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Scientific Papers in SCI

2019


Photodegradation of 2,4-dichlorophenoxyacetic acid over TiO2(B)/anatase nanobelts and Au-TiO2(B)/anatase nanobelts


Chenchana, A.; Nemamcha, A.; Moumeni, H.; Doña Rodríguez, J.M.; Araña, J.; Navío, J.A.; González Díaz, O.; Pulido Melián, E.
Applied Surface Science, 467-468 (2019) 1076-1087

ABSTRACT

In this work, novel TiO2-based nanobelts with various phases were synthesized: biphasic TiO2(B)/anatase, pure TiO2(B) and pure anatase. These catalysts were obtained via hydrothermal reaction using two nanoparticulated TiO2 photocatalysts as precursors: Aeroxide TiO2 P25 (P25) and TiO2 synthesized via a sol-gel process (SG). In addition, the surface of the photocatalysts was modified with gold using a photodeposition method. A characterization study of the different photocatalysts was performed with X-ray diffraction analysis (XRD), UV–Vis diffuse reflectance spectra (DRS), scanning electron microscopy (SEM), X-ray photoelectron spectrum analysis (XPS) and Brunauer-Emmett-Teller measurements (BET). The photocatalytic reaction of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) was investigated under UVA irradiation. A toxicity analysis was performed with the marine bioluminescent bacteria Vibrio fischeri. The highest 2,4-D removal efficiency of 99.2% was obtained with the biphasic Au-TiO2(TiO2(B)/anatase) nanobelts with anatase as predominant phase. Toxicity was mainly due to the intermediate 2,4-dichlorophenol (2,4-DCP) which was eliminated in 4 h. The TiO2 nanobelt phase structure is shown to have a significant effect on photocatalytic activity.


February, 2019 | DOI: 10.1016/j.apsusc.2018.10.175

Coupling of Ag2CO3 to an optimized ZnO photocatalyst: Advantages vs. disadvantages


P. Sánchez-Cid; C. Jaramillo-Páez; J.A. Navío; A.N. Martín-Gómez; M.C. Hidalgo
Journal of Photochemistry and Photobiology A: Chemistry, 369 (2018) 119-132

ABSTRACT

With the aim of improving the photocatalytic properties of a previously optimized zinc oxide photocatalyst, the effect of the incorporation of different amounts of Ag2CO3 on the aforementioned ZnO has been studied. For this purpose we report the synthesis, by means of simple precipitation procedures, of bare ZnO and Ag2CO3 samples as well as the coupled materials ZnO/Ag2CO3 (X) (where X = 1%, 2%, 4% and 5% in molar percentages). Both, single and coupled materials have been characterized by different techniques (XRD, XRF, N2-absorption, SEM, TEM, UV–vis/DRS and XPS). To assess the advantages or disadvantages that Ag2CO3 addition could have over the optimized ZnO, the photocatalytic properties have been established by following the photo-degradation of selected toxic molecules, both in the UV and in the visible, as well as using complementary techniques of liquid medium analyses (TOC and Atomic Emission Spectrometry with plasma ICP). Three selected substrates were chosen: Rhodamine B (RhB) as a dye, and phenol and caffeine as colourless recalcitrant toxic molecules.
Our results suggest that although the use of Ag2CO3 could be beneficial to implement the optical absorption towards the visible region, however, other effects have to be bore in mind, such as the photo-corrosion of Ag2CO3 and the chemical structure of the chosen substrate, to elucidate whether the addition of Ag2CO3 has beneficial or detrimental effects on the photocatalytic properties of the coupled ZnO/Ag2CO3 materials.


