YAG:Sm3+ (5 at.%) transparent ceramics have been obtained by reactive sintering using different starting Al2O3 powders. Effect of particle size in the 100–400 nm range, morphology and dispersity of starting alumina powders on the properties of YAG:Sm3+ powder mixtures, their densification and resulted optical ceramics have been studied. It was shown that YAG:Sm3+ powder mixtures prepared with BMA15 and AKP-50 powders possess significant reduction of a fraction of micron-sized agglomerates. The particles within the range of 100–300 nm presented in BMA15 and AKP-50 powders, act as proppant, enabling collapse of the Y2O3 agglomerates. As a result, ceramics prepared from these powder mixtures show improved densification behavior. Reactively-sintered YAG:Sm3+ transparent ceramics prepared using AKP-50 powder are characterized by optical transmittance of 83% and low concentration of residual pores.

A homogeneous MgO-Y2O3 (50:50 vol%) nanocomposite semispherical green bodies with diameter up to 90 mm and relative green density of 46 % were successfully fabricated by the slip casting using nanopowder, synthesized by the glycine-nitrate method. To advance the rheological properties of MgO-Y2O3 aqueous suspensions, the effect of Dolapix CE64 and NH4PAA as dispersants was examined by viscosity measurements, sedimentation tests. A stable MgO-Y2O3 slurry showing near-Newtonian behavior was prepared with 30 wt% solid loading and 2 wt% of Dolapix CE64. Finally, sinterability of MgO-Y2O3 composite nanopowder was studied by vacuum sintering method for the first time. MgO-Y2O3 nanocomposite of complex shape vacuum-sintered at 1300 degrees C demonstrate optical transmittance of 45 % at the wavelength of 5.3 mu m, and the average grain size of MgO and Y2O3 components of 355 nm and 342 nm, respectively.

15 at.% Yb3+:YAG/YAG transparent composite ceramics with a coaxial geometry were synthesized by a ceramic forming method combined with the reactive sintering at 1800 degrees C. Densification peculiarities, microstructure, optical properties, and laser characteristics of composite ceramic samples were studied. Powder mixtures of Yb3+:YAG and YAG stoichiometric compositions demonstrate almost the same densification enabling uniform shrinkage of composite without differential sintering. It was shown that in-line optical transmittance of 15 at.% Yb3+:YAG/YAG composite ceramics reaches 80 % at 1030 nm wavelength. The effective diffusion coefficient of Yb3+ ions in garnet structure has been determined. Efficient laser emission was generated from Yb3+:YAG/YAG composite ceramics with a slope efficiency of eta(sa) = 0.30.

The paper is devoted to studying of Si4++Mg2+ complex additive for obtaining transparent YAG ceramics for laser applications. Ceramics were fabricated by reactive vacuum sintering of commercial Y2O3, Al2O3 powders taken in a stoichiometric mixture with TEOS and MgO as sintering aids. Microstructure and optical properties of YAG:Si4+,Mg2+ ceramics were investigated as a function of the Si4+/Mg2+ ratio. It was found that the influence of complex additive does not correspond to the direct superposition of known Si4+- and Mg2+-induced sintering mechanisms and involves interaction between Si4+ and Mg2+ ions during sintering. It was shown that CSi/CMg> 1 provides more effective pore elimination and uniform microstructure when CSi/CMg< 1 gives more intense inhibition of grain grown which may be important for scaling the size of ceramics.

YAG:Sm3+ (5 аt.%) optical ceramics were obtained by the solid-state reactive sintering in the 1700–1800°C temperature range. The effect of the sintering temperature on the microstructure, phase composition and optical properties of YAG:Sm3+ ceramics has been studied. It has been shown that the optimal sintering temperature in order to produce YAG:Sm3+ transparent ceramics is 1725°С. The sintered ceramics are characterized by high optical transmittance (>82% at 808 nm), low residual porosity and the average grain size of 21 μm. It has been shown that the sintering temperature has a little effect on the average grain size of synthesized ceramics. Microstructure of YAG:Sm3+ ceramics consolidated at 1800°C is characterized by the presence of large grains up to 90 μm surrounded by the main fraction with an average grain size of 19 μm, which could be evidence of starting bimodal grain size distribution.

