Synthesis,crystal structure and characterization of trivacant-Keggin-polyoxometalate-based carbonyl manganese derivative

A novel trivacant polyoxometalate-based carbonyl manganese derivative, (NH₄)₃H₃[{Mn(CO)₃}(Mn(H₂O)₂)(Mn(H₂O)₃)(TeW₉O₃₃)]₂·31H₂O (1), was successfully isolated in the mixed solvent of acetonitrile and water at room temperature. The composition and structure of the product were characterized by elemental analysis, X-ray powder diffraction (XRPD), thermogravimetric analysis (TGA), IR, UV–vis, CV and single crystal X-ray diffraction.     

Reaction,densification, and mechanical properties of Ti2AlCx ceramics at low applied pressure and temperature

Ti₂AlC_x ceramic was produced by reactive hot pressing (RHP) of Ti:Al:C powder mixtures with a molar ratio of 2:1:1–.5 at 10–20 MPa, 1200–1300°C for 60 min. X-ray diffraction analysis confirmed the Ti₂AlC with TiC, Ti₃Al as minor phases in samples produced at 10–20 MPa, 1200°C. The samples RHPed at 10 MPa, 1300°C exhibited ≥95 vol.% Ti₂AlC with TiC as a minor phase. The density of samples increased from 3.69 to 4.04 g/cm³ at 10 MPa, 1200°C, whereas an increase of pressure to 20 MPa resulted from 3.84 to 4.07 g/cm³ (2:1:1 to 2:1:.5). The samples made at 10 MPa, 1300°C exhibited a density from 3.95 to 4.07 g/cm³. Reaction and densification were studied for 2Ti–Al–.67C composition at 10 MPa, 700–1300°C for 5 min showed the formation of Ti–Al intermetallic and TiC phases up to 900°C with Ti, Al, and carbon. The appearance of the Ti₂AlC phase was ≥1000°C; further, as the temperature increased, Ti₂AlC peak intensity was raised, and other phase intensities were reduced. The sample made at 700°C showed a density of 2.87 g/cm³, whereas at 1300°C it exhibited 3.98 g/cm³; further, soaking for 60 min resulted in a density of 4.07 g/cm³. Microhardness and flexural strength of Ti₂AlC_0.8 sample were 5.81 ± .21 GPa and 445 ± 35 MPa.     

Oxygen stoichiometry,chemical expansion or contraction,and electrical properties of rutile,TiO2±δ ceramics

Rutile, TiO₂ is increasingly oxygen-deficient on heating in air above ~700°C. The weight loss is generally too small for accurate measurement, but the electrical properties of quenched samples provide a sensitive qualitative indicator of oxygen content since their conductivity can vary by many orders of magnitude. The oxygen lost at high temperature is fully recovered if samples are cooled slowly. With rapid quenching, by dropping samples into liquid N₂, the oxygen stoichiometry at high temperature is preserved to ambient and the resulting materials are kinetically stable but thermodynamically metastable. The lattice parameters of quenched samples showed an unusual dependence on quench temperature and, by implication, on oxygen stoichiometry. Lattice parameters increased with a small oxygen loss, δ; chemical expansion of the lattice occurred and is attributed to reduction in average Ti oxidation state and increase in Ti–O bond lengths. At higher δ, lattice parameters started to decrease giving a chemical contraction effect attributed to partial collapse of columns of edge-sharing TiO₆ octahedra in the rutile structure and elimination of oxygen vacancies by crystallographic shear plane formation. Oxygen-deficient samples quenched from above 700°C were n-type, as were samples annealed and measured at 650 and 700°C. Samples measured at 450-500°C were p-type and believed to be slightly oxygen-rich; it is suggested that holes located on oxide ions at or near the sample surface arose from redox electron transfer between underbonded surface oxide ions and adsorbed O₂ molecules. Samples annealed between 550 and 600°C showed cross-over between n- and p-type behavior.     

Organosilicate coatings containing dibasic aluminophosphate for heat resistant electrical insulation

Two compositions of heat-resistant electrical insulating organosilicatephosphate coatings are developed with the use of debased aluminophosphate Al₂(HPO₄)₃ · 2.5H₂O in the composition. The coating has resistance of 600 and 700°C and thermal shock resistance from the maximum allowable to–60°C. Electrical insulating and various physical and mechanical properties of the coatings are determined.     

