Allu Amarnath Reddy

KCa4(BO3)3:Ln3+ (Ln = Dy, Eu, Tb) phosphors for near UV excited white–light–emitting diodes

A series of doped KCa4(BO3)3:Ln3+ (Ln: Dy, Eu and Tb) compositions were synthesized by solid–state reaction method and their photoluminescent properties were systematically investigated to ascertain their suitability for application in white light emitting diodes. The X–ray diffraction (XRD) and nuclear magnetic resonance (MAS–NMR) data indicates that Ln3+–ions are successfully occupied the non–centrosymmetric Ca2+ sites, in the orthorhombic crystalline phase of KCa4(BO3)3 having space group Ama2, without affecting the boron chemical environment. The present phosphor systems could be efficiently excitable at the broad UV wavelength region, from 250 to 350 nm, compatible to the most commonly available UV light–emitting diode (LED) chips. Photoluminescence studies revealed optimal near white–light emission for KCa4(BO3)3 with 5 wt.% Dy3+ doping, while warm white–light (CIE; X = 0.353, Y = 0.369) is obtained at 1wt.% Dy3+ ion concentration. The principle of energy transfer between Eu3+ and Tb3+ also demonstr...

Influence of the annealing temperatures on the photoluminescence of KCaBO3:Eu3+ phosphor

Novel red emitting KCaBO3:Eu phosphors have been synthesized by solid-state reaction at various temperatures. Systematic studies on annealing effects and consequent structural evolution and optical properties were investigated by various structural and photoluminescence studies. With an increase in annealing temperature (from 700 °C to 950 °C), these phosphors show a gradual change from a mixed low crystalline phase to a highly crystalline single phase, with minimized volatile impurities. Photoluminescence studies revealed that the low-temperature annealed phosphors showed distinct mixed emission composed of blue and red emissions upon UV excitation. Such dual emission is due to the coexistence of Eu3+ and Eu2+ ions, wherein the reduction of Eu3+ into Eu2+ was attributed to the presence of volatile impurities. Relatively high-temperature annealed phosphors exhibit strong red color photoluminescence due to homogeneously occupied Eu3+ ions in the host crystal charge-compensated (with K+ ions) sites of Ca2+ ions. The dominant red-to-orange emission intensity ratios and Judd–Ofelt parameters of Eu3+ ions support the strong covalent nature and site-occupation of higher asymmetry sites of K+ and Ca2+ ions. High emission life times and efficient and stable photoluminescence at different excitation wavelengths make these phosphors suitable for white LEDs and other display applications.

Sintering behavior of lanthanide-containing glass-ceramic sealants for solid oxide fuel cells

This article reports on the influence of different lanthanides (La, Nd, Gd and Yb) on sintering behavior of alkaline-earth aluminosilicate glass-ceramic sealants for their application in solid oxide fuel cells (SOFCs). All the glasses have been prepared by the melt–quench technique. The in situ follow up of sintering behavior of glass powders has been done by a high temperature-environmental scanning electron microscope (HT-ESEM) and a hot-stage microscope (HSM) while the crystalline phase evolution and assemblage have been analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). All the glass compositions exhibit a glass-in-glass phase separation followed by two stage sintering resulting in well sintered glass powder compacts after heat treatment at 850 °C for 1 h. Diopside (CaMgSi2O6) based phases constituted the major crystalline part in glass-ceramics followed by some minor phases. The increase in lanthanide content in glasses suppressed their tendency towards devitrification, thus resulting in glass-ceramics with a high amount of residual glassy phase (50–96 wt%) which is expected to facilitate their self-healing behavior during SOFC operation. The electrical conductivity of the investigated glass-ceramics varied between (1.19 and 7.33) × 10−7 S cm−1 (750–800 °C) while the coefficient of thermal expansion (CTE) varied between (9.4 and 11.2) × 10−6 K−1 (200–700 °C). Further experimentation with respect to the long term thermal and chemical stability of residual glassy phase under SOFC operation conditions along with high temperature viscosity measurements will be required in order to elucidate the potential of these glass-ceramics as self-healing sealants.

