Crystallization kinetics and enhanced Bi NIR luminescence of transparent silicate glass‐ceramics containing Sr2YbF7 nanocrystals

Bismuth‐doped glasses and crystals have been widely investigated due to their intriguing potential applications in superbroadband fiber amplifier and lasers in new NIR spectral range. However, few reports have been devoted so far to bismuth‐doped transparent glass‐ceramics. Here, this work reports on bismuth‐doped silicate glasses and glass‐ceramics, which were prepared by melt‐quenching and consequent annealing processes, respectively. On the basis of the analyses on crystallization kinetics, nucleation and growth rate of crystalline phase can be modulated and Sr₂YbF₇ nanophase can, therefore, be precipitated uniformly inside the glass matrix in a controlled way to maintain proper transparence especially in optical telecommunication windows. Once the nanophase comes into being, enhanced bismuth NIR luminescence can be observed by more than 40 times upon excitation of 470 nm. Similar enhancement can appear upon different excitation schemes and the mechanism is discussed accordingly. Such Bi doped transparent glass‐ceramics with improved luminescence efficiency might find application in fiber lasers for future optical fiber communication.     

Crystallization,mechanical, and optical properties of transparent,nanocrystalline gahnite glass‐ceramics

This paper describes the preparation of a transparent glass‐ceramic from the SiO₂‐K₂O‐ZnO‐Al₂O₃‐TiO₂ system containing a single crystalline phase, gahnite (ZnAl₂O₄). TiO₂ was used as a nucleating agent for the heat‐induced precipitation of gahnite crystals of 5‐10 nm. The evolution of the ZnAl₂O₄ spinel structure through the gradual formation of Al‐O bonds was examined by infrared spectroscopy. The dark brown color of the transparent precursor glass and glass‐ceramic was eliminated using CeO₂. The increase in transparency of the CeO₂‐doped glass and glass‐ceramics was demonstrated by UV‐visible absorption spectroscopy. EPR measurements confirmed the presence of Ce³⁺ ions, indicating that CeO₂ was effective in eliminating the brown color introduced by Ti³⁺ ions via oxidation to Ti⁺⁴. The hardness of the glass‐ceramic was 30% higher than that of the as‐prepared glasses. This work offers key guidelines to produce hard, transparent glass‐ceramics which may be potential candidates for a variety of technological applications, such as armor and display panels.     

Controllable synthesis of glass ceramics containing YF3:Eu3+ nanocrystals: Well-preserved Eu and prolonged lifetime

Monolithic luminescent glass-ceramic (GC) embedded with fluoride-based nanocrystals (NCs) has drawn much attention as it is stable and possesses long fluorescent lifetime, while only small partial of optically active ions could enter the fluoride NCs prepared by conventional crystallization process. In this work, YF₃:Eu³⁺ embedded GCs have been controllably synthesized by Spark Plasma Sintering at a relatively low sintering temperature (960°C) within 10 minutes. The GC samples show typical sharp reddish-orange emissions peaking at λ = 590 nm and 620 nm, which can be ascribed to the ⁵D₀ → ⁷F_j transitions of Eu³⁺ located in the tetrahedral coordination sites of the YF₃ NCs. Significantly, a small R/O ratio (photoluminescence intensity ratio of ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁) suggests that majority of Eu³⁺ ions are well preserved in YF₃ NCs, which is confirmed by the EDS and TEM results that highly crystallized YF₃:Eu³⁺ NCs are homogeneously dispersed into the silica glass matrix without interfacial reaction. Hence, the lifetime of GC sample is prolonged to 6.8 ms These results demonstrate that Eu³⁺ could be well protected and resided in YF₃ low phonon crystal by this method to fabricate GC composites with high optical performance.     

