Synthesis of alumina supported nickel nanoparticle catalysts and evaluation of nickel metal dispersions by temperature programmed desorption

The purpose of this study was to synthesize highly dispersed Ni/Al₂O₃ catalysts and to develop a suitable hydrogen-temperature programmed desorption (H₂-TPD) method for the determination of nickel metal surface area, dispersion, and crystallite sizes. Several highly dispersed Ni/Al₂O₃ catalysts with a Ni loading between 15 and 25 wt.% were synthesized. The reducibility of catalysts was determined by temperature programmed reduction (TPR) experiments. All catalysts exhibited a single reduction peak with a maximum rate of H₂ consumption (T_max in TPR) occurring below 450 °C. Three different H₂-TPD methods were employed to determine the amount of H₂ chemisorbed. In TPD-1, a 10% H₂/Ar mixture was used for catalyst pre-reduction and surface saturation by cooling down from T_max in TPR to room temperature. In TPD-2, the catalyst surface after pre-reduction was flushed with Ar at T_max in TPR + 10 °C. The TPD-3 was similar to the TPD-2, but used 100% H₂ instead of 10% H₂/Ar mixture. In all three TPD methods, the profiles exhibited 2 domains of H₂ desorption peaks, one below 450 °C, referred to as type-1 peaks, and attributed to H₂ desorbed from exposed fraction of Ni atoms, and the other above 450 °C, denoted as type-2 peaks, and assigned to the desorption of H₂ located in the subsurface layers and/or to spillover H₂. Flushing the reduced catalyst surface in Ar at T_max in TPR + 10 °C in TPD-2 and TPD-3 removed most of the H₂ located in the subsurface layers/ spillover H₂. The amount of H₂ chemisorbed to form a monolayer on the reduced Ni/Al₂O₃ catalysts was determined quantitatively from the TPD peak areas of type-1 peaks in TPD-1, and from both type-1 and type-2 peaks in TPD-2 and TPD-3. The Ni metal surface area, dispersions and crystallite sizes were calculated from the chemisorption data and the values were compared with those obtained using the static chemisorption method. Both TPD-2 and TPD-3 gave chemisorption results similar to that obtained from the static method.     

Self-induced polarization rotation of laser beam in fullerene (C70) solutions

Nonlinear self-rotation of elliptically polarized laser pulses (λ = 532 nm, τ_FWHM ~ 12 ns) in toluene, benzene and binary mixture (toluene + ethanol) solutions of fullerene C₇₀ has been investigated experimentally. Absolute values and signs of the nonlinear refractive indices (n₂) and nonlinear optical susceptibilities χ⁽³⁾(ω, ? ω, ω) of C₇₀ solutions in toluene and benzene at different values of polarization ellipse (θ = 0.2 ÷ 0.8) have been determined. High-resolution transmission electron microscope studies of C₇₀ solutions showed that in toluene + ethanol mixtures ball-shaped C₇₀ clusters are formed with particle sizes in the range ~ 100 ÷ 500 nm. It has been demonstrated, that the clusters sizes depend on the C₇₀ concentration and volume fraction of ethanol in toluene. Correlation between the processes of C₇₀ clusters formation in solutions and the values of polarization self-rotation angle of transmitted laser beam has been demonstrated. Physical mechanisms of laser induced optical activity in fullerene solutions have been discussed.     

Carbon doping of MgB2 by toluene and malic-acid-in-toluene

The decomposition of malic acid (C₄H₆O₅) in the presence of Mg and B was studied using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) which revealed that malic acid reacted with Mg but not B. Also, the addition of toluene (C₇H₈) to dissolve malic acid followed by subsequent drying resulted in no reaction with Mg, indicating that the malic acid had decomposed during the dissolution/drying stage. The total carbon contributed by toluene versus a toluene/5 wt.% malic acid mixture was measured using a LECO CS600 carbon analyzer. The toluene sample contained ～0.4 wt.% C while the toluene/malic acid mixture had ～1.5 wt.% C, demonstrating that the toluene contributed a significant amount of carbon to the final product. Resistivity measurements on powder-in-tube MgB₂ monofilamentary wires established that the toluene/malic acid doped sample had the highest B_c2. However, the toluene-only sample had the highest transport J_c over most of the magnetic field range (0–9 T), equaled only by that of toluene/malic-acid sample in fields above 9 T.     

