Henrik L. Andersen

Mechanisms for iron oxide formation under hydrothermal conditions: an in situ total scattering study.

The formation and growth of maghemite (γ-Fe2O3) nanoparticles from ammonium iron(III) citrate solutions (C(6)O(7)H(6) · xFe(3+) · yNH(4)) in hydrothermal synthesis conditions have been studied by in situ total scattering. The local structure of the precursor in solution is similar to that of the crystalline coordination polymer [Fe(H(2)cit(H2O)](n), where corner-sharing [FeO(6)] octahedra are linked by citrate. As hydrothermal treatment of the solution is initiated, clusters of edge-sharing [FeO(6)] units form (with extent of the structural order <5 Å). Tetrahedrally coordinated iron subsequently appears, and as the synthesis continues, the clusters slowly assemble into crystalline maghemite, giving rise to clear Bragg peaks after 90 s at 320 °C. The primary transformation from amorphous clusters to nanocrystallites takes place by condensation of the clusters along the corner-sharing tetrahedral iron units. The crystallization process is related to large changes in the local structure as the interatomic distances in the clusters change dramatically with cluster growth. The local atomic structure is size dependent, and particles smaller than 6 nm are highly disordered. The final crystallite size (<10 nm) is dependent on both synthesis temperature and precursor concentration.

Unraveling structural and magnetic information during growth of nanocrystalline SrFe12O19

The hydrothermal synthesis of magnetic strontium hexaferrite (SrFe12O19) nanocrystallites was followed in situ using synchrotron powder X-ray diffraction. For all the studied temperatures, the formation of SrFe12O19 happened through an intermediate crystalline phase, identified as the so-called six-line ferrihydrite (FeOOH). The presence of FeOOH has been overlooked in previous studies on hydrothermally synthesized SrFe12O19, despite the phase having a non-trivial influence on the magnetic properties of the final material. The chemical synthesis was successfully reproduced ex situ in a custom-designed batch-type reactor that resembles the experimental conditions of the in situ setup, while allowing larger quantities of material to be produced. The agreement in phase composition between the two studies reveals comparability between both experimental setups. Hexagonal platelet morphology is confirmed for SrFe12O19 combining Rietveld refinements of powder X-ray diffraction (PXRD) data with transmission electron microscopy (TEM). Room temperature magnetization curves were measured on the nanopowders prepared ex situ. The magnetic properties are discussed in the context of the influence of phase composition and crystallite size.

Structural stability and thermoelectric properties of cation- and anion-doped Mg2Si0.4Sn0.6

Undoped, anion-doped (Sb, Bi), and cation-doped (Ca, Zn) solid solutions of Mg2Si0.4Sn0.6 have been prepared by a commercially feasible large-scale solid state synthesis method. The compositional and structural stability of the prepared samples are investigated by high resolution synchrotron powder X-ray diffraction (PXRD) in the potential application temperature range of 300–750 K. Quantitative compositional and structural information are extracted from the multi-temperature PXRD data by the Rietveld method. Detailed analysis of the PXRD data reveals an irreversible thermally induced partial conversion of Mg2Si0.4Sn0.6 into a discrete Sn-rich Mg2Si1−xSnx-phase in the undoped and anion-doped samples. On the other hand, the cation-doped samples only undergo very minor compositional and structural changes with increasing temperature, indicating a stabilizing effect of Ca and Zn on the Mg2Si0.4Sn0.6 solid solution. The structural instability of the undoped and anion-doped samples is corroborated by the measured electrical resistivity as function of temperature in the same temperature range, in which a clear difference is observed between values during initial heating and subsequent cooling. In contrast, the resistivity data of the cation-doped samples exhibit good repeatability for two thermal cycles, confirming that cation doping greatly improves the thermal stability. This work highlights the importance of conducting multiple temperature cycles in the measurement of physical properties combined with a thorough structural characterization in studies of thermoelectric materials.

