Redox-Induced Microstructure and Phase Dynamics in Nickel: Insights from In Situ Synchrotron X-ray Diffraction.

The structural integrity of polycrystalline Nickel (Ni) under fluctuating chemical environments dictates its performance in catalytic and energy storage applications. Prior research has shown that the exposure of metallic surfaces to oxygen and hydrogen induces complex surface reconstructions and subsurface modifications. Understanding how these materials evolve during repeated oxidation and reduction cycles remains essential for predicting long-term stability in industrial reactors. Traditional ex situ observations often fail to capture the transient intermediates and crystallographic shifts occurring at the gas-solid interface during active processing. The specific behavior of nickel lattices when subjected to moisture-rich or hydrogen-saturated atmospheres requires high-resolution temporal tracking to resolve complex nickel phase dynamics. Advanced characterization techniques are necessary to bridge the gap between theoretical predictions and observed experimental outcomes in reactive gas environments. This absence of evidence motivated the current investigation into real-time microstructural shifts using advanced radiation sources.

This investigation interrogates the microstructural and phase evolution of polycrystalline Nickel (Ni) during redox cycling across diverse gaseous environments. Researchers sought to determine how Oxygen (O2), Hydrogen (H2), and Water (H2O) atmospheres influence the crystallographic orientation of the metal substrate and its oxide. The study evaluates the stability of the (111) texturing observed in Nickel Oxide (NiO) overlayers and the underlying metallic framework during sequential gas exposures. A secondary objective involves characterizing the localized transition from Face-Centered Cubic (FCC) to Hexagonal Close-Packed (HCP) structures under high hydrogen saturation conditions. The team also explored the dissociative adsorption of water molecules and the subsequent permeation of protons through the oxide lattice to form oxyhydroxide species. Quantifying the growth kinetics of various phases using mathematical modeling provides a deeper understanding of the underlying physical mechanisms governing these transformations. This comprehensive approach aims to clarify the relationship between environmental chemistry and the resulting structural state of the metal.

Scientists employed In Situ Synchrotron X-ray Diffraction (XRD) to monitor the specimen throughout the environmental transitions in real time. The experimental setup allowed for the precise control of O2, H2, and H2O concentrations while maintaining high-resolution diffraction patterns necessary for phase identification. Data collection focused on identifying shifts in the Bragg peaks corresponding to the Face-Centered Cubic (FCC) and Hexagonal Close-Packed (HCP) lattices of the metal. The researchers applied the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model to analyze the kinetic data derived from the diffraction intensities across different time scales. This mathematical framework distinguished between one-dimensional (1D) and three-dimensional (3D) growth modes based on the calculated Avrami exponents for each phase. Surface conditions were varied between pristine and preoxidized states to assess the impact of initial morphology on subsequent phase development and texturing. High-energy X-rays provided the penetration depth required to probe both the surface overlayers and the bulk substrate simultaneously.

Oxidation in pure O2 environments generated a strong (111) texture in both the Nickel Oxide (NiO) overlayer and the base Nickel (Ni) substrate. Subsequent reduction in Hydrogen (H2) followed by reoxidation resulted in the permanent loss of this initial crystallographic alignment, indicating irreversible damage. Hydrogen exposure triggered a phase transition from Face-Centered Cubic (FCC) to Hexagonal Close-Packed (HCP) Nickel in regions where proton dissolution reached saturation levels. In H2O-containing atmospheres, the dissociative adsorption of water led to the formation of Gamma-Nickel Oxyhydroxide (γ-NiOOH) within the oxide lattice. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) analysis revealed that preoxidized surfaces exhibit one-dimensional (1D) kinetics for the growth of NiO, γ-NiOOH, and HCP phases. Pristine surfaces followed three-dimensional (3D) kinetics, which the authors attributed to the island-like nucleation and expansion of the oxide layer across the metal surface. These distinct kinetic pathways highlight the influence of surface preparation on the rate and dimensionality of phase transformations.

The findings demonstrate that redox-driven phase transformations in nickel are highly sensitive to the specific chemical composition of the surrounding gas environment. Irreversible microstructural changes occurring after hydrogen exposure suggest that previous thermal or chemical treatments significantly alter future material behavior and performance. The identification of the Hexagonal Close-Packed (HCP) phase and Gamma-Nickel Oxyhydroxide (γ-NiOOH) provides a mechanistic basis for understanding nickel degradation in moisture-rich environments. These insights offer a pathway for the rational design of nickel-based catalysts with enhanced durability and selectivity in energy conversion devices. Engineers can utilize the established kinetic models to optimize the fabrication of hydrogen storage systems through controlled phase evolution and microstructural management. Future research should focus on the long-term stability of these metastable phases under industrial operating conditions to ensure material longevity. This study underscores the necessity of in situ monitoring for developing advanced materials capable of withstanding harsh redox environments.