Yttrium Aluminum Garnet Ceramics Research Articles

Evaluation of oxide ion diffusivity in YAG ceramics

Transparent yttrium aluminium garnet (YAG) polycrystal can be used as a host material for a solid state laser. In the present study, the authors prepared completely transparent YAG ceramics from active powder. The optical and mechanical properties of YAG ceramics are very sensitive to point defects. In order to examine the relationship between these properties and point defects, it is first necessary to characterise the defects. The authors report the effect of doping on oxygen diffusivity in several YAG ceramics, doped with various elements. Although Nd doping slightly increases the oxygen diffusion coefficient and the number of oxygen vacancies increases with increasing Nd concentration in as sintered materials, the variation of the oxygen diffusion coefficient in well annealed samples is small. These results suggest that the oxygen vacancies exist in non-equilibrium. However, grain boundary diffusivity was not influenced by preannealing before the oxygen diffusion experiments. The oxygen defect structure in transparent YAG ceramics is also discussed.

Yttrium aluminum garnet nanoparticles synthesized by nitrate decomposition and their low temperature densification behavior

Abstract A simple method for synthesizing yttrium aluminum garnet (YAG) powder based on a mixed precursor solution of yttrium nitrate and aluminum nitrate is proposed. At a temperature below 850 °C, the pure YAG phase can be obtained by simply calcining the precursors in air. Uniform granular YAG nanoparticles of about 20–30 nm in diameter are obtained. The YAG granules can be densified to become a nanoceramic fraction by calcination at 900 °C, and can be sintered at 1000 °C to become a very dense ceramic with well crystallized grains of diameter from 1 μm to 2 μm. This result indicates that YAG nanoceramics can be obtained at 900 °C without a stringent sintering process, and that transparent YAG ceramics may be obtained below 1000 °C by this simple and novel synthesis method. Based on X-ray diffraction (XRD), thermal gravimetric analysis (TGA), Fourier transform infrared (FTIR), and scanning electron microscopy analysis, a chemical mechanism for the densification of the ceramic is proposed.

Bulk optical damage thresholds for doped and undoped, crystalline and ceramic yttrium aluminum garnet

We measured the bulk optical damage thresholds of pure and Nd-doped ceramic yttrium aluminum garnet (YAG), and of pure, Nd-doped, Cr-doped, and Yb-doped crystalline YAG. We used 9.9 ns, 1064 nm, single-longitudinal mode, TEM00 pulses, to determine that the breakdown thresholds are deterministic, with multiple-pulse thresholds ranging from 1.1 to 2.2 kJ/cm2.

Temperature characteristics of small signal gain for Nd/Cr:YAG ceramic lasers

Thermal dependence on the small signal gain of Nd/Cr:YAG (yttrium aluminum garnet) ceramics was observed experimentally. Usually, Nd:YAG crystal and ceramics have remarkable gain reduction when optical pumping is performed and the temperature of the laser media is upped to 373K. A CW laser light generated in a 1064nm Nd:YAG laser oscillator was amplified by Nd/Cr:YAG ceramic amplifier, and the output power was measured at non-saturation level. Laser small signal gain of the ceramic disk was kept to 470K. This property was remarkably different from one of Nd:YAG crystals or ceramics. The peak shift of the fluorescence was observed experimentally when the temperature is high.

Microstructuration induced differences in the thermo-optical and luminescence properties of Nd:YAG fine grain ceramics and crystals

The temperature and compositional dependences of thermo-optical properties of neodymium doped yttrium aluminum garnet (YAG) crystals and fine grain ceramics have been systematically investigated by means of time-resolved thermal lens spectrometry. We have found that Nd:YAG ceramics show a reduced thermal diffusivity compared to Nd:YAG single crystals in the complete temperature range investigated (80-300 K). The analysis of the time-resolved luminescent properties of Nd(3+) has revealed that the reduction in the phonon mean free path taking place in Nd:YAG ceramics cannot be associated with an increment in the density of lattice defects, indicating that phonon scattering at grain boundaries is the origin of the observed reduction in the thermal diffusivity of Nd:YAG ceramics. Finally, our results showed the ability of the time-resolved thermal lens to determine and optimize the thermo-optical properties of Nd:YAG ceramic based lasers.

