Synthesis of Nano-sized Barium Titanate Powder by Solid-state Reaction between Barium Carbonate and Titania
U.Manzoor1)†and D.K.Kim2)
1)Department of Physics,COMSATS Institute of Information Technology,Islamabad,Pakistan
2)Department of Materials Science and Engineering,Korea Advanced Institute of Science and Technology(KAIST),
Daejeon,Korea
[Manuscript received June14,2006,in revised form April10,2007]
Size control of BaTiO3in solid-state reaction between BaCO3and TiO2was demonstrated by varying the size of TiO2and milling conditions of BaCO3.The smaller TiO2particles had higher surface area,resulting in faster initial reaction.The mechanically milled BaCO3particles accelerated the diffusion process and decreased the calcinations temperature.It can be deduced from the results that the size control is possible and nano-sized BaTiO3particles with about60nm can be synthesized by using the conventional solid-state reaction between BaCO3and TiO2.
KEY WORDS:Barium titanate;Mechanical milling;Solid-state reaction;Nano particles
1.Introduction
Oxide nanoparticles and nanostructures are good candidate for a variety of applications,such as high capacitance capacitors,thermistors,gas sensors and positive temperature coefficient resistors(PTCR)[1,2]. Alkaline earth metal titanates are becoming increas-ingly important in the ceramic and electronic indus-try.Nano-sized barium titanate has been attract-ing interest because of their application in technical and fundamental research.It is therefore important to synthesize nano-sized BaTiO3powder with nar-row particle size distribution.In general,BaTiO3 nanoparticles have been synthesized by wet chemical processe[3,4].However,high cost and difficulty in pro-cess control in these routes forced tofind other ways for the synthesis of BaTiO3nano particles.
BaTiO3powders were conventionally synthesized by solid-state reaction between BaCO3and TiO2 above1100◦C[5].The overall reaction for the BaTiO3 formation while heating equi-molar BaCO2and TiO2 mixture is
BaCO3(s)+TiO2(s)→BaTiO3(s)+CO2(g)(1) The high calcination temperature leads to many disadvantages,such as large particle size and wide size distribution[6].Balaz et al.[7]reported that the a
pparent activation energy of BaTiO3formation from BaCO3and TiO2decreases by pre-milling the mix-ture before calcinations,suggesting that calcination temperature can be decreased by using mechanically activated powders.Recently Brzozowski et al.[8] suggested that the use of mechanochemical activa-tion favors the decomposition of BaCO3at lower temperatures and improves the barium ion diffusion through the BaTiO3layer.Kong et al.[9]also re-ported the single phase BaTiO3using milled pow-ders,at temperatures as low as800◦C.Xue et al.[10] reported a breakthrough by performing a room tem-perature BaTiO3synthesis by milling a mixture of barium and titanium oxides under nitrogen[10].Howe-†Ph.D.,to whom correspondence should be addressed, E-mail:umanzoor@comsats.edu.pk.ver,this room temperature synthesis starting from the oxides has to overcome the problem of hy-dration/carbonation,barium oxide by air and moisture[11].
In this study,size control and synthesis of nano-sized barium titanate was demonstrated by using me-chanically activated BaCO3and nano-sized TiO2.
2.Experimental
Barium titanate was synthesized from commer-cially available BaCO3(99%,purity)and TiO2 (KA100,Hankook Titanium Industry Co.Ltd.,Ko-rea,d50=200nm).For comparison,smaller TiO2 (P25,De
gussa Co.Germany,d50=30nm)was used and the milling of BaCO3was done in pulverizer (Fritish,Oberstein,Germany)for50h,prior of mix-ing with titania.
