Gerald G. Johnson

Quantitative X-Ray Powder Diffraction Method Using the Full Diffraction Pattern

A new quantitative X-ray powder diffraction (QXRPD) method has been developed to analyze polyphase crystalline mixtures. The unique approach employed in this method is the utilization of the full diffraction pattern of a mixture and its reconstruction as a weighted sum of diffraction patterns of the component phases. To facilitate the use of the new method, menu-driven interactive computer programs with graphics have been developed for the VAX series of computers. The analyst builds a reference database of component diffraction patterns, corrects the patterns for background effects, and determines the appropriate reference intensity ratios. This database is used to calculate the weight fraction of each phase in a mixture by fitting its diffraction pattern with a least-squares best-fit weighted sum of selected database reference patterns. The new QXRPD method was evaluated using oxides found in ceramics, corrosion products, and other materials encountered in the laboratory. Experimental procedures have been developed for sample preparation and data collection for reference samples and unknowns. Prepared mixtures have been used to demonstrate the very good results that can be obtained with this method.

Use of full diffraction spectra, both experimental and calculated, in quantitative powder diffraction analysis

The use of the full powder diffraction trace over a selected diffraction range for quantitative analysis has advantages over using single peaks in that it compensates for the effects of peak overlap and low levels of preferred orientation. Using a data base composed of experimental and calculated traces, the phase composition of an unknown may be determined by determining the least-squares best-fit sum of the appropriate data base patterns to the pattern of the unknown. Weight fractions are calculated from the pattern weighting factors using the reference-intensity-ratio method.

Computer analysis of multi-channel SEM and X-ray images from fine particles

Abstract : Techniques involving the computer processing of scanning electron microscope (SEM) images using a binary coded map approach have been developed. For each picture from one to six different SEM signals are converted from analog to digital form and recorded on magnetic tape for subsequent computer analysis. The analysis of fine grained A1203 particles and multiphase particulate mixtures are carried through from the sample preparation, the actual examination under the SEM, the digital recording of the image and finally the computer processing of the images. The computer program and the results are viewed step by step - with an explanation of the possibilities. A listing of the entire computer program is given in the Appendix. (Author)

Computer processing of SEM images by contour analyses

Abstract Techniques involving the computer processing of scanning electron microscope (SEM) images using a contour approach have been developed. For each picture from one to six different SEM signals are converted from analog to digital form and recorded on magnetic tape for subsequent computer analysis. A program finds and analyzes coordinate arrays representing the reconstructed computer picture. Least squares fitting of the contour arrays to ellipses provides measurements of the aspect ratios and orientations of the picture fields. Line integration techniques produce areas and perimeters. Computer plotting enables both the visual comparison of the reconstructed picture with a photograph of the image on the cathode ray tube of the SEM and an estimate of the accuracy of the ellipse fits.

Interactive Dialog for Phase Identification by the Johnson/Vand Program for X-Ray Diffraction Data

A new, convenient program has been designed and implemented on a VAX computer to facilitate the use of the Johnson/Vand program (Version 21) for identifying components in crystalline mixtures by X-ray diffraction. (The data base of references is distributed solely by the JCPDS – International Centre for Diffraction Data.) This new program uses an easy-to-follow conversational mode of communication for setting up the input file for the identification program from a remote terminal. The program is menu driven with screens for input of sample information, for change of default computational parameters, and for handling the experimental diffraction data. Many of the input screens can be readily bypassed when the default parameters are acceptable. An editor feature is provided for viewing the final input file and for correcting the diffraction data.

The development of a dynamic interactive computer graphics research and educational support environment

The Pennsylvania State University Computation Center is currently performing research and development in providing an effective research and training support environment which enables a user, whether researcher or student, to become familiar with an available base of graphics hardware and software support and to proceed to original programming with confidence and effectiveness.The main consideration is to provide an interactive programming environment in which users can become self-sufficient in coping with both hardware operation and potential as well as effective utilization of software in a reasonably short period of time. Such users should not have to be computer graphics specialists to apply a graphics system to their fields of expertise. Users should be thoroughly trained to understand and handle an available hardware/software system in order to be better able to evaluate the potential system usage directly and effectively.This paper, together with a film, describes an approach developed at the Computation Center and tested and implemented in three offerings of a Computer Science advanced undergraduate course.