小角x射线散射方法研究甲基改性氧化硅干凝胶的孔结构

以正硅酸乙酯［Si(OC2H5)4，TEOS］为前驱体制备SiO2悬浮液，分别以甲基三乙氧基硅烷［CH3Si(OC2H5)3，MTES］和二甲基二乙氧基硅烷［(CH3)2Si(OC2H5)2，DDS］为前驱体制备硅氧烷聚合物溶液，通过混合法得到两种不同的甲基改性氧化硅凝胶-测量了凝胶的散射强度，计算了凝胶的孔径分布、平均孔径DSAXS、界面层厚度E等参数，结合氮气吸附实验，分析了凝胶的孔结构-发现SiO2一次簇团被MTES聚合物或DDS聚合物连接为二次簇团时产生微孔，同时甲基随聚合物连接于凝胶骨架上形成与 关键词： 小角x射线散射 甲基改性氧化硅 干凝胶 孔结构     

Time of flight grazing incidence small angle neutron scattering

Grazing incidence small angle neutron scattering (GISANS) overcomes the limitations of conventional small angle scattering with respect to extremely small sample volumes in the thin film geometry. In time of flight (TOF) mode neutrons with a broad range of wavelengths are used simultaneously and recorded as a function of their respective times of flight. The combination of both, TOF-GISANS, enables the simultaneous performance of several GISANS measurements, which differ in wavelength. As a consequence, within one measurement a full set of GISANS pattern related to different scattering vectors, different scattering depths and resolutions result. This allows the detection of nanostructures with a chemical sensitivity. The possibilities of TOF-GISANS are demonstrated by the simple example of polymer nano-dots located on top of a silicon surface. As probed with atomic force microscopy (AFM) the nano-dots exhibit a large characteristic nearest neighbour distance of 545 nm and a surface coverage of 28%. From the analysis of the wavelength dependent data in combination with AFM the mass density of the polymer nano-dots is determined to be equal to the bulk value. A comparison to common single wavelength GISANS experiments is shown.     

Intrinsic bulk vortex dynamics revealed by time resolved small angle neutron scattering

Measurements of intrinsic vortex lattice (VL) dynamics in superconductors as for instance VL melting or Bragg glass transitions are typically performed by e.g. macroscopic transport or surface sensitive measurement techniques. Therefore, usually thin superconducting films are used for microscopic measurements of VL dynamics. A direct consequence of using thin films is the strong influence of surface effects and defects, sample quality and geometry. We succeeded to combine time resolved stroboscopic small angle neutron scattering (SANS) with an advanced, time varying magnetic field setup allowing to extend the time window for slow dynamical processes to the range of 10 ms up to several minutes. The new results demonstrate that it is possible to observe directly the intrinsic dynamics of the VL in a bulk niobium single crystal on a microscopic scale without limitations due to surface effects. Field and temperature dependent relaxation times of the VL from 100 to 500 ms could be observed for the first time, allowing to directly measure the VL of the tilt modulus. This new experimental technique provides the possibility to study also the dynamical magnetic properties of various strongly correlated electron systems.     

Nano-meter-sized domain formation in lipid membranes observed by small angle neutron scattering

Using a contrast matching technique of small angle neutron scattering (SANS), we have investigated a phase separation to liquid-disordered and liquid-ordered phases on ternary small unilamellar vesicles (SUVs) composed of deuterated-saturated, hydrogenated-unsaturated phosphatidylcholine lipids and cholesterol, where the equilibrium size of these domains is constrained to less than 10nm by the system size. Below a miscibility temperature, we observed characteristic scattering profiles with a maximum, indicating the formation of nano-meter-sized domains on the SUVs. The observed profiles can be described by a multi-domain model rather than a mono-domain model. The nano-meter-sized domain is agitated by thermal fluctuations and eventually ruptured, which may result in the multi-domain state. The kinetically trapped nano-meter-sized domains grow to a mono-domain state by decreasing temperature. Furthermore, between the miscibility and disorder-order transition temperature of saturated lipid, the integrated SANS intensity increased slightly, indicating the formation of nano-meter-sized heterogeneity prior to the domain nucleation.     

纳米团聚生长的多重分形谱

采用小角x射线散射（SAXS）方法，对两类具有代表性的纳米团聚的生长分形进行了表征.一 类为用化学方法 (水合肼溶液还原法) 制备的纳米金属Ni粉；另一类通过物理方法(纳米晶 化处理)，由非晶基体相中生长纳米晶相、形成非晶/纳米晶双相结构的Finemet (Fe_{73 .5} Cu₁Nb₃Si_13.5B₉) 合金.上述两 类材料的纳米团聚在生长过程中都存在 明显元素扩散迁移，形成在1—100 nm范围内的元素分布非均匀区域.这些元素分布的非均匀 区域具有多重质量生长分形特征，其尺度大小和分布方式对最终的材料的物理性能至关重要 .SAXS方法是表征这类具有分形生长特征的纳米团聚微观结构信息的强有力手段.从方法论的 角度详述了从SAXS测量到获得多重分形谱的处理过程，这一实验研究分析手段对于定量考察 纳米微结构形貌的生长机理和性能的其他研究课题有一定的帮助作用. 关键词： 纳米材料 分形生长 小角x射线散射 磁性材料     

