Maoxu Qian

Evidence of a low compressibility carbon nitride with defect-zincblende structure

A carbon-nitride compound with defect zincblende structure (P43m) has been discovered in samples prepared by a chemical precursor route. Crystallographical (high-resolution electron microscopy and electron nanodiffraction) and electron energy loss measurements has been performed to identify the material as cubic zincblende with C3N4 composition. Nanoindentation indicates a high elastic recovery and hardness. The results agree with detailed ab initio calculations on metastable structures and compressibility. Our synthesis method is projected to be a process that could produce large quantities of material by controlling the chemical strategy. The new compound has potential applications for high hardness, elasticity and thermal conductivity materials and thin films.

Development of the EXELFS technique for high accuracy structural information

Abstract EXELFS (Extended Electron Energy Loss Fine Structure) spectroscopy contains the same local atomic structure information as XAFS in addition to having good low Z element sensitivity, much higher spatial resolution (nanoscale), and the capacity of combining other high spatial resolution TEM measurements. Due to poor quality of the EELS data, however, the EXELFS technique has not been developed to its full advantage. In this work, various new methods have been introduced to improve data acquisition technique and obtain now high quality EXELFS spectra. Also, the XAFS data analysis software has been interfaced to perform EXELFS data analysis at the same level of sophistication; this is a significant improvement over the previous cases. To develop EXELFS as a standard tool for structure determination, several additional problems must also be taken into consideration. Some of these are monitoring thickness variation, energy resolution variation, radiation damage during measurements, and better dark-current correction, as well as χ data renormalization. In this investigation, Al and SiC samples were used as calibration materials. Al K-edge, Si K-edge, and C K-edge EXELFS spectra were measured and analyzed using theoretical calculations and r -space non-linear least-squares fits to determine the structural parameters. Good agreement with the known structures was obtained.

EXELFS χ-data renormalization

Abstract In this paper, a procedure developed for accurate modeling of the background of core edges in electron energy loss spectroscopy (EELS) is described. This procedure is necessary for improving data analysis technique used in extended energy loss fine structure (EXELFS) spectroscopy via the adaptation of X-ray absorption fine structure (XAFS) analysis programs. With the improved background modeling, it now becomes possible to renormalize the EXELFS χ-data and to ensure the correct use of the XAFS programs for EXELFS analysis.

Low Temperature Hydrothermal Synthesis of Nanophase BaTiO 3 and BaFe 12 O 19 Powders

Nanocrystalline powders with an average particle size of 50 nm has been synthesized in two materials systems under hydrothermal conditions below 100°C. Processing variables, such as temperature, concentration and molar ratio of reactants and reaction time were optimized to obtain particles of reduced size and stoichiometric compositions. Hydrothermal reaction takes place between Ba(OH) 2 solution and titanium/iron precursors in sealed polyethylene bottles in the BaTiO 3 and BaFe 12 O 19 systems, respectively. While crystalline BaTiO 3 forms relatively fast within a few hours, formation of fully crystalline and stoichiometric BaFei20i9 require considerably longer reaction times up to several weeks and strongly dependent on the Ba:Fe ratio of the precursors. The structural and compositional evaluation of the nanophase powders were studied by XRD and TEM techniques.

EXELFS as a tool for quantifying phase distributions in materials

Abstract A method has been developed to quantify phase distributions in multiphase materials by using the information contained in the EXtended Energy Loss Fine Structure (EXELFS) of a probe atom in an Electron Energy Loss Spectroscopy (EELS) spectrum. The EELS and EXELFS determine, in addition to the atomic composition, the near-neighbor atomic environment about each atom type separately. The method is applicable to electron-transparent foils observed in a Transmission Electron Microscope (TEM) equipped with a parallel-detection electron energy loss spectrometer. The relative amounts of phases that have a common element can be determined even in the presence of additional phases which do not share that common element. The method is particularly applicable to layered materials and can be used, for example, to determine the local relative thickness of a metal and its oxide layer. The Mg MgO system was used to demonstrate the method, and it was found that the accuracy in relative thickness determination is about 5%. The near-edge fine structure information was consistent with the results of the EXELFS analysis.