Densification mechanisms of alumina,aluminosilicates and aluminoborosilicates gels,glasses and ceramics

The thermal evolution of gels, glasses and ceramics of various more or less refractory compositions (Al₂O₃, 3Al₂O₃2SiO₂, 7Al₂O₃3SiO₂, Al₂O₃2SiO₂, Al₂O₃2SiO₂0.7B₂O₃, Al₂O₃2SiO₂2B₂O₃, Al₂O₃2SiO₂6B₂ O₃) have been studied by dilatometry, DTA, and helium density measurements. Comparison is made for materials prepared by rapid (powder) or by very slow gelation (optically clear monoliths). The influence of atmosphere sintering (air, H₂, vacuum) is reported. Densification and kinetic laws are discussed.Also at LASIR, CNRS, 2 rue Henry Dunant, 94320 Thiais, France.     

Neutral Pentacoordinate Silicon(IV) Complexes with a Tridentate Dianionic O,N,O or N,N,O Ligand,an Anionic PhX Ligand (X = O,S, Se), and a Phenyl Group: Synthesis and Structural Characterization in the Solid State and in Solution

A series of neutral pentacoordinate silicon(IV) complexes with a SiO₃NC, SiO₂SNC, SiO₂SeNC, SiO₂N₂C, SiOSN₂C, or SiOSeN₂C skeleton was synthesized and structurally characterized by multinuclear NMR spectroscopy in the solid state and in solution and by single‐crystal X‐ray diffraction. The compounds studied contain a tridentate dianionic O,N,O or N,N,O ligand, an anionic PhX ligand (X = O, S, Se), and a phenyl group. The structures, NMR spectroscopic parameters, and chemical properties of these silicon(IV) complexes were compared with those of related compounds that contain a tridentate dianionic S,N,O ligand instead of the O,N,O or N,N,O ligand.     

Particle size,morphology, and properties of transition metal ferrospinels of the MFe<Subscript>2</Subscript>O<Subscript>4</Subscript> (M = Co,Ni, Zn) type,produced by glycine-nitrate combustion

Ferrites-spinels of the MFe₂O₄ type (M = Co, Ni, Zn), produced by glycine-nitrate combustion were studied. A physicochemical study of ferrite samples was performed by X-ray fluorescence analysis, scanning electron microscopy, X-ray phase analysis, and X-ray spectroscopy. The average size of the coherent scanning region was found to be (nm): 28 ± 2 for CoFe₂O₄, 32 ± 2 for NiFe₂O₄, and 26 ± 2 for ZnFe₂O₄. Magnetic characteristics were determined by the NMR method. The specific residual magnetization, specific saturation magnetization, and coercive force were, respectively, 14.1 A m² kg^?1, 20.1 A m² kg^?1, 31800 A m^?1 for CoFe₂O₄; 4.4 A m² kg^?1, 23.1 A m² kg^?1, 6550 A m^?1 for NiFe₂O₄; and 5.1 A m² kg^?1, 18.3 A m² kg^?1, 3200 A m^?1 for ZnFe₂O₄. Their magnetic properties show that the resulting ferrospinel powders can be used in the following fields of technology: CoFe₂O₄ in those areas where heat transfer is necessary (hyperthermia) and in development of data storage media; NiFe₂O₄ and ZnFe₂O₄ in those areas where low heat exchange is necessary, ZnFe₂O₄ for fast remagnetization and NiFe₂O₄ as a core or shell for transportation of other substances. The ferrospinel samples compare well in magnetic properties with their foreign commercial analogs, which makes these compounds commercially viable.     

Gas permeability,diffusivity, solubility,and aging characteristics of 6FDA-durene polyimide membranes

We have determined the intrinsic gas transport properties of He, H₂, O₂, N₂, CH₄, and CO₂ for a 6FDA-durene polyimide as a function of pressure, temperature and aging time. The permeability coefficients of O₂, N₂, CH₄, and CO₂ decrease slightly with increasing pressure. The pressure-dependent diffusion coefficients and solubility coefficients are consistent with the dual-sorption model and partial immobilization. All the gas permeabilities increase with temperature and their apparent activation energies for permeation increase with increasing gas molecular sizes in the order of CO₂, O₂, N₂, and CH₄.The percentages of permeability decay after 280 days of aging are 22, 32, 36, 40, 42, and 30% for He, H₂, O₂, N₂, CH₄, and CO₂, respectively. Interestingly, except for H₂ (kinetic diameter of 2.89 Å), the percentages of permeability decay increase exactly in the order of He (kinetic diameter of 2.6 Å), CO₂ (3.30 Å), O₂ (3.46 Å), N₂ (3.64 Å), and CH₄ (3.80 Å). The apparent activation energies of permeation for O₂, N₂, CH₄, and CO₂ increase with aging because of the increases in activation energies of diffusion and the decreases in solubility coefficients. The activation-energy increase for diffusion is probably due to the decrease in polymeric molar volume because of densification during aging. The reduction in solubility coefficient indicates the available sites for sorption decreasing with aging because of the reduction of microvoids and interstitial chain space.     

