Structures and superparamagnetic properties of overaged Fe–Al–Mn–C alloys

Microstructures and superparamagnetic properties in aged-hardened Fe–9%Al–30%Mn– (x)C,Si alloys, resulting from overaging at a temperature of 823 K for 48 h to 313 days, have been investigated by transmission electron microscopy (TEM), X-ray diffraction patterns, and vibrating sample magnetometry (VSM). The results reveal that the precipitate κ-phase [(Fe,Mn)₃AlC] decomposition in this alloy, overaged at 823 K for one week, resulted from two separate mechanisms: (1) wetting of the antiphase boundary segment (APBs) of D0₃ [(Fe/Mn)₃Al] domains by the B2 [(Fe/Mn)Al] phase; and (2) precipitation of the B2 [(Fe/Mn)Al] phase within the domain. A superparamagnetic behaviour was discovered when the alloy was overaged at 823 K for ≈120–313 days. The super-soft magnetic property was mainly attributable to the ferromagnetic spinel-ordered (B2 [(Fe/Mn)Al]+D0₃ [(Fe/Mn)₃Al]) phases and ordered B2 with monoclinic ′Mn structures.     

Spectroscopic and dielectric studies on MnO doped PbO–Nb2O5–P2O5 glass system

PbO–Nb₂O₅–P₂O₅ glasses containing different concentrations of MnO ranging from 0 to 2.5 mol% were prepared. A number of studies viz., differential thermal analysis, infrared, optical absorption, luminescence, Raman and ESR spectra, magnetic susceptibility and dielectric properties (constant ′, loss tan δ, ac conductivity σ_ac over a range of frequency and temperature) of these glasses have been carried out. The results have been analyzed in the light of different oxidation states of manganese ions. The analysis indicates that when the concentration of MnO is around 1.0 mol%, manganese ions mostly exist in Mn²⁺ state, occupy network forming positions with MnO₄ structural units and increase the rigidity of the glass network. When MnO is present in higher concentrations, these ions seem to exist mostly in Mn³⁺ state and occupy modifying positions.     

Characterization and hydrogenation of CeNi5−xCrx (x = 0, 1, 2) alloys

The effect of partial substitution of Ni by Cr in CeNi₅ intermetallic compound has been studied by pressure–composition isotherm measurements for different temperatures. The samples were prepared of high purity materials using the standard arc melting technique in argon atmosphere. The structure and the elemental composition of different alloys have been investigated by means of XRD, SEM and EDX techniques. The unit cell volume of the alloy was found to increase with increasing Cr content. In order to calculate the hydrogen storage capacity pressure–composition isotherm has been investigated for CeNi_5−xCr_x (x = 1, 2) alloys in the temperature and pressure ranges of 293 ≤ T ≤ 333 K and 0.5 ≤ P ≤ 35 bar, respectively. The P–C–T isotherm for different alloys clearly shows the presence of three regions ,  + β and β. The enthalpy and entropy for the systems has also been calculated using Van’t Hoff plot. The variation of enthalpy and entropy with hydrogen content has also been studied.     

Microstructure and thermoelectric properties of n-type Bi2Te2.85Se0.15 prepared by mechanical alloying and plasma activated sintering

Starting from elemental bismuth, tellurium and selenium powders, n-type Bi₂Te_2.85Se_0.15 solid solution with fine microstructure was prepared by mechanical alloying (MA) and plasma activated sintering (PAS) in the present work. The effect of PAS process on microstructure and thermoelectric properties of the sintered samples was investigated. The sintering temperature of PAS process (683 K) was 80–100 K lower than that of conventional hot pressing and the whole PAS process was also greatly shortened to about 30 min. A preferentially orientated microstructure with the basal planes (0 0 l) perpendicular to pressing direction was formed in the PASed sample and the maximum figure of merit (Z) at room temperature was 1.80 × 10⁻³ K⁻¹.     

