SYNTHESIS AND PHOTOLUMINESCENCE PROPERTIES OF RARE EARTH DOPED ALKALINE EARTH NIOBATE PHOSPHORS FOR SOLID STATE LIGHTING APPLICATIONS

Abstract

properties have been systematically studied to obtain the efficient cool white light from single phase BaNb2O6: Dy3+ phosphor. The emission spectra exhibit sharp blue (4F9/2 → 6H15/2) and yellow (4F9/2 → 6H13/2) emission bands in the visible region under n-UV excitation. The Commission Internationale de I’Eclairage (CIE) coordinates are found to be x= 0.322, y= 0.339 with CCT value 5907 K found to be in cool white region, which is close to the standard white lamp colorimetric point. The luminescent characteristics suggest that this phosphor could be the most suitable for generating an appreciable cool white light. Chapter 4 deals with the pure orange light emitting Sm3+ doped BaNb2O6 phosphor for solid state lighting applications. The strong orange emission has been observed at 599 nm corresponding to 4G5/2→6H7/2 under 405 nm n-UV excitation. The structural and morphological studies exhibit pure orthorhombic phase and irregular size dense particles in micrometre range. The CIE chromaticity coordinates are found to be (0.557, 0.440), which are close to the Nichia corporation developed amber LED. The calculated purity of orange color is 99.5% and correlated color temperature (CCT) is about 1970 K. The measured lifetime for optimized sample is found to be in microsecond range. The results suggested that BaNb2O6: Sm3+ phosphor is a potential candidate for applications in displays and solid state lighting. Chapter 5 describes the red light emitting BaNb2O6 phosphor for solid state lighting applications. X-ray diffraction pattern for the synthesized phosphor confirms the formation of pure orthorhombic phase for BaNb2O6. The particle sizes are in the range of 3–5 μm observed for the as prepared sample. In addition to this, photoluminescence studies reveal that this phosphor exhibit intense red emission due to 5D0→7F2 transitions of Eu3+ ion under n-UV and blue excitations. The CIE chromaticity coordinates evaluated from the emission spectra are falling in the pure red region of the CIE 1931 diagram, which are very much close to the commercial red (Y2O2S: Eu3+) phosphor. Based on the above results, it is concluded that Eu3+ doped BaNb2O6 phosphor may be potential candidate to use in w-LEDs as a red component. Chapter 6 focussed especially on the enhancement of photoluminescence properties of Eu3+ doped BaNb2O6 phosphor by two ways namely (i) flux assisted solid state reaction and (ii) metal citrate gel (MCG) method. In the flux assisted solid state Abstract (vii) reaction method, the photoluminescent properties enhanced significantly by using various chloride fluxes such as (NH4Cl, LiCl, KCl and NaCl). Among all fluxes used, NH4Cl was found to be a suitable flux that enhances luminescence intensity more than two times while compared to the optimized Eu3+ doped BaNb2O6 phosphor (synthesized by SSR without flux). The CIE chromaticity coordinates of the optimized sample are found to be (0.65, 0.35), which are well-matched with commercial red emitting Y2O3: Eu3+ phosphor. On the other hand, in MCG method, Eu3+ doped BaNb2O6 phosphor has been synthesized at low processing temperature relative to the solid state reaction method. FE-SEM confirms the nanofeatures of the synthesized samples. The photoluminescence spectra exhibit enhancement in emission intensity about three times when compared to the optimized Eu3+ doped BaNb2O6 phosphor (synthesized by SSR without flux). The CIE chromaticity coordinates for the optimized samples are close to the NTSC standard coordinates of the red-emitting phosphors. Finally, the conclusions have been drawn from the studies of Eu3+ doped BaNb2O6 phosphor that luminescent properties have been significantly enhanced in both the methods. Moreover, this phosphor may be a potential candidate to use as a red component for white LEDs and also to enhance the CRI of the commercially available phosphor. Finally, Chapter 7 includes the general conclusions derived from the present work and scope for the future work. The thesis lists the many noticeable features of the proposed new phosphors and their exceptional properties are also highlighted along with their potential use in various display applications. The thesis ends on a note that addresses the enormous scope of this work in future.