Effect of annealing on the structural and superconducting properties of FeSe1−xTex

<p><b>The term 'high-temperature superconductivity' has long been synonymous with copper oxide-based superconductors (cuprates) up until the recent discovery of the iron-based superconductors in 2008. This new family of superconductors exhibits fundamentally interesting properties such as the interplay between magnetism and superconductivity as well as the very recently discovered topological properties of FeSe1−xTex. Furthermore, from an application point of view, iron-based superconductors have the potential to become the new norm for low-temperature, high-field applications such as MRI and nuclear fusion.</b></p> <p>However, some of the post-processing procedures required to obtain high-quality samples, like the annealing process of FeSe1−xTex, are yet to be fully understood.</p> <p>This thesis reports on the effect of annealing on the structure and composition of FeSe1−xTex and how they manifest as changes in the superconducting properties.</p> <p>Overall, air annealing is shown to improve the critical temperature and critical current density of FeSe1−xTex for almost all investigated doping concentrations.</p> <p>These improvements are the result of a decrease in excess iron driven by the formation of thin iron oxide layers on exposed surfaces of the crystal.</p> <p>Further analysis suggests that the reduction in the excess iron concentration is largest in the region right underneath the oxide layers. Consequently, the improvement in the superconducting properties is also found to be largest in these regions. In terms of the annealing atmosphere, even in nitrogen and low-vacuum atmospheres, annealing still leads to the formation of an iron oxide layer and an improvement in the superconducting properties due to the presence of residual oxygen. In rare cases, annealing was found to induce asymmetric magnetic hysteresis loops as a result of weak bulk pinning and strong surface pinning. Whilst asymmetric hysteresis loops have occasionally been reported in the cuprates and polycrystalline iron-based superconductors, this work reports the first observation of such behaviour in FeSe1−xTex single crystals. This work has deepened the understanding of the annealing process on the intrinsic properties of FeSe1−xTexand facilitates the study of additional post-processing procedures that will further improve the properties of this family of superconductors.</p>

Effect of annealing on the structural and superconducting properties of FeSe1−xTex

<p><b>The term 'high-temperature superconductivity' has long been synonymous with copper oxide-based superconductors (cuprates) up until the recent discovery of the iron-based superconductors in 2008. This new family of superconductors exhibits fundamentally interesting properties such as the interplay between magnetism and superconductivity as well as the very recently discovered topological properties of FeSe1−xTex. Furthermore, from an application point of view, iron-based superconductors have the potential to become the new norm for low-temperature, high-field applications such as MRI and nuclear fusion.</b></p> <p>However, some of the post-processing procedures required to obtain high-quality samples, like the annealing process of FeSe1−xTex, are yet to be fully understood.</p> <p>This thesis reports on the effect of annealing on the structure and composition of FeSe1−xTex and how they manifest as changes in the superconducting properties.</p> <p>Overall, air annealing is shown to improve the critical temperature and critical current density of FeSe1−xTex for almost all investigated doping concentrations.</p> <p>These improvements are the result of a decrease in excess iron driven by the formation of thin iron oxide layers on exposed surfaces of the crystal.</p> <p>Further analysis suggests that the reduction in the excess iron concentration is largest in the region right underneath the oxide layers. Consequently, the improvement in the superconducting properties is also found to be largest in these regions. In terms of the annealing atmosphere, even in nitrogen and low-vacuum atmospheres, annealing still leads to the formation of an iron oxide layer and an improvement in the superconducting properties due to the presence of residual oxygen. In rare cases, annealing was found to induce asymmetric magnetic hysteresis loops as a result of weak bulk pinning and strong surface pinning. Whilst asymmetric hysteresis loops have occasionally been reported in the cuprates and polycrystalline iron-based superconductors, this work reports the first observation of such behaviour in FeSe1−xTex single crystals. This work has deepened the understanding of the annealing process on the intrinsic properties of FeSe1−xTexand facilitates the study of additional post-processing procedures that will further improve the properties of this family of superconductors.</p>

