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The influence of different conditions on the luminescent properties of YAG:Ce phosphor formed by combustion
ARTICLE IN PRESS
Journal of Luminescence 122–123 (2007) 707–709 www.elsevier.com/locate/jlumin
The influence of different conditions on the luminescent properties of YAG:Ce phosphor formed by combustion Zhiping Yanga,, Xu Lib, Yong Yanga, Xingmin Lia a
Physics Science and Technology College, Hebei University, Baoding 071002, China College of Electronic and Informational Engineering, Hebei University, Baoding 071002 China
b
Available online 14 March 2006
Abstract Powder phosphor yttrium aluminum garnet (YAG), activated with trivalent cerium (Ce3+) is synthesized by combustion from mixed metal nitrate reactants and urea with ignition temperature of 500–550 1C. After repeated calcinations at a given temperature for 5 h, purephase phosphor was obtained. The experiment result shows the appropriate temperature for repeated calcinations is 1000 1C. Luminescent intensity at various quantity of urea is studied in detail and the maximum value shown in the result in fact is 2.5 times larger than the theoretical value. In addition, the characteristics of particles such as crystallinity, morphology and photoluminescence were investigated. There are two peaks in its excited spectrum and the major one is a broad band around 470 nm, which matches the blue emission of GaN LED very well. The emission peak locates at about 532 nm, which can combine the blue light of GaN LED to yield white light. r 2006 Elsevier B.V. All rights reserved. Keywords: Combustion; Quantity; Excited spectrum; Emission spectrum
1. Introduction In recent years, light emitting diodes (LEDs) have been used in various fields with the improvement of LEDs, especially in the art of high-brightness LEDs [1–3]. There are three main methods to realize the white LED device. The most common method is to combine the GaN-based blue LED chip and a yellow phosphor, which can be effectively excited by GaN chip. The excited band of powder phosphor yttrium aluminum garnet (YAG) activated by cerium (YAG:Ce) is located around 470 nm, which can change the blue emission of GaN-based LED to wide band emission at longer wavelength range resulting in enough intensity to complement the residual blue emission to yield bright white light. Generally, YAG:Ce is prepared by solid-state reaction using the Y2O3, Al(OH)3 and CeO2. The raw materials are sintered at 1300 1C for 10 h under CO atmosphere, then, sintered at 1500 1C. Grinding is required in order to obtain smaller-size phosphor powders. However, the luminescent Corresponding author. Tel./fax: +86 312 5079423.
E-mail address: [email protected] (Z. Yang). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.266
intensity will reduce in some extent as this results from repeated grinding and milling in this process. For overcoming above-mentioned disadvantages, several direct phosphor synthesis techniques, such as combustion methods, sol–gel, and co-precipitation, have received more attention [4–6]. In this paper, powder phosphors of YAG:Ce are synthesized by combustion from mixed metal nitrate reactants and urea. Using combustion method, the lower starting temperature and shorter reaction time are needed than solid-state method. The raw materials are mixed in liquid phases so that better homogeneity solution can be obtained. Luminescent intensity under various quantity of urea is studied in this investigation.
2. Experimental In the following experiment, 99.99%Y(NO3)3 6H2O and 99.99%Al(NO3)3 9H2O are used as the source of Y3+ and Al3+, respectively, 99.99%Ce(NO3)3 6H2O is used as source of Ce, which is the excitant of YAG:Ce. The
ARTICLE IN PRESS 708
Z. Yang et al. / Journal of Luminescence 122–123 (2007) 707–709
urea is used as the reductant in this oxidation-resistant reaction. 2.1. Synthesis of YAG:Ce by combustion method with urea Powder phosphors of Y3xAl5O12: Cex where x is 0.005 is prepared by combustion reaction. The reactants consisting of Y(NO3)3 6H2O, Al(NO3)3 9H2O, Ce(NO3)3 6H2O are weighted in appropriate stoichiometric ratio and dissolved in the crucible with some de-ionized water. The reactants need to be stirred for a few minutes in order to obtain homogeneous solution. The mixture is transferred into a water extractor and dried so that only a little water exists in it, and then the dried mixture was put into a electric muffle furnace which had been heated to 500 1C. There is an oxidation–reduction reaction, generating much smoke of chemicals such as NO2, NH4 and CO2 during the heating process. A bright flame is observed at the same time. This reaction cannot finish until the solution becomes opaque and foamy. After the precursor is dried, it is milled gently and calcined at 1000 1C in air for several hours then efficient phosphor is obtained.
