Estudio magnetoestructural y cinético de las transiciones de fase en estado sólido de aleaciones metaestables basadas en Fe

Esta tesis se centra en el análisis de las transformaciones de fase en estado sólido que exhiben ciertas aleaciones metaestables basadas en Fe. Para ello se ha analizado la evolución estructural y magnética que experimentan diferentes sistemas durante las transiciones que presentan, así como su cinética. En primer lugar, se han analizado aleaciones amorfas binarias y tipo FINEMET, utilizadas como precursores para el desarrollo de aleaciones nanocristalinas. La comparación sistemática de los resultados obtenidos a través de diferentes técnicas de caracterización ha permitido analizar tanto el proceso de amorfización mecánica como las fases resultantes del proceso de desvitrificación. Para su caracterización cinética se ha desarrollado un modelo a partir de la teoría clásica de cristalización asumiendo varios procesos simultáneos. Este modelo permite la descripción de los procesos de desvitrificación que dan lugar a varias fases y se ha extendido a un conjunto de múltiples microprocesos para la descripción del proceso de nanocristalización. En segundo lugar, se ha evaluado la estabilidad de la transformación martensítica en sistemas Heusler Ni-Fe-Ga obtenidos por enfriamiento rápido. Aunque ésta técnica de producción permite la obtención de muestras metaestables monofásicas de la fase modulada 14M a temperatura ambiente, la transformación martensítica es especialmente sensible a la temperatura y la presión hidrostática. La alta velocidad de crecimiento de la fase austenita impide que la transformación martensítica sea descrita mediante la teoría clásica de cristalización, mostrándose que realmente responde a un proceso autocatalítico de primer orden. Finalmente, varios factores experimentales que pueden afectar a una correcta caracterización magnética de los sistemas estudiados han sido evaluados a partir de datos experimentales y cálculos numéricos. En particular, el análisis de la respuesta teórica de sistemas heterogéneos ha permitido proporcionar una serie de ecuaciones simples con las que estimar los parámetros de la distribución de temperaturas de transición

¿Qué investigación didáctica en el aula de física se publica en España? Una revisión crítica de la última década para el caso de educación secundaria

Se presenta un estudio exploratorio y descriptivo sobre la investigación didáctica publicada en España durante la última década relativa a la enseñanza de la física en educación secundaria. Al centrar la atención en aquellos estudios que analizan algún aspecto del proceso de enseñanza-aprendizaje en el aula, se obtiene que estos constituyen una minoría en el conjunto de la bibliografía consultada. Se encuentra también que la autoría de profesorado de secundaria en los estudios sobre didáctica de la física consultados es relativamente baja. Asimismo, en los estudios predomina un enfoque de la enseñanza de la física orientado por contenidos disciplinares que no sintonizan con los planteamientos basados en el desarrollo de competencias. Todo ello revela cierta distancia entre investigación y práctica en la enseñanza de la física.An explorative and descriptive study about educational research concerning physics teaching in secondary education, published in Spain during the last decade, is presented. When focusing on those studies oriented towards the analysis of some aspects of the teaching/learning processes within classroom, it is found that they constituted a minority in the frame of the consulted literature. It is also found that the number of studies on physics education authored by secondary teachers is relatively low. Likewise, an approach to physics teaching guided by disciplinary contents is predominant in these studies, which is not in tune with those approaches based on the development of competences. All this reveals a certain disconnection between research and practice in physics education

On the use of JMAK theory to describe mechanical amorphization: a comparison between experiments, numerical solutions and simulations

The kinetics of amorphization during ball milling is generally analyzed using two different approaches: the classical Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory and Delogu and Cocco’s model for which a region deterministically transforms after it reaches a certain number of collisions. The application of JMAK analysis to the latter model predicts Avrami exponents to be higher than the experimental ones (typically close to one). We develop simulations based on the probabilistic character of the nucleation phenomenon and concave growth of the amorphous phase in the core of a nanocrystal. The predictions of our simulations are in good agreement with the low Avrami exponents and with the size evolution of the remaining crystallites found experimentally. From these values, the parameters involved in the simulated model (growth rate and probability of nucleation) can be estimated.AEI/FEDER-UE (Project MAT-2016-77265-R)Junta de Andalucía (Grupo PAI

Influence of Milling Time on the Homogeneity and Magnetism of a Fe70Zr30 Partially Amorphous Alloy: Distribution of Curie Temperatures

