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.     

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.     

Conductivity of a new pyrophosphate Sn0.9Sc0.1(P2O7)1−δ prepared by an aqueous solution method

New pyrophosphate Sn_0.9Sc_0.1(P₂O₇)_1−δ was prepared by an aqueous solution method. The structure and conductivity of Sn_0.9Sc_0.1(P₂O₇)_1−δ have been investigated. XRD analysis indicates that Sn_0.9Sc_0.1(P₂O₇)_1−δ exhibits a 3 × 3 × 3 super structure. It was found that Sn_0.9Sc_0.1(P₂O₇)_1−δ prepared by an aqueous method is not conductive. The total conductivity of Sn_0.9Sc_0.1(P₂O₇)_1−δ in open air is 2.35 × 10⁻⁶ and 2.82 × 10⁻⁹ S/cm at 900 and 400 °C respectively. In wet air, the total conductivity is about two orders of magnitude higher (8.1 × 10⁻⁷ S/cm at 400 °C) than in open air indicating some proton conduction. SnP₂O₇ and Sn_0.92In_0.08(P₂O₇)_1−δ prepared by an acidic method were reported fairly conductive but prepared by similar solution methods are not conductive. Therefore, the conductivity of SnP₂O₇-based materials might be related to the synthetic history. The possible conduction mechanism of SnP₂O₇-based materials has been discussed in detail.     

Transport anisotropy and pseudo-gap state in oxygen deficient ReBa2Cu3O7−δ (Re = Y, Ho) single crystals

The temperature dependence of the transport anisotropy of ReBa₂Cu₃O_7−δ (Re = Y, Ho) single crystals is investigated. We experimentally determine the parallel and perpendicular conductivity in oxygen deficient single crystals. It is found that, in these single crystals, oxygen deficiency results to uneven oxygen distribution within their volume that leads to the formation of superconducting phases with different critical temperatures. We present an analysis regarding the agreement of the experimental data with the predictions of different theoretical models. It is determined that, the absolute values of the energy gaps along and perpendicular to the basis plane are changed, with different signs of their derivatives. When the value of Δ_c increases, the value of the pseudo-gap decreases and vice versa, testifying that the PG regime is suppressed, with a synchronous strengthening of the localization effects. In distinction to YBa₂Cu₃O_7−δ, the temperature dependence of the anisotropy of the resistivity ρ_c/ρ_ab(T) for the HoBa₂Cu₃O_7−δ samples is well described, by the universal “law of 1/2” for the thermally activated hopping conductivity.     

Deposition, microstructure and properties of texture-controlled CVD α-Al2O3 coatings

The influence of nucleation on the microstructure and properties of CVD Al₂O₃ was investigated. The experimental α-Al₂O₃ layers were deposited (a) without nucleation control and (b) with nucleation steps resulting in pronounced , and growth textures. The experimental layers were characterised using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Wear properties of the textured coatings were evaluated in turning. The chemistry of the nucleation surface appeared to be an important factor in pre-determining the phase content and growth textures of the Al₂O₃ layers. Optimised nucleation resulted in substantially improved wear properties and these kinds of α-Al₂O₃ layers were typically composed of relatively small, defect-free grains exhibiting no porosity. The textured α-Al₂O₃ layer showed the best wear resistance.     

Mechanical properties improvement related to the isothermal holding time in Si3N4 ceramics sintered with an alternative additive

In the present work, the influence of the isothermal holding time on the physical (relative density and mass loss), chemical (α–β transformation and intergranular phase crystallization) and mechanical (hardness and fracture toughness) properties of Si₃N₄ ceramics with Al₂O₃ and CTR₂O₃ as additives has been studied. CTR₂O₃ is a natural rare earth oxide mixture, produced at DEMAR-FAENQUIL from the mineral xenotime, consisting mainly of Y₂O₃, Yb₂O₃, Er₂O₃ and Dy₂O₃. The increase in hardness and fracture toughness with increasing duration of isothermal sintering is discussed in regard of densification, α–β Si₃N₄ phase transformation and microstructure. The microstructural variations were decisive for the increase of fracture toughness, because larger grains (>4 μm) with higher aspect ratios (>6) developed during increased sinter periods, enhancing crack deflection and crack-bridging mechanism. In this way longer isothermal holding times contribute to the improvement of the physical and mechanical properties of silicon nitride based ceramics.     

