The influence of Zn-dopant on the precipitation of α-FeOOH in highly alkaline media

The influence of Zn-dopant on the precipitation of α-FeOOH in highly alkaline media was monitored by X-ray diffraction (XRD), ⁵⁷Fe Mössbauer and Fourier transform infrared (FT-IR) spectroscopies and field emission scanning electron microscopy (FE SEM). Acicular and monodisperse α-FeOOH particles were precipitated at a very high pH by adding a tetramethylammonium hydroxide solution to an aqueous solution of FeCl₃. The XRD analysis of the samples precipitated in the presence of Zn²⁺ ions showed the formation of solid solutions of α-(Fe, Zn)OOH up to a concentration ratio r = [Zn]/([Zn] + [Fe]) = 0.0909. ZnFe₂O₄ was additionally formed in the precipitate for r = 0.1111, whereas the three phases α-FeOOH, α-Fe₂O₃ and ZnFe₂O₄ were formed for r = 0.1304. In the corresponding FT-IR spectra, the FeOH and FeO stretching bands were sensitive to the Zn²⁺ substitution, whereas the FeOH bending bands of α-FeOOH at 892 and 796 cm⁻¹ were almost insensitive. The Mössbauer spectra showed a high sensitivity to the formation of α-(Fe, Zn)OOH solid solutions which were monitored on the basis of a decrease in B_hf values in dependence on Zn-doping. A strictly linear decrease in B_hf for α-FeOOH doped with Zn²⁺ ions was measured up to r = 0.0291, whereas for r = 0.0476 and higher there was a deviation from linearity. The presence of α-(Fe, Zn)OOH, α-Fe₂O₃ and ZnFe₂O₄ phases in the samples was determined quantitatively by Mössbauer spectroscopy. Likewise, Mössbauer spectroscopy did not show any formation of the solid solutions of α-Fe₂O₃ with Zn²⁺ ions. FE SEM showed a strong effect of Zn-doping on the elongation of acicular α-FeOOH particles (500–700 nm in length) up to r = 0.1111. For r = 0.1304 the sizes of ZnFe₂O₄ particles were around 30–50 nm, and those of α-Fe₂O₃ particles were around 500 nm, whereas a relatively small number of very elongated α-(Fe, Zn)OOH particles was observed. A possible mechanism of the formation of α-(Fe, Zn)OOH, α-Fe₂O₃ and ZnFe₂O₄ particles was suggested.     

Influence of thickness on the material characteristics of reactively sputtered W2N layer and electrical properties of W/W2N/SiO2/Si capacitors

The material characteristics of W₂N layer and electrical properties of W/W₂N/SiO₂/Si metal–oxide–semiconductor (MOS) capacitors with different W₂N thickness upon annealing in N₂ + H₂ ambient at 500 °C for 20 min are investigated. The nitrogen concentration of W₂N for the W/W₂N stack with thin W₂N layer (≤10 nm) is lower than that for the W/W₂N stack with thick W₂N layer (≥15 nm). In addition, the crystallinity of W₂N in the W/W₂N (15 nm) stack is better than that in the W/W₂N (10 nm) stack. For all capacitors, the oxide charges decrease significantly after annealing and the amount of oxide charges is independent of the W₂N thickness. However, the work function (Φ_m) of the W/W₂N (≤10 nm) stack (4.6 eV) is smaller than that of W/W₂N (15 nm) stack (5.0 eV). The Φ_m of W/W₂N (15 nm) stack is close to that of W₂N single layer. After annealing, the Φ_m of W/W₂N (15 nm) stack and W₂N single layer decrease, especially for the W₂N single layer. But for the W/W₂N (≤10 nm) stack, the Φ_m increases after annealing.     