Thermal and mechanical stability of lanthanide-containing glass–ceramic sealants for solid oxide fuel cells

Thermal stability of lanthanide (Ln = La, Nd, Gd, Yb) containing glass and glass–ceramics (GCs) was characterized for their application as sealants for solid oxide fuel cells (SOFCs). X-ray diffraction (XRD) in conjunction with the Rietveld-RIR and solid-state NMR techniques was employed to quantify the crystalline and amorphous fractions in the glasses sintered/heat treated at 850 °C in air for 1–1000 h. The structure and crystalline phase evolution of Ln containing aluminosilicate glasses depend markedly on the Ln3+ cation field strength over both short and intermediate length scales. Along with diopside, Ln containing silicate apatites, with general formula Ln9.33+2x(Si1−xAlxO4)6O2 (Ln = La, Nd and Gd; with x varying between 0 and 0.33), were observed in the GCs after the heat treatment periods of 1 to 1000 h at 850 °C, leading to moderately higher electrical conductivity. The substantial amount of the remaining glassy phase in Gd2O3-containing GC after 1000 h at 850 °C is likely to confer self-healing properties to this composition, in accord with the oxygen leakage measurements on thermal cycling. 29Si, 27Al and 11B magic-angle spinning (MAS) NMR spectra confirmed the results of the XRD RIR analysis. The values of Weibull characteristic strength and of average flexural strengths for all the GCs are higher than those reported for G-18 commercial glass (51 MPa), with Weibull modulus varying in the range 11.6–34.4 towards good mechanical reliability. Thermal shock resistance of model electrochemical cells made of yttria-stabilized zirconia (YSZ) was evaluated employing quenching from 800 °C in air and water. All the GC seals bonded well to YSZ and Sanergy HT metallic interconnects without gap formation. Suitable thermal expansion coefficient (9.7–11.1 × 10−6 K−1), mechanical reliability, high electrical resistivity, strong adhesion to Sanergy HT interconnects and YSZ, and sufficient thermal shock resistance indicate good suitability of the lanthanide-containing sealants for SOFC applications.

Study of melilite based glasses and glass-ceramics nucleated by Bi2O3 for functional applications

Bi2O3-nucleated melilite based glasses and glass-ceramics (GCs) in the system CaO–MgO–Al2O3–SiO2–La2O3 have been appraised for solid electrolyte sealing applications in high-temperature electrochemical devices, such as solid oxide fuel cells and oxygen pumps . The structure of the glasses was assessed by Fourier transform infrared (FTIR) and 29Si and 27Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. The crystallization kinetics and sintering behaviour were investigated by differential thermal analysis and hot stage microscopy. All glass compositions exhibited single-stage shrinkage behaviour. X-ray diffraction (XRD), in conjunction with the Rietveld-RIR (reference intensity ratio) technique, was employed to quantify the crystalline and amorphous phases in GCs sintered under non-isothermal conditions for 1 h within the operating temperature range (850–900 °C). Merwinite and melilite (the solid solutions of akermanite and gehlenite) were revealed as the major crystalline phases formed in GCs. The amount of GC in the melilite phase increased significantly from 850 to 900 °C at the expense of merwinite. The coefficients of thermal expansion (CTE (200–700 °C)), 10.2–10.9 × 10−6 K−1 for the glasses and 9.8–11.2 × 10−6 K−1 for the GCs, are in good agreement with those typical for solid oxide electrolytes, such as 8 mol% yttria-stabilized zirconia (8YSZ), and metallic interconnects, such as Sanergy HT. Long term thermal stability studies demonstrate stable behaviour for the crystalline phase assemblage and chemical composition, CTE (200–700 °C) values of 9.9–11.7 × 10−6 K−1, and good adjoining performance with the metal and electrolyte. The well matched CTE values and good adhesion to the other components in both air and a reducing atmosphere allow us to propose further research into the parent compositions as stabilized zirconia sealants. Glasses demonstrating bulk nucleation may also attract interest for other functional applications in optical and electronic devices.

