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The Sral <sub> 2 </ Sub> O <sub> 4 </ Sub> Long Afterglow Materials, Preparation Of New Technology And Light-emitting Mechanism, And

Posted on:2007-06-30Degree:DoctorType:Dissertation
Country:ChinaCandidate:J HuFull Text:PDF
GTID:1111360185453831Subject:Materials science
Abstract/Summary:PDF Full Text Request
SrAl2O4:Eu2+,Dy3+ system long afterglow materials have been accepted as one of the most important luminescent materials, due to the advantages of its wide excitation spectrum, visible emission range, high luminescent intensity, non-poisonous, non-radioactivity, long afterglow and high stability. The application includes public security, fire fighting, transportation, etc.SrAl2O4:Eu2+,Dy3+ system long afterglow materials can be fabricated by several procedures such as solid phase reaction, sol-gel, combustion synthesis , hydrothermal method, microwave radiation and zone refining. Among these the solid phase reaction method has been widely applied because of its simple scheme and low cost. However it is difficult to obtain uniform single phase long afterglow materials by this method, as a result the fluorescence properties will not be satisfying. The other methods mentioned above are still in laboratory scale due to the complicated operation and high cost. Therefore the hot-spot of this research field will be looking for new synthesis routines to prepare high quality long afterglow materials with high efficiency and low cost.In this work the hydrolysis product of actived Al-Sr alloy is introduced as the precursor of preparing SrAl2O4 long afterglow materials. The precursor is mixed with rare earth oxide Eu2O3 / Dy2O3 and then calcinated at high temperature under reducing atmosphere. High quality long afterglow materials can be fabricated by this modified solid phase reaction.Laser particle size analyzer, BET, TG-DTG, XRD, EPS, SEM and TEM are employed to systematically investigate the size distribution, crystalline structure, microscopic pattern and specific area of the Al-Sr alloy powder under different activating conditions. The hydrolysis reaction and change of pH value during the reaction are also studied. The Al-Sr hydrolysis reaction is studied by analyzing the structure, particle size and specific area of the products, together with the effects of heat treatment on structure of the products. The hydrolyzed composite powder is calcinatedand used as the precursor of SrAl2O4 long afterglow materials. The solid phase reaction parameters are studied , e.g. effects of calcinations temperature, reaction atmosphere, additives and rare earth oxide on the crystal structure and fluorescence properties of SrA^Ci: Eu,Dy materials. A defect model and luminescent mechanism are set up according to the defect chemistry and the lab results.The result shows that, during activating, particle size of the alloy decreases, particle size distribution increase with injection pressure, while the particle size increased when the spraying pressure exceed 3 MPa. The particle size decreased significantly with spinning speed, but the change turns to slow when the speed exceed 6000 rps, and a wider size distribution can be observed.At a spraying pressure of 3MPa, spinning speed of 6000 rps, the Al-Sr alloy powder completely hydrolyses in 35 hours. The reaction can be described as:Sr(s) + 2H2O(1) —? Sr(OH)2(aq) + H2(g) 2Al(s) + 6H2O(1) —?2Al(OH)3(aq) + 3H2(g)The reaction produces hydrogen and heat, with white deposit can be observed. During the reaction the pH value increases from 7 to 13.2, then decreases to 11.1 at the end of reaction. The main hydrolysis product are Sr(OH)2.8H2O and A1(OH)3. The two components consist of evenly mixed sheet-shaped particles in size of 1-3 U m (narrow distribution) , according to EPS and SEM results.The hydrolysis reaction routine is analyzed, shows that Sr in the alloy powders hydrolyzed first, pH increased because of high solubility product of Sr (OH)2. The high pH value promote hydrolysis of Al component and deposition of Al (OH)3 and grow into sheet-shaped crystal. When pH value reaches 12.8, co-deposition of Al (OH)3 and Sr (OH)2 occurred. Sr2+ nucleated on Al (OH)3 deposit and form into needle crystals along Al ( OH) 3 crystal face, according to TEM results.TG— DTG results show that the hydrolysis product lose crystal water at 70- 190°C , the hydroxide start to decompose at 190- 650°C . After 700°Cthe product will be SrO, a -AI2O3 and Y -AI2O3, with the specific area of 122.9 m2/g. The microscopic pattern doesn't change much but particle size distribution enlarged. Under 1200°CT the product will be SrO and a -AI2O3, SrO (AI2O3) 6/SrAUO7 can also be seen. The specific area reaches 25.24 m2/g0The calcinated hydrolysis product can be used as precursor to produce long afterglow SrAl2O4 : Eu,Dy materials. The preparation parameters include sintering temperature, additive quantity, reducing atmosphere and rare earth oxide dopants. Effects of above parameters on crystal structure, microscopic pattern, afterglow and fluorescence properties of SrAl2O4Eu,Dy are demonstrated. By the modified reaction method, under low temperature ( 1300°C ) and less additive ( 2.5wt% H3BO3) , one can prepare long afterglow material with emission wave length of 