Interband electronic excitation-assisted atomic-scale restructuring of metal surfaces by nanosecond pulsed laser light

Interaction of high-power laser light with materials often causes irreversible damage of the near-surface region. It is shown that copper single-crystal surfaces can be patterned by laser light. Irradiation with green light produced adatoms and vacancies, which self-organized into nanoscale pyramids. This restructuring can be removed by annealing. In contrast to green light, infrared laser irradiation at equivalent absorbed energy density did not produce any structural change. This, for metallic systems, unforeseen spectral difference in laser light action points to a concerted process as the source for structural modification, which involves long-lived primary excitation of localized d-electrons through interband transition together with phonon excitation.     

Tailored surface modification by ion implantation and laser treatment

An important trend in materials science is the use of increasingly sophisticated methods to control composition and microstructure during processing. Near-surface modification by ion implantation and laser treatment is one of these new methods for tailoring material properties. Novel materials have been formed which are far from thermodynamic equilibrium and which exhibit unexpected and useful properties. The most extensively studied property changes include modified electrical properties of semiconductors and improved wear, hardness, and corrosion resistance of metals. The high degree of control available with energetic beams allows relations between microstructure and properties to be systematically investigated at the atomic level. This article illustrates how ion and laser beam modification is being applied to advance both the technology and the exploratory science of materials.     

Synthesis of fluoropolymers in supercritical carbon dioxide

Fluoropolymers are used in many technologically demanding applications because of their balance of high-performance properties. A significant impediment to the synthesis of variants of commercially available amorphous fluoropolymers is their general insolubility in most solvents except chlorofluorocarbons (CFCs). The environmental concerns about CFCs can be circumvented by preparing these technologically important materials in supercritical fluids. The homogeneous solution polymerization of highly fluorinated acrylic monomers can be achieved in supercritical carbon dioxide by using free radical methods. In addition, detailed decomposition rates and efficiency factors were measured for azobisisobutyronitrile in supercritical carbon dioxide and were compared to those obtained with conventional liquid solvents.     

烟叶不同区段质量特征差异研究进展

同一叶片不同区段在各项质量指标等方面存在一定差异,本文综述了不同区段烟叶在外观质量、物理特性、化学成分、感官质量等方面差异的研究进展,认为通过合理的分切可以实现烟叶的最优化利用,从而提高烟叶可用性,缓解优质原料紧缺现状。     

Electron acceleration by a wake field forced by an intense ultrashort laser pulse

Plasmas are an attractive medium for the next generation of particle accelerators because they can support electric fields greater than several hundred gigavolts per meter. These accelerating fields are generated by relativistic plasma waves-space-charge oscillations-that can be excited when a high-intensity laser propagates through a plasma. Large currents of background electrons can then be trapped and subsequently accelerated by these relativistic waves. In the forced laser wake field regime, where the laser pulse length is of the order of the plasma wavelength, we show that a gain in maximum electron energy of up to 200 megaelectronvolts can be achieved, along with an improvement in the quality of the ultrashort electron beam.     

Heating of basalts with a carbon dioxide laser

Basalts heated strongly with focused infrared laser radiation vaporized and splattered. Electron microprobe analyses of condensate, ejecta, and residue show strong vapor fractionation trends which, for some elements, are different from what would be expected theoretically and from previously reported data on more siliceous materials. It appears that solution effects can account for these differences. Heating of materials by a powerful focused laser beam for the purpose of study of vapor fractionation is a convenient technique that is more versatile than previous methods such as heating in solar or arc image furnaces.     

High-gain backward lasing in air

The compelling need for standoff detection of hazardous gases and vapor indicators of explosives has motivated the development of a remotely pumped, high-gain air laser that produces lasing in the backward direction and can sample the air as the beam returns. We demonstrate that high gain can be achieved in the near-infrared region by pumping with a focused ultraviolet laser. The pumping mechanism is simultaneous resonant two-photon dissociation of molecular oxygen and resonant two-photon pumping of the atomic oxygen fragments. The high gain from the millimeter-length focal zone leads to equally strong lasing in the forward and backward directions. Further backward amplification is achieved with the use of earlier laser spark dissociation. Low-divergence backward air lasing provides possibilities for remote detection.     

