During the last decade a number of ceramic materials, mostly oxides have been suggested as new thermal barrier coating (TBC) materials. These new compositions have to compete with the state-of-the-art TBC material yttria stabilized zirconia (YSZ) which turns out to be difficult due to its unique properties. On the other hand YSZ has certain shortcomings especially its limited temperature capability above 1200 °C which necessitates its substitution in advanced gas turbines. In the paper an overview is tried on different new materials covering especially doped zirconia, pyrochlores, perovskites, and aluminates. Literature results and also results from our own investigations will be presented and compared to the requirements. Finally, the double-layer concept, a method to overcome the limited toughness of new TBC materials, will be discussed.

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Overview on advanced thermal barrier coatings

Publisher Summary The intensities of intraconfigurational f–f transitions are reviewed in the chapter The chapter presents the transition mechanisms for lanthanide ions—namely, the magnetic dipole transition, the induced electric dipole transition, and the electric quadrupole transition It discusses the way experimental intensities can be determined from the spectra The chapter also focuses on the expression of the dipole strength and the oscillator strength in oriented and in randomly-oriented systems Correction factors for lanthanide ions in a dielectric medium are described in the chapter Although only a few magnetic dipole transitions exist for the trivalent lanthanide ions, magnetic dipole transitions are of interest, because their intensities are in a first approximation independent of the ligand environment and can, thus, be used as intensity standards The intensity of a magnetic dipole transition can be calculated exactly, provided that suitable wavefunctions are available These wavefunctions can be obtained from diagonalization of the energy matrix

Chapter 167 Spectral intensities of f-f transitions

In vivo fluorescence imaging in the near-infrared region between 1500–1700 nm (NIR-IIb window) affords high spatial resolution, deep-tissue penetration, and diminished auto-fluorescence due to the suppressed scattering of long-wavelength photons and large fluorophore Stokes shifts. However, very few NIR-IIb fluorescent probes exist currently. Here, we report the synthesis of a down-conversion luminescent rare-earth nanocrystal with cerium doping (Er/Ce co-doped NaYbF4 nanocrystal core with an inert NaYF4 shell). Ce doping is found to suppress the up-conversion pathway while boosting down-conversion by ~9-fold to produce bright 1550 nm luminescence under 980 nm excitation. Optimization of the inert shell coating surrounding the core and hydrophilic surface functionalization minimize the luminescence quenching effect by water. The resulting biocompatible, bright 1550 nm emitting nanoparticles enable fast in vivo imaging of blood vasculature in the mouse brain and hindlimb in the NIR-IIb window with short exposure time of 20 ms for rare-earth based probes. Fluorescence imaging in the near-infrared window between 1500–1700 nm (NIR-IIb window) offers superior spatial resolution and tissue penetration depth, but few NIR-IIb probes exist. Here, the authors synthesize rare earth down-converting nanocrystals as promising fluorescent probes for in vivo imaging in this spectral region.

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Boosting the down-shifting luminescence of rare-earth nanocrystals for biological imaging beyond 1500 nm

A novel green-emitting phosphor Ca2Al3O6F:Eu2+ was synthesized and its photoluminescence (PL) properties were investigated for application in white light-emitting diodes (w-LEDs). This phosphor shows good absorption ranging from the ultraviolet to the blue region and a broad green emission band centered at 502 nm. The concentration quenching mechanism and fluorescence lifetime of Eu2+ emission in the Ca2Al3O6F:Eu2+ phosphors have been investigated. The key parameters for the fabrication of w-LED lamps, such as the temperature-dependent photoluminescence, microstructure and morphology, CIE value and quantum efficiency of Ca2Al3O6F:Eu2+ phosphors, have also been studied. The above results indicate that Ca2Al3O6F:Eu2+ is a good candidate as a green component for near UV-excited w-LEDs.