Investigation of optical properties: Eu with Al codoping in aluminum silicate glasses and glass‐ceramics

Eu‐doped transparent oxyfluoride aluminosilicate glass was prepared by controlling with Al codoping of melt‐quenched glass fabricated under air atmosphere. In the presence of Al input, the photoluminescence emission spectra under 393 nm excitation shows a blue shift by adjusting the ratio of Eu³⁺ and Eu²⁺. After heat treatment of glass, the ratio of Eu³⁺ and Eu²⁺ of luminescence emission were changed by controlling treatment temperature. The PL intensity of Eu³⁺ and Eu²⁺ ions in the glass‐ceramics (GC) was much stronger than in the precursor glass (PG). The possible mechanism responsible for color tuneability of the ratio of Eu³⁺ and Eu²⁺ doped was discussed.     

Structural and emission properties of Eu3+‐doped alkaline earth zinc‐phosphate glasses for white LED applications

Quaternary alkaline earth zinc‐phosphate glasses in molar composition (40 ? x)ZnO – 35P₂O₅ – 20RO – 5TiO₂ – xEu₂O₃ (where x=1 and R=Mg, Ca, Sr, and Ba) were prepared by melt quenching technique. These glasses were studied with respect to their thermal, structural, and photoluminescent properties. The maximum value of the glass transition temperature (T_g) was observed for BaO network modifier mixed glass and minimum was observed for MgO network modifier glass. All the glasses were found to be amorphous in nature. The FT‐IR suggested the glasses to be in pyrophosphate structure, which matches with the theoretical estimation of O/P atomic ratio and the maximum depolymerization was observed for glass mixed with BaO network modifier. The intense emission peak was observed at 613 nm (⁵D₀→⁷F₂) under excitation of 392 nm, which matches well with excitation of commercial n‐UV LED chips. The highest emission intensity and quantum efficiency was observed for the glass mixed with BaO network modifier. Based on these results, another set of glass samples was prepared with molar composition (40 ? x)ZnO – 35P₂O₅ – 20BaO – 5TiO₂ – xEu₂O₃ (x=3, 5, 7, and 9) to investigate the optimized emission intensity in these glasses. The glasses exhibited crystalline features along with amorphous nature and a drastic variation in asymmetric ratio at higher concentration (7 and 9 mol%) of Eu₂O₃. The color of emission also shifted from red to reddish orange with increase in the concentration of Eu₂O₃. These glasses are potential candidates to use as a red photoluminsecent component in the field of solid‐state lighting devices.     

Transmission electron microscopic and optical spectroscopic studies of Ni2+/Yb3+/Er3+/Tm3+ doped dual‐phase glass‐ceramics

Ni²⁺/Yb³⁺/Er³⁺/Tm³⁺ codoped transparent glass‐ceramics (GCs) containing both hexagonal β‐YF₃ and spinel‐like γ‐Ga₂O₃ dual‐phase nanoparticles (NCs) are synthesized by melt‐quenching and subsequent heating procedures. Two techniques of transmission electron microscopy (TEM) nanoanalytics and optical spectroscopy are conjugated to understand the distribution of the rare‐earth ions (REs) and transition metals (TMs) in the nanostructured GCs. It is found that the REs are located predominantly in β‐YF₃, whereas the TMs in γ‐Ga₂O₃ NCs. As a result, energy transfer (ET) between the REs and TMs is considerably suppressed due to the large spatial separation (> 3 nm), but it is enhanced between the REs partitioned in the β‐YF₃ NCs. This has important implications for intended and demanding photoluminescence functions. For example, an ultrabroadband near‐infrared (NIR) emission in the wavelength region of 1000‐2000 nm covering the entire telecommunications window is observed for the first time. Meanwhile, intense upconversion (UC) emissions covering the 3 primary colors and locating in the first biological window can be also recorded under excitation by a single pump source at 980 nm.     

