The origin of glass formation has been one of the greatest mysteries of science. The first clues emerged in GexSe1-x glasses, where the bond-stretching and bond angle-bending constraints are countable, and it was found that the most favorable compositions for glass formation involved matching constraints with the degrees of freedom. Modulated-Differential Scanning Calorimetric (MDSC) studies on GexSe1-x chalcogenide glasses revealed two elastic phase transitions—a stiffness transition at x = 0.20 and a stress transition at x = 0.26—leading to the observation of three topological phases: a flexible phase at x < 0.20, an intermediate phase in the 0.20 < x < 0.26 range, and a stressed–rigid phase for compositions x > 0.26. The three topological phases (TPs) have now been generically observed in more than two dozen chalcogenides and modified oxide glasses. In proteins, the transition from the unfolded (flexible) to the folded (isostatically rigid intermediate) phase represents the stiffness transition. Self-organization causes proteins to display a dynamic reversibility of the folding process. The evolutions of protein dynamics may also exhibit stiffness phase transitions similar to those seen in glasses.

Raman scattering and calorimetric measurements on lithium borate, (Li2O)(x)(B2O3)(100-x), and sodium borate, (Na2O)(x)(B2O3)(100-x), glasses, 0 < x < 45 %, were undertaken to elucidate the role of glass molecular structure on the topological phases and melt dynamics. For (Na2O)(x)(B2O3)(100-x) glasses, a wide square-well like variation of the enthalpy of relaxation at the glass transition was observed, showing an intermediate phase (IP) in the 20 % < x < 40 % range of soda. Raman results reveal the IP to be correlated with the existence of six unique Isostatically Rigid Local Structures (ISRLSs) which have characteristic ring morphologies appearing sequentially with increasing soda concentration across the IP. Additionally, several narrowly defined Gaussian-like fragility index, m, minima are observed to manifest in the 15 < m < 20 range, which appear sequentially with increasing mol x% and directly correspond to the stoichiometry of the six molecular ring based ISRLS. Parallel results are observed for (Li2O)(x)(B2O3)(100-x).

Network glasses composed of chalcogenide and modified oxides display three topological phases: the flexible, intermediate, and stressed-rigid. Modulated-DSC experiments, Raman scattering, and molar volume measurements along with topological constraint theory have shown that glass compositions in the flexible and stressed-rigid phases display glass transitions that age with time. The change of the enthalpy of relaxation (Delta Hnr) at the glass transition temperature (Tg) steadily evolves with waiting time which can be traced to the presence of Open Degrees of Freedom (ODF). In the isostatically constrained intermediate phase (IP), glasses possess an enthalpy of relaxation which is substantially reduced in aging due to a lack of ODF. IP glasses are composed of self-organised Isostatically Rigid Local Structures (ISRLSs) which have no ODF. The melt fragility index (m) across the three topological phases displays a Gaussian-like global minimum of m = 15 near the IP centre composition, illustrating a super-strong melt behaviour leading to the realisation of the ideal Phillips glass condition where glass forming tendency is globally optimised as demonstrated in the present work. For melt compositions away from the IP centre and into the flexible and stressed-rigid phases, the fragility index steadily increases due to the increased presence of non-ISRLSs. The non-ISRLSs admix with the ISRLSs formed near the mid IP composition, thus promoting the melt networks to relax, age, and diffuse as one goes away from IP centre composition.

Network glasses are composed of three topological phases: flexible, intermediate, and stressed-rigid. Intermediate Phase (IP) glasses form compact, rigid, and stress-free networks with no Open Degrees of Freedom (ODF). IP melts are observed to be super-strong with the lowest fragility index, m = 15 due to the absence of ODF in the parent glass. This feature is general and observed in multiple selenide, sulfide, and modified oxide glasses can be traced to Isostatically Rigid Local Structures (ISRLSs) formed in the IP. On the other hand, the flexible and stressed-rigid phase glasses form non-compacted and stressed networks with increasing residual pressure as the count of ODF increases. Their melts are found to be fragile and display fragility indices m > 20 as the count of the ODF in the glass increases and the count of ISRLSs dissipates.