Dependence on sintering temperature of structure,optical and magnetic properties of La0.625Ca0.315Sr0.06MnO3 perovskite nanoparticles

In this study, La_0.625Ca_0.315Sr_0.06MnO₃ (LCSMO) nanoparticles were prepared by facile sol-gel method at low crystallization temperatures. Various test methods were used to characterize structure, optical and magnetic properties of LCSMO nanoparticles. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) suggested complete crystallization of LCSMO nanoparticles sintered at 700 °C. In addition, unit cell volume and grain size increased with sintering temperature. Besides, X-ray photoemission spectroscopy (XPS) fitting results of Mn2p core level peaks confirmed the increase in Mn³⁺ ion concentration with sintering temperature, mainly attributed to formation of more oxygen vacancies. Raman microscopy and Fourier transform infrared spectrometry (FTIR) jointly depicted the existence of Mn–O bond, indicating that sintering temperature definitely impacted vibration mode of Mn–O and affected both crystal structure and performance. UV–vis optical band gap width of LCSMO nanoparticles sintered at 700 °C, 1000 °C, and 1500 °C decreased from 1.2 to 0.75 eV as sintering temperature increased, suggesting the semiconducting properties of nanoscale LCSMO particles. Magnetization dependent temperature (M-T) and magnetic field (M-H) measurements revealed degradation in magnetic properties of the specimens with temperature. Overall, LCSMO nanoparticles sintered at different sintering temperatures provided novel insights into properties of rare earth doped perovskite manganites.     

Metallic monolith supported LaMnO3 perovskite-based catalysts in methane combustion

Metallic monolith supported LaMnO₃ perovskite-based catalysts are characterized by a high activity in methane combustion (95.5% conversion at 745 °C) and by a high thermal resistance. The activity of the catalysts depends on the duration and temperature of LaMnO₃ calcination. The same relation holds for the chemical composition of the catalyst surfaces when they are determined by the XPS method. The shortening of the time of LaMnO₃ perovskite calcination from 12.5 h to 8 h (700 °C) reduces the conversion of methane over a fresh catalyst. This is attributable to the lower amount of manganese (Mn:La = 0.48) on the surface of this catalyst compared to the catalyst whose perovskite was calcined for 12.5 h (Mn:La = 1.8). The extension of calcination time from 8 h to 12.5 h (at 700 °C) brings about a decrease in the specific surface area (SSA) from about 13.7 m²/g to 9.4 m²/g. After approximately 6 h on stream, the activities of the two catalysts become comparable. Aging of the catalyst with an LaMnO₃ active layer at 920 °C for 24 h reduces methane combustion to 82.5% (at 745 °C). The aging process changes the catalyst surface, where Al and C content increases and the Mn:La ratio decreases. The activity of the monolithic LaMnO₃ catalyst rises with the increase in the amount of the active layer from 11.5% to 17.8%. Methane conversion is greater over catalysts with an LaMnO₃ than with an LaCoO₃ active layer, but the LaMnO₃ catalysts show a lower resistance to thermal shocks.     

Crystal structure and temperature dependence of permittivity in barium aluminate based solid solutions

This study systematically investigated the structural, dielectric, and ferroelectric properties of BaAl_(2−2x)(Mg_0.5Ti_0.5)_2xO₄ ceramics in the 0 ≤ x ≤ 0.04 range. Single-phase solid solutions in the P6₃ space group with hexagonal crystal symmetry were confirmed in the composition range of 0 ≤ x ≤ 0.03. The bond lengths of Al1/(Mg,Ti)–O, Al₂/(Mg,Ti)–O, and Al₃/(Mg,Ti)–O increased with the increase in x, as confirmed through the Rietveld refinement and evolutions of corresponding modes in Raman spectra. The temperature stability of dielectric properties improved at a composition around x = 0.03, and the dielectric constant ε_r ascended with the increase in x. Ultrabroad temperature stability (−100°C to 700°C) was obtained, and an optimal combination (ε_r = 18.5, tan δ < 10⁻³, −22 ppm/°C ≤ TCC ≤ +20 ppm/°C, resistivity ~4.5 × 1014 Ω·cm) was achieved for the x = 0.03 ceramic sintered at 1260°C in air for 6 hours. The increase in stability was ascribed to the variations in axial bonds, and lattice distortions were determined through high-resolution transmission electron microscopy. The x = 0.03 ceramic could be a promising candidate for C0G or NP0 multilayer ceramic capacitors because of its low loss, high reliability, superior insulating properties and comparatively low-cost raw materials.     