Thermo-optical modeling of high power operation of 2 μm codoped Tm,Ho solid-state lasers

The results of coupled thermo-optical modeling of Tm,Ho solid-state laser operation are reported. A rate dynamics model integrated together with a TEM00 distribution for the total number of stimulated photons is coupled with a two-dimensional time-dependent heat transfer model. The heat transfer model includes absorption, heat release, and transfer inside the crystal as well as the thermal effect of the spontaneously emitted infrared radiation. In water cooled crystal operation this radiation is shown to be absorbed within the water boundary layer, producing significant inhibition of dissipation of the heat released inside the crystal. This effect leads to crystal superheating and significant inhibition of laser energy output. Absorption loss, in particular, due to water vapor present in the cavity is found to decrease significantly laser pulse energy. Numerical results are compared to previous (Tm,Ho:yttrium lithium fluoride) and new (Tm,Ho:ceramic yttrium aluminum garnet) experimental data and discussed within the framework of the computational model and simplified analytical approximations.

Surface and Mirror Twin Grain Boundary Segregation in Nd:YAG: An Atomistic Simulation Study

Ceramic lasers have advantages such as better optical homogeneity, cheaper production, and more freedom in shape compared with monocrystalline lasers. However, equal or better laser performances are required. Interface segregation is important as ceramics contain a high number of interfaces, segregation to which may locally alter the concentration of luminescent dopants, which in turn may influence laser performance. The present work applies atomistic simulation techniques to investigate the segregation of neodymium (Nd) dopants to surfaces and mirror twin grain boundaries in yttrium aluminum garnet (YAG). These results allow a better understanding of interfacial segregation and its influence on laser performance of Nd:YAG ceramics.

Fabrication of High Transparent YAG Ceramics by Vacuum Sintering at Low Temperature

Yttrium aluminum garnet (YAG) nanopowders were synthesized by n-amyl co-precipitation method. Transparent YAG ceramics were achieved by sintering the green body at 1800°C and then 1600°C for ten hours with TEOS as sintering additive. The transmittance of the sample was 75% at visible wavelength of 800nm.

Mechanical properties of fully dense yttrium aluminum garnet (YAG) ceramics

The mechanical properties of yttrium aluminum garnet (YAG) ceramics were studied at the temperature range of 1100°C to 1250°C using three-point flexure tests. This experiment was conducted on YAG ceramics (YAG(A) and YAG(B)) having a different average grain size. YAG(A) had an average grain size of 1.04 μm, while YAG(B) had an average grain size of 1.54 μm. In the case of crosshead speed of 1 mm/min, YAG(A) and YAG(B) failed in a brittle manner at temperatures up to 1250°C, and had a strength of 500-600 MPa. In the case of crosshead speed of 0.0016 mm/min, YAG(A) failed in a brittle manner at temperatures up to 1100-1150°C, and had strength of 500-600 MPa, but exhibited plastic deformation at 1200°C. On the other hand, YAG(B) failed in a brittle manner at temperatures up to 1150-1200°C, and had strength of about 700 MPa, but exhibited plastic deformation at 1250°C.

Freeze-dried nanometric neodymium-doped YAG powders for transparent ceramics

Abstract Nanometric neodymium-doped yttrium aluminium garnet (YAG) has been synthesised from freeze-dried precursors. The products were calcined between 900 °C and 1200 °C under flowing oxygen. Powder morphologies were observed by TEM, SEM and laser granulometer. The oxides had small crystallite size. The best sinterability under vacuum at 1700 °C for 3 h was obtained for precursors with the lowest amount of organic impurities as indicated by IR measurements. Hot isostatic pressing at 1700 °C in 160 MPa of argon was applied to complete the sintering. Under these conditions, Nd:YAG ceramics with very good transmittance were obtained.