Fig. 1. XRD patterns of 0.5% Ce-doped YAG prepared by combustion with different conditions: (a) x ¼ 1 and (b) x ¼ 2:5. x denoting the ratio of urea to Y3+ (The molecular ratio of urea to Y3+is denoted as x).
2.2. Characterization The products are characterized by using powder X-ray diffraction (XRD) collected by a D/max-rA X-ray Diffractometer using Cu Ka radiation at 40 kV and 100 mA. The excitation spectrum and the emission spectrum are measured using a SHIMADZU RF-540 ultraviolet spectrophotometer and a SPEX1404 spectrophotometer, respectively. All the luminescence properties of the phosphors are studied at room temperature. 3. Result and discussion 3.1. Crystal structure of product 3.1.1. X-ray powder diffraction The XRD patterns in Fig. 1 shows that the samples with different quantity of urea have similar XRD patterns, which are in good agreement with JCPDS Card (no.33–40). It can be proved that the various quantity of urea in raw materials cannot alter the crystal structure of pure YAG. Also, we can see from the XRD patterns that the characteristic peak of the sample with the proportion between urea and Y3+ is 2.5 is stronger than the other. As we all know, pure YAG phase can be obtained at relatively low temperature, such as 1000 1C in air by calcining the precursor in combustion with urea because of the well distribution of metal ions in the uniformed cross-link in the precursor. 3.1.2. Morphology and size of YAG:Ce phosphors Phosphor in foamy, porous agglomeration is observed using a SEM, see Fig. 2. The shape of the product particles changes slightly with altering the molar ratio of Y3+ ions
Fig. 2. SEM photograph of YAG:Ce with 0.5% Ce3+ prepared at 1000 1C for 5 h in air.
to urea ions. The formation of the porous morphology of the products is caused by the gases, such as N2, CO2 and oxides of nitrogen produced during the combustion of urea. The brittle products are easily grounded to fine powders. 3.2. Photoluminescence property of YAG:Ce It can be seen from the Fig. 3 that three excitation bands appear with peak wavelengths at 233, 340, 470 nm, respectively. There is one electron in the 4f state of Ce3+, and the ground state of Ce3+ is split into 2F7/2 and 2F5/2. The major excited spectrum peak locates at 470 nm, which matches the blue emission of GaN LED very well. The emission peak locates at about 532 nm, which can combine the blue light of GaN LED to yield white light.
ARTICLE IN PRESS Z. Yang et al. / Journal of Luminescence 122–123 (2007) 707–709
709
a a b c d e f g
b c
Emission Intensity (a.u.)
Intensity (a.u.)
Excition
d
n=2.5 n=3 n=2 n=4 n=1 n=0.5 n=6
e f g
450 200
300
400 500 Wavelength (nm)
600
500
700
550 600 Wavelength (nm)
650
Fig. 4. Variation of the luminescent intensity with the quantity of urea.
A good chance is offered to the combination of the yellow and blue emission because of the existance of a wide emission band from blue to red in the YAG:Ce phosphor. It is well known that the emission at 532 nm is attributed to the 5d–4f transition of Ce3+, which depends strongly on the crystal field. 3.3. Other process conditions affecting the luminescent properties 3.3.1. The effect of quantity of urea It can be seen from the Fig. 4 that the intensity of emission varies with the quantity of urea and reaches the maximum value at the ratio of urea to Y3+ 2.5. However, the position of spectrum peaks has a little change with the ratio of urea to Y3+.