Pitfalls and reproducibility of in situ synchrotron powder X-ray diffraction studies of solvothermal nanoparticle formation

In situ powder X-ray diffraction (PXRD) is a powerful characterization tool owing to its ability to provide time-resolved information about phase composition, crystal structure and microstructure. The application of high-flux synchrotron X-ray beams and the development of custom-built reactors have facilitated second-scale time-resolved studies of nanocrystallite formation and growth during solvothermal synthesis. The short exposure times required for good time resolution limit the data quality, while the employed high-temperature–high-pressure reactors further complicate data acquisition and treatment. Based on experience gathered during ten years of conducting in situ studies of solvothermal reactions at a number of different synchrotrons, a compilation of useful advice for conducting in situ PXRD experiments and data treatment is presented here. In addition, the reproducibility of the employed portable in situ PXRD setup, experimental procedure and data analysis is evaluated. This evaluation is based on repeated measurements of an LaB6 line-profile standard throughout 5 d of beamtime and on the repetition of ten identical in situ synchrotron PXRD experiments on the hydrothermal formation of γ-Fe2O3 nanocrystallites. The study reveals inconsistencies in the absolute structural and microstructural values extracted by Rietveld refinement and whole powder pattern modelling of the in situ PXRD data, but also illustrates the robustness of trends and relative changes in the extracted parameters. From the data, estimates of the effective errors and reproducibility of in situ PXRD studies of solvothermal nanocrystallite formation are provided.

Multi-temperature structure of thermoelectric Mg2Si and Mg2Sn

A multi-temperature structural study of Mg2Si and Mg2Sn was carried out from 100 to 700 K using synchrotron X-ray powder diffraction. The temperature dependence of the lattice parameters can be expressed as a = 6.3272 (4) + 6.5 (2) × 10−5T + 4.0 (3) × 10−8T2 A and a = 6.7323 (7) + 8.5 (4) × 10−5T + 3.8 (5) × 10−8T2 A for Mg2Si and Mg2Sn, respectively. The atomic displacement parameters (ADPs) are reported and analysed using a Debye model for the averaged Uiso giving Debye temperatures of 425 (2) K for Mg2Si and 243 (2) K for Mg2Sn. The ADPs are considerably smaller for Mg2Si than for Mg2Sn reflecting the weaker chemical bonding in the Mg2Sn structure. Following the heating, an annealing effect is observed on the lattice parameters and peak widths in both structures, presumably due to changes in the crystal defects, but the lattice thermal expansion is almost unchanged by the annealing. This work provides accurate structural parameters which are of importance for studies of Mg2Si, Mg2Sn and their solid solutions.

Approaching Ferrite-Based Exchange-Coupled Nanocomposites as Permanent Magnets

During the past decade, CoFe2O4 (hard)/Co–Fe alloy (soft) magnetic nanocomposites have been routinely prepared by partial reduction of CoFe2O4 nanoparticles. Monoxide (i.e., FeO or CoO) has often been detected as a byproduct of the reduction, although it remains unclear whether the formation of this phase occurs during the reduction itself or at a later stage. Here, a novel reaction cell was designed to monitor the reduction in situ using synchrotron powder X-ray diffraction (PXRD). Sequential Rietveld refinements of the in situ data yielded time-resolved information on the sample composition and confirmed that the monoxide is generated as an intermediate phase. The macroscopic magnetic properties of samples at different reduction stages were measured by means of vibrating sample magnetometry (VSM), revealing a magnetic softening with increasing soft phase content, which was too pronounced to be exclusively explained by the introduction of soft material in the system. The elemental compositions of the constituent phases were obtained from joint Rietveld refinements of ex situ high-resolution PXRD and neutron powder diffraction (NPD) data. It was found that the alloy has a tendency to emerge in a Co-rich form, inducing a Co deficiency on the remaining spinel phase, which can explain the early softening of the magnetic material.

Enhancement of magnetic properties by spark plasma sintering of hydrothermally synthesised SrFe12O19

Platelet-shaped nano-crystallites of SrFe12O19 were hydrothermally synthesised and compacted into bulk magnets using Spark Plasma Sintering (SPS). Pellets were obtained with densities >90% of the theoretical crystallographic density. Compaction of the as-synthesised powders at 950 °C for 2 minutes causes improved alignment of the crystallites within the pellet, compared with room-temperature compaction. Applying an external magnetic field prior to the SPS compaction further increases the degree of alignment leading to an enhanced magnetic saturation and remanence, resulting in a 25% improvement of the energy product (BHmax), from 21.5 kJ m−3 obtained without magnetic alignment to 27.0 kJ m−3 with the applied magnetic field. Post-annealing of the SPS-pressed pellets improves the magnetic saturation even further leading to an increase of the energy product to 30.4 kJ m−3. Ball milling, on the other hand, diminishes the magnetic remanence, causing a reduction of the energy product.

