Laura Esposito

Bonding of zirconia to super alloy with the active brazing technique

Abstract Zirconia stabilized with yttria was bonded to a Ni-based superalloy by active brazing. An interlayer with the composition of the silver–copper eutectic plus 1.75 wt.% titanium was used as brazing material. Various brazing heating cycles were performed using a furnace with graphite heating elements and under flowing argon. The microstructure of the bonded couples is related to the chemical reactions occurring during the brazing cycles, to the properties of the specific compounds formed and to the interdiffusion phenomena occurring across the interfaces. Critical feature for the success of the joint is the nature of the interfaces that the brazing interlayer formed with the superalloy and the zirconia. At the interface with the superalloy adhesion is obtained under a relatively wide range of experimental conditions. On the contrary, at the interface with zirconia, a good wetting and adhesion occurred only when a titanium oxide sublayer with a specific thickness formed. The best results were obtained with a maximum temperature between 870 and 900°C, a soaking time of 10 min and a fast heating rate (10°C/min). The optimal thickness of the TiO x sublayer was less than 1 μm.

Microstructure and properties of porous β-SiC templated from soft woods

Abstract Porous β-SiC with a multimodal porosity preferentially oriented along one direction is obtained by infiltration of pyrolyzed wood with Si at T > T Si Melting . Hard woods (obece, poplar and assembled poplar) are used as starting materials. The microstructure of the starting wood, of the wood after pyrolysis and after Si infiltration, is characterised in terms of overall porosity, pore size distribution and of crystallographic phases. The process is optimised for obtaining porous templates of only β-SiC with a microstructure that replicates the original wood microstructure. Features such as the presence of unreacted carbon, or conversely, of Si within the open pores of the infiltrated materials are minimized by a careful control of the amount of Si in contact with the carbon preform during the infiltration cycle. Compression tests on cubic samples are performed along the axial and longitudinal direction.

Solid state bonding of Al2O3 with Cu, Ni and Fe: characteristics and properties

A solid state bonding technique under hot pressing was used for joining alumina with thin metal sheets of Ni, Cu and Fe. The microstructure and microchemistry of the ceramic–metal interface and of the fracture interface were examined using scanning electron microscopy (SEM) energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD), in order to identify the adhesion mechanisms and the nature of strength limiting flaws. Interaction between the selected metals and alumina can be physical or physico-chemical in nature: very low amounts of interfacial compounds were formed, depending on the processing conditions and on the presence of oxygen in the system. Fracture and toughness tests indicated that high ceramic–metal interface strengths (up to 177 MPa) were achieved under the adopted processing conditions and that strength and toughness were directly related. Moreover, an increase in hardening in the metal interlayer at a distance of 2–3 μm from the interface was observed in the samples with high strength values. The mechanical behaviour was related to several factors that strongly depend on the bonding conditions: plastic deformation of the metal, metal creep, metal intrusion and diffusion into alumina, and chemical reactions at the interface.

Spatial distribution of the Yb3+ rare earth ions in Y3Al5O12 and Y2O3 optical ceramics as analyzed by TEM

There is a need to investigate the possible improvements of rare earth ion-doped transparent ceramics in terms of better optical performances for lasers, scintillators and phosphors. Especially, the spatial distribution of the rare earth ions within their host material is important for quenching processes. Results from Transmission Electron Microscopy (TEM, both conventional and high resolution TEM) and EDX (energy dispersive X-ray) quantitative chemical analysis positively show that there is no significant segregation of Yb3+ ions, the last active rare earth element, in the grain boundaries of Yb3+-doped YAG and Yb3+-doped Y2O3 polycrystalline ceramics. The large value of the Yb3+ segregation coefficient (1.09 in YAG ceramic) explains the homogeneity of this distribution, in opposition to the low values of the Ce3+ and Nd3+ segregation coefficients, the first active rare earth elements, (0.08 and 0.18 respectively in YAG ceramic), which leads to a strong segregation at grain boundaries.

Multilayered YAG-Yb:YAG ceramics: manufacture and laser performance

Thermal effects in transparent laser crystals and ceramics are generally an unwanted consequence of the pumping process: temperature gradients give rise to an unevenly distributed refractive index variation and a distortion of the optical surfaces crossed by the laser beam (thermal lens); birefringence due to thermomechanical stress can cause depolarization losses; and absorption from the ground level usually increases with temperature in quasi-three-level systems. All these effects can seriously impair laser performance, especially in high-power devices. Layered structures with a tailored modulation of the doping level can be used to reduce the peak temperature, the temperature gradients and also the thermally induced deformation of the laser material, thus mitigating the overall thermal effects. In the present work, structures comprising two and three layers of different compositions (pure YAG/10 at% Yb:YAG and pure YAG/10 at% Yb:YAG/pure YAG) were designed with a view to control deformation and stresses, and to reduce the thermal lensing effect. The multilayered samples were assembled by linear and cold isostatic pressing, and co-sintered under a high vacuum in a clean-atmosphere furnace. The microstructure of the layered samples obtained was characterized by FEG SEM, ESEM and TEM. The Yb diffusion profile across the doped/undoped interface was identified and related to the lasers output power. An internal optical transmittance up to 96% was obtained. A laser output power up to 5 W, with a slope efficiency as high as 74.3%, was also achieved.