The equi-molar,oven-dried BaCO3and TiO2pow-ders were mixed in the centrifugal mill for15h in ethanol with10mm ZrO2milling media.The pow-der mixture was dried and kept at60◦C.The soft agglomeration was broken and5g batches were cal-cined in a tube furnace using an alumina boat.The calcination temperature was600,800and900◦C,cal-cination time wasfixed for2h and heating and cooling rates were10◦C/min.All the experiments were done in air.A scanning electron microscope (FESEM,XL30SFEG,Philips,Netherlands)and a transmission electron microscope(TEM,JEM-3010, Jeol,Tokyo,Japan)were used for the characterization of morphology and size analysis,X-ray diffractome-ter(D/max-IIIC,Rigaku,Tokyo,Japan)for phase analysis and TG/DTA(TG92,SETARAM,Caluire, France)for weight loss.At least100particles were averaged for the particle size measurement by SEM.
3.Results and Discussion
The SEM micrographs in Fig.1(supportingfigure) show some of the intermediate steps during the calci-nation process.Rod like BaCO3particles are clearly visible even after800◦C and their irregular shape sug-
Table 1Starting mixtures and major phases at different calcination temperatures Sample Starting materials Phase
(temperature,
time)
1D
TiO 2
BaCO 3600◦C,2h 800◦C,2h 900◦C,2h UM-200200nm
As received BaCO 3(v)1)BaCO 3(v)BaCO 3(w)3)M-200200nm
50h milled BaCO 3(s)2)BaCO 3(w)BaTi 3M-3030nm
50h milled BaCO 3(s)BaTi 3BaTi 3
Notes:1)v—very strong intensity,2)s—strong intensity,3)w—weak intensity
Fig.1SEM micrographsreaction rod
showing
intermediate stages of the reaction between as received
BaCO
3and TiO 2:
(a)starting TiO 2and BaCO 3equi-molar mixture just after milling,(b)powder calcined at 800◦C.Con-sumption of rod like BaCO 3and increase in smaller TiO 2particle size is clearly visible,(c)powder calcined at 900◦C and difficult to distinguish between BaCO 3,TiO 2and BaTiO 3particles,(d)single phase BaTiO 3particles calcined at 1300◦C
Fig.2XRD results of (a)as-received BaCO 3and (b)
50h milled BaCO 3.The broadening of XRD
peaks in the milled powder is an indication of lat-
tice distortion.The inset shows SEM micrographs
of unmilled and 50h milled BaCO 3
gested diffusion of barium source in the TiO 2parti-
cle.The final shape of the BT particles at calcination temperature of 1300◦C clearly shows that the shape is more close to the starting TiO 2particle,with an obvious increase in the particle size [12,13].The results suggested that controlling the size and milling condi-tion of the starting powders could control the size of the final BaTiO 3.Therefore two different parameters were considered to contr
ol the particle size.One was the milling of BaCO 3and the other was particle size of TiO 2.Table 1shows the summary of the starting materials and the major phases formed after calcina-tion at different temperatures.Mechanical milling of BaCO 3powder changes the morphology from rod-like to more or less round with-out having a dominant effect on the particle size,en-suring a well-mixed powder mixture (inset in Fig.2).XRD results of as received and 50h milled BaCO 3are shown in Fig.2.The broadening and decreasing in intensity of XRD peaks in the milled powder is an indication of lattice distortion [14].Figure 3shows the XRD results of three different powder mixtures (Table 1),calcined at 600,800and 900◦C.In UM-200(as received BaCO 3and 200nm TiO 2),after calcination of powder mixtures at 600◦C,the XRD only shows the BaCO 3and TiO 2peaks with traces of BaTiO 3.With the increase in calcination temperature,the amount of BaTiO 3phase increases.
But even after calcining at 900◦C,significant amount
Fig.3XRD results of powder mixtures calcined at dif-
ferent temperatures for 2h.The XRD of M30at 800◦C shows single phase BaTiO
3
Fig.4TGA curves of UM-200,M-200and M-30.Weight
loss is an indication of consumption of BaCO 3.Milled BaCO 3clearly indicate faster reaction ki-netics
of BaCO 3is present suggesting that higher tempera-tures are needed for completion of the reaction.The small peaks may be due to the orthotitanate phases formed during the process.
