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Author or Editor: M. Pérez-Sánchez x
Development of a universal constant rate thermal analysis system for being used with any thermoanalytical instrument
Authors: J. Criado, L. Pérez-Maqueda, M. Diánez, and P. Sánchez-Jiménez
The SCTA method implies to control the temperature in such a way that the reaction rate changes with the time according to a function previously defined by the user. Constant Rate Thermal Analysis (CRTA) is one of the most commonly used SCTA methods and implies achieving a temperature profile at which the reaction rate remains constant all over the process at a value previously selected by the user. This method permits to minimize the influence of heat and mass transfer phenomena on the forward reaction. The scope of this work is to develop a universal CRTA temperature controller that could be adapted to any thermoanalytical device. The thermoanalytical signal is programmed to follow a preset linear trend by means of a conventional controller that at the time controls a second conventional temperature programmer that forces the temperature to change for achieving the trend programmed for the thermoanalytical signal. Examples of the performance of this control system with a Thermobalance and a Thermomechanical Analyser (TMA) are given.
Separation of lanthanum, cerium and samarium from uranium samples
Authors: M. Cassir, J. Cruces, G. Pérez, M. Jiménez-Reyes, and A. Sánchez-Ocampo
We report results obtained on the separation of lanthanum, cerium, and samarium from synthetic and natural uranium samples by means of solvent extraction and ion exchange. Quantitative determinations of these elements were performed using X-ray fluorescence, polarography, and chemical or neutron activation analysis.
Comparative Thermophysical Study of Some
https://doi.org/10.1023/b:jtan.0000027798.68978.43
Authors: J. Cheda, M. García Pérez, M. I. Yélamos, and A. Sánchez Arenas
Thermal characterization of montmorillonite clays saturated with various cations
Authors: V. Balek, M. Beneŝ, J. Ŝubrt, J. Pérez-Rodriguez, P. Sánchez-Jiménez, L. Pérez-Maqueda, and J. Pascual-Cosp
Emanation thermal analysis (ETA), thermogravimetry and high temperature XRD were used to characterize the thermal behavior during dehydration of natural Na montmorillonite (Upton Wyoming, USA) and homoionic montmorillonite (MMT) samples saturated with different cations, i.e. Li+, Cs+, NH4 +, Mg2+ and Al3+. ETA results characterized radon mobility and microstructure changes that accompanied the mass loss of the samples due to dehydration on heating in air. A collapse of interlayer space between the silicate sheets after water release from the MMT samples was characterized by a decrease of the radon release rate, ΔE. Decreases in c-axis basal spacing (d 001) values determined from XRD patterns for the different montmorillonite samples follow the sequence:
\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $$Mg - MMT > Al - MMT > Li - MMT > Na - MMT > NH_4 - MMT > Cs - MMT$$ \end{document}
The decrease of the radon release rate (ΔE) determined by ETA that characterized microstructure changes due to collapse of interlayer space corresponded well to differences in the c-axis basal spacing (Δd 001) values determined from the XRD patterns before and after samples dehydration.
Comparative electrochemistry of technetium(V) and rhenium(V) dioxo complexes
Authors: C. Kremer, S. Domínguez, M. Pérez-Sánchez, A. Mederos, and E. Kremer
The heavy use of99mTc in nuclear medicine and the recent development of188Re radiopharmaceuticals have encouraged the comparative study of Tc and Re coordination compounds. In this work, the electrochemistry of [MVO2 (amine)2]+ (M=Tc, Re; amine = ethylenediamine, 1,3-diaminopropane, diethylenetriamine, triethylenetetramine) complexes is studied by cyclic voltammetry and the results are compared. The voltammograms of these compounds, obtained at different pH values, show that [ReO2(amine)2]+ cations are thermodynamically stable even when protonated. On the other hand, analogous Tc compounds are not so stable and easily decompose if existing as [TcO(OH) (amine)2]2+.
Sulphamethizole-Cyclodextrin-Hydroxy Propylmethyl Cellulose Multicomponent Complexes
https://doi.org/10.1023/a:1016016525607
Authors: B. Pose-Vilarnovo, X2 Rodríguez-Tenreiro Sánchez, M. Pérez-Marcos, and J. Torres-Labandeira
The effect of cyclodextrin complexation of sulphamethizole (SM) was studied. Two systems were prepared with two cyclodextrin derivatives, β-cyclodextrin (BCD) and hydroxypropyl-β-cyclodextrin (HPBCD): binary complexes and multicomponent systems (cyclodextrins and a hydroxylpropylmethyl cellulose K4M). Inclusion complexes were prepared by freeze-drying and characterized by thermal analysis (DSC) and X-ray diffractometry. The presence of the polymer in the solution increases the effect of cyclodextrins – specially BCD – on the solubility of SM. In solid state, binary inclusion complexes enhance the dissolution behaviour of SM but, from the multi-component complexes, the polymer controls the release of the drug.