Laser and optical properties of Yb:YAG ceramics with layered doping distribution: design, characterization and evaluation of different production processes

The laser, optical and spectroscopic properties of multilayer Yb:YAG ceramic structures, differently activated, were investigated. The structures were designed by means of Finite Element Modeling, adjusting the doping distributions to reduce peak temperature, surface deformation and thermally induced stresses, depending on the pump and cooling geometry. Two ceramic processes were used, i.e. dry pressing of spray-dried powders (SD) and tape casting (TC), resulting in different defect density and size distribution: TC gives a more uniform transmission, whereas SD results in larger, unevenly scattered defects. The spectroscopic properties were found independent from the production process. The laser performance has been characterized under high intensity pumping in a longitudinally diode pumped laser cavity, comparing the behavior of the different structures in terms of slope efficiency, stability under increasing thermal load, spatial uniformity of laser emission. Slope efficiency values as high as 58% in Quasi-CW pumping conditions and 54% in CW conditions was measured in two-layers structures. The production process and the number of layers influenced the behavior of the samples, in particular regarding the spatial uniformity of the laser emission. Samples made by tape casting have shown overall a better thermal stability with respect to the samples made by spray drying.

Characterization of Yb:YAG active slab media based on a layered structure with different doping

Slabs with non-uniform doping distribution are studied with the aim of reducing thermal deformations in high-energy high-average-power Yb:YAG slab systems. We present a numerical analysis based on Finite Element Mesh (FEM) methods suitable to model non-uniform devices. The thermal variation of the refractive index, the end-faces deformations and the photo-elastic effect have been calculated in order to estimate the total thermal-lens effect. The stress distributions are also obtained. Some results of this numerical approach are compared to experimental thermal lens measurements in a simple geometry for both uniform and structured samples, in order to validate the numerical procedures. Finally we compare numerical simulations for different single- or double-sided pumping and cooling geometries. They show that structured slabs can reduce thermal gradients with respect to uniformly doped means with comparable absorption and geometry. This means a reduction of thermal lens effect and thus an increase of maximum allowed pump power loading. Previous literature reports some work made with structured slabs where higher doping was located in layers with lower pump radiation levels, in order to get a more uniform absorption. Interestingly our modeling indicates that reduced thermal effects are instead obtained when a higher doping is located close to the cooled surfaces.

Quantification of SiO 2 sintering additive in YAG transparent ceramics by laser-induced breakdown spectroscopy (LIBS)

Transparent ceramics are important optical materials with applications in street lighting, high-strength windows, electro- and magneto-optical isolators, high-power laser gain media and radiation detectors. Their fabrication most often relies on powder densification techniques carried out at high temperatures, sometimes promoted by sintering additives. Here, we describe the application of laser-induced breakdown spectroscopy (LIBS) for following the concentration levels of silica used as a sintering agent in the fabrication of yttrium aluminum garnet (YAG) transparent ceramics. The sensitivity limit of our protocol reaches a few tens of ppm of silica in YAG ceramic samples, showing that LIBS can be implemented reliably for the rapid assessment of sintering additives in advanced ceramic processing.

YAG:Pr3+ transparent ceramics for applications in photonics: synthesis and characterization

Y3Al5O12 (YAG) transparent ceramics doped with Pr3+ have been obtained by vacuum sintering of spray dried commercial powders and characterized by XRD and SEM techniques. A comprehensive spectroscopic investigation has then been carried out including: absorption spectra and decay time measurements in the visible-IR spectral region and x-ray and VUV excitation and emission properties studied with synchrotron radiation. The main processes responsible for the excited states dynamics have been identified and characterized. Comparison with the properties of the single crystal reveals that the investigated material has interesting perspectives for applications in optics and photonics.

Advances in Transient-Liquid-Phase Bonding of Ultra-high Temperature ZrC Ceramics

Abstract Full exploitation of the many attractive engineering properties of ultra-high temperature ceramics (UHTCs) requires that they can be joined. This paper explores progress in identifying joining strategies based on the use of transient liquid phases (TLPs). Wetting studies are used to explore the suitability of specific liquids for joining, while bonding studies provide the ultimate test. Sintering aids in the UHTC provide a major potential obstacle to successful joining, and dissolved impurities in the TLP can also complicate the joining process. Nonetheless, we show that well-bonding interfaces can be achieved when ZrC ceramics are bonded at 1673 K using a Ni/Nb/Ni multilayer interlayer.