January, 2019 | DOI: 10.1016/j.jphotochem.2018.10.024

2018


New concept for old reaction: Novel WGS catalyst design


Garcia-Moncada, N; Gonzalez-Castano, M; Ivanova, S; Centeno, MA; Romero-Sarria, F; Odriozola, JA
Applied Catalysis A-General, 238 (2018) 1-5

ABSTRACT

The viability of water gas shift catalytic system for mobile application passes through obligatory reactor volume reduction, achieved normally by using less charge of more efficient catalyst. Completely new concept for catalyst design is proposed: a catalytic system including classically reported WGS catalysts of different nature or active phase (Cu, Pt or Au) mechanically mixed with an ionic conductor. The influence of the later on catalyst activity is studied and discussed, more precisely its effect on the rate of the reaction-limiting step and catalysts' efficiency. It is demonstrated with this study, that the presence of an ionic conductor in contact with a WGS catalyst is essential for the water supply (dissociation and transport), thereby potentiating the water activation step, whatever the mechanism and catalyst overall performance.


December, 2018 | DOI: 10.1016/j.apcatb.2018.06.068

CO/H-2 adsorption on a Ru/Al2O3 model catalyst for Fischer Trospch: Effect of water concentration on the surface species


Jimenez-Barrera, E; Bazin, P; Lopez-Cartes, C; Romero-Sarria, F; Daturi, M; Odriozola, JA
Applied Catalysis B-Environmental, 237 (2018) 986-995

ABSTRACT

Water presence and concentration strongly influence CO conversion and CS+ selectivity in the Fischer Tropsch reaction. In this work, the influence of the water concentration was investigated using a model Ru/Al2O3 (5 wt. %) catalyst. The surface species formed after CO and H-2 adsorption in dry and wet (different water concentrations) conditions were analyzed by FTIR. Firstly, water adsorption was carried out up to complete filling of the pores and then CO was put in contact with the catalyst. The absence of adsorbed CO species in these conditions evidences that CO diffusion in water controls the access of the gas to the active sites and explains the negative effect of high water concentrations reported by some authors. Moreover, the adsorption of a mixture of CO + H-2 + H2O, being the water concentration close to that needed to have a monolayer, and a dry mixture of CO + H-2 were carried out and compared. Results evidence that water in this low concentration, is able to gasify the surface carbon species formed by CO dissociation on the metallic sites. This cleaning effect is related to the positive effect of water on CO conversion detected by some authors.


December, 2018 | DOI: 10.1016/j.apcatb.2018.06.053

Role of calcium looping conditions on the performance of natural and synthetic Ca-based materials for energy storage


Sarrion, B; Perejon, A; Sanchez-Jimenez, PE; Perez-Maqueda, LA; Valverde, JM
Journal of CO2 utilization, 28 (2018) 374-384

ABSTRACT

In this work, the multicycle activity of natural CaO precursors (limestone and dolomite) and Ca-based composites (Ca3Al2O6/CaCO3 and ZrO2/CaCO3 mixtures) has been studied for Thermochemical Energy Storage (TCES) in Concentrated Solar Power (CSP) plants by means of the Calcium-Looping process (CaL), using two integration schemes proposed elsewhere that differ in the calcination stages. Under CSP-He conditions, calcination for CaO regeneration is performed under pure He at low temperatures (725 degrees C) while under CPS-CO2 conditions calcination is carried out under pure CO2 at high temperatures (950 degrees C). The latter avoids the use of selective membranes to separate He from CO2 even though it requires the use of more expensive materials for solar receptors. Carbonation/calcination conditions drastically affect the multicycle CO2 uptake of the materials tested. Effective multicycle conversion is higher in CSP-He tests due to the mild conditions employed for calcination, which mitigates CaO sintering. On the other hand, the harsh calcination conditions used in CSP-CO2 tests enhance sintering of CaO derived from limestone and the Ca3Al2O6/CaCO3 composite due to the low Tammann temperature of Ca3Al2O6. CaO sintering is hindered by the presence of inert oxides with high Tammann temperatures, such as ZrO2 in the ZrO2/CaCO3 composite and MgO in dolomite. Dolomite derived CaO shows high effective conversion values along the carbonation/calcination cycles when tested under both types of conditions, as compared to limestone and the composites, which suggests that the integration scheme based on CSP-CO2 conditions would be a feasible alternative to CSP-He if natural dolomite were used as CaO precursor.


