Improvements in the processing of Yb:YAG ceramic materials

Laser ceramics can attain a significant role in the frame of the generation of high peak power, high energy laser pulses at high repetition rate, applications where fiber lasers or single crystals undergo fundamental or technological limitations. A ceramic material in order to be used as a laser amplifier, needs to fulfil strict requirements in terms of properties, in particular microstructure, purity level, porosity. The important role played by the production process on the transparency of ceramic materials has been explained elsewhere. The current study focuses on the optimization of the powder processing. An innovative Spray Drying process of solvent-based suspensions has been adopted for the preparation of ready-to-press powders for the reactive sintering of Yb:YAG materials. The influence of the experimental conditions on the morphology of the granulated powders and eventually on the microstructure evolution during sintering and the transparency, is describe

Transparent Yb:YAG ceramics

YAG ceramics doped with rare earth elements have been recently given a consistent attention as materials for various applications. The specific application depends on the doping element added. Addition of Yb is used for the production of active materials for solid state lasers, and YAG polycrystalline ceramics are promising materials for the replacement of single crystals, which are mostly used at present. The advantage of polycrystalline ceramics over single crystals are the lower fabrication costs, faster production or easier preparation of complicated and compositionally graded structures. The poster presents Yb:YAG polycrystalline ceramics prepared via solid-state reaction of commercial submicrometer and nanosized powders (Al2O3, Y2O3 and Yb2O3). Powders were homogenized by ball milling and two different powder drying methods were compared, rotary evaporation and spray drying. Samples were prepared by cold isostatic pressing of homogenized powders followed by calcination in flowing air and sintering in vacuum. High transparency was obtained: optical transmittance greater than 80% was achieved, while the theoretical maximum is 84% at 1064 nm. Furthermore, the effect of the dopant content on properties of sintered material was observe

Experimental features affecting the transparency of YAG materials

The important role played by the processing on the transparency of ceramic materials is often underestimated. In the literature a high level of transparency has been reported by many authors that for years focused their research on the development of polycrystalline YAG for laser applications,but the description of the experimental process is seldom thoroughly described. A detailed description of the powder treatment and shaping and of other important information that are necessary to reproduce the described results, is often missing. In order to be transparent a ceramic material must exhibit a very low concentration of defects such as secondary or grain boundary phases and residual pores. In order to fulfill this requirement specific experimental conditions must be combined together. Powders need to be nanometric or at least sub-micrometric and extremely pure. On the other hand, nanometric powders aggregate easily and the poor packing that may derived can lead to residual porosity. In addition, very fine powders are difficult to handle and tend to absorb water on the surface. Finally, the powder manipulation (weighting operations, solvent removal, spray drying, shaping, etc), easily introduces impurities. In case of transparent materials all these features must be controlled because they lead to the formation of defects that works as light scattering sources thus decreasing the transparency. This work describes the results obtained with YAG based ceramics under different experimental conditions of powder treatment and shaping. Commercial powders are used for the reactive sintering in a clean atmosphere and under high vacuum of YAG materials doped with Nd, Yb or Er. These dopants have been selected as the more appropriate for high power lasers. The powder treatment (ball milling duration and speed, suspension concentration, solvent/powder ratio, type and amount of dispersant) the solvent removal technique (spray drying conditions, rotavapor temperature, etc.) are described in detail as well as the conditions adopted during shaping. The influence of the powder process on the powder packing during shaping by pressing is also reported. Finally, the influence of the pre-sintering and sintering cycles is also described

Effect of milling parameters on the morphology and sinterability of the yttrium oxide powders for transparent ceramics

Yttrium oxide has multiple applications both as a transparent material with good optical, mechanical, and thermal properties, and for photonics when doped with rare earth ions. To achieve full transparency, a careful control of the process, from the selection of powders to the final densification by sintering, is required. In this context, the characteristics of the starting powders have a great impact on the final properties. In the present work, the effect of milling conditions of two commercial Y2O3 powders on the properties of ceramics obtained by cold isostatic pressing (CIP) and vacuum sintering was investigated. The milling rate varied between 80 and 300 rpm, and the milling time between 1 and 22 h. It was found that the optimal treatment conditions are 300 rpm for 65 min, which provided a homogeneous nano-sized Y2O3 powder. IR-transparent Y2O3 ceramics obtained by a vacuum sintering have a transmittance of 78.30% (1100 nm)