Microwave Synthesis of Co,Ni, Cu,Zn Ferrites

Solid-phase synthesis of cobalt, nickel, copper, and zinc ferrites with spinel structure was performed from oxides of these metals and natural magnetite under the action of microwave radiation. The optimal conditions in which the corresponding ferrites can be formed were determined by varying the irradiation parameters affecting the reaction (magnetron power and reaction duration). It was found that the solid-phase interaction of oxides of Ni, Zn, Cu, Co(II) metals gives in practically acceptable yields metal-substituted ferrospinels (NiFe₂O₄, CoFe₂O₄, ZnFe₂O₄, CuFe₂O₄) constituting the main phase of the samples. A high capacity of the precursors and target synthesis products for absorption of the microwave radiation energy at a frequency of 2.45 GHz and its transformation into heat was demonstrated. This gives reason to use these compounds when preparing catalysts for microwave-stimulated reactions.     

Crystal growth, structure determination, and optical properties of new potassium-rare-earth silicates K3RESi2O7 (RE=Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu)

Single crystals of K₃RESi₂O₇ (RE=Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were grown from a potassium fluoride flux. Two different structure types were found for this series. Silicates containing the larger rare earths, RE=Gd, Tb, Dy, Ho, Er, Tm, Yb crystallize in a structure K₃RESi₂O₇ that contains the rare-earth cation in both a slightly distorted octahedral and an ideal trigonal prismatic coordination environment, while in K₃LuSi₂O₇, containing the smallest of the rare earths, lutetium is found solely in an octahedral coordination environment. The structure of K₃LuSi₂O₇ crystallizes in space group P6₃/mmc with a=5.71160(10) Å and c=13.8883(6) Å. The structures containing the remaining rare earths crystallize in the space group P6₃/mcm with the lattice parameters of a=9.9359(2) Å, c=14.4295(4) Å, (K₃GdSi₂O₇); a=9.88730(10) Å, c=14.3856(3) Å, (K₃TbSi₂O₇); a=9.8673(2) Å, c=14.3572(4) Å, (K₃DySi₂O₇); a=9.8408(3) Å, c=14.3206(6) Å, (K₃HoSi₂O₇); a=9.82120(10) Å, c=14.2986(2) Å, (K₃ErSi₂O₇); a=9.80200(10) Å, c=14.2863(4) Å, (K₃TmSi₂O₇); a=9.78190(10) Å, c=14.2401(3) Å, (K₃YbSi₂O₇). The optical properties of the silicates were investigated and K₃TbSi₂O₇ was found to fluoresce in the visible.     

Surface and internal modification of composite ion exchange membranes for removal of molybdate,phosphate, and nitrate from polluted groundwater

Al₂O₃/chitosan-multiwall carbon nanotubes (MWCNTs) were created to increase the exchange capacity of polyvinylidene fluoride (PVDF) ion-exchange membranes. The composite membranes were made by mixing Al₂O₃ nanoparticles into the PVDF cast solution, then applying a thin coating of chitosan functionalized carbon nano tubes (Cs-MWCNTs) to the PVDF membrane surface. The structure and characteristics of the hybrid membranes were described using XRD, SEM, IR, and TG-DTA. The Al₂O₃-PVDF/Cs-MWCNTs membrane beat the other Al₂O₃-PVDF/Cs, Al₂O₃-PVDF, and PVDF membranes in terms of molybdate, phosphate, and nitrate adsorption. The removal efficiency, pH solution, adsorption capacity, and desorption process of molybdate, phosphate, and nitrate anions by Al₂O₃-PVDF and PVDF membranes were investigated. The removal effectiveness of molybdate, phosphate, and nitrate, according to the testing findings, was 94.3, 65.6, and 85.78 %, respectively. The adsorption of MoO₄^2?, PO₄^3?, and NO₃^? increased as the pH increased initially until the best adsorption was achieved, and then decreased significantly as the pH increased further. The total adsorption capabilities of MoO₄^2?, PO₄^3?, and NO₃^?for the Al₂O₃-PVDF/Cs-MWCNTs membrane were 65.50, 61.22, and 59.77 mg/g, respectively. Using regeneration and reuse experiments for the simultaneous adsorption of molybdate, phosphate, and nitrate during three consecutive cycles, the adsorption/desorption of Al₂O₃-PVDF/Cs-MWCNTs was assessed. Al₂O₃-PVDF/Cs-MWCNTs offer a lot of promise when it comes to eliminating MoO₄^2?, PO₄^3?, and NO₃^?from actual wastewater samples.     

