Energy transfer studies of Ce:Eu system in phosphate glasses

The preparation of sodium phosphate glasses singly and doubly doped with rare earth ions Ce³⁺ and Eu³⁺ by melt quench method is described. The spectroscopic characterizations of the samples are done using absorption, excitation and emission spectra. The nonradiative energy transfer between trivalent cerium and europium is achieved through the phosphate lattice and the results are incorporated. The main reason of quenching of Ce³⁺ ions and the mechanism of energy transfer is mainly electric dipole-dipole in nature for Ce³⁺:Eu³⁺ system.     

Mechanical behavior under nanoindentation of a new Ni-based glassy alloy produced by melt-spinning and copper mold casting

An investigation was made into the thermal stability and mechanical behavior under nanoindentation of a new glassy alloy with composition Ni₅₀Nb₂₈Zr₂₂, produced in the form of melt-spun ribbons and copper mold-cast wedges. The alloy composition was designed based on the lambda criterion combined with the electronegativity difference among the elements. X-ray diffraction and scanning electron microscopy confirmed that the ribbons and wedges (up to 200 μm in thickness) are amorphous. The thermal properties of these samples were evaluated by differential scanning calorimetry (DSC). Nanoindentation revealed that the hardness of this alloy, around 10 GPa, is among the highest reported for metallic glasses. Remarkably, the cast wedge exhibits greater hardness and higher elastic modulus than the ribbon. This correlates with the larger amount of frozen-in free volume in the ribbons than in the cast wedges, as evidenced by DSC. In addition, finite element simulations of nanoindentation curves were performed. The Mohr-Coulomb yield criterion allows for better adjustment of the experimental data than the pressure-independent Tresca yield criterion. The simulations also reveal that the cohesive stress in the ribbons is lower than in the wedges, which explains the difference in hardness and Young's modulus between the two samples.     

~ 1.2 μm near-infrared emission and gain anticipation in Ho doped heavy-metal gallate glasses

Ho³⁺-doped low-phonon-energy heavy-metal gallate glasses (LKBPBG) have been prepared and efficient 1.199 μm emission originating from the ⁵I₆ → ⁵I₈ radiative transition has been observed under 900 nm excitation. The spontaneous emission probability and the maximum stimulated emission cross-section were derived to be 294.31 s^− 1 and 3.46 × 10^− 21 cm², respectively. The ratio of quantum yields between ~ 1.2 and ~ 2.0 μm emissions was identified to be 16%, demonstrating that the ⁵I₆ → ⁵I₈ transition is favorable for optical amplification. The maximum gain coefficient of 1.84 dB/cm at 1.199 μm wavelength was anticipated in the ideal status. These results indicate that the Ho³⁺-doped LKBPBG glasses have a promising potential for the development of ~ 1.2 μm signal amplifier devices.     

Luminescence and scintillation of Eu2+‐doped high silica glass

High silica glass doped with Eu²⁺ ions was prepared as a scintillating material by impregnation of Eu ions into a porous silica glass followed by reduction sintering in CO atmosphere. A dominant emission band of the Eu²⁺ 5d–4f transition peaking around 430 nm was observed in the luminescence spectrum with the excitation peak around 280 nm and no emission from Eu³⁺ was present. Photoluminescence decay kinetics was governed by decay times of a few microseconds. The Eu²⁺‐doped high silica glass exhibited comparable energy resolution and slightly higher photoelectron yield with respect to the Bi₄Ge₃O₁₂ crystal in the pulse height spectra for X‐ray photon energies within 22–60 keV. Furthermore, a factor of 1.2 higher radioluminescence intensity was observed as well. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal characterization of Se85−xSb15Snx (10≤x≤13) chalcogenide glasses

Results of crystallization kinetics, viscosity, specific heat, thermal stability, and glass forming ability of Se_85−xSb₁₅Sn_x (x=10, 11, 12.5, and 13) chalcogenide glasses, using differential scanning calorimeter (DSC), under non-isothermal condition have been reported and discussed. The variation of the peak temperature of crystallization T_p with the heating rate β has been used to investigate the growth kinetics using Kissinger, Takhor, and Augis-Bennet models. The activation energy of crystallization E_c has been found to increase with Sn content and the crystal growth occurs in one dimension. The increasing trend of E_c is interpreted in terms of enhancement of the degree of cross-linking due to the formation of SnSe_4/2 structural units of energies higher than that of Se-Se and Se-Sb bond energies. The viscosity η against 1/T curves has also been drawn and indicated that the atoms of ternary Se-Sb-Sn glasses required more energy, with the addition of Sn, to complete the transformation from amorphous to crystalline state. The demand for thermal stability has been ensured through the calculations of the enthalpy released ΔH_c during the crystallization process and S-parameter, while the obtained values of the reduced glass transition temperature T_rg and Hurby number H_R have been used to estimate the glass forming ability (GFA). Results reveal that, both thermal stability and GFA enhanced with increasing Sn content and the studied samples were prepared from strong glass-forming liquids. The obtained values for the specific heat difference ΔC_p, between the equilibrium liquid and the glass, have been found to decrease with increasing Sn content and are in support of the results of thermal stability and GFA.     

