Elsevier

Journal of Non-Crystalline Solids

Volume 481, 1 February 2018, Pages 191-201
Journal of Non-Crystalline Solids

Effect of alkali/mixed alkali metal ions on the thermal and spectral characteristics of Dy3 +:B2O3-PbO-Al2O3-ZnO glasses

https://doi.org/10.1016/j.jnoncrysol.2017.10.047Get rights and content

Highlights

  • Alkali/mixed alkali based Dy3 +: B2O3–PbO–Al2O3ZnO glasses are evaluated for W-LEDs.

  • ΔT = Tx–Tg varied at 140–173 °C, and 132–160 °C for single, and mixed alkali glasses.

  • Dy3 +: Li–Na glass shows the highest emission bands intensity among all the glasses.

  • The Y/B intensity ratio varied within the range 1.257–1.376 for the studied glasses.

  • All Dy3 +-doped glasses CIE (x, y), and CCT values represent neutral white light region.

Abstract

Thermal and spectroscopic features of 50 B2O3–10 PbO–10 Al2O3–10 ZnO–(x) Li2O–(y) Na2O–(z) K2O–1.0 Dy2O3 (mol %) (x = 19, y = 0, and z = 0; x = 0, y = 19, and z = 0; x = 0, y = 0, and z = 19; x = 9.5, y = 9.5, and z = 0; x = 9.5, y = 0, and z = 9.5; x = 0, y = 9.5, and z = 9.5) glasses, that were fabricated by utilizing melt-quenching approach, are investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), optical absorption, photoluminescence excitation (PLE), photoluminescence (PL), and PL decay lifetimes. PL spectra for all the Dy3 +-doped samples show emission bands at 453 nm (blue), 482 nm (blue), 573 nm (yellow), 662 nm (red), and 752 nm (red) corresponding to the 4I15/2  6H15/2, 4F9/2  6H15/2, 4F9/2  6H13/2, 4F9/2  6H11/2, and 4F9/2  6H9/2 transitions, respectively, upon excitation at 350 nm. Here, Dy3 +: Li–Na glass shows the highest PL intensity for all identified emissions. The yellow-to-blue (Y/B) emission intensity ratio (varied within the range 1.257–1.376), CIE chromaticity coordinates (x,y) (slight variation between (0.3410, 0.3802) and (0.3495, 0.3872), and correlated color temperatures (CCTs) (changed from 4953 K to 5212 K) are calculated following the PL spectra. Dy3 +: 4F9/2 decay curves show non-exponential behavior and are fitted by the Inokuti-Hirayama (I–H) model, where S = 6 shows best fit, indicating dipole-dipole (d-d) interactions for Dy3 + excited (donor) and ground state (acceptor) ions.

Introduction

Usually, inorganic glasses, which are classified as amorphous materials, show several advantages compared to crystalline as well as polymer materials, such as high optical transparency, wide range of compositions and working temperatures, ease of synthesis, fabrication in different shapes, homogeneous luminescence, low manufacturing cost, and high mechanical stability [1], [2], [3]. Among several optical materials from oxide glasses, recently, rare-earth (RE) ion doped borate (B2O3) glasses gained intensive attention from the researchers for various technological applications, such as solid-state lasers [4], optical fiber amplifiers [5], radiation dosimetry [6], high quality laser illuminators [7], W-LEDs [8], and scintillators [9] etc. Here, borate glasses possess good RE ion solubility, good mechanical hardness, high spectral transparency, low melting temperature, and excellent thermal stability or lower thermal expansion coefficients with respect to phosphate glasses [4], [5], [6], [7], [8], [9], [10]. Further, RE ions offer excellent emission efficiency because of their characteristic 4f–4f and 4f–5d transitions, and these 4f-4f transitions can give luminescence covering wide spectral range starting from ultraviolet (UV) to infrared (IR) as the 4f orbitals are very effectively protected from the interaction with external fields by 5s2 5p6 shells [11]. But, high phonon energy (~ 1300–1500) cm 1 of borates increases the multi-phonon relaxation rates of the RE emission transitions, that decreases the luminescence and quantum efficiencies.

