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1.
Fullerene tubular structures can be generated by vapor condensation of carbon on an atomically flat graphite surface. Scanning tunneling microscope (STM) images revealed the presence of tubes with extremely small diameters (from 10 to 70 angstroms), most of which are terminated by hemispherical caps. Atomic resolution images of such structures showed that the tubes have a helical graphitic nature. The formation of the tubes under the quasi-free conditions suggests that the growth to tubular rather than spherical configurations is preferred for "giant fullerenes." 相似文献
2.
Stephan O Ajayan PM Colliex C Redlich P Lambert JM Bernier P Lefin P 《Science (New York, N.Y.)》1994,266(5191):1683-1685
Composite sheets and nanotubes of different morphologies containing carbon, boron, and nitrogen were grown in the electric arc discharge between graphite cathodes and amorphous boron-filled graphite anodes in a nitrogen atmosphere. Concentration profiles derived from electron energy-loss line spectra show that boron and nitrogen are correlated in a one-to-one ratio; core energy-loss fine structures reveal small differences compared to pure hexagonal boron nitride. Boron and carbon are anticorrelated, suggesting the substitution of boron and nitrogen into the carbon network. Results indicate that singlephaase CyBxNx as well as separated domains (nanosize) of boron nitride in carbon networks may exist. 相似文献
3.
外源茉莉酸类化合物系统诱导红松酚酸含量变化 总被引：1，自引：1，他引：0
冷杉梢斑螟和赤松梢斑螟常伴随发生,危害红松球果。为探讨利用红松诱导防御抵抗梢斑螟危害的可行性,以不同浓度茉莉酸甲酯、茉莉酮喷雾处理红松,采集健康、虫害及喷雾处理红松的球果和主梢,利用高效液相色谱技术测定其酚酸含量的时序变化。在健康红松球果中检测到香豆酸、苯乙酸、阿魏酸、水杨酸和肉桂酸等5种酚酸,在主梢中检测到香豆酸、苯乙酸、苯甲酸、水杨酸和肉桂酸等5种酚酸,两部位酚酸数量相同但组分存在差异。梢斑螟幼虫取食危害后,球果内出现原儿茶酸和苯酚,主梢内出现苯酚和阿魏酸,且除球果内香豆酸含量下降外,球果和主梢内其余几种酚酸含量均显著增加。相对茉莉酸甲酯,喷施茉莉酮诱导的酚酸种类更多,除原儿茶酸、绿原酸、丁香酸外其余酚酸种类与虫害诱导相同,且诱导时间与虫害取食更为接近。10 mmol/L的原位或侧枝喷雾,诱导各酚酸含量即显著高于虫害诱导,可以利用此浓度的茉莉酮喷雾处理增加红松抗性,抵御梢斑螟幼虫取食。 相似文献
4.
Sputtering of highly oriented pyrolytic graphite (HOPG) by 248-nanometer laser radiation was studied. Neutral carbon atoms and small clusters were ejected with significantly higher translational energies than were expected from a simple model of thermal vaporization in the absence of a potential barrier. The HOPG also developed a remarkable surface morphology that consists of regular cones and domes. Cone formation appears to be initiated by trace metal impurities that serve as heat shunts in this highly anisotropic material. 相似文献
5.
Two modified forms of carbon were quenched by a rapid-cooling technique from graphite sheets shock-compressed to 65 gigapascals and 3700 K. One form, ;;n-diamond," which was obtained from the most rapidly cooled part, has a crystal structure close to that of cubic diamond. The other form, found in the relatively slow-cooled part, was comparable to an i-carbon prepared by an ion-beam technique. The n-diamond is interpreted as a metastable form, the same as hexagonal diamond, converted from graphite through a martensitic transition, for which either the region or the path may be different from that of hexagonal diamond. The second form was found to be produced through reconstruction. 相似文献
6.
