YAG Nanopowders Research Articles

Synthesis of Nd:YAG nanopowder using the citrate method with microwave irradiation

Abstract Nd:YAG nanopowders were prepared using the Pechini process with microwave irradiation ( MWs ). A reference sample was also prepared using conventional heating. XRD pattern analysis showed that nanopowder obtained by means of conventional heating and calcination for 1 h at 900 °C has a structure made up of the garnet phase together with the hexagonal phase that disappeared after two additional hours of thermal treatment. The MWs powder calcined for 1 h consists of the single garnet phase. SAXS data analysis indicated that nanoparticles are characterized by a sharp interface. TEM investigation showed crystalline particles with remarkable agglomeration in both samples, although a more homogeneous size of particles was evident in the MWs sample as compared to the reference sample. The rate, the calcination time and the quality of the powder obtained make the Pechini process with microwave irradiation an attractive alternative procedure to conventional YAG preparation methods.

Co-precipitation synthesis of Nd:YAG nanopowders II: The effect of Nd dopant addition on luminescence properties

Nanopowders of Yttrium Aluminium Garnet (Y 3Al 5O 12, YAG) doped with neodymium (Nd:YAG, 0.2–24.0 at.%) were prepared using the co-precipitation method followed by an annealing treatment up to 950 °C. For a concentration of neodymium lower than 3.2 at.% the materials were found constituted by the garnet phase according to X-ray diffraction investigations. However, at higher neodymium loading the hexagonal and monoclinic forms of yttrium aluminium oxides were found together with the garnet phase. For Nd quantity lower than 0.8% the luminescence emission spectra appear to be nearly the same, indicating that in the examined range of composition the immediate surrounding of the emitting Nd 3+ ions is independent of its concentration. For Nd quantity higher than 0.8%, the emission lines have nearly the same peak position, but the band-widths are larger and the relative line intensity increases with neodymium content. A dramatic decrease of two magnitude orders of the luminescence lifetimes occurs with the Nd quantity increase. This behaviour is ascribable to the presence of a secondary phase together with the quenching effects.

Agglomeration Control of Nd:YAG Nanoparticles Via Freeze Drying for Transparent Nd:YAG Ceramics

Neodymium‐doped yttrium aluminum garnet (Nd:YAG) nanopowders were synthesized by the carbonate coprecipitation method. The effects of freeze drying and conventional oven drying of the precursor on the agglomeration of the Nd:YAG nanopowders were compared. The optical properties of the Nd:YAG nanopowders and the corresponding sintered Nd:YAG transparent ceramics were also investigated. The Nd:YAG nanopowders synthesized from freeze‐dried precursor showed better dispersion and narrower particle size distribution compared with the powders synthesized from conventional oven drying. As a result, the Nd:YAG nanopowders synthesized from freeze‐dried precursor have good sinterability, and Nd:YAG transparent ceramics were fabricated by vacuum sintering at 1750°C for 5 h.

Effect of green density and electric field direction on densification of YAG nano-powders by spark plasma sintering

Nanocrystalline YAG powders with 34-nm average particle size were cold isostatic pressed (CIP) between 10 and 240 MPa to form compacts with 43–52% green densities. Spark plasma sintering (SPS) was performed for 5 min at 1,400 °C and 90 MPa using clusters of three disc specimens packed in series with the electric field direction. The pore size and its volume fraction decreased with the increase in the CIP pressure. The final density varied between 86% and 99% and strongly depended on the specimen location in the series. Density, grain size, and pore size analyses showed that the specimens facing the electric current experienced higher SPS temperatures. Possible temperature gradient due to axial displacement of non-conducting oxide was too low to explain the observed effect. Effect of the green density on the final density was marginal.

