带电胶体粒子结晶过程的实验研究

利用带电单分散聚苯乙烯胶体粒子，通过自组装机理，制备了体积百分比为4.8%的具有多晶结构的胶体晶体，并用Kossel衍射技术和紫外可见分光光度计分别对晶体的生长过程进行了监测.通过对Kossel的图像分析检测不同阶段相应的晶格结构，发现胶体结晶过程晶体结构演变顺序为由液态—随机层结构—堆无序结构—面心立方孪晶结构到面心立方结构.定量地确定了结晶过程中晶体不同晶面的晶面间距和晶体的晶格常数，通过紫外可见分光光度计测量的晶体透射谱图，计算得到111晶面的晶面间距和晶体的晶格常数，与用Kossel衍射技术得到的结果相一致，还发现随样品放置时间的延长，衰减峰变窄和加深，并向短波方向移动，对应着晶体的晶格常数减小的现象. 关键词： 胶体晶体 自组织 Kossel环 聚苯乙烯粒子     

Colloidal monolayer trapped near a charged wall: a synchrotron x-ray diffraction study

Using x-ray diffraction from microfluidic channel arrays, we have determined concentration profiles of charge-stabilized silica colloids (radius 60+/-2 nm) confined between two like-charged dielectric walls at a few hundred nanometer distance. In solutions of very low ionic strength, strongly repulsive Coulomb interactions drive the colloids toward the central region between the walls. The addition of a small quantity of salt ions (0.2 mM) causes a dense colloidal monolayer to be trapped near the walls.     

The magnetic phase diagram of a dilute Cr---Ga system

The boundaries of the magnetic phase diagram of a dilute Cr---Ga system have been determined from measurements of the temperature and pressure dependence of the electrical resistivity. The phase boundaries separating the observed magnetic states, namely the paramagnetic state and the incommensurate and the commensurate spin density wave states, meet at about 0.5 at.% Ga, the triple point for this system.     

A model for phase transitions in a system of rod-like colloidal suspensions

The present paper describes an extension of the Maier-Saupe theory of liquid crystals to a system of charged interacting rods and shows that nematic to isotropic transitions can also be induced by increasing the charge on the rods. The effects of length of rod, concentration of salt and the volume fraction on the phase diagram are described. The concept of the charge-induced isotropic phase is described, and it is argued that the structure is expected to be glassy. Evidence to support these conclusions from the existing literature is presented.     

Phase diagram of hard colloidal platelets: a theoretical account

We construct the complete liquid crystal phase diagram of hard plate-like cylinders for variable aspect ratio using Onsager's second virial theory and employing the Parsons–Lee decoupling approximation to account for higher-body interactions in the isotropic and nematic fluid phases. The stability of the solid (columnar) state at high packing fraction is included by invoking a simple equation of state based on a Lennard–Jones–Devonshire cell model which has proven to be quantitatively reliable over a large range of packing fractions. By employing an asymptotic analysis based on the Gaussian approximation we are able to show that the nematic–columnar transition is universal and independent of particle shape. The predicted phase diagram is in qualitative agreement with simulation results.     

Kinetic pathways for the isotropic-nematic phase transition in a system of colloidal hard rods: a simulation study

We study the kinetic pathways for the isotropic-to-nematic transition in a fluid of colloidal hard rods. In order to follow the formation of the nematic phase, we develop a new cluster criterion that distinguishes nematic clusters from the isotropic phase. Applying this criterion in Monte Carlo simulations, we find spinodal decomposition as well as nucleation and growth depending on the supersaturation. We determine the height of the nucleation barrier and we study the shape and structure of the cluster. More specifically, we find ellipsoidal nematic clusters with an aspect ratio of about 1.7 and a homogeneous nematic director field. Our results are consistent with theoretical predictions on the shape and director field of nematic tactoids. Classical nucleation theory gives reasonable predictions for the height of the nucleation barrier and the critical nucleus size.     

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.     

The phase diagram of the system lithium-cadmium

Liquidus and solidus temperatures of a large number of Li-Cd alloys with compositions between 35 and 100 at % Cd have been determined by means of resistivity measurements and differential scanning calorimetry. The results are partly in disagreement with old literature data. According to our measurements the liquidus, which is bell-shaped, attains a maximum of 544°C for 59 at% Cd.     

The phase diagram of the system lithium-silicon

A large number of LiSi alloys with compositions between 5 and 50 at.% Si have been investigated by means of the Differential Scanning Calorimetry (DSC) technique. The results are in good agreement with a number of independently determined liquidus temperatures, and compatible with the compositions of the intermetallic compounds in this system. The phase diagram obtained is slightly at variance with earlier reported ones. It is shown that the latter probably are inaccurate as a consequence of corrosion of the used sample holders.     

