00 Low-temperature synthesis of BaMgAl10O17_Eu2

Low-temperature synthesis of BaMgAl10O17:Eu2tblue phosphors

Wen Zhang a,Diping He a,Gang Ma b,Shujie Cui a,Gang Li a,Huan Jiao a,n

a Key Laboratory of Macromolecular Science of Shaanxi Province,School of Chemistry&Chemical Engineering,Shaanxi Normal University,Xi'an710062,

Shaanxi Province,PR China

b Teaching and Research Department of Military Training,Border Defence Academy of PLA,Xi'an710106,Shaanxi Province,PR China

a r t i c l e i n f o

Article history:

Received3July2012

Received in revised form

27August2013

Accepted5September2013

Available online19September2013

Keywords:

D.Luminescence

a b s t r a c t

Using H3BO3as the?ux,pure BaMgAl10O17:Eu2t(BAM)blue phosphors were successfully prepared via a

solid-state reaction.By this approach,well-crystallized submicron BAM particles were obtained at

temperatures as low as11001C with a sintering duration of2h.The sintering temperature required by

this approach was at least4001C lower than that required by the conventional solid-state approach for

preparing BAM;moreover,the sintering time required by the former approach was also considerably

shorter than that required by the latter approach.These factors are expected to lower the cost for large-

scale production of BAM phosphors.Crystal structures and luminescence properties of the synthesized

samples were characterized by XRD and TG–DTA,and photoluminescence spectroscopy,respectively.The

reactivity of an intermediate,BaAl2O4,is thought to be the key factor in?uencing the synthesis

temperature for BAM.

&2013Elsevier Ltd.All rights reserved.

1.Introduction

It is important to improve the performance and lower the cost

of large-scale production of blue-emitting BaMgAl10O17:Eu2t

(BAM:Eu2t)phosphors for continuing the recent progress in

plasma display panels,tricolor lamps,and LEDs and backlights

for liquid crystal displays[1,2].

Conventionally,BAM:Eu2tphosphors are prepared by a solid-

state reaction process,i.e.,?ring a mixture of BaCO3,Eu2O3,Mg

(OH)2á4MgCO3á4H2O,and Al2O3together with a small amount of

additional?ux such as AlF3or MgF2at about16001C in a reducing

atmosphere.This approach has two intrinsic disadvantages:(1)it

imposes high cost and energy requirements on the production

process;and(2)the high-temperature process often results in

irregular-shaped particles with hard aggregation that are detri-

mental to phosphor ef?ciency[3].Thus,many alternative prepara-

tion technique such as combustion synthesis,hydrothermal

process,spray pyrolysis[5]and sol–gel route[6]have been

developed to decrease the synthesis temperature.However,the

use of organic species is often involved in these approaches and

their removal is often harmful for the environment.In addition,a

subsequent additional heat treatment(in range of1200–16001C)

is also required to achieve good crystallinity for the phosphor

particles[4–6].The fabrication of high-purity BAM at low tem-

peratures by a simple process has still not been realized.The

fabrication of BAM with well-crystallized particles by a low-

temperature solid-state reaction is still one of the most desirable

industrial processes.In the present study,H3BO3was selected as

?ux for the preparation of BAM because it decreases the synthesis

temperature for well-crystallized BAM and has potential applica-

tions in the synthesis of similar materials.

2.Experimental section

A BAM:Eu2tphosphor with a composition similar to those of

commercial phosphors(containing10mol%Eu2tions)was synthe-

sized and examined in this study.The starting materials were Eu2O3

(99.99%),Al2O3(99%),BaCO3(99%),and Mg(OH)2á4MgCO3á6H2O

(99%).H3BO3(99%)was added as?ux to improve the reaction

process.The amount of added H3BO3was5,6,7,8,and9wt%with

respect to the total weight of the materials.In order to obtain a

homogeneous mixture,distilled ethanol was added as a dispersing

liquid.All raw materials were thoroughly mixed by grinding.All

samples were?red in reductive atmospheres,and the calcination

temperature and time were in the range of600–11001C and0.5–5h,

respectively.