An experimental approach to delineate the reaction mechanism of Ti3AlC2 MAX-Phase formation

A comprehensive reaction mechanism of Ti₃AlC₂ MAX-phase formation from its elemental powders while spark plasma sintering has been proposed. Microstructural evaluation revealed that Al-rich TiAl₃ intermetallic forms at around 660 °C once Al melts. Gradual transition from TiAl₃ to Ti-rich TiAl and Ti₃Al intermetallic phases occurs between 700 °C and 1200 °C through formation of layered structure due to diffusion of Al from periphery toward the centre of Ti particles. Formation of TiC and Ti₃AlC transient carbide phases were observed to occur through two different reactions beyond 1000 °C. Initially, TiC forms due to interaction of Ti and C, which further reacts with TiAl and Ti and gives rise to Ti₃AlC. Later, Ti₃AlC also forms due to diffusion of C into Ti₃Al above 1200 °C. Above 1300 °C, Ti₃AlC phase decomposes into Ti₂AlC MAX-phase and TiC in presence of unreacted C. Finally, Ti₂AlC and TiC reacts together to from Ti₃AlC₂ MAX-phase above 1350 °C and completes at 1500 °C.     

Experimental investigations of the structural transformations induced by the heat treatment in manganese ferrite synthesized by ultrasonic assisted co-precipitation method

Manganese ferrite powder has been synthesized by ultrasonically assisted co-precipitation method using Fe(NO₃)₃·2H₂O and Mn(NO₃)₂·H₂O as precursors. The precipitate was washed and dried in oven at 80 °C for 2 h to obtain the initial MnFe₂O₄ powder (denoted by sample S1). Subsequently, other three samples (denoted by S2, S3 and S4) were prepared starting from the initial powder, which was subjected to heat treatment (for 2 h in air) at 400 °C (for sample S2), 700 °C (for sample S3) and 1000 °C (for sample S4). The structural and morphological analysis of samples has been studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). From the XRD analysis of samples one finds that the samples S1 and S2 consist of MnFe₂O₄ with cubic spinel structure, whilst the samples S3 and S4 consist of FeMnO₃ with perovskite structure and Fe₂O₃; meaning that the thermal treatment of MnFe₂O₄ induces chemical and structural transformation. The complex impedance measurements, over the frequency range 20 Hz–2 MHz and magnetic measurements have confirmed the structural transformation in the samples subjected of heat treatment.     

Reactive synthesis for porous TiVAlC ceramics by TiH2,V,Al and graphite powders

Using the mixed powder of TiH₂, graphite, aluminum and vanadium as starting materials, porous TiVAlC ceramics were fabricated by the reactive synthesis technology at 1300 °C. The chemical steadiness of porous TiVAlC along with the effects of sintering temperature on the viscous permeability coefficient, strength, porosity, pore size and volume expansion rate of the porous TiVAlC were explored, and the mechanism of pore formation was also revealed. The preparation process includes five steps as follows: (i) the complete decomposition of stearic acid at 500 °C; (ii) the pyrolysis of TiH₂ at 700 °C, converting TiH₂ into hydrogen and titanium (iii) The solid-liquid chemical reaction of solid vanadium, titanium and molten aluminum at 700 °C, converting the mixture into V–Al and Ti–Al compounds; (iv) At 900–1100 °C, Surplus V and Ti interact with graphite to synthesize carbides of TiVC₂, VC, and TiC; (v) Reactive synthesized carbides (TiVC₂, VC, and TiC), Ti₂AlC, V–Al and Ti–Al compounds that yield porous TiVAlC at 1300 °C.     