In M200(50h milled BaCO 3and 200nm TiO 2),a significant amount of BaTiO 3is formed even at 600◦C.Calcination at 800◦C results in only a small amount of BaCO 3peak,which disappear at 900◦C,clearly indicating single phase BaTiO 3.The sam-ple M-30(50h milled BaCO 3and 30nm TiO 2)shows exciting results and BaTiO 3exists as a major phase even at 600◦C.At 800◦C the XRD shows single phase BaTiO 3.The low temperature synthesis is
at-Fig.5Particle size analysis of M-200and M-30.BaTiO 3formed by reacting smaller TiO 2shows smaller size tributed to the prior milling of the starting powders and other researchers had also shown the formation of BaTiO 3at this low temperature.However,in the previously reported results,BaCO 3and TiO 2were mixed together instead of using milled BaCO 3and then mixing [15].The unwanted orthotitanates in the milled samples were either absent or present in the trace amounts.The BaCO 3decomposition is favored by the presence of more reactive TiO 2.An effective way of limiting the existence of secondary phases is to modify the reactivity of the raw materials.In this work,such goal was achieved by increasing the reac-tivity of BaCO 3by milling [16].The weight loss during calcinations is due to the CO 2gas formed according to Eq.(1).The weight loss curves in Fig.4clearly indicate the decrease in reaction temperature due to the mechanical activa-tion of BaCO 3and faster reaction kinetics in case of smaller TiO 2particles.In sample M-200,the slope is steeper than that of UM-200and
the reaction was completed at 930◦C.Brzozowski et al.[8]and Gomez-Yanez group [14]also showed similar results,suggesting that this is due to the milling of the starting powders.The weight change curve for M-30indicates that the initial reaction is very fast in the beginning.It is believed that the smaller TiO 2particles increase the contact area between TiO 2and BaCO 3resulting in faster initial reaction.A layer of primary BaTiO 3is formed on the TiO 2particle.The reaction then be-comes diffusion-controlled and the outer BaTiO 3layer on TiO 2particle makes further diffusion increasingly difficult.Thus the curve showed slower rate in the intermediate stage [8].The particle size analysis of M200and M-30with calcination temperature is presented in Fig.5.Both curves show an increase in the particle size with tem-perature.This increase in particle size may be at-tributed to the higher diffusion rate at higher tem-peratures,giving rise to strong particle growth and particle aggregation [16,17].The general trend for both of the curves in Fig.5is also similar.However,the BaTiO 3formed by reacting smaller TiO 2shows
Fig.6TEM micrograph of BaTiO3(M-30)calcined at 800◦C for2h.The results suggest that the pri-
mary particle size is less than60nm
smaller size.It can be deduced from the experimen-tal results that the nano-sized BaTiO3can be synthe-sized by the conventional solid-state reaction between BaCO3and TiO2.
Figure6shows the TEM micrograph of sample M-30calcined at800◦C for2h without post milling treatment.The micrograph also supported and con-firmed the evidence that the primary BaTiO3par-ticles were below60nm in size.It may be due to the fact that after milling,BaCO3particles become mechanically activated;sticky and round shape re-sulting in a decrease in calcination temperature and well mixed TiO2and BaCO3mixture.Therefore,by carefully controlling the milling and calcination con-ditions,the diffusion process and thus the particle size can be controlled.The unwanted secondary agglomer-ation is also present along with the primary individual particles.However,this unwanted agglomeration can be suppressed by using rotary furnace for the calcina-tion or the post milling of the calcined powder.
4.Conclusions
(1)Smaller TiO2particles resulted in smaller BaTiO3powder and enhanced the initial reaction rate by increasing the contact area between BaCO3and TiO2.Mechanical milling of BaCO3results in spheri-cal particles ensuring better mixing and decreases the reaction temperature which suppresses BaTiO3par-ticle growth and particle aggregation.
(2)The single phase nano-sized BaTiO3was ob-served at lower temperature than the conventional processing.
(3)It can be deduced from the results that size control is possible and nano-sized BaTiO3particles (about60nm)can be synthesized by the conventional solid-state reaction between BaCO3and TiO2. Acknowledgement
This work was supported by Hanchang Chemical Co. and partially by the Korea Ministry of Education through Brain Korea21Program.
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