Oxidation behavior of NixFe1−x(OH)2 in Cl-containing solution

The addition of Ni leads to the formation of protective rust layer on steel and subsequently high corrosion resistance of steel in Cl⁻-containing environment. α-FeOOH, β-FeOOH, γ-FeOOH and Fe₃O₄ are formed mainly on steels exposed to Cl⁻-containing atmosphere. It is expected that systematic investigation of the effect of Ni(II) on the formation process of each oxide in solution should lead to elucidation of the role of Ni in the formation of anticorrosive oxide layer. This study reports the oxidation behavior of Ni_xFe_1−x(OH)₂ in Cl⁻-containing solution at two different pH regions (condition I under which solution pH is allowed to decrease and condition II under which solution pH is maintained at 8) where γ-FeOOH and Fe₃O₄ are predominantly formed, respectively, upon the oxidation of Fe(OH)₂. In the presence of Ni(II) in the starting solution, the formation of Ni(II) doped β-FeOOH with very low crystalline was facilitated and the formation of γ-FeOOH was suppressed with increasing Ni(II) content and with increasing oxidation rate of Fe(II). Ni(II) was found to have Fe₃O₄-suppressing effect under condition II.     

Role of titanium valence states in optical and electronic features of PbO–Sb2O3–B2O3:TiO2 glass alloys

PbO–Sb₂O₃–B₂O₃ glasses mixed with different concentrations of TiO₂ (ranging from 0 to 1.5 mol.%) were synthesized. The samples are characterized by X-ray diffraction, scanning electron microscopy and DSC techniques. A variety of properties, i.e. optical absorption, photoluminescence, infrared, ESR spectra, magnetic susceptibility, photo-induced birefringence (PIB) and dielectric properties (constant ′, loss tan δ, a.c. conductivity σ_ac over a wide range of frequency and temperature) of these glass–ceramics have been explored. The analysis of these results indicated that Ti ion surrounding ligands play principal role in the observed PIB and the sample crystallized with 0.8 mol.% of TiO₂ is the most suitable for the applications in non-linear optical devices.     

Oxygen partial pressure dependence of memory effect of sputtered nc-Al/α-Al2O3 thin films

Nanocrystalline aluminum embedded in amorphous dielectric alumina matrix thin films (nc-Al/α-Al₂O₃) was synthesized via reactive magnetron sputtering. The nc-Al/α-Al₂O₃ films at different oxygen partial pressures were sputtered on p-type Si substrates from a pure Al target in the mixed ambient of Ar and O₂. Both deposition rate and aluminum concentration increase as the oxygen partial pressure decreases. X-ray photoelectron spectroscopy and high-resolution transmission electron microscope studies give the confirmation of nanocrystalline Al embedded in amorphous Al₂O₃ matrix. This nanocomposite thin film exhibits memory effect as a result of charge trapping. The flat band voltage value depends on the Al nanocrystal concentration which is related to oxygen partial pressure.     

Preparation of nanometric CeO2–ZrO2–Nd2O3 solid solution and its catalytic performances

A kind of nanometric CeO₂–ZrO₂–Nd₂O₃ (CZN) solid solution for a carrier in the automotive three-way catalysts was synthesized by a coprecipitation method and characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption–desorption (BET), scanning electron microscopy (SEM) and oxygen storage capacity (OSC). For the purpose of comparison, an unincorporated CeO₂–ZrO₂ (CZ) was also synthesized. The XRD measurements disclose the prepared CeO₂–ZrO₂–Nd₂O₃ have a face-centered cubic fluorite structure and nanoparticle sizes. According to the results of XPS, Nd³⁺ ions can enter the CZ lattice and form a homogenous solid solution. Oxygen storage capacity measurements reveal that CeO₂–ZrO₂–Nd₂O₃ display high oxygen mobility at a low temperature. The results of the activity tests show that the catalyst exhibits good three-way catalytic activity and fairly wide range of air-to-fuel ratios.     