Synthesis of crystalline γ-Al2O3 with high purity

Crystalline γ-AlO(OH) was synthesized by the precipitation of sodium aluminate and oxalic acids in aqueous solution. And then γ-AlO(OH) was successfully transferred to γ-Al₂O₃ after subsequent high temperature heat treatment. The effects of reaction conditions on formation of γ-AlO(OH) and γ-Al₂O₃ were further investigated in detail. The XRD analysis shows that the complete formation of crystalline γ-Al₂O₃ is at pH 8–9, reaction temperature of 93–96 °C and calcination temperature of higher than 400 °C. The product of γ-Al₂O₃ contains impurity, including iron, calcium and silicon ion with a low content of about 0.01% and has large specific surface area and high pore volume of 269.9 m²/g and 0.57 mL/g, which can be applied in catalysts and catalyst supports.     

Synthesis and luminescent properties of GdSrAl3O7:Tb phosphor under VUV/UV excitation

Superfine powder of Tb³⁺ ion-doped aluminates phosphors, GdSrAl₃O₇:Tb³⁺ was synthesized with a precursor prepared by an EDTA-sol–gel method at 900 °C. Field-emission scanning electron-microscopy (FE-SEM) observation indicated a narrow size-distribution of about 150–300 nm for the particles with elliptical shape. Upon excitation with vacuum ultraviolet (VUV) and near UV light excitation, the phosphors show a strong emission line at around 542 nm corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺, and the highest PL intensity at 542 nm was found at a content of about 12 mol% Tb³⁺. As the Tb³⁺ concentration increases, Tb ions strongly cross-relaxation interact resulting in a decrease of the lifetime. The results reveal that GdSrAl₃O₇:Tb³⁺ would be a promising green phosphor for PDP application.     

Synthesis and characterization of Al2O3-Ce0.5Zr0.5O2 powders prepared by chemical coprecipitation method

Al₂O₃-Ce_0.5Zr_0.5O₂ catalytic powders were synthesized by the coprecipitation (ACZ-C) and mechanical mixing (ACZ-M) methods, respectively. As-synthesized powders were characterized by XRD, Raman spectroscopy, surface area and thermogravimetric analyses. It was found that the mixing extent of Al³⁺ ions affected the phase development, texture and oxygen storage capacity (OSC) of the Ce_0.5Zr_0.5O₂ powder. Single phase of ACZ-C could be maintained without phase separation and inhibit α-Al₂O₃ formation up to 1200 °C. The specific surface area value of ACZ-C (81.5 m²/g) was larger than that of ACZ-M (62.1 m²/g) and Ce_0.5Zr_0.5O₂ (17.1 m²/g) powders, which were calcined at 1000 °C. In comparison with ACZ-C and Al₂O₃, which were calcined at high temperature (900–1200 °C), it was found that the degradation rate of specific surface area of ACZ-C was lower than that of Al₂O₃. ACZ-C sample showed a higher thermal stability to resist phase separation and crystallite growth, which enhanced the oxygen storage capacity property for Ce_0.5Zr_0.5O₂ powders.     

High-temperature oxidation of Fe3Al-based iron aluminides in oxygen

The isothermal high-temperature oxidation behavior of Fe₃Al-based iron aluminides in oxygen has been studied. Fe–25Al was oxidized at 1225, 1330, 1425 and 1530 K, while Fe–28Al, Fe–24Al–5Cr, Fe–24Al–5Ti, Fe–28Al–2Cr and Fe–30Al–4Cr (all compositions in atom percent) were oxidized at 1330 K. The weight gain data were analyzed and rate constants (k_p) determined by assuming a parabolic rate law. The variations of instantaneous parabolic rate constant with time reflected the complexity of the oxidation behavior. These have been attributed to the changes taking place in the nature and properties of the scale as a function of time. The values of k_p for oxidation of Fe₃Al were one to two orders of magnitude lower than those for Ti₃Al-based intermetallics. As revealed by X-ray diffraction, the scale formed on Fe–25Al was predominantly α-Al₂O₃ at higher temperatures, while θ-Al₂O₃ was observed after oxidation at lower temperatures. The observed kinetics matched with α-Al₂O₃-formation kinetics at higher temperatures and θ-Al₂O₃-formation kinetics at lower temperatures. For all the other intermetallics, only α-Al₂O₃ was identified at 1330 K. The whisker morphology of θ-Al₂O₃ and the ridged morphology of α-Al₂O₃ were confirmed by scanning electron microscopy. Alloying with Cr or Ti increased the oxidation rate of iron aluminides, especially during the initial stages. Addition of Ti changed the nature, color, and morphology of the scale, leading to improved adherence.     