Electronic Energy Structure and the Nature of Chemical Bonding in Monoferrites with a Spinel Structure M(Mg, Mn, Ni, Zn)Fe2O4

The electronic energy structure and FeK XANES in monoferrites MgFe₂O₄, MnFe₂O₄, NiFe₂O₄, and ZnFe₂O₄ were calculated with the FEFF8 program. In both normal (MnFe₂O₄, ZnFe₂O₄) and invert (MgFe₂O₄, NiFe₂O₄) spinels, hybridization of the p-states of oxygen with the 3d-states of transition metal ions leads to a similarity in the formation of the top of the valence band. In contrast to the magnetic nickel and manganese ions, the nonmagnetic zinc and magnesium ions are not actively involved in chemical bonding. It is shown that when Mn and Fe lie in the same coordination spheres of the absorbing iron atom the 3d-states are split and the curve of the densities of the p-states of transition element ions with different spin orientations changes in shape.     

DTA,TG studies of catalytic oxidation of carbon particles over M2 III O3 (MIII=Al,Cr, Fe,Ni)

Carbonizate as a model soot has been submitted to oxidation using Al₂O₃, Cr₂O₃, Ni₂O₃ and Fe₂O₃ as catalysts in the temperature range from RT up to 1000°C. The results obtained indicate that Fe₂O₃ is the most active catalyst in soot oxidation. However, all the catalysts examined are active in transformation of carbonizate components. It has been shown that DTA and TG methods can be used as fast methods testing the carbonizate oxidation. This revised version was published online in July 2006 with corrections to the Cover Date.     

The standard free energy of formation of YbFe2O4, Yb2Fe3O7, YbFeO3, and Yb3Fe5O12 at 1200°C

The standard free energy of formation of YbFe₂O₄, Yb₂Fe₃O₇, YbFeO₃, and Yb₃Fe₅O₁₂ from metallic iron, Yb₂O₃, and oxygen was determined to be ?100.38, ?158.38, ?58.17, and ?283.40 kcal/mole, respectively, at 1200°C on the basis of the phase equilibria in the FeFe₂O₃Yb₂O₃ system. The FeFe₂O₃-Lanthanoid sesquioxide systems were classified into four types with respect to the assemblage of the ternary compounds in stable existence at 1200°C, and the standard free energy of formation of YbFeO₃ was compared with those of the other lanthanoid-iron perovskites.     

Study on mechanism of hydrogen adsorption on WO3, W20O58, and W18O49

The density functional theory (DFT) calculation of hydrogen adsorption on tungsten oxides and calculation of the crystal structure of WO₃, W₂₀O₅₈, and W₁₈O₄₉ were performed_. These calculations suggest that the length of W-O bonds in WO₃ are 1.913 Å, the length of 66% W-O bonds in W₂₀O₅₈ is 1.8 to 1.9 Å, and the length of 43.48% W-O bonds in W₁₈O₄₉ is longer than 2.0 Å. The hydrate (WO₂[OH]₂), as an autocatalyst in the hydrogen reduction process, was found in the particular adsorption configuration of W₁₈O₄₉. The WO₃ and W₂₀O₅₈ were completely reduced within 40 to 60 minutes at a temperature of 1000°C and at a hydrogen flow rate of 200 mL/min, while W₁₈O₄₉ was completely reduced within 20 to 40 minutes. The phase composition and micromorphology of raw material and production were studied by both X-ray diffraction analysis (XRD) and FE-SEM technology. The differences of the mechanism of hydrogen adsorption on WO₃, W₂₀O₅₈, and W₁₈O₄₉ were explored based on the density functional theory calculation and the hydrogen reduction experiments.     