Induced crystallization and physical properties of Li2O–CaF2–P2O5:TiO2 glass system: Part II. Electrical, magnetic and optical properties

The results of various physical properties namely, dielectric properties (dielectric constant, loss tan δ, ac conductivity σ, over a wide range of frequency and temperature and dielectric breakdown strength in air medium at room temperature), optical absorption, electron spin resonance (ESR) at liquid nitrogen temperature and magnetic susceptibility at room temperature of Li₂O–CaF₂–P₂O₅:TiO₂ glass-ceramics have been reported. The optical absorption and magnetic susceptibility studies indicated that the titanium ions exist in Ti³⁺ state in addition to Ti⁴⁺ state in these samples. However, the reduction seems to be the lowest in the sample containing 0.6 mol% of TiO_2. The dielectric constant and loss variation with the concentration of TiO₂ have been explained on the basis of space charge polarization mechanism. The dielectric relaxation effects exhibited by these samples have been analyzed by a pseudo Cole–Cole plot method and the spreading of dielectric relaxation has been observed. The ac conductivity in the high temperature region seems to be related both with electronic and ionic movements. The low temperature (or the nearly temperature independent) part of conductivity could be explained on the basis of quantum mechanical tunneling model. The studies on dielectric breakdown strength indicated the highest insulating strength for the sample containing 0.6 mol% of TiO₂.     

Thermoelectric properties of A-site doped perovskites (Sr0.6Ba0.4)1−xMxPbO3 (M = La, K)

La- and K-doped perovskite-type ceramics, (Sr_0.6Ba_0.4)_1−xLa_xPbO₃ with x = 0.0−0.1 and (Sr_0.6Ba_0.4)_1−xK_xPbO₃ with x = 0.00−0.15, were prepared to modify thermoelectric properties of semi-metallic Sr_0.6Ba_0.4PbO₃ via the doping of electrons and holes, respectively. The electrical conductivity σ and Seebeck coefficient S for the ceramics were measured at temperatures of 373–1073 K in air. With the La doping, electron carriers were successively doped and the material changed from a semi-metal for the undoped Sr_0.6Ba_0.4PbO₃ to a metal for the (Sr_0.6Ba_0.4)_0.9La_0.1PbO₃. With the K doping, the thermoelectric properties were essentially unchanged probably due to the carrier compensation effect by the generation of oxygen deficiencies. The thermoelectric power factor S²σ was maximized to a value of 3.1 × 10⁻⁴ Wm⁻¹ K⁻² at 773 K for the undoped Sr_0.6Ba_0.4PbO₃ ceramic.     

Power factor of La1−xSrxFeO3 and LaFe1−yNiyO3

The electrical conductivity (σ), Seebeck coefficient (S), and power factor (σS²) of perovskite-type LaFeO₃, La_1−xSr_xFeO₃ [0.1 ≤ x ≤ 0.4] and LaFe_1−yNi_yO₃ [0.1 ≤ y ≤ 0.6] were investigated in the temperature range of 300–1100 K to explore their possibility as thermoelectric materials. The electrical conductivity of LaFeO₃ showed semiconducting behavior, and its Seebeck coefficient changed from positive to negative around 650 K with increasing temperature. The electrical conductivity of LaFeO₃ increased with the substitutions of Sr and Ni atoms, while its Seebeck coefficient decreased. The Seebeck coefficient of La_1−xSr_xFeO₃ was positive, whereas that of LaFe_1−yNi_yO₃ changed from positive to negative with increasing Ni content. The substitutions of Sr and Ni were effective in increasing the power factor of LaFeO₃; 0.0053 × 10⁻⁴ Wm⁻¹ K⁻² for LaFeO₃ (1050 K), 1.1 × 10⁻⁴ Wm⁻¹ K⁻² for La_1−xSr_xFeO₃ (x = 0.1 at 1100 K) and 0.63 × 10⁻⁴ Wm⁻¹ K⁻² for LaFe_1−yNi_yO₃ (y = 0.1 at 1100 K).     

