Order-disorder Temperature Research Articles

Formation of L10 Ordering in FeNi by Mechanical Alloying and Field-Assisted Heat Treatment: Synchrotron XRD Studies.

L10-ordered FeNi, tetrataenite, found naturally in meteorites is a predilection for next-generation rare-earth free permanent magnetic materials. However, the synthesis of this phase remains unattainable in an industrially relevant time frame due to the sluggish diffusion of Fe and Ni near the order-disorder temperature (593 K) of L10 FeNi. The present work describes the synthesis of ordered L10 FeNi from elemental Fe and Ni powders by mechanical alloying up to 12 h and subsequent heat treatment at 623 K for 1000 h without a magnetic field and for 4 h in the presence of 1.5 T magnetic field. Also, to address the ambiguity of L10 phase identification caused by the low difference in the X-ray scattering factor of Fe and Ni, synchrotron-based X-ray diffraction is employed, which reveals that 6 h milling is sufficient to induce L10 FeNi formation. Further milling for 12 h is done to achieve a chemically homogeneous powder. The phase fraction of L10-ordered FeNi is quantified to ∼9 wt % for 12 h milled FeNi, which increases to ∼15 wt % after heat treatment. Heat treatment of the milled powder in a magnetic field increases the long-range order parameter (S) from 0.18 to 0.30. Further, the study of magnetic properties reveals a decrease in magnetic saturation and a slight increase in coercivity with the increase in milling duration. At the same time, heat treatment in the magnetic field shows a considerable increase in coercivity.

Effect of the post annealing cooling rate on the martensitic transformation and the magnetocaloric effect in Ni-Mn-Sn ribbons

We report the modification of the martensitic transformation in Ni-Mn-Sn ribbons by controlling the post annealing cooling rate. Isothermal holding above the order-disorder temperature for 24 h ensured homogeneous grain growth and uniform microstructural features in all the samples. However the atomic order was tailored by varying the cooling rate. The furnace-cooled sample exhibited the highest austenite stabilization and the lowest martensitic transformation temperature of 200.2 K, whereas the air-cooled sample exhibited the highest transformation temperature of 298.5 K. The martensite evolved as ferromagnetic clusters in an anti-ferromagnetic matrix in the samples with high atomic disorder, whereas long-range anti-ferromagnetic co-relations developed in the highly ordered samples. The martensitic transformation in the as-spun ribbons generated a significant magnetocaloric effect and a large entropy change of 4.02 J/kg/K, for a ΔH of 2 T. Furthermore, furnace cooling increased the relative cooling power of the ribbons by 23.3 J/kg over the as-spun specimens.

Abnormal notch brittleness induced by short-range ordering in low-cobalt iron-cobalt alloys under tensile and impact loading: A combined experimental and molecular dynamics simulation study

According to the binary Fe-Co phase diagram, Fe-21.5/23 wt% Co alloys undergo an order-disorder transition in the temperature range of 430–470 °C. Above the order-disorder temperature, Fe-21.5/23 wt% Co alloys are generally considered as disordered solid solutions with excellent ductility and toughness. In this work, short-range ordered (SRO) structures in Fe-21.5 wt% Co alloy quenched from α phase region were directly observed via scanning transmission electron microscope (STEM). SROs are mainly the B2 structure, with a very small amount of L60 Fe3Co structure. The mechanisms of SRO-induced notch brittleness were systematically investigated by experiments and molecular dynamics (MD) simulations. Under tensile loading, SROs change slip mode from cross slip to planar slip, which significantly leads to the strengthening effects and large work hardening rates. For the notch tensile tests, different notch depths were set to investigate the effects of SROs on the notch brittleness. The Fe-23 wt% Co samples quenched from 550 °C were surprisingly found to exhibit the notch brittleness induced by SROs. Under impact loading, SROs significantly lead to impact brittleness with the cleavage fracture. MD results of notch tensile and impact tests showed that SRO-induced poor dislocation and twinning plasticity leads to larger concentrated stresses and stress concentration factor near the notch, directly resulting in the easier nucleation and rapid propagation of intergranular and intracrystalline microcracks. Our findings suggest that SRO-induced notch brittleness must be considered in engineering applications for the low-cobalt iron-cobalt alloys.

