A Pulse Combustion-Spray Pyrolysis Process for the Preparation ofNano- and Submicrometer-Sized Oxide Particles

W. Widiyastuti,z Wei-Ning Wang,z Agus Purwanto,z I. Wuled Lenggoro,y and Kikuo Okuyamaw,z

zDepartment of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima739-8527, Japan

yInstitute of Symbiotic Science and Technology, Tokyo University of Agriculture and Technology, Tokyo 184-8588,Japan

The preparation of nano- and submicrometer particles using anintegrated pulse combustion–spray pyrolysis process was inves-tigated for the first time. Zinc oxide nanoparticles with averagesizes ranging from 20 to 40 nm were synthesized using a saltprecursor with a lower decomposition temperature than the pulsecombustion (PC) temperature. Particles of a doped-type oxide,gadolinium oxide-doped with europium, were also producedfrom a precursor, whose decomposition temperature was higherthan the PC temperature, and were observed to be ellipsoidal–toroidal in shape and submicrometer in size. The high-intensityfluctuating pressures generated by PC and the precursor decom-position temperature are proposed as the reasons for the forma-tion of either nanoparticles or aspherical particles. In addition,hot gas fluctuating flow in PC enhanced energy transfer, result-ing in more crystalline particles, as compared with particlesproduced by conventional spray pyrolysis.

I. Introduction

THE spray pyrolysis (SP) method can be used for the produc-tion of high-purity particles in the nano- to micrometer-

sized ranges with a continuous operation and a high productionrate.1 Conventional SP generally produces spherical particles inthe submicrometer and micrometer-sized ranges. The spray-pyrolyzed particles consist of aggregated nano-sized crystallitesderived from nucleation and growth from dissolved precursorsinside droplets. Modifications of the SP method to preparenanoparticles have been used by our group, such as SP underlow-pressure conditions,2 flame SP,3 salt-assisted SP,4 and poly-mer-assisted SP.5 In this study, a new method is introduced thatintegrates pulse combustion (PC) and SP, called pulse combus-tion–spray pyrolysis (PCSP).

In PC, the momentum, energy, and mass transfer rates areenhanced by the generation of high-intensity acoustic waves andunsteady gas flow rates.6,7 PC has been used in drying processesbecause it can increase the drying rates significantly. The ad-vantages of PC in spray drying have been reported by Zbicinskiet al.8 and Kuts et al.9 It has also been used for drug production,as reported by Liang et al.10 SP differs from spray drying, in thatSP involves evaporation, drying, and thermal decomposition orreaction, while spray drying only involves evaporation and dry-ing. The use of PC for oxide particle synthesis has not been

studied previously, and the effect of pressure oscillations on theparticle synthesis process needs to be studied further.

A pulse engine mainly consists of three parts, i.e. the inlet, thecombustor, and the tail or exhaust part, as shown in Fig. 1, in-set.11 A combustible mixture is introduced into the combustor,which is subsequently ignited by a spark plug. During the com-bustion process, the pressure inside the combustor increases andleads to the movement of hot gas toward the tail. This move-ment produces a negative pressure in the combustion chamber;thus, the hot exhaust gas from the tail flows back into thecombustor, resulting in precompression of the mixture inside thecombustor. Hot and high-pressure gas comes in contact with thecombustible mixture and leads to ignition. The process is re-peated, producing a fluctuating flow with a high acoustic waveintensity. The pressure and temperature in the combustor there-fore vary with time.

To evaluate the effect of PC on the particle characteristicsproduced, particles of two different materials were prepared:zinc oxide (ZnO) and gadolinium oxide-doped with europium(Gd2O3:Eu

31). ZnO was selected because it has a lower decom-position temperature than that of the average flow temperaturegenerated by PC,12 and because it is an often-used semiconduc-tor.13 ZnO nanoparticles were prepared by many methods, in-cluding SP methods, such as salt-assisted SP,4 flame SP,14 andlow-pressure SP.15 However, salt-assisted SP needs an addition-al treatment after spraying to remove the salts. Flame spray andlow-pressure SP are relatively energy consumption processes. Inthis work, a PCSP was introduced, which is considered to be apromising method because it is a one-step process with a rela-tively low energy consumption. Gd2O3:Eu

31, which is wellknown as a red-emitting phosphor, was selected for compari-son. It has a higher thermal decomposition temperature thanthat of the average flow temperature generated by PC.16 Theparticle size, morphology, and crystallinity obtained for bothZnO and Gd2O3:Eu

31 particles produced by PC, PCSP, andconventional SP are presented and discussed in this work.

