Search

Directionally solidified Al2O3/Y3Al5O12 (YAG), Al2O3/YAG/ZrO2, Al2O3/GdAlO3 (GAP), Al2O3/GAP/ZrO2 and Al2O3/ZrO2 eutectic/non eutectic ceramics were prepared with laser zone melting. Their microstructure, phase component and shape were investigated. The results indicated that the molar ratio of Zr4+ to rare earth ions (Y3+, Gd3+) in the ZrO2 phase of the ternary eutectic was closed to 2:1. The free energy of chemical reaction was the key to determine the precipitation order of phase. The growth of the firstly-precipitated phase could be divided into the formation of ordered Bravais lattice composed by the oxygen ions, the diffusion of metal cations in this lattice and the change of phase morphology. The unfilled cation gap might be a fast channel for the ion transport. On the same growth plane, the nucleation of the later-precipitated phase began only after the firstly-precipitated phase was fully formed. The triangular was the main ion arrangement way for the stable nucleation and growth of phases. The effects of the precipitation temperature, the phase size, the eutectic type and the growth rate on the microstructure were discussed. This improved theory provided a more reasonable explanation for the formation of rod-shaped, layered, triangular, Chinese script (CS) and cellular microstructures, which might be of significance to regulate the morphology of microstructures.

Al2O3/Er3Al5O12(EAG)/ZrO2 directionally solidified eutectic ceramics (DSECs) were prepared with high frequency induction heating zone melting (IHZM), and heat treated at high temperature. Their phase components, microstructures, flexural and tensile strengths were investigated. The results indicated that the microstructures were composed of the mutually interpenetrated Al2O3, EAG and ZrO2 (stabilized by Er3+ ion) phases. As the growth rate increased from 2 mm/h to 20 mm/h, the phase size decreased and the irregular microstructure became regular. When the heat treatment temperature was up to 1400 °C, the ionic diffusion was triggered and the phase began to grow. Under the action of surface energy and special microstructure, the growth of phase stopped after it reached a certain threshold. The sample with smaller phase size exhibited higher strength at room temperature, which was dependent on the crack propagation resistance. However, the speed of strength reduction was faster. The formation of surface groove, microcrack and malignant growth phase was responsible for this rapidly decreasing of strength. The flexural and tensile strengths of eutectic ceramic with larger phase size remained at 671.6 MPa and 359.45 MPa after long-term heat treatment at 1500 °C, respectively. The smaller difference between the original phase size and the maximum coarsened phase size was beneficial to avoid the occurrence of the above mentioned harmful factors and was the source of strength stability.

The resonant cross relaxation processes between Yb3+ and Er3+ ions in calcium alumino-silicate glasses have been quantified under selective Er3+ excitation. The infrared emission spectra, measured under steady state conditions (CW excitation to the 4I9/2 erbium level), have allowed to obtain an experimental relationship linking the transfer (Yb3+ → Er3+) and back transfer (Er3+ → Yb3+) parameters. These measurements combined with the dynamics of the main emitting levels, measured under pulsed excitation to the 2H11/2 erbium level, have allowed the fully quantification of the energy transfer parameters. The obtained values, C25 = 5.5 × 10− 18 cm3s− 1 (Yb3+ → Er3+), C52 = 1.5 × 10− 18 cm3s− 1 (Er3+ → Yb3+) and C27 = 7.6 × 10− 18 cm3s− 1 (up-conversion mechanism, estimated from the Judd-Ofelt analysis previously reported), can be used to predict the temporal evolution of the main luminescent emission bands.