In this work, the mechanically alloyed Fe70Zr30 (at. %) composition has been used to study the influence of milling time on its homogeneity and magnetic properties. The microstructure and Fe environment results show the formation of an almost fully amorphous alloy after 50 h of milling in a mixture of pure 70 at. % Fe and 30 at. % Zr. The soft magnetic behavior of the samples enhances with the increase of the milling time, which is ascribed to the averaging out of the magnetocrystalline anisotropy as the crystal size decreases and the amorphous fraction increases. The formation of a non-perfectly homogenous system leads to a certain compositional heterogeneity, motivating the existence of a distribution of Curie temperatures. The parameters of the distribution (the average Curie temperature, T C ¯ , and the broadening of the distribution, ∆ T C ) have been obtained using a recently reported procedure, based on the analysis of the approach towards the saturation curves and the magnetocaloric effect. The decrease of ∆ T C and the increase of T C ¯ with the milling time are in agreement with the microstructural results. As the remaining α-Fe phase decreases, the amorphous matrix is enriched in Fe atoms, enhancing its magnetic response.AEI/FEDER-UE Project MAT 2016-77265-

Effect of reactive flash sintering on the magnetic and hyperfine parameters of SrFe12O19 ceramic permanent magnets

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A comparative study of magnetic and electrical properties of BiFeO3 mutiferroic ceramics sintered by electric field assisted-methods: Spark plasma sintering and flash sintering

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Reactive Flash Sintering of High Entropy (Mn0.2Co0.2Ni0.2Cu 0.2X 0.2)Fe2O4 (X=Fe, Mg) Spinel Oxides

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Distribution of transition temperatures in magnetic transformations: Sources, effects and procedures to extract information from experimental data

The presence of a distribution of transition temperatures (DTT) is ubiquitous in materials science. It is common to ascribe deviations from theoretical pure-phase behavior to this fact. To adapt the different pure phase models to systems with a DTT, the parameters of such distribution must be known or at least estimated. In this review, the different sources for the existence of such distributions and their effects on magnetothermal properties are summarized. In addition, different models proposed to extract the parameters of the corresponding DTT are discussed and extended, starting from Weiss model, to account for other phenomenologies. Experimental results on amorphous Fe-Nb-B and intermetallic MnCo(Fe)Ge systems are also reported.Ministerio de Ciencia e Innovación MAT 2016-77265-

A procedure to obtain the parameters of Curie temperature distribution from thermomagnetic and magnetocaloric data

We propose a procedure for the determination of the parameters of the Curie temperature distribution (TCD)in a compositionally inhomogeneous ferromagnetic material. Assuming a Gaussian TCD and using a mean field approach based on the Brillouin function, we report that with respect to the average value of the distribution: a)both inflection point of magnetization, Tinf, and temperature at maximum magnetic entropy change curves, Tpk MCE, shift to lower temperatures and b)temperature at maximum paramagnetic susceptibility, Tpk χ, shifts to higher temperatures. Using these evolutions as a function of the TCD broadening and fitting them to a second order polynomial function, a self-consistent procedure to determine the parameters of the distribution is supplied. These predictions have been experimentally tested for a ball milled Fe70Zr30 amorphous alloy using thermomagnetic and magnetocaloric measurements. The obtained parameters using the proposed procedure agree with those directly measured using Mössbauer spectrometry.Agencia Estatal de Investigación MAT 2016-77265-

Nanocrystallization Kinetics Understood as Multiple Microprocesses Following the Classical Theory of Crystallization

In this work, we propose a model for using the classical Johnson-Mehl-Avrami-Kolmogorov (JMAK) crystallization theory to analyze nanocrystallization processes as a set of multiple microprocesses. This model is based on the well-known microstructure observations of nanocrystalline systems for which, although the number of crystallites increases along the process, the growth of each crystallite is limited to a much shorter time than the needed one for completing the process. The very low values of the Avrami exponent, n∼1, are well reproduced assuming a set of multiple classical JMAK processes with constant nucleation rate and diffusion controlled growth, ni = 2.5 for each i individual microprocess. It is shown that the values of Avrami exponent experimentally observed and lying out of the theoretical range can be assumed as effective values derived from a complex process consisting of multiple microprocesses, which can be individually treated as classical ones.Ministerio de Economía y Competitividad MAT2013-45165-PEuropean Commission (EC). Fondo Europeo de Desarrollo Regional (FEDER) MAT2013-45165-