Competitive hydrogen bonding induced phase separation in supramolecular comb-shaped diblock copolymer

In this study, a route to prepare nanostructure in miscible diblock copolymers by utilization of competitively intermolecular hydrogen-bonding interactions is demonstrated. Block copolymers poly(4-vinylpyridine-b-4-hydroxybutyl acrylate) (P4VP-b-P4HBA) with various block ratios were synthesized and they were demonstrated to be miscible due to the intramolecular hydrogen bonding. By combination of P4VP-b-P4HBA and 3-pentadecylphenol (PDP), the miscible block copolymer forms comb-shaped di-block copolymer complexes through the competition interactions between the hydroxyl groups of PDP (proton donors) and the nitrogen of the pyridine rings of the P4VP blocks (proton acceptors). Upon blending with PDP, we observed morphology change from sphere structure to cylindrical, and lamellar, in a manner strongly dependent on the P4HBA fraction. The microstructure of this special comb-shaped di-block copolymer shows a strong sensitivity to the temperature. The results from the in situ small angle X-ray scattering experiments have demonstrated that, even below the order-disorder temperature, the size of the phase domain decreases with increasing temperature due to decrease of the strength of the hydrogen bond upon heating.

Acceleration of B2/L21 order-disorder transformation in Ni2MnAl Heusler alloys by in-magnetic-field annealing

Ferromagnetic L21 phase of Ni2MnAl with high order degree is difficult to synthesize because of low L21/B2 order-disorder transformation temperature. It is found that magnetic field accelerated the transformation from the antiferromagnetic disordered B2 to ordered L21 phase. Magnetization increased by in-magnetic-field annealing at 673 K, which indicated the acceleration of the ordering to L21 phase. Magnetic field effects on the B2/L21 order-disorder transformation were discussed based on magnetic-field-induced change of transformation temperature, enhancement of degree of order at annealing temperature, and reduction of critical radius of L21 nuclei. Growth kinetics of L21 phase were evaluated by Johnson-Mehl-Avrami equation, which indicated that 15-T annealing accelerated the growth of L21 phase 5.7 times faster than zero-field annealing. Our results showed that magnetic field can contribute to shortening the annealing time for synthesis of ferromagnetic L21-Ni2MnAl alloy.

A First Order Phase Transition Studied by an Ising-Like Model Solved by Entropic Sampling Monte Carlo Method

Two-dimensional (2D) square, rectangular and hexagonal lattices and 3D parallelepipedic lattices of spin crossover (SCO) compounds which represent typical examples of first order phase transitions compounds are studied in terms of their size, shape and model through an Ising-like Hamiltonian in which the fictitious spin states are coupled via the respective short and long-range interaction parameters J, and G. Furthermore, an environmental L parameter accounting for surface effects is also introduced. The wealth of SCO transition properties between its bi-stable low spin (LS) and high spin (HS) states are simulated using Monte Carlo Entropic Sampling (MCES) method which favors the scanning of macro states of weak probability occurrences. For given J and G, the focus is on surface effects through parameter L. It is shown that the combined first-order phase transition effects of the parameters of the Hamiltonian can be highlighted through two typical temperatures, TO.D., the critical order-disorder temperature and Teq the equilibrium temperature that is fixed at zero effective ligand field. The relative positions of TO.D. and Teq control the nature of the transition and mediate the width and position of the thermal hysteresis curves with size and shape. When surface effects are negligible (L = 0), the equilibrium transition temperature, Teq. becomes constant, while the thermal hysteresis’ width increases with size. When surface effects are considered, L ≠ 0, Teq. increases with size and the first order transition vanishes in favor of a gradual transition until reaching a threshold size, below which a reentrance phenomenon occurs and the thermal hysteresis reappears again, as shown for hexagonal configuration.

