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Lanthanide-ion-doped red upconversion (UC) fluorescent materials with excellent signal-to-noise ratio, stability, and red-green luminous intensity ratio have proven of tremendous research importance for biomedical, three-dimensional (3D) displays, sensors, and other sectors. In this study, xYb, 1Er: Sr 5 (PO 4) 3 F (S-FAP) (x=0–5 at%) transparent ceramics with outstanding optical quality were fabricated by hot pressing the co-precipitation powder with high sintering activity. The energy dispersive X-ray (EDX) results showed that the trivalent lanthanide ions were evenly doped into the S-FAP ceramic lattice. Increased Yb3+ doping considerably restricted the growth of ceramic grain size, which should be attributable to a decrease in sintering activity under high Yb doping conditions. The co-doping of Yb3+ successfully resulted in a significant improvement in up/down conversion efficiency via effective energy transfer mechanisms and the optimization of the electron population at the energy level of red-green emission. Furthermore, single-band red upconversion luminescence with the greatest red-green upconversion luminescence integral intensity ratio of 125.74 was discovered under 980 nm laser pumping at room temperature. The energy transition process involved in the phenomenon of the up-conversion luminescence intensity shift of red-green emission has been thoroughly examined. Finally, the noncontact optical thermometric ability based on fluorescence intensity ratio (FIR) technology showed that the maximum absolute sensitivity (Sa) of the 5Yb1Er sample in the temperature range of 320–556 K is 1.1×10−3 K−1, implying the possible use of Yb/Er-doped S-FAP transparent ceramics in the field of optical thermometers. [ABSTRACT FROM AUTHOR]

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]

Given the downsizing and high integration of laser diodes in recent decades, Yb: Sr 5 (PO 4) 3 F (S-FAP) crystal materials are predicted to be employed as an all-solid laser medium to produce more energy for inertial nuclear fusion. The effects of various sintering conditions on the microstructure evolution and properties of 2 at. % Yb: S-FAP transparent ceramics by hot pressing sintering were systematically investigated in this work, which including sintering temperature, sintering pressure, heating rate, and holding time. When other sintering variables are held constant, the results reveal that a sintering temperature of up to 1050 °C results in visible grain development, substantial optical scattering, and even a significant carburizing impact at 1100 °C. Excessive sintering dwell time will result in grain development, and a high sintering holding time will result in the emergence of intergranular pores, lowering the optical quality of ceramics. The optimal heating rate range for as-synthesized ceramic is found to be 5–15 °C/min. Moreover, the grain development process of Yb: S-FAP transparent ceramics is determined to be grain boundary diffusion, with an activation energy of 239.4 kJ/mol during hot pressing sintering. Meanwhile, the as-prepared sample has excellent mechanical properties such as Vickers hardness of 7.47 ± 0.045 GPa, which are highly competitive when compared to other Yb-doped optical ceramic media such as Y 2 O 3 , YAG, Lu 2 O 3 , and so on. Finally, the transmittance was interpreted based on the microstructures of the as-synthesized ceramic samples and the Rayleigh-Gans-Debye scattering theory. [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]

The 2 at.% Yb3+:Sr5(PO4)3F (S‐FAP) polycrystalline transparent ceramic with asymmetric hexagonal structures has been synthesized by vacuum hot‐pressing the nanoparticles prepared via coprecipitation method. X‐ray diffraction results of powder and ceramic indicate that their phase peaks are well matched to the crystal structure of S‐FAP. The average particle size of 35.5 nm has been exhibited by powder scanning electron microscopy images, and subsequent images of the ceramic cross section and surface morphology revealed a homogenous and compact microstructure with an average grain size of around 220 nm. The relationship between the optical loss caused by the scattering of anisotropic ceramic grains and the optical transmittance of ceramics was revealed in the hexagonal S‐FAP transparent ceramics with different thicknesses. The in‐line transmittance of hot‐pressed ceramics with 1.5‐mm thickness achieved 79.95% at 1100‐nm wavelength, and the room‐temperature absorption and emission spectra of Yb3+ in S‐FAP polycrystalline ceramic matrix were measured using a spectrofluorometer. [ABSTRACT FROM AUTHOR]

