increased from 34 to 138 MPa, the strength of the joined samples increased from 180 to 250 MPa.     

Influence of Temperature on the Properties of Polycarbosilane

A polycarbosilane precursor of SiC fiber was synthesized at high temperature under high pressure from liquid polysilane (LPS), which was obtained by thermal decomposition of poly(dimethylsilane). The effect of reaction temperature on the Si–H bond content, degree of linearity, Si–Si bond content, molecular weight, molecular weight distribution, elemental composition, softening point, and yield of the polycarbosilane (PCS) was studied spectroscopically (FTIR, UV, ¹H-NMR, and ²⁹Si-NMR) and by gel permeation chromatography (GPC). The results showed that the molecular weight, yield and softening point of the PCS increased, the molecular weight distribution broadened, and the Si–Si bond content and degree of linearity decreased when the reaction temperature increased. The Si–H bond content increased when the thermolysis reaction temperature was less than 450°C and decreased when the temperature was over 460°C. Increasing of the reaction temperature affected the composition with a general decrease in the amount of carbon, hydrogen and oxygen in the product. A middle molecular weight region of PCS in the GPC appears when the reaction temperature approaches 450°C; and, the as-synthesized PCS was stable with low Si–Si bond content. Synthetically, the conversion process is initially the formation of PCS by thermal decomposition of LPS, which is followed by an increase in molecular weight via condensation of PCS molecules.     

Rheological Behavior of Polycarbosilane Part II: Effect of Heterometal (Al) Content and Nature of Bonding with Si of Polycarbosilane

A few aluminum containing polycarbosilanes named AlOR-PCS and Alac-PCS have been synthesized by the reaction of aluminum isopropoxide (AlOR) and aluminum acetylacetonate (Alac) with polycarbosilane (PCS), respectively. These materials were characterized by Fourier Transform Spectroscopy (FTIR), Thermo gravimetric Analysis (TGA) and Gel Permeation Chromatography (GPC). The rheological properties of these compounds were studied with respect to time, temperature and atmosphere (inert & air). It has been observed that the increase in metal content enhances the crosslinking of the PCS chains. Under similar conditions, the crosslinking of AlOR-PCS derivatives was found slower than Alac-PCS. GPC analysis of the samples showed the increase in molecular weight of these compounds compared to virgin PCS. TGA showed improved ceramic yield with increasing metal content. Alac-PCS gave higher ceramic yield than AlOR-PCS for similar molar ratios of metal complexes.     

Pyrolysis Kinetic Behaviours of Polycarbosilane by Thermal Analysis Method

利用综合热分析和 Coats–Redfem 动力学模型研究了聚碳硅烷（PCS）的热裂解动力学行为。用 X 射线衍射、红外光谱分析、扫描电子显微镜等分析技术研究了 PCS 热裂解过程特点和化学反应机理。结果表明,PCS 的热解反应过程以一级热分解化学反应为主。热解过程中升温速率对 PCS 热解过程和热解产物的显微结构有较大影响,以较低升温速率（2 ℃ /min）热解时,热分解反应的活化能较高,达到 16.819kJ/mol,热解后固相残余率小,热解产物的显微结构均匀,表明较低的热解速率有利于 PCS 充分热解,可以获得结构优良的热解产物。     

用不同分子量聚碳硅烷制备含铝聚碳硅烷

分别选用分子量Mn为929、1 050、1 186的聚碳硅烷(polycarbosilane,PCS)与乙酰丙酮铝反应制备含铝聚碳硅烷(polyaluminocarbosilanes,PACS),研究PCS分子量对PACS性能及结构的影响.结果表明:随着PCS分子量的增加,PACS分子量逐渐增加,分布加宽,而活性基团Si-H键含量没有明显变化;随着PCS分子量增加,PACS陶瓷产率增加.将不同的PACS进行热交联,其陶瓷产率明显提升,增长幅度随PCS分子量增大而增大.PACS的纺丝性能随PCS分子量增大而降低.     

Effect of Vacuum Heat Treatment on Electron-Beam-Irradiation-Cured Polycarbosilane Fibers

Electron-beam-cured polycarbosilane fibers were heat-treated at 673–1773 K in a tube evacuated to 1.3 × 10⁻¹ Pa and then exposed at 1873 K in argon. The effect of vacuum heat treatment on improving the high-temperature stability of low-oxygen SiC fibers was investigated by examining gas evolution, grain growth, surface composition, tensile strength, and morphology. The fibers heat-treated at <1173 K lost strength, because of the vigorous generation of residual hydrogen. A minute amount of oxygen in the atmosphere caused the active oxidation of SiC during heat treatment at >1673 K, resulting in severe strength degradation for the as-heat-treated fibers. Vacuum heat treatment at 1573 K provided the best characteristics in low-oxygen SiC fibers.     

含乙烯基的液态聚碳硅烷的结构与性能表征

以聚二甲基硅烷为原料,热分解得到液态聚碳硅烷(LPCS);然后与四甲基四乙烯基环四硅氧烷(D4Vi)反应,合成了液态聚碳硅烷(LPVCS)。利用FT-IR、TG、XRD等分析手段对LPVCS及其陶瓷产物的组成、结构和热稳定性进行表征。结果表明,LPVCS中含有SiH及C C活性基团,可在250~300℃固化,300℃固化质量保留率为89.2%,对应1 000℃陶瓷产率为59.0%,1 200℃陶瓷产物出现β-SiC微晶。     

Synthesis of Ti3SiC2 Phase from Polycarbosilane Precursor

Ti₃SiC₂ phase was synthesized by reactive pyrolysis of three different polycarbosilane, [Si(H)₂CH₂]_m·[(H)Si(Vi)CH₂]_n·[(H)Si(Me)CH₂]_p (AHPCS), [Si(CH₃)₂CH₂]_x·[Si(H)(CH₃)CH₂]_y (PCS), and [Me(H)SiC≡C]_n, filled with metal Ti powder. The pyrolysis was carried out in argon atmosphere between 1200°C and 1400°C. The metal–precursor reactions and phase evolution during the pyrolysis were studied by means of X-ray diffraction and scanning electron microscopy. The results indicated that PCS/Ti system was more beneficial for the synthesis of Ti₃SiC₂. In addition, the high-purity Ti₃SiC₂ could be synthesized through the pyrolysis of green compact of the PCS/Ti system with CaF₂ at 1400°C.     

聚碳硅烷/二乙烯基苯陶瓷前驱体交联样品的制备

研究了聚碳硅烷交联样品的制备过程，着重讨论了用二乙烯基苯交联聚碳硅烷的条件。结果表明：选择熔点为200℃，摩尔质量为1500g/mol作为熔融纺丝，生产碳化硅纤维前驱体用的聚碳硅烷，以二乙烯基苯为交联剂，氯铂酸异丙醇溶液为催化剂，聚碳硅烷与二乙烯基苯的质量比不能低于1：0．2，二乙烯基苯的用量为20％－40％，交联反应温度120℃时，制成的聚碳硅烷／二乙烯基苯陶瓷前驱体交联样品性能良好。