Composition Segregation Research Articles

Fe-Based Powders Prepared by Ball-Milling with Considerable Degradation Efficiency to Methyl Orange Compared with Fe-Based Metallic Glasses

In this study, the degradation efficiencies of zero-valent iron (ZVI) powders with different structures and components were evaluated for methyl orange (MO). The results show that the structure is an essential factor that affects degradation, and added non-metallic elements help optimize the structure. The amorphous and balled-milled crystalline Fe70Si10B20 has comparative degradation efficiencies to MO with t1/2 values of 6.9 and 7.0 min, respectively. Increasing the boron content can create a favorable structure and promote degradation. The ball-milled crystalline Fe70B30 and Fe43.64B56.36 powders have relatively short t1/2 values of 5.2 and 3.3 min, respectively. The excellent properties are mainly attributed to their heterogeneous structure with boron-doped active sites in ZVI. Composition segregation in the nanoscale range in an amorphous FeSiB alloy and small boron particles in the microscale range embedded in large iron particles prepared by ball-milling, both constitute effective galvanic cells that promote iron electron loss and therefore decompose organic chemicals. These findings may provide a new, highly efficient, low-cost commercial method for azo dye wastewater treatment using ZVI.

Compositional segregation and electrical properties characterization of [001]- and [011]-oriented co-growth Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystal

A Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 relaxor ferroelectric single crystal boule of 3 in. diameter was grown using [001]- and [011]-oriented co-growth crystals as seeds by the modified Bridgman method. The as-grown crystal boule was divided into two growth regions [001] and [011]. The composition and the electrical properties of samples of the two regions were characterized in detail along the growth direction in the whole crystal boule. The PT content of [001]-grown samples increased from 27.7 to 36.8 mol. % along the growth direction, especially they are averagely higher about 0.7 mol. % than those of [011]-grown samples at the same growth position. The PMN content of [001]-grown samples decreased from 43.0 to 34.8 mol. %, which is lower than that of the corresponding [011]-grown samples. The compositional segregation behavior along [001] and [011] growth directions was discussed based on the crystal growth theories. Variations in the rhombohedral to tetragonal phase transition temperature (TRT), the piezoelectric properties, the coercive electric field and the remnant polarization of [001] samples were discussed according to changes in compositional segregation and crystal anisotropy. Under the exact same growth conditions, the [011]-grown crystal has a larger composition segregation effect than that of the [001]-grown crystal. This result provides a positive effect on analysing the nature and reducing the effect of compositional segregation.

In Situ Investigation of the Growth of Methylammonium Lead Halide (MAPbI3–xBrx) Perovskite from Microdroplets

The properties of organic–inorganic hybrid perovskites greatly depend on their compositions. Here, we investigate the crystallization of methylammonium lead mixed halide (MAPbI3–xBrx) from microdroplets by adjusting the molar ratio of iodine to bromine in precursor solution, which helps to control the morphology, crystallinity, and physical properties of the perovskite materials. The composition of the precursor solutions has profound effects on the crystallization and perovskite morphology. It tends to form needle intermediate crystals of MAPbI3−xBrx·DMF from the precursor solution at low bromine concentration, while it tends to form flake or cubic perovskite of MAPbI3−xBrx at high bromine concentration. There is evident composition segregation of bromine when the perovskite and its intermediate grow freely from the solution at low temperature. At a high growing temperature of 100 °C, continuous perovskite films consisting of large crystal domains with sizes up to 100 μm are fabricated from the precursor...

Impact of Heat Treatment on the Electrochemical Properties of Octahedral Pt-Ni Nanoparticles