New Method for Calculating Pairwise Effective Potential in Binary Solid Solutions

A new method for calculating the pairwise effective potential on arbitrary shells of the binary disordered solid solutions is proposed with account for the linear size effect. The method is based on the fitting of the order–disorder temperature calculated by the model potential method to its known experimental value. In disordered Cu $${}_{3}$$ Au alloy, the pairwise effective potential is calculated and the short-range order parameters on the first fourteen shells are estimated using the Krivoglaz–Clapp–Moss method. A satisfactory coincidence with the experimental data is obtained.

Thermal Stability of π-Conjugated n-Ethylene-Glycol-Terminated Quaterthiophene Oligomers: A Computational and Experimental Study.

This work represents a joint computational and experimental study on a series of n-ethylene glycol (PEOn)-terminated quaterthiophene (4T) oligomers for 1 < n < 10 to elucidate their self-assembly behavior into a smectic-like lamellar phase. This study builds on an earlier study for n = 4 that showed that our model predictions were consistent with experimental data on the melting behavior and structure of the lamellar phase, with the latter consisting of crystal-like 4T domains and liquid-like PEO4 domains. The present study aims to understand how the length of the terminal PEOn chains modulates the disordering temperature of the lamellar phase and hence the relative stability of the ordered structure. A simplified bilayer model, where the 4T domains are not explicitly described, is put forward to efficiently estimate the disordering effect of the PEO domains with increasing n; this method is first validated by correctly predicting that layers of alkyl (PE)-capped 4T oligomers (for 1 < n < 10) stay ordered at room temperature. Both 4T-domain implicit and explicit model simulations reveal that the order-disorder temperature decreases with the length of the PEO capping chains, as the associated increase in conformational entropy drives a tendency toward disorder that overtakes the cohesive energy, keeping the ordered packing of the 4T domains.

Effect of an Antimicrobial Peptide on Lateral Segregation of Lipids: A Structure and Dynamics Study by Neutron Scattering.

Antimicrobial peptides are one of the most promising classes of antibiotic agents for drug-resistant bacteria. Although the mechanisms of their action are not fully understood, many of them are found to interact with the target bacterial membrane, causing different degrees of perturbations. In this work, we directly observed that a short peptide disturbs membranes by inducing lateral segregation of lipids without forming pores or destroying membranes. Aurein 1.2 (aurein) is a 13-amino acid antimicrobial peptide discovered in the frog Litoria genus that exhibits high antibiotic efficacy. Being cationic and amphiphilic, it binds spontaneously to a membrane surface with or without charged lipids. With a small-angle neutron scattering contrast matching technique that is sensitive to lateral heterogeneity in membrane, we found that aurein induces significant lateral segregation in an initially uniform lipid bilayer composed of zwitterionic lipid and anionic lipid. More intriguingly, the lateral segregation was similar to the domain formed below the order-disorder phase-transition temperature. To our knowledge, this is the first direct observation of lateral segregation caused by a peptide. With quasi-elastic neutron scattering, we indeed found that the lipid lateral motion in the fluid phase was reduced even at low aurein concentrations. The reduced lateral mobility makes the membrane prone to additional stresses and defects that change membrane properties and impede membrane-related biological processes. Our results provide insights into how a short peptide kills bacteria at low concentrations without forming pores or destroying membranes. With a better understanding of the interaction, more effective and economically antimicrobial peptides may be designed.

Effects of Ag presence on phases separation and order-disorder transitions in Cu-xAl-Mn alloys

Abstract The effects of Ag additions on phases separation and order-disorder transitions in the Cu-7Al-10Mn, Cu-9Al-10Mn and Cu-11Al-10Mn alloys have been studied using electrical resistivity measurements with temperature, differential scanning calorimetry, scanning electron and transmission electron microscopies, X-ray diffractometry and electron diffraction analysis. The results indicated that the decrease in Al content reduces the Cu2AlMn phase amount produced on slow cooling. It was also verified that the variation in Al fraction and Ag addition modify the order-disorder transitions temperatures. A schematic phase diagram showing the shift of phases field with addition of Ag has been proposed.

