Microstructures and mechanical properties of NbCr2 and ZrCr2 Laves phase alloys prepared by powder metallurgy

Microstructures, mechanical properties and oxidation behavior were investigated on NbCr₂ and ZrCr₂ Laves phase alloys prepared by powder metallurgy (P/M), and also by arc-melting, i.e. ingot metallurgy (I/M). These properties were also evaluated, in terms of alloying, heat treatment and alloy stoichiometry. High-temperature yield strength and brittle ductile transition temperature (BDTT) were generally lower in alloys prepared by P/M process than in those prepared by I/M process while micro hardness and fracture toughness were higher in alloys prepared by P/M process than in those prepared by I/M process, irrespective of NbCr₂ or ZrCr₂ alloys. Also, high-temperature strength and micro hardness were higher in NbCr₂ alloys than in ZrCr₂ alloys while fracture toughness was lower in NbCr₂ alloys than in ZrCr₂ alloys, irrespective of P/M or I/M process. For oxidation behavior at 1223 K, NbCr₂ alloys showed linear increase with increasing time accompanied with irregular fluctuation, while ZrCr₂ alloys showed parabolic increase with increasing time. It was also found that alloy stoichiometry greatly affected micro hardness, fracture toughness and oxidation behavior in ZrCr₂ alloys.     

Kinetic aspects of the chlorination of wolframite and of scheelite with chlorine and sulphur dioxide

The chlorination of wolframite and of scheelite with chlorine (chlorination agent) and sulphur dioxide (reducing agent) was studied within the temperature range 673–1173 K. The tests were carried out in a vertical reactor with a static bed. The greatest tungsten extractions (1173 K) were 86% from wolframite and 33% from scheelite. The behaviour of the chlorination reactions was analyzed through characterization of the reaction products obtained in each case. The principal reaction products identified were WO₂Cl₂, FeCl₃, CaSO₄ and CaCl₂. Likewise, size changes, specific surface areas and surface contents of calcium, tungsten, iron and manganese of both ores were studied during chlorination. By means of kinetic models it was determined that the wolframite chlorination is controlled by the movement of an interface and that the presence of reaction products affects the scheelite chlorination. Finally, a possible stoichiometry was proposed to explain the reaction of oxygen contained in the wolframite and in the scheelite.     

Coaxial metal-silicide Ni2Si/C54-TiSi2 nanowires

One-dimensional metal silicide nanowires are excellent candidates for interconnect and contact materials in future integrated circuits devices. Novel core-shell Ni(2)Si/C54-TiSi(2) nanowires, 2 μm in length, were grown controllably via a solid-liquid-solid growth mechanism. Their interesting ferromagnetic behaviors and excellent electrical properties have been studied in detail. The coercivities (Hcs) of the core-shell Ni(2)Si/C54-TiSi(2) nanowires was determined to be 200 and 50 Oe at 4 and 300 K, respectively, and the resistivity was measured to be as low as 31 μΩ-cm. The shift of the hysteresis loop with the temperature in zero field cooled (ZFC) and field cooled (FC) studies was found. ZFC and FC curves converge near room temperature at 314 K. The favorable ferromagnetic and electrical properties indicate that the unique core-shell nanowires can be used in penetrative ferromagnetic devices at room temperature simultaneously as a future interconnection in integrated circuits.     

Composition dependence of magnetocaloric effect in Pr0.6Ca0.4Mn(1-x)Cr(x)O3 (x = 0.02-0.08)

We report the effect of varying Cr content on magnetic and magnetocaloric properties of Pr0.6Ca0.4Mn(1-x)Cr(x)O3 samples (x = 0, 0.02, 0.04, 0.06 and 0.08). While the parent compound (x = 0) is a charge ordered and antiferromagnetic insulator, Cr doped compounds are ferromagnetic metals with nearly same Curie temperature (T(c) approximately 140 K). We find unusual field-induced meta-magnetic transition above T(c) in x = 0.02 and 0.04 which is absent in x = 0.06 and 0.08. It is suggested that the paramagnetic phase in these compounds is inhomogeneous with coexistence of nano-size ferromagnetic clusters and short range charge ordered clusters. Field induced growth of ferromagnetic nano-clusters and destruction of short-range charge ordering leads to the observed metamagnetic transition, which results in large magnetic entropy change of -deltaS(M) = 5.043, 6, 5.509 and 4.375 J/kg K under deltaH = 5 T, for x = 0.02, 0.04, 0.06 and 0.08, respectively. In addition, large relative cooling power (RCP) found in these materials (327.384, 286.36, 272.22 and 279.936 J/kg) makes it interesting for practical applications. Our study suggests that creation of ferromagnetic nano-clusters in the paramagnetic phase by Mn-site doping in charge ordered compounds provides an alternative approach to achieve high AS(M) and RCP values.     