Evidence for a double doping regime in Nd:YAG nanopowders

Nanopowders of Yttrium Aluminium Garnet doped with neodymium (Nd:YAG) were investigated by X-Ray Absorption Fine Structure (XAFS) at the Nd LIII edge in the 1.3–20.8% doping range. XANES spectra appear similar in the full range of the Nd concentration. However, a significant decrease in the white line intensity of XANES is revealed as the quantity of Nd doping ions increases. Plotting the white line intensity as a function of Nd doping ions reveals two linear trends with two different slopes, identifying a threshold value where the neodymium concentration reaches 5 at.% This experimental finding provides support for the existence of a double doping regime in Nd:YAG nanopowders.

Production of Y3Al5O12 and Y2O3 nanopowders for optical ceramics

The physical and technical conditions for reproducible production of nanodispersed yttrium aluminum garnet (Y3Al5O12, YAG) and yttrium oxide (Y2O3) powders by chemical coprecipitation have been investigated. It is established that the obtained YAG nanopowders have enhanced reactivity, which significantly decreases the temperature range of interaction in the Y2O3-Al2O3 system in comparison with ceramic synthesis. It is shown that vacuum heat treatment may lead to reversible transformation of the YAG crystal structure from cubic to tetragonal.

Fabrication of Transparent YAG Ceramics from Co-Precipitation Synthesized Nanopowders

YAG nanopowders were synthesized by a co-precipitation method using ammonium hydrocarbonate and ammonia water as the precipitants respectively. The influences of precipitants on chemical compositions, phase transformation and sinterability of the prepared powders, and transmittance of the vacuum-sintered YAG ceramics were studied. The sinterability of powders synthesized using ammonium hydrocarbonate as precipitant is better than that with ammonia water. Pure YAG phase can be obtained by calcining the hydrate precursor at 1200°C, while some impurity phases exist when calcining the carbonate precursor at the same temperature. Transparent YAG ceramics were fabricated by vacuum sintering at 1700°C for 5 h using the YAG nanopowders, and their in-line transmittance is about 60% in the visible light range.

YAG:Ce<sup>3+</sup> Nanopowders: Synthesis, Characteristics and Luminescent Properties

By means of co-precipitation, pure YAG phase was synthesized from the reaction of aluminum yttrium and cerium nitrates with ammonia as precipitator. In order to obtain the special coating oxide precursor, the metal ions solution and ammonia were respectively added with fixed ratio by two peristaltic pumps to maintain an expired pH value and reaction rate. The as-received precursor was annealed at 700°C and 900°C respectively. The structure, phase evolution and morphology of the as-prepared YAG nanopowders were tested by XRD, BET and SEM. At lower temperature (900°C), coating oxide precursor was directly transformed to nanopowders of pure YAG phase. The high salt solution concentration was critical for the control of chemical homogeneity within the precursors. The YAG nanopowders were less-aggregated and better-dispersed. The luminescent properties were examined and discussed.

Time-resolved luminescence of nanocrystalline inorganic complex oxides

Two types of complex nanosized oxides – cerium doped Y3Al5O12 (YAG) and CaWO4– have been studied by means of time-resolved luminescence spectroscopy. Comparative study of time-resolve luminescence characteristics of cerium doped YAG single crystal, nanopowders and nanoceramic as well as for CaWO4 macro- and nanocrystals has been done. Two components in the decay kinetic of Ce3+ related emission in YAG nanocrystals were detected and it was suggested that a different energy transfer rate to volume and surface Ce3+ ions takes place. It is shown that the segregation of Ce3+ ions near nanoparticles surface and/or dislocation lines plays a crucial role in degradation of light yield of cerium related luminescence in YAG nanocrystals. Time-resolved properties of sol-gel synthesized CaWO4 nanocrystals depends strongly on the synthesis rout. It was shown that shallow traps have a strong influence on the luminescence decay times of nanosized CaWO4.