The object of this work is to study the relation between composition, microstructure and oxidation state of Pr2±xZr2∓xO7±y materials produced by the laser–floating zone (LFZ) technique. Three compositions are studied, nominally Pr1.7Zr2.3O7+y, Pr2Zr2O7+y and Pr2.24Zr1.76O7±y, all within the pyrochlore field in the ZrO2–PrOx phase diagram. Samples have been processed under four different atmospheres (O2, air, N2 and 5%H2(Ar)), so as to vary the environmental conditions from oxidising to reducing. Sample colouration ranged from dark brown to bright green, owing to varying Pr4+ content. A close correlation is found between the phase homogeneity, the microstructure and the Pr content. Pr–deficient samples present a homogeneous microstructural aspect and composition, whereas Pr–rich compositions always break into 5–25 µm–sized grains with pyrochlore phases at the grain centre and ill–crystallised, Pr–rich oxidised phases at the grain–boundaries. Raman spectroscopy shows that different types of oxygen disorder occur depending on composition and processing atmosphere: in Pr–poor samples oxygen interstitials are created to compensate for Zr4+ excess charge, whereas in Pr–rich samples oxygen disorder occurs around the Pr3+ or Pr4+ ions substituting for Zr4+, because of size–mismatch. Magnetic measurements showed a high Pr4+ content, which has been attributed to several factors: the highly oxidised state of the feedstock material, the segregation of Pr and O–rich grain boundaries in compositions with praseodymium molar rate> 0.5, and the lower oxide–ion conductivity for PZO compositions, compared to either Pr–poor or Pr–rich compositions. Post–processing thermal annealing in a vacuum at 1000 °C enabled total Pr reduction, with the exception of the Pr–rich P2.24 samples, where some Pr4+ ions remained in the oxidised state.

[EN] The laser is a powerful tool for materials processing, incorporated already in many industrial processes and laboratory procedures. In this work, we are concerned with laser processing applied to research and development of ceramics for electrochemical cells and other high temperature oxide ceramics for energy applications. Solidification of single crystals or composites of relevant oxides can be performed by the laser assisted floating zone method, providing samples for structural, mechanical or functional fundamental research, as well as knowledge about its manufacture by melt processes. Selective laser melting of these ceramic oxides is a very promising technology, at the research level. Successful examples of surface laser melting of oxide eutectic composites are presented. The technologies of subtractive laser processing of ceramics (cutting, drilling, structuring, cleaning, etc.) are more developed, and the research is directed towards the optimization of mechanisms, increase of resolution and efficiency and the investigation of the effects of the laser treatment on the functional performance. Different laser processes of SOC (solid oxide cell) components are shown to decrease the ohmic, concentration and activation losses. The manuscript describes the state-of-the art of the technologies as applied to oxide and composite materials present in solid oxide electrochemical devices (SOFC, SOEC, and batteries) and selective emitters for thermophotovoltaics, with emphasis on the last achievements by the authors team., [ES] El láser es una potente herramienta para el procesamiento de materiales, incorporada ya en muchos procesos industriales y procedimientos de laboratorio. En este trabajo nos ocupamos del procesamiento láser aplicado a la investigación y desarrollo de cerámicas para celdas electroquímicas y otras cerámicas de óxido de alta temperatura para aplicaciones energéticas. La solidificación de monocristales o composites de óxidos relevantes se puede realizar mediante el método de zona flotante asistida por láser, proporcionando muestras para investigación básica estructural, de propiedades mecánicas o funcionales, así como conocimiento sobre su fabricación mediante procesos de fusión. La fusión selectiva por láser de estos óxidos cerámicos es una tecnología muy prometedora, cuyo desarrollo es todavía incipiente. Se presentan ejemplos exitosos de fusión por láser de superficie de compuestos eutécticos de óxidos. Las tecnologías sustractivas de procesamiento de cerámicas con láser (corte, taladrado, estructuración, limpieza, etc.) están más desarrolladas, y la investigación se dirige hacia la optimización de procedimientos, aumento de resolución y eficiencia y la investigación de los efectos del tratamiento láser sobre el rendimiento funcional. Diferentes procesos láser de los componentes de celda de óxido sólido disminuyen las pérdidas óhmicas, de concentración y de activación. El manuscrito describe el estado actual de las tecnologías aplicadas a óxidos y materiales compuestos presentes en dispositivos electroquímicos de óxidos sólidos (SOFC, SOEC y baterías) y emisores selectivos para aplicaciones termofotovoltaicas, con énfasis en los últimos logros del equipo.