Wet-chemical synthesis and properties of CoPt and CoPt3 alloy nanoparticles

Surface-protected, air-stable nanoparticles of CoPt and CoPt3 were prepared by thermal decomposition/reduction of organometallic precursors with a long-chain aliphatic diol, also known as the polyol process. Particles 3 nm in diameter showed ferromagnetic behavior up to 350 K (Hc = 65 Oe at T = 300 K; Hc = 410 Oe at T = 5K) and underwent a disordering-ordering phase transformation after annealing that resulted in an increase in coercivity (Hc = 170 Oe at T = 300 K; Hc = 2000 Oe at T = 5 K).     

Microstructural, mechanical and magnetic properties of shape memory alloy Ni55Mn23Ga22 thin films deposited by radio-frequency magnetron sputtering

The near-stoichiometric Ni₂MnGa ferromagnetic alloys are one of the smart materials, that are of a great interest when they are deposited as a thin film by r.f. sputtering. These thin films of shape memory alloys are prospective materials for micro and nanosystem applications. However, the properties of the shape memory polycrystalline thin films depend strongly on their structure and internal stress, which develop during the sputtering process as well as during the post-deposition annealing treatment. In this study, about 1 μm Ni₅₅Mn₂₃Ga₂₂ thin films were deposited in the range 0,45 to 1,2 Pa of Ar pressure and P = 40 to 120 W. Their composition, crystallographic structure, internal stress and stress gradient, indentation modulus, hardness, deflection induced by magnetic field and magnetic properties were systematically studied as a function of the temperature of the silicon substrate ranging from 298 to 873 K and the vacuum annealing treatment at 873 K for 21,6 ks and 36 ks. A silicon wafer having a native amorphous thin SiO_x buffer layer was used as a substrate. This substrate influences the microstructure of the films and blocks the diffusion process during the heat treatment.The crystal structure of the martensitic phase in each film was changed systematically from bct or 10 M or 14 M. In addition, the evolution of the mechanical properties such as mean stress, stress gradient, roughness, hardness and indentation modulus with the temperature (of substrate or of heat treatment) were measured and correlated to crystal structure and morphology changes.Moreover, it has been shown that it is necessary to associate a high temperature (873 K) annealing during a long time (21 ks and 36 ks) to obtain good ferromagnetic properties. Thus, for the well annealed films (36 ks at 873 K) the magnetostrain is about - 170 ppm for a magnetic field of 1 MA m^- 1 applied along the beams.As a conclusion, the response of free-standing magnetic shape memory films to a magnetic field of 0,2 MA m^- 1 depends strongly on the martensitic structure, internal mechanical stress (mean and gradient) and magnetic properties. The free-standing annealed film at 873 K for 36 ks points out a considerable magnetic actuation associated with bct or 10 M or 14 M martensitic structures.     

Structure and magnetic properties of iron oxide dispersed silver based nanocluster composite

Magnetic nanocomposites composed of iron oxide and silver were fabricated by an inert gas condensation (IGC) method combined with co-evaporation, in situ oxidation, and in situ compaction techniques. The particle sizes of composite powder were controlled by varying helium gas pressure between 1 and 10 Torr, with the smallest one being about 10 nm at 1.0 Torr. The nanostructure of the composites was characterized by TEM. The magnetization behaviors were analyzed taking into account both the paramagnetic (PM) and ferromagnetic (FM) contributions to investigate the correlation between the nanostructure and the magnetic properties. It was found that some composites exhibit the superparamagnetism evidencing magnetically isolated grains as a single domain. TEM observation assisted with EDX revealed that iron nanocluster of a few nanometers size were surrounded by silver grains. Variation of the magnetic property of the nanocluster composites was also related to nanocluster size and heat treatment in an oxygen atmosphere.     