The Effect of Ethanol on Abnormal Grain Growth in Copper Foils

Single-crystal Cu not only has high electrical and thermal conductivity, but can also be used as a promising platform for the epitaxial growth of two-dimensional materials. Preparing large-area single-crystal Cu foils from polycrystalline foils has emerged as the most promising technique in terms of its simplicity and effectiveness. However, the studies on transforming polycrystalline foil into large-area single-crystal foil mainly focus on the influence of annealing temperature and strain energy on the recrystallization process of copper foil, while studies on the effect of annealing atmosphere on abnormal grain growth behavior are relatively rare. It is necessary to carry out more studies on the effect of annealing atmosphere on grain growth behavior to understand the recrystallization mechanism of metal. Here, we found that introduction of ethanol in pure argon annealing atmosphere will cause the abnormal grain growth of copper foil. Moreover, the number of abnormally grown grains can be controlled by the concentration of ethanol in the annealing atmosphere. Using this technology, the number of abnormally grown grains on the copper foil can be controlled to single one. This abnormally grown grain will grow rapidly to decimeter-size by consuming the surrounding small grains. This work provides a new perspective for the understanding of the recrystallization of metals, and a new method for the preparation of large-area single-crystal copper foils.

New Insights into the Reactivity of Detonation Nanodiamonds during the First Stages of Graphitization

The present study aims to compare the early stages of graphitization of the same DND source for two annealing atmospheres (primary vacuum, argon at atmospheric pressure) in an identical set-up. DND samples are finely characterized by a combination of complementary techniques (FTIR, Raman, XPS, HR-TEM) to highlight the induced modifications for temperature up to 1100 °C. The annealing atmosphere has a significant impact on the graphitization kinetics with a higher fraction of sp2-C formed under vacuum compared to argon for the same temperature. Whatever the annealing atmosphere, carbon hydrogen bonds are created at the DND surface during annealing according to FTIR. A “nano effect”, specific to the < 10 nm size of DND, exalts the extreme surface chemistry in XPS analysis. According to HR-TEM images, the graphitization is limited to the first outer shell even for DND annealed at 1100 °C under vacuum

The Influence of Annealing Atmosphere, Blending Ratio, and Molecular Weight on the Phase Behavior of Blend Materials

In the study of block copolymers, many parameters need to be adjusted to obtain good phase separation results. Based on block copolymer polystyrene-b-polycarbonate and homopolymer polystyrene, the effects of the annealing atmosphere, blending ratio, and molecular weight on phase separation were studied. The results show that annealing in air can inhibit the occurrence of phase separation. In addition, snowflake patterns are formed during phase separation. The blending ratio affects the quality of the pattern. The molecular weight affects the size of the pattern, and the size increases as the molecular weight increases. In this article, the influence of process conditions and materials on phase separation was discussed, which has laid a solid foundation for the development of block copolymer self-assembly in the future.

Evolution of Structural and Magnetic Properties of Fe-Co Wire-like Nanochains Caused by Annealing Atmosphere

Thermal treatment is a post-synthesis treatment that aims to improve the crystallinity and interrelated physical properties of as-prepared materials. This process may also cause some unwanted changes in materials like their oxidation or contamination. In this work, we present the post-synthesis annealing treatments of the amorphous Fe1−xCox (x = 0.25; 0.50; 0.75) Wire-like nanochains performed at 400 °C in two different atmospheres, i.e., a mixture of 80% nitrogen and 20% hydrogen and argon. These processes caused significantly different changes of structural and magnetic properties of the initially-formed Fe-Co nanostructures. All of them crystallized and their cores were composed of body-centered cubic Fe-Co phase, whereas their oxide shells comprised of a mixture of CoFe2O4 and Fe3O4 phases. However, the annealing carried out in hydrogen-containing atmosphere caused a decomposition of the initial oxide shell layer, whereas a similar process in argon led to its slight thickening. Moreover, it was found that the cores of thermally-treated Fe0.25Co0.75 nanochains contained the hexagonal closest packed (hcp) Co phase and were covered by the nanosheet-like shell layer in the case of annealing performed in argon. Considering the evolution of magnetic properties induced by structural changes, it was observed that the coercivities of annealed Fe-Co nanochains increased in comparison with their non-annealed counterparts. The saturation magnetization (MS) of the Fe0.25Co0.75 nanomaterial annealed in both atmospheres was higher than that for the non-annealed sample. In turn, the MS of the Fe0.75Co0.25 and Fe0.50Co0.50 nanochains annealed in argon were lower than those recorded for non-annealed samples due to their partial oxidation during thermal processing.