A Fourth-Order Kernel-Free Boundary Integral Method for Interface Problems

This paper presents a fourth-order Cartesian grid based boundary integral method(BIM)for heterogeneous interface problems in two and three dimensional space,where the problem interfaces are irregular and can be explicitly given by parametric curves or implicitly defined by level set functions.The method reformulates the governing equation with interface conditions into boundary integral equations(BIEs)and reinterprets the involved integrals as solutions to some simple interface problems in an extended regular region.Solution of the simple equivalent interface problems for integral evaluation relies on a fourth-order finite difference method with an FFT-based fast elliptic solver.The structure of the coefficient matrix is preserved even with the existence of the interface.In the whole calculation process,analytical expressions of Green’s functions are never determined,formulated or computed.This is the novelty of the proposed kernel-free boundary integral(KFBI)method.Numerical experiments in both two and three dimensions are shown to demonstrate the algorithm efficiency and solution accuracy even for problems with a large diffusion coefficient ratio.

Effects of metal cations on sorption-desorption of p-nitrophenol onto wheat ash

The mutual effects of metal cations （Cu2＋, Pb2＋, Zn2＋, and Cd2＋） and p-nitrophenol （NP） on their adsorption desorption behavior onto wheat ash were studied. Results suggested that Cu2＋, Pb2＋, and Zn2＋ diminished the adsorption and increased the desorption of NP remarkably, while Cd2＋ had no such effect. In contrast, NP diminished the adsorption of Cu2＋, Pb2＋, and Zn2＋ onto ash, however, this suppression effect depended on the initial concentrations of metal cations. NP had no effect on Cd〉 adsorption on ash. Fourier transform infrared （FT-IR） and X-ray absorption spectroscopic （XAS） studies suggested the following mechanisms responsible for the metal suppression effect on NP adsorption： （1） large hydrated Cu2＋, Pb2＋, and Zn〉 shells occupied the surface of ash and prevent nonspecific adsorption of NP onto ash surface; （2） Cu2＋, Pb2＋, and Zn2＋ may block the micropores of ash, resulting in decreased adsorption of NP; （3） cornplexation of Cu2＋, Pb2＋, and Zn2＋ was likely via carboxyl, hydroxylic and phenolic groups of wheat ash and these same groups may also react with NP during adsorption. As a ＂soft acid＂, Cd2＋ is less efficient in the complexation of oxygencontaining acid groups than Cu2＋, Pb2＋, and Zn2＋. Thus, Cd2＋ had no effect on the adsorption of NP on wheat ash.

A new mobile grazing-incidence X-ray absorption fine spectroscopy endstation at Beijing Synchrotron Radiation Facility

Purpose A new mobile grazing-incidence X-ray absorption fine spectroscopy(GIXAFS)endstation was developed at Beijing Synchrotron Radiation Facility(BSRF)to improve the function of general XAFS beamlines and extend their capabilities to a wider user community.Methods We developed a facile GIXAFS endstation through modifying the regular XAFS in grazing-incidence geometry.Additionally,a soller slit,filter,photographic film and tiny lead sheets were assembled to improve the signal-to-noise ratio of XAFS data.Furtherly,combined with time-resolved quick scanning XAFS(QXAFS)techniques,the systems can perform in situ XAFS measurement to study materials under operando condition.Results The GIXAFS had been used to collect the Ga K-edge XAFS of InGaN thin film on sapphire substrate,which demonstrated that signal-to-noise ratio of XAFS data had been greatly improved through suppressing the effect of substrate diffractions.Moreover,the feasibility of GIXAFS-QXAFS combination was illustrated with in situ exploring the degradation of organic-inorganic perovskites under X-ray radiation.Conclusion A new mobile and facile GIXAFS endstation has been developed for thin films study.Based on the photographic film and lead sheets,the contamination of the XAFS from the matrix is minimized.Further combined with QXAFS techniques,the systems are used to reveal the X-ray-induced organic-inorganic perovskite thin films photodegrading process,which proved their successful application in the time-resolved measurements,extending the capabilities of general beamlines available to a wider user community.