Temperature Dependence of the Electron Paramagnetic Resonance Spectrum of Asphaltenes from Venezuelan Crude Oils and Their Vacuum Residues

The electron paramagnetic resonance (EPR) technique is used to evaluate the temperature dependence of the aggregation processes in asphaltenes of heavy and extra-heavy crude oils and their vacuum residues from the Venezuelan Oil Belt. All of the EPR spectra of the samples studied show an intense central signal assigned to free radicals (FRs) and a multiplet as a result of the presence of a V+4 porphyrin. From the analyses of the inverse of the normalized area as a function of the temperature for the FR signal and the most intense peak in the vanadium multiplet, we obtain a set of temperatures related to the transition of fast-to-slow motion regions, the temperature of generation–recombination of FRs, and the cracking temperature in all of the samples. On the other hand, applying a Curie–Weiss law, we obtain the order–disorder temperatures and correlated them with the American Petroleum Institute grade of the samples. A computational simulation was performed to propose possible conformers based on the expe...

The Rheology of PEOT/PBT Block Copolymers in the Melt State and in the Thermally-Induced Sol/Gel Transition. Implications on the 3D-Printing Bio-Scaffold Process.

Poly(ethylene oxide terephthalate)/poly(butylene terephthalate) (PEOT/PBT) segmented block copolymers are widely used for the manufacturing of 3D-printed bio-scaffolds, due to a combination of several properties, such as cell viability, bio-compatibility, and bio-degradability. Furthermore, they are characterized by a relatively low viscosity at high temperatures, which is desired during the injection stages of the printing process. At the same time, the microphase separated morphology generated by the demixing of hard and soft segments at intermediate temperatures allows for a quick transition from a liquid-like to a solid-like behavior, thus favoring the shaping and the dimensional stability of the scaffold. In this work, for the first time, the rheology of a commercial PEOT/PBT material is studied over a wide range of temperatures encompassing both the melt state and the phase transition regime. Non-isothermal viscoelastic measurements under oscillatory shear flow allow for a quantitative determination of the material processability in the melt state. Additionally, isothermal experiments below the order–disorder temperature are used to determine the temperature dependence of the phase transition kinetics. The importance of the rheological characterization when designing the 3D-printing scaffold process is also discussed.

Role of the extra Fe in K2−xFe4+ySe5 superconductors

The exact superconducting phase of K2-x Fe4+y Se5 has so far not been conclusively decided since its discovery due to its intrinsic multiphase in early material. In an attempt to resolve this mystery, we have carried out systematic structural studies on a set of well-controlled samples with exact chemical stoichiometry K2-x Fe4+x Se5 (x = 0-0.3) that are heat-treated at different temperatures. Using high-resolution synchrotron radiation X-ray diffraction, our investigations have determined the superconducting transition by focusing on the detailed temperature evolution of the crystalline phases. Our results show that superconductivity appears only in those samples that have been treated at high-enough temperature and then quenched to room temperature. The volume fraction of superconducting transition strongly depends on the annealing temperature used. The most striking result is the observation of a clear contrast in crystalline phase between the nonsuperconducting parent compound K2Fe4Se5 and the superconducting K2-x Fe4+y Se5 samples. The X-ray diffraction patterned can be well indexed to the phase with I4/m symmetry in all temperatures investigated. However, we need two phases with similar I4/m symmetry but different parameters to best fit the data at a temperature below the Fe vacancy order temperature. The results strongly suggest that superconductivity in K2-x Fe4+y Se5 critically depends on the occupation of Fe atoms on the originally empty 4d site.