Barium fluoride (BaF2) crystals are attracting much attention as efficient inorganic scintillator promising for high‐energy physics, industrial inspection, and other fields because of the fast component of the decay time (0.6 ns) and high radiation resistance. However, two major drawbacks limit its practical application: (i) a slow decay time of ~600 ns is derived from self‐trapping excitons; (ii) the absolute light yield from the fast luminescence component is not competitive. The introducing of rare earth ions and preparation of BaF2 polycrystalline ceramics is considered to be effective measures to solve these bottlenecks. Pr3+ is extremely suitable as the activated ion of scintillation materials, which possess emission peaks located in visible band and the faster 5d–4f transition. In this work, highly sinterable Pr3+:BaF2 precursor powder was synthesized via the coprecipitation method by adjusting the reactant concentration and temperature. The morphology and microstructure of as‐synthesized powders were characterized using scanning electron microscopy (SEM) and transmittance electron microscopy analysis. The 5 at.% Pr3+:BaF2 transparent ceramic with a transmittance of 50.7% at the wavelength of 500 nm was fabricated by hot pressing the as‐prepared powders at 900°C for 4 h under the axial pressure of 50 MPa. The SEM images of ceramic cross‐section show that the residual pore is the main light scattering source. The absorption and emission spectrum of ceramic samples were discussed. [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]

• The xPr3+:BaF 2 (x = 0.001–0.05) transparent ceramics with highly optical quality were prepared by hot-pressed sintering for the first time. • The microstructural and luminescence characteristics of as-synthesized Pr:BaF 2 transparent ceramics were investigated. As one of the quickest inorganic scintillators, BaF 2 crystal is projected to be used in medical treatment, high energy physics, geological research, and other sectors. A series of Pr:BaF 2 polycrystalline transparent ceramics with nominal Pr3+ concentrations (0.1–5 at%) were synthesized via HP sintering in this research. XRD, FE-SEM, and TEM measurements were used to analyze the phase composition and microstructure of BaF 2 precursor powders with varying Pr-doping levels, and the results demonstrated that as concentration increases, the size of the nanopowders decreases. Furthermore, the morphology shifted from uniform square to uneven sheet. Cross-sectional SEM pictures of all samples revealed a clean grain boundary with no remaining pores or contaminants. The absorption spectra, emission spectra, and fluorescence lifetime of a group of Pr:BaF 2 transparent ceramics with excellent optical transmittance were measured and discussed after hot-pressing the precursor powders synthesized via co-precipitation method at 900 °C under an axial pressure of 50 MPa for 4 h. [ABSTRACT FROM AUTHOR]

In this study, 3 at.% Yb3+: CaxSr1−xF2 nanopowders were synthesized via the chemical co-precipitation method. Highly transparent 3 at.% Yb3+: CaxSr1−xF2 ceramics with various CaF2 concentrations were fabricated by hot-pressed sintering. The 3 at.% Yb3+: CaxSr1−xF2 nanopowders exhibited a spherical shape with slight agglomeration, and their particle size ranged from 26 nm to 36 nm. With an increase of the CaF2 concentration, the peak shape changed significantly and the width of the emission band increased inhomogeneously. The minimal fluorescence lifetime at the wavelength of 1011 nm of 3 at.% Yb3+: CaxSr1−xF2 transparent ceramics with various CaF2 concentrations was higher than 3.25 ms, which was longer than that of the 3 at.% Yb3+: CaF2 (2.6 ms) and the 3 at.% Yb3+: SrF2 (3.22 ms) reported in previous literature. The results indicate that incorporating Ca2+ ions into the SrF2 is an effective method to modulate the optical properties of transparent ceramics. [ABSTRACT FROM AUTHOR]

0.5–5 % (atom percent) Nd3+-doped SrF 2 transparent ceramics were fabricated by hot-pressed sintering and the effects of Nd3+ doping content on microstructure and spectral properties were studied. With Nd3+ content increased from 0.5 % to 5 %, in-line transmittance of Nd:SrF 2 ceramic at 1060 nm increased from 15.8 % to 85.3 %. FE-SEM micrographs showed the grain growth was inhibited and the microstructure became more compact and homogenous. Emission spectra revealed that concentration quenching was occurred at Nd3+ content of 1–2 %. Emission lifetime of 4F 3/2 energy level monotonously decreased from 1805.4 μs to 19.6 μs. Evolution profiles of emission intensity and lifetime of Nd:SrF 2 transparent ceramic with Nd3+ doping content were proposed. The appropriate Nd3+ content for spectral properties of Nd:SrF 2 transparent ceramic was found around 1 %. The results demonstrated Nd:SrF 2 transparent ceramic has large emission lifetime and mild concentration quenching effect, indicating that Nd3+-doped SrF 2 transparent ceramics could be more promising gain media for all-solid-state lasers than other Nd3+-doped alkaline-earth fluorides. • Optical quality and microstructure of Nd:SrF 2 transparent ceramic varied largely with Nd3+ content. • Concentration quenching occurred at Nd3+ content of 1–2 %. • Lifetime was more than 1 ms when Nd3+ content was below 1 %, it reached 1805.4 μs at 0.5 % Nd3+ doping. • Evolution profiles of emission intensity and lifetime against Nd3+ content was proposed. • In respect to spectral properties the optimal Nd3+ content was around 1 %. [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]

