Investigation on the structural and magnetic properties of sputtered TbFe2/Fe3Ga heterostructures

We have analyzed the structural and magnetic properties of as-grown and annealed [TbFe2/Fe3Ga]n heterostructures grown by sputtering. Evidence of the bcc structure in the Fe3Ga layers has been found. The diffraction peak related to this structure shifts to high angles with the annealing temperature. Also, we have observed a change in the microstructure of the Tb-Fe layers when the thickness layer is reduced in the as-grown heterostructures. Moreover, the Tb content is lower than 33% of the TbFe2 Laves phase and it depends on the layer thickness. The thermal treatments promote the increase of the Tb content, but only in the heterostructures with thick layers. The strong lattice mismatch between the Tb-Fe and Fe-Ga layers seems to prevent a complete Tb diffusion upon the annealing process. Thus, the crystallization of the TbFe2 Laves phase is inhibited in the heterostructures with thin layers, although our experimental results indicate the presence of potential magnetostrictive TbFeGa alloys

High precision pressure sensors based on SAW devices in the GHz range

In this paper, an AlN/free-standing nanocrystalline diamond (NCD) system is proposed in order to process high frequency surface acoustic wave (SAW) resonators for sensing applications. The main problem of synthetic diamond is its high surface roughness that worsens the sputtered AlN quality and hence the device response. In order to study the feasibility of this structure, AlN films from 150 nm up to 1200 nm thick have been deposited on free-standing NCD. We have then analysed the influence of the AlN layer thickness on its crystal quality and device response. Optimized thin films of 300 nm have been used to fabricate of one-port SAW resonators operating in the 10–14 GHz frequency range. A SAW based sensor pressure with a sensibility of 0.33 MHz/bar has been fabricated.

Confined crystallization of nanolayered poly(ethylene terephthalate) using X-ray diffraction methods

The development of crystalline lamellae in ultra-thin layers of poly(ethylene terephthalate) PET confined between polycarbonate (PC) layers in an alternating assembly is investigated as a function of layer thickness by means of X-ray diffraction methods. Isothermal crystallization from the glassy state is in-situ followed by means of small-angle X-ray diffraction. It is found that the reduced size of the PET layers influences the lamellar nanostructure and induces a preferential lamellar orientation. Two lamellar populations, flat-on and edge-on, are found to coexist in a wide range of crystallization temperatures (Tc = 117–150 °C) and within layer thicknesses down to 35 nm. Flat-on lamellae appear at a reduced crystallization rate with respect to bulk PET giving rise to crystals of similar dimensions separated by larger amorphous regions. In addition, a narrower distribution of lamellar orientations develops when the layer thickness is reduced or the crystallization temperature is raised. In case of edge-on lamellae, crystallization conditions also influence the development of lamellar orientation; however, the latter is little affected by the reduced size of the layers. Results suggest that flat-on lamellae arise as a consequence of spatial confinement and edge-on lamellae could be generated due to the interactions with the PC interface. En este trabajo se investiga mediante difracción de rayos X a ángulos bajos (SAXS) y a ángulos altos (WAXS), la cristalización de láminas delgadas de Polietilén tereftalato (PET) confinadas entre láminas de Policarbonato (PC), tomando como referencia PET sin confinar. El espesor de las capas de PET varía entre 35nm y 115 nm. Se realizaron medidas de difracción a tres temperaturas de cristalización (117ºC, 132ºC y 150ºC) encontrándose que el reducido espesor de las capas de PET influye en la estructura lamelar que se desarrolla, induciendo una orientación preferente de las láminas. Se integró la intensidad difractada alrededor del máximo en SAXS para obtener una representación de la intensidad en función del ángulo acimutal. Mediante análisis de mínimos cuadrados se separó la curva experimental obtenida en tres contribuciones diferentes: una función Gausiana que describe la distribución de las orientaciones de las lamelas, una función lorenziana asociada a los máximos meridionales (asociados a las interfases PET-PC) y un background constante. Por otra parte la cantidad de material cristalizado se estimó asumiendo que la intensidad del background en el barrido acimutal, una vez restado el background del primer difractograma (sin máximos en SAXS) se asocia con la contribución del material isotrópico que resta en la muestra cristalizada. Se observa la coexistencia de dos poblaciones de lamelas: flat-on y edge-on. A medida que el espesor de las láminas de PET disminuye la población de las lamelas flat-on experimenta los siguientes cambios: 1) la distribución de orientación se estrecha, 2) la fracción de material cristalizado orientado aumenta, 3) la cinética de cristalización se ralentiza y 4) el largo espaciado aumenta es decir las regiones amorfas entre lamelas aumentan su tamaño. Parece demostrarse que es en las primeras etapas del crecimiento lamelar cuando la restricción espacial fuerza a las lamelas a esta orientación tipo flat-on frente a la orientación edge-on.

Analysis of the surface state of epi-ready Ge wafers

The surface state of Ge epi-ready wafers (such as those used on III-V multijunction solar cells) supplied by two different vendors has been studied using X-ray photoemission spectroscopy. Our experimental results show that the oxide layer on the wafer surface is formed by GeO and GeO2. This oxide layer thickness differs among wafers coming from different suppliers. Besides, several contaminants appear on the wafer surfaces, carbon and probably chlorine being common to every wafer, irrespective of its origin. Wafers from one of the vendors show the presence of carbonates at their surfaces. On such wafers, traces of potassium seem to be present too.

Agent-based model for the effect of curing temperature on cement hydration

The agent-based model presented here, comprises an algorithm that computes the degree of hydration, the water consumption and the layer thickness of C-S-H gel as functions of time for different temperatures and different w/c ratios. The results are in agreement with reported experimental studies, demonstrating the applicability of the model. As the available experimental results regarding elevated curing temperature are scarce, the model could be recalibrated in the future. Combining the agent-based computational model with TGA analysis, a semiempirical method is achieved to be used for better understanding the microstructure development in ordinary cement pastes and to predict the influence of temperature on the hydration process.