Increase of the blue upconversion emission in YAG:Tm 3+ nanopowders by codoping with Yb 3+ ions

The YAG nanopowders were prepared by a co-precipitation method using nitrate and ammonium hydrogen carbonate as raw materials. To obtain homogenous precipitate, reverse-strike (adding salt solutions to the precipitant solution) technique was adopted. Therefore, single (Tm 3+) and codoped (Tm 3+–Yb 3+) YAG nanopowders with a size between 40–90 nm have been obtained. Blue upconversion emission at around 480 nm has been found in YAG: Tm 3+ nanopowders under excitation to the 3H 4 level of Tm 3+ at around 800 nm. However, this upconversion emission in nanopowders codoped with Tm 3+–Yb 3+ ions is increased by a factor of about 10. The analysis of the temporal evolution of the involved levels and the dependence of the upconversion intensity on the pump power at 800 nm allowed to distinguish the upconversion mechanism. In YAG: Tm 3+ nanopowders the upconversion mechanism is due to excited state absorption processes. However, in the codoped samples, Yb 3+ ions acts as the sensitizers; in consequence, the blue upconversion is strongly increased.

Optimization of Conditions of Preparation of YAG Nanopowders for Sintering of Translucent Ceramics

The agglomeration of YAG (Yttrium-Aluminum Garnet Y3Al5O12) nanopowders doped with various rare earth ions obtained by the coprecipitation and calcination route is a major problem if it is wished to exploit the nano-size properties such as the transparency of dispersions of the powders or low temperature sintering. Investigations to optimize the preparation process of powders for High Pressure-Low temperature Sintering (HPLS) of semi-translucent pellets was undertaken. The appropriate milling time to decrease the agglomeration was evaluated with the help of ZETA- potential measurement. The dependence of the agglomerate size against the time of hand milling was used to optimize the process.

Effects of Thermal Treatment on the Structure of Eu:YAG Nanopowder

Eu:YAG nanopowder precursors were obtained by co-precipitation of aluminium, yttrium and europium nitrates solution with ammonia. The hydroxides precursors were calcined at different temperatures from 900 to 1200°C as a function of holding time (1, 2 and 6 hours). The presence of Eu3+ ions in the matrix was confirmed by Energy Dispersive X-rays analysis. X-Ray Diffraction investigation by the Rietveld method shows that the sample treated at 900°C for 1 hour is essentially the garnet phase with the minor presence of hexagonal and monoclinic metastable phases. The Eu3+ ions are incorporated into the garnet phase, as is suggested by the lattice parameter value being larger than that in literature data (homogeneous strain). For the sample treated at 900°C for 1 hour, electron microscopy observations showed agglomerates of spherical particles of mean size about 50 nm. At higher temperature treatments and for longer holding times the minority hexagonal and monoclinic phases totally disappeared. However, the lattice parameters of the cubic garnet phase gradually decreased with temperature, suggesting an expulsion of Eu3+ ions from the solid solution. Simultaneous with this, it was noted that the lattice strain reached a maximum value, but to later decrease, due to the vacancies created by the Eu species initially migrating to the surface of the coherent domains of diffraction. The lattice strain definitely decreased upon more drastic thermal treatments. Meanwhile, FEG-SEM and TEM observations on the same samples confirmed the growth of the garnet particles as a function of the thermal treatment.

Time-Resolved Luminescence Characteristics of Cerium Doped YAG Nanocrystals

Time-resolved luminescence characteristics have been studied for cerium doped YAG nanopowders (NP) and nanostructured ceramics (NC). The results obtained have been compared to the luminescence characteristics for the well studied YAG:Ce3+ single crystal (SC). It was detected that the luminescence decay kinetics of Ce3+ related emission in nanocrystals can be closely approximated by two exponents, whereas a single exponential decay was observed in the single crystal. It was also found that the luminescence decay time and light yield of Ce3+ emission are strongly dependent on the cerium concentration and an unusual concentration quenching of Ce3+ emission was observed in YAG nanocrystals. The origin of the differences of luminescence characteristics for cerium doped nanocrystals and SC are discussed. It is suggested that there are special Ce3+ sites which are closely related to the surface states and that they have a strong influence on the luminescence properties of cerium doped YAG nanocrystals.