Another unusual phenomenon for Zr7Ni10: structural change in hydrogen solid solution and its conditions

Zr₇Ni₁₀ has three hydrogen occlusion phases, , β and γ, and the following unusual features are known for the phase transitions in the Zr₇Ni₁₀–H₂ system: (1) The intermediate hydride phase (β) appears only during dehydrogenation but not during hydrogenation, and (2) The continuous hydrogen solid solution phase () exhibits a much higher hydrogen solubility during hydrogenation than during dehydrogenation. In order to clarify the mechanism about the difference in the hydrogen solubility of the phase, the relation between the pressure-composition isotherms and corresponding structural change has been examined by a conventional volumetric method and X-ray diffraction. Through the examination, we discovered that the crystal structure of the phase, which undergoes hydrogenation followed by dehydrogenation, is different from that of its pure metal phase, where the crystal structure of the dehydrogenated phase changes from an orthorhombic structure to a tetragonal structure. The conditions causing the structural change were then examined, and it has been found that the phase maintains its original orthorhombic structure as long as it is hydrogenated so as not to absorb enough hydrogen to change it to the hydride with a higher hydrogen content (γ). The phenomenon can be understood as one of the hydrogen-assisted phase transitions such as hydrogen-induced amorphization (HIA) in the sense that the phase transition requires hydrogenation under special conditions.     

The ternary system Na2O–ZnO–WO3: Compounds and phase relationships

The subsolidus phase relationships of ternary system Na₂O–ZnO–WO₃ have been investigated by X-ray diffraction (XRD) and differential thermal analyzer (DTA). All the samples were synthesized in the temperature range from 530 to 850 °C in air. There are one ternary compound and five binary compounds in the Na₂O–ZnO–WO₃ system, which can be divided into eight three-phase regions. The crystal structure of the ternary compound Na_3.6Zn_1.2(WO₄)₃ is determined by single-crystal structure analysis method. It belongs to triclinic system with space group and lattice constants a = 7.237 (5) Å, b = 9.172 (6) Å, c = 9.339 (6) Å and  = 94.920 (4)°, β = 105.772 (9)°, γ = 103.531 (8)°, Z = 2. DTA analyses indicate that the compound Na₂WO₄ is not suitable to be the flux for ZnO crystal growth below 1250 °C, since no liquidus was observed in the system before 1250 °C.     

Electrical, magnetic, thermal and thermoelectric properties of the “Bergman phase” Mg32(Al,Zn)49 complex metallic alloy

The Mg–Al–Zn system of intermetallics contains an exceptional crystalline phase Mg₃₂(Al,Zn)₄₉, named the Bergman phase, whose crystal structure is based on a periodic arrangement of icosahedral Bergman clusters within the giant-unit-cell, so that periodic and quasiperiodic atomic orders compete in determining the physical properties of the material. We have investigated electrical, magnetic, thermal and thermoelectric properties of a monocrystalline Bergman phase sample of composition Mg_29.4(Al,Zn)_51.6, grown by the Bridgman technique. Electrical resistivity is in the range ρ ≈ 40 μΩ cm and exhibits positive-temperature-coefficient with T² dependence at low temperatures and T at higher temperatures, resembling non-magnetic amorphous alloys. Magnetic susceptibility χ measurements revealed that the sample is a Pauli paramagnet with a significant Landau diamagnetic orbital contribution. The susceptibility exhibits a weak increase towards higher temperature. Combined analysis of the ρ(T) and χ(T), together with the independent determination of the Pauli susceptibility via the NMR Knight shift suggests that the observed temperature dependence originates from the mean-free-path effect on the orbital susceptibility. The electronic density of states (DOS) at the Fermi energy E_F was estimated by NMR and was found to amount 72% of the DOS of the fcc Al metal, with no evidence on the existence of a pseudogap. Thermal conductivity contains electronic, Debye and hopping of localized vibrations terms, whereas thermopower is small and negative. High structural complexity of the Bergman phase does not result in high complexity of its electronic structure.     

Low temperature preparation and transport properties of ternary Pb–Bi–Te alloy

Polycrystalline samples of the ternary compound Pb₂Bi₄Te₅ were prepared by a solvothermal process based on the reaction between BiCl₃, Pb(NO₃)₂, Te, and KBH₄ in N,N-dimethylformamide (DMF), and followed by a post heat-treatment of compacted pellets. The microstructures of samples were characterized using XRD and SEM. The thermoelectric properties were determined by measuring the electric conductivity and the Seebeck coefficient. These samples show a disordered layered structure, with a maximum Seebeck coefficient of 84 μV/K around 520 K.     

