Luminescent properties of doped amorphous and polycrystalline Y3Al5O12-Al2O3

Transparent polycrystalline ceramics (TPCs) are crystalline materials with single-crystal-like transparency, which, however, have to rely on fabrication processes with a relatively high cost. Here, we produced lab-scale TPCs based on the typical refractory Y₂O₃-Al₂O₃ system, through full congruent crystallization of the parent glass prepared by aerodynamic levitation melting method. Doping of the glass and TPCs by rare-earth (RE) ions (Ce³⁺, Tb³⁺, Nd³⁺, and Yb³⁺) and transition-metal (TM) ions (Cr³⁺) results in strong visible and near-infrared (NIR) photoluminescence with high quantum yield. The dominance of Stark splitting of the emission band for RE and TM ions in the TPCs as compared with that of the glass confirms crystallization of the parent glasses.     

Thermal and optical properties of La2O3–Ga2O3– (Nb2O5 or Ta2O5) ternary glasses

La₂O₃–Ga₂O₃–M₂O₅ (M = Nb or Ta) ternary glasses were fabricated using an aerodynamic levitation technique, and their glass‐forming regions and thermal and optical properties were investigated. Incorporation of adequate amounts of Nb₂O₅ and Ta₂O₅ drastically improved the thermal stabilities of the glasses against crystallization. Optical transmittance measurements revealed that all the glasses were transparent over a wide wavelength range from the ultraviolet to the mid‐infrared. The refractive indices of the glasses increased and the Abbe number decreased upon substituting Ga₂O₃ with Nb₂O₅, and the decrease in the Abbe number was significantly suppressed when Ta₂O₅ was incorporated into the glass. As a result, excellent compatibility between high refractive index and lower wavelength dispersion was realized in La₂O₃–Ga₂O₃–Ta₂O₅ glasses. Analysis based on the single‐oscillator Drude–Voigt model provided more systematical information and revealed that this compatibility was due to an increase in the electron density of the glass.     

Microstructure of rapidly-quenched ZrO2-SiO2 glass-ceramics fabricated by container-less aerodynamic levitation technology

In this work, an aerodynamic levitation technology (ALT) was utilized to prepare ZrO₂-SiO₂ glass-ceramics with two different ZrO₂ contents, that is, 35 mol% and 50 mol%. The glass-ceramics were partially melted at ∼2000°C or fully melted at ∼3000°C by ALT, followed by rapid quenching to obtain spherical glass-ceramic beads. The phase compositions and microstructures of the glass-ceramics were characterized. Crystallization of ZrO₂ occurred during the solidification process and ZrO₂ content, processing temperature, and the addition of yttrium (3 mol%) affected the crystalline phase of ZrO₂. No ZrSiO₄ or crystalline SiO₂ were formed during the solidification process and the glass-ceramics were away from thermodynamic equilibrium due to rapid quenching. The glass-ceramics showed a microstructure of irregular-shaped ZrO₂ micro-aggregates embedded in an amorphous SiO₂ matrix, with lamellar twins and lattice defects formed within ZrO₂ crystals. For samples prepared at ∼3000°C, a liquid-liquid phase separation occurred in the melt, which eventually resulted in the formation of large and irregular-shaped ZrO₂ aggregates. In comparison, for samples prepared at ∼2000°C, pre-existed ZrO₂ crystals formed during heating acted as nucleation sites during the cooling process, followed by grain growth to form large ZrO₂ aggregates. Solidification and microstructure formation mechanisms were proposed to elucidate the solidification process during rapid cooling and the microstructure of the glass-ceramics obtained.     

Tunable ultra-wideband NIR emission and irradiation resistance in novel Er3+/Tm3+/Pr3+ tri-doped La2O3- Ga2O3-Ta2O5 glasses synthesized using aerodynamic levitation method

Glasses with ultra-wideband near-infrared emission and superior irradiation resistance are important for the potential applications in optical communications under harsh environments. Here, transparent 35La₂O₃-(65-x)Ga₂O₃-xTa₂O₅ (LGT) and Er³⁺/Tm³⁺/Pr³⁺ tri-doped LGT glasses are fabricated using the levitation method. LGT glasses exhibit a wide glass-formation region, low largest vibration energy, high refractive indices, and excellent mechanical properties. Additionally, Er³⁺/Tm³⁺/Pr³⁺ tri-doped LGT samples with varying Pr³⁺ contents are characterized by possessing good thermal stability (T_g>849°C), wide transparent optical window, strong radiation resistance, excellent compatibility between low wavelength dispersion (v_d>31.2), and large refractive index (n_d>2.048). By optimizing the doping content of Er³⁺, Tm³⁺, and Pr³⁺ in an appropriate ratio, the ultra-wideband near-infrared luminescence ranging from 1250 to 1640 nm (FWHM = 251 nm) has been acquired under 808 nm pumping. Furthermore, decay curves are measured to reveal the fluorescence dynamics, and then the related emission mechanism is elaborated systematically. Meanwhile, the effects of gamma irradiation doses on microstructure, transmittance spectra, and fluorescence characteristics are studied. This work may offer a valuable reference for doping optimization and new design strategy of multifunctional materials.     

