Hydrothermal synthesis of precursors of neodymium oxide nanoparticles

The nanometric precursors of neodymium oxide of various morphologies were prepared via a hydrothermal reaction route. The precursors and their thermal evolution to neodymium oxide phase were characterised by means of X-ray diffraction (XRD), transmission electron microscopy (TEM, HRTEM), thermal analysis (TG, DTA, EGA-MS), FTIR and atomic force microscopy (AFM). It was found that the reaction conditions (temperature, pressure) played a key role for the product formation of desired morphology and structure. At mild conditions (140 °C) precursor with unusual fibrous morphology and Nd(OH)_2.45(Ac)_0.550.45H₂O stoichiometry was obtained. Upon heating this phase transformed, via intermediate cubic oxide, into trigonal Nd₂O₃ at 800 °C. Nd(OH)₃ hydroxide obtained at severe conditions (180 °C) transformed upon heating into cubic Nd₂O₃ phase at about 500 °C and this phase was stabilised even at 800 °C. The fibrous precursors appeared to be a convenient material for preparation of homogeneous thin coatings on planar substrates is shown.     

Cobalt–silicon mixed oxide nanocomposites by modified sol–gel method

Cobalt–silicon mixed oxide materials (Co/Si=0.111, 0.250 and 0.428) were synthesised starting from Co(NO₃)₂·6H₂O and Si(OC₂H₅)₄ using a modified sol–gel method. Structural, textural and surface chemical properties were investigated by thermogravimetric/differential thermal analyses (TG/DTA), XRD, UV–vis, FT-IR spectroscopy and N₂ adsorption at −196 °C. The nature of cobalt species and their interactions with the siloxane matrix were strongly depending on both the cobalt loading and the heat treatment. All dried gels were amorphous and contained Co²⁺ ions forming both tetrahedral and octahedral complexes with the siloxane matrix. After treatment at 400 °C, the sample with lowest Co content appeared amorphous and contained only Co²⁺ tetrahedral complexes, while at higher cobalt loading Co₃O₄ was present as the only crystalline phase, besides Co²⁺ ions strongly interacting with siloxane matrix. At 850 °C, in all samples crystalline Co₂SiO₄ was formed and was the only crystallising phase for the nanocomposite with the lowest cobalt content. All materials retained high surface areas also after treatments at 600 °C and exhibited surface Lewis acidity, due to cationic sites. The presence of cobalt affected the textural properties of the siloxane matrix decreasing microporosity and increasing mesoporosity.     

Preparation and characterization of the catalytically active forms of rare earth oxides

Catalytically active forms of the rare earth oxides Ln₂O₃(Ln = La, Sm, Eu, Dy, Ho, and Yb), Pr₆O₁₁, and CeO₂ have been prepared. The dehydration behavior of the precursors of these oxides has been studied by XRD, TG, DTA, TPD-MS, IR, and adsorption-desorption isotherms of N₂ at −196°C. Thermal analyses show that in most cases the dehydration takes place through an intermediate oxyhydroxide LnO (OH) that decomposes to the respective oxide at around 400°C. Strongly held difficult to remove carbonates were present on the surface, e.g., for Yb₂O₃ it was necessary raise the outgassing temperature to 700°C to achieve carbonate decomposition. At temperatures around 500°C these oxides are well crystallized and have moderate specific surface areas (10–40 m²g⁻¹). As a representative of the series, a detailed study of the dehydration and surface decarbonation of Yb₂O₃ was carried out by means of TPD-MS and infrared spectroscopy.     

Thermal properties and combustion behaviors of a novel UV-curable flame retarded coating containing silicon and phosphorus

A silicon-based acrylate (SHEA) was synthesized via the reaction between 2-hydroxylethyl acrylate and dimethyldichlorosilane, and characterized by Fourier transform infrared (FTIR), ¹H NMR spectroscopy and ²⁹Si NMR spectroscopy. The SHEA was blended with phosphorus-containing tri(acryloyloxyethyl) phosphate (TAEP) at different ratios to obtain a series of UV-curable flame retarded resins. The final unsaturation conversion of the SHEA films was determined by FTIR. Their combustion behaviors were examined by microscale combustion calorimetry (MCC). The thermal degradations of TAEP/SHEA composites were characterized using thermogravimetric analysis/infrared spectrometry (TG–IR). The MCC results present that the addition of TAEP into SHEA was able to decrease the HRR, HRC, T_max and THC. Among the TAEP/SHEA resins, Si1 (TAEP:SHEA is 1:1) owns the highest initial decomposition temperature and leaves the most char residue at 800 °C. The change of chemical structure during the thermal degradation process was monitored by real-time FTIR analysis to study the condensed-phase flame retarded mechanism.     