The concentration dependence of luminescence of Nd:Y3Al5O12 nanoceramics

Abstract Transparent Nd:YAG ceramics composed of nanosized grains were fabricated by the low temperature high pressure (LTHP) sintering technique. The effect of concentration of ions on luminescence properties of Nd 3+ doped YAG nanoceramics and nanopowders was investigated. It was found that at room temperature the intensity ratio of 4 F 3/2 → 4 I 9/2 and 4 F 3/2 → 4 I 11/2 transitions in nanoceramics was strongly dependent on concentration of Nd 3+ . This behavior was not observed for the Nd:YAG nanopowders. The nature of such behavior was briefly discussed in terms of lowering the site symmetry of Nd 3+ ions with applied pressure.

Preparation of Nd:YAG Nanopowder in a Confined Environment

Nanopowder of yttrium aluminum garnet (YAG, Y3Al5O12) doped with neodymium ions (Nd:YAG) was prepared in the water/cetyltrimethylammonium bromide/1-butanol/n-heptane system. Aluminum, yttrium, and neodymium nitrates were used as starting materials, and ammonia was used as a precipitating agent. Coprecipitate hydroxide precursors where thermally treated at 900 degrees C to achieve the garnet phase. The starting system with and without reactants was characterized by means of the small-angle neutron scattering technique. The system, without reactants, is constituted by a bicontinuous structure laying near the borderline with the lamellar phase region. The introduction of nitrates stabilizes the bicontinuous structure, while the presence of ammonia induces a transformation from the bicontinuous phase to a lamellar phase. Nd:YAG nanopowder was characterized by wide-angle X-ray scattering, transmission electron microscopy, gas adsorption, and photoluminescence spectroscopy. By comparison with a sample prepared by the conventional coprecipitation method, the obtained Nd:YAG nanopowder is constituted by smaller crystalline nanoparticles showing a lower tendency to agglomerate. In addition, the nanoparticles present a well-defined spherical shape. Photoluminescence spectroscopy confirms that the doping Nd3+ ions substitute Y3+ ions in the YAG crystalline lattice. The Nd3+ lifetime value, obtained from the luminescence decay curves, was 286 +/- 10 micros, higher than the single-crystal value (255 micros) and much higher than the nanopowder value obtained by the conventional coprecipitation method (75 micros).

Co-precipitation synthesis of Nd:YAG nano-powders: the effect of Nd dopant addition with thermal treatment

Nanopowders of Yttrium Aluminium Garnet doped with neodymium ions were obtained by the co-precipitation method from the reaction of aluminium, yttrium and neodymium nitrate with ammonia. The amount of neodymium was selected in order to produce samples of nominal stoichiometry NdXY(3–X)Al5O12 (where X = 0.006, 0.012, 0.024, 0.048, 0.081, 0.096, 0.17, 0.19, 0.38, 0.54, and 0.72, respectively). After washing and drying, the hydroxide precursors were subjected to Thermo-Gravimetry and Differential Thermal Analysis experiments from room temperature up to 1500 °C, which showed the presence of exothermal events accompanying phase transformation phenomena. X-ray diffraction investigations conducted with a high-resolution powder diffractometer on the specimens arrested at selected temperature of the thermograms, evidenced the amorphous-to-crystalline transformation phenomena leading to the garnet phase as the main product. On increasing the concentration of Nd, the presence of the monoclinic Y4Al2O9 phase was also detected together with a variable amount of a metastable hexagonal YAlO3 phase. Precise determination of the cubic garnet lattice parameters as a function of the neodymium content according to the Rietveld method shows a change from the value of 12.016 (±2) A when X = 0 up to 12.128 (±2) A for X = 0.720 with two distinctive regimes of increase. The line broadening analysis of X-ray profiles after correction for instrumental factors indicates that the average crystallite size is in the range 50–80 nm. Field-Emission Gun-Scanning Electron Microscopy observations showed the presence of aggregation features in the powders with a rounded morphology and a relatively uniform and narrow particle size distribution, with the average size figures in substantial agreement with the diffraction analyses.