The crystal structures of all samples were investigated by X-ray

powder diffraction using Cu Kα(λ?1.5406?)radiation on a Rigaku

D/MAX2000X-ray diffractometer.The photoluminescence of the

synthesized samples was measured using a Hitachi F4600?uor-

escentometer.The thermal reactions of the raw materials were

analyzed under an ambient atmosphere by simultaneous thermo-

gravimetry(TG)and differential thermal analysis(DTA)performed

using a Q1000DSCtLNCStFACS Q600SDT instrument.For this

Contents lists available at ScienceDirect

journal homepage:https://www.360docs.net/doc/de15334356.html,/locate/jpcs

Journal of Physics and Chemistry of Solids

0022-3697/$-see front matter&2013Elsevier Ltd.All rights reserved.

https://www.360docs.net/doc/de15334356.html,/10.1016/j.jpcs.2013.09.002

n Corresponding author.Tel.:t862981530766;fax:t862981530727.

E-mail address:jiaohuan@https://www.360docs.net/doc/de15334356.html,(H.Jiao).

Journal of Physics and Chemistry of Solids75(2014)163–167

analysis,10mg of a sample at a heating rate of 101C/min was reacted in the temperature range of 25–12001C.

3.Results and discussion

3.1.Powder X-ray diffraction analysis

In order to understand the crystallization behavior of BAM by using the mixture of raw materials,temperature-resolved X-ray diffraction patterns of the BAM:Eu powders were measured and are represented in Fig.1.The compositions of all raw material samples were set as BAM:Eu 2t,to which H 3BO 3was added at a ?xed amount of 7wt%.Further,the sintering duration was set as 2h.The main reaction process was deduced with increasing calcination temperatures.The XRD pattern of the powder obtained after ?ring the mixture of raw materials at 6001C showed characteristics typical of an amorphous phase and crystallization,the main crystal being BaCO 3,whereas aluminum and magnesium compounds were not detected by XRD.The samples maintained their amorphous character until a sintering temperature of 8001C,even though a phase transformation from BaCO 3to an intermediate phase (BaAl 2O 4)occurred.When the temperature increased to 9001C,only patterns corresponding to BaAl 2O 4could be detected.When the sintering temperature was further increased up to 10001C,patterns corresponding to BaM-gAl 10O 17began to appear,whereas BaAl 2O 4could be still detected.With the temperature increasing to 11001C,a pure BAM phase was obtained.It should be noted that all the XRD patterns perfectly matched the reported one in JCPDS (26-0163).This result indicates that the use of H 3BO 3as ?ux reduces the synthesis temperature for the BAM phase to be at least 4001C lower than that required in the conventional solid-state reaction method [7,8].

Normally,the temperature required for synthesizing BAM by the solid-state method is in the range of 1400–16001C.The most signi ?cant obstacle to the low-temperature synthesis of BAM by the conventional solid-state method is that stable BaAl 2O 4inter-mediates with a spinel structure are formed at ?ring temperatures lower than 12001C [7].The transformation of BaAl 2O 4and residual Al 2O 3to BAM normally occurs from 1300to 16001C.The reactivity of BaAl 2O 4plays an important role in the sintering process.BaAl 2O 4formed at a relatively high temperature would result in a high synthesis temperature for BAM.Thus,it can be assumed that the temperature required for the transformation of BaAl 2O 4to BAM can be decreased by the formation of highly active BaAl 2O 4.In this study,BaAl 2O 4was formed at 800–9001C,which is much lower than the temperatures reported in other studies [7,8].Therefore,it can be deduced that BaAl 2O 4formed in this study was highly active and induced a signi ?cant decrease in the BAM synthesis temperature.

In order to determine the optimum ?ring time,BAM:Eu 2tpowders were prepared at 11001C and different heating times,

and were evaluated by XRD.XRD patterns of the BAM:Eu 2tparticles sintered at 11001C and different times are shown in Fig.2.It can be seen that the main phase of the product is BAM,even for samples sintered for only 0.5–1h.When the sintering time was shorter than 2h,weak diffraction peaks corresponding to BaAl 2O 4and Al 2O 3in the synthesized samples were detected,which indicates that the transformation of BaAl 2O 4to BAM was not completed and a longer reaction time was needed.When the sintering time was 2h or longer,a pure BAM phase was obtained.These results show that the time needed to obtain a pure BAM phase in this study was signi ?cantly shorter than that required by the conventional solid-state reaction method.

