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Available online at w.sciencedirect.com ScienceDirect JOURYAL OF KARE EARTHS 25 (2007) 11 - 14 www .elsevier. codlocate/jre Research on Yz03 : Eu Phosphor Coated with In203 Hu xlaoshan ($flfl%%), Huang Xiaowei (XJJG), Hu Yunsheng (#S&), Zh- Weidong (&I%)* ( .\.ational Enrlneuing Research Center f.r Rare Earth Materials , General Research Institute for Nonferrous Metals , and Grirem 4,il~anrecf Zlntwials Co. Ltd. , Reijing 100088, China) Recei\ed 10 Jaiiiian 2006; reiised 20 September 2006 Abstract: '1-203 : Eu red phosphor for FED application was prepared by high temperature solid-state reaction. The In203 coating b!- precipitation method to the phosphor was applied and the analyses of X R D , Zeta potential, SEM , EUS and low voltage cathodoluminescence (CL) were conducted for investigating the coating effect. The results showed that In203 coat- ing promoted I he low- voltage CL of the phosphor efficiently. The promotion was possibly due to the enhancement of the surface condul.tivity of the phosphor grains. Key words : I-ED: Yz03 : Eu : 111203 : Zeta potential ; coating; rare earths CLC number,: 0482.3 1 Document code: ii Article ID: 1002 - 0721 (2007 101 - 001 1 - 04 As a traditional red phosphor, Y203 : Eu is widely used in the field of display (electroluminescence) and lighting ( photoluminescence ) ~' . In the field of dis- play, field e,nission display (FED) is a kind of prom- ising flat dis day for its small bulk, light weight, fast response speed, wide visual angle, long life, high temperature ind radiation resistance ' . Phosphc.rs for FED are usually operated under low voltage( 300 - 10 kV) and high current ( 10 - 100 ,uA - cm -:) . The excitation mechanisni is low-voltage cathodolumi*irscenoe-' . Kith the development of FED devices. tht properties of phosphors for FED have been a very important factor to improve the properties of FED. -It present, most of the phosphors for FED are used for reference from traditional phosphors for CRT. in which the red phosphor adopts YzOzS: Eu or Yz03 Eu . 'YIOzS : Eu phosphor offers high luminous efficiency a id low resistance. However, there are problems niih device lifetime. During the operation of low voltage and high current of FED. sulfur may leak from the phcsphor and dispers~ into the vacuum, cent- .. ~__~ ~ * Corresponding author ( E-mail : wdzhuange 126.cum) aminating the cathode and thus hindering electron emission, even shortening device lifetime'4' . The other red phosphor, Y203 Eu has desirable stability in the operating circumstance of FED. However, it is insula- tor, and it does not emit secondary electron when ex- cited by low-energy electron beamr5]. The electron emitted by electron gun is prone to accumulating on the surface of the phosphor, which will reduce the ex- cited ability of electron beam. So there is a demand for FED to improve the conductivity of Y203 Eu'~' . In this paper, Y203 : Eu red phosphor for FED was prepared by traditional solid-state reaction, In203, a semiconductor material, was introduced to improve its conductivity. h203 was coated on the sur- face of Y203 : Eu by precipitation. The coating condi- tion was discussed in details. 1 Experimental Red emitting phosphor Y203 : ELI was prepared by high temperature solid-state reaction. The starting ma- Foundation item: Projrct .upported b\ the National haturd Science Foundation of China (50272013, 50372086) Biography : Hu Xinmhan ( 1981 - ), Female, Master candidate Copyigk t 92007. b!- Editorial Committee of Journal of the Chinese Rare Earths Society. Published by Elsevier B. V. All rights reserved

Research on Y2O3:Eu Phosphor Coated with In2O3

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Page 1: Research on Y2O3:Eu Phosphor Coated with In2O3

Available online at w.sciencedirect.com

ScienceDirect JOURYAL OF KARE EARTHS 25 (2007) 11 - 14 www .elsevier. codlocate/jre

Research on Y z 0 3 : Eu Phosphor Coated with In203 Hu xlaoshan ( $ f l f l % % ) , Huang Xiaowei (XJJG), Hu Yunsheng (#S&), Zh- Weidong (&I%)* ( .\.ational Enrlneuing Research Center f . r Rare Earth Materials , General Research Institute for Nonferrous Metals , and Grirem 4,il~anrecf Zlntwials Co. Ltd . , Reijing 100088, China)

Recei\ed 10 Jaiiiian 2006; reiised 20 September 2006

Abstract: ' 1 -203 : Eu red phosphor for FED application was prepared by high temperature solid-state reaction. The In203

coating b!- precipitation method to the phosphor was applied and the analyses of XRD, Zeta potential, SEM , EUS and low voltage cathodoluminescence ( C L ) were conducted for investigating the coating effect. The results showed that In203 coat- ing promoted I he low- voltage CL of the phosphor efficiently. The promotion was possibly due to the enhancement of the surface condul.tivity of the phosphor grains.