December, 2018 | DOI: 10.1016/j.jcou.2018.10.018

Molten carbonate salts for advanced solar thermal energy power plants: Cover gas effect on fluid thermal stability


Fereres, S; Prieto, C; Gimenez-Gavarrell, P; Rodriguez, A; Sanchez-Jimenez, PE; Perez-Maqueda, LA
Solar Energy Materials and Solar Cells, 188 (2018) 119-126

ABSTRACT

The eutectic mixture Li2CO3-Na2CO3-K2CO3 is investigated as a high temperature heat transfer fluid and storage medium alternative for molten salt solar thermal power plants. This salt has an operating temperature range of 400–700 °C, enabling the use of higher temperature/efficiency power cycles. However, this carbonate mixture is known to thermally decompose in air. This study evaluates the thermal stability of the salt mixture under different cover gases: air, nitrogen, carbon dioxide, and an 80/20 carbon dioxide/air mixture. Initial characterization is performed through thermogravimetric analysis (TGA), followed by larger scale testing in a custom-made reactor to simulate conditions closer to its practical use. The results show improved thermal stability with a CO2 atmosphere. The decomposition kinetics under different cover gases are estimated from TGA data. However, larger-scale, longer duration experiments show much slower decomposition rates compared to the classical TGA approach. These findings indicate that the main contribution to mass loss in TGA is due to vaporization rather than thermal decomposition. Thus, a proper evaluation of the molten salt´s thermal stability can only be obtained from reactor experiments where vaporization is inhibited. Very long induction periods (of the order of days) are observed, suggesting that the kinetic decomposition mechanism is a nucleation and growth type. Other considerations for future plants incorporating these high temperature salts are discussed.


December, 2018 | DOI: 10.1016/j.solmat.2018.08.028

Selective CO methanation with structured RuO2/Al2O3 catalysts


Munoz-Murillo, A; Martinez, LM; Dominguez, MI; Odriozola, JA; Centeno, MA
Applied Catalysis B-Environmental, 236 (2018) 420-427

ABSTRACT

Active and selective structured RuO2/Al2O3 catalysts for CO methanation using a flow simulating CO2-rich reformate gases from WGS and PROX units (H-2 excess, CO2 presence and 300 ppm CO concentration) were prepared. Both, the RuO2/Al2O3 powder and the slurry prepared from it for its structuration by washcoating of the metallic micromonolithic structure, were also active and selective. Both the slurry (S-RuAl) and micro monoliths (M-RuAl) were able to completely and selectively methanate CO at much lower temperatures than the parent RuAI powder. The optimal working temperature in which the CO conversion is maximum and the CO2 conversion is minimized was determined to be from 149 degrees C to 239 degrees C for S-RuAl and from 165 degrees C to 232 degrees C for M-RuAl, whilst it was from 217 degrees C to 226 degrees C for RuAI powder. TPR, XRD and TEM measurements confirmed that the changes in the activity and selectivity for CO methanation among the considered catalysts can be related with modifications in the surface particle size of ruthenium and its reducibility. These were ascribed to the metallic substrate, the presence of PVA and colloidal alumina in the slurry preparation, the aqueous and acidic media and the thermal treatment used, resulting in a more active and selective catalysts than the parent powder.


November, 2018 | DOI: 10.1016/j.apcatb.2018.05.020

Sustainable Fabrication of Plant Cuticle-Like Packaging Films from Tomato Pomace Agro-Waste, Beeswax, and Alginate


Tedeschi, G; Benitez, JJ; Ceseracciu, L; Dastmalchi, K; Itin, B; Stark, RE; Heredia, A; Athanassiou, A; Heredia-Guerrero, JA
ACS Sustainable Chemistry & Engineering, 6 (2018) 14955-14966