Features intrinsic to disorder and network aspects are ubiquitous in structural glasses. Among this important class of materials, chalcogenide glasses are special-they are built of short-range covalent forces, making them simpler than silicate glasses that possess mixed ionic and covalent forces. Selenium-based glasses also display complex elastic phase transitions that have been described from various models, including mean-field approaches to molecular simulations. These point to the presence of two sharply defined elastic phase transitions, a rigidity and stress transition that are non-mean-field in character, and separate the three distinct topological phases of flexible, isostatically rigid, and stressed-rigid. This article reviews the physics of these glassy networks. The elastic phases and glass transition temperature are explained on a molecular level in terms of topological constraint theory (TCT), connectivity, and the open degrees of freedom. The broader aspects of TCT in relation to phase change materials, high-k dielectrics, and cements are also commented upon.

In this work, Ce and Cr/Co co-doped iron oxide (FeCeMOx, where M = Cr or Co) ferrites were investigated for high-temperature water-gas shift (HT-WGS) under industrially relevant pressures (20 bars). The Ce and Cr/Co bi-doping has greatly promoted the catalytic performance in comparison to the single Ce-doping. Especially, the FeCeCoOx ferrite demonstrated the best HT-WGS activity without any deactivation at a steam to CO ratio of 3.5. The ternary FeCeCrOx and FeCeCoOx catalysts also exhibited reasonably stable performance with a slight methanation activity (<2.5%) even under low steam to CO ratio of 1.5. The hematite phase (Fe2-xCex/2Mx/2O3, where M = Cr or Co and x = 0.33) of the fresh (calcined) catalysts became [A((1-delta))B(delta)](T) [A(delta)B((2-delta))]O-O(4) type magnetite spinel structure (Fe3-xCex/2Mx/2O4, where M = Cr or Co and x = 0.5) during the activation process, which is regarded as the active phase for the WGS reaction. The co-doping of Cr or Co increased the stability of the active magnetite phase against sintering and suppressed the coke formation during the WGS reaction, which should be responsible for the stable activity. The addition of Cr or Co to iron oxide also delays the formation and over-reduction of active magnetite phase. Mossbauer spectra confirmed that the Cr or Co ions were incorporated into the octahedral sites of [A((1-delta))B(delta)](T)[A(delta)B((2-delta))]O-O(4) spinel framework during the activation and modifies the local structure. The superparamagnetic behavior of the spinel ferrites has significantly enhanced upon the co-doping of Ce and Cr/Co into the iron oxide lattice, indicating a high fraction of nanoparticles in the ternary spinel ferrites. The Cr or Co addition resulted in the decreased surface Fe3+/Fe2+ ratio which could be due to the replacement of Fe3+ ions by Cr or Co ions. Most importantly, no evident change was observed in the surface structure of ternary spinel ferrites even after the reaction, which could be responsible the stable performance of the catalysts. Published by Elsevier Inc.

The molecular structure of binary PxSe100-x glasses over the 0 < x < 57% range is examined in Raman scattering, Modulated Differential Scanning Calorimetry (MDSC) and Volumetric measurements. Raman scattering and trends in glass transition temperature T-g(x), provide evidence of two network backbones (BB) that are decoupled from each other, with T-g in the 0 < x < 40% range fixed by BB1 and in the 40% < x < 54% range by BB2. The 3D Backbone, BB1, is composed of Se-n chains cross-linked by P-centered pyramidal (PSe1/2)(3) units and Quasi-Tetrahedral [Se-P(Se-1/2) units and their 3-membered Pyramidal [(PSe1/2)(3)](3) ring, and Quasi Tetrahedral [Se-P(Se-1/2)(3)](3) ring counterparts. Backbone BB2, is composed of quasi-1D Ethylene-like P2Se2+x (x = 2,1,0) chains forming in the approximate 20% < x < 54% range. The enthalpy of relaxation at T g shows a square-well like variation in the 28% < x < 40% range determined by BB1, thus fixing the Flexible- and Intermediate- phase (IP). Compositional trends in melt Fragility index, m (x), show anomalously low value of 13.1 at x = 15% and of 7.7 at x = 34%. These minima in m(x) coincide with minima in the liquidus near the same compositions x and are traced respectively to the formation of the 3-membered rings and the decoupling of BB1 from BB2. 2021 Published by Elsevier B.V.

