High-resolution neutron imaging: a new approach to characterize water in anodic aluminum oxides

TL;DR

This study introduces high-resolution neutron imaging as a novel method to visualize and quantify water in anodic aluminum oxides, revealing how water content relates to growth conditions and structural features.

Contribution

The paper presents a new application of neutron imaging for analyzing water in porous Al oxides, providing insights into water incorporation, reversibility, and structural disorder.

Findings

Water content varies with anodizing conditions.

Water incorporation is partially reversible with heat treatment.

Neutron imaging detects structural disorder and crystallographic orientations.

Abstract

During the growth of anodic Al oxide layers water incorporates in the film and therefore influences the intrinsic properties of the oxide formed. In this study, we propose a new approach, based on the use of high-resolution neutron imaging, to visualize and quantify the water content in porous Al oxides as a function of anodizing conditions. Water in these porous films is either incorporated directly in the oxide structure (structural) and/or fills the pores (morphological). This preliminary study demonstrates that the differences in water content of porous anodic Al oxide layers are strongly related to the oxide growth parameters but interestingly cannot be directly correlated to a specific change in the amorphous oxide structure or in the pore morphology. Due to the high sensitivity of high-resolution neutron imaging to small changes in the water content, we furthermore show that the morphological water content in Al oxides formed in sulfuric acid as well as in phosphoric acid is partially reversible upon heat treatment and immersion. High-resolution neutron imaging is also found to be highly sensitive to structural disorder and crystallographic orientations, allowing to identify different crystalline Al oxide samples based on their structural and morphological defect content. This offers new perspectives to study the effect of the hydrogen and/or water incorporation as well as oxide-related structural modifications on Al oxihydroxides in relation to growth parameters, stability, functionalization as well as properties tuning, highly relevant to surface protection and their use as templates.