Co-rich Side Research Articles

Experimental Investigation of Diagram Equilibria in the Co-Nb-Re Ternary System

In this study, the isothermal sections of the Co-Nb-Re ternary system at 1200, and 1300 °C have been experimentally determined combining the means of electron probe microanalysis (EPMA) and x-ray diffraction (XRD). The obtained experimental results showed that: (1) The Laves phase of λ3-Co2Nb (C36) was stable at 1300 °C. The temperature was beyond its stability limit in Co-Nb binary system. (2) The solubility of Re in the λ3 phase was so large that the nearest λ2-Co2Nb (C15) phase was essentially surrounded. (3) The solubility of Re in the μ-Co7Nb6 phase was 34.0 at.% at 1200 °C and 35.2 at.% at 1300 °C, respectively. (4) The liquid phase existed at 1300 °C dissolving about 4.0 at.% Re, but it was absent at 1200 °C. (5) The solid solution phase of (eCo, Re) extended from Re-rich to Co-rich side.

Phase Equilibria Among γ/TCP/GCP Phases in Nb-Poor Region of Ni-Nb-Co Ternary System at Elevated Temperatures

Phase equilibria among the γ-(Ni, Co), topologically close-packed and geometrically close-packed phases in the Ni-Nb-Co ternary system were investigated using x-ray diffraction, transmission electron microscopy, and electron probe microanalysis for equilibrated bulk alloys at 1100 and 1200 °C. A two-phase equilibrium between the γ (A1) and D0a phases (Ni3Nb) is confirmed to exist on the Ni-rich side with the Ni/(Ni + Co) ratio above ~ 0.8, which agrees well with literature data. The phases in equilibrium with the A1 phase change into a ternary-compound D019 phase ((Ni, Co)3Nb) and an mC18 phase (Co7Nb2) with increasing Co content. The C15 phase (Co2Nb) is not in equilibrium with the A1 phase. The three-phase tie-triangle of the A1/mC18/D019 moves toward the Co-rich side with decreasing temperature at an equivalent Nb concentration, while that of A1/D019/D0a almost remains at the chemical composition of Ni-20Co-20Nb in the temperature range investigated. Because the mC18 phase is in equilibrium with the A1 phase up to 1086 °C in the Co-Nb binary system, Ni addition should stabilize the mC18 phase and expand the mC18 phase region toward high temperatures.

Electrochemical Synthesis of Sm-Co Metal Magnetic Materials by Co-reduction of Sm(III) and Co(II) in LiCl-KCl-SmCl3-CoCl2 Melt

High magnetism and magnetization intensity are required for preparing rare-earth-based single-crystal flaky magnetic intermetallic compound materials. Herein, we report high coercivity Sm-Co (Sm2Co17, SmCo5, Sm2Co7, SmCo3, and SmCo2) intermetallic compounds, which have been synthesized by electrochemical synthesis in LiCl-KCl-SmCl3-CoCl2 melt. The electrochemical co-reduction behaviors of Sm(III) and Co(II) ions, and the formation of intermetallic compounds in the melt were investigated by cyclic voltammetry (CV), square wave voltammetry (SWV) and open-circuit chronopotentiometry techniques (OCP), respectively. The crystal nature of Sm-Co intermetallic compounds was characterized by X-ray diffraction (XRD), selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM) and Fast Fourier transform analysis (FFTs) method. At the same time, multilayer planar structure of Sm-Co intermetallic compounds was investigated by transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). The results showed that as the mass percentage of SmCl3 changed from 0 to 12wt.%, the phase of Sm-Co intermetallic compounds transferred from Co-rich side to Sm-rich side, and the crystal structures were found to be single-crystal ones. The valence bands and core levels of Sm-Co intermetallic compounds were evolved in the crystal growth process. A structural change in the core levels Sm 3d and Co 2p during the course of transition from pure components to intermetallic compounds occurred, which could be explained by the hybridization of f and d electrons of samarium and cobalt. The results of Sm-Co intermetallic compounds for vibrating sample magnetometer (VSM) test showed that the magnetic densities of Sm2Co17 and SmCo5 were 157 emu/g and 145 emu/g, respectively.