同步辐射小角x射线散射方法研究由城市固体垃圾制备的活性炭

应用同步辐射小角x射线散射方法研究了由不同城市固体垃圾制备而成的活性炭的孔结构-结果发现利用木类、纸张、塑料这三类典型垃圾组分的热解残余物为原料制备中孔发达的活性炭是可行的-活性炭的形态和结构取决于垃圾热解残余物的组分和热解程度等因素- 关键词： 小角x射线散射 活性炭 分形维数 平均孔径     

Demonstration of spectrally resolved x-ray scattering in dense plasmas

We present the first spectrally resolved x-ray scattering measurements from solid-density plasmas. The scattering spectra show the broadened Compton down-shifted feature allowing us to determine the electron temperature and density with high accuracy. In the low temperature limit, our data indicate that the ionization balance reflects the electrons in the conduction band consistent with calculations that include quantum mechanical corrections to the interaction potential.     

Time resolved scattering relaxation mechanisms of microcavity polaritons

We study the polariton relaxation dynamics for different scattering mechanisms as: Phonon and electron scattering procesess. The relaxation polariton is obtained at very short times by solving the Boltzman equation. Instead of the well-known relaxation process by phonons, we show that the bottleneck effect relaxes to the ground state more efficiently at low pump power intensity when the electron relaxation process is included. In this way, we clearly demonstrate that different relaxation times exist, for which any of these two mechanism is more efficient to relax the polariton population to the ground state.     

Time resolved photodissociation of small peptide ions

A new instrument has been constructed whose purpose is to study time-resolved photodissociation (TRPD) of small peptide ions. Laser desorption of neutral peptides is combined with laser photoionization in an ion trap followed by thermalization, laser photodissociation and time of flight mass analysis. Ionization and excitation take place through an aromatic chromophore at the C-terminus of the peptide whereas dissociation produces the immonium ion at the N-terminus. The long-range purpose is to uncover the role of IVR in unimolecular fragmentations of peptide radical cations. One avenue is the determination of microcanonical rate constants at a well-defined internal energy E for a series of peptides possessing the same chromophore, undergoing the same fragmentation but having a variable number of degrees of freedom. This paper demonstrates the feasibility of the method and provides the first attempts at rate measurements for the peptide leucyl tyrosine (LeuTyr). Received 6 January 2002 Published online 13 September 2002     

Dynamical self-assembly of nanocrystal superlattices during colloidal droplet evaporation by in situ small angle x-ray scattering

The nucleation and growth kinetics of highly ordered gold nanocrystal superlattices during the evaporation of nanocrystal colloidal droplets was elucidated by in situ time-resolved small-angle x-ray scattering. We demonstrated for the first time that the evaporation rate can affect the dimensionality of the superlattices. The formation of two-dimensional nanocrystal superlattices at the liquid-air interface of the droplet has exponential growth kinetics that originates from interface crushing.     

Homogeneous and inhomogenous polyacrylamide gels as observed by small angle neutron scattering: A connection with elastic properties

Small angle neutron scattering of polyacrylamide solutions and gels is presented for different polymer and cross link concentrations. The gels are analyzed in connection with their elastic properties. For less than 11% of polymer concentration and for small cross link contents , it was found that the gels are no more heterogeneous than the solutions. This corresponds to the range of where the elastic modulus increases with . For larger cross link contents when the elastic modulus decreases if increases, the sample appears to be strongly heterogeneous with a large size distribution of the inhomogeneities. The results are compared with the recent model of Oshmyan and Benguigui of the elastic properties of the polyacrylamide gels. Thus a relation can be proposed between the observation of the inhomogeneities by scattering and the mechanical properties Received 30 December 1997 and Received in final form 26 June 1998     

Time resolved three-dimensional acousto-optic imaging of thick scattering media

Acousto-optic imaging is based on light interaction with focused ultrasound in a scattering medium. Thanks to photorefractive holography combined with pulsed ultrasound, we perform a time-resolved detection of ultrasound-modulated photons in the therapeutic window (780 nm). A high-gain SPS:Te crystal is used for this purpose and enables us to image through large optical thickness (500 mean free paths). We are able to generate three-dimensional (3D) acousto-optic images by translating a multielement ultrasound probe in only one direction. A 3D absorbing object is imaged through a 3 cm thick phantom.     

Specific and nonspecific collapse in protein folding funnels

Experiments with fast folding proteins are beginning to address the relationship between collapse and folding. We investigate how different scenarios for folding can arise depending on whether the folding and collapse transitions are concurrent or whether a nonspecific collapse precedes folding. Many earlier studies have focused on the limit in which collapse is fast compared to the folding time; in this work we focus on the opposite limit where, at the folding temperature, collapse and folding occur simultaneously. Real proteins exist in both of these limits. The folding mechanism varies substantially in these two regimes. In the regime of concurrent folding and collapse, nonspecific collapse now occurs at a temperature below the folding temperature (but slightly above the glass transition temperature).     