Electron-transfer transitions during the collision of O2, N2, NO and CO with their respective ions

An improved theory of electron transfer absorption is proposed. The possibility of such absorption during the collision of ion-molecule pairs is discussed and frequencies for the O₂O₂⁺, O₂O₂^?, NONO^?, COCO⁺ and N₂N₂⁺ pairs are estimated. Oscillator strengths are also estimated for the O₂O₂⁺ pair.     

Aluminum, gallium, and indium oxopivalates: Thermodynamic characteristics of gallium oxopivalates

Aluminum, gallium, and indium oxopivalates were synthesized by a heterophase method, and their evaporation was studied by the Knudsen effusion method with the mass-spectral analysis of the gas phase. It was shown that oxopivalates can be considered within binary systems as compounds of the type mM(piv)₃ · nM₂O₃. The standard enthalpies of formation of gallium oxopivalates Ga₃O(piv)_7(gas), 7Ga(piv)₃ · Ga₂O_3(solid), 4Ga(piv)₃ · Ga₂O_3(solid), and 7Ga(piv)₃ · 2Ga₂O_3(solid) were found.     

Electrical resistivity of pyrochlore compounds R2Mo2O7(R =Nd,Sm, Gd,Tb, Y)

The electrical resistivity ofR₂Mo₂O₇(R =Nd, Sm, Gd, Tb, Y) pyrochlores was measured in the temperature range 4.2 to 300 K. Metallic behavior is observed for Nd₂Mo₂O₇, Sm₂Mo₂O₇, and Gd₂Mo₂O₇ which have relatively high magnetic ordering temperatures due to ferromagnetism on the molybdenum sublattice. A clear drop in resistivity is observed near the magnetic ordering point for all metallic materials. A resistance minimum observed at low temperatures, which may be due to the magnetic ordering of the rare-earth sublattice, is not completely understood. Tb₂Mo₂O₇ and Y₂Mo₂O₇, which show no molybdenum sublattice ordering down to 4.2 K, are semiconductors. However, the temperature dependence of the resistivity is not exponential. This unusual resistivity behavior is typical of degenerate semiconductors.     

The application of differential thermal analysis technique to the study of single,binary and ternary oxide catalyst systems

The authors have reviewed the salient features of the thermal behavior of the following systems:

1. Single oxide systems: (i) Cr₂O₃, (ii) Fe₂O₃, (iii) Al₂O₃, (iv) MnO₂, (v) ZrO₂, (vi) NiO, (vii) ZnO, (viii) TiO₂, (ix) SiO₂, (x) ThO₂.
2. Binary oxide systems: (i) Cr₂O₃-Al₂O₃, (ii) Cr₂O₃-Fe₂O₃, (iii) Cr₂O₃-ZnO, (iv) Al₂O₃-SiO₂, (v) Al₂O₃-Fe₂O₃, (vi) MnO-Cr₂O₃, (vii) Cu-Al₂O₃, (viii) ZrO₂-Cr₂O₃, (ix) NiO-Cr₂O₃, (x) ZrO₂-NiO, (xi) ThO₂-Al₂O₃.
3. Ternary oxide systems: (i) NiO-Cr₂O₃-ZrO₂, (ii) Fe₂O₃-Cr₂O₃-Al₂O₃.
4. Vanadates: (i) tin vanadate, (ii) copper vanadate, (iii) lead vanadate, (iv) cobalt vanadate and (v) silver vanadate.

Excellent correlations have been obtained in most of the systems between the thermal characteristics of the solids, as revealed by DTA, and their specific surface areas and catalytic activity.     

Derivatographic study of the thermal behaviour of 13 rare earth element oxides

The simultaneous TG-DTG-DTA thermoanalytical curves of 13 rare earth (4f) element oxides, namely, CeO₂, Dy₂O₃, Er₂O₃, Gd₂O₃, HfO₂, Ho₂O₃, Nd₂O₃, Pr₆O₁₁, Sm₂O₃, Tb₄O₇, Tm₂O₃, Yb₂O₃ and Y₂O₃, were recorded with a MOM derivatograph under static air atmosphere over the temperature range from ambient to 1050°C. Only HfO₂, Nd₂O₃, Pr₆O₁₁, Sm₂O₃ and Tb₄O₇ showed appreciable weight losses due to the liberation of small amounts of oxygen. X-ray diffractometry was used to identify the thermal degradation products and their precursors.     

