synthesis of ce: yag phosphor via homogeneous precipitation under microwave irradiation

JOURNAL OF RARE EARTHS V o l . 2 4 , Spec. Issue, Dec. 2006, p . 1 8 3

Synthesis of Ce : YAG Phosphor via Homogeneous Precipitation under Microwave Irradiation Wang Yong (3. ( School of Material Science and Engineering, Jinan University, Jinan 250022 , China )

3 ) , Yuan Peng (I$; @) , Xu Hongyan (@&&), Wang Jieqiang (3@%) *

Abstract: Ultra-fine Ce : YAG phosphors were prepared by homogeneous precipitation under microwave irradiation method. The formation of Ce : YAG was investigated by means of XRD and DTA/TG. The purified YACT crystallized phase was obtained at a lower temperature ( 1 100 "c) . Basically spherical Ce:YAG powders were indicated from TEM images, and the size of the particles is about 80 nm. Two peaks of 436 and 473 nm can be seen from the excitation spectrum in the range of 402 - 5 10 nm . A broad emission band located at 480 - 630 nm shows the phosphors prepared by this method have good emission properties.

Key words : Ce : YAG ; homogeneous precipitation under microwave irradiation ; phosphors ; rare earths CLC number : TQ 1 74 Document code : A Article ID : I002 - 072 1 ( 2006) - 01 83 - 04

In 1993, Nichia Chemical Company developed the GaN-LED with blue emission, which made semi- conductor diode solid lighting possible. Three years later, the company prepared a series of Ce-doped yttrium aluminum garnet (YAG) phosphors for the first time. These phosphors combined with GaN-LED can fabricate high efficient W-LED, which can replace daylight lamp used as lighting resource. This lighting resource has many advantages compared with conventional lighting resource, such as innocuity , high eff- ciency, energy saving, longevity ( 1 x I d h ) , low driving voltage. Therefore, W-LED is called a new generation lighting resource.

One of the most important methods generating white light is combined GaN/InGaN-LED with Ce : YAG phosphor. So the characteristics of Ce : YAG phosphor wil l influence the property of the W-LED ob- viously. Solid-state reaction"' is the primary method to produce Ce : YAG phosphor, but the method needs long reaction time and high reaction temperature, and the particle size prepared by this method is large, furthermore the post-processing to the large particles will reduce the luminescent properties. Therefore, wet chemical methods have been studied, such as sol-gel rnethod12', co-precipitation method'31 , combustion process'41, and solvothermal . A number of investigations proved that the wet chemical methods could improve homogeneity, decrease calcining temperature and time, and reduce particle size.

Received date: 2006 - 06 - 26; revised date: 2006 - 09 - 11

It is well known that microwave technique is widely used in inorganic synthesis'" . During heating process, the temperature gradient which leads to seg- regation in solution, can be avoided by microwave heating technique. Meanwhile, it is beneficial to make the formation and growth of crystal nucleus progressed at the same time. In addition, numerous nucleus erupt from solution in a short time. By this way, ultra-fine powders will be produced in a lower temperat~re '~] . In this means, microwave technique was introduced to prepare Ce : YAG phosphor.

1 Experimental The staring materials for Ce : YAG phosphors were

rare earth oxide and nitrates. Y203 ( > 9 9 . 9 9 % ) was dissolved in reagent grade nitric acid. Then according to [Y3+]:[A13+]:[Ce3' ] molar ratio of ( 3 - x ) : 5 : x , A1(N03),.9H20( > 9 9 . 0 % ) a n d Ce(N03)3*6H20 ( > 99.0% ) weighed precisely were added into yttrium nitrate solution with agitation. The mixed solution was used as mother salt solution.