The characterization of mixed titanate Ba1−xSrxTiO3 phase formation from oxalate coprecipitated precursor

Mixed titanates Ba_1−xSr_xTiO₃ were synthesized via calcination of oxalate coprecipitated precursors. On heating there were three thermal event occurred: T<250°C corresponds to the evaporation of trapped water and dehydration, T=250°C–450°C corresponds to the decomposition of oxalate and the formation of an intermediate phase where the composition is close to [Ba_1−xSr_x]₂Ti₂O₅.CO₃ and T=600°C–700°C corresponds to the carbonate decomposition and the formation of Ba_1−xSr_xTiO₃ phase. Powders calcined at T=700°C for 2 h are single phase, have grain size ranges of 0.2–2 μm and elongated morphology. Rietveld refinements of the XRD data showed single phase of perovskite structure in which their tetragonality decreased with increasing concentration of Sr²⁺ incorporated in Ba²⁺ site. The transition temperature showed strong correlation with the tetragonality.     

Melting Relations of CaO-Manganese Oxide and MgO-Manganese Oxide Mixtures in Air

Melting relations in the systems CaO-manganese oxide and MgO-manganese oxide in air have been determined at temperatures up to 1705°C. In the system CaO-manganese oxide four crystalline phases have stable existence in equilibrium with liquids: lime (approximate composition CaO-MnO), spinel (approximate composition Mn₃O₄-CaMn₂O₄), and two ternary solid solution phases in which Ca/Mn ratios as well as oxygen contents vary over considerable ranges. One of these ternary solid solution phases may for the sake of simplicity be represented approximately by the formula CaMnO₃ and the other by the formula CaMn₂O₄. Three isobaric invariant situations exist, with temperatures and phase assemblages as follows: At 1588°± 10°C the two crystalline phases lime and CaMnO₃ coexist in equilibrium with liquid (40 wt% CaO, 60 wt% manganese oxide) in a peritectic situation. Another peritectic at 1455°± 5°C is characterized by the equilibrium coexistence of CaMnO₃, CaMn₂O₄, and liquid (25 wt% CaO, 75 wt% manganese oxide). A eutectic situation exists at 1439°± 5°C with CaMn₂O₄, spinel, and liquid (18 wt% CaO, 82 wt% manganese oxide) present together in equilibrium. In the system MgO-manganese oxide in air periclase-manganosite solid solution (approximate composition MgO-MnO) and spinel (approximate composition Mn₃O₄-MgMn₂O₄) are the only crystalline phases present in equilibrium with liquids. Liquidus and solidus temperatures increase with increasing MgO content. A peritectic situation exists at 1587°± 10°C, with the two crystalline phases coexisting in equilibrium with liquid (1 wt% MgO, 99 wt% manganese oxide).     

Bi-metallic catalysts of mesoporous Al2O3 supported on Fe,Ni and Mn for methane decomposition: Effect of activation temperature

Methane decomposition reaction has been studied at three different activation temperatures (500 °C, 800 °C and 950 °C) over mesoporous alumina supported Ni–Fe and Mn–Fe based bimetallic catalysts. On co-impregnation of Ni on Fe/Al₂O₃ the activity of the catalyst was retained even at the high activation temperature at 950 °C and up to 180 min. The Ni promotion enhanced the reducibility of Fe/Al₂O₃ oxides showing higher catalytic activity with a hydrogen yield of 69%. The reactivity of bimetallic Mn and Fe over Al₂O₃ catalyst decreased at 800 °C and 950 °C activation temperatures. Regeneration studies revealed that the catalyst could be effectively recycled up to 9 times. The addition of O₂ (1 ml, 2 ml, 4 ml) in the feed enhanced substantially CH₄ conversion, the yield of hydrogen and the stability of the catalyst.     