Effects of K2O-Li2O doping on surface and catalytic properties of Fe2O3/Cr2O3 system

The effects of K₂O and Li₂O-doping (0.5, 0.75 and 1.5 mol%) of Fe₂O₃/Cr₂O₃ system on its surface and the catalytic properties were investigated. Pure and differently doped solids were calcined in air at 400-600 °C. The formula of the un-doped calcined solid was 0.85Fe₂O₃:0.15Cr₂O₃. The techniques employed were TGA, DTA, XRD, N₂ adsorption at −196 °C and catalytic oxidation of CO oxidation by O₂ at 200-300 °C. The results revealed that DTA curves of pure mixed solids consisted of one endothermic peak and two exothermic peaks. Pure and doped mixed solids calcined at 400 °C are amorphous in nature and turned to α-Fe₂O₃ upon heating at 500 and 600 °C. K₂O and Li₂O doping conducted at 500 or 600 °C modified the degree of crystallinity and crystallite size of all phases present which consisted of a mixture of nanocrystalline α- and γ-Fe₂O₃ together with K₂FeO₄ and LiFe₅O₈ phases. However, the heavily Li₂O-doped sample consisted only of LiFe₅O₈ phase. The specific surface area of the system investigated decreased to an extent proportional to the amount of K₂O and Li₂O added. On the other hand, the catalytic activity was found to increase by increasing the amount of K₂O and Li₂O added. The maximum increase in the catalytic activity, expressed as the reaction rate constant (k) measured at 200 °C, attained 30.8% and 26.5% for K₂O and Li₂O doping, respectively. The doping process did not modify the activation energy of the catalyzed reaction but rather increased the concentration of the active sites without changing their energetic nature.     

Formation and properties of BaxFe3−xO4 with spinel structure by mechanochemical reaction of α-Fe2O3 and BaCO3

Magnetic Ba_xFe_3−xO₄ (x  0.23) with spinel structure was fabricated by ball milling of mixture of BaCO₃ and nonmagnetic α-Fe₂O₃ powders, and the molar ratio of BaCO₃ and α-Fe₂O₃ is 1:6. In the milling process, a mechanochemical reaction took place between BaCO₃ and α-Fe₂O₃, and Ba cation incorporated into α-Fe₂O₃ with rhombohedral structure to form a α-(Fe,Ba)₂O₃ solid solution. The Ba content in the α-(Fe,Ba)₂O₃ increased with increasing milling time, when the Ba content exceeded a limited solubility, the α-(Fe,Ba)₂O₃ transformed into a phase of Ba_xFe_3−xO₄ with spinel structure, where the Ba cation occupied an octahedral site or tetrahedral site. The product obtained in the balling process was different from that prepared in the annealing process at atmospheric pressure, which was BaFe₂O₄ with orthorhombic structure. Accompanying the crystal structure transition from α-(Fe,Ba)₂O₃ to Ba_xFe_3−xO₄, the magnetic properties also changed from nonmagnetism into ferromagnetism. The saturation magnetization was 53.3 emu/g and coercivity was 113.7 Oe. The mechanism of transitions of the crystal structure was discussed in the present work.     