Optical properties,thermal stability,and forming region of high refractive index La2O3–TiO2–Nb2O5 glasses

The high refractive index La₂O₃–TiO₂–Nb₂O₅ glasses were prepared by containerless processing, and the glass‐forming region was determined. The refractive index showed the range from 2.20 to 2.32, and the values were much higher than those of most optical glasses. The completely miscible 30LaO_3/2–(70?x)TiO₂–xNbO_5/2 (0 ≤ x ≤70) system was fabricated to study the compositional dependence of refractive index and optical transmittance. The crucial determinants of the refractive index of oxide glasses, oxygen molar volume, and electronic polarizability of oxygen ions were calculated. The principle of additivity of glass properties was suitable for the calculation of refractive index between glass and compositional oxides. All the glasses were colorless and transparent in the visible to 6.5 μm middle infrared (MIR) region. These results are useful for designing new optical glasses with high refractive index and low wavelength dispersion in wide optical window.     

Effect of topological structure on photoluminescence of PbSe quantum dot‐doped borosilicate glasses

Borosilicate glasses doped with PbSe quantum dots (QDs) were prepared by a conventional melt‐quenching process followed by heat treatment, which exhibit good thermal, chemical, and mechanical stabilities, and are amenable to fiber‐drawing. A broad near infrared (NIR) photoluminescence (PL) emission (1070‐1330 nm) band with large full‐width at half‐maximum (FWHM) values (189‐266 nm) and notable Stokes shift (100‐210 nm) was observed, which depended on the B₂O₃ concentration. The PL lifetime was about 1.42‐2.44 μs, and it showed a clear decrease with increasing the QDs size. The planar [BO₃] triangle units forming the two‐dimensional (2D) glass network structure clearly increased with increasing B₂O₃ concentration, which could accelerate the movement of Pb²⁺ and Se^2? ions and facilitate the growth of PbSe QDs. The tunable broadband NIR PL emission of the PbSe QD‐doped borosilicate glass may find potential application in ultra‐wideband fiber amplifiers.     

Effect of sintering conditions on optical and mechanical properties of MgAl2O4/Al2O3 laminated transparent composite fabricated by spark-plasma-sintering (SPS) processing

To realize a high hardness in transparent MgAl₂O₄, the MgAl₂O₄/Al₂O₃ laminated composite was fabricated by a one-step spark-plasma-sintering (SPS) method. By sintering at a temperature of 1225 °C for 10 min and at a heating rate of ≤ 10 °C/min under a pressure of 300 MPa, the MgAl₂O₄/Al₂O₃ laminated composites can attain a high hardness with maintaining the wide band transparency. The in-line and IR transmission were ～50 % at the visible wavelength of 500 nm and >77 % at the wavelength of 4 μm, respectively. The Vickers hardness measured on the surface of the Al₂O₃ layer perpendicular to the MgAl₂O₄/Al₂O₃ stacking exhibited 29 GPa, which is higher than those of the monolithic Al₂O₃ (26.6 GPa) and MgAl₂O₄ (17.2 GPa). The wide band transparency and mechanical properties can be realized by simultaneously attaining smaller grain sizes and higher densities of both the MgAl₂O₄ and Al₂O₃ phases in the laminated composite by optimizing the SPS conditions.     

Microwave dielectric and nonlinear optical studies on radio‐frequency sputtered Dy2O3‐doped KNN thin films