The Topological Phases (TPs) in specially homogenized equimolar GexAsxS100-2x ternary glasses are established here by performing detailed Raman scattering, together with Modulated-DSC and Volumetric measurements over a wide range of compositions, 5% < x < 25%. Our results show the presence of an Intermediate Phase (IP) residing in the 9.0% < x < 16.0% range, with compositions x < 9.0% in the flexible phase, and compositions x > 16% in the stressed-rigid phase. The novel use of ex-situ Raman profiling on the entire batch of compositions indicates very slow dynamics that is tracked with time and composition and reveals the impact of homogenization on physico-chemical properties of glasses. Stressed-Rigid glasses exceeding the chemical threshold composition are all found to be nanoscale phase separated. In the presently synthesized specially homogenized melts/glasses, we observe (i) evidence for the elusive 537 cm(-1) Raman active mode of the S = As stretch in quasi-tetrahedral S = As(S-1/2)(3) local structure, (ii) a square-well like variation of the non-reversing enthalpy of relaxation Delta H-nr(x) at T-g displaying the Reversibility window and defining the Intermediate Phase (IP), (ii) a square-well like variation of molar volumes, V-m(x) coinciding with the IP composition range defining a Volumetric window, (iii) Melt fragility index, m(x), variation showing a global minimum in the IP compositions with m(x) < 20, and with the fragility index, m(x) > 20 for non-IP compositions, defining a Fragility Anomaly, and finally a variation in the specific heat jump near Tg, Delta(Cp)(x), that tracks part of the observed anomalies in m(x) and Delta H-nr(x). The location of each of these anomalies/windows coinciding with those of the IP highlights the privileged nature of the window edge compositions that represent respectively rigidity- (x(r) = 9.0%)- and the stress- (x(s) = 16.0%) elastic phase transitions determined within the Topological Constraint Theory of glasses. (C) 2021 Elsevier B.V. All rights reserved.

Raman scattering is a powerful probe oflocal structure (LS)of glasses. In Sodium Phosphate Glasses (SPGs), we show that bothLScomposed of Q(n)species andExtended Range Structures (ERS)composed of Long Chains (LCs), Large Rings (LRs), and Small Rings (SRs) can be decoded by Raman scattering. The trimodal distribution of P-O(terminal)stretch modes of Q(2)species and P-O(bridging)atx ERS. These two pairs of triads of modes are uniquely identified with Q(2)units present in either LCs, or LRs, or SRs. The existence three phases of c-NaPO(3)composed of 3-membered rings, 6-membered rings, and infinitely long chains has facilitated the identification. The Intermediate Phase (IP) in SPGs extends in the 37.5 x < 46.0% range, the Stressed-rigid Phase in the 46.0% x < 50%, and the Flexible Phase in the 18% x < 37.5% range of soda. We show the IP consists predominantly of LCs (82%), with a minority of LRs (15%) and SRs (3%). The LR- and SR-fractions increase measurably in the non-IP phases. The structural finding is in harmony with the high configurational entropy of the IP glasses that leads aging to be qualitatively suppressed.

Pure Se glass and ternary AxBySe1−x−y glasses where A = Ge and B = P or As, in the low mean coordination number, ⟨r⟩, range of 2.00 < ⟨r⟩ < 2.12 display a 3‐ to 5‐fold reduction in the width of the glass transition when aged at room temperature over 4–8 months. Group IV (Ge) and group V (P, As) additives serve to crosslink the base Se glass polymeric chains, Sen, with n > 250 atoms and to reduce the length “n” between cross‐link points as x and y are increased. Herein, it is shown that in such weakly crosslinked glasses, the continued narrowing of the glass transition width Tg by a factor of 3–5, as in pure Se, stems from the fact that polymeric Sen chain segments between the crosslink points continue to have a length n of at least eight atoms or more. Such polymeric Sen chains are super‐flexible and reconstruct with each, as in pure Se, promoting structural ordering responsible for Tg narrowing. When n < 8, super flexibility is steadily lost. Along with the flexible, intermediate, and stressed‐rigid phases, a new super‐flexible phase of Se and Se‐rich glasses is obtained.

