A structure model and growth mechanism for multishell carbon nanotubes

A model that postulates a mixture of scroll-shaped and concentric, cylindrical graphene sheets is proposed to explain the microstructure of graphite multishell nanotubes grown by arc discharge. The model is consistent with the observed occurrence of a relatively small number of different chiral angles within the same tubule. The model explains clustering in a natural way and is consistent with the observation of asymmetric (0002) lattice fringe patterns and with the occurrence of singular fringe spacings larger than c/2 (c is the c parameter of graphite) in such patterns. Anisotropic thermal contraction accounts for the 2 to 3 percent increase in the c parameter of nanotubes, compared with bulk graphite, but is too small to explain the singular fringe spacings. The model also explains the formation of multishell closure domes. Nucleation is attributed to the initial formation of a fullerene "dome."     

A new mechanism for the formation of meteoritic kerogen-like material

The carbon in ancient carbonaceous chondritic meteorites is mainly in a hydrocarbon composite similar to terrestrial kerogen, a cross-linked structure of aliphatic and aromatic hydrocarbons. Until recently, the composite has been commonly thought to have been produced in the early solar nebula by a Fischer-Tropsch-type process, involving the catalytic synthesis of hydrocarbons from carbon monoxide and hydrogen on grain surfaces. Instead, the aromatic hydrocarbons may form in gas-phase pyrolysis of simple aliphatics like acetylene and methane by a mechanism developed recently to explain formation of soot in combustion and of aromatic molecules in circumstellar envelopes. Nonequilibrium chemical kinetic calculations indicate that this mechanism can produce meteoritic aromatics if the initial concentration of simple hydrocarbons in the solar nebula was sufficiently but not unreasonably high.     

Impurity-driven cone formation during laser sputtering of graphite

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.     

制备低硫可膨胀石墨的新工艺

用硫酸(98％)、硝酸(65％)和草酸的混合液与天然鳞片石墨反应制备低硫可膨胀石墨。最佳反应条件是：石墨、硫酸(98％)、硝酸(65％)、草酸的重量比为1：2．3：1．7：0．05，反应时间45min，反应温度为25℃，可膨胀石墨的含硫量为1．63％时，膨胀容积为250mL/g，其终端产品柔性石墨的含硫量为780mg/kg。     

边材生理机能及心材形成机理的研究进展

树木的边材是木质部内具有生理功能的组织,心材虽无生理功能,但对木材的利用却有非常重要的影响。树木心材形成机制是木材科学中几个尚未完全了解的问题之一。本文总结了有关边材生理机能与心材形成机理研究的重点成果,据此来讨论特异性带色心材的形成及调控机制。鉴于含水率对边材薄壁细胞生理机能的重要性,通过揭示木射线组织如何在心边材中间区调控水分导致心材形成的机理,为心材的人工调控提供一定的理论基础。      

Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load

The tensile strengths of individual multiwalled carbon nanotubes (MWCNTs) were measured with a "nanostressing stage" located within a scanning electron microscope. The tensile-loading experiment was prepared and observed entirely within the microscope and was recorded on video. The MWCNTs broke in the outermost layer ("sword-in-sheath" failure), and the tensile strength of this layer ranged from 11 to 63 gigapascals for the set of 19 MWCNTs that were loaded. Analysis of the stress-strain curves for individual MWCNTs indicated that the Young's modulus E of the outermost layer varied from 270 to 950 gigapascals. Transmission electron microscopic examination of the broken nanotube fragments revealed a variety of structures, such as a nanotube ribbon, a wave pattern, and partial radial collapse.     

Famine edema and the mechanism of its formation

Famine edema was produced experimentally in 34 normal men who lost a quarter of their body weight while subsisting for 6 months on a European type of semi-starvation diet. The ratio of extracellular water to cellular tissue was roughly doubled. Their clinical state closely resembled that seen in Europe in 1945. There were no signs of renal or cardiac failure. The plasma protein concentration fell only slightly and the A/G ratio remained within normal limits. The venous pressure was roughly 50 per cent below normal. Data from the field lend support to these indications that famine edema is not simply a result of hypoproteinemia or of renal or cardiac failure. It is concluded that there is a dynamic nonequilibrium state of the capillary wall and, accordingly, calculations from equilibrium equations are inadmissible.     