On the rate constants of OH + HO2 and HO2 + HO2: A comprehensive study of H2O2 thermal decomposition using multi-species laser absorption

Hydrogen peroxide (H₂O₂) and hydroperoxy (HO₂) reactions present in the H₂O₂ thermal decomposition system are important in combustion kinetics. H₂O₂ thermal decomposition has been studied behind reflected shock waves using H₂O and OH diagnostics in previous studies (Hong et al. (2009) [9] and Hong et al. (2010) [6,8]) to determine the rate constants of two major reactions: H₂O₂ + M → 2OH + M (k₁) and OH + H₂O₂ → H₂O + HO₂ (k₂). With the addition of a third diagnostic for HO₂ at 227 nm, the H₂O₂ thermal decomposition system can be comprehensively characterized for the first time. Specifically, the rate constants of two remaining major reactions in the system, OH + HO₂ → H₂O + O₂ (k₃) and HO₂ + HO₂ → H₂O₂ + O₂ (k₄) can be determined with high-fidelity.No strong temperature dependency was found between 1072 and 1283 K for the rate constant of OH + HO₂ → H₂O + O₂, which can be expressed by the combination of two Arrhenius forms: k₃ = 7.0 × 10¹² exp(550/T) + 4.5 × 10¹⁴ exp(?5500/T) [cm³ mol^?1 s^?1]. The rate constants of reaction HO₂ + HO₂ → H₂O₂ + O₂ determined agree very well with those reported by Kappel et al. (2002) [5]; the recommendation therefore remains unchanged: k₄ = 1.0 × 10¹⁴ exp(?5556/T) + 1.9 × 10¹¹⁺exp(709/T) [cm³ mol^?1 s^?1]. All the tests were performed near 1.7 atm.     

Interface Mn nanoclusters in YMnO3/Si ferroelectric gate structures revealed by electron magnetic resonance

In this work, systematic X-band electron magnetic resonance (EMR) studies for YMnO₃/Si ferroelectric gate structures were performed to trace a variation of interface characteristics as different sputtering condition of O₂/(Ar + O₂) ratio. Our result showed that the EMR signal intensities were increased with increasing O₂/(Ar + O₂) ratio. In addition, it was suggested from detailed analyses that the observed EMR signals could be originated from Mn nanoclusters existing in both the polycrystalline Y₂O₃ layer and the amorphous Si-enriched Y–Si interface layer in YMnO₃/Si thin film structure. And also, a correlation between the decrease of crystallinity in YMnO₃/Si film and the content of Mn nanoclusters within the polycrystalline Y₂O₃ layer and/or the amorphous Y–Si layer was discussed.     

Influence of fabrication processing on phase transition and electrical properties of 0.8Pb(Zr1/2Ti1/2)O3–0.2Pb(Ni1/3Nb2/3)O3 ceramics

The binary system of 0.8Pb(Zr_1/2Ti_1/2)O₃–0.2Pb(Ni_1/3Nb_2/3)O₃ ceramics were synthesized by conventional mixed oxide and columbite method. X-ray diffraction analysis demonstrated the coexistence of both the rhombohedral and tetragonal phases for the columbite prepared sample. Rhombohedral to tetragonal phase transition for columbite method was different compared with those of the mixed oxide method. The permittivity shows a shoulder at the rhombohedral to tetragonal phase transition temperature T_Rho–Tetra = 195 °C, and then a maximum permittivity (36,000 at 10 kHz) at the transition temperature T_m = 277 °C on ceramics prepared with the columbite method. However, piezoelectric coefficient (d₃₃) was measured to be 282 pC/N for the conventional method and higher than the columbite method. The results were related to the phase compositions and porosity of the ceramics.     

