Preparation Of Nitride/Oxynitride Nanofiber Mats Via Eletrospinning For White LEDs

It has been widely accepted that white light-emitting diodes (LEDs) will substitute the traditional light sources due to its excellent properties such as energy saving, long lifespan and positive environmental effects. Generally, there are three different approaches which can be used for generating white light based on LEDs. The conventional approach is the combination of near ultraviolet or blue light LED chips with photo convert phosphors. The optical properties of the phosphor layer determine the performance of white-light LEDs such as color temperature, color rendering index, luminous efficiency and lifespan. There are three approaches can be used to improve the optical properties of the phosphor layer:using novel high performance phosphors, secondary optical design for the phosphor layer and the improvements of phosphor packaging process. In this thesis, AlN:Eu2+and La3Si8O4N11:Ln3+(Ln=Ce3+, Sm3+) fiber mats were prepared by a simple and cost-effective electrospinning process. Patterned fiber mat was prepared by electroconductive templates assistant electrospinning and parallel nanofiber was prepared by using a wire drum collector. The effect of the fiber arrangement on optical properties of electrospinning mats was also studied.Eu2+doped AlN nanofibers and microbelts were prepared by electrospinning. The obtained samples were characterized by X-ray powder diffraction (XRD), filed emission scanning electron microscopy (FE-SEM) and photoluminescence (PL) spectrum. XRD patterns show samples nitrided at1400℃for9h were crystallized well and the patterns assigned to JCPDF card of AlN with wurtzite structure. The morphology of sample can be controlled by adjusting the concerntration of PVP (130k) in the precursor solusion. And the obtained sample not only keeps the micromorphology of nanofiber but also assemble as a film macroscopically. Comparing to the emission of AlN:Eu2+, the emision of Si4+co-doped AlN:Eu2+shows a blue shift. In comparison with AlN:Eu2+phosphors without Y3+, Y3+co-doped AlN shows higher PL intensity, resulting from the appearance of liquid assistant reaction.Ce3+doped La3Si8O4N11and Sm3+doped La3Si8ON11nanofibers and microbelts were prepared by electrospinning. The obtained samples were characterized by XRD, FE-SEM, TGA, energy-dispersive X-ray spectrometry (EDS) and PL spectrum. XRD patterns show Sample nitrided under1400℃for9h were crystallized well and the pattern assigned to JCPDF card of the monoclinic La3Si8O4N11crystal and the sample maintained La3Si8O4N11phase after doping Ce3+or Sm3+ions. The morphology of sample can be controlled by adjusting the concerntration of PVP (k-30) in the precursor solusion. And the obtained sample not only keeps the micromorphology of nanofiber but also assemble as a film macroscopically Under ultraviolet excitation (343nm), the emission spectra of Ce3+doped La3Si8O4N11sample consists a board peak near430nm and a shoulder at460nm, resulting from Ce3+5d-4f transition. The dacay curve of Ce3+doped La3Si8O4N11nanofibers is a kind of double-exponential decay curve. The fast and slow decay lifetimes of Ce3+doped La3Si8O4N11nanofibers are about8.27and32.72ns. Under409nm, the emission spectra of Sm3+doped La3Si8O4N11shows four sharp peaks situated at575nm,600nm,610nm and650nm, resulting from Sm3+4f-4f transition. This kind of phosphor has great potential for LEDs which emits light of different color temperature.Aligned La-Si-O/PVP fiber membranes with designed topological structures were prepared by electrospinning using different collectors. The samples were characterized by scanning electron microscopy (SEM), optical profiler and UV-Vis spectroscopy. The majority of the electrospun fibers in La-Si-O/PVP membranes, collected by Metal mesh directly, assembled along the cross intersection of the expanded metal. The impact of spinning collection time and the mesh number of expanded metal to the structure and transmissivity of membranes were studied. When collection time is short, the menbrane collected by expanded metal of100meshes shows a higher transmissivity compared to the menbrace collected by expanded metal of60meshes. While increasing the collection time, the result will be opposite. Parallel nanofiber was prepared with a wire drum collector. The effect of inorganic salts content and the revolving speed of wire drum on the Alignment of Electrospun fibers were also studied. The orientation degree of fibers increases with the increase of the revolving speed of wire drum. While, with the increase of inorganic salts content, the orientation degree of fibers increases first, then decreases subsequently.