First-Principles Calculations of Defect Structures in B2 AlCo and GaCo

First-principles electronic structure calculations have been performed for defect structures in nonstoichiometric B2 AlCo and GaCo. To determine the type of constitutional defects, the compositional dependence of the energy of formation and lattice parameter was obtained by calculations employing supercells of various sizes (16 and 32 atoms) as well as special quasirandom structures (SQSs) developed for random pseudobinary A1−xBxC with compositions x = 0.25 and 0.5. According to the results, Co vacancies are the constitutional point defects in the Al-rich side of both B2 AlCo and B2 GaCo, while Co vacancies present the minimum energy for the Ga-rich side. For the Co-rich side of both B2 AlCo and B2 GaCo, the Co antisite is the most stable defect. To investigate the thermal defect concentrations at finite temperature, we adopted the Wagner–Schottky model using enthalpies of formation of point defects obtained from the SQS approach. The present results suggest that the predominant thermal defects in AlCo are of complex type whereas for GaCo they are of interbranche Co type. The results of these calculations show agreement with available theoretical and experimental data.

Magnetic Properties of Liquid RE-Co (RE = Dy, Er) Alloys

The magnetic susceptibilities χ of liquid Dy-Co and Er-Co alloys have a large and negative temperature coefficient, which suggests that the Dy, Er, and Co ions in their liquid alloys are in the magnetic state. However, the temperature dependence of χ in both systems becomes weak near the content of 70 at% Co. It is interesting that the compositional dependence of χ for liquid Dy-Co and Er-Co alloys has a minimum at content of 80 at% Co, respectively. On the Co-rich side, the magnetic susceptibility of liquid Dy-Co and Er-Co alloys obeyed the Curie-Weiss law with regard to their temperature dependence of χ . On the rare earth-rich side, the magnetic susceptibilities of liquid Dy-Co and Er-Co alloys also exhibited Curie behavior with a reasonable value for the effective number of Bohr magnetons. The compositional dependence of χ 4f for liquid Dy-Co and Er- Co alloys was extracted by subtracting the corresponding data for liquid La-Co alloys from χ for the liquid Dy-Co and Er-Co alloys, respectively.

A thermodynamic description of the Al–Co–Ni system and site occupancy in Co + AlNi3 composite binder phase

Phase equilibria of the Al–Co–Ni system is extremely useful for understanding phase behavior of AlNi3-based L12 phases, regarding its attractive properties, such as high-temperature oxidation resistance and corrosion resistance, high hardness and strength. In the present work, all the experimental data about the phase equilibria and thermodynamic properties of the Al–Co–Ni system were critically evaluated, and a thermodynamic description for this system over the whole composition and temperature ranges was obtained. Most importantly, the ordered/disordered transition between fcc_A1 and L12 phases was described using a four-sublattice (4SL) model. Based on the crystallographic and experimental data, three stable ternary intermetallic phases were described using sublattice model. Two of them were treated as semi-stoichiometric compounds with homogeneity ranges for Co and Ni, and the other one was treated as a stoichiometric compound. The other phases were described by using substitutional solution model. The present work provides a more complete and accurate thermodynamic description compared with previous ones. A reasonable agreement was obtained between calculations and experimental data in the ternary system. The occupancy behavior of Co in AlNi3 is thermodynamically predicted, and the computed occupancy shows a much better fit to the experimental data than previous first-principles calculations. In addition, the composition range of the Co + AlNi3 two-phase region, concerning a new type of composite binder for cemented carbides, was found to shift to the Co-rich side with the decrease of the temperature.