Ca2Al3O6F:Eu2+: a green-emitting oxyfluoride phosphor for white light-emitting diodes

Yb3+/Er3+/Cr3+ triply doped transparent bulk glass ceramic containing orthorhombic YF3 and cubic Ga2O3 nanocrystals was fabricated by a melt-quenching route to explore its possible application in optical thermometry with high spatial and temperature resolution. It was experimentally observed that Yb3+/Er3+ ions incorporated into the precipitated YF3 nanophase, while Cr3+ ions partitioned into the crystallized Ga2O3 nanophase after glass crystallization. Importantly, such spatial isolation strategy efficiently suppressed adverse energy transfer among different active ions. As a consequence, intense green anti-Stokes luminescence originated from Er3+: 2H11/2,4S3/2 → 4I15/2 transitions, and deep-red Stokes luminescence transitions assigned to Cr3+: 2E → 4A2 radiation were simultaneously realized. Impressively, the intermediate crystal-field environment for Cr3+ in Ga2O3 made it possible for lifetime-based temperature sensing owing to the competition of radiation transitions from the thermally coupled Cr3+ 2E...

Dual-Phase Glass Ceramic: Structure, Dual-Modal Luminescence, and Temperature Sensing Behaviors.

Influence of M2+ ions substitution on the structure of lanthanum hexaaluminates with magnetoplumbite structure

Yb3+ to Er3+ energy transfer and rate-equations formalism in the eye safe laser material Yb:Er:Ca2Al2SiO7

Room-temperature CW laser operation at 1.55 /spl mu/m of Yb:Er:Ca/sub 2/Al/sub 2/SiO/sub 7/ (CAS) single crystal pumped at 940 nm and 975 nm has been achieved for the first time. Introduction of a third doping ion, Ce/sup 3+/, decreases the Er/sup 3+4/I/sub 11/2/ excited-state lifetime and improves the laser properties. For Yb:Er:Ce:CAS single crystal, a maximum of 20 mW output power is produced for 285 mW absorbed power. With this material, a low threshold of 20 mW and a relatively high slope efficiency of /spl sim/5.5% are obtained. Preliminary results indicate possible improvement in the near future. Experimental threshold values and laser properties of CAS crystals with various compositions are in good agreement with calculations, performed using the rate-equations modeling. Comparison with a Yb:Er:phosphate glass laser is also presented.

Room-temperature CW laser operation at /spl sim/1.55 /spl mu/m (eye-safe range) of Yb:Er and Yb:Er:Ce:Ca/sub 2/Al/sub 2/SiO/sub 7/ crystals

Synthesis, ESR investigation, and optical properties of the potential vibronic laser material LaMgAl11−xCrxO19

Single crystals of the aluminosilicate Ca 2 Al 2 SiO 7 (CAS) activated with Er 3+ or Yb 3+ were grown by the floating zone method for several dopant contents. The crystals of good optical quality, were investigated using optical spectroscopy with the aim to assess their potential interest as laser materials in the 1.5 μm (Er:CAS) and 1 μm (Yb:CAS) range, respectively. In both cases, absorption, stimulated emission, and gain cross sections were determined. Radiative and experimental lifetimes were also calculated. In the two compounds, as the two levels involved in the transitions are the ground and first excited states, radiation trapping and energy migration occur. For Er:CAS, emission around 1.55 μm (in the middle of the eye-safe range) may be obtained for a reasonable value of the population inversion ratio β (β = 0.4). The Judd-Ofelt parameters are significantly higher than those of Er:YAG and the nonradiative relaxation of the excited states located above 4 I 13/2 is fast (phonon energy cut-off around 1020 cm -1 ) ensuring efficient population of the 4 I 13/2 emitting level. For Yb:CAS, the absorption and emission cross sections are higher than for Er:CAS. Positive gain cross section is possible for very low β values, for instance β = 0.04 at 1045 nm. Taking into account the pump saturation intensity and the β value, it appears that this compound should be one of the best ytterbium activated laser crystal (if one excepts the apatite-type hosts).

Anne-Marie Lejus

Papers

Influence of M2+ ions substitution on the structure of lanthanum hexaaluminates with magnetoplumbite structure

Yb3+ to Er3+ energy transfer and rate-equations formalism in the eye safe laser material Yb:Er:Ca2Al2SiO7

Room-temperature CW laser operation at /spl sim/1.55 /spl mu/m (eye-safe range) of Yb:Er and Yb:Er:Ce:Ca/sub 2/Al/sub 2/SiO/sub 7/ crystals

Synthesis, ESR investigation, and optical properties of the potential vibronic laser material LaMgAl11−xCrxO19

Optical Investigation of Er:Ca2Al2SiO7 and Yb:Ca2Al2SiO7 for Laser Applications in the Near Infrared