The effect of the solid loading (41-50 wt%) of the slurry on granulometric composition and physico-chemical characteristics of Y2O3-Al2O3-Nd2O3 powder mixtures obtained by planetary ball milling has been studied for the first time. It was shown that the particle size distribution of powder, its Zeta potential, and specific surface area depend on the solid loading of the milled slurry and, consequently, on the interparticle distance during milling. The interparticle distance decreases from 200 nm to 142 nm with an increase of solid loading in the range of 41-50 wt%. It was shown that for the solid loading of 47 wt%, the convergence of particles to a distance comparable to their median diameter promotes subsequent clustering of particles. This facilitates the sintering of highly-homogenous ceramics. It was found that solid loadings in the 46-50 wt% range is useful for obtaining high-quality Nd:YAG transparent ceramics. The lowest optical losses optical losses of 1 x 10-3 cm-1 and the highest in-line transmittance of 84.1%@1064 nm were obtained for 1 at.% Nd:YAG transparent ceramics (22 x 3 x 4 mm3) prepared from slurries with 47 wt% solid loading (taking all other ball milling parameters fixed). If the interparticle distance in the powder is higher (solid loading of 41 wt%) than the median particle diameter, the ceramics are characterized by significant residual porosity due to the survival of large particles (insufficient milling).

The effect of dispersant concentration on the sedimentation stability of aqueous suspensions of Y2O3 nanopowders, as well as the influence of the slurry composition, such as the dispersant content and solid loading, on their rheological properties were investigated. It was found that 30 wt.% Y2O3 and 1.5 wt.% Dolapix CE64 water suspension with the highest solid loading has rheological properties close to Newtonian fluids and low viscosity. Increasing the dispersant or solid content leads to increase in slurry viscosity and loss of Newtonian behavior. Hemispherical samples of infrared transparent Y2O3 ceramics were obtained by the slip casting method followed by vacuum sintering at 1750 degrees C. The obtained ceramics are characterized by a relative density of 99 +/- 1 %, the average grain size of 10-15 mu m, and in -line transmittance of 30 % and 63 % at the wavelengths of 800 and 2000 nm, respectively.

Fine-grained 4 at% Nd3+:YAG transparent ceramics were prepared by reactive spark plasma sintering using a two-step heating profile. The effect of key sintering parameters (heating rate, external pressure, sintering and post-annealing temperatures) on densification peculiarities, optical and mechanical properties was investigated. Special attention was paid to the behavior and nature of carbon contaminations, dispersed in the yttrium aluminum garnet matrix. It was established that the optimal heating rate to the isothermal exposure temperature was 15 ?C min-1. In this case, supporting the uniformity of temperature field distribution between the punches ensures the synthesis of the single-phase product and positive densification dynamics throughout the sintering trajectory for the disk geometry of samples. It was shown that an increase in applied pressure from 30 to 70 MPa changed the color of ceramics: the samples darkened and became dark brown. This may be due to an increase in the concentration of oxygen vacancy defects (color centers). The formation of silicon oxycarbides SiOxCy (where x + y = 4; x,y >= 1) in ceramics at sintering temperatures T > 1400 ?C was revealed by X-ray photoelectron spectroscopy. It was found that these impurity phases demonstrated a thermal stability and resistance to decomposition during post-annealing. The study showed that 4 at% Nd3+:YAG ceramics obtained by reactive spark plasma sintering at 1350?C//10 min//30 MPa//15 ?C min-1 and post-annealed at 900 ?C for 1 h possessed a mean grain size of 0.74 mu m, a microhardness of 13.2 GPa, a residual porosity of 0.0192 vol% and in-line optical transmittance 73.7% at lambda = 1064 nm, which is 0.87 from the theoretical value for a single crystal.