Innovative methodology for co-treatment of mill scale scrap and manganese ore via oxidization roasting-magnetic separation for preparation of ferrite materials

Mill scale scrap, which contains vast amounts of valuable metals, is a solid waste produced in the iron and steel industry. Conventional mill scale scrap treatment methods for metal extraction are characterized by high energy consumption and low value addition. In this study, co-treatment of mill scale scrap and manganese ore via the oxidization roasting-magnetic separation process was investigated for the synchronous preparation of higher-value materials and recovery of valuable metals. Thermodynamic and magnetism analyses indicated that a higher temperature (>1100 °C) and a MnO₂/Fe₂O₃ molar ratio of 0.75–1 are essential for the preparation of manganese ferrite. The experimental validation revealed that soft magnetic manganese ferrite powders with a purity of 97.5 wt% were obtained when the test was conducted at 1300 °C for 120 min, followed by a two-stage grinding and magnetic separation process; the corresponding yield and the Mn and Fe recoveries were 78.99 wt%, 86.14 wt%, and 84.60 wt%, respectively. During the oxidization process, [Fe²⁺]O was initially oxidized to the anti-form spinel-type structure of [Fe³⁺][Fe²⁺Fe³⁺]O₄, and thereafter, it reacted with the decomposition product of [Mn³⁺][Mn²⁺Mn³⁺]O₄ to form a hybrid spinel-type structure [Me²⁺_xMe³⁺_l-x][Me²⁺_1-xMe³⁺_1+x]O₄ (Me refers to Mn and Fe) via the Mn²⁺/Fe²⁺/Mn³⁺/Fe³⁺ ions exchange at the tetrahedral and octahedral sites. Moreover, the as-purified ferrite can be used as an ingredient for the preparation of high-performance MnZn ferrite.     

Methane dehydrogenation and aromatization over 4 wt% Mn/HZSM-5 in the absence of an oxidant

Methane dehydrogenation and aromatization over 4 wt% Mn/HZSM-5 in the absence of an oxidant (GHSV = 1600 mL h⁻¹ g⁻¹) was investigated. Mn/HZSM-5 was prepared by impregnation of HZSM-5 with manganese acetate tetrahydrate solution; Mn₃O₄ formed was the precursor of active phase for methane activation. The induction period over Mn/HZSM-5 catalyst before aromatic products appear was long at 700 °C. This period shortened with a rise of the reaction temperature to 800 °C. XPS and TPO results showed that the partly carburized or carburized Mn species formed are probably responsible for methane activation.     

Solution Chemistry,Substrate, and Processing Effects on Chemical Homogeneity in Lead Zirconate Titanate Thin Films

The effects of chemistry, substrate, and processing conditions on through‐thickness cation distributions are explored in solution‐derived morphotropic composition lead zirconate titanate (PZT) films. Films prepared from chelate‐based and conventional sol–gel chemistries were spin cast onto Pt/ZnO/SiO₂/Si and Pt/Ti/SiO₂/Si substrates and pyrolyzed at 300°C, 350°C, and 400°C prior to crystallization at 700°C either in a preheated furnace or via rapid thermal processing. For films crystallized within a conventional furnace on Pt/ZnO/SiO₂/Si substrates no chemical gradients were observed. All films prepared on Pt/Ti/SiO₂/Si substrates had increased titanium concentrations near the PZT/Pt interfaces, and the source is shown to be titanium diffusing from the substrate metallization stack. The effect of heating method and rate was explored in films prepared on Pt/ZnO/SiO₂/Si substrates with 15°C, 50°C, and 100°C/s heating rates within a rapid thermal annealer. Only one solution chemistry‐heating rate combination resulted in the formation of a chemical gradient: a conventional sol–gel chemistry and a 50°C/s heating rate. Infrared spectroscopy of pyrolyzed gel films showed absorption spectra differences in the bonding structure between the two chemistries with the conventional sol–gel‐derived films exhibiting a signature more similar to that of a PbTiO₃ gel, suggestive of a gel‐structure source of gradient formation during crystallization.     

Promoting Influence of Doping Indium into BaCe0.5Zr0.3Y0.2O3‐δ as Solid Proton Conductor

