Comparison Between Sol-Gel,Coprecipitation and Wet Mixing Synthesis of ZnAl2O4

ZnAl₂O₄ was prepared by hydrolyzing a mixture of aluminum alkoxide with zinc nitrate dissolved in hexylene glycol and calcining at 800°C. The results are compared with those obtained by wet mixing and coprecipitation. The sol-gel method produces solids whose surface areas and pore volumes are 100% larger and with a more homogeneous pore size distribution.     

HCOOH high-resolution spectroscopy in the 9.18 μm region

We report on highly accurate absolute frequency measurement against a femtosecond frequency comb of six saturated absorption lines of formic acid (HCOOH) with an accuracy of 1 kHz. We also report the frequency measurement of 17 other lines with an accuracy of 2 kHz. Those lines are in quasi coincidence with the 9R(36) to 9R(42) CO₂ laser emission lines and are probed either by a CO₂ or a widely tunable quantum cascade laser phase locked to a master CO₂ laser. The stability of HCOOH stabilized lasers is characterized by a fractional Allan deviation of 3.1 × 10⁻¹² τ^−1/2. They give suitable frequency references for Doppler-free two-photon spectroscopy.     

Thermomagnetic transitions and coercivity mechanism in bulk composite Nd60Fe30Al10 alloys

The thermomagnetic behaviour (within the temperature range 553-300 K) for the bulk composite Nd₆₀Fe₃₀Al₁₀ alloy is described in terms of a transition from paramagnetic to superferromagnetic state at T=553 K, followed by a ferromagnetic ordering for T<473 K. For the superferromagnetic regime, the alloy thermomagnetic response was associated to a homogeneous distribution of magnetic clusters with mean magnetic moment and size of 1072 μ_B and 2.5 nm, respectively. For T<473 K, a pinning model of domain walls described properly the alloy coercivity dependence with temperature, from which the domain wall width and the magnetic anisotropy constant were estimated as being of ≈8 nm and ≈10⁵ J/m³, typical values of hard magnetic phases. Results are supported by microstructural and magnetic domain observations.     

Frequency evolution of the magnetoimpedance effect in stress annealed Co-rich amorphous ribbons

Complex impedance measurements in the 1 kHz–10 MHz frequency range have been performed on CoFeSiB ribbons, subjected to different annealing treatments in order to modify their magnetic properties. The different impedance responses as a function of the applied magnetic field are explained by the magnetization processes that take place in the ribbons at different selected frequencies. In particular, an evolution from domain wall to spin rotation is observed in the 50 kHz–2 MHz frequency range, modulated by the changes introduced by the annealing treatments. PACS 75.30.Gw; 75.50.Kj; 75.60.Ch; 75.60.Nt; 81.05.Kf     

Synthesis and pyrolysis of silicon and tin containing poly(2,2′-dioxy-1,1′-biphenoxy-phosphazenes)

The polyphosphazene {[NP(O₂C₁₂H₈)]_0.5[NP(OC₆H₄Br)₂]_0.5}_n (1) [(O₂C₁₂H₈) = 2,2′-dioxy-1,1′-biphenyl] that, as an strictly alternating copolymer, can be considered nearly as the homopolymer [NP(O₂C₁₂H₈)NP(OC₆H₄Br)₂]_n, was reacted first with ^tBuLi in THF at −78 °C to give the intermediate [NP(O₂C₁₂H₈)NP(OC₆H₄Li)₂]_n (2) and subsequently with the chlorosilanes SiMe₃Cl and SiMe₂(C₆H₅)Cl or with the chlorostannane SnMe₃Cl, to obtain the new polyphosphazenes {(NP[O₂C₁₂H₈])_0.5[NP(OC₆H₄SiMe₃)₂]_0.5−x[NP(OC₆H₅)(OC₆H₄SiMe₃)]_x}_n (3a) (x = 0.15-0.5), {(NP[O₂C₁₂H₈])_0.5[N(POC₆H₄SiMe₂Ph)₂]_0.2[NP(OC₆H₅)(OC₆H₄SiMe₂Ph)]_0.3}_n (3b), and {(NP[O₂C₁₂H₈])_0.5[NP(OC₆H₅)(OC₆H₄SnMe₃)]_0.5}_n (4), having a very regular distribution of the silicon or tin organometallic sites along the chains. The pyrolysis of the polymers in air at 800 °C gave microcrystalline residues (characterized by IR, XRD, SEM and TEM-EDXA) consisting on phases of SiO₂ · P₂O₅ · P₂O_7.9 · SiP₂O₇, or, in the case of the tin derivative, almost pure SnP₂O₇. The results indicate that, while part of the Si content is lost during the pyrolysis, almost all the tin in the original polymer was incorporated to the final residue.     

Spatial distributions of carbon, nitrogen and sulfur isotope ratios in human hair across the central United States

We present data on the carbon (δ(13)C), nitrogen (δ(15)N) and sulfur (δ(34)S) isotope ratios of human hair collected in the central portions of the USA. These elements are incorporated into hair from the diet and thus provide a record of dietary inputs that may also document geospatial patterns. We detected regional differences in hair δ(34)S values across the USA, with the lowest values in the northern Great Plains and increasing values towards the east, west and south. In contrast, no statistically significant patterns were detected in the spatial variation of human hair δ(13)C and δ(15)N values. Using δ(34)S values and a Geographic Information System approach, we created a map ('sulfur isoscape'). The accuracy of the map was tested using hair samples not included in its generation. We conclude that sulfur isotope analysis may represent a new tool to investigate the movements and/or region-of-origin of humans.     

Development of a simple desulfurization procedure for the determination of butyltins in complex sediment samples using gas chromatography-pulsed flame photometric detection

In this study a rapid solid phase extraction (SPE) procedure was developed to minimize the effect of different sulfur species for the determination of butyltin in sediments. The organosulfur species and organotins were firstly retained on C8 cartridges and then organotins were selectively eluted and analyzed by gas chromatography-pulsed flame photometric detection (GC-PFPD). Optimal conditions for the SPE procedure were obtained using an experimental design approach. The method's accuracy was established by analyzing a certified reference material (CRM), BCR-646 freshwater sediment. The experimental values were found to be in agreement with the assigned values for butyltins. Finally, complex sediment samples collected from a Chilean harbor were analyzed using this methodology to demonstrate its analytical potential for the determination of butyltin in environmental samples.     

Remanence of the interparticle interactions and its influence on the microwave absorption in Co-ferrite

Cobalt ferrite nanoparticles were obtained by the sol–gel method at several annealing temperatures: 400, 450, 500, 550 and 600 °C. X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed the formation of the spinel phase with a nanoparticle size in the 17–26 nm range as function of the annealing temperature. The Mössbauer spectra at room temperature showed the presence of a partial inverse spinel structure. Saturation magnetization and the coercive field are strongly dependent on the annealing temperature and they can be associated with variations of the nanoparticles size. Microwave power absorption (MPA) (dP/dH) measurements were carried out as a function of DC field (H_DC) in asymmetric sweeps in the 0 kOeH_DC9 kOe range, at X-band (9.4 GHz), for all annealing temperatures. The large hysteresis in the MPA is due to interparticle interaction associated with its demagnetizing-like nature.     

Structure of Pt/ZnAl2O4 catalysts

The effect of the support preparation technique (solgel and coprecipitation) on the final Pt/ZnAl₂O₄ catalyst is presented. The structural properties of the solids obtained are correlated to the selectivity and activity for isobutane dehydrogenation in H₂ and He reaction media. If a highly dispersed catalyst is suitable, the support has to be prepared by the sol-gel method.

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