This paper is devoted to the study of the electrical conductivity of tantalum-doped zirconia ceramics prepared by spark plasma sintering. In this study, the temperature dependence of conductivity in as-prepared specimens and in those previously annealed in air is determined and compared. A semi-empirical model, which is based on the oxidation states of the cations, has been developed and successfully assessed. According to this, the conductivity is basically controlled by the diffusion of tetravalent zirconium cations in both cases, although the concentration of these species varies drastically with the amount of induced oxygen vacancies. This is a quite unexpected fact, since conductivity is normally controlled by anionic diffusion in zirconia ceramics. This option is forbidden here due to the presence of substitutional pentavalent cations. Therefore, conductivity values are much lower than those reported in trivalent or divalent substitutional cation doped zirconia ceramics.

In vitro activity is important when considering the choice of a multiphasic bioceramic scaffold as phases can dissolve or transform at different rates. The aim of this study is focused on the synthesis and in vitro analysis of multiphasic ceramics obtained from the melt by directional solidification. Depending on the growth rate of the new composition different bioactive phases coexist in the same sample: akermanite, monticellite, tricalcium phosphate and Nurse's A phase, all of them with potential in the medical area as implant for bone or dental repair. With the knowledge of what and how phases dissolve first, it was possible to design materials to get porous scaffolds or more stable ceramics.

Mg2SiO4-MgAl2O4 eutectic ceramics have been fabricated by means of the laser floating zone (LFZ) technique. The microstructure has revealed as an unusual one at lower growth rate, composed of broken lamellae of MgAl2O4 distributed randomly along one matrix, composed of Mg2SiO4. At higher growth rates, a cell structure with intra-cell lamella structure is dominant. Contrary to most eutectic systems, hardness is not dependent upon the inter-spacing, but it does depend on one characteristic length of lamellae: their perimeter. One simple model based upon the dislocation is proposed, which successfully accounts for such extraordinary hardness law. Accordingly, Mg2SiO4-MgAl2O4 eutectic ceramics fabricated at 50 mm/h growth rate with the smallest MgAl2O4 lamella perimeter favorably showed more elevated hardness (13.4 GPa from Vickers indentation and 15.3 GPa from nanoindentation) and strength (∼430 MPa) than those found in the monolithic Mg2SiO4 matrix.

This work is concentrated on getting a reply to the following question: how does the grain size of boron carbide specimens influence on their mechanical and electrical response? It is a common issue that both essential properties are usually affected by the grain boundaries. To this purpose, a set of near fully-dense boron carbide specimens were prepared by spark plasma sintering. In order to reduce residual porosity and grain-size effects, nanoindentation tests at room temperature were conducted. DC conductivity was measured through four-point test technique from room temperature up to 800 °C. The results show that hardness can reach values as high as ∼60 GPa and plasticity onset takes place at around 23 GPa by dislocation nucleation. Regarding the conductivity, it is found that grain boundaries can block the mobility of bipolarons in an effective way. A simple additive law is provided to account for the resistivity of boron carbide polycrystals.

Selective emitters for thermophotovoltaic systems consisting of directionally solidified Al2O3-ME3Al5O12 (ME: Y, Er and Yb) eutectic coatings on Al2O3 substrates were produced and characterizated. Coatings were deposited by dip-coating on cylindrical substrates. After sintering, a continuous-wave CO2 laser was used to produce the surface resolidification. The optimization of the processing parameters yielded dense eutectic coatings with good adhesion to the substrate and with 90–200 µm in thickness. All coatings were free of voids and showed a eutectic microstructure consisting of a three dimensional interpenetrated network of Al2O3 and ME3Al5O12. The mechanical properties of the coatings (hardness and fracture toughness) were evaluated by indentation techniques. Thermal emission was studied by heating the rods with a CO2 laser at temperatures between 1000 and 1400 °C. Selective emission was observed in Er3+ and Yb3+ based coatings and attributed to the electronic transitions of the rare earth ions. Er3+-coatings showed the best emission properties as selective emitters for thermophotovoltaic converters.