Key words : I-ED: Yz03 : Eu : 111203 : Zeta potential ; coating; rare earths CLC number,: 0482.3 1 Document code: ii Article ID: 1002 - 0721 (2007 101 - 001 1 - 04

As a traditional red phosphor, Y 2 0 3 : Eu is widely used in the field of display (electroluminescence) and lighting ( photoluminescence ) ~' . In the field of dis- play, field e,nission display (FED) is a kind of prom- ising flat dis day for its small bulk, light weight, fast response speed, wide visual angle, long life, high temperature ind radiation resistance ' .

Phosphc.rs for FED are usually operated under low voltage( 300 - 10 kV) and high current ( 10 - 100 , u A - cm -:) . The excitation mechanisni is low-voltage cathodolumi*irscenoe-' . K i t h the development of FED devices. tht properties of phosphors for FED have been a very important factor to improve the properties of FED. - I t present, most of the phosphors for FED are used for reference from traditional phosphors for CRT. in which the red phosphor adopts YzOzS: Eu or Y z 0 3 Eu . 'YIOzS : Eu phosphor offers high luminous efficiency a id low resistance. However, there are problems niih device lifetime. During the operation of low voltage and high current of FED. sulfur may leak from the phcsphor and dispers~ into the vacuum, cent-

. .

~ _ _ ~ ~

* Corresponding author ( E-mail : wdzhuange 126. cum)

aminating the cathode and thus hindering electron emission, even shortening device lifetime'4' . The other red phosphor, Y 2 0 3 Eu has desirable stability in the operating circumstance of FED. However, it is insula- tor, and it does not emit secondary electron when ex- cited by low-energy electron beamr5]. The electron emitted by electron gun is prone to accumulating on the surface of the phosphor, which will reduce the ex- cited ability of electron beam. So there is a demand for FED to improve the conductivity of Y 2 0 3 E u ' ~ ' .

In this paper, Y203 : Eu red phosphor for FED was prepared by traditional solid-state reaction, In203, a semiconductor material, was introduced to improve its conductivity. h203 was coated on the sur- face of Y203 : Eu by precipitation. The coating condi- tion was discussed in details.

1 Experimental Red emitting phosphor Y203 : ELI was prepared by

high temperature solid-state reaction. The starting ma-

Foundation item: Projrct .upported b\ the National haturd Science Foundation of China (50272013, 50372086)

Biography : Hu Xinmhan ( 1981 - ) , Female, Master candidate

Copyigk t 9 2 0 0 7 . b!- Editorial Committee of Journal of the Chinese Rare Earths Society. Published by Elsevier B. V . All rights reserved

Page 2: Research on Y2O3:Eu Phosphor Coated with In2O3

12 JOURNAL OF RARE EARTHS, Vol. 2.5, N o . I , Feb. 2007

terials Y 2 0 3 ( 99 .99% ) and Eu203 ( 99 .99% ) were weighed with an appropriate stoichiometric ratio. These powders were blended and grounded thoroughly in an agate mortar, then were filled into an alumina crucible and calcined in a muffle furnace at the tem- perature of 1100 - 1400 T in air. After a series of post-treatment of fragmentation , washing, drying and sieving, the uncoated Y,03 : Eu was obtained.

Y,03 : Eu was added into the In ( NO3 ), solution which was prepared by dissolving indium into concen- trated nitric acid. Dilute ammonia was added drop- wise into above agitating suspension in order to control the precipitate pH. At the same time, I n ( OH), was formed and deposited on the Y20, : Eu phosphor parti- cles. Subsequently , the resultant deposition was fil- tered, washed and dried, then fired at 850 "c to make In ( OH ) 3 dehydrate into In203, thus the Y,O, : Eu coated wTith In20, was obtained.

XRD pattern of the phosphor sample was recorded on an MXP21VAHF-M21X X-ray diffractometer run- ning Cu Ku radiation at 40 kV and 250 mA. Zeta po- tentiometer was employed to measure the potential of particles in disperse system. JSM-6400 SEM with van- tage X-ray EDS was used to characterize the morpholo- gy and the component of particles. The luminescence of phosphor was examined by low-voltage cathode phosphor test system which is the first one in China, and was developed by Grirem Advanced Materials Co. Ltd. and Xi' an Jiaotong University. This system is composed of vacuum system, electron gun, power, control system and CCD. The voltage of electron gun can be adjusted from - 200 to - 5 k V , and its current density from 0 - 100 p A . ~ m - ~ .