Al-Zn-Mg-Cu合金时效过程的小角度x射线散射研究

通过同步辐射小角度x射线散射方法（SAXS），研究了三种Al_Zn_ Mg_Cu合金沉淀析出过程显微结构参数（析出相尺寸和体积分数），随时效温度和时效时间 的演化，同时分析了Zn含量对合金沉淀析出过程的影响.结果表明，三种合金（A，B，C） 在实验条件下析出相均属于纳米尺度，析出相的最大体积分数随Zn含量的增加而增加，最大体积分数分别为0.023—0.028，0.052—0.054和0.04.在一定时效温度下，体积分数随时效时间的变化规律，符合析出相的形核、核长大和粗化动力学过程.  关键词： 小角度x射线散射 Al_Zn_Mg_Cu合金 时效 析出相尺寸 析出相体积分数     

Small angle neutron scattering and small angle X-ray scattering studies of platinum-loaded carbon foams

The morphology of carbon nanofoam samples comprising platinum nanoparticles dispersed in the matrix was characterized by small angle neutron scattering (SANS) and small angle X-ray scattering (SAXS) techniques. Results show that the structure of pores of carbon matrix exhibits a mass (pore) fractal nature and the average radius of the platinum particles is about 2.5 nm. The fractal dimension as well as the size distribution parameters of platinum particles varies markedly with the platinum content and annealing temperature. Transmission electron micrographs of the samples corroborate the SANS and SAXS results.      

Synchrotron small angle x-ray scattering study of dye-sensitized／-unsensitized TiO2 nanoparticle colloidal solution

The wide-gap semiconductor TiO_2 nanoparticles with and without dye sensitization have been studied by small angle x-ray scattering using synchrotron radiation. Surface properties of the colloidal TiO_2 nanoparticles have been analysed in terms of the surface fractal dimensions (D_s), showing that D_s changes from 3.25 to 2.34 when TiO_2 nanoparticles are sensitized by ATRA (all-trans-retinoic acid), which reveals that the surface of the particles become relatively smooth after dye sensitization. The size distribution of gyration radius of TiO_2 nanoparticles in the colloids M(R_g) has been successfully determined by the Shull－Roess method. The main peak of M(R_g) for the unsensitized TiO_2 colloid is located at 2.1nm, corresponding to a spherical diameter of 5.4nm, and this value for the ATRA sensitized TiO_2 increases to 2.4nm, indicating a spherical diameter of 6.4nm. Such a size enlargement of TiO_2 nanoparticles suggests that there is a coating of ATRA on the TiO_2 surface, supporting the view that a monolayer of the dye has been attached to the surface of the TiO_2 nanoparticle.     

Evidence for nanoparticles in microwave-generated fireballs observed by synchrotron x-ray scattering

The small-angle x-ray scattering method has been applied to study fireballs ejected into the air from molten hot spots in borosilicate glass by localized microwaves [V. Dikhtyar and E. Jerby, Phys. Rev. Lett. 96 045002 (2006)10.1103/PhysRevLett.96.045002]. The fireball's particle size distribution, density, and decay rate in atmospheric pressure were measured. The results show that the fireballs contain particles with a mean size of approximately 50 nm with average number densities on the order of approximately 10(9). Hence, fireballs can be considered as a dusty plasma which consists of an ensemble of charged nanoparticles in the plasma volume. This finding is likened to the ball-lightning phenomenon explained by the formation of an oxidizing particle network liberated by lightning striking the ground [J. Abrahamson and J. Dinniss, Nature (London) 403, 519 (2000)10.1038/35000525].     

Magnetic domain fluctuations in an antiferromagnetic film observed with coherent resonant soft x-ray scattering

We report the direct observation of slow fluctuations of helical antiferromagnetic domains in an ultrathin holmium film using coherent resonant magnetic x-ray scattering. We observe a gradual increase of the fluctuations in the speckle pattern with increasing temperature, while at the same time a static contribution to the speckle pattern remains. This finding indicates that domain-wall fluctuations occur over a large range of time scales. We ascribe this nonergodic behavior to the strong dependence of the fluctuation rate on the local thickness of the film.     

Quenching of exchange splitting in face centred cubic Fe observed by angle resolved photoemission

The energy bands along the (001)-direction of face centered cubic iron have been determined by means of angle resolved photoemission utilising synchrotron radiation. Bulk Fe is bcc at room temperature: the metastable fcc-phase has been obtained by thin film epitaxy on a Cu(001)-surface. In contrast to bcc Fe the bands of fcc Fe show no exchange splitting (to within the limits of our experimental resolution of 0.3 eV), indicating a loss of ferromagnetic order in the fcc phase at room temperature. An as yet unidentified feature is reported which is not consistent with the one-electron band theory.