Intensity(a.u.)
Fig. 3. Photoluminescence spectra of 0.5% Ce-doped YAG prepared by combustion. Excitation spectrum (lem ¼ 532 nm) and emission spectrum (lex ¼ 470 nm).
2
4
6 time of sintering (h)
8
10
Fig. 5. Variation of the luminescent intensity with the calcining time.
3.3.2. The effect of calcining time In order to improve the crystallinity of YAG particles and favor the doping Ce ions into YAG lattice, the calcining temperature was increased. Consequently the luminescent intensity is improved. Fig. 5 shows that the luminescent intensity of YAG:Ce reaches its maximum when the calcining is 5 h and the temperature is 1000 1C. 4. Conclusion We have synthesized YAG:Ce with different quantity of urea by combustion method. The spectrum peaks have a little change with the ratio variation of urea to Y3+ although the intensity of emission changes clearly. Calcination of the precursor can promote the luminescence intensity because it improves the crystallinity of YAG
particles and favors the doping Ce ions into YAG lattice. The optimized calcinating condition is calcining for 5 h at 1000 1C. References [1] Y. Pan, M. Wu, Q. Su, J. Mater .Sci. Eng. B 106 (2004) 251. [2] Y. Pan, M. Wu, Q. Su, J. Phys. chem. Solid 65 (2004) 845. [3] N. Narendran, Y. Gu, J.P. Freyssinier, H. Yu, L. Deng, J. cryst. Grow. 268 (2004) 449. [4] J. Bang, M. Abboudi, B. Abrams, P. Holloway, J. Lumin. 106 (2004) 177. [5] X. Yu, C. Zhou, X. He, Z. Peng, S. Yang, J. Mater. Lett. 58 (2004) 1087. [6] S.K. Shi, J.Y. Wang, J. Alloys. Compounds. 327 (2001) 82.
Journal of Luminescence 122–123 (2007) 707–709 www.elsevier.com/locate/jlumin
The influence of different conditions on the luminescent properties of YAG:Ce phosphor formed by combustion Zhiping Yanga,, Xu Lib, Yong Yanga, Xingmin Lia a
Physics Science and Technology College, Hebei University, Baoding 071002, China College of Electronic and Informational Engineering, Hebei University, Baoding 071002 China
b
Available online 14 March 2006
Abstract Powder phosphor yttrium aluminum garnet (YAG), activated with trivalent cerium (Ce3+) is synthesized by combustion from mixed metal nitrate reactants and urea with ignition temperature of 500–550 1C. After repeated calcinations at a given temperature for 5 h, purephase phosphor was obtained. The experiment result shows the appropriate temperature for repeated calcinations is 1000 1C. Luminescent intensity at various quantity of urea is studied in detail and the maximum value shown in the result in fact is 2.5 times larger than the theoretical value. In addition, the characteristics of particles such as crystallinity, morphology and photoluminescence were investigated. There are two peaks in its excited spectrum and the major one is a broad band around 470 nm, which matches the blue emission of GaN LED very well. The emission peak locates at about 532 nm, which can combine the blue light of GaN LED to yield white light. r 2006 Elsevier B.V. All rights reserved. Keywords: Combustion; Quantity; Excited spectrum; Emission spectrum
1. Introduction In recent years, light emitting diodes (LEDs) have been used in various fields with the improvement of LEDs, especially in the art of high-brightness LEDs [1–3]. There are three main methods to realize the white LED device. The most common method is to combine the GaN-based blue LED chip and a yellow phosphor, which can be effectively excited by GaN chip. The excited band of powder phosphor yttrium aluminum garnet (YAG) activated by cerium (YAG:Ce) is located around 470 nm, which can change the blue emission of GaN-based LED to wide band emission at longer wavelength range resulting in enough intensity to complement the residual blue emission to yield bright white light. Generally, YAG:Ce is prepared by solid-state reaction using the Y2O3, Al(OH)3 and CeO2. The raw materials are sintered at 1300 1C for 10 h under CO atmosphere, then, sintered at 1500 1C. Grinding is required in order to obtain smaller-size phosphor powders. However, the luminescent Corresponding author. Tel./fax: +86 312 5079423.