Behavior analysis of silica in aluminum production by alumina carbothermic reduction-chlorination process in vacuum

In present work, the behavior of silica was investigated experimentally in the alumina carbothermic reduction process and chlorination process in the temperature range of 1173 K-1763 K. The phase of slags, surface morphology and composition of condensate were examined by means of XRD, SEM and EDS. The results showed that SiC was produced by SiO₂ and C in the temperature range of 1573 K-1673 K, while alumina transformed into corundum completely, then Al₄SiC₄ was produced by SiC and intermediates (Al₄C₃ and Al₄O₄C) obtained during carbothermic reduction of alumina at about 1763 K. In the chlorination process, the silicon-containing materials in the condensate were not found. Because SiO₂ consumed Al₃C₄ and Al₄O₄C, then the recovery rate of aluminum decreased. It was inferred that silica may be unfavourable for aluminum extracted from alumina by carbothermic reduction-chlorination process in vacuum.     

Electro-codeposition synthesis and room temperature ferromagnetic anisotropy of high concentration Fe-doped ZnO nanowire arrays

Fe-doped ZnO dilute magnetic semiconductor (DMS) nanowire arrays were fabricated in anodic aluminum oxide (AAO) membranes using electro-codeposition followed by long-time anneal process. The morphology, chemical composition and crystal structure were characterized by field emission scanning electron microscope (FE-SEM), high resolution transmission electron microscope (HRTEM) equipped with an energy dispersive x-ray spectrometer, and X-ray diffraction (XRD) spectroscopy. The results prove that the Fe has been successfully doped in the lattice of ZnO nanowire arrays and the estimated Fe atomic ratio is around 22%. Micro-superconducting quantum interference device (SQUID) shows that the nanowire arrays exhibit room temperature (300 K) ferromagnetic and anisotropic ferromagnetic behavior which may be a consequence of the easy magnetization direction along the wire axes and magnetostatic interaction.     

High-pressure torsion for production of magnetoresistance in Cu–Co alloy

The process of high-pressure torsion (HPT) was applied to control the size and distribution of ferromagnetic Co particles in a Cu–Co alloy. Electron probe microanalysis, X-ray diffraction analysis, and transmission electron microscopy confirmed that the Co particles were significantly refined through fragmentation and dissolved with intense straining by HPT. Magnetoresistance appeared by ~2.5% at 77 K with an isotropic feature corresponding to giant magnetoresistance (GMR). It is demonstrated that HPT is a potential process for creating GMR in the Cu–Co alloy prepared by conventional ingot metallurgy.     

Effects of pH on the chlorination process of phenols in drinking water

Toxic organic compounds detected generally in source water could combine with chlorine and contribute significantly to chlorination disinfection by-products (CDBPs). The effects of pH on species distribution of CDBPs and the kinetics of chlorination were investigated using phenol as a model of ionizable toxic organic compounds in the pH range of 6.0-9.0. It was found that five chlorination products including 2-monochlorophenol (2-MCP), 4-monochlorophenol (4-MCP), 2,6-dichlorophenol (2,6-DCP), 2,4-dichlorophenol (2,4-DCP) and 2,4,6-trichlorophenol (TCP) were produced by successive chlorination substitution. MCP (2-MCP and 4-MCP) were the dominant products and phenol partly remained in acid media, while TCP and DCP (2,6-DCP and 2,4-DCP) were the main components in neutral and alkaline media. A steady equilibrium of phenol and its chlorination products was reached in 20-30 min in acid-, neutral- and slightly alkaline media, and was delayed to 60-180 min in alkaline media. The difference in properties between phenols and phenolates, and those between HOCl and ClO(-) should be considered simultaneously in explaining the effects of pH on the chlorination process with the theory of electrophilic substitution. These results show that pH plays an important regulating role in the species distribution of CDBPs and the kinetics of chlorination for ionizable toxic organic compounds in chlorination.     

Local oxidation using scanning probe microscope for fabricating magnetic nanostructures

Local oxidation technique using atomic force microscope (AFM) was studied. The local oxidation of ferromagnetic metal thin films was successfully performed by AFM under both contact and dynamic force modes. Modification of magnetic and electrical properties of magnetic devices fabricated by the AFM oxidation was achieved. Capped oxide layers deposited on the ferromagnetic metal films are advantageous for stable oxidation due to hydrophilic surface of oxide. The oxide layer is also expected to prevent magnetic devices from degradation by oxidation of ferromagnetic metal. As for modification of magnetic property, the isolated region of CoFe layer formed by nanowires of CoFe-oxide exhibited peculiar characteristic attributed to the isolated magnetization property and pinning of domain wall during magnetization reversal. Temperature dependence of current-voltage characteristic of the planar-type tunnel junction consisting of NiFe/NiFe-oxide/NiFe indicated that the observed current was dominated by intrinsic tunneling current at the oxide barrier.     

