Thermal, structural and luminescent properties of long after-glow MAlxOy:Eu²+,Dy³+ (M: Sr, Ba) phosphors
The optimization of properties for new and potentially useful materials becomes a continuous and sometimes a lifelong process if future applications are anticipated. Research on luminescent materials is a good example of this statement and rare earth-doped alkaline earth aluminates is at the epicentre of this focus due to the anticipated superior qualities, vis a vis those of classical sulphide phosphorescent materials. The focus in these developments has been to produce a phosphor with high emission intensity, high colour purity, longer afterglow and that is safe and chemically stable. To address some of the issues in these efforts, this study had three major aims: (1) The investigation of the surface morphology, crystallinity, particle size, luminescence, and thermal properties of commercial phosphors by various techniques. (2) The preparation and characterization of two types of phosphors by standard techniques. (3) The preparation and characterization of phosphor/polymer composites. The first commercial phosphor was a green-emitting aluminate phosphor. The properties of this phosphor as well as those of LDPE/phosphor and PMMA/phosphor composites were studied. Polymer/phosphor composite samples were prepared with phosphor concentrations ranging from 1 to 5 volume% and subsequently hot-melt-pressed. Sharp and broad XRD peaks were observed for the LDPE and PMMA composites respectively, reflecting configuration characteristics similar to those of the respective pure polymers. TEM micrographs show a transition from nanosized particles to cluster formation with increase in phosphor concentration. PL was observed in the composites of both polymers for phosphor volume concentrations above 1.0% for PMMA and above 0.5% for LDPE. For each of these samples, a broad PL peak at about 505 nm wavelength was observed after excitation at 350 nm with a xenon lamp. For the LDPE composites, the DSC results show that the presence of the phosphor in the polymer had no major influence on either the melting temperature or enthalpy values of the polymer. LDPE/phosphor composite samples, based on blue-emitting commercial aluminate phosphor, were similarly prepared and characterized for structural, luminescent and thermal properties. XRD analysis revealed the presence of the Sr4Al14O25 phase in the composites. PL spectra have two sets of peaks, major broad bands peaking at about 486 nm and minor ones between 412 nm and 418 nm, attributed to the 4f–5d transition of Eu2+. DSC and TGA results show that the introduction of the phosphor in LDPE matrix caused a slight reduction in the crystallinity of LDPE but a strong increase in the stability of the composites. SrAlxOy:Eu2+,Dy3+ phosphor was synthesized by a combustion method and characterized for luminescent and thermal properties. Phosphor nanocrystallites with high brightness were obtained. The average crystallite sizes, calculated from the Scherrer equation ranged between 34 and 43 nm. Emissions arising from transitions between the 5d and 4f orbital gaps of Eu2+ are manifested in the broad-band excitation and emission spectra with major peaks at 360 and 515 nm, respectively. The decay curves and half-life times show a clear trend in the influence of the phosphor in the improvement of the initial brightness and the afterglow times, which are ascribed to the presence of shallow and deep traps. Thermal results indicate that the phosphor nanoparticles acted as nucleating agents and improved the overall crystallinity in the LDPE/ SrAlxOy:Eu2+,Dy3+ phosphor system. The temperature-dependence of the structural and luminescent properties of sol-gel derived SrAlxOy:Eu2+,Dy3+ phosphor was investigated. Calculations based on XRD results, by means of the Scherrer equation showed the average crystallite sizes increasing from about 42 to 47 nm. Reflections corresponding to both SrAl2O4 and Sr2Al3O6 phases were observed at the various annealing temperatures but with a diminishing contribution from the Sr2Al3O6 phase. PL characterization also shows temperature-dependence through variation of both the peak position and intensity, which indicate emission at low and high annealing temperatures originating from Eu2+ and Eu3+ ions respectively. BaAlxOy:Eu2+,Dy3+ was the second phosphor synthesized by a combustion method. PL results indicate that the LDPE/BaAlxOy:Eu2+,Dy3+ interface, which is considered to have an influence on the composite behaviour, did not significantly change the spectral positions of the phosphor materials, whose major emission peaks occurred at about 505 nm. The improved afterglow results for the composites are probably due to morphological changes due to the increased surface area and defects. Thermal results indicate that the BaAlxOy:Eu2+,Dy3+ particles acted as nucleating centres and enhanced the overall crystallinity in the LDPE nanocomposite while preventing lamellar growth, hence reducing the crystallite sizes in LDPE.