ABSTRACT

Plant cuticles have been used as models to produce hydrophobic films composed of sodium alginate, the fatty acid fraction of tomato pomace agrowaste, and beeswax. The fabrication process consisted of the blending of components in green solvents (water and ethanol) and a subsequent thermal treatment (150 degrees C, 8 h) to polymerize unsaturated and polyhydroxylated fatty acids from tomato pomace. When sodium alginate and tomato pomace fatty acids were blended, free-standing films were obtained. These films were characterized to evaluate their morphological (SEM), chemical (solid-state NMR, ATR-FTIR), mechanical (tensile tests), thermal (TGA), and hydrodynamic (water contact angle, uptake, and permeability) properties. A comparison between nonpolymerized and polymerized samples was carried out, revealing that the thermal treatment represents a sustainable route to create structured, composite networks of both components. Finally, beeswax was added to the blend with the same amounts of sodium alginate and tomato pomace fatty acids. The presence of the wax improved the hydrophobicity and the mechanical and water barrier properties as well as decreased the water uptake. These results indicate that polymerized plant cuticle-like films have valuable potential for packaging applications.


November, 2018 | DOI: 10.1021/acssuschemeng.8b03450

The Role of Surface Recombination on the Performance of Perovskite Solar Cells: Effect of Morphology and Crystalline Phase of TiO2 Contact


Idigoras, J; Contreras-Bernal, L; Cave, JM; Courtier, NE; Barranco, A; Borras, A; Sanchez-Valencia, JR; Anta, JA; Walker, AB
Advanced Materials Interfaces, 5 (2018) art. 1801076

ABSTRACT

Herein, the preparation of 1D TiO2 nanocolumnar films grown by plasma-enhanced chemical vapor deposition is reported as the electron selective layer (ESL) for perovskite solar devices. The impact of the ESL architecture (1D and 3D morphologies) and the nanocrystalline phase (anatase and amorphous) is analyzed. For anatase structures, similar power conversion efficiencies are achieved using an ESL either the 1D nanocolumns or the classical 3D nanoparticle film. However, lower power conversion efficiencies and different optoelectronic properties are found for perovskite devices based on amorphous 1D films. The use of amorphous TiO2 as electron selective contact produces a bump in the reverse scan of the current-voltage curve as well as an additional electronic signal, detected by impedance spectroscopy measurements. The dependence of this additional signal on the optical excitation wavelength used in the IS experiments suggests that it stems from an interfacial process. Calculations using a drift-diffusion model which explicitly considers the selective contacts reproduces qualitatively the main features observed experimentally. These results demonstrate that for a solar cell in which the contact is working properly the open-circuit photovoltage is mainly determined by bulk recombination, whereas the introduction of a "bad contact" shifts the balance to surface recombination.


November, 2018 | DOI: 10.1002/admi.201801076

Influence of irrigation conditions in the germination of plasma treated Nasturtium seeds


Molina, R; Lopez-Santos, C; Gomez-Ramirez, A; Vilchez, A; Espinos, JP; Gonzalez-Elipe, AR
Scientific Reports, 8 (2018) art. 16442

ABSTRACT

Plasma treatments had emerged as a useful technique to improve seed germination. In this work we investigate the influence of different irrigation conditions and plasma treatments on the germination of nasturtium seeds. During plasma treatment, seeds experience a progressive weight loss as a function of treatment time that has been associated to water release, a process that is more pronounced after longer plasma treatment times. Seeds treated for short times (<30 s) are able to germinate more efficiently than untreated specimen under hydric stress (drought conditions), while plasma treatments for longer times (up to 300 s) impaired germination independently on irrigation conditions. Characterization analysis of plasma treated seeds by FTIR-ATR, SEM/EDX and XPS showed that plasma treatment affected the chemical state of pericarp while, simultaneously, induced a considerable increase in the seeds water uptake capacity. The decrease in germination efficiency found after plasma treatment for long times, or for short times under optimum irrigation conditions, has been attributed to that the excess of water accumulated in the pericarp hampers the diffusion up to the embryo of other agents like oxygen which are deemed essential for germination.


November, 2018 | DOI: 10.1038/s41598-018-34801-0

Exotic grain growth law in twinned boron carbide under electric fields


Moshtaghioun, BM; Gomez-Garcia, D; Rodriguez, AD
Journal of the European Ceramic Society, 38 (2018) 4590-4596

ABSTRACT

Grain growth is a ubiquitous phenomenon in all materials, and it affects both structural and functional properties. Despite its intrinsic importance, a full comprehension of grain growth from a fundamental point of view-i.e., from the nanoscale to the macroscale-is still a pending issue. In practical terms, our knowledge relies on the classical kinetic laws reported sixty years ago. 