Austenite phase formation in rapidly solidified Fe–Cr–Mn–C steels

Steels having compositions (wt%) 0.05–0.5C, 12.5–20Cr, 8–25Mn and 0–0.51N have been chill-block melt-spun to ribbons in order to investigate systematically, by X-ray diffractometry and electron microscopy, the effects of rapid solidification and of solute concentrations on the formation of the austenite phase. The austenite is most easily formed at (wt%) 16Cr–8Mn for 0.3C ribbons while ′-martensite or -martensite was observed at lower concentrations of Cr or Mn and -ferrite appeared for Cr >18 wt%. The volume fraction of austenite in the steel ribbons studied was found, by multiple regression analysis, to obey the equation

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. Thus, the effect of Mn on γ formation followed a non-linear function, containing an interaction term including the Cr and Mn contents, and first- and second-order terms involving the Mn concentration. This indicates the ranges over which Mn is a γ-former or an -former. Iso-austenitic lines, constructed on the basis of this new equation, are nearly orthogonal to those in the Schaeffler diagram for Cr–Mn steels so that use of the latter for prediction of the austenite content in the present case would be inappropriate.     

Influence of A-site cation size-disorder on structural, magnetic and magnetocaloric properties of La0.7Ca0.3−xKxMnO3 compounds

The structural and magnetic properties of perovskite oxides La_0.7Ca_0.3−xK_xMnO₃ (0 ≤ x ≤ 0.15) have been investigated to explore the influence of the A-site cation size-disorder (σ²). The materials were prepared by the solid-state method and then characterized by X-ray diffraction (XRD). The XRD data have been analyzed by Rietveld refinement technique. For K doping concentration x ≤ 0.075, the samples crystallize in the orthorhombic structure, while for x ≥ 0.1, the structure becomes rhombohedral. The variation of the magnetization M as a function of the applied magnetic field μ₀H reveals the presence of a structural distortion leading to a reduction of the magnetization at low μ₀H values. When increasing μ₀H, the structural distortion decreases and for a high applied magnetic field, the M (μ₀H) curves saturate indicating the disappearance of the structural distortion. The influence of K doping concentration and the applied magnetic field on the magnetocaloric properties has been considered. A large magnetic-entropy change (|ΔS_M|  5 J/kg K) is obtained in all samples at Curie temperatures between 270 and 280 K for an applied magnetic field of 3 T. These results show that these materials can be used as candidates for magnetic refrigerants near room temperature.     

The ternary system: silicon–titanium–uranium

Phase equilibria in the system Si–Ti–U were established at 1000 °C by optical microscopy, EMPA and X-ray diffraction. Two ternary compounds were observed and were characterised by X-ray powder data refinement: (1) stoichiometric U₂Ti₃Si₄ (U₂Mo₃Si₄-type) with a small homogeneity region of about 3 at.% exchange U/Ti and (2) U_2−xTi_3+xSi₄ (Zr₅Si₄-type) extending at 1000 °C for 0.7<x<1.3. Mutual solubility of U-silicides and Ti-silicides was found to be below about 1 at.%. The Ti,U-rich part of the diagram was also investigated at 850 °C establishing the tie-lines to the low temperature compounds U₂Ti and U₃Si. U₂Ti₃Si₄ is weakly paramagnetic following a Curie–Weiss law above 50 K with μ_eff.=2.67 μ_B/U, Θ_P=−150 K and χ₀=1.45×10⁻³ emu/mol (18.2×10⁻⁹ m³/mol).     

Synthesis and magnetic structure of the YbMn2Sb2 compound

A neutron diffraction investigation has been carried out on the trigonal La₂O₃-type (hP5, space group , No. 164; also CaAl₂Si₂-type) YbMn₂Sb₂ intermetallic. A two-step synthesis route has been tried in this work, and successfully utilised to prepare single phase samples of this compound. This study shows that YbMn₂Sb₂ presents antiferromagnetic ordering below 120 K. The magnetic structure of this intermetallic consists of antiferromagnetically coupled magnetic moments of the manganese atoms, in the Mn1 (1/3, 2/3, Z_Mn) and Mn2 (2/3, 1/3, 1 − Z_Mn) sites; the direction of magnetic moments of manganese atoms forming a φ and a θ angle, respectively with the X- and the Z-axis. At 4 K the magnetic moment of the Mn1 atom is μ_Mn = 3.6(1) μ_B, with φ = 0° and θ = 62(4)°, whilst the Mn2 atom has a magnetic moment μ_Mn = 3.6(1) μ_B, with φ = 0° and θ = 242(4)°. On the other hand, in this compound no local moment was detected on the Yb site.     