Pt is still the most used catalyst for various electrocatalytic reactions. But its high cost and scarcity hinder its wide application. In order to address this problem, researchers found that alloying Pt with one or more transition metals such as Ni, Fe or Co can enhance its activity via maximizing the exposure of (111) facets.1 Octahedral Pt-Ni nanoparticles have been regarded as promising electrocatalyst for various reactions.2-4 It has been found that post heat annealing on the Pt-Ni octahedra plays a significant role in altering their activity and durability during oxygen reduction reaction. 5 In this study, the structure evolution and compositional segregation were systematically studied during annealing in various atmosphere (vacuum, N2, H2, Ar and air) by XRD, TEM, XPS and electrochemical techniques. The impact of the heat treatment on the activities toward oxygen reduction reaction, hydrogen evolution reaction, and ethanol electro-oxidation reaction were assessed. It is interesting to find that the carbon support might be unstable in certain atmosphere, for example, Ar, and can be deposited on the catalyst surface. Figure 1 compares the CVs of Pt-Ni treated in Ar and N2 at 400 °C with the unannealed one. It is clear that the surface area of Pt-Ni treated in Ar diminished due to carbon coating. In this talk, the composition and structural evolution result upon annealing will be presented. Their effects on various electrochemical reaction activities will be discussed. Figure 1. Comparisons of cyclic voltammograms of Pt-Ni annealed in N2 (blue), Ar (red) at 400°C for 30 min, and un-annealed one in an N2-saturated 0.1 M HClO4 solution. Reference: Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Marković, N. M. Science 2007, 315 (5811), 493-497. Sulaiman, J. E.; Zhu, S.; Xing, Z.; Chang, Q.; Shao, M. ACS Catalysis 2017, 7 (8), 5134-5141. Choi, S.-I.; Xie, S.; Shao, M.; Odell, J. H.; Lu, N.; Peng, H.-C.; Protsailo, L.; Guerrero, S.; Park, J.; Xia, X.Nano letters 2013, 13 (7), 3420-3425. Kavian, R.; Choi, S.-I.; Park, J.; Liu, T.; Peng, H.-C.; Lu, N.; Wang, J.; Kim, M. J.; Xia, Y.; Lee, S. W. Journal of Materials Chemistry A 2016, 4 (32), 12392-12397. Gan, L.; Heggen, M.; Cui, C.; Strasser, P.ACS catalysis 2015, 6 (2), 692-695. Figure 1

A surface energy driven dissolution model for immiscible Cu-Fe alloy

Cu-Fe alloy processed by conventional preparation methods has serious large-scale composition segregation or spatial separation of individual Cu and Fe phases prior to solidification. For immiscible Cu-Fe alloy, a homogeneous melt is the prerequisite to obtain the subsequent uniform solidification microstructure. In this paper, the uniform metastable immiscible Cu100−xFex alloys (x=10, 20, 30, 40wt%) were prepared by arc-melting. The results indicated that with increasing melting times, the initial aggregated Fe-rich melt gradually dissolves into the bulk copper matrix and a homogeneous melt is obtained. Herein a thermodynamic model was constructed to discuss the surface energy driven melting process and the formation of uniform Cu-Fe alloy. During melting process, if the radius of detached liquid Fe-rich droplet is less than the calculated critical radius, the surface energy is high enough to compensate for the partial molar excess Gibbs energy of iron, thereby leading to the spontaneously dissolution of detached liquid Fe-rich droplet into the bulk liquid. As a result, the aggregated Fe-rich melt dissolves step by step eventually forming a homogeneous melt. After subsequent solidification, the immiscible Cu-Fe alloy exhibits homogeneous microstructure. The rapid melt quenching experiments are further performed to validate the proposal model. The results provide strong evidence and support for the surface energy driven dissolution model. The findings give us a basic understanding for the evolution of melting process in other immiscible alloys.

Grain boundary engineering to control the discontinuous precipitation in multicomponent U10Mo alloy

We demonstrate here that locally stabilized structure and compositional segregation at grain boundaries in a complex multicomponent alloy can be modified using high temperature homogenization treatment to influence the kinetics of phase transformations initiating from grain boundaries during subsequent low temperature annealing. Using aberration-corrected scanning transmission electron microscopy and atom probe tomography of a model multicomponent metallic alloy —uranium-10 wt% molybdenum (U-10Mo) a nuclear fuel, that is highly relevant to worldwide nuclear non-proliferation efforts, we demonstrate the ability to change the structure and compositional segregation at grain boundary, which then controls the subsequent discontinuous precipitation kinetics during sub-eutectoid annealing. A change in grain boundary from one characterized by segregation of Mo and impurities at grain boundary to a phase boundary with a distinct U2MoSi2C wetting phase precipitates introducing Ni and Al rich interphase complexions caused a pronounced reduction in area fraction of subsequent discontinuous precipitation. The broader implication of this work is in highlighting the role of grain boundary structure and composition in metallic alloys on dictating the fate of grain boundary initiated phase transformations like discontinuous precipitation or cellular transformation. This work highlights a new pathway to tune the grain boundary structure and composition to tailor the final microstructure of multicomponent metallic alloys.

Compositional heterogeneity near the base of the mantle transition zone beneath Hawaii

Global seismic discontinuities near 410 and 660 km depth in Earth’s mantle are expressions of solid-state phase transitions. These transitions modulate thermal and material fluxes across the mantle and variations in their depth are often attributed to temperature anomalies. Here we use novel seismic array analysis of SS waves reflecting off the 410 and 660 below the Hawaiian hotspot. We find amplitude–distance trends in reflectivity that imply lateral variations in wavespeed and density contrasts across 660 for which thermodynamic modeling precludes a thermal origin. No such variations are found along the 410. The inferred 660 contrasts can be explained by mantle composition varying from average (pyrolitic) mantle beneath Hawaii to a mixture with more melt-depleted harzburgite southeast of the hotspot. Such compositional segregation was predicted, from petrological and numerical convection studies, to occur near hot deep mantle upwellings like the one often invoked to cause volcanic activity on Hawaii.