Intermediate Co/Ni-base model superalloys — Thermophysical properties, creep and oxidation

The mechanical properties of γ′-strengthened Co–Ni–Al–W–Cr model superalloys extending from pure Ni-base to pure Co-base superalloys have been assessed. Differential scanning calorimetry measurements and thermodynamic calculations match well and show that the γ′ solvus temperature decreases with increasing Co-content. The γ/γ′ lattice misfit is negative on the Ni- and positive on the Co-rich side. High Ni-contents decelerate the oxidation kinetics up to a factor of 15. The creep strength of the Ni-base alloy increases by an order of magnitude with additions of Co before it deteriorates strongly upon higher additions despite an increasing γ′ volume fraction.

Composition effects of spinel MnxCo3−xO4 nanoparticles on their electrocatalytic properties in oxygen reduction reaction in alkaline media

In this work, we synthesized a series of manganese cobalt spinel oxide (MnxCo3−xO4) nanoparticles (NPs) covering the whole composition range (x = 0.0. 0.4, 0.8, 1.4, 1.9, and 3.0) and investigated their electrocatalytic properties in relation with the oxygen reduction reaction (ORR) in alkaline media. The NP samples were synthesized by sonochemical reactions of Mn(OAc)2 and Co(OAc)2 (Ac = acetyl) in a water-dimethylformamide mixed solvent. The four samples in the Co-rich side have the cubic structure whereas the other two samples in the Mn-rich side have the tetragonal structure. The X-ray photoelectron spectroscopy and electrochemical analyses data indicate that the distribution of manganese and cobalt ions between the two metal ion sites of the spinel structure in our NP samples conforms to that of the bulk counterparts in the literature. The electrocatalytic data show that the ORR mechanism is changed when the structure is changed from cubic to tetragonal. The highest ORR activity was observed with the x = 0.4 sample. The electrochemical stability of this sample is higher than that of commercial Pt/C.

Phase diagram of In–Co–Sb system and thermoelectric properties of In-containing skutterudites

In-containing skutterudites have long attracted much attention and debate partly due to the solubility limit issue of indium in CoSb3. The isothermal section of the equilibrium phase diagram for the In–Co–Sb system at 873 K is proposed using knowledge of the related binary phase diagrams and experimental data, which explains the debated indium solubility that depends on Sb content. In this paper, a series of In-containing skutterudite samples (InxCo4Sb12−x/3 with x varying from 0.075 to 0.6 and In0.3Co4−ySb11.9+y with y changing from −0.20 to 0.20) are synthesized and characterized. X-ray analysis and scanning electron microscopy images indicate that, up to x = 0.27, single-phase skutterudites are obtained with lattice constant increasing with In fraction x. A fixed-composition skutterudite In0.27±0.01Co4Sb11.9 was determined for the Co-rich side of In–CoSb3 which is in coexistence with liquid InSb and CoSb2. Indium, like Ga, is expected, from DFT calculations, to form compound defects in In-containing skutterudites. However, relatively higher carrier concentrations of In-containing skutterudites compared to Ga-containing skutterudites indicate the existence of not fully charge-compensated compound defects, which can also be explained by DFT calculations. The net n-type carrier concentration that naturally forms from the complex defects is close to the optimum for thermoelectric performance, enabling a maximum zT of 1.2 for the fixed skutterudite composition In0.27Co4Sb11.9 at 750 K.

