1,721,003 research outputs found
Black TiO<sub>2</sub> and Oxygen Vacancies: Unraveling the Role in the Thermal Anatase-to-Rutile Transformation
No full textUnderstanding the role of oxygen vacancies in the phase transformation of metal oxide nanomaterials is fundamental to design more efficient opto-electronic devices for a variety of applications, including sensing, spintronics, photocatalysis, and photo-electrochemistry. However, the structural mechanisms behind the phase transformation in reducible oxides remain poorly described. Here, we compare P25 and black TiO2 during the thermal anatase-to-rutile transformation using in situ synchrotron powder diffraction. The precise measurement of the phase fractions, unit cell parameters, and Ti-O bond sheds light on the phase transformation dynamics. Notably, we observe distinct temperature-dependent shifts in the relative phase fractions of anatase and rutile in both materials highlighting the role of the oxygen vacancy in promoting the phase transformation. We employ bond valence concepts for structural modeling, revealing unique trends in temperature evolution of Ti-O distances of black rutile, confirming that this TiO2 phase is preferentially reduced over anatase. These findings not only enhance our understanding of phase transitions in TiO2 but also open new ways for the design of advanced photocatalytic materials through targeted phase control
From nano to microcrystals: effects of different synthetic pathways on the defect architecture in heavily Gd-doped ceria
No full textThe evolution of the defect structure and microstructure of heavily Gd-doped ceria (Ce1-μREμO2-y, 0.313 ≤ μ ≤ 0.438) for different synthetic pathways is investigated here to explore the way defects interact with each other in a composition range known to effectively hamper the application of the material as an electrolyte. Synchrotron radiation powder diffraction is exploited by combining conventional Rietveld analysis with the Pair Distribution Function to get a multiscale picture of defect structures, and it is combined with Raman spectroscopy to assess local scale interactions. Samples were prepared via both the sol-gel route and coprecipitation of oxalates by sintering the powders at different temperatures to obtain samples with different defect distributions and crystallite sizes, investigated using electron microscopy and Whole Powder Pattern Modelling from diffraction data. As a general scheme, increasing the doping amount transforms the fluorite structure of ceria into C-type Gd2O3. For samples annealed at and above 900 °C, containing crystals at least ∼100 nm in size, this transformation occurs through a mechanism involving first the formation of distorted Gd-rich droplets on the local scale, then the growth of extended C-type nanodomains. Nanoparticles, resulting from thermal treatments at lower temperature, are less distorted on the local scale and transform abruptly upon doping, without forming larger dopant-rich aggregations, from fluorite to the C-type. The annealing temperature not only acts on the sintering of the crystallites, it is also found to promote a radical change in the microstructure as a consequence of the preferential aggregation of oxygen vacancies
Phase Stability of High Entropy (Mg,Ni,Co,Cu,Zn)O from Temperature‐Resolved Synchrotron Diffraction: Tetragonal Distortion and Guggenite Phase
No full textLithium intercalation mechanisms and critical role of multi-doping in LiFexMn2-x-yTiyO4 as high-capacity cathode material for Lithium-ion batteries
No full textIs configurational entropy the main stabilizing term in rock-salt Mg(0.2)Co(0.2)Ni(0.2)Cu(0.2)Zn(0.2)O high entropy oxide?
No full textn their paper “Entropy-Stabilized Oxides,” Rost et al. reported the synthesis of a new Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O phase (E1) with rock-salt structure as a test case for the concept of high entropy in oxides. Their claim is that “entropy predominates the thermodynamic landscape and drives a reversible solid-state transformation between a multiphase and single-phase state.” Here we use the same thermodynamic considerations by Rost et al. and replicate their experiments but reducing appropriately the configurational entropy. We demonstrate that configurational entropy does not dominate the thermodynamic stability of E1
Microstructure and mechanical behavior of hot-work tool steels processed by Selective Laser Melting
No full textThe present study is aimed at identifying and testing high-strength alloys for tooling applications featuring suitable processability for laser-based additive manufacturing technologies. The microstructure and mechanical properties of the H11 hot-work tool steel and a leaner version of the same alloy (L-H11) processed by Selective Laser Melting were assessed as a function of specific microstructural conditions obtained by performing different heat treatments. Tempering was performed on quenched alloys or simply from as built material. The rapidly solidified microstructures revealed able to respond directly to precipitation hardening treatment without performing any prior solution annealing. The microstructure of the as-built alloys revealed characterized by α-Fe dendritic cells decorated at boundaries by C-rich Î3-Fe regions. Air quenching was responsible for the transformation of the solidification cells into lath martensitic structures and for the formation of the M3C phase, which transformed into more complex carbide species on tempering. The hardness of quenched and tempered H11 steel is similar to that obtained by processing the alloy with conventional routes, and the final hardness gap between the two SLM processed H11 and L-H11 alloys treated according to optimal tempered condition was limited to 62 HV
Beyond configurational entropy: the role of solubility equilibria in the stability of the system (Co,Cu,Mg,Ni,Zn)O
No full textHigh entropy oxides are a novel class of materials, where multiple cations can be incorporated in a single-phase structure. Since the discovery of the prototypical compound Co0.2Cu0.2Mg0.2Ni0.2Zn0.2O, the research on these materials has shown an impressive boost. This compound adopts a rock-salt structure, even if CuO and ZnO are usually stable as tenorite and wurtzite. The attainment of a single phase is usually ascribed to the substantial value of configurational entropy that counterbalances unfavourable enthalpy terms. Here, we reconsider the effective role of configurational entropy in the stabilization in view of the solubility and redox equilibria involved. A careful examination of the high-temperature solubility limit of CuO in each native rock salt oxide proves to be useful to predict the phase stability. However, when the number of components is high, the behaviour becomes, at least to some extent, distinct from that of the constituent rock-salt oxides and difficult to predict
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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