Solid-solid interactions in pure and Li2O-doped manganese and magnesium mixed oxides system

The solid-solid interactions between manganese and magnesium oxides in absence and in presence of small amounts of Li₂O have been investigated. The molar ratios between manganese and magnesium oxides in the form of Mn₂O₃ and MgO were varied between 0.05:1 to 0.5:1. The mixed solids were calcined in air at 400-1000°C. The techniques employed were DTA, XRD and H₂O₂ decomposition at 20-40°C.The results obtained revealed that solid-solid interactions took place between the reacting solids at 600-1000°C yielding magnesium manganates (Mg₂MnO₄, Mg₆MnO₈, MgMnO₄ besides unreacted portions of MgO, Mn₂O₃ and Mn₃O₄). Li₂O-doping (0.75-6 mol%) of the investigated system followed by calcination at 600 and 800°C decreased progressively the intensity of the diffraction lines of Mn₂O₃ (Bixbyite) with subsequent increase in the lattice parameter 'a' of MgO to an extent proportional to the amount of Li₂O added. This finding might suggest that the doping process enhanced the dissolution of Mn₂O₃ in MgO forming solid solution. This treatment led also to the formation of Li₂MnO₃. Furthermore, the doping with 3 and 6 mol% Li₂O conducted at 800°C resulted in the conversion of Mn₂O₃ into Mn₃O₄, a process that took place at 1000°C in absence of Li₂O. The produced Li₂MnO₃ phase remained stable by heating at up to 1000°C. Furthermore, Li₂O doping of the investigated system at 400-1000°C resulted in a progressive measurable increase in the particle size of MgO.The catalytic activity measurements showed that the increase in the molar ratio of Mn₂O₃ in the samples precalcined at 400-800°C was accompanied by a significant increase in the catalytic activity of the treated solids. The maximum increase in the catalytic activity expressed as reaction rate constant measured at 20°C (k _20°C) attained 3.14, 2.67 and 3.25-fold for the solids precalcined at 400, 600 and 800°C, respectively. Li₂O-doping of the samples having the formula 0.1 Mn₂O₃/MgO conducted at 400-600°C brought a progressive significant increase in its catalytic activity. The maximum increase in the value of k _20°C due to Li₂O attained 1.93 and 2.75-fold for the samples preheated at 400 and 600°C, respectively and opposite effect was found for the doped samples preheated at 800°C.This revised version was published online in November 2005 with corrections to the Cover Date.     

Preparation and performances of nanosized Ta2O5 powder photocatalyst

Nanosized-Ta₂O₅ powder photocatalyst was successfully synthesized by using sol-gel method via TaCl₅ butanol solution as a precursor. Ta₂O₅ species can be formed under 500 °C via the decomposition of the precursor. The crystalline phase of Ta₂O₅ powder photocatalyst can be obtained after being calcined above 600 °C for 4 h. The crystal size and particle size of Ta₂O₅ powder photocatalyst was about 50 nm. A good photocatalytic performance for the degradation of gaseous formaldehyde was obtained for the nanosized-Ta₂O₅ powder. The Ta₂O₅ powder formed at 700 °C for 4 h and at 650 °C for 12 h showed the best performance. The calcination temperature and time play an important role in the crystallization and photocatalytical performance of nanosized-Ta₂O₅ powder.     

Crystallization behaviors of nanosized MgO particles from magnesium alkoxides

The effects of the size of the alkoxy group on the thermal decomposition behavior of magnesium alkoxides (magnesium methoxide and ethoxide) and the crystallization behavior of MgO was investigated using thermogravimetry, Fourier-transformed infrared spectroscopy, X-ray powder diffraction, and transmission electron microscopy. As the size of the alkyl group increased, the decomposition temperature decreased and resultant MgO crystallization of the alkoxide precursor was enhanced. In an inert N(2) atmosphere, the decomposition temperature of magnesium ethoxide was about 260 degrees C, which was lower than that of magnesium methoxide by approximately 70 degrees C. The degree of the crystallization of MgO particles from the ethoxide was also significantly higher than that of the methoxide. This result is explained in terms of the O-R bonding strength of the alkoxide. With use of the Kissinger method, the activation energy for the thermal decomposition of magnesium alkoxide was found to be dependent on the size of the alkyl group. The activation energies were 161+/-23 and 130+/-24 kJ/mol for the magnesium methoxide and the magnesium ethoxide, respectively.     