Role of Al2O3 layer in oxidation resistance of Cu-Al dilute alloys pre-annealed in H2 atmospheres

Copper was alloyed with small amounts of Al (0.2, 0.5, 1.0 and 2.0 mass%) to improve the oxidation resistance. Copper (6 N) and the Cu-Al alloys were oxidized at 773-1173 K in 0.1 MPa oxygen atmosphere after hydrogen annealing at 873 K. Continuous very thin Al₂O₃ layers were formed on the surface of all Cu-Al dilute alloys during the hydrogen annealing. Oxidation resistance of Cu-Al alloys was improved especially for Cu-2.0Al at 773-973 K, while it decreases on increasing the oxidation temperature. Cu-Al alloys followed the parabolic rate law at 1173 K, but most of other cases do not at and below 1073 K. Oxidation resistance for Cu-Al alloys was found relevant to the maintenance of the thin Al₂O₃ layer at the Cu₂O/Cu-Al alloy interface.     

An investigation of the thermal stability, crystal structure and catalytic properties of bulk and alumina-supported transition metal nitrides

A series of bulk and alumina-supported nitrides of metals (V, Mo, Fe and Co) were synthesized by NH₃-temperature-programmed reaction and characterized by X-ray diffraction (XRD) and temperature-programmed desorption techniques. The formation of bulk Co₄N and Co₄N/γ-Al₂O₃ was further confirmed using X-ray photoelectron spectroscopic analysis. The catalytic activities of these metal nitrides for NO decomposition were evaluated. Their activities for NO decomposition ranked in the order of Co₄N > Fe₃N > Mo₂N > VN and Co₄N/γ-Al₂O₃ > Fe₃N/γ-Al₂O₃ > Mo₂N/γ-Al₂O₃ > VN/γ-Al₂O₃. The relationship between crystal structures and catalytic activities was investigated. The results indicated that metal nitrides with higher vacancy concentration exhibited higher activities for NO decomposition. There was a stronger interaction between the metal nitride phases and γ-Al₂O₃ support. It was suggested that Co₄N/γ-Al₂O₃ exhibited thermal stability significantly higher than that of bulk counterpart, owing to the strong interaction between the Co₄N phase and γ-Al₂O₃ support. We applied the XRD technique to examine the structural changes of Co₄N/γ-Al₂O₃ catalysts during the reactions. The results indicated that the rapidly loss in catalytic activity was due to the bulk oxidation of Co₄N/γ-Al₂O₃. In the NO–H₂ reaction, the oxygen generated during NO dissociation was partly reduced by H₂ and partly incorporated into the nitride lattice. By the addition of H₂ in feed gas at 600 °C, one can retain the active Co₄N/γ-Al₂O₃ phase by minimizing the presence of surface oxygen.     

Electrical and electrochemical properties of γ-Li2CuZrO4

The γ-Li₂CuZrO₄ with a double rock-salt structure, as a lithium-ion compound, has never been reported on their physical and physicochemical properties. The γ-Li₂CuZrO₄ was prepared by a solid-state reaction process. X-ray diffraction was used to analyze the structure of the products. Its electrical properties were characterized by both DC and AC measurements in the temperature range from 133 to 1273 K. Cyclic voltammetry and galvanostatic cell cycling were also employed to evaluate its electrochemical performance. It is found to be a pure electronic semiconductor with a fairly high conductivity (10⁻⁵ S/cm at 133 K, 10⁻² S/cm at 300 K and 10⁻¹ S/cm at 1173 K). The average activation temperature is 14.4 kJ/mol. When increasing the temperature above 1073 K, a phase transition from γ to β takes place. When reacting with lithium in an electrochemical cell, γ-Li₂CuZrO₄ decomposes into three phases during the initial discharge process, and possesses a reversible capacity of about 70 mAh/g.     

