Scientific Papers in SCI


Title: Influence of plasma-generated negative oxygen ion impingement on magnetron sputtered amorphous SiO2 thin films during growth at low temperatures
Author(s): Macias-Montero, M; Garcia-Garcia, FJ; Alvarez, R; Gil-Rostra, J; Gonzalez, JC; Cotrino, J; Gonzalez-Elipe, AR; Palmero, A
Source: Journal of Applied Physics, 111 (2012) 054312 (6 pages)

abstract | fulltext

Growth of amorphous SiO2 thin films deposited by reactive magnetron sputtering at low temperatures has been studied under different oxygen partial pressure conditions. Film microstructures varied from coalescent vertical column-like to homogeneous compact microstructures, possessing all similar refractive indexes. A discussion on the process responsible for the different microstructures is carried out focusing on the influence of (i) the surface shadowing mechanism, (ii) the positive ion impingement on the film, and (iii) the negative ion impingement. We conclude that only the trend followed by the latter and, in particular, the impingement of O- ions with kinetic energies between 20 and 200 eV, agrees with the resulting microstructural changes. Overall, it is also demonstrated that there are two main microstructuring regimes in the growth of amorphous SiO2 thin films by magnetron sputtering at low temperatures, controlled by the amount of O2 in the deposition reactor, which stem from the competition between surface shadowing and ion-induced adatom surface mobility.

March, 2012 | DOI: 10.1063/1.3691950

Title: Nanoscale mechanically induced structural and electrical changes in Ge 2Sb 2Te 5 films
Author(s): Cecchini, R; Benitez, JJ; Sanchez-Lopez, JC; Fernandez, A
Source: Journal of Applied Physics, 111 (2012) 016101 (3 pages)

abstract | fulltext

We demonstrate that the microstructure and electrical properties of Ge2Sb2Te5 films can be changed by a nanoscale mechanical process. Nanoscratching is used to define modified areas onto an as-deposited crystalline Ge2Sb2Te5 film. Scanning tunneling microscopy measurements show that the modified areas have a very low electrical conductivity. Micro-Raman measurements indicate that the mechanically induced microstructural changes are consistent with a phase transformation from crystalline to amorphous, which can be reversed by laser irradiation.

January, 2012 | DOI: 10.1063/1.3673592

Title: Aluminum solubility in TiO2 rutile at high pressure and experimental evidence for a CaCl2-structured polymorph
Author(s): Escudero, A; Langenhorst, F; Muller, WF
Source: American Mineralogist, 97 (2012) 1075-1082

abstract | fulltext

Aluminum incorporation into TiO 2 has been studied in the TiO 2-Al 2O 3 system as a function of pressure at temperatures of 900 and 1300 °C using commercial Al 2TiO 5 nanopowder as starting material. A new orthorhombic TiO 2 polymorph with the CaCl 2 structure has been observed in the recovered samples synthesized from 4.5 to 7 GPa and 900 °C and from 2.5 to 7 GPa at 1300 °C. The phase transition to the α-PbO 2 type TiO 2 phase takes place between 7 and 10 GPa at both temperatures. Two mechanisms of Al incorporation in TiO 2 rutile have been observed in the recovered samples. The substitution of Ti 4+ by Al 3+ on normal octahedral sites is dominant at lower pressures. High pressure induces the incorporation of Al 3+ into octahedral interstices of the rutile structure, which is responsible for an orthorhombic distortion of the TiO 2 rutile structure and gives rise to a (110) twinned CaCl 2 type structure. This phase is probably a result of temperature quench at high pressure. Aluminum solubility in TiO 2 increases with increasing pressure. TiO 2 is able to accommodate up to 9.8 wt% Al 2O 3 at 7 GPa and 1300 °C. Temperature has a large effect on the aluminum incorporation in TiO 2, especially at higher pressures. High pressure has a strong effect on both the chemistry and the microstructure of Al-doped TiO 2. Enhanced aluminum concentration in TiO 2 rutile as well as TiO 2 grains with a microstructure consisting of twins are a clear indication of high-pressure conditions.