AA Zakhidov RH Baughman Z Iqbal C Cui I Khayrullin SO Dantas J Marti VG Ralchenko 《Science (New York, N.Y.)》1998,282(5390):897-901
Porous carbons that are three-dimensionally periodic on the scale of optical wavelengths were made by a synthesis route resembling the geological formation of natural opal. Porous silica opal crystals were sintered to form an intersphere interface through which the silica was removed after infiltration with carbon or a carbon precursor. The resulting porous carbons had different structures depending on synthesis conditions. Both diamond and glassy carbon inverse opals resulted from volume filling. Graphite inverse opals, comprising 40-angstrom-thick layers of graphite sheets tiled on spherical surfaces, were produced by surface templating. The carbon inverse opals provide examples of both dielectric and metallic optical photonic crystals. They strongly diffract light and may provide a route toward photonic band-gap materials. 相似文献
7.
Transient pioneer neurons are essential for formation of an embryonic peripheral nerve 总被引：6，自引：0，他引：6
In developing nervous systems, many peripheral and central pathways are established by early arising populations of pioneer neurons. The growth cones of these pioneer neurons can migrate while embryonic distances are short and while intervening tissue is relatively uncomplicated. Are these pioneers necessary? In grasshopper embryos, a pair of pioneer neurons arise at the tips of limb buds and extend axons through the limb to the central nervous system. Growth cones of later arising sensory neurons migrate along the pioneer axons. After ingrowth of sensory axons, the pioneer neurons die. If the pioneer neurons are prevented from differentiating by heat shock, then the sensory growth cones that would have migrated along them are blocked and fail to reach the central nervous system. Thus, the pioneer axons are necessary for successful migration of these sensory growth cones. By crossing a segment boundary early in embryogenesis, the pioneers circumvent an incompatibility between differentiated segment boundary cells and growth cone migration. Pioneer neurons may resolve similar problems in many systems. 相似文献
8.
樟子松枯梢病的侵染规律 总被引：3，自引：4，他引：3
对樟子松枯梢病、健组织病菌的分离、孢子萌发和接种试验等研究结果表明，樟子松枯梢病是一种寄主主导性病害。引起该病的松壳色二孢[Diplodia pinea(Dem.)Kickx]可在历年病树上的针叶、芽、梢、枝和果鳞上越冬，又可在健康树上的针叶、芽、梢、枝和果鳞上潜代侵染。其潜伏侵染带菌率：健康樟子松越冬各部位比当年生各部位潜伏带菌率高，分别为20％－70％和6％－30％，其中以针叶、梢、枝和每年4，5月份、9，10月份潜伏带菌率最高。病害的初次侵染源主要来自历年病树上病菌子实体飞散出来的分生孢子，借风和雨水淋洗传播，种子上不带菌。病菌可直接侵入当年嫩梢和嫩针叶，也可通过伤口、气孔侵入针叶、芽、梢和枝。潜伏期7－14d,繁殖期23－28d。 相似文献
9.
A protein induced during nerve growth (GAP-43) is a major component of growth-cone membranes 总被引：24，自引：0，他引：24
J H Skene R D Jacobson G J Snipes C B McGuire J J Norden J A Freeman 《Science (New York, N.Y.)》1986,233(4765):783-786
Growth cones are specialized structures that form the distal tips of growing axons. During both normal development of the nervous system and regeneration of injured nerves, growth cones are essential for elongation and guidance of growing axons. Developmental and regenerative axon growth is frequently accompanied by elevated synthesis of a protein designated GAP-43. GAP-43 has now been found to be a major component of growth-cone membranes in developing rat brains. Relative to total protein, GAP-43 is approximately 12 times as abundant in growth-cone membranes as in synaptic membranes from adult brains. Immunohistochemical localization of GAP-43 in frozen sections of developing brain indicates that the protein is specifically associated with neuropil areas containing growth cones and immature synaptic terminals. The results support the proposal that GAP-43 plays a role in axon growth. 相似文献
10.
侧柏球花的发端发育及物候学 总被引：1，自引：0，他引：1
侧柏营养技端4月下旬萌动,5～10月为营养生长期,10月以后休眠。7月中旬至8月上旬,长日照中日照惭短的时期,营养枝端转化为生殖枝端。雄球花端圆球形,雌球花端圆柱状。8～10月为球花发育期,10月进入休眠。翌年3月末传粉受精,10月初球果成熟。成熟的雌球花枝较雄球花枝长,90°弯曲。雌球花数量少,多着生在当年生枝条基部近轴面的侧枝上。对侧柏球花发端的时间、特点和其他针叶树种进行比较与讨论。 相似文献
11.