The work is devoted to obtaining of transparent nanocomposite materials as low loss, highly thermally conductive materials for potential laser applications. We report Ho3+:Y2O3–MgO nanocomposite ceramics with excellent mechanical and optical properties by combining glycine-nitrate process and spark plasma sintering. Morphology, structural-phase state, infrared transmittance and luminescence depending on the holmium concentration (0…12 at.%) were studied for the first time. It was found that optical transmittance reaches 75% at 6000 nm for 3 at.% Ho3+:Y2O3–MgO ceramics.

Low-agglomerated (LuO3)-O-2:Eu3+ 5 at.% nanopowders, as well as transparent ceramics were fabricated by co-precipitation with ammonium hydrogen carbonate and vacuum sintering method, respectively. It was determined that transition of a part of europium ions into the divalent state enhanced sinterability due to formation of anionic vacancies and decrease of the RE-O bonds covalency. The luminescent properties of the Lu2O3:Eu3+ 5 at. % nanopowders and ceramics were studied under excitation by synchrotron radiation. The difference in the excitation spectra of (LuO3)-O-2:Eu3+ nanopowders and ceramics are caused by participation of the F+ centers in the excitation processes of Eu3+ luminescence in Lu2O3 host. Finally, the locations of the energy levels related to the Eu3+ dopant in Lu2O3 matrix were determined.

Y 2 O 3 :Yb 3+ 5 at% ceramics have been synthesized by the reactive sintering method using different commercial yttria powders (Alfa-Micro, Alfa-Nano, and ITO-V) as raw materials. It has been shown that all Y 2 O 3 starting powders consist from agglomerates up to 5–7 µm in size which are formed from 25–60 nm primary particles. High-energy ball milling allows to significantly decreasing the median particle size D 50 below 500 nm regardless of the commercial powders used. Sintering experiments indicate that powder mixtures fabricated from Alfa-Nano yttria powders have the highest sintering activity, while (Y 0.86 La 0.09 Yb 0.05 ) 2 O 3 ceramics sintered at 1750 °C for 10 h are characterized by the highest transmittance of about 45%. Y 2 O 3 :Yb 3+ ceramics have been obtained by the reactive sintering at 1750–1825°C using Alfa-Nano Y 2 O 3 powders and La 2 O 3 +ZrO 2 as a complex sintering aid. The effects of the sintering temperature on densification processes, microstructure, and optical properties of Y 2 O 3 :Yb 3 + 5 at% ceramics have been studied. It has been shown that Zr 4 + ions decrease the grain growth of Y 2 O 3 :Yb 3+ ceramics for sintering temperatures 1750–1775 °C. Further increasing the sintering temperature was accompanied by a sharp increase of the average grain size of ceramics referred to changes of structure and chemical composition of grain boundaries, as well as their mobility. It has been determined that the optimal sintering temperature to produce high-dense yttria ceramics with transmittance of 79%–83% and average grain size of 8 µm is 1800 °C. Finally, laser emission at ∼1030.7 nm with a slope efficiency of 10% was obtained with the most transparent Y 2 O 3 :Yb 3+ 5 at% ceramics sintered.

The "metallic" temperature variation of the electrical resistance of chemical coatings of starlike gold-silver nanoparticles with a silver content of 6.6% at. to 13.1% at. is carefully analyzed in the temperature range 4.2-300 K. Features of low-temperature measurements of the resistance of nanocoatings were found and explained. The possibility of the superconductivity appearance in such structures is discussed.