Thermal Analysis of Acrylonitrile Polymerization and Cyclization in the Presence of N,N-Dimethylformamide
Authors: M. Avilés, J. Ginés, J. del Rio, J. Pascual, J. Pérez-Rodríguez, and P. Sánchez-Soto
This paper examines the polymerization of acrylonitrile to poly(acrylonitrile)(PAN), and its cyclization, in bulk form and using N,N-dimethylformamide (DMF) as solvent in which both monomer and polymer are soluble. Thermal analysis of the resultant products after polymerization has been performed by DSC and pyrolysis gas chromatography/mass spectrometry (Py-GC/MS). Scanning electron microscopy has been used to study the morphology of the resultant products and after thermal treatments. The DSC thermal curve of PAN-DMF sample is quite different from the PAN bulk sample, showing a single sharp exothermic peak associated with nitrile group polymerization (cyclization) of PAN at lower temperature (240°C) than that of bulk PAN sample (314°C). Cyclization of PAN was confirmed by IR spectroscopy. It was found that the amide molecules are difficult to eliminate completely in the product obtained after the polymerization reaction, even after prolonged heating at 110°C, and remain occluded. The formation of a complex by dipolar bonding is also possible and it is discussed. It is concluded that the amount of heat evolved as well as the temperature interval over which it is released are influenced by the chemical processing of PAN when using DMF as solvent of both monomer and polymer. Pyrolysis of these PAN samples revealed the release of occluded molecules of DMF, and several compounds containing nitrogen produced from the thermal degradation processes. All these results are interesting to know the chemical processing of carbon fibres and activated carbon fibres from PAN modified precursors.
Dependence of the preexponential factor on temperature
Errors in the activation energies calculated by assuming that Ais constant
Authors: J. M. Criado, L. A. Pérez-Maqueda, and P. E. Sánchez-Jiménez
The dependence of the preexponential factor on the temperature has been examined and the errors involved in the activation energy calculated from isothermal and non-isothermal methods without considering such dependence have been estimated. It has been shown that the error in the determination of the activation energy calculated ignoring the dependence of Aon Tcan be rather large and it is dependent on x=E/RT, but independent of the experimental method used. It has been also shown that the error introduced by omitting the dependence of the preexponential factor on the temperature is considerably larger than the error due to the Arrhenius integral approach used for carrying out the kinetic analysis of TG data.
Thermal mass changes of portland cement and SLAG cements after water sorption
Authors: E. Stepkowska, J. Bijen, J. Perez-Rodriguez, A. Justo, P. Sanchez-Soto, and M. Aviles
A simple water sorption/retention (WS/WR) test, followed by stepwise static heating, was applied to the study of cement quality and the reactivity of its grain surface. The physically bound water and hence the specific surface both in the unhydrated and in the hydrated state were estimated as a function of the hydration time. Rehydration after heating at 220°C and contact with air was different inWS from that inWR samples, which indicates a difference in microstructure. XRD proved the formation of portlandite during the sorption test and eventual heating at 200°C, and its transformation into carbonates on contact with air, especially on heating at 400°C. The contents of these compounds were estimated from the mass difference between 400 and 800°C, which was compatible with the mass change between 220 and 400°C and this indicates surface reactivity. The test may serve for the routine study of cement.
Chemical characterization and quality of the waters used for hemodialyses in the INEF from 2002 to 2004
Authors: N. Alberro Macias, I. Pupo González, L. Valcárcel Rojas, D. Frías Fonseca, J. R. Estévez Álvarez, D. López Sánchez, A. Montero Álvarez, D. Simón Pérez, M. A. Isaac Tejera, and J. F. Pérez Oliva
The quality of the potable and purified for haemodialysis waters used in the National Institute of Nephrology was evaluated since 2002 up to now. A total of 20 chemical components were analyzed. The analytical results were compared with the admissible maximum concentrations according to the Cuban Standard NC 92-02:85 for potable water and with the Spanish Standard UNE 111-301-90, related to the quality of water for use in haemodialysis. The quality of both types of water was found to comply with the Standards regulations. The CEADEN analytical chemistry laboratory operates a quality management system since 1992, that was credited according to ISO/IEC 17025 requirements.