Flux plays an important role in the synthesis of phosphors.In this study,H 3BO 3was used as the ?ux,which in ?uenced not only the formation of BaAl 2O 4but also the formation of BAM.Experimental results indicated that BaAl 2O 4could not be formed after the raw materials were sintered at 9001C for 2h without using H 3BO 3.The content of added H 3BO 3also in ?uences the formation of BaAl 2O 4;high-purity BaAl 2O 4could be obtained only when the H 3BO 3content was higher than 5wt%.On the other hand,a high H 3BO 3content could induce impurities.Therefore,the H 3BO 3content was varied in a narrow range of 5–8wt%.BAM:Eu 2tphosphors doped with different amounts of H 3BO 3were prepared in this study,and their photoluminescence properties were investigated.The results exhibited that the phosphor pre-pared with 7wt%H 3BO 3showed the highest PL intensity.Thus,the content of H 3BO 3was set as 7wt%.

Normally,?ux plays an important role in phase formation,ameli-oration properties,and control of the particle shape of materials.For inorganic luminescent materials,the effect of ?ux on synthetic techniques and luminescent properties has become a hot research topic.The mechanism of improvement in the photoluminescence characteristics of BAM by the use of ?ux in this study can be explained as follows.At the synthesis temperatures of (1073and 1373K)employed in this study,H 3BO 3decomposed and it was one of the products,B 2O 3,that played an important part in the reaction.The melting point of B 2O 3is 718.13K.At the reaction temperatures,it melted and a certain amount of liquid was introduced in the reaction system.The liquid phase caused a strong ?uxing action,enabling an increase in process ?exibility,accelerated the reaction with the addition of ?ux,and decreased the sintering temperature.Initially,it was thought that H 3BO 3promotes only crystallization by acting as a high-temperature solvent.The liquid phase surrounded the solid particles in the reaction system.With an increase in the temperature,the solid particles dissolved in the liquid.This means that the reactions between solid particles were replaced by reactions between ions in the liquid,which inevitably accelerated the chemical reaction and decreased the reaction temperature.At the same time,the liquid phase also facilitated the uniform distribution of the activator ions,Eu 2t,which improved the PL intensity.Furthermore,it was easy to obtain regularly shaped BAM particles from the liquid phase [9–13]

.

Fig.1.XRD patterns of the raw materials with a composition of BAM:Eu powders calcined at different temperatures for 2

h.

Fig. 2.XRD patterns of the BAM:Eu 2tparticles sintered at 11001C for

different times.

W.Zhang et al./Journal of Physics and Chemistry of Solids 75(2014)163–167

164

For BAM:Eu phosphors,the introduction of H 3BO 3can change the sublattice structure and in ?uence the crystal ?eld around Eu 2tions.It was thus expected that the introduction of H 3BO 3would optimize the photoluminescence properties of these phosphors.Hence,the BAM:Eu 2tphosphors doped with different amounts of H 3BO 3were prepared,and their photoluminescence properties were investigated.However,the usage of H 3BO 3was limited to a narrow range of 5–9wt%in order to ensure the purity of BAM.The related results indicated that the sample with 7wt%H 3BO 3showed the highest PL intensity.However,further research is needed to explain the speci ?c mechanism for this increase.