We report the effect of oxygen mixing percentage (OMP) on structural, microstructural, dielectric, linear, and nonlinear optical properties of Dy₂O₃‐doped (K_0.5Na_0.5)NbO₃ thin films. The (K_0.5Na_0.5)NbO₃ + 0.5 wt%Dy₂O₃ (KNN05D) ferroelectric thin films were deposited on to quartz and Pt/Ti/SiO₂/Si substrates by RF magnetron sputtering. An increase in the refractive index from 2.08 to 2.21 and a decrease in the optical bandgap from 4.30 to 4.28 eV indicate the improvement in crystallinity, which is also confirmed from Raman studies. A high relative permittivity (ε_r=281‐332) and low loss tangent (tanδ=1.2%‐1.9%) were obtained for the films deposited in 100% OMP, measured at microwave frequencies (5‐15 GHz). The leakage current of the films found to be as low as 9.90×10^?9 A/cm² at 150 kV/cm and Poole‐Frenkel emission is the dominant conduction mechanism in the films. The third order nonlinear optical properties of the KNN05D films were investigated using modified single beam z‐scan method. The third order nonlinear susceptibility (?χ⁽³⁾?) values of KNN05D films increased from 0.69×10^?3 esu to 1.40×10^?3 esu with an increase in OMP. The larger and positive nonlinear refractive index n₂=7.04×10^?6 cm²/W, and nonlinear absorption coefficient β=1.70 cm/W were obtained for the 100% OMP film, indicating that KNN05D films are good candidates for the applications in nonlinear photonics and high‐frequency devices.     

Effect of B2O3 addition on structure and properties of Yb3+/Al3+/B3+-co-doped silica glasses

The homogeneous Yb³⁺/Al³⁺/B³⁺-co-doped silica glasses were prepared via a sol-gel method. The impact of B₂O₃ addition on the physical and optical properties and network structure was systematically studied. The network structure was investigated by the Fourier Transform Infrared (FT-IR), Raman spectra, and Solid State Nuclear Magnetic Resonance (SSNMR). Herein, B₂O₃ addition can continuously decrease the refractive index and density. When B₂O₃ is lower than 2 mol%, B₂O₃ addition can obviously decrease the scalar crystal field parameters, Yb³⁺ asymmetry degree, Yb³⁺ cross-sections, due to the generation of Yb–O–B bonds at the cost of partial Yb–O–Al/Si ones. When B₂O₃ is more than 2 mol%, FT-IR, Raman spectra, and SSNMR results indicate that further increased B atoms prefer to connect with Si and Al rather than Yb. Consequently, the above parameters are basically unchanged. Based on the results, an intuitive model of structure and properties evolution during the substitution of SiO₂ by B₂O₃ has been established.     

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.     

Ni/CaO‐Al2O3 bifunctional catalysts for sorption‐enhanced steam methane reforming

Ni/CaO‐Al₂O₃ bifunctional catalysts with different CaO/Al₂O₃ mass ratios were prepared by a sol–gel method and applied to the sorption‐enhanced steam methane reforming (SESMR) process. The catalysts consisted mainly of Ni, CaO and Ca₅Al₆O₁₄. The catalyst structure depended strongly on the CaO/Al₂O₃ mass ratio, which in turn affected the CO₂ capture capacity and the catalytic performance. The catalyst with a CaO/Al₂O₃ mass ratio of 6 or 8 possessed the highest surface area, the smallest Ni particle size, and the most uniform distribution of Ni, CaO, and Ca₅Al₆O₁₄. During 50 consecutive SESMR cycles at a steam/methane molar ratio of 2, the thermodynamic equilibrium was achieved using the catalyst with a CaO/Al₂O₃ mass ratio of 6, and H₂ concentration profiles for all the 50 cycles almost overlapped, indicating excellent activity and stability of the catalyst. Moreover, a high CO₂ capture capacity of 0.44 was maintained after 50 carbonation–calcination cycles, being almost equal to its initial capacity (0.45 ). © 2014 American Institute of Chemical Engineers AIChE J, 60: 3547–3556, 2014     

Tb3Al2.5 Ga2.5O12 transparent ceramic for magneto‐optical application

The optical quality and microstructure of the Tb₃Al_2.5 Ga_2.5O₁₂ transparent ceramic were investigated and compared with those of terbium gallium garnet (Tb₃Ga₅O₁₂ TGG) and terbium aluminum garnet (Tb₃Al₅O₁₂ TAG). The transmission of the Tb₃Al_2.5 Ga_2.5O₁₂ ceramic was as high as 80% in the visible and near‐infrared (IR) regions, which was close to the theoretical value. The Verdet constant of the Tb₃Al_2.5 Ga_2.5O₁₂ ceramic measured at 632.8 nm and room temperature was ?150.6 rad/Tm, which was 12% larger than that of the TGG single crystal and 13% smaller than that of the TAG single crystal, and the Tb₃Al_2.5 Ga_2.5O₁₂ ceramic showed the highest transparency among these three samples.     