[目的]研究玉米秸秆制备活性炭的吸附性能。[方法]以玉米秸秆制备的粒状活性炭为研究对象,搭建了吸附性能模拟试验装置,采用静态重量法测试制备的活性炭对甲醇的吸附能力,并研究吸附床结构、吸附床内盛装粒径不同炭粒、活性炭中添加不同量的石墨粉以及改性活性炭等对系统吸附性能的影响。[结果]床内盛装同种试样炭料在同一吸附温度下,新型吸附床A(内置膜片式刺孔吸附质管)的吸附性能明显优于未进行结构改进的吸附床B,达到相同吸附量0.22 g/g时,A床吸附提前5 min;床内盛装不同粒径与同一粒径活性炭的对比试验,在同一吸附温度下,其吸附性能明显优于盛装同一粒径的,达到同一吸附量0.22 g/g时,吸附提前16 min;床内活性炭添加适量石墨粉可增强导热、强化吸附性能,最佳添加量为活性炭总量的20%;改性活性炭试验中,相比对照组经弱酸性溶液浸泡后活性炭可增强吸附性能,达到平衡吸附量87.1%时,吸附提前了3 min。[结论]该研究可为优化吸附床的结构设计和吸附式制冷系统提供参考。 相似文献
12.
Zhou O Fleming RM Murphy DW Chen CH Haddon RC Ramirez AP Glarum SH 《Science (New York, N.Y.)》1994,263(5154):1744-1747
Previous high-resolution electron microscopy (HREM) observations of the carbon nanotubes have led to a "Russian doll" structural model that is based on hollow concentric cylinders capped at both ends. The structures of the carbon nanotubes and particles were characterized here by bulk physical and chemical property measurements. The individual nanostructure is as compressible as graphite in the c axis, and such nanostructures can be intercalated with potassium and rubidium, leading to a saturation composition of "MC(8)." These results are counter to expectations that are based on a Russian doll structure. HREM after intercalation with potassium and deintercalation indicates that individual nanoparticles are a "paper-mache" of smaller graphite layers. Direct current magnetization and electron spin resonance measurements indicate that the electronic properties of the nanostructures are distinctly different from those of graphite. Although the nanostructures have distinct morphologies and electronic properties, they are highly defective and have a local structure similar to turbostratic graphite. 相似文献
13.
Bunch JS van der Zande AM Verbridge SS Frank IW Tanenbaum DM Parpia JM Craighead HG McEuen PL 《Science (New York, N.Y.)》2007,315(5811):490-493
Nanoelectromechanical systems were fabricated from single- and multilayer graphene sheets by mechanically exfoliating thin sheets from graphite over trenches in silicon oxide. Vibrations with fundamental resonant frequencies in the megahertz range are actuated either optically or electrically and detected optically by interferometry. We demonstrate room-temperature charge sensitivities down to 8 x 10(-4) electrons per root hertz. The thinnest resonator consists of a single suspended layer of atoms and represents the ultimate limit of two-dimensional nanoelectromechanical systems. 相似文献
14.
油用型红花主要农艺性状与单株籽粒产量间的相关及通径分析 总被引：9，自引：0，他引：9
对来自世界各地的50份油用型红花品种(系)的主要农艺性状进行相关和通径分析。相关分析结果表明,10个相关性状对单株粒重的影响顺序是单株有效果球数,总果球数,单株无效果球数,百粒重,总分枝数,二级分枝数,一级分枝数,株高,分枝高,顶花球直径。多元回归分析表明,单株有效果球数,总果球数,单株无效果球数,百粒重,顶花球直径是影响单株粒重的主要因素。通径分析表明,单株有效果球数,单株总果球数对产量的直接影响较大,是红花品种选育的主攻方向。综合考虑,单株产量高的油用型红花株型应是:株高和分枝数适中,单株有效果球数和总果球数多,无效果球少,顶果球直径和百粒重较大者。 相似文献
15.