This paper reports the violation of such classical laws in boron carbide ceramics consolidated by spark plasma sintering. The conjunction of high temperature gradients with large compressive stress when a pulse electric current passes through the ceramic powders gives rise to an intense twinning-detwinning formation. These forming steps at the grain boundaries change the grain mobility drastically. Therefore, a new 'exotic' law for grain-growth kinetics is found and validated at different temperatures and dwell times.


November, 2018 | DOI: 10.1016/j.jeurceramsoc.2018.06.029

Scalable synthesis of potential solar cell absorber Cu2SnS3 (CTS) from nanoprecursors


Hegedus, M; Balaz, M; Tesinsky, M; Sayagues, MJ; Siffalovic, P; Krulakova, M; Kanuchova, M; Briancin, J; Fabian, M; Balaz, P
Journal of Alloys and Compounds, 768 (2018) 1006-1015

ABSTRACT

The present paper demonstrates an easy and scalable mechanochemical synthesis of ternary sulfide Cu2SnS3 (CTS) as a promising solar cell absorber. For the synthesis, pre-milled nanoparticles of CuS and SnS were used. The pure CTS phase was readily obtained after 60 min of milling in a laboratory planetary ball mill and 240 min in an industrial eccentric vibration industrial mill, respectively. The reaction progress of laboratory scale synthesis was studied by the quantitative Rietveld analysis. The reaction speed reaches its maximum at 4.6 min and the reaction is completed at approximately 60 min, according to the fitted data. The products of the syntheses were further characterized by X-ray powder diffractometry, Raman spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy and UV-Vis spectroscopy. The results revealed formation of near-stoichiometric CTS nanoparticles with tetragonal I-42m symmetry. An average crystallites size of approximately 10-15 nm was determined for CTS phase. The SEM images support quintessential polydisperse character of the powders obtained by ball-milling approach. The materials seem to be suitable for photovoltaic applications with the band-gap energies of approximately 1.16-1.19 eV.


November, 2018 | DOI: 10.1016/j.jallcom.2018.07.284

Mechanically induced combustion synthesis of niobium carbonitride nanoparticles


Jalaly, M; Gotor, FJ; Sayagues, MJ
Journal of Solid State Chemistry, 267 (2018) 106-112

ABSTRACT

Niobium carbonitride [Nb(C,N)] nanoparticles were synthesized by a combustive mechanochemical reaction in the Mg/Nb2O5/C3H6N6 system. High-energy ball milling was used to promote a mechanically induced self-sustaining reaction (MSR). Combustion occurred after a very short milling period of 5 min. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) analyses revealed that the nature of the product is an intermixed carbonitride material. The formation mechanism of Nb(C,N) resulted from the magnesiothermic reduction of niobium oxide to generate elemental Nb, which then reacted with the species generated from the melamine decomposition.


November, 2018 | DOI: 10.1016/j.jssc.2018.08.027

Environmentally friendly monolithic highly-porous biocarbons as binder-free supercapacitor electrodes


Orlova, TS; Shpeizman, VV; Glebova, NV; Nechitailov, AA; Spitsyn, AA; Ponomarev, DA; Gutierrez-Pardo, A; Ramirez-Rico, J
Reviews on Advanced Materials Science, 55 (2018) 50-60

ABSTRACT

A simple, low-cost and environmentally friendly method has been used to obtain highly porous biomorphic carbon monoliths with a good combination of interconnected macro-, meso- and microporosity, and good electrical conductivity and mechanical strength, making these biocarbon materials interesting for electrochemical applications as binder-free electrodes. Highly porous monolithic biocarbons were obtained from beech wood precursors through pyrolysis and subsequent surface modification in a steam heated to 970 degrees C with different activation times. The obtained biocarbons demonstrated good electrical conductivity and mechanical strength. They were studied as electrodes for supercapacitors in half cell experiments, demonstrating maximum gravimetric capacitance of 200 F g(-1) in a basic media at scan rate 1 mV s(-1). Galvanostatic charge-discharge experiments showed maximum capacitance of 185 F g(-1) at current density of 0.15 A g(-1) and similar to 100 F g(-1) at current density of 0.75 A g(-1). It has been shown that in addition to the developed porous surface, the micropores with diameters exceeding 1 nm play a key role for the enhanced electrochemical capacity. Long-cycling experiments demonstrated excellent stability of the monolithic biocarbon electrodes with no reduction of the initial capacitance values after 600 cycles in voltammetry.