TG –DTA measurements for a mixture of the starting materials were carried out to study the mechanism of the low-temperature solid-state reaction,and the results are shown in Fig.3.The mole ratio of BaCO 3,Mg(OH)2á4MgCO 3á6H 2O,and Al 2O 3as the starting materials was 1:0.2:5,which was equal to that of BAM;7wt%H 3BO 3was added as ?ux.A continuous weight loss was detected in the reaction process,which can be divided into four tempera-ture regions:room temperature to 5001C,500–7401C,740–10301C,and higher than 10301C.A gradual weight loss of about 5.21%in the TG curve and endothermic peaks at 751C and 1251C in the DTA curve were attributed to the dehydration of free and crystalline water in Mg(OH)2á4MgCO 3á6H 2O (theoretical weight loss is 2.54%)[14]and decomposition of H 3BO 3(theoretical weight loss is 2.52%).In the 500–7401C range,a rapid weight loss of about 4.44%and an endothermic peak at 7401C in the DTA curve were observed and were attributed to the decomposition of Mg (OH)2á4MgCO 3(theoretical weight loss is 4.56%).In the 740–10301C range,a weight loss of about 4.45%and an endother-mic peak at 8201C in the DTA curve were detected and were ascribed to the decomposition of BaCO 3(theoretical weight loss is 4.65%).Moreover,a persistent exothermic process was observed when the temperature was higher than 8001C,which corre-sponded to the formation of BaAl 2O 4.The exothermic process later resulted in a prominent exothermic peak at 10501C.Since there was no distinct weight change in this temperature region,this exothermic reaction was assigned to the phase transformation of BaAl 2O 4to BAM,which was also con ?rmed by the XRD measurements for the sample heated at different temperatures.

Similar results have been reported in a previous study [1].Normally,the heating process of the BAM raw materials is divided into three to four temperature regions based on the weight loss,which is ascribed to the removal of free and crystalline water,the evaporation and decomposition of ?uxes,and the decomposition of remaining carbonate.However,in Ref.[1],the temperature

range employed for measurements was from room temperature to 11001C,and the exothermic process ascribed to the phase trans-formation of BaAl 2O 4to BAM was not recorded.However,in our study,this exothermic process is demonstrated,which is a direct proof of decrease in the synthesis temperature.

To understand the mechanism of decrease in the synthesis temperature for BAM,samples with different baryta/alumina ratio (1:1,1:2,1:3,1:4,and 1:5)were prepared at 9001C for 2h.Here too 7wt%H 3BO 3was used as ?ux for all samples.The XRD patterns of these samples are shown in Fig.4.After sintering at 9001C for 2h,the XRD patterns of these samples were quite different from one another,depending on the baryta/alumina ratio.For samples with baryta/alumina ratios of 1:1and 1:2,the XRD patterns were identical or similar to that for BaAl 2O 4;BaAl 2O 4was obtained as the dominant phase after the reaction.However,impurities,namely,BaCO 3,Ba 17Al 3O 7,and Ba 1.17Al 10.67O 17.2,were also detected.When the baryta/alumina ratios in the samples were increased from 1:3to 1:5,only the BaAl 2O 4phase was detected.This comparison indicates that redundant alumina favors the formation of BaAl 2O 4.

Normally,the reaction of Al 3tand Ba 2tions occurs at the alumina particles surface.Therefore,the reaction area is an important factor in ?uencing the reaction rate between solids.The reaction process is likely to be accelerated if the reaction area between the particles is increased.When the ratio of a raw material with a certain particle size is increased,the reaction area between particles is increased accordingly.Moreover,the transfer path for matter away from the reaction surface,which is thought to be one of the rate-controlling steps in a reaction,is also omitted or shortened,thereby the reaction rate is increased.The sintering temperature to obtain BaAl 2O 4in this study was in range of 800–9001C,which was about 4001C lower than that required in a conventional solid-state reaction [7,8].BaAl 2O 4prepared under such a condition should be highly reactive.As an intermediate,this may decrease the synthesis temperature for BAM.On the other hand,the composition of intermediate BaAl 2O 4is complex.Though only the BaAl 2O 4phase was detected,super ?uous Al 2O 3also co-existed,which resulted in a large amount of lattice defects.This also enhances the reaction activity of the intermediate.This result indicates that the key to decrease the synthesis temperature for BAM is to decrease the synthesis temperature of the BaAl 2O 4intermediate.