The influence of indium doping on chemical stability, sinterability, and electrical properties of BaCe_0.5Zr_0.3Y_0.2O_3‐δ was investigated. The phase purity and the chemical stability of the powders in humid pure CO₂ were evaluated by XRD. The dense electrolyte pellets were formed after the calcination at 1450°C for 8 h. SEM images and shrinkage plot showed that the sinterability of the samples was apparently improved by doping indium. The electrical conductivity was measured by impedance test through two‐point method, at both low (200–350°C) and high temperature ranges (450–850°C) in different atmospheres. BaCe_0.4Zr_0.3In_0.1Y_0.2O_3‐δ has been proved to be the optimal composition which simultaneously maximized the chemical stability, sinterability, and electrical conductivity which reached 1.1 × 10^?2 S/cm in wet hydrogen at 700°C, comparing with the 1.3 × 10^?2 S/cm for original BaCe_0.5Zr_0.3Y_0.2O_3‐δ. Anode support fuel cell with a thin BaCe_0.4Zr_0.3In_0.1Y_0.2O_3‐δ electrolyte (15 μm) was fabricated by spin coating method. Maximum power density of 0.651 W/cm² was obtained when operating at 700°C and fed by humid H₂ (containing H₂O 3 vol%). The obtained fuel cell could efficiently run at 650°C for more than 100 h without any attenuation.     

Hierarchically nanoporous Co–Mn–O/FeOx as a high performance catalyst for CO preferential oxidation in H2-rich stream

A novel and highly-efficient hierarchically nanoporous Co–Mn–O/FeOx catalyst fabricated by a hard/soft dual-templating and subsequent deposition–precipitation (HSDT/DP) approach demonstrates unexpectedly high catalytic activity with 100% CO conversion at 75 °C and wide temperature window of 75–200 °C with complete CO removal for CO preferential oxidation (CO PROX), ascribed to the unique microstructure and strong interaction between finely dispersed cobalt–manganese and FeO_x. The excellent catalytic performance allows it to be a practical candidate for CO elimination from H₂-rich stream.     

Conduction properties of Ti-doped NaTaO3 at intermediate temperature

In this work, the ionic conductivity and charge carriers of acceptor-doped sodium tantalate (NaTaO₃) with perovskite structure were investigated at intermediate temperatures. The Ta-site of NaTaO₃ was doped with up to 20% titanium (Ti) with the conventional solid-state reaction method. After calcination at 900°C, samples nominally doped with 5, 10% Ti show X-ray diffraction (XRD) pattern of orthorhombic NaTaO₃ only, while peaks of Na₂Ti₃O₇ can be observed in those doped with 15, 20% Ti. The conductivity of undoped, 5% Ti and 10% Ti-doped NaTaO₃ at 300°C–700°C was measured with electrochemical impedance spectroscopy (EIS) under dry and wet O₂ atmospheres. Ti-doped NaTaO₃ samples have higher conductivity in the wet atmosphere than in the dry atmosphere, reaching 3 × 10⁻⁴ S/cm at 700°C (10% Ti-doped NaTaO₃), which confirms the hydration and proton conduction in Ti-doped NaTaO₃. Through the investigation on the dependence of conductivity on oxygen partial pressure, hole conduction in an oxidizing atmosphere, and electron conduction in reducing atmosphere can be verified. Na⁺ conduction was proven to be negligible with direct current polarization.     

Rapid Synthesis of Mesoporous,Nano‐Sized MgCr2O4 and Its Catalytic Properties

Monophasic MgCr₂O₄ has been synthesized by calcining the gel formed by the addition of epoxide to an ethanolic solution containing MgCl₂·6H₂O and CrCl₃·6H₂O. The sample has been characterized by a variety of analytical techniques including powder X‐ray diffraction (PXRD), FT‐IR, Raman, UV–Visible spectroscopy, transmission electron microscopy, and magnetic measurements at room temperature. Calcining the xerogel at 500°C and 700°C for 2 h yielded MgCr₂O₄ (yield of almost 61% by weight). BET surface area of 33.95 m²/g with an average pore diameter of 28.45 nm was obtained for the sample after calcination at 700°C. Square facets of the cubic spinel structure were observed in TEM images with an average crystallite diameter of 18 nm. HR‐TEM experiments and SAED measurements confirmed the spinel structure and negated the presence of other phases. The presence of MO₄ tetrahedral and MO₆ octahedral units in MgCr₂O₄ has also been evidenced from FTIR and Raman spectra. The sample showed paramagnetic behavior at room temperature with μ_eff of 3.54 B.M suggesting the presence of Cr in III oxidation state. Its use as an efficient catalyst for the oxidative degradation of Xylenol Orange (XO) and the photo degradation of Rhodamine‐6G (Rh‐6G) dyes have been demonstrated as these dye molecules are environmental pollutants.