Study on Chemical Coprecipitation Synthesis of Sr4Al14O25: Eu～(2+), Dy～(3+) Long Afterglow Phosphors

Study on Chemical Coprecipitation Synthesis of Sr4Al14O25: Eu～(2+), Dy～(3+) Long Afterglow Phosphors
　　The aluminate-based phosphors have intensely interested people becausethey have excellent properties such as high luminescent efficiency, stablechemical properties. Before the 1970s the most phosphors studied were silicate,phosphate and sulfide. Because it was easy to get high-purity silicate, alkalineearth carbonate and alkaline earth phosphate but lack of high-purityaluminium oxide in fine grain size. In 1974 Dutch Verstegen synthesized(BaMgEu)3Al16O27 blue phosphor activated by rare earth, this made the usageof phosphors used as by fluorescent lamp and promoted the development ofaluminate based phosphors with long afterglow.At present, CaAl2O4:Eu,Nd, SrAl2O4:Eu,Dy and Sr4Al14O25:Eu,Dyprepared by solid state method have been applied in practical living. Block Neodymium magnets Hightemperature solid state method is the earliest and the most common method toprepare the strontium aluminates based phosphors with long afterglow, and itis the only method to realize commercial process. The preparing ofSr4Al14O25:Eu,Dy is mostly via solid state method. But this method has itselfdefect.
　　The sintering temperature is higher, from 1300℃ to 1600℃, sometimesit can arrive 1700℃;The reaction needs long time, about 6 to 8 hours;Thephosphor has large grain size , high density, high hardness. In the practicalapplication, the phosphors have to be ground and this impossibly destroy thecrystal lattice structure, so it can impact luminescent brightness and afterglowproperties, even make luminescent properties descend obviously. In order toovercome these defect of high temperature solid state method, in this paper wesynthesized the precursor of phosphor via chemical coprecipitation and thenthe precursor was sintered in reducing atmosphere for a period of time, coolingin the high temperature furnace, at last the phosphor Sr4Al14O25:Eu,Dy wasprepared.The paper consists of five sections, including introduction, experimental,study of precursor, results and discussion of Sr4Al14O25:Eu,Dy phosphor andconclusions.After discussing the effect of PH value and ageing time on the phasecomposition of the precursor, we determined the optimal technologicalconditions. The PH value of system should be 10 and the ageing time shouldbe 24 hours. Under these conditions, the precursor synthesized was SrCO3 andNH4AlO(OH)HCO3. In the paper we investigated the sorts of factors whicheffect the phase composition and the luminescent performance ofSr4Al14O25:Eu,Dy phosphor with long afterglow, such as the mole ratio ofSr/Al, sintering temperature, sintering time, the quantity of H3BO3 andactivator. The interrelated conclusion in this paper is come to as follows.The phase of phosphors change from Sr3Al2O6 、SrAl2O4 、Sr4Al14O25 、SrAl12O19 with varying the mole ratio of Sr(NO3)2/AlCl3·6H2O from 1/0.6 to1/4.4.The phase of phosphor is Sr4Al14O25 without any other crystalline phaseswhen the mole ratio is 1/3.2.We synthesized phosphors at different temperature and analyzed theirX-ray diffraction patterns and excitation spectrum and emission spectrum. Theresults show that the optimal sintering temperature is1200℃ to synthesizeSr4Al14O25:Eu,Dy phosphor which has the highest luminescent intensitywithout any other crystal phase. During the sintering, the optimal sinteringtime is 3 hours. On this condition, the luminescent properties of the phosphorare best, being Sr4Al14O25 crystal phase, having the highest luminescentintensity. As are indicated by the X-ray http://www.999magnet.com/ diffraction patterns and emissionspectra.The role of H3BO3 addition on the phosphors has been investigated.H3BO3 acts as solvent and with increasing the concentrations of H3BO3, thecrystalline phase and luminescent properties of phosphors are changed. Whenthe concentration is 46%, the phosphor belongs to Sr4Al14O25 crystallinephase with the highest luminescent intensity, according to X-ray diffractionand emission spectra results.
　　The effect of Eu2+ concentration on the luminescent properties ofSr4Al14O25:Eu,Dy was discussed by changing the mole ratio of Eu2+ / Sr2+. Theresults show the relative intensity of tow emission peaks in the emissionspectrum transfers when the mole ratio of Eu2+ / Sr2+ changes. When the moleratio is less than 0.008, two emission peaks exist in the emission spectrum, oneat about 410nm, the other at about 490nm. With increasing the mole ratio, theformer lowers and the latter heighten. The two main emission peaks ofSr4Al14O25:Eu,Dy belong to two luminescent center respectively, which areEu1 and Eu2. They both have a concentration quench process,in which theemission intensity increase first, and then weaken. The critical concentrationof Eu1 is at Eu2+/Sr2+= 0.006/1, and Eu2 at Eu2+/Sr2+=0.012/1. Dy3+ was addedas auxiliary activator, which improved the luminescent properties of phosphors.With the increasing of Dy3+ concentration appropriately, the initial luminescentintensity of phosphor will descend. After the excited light source was removed,the phosphors with high concentration Dy3+ have high luminescence intensityand long afterglow time.Sr4Al14O25:Eu2+,Dy3+ phosphors with long afterglow were synthesizedsuccessfully via chemical coprecipitation method. The sintering temperaturewas lower than that of solid state method obviously and the sintering time wasshortened greatly. http://www.999magnet.com/ The phosphor with small size particle, homogenousgranularity can be applied in many fields. The luminescent properties do notdescend obviously after grinding because grinding can hardly destroy thelattice structure. So it is a kind of soft chemical preparation method withenormous prospects to prepare phosphors with long afterglow via chemicalcoprecipitation method. This method can be easily applied in commercial run,but the initial brightness of phosphor synthesized by this method is lower thanthat by solid state method, which needs being studied further.