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Carbothermal synthesis of silicon nitride(Si3N4): Kinetics and diffusion mechanism

Ortega, A.; Alcala, M. D.; Real, C.
Journmal of Materials Processing Technology, 195 (2008) 224-231
Reactividad de Sólidos


Sample controlled reaction temperature (SCRT) has been used for the first time to study the kinetics of the carbothermal reduction of silica in nitrogen to obtain silicon nitride (Si3N4). It is noteworthy to point out that SCRT method allows both a good control of pressure in the sample surroundings and the use of reaction rates low enough to keep temperature gradients at a negligible level to avoid any heat or mass transfer phenomena. It has been shown that the carbothermal synthesis of silicon nitride is best described by a two-dimensional diffusion kinetic model with an activation energy of about 306 kJ/mol. Effects of experimental parameters on the activation energy has been investigated. By using SCRT it has been possible to obtain ceramic powder with a determined phase composition and a controlled microstructure. 

Enero, 2008 | DOI: 10.1016/j.jmatprotec.2007.05.004

Influence of amine template on the photoactivity of TiO2 nanoparticles obtained by hydrothermal treatment

Colon, G; Hidalgo, MC; Navio, JA; Melian, EP; Diaz, OG; Dona, JM
Applied Catalysis B-Environmental, 78 (2008) 176-182
Fotocatálisis Heterogénea: Aplicaciones - Materiales y Procesos Catalíticos de Interés Ambiental y Energético


TiO2 nanoparticles have been prepared by amine template-assisted sol-gel precipitation and further hydrothermal treatment. We have investigated the effect of different amines (hydrazine and triethylamine) in the final surface and structural properties. It has been stated that the different amounts of amine could act as an interesting template upon hydrothermal treatment. Further thermal treatment also leads to a significant improvement in the photocatalytic properties of the studied systems. Surface and morphological features notably differ from TiO2 prepared using different synthetic routes. Wide surface and structural characterization of the samples have been carried out, and correlations with precipitation pH are pointed out from this characterization. In all cases, amine template TiO2 obtained exhibit high conversion values for phenol photo-oxidation reaction. Further calcination treatment of all the studied systems clearly leads to photocatalytic conversions higher than that exhibited by TiO2 Degussa P25. 

Enero, 2008 | DOI: 10.1016/j.apcatb.2007.09.019

Thermal Behavior of Ground Talc Mineral

Balek, V; Subrt, J; Perez-Maqueda, LA; Benes, M; Bountseva, IM; Beckman, IN; Perez-Rodriguez, JL
Journal of Mining and Metallurgy Section B-Metallurgy, 44 (2008) 7-17
Reactividad de Sólidos


Microstructure development and thermal behavior of ground talc mineral samples (from locality Puebla de Lillo,Spain) was characterized by X-ray diffraction, scanning electron microscopy(SEM),surface area measurements differential thermal analysis, thermogravimetry and emanation thermal analysis (ETA). A vibratory mill and grinding time 5 min. was used to prepare the ground talc sample. It was found that grinding caused an increase in the surface area of the natural talc from 3 m(2)g(-1) to 110 m(2) g(-1). A decrease of particle size after sample grinding was observed by SEM. The increase of structure disorder of the ground sample and the crystallite size reduction from 40 to 10 nm were determined from the XRD results. ETA results revealed a closing up of surface micro-cracks and healing of microstructure irregularities on heating in the range 200 - 500 degrees C of both un-ground and ground talc samples. For the ground talc sample a crystallization of non-crystalline phase into orthorhombic enstatite was characterized as by a decrease of radon mobility in the range 785-825 degrees C and by a DTA exothermal effect with the maximum at 830 degrees C. ETA results were used for the assessment of transport properties of the talc samples on heating.

Enero, 2008 | DOI: 10.2298/JMMB0801007B

Syntesis by reactive grinding of molibdenum iron bimetallic nitride

Roldan, MA; Ortega, A; Palencia, I; Real, C
Revista de Metalurgia, 44 (2008) 13-18
Reactividad de Sólidos


The transition metal nitride ternary show similar properties to the binary nitride and some times this behaviour are improved. In the present work, the molybdenum-iron nitride has been prepared by reactive grinding from the two metals under nitrogen atmosphere at a pressure of 11 bar. The characterization of the compound is presented and it is also shown a study of the stability of the nitride under several atmospheres.