Shock tube/laser absorption studies of the decomposition of methyl formate

Reaction rate coefficients for the major high-temperature methyl formate (MF, CH₃OCHO) decomposition pathways, MF → CH₃OH + CO (1), MF → CH₂O + CH₂O (2), and MF → CH₄ + CO₂ (3), were directly measured in a shock tube using laser absorption of CO (4.6 μm), CH₂O (306 nm) and CH₄ (3.4 μm). Experimental conditions ranged from 1202 to 1607 K and 1.36 to 1.72 atm, with mixtures varying in initial fuel concentration from 0.1% to 3% MF diluted in argon. The decomposition rate coefficients were determined by monitoring the formation rate of each target species immediately behind the reflected shock waves and modeling the species time-histories with a detailed kinetic mechanism [12]. The three measured rate coefficients can be well-described using two-parameter Arrhenius expressions over the temperature range in the present study: k₁ = 1.1 × 10¹³ exp(?29556/T, K) s^?1, k₂ = 2.6 × 10¹² exp(?32052/T, K) s^?1, and k₃ = 4.4 × 10¹¹ exp(?29 078/T, K) s^?1, all thought to be near their high-pressure limits. Uncertainties in the k₁, k₂ and k₃ measurements were estimated to be ±25%, ±35%, and ±40%, respectively. We believe that these are the first direct high-temperature rate measurements for MF decomposition and all are in excellent agreement with the Dooley et al. [12] mechanism. In addition, by also monitoring methanol (CH₃OH) and MF concentration histories using a tunable CO₂ gas laser operating at 9.67 and 9.23 μm, respectively, all the major oxygen-carrying molecules were quantitatively detected in the reaction system. An oxygen balance analysis during MF decomposition shows that the multi-wavelength laser absorption strategy used in this study was able to track more than 97% of the initial oxygen atoms in the fuel.     

Polymorphic phase transition and thermal stability in squaric acid (H2C4O4)

Phase transformations in squaric acid (H₂C₄O₄) have been investigated by thermogravimetry and differential scanning calorimetry with different heating rates β. The mass loss in TG apparently begins at onset temperatures T_di=245±5 °C (β=5 °C min^?1), 262±5 °C (β=10 °C min^?1), and 275±5 °C (β=20 °C min^?1). A polymorphic phase transition was recognized as a weak endothermic peak in DSC around 101 °C (T_c⁺). Further heating with β=10 °C min^?1 in DSC revealed deviation of the baseline around 310 °C (T_i), and a large unusual exothermic peak around 355 °C (T_p), which are interpreted as an onset and a peak temperature of thermal decomposition, respectively. The activation energy of the thermal decomposition was obtained by employing relevant models. Thermal decomposition was recognized as a carbonization process, resulting in amorphous carbon.     

Superconductivity and thermal properties of sulphur doped FeTe with effect of oxygen post annealing

Here, we report the synthesis and characterization of sulphur-substituted iron telluride i.e. FeTe_1?xS_x; (x = 0–30 %) system and study the impact of low temperature oxygen (O₂) annealing as well. Rietveld analysis of room temperature X-ray diffraction (XRD) patterns shows that all the compounds are crystallized in a tetragonal structure (space group P4/nmm) and no secondary phases are observed. Lattice constants are decreased with increasing S concentration. The parent compound of the system i.e. FeTe does not exhibit superconductivity but shows an anomaly in the resistivity measurement at around 78 K, which corresponds to a structural phase transition. Heat capacity C_p(T) measurement also confirms the structural phase transition of FeTe compound. Superconductivity appears by S substitution; the onset of superconducting transition temperature is about 8 K for FeTe_0.75S_0.25 sample. Thermoelectric power measurements S(T) also shows the superconducting transition at around 7 K for FeTe_0.75S_0.25 sample. The upper critical fields H_c2(10%), H_c2(50%) and H_c2(90%) are estimated to be 400, 650 and 900 kOe respectively at 0 K by applying Ginzburg Landau (GL) equation. Interestingly, superconducting volume fraction is increased with low temperature (200 °C) O₂ annealing at normal pressure. Detailed investigations related to structural (XRD), transport [S(T), R(T)H], magnetization (AC and DC susceptibility) and thermal [C_p(T)] measurements for FeTe_1?xS:O₂ system are presented and discussed.     