Production and characterization of geopolymer blocks based on hydroxyapatite rich biomass ashes

Vegetal and animal derived biomass ashes were selected to realize new sustainable chemically consolidated ceramics via acid or alkaline activation. Mixed biomass ashes are mainly based on calcium phosphate and secondly on aluminosilicate compounds ;they still represent an unexplored source material to be used in alternative ceramics production for building industry by low-cost manufacturing processes.Mixed biomass ashes were used to produce room-temperature hardened materials by chemical activation with solutions of phosphoric acid or potassium poly-silicate. Reactive mineral powders (metakaolin, magnesia and wollastonite) were also added to fasten the consolidation and the formation of a binding phase. The obtained formulations were finalized to maximize the amount of ashes disposed. Microstructural, physical and mechanical properties of the obtained materials were evaluated, along with their waterstability. Acid-activated samples, even in presence of secondary reactive precursors, maintained a crystalline structure,while alkali-activated ones were able to form a partially amorphous and wellc onsolidated matrix, with compressive strength of about 20 MPa

Pore evolution and compaction behaviour of spray-dried bodies for porcelain stoneware slabs

The compaction behavior of spray-dried powders has turned into concern in porcelain stoneware manufacturing due to the increasing diffusion of large slabs. It is necessary to fill a knowledge gap between the compaction behavior with conventional presses and novel technologies. For this purpose, eighteen industrially-manufactured spray dried bodies were characterized for specific properties connected to the compaction behavior (curves of bulk density, intergranular and intragranular porosity in function of applied load, apparent yield strength). In addition, the firing behavior was investigated in order to reveal any effect of dry bulk density on firing shrinkage and bulk density of fired samples. Powder compressibility is within 50% and 55% (Carr index) and is primarily controlled by moisture. Two regimes are found: low pressure (fast density increasing by granule cave in and closure of intergranular porosity) and high pressure (slow density gain by downsizing microporosity). A peculiar mechanism is unveiled: granules squeeze in the low-P regime and further densification is achieved through microfracture around individual agglomerate. A phenomenological model is illustrated for the compaction of spray-dried powders. In conclusion, the performance of spray dried bodies during compaction is crucial to control the uniformity, in terms of porosity and bulk density, which has important repercussions on the properties of final slabs, especially differential shrinkages and deformation during firing due to density gradients

Design of TiO₂-Surfactin Hybrid Systems with Multifunctional Properties

In recent years, multifunctional inorganic−organic hybrid materials have been widely investigated in order to determine their potential synergetic, antagonist, or independent effects in terms of reactivity. The aim of this study was to design and characterize a new hybrid material by coupling well-known photocatalytic TiO2 nanoparticles with sodium surfactin (SS), a biosurfactant showing high binding affinity for metal cations as well as the ability to interact with and disrupt microorganisms’ cell membranes. We used both chemical and colloidal synthesis methodologies and investigated how different TiO2:SS weight ratios affected colloidal, physicochemical, and functional properties. We discovered a clear breaking point between TiO2 and SS single-component trends and identified different ranges of applicability by considering different functional properties such as photocatalytic, heavy metal sorption capacity, and antibacterial properties. At low SS contents, the photocatalytic properties of TiO2 are preserved (conversion of organic dye = 99% after 40 min), and the hybrid system can be used in advanced oxidation processes, taking advantage of the additional antimicrobial SS properties. At high SS contents, the TiO2 photoactivity is inhibited, and the hybrid can be usefully exploited as a UV blocker in cosmetics, avoiding undesired oxidative effects (UV adsorption in the range between 300–400 nm). Around the breaking point (TiO2:SS 1:1), the hybrid material preserves the high surface area of TiO2 (specific surface area around 180 m2/g) and demonstrates NOx depletion of up to 100% in 80 min, together with improved adhesion of hybrid antibacterial coating. The last design demonstrated the best results for the concurrent removal of inorganic, organic, and biological pollutants in water/soil remediation applications

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

International audienceThermal 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 laser's 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