Herein, the catalytic performance of Fe/Nb/M (M = Mn, Co, Ni, and Cu) spinel ferrites prepared by a co-precipitation method for the high-temperature water-gas shift (HT-WGS) was investigated. Incorporation of Nb into the iron oxide lattice was found to moderately improve the catalytic activity. Conversely, the co-doping with Nb and transition metals (Mn, Co, Ni, and Cu) into the iron oxide matrix drastically enhanced the HT-WGS activity. The high lattice strain/disorder and facile Fe3+/Fe2+ redox cycle induced by the enhanced synergism in the Fe/Nb/M ternary catalysts serve as active sites to efficiently catalyze the WGS reaction. The results also indicate that the Nb acts as a textural promoter to improve the thermal stability of the active magnetite phase, while the transition metals act as structural promoters to enhance the WGS activity. The Fe/Nb/Ni exhibited the higher catalytic performance among the co-doped spinel ferrite catalysts. The high lattice strain/disorder, facile reduction of hematite to magnetite and highly facilitated surface Fe3+/Fe2+ redox pair by strong synergistic effect could be responsible for the better activity and stability of Fe/Nb/Ni in HT-WGS reaction. The characterizations of the spent Fe/Nb/Ni after 100 h of reaction revealed that the catalyst exhibited an excellent structural and surface stability during the reaction.

In all previous studies of soft magnetic alloys, magnetic softness is obtained through forming a completely amorphous state via rapid solidification, such as by melt spinning at a high cooling rate followed by annealing, typically at 600 °C, to develop a magnetically isotropic nanostructure. Fine powdering of the annealed alloy via ball milling is then required for manufacturing, net shaping, and 3D printing. However, the soft magnetic properties are susceptible to the subsequent processing conditions, characterized by significantly increased coercivity. Herein, nanoscale crystallites are obtained directly from the melt‐spun Fe77Ni5.5Co5.5Zr7B4Cu ribbon (i.e., not through annealing of a completely amorphous ribbon) that exhibits structural stability during the annealing and ball‐milling processes. The melt‐spun ribbon annealed at high temperatures (700 °C) remains magnetically soft with Hc of ≈0 Oe, which is a key property for high‐temperature applications. Ball milling of the annealed melt‐spun samples results in fine powders with low Hc values over a wide temperature range up to 427 °C. It is shown that the rapidly solidified crystalline ribbon provides an ideal precursor for the manufacture of high‐temperature soft magnetic materials. This new approach provides a straightforward method of making soft magnetic alloy powders.

Equimolar GexPxSe100-2x ternary glasses have been synthesized over a wide composition range, 4% < x < 25%, and examined in Raman scattering, modulated DSC, and volumetric experiments. Modulated DSC experiments show the enthalpy of relaxation at T-g to display a square-well-like reversibility window with an onset (end) of 9.0% (18.0%) respectively, thus, fixing the onset of the rigidity transition, near x(r) = 9.0% and the stress transition, near x(s) = 18.0%. These findings show that the Intermediate-Phase (IP) resides in the 9% < x < 18% range. Melt fragility index, m(x) display a Gaussian-like minimum with m(x) < 20 for IP compositions and with m > 20 for non-IP ones. Fragility index results show a global minimum of m = 14 near the center of the window, x = 14, underscoring IP melts to be superstrong. Molar volumes, V-m(x), of glasses reveal a Gaussian-like minimum, for IP compositions in relation to the non-IP ones. Melt fragility indices are closely correlated to the glass enthalpy of relaxation, and they show that superstrong melts yield IP glasses, while fragile melts yield flexible phase (FP) or stressed-rigid phase (SRP) glasses upon cooling. Special synthesis of glasses permitted the variance of Ge or P content, , across 1.5-g-sized batch compositions could be reduced to less than 0.1%. The homogeneous nature of glasses led to abrupt rigidity- and stress-elastic phase transitions in harmony with the percolative nature of these transitions as predicted by theory. Availability of the topological phases (TPs) in GexSe100-x glasses, and PxSe100-x glasses from earlier work, when combined with the presently measured TPs in the equimolar GexPxSe100-2x glasses, has permitted constructing a global TP diagram for the Ge-P-Se composition triangle. The global plot will assist in the choice of appropriate glass compositions for select applications of these materials. In spite of homogenization of these melts/glasses, compositional trends in T-g(x) and Raman vibrational modes show that the P-P bearing local structural units of P4Se3 monomers and ethylene-like P-2(Se-1/2)(4) units are decoupled from the network backbone in the IP and in the SRP.

