Measurements on thin sections of soils with the Eindhoven proton microprobe

In thin sections of soils inclusions of various sizes with various components and with different chemical composition are present. The chemical composition of these small inclusions can be measured with a proton microprobe using the SPIXE technique (Scanning Particle Induced X-Ray Emission). By scanning the thin section of soil with the Eindhoven proton microprobe (40 × 60 μm beam diameter) trace element mapping can be achieved with an optimum sensitivity of 20 ppm for elements with atomic number Z = 30(Zn). The result is shown with 1024 picture elements of a bauxite thin section.     

Diastereoselective noncovalent synthesis of hydrogen-bonded double-rosette assemblies

Chiral centers present either in the dimelamine components of calix[4]arene 1 or in the cyanurate components CA quantitatively induce one handedness (P or M) in the corresponding hydrogen-bonded assemblies 1(3).(CA)(6) (de>98 %). The high degree of chiral induction results from the presence of six chiral centers in close proximity (C(alpha)) to the core of the assembly. A much lower level of chiral induction is observed for assemblies with chiral centers that are more remote (C(beta)). All diastereomerically pure assemblies 1(3).(CA)(6) exhibit very high CD activities (deltavarepsilon(max) approximately 100 L mol(-1) cm(-1)), in sharp contrast to the low CD activities (deltavarepsilon(max)相似文献   

Determination of the ambipolar field in medium pressure neondischarges using radioactive tracers

Measurements of the ambipolar field in the positive column o fneondischarges with pressures of 100 and 200 torr and currents between 5 and 100 mA are presented. For these measurements a new diagnostic method which involves the use of radioactive tracers was developed. The experimental results are compared with predictions derived from a theoretical model of the positive column. The agreement appears to be fairly good.     

Two-phase method and Lagrangian relaxation to solve the Bi-Objective Set Covering Problem

This paper deals with the Bi-Objective Set Covering Problem, which is a generalization of the well-known Set Covering Problem. The proposed approach is a two-phase heuristic method which has the particularity to be a constructive method using the primal-dual Lagrangian relaxation to solve single objective Set Covering problems. The results show that this algorithm finds several potentially supported and unsupported solutions. A comparison with an exact method (up to a medium size), shows that many Pareto-optimal solutions are retrieved and that the other solutions are well spread and close to the optimal ones. Moreover, the method developed compares favorably with the Pareto Memetic Algorithm proposed by Jaszkiewicz.     

The Formation of NaYF4 : Er3+, Yb3+ Nanocrystals Studied by In Situ X-ray Scattering: Phase Transition and Size Focusing

β-NaYF₄ nanocrystals are a popular class of optical materials. They can be doped with optically active lanthanide ions and shaped into core-multi-shell geometries with controlled dopant distributions. Here, we follow the synthesis of β-NaYF₄ nanocrystals from α-NaYF₄ precursor particles using in situ small-angle and wide-angle X-ray scattering and ex situ electron microscopy. We observe an evolution from a unimodal particle size distribution to bimodal, and eventually back to unimodal. The final size distribution is narrower in absolute numbers than the initial distribution. These peculiar growth dynamics happen in large part before the α-to-β phase transformation. We propose that the splitting of the size distribution is caused by variations in the reactivity of α-NaYF₄ precursor particles, potentially due to inter-particle differences in stoichiometry. Rate equation modeling confirms that a continuous distribution of reactivities can result in the observed particle growth dynamics.