Scientific Papers in SCI

2013


Title: Thermal Expansion of Rare-Earth Pyrosilicates
Author(s): Fernandez-Carrion, AJ; Allix, M; Becerro, AI
Source: Journal of the American Ceramic Society, 96 (2013) 2298-2305

abstract | fulltext

The use of RE2Si2O7 materials as environmental barrier coatings (EBCs) and in the sintering process of advanced ceramics demands a precise knowledge of the coefficient of thermal expansion of the RE2Si2O7. High-temperature X-ray diffraction (HTXRD) patterns were collected on different RE2Si2O7 polymorphs, namely A, G, α, β, γ, and δ, to determine the changes in unit cell dimensions. RE2Si2O7 compounds belonging to the same polymorph showed, qualitatively, very similar unit cell parameters behavior with temperature, whereas the different polymorphs of a given RE2Si2O7 compound exhibited markedly different thermal expansion evolution. The isotropy of thermal expansion was demonstrated for the A-RE2Si2O7 polymorph while the rest of polymorphs exhibited an anisotropic unit cell expansion with the biggest expansion directed along the REOx polyhedral chains. The apparent bulk thermal expansion coeficcients (ABCTE) were calculated from the unit cell volume expansion for each RE2Si2O7 compound. All compounds belonging to the same polymorph exhibited similar ABCTE values. However, the ABCTE values differ significantly from one polymorph to the other. The highest ABCTE values correspond to A-RE2Si2O7 compounds, with an average of 12.1 × 10−6 K−1, whereas the lowest values are those of β- and γ-RE2Si2O7, which showed average ABCTE values of ~4.0 × 10−6 K−1.

July, 2013 | DOI: 10.1111/jace.12388

Title: Dissociation of basal dislocations in 4 H - SiC single crystals deformed around the transition temperature
Author(s): Castillo-Rodriguez, M; Lara, A; Munoz, A; Dominguez-Rodriguez, A
Source: Journal of the American Ceramic Society, 96 (2013) 2921-2925

abstract | fulltext

The dislocation microstructure was studied in 4H–SiC samples plastically deformed by basal slip activation around the transition temperature (1000°C–1100°C). Dissociation of basal dislocations takes place over a wide temperature range (800°C–1300°C), but its influence on dislocation motion is different in the high- and low-temperature regimes due to the difference in mobility of partials. Consequently, this material exhibits a completely different mechanical behavior below and above its transition temperature, indicating a change in the deformation mechanism. In this work, the dislocation microstructure was studied around the transition temperature at which both mechanisms are still operative, thus providing a richer number of different configurations generated by dissociation of basal dislocations. They were observed and analyzed by means of the complementary use of weak-beam dark-field imaging and high-resolution transmission electron microscopy. Firstly, 3C band nucleation in the 4H–SiC matrix was identified and its appearance discussed from an energy standpoint. Secondly, the attractive interaction between partials in dipoles and the difference in mobility between the leading and the trailing partial have remarkable effects on the dissociation width, and explain the absence of work hardening above the transition temperature.

September, 2013 | DOI: 10.1111/jace.12434

Title: Mechanical properties of ZrB2- and HfB2-based ultra-high temperature ceramics fabricated by spark plasma sintering
Author(s): Zapata-Solvas, E; Jayaseelan, DD; Lin, HT; Brown, P; Lee, WE
Source: Journal of the European Ceramic Society, 33 (2013) 1373-1386

abstract | fulltext

Flexural strengths at room temperature, at 1400 °C in air and at room temperature after 1 h oxidation at 1400 °C were determined for ZrB2- and HfB2-based ultra-high temperature ceramics (UHTCs). Defects caused by electrical discharge machining (EDM) lowered measured strengths significantly and were used to calculate fracture toughness via a fracture mechanics approach. ZrB2 with 20 vol.% SiC had room temperature strength of 700 ± 90 MPa, fracture toughness of 6.4 ± 0.6 MPa, Vickers hardness at 9.8 N load of 21.1 ± 0.6 GPa, 1400 °C strength of 400 ± 30 MPa and room temperature strength after 1 h oxidation at 1400 °C of 678 ± 15 MPa with an oxide layer thickness of 45 ± 5 μm. HfB2 with 20 vol.% SiC showed room temperature strength of 620 ± 50 MPa, fracture toughness of 5.0 ± 0.4 MPa, Vickers hardness at 9.8 N load of 27.0 ± 0.6 GPa, 1400 °C strength of 590 ± 150 MPa and room temperature strength after 1 h oxidation at 1400 °C of 660 ± 25 MPa with an oxide layer thickness of 12 ± 1 μm. 2 wt.% La2O3 addition to UHTCs slightly reduced mechanical performance while increasing tolerance to property degradation after oxidation and effectively aided internal stress relaxation during spark plasma sintering (SPS) cooling, as quantified by X-ray diffraction (XRD). Slow crack growth was suggested as the failure mechanism at high temperatures as a consequence of sharp cracks formation during oxidation.