Slurry sampling for determination of lead in marine plankton by electrothermal atomic absorption spectrometry

A slurry sampling method has been developed for the determination of Pb in marine plankton by ETAAS using a freshwater plankton certified reference material (CRM 414). Slurries were prepared in 1–3% m/v range with 1% v/v HNO₃ by ultrasonic agitation for 5 min. The effects of several chemical modifiers, including Ir(NO₃)₂, Mg(NO₃)₂, Pd(NO₃)₂, Pd(NO₃)₂ + Mg(NO₃)₂, and Mg(NO₃)₂ + NH₄H₂PO₄, were investigated for the stabilization of Pb during thermal pretreatment. Lead in slurries was effectively stabilized up to 1000 °C with Ir, Pd and Pd + Mg modifiers among which Pd + Mg provided the best results with complete atomization at 1850 °C. Firings in the presence of Ir were, problematic due to ash formation inside the atomizer. Water, dilute HNO₃ and HF were examined as suspension medium. Dilute HNO₃ (1–2% v/v) proved to be advantageous over water as it afforded extraction of Pb from plankton almost quantitatively in 5 min agitation. Hydrofluoric acid was the least suitable medium. Increasing HF concentration up to 5% v/v resulted in inaccuracy and substantial background absorption. Fast-heating furnace method provided comparable accuracy and precision to that of conventional-heating in slurries of CRM 414. Detection limits and characteristic masses were, respectively, 0.49 μg L^− 1 and 32 pg for the conventional method and 0.62 μg L^− 1 and 37 pg for the fast-heating method. However, fast-heating approach suffered from distorted peaks at high temperatures and incomplete pyrolysis of matrix at lower temperatures. Analysis of marine plankton samples for Pb was performed by using the conventional furnace program. The results showed a high correlation with those obtained by solution ICP-MS. Differences were statistically insignificant within 95% confidence interval.     

Real-time monitoring of metal deposition and segregation phenomena during preparation of PdCu membranes

The N₂ and H₂ evolution, respectively, were monitored during deposition of Pd and Cu from electroless plating baths to obtain in-process control of the composition during preparation of 3–7 μm thick PdCu membranes on tubular ceramic substrates. Compositions estimated by gas evolution compare favorably to those measured in post-mortem XRD and EDS analyses, mostly differing by not more than 1 at.%. This result suggests that use of gas evolution measurements to enable in-process control of composition to within 1 at.% is feasible. Annealing experiments in an H₂ atmosphere demonstrated that, at 893 K, only 48 h are needed to form a stoichiometrically homogeneous, 9.5 μm thick, face centered cubic (fcc) Pd₆₃Cu₃₇ membrane from sequentially deposited layers; at 723 K, the same transformation requires over 2 weeks. The appearance of transient body centered cubic (bcc) and fcc phases with lower Pd contents signaled compositional segregation in the initial stages of alloy formation at 723 and 773 K and could be a source of persistent stoichiometric heterogeneity particularly in bcc PdCu membranes. The H₂ fluxes of fcc Pd₅₈Cu₄₂ and Pd₇₀Cu₃₀ membranes were J_H2=(1.6±1.1) mol m⁻² s⁻¹ exp[(−24.8±0.4)kJ mol⁻¹/RT] and J_H2=(3.7±0.6) mol m⁻² s⁻¹ exp[(−21.3±1.0)kJ mol⁻¹/RT], respectively, at 100 kPa H₂ pressure difference.     

High solubility and optical transparency of novel polyimides containing 3,3′,5,5′-tetramethyl pendant groups and 4-tert-butyltoluene moiety

A series of novel polyimides (3a–d) were prepared from 3,3′,5,5′-tetramethyl-4,4′-diaminodiphenyl-4”-tert-butyltoluene (1) with four aromatic dianhydrides via a one-step high-temperature polycondensation procedure. The obtained polyimides showed excellent solubility, with the dissolvability at a concentration of 10 wt% in most amide polar solvents and chlorinated solvents. Their films were nearly colorless and exhibited high optical transparency, with the UV cutoff wavelength in the range of 322–350 nm and the wavelength of 80% transparency in the range of 395–414 nm. They also showed low dielectric constant (2.72–2.91 at 1 MHz) and low water absorptions (0.37–0.62%). Moreover, these polyimides possessed high glass transition temperatures (T_g) (above 321 °C) and good thermal stability with 10% weight loss temperatures in the range of 526–547 °C in nitrogen atmosphere. In comparison with the analogous polyimides non-containing 3,3′,5,5′ -tetramethyl pendant groups, the resultant polyimides 3a–d showed better solubility, higher optical transparency and lower dielectric constant.     