Functional properties of individual neuronal branches isolated in situ by laser photoinactivation

The functional properties of an isolated dendritic branch of an identified sensory interneuron in the cricket were studied. The branch responded to wind stimuli directed at the animal and displayed a distinct directional sensitivity to those stimuli. A technique was used that allows a neuron to be specifically lesioned in a semi-intact preparation during intracellular recording. Lesioning was achieved by dye-sensitized photoinactivation with a laser epi-illumination stereomicroscope.     

Hardening by annealing and softening by deformation in nanostructured metals

We observe that a nanostructured metal can be hardened by annealing and softened when subsequently deformed, which is in contrast to the typical behavior of a metal. Microstructural investigation points to an effect of the structural scale on fundamental mechanisms of dislocation-dislocation and dislocation-interface reactions, such that heat treatment reduces the generation and interaction of dislocations, leading to an increase in strength and a reduction in ductility. A subsequent deformation step may restore the dislocation structure and facilitate the yielding process when the metal is stressed. As a consequence, the strength decreases and the ductility increases. These observations suggest that for materials such as the nanostructured aluminum studied here, deformation should be used as an optimizing procedure instead of annealing.     

N_2与He-Ne激光诱变小麦后代的生物学效应探讨

本试验采用Ｈｅ－Ｎｅ激光和Ｎ２激光辐照“汉源小麦”等四个材料的干种子,采用随机区组设计,重复３次,利用生物统计学方法,从个体水平上连续两年考查了激光诱变小麦Ｌ１代和Ｌ２两代的单株籽粒产量等５７个性状的变异,结果表明：辐照材料不同,激光诱变后代的变异程度不同,差异明显;Ｈｅ－Ｎｅ激光和Ｎ２激光诱变后代的变异随剂量增加而加大;激光种类不同,后代变异差异不大;不同激光诱变处理后代的变异大小顺序是：激光与６０Ｃｏ－γ复合诱变＞６０Ｃｏ－γ处理＞激光处理。     

A laser ablation method for the synthesis of crystalline semiconductor nanowires

A method combining laser ablation cluster formation and vapor-liquid-solid (VLS) growth was developed for the synthesis of semiconductor nanowires. In this process, laser ablation was used to prepare nanometer-diameter catalyst clusters that define the size of wires produced by VLS growth. This approach was used to prepare bulk quantities of uniform single-crystal silicon and germanium nanowires with diameters of 6 to 20 and 3 to 9 nanometers, respectively, and lengths ranging from 1 to 30 micrometers. Studies carried out with different conditions and catalyst materials confirmed the central details of the growth mechanism and suggest that well-established phase diagrams can be used to predict rationally catalyst materials and growth conditions for the preparation of nanowires.     

戊唑醇硅酸钙纳米胶囊缓释剂的制备及其缓释性能研究

以正硅酸乙酯(TEOS)和氯化钙为壁材固化包裹戊唑醇,制备得到2.5%的戊唑醇硅酸钙纳米胶囊缓释剂.并利用扫描电镜、激光粒度分布仪、傅里叶红外色谱仪、高效液相色谱对其理化性能进行详细表征,并对其缓释性能进行了探讨.结果表明:该方法能够得到硅酸钙包裹的戊唑醇纳米胶囊缓释剂;胶囊形状为规则的球形,平均粒径为209.8nm,粒径分布窄,且呈正态分布,形态稳定;较高温度和pH碱性能够提升戊唑醇的缓释速率,壁材的增厚会降低戊唑醇的缓释速率.戊唑醇微胶囊的制备对提高戊唑醇的使用效率、降低残留、减少环境污染具有重要的意义.     

Multipolymer systems

Different polymers can be combined to yield a wide variety of composite materials: layered sheets and films, homogeneous and heterogeneous blends, interpenetrating polymer networks, bicomponent fibers, and others. Some properties of a multipolymer material are roughly additive, but synergistic interactions can yield properties and performances superior to those of the individual constituents. Consequently, the use of polymers in combination is a rapidly growing component of polymer materials technology.     