New binary systems Mg-M-O (M=Y, La, Ce): Synthesis and physico-chemical characterization

New binary oxide Mg-M-O (M=Y, La, Ce) systems are obtained by co-precipitation and characterized by adsorption methods, X-ray diffraction method (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy of adsorbed probe molecules (CO and CDCl₃). It is shown that Mg-Y-O systems after calcination at 450-750 °C represent the physical mixtures of MgO and Y₂O₃, while the components of Mg-La(Ce)-O systems interact to form La₂MgO_x and (Ce,Mg)O₂ solid solutions, respectively. From XPS data, the surface of the binary systems is enriched with lanthanide ions. Addition of ≈5 mol% M₂O₃ to MgO results in an increase in concentrations of strong and weak Lewis acidic sites, the content of the latter being much higher and changing in the series: MgO<Mg-Ce-O<Mg-La-O<Mg-Y-O. At the same time basic sites become stronger in the binary systems but their total content decreases in comparison to that in individual MgO. Mg-M-O samples containing ≈5 mol% M₂O₃ are highly-dispersed and characterized by bimodal porous texture.     

Barium Sodium Niobate Powders and Thin Films: Preparation and Ferroelectric Behavior

Powders and thin films of barium sodium niobate, Ba₄Na₂Nb₁₀O₃₀, of filled tungsten bronze type ferroelectric were processed by a sol-gel route using barium metal, niobium ethoxide and sodium methoxide as precursors. Polycrystalline powder resulted after heat treating the gel powder at or above 650°C. Thin films of Ba₄Na₂Nb₁₀O₃₀ showed no preferred orientation on Si(100), Pt/Si(100) and sapphire substrates. Hysteresis measurements at 1 kHz for the thin films annealed at 750°C, obtained using a prehydrolyzed precursor solution, and gave remanent polarization of 17.34 µC/cm² and coercive field of 62.5 kV/cm. Microstructural investigation of surface morphology of these films revealed grains about 0.3 µm in size. Prehydrolysis of the precursor solution was found to be necessary to achieve dense films with ferroelectric properties.     

Chemistry of borate in salt lake brine (XXXIV)

The crystallization processes of hydrated Mg-borates, boric, magnesium hydroxide and Mg-oxychloride from MgO-B₂O₃-18%MgCl₂-H₂O supersaturated solution at 20°C have been studied by kinetic method. The crystallization solid phases were characterized by X-ray powder diffraction, IR spectra, thermal analysis and chemical analysis. The liquid-solid phase diagram of thermodynamic nonequilibrium state has been given. In this phase diagram, there exist eight crystallization fields, boric acid(H₃BO₃), trigomagneborite(MgO · 3B₂O₃ · 7.5H₂O, MgO · 3B₂O₃ · 7H₂O), hungchaoite(MgO · 2B₂O₃ · 9H₂O), inderite(2MgO · 3B₂O₃ · 15H₂O), chloropinnoite(2MgO · 2B₂O₃ · MgCl₂ · 14H₂O), magnesium hydroxide(Mg(OH)₂) and magnesium oxychloride (5Mg(OH)₂ · MgCl₂ · 8H₂O).     

Surface characteristics and corrosion resistance of sol–gel derived CaO–P2O5–SrO–Na2O bioglass–ceramic coated Mg alloy by different heat-treatment temperatures