On the phase stability of charged colloidal solutions modelled in terms of a simplified rigid-ion potential

Summary The phase stability of ionic multicomponent fluids is investigated through the Mean Spherical Approximation in the framework of the well-known Restricted Primitive Model of electrolytes. Phase stability conditions are obtained by studying the behaviour of the isothermal compressibility which diverges at the liquid-vapour spinodal. The model parameters are chosen so as to represent charged colloidal solutions with a big polyion and small counterions; the case in which a third neutral-particle species is present, possibly modelling the solvent, is also investigated. The MSA predictions turn out to be in good qualitative agreement with previous hypernetted-chain calculations for the same system; the simplicity and flexibility of the theory also allows a determination of the stability conditions for various assignments of the physical parameters which can reasonably mimic realistic configurations of charged colloidal solutions. Possible extensions and improvements of the present theoretical study, and a systematic procedure to assess its predictions for practical applications, are also shortly discussed. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Complete phase diagram of DNA unzipping: eye, Y fork, and triple point

We study the unzipping of double stranded DNA by applying a pulling force at a fraction s (0< or =s < or =1) from the anchored end. From exact analytical and numerical results, the complete phase diagram is presented. The phase diagram shows a strong ensemble dependence for various values of s. In addition, we show the existence of an eye phase and a triple point.     

The two-dimensional alternative binary L-J system: liquid——gas phase diagram

A two-dimensional (2D) binary system without considering the　Lennard-Jones (L-J) potential has been studied by using the　Collins model. In this paper, we introduce the L-J potential　into the 2D binary system and consider the existence of the　holes that are called the ``molecular fraction". The liquid--gas　phase diagram of the 2D alternative binary L-J system is obtained. The　results are quite analogous to the behaviour of 3D substances.     

Quantum orientational melting and the phase diagram of a mesoscopic system

The “phase diagram” of a two-dimensional mesoscopic system of bosons is investigated. An example of such a system is a system of indirect magnetoexcitons in semiconductor double quantum dots. Quantum Monte Carlo calculations show the existence of quantum orientational melting. At zero (quite low) temperature, as quantum fluctuations of the particles intensify, two quantum disordering phenomena occur with increasing de Boer parameter q. First, at q≈10⁻³ the system passes to a radially ordered but orientationally disordered state, where different shells of a cluster rotate relative to one another. Then at q≈0.16 a transition to a superfluid state occurs. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 11, 817–822 (10 December 1998)     

The magnetic phase diagram of the up-USe system

The magnetic phase diagram of the UP-USe solid solution was determined using neutron diffraction and magnetic susceptibility data obtained from polycrystalline samples. The lattice constants obey Vegard's Law. As the Se content increases magnetic phases with long periodicity gradually appear. The characteristic for UP magnetic structure (type I) consisting of ferromagnetic sheets stacked in the sequence +-+- is replaced by the type IA structure, ++--, which is stable over a fairly large composition range. A new phase transition called “step-like” is observed in this composition range at about 0.5T_N in both the neutron diffraction and the susceptibility data. In the UP_0.76Se_0.24 sample the type IA phase transforms to a new magnetic phase consisting of ferromagnetic sheets stacked nearly in the sequence +++--- or (3+, 3-). The repeat distance in real space is almost 3 chemical unit cells in one direction and the neutron diffraction data suggests that the square wave modulation is not fully developed. For compositions close to UP_0.70Se_0.30 the (3+,3-) type magnetic ordering coexists with ferromagnetism at low temperatures. The latter is the only magnetic phase obseved in compositions with more than 30 mol% of USe. Magnetization measurements performed at field strengths up to 5 T show that the samples with Se content around 30 mol% of USe exhibit typical metamagnetic behavior.     

Ground state phase diagram of the infinite dimensional Hubbard model: A variational study

We use the Gutzwiller variational method to study the ground state phase diagram of the infinite dimensional Hubbard model, paying special attention to features associated with the Gaussian form of the tight binding band density of states. We only consider trial states for which the validity of the Gutzwiller approximation has been proven, i.e., the paramagnetic, ferromagnetic, and two-sublattice antiferromagnetic states. We map out the phase diagram numerically, and give approximate analytic arguments to explain the behaviour in the small-U, and large-U limits. We give two versions of the phase diagram: one restricted to homogeneous phases, and another when phase separation is allowed. In the latter case, a homogeneous antiferromagnetic state is found only at exact half-filling, where the state is also insulating. Off half-filling, four different regions are found: pure paramagnetic, and ferromagnetic states, as well as antiferromagnetic-paramagnetic, and antiferromagnetic-ferromagnetic mixed phases.Dedicated to Professor W. Brenig on the occasion of his 60th birthdayResearch performed within the program of the Sonderforschungsbereich 341 supported by the Deutsche Forschungsgemeinschaft