Crystallographic and morphological relationships between β phase and the Widmanstätten and allotriomorphic α phase at special β grain boundaries in an α/β titanium alloy

In the present study, the relationship between the crystallographic orientations and growth directions of grain boundary-allotriomorphic-α (GB α) and secondary Widmanstätten α laths growing from the GB α at grain boundaries separating β grains with specific misorientations has been examined. These relationships have been determined using a variety of characterization techniques, including scanning electron microscopy, orientation imaging microscopy, transmission electron microscopy (TEM) and a dual-beam focused ion beam instrument to provide site-selected TEM foils. Two very interesting cases, one in which the two adjacent β grains are rotated mutually by approximately 10.5° about a common 1 1 0 direction and the other in which the two β grains are in a twin relationship, i.e. a 60° rotation about a common 1 1 1 direction, have been studied. It was discovered that the α laths growing into two adjacent β grains from the common grain boundary may have the same orientation in both grains, while they may have either large (88.8°) or small (28.8°) angular differences in growth directions in the two adjacent β grains, depending on the relative misorientation of the β grains. The growth directions of the α laths growing from such boundaries are explained on the basis of the Burgers orientation relationship between the Widmanstätten α and the β phases and the interfacial structure proposed previously by various workers.     

Hot corrosion behavior of detonation gun sprayed Cr3C2–NiCr coatings on Ni and Fe-based superalloys in Na2SO4–60% V2O5 environment at 900 °C

The present work investigates the hot corrosion resistance of detonation gun sprayed (D-gun) Cr₃C₂–NiCr coatings on Superni 75, Superni 718 and Superfer 800 H superalloys. The deposited coatings on these superalloy substrates exhibit nearly uniform, adherent and dense microstructure with porosity less than 0.8%. Thermogravimetry technique is used to study the high temperature hot corrosion behavior of bare and Cr₃C₂–NiCr coated superalloys in molten salt environment (Na₂SO₄–60% V₂O₅) at high temperature 900 °C for 100 cycles. The corrosion products of the detonation gun sprayed Cr₃C₂–NiCr coatings on superalloys are analyzed by using XRD, SEM, and FE-SEM/EDAX to reveal their microstructural and compositional features for elucidating the corrosion mechanisms. It is shown that the Cr₃C₂–NiCr coatings on Ni- and Fe-based superalloy substrates are found to be very effective in decreasing the corrosion rate in the given molten salt environment at 900 °C. Particularly, the coating deposited on Superfer 800 H showed a better hot corrosion protection as compared to Superni 75 and Superni 718. The coatings serve as an effective diffusion barrier to preclude the diffusion of oxygen from the environment into the substrate superalloys. It is concluded that the hot corrosion resistance of the D-gun sprayed Cr₃C₂–NiCr coating is due to the formation of desirable microstructural features such as very low porosity, uniform fine grains, and the flat splat structures in the coating.     

Thermoelectric properties of hot-pressed Zn4Sb3−xTex

A series of samples have been fabricated through vacuum melting method followed by hot-pressing for Zn₄Sb_3−xTe_x (x = 0.02–0.08), XRD patterns indicated that all the samples were single-phased β-Zn₄Sb₃. Electrical conductivity and Seebeck coefficient were evaluated in the temperature range of 300–700 K, showing p-type conduction. The thermoelectric figure of merit (ZT) was increased with the increase of Te content. ZT values of 0.8 and 1.0 were obtained at 673 K for Zn_4.08Sb₃ and Zn₄Sb_2.92Te_0.08, respectively.     

Toughening and strengthening mechanism of plasma sprayed nanostructured Al2O3–13 wt.%TiO2 coatings

The starting materials of Al₂O₃, TiO₂, ZrO₂ and CeO₂ nanoparticles were agglomerated into sprayable feedstock powders and plasma sprayed to form nanostructured coatings. There were net structures and fused structures in plasma sprayed nanostructured Al₂O₃–13 wt.%TiO₂ coatings. The net structures were derived from partially melted feedstock powders and the fused structures were derived from fully melted feedstock powders. The nanostructured Al₂O₃–13 wt.%TiO₂ coatings possessed higher hardness, bonding strength and crack growth resistance than conventional Metco 130 coatings which were mainly composed of lamellar fused structures. The higher toughness and strength of nanostructured Al₂O₃–13 wt.%TiO₂ coatings were mainly related to the obtained net structures.     