2 Results and Discussion

2 .1 X-ray diffraction analysis Fig. 1 shows the XRD patterns of the uncoated

Y,O,: Eu ( 1 ) and the YIO, : Eu sample coated with In,O,(2) ( 3 % (mass fraction) ) . Compared with the JCPDS card of Y203 : Eu (No. 29-0351 ) , all observ- able peaks in Fig. 1 nearly perfectly coincide with characteristic diffraction lines of the Y,O, : Eu phase. That is to say, the coating treatment has no remark- able influence on the crystal lattice and basic structure of Y203 : Eu. On the other hand, a low-intensity peak (marked by sign "0") is founded near 30" in Fig. I (2) , which is corresponding to the highest diffraction peak of In203. Combined with the SEM analysis, it al- so proves that the surface of phosphor has been coated with In203 semiconductor.

-.. 0

0 1 20 4 0 60

2 0 '(, )

Fig. 1 XRD patterns of uncoated Yz03 : Eu I ) and coated

Y207:Eu(2)

2 . 2 Effect of Zeta potential on Yz03 : Eu coated with In203

The electrical charge on the surface of the parti- cles in solution can be changed by adjusting pH val- ue. The particle shows negativr. charge when the pH value of the solution is higher than the equipotential point of the particle, whereas it shows positive charge when the pH value of the solution is lower than the equipotential point of the particle. If the pH value of the solution is controlled between the equipotential points of two kinds of particles, the electrical charge on the surface of them will be opposite, and this two kinds of particles will be tightly absorbed together'" .

For Zeta potential measurement of 1 20, : Eu, 4 g

Y203 : Eu phosphor was added into 100 ml deionized water and dispersed by ultrasonic. The value of pH was adjusted by ammonia or nitric acid (mole ratio 1 : 1 ) .

Fig. 2 illustrates the relations between pH value and Zeta potentials ( ) of In ( OH ) 3 and uncoated Y20, : Eu . The equipotential point of In( OH), is about 9 . 3 , and the equipotential point of Y203 : Eu is about 7 . 5 . The electrical charge on the surface of In ( OH ) 3

Fig. 2 Zeta potentials of In (OH)3 and uncoated YzO3 : Eu

versus pH value of solution

Page 3: Research on Y2O3:Eu Phosphor Coated with In2O3

Hu X S et al Research on Y20, : Eu Phosphor Coated with In2O., 13

and Yz03 : Eu s opposite if pH is between 7 .5 - 9.0. Consequently. I n ( OH ) 3 precipitated from the solution can be adsorhd tightly on the surface of YzO, : E u . iZfter sintered. the In(OH)3 was dehydrated into In2& and could still adhere to the surface of the phosphor.

Fig. 3 co npares the < - pH relatioris of InZ@, uncoated IT2O: Eu and 1-?03 : Eu coated with In203. Obviouslv, afti.r Y20, : Eu coated with In203, its equi- potential point falls from 7 . 5 to 7 , very close to that of the coating mi,terial In203. As known, the equipoten- tial point of coated particles is inclined to the equipo- tential point o ’ the coating material, so the fall of the equipoteritial Ioint rereals to some extent that In203 has been c0at.d successfully on the surface of Y203 : Eu phosphor.

2.3 Morphology of synthesized Y 2 0 3 : Eu and EDS analysis

The SEM micrographs of uncoated ( a ) and coated ( b ) YZO3 : Eu are shown in Fig. 4 . Obviously, the sur- face of uncoated YzO, : ELI is smooth and clean. After coated. the siirface of Y203 : Eu is covered with a dis- continuous la! er . Compared with continuous and com- pact film. thi, discontinuous layer is more suitable to FED : on the one hand. as a semiconductor, In203 on the surface of phosphor can reduce the electrons which accumulate oil the surface of phosphor; on the other hand. In103 itself cannot emit visible light when it is excited by elt.ctron beam, and the In20, film must he a barrier wh 111 electron beam bombards Y 2 0 3 : Eu phosphor. an 1 this will weaken excitation efficiency to some extent. .it the same time, In2O3 film will reduce the output of luminescence of Y20, : Eu phosphor, but the encumbrance effect will lower with decline of com- pact degree. I’herefore , the influence of discontinuous coating film c n excitation source and emission light is relative small. At the same time. In203 can improve the conductii-ity of Y?03 :ELI particles, accordingly ad-

t

Fig. 3 Zeta prtrntials of InzO,, Y>O? :ELI and coated Y20, : Eu versus :,H value of solution

vance the excitation efficiency. Fig. 5 shows the EDS spectrum of Y 2 0 3 : Eu coat-

ed with 3% In,O,. Besides the primary elements Y and Eu , the peak of In can be seen obviously in Fig. 5 . That is to say, In20, is coated on the surface of Y,O, :Eu phosphor.