Structural investigation of sol–gel derived SiO2–GeO2 noncrystalline and nanocomposite materials

Noncrystalline and nanocomposite materials of (1?x)SiO₂·xGeO₂ system, with Si:Ge ratio from 8:1 to 2:1 (x?=?0.111; 0.142; 0.2; 0.333), initially obtained by sol?Cgel method, were characterized by thermal analyses, X-ray diffraction, nuclear magnetic resonance and Raman spectroscopy. According to DTA and XRD results, the noncrystalline state of the as-prepared samples is stable up to 1,000?°C and only after 30?min heat treatment at 1,200?°C the samples become partial crystalline, due to development of cristobalite and quartz nanocrystals. Solid-state ²⁹Si MAS-NMR was employed in order to characterize the local structure around silicon as a function of composition and thermal history of the samples. The NMR data indicate the presence of Q², Q³ and Q⁴ units in all samples. The fraction of the highly interconnected SiO₄ tetrahedra increases both with germanium content and with annealing temperature. The Raman spectroscopy results evidence structural changes related to silicon- and germanium-oxygen units but also to their interconnection, that depend on Si:Ge ratio and annealing temperature.     

Sur un nouvel hétéropolytungstate lacunaire à squelette X2W21 dérivé du polytungstate [H6X2W22O76] (X = Sb,Bi)

A novel vacant heteropolytungstate of skeleton X₂W₂₁ derived from the polytungstate [H₆X₂W₂₂O₇₆]^8– (X = Sb,Bi). Five heteropolytungstates Na₈[H₆Sb₂W₂₁O₇₃]·16.5 H₂O, Na₈[H₆Sb₂W₂₁O₇₃]·19 H₂O, Na₈[H₆Bi₂W₂₁O₇₃]·16.5 H₂O, Li₂Na₆[H₆Sb₂W₂₂O₇₆]·14.5 H₂O, and Na₅[H₇Sb₂NiW₂₁O₇₃(H₂O)₃]·20 H₂O were prepared and studied by X-ray diffraction. The values of isotropic temperature factors of the external tungsten atom and of its three terminal oxygen atoms were compared to those of same-type atoms, firstly of the same polytungstate, and secondly inside this five-compound family. This analysis led to the conclusion that the vacant assembly X₂W₂₁O₇₃ (X = Sb,Bi) does exist and that the vacancy may be filled up with a first row transition metal such as nickel. To cite this article: Y. Jeannin, C. R. Chimie 7 (2004).     

Voltammetric studies on the α-tocopherol anion and α-tocopheroxyl (Vitamin E) radical in acetonitrile

The α-tocopheroxyl radical was generated voltammetrically by one-electron oxidation of the α-tocopherol anion (E ^r_1/2=−0.73 V versus Ag|Ag⁺) that was prepared by reacting α-tocopherol with Et₄NOH in acetonitrile (with Bu₄NPF₆ as the supporting electrolyte). Cyclic voltammograms recorded at variable scan rates (0.05–10 V s⁻¹), temperatures (−20 to 20°C) and concentrations (0.5–10 mM) were modelled using digital simulation techniques to determine the rate of bimolecular self-reaction of α-tocopheroxyl radicals. The k values were calculated to be 3×10³ l mol⁻¹ s⁻¹ at 20°C, 2×10³ l mol⁻¹ s⁻¹ at 0°C and 1.2×10³ l mol⁻¹ s⁻¹ at −20°C. In situ electrochemical-EPR experiments performed at a channel electrode confirmed the existence of the α-tocopheroxyl radical.     

Structure determination from powder diffraction data and thermal behaviour of layered lead nitrate oxalate hydrate, Pb2(NO3)2(C2O4).2H2O