Synthesis of Eu3+‐doped BaBi2Ta2O9 based glass‐ceramic nanocomposites: Optical and dielectric properties

In this paper we report for the first time synthesis of Eu³⁺‐doped transparent glass‐ceramics (TGC) with BaBi₂Ta₂O₉ (BBT) as the major crystal phase using the glass system SiO₂–K₂O–BaO–Bi₂O₃–Ta₂O₅ by melt quenching technique followed by controlled crystallization through ceramming heat treatment. DSC studies were conducted in order to determine a novel heat‐treatment protocol to attain transparent GCs by controlling crystal growth. The structural properties of the BBT GCs have been investigated using XRD, FE‐SEM, TEM and FTIR reflectance spectroscopy. Optical band gap energies of the glass‐ceramic samples were found to decrease with respect to the precursor glass. An increased intensity of emission along with increase in the average lifetime of Eu³⁺ was observed due to incorporation of Eu³⁺ ions into the low‐phonon energy BBT crystal site. The local field asymmetric ratios of all the samples were observed greater than unity. The dielectric constant (ε_r), dielectric loss, and dissipation factor values of both the base glass and ceramized samples were found to decrease with increase in frequency.     

Second harmonic generation in germanotellurite bulk glass‐ceramics

In this study, we have investigated the use of silver cation as nucleating agent in germanotellurite glass matrix of compositions (100?x) [70TeO₂–10GeO₂–10Nb₂O₅–10K₂O]–xAg₂O (x=0‐6 mol%), in order to promote bulk crystallization. Density measurements, differential scanning calorimetry, X‐ray diffraction, UV‐Vis, and Raman spectroscopies have been performed to study the crystallization process. We have observed bulk crystallization of a unique noncentrosymmetric phase, K[Nb_1/3Te_2/3]₂O_4.8, which has been investigated for its second‐order optical activity. Transparent to translucent glass‐ceramics have been successfully tailored under thermal treatment and second harmonic generation signals were recorded on the glass‐ceramic samples as a function of their synthesis procedure. It is suggested that the second‐order optical properties observed are strongly related to the organization of crystallites within phase‐separated domains.     

Cordierite glass‐ceramic used for transparent tile glazes

This study aimed at producing a high transparent and colorless cordierite glass‐ceramic that is used for a tile glaze with a single firing. Crystallization path of glaze, microstructure development, abrasion performance, microhardness, and the transparency of cordierite glass‐ceramic were tested. The results suggested that the high transparency was significantly affected by the number of internal air holes and the crystal scattering loss. The maximum crystalline content of cordierite was nearly 50 wt% at 1200°C. Due to the high crystallization content of cordierite and the bulk crystallization mechanism, the cordierite crystalline glaze has a higher microhardness and abrasion resistance than noncrystalline glaze.     

Insulating characteristics of zinc niobium borate glass‐ceramics

Zinc borate glasses with different concentrations of Nb₂O₅ were prepared and later were heat treated for prolonged times. Prepared samples were characterized by XRD, SEM, DSC, IR and optical transmission spectroscopy techniques. Later, dielectric properties viz., dielectric constant, loss tangent, electric modulii, electrical impedance and a.c. conductivity over wide ranges of frequency and temperature, were investigated as a function of Nb₂O₅ concentration. Finally, the dielectric breakdown strength was measured in air medium at ambient temperature. The results of characterization techniques viz., XRD, SEM and DSC indicated that multiple crystal grains (with sizes varying from 0.1 to 1 μm) are dispersed in the residual glass phase. The concentration of crystal grains found to increase with increase in Nb₂O₅ content. The XRD studies have further revealed that the bulk samples are composed of columbite ZnNb₂O₆ crystal phases. Using generalized gradient approximation (GGA) quantitative information on these crystal phases viz., the lattice parameters, optical band gap and band structure were evaluated. The analysis of results of IR spectral studies have indicated that there is an increasing degree of polymerization of glass network with increase in Nb₂O₅ content due to the increased connectivity between various structural groups in the glass network. The optical absorption spectra indicated an increase in optical transmittance of the bulk samples with increase in Nb₂O₅ content. The dielectric parameters are observed to decrease, whereas the dielectric breakdown strength is observed to increase to a large extent due to the crystallization of the glass with the Nb₂O₅. The increase is attributed to the formation of ZnNb₂O₆ crystalline phases that contain intertwined ZnO₆ and NbO₆ structural units. As a whole, zinc borate glasses exhibited a significant increase in the electrical insulating strength due to the crystallization with Nb₂O₅ as the crystallizing agent. Further, the value of dielectric constant is also found to be the optimal with no dispersion with frequency up to 450 K. Overall, the studied glass‐ceramics meet the necessary physical conditions to be used as insulating layers in the display panels and hence may be considered for such applications.     