A supershear transition mechanism for cracks

Seismic data indicate that fault ruptures follow complicated paths with variable velocity because of inhomogeneities in initial stress or fracture energy. We report a phenomenon unique to three-dimensional cracks: Locally stronger fault sections, rather than slowing ruptures, drive them forward at velocities exceeding the shear wave speed. This supershear mechanism differentiates barrier and asperity models of fault heterogeneity, which previously have been regarded as indistinguishable. High strength barriers concentrate energy, producing potentially destructive pulses of strong ground motion.     

多壁碳纳米管和重金属镉的细菌毒性及影响机制

  目的  探讨多壁碳纳米管和重金属的相互作用,对复合生态毒性评价具有重要意义。  方法  测定3种不同官能团多壁碳纳米管（短、短羟基和短羧基）与镉离子（Cd2+）对大肠埃希菌Escherichia coli的单独及复合毒性,并通过碳纳米管zeta电位测定、沉降和吸附实验揭示短多壁碳纳米管-Cd2+混合物的细菌毒性及影响机制。  结果  3种多壁碳纳米管细菌毒性从强到弱依次为短多壁碳纳米管、短羧基多壁碳纳米管、短羟基多壁碳纳米管。100 mg·L-1短多壁碳纳米管混合不同质量浓度（0、1、2、4、8、10 mg·L-1）Cd2+,复合毒性从强到弱依次为短多壁碳纳米管-Cd2+、短羧基多壁碳纳米管-Cd2+、短羟基多壁碳纳米管-Cd2+,且复合毒性都小于两者叠加毒性。不同质量浓度（10、20、50、100、200 mg·L-1）多壁碳纳米管混合1 mg·L-1Cd2+,复合毒性从强到弱依次为短多壁碳纳米管-Cd2+、短羧基多壁碳纳米管-Cd2+、短羟基多壁碳纳米管-Cd2+。  结论  多壁碳纳米管-Cd2+混合物的细菌毒性主要取决于多壁碳纳米管对Cd2+的吸附状况。     

A new astrophysical setting for chondrule formation

Chondrules in the metal-rich meteorites Hammadah al Hamra 237 and QUE 94411 have recorded highly energetic thermal events that resulted in complete vaporization of a dusty region of the solar nebula (dust/gas ratio of about 10 to 50 times solar). These chondrules formed under oxidizing conditions before condensation of iron-nickel metal, at temperatures greater than or equal to 1500 K, and were isolated from the cooling gas before condensation of moderately volatile elements such as manganese, sodium, potassium, and sulfur. This astrophysical environment is fundamentally different from conventional models for chondrule formation by localized, brief, repetitive heating events that resulted in incomplete melting of solid precursors initially residing at ambient temperatures below approximately 650 K.     

A radically different mechanism for S-adenosylmethionine-dependent methyltransferases

Methylation of small molecules and macromolecules is crucial in metabolism, cell signaling, and epigenetic programming and is most often achieved by S-adenosylmethionine (SAM)-dependent methyltransferases. Most employ an S(N)2 mechanism to methylate nucleophilic sites on their substrates, but recently, radical SAM enzymes have been identified that methylate carbon atoms that are not inherently nucleophilic via the intermediacy of a 5'-deoxyadenosyl 5'-radical. We have determined the mechanisms of two such reactions targeting the sp(2)-hybridized carbons at positions 2 and 8 of adenosine 2503 in 23S ribosomal RNA, catalyzed by RlmN and Cfr, respectively. In neither case is a methyl group transferred directly from SAM to the RNA; rather, both reactions proceed by a ping-pong mechanism involving intermediate methylation of a conserved cysteine residue.     

A mechanism for social selection and successful altruism

Within the framework of neo-Darwinism, with its focus on fitness, it has been hard to account for altruism behavior that reduces the fitness of the altruist but increases average fitness in society. Many population biologists argue that, except for altruism to close relatives, human behavior that appears to be altruistic amounts to reciprocal altruism, behavior undertaken with an expectation of reciprocation, hence incurring no net cost to fitness. Herein is proposed a simple and robust mechanism, based on human docility and bounded rationality that can account for the evolutionary success of genuinely altruistic behavior. Because docility-receptivity to social influence-contributes greatly to fitness in the human species, it will be positively selected. As a consequence, society can impose a "tax" on the gross benefits gained by individuals from docility by inducing docile individuals to engage in altruistic behaviors. Limits on rationality in the face of environmental complexity prevent the individual from avoiding this "tax." An upper bound is imposed on altruism by the condition that there must remain a net fitness advantage for docile behavior after the cost to the individual of altruism has been deducted.     