E-mail address: [email protected] (Z. Yang). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.266
intensity will reduce in some extent as this results from repeated grinding and milling in this process. For overcoming above-mentioned disadvantages, several direct phosphor synthesis techniques, such as combustion methods, sol–gel, and co-precipitation, have received more attention [4–6]. In this paper, powder phosphors of YAG:Ce are synthesized by combustion from mixed metal nitrate reactants and urea. Using combustion method, the lower starting temperature and shorter reaction time are needed than solid-state method. The raw materials are mixed in liquid phases so that better homogeneity solution can be obtained. Luminescent intensity under various quantity of urea is studied in this investigation.
2. Experimental In the following experiment, 99.99%Y(NO3)3 6H2O and 99.99%Al(NO3)3 9H2O are used as the source of Y3+ and Al3+, respectively, 99.99%Ce(NO3)3 6H2O is used as source of Ce, which is the excitant of YAG:Ce. The
ARTICLE IN PRESS 708
Z. Yang et al. / Journal of Luminescence 122–123 (2007) 707–709
urea is used as the reductant in this oxidation-resistant reaction. 2.1. Synthesis of YAG:Ce by combustion method with urea Powder phosphors of Y3xAl5O12: Cex where x is 0.005 is prepared by combustion reaction. The reactants consisting of Y(NO3)3 6H2O, Al(NO3)3 9H2O, Ce(NO3)3 6H2O are weighted in appropriate stoichiometric ratio and dissolved in the crucible with some de-ionized water. The reactants need to be stirred for a few minutes in order to obtain homogeneous solution. The mixture is transferred into a water extractor and dried so that only a little water exists in it, and then the dried mixture was put into a electric muffle furnace which had been heated to 500 1C. There is an oxidation–reduction reaction, generating much smoke of chemicals such as NO2, NH4 and CO2 during the heating process. A bright flame is observed at the same time. This reaction cannot finish until the solution becomes opaque and foamy. After the precursor is dried, it is milled gently and calcined at 1000 1C in air for several hours then efficient phosphor is obtained.
Fig. 1. XRD patterns of 0.5% Ce-doped YAG prepared by combustion with different conditions: (a) x ¼ 1 and (b) x ¼ 2:5. x denoting the ratio of urea to Y3+ (The molecular ratio of urea to Y3+is denoted as x).
2.2. Characterization The products are characterized by using powder X-ray diffraction (XRD) collected by a D/max-rA X-ray Diffractometer using Cu Ka radiation at 40 kV and 100 mA. The excitation spectrum and the emission spectrum are measured using a SHIMADZU RF-540 ultraviolet spectrophotometer and a SPEX1404 spectrophotometer, respectively. All the luminescence properties of the phosphors are studied at room temperature. 3. Result and discussion 3.1. Crystal structure of product 3.1.1. X-ray powder diffraction The XRD patterns in Fig. 1 shows that the samples with different quantity of urea have similar XRD patterns, which are in good agreement with JCPDS Card (no.33–40). It can be proved that the various quantity of urea in raw materials cannot alter the crystal structure of pure YAG. Also, we can see from the XRD patterns that the characteristic peak of the sample with the proportion between urea and Y3+ is 2.5 is stronger than the other. As we all know, pure YAG phase can be obtained at relatively low temperature, such as 1000 1C in air by calcining the precursor in combustion with urea because of the well distribution of metal ions in the uniformed cross-link in the precursor. 3.1.2. Morphology and size of YAG:Ce phosphors Phosphor in foamy, porous agglomeration is observed using a SEM, see Fig. 2. The shape of the product particles changes slightly with altering the molar ratio of Y3+ ions
Fig. 2. SEM photograph of YAG:Ce with 0.5% Ce3+ prepared at 1000 1C for 5 h in air.