The evolution of the pore structure of ceramics 0-4 at% Nd3+:Y3Al5O12 at the initial and intermediate stages of sintering was studied by the nitrogen physical adsorption. It was shown that the maximum decrease in the specific surface area (by 70 divided by 83 %) and the total volume of nanopores (by 84 divided by 92 %) at the sintering temperature range of 1100-1500 degrees C is observed for systems with (Nd3+) = 1 divided by 2 at%. The nonlinear nature of the effect of neodymium on the formation of the Nd3+:Y(3)Al(5)O(12 )microstructure is established, which is explained by the presence of several competing mechanisms, the magnitude of which varies differently with the activator concentration. Differences in the pore structure and transparency of ceramics 1-4 at% Nd3+:Y3Al5O12 at the final sintering stage are shown. A possible reason for the slowdown in densification of samples with (Nd3+) >= 3 at% is a decrease in diffusion mobility in the cationic sublattice of garnet. 1.2 at% Nd3+:Y3Al5O12 laser ceramics obtained by reactive sintering at 1750 degrees C for 10 h are characterized by a residual porosity of 0.0009 and 0.0026 vol% with an average pore size of 182 and 161 nm, respectively.

The luminescence kinetics of Er ions in Y2O3:Er nanospheres with diameters of 75–270 nm has been studied. A pronounced decrease in the luminescence decay times for Er3+ ion transitions in the green-red visible range is observed with increasing diameter up to 270 nm, unlike near-infrared emission at ∼1.5 μm. This finding is attributed to the appearance of photonic modes accelerating spontaneous luminescence, which is confirmed by a model calculation of bound optical states in the studied nanospheres.

Formation peculiarities of highly-doped (Y(0.86)La(0.09)Vb(0.05))(2)O-3 transparent ceramics have been studied by X-ray diffraction and electron microscopy methods. The phase composition evolution of 1.81Y(2)O(3).0.18La(2)O(3)0.01Yb(2)O(3) powder mixtures annealed at the temperatures of 1100, 1200, 1300, and 1400 degrees C has been studied by XRD. It has been shown that Yb2O3 phase dissolves in Y2O3 matrix in the calcination temperature range of 1300-1400 degrees C. Complete dissolution of La2O3 in Y2O3 matrix occurs at temperatures above 1400 degrees C. La3+ ions enter in Y2O3 and Yb2O3 crystal structures simultaneously in the 1200-1300 degrees C range, which leads to a remarkable increase in the volume of the corresponding crystal lattices. The possible reasons for suppressing the crystalline growth of Y2O3 and Yb2O3 cubic phases have been discussed. Finally, (Y(0.86)La(0.09)Vb(0.05))(2)O-3 transparent ceramics have been obtained by solid-state vacuum sintering at 1650-1750 degrees C. Ceramics synthesized at a temperature of 1750 degrees C have been characterized by an in-line optical transmittance of 60% and a homogeneous distribution of constituent components within the volume and along the grain boundaries.

Composite nanopowders Y2O3-MgO (1:1 by volume) were synthesized by the method of self-propagating glycine-nitrate synthesis with an excess of glycine and nitric acid. It was shown that freshly prepared powder (precursor) contains about 19 % of unreacted components and intermediate reaction products, which are removed by subsequent calcining. Crystallization of the precursor starts at calcination temperature above 600 degrees C and leads to nucleation of the crystalline phases MgO and Y2O3. It was shown that calcining at temperatures from 800 to 1000 degrees C leads to the formation of nanocrystals with sizes from 20 to 90 nm, respectively. The specific surface area of composite nanopowders decreases from 48 to 16 m(2)/g with increasing calcination temperature in the range T = 700-1000 degrees C. It was shown that during Y2O3-MgO calcination in the air, intense chemisorption of CO2 occurs on the surface of nanopowders. According to calculations, about 5 % of MgO is converted to magnesium carbonate. Finally, Y2O3-MgO composite ceramics with average grain size of 255 nm and transmittance of 71 % at X = 6000 nm have been obtained by spark plasma sintering of synthesized nanopowders.

