Preparation and photoluminescence study of Alumin o-boro silicate YAG uorescent glass for White LED

The YAG: Eu 3+ uorescent glass for NUV(near ultraviolet) white LEDs was obtained rstly by synthesizing Y 3 Al 5 O 12 (YAG): Eu 3+ precursors through a simple co-precipitation method, and then mixing precursor with B 2 O 3 -Al 2 O 3 -SiO 2 -Na 2 O-BaO glass powder calcined at 1400°C for 2.5 hours. The as-prepared YAG glass-ceramic phosphor was investigated by DTA, XRD, SEM, and photoluminescence (PL). Inuence of YAG: Eu 3+ precursor and Eu 3+ doping on excitation and emission spectra also have been studied. The results show that: the phosphor's emission peak located at 393nm is correspond to the 7 F 0 - 5 L 6 transition of Eu 3+ ions, which matches good with UV LED chips; the phosphor gives intense emission at 593nm originating from the 5 D 0 - 7 F 1 transition of Eu 3+ ions. When the YAG precursor is 0.9g and amount of Eu 3+ doped is 0.08, the uorescence excitation spectra of glass and emission spectra of the peak intensity reach its maximum value. The YAG: Eu 3+ uorescence glass could be a promising material for the production of near ultraviolet chip white light-emitting diode.


Introduction
White light emitting diode light source with a low driving voltage, applicability, high stability, short response time, mercury and lead pollution, monochromatic light etc., will be the most promising lightning source, which will replace the incandescent, uorescent, high intensity gas discharge lamp in the 21 century [1,2]. Currently people use blue LED chip coated with yellowish phosphor of YAG to achieve white LED, its color temperature is usually higher than 5000K, the general color rendering index also is less than 80. Optical parameter of white LEDs will change after a period time of using as the light-emitting chip and phosphors have different life span; additionally, when the phosphor is heated by LED chips, it will accelerate phosphor deterioration, luminousness of organic encapsulating material also will decline, making the life span of white LED decrease [3,4,5]. Although RGB LED chips of white LED have relatively wide and low color temperature range, it can easily achieve high color rendering index, but the RGB chips have different light attenuation coe cient, it needs to add appropriate control circuit [1,6,7].
Using near-UV chips as excitation source to excite tribasic uorescent powder has become a n international hot spot recently [8,9,10]. As the visual sensitivity of the near-UV is not obvious to human, we can achieve high color rendering index in a large range of color temperature. Near-UV excitation white LED is considered to lead new generation of white LED lighting direction [11,12]. YAG is widely used as laser and light-emitting substrate material for its excellent physical and chemical properties [13]; YAG phosphors doped with Eu 3+ ions have a strong excitation peak in 395nm, which matches good with exciting wave band of near-UV LED chips and emit bright orange-red shade. The phosphor uorescent glass makes excellent heat distribution in the glass material, which approximately equal to phosphor remote technology. Fluorescent glass has following advantages: simple production process, high light e ciency, low luminescence decay [14,15,16,17]. In this work we have prepared YAG: Eu 3+ uorescence glass in B 2 O 3 -Al 2 O 3 -SiO 2 -Na 2 O-BaO system, its spectral properties, light-emitting properties also have been studied. First yttrium oxide and europium oxide were dissolved in dilute nitric acid respectively to obtain yttrium nitrate, europium nitrate and aluminum nitrate with concentration of 1mol/L metal ions solution. Then mix the above three kinds of solutions according to yttrium aluminum garnet Y 3-x Eu x Al 5 O 12 (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1) stoichiometric. Prepare 100mL mixed precipitant with the composition of NH 3 ·H 2 O 2mol/L and NH 4 HCO 3 2mol/L, then mix the three metal ion source by fap funnel with 3mL/min into mixed precipitator with constant temperature at 30℃. Adjust mixture pH to 6.5 with ammonia swap around, then continue stirring 2h, aging for 12h; Discard the supernatant after centrifugation, wash precipitate precursor with deionized water four times, with ethanol three times, then put precipitate precursor into constant temperature oven with 120℃ for 12h, obtain YAG: Eu 3+ precursors by grinding at last [16].

Characterization of YAG: Eu 3+ Glass-ceramic
Sample's crystal structure was scanned by ARL TRA X-ray diffraction (XRD). YAG precursor, the glass base and their mixture were analyzed with differential thermal analysis (DTA) by Netzsch STA-449F3 thermal analyzer. Sample morphology was scanned with Hitachi S3400 scanning electron microscope (SEM). Sample's excitation and emission spectra were measured by RF-5301PC uorescence spectrophotometer.
Results And Discussion 3.1 DTA-TGA analyze of YAG: Eu 3+ uorescent glass Fig.1 shows the DTA-TGA curves of precursor during the calcination process. The precursor sample was synthesized by 1.74g H 3 BO 3 -SiO 2 -Al 2 O 3 -Na 2 CO 3 -BaCO 3 adding 0.9g Y 2.94 Eu 0.06 Al 5 O 12 by co-precipitation method. We can see that YAG precursor materials and glass initial homogeneous mixture loss 38% weight from room temperature to 300℃, there is an endothermic peak at 68.4℃ corresponding to the initial precursor materials and glass adsorption water loss; and there is a sharp endothermic peak at 203.8℃, corresponding to the apparent weight loss TGA curve, which is the initial glass material loss of the crystal water. From 300℃ to 1400℃, there are about 40% weight loss and endothermic peak appeared separately at 662.7℃, 818.2℃ and 938.7℃, etc. There is more than one endothermic peak, which ascribe to the decomposition of boric acid and carbonate compounds, subsequent exothermic peak and formation of YAG crystal phase.
3.2 XRD analyze of YAG: Eu 3+ uorescent glass The Y 2.94 Eu 0.06 Al 5 O 12 uorescent glass was synthesized by 1.74g glass material and 0.9g precursor, its XRD patterns was correspond with Y 3 Al 5 O 12 standard map (PDF:79-1892). We can certi cate that the crystal in glass-ceramic was YAG, which belong to body-centered cubic crystal system. The diffraction have relatively sharp peaks, which indicate that a small amount of rare earth ions Eu 3+ replaces Y of the crystal lattice and crystallization of YAG phase was well mixed, and it does not affect the formation of YAG phase [15,16,18]; Fig.2 shows the XRD patterns of glass material without YAG: Eu 3+ glass precursor. There are not obvious diffraction peaks in the photo, which certi cated that products have amorphous form. Fig.3 and Fig.4 show the excitation and emission spectra of uorescent glass with different quality (0.7g, 0.8g, 0.9g, 1.0g and 1.1g) of Y 2.94 Eu 0.06 Al 5 O 12 precursor. The excitation spectra of YAG: Eu 3+ uorescence glass was detected at wavelength of 593nm, test range was 350 ~ 420nm. The strongest excitation peak was at 393nm, correspond to 7 F 0 -5 L 6 transition of Eu 3+ ions, which belong to Eu 3+ Characteristic absorption lines. The excitation spectra matches good with exciting wave band of near-UV LED chips, the other weak excitation peaks were 7 D 0 -5 G 2 transition (peak at 381nm) and 7 F 0 -5 D 3 transition (peak at 416nm), etc. [17].

