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Fluoride-upconversion transparent ceramic materials doped with Er ions have attracted extensive interest due to their higher stability, lower phonon energy, and higher upconversion efficiency. xEr:Sr 5 (PO 4) 3 F (S-FAP) (x=0.5–5 at%) transparent ceramics have been synthesized by hot pressing sintering in this work, and the maximum optical transmittance of the sample reached 74.52 and 82.45% at 500 and 1000 nm, respectively. The XRD results of the powder and ceramic samples were consistent with the diffraction characteristics of the hexagonal S-FAP crystal structure. The SEM and TEM images of powder revealed that the morphology of the sample was composed of mostly rice grains with a diameter of around 20 nm. The XPS and EDS results confirmed the presence of Er3+ and the uniform distribution of Er elements. The thermally etched surface and cross section of ceramics revealed residual pores as the most important scattering source, and the SEM images of the ceramics show that Er doping can obviously promote the growth of the average grain size of S-FAP transparent ceramics. Furthermore, the upconversion luminescence characteristics and fluorescence lifespan of S-FAP transparent ceramics with varied Er doping concentrations were examined, as well as the upconversion luminescence process. The findings in this work indicate that Er:S-FAP transparent ceramics can be employed as a potential upconversion fluorescent material in display technology, biomedicine, temperature sensing, and other applications. • The xEr:S-FAP (x=0–0.05) transparent nanostructured ceramics with highly optical quality were prepared by hot-pressed sintering for the first time. • The microstructural characteristics, upconversion luminescence property, and mechanism of as-synthesized xEr:S-FAP transparent ceramics were investigated for the first time. • The wide visible light spectrum from red light to green light was achieved by changing the doping amount of Er3+ from 0.5 to 5 at%. [ABSTRACT FROM AUTHOR]

Transparent hexagonal 2 at.% Yb:Sr 5 (PO 4) 3 F (S -FAP) ceramics were effectively fabricated by hot pressing the coprecipitated powder employing LiF as the sintering additive. The effects of LiF doping levels (0–0.5 wt%) and sintering temperature (820–940 °C) on microstructural development and optical characteristics of samples were examined. The results showed that the addition of LiF and the evaluated sintering temperature accelerated the grain development and densification process of Yb:S-FAP ceramics. Also, the full densification process of low-doped LiF ceramic samples was found to be strongly dependent on the reduced sintering temperature. For the Yb:S-FAP transparent ceramic sintered at the lowest temperature of 850 °C than previously reported with 0.1 wt% LiF, which have an optical in-line transmittance of 59.7% and 79.66% at 500 and 1100 nm respectively. The corresponding scattering sources of ceramic samples were analyzed by the surface SEM of different magnification at different LiF doping concentrations, and residual LiF, residual porosity, and grain boundary scattering were considered to be the main scattering sources. The spectral results showed that the LiF-doped Yb:S-FAP transparent ceramics had a wide absorption peak at 975 nm with the full width at half maximum (FWHM) of 15 nm and possessed a fluorescence lifetime of 1.12 ms. [ABSTRACT FROM AUTHOR]

The asymmetric hexagonal Sr 5 (PO 4) 3 F (S-FAP) crystal material is considered to be the most suitable solid state laser gain medium for small laser diode pumping in the future due to its large absorption, emission interface, and long fluorescence lifetime. However, the mediocre optical transmittance of S-FAP transparent ceramics and the degradation of luminescence properties due to the doping of Yb activated ions seriously hinder its application prospects. In view of this, a series of 0.02Yb, xLu: S-FAP (x = 0–0.02) transparent ceramics with excellent optical properties were synthesized by hot pressing sintering. The powder SEM results show that Lu doping has no obvious effect on the morphology, grain size, and dispersion of powder. The linear transmissivity curves show that the ultraviolet (200 nm) and visible (500 nm) transmissivity increase by 54 % and 17 %, respectively, with Lu doping compared with the undoped ceramic samples. The surface SEM of ceramics revealed that Lu3+ promoted the increase of ceramic grain size significantly. The emission spectrum and fluorescence decay curves at room temperature also show that the emission intensity and fluorescence lifetime of ceramic samples increase significantly with Lu co-doping. • The 0.02Yb, xLu:S-FAP (x = 0–0.02) transparent ceramics with highly optical quality were prepared by hot-pressed sintering for the first time. • The microstructural characteristics of as-synthesized 0.02Yb,xLu:S-FAP transparent ceramics were investigated. • The luminescence property of as-synthesized 0.02Yb,xLu:S-FAP transparent ceramics were investigated. • The tremendous optical property of as-synthesized 0.02Yb,xLu:S-FAP transparent ceramics were found. [ABSTRACT FROM AUTHOR]