July, 2013 | DOI: 10.1016/j.jeurceramsoc.2012.12.009

Title: Preliminary investigation of flash sintering of SiC
Author(s): Zapata-Solvas, E; Bonilla, S; Wilshaw, PR; Todd, RI
Source: Journal of the European Ceramic Society, 33 (2013) 2811-2816

abstract | fulltext

The feasibility of flash sintering a covalent ceramic, SiC, has been investigated for the first time. Flash sintering involves the application of an electrical potential difference across a powder compact during heating, which leads to sintering at low furnace temperatures in a few seconds and has only been demonstrated with ionic ceramics previously. Near-theoretical density was achieved using Al2O3 + Y2O3 sintering aids at a furnace temperature of only 1170 degrees C and in a time of 150 s. Specimen temperatures were significantly higher than the furnace temperature owing to Joule heating and consequently heat loss limited densification in the near surface region. It was not possible to reach high densities using "ABC" sintering aids (aluminium-boron-carbon) or pure SiC. The mechanisms involved and potential commercial advantages are briefly discussed.

November, 2013 | DOI: 10.1016/j.jeurceramsoc.2013.04.023

Title: Effects of thermal and mechanical treatments on montmorillonite homoionized with mono- and polyvalent cations: Insight into the surface and structural changes
Author(s): Fernandez, M; Alba, MD; Sanchez, RMT
Source: Colloids and Surfaces A: Physicochemical and Engineering Aspects, 423 (2013) 1-10

abstract | fulltext

Smectite is a family of clay minerals that have important applications. In the majority of these clay minerals, the hydrated interlayer cations play a crucial role on the properties of the clay. Moreover, many studies have revealed that both thermal and grinding treatments affect the MMT structure and that interlayer cations play an important role in the degradation of the structure, primarily after mechanical treatment. In this study, the effects of these treatments on MMTs homoionized with mono (Na+, Li+ or K+) or polyvalent (Ca2+ or Al3+) cations were analyzed by the combination of a set of techniques that can reveal the difference of bulk phenomena from those produced on the surface of the particles. The thermal and mechanical (in an oscillating mill) treatments affected the framework composition and structure of the MMT, and the thermal treatment caused less drastic changes that the mechanical one. The effect of the interlayer cations is primarily due to the oxidation state and, to the size of the cations, which also influenced the disappearance of aluminum in the MMT tetrahedral sheet. These treatments caused a decrease in the surface area and an increase in the particle agglomeration and the isoelectric point. Both treatments caused the leaching of the framework aluminum. Furthermore, the mechanical treatment induced structural defects, such as the breakup of the particles, which favored the dehydroxylation and the increase of the isoelectric points of the montmorillonites.

April, 2013 | DOI: 10.1016/j.colsurfa.2013.01.040

Title: Cystine-capped CdSe@ZnS nanocomposites: mechanochemical synthesis, properties, and the role of capping agent
Author(s): Balaz, M; Balaz, P; Tjuliev, G; Zubrik, A; Sayagues, MJ; Zorkovska, A; Kostova, N
Source: Journal of Materials Science, 48 (2013) 2424-2432

abstract | fulltext

Cystine-capped CdSe@ZnS nanocomposites were synthesized mechanochemically with the aim to prepare a material which could be used in medicine for biosensing applications. Although synthesized CdSe@ZnS nanocomposites were capped with l-cysteine, cystine was formed from l-cysteine during the milling process. It was proven that water plays the key role in this oxidative transformation. The novel material was characterized by the complex of physico-chemical methods (FTIR, XPS, SEM, EDX, surface area measurements) and CHNS analysis. The leakage of Cd2+ and Zn2+ ions into physiological solution was also studied.

