Bob Baoping He

Introduction to two-dimensional X-ray diffraction

Two-dimensional X-ray diffraction refers to X-ray diffraction applications with two-dimensional detector and corresponding data reduction and analysis. The two-dimensional diffraction pattern contains far more information than a one-dimensional profile collected with the conventional diffractometer. In order to take advantage of two-dimensional diffraction, new theories and approaches are necessary to configure the two-dimensional X-ray diffraction system and to analyze the two-dimensional diffraction data. This paper is an introduction to some fundamentals about two-dimensional X-ray diffraction, such as geometry convention, diffraction data interpretation, and advantages of two-dimensional X-ray diffraction in various applications, including phase identification, stress, and texture measurement.

Microdiffraction using two-dimensional detectors

X-ray diffraction analysis on small samples or micro-area of large samples is always a challenge due to weak diffraction and poor statistics, especially when dealing with samples containing large grain size, inhomogeneous phase distribution, and preferred orientation. Two-dimensional X-ray diffraction has many advantages in microdiffraction analysis. A two-dimensional detector can collect a large amount of data both in terms of speed and angular coverage. This paper covers some aspects about instrumentation of two-dimensional X-ray diffraction and its applications in phase identification and stress analysis on small samples and micro-area of large samples.

A parallel-plate resistive-anode gaseous detector for X-ray imaging

A novel gaseous detector, based on parallel-plate gas amplification with a resistive anode and remote readout electrode, is described. The detector is significantly less prone to discharges than other gaseous wireless detectors and unique in its ability to achieve high gain at high counting rates. A local counting rate > 5 /spl times/ 10/sup 5/ counts/mm/sup 2/-sec at a gas gain of up to 10/sup 5/ has been achieved in a 14 /spl times/ 14 cm/sup 2/ area detector for diffraction applications. The detector radiation hardness is characterized by the anode accumulated charge > 100 Coul/cm/sup 2/. The detector is sealed and its re-gassing period is estimated to be 5-10 years. The operating characteristics of the new detector, its spatial resolution, and linearity are described and preliminary X-ray diffraction data are presented.

Two-Dimensional X-Ray Diffraction for Structure and Stress Analysis

This paper introduces the recent progress in two-dimensional X-ray diffraction as well as its applications in microstructure and residual stress analysis. Based on the matrix transformation between diffraction space, detector space and sample space, the unit vector of the diffraction vector can be expressed in the sample space corresponding to all the geometric parameters and Bragg conditions. The same transformation matrix can be used for texture and stress analysis. The fundamental equations for both stress measurement and texture measurement are developed with the matrix transformation defined for the two-dimensional diffraction. Stress measurement using twodimensional detector is based on a direct relationship between the stress tensor and the diffraction cone distortion. The two-dimensional detector collects texture data and background values simultaneously for multiple poles and multiple directions.

An Investigation of Residual Stress of Porous Titania Layer by Micro-Arc Oxidation under Different Voltages

The surface modification of titanium by micro-arc oxidation under different voltages was processed to achieve good direct oseointegration. The new technique of two-dimensional X-ray diffraction was used to measure the residual stress of the layer. The results show that a porous titania layer containing Ca and P is obtained by micro-arc oxidation. The pore size and Ca/P of the layer are affected by the voltage. The high voltage can induce forming CaTiO3. The residual stress under different voltage is compressive stress and increases with the improvement of the voltage.

Materials characterization from diffraction intensity distribution in the γ -direction

Two-dimensional X-ray diffraction (XRD 2 ) pattern can be described by the diffraction intensity distribution in both 2θ and γ-directions. The XRD 2 images can be reduced to two kinds of profiles: 2θ-profile and γ-profile. The 2θ-profile can be evaluated for phase identification, crystal structure refinement, and many applications with many existing algorithms and software. In order to evaluate the materials structure associated with the intensity distribution along γ-angle, either the XRD 2 pattern should be directly analyzed or the γ-profile can be generated by 2θ-integration. A γ-profile contains information on texture, stress, crystal size, and crystal orientation relations. This paper introduces the concept and fundamental algorithms for stress, texture, and crystal size analysis by the γ-profile analysis.

MicroGap Area Detector for Stress and Texture Analysis

Two-dimensional x-ray diffraction is an ideal method for examining the residual stress and texture. The most dramatic development in two-dimensional x-ray diffractometry involves three critical devices, including x-ray sources, x-ray optics and detectors. The recent development in brilliant x-rays sources and high efficiency x-ray optics provided high intensity x-ray beam with the desired size and divergence. Correspondingly, the detector used in such a high performance system requires the capability to collect large two-dimensional images with high counting rate and high resolution. This paper introduces the diffraction vector approach in two-dimensional x-ray diffraction for stress and texture analysis, and an innovative large area detector based on the MikroGap™ technology.

D009 Retractable Knife-Edge for XRD Combinatorial Screening

A motorized retractable knife-edge (patent pending) for two-dimensional X-ray diffraction is introduced for combinatorial screening at low Bragg angles. The knife-edge can control the size of the viewed diffracting sample surface, so as to improve diffraction resolution and eliminate cross contamination between adjacent cells in the materials library. It also reduces the background by blocking off the primary beam air scatter. The retractable knife-edge can be driven into two positions, retracted position for automatic sample alignment and sample viewing, and extended position for data collection.

XRD2 Stress Measurement for Samples with Texture and Large Grains

Stress measurement on samples with texture and large grains is always a challenge. The diffraction peak intensity varies dramatically with different sample orientation. The macroscopic elasticity becomes anisotropic due to strong preferred orientation. The large grains may results in a big error in 2θ due to poor sampling statistics. The fitting results of the conventional sin2ψ method is extremely sensitive to texture and large grains. When stress is measured with a 2D detector, most of the above adverse effects can be minimized or eliminated. The data integration helps to smooth out rough diffraction profiles due to large grain size, texture, small sample area or weak diffraction. The large angular coverage and multiple diffraction rings can minimize the effect of the macroscopic anisotropy. The weighted least squares regression and intensity threshold can further reduce the effect of poor statistics associated with texture and large grains. Multiple {hkl} rings may be used to measure the stress to improve the statistics and minimize the elastic anisotropy effect.

Recent Advances in Texture Measurement Using Two-Dimensional Detector

The two most important advances in two-dimensional x-ray diffraction (XRD2) are area detectors for collecting 2D diffraction patterns and algorithms in analyzing 2D diffraction patterns. The VÅNTEC-500 area detector represents the innovation in detector technology. The combination of its large active area, high sensitivity, high count rate, high resolution and low noise, makes it the technology of choice for many applications, including texture analysis. A 2D diffraction pattern contains information in a large solid angle which can be described by the diffraction intensity distribution in both 2θ and g directions. The texture information appears in a 2D diffraction pattern as intensity variation in g direction. The intensity variation represents the orientation distribution of the crystallites in a polycrystalline material. The diffraction vector orientation regarding to the sample orientation can be obtained by vector transformation from the laboratory space to the sample space. The fundamental equations for texture analysis are derived from the unit vector expression in the sample space.