2.4 Effect of coating quantity on lumines- cent properties

In order to optimize the coating process, the in- fluence of coating quantity on luminescent properties was investigated. Emission spectrum of the coated Y20, :Eu under 900 V excitation is presented in Fig. 6 . It illustrates that the coating treatment has nearly no influence on spectrum shape and main peak posi -

Fig.4 SEM images of uncoated Yz03 : Eu ( a ) and coated Y z 0 3 : E u ( b )

J , , .

2 4 6 8 10

t / k c V

Fig.5 EDS spectrum of Y203 :Eu coated with 3% In203

Page 4: Research on Y2O3:Eu Phosphor Coated with In2O3

14 JOURNAL OF RARE EARTHS. Vol. 25 , N o . I , Feb . 2007

( I j u u u > - 1 350 400 450 500 550 600 650 700 750800

h~avelcngth ‘nm

Fig. 6 Emission spectra of ccated and uncoated Y,03 : Eu (excited by 900 V)

tion of the phosphor. The coated Y 2 0 3 :Eu still exhib- its linear spectrum emission, and the main peak corre- sponding to 5D0+7F2 transitions of Eu3+ is still at 61 1

Fig. 7 illustrates the relations of the relative brightness of the phosphor and the coating quantity of In,O, ( 0 - 5 % ) under different voltage excitation, where the brightness of the uncoated Y203 : Eu under 1200 V is set as 100%. It can be seen that the bright- ness of all coated samples has been promoted to some different extent. The relative brightness of the sample coated with 3% h,03 is promoted at maximum. This is because that in low-coating quantity, the improve- ment of conductivity of Y203 is not enough so that there are still large numbers of accumulative electrons on the surface of the phosphor. Nevertheless, the film will be more compact with the increase of coating quantity, as mentioned above, this will lead to weaken excitation efficiency and emission efficiency to some extent. Accordingly, there exists an optimized coating quantity depending on the trade-off of the above two factors, and the coating quantity of 3 % is so appropri- ate that it improves the conductivity but not influence excitation and emission. It can be seen from Fig. 6 that the relative brightness of the sample coated with 3% In203 reaches 107.6 under 1000 V excitation. In addition, when excited by 900 V , the promotion rate of brightness is the highest, about 70% (from 44% to 7 8 % ) . Although with the step-down of voltage, the penetrate ability of electron is weakened thus results in the increase of electrons on the surface of phosphor, the brightness of coated Y z 0 3 in low-voltage enhances observably just for the In203 film improving the con- ductivity in low voltage. That is to say, appropriate coating quantity can make Y203 : Eu phosphor more suitable to the requirement of low-voltage for FED.

nm[81.

+ 1200 L 0 ’ I . I . I . t . I . I

0 1 2 3 4 5

In,O contcnt’O ,I

Fig. 7 Relative brightness of phosphor versus In2@ coating quantity under different voltage

3 Conclusion Y203 :Eu red phosphor for FED was prepared by

high temperature solid-state reaction, and then was coated with In20, to improve its conductivity by pre- cipitation method.

When the precipitation pH was controlled be- tween 7 . 5 - 9.0, In203 can be coated tightly on the surface of Y203 : Eu phosphor. The spectrum shape and CIE chromaticity coordinates of Y 2 0 3 : Eu did not change after coating treatment. The coating quantity at 3% could effectively improve the conductivity of Y2O3

:Eu in low voltage, and the brightness in low voltage was also enhanced.

References :

[ 1 ] Zhai Y Q , Yao Z H , Liu B S . Luminescent properties of Y,03 : Eu nanocrystalls synthesized by EDTA complexing sol-gel process [ J ] . Journal of Rare Earths , 2002, 20 ( 5 ) : 465.

[2] Ke C H , Peng Z. Field emission display [ J ] . Vacuum Elecmnics (in Chin. ) , 1996, (6) : 53.

[ 31 Liu X R , Wang X J , Xie Y H . Recent developments of luminescent materials for PDP, FED and LED [ J ] . Chi- nese Journal of Liquid Crystals and Displays (in Chin. ) , 1998, 13(3): 158.

[4] Shigeo Shionoya, William M ken. Phosphor handbook [ M I . CRC Press, 1998. 576.

[ 5 ] Luo N Z, Xu L , Liu W D. Basic principle and structure design of FED [ J ] . Modern Display (in Chin. ) , 1996,

[6] Markku Leskela. Rare earths in electroluminescent and field emission display phosphors [ J ] . J . .-lLloys and Corn- pounds, 1998, 275 - 277: 703.

Surface Modification of Powder [ M I . Bei- jing: China Building Material Industry Press (in Chin. ) , 2004. 16.

[8] Zhang J H , Jia M L , Lu S Z . Study on UV excitation properties of Eu3+ at S6 site in bulk and nanocrystalline cubic Y,03 [ J ] . Journal of Rare Earths, 2004, 22 ( 1 ) : 45.

(4): 8.

[7] Zheng S L .