In this work, results obtained by laser ablation of advanced ceramics and glass-ceramic materials assisted by liquids are reported. A Q-switched Nd:YAG laser at its fundamental wavelength of 1064 nm with pulse-width in the nanosecond range was used to machine the materials, which were immersed in water and ethylene glycol. Variation in geometrical parameters, morphology, and ablation yields were studied by using the same laser working conditions. It was observed that machined depth and removed volume depended on the thermal, optical, and mechanical features of the processed materials as well as on the properties of the surrounding medium in which the laser processing was carried out. Variation in ablation yields was studied in function of the liquid used to assist the laser process and related to refractive index and viscosity. Material features and working conditions were also related to the obtained results in order to correlate ablation parameters with respect to the hardness of the processed materials.

Directionally solidified eutectics in the 8 mol% Yttria stabilised zirconia and manganese oxide system were fabricated by the laser assisted melt float zone (LFZ) method at different growth rates. The eutectic composition was determined to 0.82 mol% of MnOx. The crystal structure and composition of the eutectic phases were determined by XRD, EBSD, XRF and Raman spectroscopy. An eutectic consisting of cubic Zr0.66Y0.16Mn0.18Ox (YMnSZ) fibres embedded in the twinned tetragonal Hausmannite Mn3O4 matrix was obtained in air. The fibre diameter decreased as the growth rate increased, with values between 80 and 200 nm. YMnSZ fibres grew along the [1 1 1] cubic axis and tetragonal Mn3O4 along [1 1 2], equivalent to the [1 1 0] cubic axis. The eutectic is a mixed conductor with values of 0.2 S/cm at 1100 K. The structure withstands oxide-reduction, yielding new well ordered structures of MnO/YMnSZ and Mn2O3/YMnSZ composites. Potential functional applications of these composites in the fields of magnetism, electroceramics and photonics are suggested.

Nanofibrillar Al2O3-Y3Al5O12-Zr O2 eutectic rods were manufactured by directional solidification from the melt at high growth rates in an inert atmosphere using the laser-heated floating zone method. Under conditions of cooperative growth, the ternary eutectic presented a homogeneous microstructure, formed by bundles of single-crystal c-oriented Al2O3 and Y3Al5O12 (YAG) whiskers of ≈100nm in width with smaller Y2O3-doped ZrO2 (YSZ) whiskers between them. Owing to the anisotropic fibrillar microstructure, Al2O3-YAG-YSZ ternary eutectics present high strength and toughness at ambient temperature while they exhibit superplastic behavior at 1600K and above. Careful examination of the deformed samples by transmission electron microscopy did not show any evidence of dislocation activity and superplastic deformation was attributed to mass-transport by diffusion within the nanometric domains. This combination of high strength and toughness at ambient temperature together with the ability to support large deformations without failure above 1600K is unique and shows a large potential to develop new structural materials for very high temperature structural applications. © 2013 Elsevier Ltd.

Lamellar cermets produced from directionally solidified eutectic ceramic precursors are now being considered an alternative option to conventional cermets in SOFC technology. The reduction behaviour of lamellar and conventional oxide–oxide ceramics of the same composite, but prepared through two different methods, as well as the ageing effects of the resulting cermet have been analysed with a view to its possible application as the SOFC anode. On the one hand, lamellar ceramics composed of gadolinium doped ceria (GDC) and cobalt oxide (CoO) with their eutectic composition have been produced by directional solidification from laser floating zone technique (LFZ). On the other hand, fine ceramics with the same composition have been prepared from a conventional wet chemistry route in order to assure the production of small and homogeneous grain size samples (conventional cermets). In both cases, reduction kinetic experiments in the 500–750 °C temperature range have been performed. The lamellar cermets show a competing reduction process that is not thermally activated and this has been linked to the existence of a high number of low energy interfaces in the lamellar material. Evolution of the electronic conductivity and porosity with ageing was used to evaluate the stability of the samples under operating conditions. In this sense, lamellar cermets show a better performance.