Luminescence of cerium doped YAG nanopowders

Cerium doped YAG nanopowders with grain size ∼ 20 nm have been synthesized by co-precipitation method. Time-resolved luminescence characteristics have been studied. The results obtained have been compared with the luminescence characteristics for well-studied YAG single crystals. In contrast to cerium doped YAG crystals, cerium doped nanocrystal does not reveal luminescence bands due to antisite defects under electron beam pulse and X-ray excitations at 80–300 K temperature range. It was detected that decay kinetic of Ce 3 + related emission in the nanocrystal can be approximated by two exponents with time constants ∼ 9 and ∼ 47 ns at 290 K. It is faster than single exponential decay ( ∼ 80 ns ) of cerium ions emission in YAG crystals. The origin of differences in luminescence characteristics for cerium doped nanocrystals and single crystals will be discussed. It is suggested that special Ce 3 + sites which are closely related to surface states have an influence on luminescence properties of cerium doped YAG nanocrystals.

Synthesis of an alumina–YAG nanopowder via sol–gel method

Alumina–YAG (Y 3Al 5O 12) composite has attracted a great deal of attention as a high temperature structural ceramic due to its good creep resistance. In the present article, an Al 2O 3–YAG nanopowder was synthesized by aqueous sol–gel method using AlCl 3·6H 2O, aluminum and Y 2O 3 powders as raw materials. The dried amorphous gel was heat treated in the range of 600–1500 °C. The influence of heat treatment on crystallization and phase transformation of the dried gel was investigated using X-ray diffractometery (XRD), scanning electron microscopy (SEM), thermogravimetry and differential thermal analysis (TG–DTA) and Fourier transform infrared spectroscopy (FTIR). It was shown that the gel heat treated at 1300 °C resulted in the formation of a crystalline Al 2O 3–YAG nanopowder with particle size distribution ranges from 70 to 130 nm.

Influence of local-field effects on the radiative lifetime of liquid suspensions of Nd:YAG nanoparticles

We measured the radiative lifetime of Nd:YAG nanopowder with an average particle size of 20nm suspended in different organic and inorganic liquids. To extract information regarding local-field effects, we fitted the experimental data to three different local-field models: the virtual-cavity (or Lorentz) model, the real-cavity model, and the no-local-field-effects model. The real-cavity model and the no-local-field-effects model can both be adequately fitted to our experimental results, while the virtual-cavity model can be ruled out.

Effect of the addition of YAG (Y 3Al 5O 12) nanopowder on the mechanical properties of lanthanum zirconate

La 2Zr 2O 7 (LZ) is a promising thermal barrier coating material for the high-temperature applications, which could be significantly toughened by the YAG nanopowder incorporated into the matrix. The composites of xYAG/(1 − x)LZ ( x = 10, 15, 20 vol.%, LZ- x-YAG) were densified by means of high-pressure sintering (HPS) under a pressure of 4.5 GPa at 1650 °C for 5 min, by which a high-relative density above 93% could be obtained. The morphologies of the fractured surfaces were investigated by the scanning electron microscope, and the fracture toughness and Vicker's-hardness of the composites were evaluated by the microindentation. The grain size of the LZ matrix drops significantly with the addition of YAG nanoparticles and the fracture type changes from the intergranular to a mixture type of the transgranular and intergranular in the nanocomposites. The LZ-20-YAG nanocomposite has a fracture toughness of 1.93 MPa m 1/2, which is obviously higher than that of the pure LZ (1.57 MPa m 1/2), and the toughening mechanism is discussed in this paper.