Nathan A. S. Webster

Quantitative Phase Analysis

The most common use of powder diffraction in analytical science is the identification of crystalline components, or phases, present in a sample of interest. The near universal applicability of the method for this purpose is derived from the fact that a diffraction pattern is produced directly from the components’ crystal structure. However, for multi-phase samples, once the nature of phases present has been established, the next question usually asked of the diffractionist is “how much of each phase is there?” This chapter provides an overview of the basis and application of commonly used methods of quantitative phase abundance determination as well as references to the extensive literature on the subject.

Phase and morphology evolution during the solvothermal synthesis of VO2 polymorphs

The phase and morphological features of materials are often tunable by adjusting the reaction parameters of solvothermal synthesis but this versatility also poses a challenge for preparing materials with a desired phase and morphology if the behaviors of phase and morphological evolution during the solvothermal synthesis are not known. In this work, the formation and growth of VO2 nanomaterials in the solvothermal systems via the reduction of V2O5 by ethylene glycol (EG) were investigated by in situ powder X-ray diffraction (PXRD). The results show that both fast and slow heating produce the same VO2(B) final product but the phase evolution during the synthesis is very sensitive to the heating rate. Fast heating (10 °C min−1) involves an unknown intermediate while V3O7·H2O is the intermediate phase at slow heating (2 °C min−1). The formation mechanism was employed to design the synthesis of VO2(B) nanorods and the phase transformation paths were verified by large-scale batch synthesis. Furthermore, ex situ PXRD and SEM were employed to follow the structure and morphology evolution during growth. This research indicates that in situ PXRD, as a powerful tool to monitor the whole reaction process and to collect information such as phase evolution and the fate of the transient intermediates, can be used to direct the controlled synthesis of materials.

Controllable synthesis of VO2(D) and their conversion to VO2(M) nanostructures with thermochromic phase transition properties

VO2(M) nanostructures of various shapes were synthesized by a hydrothermal-calcination method. First, VO2(D) nanoparticles were synthesized by the surfactant-free hydrothermal reduction of ammonium metavanadate by oxalic acid at 160–220 °C. Then, the produced VO2(D) was further calcined at 250–600 °C to obtain the VO2(M) nanoparticles. To understand the hydrothermal reduction processes, both in situ powder X-ray diffraction (PXRD) and ex situ characterization were carried out. The results indicate a sequential process starting from the reduction of ammonium metavanadate and nucleation of the vanadium precursor, followed by the formation of intermediate VO2(B) nanosheets or nanorods, and finally phase transformation from VO2(B) to VO2(D) with a variety of morphologies. A crystal growth mechanism based on self-assembly and Ostwald ripening was proposed to explain the formation process of these unique nanostructures. The as-prepared VO2(M) nanoaggregates exhibited a lower thermochromic phase transition temperature (41.0 °C) and a narrower thermal hysteresis width (6.6 °C) than those nanopowders prepared by other methods.

Synthesis and formation mechanism of VO2(A) nanoplates with intrinsic peroxidase-like activity

Monocrystalline VO2(A) nanoplates have been synthesized via a one-pot hydrothermal process. In situ powder X-ray diffraction was used to monitor the hydrothermal synthesis and it was found that VO2(A) nucleates and grows directly from solution after the complete hydrolysis of a 2.0 M VO(acac)2 precursor solution, rather than involving a previously reported intermediate phase VO2(B). A hydrating–exfoliating–splitting mechanism was established to explain the formation of the nanoplate architecture. The synthesized VO2(A) nanoplates showed outstanding peroxidase-like activity and hence are a promising candidate for artificial peroxidase.

Microstructure Observation of Oxidation of Nd-Magnet at High Temperatures

There is a growing interest in recycling/recovery of rare earth elements from permanent magnets. A number of processing techniques are currently being developed but highly sensitive to the oxidation state of rare earth in the magnetic waste. This study investigated the microstructural changes of thermal oxidation of an Nd-based magnet and the behaviour of its oxides under high temperature recycling/recovery process. XRD analyses were carried out on a powdered sample (~10 µm) heated to 1273 K. SEM-EDS analysis was conducted on the heated bulk samples to provide detailed metallographic information. Metallographic analysis revealed multiple oxidation zones where the outer scale did not effectively inhibit further diffusion of oxygen. The thickness of this scale was found to be grown quite rapidly at temperatures higher than 973 K. The results indicated that the micro-mechanism of oxidation at higher temperature are more complex than at temperature below 773 K.

The influence of ore composition on sinter phase mineralogy and strength

The physical properties of iron ore sinter are largely influenced by raw material properties, in particular the bulk ore composition and its associated mineralogy. The levels of Fe, SiO2, Al2O3, MgO and other elements, together with the nature of the minerals in the fine ore and associated sintering conditions play a major role in determining the abundance and type of high-temperature bonding phases that form during sintering. This study using natural ores combines in situ X-ray diffraction experiments in model sinter systems with laboratory-based compact sinter tests to examine the links between iron ore composition and sintering conditions and their effects on sinter strength. Results help to establish the critical compositional and thermal parameters that control the bonding phase chemistry, which in turn influences the strength, a key sinter quality parameter of the sinter matrix.

A flow cell for the study of gas-solid reactions via in situ powder X-ray diffraction

This paper describes the development and testing of a novel capillary flow cell for use in in situ powder X-ray diffraction experiments. It is designed such that it achieves 200° of rotation of the capillary whilst still allowing the flow of gas through the sample and the monitoring of off gas via mass spectrometry, gas chromatography, or other such analytical techniques. This high degree of rotation provides more uniform heating of the sample than can be achieved in static cells or those with lower rotational ranges and consequently also improves particle statistics. The increased uniformity of heating provides more accurate temperature calibration of the experimental setup as well. The cell is designed to be held in a standard goniometer head and is therefore suitable for use in many laboratory and synchrotron instruments.

In situ synchrotron X-ray diffraction investigation of the evolution of a PbO2/PbSO4 surface layer on a copper electrowinning Pb anode in a novel electrochemical flow cell

Figures 7 and 8 of the article by Clancy et al. [(2015), J. Synchrotron Rad. 22, 366-375] are corrected.

Weathered Ilmenite: Diverse Mechanisms of Sintering and Association with Contaminants

We briefly review the nature and provenance of extremely weathered ilmenite, then investigate its fate when it is 10 % of a blended feed in the reduction kilns of the Becher process. There it is bound, unintentionally and intermittently, in sinter, which is discarded. It takes with it discrete aluminosilicate grains as well as contaminants, adsorbed and occluded within the ilmenite. Two distinct sintering mechanisms are identified. A layered wall accretion forms purely by solid state reaction, and is thickest early in the kiln. It occasionally detaches under its own weight or is eroded, and is tolerated between scheduled shutdowns for maintenance. However, if the weathered ilmenite and accompanying silica escape earlier wall accretion, a transitory aluminosilicate melt may later promote sintering of this material within the bed, adhering catastrophically to constrictions and obstructions on the kiln wall. The mechanisms and controlling factors are discussed.