A prototype of YAG: Ce (Y3Al5O12) luminous bulk ceramic as a remote phosphor for white LED illumination was fabricated in air through a strategy of silica addition. With increasing the amount of silica in a specific range, the opaque sample turns to be semi-transparent. The precipitation of crystals is verified to be in pure YAG phase by X-ray diffraction (XRD). Beyond the limit of silica amount, the dominant phase of YAG crystal is found to coexist with a small amount of newly-formed Y2Si2O7, Al2O3 and the amorphous phase. The YAG crystals are with a grain size of approximately 2 mm and distribute evenly. The YAG hosts after structural modification via addition of silica result in yellow band emission of 5d→4f transition peaked around 535 nm as excited by a blue LED, owing to the self-reduction of Ce4+ to Ce3+ even in the absence of reductive atmosphere.
We report a novel approach to obtaining a classical blue-green excitable CaS：Eu2＋ phosphor with desired red emission by microwave （MW） firing procedure in the absence of adding elemental sulphur. The disturbing effect of MW electro- magnetic field on decomposition of CaSO4 into CaS activated by europium is distinctly observed to give pure host phase without adding any elemental sulphur and carbon. The host phase evolution is observed to be highly dependent on the variation of applied MW power from X-ray diffraction （XRD） patterns and the corresponding photoluminescence （PL）, and a maximum PL intensity at 1100 W of MW power is acquired for the obtained purer host phase. The non-thermal and non-equilibrium effects by MW are revealed to correlate with the interaction between polar structure of the host and applied electromagnetic field. The results demonstrate an optional procedure to prepare this red-emitting phosphor in an effective, environment-friendly and scalable approach for phosphor production in the application of bio-illumination for plant cultivation and artificial photosynthesis.
Morphology control of cage-like （Ba,Sr）3MgSi2O8：Eu,Mn luminous sphere in micrometer size with a simultaneous 660 nm/430 nm-featured band emission was investigated via microwave （MW） firing procedure. A firing temperature range associated with distinct reaction of xerogel particles was determined by thermal analysis, at which the pure host phase of （Ba,Sr）3MgSi2O8 was formed and the release of decomposed gas from the precipitated nitrates played a key role in controlling the multi-scale structured morphology. As-prepared Ba1.14Sr1.7MgSi2O8：0.06Eu2＋,0.1Mn2＋ samples featured in a band emission simultaneously emitting at both 660 and 430 nm under 350 nm light excitation by MW procedure with an enhancement emission compared to the sample by solid state procedure. The results suggested that MW firing procedure affected assembling cage-like particle in meso-, nano- and submicro- meters to achieve photoluminescence （PL） enhancement of the simultaneous red/blue emission.
Photoluminescence （PL） and colorimetric properties of white-light emission SrAl2Si2O8：Eu2＋,Mn2＋ phosphor were tuned effectively through incorporating Si–N bond to the host in the form of Si3N4. A maximum solubility of Si–N bond in SrAl2–xSi2＋xO8–xNx was estimated theoretically to be in a value of x=1.0. Under 365 nm irradiation, a distinct red-shift of blue band emission from 406 to 473 nm for Eu2＋ and an enhancement of yellow band emission peaked at ~565 nm for Mn2＋ were observed with the increase of Si–N content. These effects resulted from partial substitution of Si–N for Al1–O1 and Al4–O2 sites in SrAl2Si2O8 lattice. Eventually, a white emission with CIE chromaticity coordinates （0.287, 0.337） and color rendering index （CRI） 78.3 for SrAl2Si2O8：Eu2＋,Mn2＋ phosphor were achieved upon optimization to a suitable amount of Si–N.
Eu2+ -activated reddish-orange-emitting Ca3Si2O7 phosphors were synthesized with the addition of NH4Cl flux.When the phosphors were synthesized in a nominal composition of (Ca0.99Eu0.01)3Si2O7 without flux addition,a Ca3Si2O7 phase responsible for reddish-orange emission was identified to coexist with an intermediate phase of a-Ca2SiO4 for green emission.With the addition of NH4Cl flux,a-Ca2SiO4 was suppressed while the pure phase Ca3Si2O7 was obtained as the flux content was 3 wt%.Through varying the amount of flux,the emission color of samples can be tuned from green to reddish-orange,corresponding to the phase transformation from a-Ca2SiO4 to Ca3Si2O7.Through optimizing the doping concentration of Eu2+ ,the optimized photoluminescence (PL) properties for reddish-orange emission can be achieved,which makes this kind of phosphor prospective in the applications of the phosphor-converted white light emitting diodes (PC-WLEDs).
A method of color mixture for white light is presented with Sr3MgSi2O8:Eu2+, Mn2+ shell coated on Sr2SiO4:Eu2+ core by spray pyrolysis procedure. Upon near ultraviolet (NUV) excitation, a 550 nm band emission of Eu2+ from core host combines with the simultaneous emissions of Eu2+ at 457 nm and Mn2+ at 683 nm based on energy transfer in the shell lattice to generate warm white light with color rendering index (CRI) of 91. With such a core-shell-like structure, the re-absorption of blue light from shell layer can be effectively suppressed, and the chemical stability of the phosphor is verified experimentally to be superior to that of the Sr2SiO4:Eu2+. This new proposed phosphor provides great potential in the color mixture of blending-free phosphor converted white NUV light emitting diode (LED) devices.