To improve the initial corrosion resistance and then make the degradation rate of magnesium alloys to meet the biomedical application, crack-free CaO–P₂O₅–SrO–Na₂O bioglass-ceramic coatings were synthesized on AZ31 magnesium alloy substrates using a sol–gel dip-coating technique followed by a heat-treatment in the temperature range of 400–500 °C. The effects of heat-treatment on the phase constituents, surface characteristics and corrosion resistances of the coatings were investigated. It was shown that the crystallization of Ca₂P₂O₇ occurred after the glass was treated at 400 °C. As the temperature increased from 400 °C to 450 °C, besides main phase Ca₂P₂O₇, β-Ca(PO₃)₂ and Ca₄P₆O₁₉ were identified as minor crystal phases in the glass–ceramic. No new phase was detected with the temperature increasing to 500 °C except for the further crystallization. Meanwhile, the water contact angles of the coatings decreased with the increase of heat-treatment temperature due to the great crystallization. The corrosion resistances of the coated magnesium alloys were studied by electrochemical corrosion techniques in the simulated body fluid. The results revealed that the coating heat-treated at 400 °C exhibited superior corrosion resistance because of less crystallization, suggesting that the calcium phosphate bioglass–ceramic coating can provide effective protection for magnesium alloy substrate to control its initial degradation in vivo and maintain the desired mechanical properties.     

Bi4Ti3O12 Thin Films from Mixed Bismuth-Titanium Alkoxides

Bi₄Ti₃O₁₂ thin films were obtained by the sol-gel method. The precursor solution was prepared by allowing the two metallic alkoxides, Bi(OC₂H₄OCH₃)₃ and Ti(OC₂H₄OCH₃)₄, to react in 2-methoxy-ethanol to form the mixed alkoxide. This stable sol was deposited by spin-coating onto platinized silicon substrates. X-Ray diffraction patterns indicate that the perovskite initial crystallization temperature is 460°C for powder samples and it ranges between 400 and 500°C, for thin films, as a function of the number of coating layers. Dense, smooth and crack free thin films with grain sizes ranging from 20 nm to 500 nm are obtained, depending on the number of coating layers and on the post-deposition temperature annealing.     

Titania-lanthanum phosphate photoactive and hydrophobic new generation catalyst

Titania-lanthanum phosphate nanocomposites with multifunctional properties have been synthesized by aqueous sol-gel method. The precursor sols with varying TiO₂:LaPO₄ ratios were applied as thin coating on glass substrates in order to be transparent, hydrophobic, photocatalytically active coatings. The phase compositions of the composite powders were identified by powder X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). The anatase phase of TiO₂ in TiO₂-LaPO₄ composite precursors was found to be stable even on annealing at 800 °C. The glass substrates, coated with TL1 (TiO₂-LaPO₄ composition with 1 mol% LaPO₄) and TL50 (composite precursor containing TiO₂ and LaPO₄ with molar ratio 1:1) sols and annealed at 400 °C, produced contact angles of 74° and 92°, respectively, though it is only 62° for pure TiO₂ coating. The glass substrates, coated with TL50 sol, produced surfaces with relatively high roughness and uneven morphology. The TL1 material, annealed at 800 °C, has shown the highest UV photoactivity with an apparent rate constant, k_app=24×10⁻³ min⁻¹, which is over five times higher than that observed with standard Hombikat UV 100 (k_app=4×10⁻³ min⁻¹). The photoactivity combined with a moderate contact angle (85.3°) shows that this material has a promise as an efficient self-cleaning precursor.     

Hydrothermal preparation and visible-light photocatalytic activity of Bi2WO6 powders

Bi₂WO₆ powder photocatalyst was prepared using Bi(NO₃)₃ and Na₂WO₄ as raw materials by a simple hydrothermal method at 150 °C for 24 h, and then calcined at 300, 400, 500, 600 and 700 °C for 2 h, respectively. The as-prepared samples were characterized with UV-visible diffuse reflectance spectra, fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and N₂ adsorption-desorption measurement. The photocatalytic activity of the samples was evaluated using the photocatalytic oxidation of formaldehyde at room temperature under visible light irradiation. It was found that post-treatment temperature obviously influenced the visible-light photocatalytic activity and physical properties of Bi₂WO₆ powders. At 500 °C, Bi₂WO₆ powder photocatalyst showed the highest visible-light photocatalytic activity due to the samples with good crystallization and high BET surface area.     