July, 2012 | DOI: 10.2138/am.2012.4049

Title: Incorporation of Si into TiO2 phases at high pressure
Author(s): Escudero, A; Langenhorst, F
Source: American Mineralogist, 97 (2012) 524-531

abstract | fulltext

Silicon incorporation in TiO 2 phases at increasing pressures until 20 GPa at 1300 °C has been studied by XRD and TEM. Rutile is the stable Si-doped TiO 2 phase until at least 7 GPa, transforming into α-PbO 2 structured TiO2 between 7 and 10 GPa. The further transformation to the TiO 2 polymorph with the baddeleyite structure, akaogiite, has not been observed on the quenched samples. XRD and TEM-EDX data suggest that the Si-doped TiO 2 akaogiite polymorph is non-quenchable and reverts to a-PbO2 structured TiO 2 when releasing the pressure. This transformation gives rise to α-PbO 2 structured TiO 2 grains decorated with p fringes stacking faults. Silicon solubility in TiO 2 phases increases with increasing the synthesis pressure until 16 GPa, implying the substitutional solid solution to be the mechanism of solubility. The influence of the dopants on the stability of the rutile and the α-PbO2 structured TiO 2 has also been analyzed.

April, 2012 | DOI: 10.2138/​am.2012.3941

Title: Changes on the nanostructure of cementitius calcium silicate hydrates (C-S-H) induced by aqueous carbonation
Author(s): Morales-Florez, V; Findling, N; Brunet, F
Source: Journal of Materials Science, 47 (2012) 764-771

abstract | fulltext

The nanostructure of the main binding phase of the hydrated cements, the calcium silicate hydrates (C–S–H), and their structural changes due to aqueous carbonation have been characterized using TEM, nitrogen physisorption, and SAXS. Synthetic C–S–H has been used for this purpose. Two different morphologies were identified, similar to the high density and low density C–S–H types. When submitting the sample to a CO2 flux, the low density phase was completely carbonated. The carbonation by-products, calcium carbonate, and silica gel were also identified and characterized. The precipitation of the silica gel increased the specific surface area from 95 to 132 m2/g, and its structure, formed by particles of ~5 nm typical radius, was observed by small angle X-ray scattering. In addition, the resistance of the high density C–S–H to carbonation is reported, and the passivating effect of the precipitated calcium carbonate is also discussed. Finally, the results have been compared with carbonation features observed in Portland cement carbonated experimentally at downhole conditions.

January, 2012 | DOI: 10.1007/s10853-011-5852-6

Title: Chemical–physical characterization of isolated plant cuticles subjected to low-dose γ-irradiation
Author(s): Heredia-Guerrero, Jose A; de Lara, Rocio; Dominguez, Eva; Heredia, Antonio; Benavente, Juana; Benitez, Jose J
Source: Chemistry and physics of lipids, 165 (2012) 803-808

abstract | fulltext

Isolated tomato fruit cuticles were subjected to low dose (80 Gy) γ-irradiation, as a potential methodology to prevent harvested fruit and vegetables spoilage. Both irradiated and non-irradiated samples have been morphologically and chemically characterized by scanning electron (SEM), atomic force (AFM), attenuated total reflectance Fourier transform infrared (ATR-FTIR) and X-ray photoelectron (XPS) spectroscopies. Additionally, electrochemical measurements comprising membrane potential and diffusive permeability were carried out to detect modifications in transport properties of the cuticle as the fruit primary protective membrane. It has been found that low dose γ-irradiation causes some textural changes on the surface but no significant chemical modification. Texture modification is found to be due to a partial removal of outermost (epicuticular) waxes which is accompanied by mild changes of electrochemical parameters such as the membrane fixed charge, cation transport number and salt permeability. The modification of such parameters indicates a slight reduction of the barrier properties of the cuticle upon low dose γ-irradiation.