Melilite glass–ceramic sealants for solid oxide fuel cells: effects of ZrO2 additions assessed by microscopy, diffraction and solid-state NMR

The influence of adding 0–5 mol% zirconia (ZrO2) to a series of melt-quenched alkaline-earth aluminosilicate glasses designed in the gehlenite (Ca2Al2SiO7)–akermanite (Ca2MgSi2O7) system has been investigated for their potential application as sealants for solid oxide fuel cells (SOFCs). The work was implemented with a dual aim of improving the sintering ability of the glass system under consideration and gaining insight into the structural changes induced by ZrO2 additions in the glasses consequentially leading to their enhanced long-term thermal stability. That the degree of condensation of SiO4 tetrahedra increased with increasing amounts of zirconia was confirmed by 29Si magic-angle (MAS) NMR. 1D 27Al, 11B MAS as well as two-dimensional (2D) 11B MQMAS/STMAS NMR experiments gave structural insight into the number and nature of aluminum and boron sites found in the glass and glass–ceramic (GC) samples. Irrespective of the heat treatment time, increasing the zirconia content in glasses suppressed their tendency towards devitrification, while the glasses exhibited good sintering behavior resulting in mechanically strong GCs with higher amounts of residual glassy phase making them suitable for self-healing during SOFC operation. All the GCs exhibited low total electrical conductivity; appropriate coefficients of thermal expansion (CTE), good joining and minimal reactivity with SOFC metallic components at the fuel cell operating temperature, thus, qualifying them for further appraisal in SOFC stacks.

Optical Amplifiers from Rare-Earth Co-Doped Glass Waveguides

Optical amplifiers are of potential use in wide variety of optoelectronic and optical communication applications, particularly for Wavelength Division Multiplexing (WDM) to increase the number of channels and transmission capacity in optical network systems. For efficient performance of WDM systems, essential requirements are larger bandwidth, higher output power and flat gain over entire region of operation. Recent research is focused on design and development of fiber/MEMS-compatible optical amplifiers. Some examples of such sources are semiconductor quantum dot light-emitting diodes, super-luminescent diodes, Erbium doped fibre amplifier (EDFA, 1530-1625nm), Erbium doped planar amplifier (EDWAs), Fibre Raman amplifier, Thulium doped fibre amplifier (1460-1510nm). However, for many applications covering the total telecommunication window (1260-1700nm) is highly desirable and as such it is not yet realized. Typical attenuation spectrum for glassy host is shown in Figure 1. Specially the low loss region extending from 1450 to 1600 nm, deemed the 3rd telecommunication window, emerged as the most practical for long haul telecommunication systems. This window has been split into several distinct bands: Shortband (S-band), Centre-band (C-band) and Long-band (L-band). With several generations of development, the transmission rates have increased dramatically so that several Terabits per second data can be transmitted over a single optical fiber at carrier wavelengths near 1550 nm, a principal optical communication window in which propagation losses are minimum. EDFAs are attractive to WDM technology to compensate the losses introduced by WDM systems and hence has grown as a key to upgrade the transmission capacity of the present fiber links. EDFAs are widely used in long-haul fiber optic networks where the fiber losses are limited to 0.2 dB/km, is compensated periodically by placing EDFAs in the transmission link with spacing of up to 100 km. EDFAs make use of trivalent erbium (Er3+) ions to provide the optical amplification at wavelengths near 1550 nm, the long wavelength window dominantly used in optic networks since the fiber losses are found minimum around this wavelength. Light from an external energy source at a wavelength of 980 nm or 1450 nm, coupled along with the information signal, and is passed through the EDFA to excite the Er3+ ions in order to produce the optical amplification through stimulated emission of photons at the signal wavelength. Er3+ doped waveguides (EDWA) have