Tunable interplay between3dand4felectrons in Co-doped iron pnictides

We study the interplay of $3d$ and $4f$ electrons in the iron pnictides CeFe${}_{1\ensuremath{-}x}$Co${}_{x}$AsO and GdFe${}_{1\ensuremath{-}y}$Co${}_{y}$AsO, which correspond to two very different cases of $4f$-magnetic moment. Both CeFeAsO and GdFeAsO undergo a spin-density-wave (SDW) transition associated with Fe $3d$ electrons at high temperatures, which is rapidly suppressed by Fe/Co substitution. Superconductivity appears in a narrow doping range: $0.05<x<0.2$ for CeFe${}_{1\ensuremath{-}x}$Co${}_{x}$AsO and $0.05<y<0.25$ for GdFe${}_{1\ensuremath{-}y}$Co${}_{y}$AsO, showing a maximum transition temperature ${T}_{\mathrm{sc}}$ of about 13.5 K for Ce and 19 K for Gd. In both compounds, the $4f$ electrons form an antiferromagnetic (AFM) order at low temperatures over the entire doping range and Co $3d$ electrons are ferromagnetically ordered on the Co-rich side; the Curie temperature reaches ${T}_{\mathrm{C}}^{\mathrm{Co}}\ensuremath{\approx}$ 75 K at $x=1$ and $y=1$. In the Ce compounds, the N\'eel temperature ${T}_{\mathrm{N}}^{\mathrm{Ce}}$ increases upon suppressing the SDW transition of Fe and then remains nearly unchanged with further increasing Co concentration up to $x\ensuremath{\simeq}0.8$ (${T}_{\mathrm{N}}^{\mathrm{Ce}}\ensuremath{\approx}$ 4 K). Furthermore, evidence of Co-induced polarization on Ce moments is observed on the Co-rich side. In the Gd compounds, the two magnetic species of Gd and Co are coupled antiferromagnetically to give rise to ferrimagnetic behavior in the magnetic susceptibility on the Co-rich side. For $0.7\ensuremath{\le}y<1.0$, the system undergoes a possible magnetic reorientation below the N\'eel temperature of Gd (${T}_{\mathrm{N}}^{\mathrm{Gd}}$). Our results suggest that the effects of both electron hybridizations and magnetic exchange coupling between the $3d$-$4f$ electrons give rise to a rich phase diagram in the rare-earth iron pnictides.

Phase diagram of the SmCo1−xFexAsO system

A series of SmCo1−xFexAsO (x = 0, 0.05, 0.1, 0.2, 0.3) polycrystalline samples were synthesized and their transport and magnetic properties were studied. Similar to the undoped SmCoAsO, these Fe-doped samples of SmCo1−xFexAsO also undergo successive magnetic phase transitions associated with the Co or Fe magnetic moments below 100 K, i.e., the system become ferromagnetic (FM) ordered first below the Curie temperature Tc and then become antiferromagnetic (AFM) ordered with further decreasing temperature. With Fe doping on the Co site, both FM (Tc) and AFM (TN1) transition temperatures are rapidly suppressed and then both Tc and TN1 approaching zero temperature at x = 0.3, whereas the AFM order (TN2) of Sm3+ moments below 5.1 K is still robust. Based on these results, the magnetic phase diagram in the Co-rich side of the SmCo1−xFexAsO system is established.

Magnetic phase diagram in the Co-rich side of theLCo1−xFexAsO (L=La, Sm) system

The magnetic phase diagram has been mapped out via the measurements of electronic resistivity, magnetization and specific heat in the cobalt-based layered LnCo1-xFexAsO (Ln=La, Sm) compounds. The ferromagnetic (FM) transition at 63 K for LaCoAsO is rapidly suppressed upon Fe doping, and ultimately disappears around x=0.3 in the LaCo1-xFexAsO system. When La is replaced by magnetic rare earth element Sm, the 3d electrons first undergo a FM transition at Tc = 75 K, followed by an antiferromagnetic (AFM) transition at a lower temperature TN1 = 45 K. With partial Fe doping on the Co site, both FM (Tc) and AFM (TN1) transition temperatures are significantly suppressed, and finally approach zero kelvin at x = 0.3 and 0.2, respectively. Meanwhile, a third magnetic transition at TN2 = 5.6 K for SmCoAsO, associated with the AFM order of the Sm3+ 4f-oments, is uncovered and TN2 is found to be almost robust against the small Fe-doping. These results suggest that the 4f electrons of Sm3+ have an important effect on the magnetic behavior of 3d electrons in the 1111 type Co-based LnCo1-xFexAsO systems. In contrast, the magnetism of the f-electrons is relatively unaffected by the variation of the 3d electrons. The rich magnetic phase diagram in the Co-rich side of the LnCo1-xFexAsO system, therefore, is established.