High-temperature reactions in systems of interest for spectrochemical analysis

Summary Reactions between graphite and magnesium, silicon, vanadium and aluminium oxides in graphite electrodes have been investigated by spectrochemical and X-ray diffraction methods. Samples were ignited by means of an evaporator up to controlled temperatures. In the range of 1000–1900°C magnesium oxide does not react with graphite. Aluminium trioxide first forms -Al₂O₃ and at 1900°C Al₄C₃. Silicon dioxide forms silicon carbide at about 1400°C. Vanadium pentoxide is first reduced to VO₂ and than at higher temperatures (1200° C) forms -VC. At about 1400° C MgO and SiO₂ mixed with graphite powder form magnesium silicate, Mg₂SiO₄, and this silicate was stable at higher temperatures (up to 2000°C).

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Hochtemperaturreaktionen in spektralanalytisch wichtigen SystemenII. Reaktionen von Graphit mit den Oxiden von Magnesium, Silicium, Vanadium und Aluminium

Zusammenfassung Die Reaktionen wurden durch Spektralanalyse und Röntgendiffraktometrie untersucht. Die Proben wurden in einem Evaporator auf kontrollierte Temperaturen erhitzt. Im Bereich von 1000–1900° C reagiert MgO nicht mit Graphit. Al₂O₃ bildet zunächst -Al₂O₃ und bei 1900°C Al₄C₃. SiO₂ bildet bei etwa 1400°C SiC. V₂O₅ wird zunächst zu VO₂ reduziert und geht dann bei höheren Temperaturen (1200°C) in -VC über. MgO und SiO₂ im Gemisch mit Graphit bilden Mg₂SiO₄, das bei hohen Temperaturen (bis 2000°C) noch beständig ist.

     

Thermal and X-ray diffraction studies of the solid–solid interactions in CuO–MoO3/MgO system

The solid–solid interactions in pure and MoO₃-doped CuO/MgO system were investigated using TG, DTA and XRD. The composition of pure mixed solids were 0.1CuO/MgO, 0.2CuO/MgO and 0.3CuO/MgO and the concentrations of MoO₃ were 2.5 and 5 mol%. These solids were prepared by wet impregnation of finely powdered basic magnesium carbonate with solutions containing calculated amounts of copper nitrate and ammonium molybdate followed by heating at 400–1000°C. The results revealed that ammonium molybdate doping of the system investigated enhanced the thermal decomposition of copper nitrate and magnesium hydroxide which decomposed at temperatures lower than those observed in case of the undoped mixed solids by 70 and 100°C, respectively. A portion of CuO present dissolved in the lattice of MgO forming CuO–MgO solid solution with subsequent limited increase in its lattice parameter. The other portion interacted readily with a portion of MoO₃ at temperatures starting from 400°C yielding CuMoO₄ which remained stable up to 1000°C. The other portion of MoO₃ interacted with MgO producing MgMoO₄ at temperatures starting from 400°C and remained also stable at 1000°C. The diffraction peaks of Cu₂MgO₃ phase were detected in the diffractograms of pure and MoO₃-doped 0.3CuO/MgO precalcined at 1000°C. The formation of this phase was accompanied by an endothermic peak at 930°C.     

A low-temperature route for the synthesis of nanocrystalline LaB6

Nanocrystalline lanthanum hexaboride (LaB₆) with mean particle size of 30 nm has been successfully synthesized at 400 °C in an autoclave starting from metallic magnesium powder, NaBH₄ and LaCl₃. In this case, by using B₂O₃ instead of NaBH₄, LaB₆ nanocubes with mean size of ∼200 nm were formed at 500 °C. The X-ray diffraction (XRD) pattern can be indexed as cubic LaB₆ with the lattice constant of a=4.151 Å for LaB₆ nanoparticles and 4.154 Å for LaB₆ nanocubes. An atomic ratio of La and B as 1:5.94 was determined from EDS for LaB₆ nanoparticles. XPS data of LaB₆ nanocubes indicate the atomic ratio of La to B as 1:5.95. Raman spectra indicate the formation of LaB₆.     

