Year: 2006
Type: dissertation
Abstract: A novel technique to obtain crystallographic information on a sample with a temporal resolution in the order of ten seconds is presented.The crystallographic information is derived from neutron time-of-flight (TOF) transmission patterns utilising neutrons traveling through a sample without interaction.Neutron source, sample and detector are aligned on a single axis.The interaction of neutrons when penetrating a sample depends on the nuclear properties of the sample atoms and the spatial distribution of the atoms.For crystalline samples, neutron intensity is diffracted away from the direction of the incident beam, leaving a characteristic pattern in the transmitted signal from which structural information can be derived.Transmission geometry provides under certain circumstances several advantages over the conventional diffraction setup: The application of a pulsed neutron source provides Bragg-edge transmission patterns over a large wavelength range with sufficient statistics within seconds as the transmitted intensity is high compared to the neutron intensity diffracted into a certain solid angle and the neutron time-of-flight readily allows to determine the neutron wavelength.This allows to study kinetics of structural phase transitions in solids lasting for much less than one hour.As the 'reflections', the so-called Bragg-edges, result from backscattering (i.e. the Bragg angle ϑ is equal to 90 • ), there is no contribution of the scattering angle to the error of the d-spacing, making this technique an interesting alternative for neutron applications needing a high d-spacing resolution like strain measurements.Since no beam-path for diffracted intensity is necessary, equipment for special environments, e.g.temperature, atmosphere or pressure, can be designed much easier than for a diffraction experiment.Precise sample positioning, as is mandatory for a diffraction setup, is not required for transmission setup and further simplyfies experimentation.The acquired patterns are naturally based on different laws than conventional powder diffraction data, thus, the available Rietveld software for the analysis of powder diffraction data cannot be used or even easily adapted for the analysis of transmission data.The large number of spectra acquired during the course of a phase transition, typically in the order of 1000, requires automated analysis.Consequently, a software package 'BETMAn', (Bragg-Edge Transmission Measurements Analysis) was developed.BETMAn allows for the first time to apply the Rietveld method to neutron transmission patterns.BETMAn and its application for studying the mechanisms and kinetics of structural phase transitions and strain measurements is presented in this work.Application examples include tracking of the carbon content in the austenitic and ferritic phase during the temperature-induced isothermal decomposition of austenite to bainite in a Si-rich steel as well as a study of kinetics of nickeloxide reduction at temperatures above 1000 • C. Calibration and benchmarking experiments are described also.
Author: Sven C. Vogel
Language: English
Subfield: Radiation
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W893036653