High-efficiency separation of Ni from Cu-Ni alloy by electrorefining in choline chloride-ethylene glycol deep eutectic solvent

High-purity Cu powders (99.967%) can be obtained by the electrorefining of Cu-Ni alloy (Ni ≤ 5%) in choline chloride-ethylene glycol deep eutectic solvent (ChCl-EG DES) at near room temperature. The electrochemical separation behaviors of Cu and Ni have been investigated by linear sweep voltammetry in 0.1 M CuCl + ChCl-EG DES. The results show that the oxidation potential of Cu is more positive than that of Ni, which implies Cu and Ni can be separated by electrochemistry. And the increasing of temperature is beneficial to the decontamination of Ni from Cu-Ni alloy. Besides, the effects of current density (2–10 mA·cm⁻²) and reaction temperature (323–363 K) on the current efficiency and specific energy consumption during the electrorefining process are also analyzed, respectively. The highest current efficiency (99.89%) and the corresponding specific energy consumption (281.492 kW·h·t⁻¹) can be achieved at 10 mA·cm⁻² and 363 K. The morphologies of Cu powders change from cauliflower-like to wheat ear-like with the increasing of current density. This finding provides a theoretical guidance for the separation of Ni from Cu-Ni alloy by an eco-friendly and facile electrorefining in ChCl-EG DES.     

Oxidation of chlorinated ethenes by potassium permanganate: a kinetics study

The kinetics of oxidation of perchloroethylene (PCE), trichloroethylene (TCE), three isomers of dichloroethylene (DCE) and vinyl chloride (VC) by potassium permanganate (KMnO(4)) were studied in phosphate-buffered solutions of pH 7 and ionic strength approximately 0.05 M and under isothermal, completely mixed and zero headspace conditions. Experimental results have shown that the reaction appears to be second order overall and first order individually with respect to both KMnO(4) and all chlorinated ethenes (CEs), except VC. The degradation of VC by KMnO(4) is a two-consecutive-step process. The second step, being the rate-limiting step, is of first order in VC and has an activation energy (E(a)) of 7.9+/-1 kcal mol(-1). The second order rate constants at 20 degrees C are 0.035+/-0.004 M(-1) s(-1) (PCE), 0.80+/-0.12 M(-1) s(-1) (TCE), 1.52+/-0.05 M(-1) s(-1) (cis-DCE), 2.1+/-0.2 M(-1) s(-1) (1,1-DCE) and 48.6+/-0.9 M(-1) s(-1) (trans-DCE). The E(a) and entropy (DeltaS(*)) of the reaction between KMnO(4) and CEs (except VC) are in the range of 5.8-9.3 kcal mol(-1) and -33 to -36 kcal mol(-1) K(-1), respectively. Moreover, KMnO(4) is able to completely dechlorinate CEs, and the increase in acidity of the solution due to CE oxidation by KMnO(4) is directly proportional to the number of chlorine atoms in CEs.     

Substrate Effects on the High-Temperature Oxidation Behavior of Thermal Barrier Coatings

The high-temperature oxidation behaviors of the NiCrAlYSi/P-YSZ thermal barrier coatings (TBCs) produced by electron beam-physical vapor deposition (EB-PVD) on directionally solidified (DS) and single crystalline (SC) Ni-based superalloy substrates were investigated. The cross-sectional microstructure investigation, isothermal and cyclic oxidation tests were conducted for the comparison of oxidation behaviors of TBCs on different substrates. Although TBC on DS substrate has a relatively higher oxidation rat...     

Study of magnetic and magnetocaloric properties of La0.6Pr0.1Ba0.3MnO3 and La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 perovskite-type manganese oxides

The compositional dependence of the magnetic and magnetocaloric properties of La_0.6Pr_0.1Ba_0.3MnO₃ (LPBMO) and La_0.6Pr_0.1Ba_0.3Mn_0.9Fe_0.1O₃ (LPBMFO) were investigated. Polycrystalline samples were prepared by the standard solid-state reaction method. Temperature-dependent magnetization measurements and Arrott analysis reveal second-order ferromagnetic transitions in both samples with Curie temperature increasing with doping iron from 94 K for LPBMO to 277 K for LPBMFO. Magnetic entropy change \( | {\Delta S_{\text{M}} } | \) was calculated by applying the thermodynamic Maxwell equation to a series of isothermal field-dependent magnetization curves. However, the analysis of the magnetocaloric effect (MCE) using Landau theory of phase transition shows that the contributions to the free energy from the presence of ferromagnetic clusters are strongly influencing the MCE by coupling with the order parameter around the Curie temperature.     