The mixed lead nitrate oxalate, Pb₂(NO₃)₂(C₂O₄).2H₂O, has been obtained in a polycrystalline form in the course of a study on precursors of nanocrystalline PZT-type oxides. Its crystal structure has been solved from powder diffraction data collected using a monochromatic radiation from a conventional X-ray source. The symmetry is monoclinic, space group P2₁/c (No. 14), the cell dimensions are a=10.623(2) Å, b=7.9559(9) Å, c=6.1932(5) Å, β=104.49(1)° and Z=4. The structure consists of a stacking of complex double sheets parallel to (1 0 0), forming layers held together by hydrogen bonds. The sheets result from the condensation of PbO₁₀ polyhedra, in which the oxalate and nitrate groups, as well as water molecules, play a major role. The structure is discussed in terms of Pb---O distances, polyhedra shape and lead coordination, with emphasis on the dimensional polymerisation role of water molecules. The thermal behaviour of this layered compound is carefully described from temperature-dependent powder diffraction and thermogravimetric measurements. The enthalpy, Δ_rH=232(3) kJ mol⁻¹, and entropy, Δ_rS=532(8) J K⁻¹ mol⁻¹, of the dehydration reaction have been determined. The high value of Δ_rH demonstrates that the water molecules are strongly bonded in the structure. The complex decomposition proceeds through the crystallisation and decomposition of Pb(NO₃)₂(C₂O₄) into Pb(NO₃)₂ and PbC₂O₄, and, finally, various lead oxides.     

Synthesis and structural characterization of a new layered gallium phosphate using triethylenetetramine as structure-directing agent

A new layered gallium phosphate Ga₁₀(PO₄)₂(HPO₄)₁₂(OH)₅2.5N₄C₆H₂₀4.5H₂O was hydrothermally synthesized at 180 °C for 3 days by using triethylenetetramine (teta) as structure-directing agent. Its structure was determined by single-crystal X-ray diffraction with a triclinic cell in the space group P-1 (n^o 2) with a=8.4718(1) Å, b=18.5915(1) Å, c=24.1994(3) Å, α=110.538(1)°, β=93.656(1)°, γ=93.549(1)° and V=3547.45(6) Å³. It consists of [Ga₁₀(PO₄)₂(HPO₄)₁₂(OH)₅]⁵⁻ macroanionic sheets composed of infinite chains of GaO₄(OH)₂ octahedra connected via corner-sharing to PO₄ and GaO₄ tetrahedra. They contain four-, seven- and nine-membered rings. The inorganic layers are held together through hydrogen bond between the terminal PO bondings, the water molecules and the terminal ammonium groups of the intercalated teta molecules.     

Structural analysis of the microporous semiconductor K-SBC-1 during its reversible sorption of water

The reversible sorption of water molecules in the crystalline microporous semiconductor K-SBC-1 was investigated using temperature-resolved single-crystal XRD analysis. Three crystallographic sites of adsorbed water molecules, differing in adsorption strength, were discovered in the pores of K-SBC-1. The least tightly bound is located at the centre of the {Sb₁₂O₁₈} tube and begins to desorb around 50 °C. Above 200 °C the more strongly bound water molecules rearrange from their potassium-coordinating positions to the centre of the tube, thus obtaining the characteristics of the loosely bound water, and desorb thereafter. At 240 °C approximately 10% of the water has desorbed, leaving the host framework of K-SBC-1 intact. Upon re-adsorption of water at room temperature the molecules preferentially adsorb at sites in the centre of the {Sb₁₂O₁₈} tube. This shows that a heat treatment of 240–300 °C activates K-SBC-1 for sorption and explains the observed facile desorption of water from activated samples.     

Phase diagrams of the ternary systems Mn, Fe, Co, Ni---Si---N

Phase equilibria in the ternary systems Mn, Fe, Co, and Ni---Si---N are investigated and isothermal sections at 900°C (Fe---Si---N, Ni---Si---N), at 1000°C (Mn---Si---N, Co---Si---N) and at 1150°C (Fe---Si---N) are presented. In the system Mn---Si---N, Si₃N₄ coexists with MnSiN₂, Mn₃Si, Mn₅Si₃, MnSi, and MnSi_2−x. In the systems Fe, Co, Ni---Si---N, Si₃N₄ coexists with all binary silicides but reacts rapidly with iron above 1120 ± 10°C, and cobalt and nickel above 1170 ± 10°C to form binary silicides and nitrogen gas.     

Low-temperature, solution processing of TiO2 thin films and fabrication of multilayer dielectric optical elements

High-quality TiO₂ thin films have been deposited from aqueous titanium-peroxo solutions via spin coating. The effects of precursor solution pH on the crystallization behavior, morphology, density, and refractive index of the films are reported. From X-ray diffraction measurements, the amorphous as-deposited films are found to crystallize in the anatase phase at 250 °C. Surface and cross-section SEM images reveal that films deposited from an acidic precursor are more uniform and denser than those deposited from a basic precursor. X-ray reflectivity measurements show that films with smooth surfaces and high densities (up to 87% of single-crystal anatase) can be produced at temperatures as low as 300 °C. Measured densities are consistent with high refractive indices at 633 nm of 2.24 and 2.11 for films derived from acidic and basic precursors, respectively. The uniformity and dense nature of the films have allowed fabrication of multilayer dielectric optical elements with thermal processing at only 300 °C. The distributed Bragg reflector with four bilayers exhibits a reflectance of 92% and a stop band width of 150 nm. The optical microcavity has a quality factor of 20. The optical properties of all elements agree well with theoretical models, indicating good optical quality. Use of the precursor chemistry for direct photopatterning of TiO₂ films without a polymer resist is also demonstrated.     