Effect of CuO addition on the optical and electrical properties of sodium zinc borophosphate glasses

Sodium borophosphate glasses doped with copper ions having general composition 20Na₂O-20ZnO-25B₂O₃-(35-x) P₂O₅-x CuO (x=1-8 mol %) were prepared using conventional melt-quench method and characterized by density, UV-visible optical absorption, photoluminescence and conductivity measurements. E_optical values for different glass samples are found to decrease systematically from 3.5 to 2.5 eV with increase in CuO content in the glass. Network modifying action of CuO with the glass network has been confirmed from the UV-visible optical absorption studies. Presence of Copper in the form of Cu⁺ species has been confirmed from photoluminescence measurements. The electrical conductivity (σ) increases with increase in copper oxide content in the glass and temperature dependence of electrical conductivity confirmed the semiconducting nature of the samples.     

Emission features of Ho ion in Nb2O5, Ta2O5 and La2O3 mixed Li2O-ZrO2-SiO2 glasses

Li₂O-ZrO₂-SiO₂: Ho³⁺ glasses mixed with three interesting d-block elemental oxides, viz., Nb₂O₅, Ta₂O₅ and La₂O₃, were prepared. Optical absorption and photoluminescence spectra of these glasses have been recorded at room temperature. The luminescence spectra of Nb₂O₅ and Ta₂O₅ mixed Li₂O-ZrO₂-SiO₂ glasses (free of Ho³⁺ ions) have also exhibited broad emission band in the blue region. This band is attributed to radiative recombination of self-trapped excitons (STEs) localized on substitutionally positioned octahedral Ta⁵⁺ and Nb⁵⁺ ions in the glass network. The Judd-Ofelt theory was successfully applied to characterize Ho³⁺ spectra of all the three glasses. From this theory various radiative properties, like transition probability A, branching ratio β_r and the radiative lifetime τ_r, for ⁵S₂ emission levels in the spectra of these glasses have been evaluated. The radiative lifetime for ⁵S₂ level of Ho³⁺ ions has also been measured and quantum efficiencies were estimated. Among the three glasses studied the La₂O₃ mixed glass exhibited the highest quantum efficiency. The reasons for such higher value have been discussed based on the relationship between the structural modifications taking place around the Ho³⁺ ions.     

Effect of Li2O content on physical and structural properties of vanadyl doped alkali zinc borate glasses

The effect of Li₂O content in vanadyl doped 20ZnO+xLi₂O+(30−x)Na₂O+50B₂O₃ (5≤x≥25) glasses has been studied with respect to their physical and structural properties. The absence of sharp peaks in XRD spectra of these glass samples confirms the amorphous nature. The physical parameters like density, refractive index, ionic concentration and electronic polarizability vary non-linearly with x mol% depending on the diffusivities of alkali ions. EPR and optical absorption spectra reveal that the resonance signals are characteristics of VO²⁺ ions in tetragonally compressed octahedral site. Spin-Hamiltonian, crystal field, tetragonal field and bonding parameters are found to be in good agreement with the other reported glass systems. The tetragonal distortion (g_⊥-g_∥) and Dt reveals that their values vary non-linearly with Li₂O content and reaches a minimum at x=10 mol%. An anomaly of character has been observed in all the properties of vanadyl doped glass systems, which gives a clear indication of mixed alkali effect.     

Effect of WO3 on EPR, structure and electrical conductivity of vanadyl doped WO3·M2O·B2O3 (M=Li, Na) glasses

Glasses with composition xWO₃·(30−x)M₂O·70B₂O₃ (M=Li, Na; 0≤x≤15) doped with 2 mol% V₂O₅ have been prepared using the melt-quench technique. The electron paramagnetic resonance spectra have been recorded in X-band (ν≈9.14 GHz) at room temperature (RT). The spin Hamiltonian parameters, dipolar hyperfine coupling parameter and Fermi contact interaction parameter have been calculated. It is observed that the resultant resonance spectra contain hyperfine structures (hfs) only due to V⁴⁺ ions, which exist as VO²⁺ ions in octahedral coordination with a tetragonal compression in the present glass system. The tetragonality increases with WO₃:M₂O ratio and also there is an expansion of 3d_xy orbit of unpaired electron in the vanadium ion. The study of IR transmission spectra over a range 400-4000 cm⁻¹ depicts the presence of WO₆ group. The DC conductivity (σ) has been measured in the temperature range 423-623 K and is found to be predominantly ionic.     

High strain rate sensitivity of hardness in quinary Ti-Zr-Hf-Cu-Ni high entropy metallic glass thin films

Quinary Ti-Zr-Hf-Cu-Ni high-entropy metallic glass thin films were produced by magnetron sputter deposition. Nanoindentation tests indicate that the deposited film exhibits a relatively large hardness of 10.4±0.6 GPa and a high elastic modulus of 131±11 GPa under the strain rate of 0.5 s⁻¹. Specifically, the strain rate sensitivity of hardness measured for the thin film is 0.05, the highest value reported for metallic glasses so far. Such high strain rate sensitivity of hardness is likely due to the high-entropy effect which stabilizes the amorphous structure with enhanced homogeneity.