Solubilities of lanthanum oxide in fluoride melts: Part I. Solubility in M3AlF6 (M = Li, Na, K)

Solubility of lanthanum oxide was measured by thermal analysis. The solubility in alkali cryolites is rather high, because of chemical reactions between lanthanum oxide and cryolites. In Li₃AlF₆-La₂O₃, alumina precipitates, in the other systems the mixed oxide LaAlO₃ is formed. In La₂O₃-Li₃AlF₆ the eutectic point is at 9.5 mol% La₂O₃ and 755 °C. The eutectic points in La₂O₃-Na₃AlF₆ and La₂O₃-K₃AlF₆ are at 11.5 mol% La₂O₃, and at 937 and 934 °C, respectively.     

Effect of alumina concentration on thermal and structural properties of mas glass and glass-ceramics

Magnesium aluminum silicate (MAS) glass samples with different concentrations of alumina (7.58 to 14.71 mol%) were prepared by melt and quench-technique. Total Mg content in the form of MgF₂+MgO was kept constant at 25 mol%. MAS glass was converted into glass-ceramics by controlled heat treatment at around 950°C. Crystalline phases present in different samples were identified by powder X-ray diffraction technique. Dilatometry technique was used to measure the thermal expansion coefficient and glass transition temperature. Scanning electron microscopy (SEM) was employed to study the microstructure of the glass-ceramic sample. It is seen from X-ray diffraction studies that at low Al₂O₃ concentrations (up to 10.5 mol%) both MgSiO₃ and fluorophlogopite phases are present and at higher Al₂O₃ concentrations of 12.3 and 14.7 mol%, fluorophlogopite and magnesium silicate (Mg₂SiO₄), respectively are found as major crystalline phases. The average thermal expansion co-efficient (_avg) of the glass samples decreases systematically from 9.8 to 5.5·10^–6 °C^–1 and the glass transition temperature (T g) increases from 610.1 to 675°C with increase in alumina content. However, in glass-ceramic samples the _avg varies in somewhat complex manner from 6.8 to 7.9·10^–6 °C^–1 with variation of Al₂O₃ content. This was thought to be due to the presence of different crystalline phases, their relative concentration and microstructure.Authors wish to thank Dr V. C. Sahni, Director Physics Group and Dr J. V. Yakhmi, Head TPPED, BARC for encouragement and support to the work. They would like to thank Dr S. K. Kulshreshtha for many useful discussions. Technical assistance from Shri B. B. Sawant, Mrs Shobha Manikandan, Mr Rakesh Kumar and Shri P. A. Wagh is gratefully acknowledged. One of authors (BIS) would like to thank BRNS-DAE for awarding him KSKRA fellowship.     

A chemical route for the synthesis of cubic bismuth zinc niobate pyrochlore nanopowders

Cubic bismuth zinc niobate pyrochlore (base composition (Bi_1.5Zn_0.5)(Zn_0.5Nb_1.5)O₇) powders were successfully prepared by a chemical method. The formation mechanism of the pyrochlore phase was investigated by TG-DSC, FT-IR, Raman, and X-ray diffraction (XRD). The optical bandgap for the powders treated at temperatures ranging from 500 to 700 °C is 3.0-3.1 eV, indicating low crystallization temperature for the pyrochlore phase. No detectable intermediary phases as BiNbO₄ or a pseudo-orthorhombic pyrochlore were observed at any time and the cubic-BZN phase was already formed after thermal treatment at temperatures as low as 500 °C. The phase formation study reveals that a well-crystallized single-phased nanopowder is obtained after calcination at 700 °C, indicating that the chemical synthesis conferred a higher chemical homogeneity and reactivity on the powder, modifying the crystallization mechanism.     