to urea ions. The formation of the porous morphology of the products is caused by the gases, such as N2, CO2 and oxides of nitrogen produced during the combustion of urea. The brittle products are easily grounded to fine powders. 3.2. Photoluminescence property of YAG:Ce It can be seen from the Fig. 3 that three excitation bands appear with peak wavelengths at 233, 340, 470 nm, respectively. There is one electron in the 4f state of Ce3+, and the ground state of Ce3+ is split into 2F7/2 and 2F5/2. The major excited spectrum peak locates at 470 nm, which matches the blue emission of GaN LED very well. The emission peak locates at about 532 nm, which can combine the blue light of GaN LED to yield white light.
ARTICLE IN PRESS Z. Yang et al. / Journal of Luminescence 122–123 (2007) 707–709
709
a a b c d e f g
b c
Emission Intensity (a.u.)
Intensity (a.u.)
Excition
d
n=2.5 n=3 n=2 n=4 n=1 n=0.5 n=6
e f g
450 200
300
400 500 Wavelength (nm)
600
500
700
550 600 Wavelength (nm)
650
Fig. 4. Variation of the luminescent intensity with the quantity of urea.
A good chance is offered to the combination of the yellow and blue emission because of the existance of a wide emission band from blue to red in the YAG:Ce phosphor. It is well known that the emission at 532 nm is attributed to the 5d–4f transition of Ce3+, which depends strongly on the crystal field. 3.3. Other process conditions affecting the luminescent properties 3.3.1. The effect of quantity of urea It can be seen from the Fig. 4 that the intensity of emission varies with the quantity of urea and reaches the maximum value at the ratio of urea to Y3+ 2.5. However, the position of spectrum peaks has a little change with the ratio of urea to Y3+.
Intensity(a.u.)
Fig. 3. Photoluminescence spectra of 0.5% Ce-doped YAG prepared by combustion. Excitation spectrum (lem ¼ 532 nm) and emission spectrum (lex ¼ 470 nm).
2
4
6 time of sintering (h)
8
10
Fig. 5. Variation of the luminescent intensity with the calcining time.
3.3.2. The effect of calcining time In order to improve the crystallinity of YAG particles and favor the doping Ce ions into YAG lattice, the calcining temperature was increased. Consequently the luminescent intensity is improved. Fig. 5 shows that the luminescent intensity of YAG:Ce reaches its maximum when the calcining is 5 h and the temperature is 1000 1C. 4. Conclusion We have synthesized YAG:Ce with different quantity of urea by combustion method. The spectrum peaks have a little change with the ratio variation of urea to Y3+ although the intensity of emission changes clearly. Calcination of the precursor can promote the luminescence intensity because it improves the crystallinity of YAG
particles and favors the doping Ce ions into YAG lattice. The optimized calcinating condition is calcining for 5 h at 1000 1C. References [1] Y. Pan, M. Wu, Q. Su, J. Mater .Sci. Eng. B 106 (2004) 251. [2] Y. Pan, M. Wu, Q. Su, J. Phys. chem. Solid 65 (2004) 845. [3] N. Narendran, Y. Gu, J.P. Freyssinier, H. Yu, L. Deng, J. cryst. Grow. 268 (2004) 449. [4] J. Bang, M. Abboudi, B. Abrams, P. Holloway, J. Lumin. 106 (2004) 177. [5] X. Yu, C. Zhou, X. He, Z. Peng, S. Yang, J. Mater. Lett. 58 (2004) 1087. [6] S.K. Shi, J.Y. Wang, J. Alloys. Compounds. 327 (2001) 82.