Synthesis of spherical COA12O4 pigment particles with high reflectivity by polymeric-aerosol pyrolys

RARE METALSAvailable online at www.sciencedirect.comVol. 26, No.3, Jun 2007, p. 236E-mail: rm@ ustb.edu.cn。" ScienceDirectSynthesis of spherical CoAl2O4 pigment particles withhigh reflectivity by polymeric-aerosol pyrolysisHU Guorong, DENG Xinrong, CAO Yanbing, and PENG ZhongdongSchool of Metllurgical Science and Engineering, Central South University, Changsha 410083, China(Received 2006-02-26)Abstract: Spherical cobalt blue particles with good reflectivity characteristics were synthesized by spray pyrolysis. Twodifferent spray solutions were prepared to investigate the differences in the morphology and the reflectivity properties ofcobalt blue particles. One was an aqueous solution, and the other was a polycation solution that was obtained by chemicallymodifying the aqueous solution with NH2OH. The cobalt blue particles prepared with the aqueous solution had an iregularmorphology after heat treatment at 1000C for 2 h. On the contrary, spherical and dense particles were obtained from thepolycation solution. The spherical and dense cobalt blue particles showed remarkable improvement in reflectivity comparedwith that of iregular morphology particles as well as the commercial.Key words: inorganic compounds; optical materials; spray pyrolysis; polymeric aerosol1. Introductionare prepared by the solid-state route, which requiresflux additives, high reaction temperature, long heat-Most ceramic dyeing materials are transitioning time， and post-milling process to obtainmetal oxides crystallizing in the spinel structure,high-purity and small-sized particles [7-10]. As awhich provide high thermal stability and chemicalresult, the final obtained pigment particles have ir-resistance. Amongst these, cobalt aluminateregular shape and high aggregated structure. ToCoA12O4 is well known as Thenard's blue. It exhib-solve this problem, it is necessary to develop a novelits the normal spinel structure and is widely used inmethod that can control the shape of the CoAl2O4ceramic, glass, and paint industries and is used forpigment particles with high reflectivity properties.color TV tubes as contrast-enhancing luminescentSpray pyrolysis is a promising technique for pro-pigment [1-2]. Cobalt aluminate has been paid atten-ducing spherical phosphor particles, which havetion to as an advanced pigment because of its tech-nonaggregated structure, fine size, and narrow sizenological significance [3]. The pigment materialdistribution [11-14]. The particles prepared by con-with fine particle size, nonaggregation, and a narrowventional spray pyrolysis have a hollow structure,size distribution is known to have good reflectivewhich is known as a fatal disadvantage. Hence, it ischaracteristics in terms of the contrast enhancingcrucial to develop spray techniques to overcome thisluminescence. Particularly, the particle morphologyweakness of spray pyrolysis [15- 17]. Kang et al. [18]can influence the optical property of the pigment.showed that the colloidal solution was useful forThe spherical morphology of pigments is requiredcontrolling the morphology of the oxide phosphorfor high-reflectivity, because pigment particles withparticles in spray pyrolysis. Flame spray pyrolysisspherical morphology decrease the scattering of lightwas中国煤化工e particles havingfrom pigment surfaces [4-6].flle!YHC N M H Gr, these techniquesHowever, most commercialized pigment particlesin spray pyrolysIs were only applied to the prepara-Corresponding author: DENG XinrongE-mail: csudxr@ mail.csu.edu.cnHu G.R. et al, Synthesis of spherical CoAlO pigment particles with high reflectivity by...237tion of phosphor particles.crystal structure was identified from an X-ray dif-In the present study, the possibility of high reflc-fraction (XRD) pattern obtained by diffactometrytivity of the CoAl2O4 pigment particles by the spray(Rigaku D/max 2550 VB). The morphology waspyrolysis technique was investigated. To control theobserved by scanning electron microscopy (SEM,morphology and reflectivity characteristics of theJEOL JSM-6360LV). The reflectivity spectra of theCoAl2O4 pigment particles, polycation precursorprepared particles were