Dissociation of dimethyl ether at high temperatures

The decomposition of dimethyl ether (CH₃OCH₃) has been investigated behind incident shock waves in a diaphragmless shock tube using laser schlieren densitometry, LS (T = 1500–2450 K, P = 57 ± 4, 125 ± 5 and 253 ± 12 Torr). The LS density gradient profiles were simulated and excellent agreement was found between the simulations and experimental profiles. Rate coefficients for CH₃OCH₃ → CH₃O + CH₃ were obtained. They showed strong fall-off, and at the lower end of the experimental temperature range are close to the low pressure limit. First order rate coefficient expressions were determined over 1500 < T < 2450 K. k_57Torr = (3.10 ± 1.0) × 10⁷⁹T^?19.03 exp(?54417/T) s^?1, k_125Torr = (1.12 ± 0.3) × 10⁸³T^?19.94 exp(?55554/T) s^?1and k_253Torr = (1.02 ± 0.3) × 10⁷³T^?17.09 exp(?51500/T) s^?1. The effect of a roaming channel for decomposition of dimethyl ether was assessed and the best agreement was obtained with 1% dissociation of DME via the roaming path.     

Rate limiting steps of the porous La0.6Sr0.4Co0.8Fe0.2O3−δ electrode material

The electrode reaction of porous La_0.6Sr_0.4Co_0.8Fe_0.2O_3?δ films deposited onto Ce_0.9Gd_0.1O_1.95 (CGO) was investigated by impedance spectroscopy within the temperature and oxygen partial pressure (pO₂) ranges of 500 ≤ T ≤ 700 °C and 10^? 4 < pO₂ < 1 atm, respectively, using Ar and He as gas carriers. The electrochemical impedance spectroscopy (EIS) measurements reveal a high frequency (HF) and a low frequency (LF) regions in the Nyquist plane. The high frequency (HF) region was fitted with a Warburg-type impedance element, and the low frequency (LF) region was reproduced with a resistance in parallel to a constant phase element. Both, the slight dependence of the polarization resistance (R_W) and the small variation of the apex frequency (f_v) of the HF Warburg-type element, on pO₂, suggest that this contribution corresponds to the oxygen diffusion in the bulk of the La_0.6Sr_0.4Co_0.8Fe_0.2O_3?δ electrode material. The variation of the polarization resistance of the LF region (R_rcpe) with pO₂ indicates that as T increases, the limiting step evolves from dissociative oxygen adsorption to oxygen gas diffusion in the pores of the mixed ionic/electronic conductor (MIEC) electrode.     

The thermopower of transition-metal double-perovskites: A sensitive probe of their electronic structures around the Fermi level

We report on a comparative study of S(T) for a series of transition-metal double-perovskites A₂BB′O₆ (A – Ca, Sr, Ba, and B, B′ = transition metal ions), some of them known to have half-metallic ground states. For Sr₂BB′O₆ with BB′ = CrMo, CrW, CrRe, FeMo, and FeRe (ferrimagnetic with high Curie temperatures), S(T) is metallic, for B′ = Mo and W it is n-type and for B′ =  Re, p-type. For A₂FeMoO₆ (A = Ca, Sr, Ba), the crystallographic differences (monoclinic, tetragonal and cubic space-groups, respectively) are accompanied by prominent differences in their (metallic) S(T). For the insulating Sr₂MnReO₆ and Ba₂MnReO₆, the onset of ferromagnetic order below T_c ∼ 120 K is marked by a steep drop of S(T) accompanied by only a slight change in the slope of ln ρ versus 1/T^1/2. Significant conclusions were drawn from the experimental results without the need for elaborate models.     