EDITORIAL article Front. Mater., 17 July 2020Sec. Ceramics and Glass Volume 7 - 2020 | https://doi.org/10.3389/fmats.2020.00175

A key property for high-frequency inductive applications is the magnetic softness, ideally characterized by complete reversibility of the hysteresis curve. Magnetic softness of alloys depends on the dimensional ratio of grain size to the correlation length. The random anisotropy model predicts the optimum magnetic softness if the crystallite size is well below the correlation length. Based on this theory, a common strategy in processing is to obtain extremely fine nanoscale grains via rapidly solidified alloys, followed by a controlled vacuum anneal. As is well known, this conventional approach requires two demanding experimental conditions: 1) an extremely high quench rate, & x003E;10(5) K/s, and 2) a high vacuum anneal near 600 & x00B0;C to avoid oxidation. Furthermore, the completely amorphous ribbon is extremely elastic and mechanically strong and is not easily made into the fine powder form for net-shaping and 3-D printing of the soft magnets. Therefore, annealing to induce crystallization is a necessary step before using any conventional powdering techniques, such as ball milling. In this article, we report a new processing strategy of making Fe77Ni5.5Co5.5Zr7B4Cu powders directly from the as-spun ribbons without any crystallization annealing step. This is achieved by melt spinning of the alloy at a relatively low quenching rate, resulting in partially crystallized ribbons that deform plastically, a behavior that is in sharp contrast to its amorphous counterpart. The ribbon samples are ball milled into fine powders by tungsten carbide (WC) at various time intervals. Magnetization measurements show extremely low coercive field (Hc) of 0.51 Oe for the powder that is ball milled for 60 min. However, Hc consistently increases with ball milling time, as the samples are strain-hardened, introducing more magneto anisotropy. The coercive fields of these samples are easily recovered during the high-temperature magnetization measurement at 900 K, and an Hc of 3.78 Oe is obtained upon recovery. This strategy paves a new way for straightforwardly making soft magnetic powders while avoiding high quenching rate requirement and the possibility of oxidation during annealing.

In this work, evidence for ligand formation between Sn and Bi during the colloidal synthesis of Bi metal nanoparticles (NP) in an aqueous suspension is provided utilizing time-resolved X-ray absorption and 119Sn Mössbauer spectroscopy .

The observation of reversibility windows (RWs) in binary Ge x S 100− x , As x S 100− x and ternary Ge x As x S 100−2 x glass systems permits constructing a global topological phase diagram in the GeAsS composition triangle. The RW denotes glass compositions for which the enthalpy of relaxation at T g vanishes, leading to thermally reversing glass transitions. The phase diagram permits delineating glass compositions that are in the flexible phase (FP), intermediate phase (IP), and the stressed rigid phase (SRP) for all ternary Ge x As x S 100−2 x glass compositions. For the IP compositions examined, a general lowering of the molar volumes, V m , is observed in relation to those for non‐IP compositions, giving rise to a volumetric window. Melt dynamics of IP compositions reveal a fragility index, m < 20, lower than that for non‐IP melts for which m > 20 displays a fragility window, underscoring in part the reason for the delayed homogenization of melts in these sulfide glasses. The observations of the three types of window characteristics of IP compositions are feasible only when the homogeneous bulk glasses are synthesized, in which the variance ⟨Δ x ⟩ in As and Ge concentrations, x , across batch compositions is less than 0.01%. This is established by Fourier transform–Raman profiling of each batch of composition that is synthesized.