November, 2018 | DOI: ---

Behavior of High-Strength Polypropylene Fiber-Reinforced Self-Compacting Concrete Exposed to High Temperatures


Rios, JD; Cifuentes, H; Leiva, C; Garcia, C; Alba, MD
Journal of Materials in Civil Engineering, 30 (2018) 04018271

ABSTRACT

In this study we analyzed the use of high-performance structural concrete reinforced with polypropylene fibers in applications requiring long exposure times to high temperatures, such as thermal energy storage systems. We analyzed the behavior of the concrete at different temperatures (hot tests: 100 degrees C, 300 degrees C, 500 degrees C and 700 degrees C), cooled-down states (cold tests) and exposure times (6, 24, and 48h). We also experimentally determined the thermogravimetric analysis, fracture behavior, compressive strength, Young's modulus, and tensile strength of concrete. Subsequently, we performed a comprehensive analysis of the thermal and mechanical behavior of high-performance concrete under different thermal conditions. We applied longer exposure times to broaden the available results on the behavior of high-performance fiber-reinforced concrete when subjected to high temperatures. Results show that, once thermal and moisture equilibriums are reached, exposure time does not have any influence on mechanical properties. They also provide useful information about the influence of high temperatures on the different parameters of fiber-reinforced concrete and its application for thermal energy storage structures.


November, 2018 | DOI: 10.1061/(ASCE)MT.1943-5533.0002491

Quantitative analysis of Yb 4d photoelectron spectrum of metallic Yb


Pauly, N; Yubero, F; Tougaard, S
Surface & Coatings Technology, 50 (2018) 1168-1173

ABSTRACT

The measured Yb 4d(3/2) intensity is larger than the Yb 4d(5/2) in X-ray photoelectron (XPS) emission of metallic Yb, which is unexpected. The shape and intensity of photoelectron peaks are strongly affected by extrinsic excitations due to electron transport out of the surface (including bulk and surface effects) and to intrinsic excitations due to the sudden creation of the static core hole. To quantitatively extract from experimental XPS the primary excitation spectrum (ie, the initial excitation process) of the considered transition, these effects must be included within the theoretical description. The combined effect of both extrinsic and intrinsic excitations can be described by an effective energy-differential inelastic electron scattering cross section for XPS evaluated by a dielectric response model with the dielectric function as only input. Then, using this cross section, a direct evaluation of the primary excitation spectrum is performed by standard peak shape analysis for thick homogeneous samples. We use this approach in the present paper to determine the Yb 4d photoemission spectrum for metallic Yb. We show that the unexpected larger intensity of Yb 4d(3/2) compared to 4d(5/2) can be fully accounted for by our model and that the total spectrum consists of a sum of symmetric primary excitation peaks.


November, 2018 | DOI: 10.1002/sia.6402

Analysis of the variables that modify the robustness of Ti-SiO2 catalysts for alkene epoxidation: Role of silylation, deactivation and potential solutions


Plata, JJ; Pacheco, LC; Remesal, ER; Masa, MO; Vega, L; Marquez, AM; Odriozola, JA; Sanz, JF
Molecular Catalysis, 459 (2018) 55-60

ABSTRACT

Catalytic epoxidation of alkenes plays an essential role in the transformation and synthesis of many organic chemicals. Ti atoms grafted on mesoporous silica, silylated on the surface, is considered the most active and selective catalyst for these reactions. However, the durability and robustness of the active centers remain as the main drawback in industry. In this paper, the characterization of industrial samples is combined with DFT calculations to rationalize the deactivation process of the catalyst and improve its performance. Silylating agents are characterized by experimental and simulated 29Si-NMR and their role in the catalytic mechanism is analysed. Potential deactivation processes are identified before, during and after the reaction. Modifications of the silylating agents and of the active center are proposed to improve the durability of the catalyst.