December, 2012 | DOI: 10.1016/j.chemphyslip.2012.10.003

Title: Spark plasma sintering of Ti yNb 1-yC xN 1-x monolithic ceramics obtained by mechanically induced self-sustaining reaction
Author(s): Borrell, A; Salvador, MD; Garcia-Rocha, V; Fernandez, A; Chicardi, E; Gotor, FJ
Source: Materials Science and Engineering A, 543 (2012) 173-179

abstract | fulltext

Nanometer-sized titanium-niobium carbonitride powders (Ti yNb 1-yC xN 1-x) with different Ti/Nb atomic ratios were obtained by a mechanically induced self-sustaining reaction, and sintered by spark plasma sintering technique at 1500°C for 1min in a vacuum atmosphere. Mechanical properties such as hardness and Young's modulus were determined by nanoindentation technique and friction and wear coefficients assessed by ball-on-disk testing using alumina ball in dry sliding conditions. The fracture surface and wear tracks of samples were examined by scanning electron microscopy. Results showed that it is possible to obtain dense monolithic ceramics from the solid solution (Ti yNb 1-yC xN 1-x) with good mechanical properties and excellent wear resistance. The optimum values of nanomechanical properties were found for the Ti 0.3Nb 0.7C 0.5N 0.5 ceramic composition, which exhibited a high hardness over 26.0GPa and Young's modulus around 400GPa.

May, 2012 | DOI: 10.1016/j.msea.2012.02.071

Title: Residual stresses in Al2O3-ZrO 2 (3mol.% Y2O3) directionally solidified eutectic ceramics as a function of temperature
Author(s): Ramirez-Rico, J; Martinez-Fernandez, J; Pena, JI; Singh, D; Routbort, J
Source: Materials Science and Engineering A, 541 (2012) 61-66

abstract | fulltext

Directionally solidified eutectics are in situ composites grown from the melt. Due to the differences in the thermoelastic properties of the different phases present in the material, these composites often exhibit residual stresses that can affect their mechanical properties. In this work we use neutron diffraction to investigate residual stresses in Al 2O 3-ZrO 2 eutectic composites as a function of temperature, for samples processed at two different growth rates, 10mm/h and 750mm/h. Our results show that the stress-free temperature is in the range of 1200±200°C. We explain the experimental observations based on the thermoelastic properties of the phases in the material and confirm our measurements using a simple, self-consistent model.

April, 2012 | DOI: 10.1016/j.msea.2012.02.001

Title: Effect of oxidation on the compressive strength of sintered SiC-fiber bonded ceramics
Author(s): Ramirez-Rico, J; Martinez-Fernandez, J; Singh, M
Source: Materials Science and Engineering A, 534 (2012) 394-399

abstract | fulltext

The compressive strength of SiC-fiber bonded ceramics obtained from hot-pressed amorphous Si-Al-C-O fibers and its degradation by high temperature exposure to an oxidizing environment was studied. Compressive strength was measured at room temperature as a function of strain rate, orientation, and oxidation temperature. Weight loss was monitored as a function of exposure time in atmospheric air at temperatures ranging from 800 to 1600°C, for times ranging from 0.5 to 5. h. Room-temperature compressive strength had a moderate decrease after exposures at 800°C associated to carbon burnout; increased for exposures in the range 1000-1500°C due to a defect-blunting action of the silica scale; and decreased significantly at 1600°C due to extensive surface recession.

February, 2012 | DOI: 10.1016/j.msea.2011.11.085

Title: Microcalorimetric study of the annealing hardening mechanism of a Cu–2.8Ni–1.4Si (at%) alloy
Author(s): Donoso, E; Espinoza, R; Dianez, MJ; Criado, JM
Source: Materials Science and Engineering: A, 556 (2012) 612-616

abstract | fulltext

The precipitation processes in a Cu–2.8 at% Ni–1.4 at% Si alloy were studied using differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and microhardeness measurements. The analysis of the calorimetric curves from room temperature to 900 K shows the presence of one exothermic reaction attributed to the formation of δ-Ni2Si particles in the copper matrix that was confirmed by Transmission Electron Microscopy (TEM) and EDS microanalysis. The activation energies calculated for the precipitation of δ-Ni2Si was lower than the ones corresponding to diffusion of nickel and silicon in copper. A correlation between of microhardness of the alloy and the formation of δ-Ni2Si particles has been found.

October, 2012 | DOI: 10.1016/j.msea.2012.07.035

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