Enero, 2008 | DOI:

Spectral Response of Opal-Based Dye-Sensitized Solar Cells

Mihi, A.; Calvo, M. E.; Anta, J. A.; Miguez, H.
Journal of Physical Chemistry C, 112 (2008) 13-17
Materiales Ópticos Multifuncionales


Herein we present an experimental study of the spectral dependence of the photogenerated current of opal-based solar cells. We analyze the incident photon-to-current conversion efficiency (IPCE) for dye-sensitized solar cells in which colloidal crystals are introduced in different configurations. We prove that a dye-sensitized nanocrystalline titanium oxide electrode moulded in the shape of an inverse opal shows a decrease of efficiency for the spectral region in which a photonic stop band opens up. Contrarily, when a standard thin film of disordered titania nanocrystallites is coupled to an inverse opal, the mirror effect of the photonic crystal at band gap frequencies increases the light harvesting efficiency of the cell and thus the IPCE. This effect is further demonstrated by coupling an inverse opal multilayer to a homogeneous electrode, with two well-defined spectral ranges of increased photogenerated current being detected.

Enero, 2008 | DOI: 10.1021/jp7105633

Microstructural and chemical characterisation techniques for nanostructured and amorphous coatings

Godinho, V; Fernandez-Ramos, C; Martinez-Martinez, D; Garcia-Lopez, J; Sanchez-Lopez, JC; Fernandez, A
European Physical Journal-Applied Physics, 43 (2008) 333-341
Materiales Nanoestructurados y Microestructura - Tribología y Protección de Superficies


A full characterization of amorphous or nanostructured coatings at the microstructural level has some intrinsic difficulties associated with the lack of long range order and reference compounds, which often make difficult their study. Only by the combination of different characterization techniques is possible in many cases to achieve valuable chemical and structural information. In this paper, three different systems were used to illustrate how the combination of characterization techniques, as TEM associated to ED or EELS, EFTEM, SEM, XPS, RBS and XRD was determinant to correlate microstructure with deposition parameters and properties in such complex systems. The coatings were deposited on silicon and AISI M2 steel substrates by magnetron sputtering under different Ar/N-2 gas mixtures from Ti and C targets (system 1 and 2) or a Si target (system 3). In each case, the performed characterization allowed to get a deeper understanding of the whole system and explain their mechanical response. The studied systems are: (i) Ti-TiN-CNx multilayered coatings: the chemical and structural analysis shows that a gradual enrichment in nitrogen and nitride phases from the metallic substrate to the CNx top layer is responsible for the improvement of the adhesion properties. (ii) Ti-C-N: the existence of a nanocrystalline TiC phase embedded in an amorphous carbon matrix is demonstrated by the microstructural and chemical analysis for samples prepared under pure Ar. When N-2 is introduced in the gas phase, the nanocrystalline structure is not seen and the chemical composition is enriched in amorphous non-stoichiometric CNx. (iii) SiOxNy: although the coatings present similar composition, small differences in microstructure are observed, which can be responsible for different mechanical properties.

Septiembre, 2008 | DOI: 10.1051/epjap:2008114

Degradation of phenylmercury compounds by heterogeneous photocatalysis over alpha-Fe2O3

Navio, JA; Cerrillos, C; Macias, M; Pradera, MA
Journal of Advanced Oxidation Technologies, 11 (2008) 411-416
Fotocatálisis Heterogénea: Aplicaciones


We report here on the degradation of diphenylmercury and monophenylmercury chloride in air-equilibrated aqueous solutions by heterogeneous photocatalysis over alpha-Fe2O3. Photocatalysis proceeded via sequential de-phenylation leaving inorganic mercury. GC-MS analysis of the products revealed that photocatalytic degradation involved hydroxylation of the phenyl rings. Mechanistic features and photocatalytic activity were also studied and are reported upon.

Julio, 2008 | DOI: