In-situ investigation of the reduction of NiO by a neutron transmission method

A novel neutron Bragg-edge transmission method was used to study the kinetics of a model solid-state reaction: NiO→Ni+1/2O2 at 1100 and 1400 °C. This method records variations in the wavelength dependence of the neutron beam transmission due to changes in Bragg diffraction in a test specimen. Since the beam transmission depends on diffraction scattering over 4π radians, the technique is more effective in detecting change in diffraction than a measurement of scattering into a detector subtending a limited solid angle. Therefore, the Bragg-edge transmission method permits kinetic study of chemical reactions and phase transformations with a better time resolution than can be achieved with powder diffraction. The advantages of this method compared with other techniques that yield kinetics information from solid-state reactions or phase transformations (e.g. thermogravimetry, dilatometry and metallography) include sensitivity to phase and crystallographic structure. In this study, we report volume fraction evolution with temporal resolution as low as 30 s. To obtain information about the rate by controlling process during the reaction, the diffraction data were analyzed using the traditional thermogravimetric approach and by employing a number of kinetics models. The analysis suggests that the reaction rate is largely controlled by diffusion at 1100 °C, but is influenced more by an interface reaction at 1400 °C.