J. M. Criado

Universidad de Sevilla, Hispalis, Andalusia, Spain

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Publications (179)222.28 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: In this work, a mechanochemical procedure is proposed as a simple and fast method to synthesize the pure BiFeO3 perovskite phase as a nanostructured material without the need for purification treatments, while the mechanochemical reaction mechanism has been investigated and correlated with that of the conventional solid-state reaction. Thus, different milling conditions have been used as a tool for tailoring the crystallite size of the resulting BiFeO3 nanoparticles. The materials prepared by the mechanochemical reaction could be annealed or sintered without the formation of undesirable phases. Both the ferroelectric and ferromagnetic transitions were observed by DSC. Finally, the dielectric constants of the prepared material at different frequencies as a function of the temperature have been measured, showing that the material is clearly an isolator below 200 °C, characteristic of a high quality BiFeO3 material.
    J. Mater. Chem. C. 05/2013; 1(22):3551-3562.
  • Rapid Communications in Mass Spectrometry 02/2013; 27(3):500-2. · 2.51 Impact Factor
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    ABSTRACT: BACKGROUND: This paper provides some clarifications regarding the use of model-fitting methods of kinetic analysis for estimating the activation energy of a process, in response to some results recently published in Chemistry Central journal. FINDINGS: The model fitting methods of Arrhenius and Savata are used to determine the activation energy of a single simulated curve. It is shown that most kinetic models correctly fit the data, each providing a different value for the activation energy. Therefore it is not really possible to determine the correct activation energy from a single non-isothermal curve. On the other hand, when a set of curves are recorded under different heating schedules are used, the correct kinetic parameters can be clearly discerned. CONCLUSIONS: Here, it is shown that the activation energy and the kinetic model cannot be unambiguously determined from a single experimental curve recorded under non isothermal conditions. Thus, the use of a set of curves recorded under different heating schedules is mandatory if model-fitting methods are employed.
    Chemistry Central Journal 02/2013; 7(1):25. · 1.31 Impact Factor
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    ABSTRACT: Hematite nanoparticles of narrow size distribution were prepared by grinding of goethite. Intermediate and final products of grinding were characterised by different techniques, including the less-common emanation thermal analysis (ETA). ETA was shown to be a useful technique for characterising processes of surface annealing, initial sintering and growth of hematite particles under in situ conditions of thermal treatment. A good agreement was found between results of ETA, TG, XRD, IR spectrometry, transmission electron microscopy and scanning electron microscopy, used for characterisation of thermal behaviour of the goethite samples ground for varying time (0–70 h).
    Journal of Thermal Analysis and Calorimetry 04/2012; 60(3):997-1007. · 1.98 Impact Factor
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    ABSTRACT: The present recommendations have been developed by the Kinetics Committee of the International Confederation for Thermal Analysis and Calorimetry (ICTAC). The recommendations offer guidance for reliable evaluation of kinetic parameters (the activation energy, the pre-exponential factor, and the reaction model) from the data obtained by means of thermal analysis methods such as thermogravimetry (TGA), differential scanning calorimetry (DSC), and differential thermal analysis (DTA). The recommendations cover the most common kinetic methods, model-free (isoconversional) as well as model-fitting. The focus is on the problems faced by various kinetic methods and on the ways how these problems can be resolved. Recommendations on making reliable kinetic predictions are also provided. The objective of these recommendations is to help a non-expert with efficiently performing analysis and interpreting its results.Highlights► Kinetics concepts are introduced and advices on collecting data are given. ► Methods based on multiple temperature programs are recommended. ► Evaluation of log A and f(α) or g(α) for isoconversional methods is described. ► Principles of securing reliability of model-fitting computations are offered. ► Problems of kinetic predictions and approaches to solving them are discussed.
    Thermochimica Acta 06/2011; 520:1-19. · 1.99 Impact Factor
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    ABSTRACT: The kinetic analysis of complex solid-state reactions that involve simultaneous overlapping processes is challenging. A method that involves the deconvolution of the individual processes from the overall differential kinetic curves obtained under linear heating rate conditions, followed by the kinetic analysis of the discrete processes using combined kinetic analysis, is proposed. Different conventional mathematical fitting functions have been tested for deconvolution, paying special attention to the shape analysis of the kinetic curves. It has been shown that many conventional mathematical curves such as the Gaussian and Lorentzian ones fit kinetic curves inaccurately and the subsequent kinetic analysis yields incorrect kinetic parameters. Alternatively, other fitting functions such as the Fraser-Suzuki one properly fit the kinetic curves independently of the kinetic model followed by the reaction and their kinetic parameters, and moreover, the subsequent kinetic analysis yields the correct kinetic parameters. The method has been tested with the kinetic analysis of complex processes, both simulated and experimental.
    The Journal of Physical Chemistry B 02/2011; 115(8):1780-91. · 3.61 Impact Factor