measured using a UV spec-solutions were introduced in spray pyrolysis. For thetrophotometer (Shimadzu UV-365).first time, the spherical and dense CoAl2O4 pigmentparticles were investigated in spray pyrolysis.AI nitrate pecursors+H.o[Al(H2O)]"*Excess NH,OH - 》Precipitation2. Experimentala-Al(OH))| Aluminum hydroxideThe spray equipment consisted of an ultrasonicFitering - >aerosol generator with six vibrators (1.7 MHz), aHydrolysiscondensationquartz tube (length, 1000 mm and diameter, 50 mm),| Aluminum polycationand a particle collctor. The flow rate of air used as a[ Co precursorscarrier gas was 45 Lmin. The residence time ofSpray solution[AIOAIL(OH),(H2O).J]*droplets inside the reactor was about 0.6 s.Two dfferent precursor solutions were preparedFig. 1. Schematic diagram displaying the procedurewith cobalt and aluminum nitrate salts. The first so-for the preparation of the spray solution containinglution, denoted NS, was obtained by only dissolvingaluminum polycations.the nitrate salt precursors of Co and Al in deionizedwater. The second solution, denoted PS, was pre-3. Results and discussionpared as follows. First, aluminum nitrate was dis-solved in deionized water and precipitated as a formThe as-prepared particles had poor crystallinity inof bayerite 0x aluminum hydroxide by the addition ofspray pyrolysis. Since high crystallinity of pigmentexcess NH4OH. The precipitate (O- Al(OH)3) wasand removal of impurities such as OH and NOzfiltrated and dispersed again in deionized water,ions are generally required for proper energy transferwhile HNO3 was added until the solution tumed blueprocessing, the as-prepared particles must be an-wherein the aluminum hydroxide molecules werenealed at high temperature. To investigate the effectshydrolyzed and condensed to form embryos as aof the annealing process, the annealing temperaturetype of polycation [AL,(OH),(H2O)]_--, of which xwas controlled over the range 800- 1000°C and theand y depended on the concentration of aluminumannealing time was set to 2 h. Fig. 2 shows the X-rayhydroxide and the pH of the solution. Thereafter,diffaction (XRD) patters of as-prepared and an-cobalt nitrate precursors were added in the alumi-nealed particles. In the CoAl2O4 pigment particlesnum polycation solution. The overall procedure forprepared from aluminum polycation solution, thethe preparation of polycation solution is displayedcrystallization begins at 800°C. The intensity of theschematically in Fig. 1. In the two different sprayXRD peaks increase with increasing annealing tem-solutions, the total salt concentration was maintainedperature. The annealed particles at 1000°C haveat1 M.good crystallinity. These XRD patterns coincideThe prepared spray solutions were atomized bywith the pattern of CoAl2O4 registered in the Jointthe ultrasonic nebulizer to produce droplets, whichCommittee on Powder Diffraction Standardswere carried into the quartz tube maintained at(JCP中国煤化工peak intensity of900°C. The produced CoAl2O4 particles were col-CoAIm the aluminumlected by a special collector and post-treated atYHCNMHGaterhan hatofte800-1000°C for 2 h. The formation of a CoAl2O4particles obtained from aqueous nitrate solution as238RARE METALS, Vol. 26, No.3, Jun 2007shown in Fig. 3, when these were annealed atpletely lose their sphericity after heat treatment at1000°C for 2 h. Thus, the NH4OH addition is helpful1000°C, whereas the CoAl2O4 particles preparedfor enhancing the crystallinity of CoAl2O4 particles.from the aluminum polycation solution were denseThe crystallinities obviously have influences on theand maintained their spherical shape perfectly evenreflectivity propertes, as shown in Fig. 4. As de-after heat-treatment.scribed, this low reflectivity intensity under 450 nmeflects the insufficient crystal formation of the80CoAl2O4 phase. The higher the cysallinity, thehigher is the reflectivity under 450 nm.g2 60→a311220系404004233120 tL400 450 500 550600 650 700Wavelength / nmFig. 