Synthesis, structure determination and magnetic susceptibilities of the oxides Ln3Li5Sb2O12 (Ln ≠ Pr, Nd, Sm)

The oxides Ln₃Li₆Sb₂O₁₂ (Ln≠Pr, Nd, Sm) were prepared in air by heating a mixture of Ln₂O₃, LiNO₃and Sb₂O₃ in the temperature range 1023–1243 K. Lattice parameters as well as atomic coordinates in the space group 12,3 (Z≠8) are established from X-ray powder diffraction data by the Rietveld method. Magnetic susceptibilities from 4.2 to 300 K follow a Curie-Weiss law above 50 K (Ln≠Pr) or 70 K (Ln≠Nd). This is attributed to the splitting of the ground state associated with the Ln³⁺ ions by the influence of the crystal field. The magnetic moments, 3.54 and 3.61 μ_D for praseodymium and neodymium respectively, agree with those calculated by Hund's formula. The Sm³⁺ behaviour can be explained by taking into account that the splitting of the multiplets is not too large compared to kT. The magnetic moment observed at room temperature for this cation is 1.6 μ_B.     

Moderate temperature and high-speed synthesis of α-Al2O3 films by laser chemical vapor deposition using Nd:YAG laser

Al₂O₃ films were prepared at deposition temperatures (T_dep) from 980 to 1230 K by laser chemical vapor deposition (LCVD) using the continuous wave of a Nd:YAG laser with laser power (P_L) up to 260 W. γ-Al₂O₃ films were obtained at T_dep < 1100 K, whereas α-Al₂O₃ films were obtained at T_dep > 1100 K. γ-Al₂O₃ films were morphologically characterized by a cone-like structure, while α-Al₂O₃ films had hexagonal faceted grains. The highest deposition rate (R_dep) of γ-Al₂O₃ film was 570 μm h^− 1, while that of α-Al₂O₃ film was 250 μm h^− 1. α-Al₂O₃ films in a single phase were obtained at 170 K lower in T_dep and 100 times higher in R_dep than those by conventional thermal CVD.     

Texture and orientation characteristics of α-Al2O3 films prepared by laser chemical vapor deposition using Nd:YAG laser

α-Al₂O₃ films were prepared by laser chemical vapor deposition (LCVD) and the effects of precursor vaporization temperature (T_vap), total chamber pressure (P_tot), laser power (P_L) and deposition temperature (T_dep) on the phase, orientation and texture of Al₂O₃ film were investigated. At P_tot = 0.93 kPa, α-Al₂O₃ films were obtained in the region of T_vap > 423 K and T_dep > 1100 K. The orientation of α-Al₂O₃ film changed from (1 1 0) to (0 1 2) to (1 0 4) to (0 0 6) with increasing P_tot. Porous α-Al₂O₃ films were formed at high T_vap (443 K) and low P_tot (0.47 kPa). At T_vap = 413 K, α-Al₂O₃ film had hexagonal and rectangular plate-like grains with finely faceted edges. With increasing P_tot = 0.93–1.4 kPa, (0 0 6)-oriented α-Al₂O₃ film with a hexagonal terrace texture was obtained.     

Phase Equilibria in the System Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-CaO-CoO and Gibbs Energy of Formation of Ca<Subscript>3</Subscript>CoAl<Subscript>4</Subscript>O<Subscript>10</Subscript>