November, 2018 | DOI: 10.1016/j.mcat.2018.08.010

Optimizing the homogenization technique for graphene nanoplatelet/yttria tetragonal zirconia composites: Influence on the microstructure and the electrical conductivity


Lopez-Pernia, C; Munoz-Ferreiro, C; Gonzalez-Orellana, C; Morales-Rodriguez, A; Gallardo-Lopez, A; Poyato, R
Journal of Alloys and Compounds, 767 (2018) 994-1002

ABSTRACT

3 mol% yttria tetragonal zirconia polycrystalline (3YTZP) ceramic composite powders with 10 vol% nominal content of graphene nanoplatelets (GNPs) were prepared using four different homogenization routines: dispersion of the powder mixture by ultrasonication in isopropyl alcohol, homogenization in a high-energy planetary ball mill in wet or dry conditions after ultrasonication, and milling of the powders in a high-energy planetary ball mill in dry conditions. A significant effect of the homogenization routine on the powders particle size distribution was revealed by laser granulometry and Raman spectroscopy. Highly densified composites were obtained after spark plasma sintering (SPS) and remarkable differences on the GNP size, shape and distribution throughout the ceramic matrix and also in the electrical conductivity were observed in the four different composites. The composite with the best performance in terms of electrical conductivity was the one prepared after planetary ball milling of the powders in dry conditions as a consequence of the reduced dimensions of the GNPs and their excellent distribution throughout the ceramic matrix. 


October, 2018 | DOI: 10.1016/j.jallcom.2018.07.199

Isosymmetric structural phase transition of the orthorhombic lanthanum gallate structure as a function of temperature determined by Rietveld analysis


Tang, Y. Q.; Lopez-Cartes, C.; Aviles, M. A.; Cordoba, J. M.
CRYSTENGCOMM, 20 (2018) 5562-5569

ABSTRACT

High energy planetary ball milling has been used to synthesize pseudo-cubic highly-pure LaGaO3 in one hour from its oxide components in an air atmosphere. Calcination at different temperatures led to the crystallization of lanthanum gallate in an orthorhombic structure with its local lanthanum coordination number environment changing from 12 to 7 when the temperature was increased. This change was attributed to the thermal expansion of the Ga-O bonds that varied non-monotonically inducing GaO6 tilting. Rietveld analysis, Raman spectroscopy, and transmission electron microscopy were used to elucidate the LaGaO3 structures at different temperatures.


October, 2018 | DOI: 10.1039/c8ce00726h

The role of carbon nanotubes on the stability of tetragonal zirconia polycrystals


Morales-Rodriguez, A; Poyato, R; Gutierrez-Mora, F; Munoz, A; Gallardo-Lopez, A
Ceramics International, 44 (2018) 17716-17723

ABSTRACT

The effect of single walled carbon nanotubes (SWNT) at zirconia grain boundaries on the stability of a tetragonal zirconia polycrystalline matrix has been explored in as-sintered composites and after low temperature hydro thermal degradation (LTD) experiments. For this purpose, highly-dense 3 mol% Y2O3-doped tetragonal zirconia polycrystalline (3YTZP) ceramics and SWNT/3YTZP composites were prepared by spark plasma sintering (SPS). Quantitative X-ray diffraction analysis and microstructural observations point out that an increasing amount of well-dispersed SWNT bundles surrounding zirconia grains decreases the metastable tetragonal phase retention in the ceramic matrix after sintering. In contrast, the tetragonal ceramic grains in composites with SWNTs are less sensitive to the presence of water, i.e. to undergo a martensitic transformation under LTD conditions, than monolithic 3YTZP ceramics. The SWNT incorporation diminishes micro-cracking due to tetragonal to monoclinic ZrO2 phase transformation in the composites.


October, 2018 | DOI: 10.1016/j.ceramint.2018.06.238

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