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    ABSTRACT: Phase transition of anatase nanoparticles into the phases TiO2-II and rutile under grinding was studied. The addition of ammonium carbamate to the reaction mixture inhibits the phase conversion and the cold welding of particles. The UV-visible absorption spectrum showed narrowing the band gap width after grinding with an ammonium carbamate additive resulting in shift of the light absorption of the ground sample towards the visible region. By EPR, intensive formation of OH• radical at irradiation of the sample with both UV (λ > 300 nm) and visible (λ > 435 nm) light was observed. High photocatalytic activity of the ground sample in visible light region was demonstrated also by measurement of kinetics of the photocatalytic decomposition of 4-chlorophenol.
    International Journal of Photoenergy 01/2011; 2011. · 2.66 Impact Factor
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    ABSTRACT: In this paper, the kinetic of precipitation process in mechanically alloyed Cu-1wt.% Cr and Cu-1wt.% Cr/3wt.% Al2O3 solid solution was compared using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The ageing kinetics in Cu–Cr and Cu–Cr/Al2O3 can be described using Johnson–Mehl–Avrami (JMA) and Sestak–Berggren (SB) models, respectively. These different behaviors have been discussed in details. It was found that in presence of Al2O3 reinforcement, the ageing activation energy is decreased and the overall ageing process is accelerated. This behavior is probably due to higher dislocation density previously obtained during ball milling and Al2O3–Cu interface. TEM observations confirm that Al2O3–Cu interface and structural defects act as a primary and secondary nucleation sites, respectively.
    Thermochimica Acta - THERMOCHIM ACTA. 01/2011; 526(1):222-228.
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    ABSTRACT: This paper explores the relationship between the shapes of temperature–time curves obtained from experimental data recorded by means of constant rate thermal analysis (CRTA) and the kinetic model followed by the thermal degradation reaction. A detailed shape analysis of CRTA curves has been performed as a function of the most common kinetic models. The analysis has been validated with simulated data, and with experimental data recorded from the thermal degradation of polytetrafluoroethylene (PTFE), poly(1,4-butylene terephthalate) (PBT), polyethylene (PE) and poly(vinyl chloride) (PVC). The resulting temperature–time profiles indicate that the studied polymers decompose through phase boundary, random scission, diffusion and nucleation mechanisms respectively. The results here presented demonstrate that the strong dependence of the temperature–time profile on the reaction mechanism would allow the real kinetic model obeyed by a reaction to be discerned from a single CRTA curve.
    Polymer Degradation and Stability 01/2011; 96(5):974-981. · 2.77 Impact Factor
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    ABSTRACT: This paper explores the potential of sample-controlled thermal analysis (SCTA) in order to perform compositional analysis of multicomponent polymeric materials by means of thermogravimetric experiments. In SCTA experiments, the response of the sample to the temperature determines the evolution of the temperature by means of a feedback system; thus, what is controlled is not the temperature-time profile, as in conventional analysis, but rather the evolution of the reaction rate with time. The higher resolving power provided by the technique has been used for determining the composition of polymer blends composed of polyvinyl chloride (PVC) and different commercial plasticizers, a system where the individual components have very similar thermal stabilities, thereby rendering useless thermogravimetric experiments run under conventional conditions. Different SCTA procedures, such as constant rate thermal analysis (CRTA), which has received special attention, and high-resolution and stepwise isothermal analysis have been tested, and the results obtained have been compared with linear heating rate technique. It has been proven that CRTA can be used to effectively determine the exact composition of the blend.
    Analytical Chemistry 10/2010; · 5.70 Impact Factor
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    ABSTRACT: In this paper, the f(alpha) conversion functions for random scission mechanisms have been proposed to allow for the construction of generalized master plots suitable for these kinds of mechanisms. The master plots have been validated by their application to simulated data and to the thermal degradation of poly(butylene terephthalate), polyethylene, and poly(tetrafluoroethylene).
    The Journal of Physical Chemistry A 08/2010; 114(30):7868-76. · 2.77 Impact Factor
  • C. REAL, M. D. ALCALA, J. M. CRIADO
    ChemInform 01/2010; 27(49).
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    ABSTRACT: In this paper, a series of f(α) kinetic equations able to describe the random scission degradation of polymers is formulated in such a way that the reaction rate of the thermal degradation of polymers that go through a random scission mechanism can be directly related to the reacted fraction. The proposed equations are validated by a study of the thermal degradation of poly(butylene terephthalate) (PBT). The combined kinetic analysis of thermal degradation curves of this polymer obtained under different thermal pathways have shown that the proposed equation fits all these curves while other conventional models used in literature do not.
    Polymer Degradation and Stability - POLYM DEGRAD STABIL. 01/2010; 95(5):733-739.
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    ABSTRACT: A mechanochemical procedure is proposed for an easy preparation of a BaTiO3–Ni composite in a single step. BaTiO3 and Ni powders available in the market are mixed by dry ball milling producing a decrease of particle size and an evenly distribution of both phases. In the sintered pellets the nickel particles are homogeneously distributed into the BaTiO3 matrix and isolated from others Ni particles. The dielectric constant of the composite is considerably higher than that of the barium titanate. Moreover, the temperature of the ferroelectric ↔ paraelectric transition of the BaTiO3–Ni composite here prepared is much lower than the one of the pure BaTiO3 single phase.