Understanding the structure,thermal, and optical properties in Al2O3-incorporated La2O3-TiO2-Nb2O5 glasses

Glasses in the 30La₂O₃-40TiO₂-30Nb₂O₅ system are known to have excellent optical properties such as refractive indices over 2.25 and wide transmittance within the visible to mid-infrared (MIR) region. However, titanoniobate glasses also tend to crystallize easily, significantly limiting their applications in optical glasses due to processing challenges. Therefore, the 30La₂O₃-40TiO₂-(30−x) Nb₂O₅-xAl₂O₃ (LTNA) glass system was successfully synthesized using a aerodynamic containerless technique, which improves glass thermal stability and expands the glass-forming region. The effects of Al₂O₃ on the structure, thermal, and optical properties of base composition glasses were investigated by XRD, DSC, NMR, Raman spectroscopy, and optical measurements. DSC results indicated that as the content of Al₂O₃ increased, the thermal stability of the glasses and glass-forming ability increased, as the 30La₂O₃-40TiO₂-25Nb₂O₅-5Al₂O₃ (Nb-Al-5) glass obtained the highest ΔT value (103.5°C). Structural analysis indicates that the proportion of [AlO₄] units increases gradually and participates in the glass network structure to increase connectivity, promoting more oxygen to become bridging oxygen and form [AlO₄] tetrahedral linkages to [TiO₅] and [NbO₆] groups. The refractive index values of amorphous glasses remained above 2.1 upon Al₂O₃ substitution, and a transmittance exceeding 65% in the visible and mid-infrared range. The crystallization activation energies of 30La₂O₃-40TiO₂-30Nb₂O₅ (Nb-Al-0) and Nb-Al-5 glasses were calculated to be 611.7 and 561.4 kJ/mol, and the Avrami parameters are 5.28 and 4.96, respectively. These results are useful to design new optical glass with good thermal stability, high refractive index and low wavelength dispersion for optical applications such as lenses, endoscopes, mini size lasers, and optical couplers.     

Porosity‐dependent properties of Nb‐doped Pb(Zr,Ti)O3 ceramics

In the present work, it is shown how the controlled porosity can be exploited to obtain a compromise between a reduced permittivity down to a few hundreds and maintaining a high tunability level as in the dense material, to fulfill requirements for tunable applications. Nb‐doped Pb(Zr,Ti)O₃ ceramics with porosity in the range 5%‐30% have been prepared by direct sintering method. X‐ray diffraction analysis and Rietveld refinement indicated a co‐existence of tetragonal and monoclinic phases in the porous ceramics. Dielectric properties revealed a gradual reduction in permittivity when increasing the porosity level, while maintaining low dielectric losses below 3%. The ferroelectric switching behavior is also influenced by the porosity level: a continuous reduction in the saturation and remnant polarization is observed with increasing porosity. The nonlinear dielectric properties of all the investigated ceramics preserve a high level of tunability in comparison with one of the dense material, irrespective of the porosity level, while zero field permittivity was decreased below 1000. An optimum behavior is found for the ceramic sample with 25% porosity, which shows a high tunability, smaller losses, and moderate dielectric constant (ε ~600).     

Structure and properties of Al2O3-bonded porous fibrous YSZ ceramics fabricated by aqueous gel-casting

To meet requirements for high porosity and high strength, novel aqueous gel-casting process has been successfully developed to fabricate Al₂O₃-bonded porous fibrous YSZ ceramics with ρ-Al₂O₃ and YSZ fibers as raw materials. Microstructure, phase composition, apparent porosity, bulk density, thermal conductivity, and compressive strength of fabricated porous ceramics were investigated, and effects of fiber content on properties were discussed. According to results, bird nest 3D mesh with interlaced YSZ fibers and Al₂O₃ binder was formed, ensuring the ability to obtain high performance, lightweight ceramics. An increase in the number of YSZ fibers led to more complex interlaced arrangement of fibers and denser network structure of porous ceramics at retaining their stability. Furthermore, their apparent porosity and bulk density increased, whereas thermal conductivity and compressive strength decreased with increasing the fiber content. In particular, comparatively high porosity (71.1–72.7%), low thermal conductivity (0.209–0.503 W/mK), and relatively high compressive strength (3.45–4.24 MPa) were obtained for as-prepared porous ceramics, making them promising for applications in filters, thermal insulation materials, and separation membranes.     