The effect of green bodies' mesostructure on the porosity, optical properties and laser performance of reactive sintered Y3Al5O12:Nd3+ transparent ceramics was studied. Only minor changes in microstructure were revealed for green bodies without annealing and those annealed at 600, 800, 1000 degrees C, while average pore size increases to 140 nm for sample annealed at 1200 degrees C. Y3Al5O12:Nd3+ ceramics sintered at 1750 degrees C for 10 hours possess significant differences in the final porosity, optical and laser characteristics. Despite all green bodies exhibit a similar phase evolution and sintering behavior on heating, the differences appear in the final stage, when the latest percentage of porosity is removed. The green bodies annealed at 600 degrees C have an optimal mesostructure from the standpoint of uniform densification. Y3Al5O12:Nd3+ ceramics prepared using these green bodies exhibit porosity <= 0.001 vol% and yield efficient laser emission at 1.06 mu m with slope efficiency as high as 67% in quasi continuous pumping at 807 nm.

YAG:Cr, Mg optical ceramics were prepared by the solid-state reactive sintering method within the temperature range of 1750-1800 degrees C. The influence of the sintering temperature on the microstructure, phase composition, and optical properties of YAG:Cr, Mg ceramics was studied. It has been shown that optimal obtaining temperature of YAG:Cr, Mg ceramics doped with 0.5 at.% of chromium ions and 0.5 at.% of magnesium ions is 1765 degrees C. YAG:Cr, Mg ceramics sintered at T=1765 degrees C possess the lowest porosity and optical losses in the visible wavelength range. They are characterized by the average grain size of about 3 pm and in-line transmittance above 80%. Thermal annealing of ceramics results in transition of a part of Cr3+ ions to Cr4+ charge state. A decrease in concentration of Cr4+ ions in tetrahedral coordination with increase the sintering temperature has been observed. This effect was attributed to different charge compensation mechanisms of Mg2+ ions taking place in YAG:Cr, Mg ceramics obtained at various temperatures. Finally, laser performance of YAG:Cr, Mg ceramics was estimated in the passive Q-switch mode.

(Y0.99Eu0.01)(2)O-3 nanoceramics have been produced by sintering of stable cubic nanopowders under 8 GPa at temperature in the range of 25-500 degrees C with the use of Low Temperature High Pressure (LTHP) technique. During consolidation step irreversible phase transition from cubic to monoclinic yttria occurs resulting in two-phase nanoceramics with a grain size in the 10-40 nm range. It has been demonstrated that composite nanoceramics possess a high transmittance in the visible and mid IR ranges due to small light scattering on the nanoscale pores and low birefringence due to extremely small grain size. It has been shown that Eu3+ ions act as a luminescent probe in composite (Y0.99Eu0.01)(2)O-3 nano-ceramics since their 4f-4f luminescence strongly depends on the crystallographic environment. The luminescence spectra excited in the charge transfer band (CTB) are presented by superposition of emission from europium ions in cubic and monoclinic yttria. A new wide emission band of (Y0.99Eu0.01)(2)O-3 ceramics in the lambda = 500-650 nm wavelengths range (lambda(ex). = 307 nm) were attributed to luminescence of Eu3+ ions located in perturbed sites at grain boundaries or interfaces. (C) 2018 Elsevier B.V. All rights reserved.

Luminescent properties of Yb,Er:YAG transparent ceramics containing 5 at% Yb3+ and 0.5, 1 and 1.5 at%, respectively, Er3+ ions have been studied. It has been found that increasing of erbium ions concentration increases both efficiency of nonradiative energy transfer Yb3+ -> Er3+ (which reaches 72% for 1.5 at% Er3+) and the luminescence in the range of 650-700 nm associated with F-4(9/2), (11/2) -> I-4(15/2) transitions of Er3+ ions. It was also determined that the concentration quenching of sensitized luminescence of Er3+ ions at 1532 nm is associated with secondary excitation of metastable energy level of Er3+ followed by up conversion emission. (C) 2018 Elsevier B.V. All rights reserved.