The structure of BaMgAl 10O 17mainly consists of Al spinel blocks and BaO layers.Mg atoms,however,may partially substitute for two kinds of Al sites surrounded by oxygen atoms,namely,the

tetrahedral

Fig. 3.TG –DTA curve (heating rate 101C/min)of the raw materials with a composition of BAM:Eu 2t

.

Fig.4.XRD patterns of the samples with different baryta/alumina ratio sintered at 9001C for 2h.

W.Zhang et al./Journal of Physics and Chemistry of Solids 75(2014)163–167165

and octahedral sites,in the spinel block.Furthermore,there is a probability that both sites have partial occupation.In Ref.[15],it was assumed that Mg atoms partially substitute Al atoms,which occupy tetrahedral sites 4f (1/3,2/3,z ),surrounded by oxygen atoms in the spinel block.Therefore,Mg 2tis inserted into the spinel block to maintain the charge balance.

In order to verify the reaction ability of the intermediate,samples with different Al/Ba ratios were synthesized (Fig.4).After re-calculation,re-weighting,re-mixing,and re-sintering at 11001C for 2h,samples with different compositions of BAM:Eu 2twere prepared.Fig.5shows the XRD patterns of the BAM:Eu 2tpowders prepared using the intermediates with different baryta/alumina ratios.It was observed that the XRD patterns of the samples match the standard JCPDS (26-0163)perfectly.This demonstrates that even though intermediates with different Al/Ba ratios were used,the BAM phase was formed and no peaks of impurity were detected.The XRD peaks in Fig.5are widened slightly,which might be interpreted by the lattice distortion caused by the low synthesis temperature.By increasing the sintering temperature,the crystal quality can be improved greatly.

Additionally,comparative experiments were designed to verify our hypothesis that a highly active BaAl 2O 4intermediate is the key to decrease the BAM synthesis temperature.If the raw materials are heated at 15001C with a heating speed of 501C/min,the time needed to form a pure BAM phase is longer than 6h.When the reaction is used to obtain BaAl 2O 4intermediates at a higher temperature,a long sintering time is needed.This implies that a low-temperature sintering process is essential to decrease the synthesis temperature for BAM.

Fig.6a shows the room-temperature PL spectrum of the BAM:Eu 2tphosphors prepared at 11001C for 2h.The blue-emission peak at 430nm was attributed to the 5d –4f transition from Eu 2t,whose ions partially replaced the Ba 2tions in BaMgAl 10O 17[7].The BAM:Eu 2tsamples synthesized below 15001C are thought to include the BaAl 2O 4:Eu 2tphosphor,which exhibits a bluish-green emission peak at 500nm,and would cause the longer wavelength side of the spectrum of BAM:Eu 2tto expand [8].In this study,no such phenomenon was observed,and this is also a proof that high-purity BAM:Eu 2twas obtained,again con ?rming that BaAl 2O 4and Al 2O 3were transformed to BAM at 11001C thoroughly.The excitation spectrum of the sample shows a broad band,which corresponds to the 4f –5d transition of Eu 2tions,implying that this phosphor can be well excited by UV light.

In order to optimize the H 3BO 3usage to maximize the PL intensity,the H 3BO 3usage was varied in the range of 5–9wt%.

Fig.6b shows the dependence of the PL intensity of the BAM:Eu 2tphosphors on the concentration of H 3BO 3?ux excited by a wavelength of 304nm.It can be observed that the PL emission intensities of BAM:Eu 2tinitially increased as the concentration of H 3BO 3increased.However,the PL intensity decreased when the H 3BO 3concentration was further increased beyond 7wt%.These results indicate that the optimum concentration of H 3BO 3for preparing good BAM:Eu 2tphosphors is about 7wt%.It should also be noted that the shape and position of the emission peaks were not in ?uenced by the H 3BO 3?ux concentration.

Fig.7shows the SEM images of BAM:Eu particles prepared at temperatures as low as 11001C with a sintering duration of 2h,using 7wt%H 3BO 3as ?ux.Particles of the as-prepared BAM:Eu phosphor,with a smooth surface,were irregular with a size of around 5μ

m.

Fig.5.XRD patterns of BAM:Eu 2tpowders prepared using intermediates with different baryta/alumina

ratios.