A common explosion mechanism for type Ia supernovae

Type Ia supernovae, the thermonuclear explosions of white dwarf stars composed of carbon and oxygen, were instrumental as distance indicators in establishing the acceleration of the universe's expansion. However, the physics of the explosion are debated. Here we report a systematic spectral analysis of a large sample of well-observed type Ia supernovae. Mapping the velocity distribution of the main products of nuclear burning, we constrain theoretical scenarios. We find that all supernovae have low-velocity cores of stable iron-group elements. Outside this core, nickel-56 dominates the supernova ejecta. The outer extent of the iron-group material depends on the amount of nickel-56 and coincides with the inner extent of silicon, the principal product of incomplete burning. The outer extent of the bulk of silicon is similar in all supernovae, having an expansion velocity of approximately 11,000 kilometers per second and corresponding to a mass of slightly over one solar mass. This indicates that all the supernovae considered here burned similar masses and suggests that their progenitors had the same mass. Synthetic light-curve parameters and three-dimensional explosion simulations support this interpretation. A single explosion scenario, possibly a delayed detonation, may thus explain most type Ia supernovae.     

广西农业区位优势空间分异及形成机制

[目的]探索广西农业区位优势空间分异及其形成机制,为广西农业区划与相关政策制定提供参考.[方法]采用区位商对广西14个地级市的农业区位优势进行测算,并利用固定效应模型对不同优势层次地区区位商的形成机制进行研究.[结果]总体上,广西各地区的农业区位优势横向空间异质性强烈,且不同层次地区的形成机制各异.率先实现农业生产与技术服务规模化是加快现代规模化农业进程的关键点,财政支农力度不足、教育水平不高、农业机械化水平低可能成为制约广西现代化农业规模化发展的瓶颈.[建议]应强化总体规划布局,推进产业聚集,提高产业溢出效应;因地制宜,完善交通与基础生产设施;完善农业技术服务体系,突破规模化生产限制;提高财政支农效率,提升农民科技素养,实现广西农业合理布局与良性发展.     

Effects of seedling age on the growth stage and yield formation of hydroponically grown long-mat rice seedlings

Understanding the characteristics of rice productivity is of great importance for achieving high yield formation. However, such traits have not yet been studied for different ages of hydroponically grown long-mat rice seedlings(HLMS), which constitutes a new method of seedling cultivation. Field experiments were conducted to evaluate the effects of seedling age on the growth stage, photosynthesis characteristics, dry matter production, and yield of HLMS. A conventional japonica rice cultivar(Wuyunjing 24) and an indica hybrid rice cultivar(6 Liangyou 9368) were used as test materials. The results showed that the whole phase was shortened by 13–15 days for young seedlings(13-day-old) compared with old seedlings(27-day-old), which occurred because the growth process accelerated with the transplantation of young seedlings. As seedling age increased, the dry matter weight of stems of individual plants and of the population increased at the transplanting stage but decreased at the maturity stage(MS). Compared with that of 27-day-old seedlings, the average ratio of panicle weight to total plant dry weight of 13-day-old seedlings during a 2-year period increased by 3.71% for Wuyunjing 24 and by 3.78% for 6 Liangyou 9368 at the MS. Moreover, as seedling age increased, the leaf area index and photosynthetic potential decreased for both cultivars, and the photosynthetic rate markedly decreased at the heading stage(HS). With the exception of that of Wuyunjing 24 from the jointing stage to the HS in 2014, the crop growth rate was higher for young seedlings than for old seedlings. Grain yield significantly decreased with seedling age, but no significant difference was detected between the 13-and 20-day-old seedlings for either cultivar. Therefore, equilibrious and high biological yield formation, vigorous growth in the late stages, and high photosynthetic production capacity are important characteristics and causes of the efficient and sustainable output of photosynthetic systems and for achieving high yield formation in young transplanted seedlings(13–20-day-old).     

A requirement for two cell types for antibody formation in vitro

A suspension of mouse spleen cells can be separated into two populations on the basis of their ability or inability to adhere to plastic dishes. It was found that both adherent and nonadherent cells were necessary for the induction of antibody formation to sheep red blood cells in vitro. Exposure of adherent cells to antigen for brief periods of time was sufficient to initiate a maximal in vitro response.