Neuronal cytomechanics: the actin-based motility of growth cones 总被引：43，自引：0，他引：43
S J Smith 《Science (New York, N.Y.)》1988,242(4879):708-715
The patterns of synaptic connection that underlie brain function depend on the elaborate forms characteristic of neurons. It is therefore a central goal of neuroscience to understand the molecular basis for neuronal shape. Neuronal pathfinding during development is one major determinant of neuronal shape: growing nerve axons and dendrites must navigate, branch, and locate targets in response to extracellular cue molecules within the embryo. The leading tips of growing nerve processes, structures known as growth cones, contain especially high concentrations of the ubiquitous mechanochemical protein actin. Force generation involving this cytoskeletal molecule appears to be essential to the ability of growing nerve fibers to respond structurally to extracellular cues. New results from electronically enhanced light microscopy of living growth cones are helping to show how actin-based forces guide neurite growth and synapse formation. 相似文献
16.
[目的]研究油茶黑膜覆盖栽培技术以提高造林效果。[方法]研究黑膜覆盖和不覆膜2种处理对油茶苗木成活率、地径、苗高、分枝数、新梢长度的影响,并采用入户调查的方式调查油茶种植户的收入和成本。[结果]黑膜覆盖与对照相比,苗木成活率提高了5.94个百分点,苗木平均高增幅为13.06%,苗木地径增幅为17.31%,平均分枝数增幅为13.55%,新梢平均长度增幅为17.06%。[结论]黑膜覆盖能显著提高油茶苗生长量和存活率,促进油茶苗木的生长;与未覆膜相比,黑膜覆盖在造林成活率、苗高、地径、分枝数及新梢长度等方面均达到显著水平。 相似文献
17.
Zhang M Fang S Zakhidov AA Lee SB Aliev AE Williams CD Atkinson KR Baughman RH 《Science (New York, N.Y.)》2005,309(5738):1215-1219
Individual carbon nanotubes are like minute bits of string, and many trillions of these invisible strings must be assembled to make useful macroscopic articles. We demonstrated such assembly at rates above 7 meters per minute by cooperatively rotating carbon nanotubes in vertically oriented nanotube arrays (forests) and made 5-centimeter-wide, meter-long transparent sheets. These self-supporting nanotube sheets are initially formed as a highly anisotropic electronically conducting aerogel that can be densified into strong sheets that are as thin as 50 nanometers. The measured gravimetric strength of orthogonally oriented sheet arrays exceeds that of sheets of high-strength steel. These nanotube sheets have been used in laboratory demonstrations for the microwave bonding of plastics and for making transparent, highly elastomeric electrodes; planar sources of polarized broad-band radiation; conducting appliqués; and flexible organic light-emitting diodes. 相似文献
18.
Direct excitatory interactions between cones of different spectral types in the turtle retina 总被引：2，自引：0，他引：2
Cone linear sensitivities to red and green stimuli were measured intracellularly in the dark- and light-adapted turtle retina. Test flashes of small diameter were used to minimize horizontal cell feedback. Light adaptation was achieved with either green or red background illumination. The ratio of cone sensitivities to red and green light depended on the color of the background light and differed from the ratio measured in the dark. Electron microscope studies of Golgi-stained turtle cones revealed direct synaptic connections between red and green cones mediated by cone telodendria. These data indicate that the red cone photoresponse is not univariant as has been previously supposed and suggest that mixing of signals from different spectral classes of cones can occur via direct excitatory connections between cones. 相似文献
19.
Mao WL Mao HK Eng PJ Trainor TP Newville M Kao CC Heinz DL Shu J Meng Y Hemley RJ 《Science (New York, N.Y.)》2003,302(5644):425-427
Compressed under ambient temperature, graphite undergoes a transition at approximately 17 gigapascals. The near K-edge spectroscopy of carbon using synchrotron x-ray inelastic scattering reveals that half of the pi-bonds between graphite layers convert to sigma-bonds, whereas the other half remain as pi-bonds in the high-pressure form. The x-ray diffraction pattern of the high-pressure form is consistent with a distorted graphite structure in which bridging carbon atoms between graphite layers pair and form sigma-bonds, whereas the nonbridging carbon atoms remain unpaired with pi-bonds. The high-pressure form is superhard, capable of indenting cubic-diamond single crystals. 相似文献