Comparison of Mechanical Properties and Effects in Micro- and Nanocomposites with Carbon Fillers (Carbon Microfibers,Graphite Microwhiskers,and Carbon Nanotubes)

The mechanical properties and effects in fibrous composite materials are compared. The materials are based on the same matrix (EPON-828 epoxy resin) and differ in the type of fibers: Thornel-300 carbon microfibers, graphite microwhiskers, carbon zigzag nanotubes, and carbon chiral nanotubes. Two material models are considered: a model of elastic medium (macrolevel model) and a model of elastic mixture (micro-nanolevel model). Mechanical constants of 40 materials (4 types + 10 modifications) are calculated and compared. The theoretical ultimate compression strength along the fibers is discussed. The effects accompanying the propagation of longitudinal waves in the fiber direction are investigated.     

催化热解C2H2制备螺旋形碳晶须

以Ni粉为催化剂，在高温下催化热解乙炔(C2H2)，制备出了螺旋形碳晶须材料．实验结果表明：催化热解乙炔(C2H2)制备螺旋形碳晶须的最佳温度范围为450～600℃，恒温时间为60～70min，气体流量比分别为ρ(C2H2)：ρ(Ar)=7：6，ρ(C2H2)：ρ(Ar)=25：3及ρ(C2H2)：ρ(Ar)：ρ(H2)=3：4：7．经透射电镜、扫描电镜、X射线衍射仪分析表明，螺旋形碳晶须的螺径为130～1000nm，螺距为100～350nm，晶须直径在100nm左右，晶须长度达几百微米．在加入氢气的情况下，出现了少量的直径在500nm左右的管状碳晶须．将螺旋形碳晶须放入Ar气中，850℃温度下经历1h，成分结构未变，结晶性更好．     

Whiskers of poly(3-alkylthiophene)s

It is shown that poly(3-alkylthiophene)s may readily crystallize from poor solvents in the form of whiskers. The experimental conditions required for the formation of whiskers were found to depend upon the alkyl side-chain length, solvent quality, and temperature. In all cases studied for alkyl side-chain lengths of 3-12 carbon atoms, the widths of the whiskers were of the order of 15nm, while their lengths often exceeded tens of microns. The thickness of the whiskers formed under the experimental conditions employed was found to scale with side-chain length, and was approximately 2 or 3 times the a/2 lattice dimension of the polymer unit cell. Packing of the macromolecules within the whiskers was such that the polymer backbones were normal to the whisker length; that is, the b-axis was oriented parallel to the long axis of the whiskers. These results are thought to be relevant to known thermochromism phenomena associated with poly(3-alkylthiophene)s. © 1993 John Wiley & Sons, Inc.     

Preparation of copper aluminum borate whiskers via flux method

Single‐crystal and uniform copper aluminum borate whiskers have been synthesized by heating a mixture of boric acid, copper sulfate and aluminum sulfate with potassium sulfate as flux at 870 °C for 4 h. The synthesized whiskers exhibit a well‐crystallized, one‐dimensional structure with diameters ranging from 100 nm to 5 μm, lengths from 5 to 100 μm. Heating temperature and flux addition affect the aspect ratio and morphology of the copper aluminum borate whiskers. A possible growth mechanism of the whiskers is proposed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of disodium hydrogen phosphate dodecahydrate on hydrothermal formation of hemihydrate calcium sulfate whiskers

The influence of Na_2HPO_4·12H_2O on the hydrothermal formation of hemihydrate calcium sulfate(CaSO_4·0.5H_2O) whiskers from dihydrate calcium sulfate(CaSO_4·2H_2O)at 135 ℃ was investigated.Experimental results indicate that the addition of phosphorus accelerates the hydrothermal conversion of CaSO_4·2H_2O to CaSO_4·0.5H_2O via the formation of Ca_3(PO_4)_2 and produces CaSO_4-0.5H_2O whiskers with thinner diameters and shorter lengths.Compared with the blank experiment without Na_2HPO_4·12H_2O,the existence of minor amounts(8.65 ×10~(-4)-4.36 × 10~(-3) mol/L) of Na_2HPO_4·12H_2O led to a decrease in the diameter of CaSO_4·0.5H_2O whiskers from 1.0-10.0 to 0.5-2.0 μm and lengths from 70-300 to50-200 μm.     