Defect equilibria and partial molar properties of (La,Sr)(Co,Fe)O3−δ

The oxygen nonstoichiometry δ of La_1−xSr_xCo_1−yFe_yO_3−δ (x = 0.6 and y = 0.2, 0.4) was investigated by thermogravimetry in the range 703 ≤ T/°C ≤ 903 and 1E−5 < pO₂/atm < 1. The oxygen deficit increases with increasing T and decreasing pO₂. Electronic conductivities σ were measured as a function of pO₂ in the range 1E−5 < pO₂/atm < 1 at 700 ≤ T/°C ≤ 900. At constant T, a p-type pO₂-dependence of σ is observed. Oxygen nonstoichiometry data are analyzed with regard to the enthalpy and entropy of oxidation ΔH_ox^θ and ΔS_ox^θ, as well as to the partial molar enthalpy and entropy of oxygen with respect to the standard state of oxygen (pO₂^θ = 1 atm), (h_O − H_O^θ) and (s_O − S_O^θ), respectively. For 2.67 ≤ (3 − δ) ≤ 2.79, (h_O − H_O^θ) decreases with increasing δ, while (s_O − S_O^θ) is constant within the limits of error. Defect chemical modelling was performed by an ideal solution model under consideration of three different valence states for B-site ions (Co or Fe). The dependence of σ on δ is modelled, using calculated defect concentrations as functions of δ. Deviations from the ideal behaviour suggest an immobilization of n-type charge carriers by oxygen vacancies.     

Analysis of temperature-dependent electrical resistivity of ZnO nano-structures

The temperature–dependent electrical resistivity ρ(T) in metallic and semiconducting phase of ZnO nanostructures is theoretically analysed. ρ(T) shows semiconducting phase in low temperature regime (140 K<T<180 K), shows an absolute minimum near 180 K and increases linearly with T at high temperatures (200 K<T<300 K). The resistivity in metallic phase is estimated within the framework of electron–phonon and electron–electron scattering mechanism. The contributions to the resistivity by inherent acoustic phonons (ρ_ac) as well as high frequency optical phonons (ρ_op) were estimated using Bloch–Gruneisen (BG) model of resistivity. The electron–electron contributions ρ_e?e=BT² in addition with electron–phonon scattering is also estimated for complete understanding of resistivity in metallic phase. Estimated contribution to resistivity by considering both phonons, i.e., ω_ac and ω_op and the zero limited resistivity are added with electron–electron interaction ρ_e–e to obtain the total resistivity. Resistivity in Semiconducting phase is discussed with small polaron conduction (SPC) model. The SPC model consistently retraces the low temperature resistivity behaviour (140 K<T<180 K). Finally the theoretically calculated resistivity is compared with experimental data which appears favourable with the present analysis in wide temperature range.     

Effect of Cr substitution on the superconducting and magnetic properties of RuSr2Eu1.5Ce0.5Cu2O10−δ

In this paper we investigate the properties of polycrystalline series of Ru_1?xCr_xSr₂Eu_1.5Ce_0.5Cu₂O_10?δ (0.0 ? x ? 0.40) by resistivity, XRD and dc magnetization measurements. EuRu-1222 is a reported magneto superconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity occurs in Cu–O₂ planes below T_c ? 40 K. The exact nature of Ru spins magnetic ordering is still being debated and no conclusion has been reached yet. In this work, we found the superconducting transition temperature T_c = 20 K from resistivity and dc magnetization measurements for pristine sample. DC magnetization measurements exhibited ferromagnetic like transition for all samples.     

Magnetic properties of ball-milled Mn nanoparticles

Nanoparticles of Mn of sizes  < 500 Å were prepared by the ball-milling technique. The temperature dependence of the magnetic susceptibility χ showed systematic variation with particle size. Peaks observed in χ were attributed to the magnetic ordering of the oxides Mn₃O₄and MnO. Peaks found in ∂(χT) / ∂T were associated with the Neel temperature ofα -Mn. We estimated that our samples contain about 0.4% of Mn₃O₄. This low concentration of Mn₃O₄was not detected by X-ray diffraction experiments but contributed significantly to the magnetization measurements.     

Stable barium compounds in YBa2Cu3Oy superconductors

The stability of various amounts of Ba₃Cu₃In₄O₁₂ (334) or BaTbO₃ (BTO) in a sintered YBa₂Cu₃O_y (YBCO) matrix was examined. Samples with added 334 or BTO exhibited critical temperatures (T_c) above 90 K for up to 20 vol.% addition and improved critical current densities (J_c) under a magnetic field. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analysis indicated that 334 and BTO did not react with the YBCO matrix under the sintering conditions used. The normalized J_c under a magnetic field of 1 T reached a maximum at 14 vol.% of 334 addition and 20 vol.% BTO addition. YBCO thin films with added BTO showed a gradual decrease in the T_c with increasing BTO content. YBCO films with added 334 showed a constant T_c of 87 K up to a 334 content of 4 vol.%.     