First-principles study of phase stability of bccXZn(X=Cu, Ag, and Au) alloys

First-principles density-functional theory is used to study the phase stability/instability and anomalies in the formation of the high-temperature bcc phases of $X\mathrm{Zn}$ ($X$ = Cu, Ag, and Au) alloys. Although from perhaps a naive point of view, their properties are expected to monotonically depend on the noble-metal ($X$) column position in the periodic table, this is not the case. For example, the middle-column AgZn alloy has a lower bcc order-disorder (critical) temperature than the CuZn and AuZn alloys above and below in the column. It is shown that this and other nonmonotonic behaviors can be explained in terms of a competition between atomic-size effects and $X$-atom $d$-orbital spatial extent. For example, charge-density studies and pair-potential modeling of $X\mathrm{Zn}$ alloys show that the effective Ag-Zn bond is significantly weaker than either the Cu-Zn or Au-Zn bond at their respective equilibrium lattice constants. We find that an increased atomic-core size effect initially weakens the $X$-Zn bonding as one goes from CuZn to AgZn, but then the larger $d$-orbital spatial extent for higher principal quantum numbers becomes a more dominant effect and increases the bonding from AgZn to AuZn. This study is focused on the highly symmetric cubic high-temperature phases, where relative bond-strength magnitudes should be far more important than any bond-directionality effects; the lattice parameters, bulk moduli, elastic constants, Debye temperatures, heats of formation, and order-disorder temperatures for the bcc phases of the three $X\mathrm{Zn}$ alloys are calculated and compared with experiment.

Dissolution of thin TaV2 during annealing of Ta/TaV2/V tri-layer below the order-disorder temperature

In this research, we provide first experimental evidence on the dissolution of a thin compound layer sandwiched between the parent materials when it is heated below the order-disorder temperature. The Ta(10 nm)/TaV2(6 nm)/V(30 nm) system, prepared by DC magnetron sputtering, has been chosen to prove the expected simulation results. The samples were investigated mainly by secondary neutral mass spectrometry. The about 6 nm thick TaV2 compound layer was dissolved by annealing at 1025 °C for 1 h. Then, it was reformed by increasing the annealing time to 2–3 h. These results prove our previous computer kinetic Monte Carlo and Kinetic mean field calculations. These findings are important for nanotechnologies utilizing early stages of solid state reactions.

Pattern formation mechanisms in sphere-forming diblock copolymer thin films

The order-disorder transition of a sphere-forming block copolymer thin film was numerically studied through a Cahn-Hilliard model. Simulations show that the fundamental mechanisms of pattern formation are spinodal decomposition and nucleation and growth. The range of validity of each relaxation process is controlled by the spinodal and order-disorder temperatures. The initial stages of spinodal decomposition are well approximated by a linear analysis of the evolution equation of the system. In the metastable region, the critical size for nucleation diverges upon approaching the order--disorder transition, and reduces to the size of a single domain as the spinodal is approached. Grain boundaries and topological defects inhibit the formation of superheated phases above the order--disorder temperature. The numerical results are in good qualitative agreement with experimental data on sphere-forming diblock copolymer thin films.&#x0D; Received: 22 August 2017, Accepted: 12 December 2017; Edited by: R. Dickman; Reviewed by: A. Peters, Dept. Chemical Engineering, Louisiana Tech Univ., Ruston, USA; DOI: http://dx.doi.org/10.4279/PIP.100001&#x0D; Cite as: L R Gómez, N A García, R A Register, D A Vega, Papers in Physics 10, 100001 (2018)&#x0D; This paper, by L R Gómez, N A García, R A Register, D A Vega, is licensed under the Creative Commons Attribution License 4.0.

The Order–Disorder Transition in Graft Block Copolymers

The order–disorder transitions of diblock copolymers grafted to a common backbone were examined by oscillatory shear rheology and small-angle X-ray scattering. The effect of grafting density, graft molecular weight, number of grafts, and backbone dispersity were studied using poly[(styrene-alt-N-hydroxyethylmaleimide)-random-(styrene-alt-N-ethylmaleimide)]-graft-poly(4-methylcaprolactone-block-d,l-lactide) [PSHE-g-(P4MCL-PLA)] as a model graft block copolymer. At high grafting densities (25–50%), the order–disorder temperature (TODT) of the graft polymers was nearly identical to the analogous linear diblock. At lower grafting densities (<25%), the TODT was found to systematically decrease. The number of grafts did not significantly change the value of the TODT; however, increasing the number of grafts resulted in broad, ill-defined transitions. Backbone dispersity was found to have little impact on the TODT. Ordered morphologies were imaged by transmission electron microscopy. Long-range order was observe...