Heavy metal oxides (HMO) (e.g. PbO), which possess relatively high mass, low crystal field strength, and large electronic polarizability inclusion into borate glass matrix could produce lower non-radiative (NR) relaxation rates, which causes enhanced luminescence from RE excited states due to their low vibrational frequencies and high refractive index [11], [12] and further, due to the allowed s-p electronic transition of Pb2 + ions, they show excellent UV radiation absorption features. Usually, at low content, forming PbO6 groups with Pbsingle bondO ionic bonds, PbO acts as a network modifier, while at high concentration it plays the network former role due to PbO4 functional groups and here, Pbsingle bondO bonds exhibit covalent nature [13]. In the glass network, Al2O3 improves mechanical, chemical, electrical resistance and de-cluster RE dopant ions, which in turn enhance the emission efficiency by reducing the cross-relaxation (CR) losses [14]. Similarly to PbO, depending on the concentration, ZnO also acts in two ways like (i) glass former (at higher ZnO content) and (ii) network modifier (at low ZnO concentration), respectively, and causes improved stability, moisture resistance, transparency, and low rates of crystallization in the glasses [15], [16]. Glass modifiers such as Li+, Na+, and K+ ions affect the physical and optical features, decrease viscosity and melting points of the glasses by disrupting the glass network and creating non-bridging oxygens (NBOs) [16], [17].

Because of their attractive features like less power utilization, enhanced energy capability, excellent chromatic stability, high luminous efficacy, environmental friendliness, small volume, longer lifetime, and good stability, white light emitting diodes (W-LEDs), recently, showed their potentiality as an alternative to fluorescent lamps and incandescent bulbs [18], [19], [20]. (i) In a suitable ratio, the mixture of red (R), green (G), and blue (B) (i.e. RGB) color emitting phosphors or (ii) the combination of a blue laser diode with YAG: Ce3 + phosphor [21] are the two well-established methods for the design of W-LEDs. But the above two methods show inherent drawbacks such as re-absorption, the shift of chromaticity, low CRI, Ra ~ 70–80, poor heat dissipation, degradation of luminous efficacy, high correlated color temperature, CCT, ~ 7750 K, and reduced lifetimes etc. [8], [21]. To overcome these complications that arise from common phosphors used for W-LEDs, very recently, RE-doped glasses are becoming a hot spot for researchers as an alternative source for W-LEDs. The RE-doped glasses possess lower production cost, simple manufacturing procedure, higher transparency, better thermal stability, homogeneous light-emitting capacity, free of halo effect, and epoxy resin free assembly [8], [9], [10], [22].

Among various RE ions in Ln3 + group, recently, Dy3 + ion has attracted a lot of attention from the researchers due to its ability to generate white emission color by efficiently absorbing the emission from the UV and blue based LED chips, and also shows an application in telecommunications [8], [9], [10], [22], [23]. Dy3 +: 4F9/2 and 6H9/2 levels with wide ranges of visible and NIR region emissions, give distinct yellow emission (4F9/2  6H13/2 transition, 570–600 nm), blue emission (4F9/2  6H15/2 transition, 470–500 nm), weak red emission (4F9/2  6H11/2 transition, (660–670 nm), and near-infrared (NIR) emission at 1.32 μm (6H9/2  6H15/2), respectively. Further, the yellow emission intensity depends on the selected host glass composition due to its hypersensitive and forced electric-dipole (ED) nature, for which |∆J | = 2 (i.e. Dy3 +: 4F9/2  6H13/2 transition is suitable for lasing action owing to its large cross-section “σ”). The 4F9/2  6H15/2 magnetic-dipole (MD) transition intensity shows less sensitivity to Dy3 + ion surroundings in a host matrix. With this intention, the optimum yellow/blue (Y/B) intensity ratio (at around unity) from Dy3 + ion can offer white light emission [8], [9], [10], [22], [23], [24], [25], [26].