March, 2013 | DOI: 10.1007/s10853-012-7029-3

Title: Structure of supercritically dried calcium silicate hydrates (C–S–H) and structural changes induced by weathering
Author(s): Morales-Florez, V; de la Rosa-Fox, N
Source: Journal of Materials Science, 48 (2013) 5022-5028

abstract | fulltext

The nanostructure of supercritically dried calcium silicate hydrates was researched. This particular drying procedure was used to avoid nanostructure modifications due to conventional drying processes. Thus, in this study, the as-precipitated cementitious C–S–H structure was obtained for the first time. A specific surface area 20 % larger than conventionally dried C–S–H was measured. Given the importance of this nanostructured phase for the properties of hydrated cements, especially when in contact with CO2-rich environments, the supercritically dried C–S–H was weathered for 2 weeks. The structural effects of this weathering process on the C–S–H were researched and calcium carbonate microcrystal precipitation or the presence of silica by-product are reported. Calcite and aragonite polymorphs were observed, as well as nanoporous silica forming globular arrangements. In addition, 2 weeks of weathering was not enough to carbonate the entire C–S–H sample.

July, 2013 | DOI: 10.1007/s10853-013-7289-6

Title: Formation mechanism of ZrB2–Al2O3 nanocomposite powder by mechanically induced self-sustaining reaction
Author(s): Jalaly, M; Bafghi, MS; Tamizifar, M; Gotor, FJ
Source: Journal of Materials Science, 48 (2013) 7557-7567

abstract | fulltext

ZrB2–Al2O3 nanocomposite powder was produced by aluminothermic reduction in Al/ZrO2/B2O3 system. In this research, high energy ball milling was used to produce the necessary conditions to induce a mechanically induced self-sustaining reaction. The ignition time of the composite formation was found to be about 13 min. The synthesis mechanism in this system was investigated by examining the corresponding sub-reactions as well as changing the stoichiometry of reactants. Thermal behavior of the system was also studied.

November, 2013 | DOI: 10.1007/s10853-013-7571-7

Title: Improved photocatalytic activity of g-C3N4/TiO2 composites prepared by a simple impregnation method
Author(s): Miranda, C; Mansilla, H; Yanez, J; Obregon, S; Colon, G
Source: Journal of Photochemistry and Photobiology A: Chemistry, 253 (2013) 16-21

abstract | fulltext

g-C3N4 and TiO2 hybrid structures are synthesized by means of a simple impregnation method having good photoactivities for the degradation of phenol under UV irradiation. From the wide structural and surface characterization we have stated that the presence of g-C3N4 notably affect the surface feature of TiO2 (surface area and pore size distribution). Enhanced photoactivities have been obtained for composites systems. The best result was obtained for 2 wt% loading of g-C3N4 leading to a 70% of improvement with respect to bare TiO2 in the reaction rate. The effective charge carrier separation was proposed as the responsible of such improved photoactivity.

February, 2013 | DOI: 10.1016/j.jphotochem.2012.12.014

Title: Strong quantum confinement effects in SnS nanocrystals produced by ultrasound-assisted method
Author(s): Azizian-Kalandaragh, Y; Khodayari, A; Zeng, ZP; Garoufalis, CS; Baskoutas, S; Gontard, LC
Source: Journal of Nanoparticle Research, 15 (2013) 1388

abstract | fulltext

Nanocrystalline SnS powder has been prepared using tin chloride (SnCl2) as a tin ion source and sodium sulfide (Na2S) as a sulfur ion source with the help of ultrasound irradiation at room temperature. The as-synthesized SnS nanoparticles were quantitatively analyzed and characterized in terms of their morphological, structural, and optical properties. The detailed structural and optical properties confirmed the orthorhombic SnS structure and a strongly blue shifted direct band gap (1.74 eV), for synthesized nanoparticles. The measured band gap energy of SnS nanoparticles is in a fairly good agreement with the results of theoretical calculations of exciton energy based on the potential morphing method in the Hartree–Fock approximation.

January, 2013 | DOI: 10.1007/s11051-012-1388-1

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