In situ crystallization and elastic properties of transparent MgO–Al2O3–SiO2 glass‐ceramic

Glass‐ceramics (GC) generally possess enhanced mechanical properties compared to their parent glasses. The knowledge of how crystallization evolves and affects the mechanical properties with increasing temperature is essential to optimize the design of the crystallization cycle. In this study, we crystallized a glass of the MgO–Al₂O₃–SiO₂ system with nucleating agents TiO₂ and ZrO₂. The crystallization cycle comprised a 48 hour nucleation treatment at the glass‐transition temperature followed by a 10 hour growth step at a higher temperature. During this cycle, the evolution of crystalline phases was followed by high‐temperature X‐ray diffraction (HTXRD), which revealed the presence of karooite (MgO·2TiO₂), spinel (MgO·Al₂O₃), rutile (TiO₂), sillimanite (Al₂O₃·SiO₂), and sapphirine (4MgO·5Al₂O₃·2SiO₂). The same heat treatment was applied for in situ measurement of elastic properties: elastic modulus, E, shear modulus, G, and Poisson's ratio, ν. The evolution of these parameters during the heating path from room temperature to the final crystallization temperature and during the nucleation and the crystallization plateaus is discussed. E and G evolve significantly in the first two hours of the growth step. At the end of the crystallization process, the elastic and shear moduli of the GC were approximately 20% larger than those of the parent glass.     

Novel Er3+/Ho3+‐codoped glass‐ceramic fibers for broadband tunable mid‐infrared fiber lasers

It is well recognized that a widely wavelength‐tunable mid‐infrared (MIR) fiber laser plays an important role in the development of compact and efficient coherent sources in the MIR range. Herein, the optimizing Er/Ho ratio for enhancement of broadband tunable MIR emission covering 2.6‐2.95 μm in the Er³⁺/Ho³⁺‐codoped transparent borosilicate glass‐ceramic (GC) fibers containing NaYF₄ nanocrystals under 980 nm excitation was investigated. Specifically, the obtained GC fibers with controllable crystallization and well fsd‐maintained structures were prepared by the novel melt‐in‐tube approach. Owing to the effective energy transfer between Er³⁺ and Ho³⁺ after crystallization, the 2.7 μm MIR emission was obviously enhanced and the emission region showed a notable extension from 2.6‐2.82 μm to 2.6‐2.95 μm after the addition of Ho³⁺. Importantly, we conducted a theoretical simulation and calculation related to the MIR laser performance, signifying that the GC fiber may be a promising candidate for MIR fiber laser. Furthermore, the melt‐in‐tube approach will provide a versatile strategy for the preparation of diverse optical functional GC fibers.     

Rare‐earth‐doped SiO2‐CaF2 glass ceramic nano‐particle with upconversion properties

Rare‐earth‐doped upconversion nano‐phosphor shows new possibilities in the field of bioimaging because of its unique properties like higher penetration depth, low signal to noise ratio (SNR), good photo stability, and zero auto fluorescence. The oxyfluoride glass system is the combination of both fluoride and oxide where fluoride host offers high optical transparency due to low phonon energy and oxide network offers high physical stability. Thus, in the present work, an attempt has been made to synthesize 1 mol% Er³⁺ doped SiO₂‐CaF₂ glass ceramic nano‐particles through sol‐gel route. The synthesized glass ceramic particles were heat treated at 4 different temperatures starting from 600°C to 900°C.The X‐ray diffraction (XRD) analysis and Transmission electron microscopy (TEM) analysis confirmed the formation of CaF₂ nano‐crystals in the matrix which is 20‐30 nm in size. The vibrational spectroscopic analysis of the glass ceramics sample has been investigated by Fourier transform infrared (FTIR) spectroscopy. The UV‐Visible‐NIR spectroscopy analysis was carried out to analyze the absorption intensity in the near infrared region. Upon 980 nm excitation, the sample shows red emission corresponds to ⁴F_9/2→⁴I_15/2 energy level transition. The prepared nano‐particles showed excellent biocompatibility when tasted on MG‐63 osteoblast cells.     