SrF 2 transparent ceramic is a promising upconversion material due to the low phonon energy. The effect of different sintering temperatures on Er:SrF 2 transparent ceramics was investigated. The suitable sintering temperature for Er:SrF 2 transparent ceramics was 900 °C by hot-pressed sintering in this study. High quality of Er:SrF 2 transparent ceramics with different doping concentrations were obtained. The upconversion luminescence spectra and decay behavior were compared between Er:SrF 2 and Er:CaF 2 transparent ceramics with different Er3+ doping concentration. The green emission of 5 at.% Er:SrF 2 ceramic was much stronger than that of 5 at.% Er:CaF 2 ceramic, while the red emission of Er:SrF 2 ceramic was almost the same as that of Er:CaF 2 ceramic. The upconversion luminescence lifetime of Er:SrF 2 transparent ceramics was longer than that of Er:CaF 2.All the results indicated Er:SrF 2 transparent ceramics was a candidate for green fluorescent upconversion materials. [ABSTRACT FROM AUTHOR]

High quality of x at.% Tb:CaF 2 (x = 1, 3, 5, 7, 9) transparent ceramics prepared by hot-pressed sintering achieved an average transmittance at 1500 nm of 88%. The effect of Tb3+ concentrations on the phase composition, microstructure, emission spectra, and fluorescence decay dynamics of Tb:CaF 2 transparent ceramics were investigated. XRD and FESEM results demonstrated that the incorporation of Tb3+ did not cause significant changes in the crystal structure of CaF 2 , and the grain size gradually decreased with increasing Tb3+ doping concentration. Under the excitation of 350 nm, the strongest emission peak was located at 542 nm, which was attributed to the 5D 4 →7F 5 transition of Tb3+. The green emission intensity enhanced linearly with increasing Tb3+ concentration from 1 at.% to 9 at.%. The fluorescence lifetime of 5D 4 level was also investigated, and the values were comparable to the corresponding single crystals. All the results suggest that Tb:CaF 2 transparent ceramic is a kind of potential green luminescence material. • Tb:CaF 2 transparent ceramics were fabricated by hot-pressed sintering. • The in-line transmittance of 7 at.% Tb:CaF 2 ceramic is 94% at 1500 nm. • Luminescence intensity of Tb:CaF 2 ceramics increased with increasing Tb3+ content. • The fluorescence lifetimes of Tb:CaF 2 ceramics is equivalent to the corresponding single crystals. [ABSTRACT FROM AUTHOR]

• Eu3+:Sr 5 (PO 4) 3 F transparent ceramics were fabricated by hot-pressed sintering. • The in-line transmittance of 3 at.% Eu3+:Sr 5 (PO 4) 3 F transparent ceramics exceeds 80% at 2300 nm. • Eu3+:Sr 5 (PO 4) 3 F nanoparticles microscopic morphology changes from short rod-like to nearly spherical. • Luminescence intensity of Eu3+:Sr 5 (PO 4) 3 F nanoparticles increases with increasing Eu3+ concentrations. The Eu3+:Sr 5 (PO 4) 3 F (Eu3+:S-FAP) nanoparticles were prepared via the co-precipitation method. The XRD and FT-IR results demonstrated different Eu3+ concentrations doped S-FAP nanoparticles were pure phase. The TEM results demonstrated the microscopic morphology of Eu3+:S-FAP changed from short rod-like to nearly spherical when more Eu3+ were introduced. Under 465 nm excitation, the strong red and orange light emissions (579nm, 594 nm, 616 nm, 654 nm, and 712 nm) were presented in the emission spectra, which were corresponding to the transitions of the 5D 0 →7F 0 , 7F 1 , 7F 2 , 7F 3 and 7F 4 , respectively. When more Eu3+ were introduced, the emission intensity of Eu3+:S-FAP nanoparticles was greatly enhanced and the fluorescence lifetimes of Eu3+:S-FAP nanoparticles gradually reduced. The Eu3+:S-FAP transparent ceramics were fabricated by vacuum hot pressing sintering method and its transmittance exceeded 80 % in the near-infrared band. These results above indicate Eu3+:S-FAP is a kind of potentially visible luminescent material. [Display omitted] [ABSTRACT FROM AUTHOR]