Fabrication of YAG transparent ceramics by two-step sintering

Yttrium aluminum garnet (YAG) nanopowders with mean particle size of about 50 nm synthesized by a modified co-precipitation method were used to sinter bulk YAG ceramic by two-step sintering method. Full densification was achieved by heating the sample up to 1800 °C followed by holding at 1550 °C for 10 h. Transparent YAG ceramics were obtained by suppressing grain-boundary migration while promoting grain-boundary diffusion during the two-step sintering process. The microstructure of the YAG ceramic is homogeneous without abnormal grain growth and the transmittance of the sintered sample is 43%.

Transparent YAG ceramics by surface softening of nanoparticles in spark plasma sintering

Rapid heating of 34 nm yttrium aluminum garnet (YAG) nanoparticle compacts using pulsed high direct electric current, during the spark plasma sintering (SPS), resulted in transparent ceramics, after 3 min at 100 MPa and 1400 °C. This phenomenon is in contrast to conventional sintering at much higher temperatures that result in translucent or opaque ceramics. Grain growth analysis and the evolved microstructure during the SPS showed that the YAG nanoparticle surfaces soften due to the pulsed direct electric current. Dynamic densification to transparency occurs by nano-grain rotation and sliding, aided by the viscous layer, and followed by curvature driven grain boundary diffusional processes at higher densities and grain sizes.

Structural and Optical Properties of Highly Nd‐Doped Yttrium Aluminum Garnet Ceramics from Alkoxide and Glycolate Precursors

Yttrium aluminum garnet (YAG) nanopowders doped with high neodymium (Nd) content (3 at.%) were synthesized by the sol–gel processing of (i) alkoxide precursors and (ii) metal chelates formed by complexing the cations with polyethylene glycol. A stoichiometric YAG composition was obtained following both procedures; however, the agglomeration of particles was significantly higher in glycolate synthesis, which shielded residual organics from oxidation (elemental analyses). Distribution of Nd3+ ions in the YAG matrix, as shown by the absorption of pump energy and photoluminescence spectra of Nd:YAG ceramics, was more homogeneous in alkoxide‐derived powders. The segregation of Nd centers in the glycolate‐derived sample was supported by the precipitation of a crystalline Nd2O3 phase (X‐ray diffraction) during sintering. High‐resolution absorption spectra (4I9/2(1)→4F9/2(1)) of the powders showed that a higher absorption cross‐section of glycolate‐derived powders is due to Nd3+–Nd3+ ion pairing, which leads to the quenching of photoluminescence. Owing to the better dispersion of optically active centers, the photoluminescence signal was found to be substantially enhanced in alkoxide‐derived Nd:YAG ceramics.

Synthesis of Nd3+ doped nano-crystalline yttrium aluminum garnet (YAG) powders leading to transparent ceramic

Abstract Nd-doped yttrium aluminum garnet (Nd:YAG) nano-crystallites were prepared by the co-precipitation method from a mixed solution of yttrium and aluminum nitrate using ammonium hydrogen carbonate as precipitant. The phase transition and structure of the prepared powder were investigated by means of XRD and TEM. The powder showed significantly less agglomeration of crystallites, indicating a high degree of sinterability. Transparent YAG ceramics were fabricated by vacuum sintering at 1700 °C for 5 h using as prepared powders. The results show that an amount of added TEOS less than 0.5 wt% is suitable for the preparation of transparent ceramics. The average grain size of YAG ceramic obtained is about 2–3 μm. The transmittance of the YAG ceramic increase as the sintering temperature and wavelength increase. The optical transmittance is about 55% in the visible range, and reaches almost 70% in the infrared.

Novel synthesis of YAG by solvothermal method

Yttrium aluminum garnet (YAG) powder was synthesized by a mixed solvothermal method under moderate conditions ( 260 – 290 ∘ C , 2–4 h) with inexpensive aluminum and yttrium hydroxides as the precursors, and a low-cost ethanol–water solution (the volume ratio of ethanol to water ⩾ 2 : 1 ) as the solvent. The presence of ethanol in mixed solvent is beneficial to the formation of YAG. The synthesized powder consists of well-dispersed and nearly spherical fine grains (80 nm on average) with a relatively narrow grain size distribution. The powder form is the most desirable for the compacting and sintering of YAG ceramics.