In situ reduction of as-prepared γ-iron oxide nanoparticles

Magnetic materials are a hot topic among energy-materials and they find applications in nearly all everyday consumer electronics. Advances in magnetic performance have in particular been made for thin film and nanosized particles, because the magnetic properties are strongly related to the size. Bulk iron is relatively unreactive, however iron on the form of nanoparticles are highly reactive due to the enlarged surface area and the oxidation potential of iron. Iron oxides are cheap and unreactive precursors for the production of nanosized iron particles. Understanding the mechanisms behind the structural development [1, 2] adds to the fundamental understanding of materials’ formation and can lead to new synthesis pathways. In this study, iron oxide (γ-Fe2O3) particles were heated to 400°C under a flow of H2/Ar mixture, while the process was followed by in situ synchrotron powder X-ray diffraction measurement. The as-prepared maghemite nanoparticles were synthesized by the continuous decomposition of solutes in supercritical hydrothermal flow synthesis [3, 4]. The reagent used was ferric ammonium citrate (C6H8O7•xFe(III)•yNH3) that under hydrothermal flow synthesis decomposes into the γ-iron oxide Fe2O3. The reduction of maghemite to body centered cubic (BCC) iron does not go through a detectable intermediate state. 1. Jensen, K.M., et al., Mechanisms for iron oxide formation under hydrothermal conditions: an in situ total scattering study. ACS nano, 2014. 8(10): p. 10704-10714. 2. Andersen, H.L., et al., Size and Size Distribution Control of γ-Fe2O3 Nanocrystallites: An in Situ Study. Crystal Growth & Design, 2014. 14(3): p. 1307-1313. 3. Bondesgaard, M., et al., Guide to by-products formed in organic solvents under solvothermal conditions. The Journal of Supercritical Fluids, 2016. 113: p. 166-197. 4. Bremholm, M., M. Felicissimo, and B.B. Iversen, Time‐Resolved In Situ Synchrotron X‐ray Study and Large‐Scale Production of Magnetite Nanoparticles in Supercritical Water. Angewandte Chemie, 2009. 121(26): p. 4882-4885.

Formation of γ-Fe2O3 in hydrothermal synthesis: In situ total scattering studies

The properties of metal oxide nanoparticles are highly dependent on particle characteristics such as size, crystallinity, and structural defects. To obtain particles with tailormade properties, it is crucial to understand the mechanisms that govern these characteristics during material synthesis. For this purpose, in situ studies of particle synthesis have proven powerful.[1] Here, in situ Total Scattering (TS) combined with in situ PXRD studies of the hydrothermal synthesis of γ-Fe2O3 (maghemite) from ammonium iron citrate will be presented. In situ TS with Pair Distribution Function (PDF) analysis has recently shown to be an efficient tool for understanding the fundamental chemical processes in particle crystallization.[2,3] The full γ-Fe2O3 crystallization process from ionic complexes over nanoclusters to crystalline particles is followed and material formation mechanisms are suggested. The study shows that the local atomic structure of the precursor solution is similar to that of the crystalline coordination polymer [Fe(H2citrate)(H2O)]n where corner sharing [FeO6] octahedra are linked by citrate. As hydrothermal treatment of the solution is initiated, clusters of edge sharing [FeO6] units form. Tetrahedrally coordinated iron subsequently appears in the structure and as the synthesis continues, the clusters slowly assemble into nanocrystalline maghemite. The primary transformation from amorphous clusters to nanocrystallites takes place by condensation of the large clusters along corner sharing tetrahedral iron units. The crystallization process is related to large changes in the local structure as the interatomic distances in the clusters change dramatically with cluster growth. The local atomic structure is size dependent, and particles below 6 nm are highly disordered. Whole Powder Pattern Modelling of the PXRD data shows that the final crystallite size (<10 nm) is dependent on synthesis temperature and that the size distribution of the particles broadens with synthesis time.