Contrasting chemiluminescent metal oxide formation: In the single collision reactions of boron with O2, NO2, N2O,O3 and ClO2

An intense source has been developed to study the reactions of boron (²P) atoms with O₂, NO₂, N₂O, ClO₂, and O₃. The chemiluminescent emissions which characterize the bimolecular reaction of boron with O₂, N₂O, ClO₂, and O₃ are dominated by the A ²Π-X ²Σ⁺ band system of BO. In contrast the chemiluminescence from the boron- NO₂ reaction is dominated by strong BO₂ A ²Π_u-X ²Σ_g emission.     

Synthesis,characterization, and optimum reaction conditions of oligo‐2‐[(pyridine‐2‐yl‐methylene) amino] phenol

The reaction conditions of the oxidative polycondensation of 2‐[(pyridine‐2‐yl‐methylene) amino] phenol (2‐PMAP) with air O₂, H₂O₂, and NaOCl were studied in an aqueous alkaline medium between 60 and 90 °C. Oligo‐2‐[(pyridine‐2‐yl‐methylene) amino] phenol (O‐2‐PMAP) was characterized with ¹H NMR, Fourier transform infrared, ultraviolet–visible, size exclusion chromatography (SEC), and elemental analysis techniques. Moreover, solubility testing of the oligomer was performed in polar and nonpolar organic solvents. With the NaOCl, H₂O₂, and air O₂ oxidants, the conversions of 2‐PMAP into O‐2‐PMAP were 98, 87, and 62%, respectively, in an aqueous alkaline medium. According to SEC, the number‐average molecular weight, weight‐average molecular weight, and polydispersity index of O‐2‐PMAP were 2262 g mol^?1, 2809 g mol^?1, and 1.24 with NaOCl, 3045 g mol^?1, 3861 g mol^?1, and 1.27 with air O₂, and 1427 g mol^?1, 1648 g mol^?1, and 1.16 with air H₂O₂, respectively. Also, thermogravimetric analysis showed that O‐2‐PMAP was stable against thermooxidative decomposition. The weight loss of O‐2‐PMAP was 96.68% at 900 °C. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2717–2724, 2004     

Living cationic polymerization of isobutyl vinyl ether using a variety of metal oxides as heterogeneous catalysts: Robust,reusable, and environmentally benign initiating systems

Cationic polymerization of isobutyl vinyl ether (IBVE) was examined using a variety of metal oxides in conjunction with IBVE–HCl adduct as a cationogen in toluene at 0 °C. Iron oxides (α‐Fe₂O₃, γ‐Fe₂O₃, and Fe₃O₄) induced living polymerization in the presence of an added base, ethyl acetate or 1,4‐dioxane, to give polymers with very narrow molecular weight distributions (MWDs). Conversely, with other metal oxides such as Ga₂O₃, In₂O₃, ZnO, Co₃O₄, and Bi₂O₃, polymers with bimodal MWDs, including long‐lived species along with uncontrolled higher molecular weight portions, were produced in the presence of an added base. A small amount of nBu₄NCl or 2,6‐di‐tert‐butylpyridine (DTBP) suppressed the uncontrolled portion to induce controlled reactions with Ga₂O₃, In₂O₃, and ZnO. The roles of these reagents are discussed in terms of the nature of the active sites of the catalyst surface and the polymerization mechanisms. In addition, the reusability of the catalyst, the effect of stirring before and during polymerization, and the estimation of the number of active sites are also described. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 916–926, 2010     

Investigation of s, p, d-element Niobates and Their Solid Solution Formation Processes by Thermal Analysis

The binary and ternary systems M'O(M'CO₃)-Nb₂O₅ and M'O(M'CO₃)-MO-Nb₂O₅, where M'=Ca,Sr and Ba and M=Cu, Ni, Cd, Zn and Pb, were investigated by means of thermal analysis in the temperature range 20–1500°C. The boundaries of stability of the solid solutions Sr_2−xMe_xNb₂O₇,Sr_2−xM_xNb₂O₇, Sr_4−xM_xNb₂O₉ and Sr_6−xM_xNb₂O₁₁ were determined by means of X-ray diffraction, and IR and Raman spectroscopy. The possibility of prognostication of the phase fields of stable solid solutions by calculation from the diagrams of the ‘comparative electronegativity of atoms vs. tolerance factor’ was demonstrated. The kinetic parameters of the interactions in theSrCO₃+MO+Nb₂O₅ powder mixtures were established. This revised version was published online in August 2006 with corrections to the Cover Date.