The Role of Cerium Valence in the Conversion Temperature of H2_2Ti3_3O7_7 Nanoribbons to TiO2_2-B and Anatase Nanoribbons, and Further to Rutile

CeO$_2$-TiO$_2$ is an important mixed oxide due to its catalytic properties, particularly in heterogeneous photocatalysis. This study presents a straightforward method to obtain 1D TiO$_2$ nanostructures decorated with CeO$_2$ nanoparticles at the surface. As the precursor, we used H$_2$Ti$_3$O$_7$ nanoribbons prepared from sodium titanate nanoribbons by ion exchange. Two cerium sources with an oxidation state of +3 and +4 were used to obtain mixed oxides. HAADF–STEM mapping of the Ce$^{4+}$-modified nanoribbons revealed a thin continuous layer at the surface of the H$_2$Ti$_3$O$_7$ nanoribbons, while Ce$^{3+}$ cerium ions intercalated partially between the titanate layers. The phase composition and morphology changes were monitored during calcination between 620 °C and 960 °C. Thermal treatment led to the formation of CeO$_2$ nanoparticles on the surface of the TiO$_2$ nanoribbons, whose size increased with the calcination temperature. The use of Ce$^{4+}$ raised the temperature required for converting H$_2$Ti$_3$O$_7$ to TiO$_2$-B by approximately 200 °C, and the temperature for the formation of anatase. For the Ce$^{3+}$ batch, the presence of cerium inhibited the conversion to rutile. Analysis of cerium oxidation states revealed the existence of both +4 and +3 in all calcined samples, regardless of the initial cerium oxidation state

A Novel Magnetic Hardening Mechanism for Nd‐Fe‐B Permanent Magnets Based on Solid‐State Phase Transformation

Permanent magnets based on neodymium-iron-boron (Nd-Fe-B) alloys provide the highest performance and energy density, finding usage in many high-tech applications. Their magnetic performance relies on the intrinsic properties of the hard-magnetic Nd$_2$Fe$_{14}$B phase combined with control over the microstructure during production. In this study, a novel magnetic hardening mechanism is described in such materials based on a solid-state phase transformation. Using modified Nd-Fe-B alloys of the type Nd$_{16}$Fe$_{bal-x-y-z}$Co$_x$Mo$_y$Cu$_z$B$_7$ for the first time it is revealed how the microstructural transformation from the metastable Nd$_2$Fe$_{17}$B$_x$ phase to the hard-magnetic Nd$_2$Fe$_{14}$B phase can be thermally controlled, leading to an astonishing increase in coercivity from ≈200 kAm$^{−1}$ to almost 700 kAm$^{−1}$. Furthermore, after thermally treating a quenched sample of Nd$_{16}$Fe$_{56}$Co$_{20}$Mo$_2$Cu$_2$B$_7$, the presence of Mo leads to the formation of fine FeMo$_2$B$_2$ precipitates, in the range from micrometers down to a few nanometers. These precipitates are responsible for the refinement of the Nd$_2$Fe$_{14}$B grains and so for the high coercivity. This mechanism can be incorporated into existing manufacturing processes and can prove to be applicable to novel fabrication routes for Nd-Fe-B magnets, such as additive manufacturin

Domain morphology of newly designed lead‐free antiferroelectric NaNbO₃‐SrSnO₃ ceramics

Reversible antiferroelectric‐ferroelectric phase transitions were recently observed in a series of SrSnO₃‐modified NaNbO₃ lead‐free antiferroelectric materials, exhibiting well‐defined double polarization hysteresis loops at ambient conditions. Here, transmission electron microscopy was employed to investigate the crystallography and domain configuration of this newly designed system via electron diffraction and centered dark‐field imaging. It was confirmed that antiferroelectricity is maintained in all compositions, manifested by the characteristic ¼ superlattice reflections in the electron‐diffraction patterns. By investigating the antiferroelectric domains and domain boundaries in NaNbO₃, we demonstrate that antiphase boundaries are present and their irregular periodicity is responsible for the streaking features along the ¼ superlattice reflections in the electron‐diffraction patterns. The signature domain blocks observed in pure NaNbO₃ are maintained in the SrSnO₃‐modified ceramics, but disappear when the amount of SrSnO₃ reaches 7 mol.%. In particular, a well‐defined and distinct domain configuration is observed in the NaNbO₃ sample modified with 5 mol.% SrSnO₃, which presents a parallelogram domain morphology