An isothermal section of the system Al₂O₃-CaO-CoO at 1500 K has been established by equilibrating 22 samples of different compositions at high temperature and phase identification by optical and scanning electron microscopy, X-ray diffraction, and energy dispersive spectroscopy after quenching to room temperature. Only one quaternary oxide, Ca₃CoAl₄O₁₀, was identified inside the ternary triangle. Based on the phase relations, a solid-state electrochemical cell was designed to measure the Gibbs energy of formation of Ca₃CoAl₄O₁₀ in the temperature range from 1150 to 1500 K. Calcia-stabilized zirconia was used as the solid electrolyte and a mixture of Co + CoO as the reference electrode. The cell can be represented as: From the emf of the cell, the standard Gibbs energy change for the Ca₃CoAl₄O₁₀ formation reaction, CoO + 3/5CaAl₂O₄ + 1/5Ca₁₂Al₁₄O₃₃ → Ca₃CoAl₄O₁₀, is obtained as a function of temperature: /J mol⁻¹ (±50) = −2673 + 0.289 (T/K). The standard Gibbs energy of formation of Ca₃CoAl₄O₁₀ from its component binary oxides, Al₂O₃, CaO, and CoO is derived as a function of temperature. The standard entropy and enthalpy of formation of Ca₃CoAl₄O₁₀ at 298.15 K are evaluated. Chemical potential diagrams for the system Al₂O₃-CaO-CoO at 1500 K are presented based on the results of this study and auxiliary information from the literature.     

Effect of Platinum on the Growth Rate of the Oxide Scale Formed on Cast Nickel Aluminide Intermetallic Alloys

Thermo-Gravimetric Analysis (TGA) and Scanning Electron Microscopy (SEM) data of the initial stages of oxidation of Ni₅₀Al₅₀ and Ni₄₀Pt₁₀Al₅₀ alloys of low and high sulfur content at 900°C and 1100°C are reported. The results show that the addition of Pt promotes the growth of the transient θ-Al₂O₃ oxide scale. This effect is particularly sensitive in the initial stages of oxidation at 1100°C where Pt considerably increases the total mass gain. It is attenuated in the presence of a high sulfur content in the alloy, indicating a competitive effect of Pt and S on the segregation of Al. The slower θ-to-α transition observed in the presence of Pt leads to an extended lifetime of the θ phase layer, which is proposed to be beneficial to the relaxation of the stresses created by the growth of α-Al₂O₃.     

Synthesis, characterization and magnetic properties of complex oxides Ln2Mn2/3Re4/3O7 (Ln = Er, Y) and Y2Zn2/3Re4/3O7

Complex oxides Ln₂Mn_2/3Re_4/3O₇ (Ln = Y, Er) and Y₂Zn_2/3Re_4/3O₇ with a zirkelite structure and hexagonal unit cells (space group P3₁21, z = 6) have been obtained. Static and dynamic magnetic susceptibility measurements show that these oxides possess spin-glass behavior at low temperatures. Valence combinations of d-metals in the oxides are Mn²⁺(Zn²⁺)–Re⁵⁺. It is supposed that the examined specimens Ln₂Mn_2/3Re_4/3O₇ (Ln = Y, Er) contain the second magnetic phase of an unknown composition.     

Isothermal oxidation behavior of electrospark deposited MCrAlX-type coatings on a Ni-based superalloy

A MCrAlX-type coating has been prepared by electrospark deposition (ESD) and its isothermal oxidation behavior studied. The results indicate that deposition rate and surface roughness of the coatings increase with increasing spark pulse energy. A splattered porous morphology was observed in the surface layer, and underneath this, a uniform superfine columnar γ phase structure with a column width of about 0.6 μm. When exposed at 1000 °C, θ-Al₂O₃ formed rapidly in the early oxidation stage. After 100 h oxidation, a large amount of θ-Al₂O₃ was still present, and a dense and adherent, thin α-Al₂O₃ scale had formed beneath it.     