Control of the multiferroic transition in Ni3V2O8by transition metal doping

We have studied the effects of doping magnetic ions on the phase transitions in polycrystalline Ni${}_{3}$V${}_{2}$O${}_{8}$ samples using magnetic, dielectric, and specific heat measurements. Substituting spin-1/2 Cu results in a larger suppression of the transition temperatures than observed on doping with spin-3/2 Co. The multiferroic-ordering transition persists at small concentrations for both Co and Cu dopants, with this phase still developing at concentrations of almost 30 at% Co. However, the multiferroic-magnetic structure is completely suppressed at only 10 at% Cu. On the Co-rich side of the composition, we find that the magnetic-ordering temperatures for Co${}_{3}$V${}_{2}$O${}_{8}$ are suppressed rapidly with Ni doping. We present phase diagrams for (Ni${}_{1\ensuremath{-}x}$M${}_{x}$)${}_{3}$V${}_{2}$O${}_{8}$ (M = Cu and Co). These studies suggest that the spin structures in Ni${}_{3}$V${}_{2}$O${}_{8}$ responsible for the development of ferroelectric order are relatively robust against perturbations produced by magnetic dopants, with the most significant disruption of the magnetic structure developing for Cu doping.

Iron substitution in NdCoAsO: Crystal structure and magnetic phase diagram

The effects of replacing small amounts of Co with Fe in NdCoAsO are reported. Polycrystalline materials with compositions NdCo1-xFexAsO (x = 0.05, 0.10, 0.15, and 0.20) are studied and the results compared to previous reports for NdCoAsO. Rietveld analysis of powder x-ray diffraction data shows that as Fe replaces Co on the transition metal (T) site, the T-As distance increases, and the As tetrahedra surrounding the T-site become more regular. Electrical resistivity and magnetization measurements indicate that the three magnetic phase transitions in NdCoAsO are suppressed as Co is replaced by Fe, and these transitions are not observed above 1.8 K for x = 0.20. Based on these results, the magnetic phase diagram for the Co-rich side of the NdCoAsO-NdFeAsO system is constructed.

The thermodynamic assessment of the Co–Sb system

The Co–Sb system has been assessed thermodynamically based on the experimental data of both the phase equilibria and the thermochemical properties. The reasonable models were constructed for all the phases of the system. The liquid and the terminal solid solution phases, ( α Co ) and ( ε Co ), were described as the substitutional solution model while the (Sb) phase was treated as the pure element Sb for lack of the solubility data of Co in (Sb). The ( β CoSb ) phase was modelled by three-sublattices, Sb: Co, Va: Va, Co, since its structure is of the NiAs-type (B8 1) with the Co vacancies on the Sb-rich side and the interstitial Co atoms on the Co-rich side. The compounds γ CoSb 2 and δ CoSb 3 were expressed as the stoichiometric compounds. Much attention has been paid to the consistence between the experimental measurements and the theoretical calculations. Using the present thermodynamic data, the calculation results can reproduce the experimental data well.

First Principles Calculation of Defect Structure in Non-stoichiometric CoAl and CoTi

First-principles electronic structure calculations have been performed for defect structure in non-stoichiometric CoAl and CoTi. In order to determine the type of constitutional defects, the compositional dependence curves both of formation energies and of lattice parameters are obtained by the calculations employing supercells in various sizes. The defect formation energies are calculated with taking into account the compositional dependence of the chemical potential. The calculated results suggest that the Co vacancy is the dominant thermal-excitation defect even in the Co-rich side near the stoiciometry in CoTi. Intermetallic compounds with the B2 structure have attracted considerable attention as promising candidates for high-temperature materials. One of the important features of intermetallic compounds with the B2 structure is having a wider range of compositions than other phases. In order to compensate the deviation from the stoichiometry, constitu- tional defects are introduced. Several properties such as mechanical and magnetic properties, therefore, show com- positional dependence originating from the constitutional defects. In addition, thermally excited defects play an important role in the diffusion of atoms and therefore the understanding of the defects are required for the alloy design and optimization of heat treatment. Electronic structure calculations using supercell models have been performed for non-stoichiometric B2 intermetallic compounds to understand the defect structure. The size of the supercell is one of the important factors for electronic structure calculations including defects. The defect formation energies in FeAl were calculated using the 8, 16 and 32 atoms supercells. 1) The calculations with the 16 and 54 atoms supercells were performed for convergence tests of the effective formation energies and the structural relaxation of the constitutional defects in NiAl. 2) In our previous work, we