Development of ZrO2‐MgO nanocomposite powders by the modified sol‐gel method

The ZrO₂‐MgO nanocomposites were synthesized using a new sol‐gel method with sucrose and tartaric acid as a gel agent. The samples were characterized by thermal analysis (TG/DTA), X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), energy‐dispersive X‐ray mapping (EDX mapping), and Ultraviolet‐visible spectroscopy (UV‐vis). The results showed that the cubic phase of ZrO₂‐MgO was formed in the presence of both gel agents. The average particle size of the samples synthesized with sucrose was lower (30 nm) than that of tartaric acid (50 nm). Finally, the formation mechanism and the optical properties of zirconia‐magnesia have been discussed.     

Photoluminescence and optical properties of Eu3+/Eu2+-doped transparent Al2O3 ceramics

Transparent Eu³⁺-doped (0.05–0.15 at. %) alumina ceramics with fine-grained microstructure were prepared and studied in terms of optical properties and photoluminescence (PL). The light transmission through ceramics up to dopant concentrations 0.125 at. % is dominated by birefringence scattering at grain boundaries. As confirmed by HRTEM/EDS element mapping, high photoluminescence intensity was achieved as the result of the dopant segregation at grain boundaries. The PL emission spectra of Al₂O₃:Eu³⁺ ceramics exhibited red light emissions with the highest intensity (394 nm excitation) for material containing 0.125 at. % of Eu³⁺. The luminescence decay was single-exponential with a lifetime ～1.5 ms. The post-sintering reduction of Eu³⁺→Eu²⁺ under an H₂ atmosphere (at 1300 °C) was difficult. Two simultaneously coexisting Eu²⁺ emitting PL centers were identified, one emitting blue light with average decay constant of 150 ns, and the other green light (more intense) with average decay constant of 1.3 μs.     

Processing and characteristics of transparent Gd3TaO7 polycrystalline ceramics

Transparent polycrystalline Gd₃TaO₇ ceramics were successfully developed. A sol‐gel process was used to synthesize Gd₃TaO₇ powder with a uniform composition and an estimated average particle size of 100 nm. Simultaneous thermal gravimetric analysis and differential thermal analysis (TGA/DTA) was used to identify the decomposition sequence as a function of temperature for the as‐synthesized sol‐gel powders. Crystallization was confirmed by X‐ray diffraction (XRD) and a single phase was achieved by calcining at 1000°C. The calcined powders were hot‐pressed at 1400°C to achieve >96% theoretical density with closed pore structure followed by a hot isostatic pressing at 1400°C at 207 MPa to achieve a fully dense structure. Microstructural characterization shows a uniform grain size distribution with an average grain size of about 7 μm. In‐line transmission measurements revealed high transparency in the red and infrared. Thermal conductivity was measured to be >1.6 W/mK at room temperature, decreasing to ~1.3 W/mK by 500°C. Dielectric properties remain stable with relative permittivity values just above 200 and loss tangents <0.005 up to 350°C.     

Up-conversion luminescence and optical thermometry properties of transparent glass ceramics containing CaF2:Yb3+/Er3+ nanocrystals

A series of Yb³⁺/Er³⁺ codoped transparent oxyfluoride glass ceramics with various amounts of Yb³⁺ have been successfully fabricated and characterized. Under 980 nm laser prompting, the samples produce intense red, green and blue up-conversion emissions, and the emission intensities increase with Yb³⁺ concentration and heat treatment temperature. Before losing good transparency in the visible region, optimum emission intensities are obtained for the sample with 25 mol% of Yb³⁺ at a heat treatment temperature of 680 °C. A possible up-conversion mechanism is proposed from the dependence of emission intensities on pumping power. The fluorescence intensity ratio between the two thermally coupled levels ²H_11/2 versus ⁴S_3/2 was measured with the laser output power of 57 mW to avoid the possible laser induced heating effect. The fluorescence intensity ratio values in the temperature range from 295 K to 723 K can be well fitted with the equation: A exp (−∆E/k_BT), where A = 6.79 and ∆E=876 cm⁻¹. The relative temperature sensitivity at 300 K was evaluated to be 1.4% K⁻¹. All the results suggest that the Yb³⁺/Er³⁺ codoped CaF₂ glass ceramics is an efficient up-conversion material with potential in optical fiber temperature sensing.     