The Sr 5 (PO 4) 3 F (S-FAP) crystal material is regarded as one of the most ideal optical materials for diode pumping owing to its huge absorption and emission cross sections and long fluorescence lifespan. In this investigation, S-FAP powders with varying Yb concentrations (0.1–5%) were produced using the coprecipitation method. Then a variety of S-FAP transparent ceramics with varying Yb content were fabricated using hot-pressing sintering. The crystalline phase structure of hexagonal Sr 5 (PO 4) 3 F was verified by XRD analysis of the precursor powder and the final ceramics. According to the powder SEM, the average grain size and the long axial-radial ratio of powders are decreasing as the Yb3+ concentration increases. Thermal-etched surface SEM reveals nanostructured S-FAP transparent ceramics with an average grain size of less than 200 nm were synthesized. The highest transmittances of the 3% ceramics at 500 and 1100 nm wavelengths are 51% and 79.78%, respectively. The ceramic cross-sectional SEM demonstrated that porosity is the primary scattering source influencing the enhancement of optical characteristics. The absorption, emission, and fluorescence lifetimes of S-FAP transparent ceramics with varying Yb concentrations were tested and discussed, and the absorption and emission cross sections corresponding to the major peak were reported. Some physical parameters of this set of ceramic samples were shown, including thermal diffusivity, specific heat capacity, and thermal diffusivity data, as well as micro-hardness. [ABSTRACT FROM AUTHOR]

2 at.% Yb:Sr 5 (PO 4) 3 F nanoparticles were synthesized by co-precipitation method. By using XRD, FT-IR and FE-SEM measurements, the influence of reactive conditions, including pH value, reactive temperature and mixing mode on the phase composition and microstructure of these nanoparticles were investigated. The results showed that pure Yb:Sr 5 (PO 4) 3 F nanopowders were produced by adding sodium hydroxide solution into S-FAP emulsion under alkaline environment or neutral environment. Agglomeration degree of the nanoparticles decreases and then increases with the increase of pH value and the aggregation was lowest when the pH value was 12. Spherical or nearly spherical nanoparticles with mean size of 24.0 nm were obtained in the environment of 20 °C and pH = 12. TEM results showed that the powders produced by fast mixing model (fast stirring and fast mixing) have more homogenous microstructure. The Yb:Sr 5 (PO 4) 3 F ceramic with a transmittance of 45% at 1500 nm was fabricated by hot-pressing the synthesized nanopowders. SEM results showed that residual pores and unsintered second phase are the main scattering sources of the as-prepared Yb:S-FAP ceramics. [ABSTRACT FROM AUTHOR]

3 at.% Er3+, x at.% Ho3+: SrF 2 (x = 0, 0.05, 0.1, 0.5, 1, 2) transparent ceramics, as the potential material for the 2.7 μm solid-state laser, were fabricated by hot-pressed sintering. XRD, TEM, SEM, and EDS measurements were used to investigate the phase composition, morphology, microstructure, and distribution of the elements of the nanoparticles and transparent ceramics. Results showed that the Er3+ ions and Ho3+ ions do not alter the SrF 2 crystal structure, and they are distributed uniformly in the sample. With the increase of the Ho3+ doping concentration, the lattice parameter decreased from 5.799 Å to 5.784 Å, and the average grain size decreased gradually. The maximum transmittance of as-obtained ceramics is approximately 93 % which is close to the theoretical transmittance of SrF 2. Moreover, the absorption spectra, emission spectra, and the lifetime of Er3+ and Ho3+ were investigated. The energy transfer processes between Er3+ and Ho3+ were discussed. After co-doping Ho3+, the lifetime difference between Er3+:4I 11/2 and Er3+:4I 13/2 levels was shortened from 8.50 ms to 1.12 ms. All the results show that the incorporation of Ho3+ with proper doping concentration is beneficial for achieving 2.7 μm laser output in Er3+: SrF 2 transparent ceramics. [ABSTRACT FROM AUTHOR]

x at. % Er3+, 3 at. % Dy3+: CaF 2 transparent ceramics (x=1-5) with good transparency were fabricated by hot-pressed sintering. The phase composition of nanoparticles and transparent ceramics, microstructure, in-line transmittance, upconversion spectra and lifetime of transparent ceramics, as well as energy transfer mechanism between Er3+ and Dy3+ were investigated. The mean grain sizes of nanoparticles decreased from 33.0 nm to 26.2 nm with the Er3+ doping concentration increasing from 1 to 5 at.%. The microstructure of ceramic samples presented nearly dense microstructure and EDS analysis indicated Er3+ and Dy3+ were uniformly incorporated into CaF 2 lattice. Under 900 nm excitation, the emission intensity for 4F 9/2 →6H 15/2 transition of Dy3+ decreased and for 4S 3/2 →4I 15/2 transition of Er3+ increased, the lifetime for the 4F 9/2 level of Dy3+ decreased while the 4F 7/2 level of Er3+ increased with the raise of Er3+ doping concentration. The energy transfer mechanism was proved to be the dipole-dipole interaction. The upconversion luminescence color was tuned from orange through yellow to green by changing the Er3+/Dy3+ ratio. In addition, the Vickers hardness, fracture toughness, and the thermal conductivity of Er3+, Dy3+: CaF 2 transparent ceramics were discussed. All the results showed the Dy3+ could be used as a sensitizer for Er3+: CaF 2 transparent ceramic in the upconversion field. [ABSTRACT FROM AUTHOR]