Experimental Verification of Magnetically Induced Phase Separation in αCo Phase and Thermodynamic Calculations of Phase Equilibria in the Co–W System

Phase equilibria of solid phases in the Co-W system were investigated using two-phase alloys and the diffusion couple technique. In addition, systematic studies of magnetic and martensitic transitions were conducted by Differential Scanning Calorimetry (DSC), Vibrating Sample Magnetomety (VSM) and dilatometric measurement. Phase separation into ferromagneticCo and paramagneticCo was confirmed on the Co-rich side. The Curie temperature and saturation magnetization decreased with increasing W content and vanished at around 25 at%W. Furthermore, thermodynamic assessment by the CALPHAD approach was performed. A set of thermodynamic values describing the Gibbs energy of the Co-W system was in good agreement with the experimental phase diagram. Calculation of metastable magnetically induced phase separation in the hcp phase along the Curie temperature was also performed.

Study of the short-range order of FeCoSiAlGaPCB amorphous alloys by EXAFS spectroscopy

The short-range order of (Fe x Co y B z C u ) 80Si 3Al 5Ga 2P 10 with x = 5 – 70 , y = 0 – 63 , z = 5 – 12 and u = 0 – 5 amorphous alloys has been investigated by X-ray absorption spectroscopy. The existence of some chemical preferences leading the B atoms to locate near Fe atoms and Si atoms near Co atoms can be concluded which allows to explain the small increase of the short-range order previously found for these alloys in the Co-rich side by Mössbauer spectrometry. Co and Fe neighbourhood in the amorphous was found to be similar to that of the crystalline phases formed during the primary crystallization.

Phase separation of BCC phase in the Co-rich portion of Co-Fe-Al system

The α(A2)+ α′(B2) and γ(A1)+ α′(B2) phase equilibria, and the A2/B2 and the ferromagnetic/paramagnetic transitions in the Co-rich portion of Co-Fe-Al system were examined by electron probe microanalysis (EPMA) and differential scanning calorimetric (DSC) measurement. The α(A2)+ α′(B2) two-phase region appears in the Co-rich side of the isothermal section even at high temperatures over 800 °C and a kind of LCST (Lower Critical Solution Temperature)-type phase separation is detected in the Co-30at.%Fe-Al section. Since this phase separation occurs in the compositional range in the vicinity of the intersection of the A2/B2 order-disorder and the magnetic transition boundaries, it seems that the formation of the LCST-type separation results from the enhancive interaction between the chemical ordering and magnetic ordering.

計算状態図と材料設計への展開 Ｃｏ‐Ｗ系αＣｏ相中の磁気変態により誘起される２相分離の実験的検討と相平衡の熱力学的計算

Phase equilibria of solid phases in the Co-W system have been investigated by using two-phase alloys and diffusion couple technique. In addition, systematic studies of magnetic and martensitic transitions have been conducted by using Differential Scanning Calorimeter (DSC), Vibrating Sample Magnetometer (VSM) and dilatometric measurement. Two-phase separation into ferromagnetic αCo and paramagnetic αCo was confirmed in the Co-rich side. The Curie temperature and saturation magnetization decrease with increasing W content and those disappear at around 25 at%W. Furthermore, thermodynamic assessment by using CALPHAD approach has been performed. A set of thermodynamic values for describing the Gibbs energy of Co-W system yields a good agreement with experimental phase diagram. The calculation of metastable magnetically induced phase separation along the Curie temperature is also presented.