The effect of first-stage polymer Tg on the morphology and thermomechanical properties of structured polymer latex particles

A series of heterogeneous latexes having stage ratios of 40:60 between the first and second stage polymers were prepared by emulsion polymerization. The first-stage polymers were non-polar S-BuA with T_gs ranging from + 100 °C to + 20 °C and the second stage polymer was polar MMA–BuA–MAA having a T_g of 20 °C. The latex particle morphologies were studied using TEM and the thermomechanical properties of the resulting latex films were studied with DSC and DMA. Calculated diffusion rates for propagating species during the reactions were correlated to the observed morphologies and to the amount of interphase in the latex particles. To cite this article: O.J. Karlsson et al., C. R. Chimie 6 (2003).     

Oxygen production at low temperature using dense perovskite hollow fiber membranes

A dense perovskite hollow fiber made of BaCo_xFe_yZr_zO_3−δ (BCFZ) was evaluated for the oxygen separation at low temperatures (400–500 °C). An oxygen permeation flux of 0.45 ml/min cm² was obtained at 500 °C, which is the first oxygen permeation data reported at such low temperature so far. A degradation of the oxygen permeation at 500 °C was observed, but the oxygen fluxes through the hollow fiber membrane can be regenerated by thermal treatment at 925 °C for 1 h in air. Energy-dispersive X-ray spectroscopy (EDXS) shows that a strong element segregation occurs in the membrane during operation at low temperature.     

Air diffusion biocathode with CueO as electrocatalyst adsorbed on carbon particle-modified electrodes

The current density of biofuel cells which use dissolved O₂ as electron acceptor is limited by O₂ supply to the electrode surface due to the low solubility and small diffusion coefficient of O₂ in the electrolyte solution. In order to increase the current density, we constructed an air diffusion biocathode which uses O₂ directly from the air. As cathodic biocatalyst, we utilized CueO from Escherichia coli, which belongs to the family of multi-copper oxidases. O₂ reduction was catalyzed by CueO adsorbed on Ketjen black-modified carbon paper electrodes. The hydrophobic electrode surface was obtained by optimizing the weight ratio of polytetrafluoroethylene binder to Ketjen black. The current density of O₂ reduction reached values as high as 20 mA cm^− 2 at 0 V vs. Ag|AgCl, KCl(sat.) in a citrate buffer (1.0 M, pH 5.0, 25 °C).     

Chemical reaction in chromatographic columns : Dehydrogenation of ethane over cadmium-exchanged zeolite 4A

Pulsed-flow techniques were used to detect considerable differences in the heats of adsorption of ethane and ethylene on various cadmium-exchanged zeolites 4A at temperatures up to 500°C. Higher values (about 10.0 kcal/mole) were observed for ethylene than for ethane (5.0 kcal/mole) at 300–400°C. Experimental verification is provided pertaining to the dehydrogenation of ethane in a gas chromatographic reactor. By appropriate choice of the reaction conditions, conversions up to 80% per pass could easily be obtained at temperatures (400–500°C) at which the thermodynamic equilibrium for a diluted ethane stream (Pc⁺₂ = 0.01-0.1 P_tot.) would not permit more than 25%.     

Solid-state synthesis of boron subnitride, B6N: myth or reality?

Solid-state synthesis of boron subnitride, B₆N, as a result of chemical interaction between boron and boron nitride at 7.5 GPa and 1700 °C has been previously reported by Hubert et al. However, a critical analysis of the results has shown that the evidence for the formation of boron subnitride with B₆O-like structure is inconclusive. We have studied in situ the interaction between boron and BN at the same p–T conditions using X-ray diffraction with synchrotron radiation. At 7.4 GPa and 1700 °C the formation of a new phase has not been observed. At the same time, HP–HT treatment has resulted in strong and unpredictable preferred orientation of boron crystallites. This leads to the rise of some weak boron reflections that might be erroneously attributed to the appearance of a new phase. To cite this article: V.L. Solozhenko et al., C. R. Chimie 9 (2006).