Effect of volume ratio on optical and mechanical properties of Y2O3-MgO composites fabricated by spark-plasma-sintering process

The effect of Y₂O₃:MgO ratio on the microstructures, optical and mechanical properties of the Y₂O₃-MgO composites were investigated. Although the dense Y₂O₃-MgO composites were successfully fabricated in various Y₂O₃:MgO ratios using the spark-plasma-sintering (SPS) technique, the Y₂O₃:MgO ratio significantly influenced the microstructures and the optical/mechanical properties of the composites. Fine grain size was obtained in the composite with Y₂O₃:MgO = 50:50 owing to the effective pinning force caused by the homogenous two phase microstructure. The SPSed dense composites showed good transmittance in the wide wavelength range from visible to infrared (IR). The monolithic Y₂O₃ polycrystal, that is Y₂O₃:MgO = 100:0, showed the highest transmittance of 62.3 % at 600 nm and 84.3 % at 5 μm. Although the IR transmittance is independent of the Y₂O₃:MgO ratio, the visible transmittance decreased with the MgO particle dispersion. Among the Y₂O₃-MgO composites, the higher visible transmittance was obtained in the composite with Y₂O₃:MgO = 50:50 than those of the other two composites with Y₂O₃:MgO = 30:70 and 70:30 due to its smallest grain size. In contrast to the transmittance, the hardness H_v and toughness K_IC tend to increase with increasing the MgO fraction irrespective of the grain size; both H_v and K_IC increased from 9.6 GPa and 1.1 MPa m^1/2 for the monolithic Y₂O₃ to 12.7 GPa and 2.5 MPa m^1/2 for the composite with Y₂O₃/MgO = 30:70, respectively. The enhanced hardness and toughness of the composite can be interpreted dominantly by the mixture rule as a function of the volume fraction of the MgO phase.     

Magneto‐optical effects of Ge‐Ga‐Sb(In)‐S chalcogenide glasses with diamagnetic responses

The Faraday effects of Ge‐Ga‐Sb(In)‐S serial chalcogenide glasses were investigated at the wavelengths of 635, 808, 980, and 1319 nm, respectively. The compositional dependences were analyzed and associated influencing factors including the absorption edge, the concentration of Sb³⁺/In³⁺ ions, and the wavelength dispersion of refraction index were discussed. 80GeS₂·20Sb₂S₃ composition glass was found to have the largest Verdet constant (V=0.253, 0.219, 0.149, and 0.065 min·G^?1·cm^?1 for wavelengths 635, 808, 980, and 1319 nm, respectively) in these glasses, which is larger than that of commercial diamagnetic glasses (Schott, SF 6, V=0.069 min·G^?1·cm^?1@633 nm, for example). Sb³⁺ ions with high polarizability possessing s²‐sp electron jumps involving ¹S₀→¹P₁, ³P_0,1,2 transitions are responsible for large Verdet constant, and Becquerel rule is proved to be an effective guidance for estimating the Verdet constant and further optimizing the compositions in chalcogenide glasses.     

Carbon nanotube/graphene oxide‐added CaO‐B2O3‐SiO2 glass/Al2O3 composite as substrate for chip‐type supercapacitor

A CaO‐B₂O₃‐SiO₂ (CBS) glass/40 wt% Al₂O₃ composite sintered at 900°C exhibited a dense microstructure with a low porosity of 0.21%. This composite contained Al₂O₃ and anorthite phases, but pure glass sintered at 900°C has small quantities of wollastonite and diopside phases. This composite was measured to have a high bending strength of 323 MPa and thermal conductivity of 3.75 W/(mK). The thermal conductivity increased when the composite was annealed at 850°C after sintering at 900°C, because of the increase in the amount of the anorthite phase. 0.25 wt% graphene oxide and 0.75 wt% multi‐wall carbon nanotubes were added to the CBS/40 wt% Al₂O₃ composite to further enhance the thermal conductivity and bending strength. The specimen sintered at 900°C and subsequently annealed at 850°C exhibited a large bending strength of 420 MPa and thermal conductivity of 5.51 W/(mK), indicating that it would be a highly effective substrate for a chip‐type supercapacitor.     

Phase equilibria study and thermodynamic description of the BaO‐CaO‐Al2O3 system

A combined experimental investigation and thermodynamic assessment was performed for the BaO‐CaO‐Al₂O₃ system. By using a high‐temperature equilibration/quenching technique and scanning electron microscopy, electron probe microanalysis, and X‐ray powder diffraction analysis, the phase equilibria at 1500°C and phase stability of BaCa₂Al₈O₁₅ phase were determined. An extensive literature survey was conducted for the experimental and thermodynamic modeling data of the BaO‐CaO‐Al₂O₃ system. According to the literature data and the present measurements, a thermodynamic assessment was made in order to obtain a set of self‐consistent thermodynamic parameters to describe the BaO‐CaO‐Al₂O₃ system. Based on the thermodynamic parameters acquired in this work, isothermal sections at 1100°C, 1250°C, 1400°C, 1475°C, and 1500°C and the BaO·Al₂O₃‐CaO·Al₂O₃ and BaO·6Al₂O₃‐CaO·6Al₂O₃ joints were calculated and compared with the available experimental data.