4. Reflectivity spectra of CoAl2O4 powders as a23010506070.function o{ the annealing temperature: as-preparedwith NHzOH (a) and after calcinations at 800°C (b),20/()900°C (C), and 1000C (d); without NH4OH after cal-Fig.2. XRD patterms of CoAl204 powderscination at 1000°C (e).as-prepared with NH4OH (a) and after calcinations at800°C (b), 900°C (c), and 1000°C (d).The different morphologies of the CoAl2O4 parti-cles prepared from solution NS and PS are due todifferent mechanisms in the drying and particle for-mation. In the nitrate solution, the formation of a .吉|dense particle was possible only when the volumeprecipitation occurred during the drying step. Tomake the volume precipitation possible, the saltconcentration along with the radial direction of thedroplet must be placed higher than the equilibriumsaturation point before the surface concentration of056070the droplet reaches the supercritical saturation point20/(0)and before any precipitation occurs on the surface.Fig. 3. XRD patters of CoAl2O4 powders withHowever, in most cases, the generation of the con-NH,OH (d) and without NH2OH (e) after calcination atcentration gradient of salts across a droplet is inevi-1000C.table because water evaporation occurs rapidly onthe surface of droplets passing through the hot fur-The morphology of CoAl2O4 pigment particlesnace. As a result, the salt concentration on the sur-depends greatly on the type of the precursor solutionface of the droplet first reaches the critical suusedFig.5 shows the SEM photographs ofper-saturation point resulting in precipitation in theCoAl2O4 pigment particles, which were sintered atsurface laver and the. formatinnof a surface shell.1000C for 2 h. The CoAl2O4 particles preparedThis中国煤化工a pssay for thefrom nitrate solution have a hollow structure. Con-evapcTYHC N M H Gpyrolyzed organicsequently, the particles were fragmented and com-or inorganic molecules to exit the droplet. Therefore,Hu G.R. et al, Synthesis of spherical CoAl2O4 pigment particles with high rfltivityt by...239(a)2ekU X1e 0e0 py.92e7 19/AUG/0520kU xi0.O0E Lum 112521/SEP/8c)(d)20kU 29F 008/ Aum 9206 19/AUG/052日, eee Lum 1731 LZ.DEC/e5Fig. 5. SEM photographs of CoAl2O4 particles prepared by spray pyrolysis: (a) as-prepared, from aqueous nitratesolution; (b) as-prepared, from NH4OH-added polycation solution; (C) calcined at 1000°C, from aqueous nitrate so-lution; (d) calcined at 1000°C, from NH0H- added polycation solution.porous particles are produced in most spray pyroly-sis processes. However, in the aluminum polycationsolution, an obvious different mechanism takes partin the particle formation process. The droplets al-ready have several embryos, which easily take partin nucleation and growth. In addition, these embryosexist as a form of polycation, which is easily polym-erized to form a three-dimensional networked gel.As the droplet increases and water is evaporated, thepolycations become more energetic and the surface018,080concentration increases. Consequently, the polym-erization of aluminum polycations is initiated at theFig. 6. SEM photograph of commercial CoAl2O4 par-surface of a droplet and instantaneously spreadsticles.across the droplet. Thus, the polymeric gelation ofaluminum polycation makes it possible to produce aGenerally, high reflectivity requires the pigmentdense and spherical particle after the pyrolysis.particles to have high crystallinity and a clean sur-When compared with commercial CoAl2O4 pigment_According to thisparticles, as shown in Fig. 6, the CoAl2O4 particlesviewp中国煤化工pigment particlesprepared by spray pyrolysis from polycation solutionare beMYHCNMHGdonesintermsofhad a cleaner surface.obtaining high reflectivity. Fig. 7 shows the reflc-240RARE METALS, Vol. 26, No. 3, Jun 2007tivity intensity measured for the prepared CoAl2O4and dense CoAl2O4 pigment particles prepared frompigment particles and the commercial one. Thethe aluminum polycation showed remarkable im-spherical and dense CoAl2O4 particles prepared fromprovement in rflectivity intensity under 450 nmaluminum polycation had higher reflectivity inten-when compared with the sample prepared from thesity under 450 nm than the porous and iregularlynitrate solution as well as the commercial. Therefore,shaped ones obtained from the nitrate solution. Also,the spherical CoAl2O4 particles prepared in the pi-the spherical CoAl2O4 particles prepared by spraylot-scale spray pyrolysis using the spray solutionpyrolysis from the aluminum polycation have higherdeveloped in