Investigation of Solid-Solid Interactions between Pure and Li2O-Doped Magnesium and Ferric Oxides

The results obtained showed that the addition of small amounts of LiNO₃ to the reacting mixed solids, consisting of equimolar proportion of Fe₂O₃ and basic MgCO₃ much enhanced the thermal decomposition of magnesium carbonate. The addition of 12 mol% LiNO₃ (6 mol% Li₂O) decreased the decomposition temperature of MgCO₃ from 525.5 to362°C. MgO underwent solid–solid interaction with Fe2O3 at temperatures starting from800°C yielding MgFe₂O₄. The amount of ferrite produced increased by increasing the precalcination temperature of the mixed solids. However, the completion of this reaction required prolonged heating at elevated temperature above 1100°C. Doping with Li₂O much enhanced the solid–solid interaction between the mixed oxides leading to the formation of MgFe₂O₄ phase at temperatures starting from 700°C. The addition of 6 mol% Li₂O to the mixed solids followed by precalcination at 1050°C for 4 h resulted in complete conversion of the reacting oxides into magnesium ferrite. The heat treatment of pure and doped solids at 900–1050°C effected the disappearance of most of IR transmission bands of the free oxides with subsequent appearance of new bands characteristic for MgFe₂O₄ structure. The promotion effect of Li2O towards the ferrite formation was attributed to an effective increase in the mobility of the various reacting cations. The activation energy of formation (ΔE) of magnesium ferrite was determined for pure and variously doped solids and the values obtained were 203, 126, 95 and 61 kJ mol⁻¹ for pure mixed solids and those treated with 1.5, 3.0 and 6.0 mol% Li₂O, respectively. This revised version was published online in August 2006 with corrections to the Cover Date.     

Y2O3:Eu (5 mol%) with Ag nanoparticles prepared by citrate precursor

Y₂O₃:Eu³⁺ (5 mol% Eu³⁺) and Y₂O₃:Eu³⁺ (5 mol% Eu³⁺) containing 1 mol% of Ag nanoparticles were prepared by heat treatment of a viscous resin obtained via citrate precursor. TEM and EDS analyses showed that Y₂O₃:Eu³⁺ (5 mol% Eu³⁺) is formed by nanoparticles with an average size of 12 nm, which increases to 30 nm when Ag is present because the effect of metal induced crystallization occurs. Ag nanoparticles with a size of 9 nm dispersed in Y₂O₃:Eu³⁺ (5 mol% Eu³⁺) were obtained and the surface plasmon effect on Ag nanoparticles was observed. The emission around 612 nm assigned to the Eu³⁺ (⁵D₀→⁷F₂) transition enhanced when the Ag nanoparticles were present in the Y₂O₃:Eu³⁺ luminescent material.     

Fully reversible hydrogen absorption and desorption reactions with Sc(Al1−xMgx), x=0.0, 0.15, 0.20

The hydrogen storage properties of Sc(Al_1−xMg_x), x=0.0, 0.15, 0.20, have been studied by X-ray powder diffraction, thermal desorption spectroscopy, pressure-composition-isotherms and scanning electron microscopy techniques. Hydrogen is absorbed from the gas phase at 70 kPa and 400 °C under the formation of ScH₂ and aluminium with magnesium in solid solution. The reaction is fully reversible in vacuum at 500 °C and shows the hydrogenation-disproportionation-desorption-recombination (HDDR) behaviour. The activation energy of desorption was determined by the Kissinger method to 185 kJ/mol. The material is stable up to at least six absorption-desorption cycles and there is no change in particle size during cycling.     

Preparation and characterization of proton-conducting sulfonated poly(ether ether ketone)/phosphatoantimonic acid composite membranes

The solid proton conductor, phosphatoantimonic acid, HSbP₂O₈ · H₂O was prepared by ion exchange of the corresponding potassium salt. The composite membranes of SPEEK with up to 40 wt% of HSbP₂O₈ · H₂O were prepared by introducing the solid proton conductor from the aqueous suspension. The composite membranes were characterized using FT-IR, powder X-ray diffraction, SEM, DSC/TGA. Thermal stability of the composite membranes was slightly lower than that of SPEEK. The composite membranes had higher water uptake when compared with SPEEK and the membranes exhibited controlled swelling up to 50 °C. The proton conductivity of the membranes was measured under 100% relative humidity up to 70 °C. The composite membranes showed enhanced proton conductivity up to 20 wt% of HSbP₂O₈ · H₂O and the conductivity was reduced with further increase of HSbP₂O₈ · H₂O loading. A maximum of four-fold increase in proton conductivity at 70 °C was observed for the composite membrane with 20 wt% of solid proton conductor.