80 ml mother salt solution was diluted with dis- tilled water to 400 ml. Urea was added to solution according to the molar rate of [urea]/[ total metal cations ( Y3+ + A13' + Ce3' ) ] = 20"' . Furthermore, ammonium sulfate was added to the solution in terms of the mass ratio of [ammonium sulfate]/[ mother salt Then the solution was heated for 30 min in refifted do- mestic micro-wave oven by 2.45 GHz microwave irra-

Foundation item: Project supported by Foundation for the Excellent Middle-Aged or Young Scientists of Shandong Province (02BS049) Biography: Wang Yong (1981 - ), Male, Master

* Corresponding author ( K-mail : nise - wangjq @ujn . edu . r n )

184 JOURNAL OF RARE EARTHS, Vol. 24, Spec. Issue, Dec . 2006

diation under 900 W output power. The precursor was washed three times with dis-

tilled water and ethanol respectively, then dried at 80 “c for 12 h in air. Subsequently, the precursor was calcined at different temperatures from 900 to 1200 “c for 4 h in air.

The composition and transformation during calcining of precursor were analyzed by IR (Model FIS165, Bio-Rad, America), DTA/TG (Model STA- 409-EP, NETZSCH, Germany) and XRD (Model D/ max-RA, RIKEN , Japan) . The size distribution of Ce :YAG phosphor was characterized by LD (laser diffraction) (Model LS13320, America). The size and morphology of Ce : YAG phosphor were observed by TEM ( Model JEOL-2010, Japan ) . The luminescent spectra of Ce : YAG phosphor were measured by Lumi- nescence Spectrophotometer ( Model LS-55, PE , America).

2 Results and Discussion

2 .1 Composition and phase transformation of precursor

Fig. 1 shows the IR spectrum of Ce : YAG precursor, in which the strongest band at 3000 - 3750 cm-’ should be due to the stretching of 0 - H in water, and the NH4+ can be detected at 2924 cm-’ . The band at 1350 - 1700 cm-’ indicates the presence of C03’- , and the band about 1129 cm-’ should be attributed to SO>-. In addition the band about 618 cm-’ should be assigned to the bending of A1 - 0, Y - 0 and Ce - 0.

Fig. 2 shows the results of DTA/TG measurement of the precursor. As we can see, under 420 “c , the weight loss reaches to 4 2 % , corresponding to a broad- ened endothermic area on DTA curve, which should be caused by the removal of absorbed water, NH4+ , OH- and C03’- . The endothermic peak at about 555 “c can be attributed to decomposition of carbonate, at

5 0 ! . . , . , . , . , . , . , I 4000 3500 3000 2500 ZOO0 1500 1000 500

Wavenum bcdcm-’

Fig. 1 IR spectrum of Ce : YAG precursor

200 400 600 800 1000 1200 TempcraturdC

Fig. 2 DTAITG curves of Ce : YAG precursor

the same time, about 10% weight loss can be seen on TG curve. The decomposition of SO:- can be observed at the peek of 1200 “c on DTA curve, which causes very little weight loss on TG curve. A total weight loss is 52% during heating, After 650 “c the TG curve became smoothly with inapparent weight loss.

On the ground of IR spectrum and DTA/TG curve, it can be concluded that the composition of the amorphous precursor is carbonate of aluminum, yttrium and cerium.

2.2 Calcining powders of Ce:YAG Fig. 3 shows the XRD patterns of Ce : YAG pow-

ders calcined at different temperatures. The XRD patterns indicate that YAG is the only crystallized component at calcining temperature over 1100 “c . There are three phases in the Y203-A1203 binary systems: YAG, YAP (YA10,) and YAM (Y4A1209).

It has been reported‘“] that YAG phase can be obtained at higher temperature over 1600 “c for a long time by the conventional solid-state method. However, phase-pure YAG is obtained at 1100 “c by homogeneous precipitation method under microwave irradiation, which is nearly 500 “c lower than conventional

Fig. 3

900 ‘C 4 h

10 20 30 40 50 60 70

2 (1 I( c’ )

XRD patterns of Ce:YAG precursor calcined for 2 h at various temperatures

Wang Y et al . Synthesis of Ce : YAG Phosphor via Homogeneous Precipitation 185

Fig. 4 TEM images of Ce : YAG powders calcined at 1100 T (a) and 1200 T (h , c )

solid-state method. As the XRD patterns show that nothing changes in diffraction peaks but the enhance- ment of the diffraction intensity in further calcining at higher temperature.