A machine learning framework for quantifying chemical segregation and microstructural features in atom probe tomography data

Atom probe tomography (APT) is ideally suited to characterize and understand the interplay of chemical segregation and microstructure in modern multicomponent materials. Yet, the quantitative analysis typically relies on human expertise to define regions of interest. We introduce a computationally efficient, multistage machine learning strategy to identify chemically distinct domains in a semi automated way, and subsequently quantify their geometric and compositional characteristics. In our algorithmic pipeline, we first coarse grain the APT data into voxels, collect the composition statistics, and decompose it via clustering in composition space. The composition classification then enables the real space segmentation via a density based clustering algorithm, thus revealing the microstructure at voxel resolution. Our approach is demonstrated for a Sm(Co,Fe)ZrCu alloy. The alloy exhibits two precipitate phases with a plate-like, but intertwined morphology. The primary segmentation is further refined to disentangle these geometrically complex precipitates into individual plate like parts by an unsupervised approach based on principle component analysis, or a U-Net based semantic segmentation trained on the former. Following the chemical and geometric analysis, detailed chemical distribution and segregation effects relative to the predominant plate-like geometry can be readily mapped without resorting to the initial voxelization

Electrochemical Li Storage Properties of Carbon-Rich B–C–N Ceramics

Amorphous BCN ceramics were synthesized via a thermal conversion procedure of piperazine–borane and pyridine–borane. The synthesized BC₂N and BC₄N ceramics contained, in their final amorphous structure, 45 and 65 wt % of carbon, respectively. Elemental analysis revealed 45 and 65 wt % of carbon for BC₂N and BC₄N, respectively. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirmed the amorphous nature of studied compounds. Lateral cluster size of carbon crystallites of 7.43 and 10.3 nm for BC₂N and BC₄N, respectively, was calculated from Raman spectroscopy data. This signified a higher order of the carbon phase present in BC₄N. The electrochemical investigation of the low carbon BC₂N composition as anodes for Li-ion batteries revealed initial capacities of 667 and 235 mAh·g⁻¹ for lithium insertion/extraction, respectively. The material with higher carbon content, BC₄N, disclosed better reversible lithium storage properties. Initial capacities of 1030 and 737 mAh·g⁻¹ for lithium insertion and extraction were recovered for carbon-rich BC₄N composition. Extended cycling with high currents up to 2 C/2 D revealed the cycling stability of BC4N electrodes. Cycling for more than 75 cycles at constant current rates showed a stable electrochemical behavior of BC₄N anodes with capacities as high as 500 mAh·g⁻¹

Designing properties of (Na1/2Bix) TiO3-based materials through A-site non-stoichiometry

Point defects largely determine the properties of functional oxides. So far, limited knowledge exists on the impact of cation vacancies on electroceramics, especially in (Na1/2Bi1/2)TiO3 (NBT)-based materials. Here, we report on the drastic effect of A-site non-stoichiometry on the cation diffusion and functional properties in the representative ferroelectric (Na1/2Bi1/2)TiO3–SrTiO3 (NBT–ST). Experiments on NBT/ST bilayers and NBT–ST with Bi non-stoichiometry reveal that Sr2+-diffusion is enhanced by up to six orders of magnitude along the grain boundaries in Bi-deficient material as compared to Bi-excess material with values of grain boundary diffusion B108 cm2 s 1 and B1013 cm2 s 1 in the bulk. This also means a nine orders of magnitude higher diffusion coefficient compared to reports from other Sr-diffusion coefficients in ceramics. Bi-excess leads to the formation of a material with a core–shell microstructure. This results in 38% higher strain and one order of magnitude lower remanent polarization. In contrast, Bi-deficiency leads to a ceramic with a grain size six times larger than in the Bi-excess material and homogeneous distribution of compounds. Thus, the work sheds light on the rich opportunities that A-site stoichiometry offers to tailor NBT-based materials microstructure, transport properties, and electromechanical properties.T. F., A. A., and K. G. W. gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft under WE 4972/2 and FR 3718/1-1. T. F. thanks Dr Edvinas Navickas for his help with the ToF-SIMS measurements. M. A. acknowledges the support of the Feodor Lynen Research Fellowship Program of the Alexander von Humboldt Foundation. M. D. and L. M.-L. acknowledge financial support from the Hessen State Ministry of Higher Education, Research and the Arts via LOEWE RESPONSE. L. M.-L. acknowledges financial support from DFG Grant MO 3010/3-1