Processing and characterization of new thermoset nanocomposites based on cellulose whiskers

In the present study, the processing and the mechanical properties of new thermoset nanocomposites prepared from aqueous suspensions of microcrystalline cellulose fillers and epoxy are described. The nature of cellulose fibers, which display a large aspect ratio and the ability to associate by means of H-bonds implies that the processing method chosen in this study avoids the problem of a high level of viscosity of the epoxy reactive system-whiskers mixture. The reinforcing effect of this type of natural fiber in an epoxy matrix is mainly shown from the dynamic mechanical properties in the rubbery state. This unusual reinforcement is due to (i) the strong interactions existing between the cellulose whiskers and the epoxy network and, (ii) the creation of a percolating network linked by H-bonds between cellulose fibers. The existence of such a percolation effect is evidenced from the analysis of the rubbery shear modulus of nanocomposites based on various volume fractions of whiskers with mechanical modeling such as Halpin-Kardos and percolation approaches.     

Synthesis of bamboo‐like SiC whiskers from waste silica fume

A process for the utilization of wasted silica fume is proposed in this work. Silicon carbide (SiC) whiskers several tens of micrometers in length and with a bamboo‐like morphology have been successfully synthesized by a carbothermal reduction process using purified silica fume as the silicon source. The morphology and structure of SiC whiskers were investigated by X‐ray diffraction, Raman spectroscopy, scanning electron microscopy, and high‐resolution transmission electron microscopy. Studies found that the as‐synthesized whiskers were grown as single‐crystalline β‐SiC along the (111) growth direction. The whiskers consisted of hexagonal stems randomly decorated with larger‐diameter knots along their whole length. On the basis of the characterization results, a vapor–solid process was discussed as a possible growth mechanism of the β‐SiC whiskers.     

Carbon Whisker Membrane

We report a recent developed membrane called carbon whisker membrane (CWM). The CWMs consist of a tubular ceramic membrane covered by a layer of carbon film and carbon whiskers formed on the surface of the carbon film. Hydrocarbons such as methane and a modified chemical vapor deposition apparatus were employed for fabricating uniformed CWMs. Because of the unique feature of the CWMs, the filtration performance of the CWMs was investigated and it was found that the CWMs possess a function of anti-attachment of particles and/or biomaterials on the membranes so that the permeate flux and the cleaning process of the membrane can be improved.     

合成温度对碳热还原法合成碳化硅晶须形貌的影响

以SiO2微粉为硅源,炭黑为碳源,氧化硼为催化剂,采用碳热还原法分别在1500 ℃、1550 ℃、1600 ℃制备了SiC晶须.通过扫描电镜,电子探针和透射电镜等分析手段,研究了合成温度对SiC晶须形貌的影响,探讨了晶须的生长机理.结果表明:当合成温度为1500 ℃时,所合成的SiC晶须形貌呈竹节状,选区电子衍射分析发现孪晶等面缺陷在晶须的生长方向上周期性出现;当合成温度在1550 ℃以上时,哑铃状晶须的数量会急剧增多,分析表明晶须表面包裹的串珠小球为β-SiC.在晶须的顶端发现催化剂熔球,由此推测生长机理为VLS机理,但当合成温度超过1550 ℃时,SiC会以VS生长机理沿径向沉积生成哑铃状晶须.     

不同氧化硅前驱体熔盐反应制备莫来石晶须

利用熔盐反应制备了莫来石晶须,采用X射线粉末衍射(XRD)、扫描电子显微镜(SEM)、热重分析(TG-DSC)、高分辨透射电镜(HRTEM)等分析表征技术对所制备的莫来石晶须进行了测定。SEM研究表明莫来石晶须的直径在200～400 nm范围,长度可达到几个微米。HRTEM照片显示所制备晶须的晶面间距为0.539 nm,正好与莫来石(110)晶面数据吻合,证明熔盐法制备的晶须为莫来石。随着反应物中氧化硅物种的引入,γ-Al_2O_3不断消耗和莫来石相不断生长,获得了各向异性的莫来石晶须。硫酸铝的分解反应是最重要的反应控制步骤,如果没有硅物种参与反应,可以在900℃下硫酸钠-硫酸铝的复合熔盐体系中得到α-Al_2O_3。熔盐反应的热力学计算表明,相比于γ-Al_2O_3,α-Al_2O_3作为硫酸铝分解的产物具有更稳定的能态。采用Kissinger-AkahiraSuno方法对硫酸铝分解过程的动力学进行研究,硫酸铝分解反应的表观活化能(Ea)为257.2 kJ·mol~(-1)。