520nm, initial emission intensity of 1804mcd/m2, afterglow time of 46 hours. The prepared material consists of single SrAl2O4 phase. Comparing with traditional solid phase reaction method, the modified routine has the advantages of less additives, lower sintering temperature, higher initial emission intensity and afterglow time.In the composite SrO-A^Os precursor, thanks to the special hydrolysis procedure of Al-Sr alloy powder, SrO and AI2O3 can be mixed evenly in a microscopic scale, lead to a larger contact area between these two components. Therefore the solid phase reaction can be promoted by a shorter diffusion route. The existence of sintering additive introduces liquid phase into the solid phase system, ion transition can be improved, and thus effectively decline the sintering temperature, eliminate impurity phase, and then ensure a pure SrAl2O4 crystalline phase. This effect also helps rare earth ions to enter Sr2+ lattice point, improve long afterglow fluorescence properties.It is also showed that with increasing H3BO3 , the fluorescence properties of SrAl2O4:Eu,Dy can be improved significantly, while the best performance achieves at 2.5wt%of H3BO3. when the amount of H3BO3 is higher then 3.5wt % , the performance declined rapidly due to theappearance of impurity phase. In the reducing atmosphere, fluorescence performance improves with H2 content. However when the atmosphere consists of 100% H2, the Sr2+vacancy density decreases and O vacancy increases. 0 vacancy may not have a positive effect on the performance of long afterglow materials.Rare earth oxide dopants affect the fluorescence performance of SrAl2O4:Eu,Dy system. For those samples without Eu2O3, the excitation spectrum and emission spectrum show that there is no excitation in the range 270nm—500nm, no emission in the range 400 nm—750nm. With different amount of EU2O3 dopant, samples have almost the same excitation spectrum and emission spectrum, and same excitation / emission peaks. The initial emission intensity and afterglow time increase with amount of EU2O3 dopant. When EU2O3 dopant exceed 0.03mol% , the increasing of performance turns to be slow, and afterglow time decreased.Samples without Dy2C>3 dopant will easily be excited by visible light in the range of 260nm450nm. Those doped by Dy2C>3 keep the same excitation peak but the intensity decreases a lot. This is because Dy3+ changes the crystal field of Eu2+ and thus affect the excitation state of Eu2+. The main emission peak is 520nm, which is the intrinsic peak of Eu2+. No intrinsic peak of Dy3+ can be found in the emission spectrum. The initial emission intensity and afterglow time increase with amount of Dy2O3dopant. Highest initial emission intensity (1804mcd/m2) and afterglow time (46 hours) can be seen at Dy ion concentration of 0.03mol. The result demonstrated that Eu2+ acts as the emission center in the matrix, while Dy3+ enters the matrix lattice, changes crystal field structure and energy level, affects the structure and distribution of trap energy level, and thus affects the fluorescence and afterglow performance of SrAhO^Eu.Dy materials.Based on defect chemistry principles and lab results, the defect chemistry models for systems of Sri.xAl2C>4 , Sri.xA^C^ : xEu2+ , Sri.x.yAl204: xEu2+ , yDy3+ has been set up. In SrAl2O4:Eu,Dy system, EuLacts as the emission center, its concentration and distribution in the matrix lattice determine the main emission peak and initial emissionintensity of the materials. Results of SrAhC^ crystal structure shows that, Eu2+ substitute different Sr2+ ions may lead to two different emission center, one emission peak at 520nm, and the other may not emit due to quenching mechanism. Sr2+ vacancy (y^) w*^ ^e ^e caPturing vacancy (negative charged dominant defects). Concentration, distribution and energy level of the vacancy dominate the afterglow time of the materials. Co-doping of Dy2C>3 significantly modify the defect state of the matrix lattice, increase the controlling defect in Dy system, and concentration of y^ and EuL' therefore improve the afterglow performance of SrAl2C?4:Eu,Dy system. The model can explain the effects of reducing atmosphere and rare earth dopants on the fluorescence properties.The vacancy transportation model of long afterglow mechanism is modified. A new vacancy transportation luminescent model has been built. According to the new mechanism, Eu2+ of the SrAl2O4:Eu,Dy system under illumination will undergo 4f-5d transition, produce a electron-vacancy pair. The electron enters conduction band, the vacancy enters valence band and transport. During transportation the vacancy will be captured by negative charged Sr2 + ion vacancy y^ trap. After illumination, the captured vacancy will be released by heat excitation, transport to emission center, compound with free electron, and then laminate. There are different source for y^: intrinsic defects distribute evenly in the system, y^ coming from EU2O3 dopants locate around defect Eulr? Vsrcom*n8 from Dy2C>3 dopants locate around defect Dy ? Energy levels are diferent, captured vacancy density will be changed largely.
Keywords/Search Tags:Al-Sr alloy, active hydrolysis, long afterglow materials, SrAl2O4:Eu, Dy, luminescent mechanism
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