BST铁电移相器材料的制备及其性能研究

基于铁电移相器材料的应用,通过掺入MgO和La2O3,以进一步优化BST材料性能。采用XRD、SEM测试手段对La2O3掺杂的BST/MgO复合陶瓷材料进行了微观结构分析。用电容法和谐振腔微扰法测试其介电性能。测试结果表明,掺入MgO和La2O3能显著降低BST材料的介电常数和损耗,而且样品在2kV/mm的电场下调谐率能达到10%以上,基本能满足用于制备铁电移相器材料的要求。     

Quantum annealing of a disordered magnet

Traditional simulated annealing uses thermal fluctuations for convergence in optimization problems. Quantum tunneling provides a different mechanism for moving between states, with the potential for reduced time scales. Thermal and quantum annealing are compared in a model disordered magnet, where the effects of quantum mechanics can be tuned by varying an applied magnetic field. The results indicate that quantum annealing hastens convergence to the optimum state.     

Cooling, heating, generating power, and recovering waste heat with thermoelectric systems

Thermoelectric materials are solid-state energy converters whose combination of thermal, electrical, and semiconducting properties allows them to be used to convert waste heat into electricity or electrical power directly into cooling and heating. These materials can be competitive with fluid-based systems, such as two-phase air-conditioning compressors or heat pumps, or used in smaller-scale applications such as in automobile seats, night-vision systems, and electrical-enclosure cooling. More widespread use of thermoelectrics requires not only improving the intrinsic energy-conversion efficiency of the materials but also implementing recent advancements in system architecture. These principles are illustrated with several proven and potential applications of thermoelectrics.     

Electronic Properties of Amorphous Materials: Changes are considered which occur when the long-range order typical for crystals disappears

A possible approach to the understanding of electronic properties of amorphous materials is to compare them with the corresponding crystalline materials, whose properties have been well explained. This approach has been exploited in the simple case of amorphous germanium, and I have indicated how the observed optical properties can be used to obtain information on the changes of electronic states, and what complications arise when we try to understand the transport properties.     

Synthesis of Novel Thin-Film Materials by Pulsed Laser Deposition

Pulsed laser deposition (PLD) is a conceptually and experimentally simple yet highly versatile tool for thin-film and multilayer research. Its advantages for the film growth of oxides and other chemically complex materials include stoichiometric transfer, growth from an energetic beam, reactive deposition, and inherent simplicity for the growth of multilayered structures. With the use of PLD, artificially layered materials and metastable phases have been created and their properties varied by control of the layer thicknesses. In situ monitoring techniques have provided information about the role of energetic species in the formation of ultrahard phases and in the doping of semiconductors. Cluster-assembled nanocrystalline and composite films offer opportunities to control and produce new combinations of properties with PLD.     

Porous semiconductor chalcogenide aerogels

Chalcogenide aerogels based entirely on semiconducting II-VI or IV-VI frameworks have been prepared from a general strategy that involves oxidative aggregation of metal chalcogenide nanoparticle building blocks followed by supercritical solvent removal. The resultant materials are mesoporous, exhibit high surface areas, can be prepared as monoliths, and demonstrate the characteristic quantum-confined optical properties of their nanoparticle components. These materials can be synthesized from a variety of building blocks by chemical or photochemical oxidation, and the properties can be further tuned by heat treatment. Aerogel formation represents a powerful yet facile method for metal chalcogenide nanoparticle assembly and the creation of mesoporous semiconductors.     

Synthesis and properties of metallic glasses that contain aluminum

The synthesis and properties of a class of metallic glasses containing up to 90 atomic percent aluminum are reported. The unusual formability of the glasses and their structural features are pointed out. Mechanical properties including tensile fracture strength and Young's modulus are reported along with crystallization temperatures. The unusually high strengths of the aluminum glasses can be of significant importance in obtaining high-strength low-density materials.