Influence of plastic deformation on the velocity and ultrasonic attenuation of copper single crystals

High-purity copper single crystal samples were plastically deformed between 0.02% and 1% in the direction. The absolute ultrasonic velocity and attenuation related to the dislocations in copper single crystal were measured as a function of the plastic deformation. The amplitude-independent Swing Model for dislocation resonance was considered to obtain information about the evolution of the dislocation density (Λ) with the degree of plastic deformation (%). The experimental data were interpreted as resulting from the contribution of two kinds of dislocations that lie in a cell structure. The results show a linear increase with the deformation of the density of dislocations (Λ_I) that lies in the cell walls [Λ_I (cm⁻²)≈1.5×10⁹ %]. The dislocation density inside the cell (Λ_II) is about two orders of magnitude lower than Λ_I.     

Effect of ZnO additive on sintering behavior and microwave dielectric properties of 0.95MgTiO3–0.05CaTiO3 ceramics

The effects of ZnO additive on the microstructures, the phase formation and the microwave dielectric properties of MgTiO₃–CaTiO₃ ceramics were investigated. The sintering temperature of ZnO-doped 0.95MgTiO₃–0.05CaTiO₃ ceramics can be lowered to 1300 °C due to the liquid phase effect. Formation of second phase MgTi₂O₅ can be effectively restrained through the addition of ZnO. The microwave dielectric properties are found strongly correlated with the sintering temperature as well as the amount of ZnO addition. At 1300 °C, 0.95MgTiO₃–0.05CaTiO₃ ceramics with 1 wt% ZnO addition possesses a dielectric constant _r of 20, a Q × f value of 65,000 (at 7 GHz) and a τ_f value of −5.8 ppm/°C. In comparison with pure 0.95MgTiO₃–0.05CaTiO₃ ceramics, the doped sample shows not only a 16% loss reduction but also a lower sintering temperature. That makes it a very promising material to replace the present one for GPS patch antennas.     

Theory of the nonlinear internal friction associated with bulk diffusion of solute atoms around dislocations in Al–Mg alloys

In this paper the nonlinear (amplitude-dependent) internal friction (P₃ peak) in cold-worked Al–Mg alloys is theoretically studied by solving the bulk diffusion equations of the solute atoms (Mg atoms) under the action of dislocation drag. The results in the case of a constant external stress show that the bow-out distance of the dislocation has an exponential relation with time, which can be well described by an exponential creep function with a Gaussian distribution in τ. With the increasing strain amplitude, the relaxation strength Δ and relaxation time τ decrease, while the distribution parameter increases. Both activation energy H and pre-exponential factor τ₀ deduced from τ through Arrhenius relation are dependent on strain amplitude.     

Self-propagating high-temperature synthesis of La(Sr)Ga(Mg)O3−δ for electrolyte of solid oxide fuel cells

This paper describes self-propagating high-temperature synthesis (SHS) of an electrolyte for solid oxide fuel (SOFC), in comparison to a conventional solid-state reaction method (SRM). Doped-lanthanum gallate: La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM9182) and LSGM9173 as the SOFC electrolyte, was prepared by the SHS and sintered at different temperatures, for measuring the electrical conductivity of the sintered LSGM and the power generating performance at 1073 K, in comparison to the SRM. In the SHS, the LSGM powders with smaller size were obtained and easily sintered at the 100 K-lower temperature, 1673 K, than in the SRM. Most significantly, the electrical conductivity of the sintered LSGM9182 was as high as 0.11 S cm⁻¹ and its maximum power density was a value of 245 mW cm⁻² in the cell configuration of Ni/LSGM9182 (0.501 mm in thickness)/Sm_0.5Sr_0.5CoO₃. The conclusion was that the proposed SHS-sintering method with many benefits of minimizing the energy requirement and the processing time in the production, easing temperature restriction for the sintering, and improving the electrolyte performance up to a conventional level is practicable for producing the LSGM-electrolyte of SOFC at an intermediate-temperature application.