Considering the above features of B2O3, PbO, Al2O3, ZnO, alkali metal ions, W-LEDs, and Dy3 + ion, in the present work, we made an attempt to evaluate the suitability of different compositions of single and mixed alkali (Li2O, Na2O, K2O and Li2O/Na2O, Li2O/K2O, Na2O/K2O) zinc alumino lead borate glasses doped with 1.0 mol% Dy2O3 for their possible applications for W-LEDs. The glasses were studied to understand different glass modifier incorporation effects mainly on the thermal and spectroscopic features. Thermal features were analyzed through TGA/DSC. The optical features like absorption, PLE, and visible PL including decay lifetimes for the prepared glasses were explored to determine the potentiality of the glasses as luminescence materials for solid-state W-LED applications by exciting with UV light. The Ωλ = 2, 4 and 6) Judd–Ofelt (J–O) intensity parameters, along with some radiative features like radiative emission transition probability (AR), radiative lifetime (τR), and branching ratio (βR) are determined from the absorption and PL spectra of the respective Dy3 +-doped glasses. The yellow to blue luminescence intensity (Y/B) ratios, the Commission Internationale de I'Eclairage (CIE) chromaticity (x, y) coordinates, and CCTs are computed following the visible PL spectra and application of the prepared glasses for W-LEDs is discussed.

Section snippets

Glass synthesis

Six zinc alumino lead borate glasses with different alkali and mixed-alkali content at fixed 1.0 mol% Dy2O3 doping, investigated in this work, have been fabricated using melt quenching method. In Table 1 the nominal compositions of the glasses are given and labeled like “a”, “b”, “c”, “d”, “e”, and “f”, respectively, for convenience. For the glass synthesis, B2O3 (99.98%), PbO (≥ 99%), Al2O3 (≥ 98%), ZnO (99.99%), Li2CO3 (99.99%), Na2CO3 (99.5%), K2CO3 (≥ 99%), and Dy2O3 (99.99%) chemicals,

TGA

For understanding the thermal stability and phase transformation of “a” to “f” glasses, TGA and DSC experiments were performed within the temperature range 30–1000 °C and the respective patterns are depicted in Fig. 1(a) for TGA and Fig. 1(b) for DSC. Below 100 °C, a small weight losses 0.09%, 0.03%, 0.15%, 0.02%, and 0.03% for the samples “a”, “b”, “c”, “d” and “e” were identified, respectively, from Fig. 1(a) and for the glass “f”, no weight loss is observed at ≤ 100 °C. This initial

DSC analysis

From Table 2, it can be observed that the Tg value shows decreasing trend for “a”, “b”, and “c” glasses within the temperature range 423–410 °C and this is because of the decreasing ionic radii (K+ = 1.38 Å, Na+ = 1.02 Å, and Li+ = 0.76 Å), including decreasing crystal field strength (Li+ = 1.731 (Å)2, Na+ = 0.961 (Å)2, and K+ = 0.525 (Å)2) of alkali metal modifiers. Here, field strength, F = Z/ri2, (Z = cation oxidation number, and ri = ionic radius). Thus, in the synthesized glasses with the addition of K2O in

Conclusions

Dy3 +: B2O3–PbO–Al2O3ZnO glasses containing Li, Na, K, Li–Na, Li–K, and Na–K alkali cations have been synthesized. The (Tg), (Tx), (Tc), and (Tm) values were obtained from the DSC profiles, and ΔT increased within the range 140–173 °C, and 132–160 °C for the Li to K (single alkali), and Li–Na to Na–K (mixed alkali) glasses, respectively. The Dy3 +: Li–Na glass possessed the strongest PL bands intensity for all the five emission bands noticed at 453, 482, 573, 662, and 752 nm wavelengths. The (Y/B)

Acknowledgments

The authors extend their appreciation to the International Scientific Partnership Program ISPP at King Saud University for funding this research work through ISPP#0106.

References (34)

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