The Effect of Silicon Addition on High-Temperature Tensile Deformation of Superplastic Yttrium Aluminum Garnet (YAG) Ceramics.

The effect of doping Silicon on the high-temperature deformation of superplastic Yttrium Aluminum Garnet (YS-YAG) was studied under conditions of uniaxial tension. In the preceding paper, the elongation and strain rate of YAG ceramics was described as having decreased by the addition of Si as a sintering aid. This was one of the reasons that the grain size of Si-doped YAG ceramics greatly increased compared with that of non-doped YAG ceramics during deformation. Creep deformation of Y2O3-excess YAG, with an average grain size of 1.6μm, was shown to have an elongation of over 210% (=en) at 1773 K. Accordingly, we prepared Si-doped Y2O3-excess YAG ceramics (YS-YAG) having the above two chemical composition, and developed YAG materials having two characteristics; remarkable grain growth and elongation. The deformation of YS-YAG reached 200% (=en) at 1773 K under a constant stress creep tensile test, in spite of the grain size of YS-YAG changed from 1.5 μm to 6μm during deformation.

Scattering in Polycrystalline Nd:YAG Lasers

For the present study, 1.1‐at.%‐neodymium‐doped yttrium aluminum garnet (YAG) ceramics with a controlled pore volume (150‐930 vol. ppm) were fabricated via a solid‐state reaction method using high‐purity powders. The scattering coefficients of neodymium‐doped YAG (Nd:YAG) ceramics, obtained from Fresnel's equation, increased simply as the pore volume increased. The effective scattering coefficient and the laser‐oscillation performance of Nd:YAG ceramics with a pore volume of ∼150 vol. ppm were almost equivalent to those of a 0.9‐at.%‐Nd:YAG single crystal fabricated via the Czochralski method.

Electrical transport and defect properties of Ca- and Mg-doped yttrium aluminum garnet ceramics

Electrical transport properties of a sintered yttrium aluminum garnet (YAG) doped with Ca or Mg have been investigated as a function of temperature and partial pressure of oxygen. Conductivity and ionic transport measurements show Ca- and Mg-doped YAG to be mixed ionic-electronic p-type conductors exhibiting an activation energy of about 2 eV. Thermally stable inclusions of alumina, which were found to be present in sintered YAG powders, act as a source of Al vacancies. Al vacancies are dominating the defect chemistry at high oxygen partial pressure. The electronic contribution to total conductivity becomes more pronounced when the temperature increases. At low pO2 a hole-release from O⋅O centers (trapped holes) influences the transport behavior. We developed a possible defect model which explains the experimental data qualitatively. This model considers the influence of dispersed particles of constant activity over a pO2 regime ranging from 105 to 10−12 Pa and their influence on the defect chemistry.

Explosion of carbonic acid in a coal mine

Stereological relations that can be routinely applied for the quantitative characterization of microstructures of heterogeneous single- and two-phase materials via global microstructural descriptors are reviewed. It is shown that in the case of dense, single-phase polycrystalline materials (e.g., transparent yttrium aluminum garnet ceramics) two quantities have to be determined, the interface density (or, equivalently, the mean chord length of the grains) and the mean curvature integral density (or, equivalently, the Jeffries grain size), while for two-phase materials (e.g., highly porous, cellular alumina ceramics), one additional quantity, the volume fraction (porosity), is required. The Delesse–Rosiwal law is recalled and size measures are discussed. It is shown that the Jeffries grain size is based on the triple junction line length density, while the mean chord length of grains is based on the interface density (grain boundary area density). In contrast to widespread belief, however, these two size measures are not alternative, but independent (and thus complementary), measures of grain size. Concomitant with this fact, a clear distinction between linear and planar grain size numbers is proposed. Finally, based on our concept of phase-specific quantities, it is shown that under certain conditions it is possible to define a Jeffries size also for two-phase materials and that the ratio of the mean chord length and the Jeffries size has to be considered as an invariant number for a certain type of microstructure, i.e., a characteristic value that is independent of the absolute size of the microstructural features (e.g., grains, inclusions or pores).