Reproducible X-ray excited luminescence performance with transparent Tb3+-doped NaGd2F7 scintillating glass ceramics

Oxyfluoride glass ceramics with low toxicity, high stability and strong X-ray excited luminescence intensity are introduced as scintillating material. Herein, a series of novel NaGd₂F₇:xTb³⁺ (x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized through in situ growth self-crystallization. X-ray diffraction, fourier transform infrared spectrometer and transmission electron microscopy were measured to investigate the structure and morphology of NaGd₂F₇ glass ceramics. These investigations demonstrate that NaGd₂F₇ nanocrystals are uniformly distributed in glass matrix with 20–30 nm diameter. Photoluminescence and X-ray excitation spectra are greatly enhanced after further thermal treatment which verify better crystallization of NaGd₂F₇ glass ceramics. The X-ray excited luminescence of the GC2–640 sample can reach 143.8% of the commercial BGO. More importantly, the damaged material can be completely repaired by annealing at 350 °C for 30 min. Our investigation indicates that Tb-doped transparent NaGd₂F₇ glass ceramics are potential candidates of X-ray scintillating material.     

Silicon oxycarbide glasses and glass‐ceramics: “All‐Rounder” materials for advanced structural and functional applications

Silicon oxycarbides can be considered as being carbon‐containing silicates consisting of glass networks in which oxygen and carbon share bonds with silicon. The carbon‐for‐oxygen substitution in silicate glass networks has been shown to induce significant changes in the network connectivity and consequently strong improvements in the properties of the silicate glass network. For instance, SiOC glasses exhibit Young's moduli, hardness values, glass transition, and crystallization temperatures which are superior to those of vitreous silica. Moreover, the silicon oxycarbide glass network exhibits unique structural features such as reduced mass fractal dimension and nano‐heterogeneity, which significantly affect and/or dictate its properties and behavior. In the present Review, a consideration of the current state of the art concerning the synthesis, processing, and various structural and functional properties of silicon‐oxycarbide‐based glasses and glass‐ceramics is done. Thus, the synthesis of silicon oxycarbides starting from macromolecular precursors such as polysiloxanes or alkoxysilanes‐based sol‐gel systems as well as current advances related to their processing will be critically reviewed. In addition, various structural and functional properties of silicon oxycarbides are presented. Specific emphasis will be put on the intimate correlation between the molecular architecture of the precursors and the structural features and properties of the resulting silicon oxycarbides.     

Near‐zero thermal expansion transparent lithium aluminosilicate glass‐ceramic by microwave hybrid heat treatment

The material of choice for space applications which demand very high dimensional stability is lithium aluminosilicate (LAS) based Ultra Low thermal Expansion Glass‐Ceramic (ULEGC). Generally, the controlled crystallization process recommended for the processing of transparent ULEGC involves a long soaking duration to achieve the required crystal number density. This paper brings out the process optimization procedure adopted for realizing transparent and nanocrystalline ULEGC from conventionally processed LAS glass using microwave‐assisted (hybrid) crystallization. The experimental strategy involves two stages (i) identification of the optimum crystallization temperature (T_c) under a microwave field (ii) optimization of a microwave‐assisted crystallization process to achieve near zero Coefficient of Thermal Expansion (CTE).. Optimum heat‐treatment schedules for nucleation and crystallization under a microwave environment were found to be 720°C/24 hours and 775°C/0.3 hours, respectively. The optimized heat‐treatment condition revealed the efficacy of the microwave hybrid heating, by producing nanocrystalline (35‐50 nm) and transparent (>82%) ULEGC having a thermal expansion of ?0.03 × 10^?6 K (0°C to 50°C).