The Relationship between Chemical Flexibility and Nanoscale Charge Collection in Hybrid Halide Perovskites

AbstractHybrid organometal halide perovskites are known for their excellent optoelectronic functionality as well as their wide‐ranging chemical flexibility. The composition of hybrid perovskite devices has trended toward increasing complexity as fine‐tuned properties are pursued, including multielement mixing on the constituents A and B and halide sites. However, this tunability presents potential challenges for charge extraction in functional devices. Poor consistency and repeatability between devices may arise due to variations in composition and microstructure. Within a single device, spatial heterogeneity in composition and phase segregation may limit the device from achieving its performance potential. This review details how the nanoscale elemental distribution and charge collection in hybrid perovskite materials evolve as chemical complexity increases, highlighting recent results using nondestructive operando synchrotron‐based X‐ray nanoprobe techniques. The results reveal a strong link between local chemistry and charge collection that must be controlled to develop robust, high‐performance hybrid perovskite materials for optoelectronic devices.

Computational alloy design of (Co1-xNix)88Zr7B4Cu1 nanocomposite soft magnets

The dependence of coercivity on composition is an important factor for establishing optimized soft magnetic properties. In this study, we have used the random anisotropy and coherent rotation models to estimate the variation of coercivity with composition in (Co1-xNix)88Zr7B4Cu1 nanocomposite alloys. Our calculations that the magnetoelastic anisotropy contribution to coercivity dominates for Ni rich compositions (x > 0.5). A small range of compositions (0.65 < x < 0.75) is predicted to result in low values of coercivity (<10 A/m). To validate this prediction, (Co1-xNix)88Zr7B4Cu1 nanocomposites in this range were prepared by melt spinning followed by 3600 s isothermal annealing at the primary crystallization peak temperature (∼673 K). Hysteresis loops were measured using vibrating sample magnetometry at room temperature and saturation magnetostriction was measured using a strain gage based magnetostrictometer. Moderately small coercivities (30-40 A/m) and magnetostrictions (3-4 ppm) were measured at for samples with 0.685 < x < 0.725. Our measured coercivity had a minimum value of 32 A/m at x = 0.725, a shift in composition of about 5 at% in the direction of higher Ni content and without the anticipated low value of coercivity. Several reasons for the inaccuracy of this approach are described, including: ignored contributions from amorphous phase (especially in magnetoealstic anisotropy), composition segregation during crystallization leading to unpredictable compositional shifts in prediction, and the general observation that the predictability of minimum coercivity from minimal combined anisotropies has unexplained deviation even in far less complicated materials.

Effect of segregation on Mn-doped relaxor-PT single crystal

The effect of compositional segregation on the properties of relaxor ferroelectric single crystal are attracting increasing attention for its serious limitation on the application field. In this paper, the composition, phase structure, optical properties and electric properties of Mn-doped Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) single crystal were investigated. The real composition of the single crystal fluctuated around the original ratio, except for MnO2. The single crystal contained less Mn element after removing inclusions compared with the starting ratio. The single crystal samples exhibited rhombohedral phase (R phase), orthorhombic phase (O phase) and tetragonal phase (T phase) structures with different composition due to the segregation. The rhombohedral to tetragonal phase transition temperature (TR-T) shifted to a lower value along the growth direction, while the Curie Temperature (TC) shifted to a higher value instead. The relaxation level of the single crystal went down to a lower level when the crystal structure changed from R phase to T and O phase. The coercive field (EC) with [001] poling in R phase remained stable before a rise in O phase and T phase. However, the composition segregation didn't obviously affected the internal bias field and kept around 0.2 kV/cm. The piezoelectric coefficient d33 and unipolar strain with poling direction along [001] had the same trend along growth direction. The curves of them both took a rapid fall at the top of the single crystal boule, which related to the T phase.