    Composite Structures - COMPOS STRUCT. 01/2010; 92(9):2236-2240.
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    ABSTRACT: In this paper, a novel method for calculating degradation kinetics is presented. The method has been applied to the thermal dehydrochlorination of two different samples of PVC. It has been observed that this dehydrochlorination is complex and involves two different processes. A model that accounts for the entire dehydrochlorination is proposed. This model involves nucleation and growth and diffusion controlled mechanisms. The kinetic parameters are obtained from linear heating rate, isothermal and sample controlled thermal analysis experiments. Kinetic results obtained from the macroscopic thermal analysis measurements demonstrate the correlation between the kinetics of the thermal dehydrochlorination of PVC and the structure of this macromolecule.
    Polymer. 01/2010; 51(17):3998-4007.
  • M. D. ALCALA, C. REAL, J. M. CRIADO
    ChemInform 01/2010; 27(43).
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    ABSTRACT: The micro and nanostructure of Cu-Al, Cu-V and Cu-Ti alloys produced by reactive milling were analyzed using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Samples with different milling times (t= 0, 10, 20 and 30 h) were considered. The grain size, dislocation density and residual microstrain were evaluated form the XRD data using the Williamson-Hall and Klug-Alexander methods. The evolution of texture as a function of milling time was also studied using XRD. It was found, using TEM, that the grain size and dispersoid size were nanometric in all three alloys considered.Se analizó la micro y nano estructura de aleaciones Cu-Al, Cu-V y Cu-Ti obtenidas por molienda reactiva, mediante difracción de rayos X (XRD) y microscopía electrónica de transmisión (TEM). Se consideraron muestras con distintos tiempos de molienda (t= 0, 10, 20 y 30 h). A partir de los datos XRD, usando los métodos de Williamson- Hall y Klug-Alexander, se evaluaron el tamaño de grano, la densidad de dislocaciones y la microdeformación residual; también se estudió la evolución de la textura de la matriz de cobre en función del tiempo de molienda. En los polvos molidos durante 30 h, de las tres aleaciones consideradas, se encontró, por TEM, que los tamaños de grano y de los dispersoides desarrollados son nanométricos.
    Revista de Metalurgia. 01/2010;
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    ABSTRACT: Combined kinetic analysis has been applied for the first time to the thermal degradation of polymeric materials. The combined kinetic analysis allows the determination of the kinetic parameters from the simultaneous analysis of a set of experimental curves recorded under any thermal schedule. The method does not make any assumptions about the kinetic model or activation energy and allows analysis even when the process does not follow one of the ideal kinetic models already proposed in the literature. In the present paper the kinetics of the thermal degradation of both polytetrafluoroethylene (PTFE) and polyethylene (PE) have been analysed. It has been concluded, without previous assumptions on the kinetic model, that the thermal degradation of PTFE obeys a first order kinetic law, while the thermal degradation of PE follows a diffusion-controlled kinetic model.
    Polymer Degradation and Stability. 01/2009;
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    ABSTRACT: Silicon nitride (Si3N4) has been obtained from the carbothermal reduction of rice husks by applying the constant-rate-thermal-analysis (CRTA) method. Under this method, the reaction rate of carbothermal reduction can be controlled and at the same time the CO concentration generated is maintained constant at a previously selected value by the user. By using this synthesis technique it has been possible to obtain ceramic powder from rice husks with a determined phase composition and a controlled microstructure. Therefore, rice husks have great potential as a raw material for producing Si3N4 by the CRTA method.
    Journal of the American Ceramic Society 06/2008; 87(1):75 - 78. · 2.11 Impact Factor
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    ABSTRACT: The dynamic heating rate method developed by TA Instruments (Hi-ResTM) is a kind of sample controlled thermal analysis in which a linear relationship between the logarithm of the heating rate and the rate of mass change is imposed. It is shown in this paper that the reacted fraction at the maximum reaction rate strongly depends on the parameters selected for the Hi-Res heating algorithm, what invalidates the use of the Kissinger method for analysing Hi-Res data unless that the reaction fits a first order kinetic law. Only in this latter case, it has been demonstrated that it is not required that a constant value of the reacted fraction at the maximum reaction rate is fulfilled for determining the activation energy from the Kissinger method. In such a case the Kissinger plot gives the real activation energy, independently of both the heating schedule used and the value of the reacted fraction, αm, at the maximum.
    Journal of Thermal Analysis and Calorimetry 01/2008; 94(2):427-432. · 1.98 Impact Factor

Publication Stats

916 Citations
222.28 Total Impact Points

Institutions

  • 1971–2013
    • Universidad de Sevilla
      • Inorganic Chemistry
      Hispalis, Andalusia, Spain
  • 1999–2012
    • Scientific Research Centre "Isla de la Cartuja"
      • Institute of Materials Science
      Hispalis, Andalusia, Spain
  • 2010–2011
    • Instituto de Ciencia de Materiales de Madrid
      Madrid, Madrid, Spain
  • 2004
    • Universidad Católica de Córdoba
      Córdoba, Córdoba, Argentina
  • 1990–1992
    • Materials Science Institute of Barcelona
      Barcino, Catalonia, Spain