The paper reviews the basis for directional solidification processing of eutectic ceramics (DSEC) with particular attention to recent developments in growth techniques and methods. We stress on the relationship between crystal growth parameters, eutectic microstructure and sample dimension. Some of the eutectic ceramic systems prepared to date are described and their microstructure, crystallography and properties discussed. Potential functional applications in the fields of electroceramics and photonics are also described.

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. © 2012 Elsevier B.V.

In this work we present the study of the interaction between NIR pulsed laser and Al2O3-ZrO2 (3%Y2O3) eutectic composite. The effect produced by modifying the reference position as well as the working conditions and laser beam features has been studied when the samples are processed by means of pulse bursts. The samples were obtained by the laser floating zone technique using a CO2 laser system. The laser machining was carried out with a Q-switched Nd:YAG laser at its fundamental wavelength of 1064nm with pulse-widths in the nanosecond range. Geometric dimensions, i.e. ablated depth, machined width and removed volume as well as ablation yield of the resulting holes have been studied. We have described and discussed the morphology, composition and microstructure of the processed samples. © 2011 Elsevier Ltd.

In this work, we present the effect produced by modifying the referenceposition as well as the method of machining on the results obtained when advancedceramics and glass-ceramicmaterials are machined by laserablation. A Q-switched Nd:YAG laser at its fundamental wavelength of 1064 nm with pulsewidths in the nanosecond range has been used. Morphology, depth and volume obtained by means of pulse bursts and grooves have been studied. Working within the same laser conditions, it has been shown that these values depend on the thermal, optical and mechanical features of the material processed. We have also studied the variation in the ablation yield when the position of the surface to be machined is modified. Material properties and work conditions are related to the results obtained. We have described and discussed the morphology, composition, microstructure and hardness of the materials processed.

In this work a glass ceramic substrate was processed by focusing a laser beam inside the said material. The crystal phase within the amorphous matrix provides mechanical properties to the glass ceramic substrate in such a way that dots can be patterned inside the fore-mentioned material without producing any cracks. These marks are made up of crystals, the growth of which has been induced by the laser beam. These inner structures can modify the optical, thermal and mechanical properties of the glass ceramic substrate. A Q-switched Nd: YAG laser at its fundamental wavelength of 1064 nm with pulsewidths in the nanosecond range has been used. Morphology, composition, microstructure, mechanical and thermal properties of the processed material are described.

A series of analytical calculations is used to study both the effect of the thermal gradients and the stability of the molten zone in the laser floating zone growth of Al2O3–Y3Al5O12 eutectic composite. The thermal gradients in the solidification interface have been calculated and the axial gradient compared with the experimental one of 4.5 × 105 K/m. For these calculations the coefficients of heat transfer from the molten zone to the ambient at the solid-melt interface have been previously obtained. The thermal stresses generated by the high thermal gradients can induce crack formation during the cooling depending on the rod diameter. The theory predicts that it is possible to grow rods free of cracks up to R = 1.7 mm, at low rates (10 mm/h) in close agreement with the experimental critical radius of 1.6 mm.The dependence of the zone length on the input laser power used to carry out the growth is shown. The study of the floating zone profile allows determining the maximum stable zone length, verifying the stability criterion established by some authors.

Laser surface melting has been applied to near eutectic NiO–YSZ sintered ceramics. The objective is to generate a functional gradient composite material with graded microstructure and composition. At low solidification rates the resultant material has a graded composition, with a severe NiO segregation towards the surface. A thick NiO layer whose thickness depends on the travelling speed is formed. Below this layer the microstructure is eutectic like with composition varying with depth. In contact with the ceramic, excess YSZ coming from the hypereutectic composition forms an almost continuous YSZ layer. The thickness of both segregated layers, NiO and YSZ can be controlled by traverse speed. Thickness decreases as travelling speed increases until a limiting travelling rate of 110 mm/h, at which no more segregation is found. Possible causes to explain the relevant NiO segregation towards the surface are discussed.