Design of highly efficient Ni-based water-electrolysis catalysts by a third transition metal addition into Ni3Mo

The surface composition segregation can have important effects on the catalytic and electrochemical properties of an alloy catalyst. We have performed density-functional theory (DFT) calculations to investigate the effects of a third transition metal (M) addition into the Ni3Mo alloy on the surface segregation and the stability of bulk-terminated and Ni-segregated Ni3Mo/M(111) surfaces. The third transition metals (MTc, Ru, W, Re, or Os) are found to enhance the Ni surface segregation, while MTi, V, Cr, Mn, Fe, Co, Cu, Nb, Rh, Hf, Ta, or Ir retain the Ni-segregated surface but weaken the Ni surface segregation tendency. The remaining transition metals (MSc, Zn, Y, Zr, Pd, Ag, Cd, Pt, or Au) show a complete suppression of the Ni surface segregation. We find the transition metals (Ti, V, Cr, Mn, Fe, Co, Cu, Hf, and W) can stabilize a special atomic configuration that is predicted to have an outstanding catalytic performance for water electrolysis. Furthermore, we analyze the factors that control the Ni surface segregation behavior in the Ni3Mo/M(111) surface. The present study provides useful theoretical guide for developing highly active Ni-Mo-M hydrogen evolution electrodes.

Novel Method for Making Biomedical Segregation-Free Ti-30Ta Alloy Spherical Powder for Additive Manufacturing

Ti-Ta alloys offer a good combination of high strength and low modulus among Ti-based alloys, and are ideal for biomedical applications. However, making Ti-Ta alloys has always been challenging because they tend to suffer from compositional segregation during melting due to the large difference between the melting points of Ti and Ta. This article describes a novel process for making spherical Ti-30Ta alloy powder through a unique powder metallurgy technique, namely the granulation–sintering–deoxygenation process. The results indicate that the compositional segregation problem can be overcome using this process technology. Combined with use of a deoxygenation process, the critical interstitial element, oxygen, can be controlled to < 400 ppm for powder with particle size < 75 µm. The destabilization effect of Ta on Ti-O solid solutions, and the resulting improved deoxygenation process for Ti-Ta, are discussed, as well as the phase composition and microstructure of the powders.

Effects of solution temperature and Cu content on the properties and microstructure of 2:17-type SmCo magnets

The effects of solution temperature and Cu content on the properties and microstructure of Sm (CobalFe0.21CuxZr0.03)7.6 (x = 0.06, 0.08, 0.10) permanent magnets were investigated systematically. The results showed that TbCu7 single-phase was obtained after solution treatment at optimum temperature. The best magnetic properties of Br = 1.010 T and (BH)max = 190.2 kJ/m3 were obtained for Sm (CobalFe0.21Cu0.08Zr0.03)7.6 by a proper solution treatment at 1478 K. At a lower solution temperature, the Co-rich FeCoZr phase was monitored, and a “step” was observed in the demagnetization curves of aged samples. At a higher solution temperature, composition segregation of the matrix gradually occurs, and a Zr-rich SmCoZr phase was found. However, at a slightly higher solution temperature with superabundant Cu, the precipitated phase remained FeCoZr phase, resulting in a worse remanence. All the above changes were related to the cellular structures.

Effect of heat input on microstructure and mechanical properties of pulsed laser welded joints in Ti6Al4V/Nb dissimilar alloys

Pulsed laser welding was used to weld dissimilar joints in Ti6Al4V/Nb alloys. The effect of heat input on the microstructural characterizations and mechanical properties of the joints was investigated. The experimental results show that underfill defects disappeared in the fusion zone (FZ) as the heat input increased. No intermetallic compounds were formed in the FZ, but a dendritic zone of Ti-rich phases and an island zone of Nb-rich phases were present for all welded joints, indicating that composition segregation occurred in the Ti6Al4V/Nb dissimilar joint. Moreover, the microstructure and mechanical heterogeneity in the FZ of the welded joint increased with increasing heat input. The microhardness of the island area was lower than that of the dendritic zone in the FZ, because it possessed higher levels of Nb. The results of the tensile tests show that the joints fractured at the Nb base metal side, suggesting that the strength of the FZ is higher than that of the Nb base metal.

Maximizing thermoelectric performance of AgPbmSbTem+2 by optimizing spark plasma sintering temperature

AgPbmSbTem+2 (abbreviation as LAST) is a promising thermoelectric material based on PbTe for mid-temperature applications. The electrical and thermal transport properties of LAST alloys are greatly affected by the formation of nanostructured precipitation depending on the processing conditions, especially for spark plasma sintering (SPS) combined with mechanical alloying (MA). This study was focused on the optimization of SPS temperature (TSPS) and found that its variation in a narrow range (100 K) leads to apparent differences of thermoelectric performances. The sample sintered at an optimized temperature (TSPS = 923 K) shows a low κtotal (0.829 Wm−1K−1) and a high PF (14.9 Wm−1K−2), which is mainly attributed to the nanoscale composition segregation of Ag and Sb. The highest ZT value of 1.28 at 773 K was obtained when TSPS = 923 K, along with the highest ZTeng value of 0.26 and the corresponding ηmax value of 5.8% at the range of 323 K (Tcold)–773 K (Thot), which were increased almost twice by the optimization of sintering temperature.