Phase diagram of In2Te3–Cr3Te4

Based on the results of a physico-chemical analysis, the equilibrium diagram of the In₂Te₃–Cr₃Te₄ section of the In–Cr–Te ternary system has been constructed. The section is quasi-binary and at the basic component ratio of 1:1 the ternary compound In₂Cr₃Te₇ with a peritectic melting character is formed. Both basic components at 300 K have homogeneity regions with limits of 5.5 and 2 mol% from the In₂Te₃ and Cr₃Te₄ sides, respectively.     

Subsolidus phase relations in the system ZnO–P2O5–MoO3

The subsolidus phase relations of the ternary system ZnO–P₂O₅–MoO₃ were investigated by means of X-ray diffraction (XRD). Seven binary compounds and eight 3-phase regions were determined, and no ternary compound was found in this system. The phase diagram of pseudo-binary system Zn₃(PO₄)₂–Zn₃Mo₂O₉ was also constructed through XRD and differential thermal analysis (DTA) methods, and the result reveals this system is eutectic system. The eutectic temperature is 904 °C and the corresponding component is 30% Zn₃Mo₂O₉ and 70% Zn₃(PO₄)₂.     

Fabrication of V3O7·H2O@C core-shell nanostructured composites and the effect of V3O7·H2O and V3O7·H2O@C on decomposition of ammonium perchlorate

V₃O₇·H₂O@C core-shell materials have been synthesized using V₃O₇·H₂O nanobelts as the cores and glucose as the source of carbon via an environmental hydrothermal method. The as-obtained V₃O₇·H₂O@C core-shell materials were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), elemental analysis (EA), Fourier transform infrared spectroscopy (FT-IR) and Raman spectrum. The influences of the reaction temperature, concentration of glucose and reaction time on the morphologies of the samples were respectively discussed in detail. The possible formation mechanism of V₃O₇·H₂O@C was proposed according to our experimental results. Furthermore, the effect of V₃O₇·H₂O and V₃O₇·H₂O@C on the thermal decomposition of ammonium perchlorate (AP) were investigated by thermal gravimetric analyzer (TG) and differential thermal analysis (DTA). The thermal decomposition temperatures of AP in the presence of V₃O₇·H₂O and V₃O₇·H₂O@C were reduced by 70 and 89 °C, respectively, which indicates that V₃O₇·H₂O@C core-shell composites have higher activity than V₃O₇·H₂O.     

Self-propagating high-temperature synthesis with post-heat treatment of La1−xSrxFeO3 (x = 0–1) perovskite as catalyst for soot combustion

La_1−xSr_xFeO₃ (x = 0–1) perovskite, Sr-substituted LaFeO₃, was prepared by Self-propagating high-temperature synthesis (SHS) and its catalytic activity for soot combustion was experimentally examined in comparison with that of a conventional Pt/Al₂O₃ catalyst. The products were also characterized by XRD, FE-SEM, and BET specific surface area. The XRD analysis revealed that all the products had a perovskite phase as the major compound, together with intermediate phases with higher x values (x = 0.7–1). The BET specific surface area of the products increased with x. Moreover, the catalytic activity for soot combustion also increased with x, wherein the BET specific surface area appeared an appropriate index for explaining the observed activity. The sample with x = 0.8 exhibited the highest activity for soot combustion among all the SHS products. The soot combustion temperature of this product was as much as 100 °C lower than that of non-catalytic soot combustion. In other words, it had the same activity as that at only 20 °C higher, in comparison to conventional Pt/Al₂O₃ catalyst. More significantly, average apparent activation energy of sample with x = 0.8 calculated by Friedman method using TG/DTA was approximately 15 kJ/mol lower than that of Pt/Al₂O₃ catalyst. This result suggested that La_1−xSr_xFeO₃ has the possibility to be an alternative catalyst to Pt/Al₂O₃ catalyst.