In this work we report the study of site-selective laser spectroscopy of Nd3+ions in glasses with the0.8CaSiO3–0.2Ca3(PO4)2eutectic composition together with a dynamic study of energy transfer amongNd3+ions by using time-resolved fluorescence line narrowing (TRFLN) spectroscopy. The spectral featuresof the time-resolved fluorescence line narrowed4F3=2!4I9=2emission spectra obtained under resonantexcitation reveal the existence of spectral migration of excitation among the Nd3+ions. The analysis ofthe time evolution of the4F3=2!4I9=2narrowed emission shows that the electronic mechanism respon-sible for the ion–ion interaction can be identified as a dipole–dipole energy transfer process.

Al2O3–Y3Al5O12 eutectic rods grown by the laser floating zone method at 25, 350 and 750 mm/h have been studied by means of constant strain rate and constant stress compressive tests. Creep exponents were in the range n 1.9–3, and the activation energy was 720 ± 20 kJ/mol for rods grown at 350 mm/h. The experimental results indicate that to explain creep behaviour in these materials the bulk diffusion contribution to creep must be considered. We propose a creep model based on the isostrain approach to explain the plastic behaviour, attending to the interphase spacing. The model successfully predicts stress exponents, activation energy and load transfer effects within experimental error.

We present here a new manufacturing procedure for the anode Ni-zirconia cermet. It is based on the modification of the surface of a NiO-CaSZ (calcia stabilized zirconia) pellet of eutectic composition by surface laser melting and resolidification. A smooth, continuous and dense NiO-CaSZ layer is obtained on top of the ceramic pellet. Its depth can be varied from less than 200 οm to more than 570 οm, depending on the processing conditions. Its microstructure consists mainly of lamellar eutectic grains with interlamellar spacing ranging from 0.4 to 1.6 οm. The interspacing diminishes towards the surface, where a very fine microstructure is developed. Chemical reduction treatment transforms NiO to metallic Ni with the accompanying volume reduction. Complete reduction results into a cermet with 43 CaSZ+33.5 Ni+23.5 pores (%vol). Electrical conductivity is mainly electronic and proceeds along NiO lamellae or through percolating Ni particles. © 2003 Elsevier Ltd. All rights reserved.

The fabrication of thin Bi-2212 rods by laser floating zone techniques gives high critical currents, Ic, but strong radial inhomogeneities in microstructure and physical properties. By changing the precursor stoichiometry or the processing parameters, we have succeeded in improving the homogeneity and the Ic values. Towards the centre of the rods, the superconducting material tends to have lower Tc values associated to higher Bi content in the Bi-2212 superconducting grains. At 77 K, this gradient of Bi originates strong variations of critical current density across the rod.

A comparison was made between Medicago sativa L. cv. Tierra de Campos plants in tripartite symbiosis with Rhizobium meliloti strain 102F51 and the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe, irrigated with N- and P-free nutrient solution, and nodulated plants plus soluble added phosphorus. When 45 days old, both groups of plants were stressed by cyclic withholding of water and rewatering at different soil water potentials of −0.1, −0.3, −0.5 and −0.7MPa. After a subsequent 1 month drought-treatment period, the following parameters were evaluated: leaf water potential, intensity of VAM infection, nodule dry weight, total plant growth, phosphorus content and acetylene reduction activity. Drought stress significantly reduced the dry weight of total plants and nodules as well as P content of non-VAM plants when compared with the tripartite symbiosis. The intensity of VAM infection was not affected by drought. During drought, nodule activity was significantly higher in VAM than in non-VAM plants; this higher activity cannot be explained by improved P uptake caused by the fungus since P-concentration in mycorrhizal plants was always lower than in phosphorus fertilized plants.