Evaluation of Sr- and Mn-substituted LaAlO3 as potential SOFC anode materials

Lanthanum–aluminate-based oxides, (La_0.8Sr_0.2)_1−yAl_1−xMn_xO_3−δ (x = 0, 0.3, 0.5; y = 0 or 0.06) (LSAM), were synthesized and evaluated in detail as potential anode materials for solid oxide fuel cells (SOFCs). The electrical conductivity of LSAM (Mn ≥ 30 mol%) is dominated by p-type electronic conduction and can be treated as a diluted system of lanthanum manganites, (La,Sr)MnO₃. At 810 °C, the electrical conductivity of (La_0.8Sr_0.2)_0.94Al_0.5Mn_0.5O_3−δ (LSAM8255b) reaches 12 S/cm in air and 2.7 S/cm in humidified Ar/4% H₂ (p(O₂) ≈ 10^− 18 bar). The thermal expansion coefficients of LSAM8255a and LSAM8255b match YSZ very well and no chemical reaction was observed between these two perovskite materials and YSZ up to at least 1400 °C. Fairly good electrochemical performance was observed for an LSAM8255b–YSZ composite anode. At 850 °C, the polarization resistances are only 0.34 and 0.50 Ω cm² in wet (∼3% H₂O) Ar/20% H₂ and wet Ar/20% CH₄, respectively. In addition, an exposure to Ar/20% CH₄/3% H₂O for 35 h did not cause any apparent carbon deposition on the electrode. However, the chemical stability of LSAM8255a and LSAM8255b in a typical anode environment under open circuit conditions does not seem sufficient, leading to performance degradation with time in wet Ar/20% H₂ or wet Ar/20% CH₄. Furthermore, relatively large chemical expansion (0.3–0.5%) was observed when the atmosphere was switched from air to wet Ar/4% H₂, which might cause intolerable stress on the thin film electrolyte layer for a large-area anode-supported planar SOFC, but which might be tolerable for small geometries or electrolyte-supported SOFCs.     

Effective demagnetizing factors in ferromagnetic resonance equations

A simple method of deriving effective demagnetizing factors (N_x^e,N_y^e) for the use of Kittel's ferromagnetic resonance formula is reported. The effective demagnetizing factors are expressed by an anisotropy energy (G) and a static field direction (θ,φ). By this derivation method, the resonance equations of thin films having a uniaxial or a four-fold anisotropy are obtained when a static field is rotated in the film plane. Six arrangements are calculated: (1) perpendicular anisotropy, (2) in-plane anisotropy, (3) cubic-crystal (0 0 1) face, (4) cubic-crystal (0 1 1) face, (5) cubic-crystal (1 1 1) face, and (6) oblique anisotropy films.     

Coexistence of magnetism and superconductivity in R1.4Ce0.6RuSr2Cu2O10 − δ(R = Eu,Sm and Gd)

The superconducting R_1.4Ce_0.6RuSr₂Cu₂O_10 − δ(R = Sm, Eu and Gd) withT_c ∼ 28, 32 and 42 K are also magnetically ordered atT_N ∼ 220, 122 and 180 K, respectively, thus,T_N ⪢ T_c. This is in contrast to intermetallic magnetic superconductors (such as RNi₂B₂C) in whichT_c ⪢  T_N. Magnetic susceptibility and Mossbauer spectroscopy show that superconductivity is confined to the CuO₂planes, whereas magnetism is due to the Ru sublattice. Irreversibility phenomena and magnetic anomalies, observed at low magnetic fields originate from antisymmetric exchange coupling of the Dzyaloshinsky–Moria type, and from spin reorientation of the Ru moments. The shielding fraction is about 100%, supporting the conclusion that the materials consist of a single phase, manifesting both magnetism and superconductivity at once.