Influence of Na Addition on Magnetic and Magnetocaloric Effects of La<Subscript>0.67</Subscript>Pb<Subscript>0.13</Subscript>Na<Subscript>0.2</Subscript>MnO<Subscript>3</Subscript> Ceramics

Structural, magnetic, magnetocaloric, and electrical properties are reported for mixed-valence manganite La_0.67Pb_0.13Na_0.2MnO₃. X-ray diffraction reveals that the sample crystallizes in the rhombohedric structure with the R-3c space group. The magnetic properties of the polycrystalline La_0.67Pb_0.13Na_0.2MnO₃ compound are discussed in detail, based on the susceptibility, magnetization, and isotherm. The sample presents a ferromagnetic property with T _C= 275 K and a Griffiths phase at T _G= 325 K which gives the existence of ferromagnetic clusters in the paramagnetic domain. A large deviation is usually observed between field cooled (FC) and zero field cooled (ZFC). M(T) is a low temperature below the blocking temperature. At 40 K, a spin-glass or a cluster-glass state is seen to arise from a ferromagnetic state. This is caused by the competition between the antiferromagnetic and ferromagnetic interactions. The electrical properties show the presence of a metal–semiconductor transition at T _M?Sc. To understand the dependence of disorder with the transport mechanism, we used the phenomenological equation for resistivity under a percolation approach, which is dependent on the phase segregation of a paramagnetic semiconductor and ferromagnetic metallic regions.     

Enhanced oxidation of estrone at multi-wall carbon nanotubes film electrode: direct evidence for the advantage of carbon nanotubes over other carbonaceous materials

The electrochemical behaviors of estrone at various carbonaceous nanomaterials-dihexadecyl hydrogen phosphate (DHP) composite films coated glassy carbon electrodes were investigated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronocoulometry (CC) and electrochemical impedance spectroscopy (EIS). Comparing with the composite films of acetylene black (AB) and carbon nanofiber (CNF), multi-wall carbon nanotubes (MWCNTs)-DHP film showed the best electrochemical performance towards the oxidation of estrone, reflected by the significantly enhanced oxidation current in the voltammograms as well as the apparently reduced charge transfer resistance in EIS. Studies on the active surface area, the surface coverage and EIS suggested that the apparently improved electrochemical responses of estrone at MWCNTs should arise from their large surface area, good conductivity and the ability to facilitate the charge transfer process. The reaction mechanisms of estrone oxidation at the three composite films were also discussed, which were expected to follow a process involved the total loss of two electrons and two protons.     

氢化锆在高温水蒸气中的氧化行为

通过恒温恒压氧化实验研究了氢化锆在300~700℃高温水蒸气中的氧化行为。结果表明,氢化锆的质量增重随着氧化温度的升高而增大。在氧化过程进行15h以后,OH-/O在氧化膜中的内扩散成为氧化反应的控速步骤。水蒸气中的H抑制了氢化锆在高温氧化条件下的氢损失。氧化膜主要由单斜相M-ZrO2组成,氧化膜的最外层由四方相T-ZrO2和立方相C-ZrO2组成。     

Magnetism induced by nonmagnetic dopant in Li<Subscript>2</Subscript>O,Na<Subscript>2</Subscript>O,K<Subscript>2</Subscript>O and Rb<Subscript>2</Subscript>O: first-principles calculations

First-principles calculations based on the tight-binding linear muffin-tin orbital (TB-LMTO) method are performed to investigate the occurrence of spin polarization in the alkali-metal oxides (M₂O) [M: Li, Na, K, Rb] in antifluorite (anti-CaF₂-type) structure with non-magnetic sp (F, Cl, Br and I) dopants. The calculations reveal that non-magnetic substitutional doping at cation site can induce stable half-metallic ferromagnetic ground state in I₂-VI compounds. Total energy calculations show that the antifluorite ferromagnetic state is energetically more stable than the antifluorite non-magnetic state at equilibrium volume. Ground state properties such as equilibrium lattice constant and bulk modulus were calculated. The calculated magnetic moment is found to be 2.00 μB per dopant atom.