Color-conversion and photoluminescence properties of Ba2MgW(Mo)O6:Eu phosphor

Compounds B₂AXO₆:Eu (B = Ba, Sr; A = Ca; and X = W, Mo) have recently been investigated and suggested to be color-conversion phosphor for WLED devices. In this work, we investigated the photoluminescence properties of the analogues Ba₂MgXO₆:Eu (X = W and Mo) and the energy transfer from W(Mo)O₆ groups to Eu³⁺ ions within the phosphors. The phase structure, UV-vis diffuse reflectance spectrum, photoluminescence properties and decay of Ba₂MgW_(1−x)Mo_xO₆:Eu were studied as a function of the W/Mo ratio. It was found that these phosphors showed excellent color-conversion capability from near-UV to orange-red light. The color-conversion process was considered to be performed by energy transferring from MoO₆ groups to doped Eu³⁺ ions. The MoO₆ → Eu³⁺ energy transfer efficiency could be greatly enhanced by partial substitution of Mo by W. The structure and photoluminescence properties of Ba₂AW_0.5Mo_0.5O₆:Eu (A = Ca and Mg, respectively) compounds were also investigated to reveal the effect of the Eu³⁺ ion coordination environment on its photoluminescence properties.     

Thermal expansion of the W5Si3 and T2 phases of the W–Si–B system investigated by high-temperature X-ray diffraction

The coefficients of thermal expansion (CTE) of the W₅Si₃ and T₂ phases of the W–Si–B system were determined using high-temperature X-ray diffraction in the 298–1273 K temperature interval. Alloys with nominal compositions 62.5W37.5Si (at%) and 58W21Si21B (at%) were prepared from high-purity materials through arc melting followed by heat treatment at 2073 K for 12 h under argon atmosphere. The highly different thermal expansion coefficients of W₅Si₃ along the a (5.0 × 10⁻⁶ K⁻¹) and c (16.3 × 10⁻⁶ K⁻¹) axes lead to a high thermal expansion anisotropy (α_c/α_a ≅ 3.3). On the other hand, the T₂-phase exhibits similar thermal expansion coefficients along the a (6.9 × 10⁻⁶ K⁻¹) and c (7.6 × 10⁻⁶ K⁻¹) axes, indicating a behavior close to isotropic (α_c/α_a ≅ 1.1).     

High-temperature oxidation of Pt-45Pd-10Rh

The surfaces of Pt-45Pd-10Rh foils oxidized over the range 875–1075 K in a 20% O₂-Ar mixture at atmospheric pressure were examined by Auger electron, X-ray photoelectron, and Raman spectroscopy. The composition of the oxide formed on the surface was found to vary with temperature from predominantly PdO at 875 K to PdRhO₂ at 1075 K. Only a few atomic percent Pt was observed, present in both the metallic and (apparently) +1 oxidation states at 875 K and in the metallic state at 1075 K. The formation of PdRhO₂ (and no Rh₂O₃) at 1075 K was found to persist upon reoxidation following a low-temperature reduction cycle in which the increased Rh concentration on the surface was retained. An oxidation-induced Rh enrichment of the surface of the alloy foil beyond 50 at. % does not appear likely within the temperature/pressure regime investigated.     

Thermodynamic studies on LaFeO3(s)

The enthalpy increments and the standard molar Gibbs energies in the formation of LaFeO₃(s) have been measured using a high-temperature Calvet micro-calorimeter and a solid oxide galvanic cell, respectively. The corresponding expression for enthalpy increments is given as:

H°(T)−H°(298.15 K)(J mol⁻¹)(±1.2%)=−36887.27+103.53 T(K)+25.997×10⁻³T²(K)+11.055×10⁵/T(K).

The heat capacity, the first differential of H°(T)−H°(298.15 K) with respect to temperature, is given as:

C_p,m°(T)(JK⁻¹mol⁻¹)=103.53+51.994×10⁻³T(K)−11.055×10⁵/T²(K).

From the measured e.m.f. of the cell, (−)Pt/(LaFeO₃(s)+La₂O₃(s)+Fe(s))//CSZ//(Ni(s)+NiO(s))/Pt(+), and the relevant Δ_fG_m°(T) values from the literature, the Δ_fG_m°(LaFeO₃, s, T) was calculated, and is given as:

Δ_fG_m°(LaFeO₃, s, T)(kJmol⁻¹)(±0.72)=−1319.2+0.2317T(K).

The calculated Δ_fH_m°(LaFeO₃, s, 298.15 K) and S°(298.15 K) values obtained using the second law method are −1334.7 kJ mol⁻¹ and 128.9 J K⁻¹ mol⁻¹, respectively.