The result shows the phase-pure YAG can be obtained at 1100 “c . The absence of the intermediate phases (YAM, YAP) during the heating process is be- cause the component of the precursor is more uniform compared with the conventional heating techniques by urea method“’] . Under microwave irradiation, a large number of precursor nucleus burst quickly at the same time, avoiding Y3’ , AP + , Ce3’ precipitate sequential- ly. Besides, the distribution of Y3+ , A13+ , Ce3’ on the ion level also shortens the diffusion distance and lowers the synthesizing temperature of phase-pure YAG .

Fig. 4 shows TEM images of Ce : YAG powders calcined at 1100 9c ( a ) and 1200 “c ( b , c ) . It is clear that the size of samples calcined at 1100 “c (about 80 nm) is smaller than that calcined at 1200 “c (about 150 nm) . This is further confirmed by the LD result in Fig. 5 . As Fig. 4 indicates, particles slightly aggregate and grow at 1100 “c , and it is initial growth for powders prepared by homogeneous precipitation method under microwave irradiation. However, two or more particles grow to one big particle for powders calcined at 1200 “c .

It can also be seen from Fig. 4 that particles are easy to aggregate and grow during calcining, which is due to finer particles and higher activation of the precursors. From Fig. 4 we also can see the shape of the particles was nearly spherical. Spherical particles would improve the accumulation density, which would influence the luminescent properties in the following process.

Fig. 5 shows size distribution of Ce : YAG powders obtained by calcining precursors at 1100 “c (1) and 1200 “c ( 2 ) . As shown in Fig. 5 , the size of the powders calcined at 1100 “c is smaller than that calcined at 1200 “c . For the powders calcined at 1100 “c ,

more than 70% of the particles size is less than 100 nm. But for the powders calcined at 1200 “c , 90% of the particles size distributes between 100 and 200 nm, which can be inter-validate with Fig. 4 .

Generally, the luminescent properties of phosphor particles depend on the characteristics of the particles, such as crystallization, shape, defect and so on. The luminescent properties of the 2% mol Ce : YAG phosphor calcined at 1100 “c are shown in Fig. 6. The Ce3 + shows blue excitation and green-yellow emission in YAG. As is shown in Fig.6, two excitation bands at 436 and 473 nm and a broad emission band from 500 to 600 nm can be seen from luminescent spectra. The excitation bands match well with the emission bands of the GaN/InGaN chip. The result is similar to Zhou Yonghui[l2] who studied the Ce : YAG phosphor using solid-state method and co-precipitation .

The excitation bands located 436 and 473 nm are result the electron transitions from the ground state of Ce3 + ( 2F2,5) to the different crystal filed splitting com- ponents of excited 5d state of Ce3’ . The broad emission band from 500 to 600 nm is ascribed to the elec-

Particle diarnated v m

Fig. 5 Size distribution of Ce : YAG powders calcined at 1100 “c ( 1 ) and 1200 “c ( 2 )

186 JOURNAL OF RARE EARTHS, Vol. 24 , Spec. Issue, Dee. 2006

40U 450 500 550 600 650

Wavclength/nm

Fig.6 Excitation ( A e x = 436 nm) and emission ( A , = 517 nm) spectra of Ce:YAG

tron transitions from the lowest splitting component of 5d level to the ground state Ce3’ (2F5/2, *F7/*), which is separated by some 2000 cni-’ due to spin-orbit coupling[”’ . 3 Conclusion

Ce : YAG ultra-fine phosphor particles were prepared by homogeneous precipitation under microwave irradiation at 1100 “c in air. The amorphous precursor transformed into phase-pure YAG directly. TEM images show that the particles shape is nearly spherical, moreover particle size is about 80 nm calcined at 1100 “c and about 150 nm calcined at 1200 “c . The 2 % mol Cr : YAG phosphors with nearly spherical morphology have good luminescent properties, which match well to the emission of the GaNhGaN chip and can be used as phosphor in W-LED.

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synthesis of ce: yag phosphor via homogeneous precipitation under microwave irradiation

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