Fig.3 Excitation spectra of uorescent glass with different content of precursor
Emission spectra was detected at wavelength of 393nm, test the range was 575 ~ 635nm. The strongest peak appears at 593nm ( 5 D 0 -7 F 1 ), which belong to the same transition with 5 D 0 -7 F 1 transition at 598nm, emission spectra was orange. There are also two weak emission peak at 611nm and 631nm, correspond to the 5 D 0 -7 F 2 transition and the 5 D 0 -7 F 4 transition. Symmetrical Y 3+ ions were replaced by Eu 3+ ions In YAG crystal, so the emission peak is based on the magnetic dipole 5 D 0 -7 F 1 transition (at 593nm) [17].
The peak wavelength of excitation and emission spectra of YAG uorescent glass with different content of YAG precursor almost not change, but the spectra intensity have changed.
When the content of YAG precursor is between 0.7 ~ 0.9g, the intensity of excitation and emission peaks increased; while the content of YAG precursor is between 0.9 ~ 1.1g, the intensity of excitation and emission peaks decreased. When the content of YAG precursor is 0.9g, the intensity of excitation and emission peaks reaches their maxium value. The change of precursor content lead to the excitation and emission peaks rise and fall as the initial mixture of materials and YAG glass precursor composition contains Al 2 O 3 . When the YAG precursor content decreased, during the high-temperature calcination process, Al 2 O 3 in precursor will partly enter the glass base and become glass components, resulting in YAG crystal phase formation reducing. When the content of YAG: Ce 3+ precursor is more than 0.9g, it led glass formation process decline. When the content of YAG: Ce 3+ precursor is 1.0 and 1.1g, the vitreous loss through impact excitation and emission light reaches phosphor escape uorescent glass surface, resulting in excitation and emission spectra intensity decreased [16] It can be seen from the Fig.5 and Fig.6 that the excitation and emission spectra of the sample is composed by a set of spikes. As the x value increases, the diffraction peaks corresponding to the wavelength unchanged, only the diffraction peak intensity increased, and no new diffraction peaks appears follow the sharp peak. When the x value is less than 0.08, the sample excitation and emission intensity will increase as x increases; when x is greater than 0.08, the sample excitation and emission intensity will decrease as the x value increases; when the amount of the x value is equal to 0.08Eu 3+ , the excitation and emission spectra of the YAG: Eu 3+ glass reach the maximum value, the peak value correspond to the 7 F 0 -5 L 6 and the 5 D 0 -7 F 1 transition.
When the x value is less than 0.08, Eu 3+ ions will enter into the stroma and replace Y 3+ ions as electron traps. The number of luminescent centers will increase with the Eu 3+ ions concentration increases, it will make the electron trap in electron impact ionization of luminescence centers and luminescence centers with radiation or recombination probability of luminescence centers increase. When the x value is greater than 0.08, the excitation energy will deliver to the Eu 3+ ions. The high energy will not stop in a step and energy transfer will proceed continuously due to the energy transfer between Eu 3+ ions is very big. This will take excitation energy far away from the absorption site, energy migration appears. When the excitation energy reach the non-radiative energy (extinction or quenching site), the system luminous e ciency will decline due to the concentration quenching [19,20,21].

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The YAG: Eu 3+ uorescent glass was obtained rstly by synthesizing Y 3 Al 5 O 12 (YAG): Eu 3+ precursors through a simple co-precipitation method, and then mixing YAG: Eu 3+ precursor with B 2 O 3 -Al 2 O 3 -SiO 2 -Na 2 O-BaO glass powder calcined at 1400℃. The content of YAG precursor and Eu 3+ ions doped in initial materials do not affect the uorescence excitation and emission spectra of peak wavelength, but the value of the peak wavelength; when the YAG precursor is 0.9g and amount of Eu 3+ doped is 0.08, the uorescence excitation spectra of glass and emission spectra of the peak intensity reach its maximum value. The uorescence excitation peak of the YAG: Eu 3+ uorescence glass is at 593nm, the emission peak is located at 393nm, and they basically have the same spectral characteristics with YAG: Eu 3+ phosphors. The YAG: Eu 3+ uorescence glass could be a promising material for the production of near ultraviolet chip white light-emitting diode.