Crystallization and microstructural evolution of an amorphous Ni–Ti–Hf film

An amorphous Ni–Ti–Hf film was fabricated through DC magnetron sputtering deposition. The crystallization and microstructural evolution of the amorphous Ni–Ti–Hf film was observed via in situ transmission electron microscopy. Composition segregation occurred after crystallization for different grains. The grains showed two phases, namely, B19′ martensite for Ni49.78Ti35.13Hf15.09 and B2 austensite for Ni55.36Ti31.08Hf13.56 at room temperature.

Arc erosion behavior and mechanism of Cu/Cr20 electrical contact material

Cu/Cr20 material was prepared by spark plasma sintering. And then the arc-erosion tests of electrical contacts were carried out under the condition of constant current load with JF04C contact material test system. The trend of mass change (loss and transfer) of the Cu/Cr20 contact material was analyzed, when the current is less than 35 A, mass loss of material increases with the increase of current, when the current is 40 A, material is transferred from cathode to anode. The SEM was employed to investigate the surface morphologies of arc-eroded specimens of the Cu/Cr20 electrical contact materials. In accordance with the relevant experimental data, the relationship among arcing time, arc energy, and welding force was analyzed, and there is a linear relationship between the arcing time (t) and arc energy(E) (E=143.9t−158.3). The results indicate that the phenomena of material melting and transfer are caused by DC arc. There are molten pools, pits, porosity, cracks and coral structure distributed on the surface of contacts. And composition segregation phenomenon occurred on the arc erosion layer at the same time. With the increasing of operation times, there is a positive linear correlation between arc energy and the arcing time.

Common Defect Analysis for Large Section Special Steel Forging

In order to Figure the common defect in large section special steel forging and find the solution, systematic study was carried out on hundreds of large section special steel forgings in a domestic famous steel mill. The steels included: low-carbon steel Q345D/E, medium-carbon steel 27SiMn, high-carbon steel GCr15SiMn, stainless steel 20Cr13. Both the amount and type of all the defect in the above steel were calculated and analyzed. The results showed that the common defects of the steel were slags, inclusions, loose (cavity) and inner cracks. The evolution of the cavity in the ingot during forging process was simulated by a numerical simulation software Deform-3D. The inner cracks in Q345D/E and 27SiMn initiated after A→F+P transformation The cracks in GCr15SiMn formed after the precipitation of net-like proeutectoid carbides. The cracks in 20Cr13 formed after the precipitation of net-like carbides. The internal cause of the cracks was relevant to composition segregation and internal stress in the forging. The external cause was connected with effect of slow cooling. Based on the above study, a set of new process was proposed and put into industrial application, with the result that the qualified ratio of flaw inspection in the above steel mill was improved from 20% to above 87%.

Shift Happens. How Halide Ion Defects Influence Photoinduced Segregation in Mixed Halide Perovskites

Minimizing photoinduced segregation in mixed halide lead perovskites is important for achieving stable photovoltaic performance. The shift in the absorption and the rate of formation of iodide- and bromide-rich regions following visible excitation of mixed halide lead perovskites is found to strongly depend on the halide ion concentration. Slower formation and recovery rates observed in halide-deficient films indicate the involvement of defect sites in influencing halide phase segregation. At higher halide concentrations (in stoichiometric excess), segregation effects become less prominent, as evidenced by faster recovery kinetics. These results suggest that light-induced compositional segregation can be minimized in mixed halide perovskite films by using excess halide ions. The findings from this study further reflect the importance of halide ion post-treatment of perovskite films to improve their solar cell performance.

Palladium-copper membranes for hydrogen separation

Abstract This communication summarizes key elements and facts of PdCu alloy membranes for H2 separation by reviewing fundamentals and literature reports, such as the PdCu phase diagram and its implications toward membrane permeability and stability, the phase and composition segregation behavior, and the intricate phase structure-composition-stability-permeability relationship. The inconvenient challenges for applying PdCu alloy membranes due to its complex alloy phase diagram consisting of more than one phase structure are identified. The sulfur resistance and H2 permeation hysteresis will also be discussed accordingly. Previously reported permeation data will be revisited by using the insights outlined to explain some of the previous observations of instability for PdCu alloys operating in pure H2 atmosphere. We conclude the manuscript with a summary, as well as our perspective and outlook on this topic.