Magnetic Properties of L10 FeNi Phase Developed Through Annealing of an Amorphous Alloy

Chemically ordered L10 FeNi phase observed in Fe-based meteorite has the potential to replace high-cost rare-earth-based permanent magnets in the future. However, artificial production of this phase is extremely difficult due to negligible atomic diffusion around order–disorder transition temperature (~320 °C). Here, we report a method for producing high-quality L10 FeNi phase and its magnetic properties. We show that a highly disordered metastable state, that is, amorphous can be utilized to produce a highly ordered state, which is not possible with the conventional processing techniques. Amorphous Fe42Ni41.3Si x B12− x P4Cu0.7 ( $x=0$ to 8 at.%) alloy ribbons were studied. Crystallization of amorphous ribbons at 400 °C results in adequate atomic diffusion at low temperatures to precipitate L10 FeNi grains. Structural characterization revealed a high degree of chemical ordering ( $S \ge 0.8$ ), but the volume fraction of precipitated L10 grains is low. The crystallized ribbons of FeSiBPCu are composed of two magnetic phases (hard magnetic L10 FeNi grains embedded in a soft magnetic matrix). Alloys with higher concentration of Si are shown to produce high coercivity ( $H_{c}\sim 700$ –750 Oe). The soft magnetic matrix strongly influences the H c . The actual switching field (≥3.7 kOe) of L10 FeNi has been found to be much higher than that of H c . In this paper, the L10 FeNi phase is shown to form at temperatures higher than the reported order–disorder temperature. Our results of temperature-dependent magnetization and thermal analyses suggest that the L10 FeNi phase can survive at temperatures ≤550 °C. The magnetization reversal mechanism was understood by angular dependence of $H_{c}$ , and it is shown to be a domain-wall pinning type. Due to structural and magnetic similarities between L10 FeNi and L10 FePt, ribbon samples with low-volume fraction of L10 FePt grains in a soft magnetic matrix were prepared with a similar technique. Magnetization behavior of L10 FeNi is shown to be similar to that of L10 FePt.

Nanoporous Thermosets with Percolating Pores from Block Polymers Chemically Fixed above the Order-Disorder Transition.

A lamellar diblock polymer combining a cross-linkable segment with a chemically etchable segment was cross-linked above its order–disorder temperature (TODT) to kinetically trap the morphology associated with the fluctuating disordered state. After removal of the etchable block, evaluation of the resulting porous thermoset allows for an unprecedented experimental characterization of the trapped disordered phase. Through a combination of small-angle X-ray scattering, nitrogen sorption, scanning electron microscopy, and electron tomography experiments we demonstrate that the nanoporous structure exhibits a narrow pore size distribution and a high surface to volume ratio and is bicontinuous over a large sample area. Together with the processability of the polymeric starting material, the proposed system combines attractive attributes for many advanced applications. In particular, it was used to design new composite membranes for the ultrafiltration of water.

Cationic and Anionic Disorder in CZTSSe Kesterite Compounds: A Chemical Crystallography Study

The cationic and anionic disorder in the Cu2ZnSnSe4-Cu2ZnSnS4 (CZTSe-CZTS) system has been investigated through a chemical crystallography approach including X-ray diffraction (in conventional and resonant setup), 119Sn and 77Se NMR spectroscopy, and high-resolution transmission electron microscopy (HRTEM) techniques. Single-crystal XRD analysis demonstrates that the studied compounds behave as a solid solution with the kesterite crystal structure in the whole S/(S + Se) composition range. As previously reported for pure sulfide and pure selenide compounds, the 119Sn NMR spectroscopy study gives clear evidence that the level of Cu/Zn disorder in mixed S/Se compounds depends on the thermal history of the samples (slow cooled or quenched). This conclusion is also supported by the investigation of the 77Se NMR spectra. The resonant single-crystal XRD technique shows that regardless of the duration of annealing step below the order-disorder critical temperature the ordering is not a long-range phenomenon. Finally, for the very first time, HREM images of pure selenide and mixed S/Se crystals clearly show that these compounds have different microstructures. Indeed, only the mixed S/Se compound exhibits a mosaic-type contrast which could be the sign of short-range anionic order. Calculated images corroborate that HRTEM contrast is highly dependent on the nature of the anion as well as on the local anionic order.