this study can be successfully used aseflectivity intensity under 450 nm than the reflec-an advanced blue pigment.tivity intensity of the commercial one. From theabove results, in this study, it is proved that sphericalReferencesand dense CoAl2O4 pigment particles with improvedreflectivity intensity can be prepared successfully byl] Koroleva L.F, Synthesis of ceramic pigments basedchemically modifying the spray solution usingon spinels from hydroxocarbonates, Steklo. Keram.,NHOH in a pilot-scale spray pyrolysis process.2004, (9): 21.2] Ouahdia N., Guillemet S.. Demai JJ, Durand B.,80 rRakho L. Er, Moussa R.. and Sarndi A. Investigationr▲With NH,OHof the reactivity of AlCl3 and CoCl2 toward molten70 r;◆Without NH,OHalkali- metal nitrates in order to synthesize CoAl2O4i..Commercial.. iMater Lett, 2005, 59 (2-3): 334.60 t3] Eric-Antonic S., Kostic-Gvozdenovic L.. DimitrjevicR, Despotovic S.. and Filipovic Petrovic L, Sol-gelmethod used for synthesis of ceramic pigments, KeyEng. Mater, 1997, 132- 136(Pt 1): 30.40下4] Maslennikova G.N, Pigments of the spinel type,Glass Ceram, 2001, 58 (5-6): 216.30 F▲5] Al-Turaif A.H., and Bousfield W.D.. The influenceof pigment size distribution and morphology on20 tcoating binder migration, J. Nordic Pulp Paper Res,2005, 20 (3): 335.50叶6] Ul Haq I, Fraser I, and Matijevic E, Preparation andcharacterization of finely dispersed pigment particles,400500600700Colloid Polymer Sci, 2003, 281 (8): 542.Wavelength 1 nm7] Fukuda M.. Kodama K., Yarmamoto H., and Mito K.Fig. 7. Retlectivity spectra of prepared and commer-Solid-state laser with newly synthesized pigment,cial CoAl2O, particles.Dyes Pigm, 2002, 53(1): 67.8] Aghababazadeh R., Mirhabibi A.R., Moztarzadeh F.and Salehpour Z, Synthesis and characterisation of4. Conclusionchromjum oxide as a pigment for high temperatureapplication, Pigm. Resin Technol, 2003, 32 (3): 160.Spherical and dense CoAl2O4 pigment particles9] Maso N, Beltran H, Munoz R, Julian B.. Carda J.B..were sucessfully prepared by pilot-scale spray pyEscribano P. and Cordoncillo E, Optimization ofrolysis. It was found that chemically modifying thepraseodymium-doped cerium pigment synthesisaluminum nitrate solution with NH4OH can producetemperaure, J. Am. Ceram. Soc., 2003. 86 (3): 425.the spherical and dense CoAl2O4 particles, which are[10] Che wsKnkihang M_ Crytallization of cehardly obtainable from the general nitrate precursor中国煤化工:xystals from Co-AIsolution. The spherical shape was maintained after[HC NM H G: Compd, 199, 287heat treatment at 1000°C for 2 h. Also, the spherical(1-2): 81.Hu G.R. et al, Synthesis of spherical CoAl2O pigment particles with high reflectivity by...241[1I] Jung K.Y, Lee D.Y.. and Kang Y.C, Morphologyphosphor prepared by spray pyrolysis, J. Lumin,control and luminescent property of YzAl sO12:Tb2003, 105 (2-4);: 127.particles prepared by spray pyrolysis, Mater. Res.[16] Kozhukharov V, Brashkova N. Machkova M.,Bull, 2005, 40(12): 2212.Carda J., and Ivanova M., UItrasonic spray pyrolysis[12] Kim EJ, Kang Y.C., Park H.D.. and Ryu S.K., UVfor powder synthesis, Difus. Defect Data Pt. B, 2003,and VUV characerstics of (YGd)2O;Eu phosphor90-91 (3): 553.particles prepared by spray pyrolysis from polymeric[17] Kang H.S.. Kang Y.C, Park H.D.. and Shul Y.G.precursors, Mater. Res. Bull, 2003, 38 (3): 515.Morphology of particles prepared by spray pyrolysis[13] Kang Y.C., Seo D.J, Park S.B., and Park H.D.. Di-from organic precursor solution, Mater. Lelt, 2003,rect synthesis of strontium titanate phosphor particles57 (7): 1288.with high luminescence by flame spray pyrolysis,[18] Kang Y.C., Roh H.S., Park S.B., and Park H.D..Mater. Res. Bul, 2002, 37 (2): 263.High luminescence Y2O3;:Eu phosphor particles pre-[14] Jokanovic V., Mioc U.B., and Nedic ZP, Nanos-pared by mdified spray pyrolysis, J. Mater. Sci. Lett,tructured phosphorous tungsten bronzes from ultra-2002, 21 (3):1027.sonic spray pyrolysis, Solid State lonics, 2005, 176[19] Camenzind A., Strobel R., and Pratsinis S.E, Cubic(39 40): 2955.or monoclinic Y2O;:Eu"+ nanoparticles by one step[15] Jung K.Y, Lee D.Y, Kang Y.C., and Park H.D.,flame spray pyrolysis, Chem. Phys. Lelt, 2005, 415Improved photoluminescence of BaMgAloO17 blue(4-6): 193.中国煤化工MYHCNMHG