Ferromagnetic Mn–Al–C L1₀ Formation by Electric Current Assisted Annealing

The ferromagnetic Mn–Al–C τ‐phase (L1₀ tetragonal structure) shows intrinsic potential to be developed as a permanent magnet; however, this phase is metastable and is easily decomposed to nonmagnetic stable phases, affecting negatively the magnetic properties. Giving the necessity to careful control of its synthesis, the use of a novel approach is investigated using electric current–assisted annealing to obtain pure τ‐phase samples. The temperature and electrical resistance of the samples are monitored during annealing and it is shown that the change in resistance can be used to probe the phase transformation. Upon increase of electric current density, the required temperature for the ferromagnetic phase formation is reduced, reaching a maximum shift of 140 °C at 45 A mm⁻². Even though this noticeable shift is achieved, the magnetic properties are not affected showing coercivity of 0.13 T and magnetization of 90 Am² kg⁻¹, independently from the electric current density used during annealing. Microstructural investigation reveals the nucleation of the τ‐phase at the grain boundaries of the parent ε‐phase. In addition, the existence of twin boundaries upon nucleation and growth of the metastable phase for all evaluated annealing conditions is observed, resulting in similar extrinsic magnetic properties

Gradual reset and set characteristics in yttrium oxide based resistive random access memory

This paper addresses the resistive switching behavior in yttrium oxide based resistive random access memory (RRAM) (TiN/yttrium oxide/Pt) devices. We report the coexistence of bipolar and unipolar resistive switching within a single device stack. For bipolar DC operation, the devices show gradual set and reset behavior with resistance ratio up to two orders of magnitude. By using nanosecond regime pulses (20 to 100 ns pulse width) of constant voltage amplitude, this gradual switching behavior could be utilized in tuning the resistance during set and reset spanning up to two orders of magnitude. This demonstrates that yttrium oxide based RRAM devices are alternative candidates for multibit operations and neuromorphic applications

Enhanced Conductivity and Microstructure in Highly Textured TiN1–x/c-Al2O3 Thin Films

Titanium nitride thin films are used as an electrode material in superconducting (SC) applications and in oxide electronics. By controlling the defect density in the TiN thin film, the electrical properties of the film can achieve low resistivities and a high critical temperature (Tc) close to bulk values. Generally, low defect densities are achieved by stoichiometric growth and a low grain boundary density. Due to the low lattice mismatch of 0.7%, the best performing TiN layers are grown epitaxially on MgO substrates. Here, we report for the first time a Tc of 4.9 K for ultrathin (23 nm), highly textured (111), and stoichiometric TiN films grown on 8.75% lattice mismatch c-cut Al₂O₃ (sapphire) substrates. We demonstrate that with the increasing nitrogen deficiency, the (111) lattice constant increases, which is accompanied by a decrease in Tc. For highly N deficient TiN thin films, no superconductivity could be observed. In addition, a dissociation of grain boundaries (GBs) by the emission of stacking faults could be observed, indicating a combination of two sources for electron scattering defects in the system: (a) volume defects created by nitrogen deficiency and (b) defects created by the presence of GBs. For all samples, the average grain boundary distance is kept constant by a miscut of the c-cut sapphire substrate, which allows us to distinguish the effect of nitrogen deficiency and grain boundary density. These properties and surface roughness govern the electrical performance of the films and influence the compatibility as an electrode material in the respective application. This study aims to provide detailed and scale-bridging insights into the structural and microstructural response to nitrogen deficiency in the c-Al₂O₃/TiN system, as it is a promising candidate for applications in state-of-the-art systems such as oxide electronic thin film stacks or SC applications

Atomic structure and domain wall pinning in samarium-cobalt-based permanent magnets

A higher saturation magnetization obtained by an increased iron content is essential for yielding larger energy products in rare-earth Sm₂Co₁₇-type pinning-controlled permanent magnets. These are of importance for high-temperature industrial applications due to their intrinsic corrosion resistance and temperature stability. Here we present model magnets with an increased iron content based on a unique nanostructure and -chemical modification route using Fe, Cu, and Zr as dopants. The iron content controls the formation of a diamond-shaped cellular structure that dominates the density and strength of the domain wall pinning sites and thus the coercivity. Using ultra-high-resolution experimental and theoretical methods, we revealed the atomic structure of the single phases present and established a direct correlation to the macroscopic magnetic properties. With further development, this knowledge can be applied to produce samarium cobalt permanent magnets with improved magnetic performance