Fig. 6.(a)Excitation and emission spectra of the BAM:Eu 2tphosphor.(b)The emission spectra of the BAM:Eu 2tphosphors with various H 3BO 3

usage.

Fig.7.SEM of the as prepared BAM:Eu particles.

W.Zhang et al./Journal of Physics and Chemistry of Solids 75(2014)163–167

166

They were assembled into three or more thin layers,although the shapes of the thin layers were different.The thickness of the thin layers was similar and was less than200nm.In Ref.[1],the effect of ?ux on the morphology of the BAM phosphor during the particle formation was discussed;the?ux system was believed to bene?t the particles size and facilitate the control of the phosphor morphology. Though the synthesis conditions employed in Ref.[1]were different from those used in this study,the results of the two studies are similar.As is well known,the morphology of particles is mainly dependent on the temperature.The synthesis temperature for the BAM phosphor decreased to11001C in this study.Although the morphology of the as-prepared BAM particles was still not satisfac-tory,it still provided the opportunity to control the particle morphology of the BAM phosphor.

4.Conclusions

A new low-temperature method for preparing blue-emitting BaMgAl10O17:Eu2t(BAM:Eu2t)phosphors was developed.By this method,a well-crystallized BaMgAl10O17:Eu2tphosphor without any impurity phase could be prepared at11001C after2h using H3BO3as?ux.The synthesis temperature required in this method was at least4001C lower than that required in the conventional solid-state reaction method.The formation of BAM was signi?-cantly accelerated by the highly active BaAl2O4intermediate.It is believed that these results could provide insight on the reaction process and lowering of the synthesis temperature for BAM:Eu2tand other phosphors.Acknowledgments

We are grateful to the National Natural Science Foundations of China(21141001,51272151)and the Fundamental Research Funds for the Central Universities(GK20111004,GK201305013)for their ?nancial support.

References

[1]J.Zhou,Y.H.Wang,B.T.Liu,J.D.Liu,Journal of Physics and Chemistry of Solids

72(2011)995–1001.

[2]Y.Zhou,J.Lin,Journal of Solid State Chemistry178(2005)441–447.

[3]S.Oshio,T.Matsuaka,S.Tanaka,H.Kobayashi,Journal of the Electrochemical

Society145(1998)3898–3903.

[4]S.Kunimi,S.Fujihara,Dyes and Pigments91(2011)49–54.

[5]Y.C.Kang,S.B.Park,I.W.Lenggoro,K.Okuyama,Journal of the Electrochemical

Society146(1999)2744–2747.

[6]C.H.Lu,W.T.Hsu,C.H.Huang,S.V.Godbole,B.M.Cheng,Materials Chemistry

and Physics90(2005)62–68.

[7]S.Oshio,K.Kitamura,T.Shigeta,T.Hori,T.Matsuoka,S.Tanaka,H.Kobayashi,

Journal of the Electrochemical Society146(1999)392–399.

[8]J.Y.Zhang,Z.T.Zhang,Z.L.Tang,Y.Tao,X.Long,Chemistry of Materials14

(2002)3005–3008.

[9]J.Zhou,Y.H.Wang,B.T.Liu,Y.H.Lu,Journal of Alloys and Compounds484

(2009)439–443.

[10]Y.J.Park,Y.J.Kim,Materials Science and Engineering:B146(2008)84–88.

[11]A.Nag,T.Kutty,Journal of Alloys and Compounds354(2003)221–231.

[12]I.C.Chen,T.M.Chen,Journal of Materials Research16(2001)644–651.

[13]Y.C.Kang,H.S.Roh,S.B.Park,H.D.Park,Journal of the European Ceramic

Society22(2002)1661–1665.

[14]C.Y.Wang,Z.M.Du,X.M.Cong,X.L.Tian,Applied Chemical Industry37(2008)

657–660.

[15]Y.Kim,S.O.Kang,J.S.Lee,M.J.Jung,K.H.Kim,Journal of Materials Science

Letters21(2002)219–222.

W.Zhang et al./Journal of Physics and Chemistry of Solids75(2014)163–167167