II. Experimental Procedures

The experimental setup is schematically shown in Fig. 1. It con-sisted of a pulse generator, a nebulizer, a furnace, and a particle-collecting system. The precursor was sprayed using an ultrasonicnebulizer (1.7 MHz, NE-U17, Omron Healthcare Co. Ltd., To-kyo, Japan), with air as the carrier gas. The droplet distributionand the number concentration were measured by a laser diffrac-tion technique (Spraytec, Malvern Instruments Ltd., Malvern,U.K.). The average volume diameters of the droplets of 0.1Maqueous zinc acetate and 0.1M aqueous gadolinium nitratesprayed by a nebulizer were 4.22 and 4.38 mm, respectively.The droplets carried by an airflow of 10 L/min were preventedfrom evaporating and precipitating using a double pipe coolingsystem before entering the furnace. An electrostatic precipitatorwas used to collect the particles.

S. Bhandharkar—contributing editor

This work was supported in part by Grants-in-Aid from the Ministry of Education,Culture, Sports, Science and Technology (MEXT) of Japan, Japan Society for thePromotion of Science (JSPS) and Japan Science and Technology (JST) Agency.

wAuthor to whom correspondence should be addressed. e-mail: okuyama@hiroshima-u.ac.jp

Manuscript No. 23248. Received May 23, 2007; approved July 31, 2007.

Journal

J. Am. Ceram. Soc., 90 [12] 3779–3785 (2007)

DOI: 10.1111/j.1551-2916.2007.02045.x

r 2007 The American Ceramic Society

3779

A PC system was provided by Pultech Co. (Kobe, Japan).0.67 L/min of propane and 20 L/min of air were used as a fuel inthe PC system, which was recommended as an optimum condi-tion. This system produced 155 dB at a frequency of 700 Hz. Tomaintain a similar residence time in the furnace, a vacuum pumpequipped with a flow meter controller was used to maintain theflow at 35 L/min (GCD-136X, Ulvac Kiko Inc., Kanagawa, Ja-pan). The temperature at the outlet region of the pulse comb-ustor tail was recorded using a data logger (midi logger GL200,Graphtec Corp., Yokohama, Japan) every second. Carried bycombustion flow, the droplets vertically passed through the fur-nace. A commercial computational fluid dynamics code (FLU-ENT 6.2, Fluent Asia Pacific Co., Ltd., Tokyo, Japan) was alsoused to analyze the steady-state temperature profile inside thefurnace and the combustion process. Three systems were usedfor comparison: PC, in which the energy source only arose frompulse combustion; SP, in which the energy only arose from aheated furnace; and PC integrated with SP (PCSP).

For precursor preparation, zinc acetate dihydrate (99%,Wako Pure Chemical, Tokyo, Japan) was dissolved in ultra-pure water, resulting in 0.1M of zinc acetate solution as a pre-cursor. 0.1M gadolinium nitrate precursor was also prepared bydiluting gadolinium nitrate hexahydrate (99%, Wako PureChemical) with ultrapure water. Europium nitrate with a con-centration of 10 at.% of Gd(NO3)3 was mixed with the gado-linium solution to determine the PC effect on the luminescencecharacteristics.

Thermogravimetric (TG) and differential thermogravimetric(DTG) analyses (TG/DTA 6200, SII Exstar 600, Seiko Instru-mentation Inc., Tokyo, Japan) were used to analyze the precur-sors. Fifteen miiligrams of zinc acetate dihydrate andgadolinium nitrate hexahydrate without pretreatment were heat-ed at a 101C/min heating rate with 200 mL/min of air flow forthermal analysis measurements. Zinc acetate dihydrate and gad-olinium nitrate hexahydrate were placed in an Al cell and a Ptcell in the temperature ranges of 301–6001C and 301–10001C,respectively. The crystallinity of the prepared particles was char-

acterized by an X-ray diffractrometer (XRD, RINT 2200 V,Rigaku, Tokyo, Japan) using nickel-filtered CuKa radiation(l5 1.54 A) at 40 kV and 30 mA. Their morphology was char-acterized by a field-emission scanning electron microscope(FE-SEM S5000, Hitachi, Tokyo, Japan) operated at 20 kV.A field emission transmission electronic microscope (FE-TEM,JEM-3000F, JEOL, Tokyo, Japan) operated at 300 kV was alsoused to analyze the crystal structure and morphology. The pho-toluminescence (PL) intensity and surface area were measured tocharacterize Gd2O3:Eu

31. PL spectra were measured using aluminescence spectrometer (RF-5300PC, Shimadzu Corp., Kyo-to, Japan) with a xenon lamp at an excitation wavelength of 254nm. The optical spectrum was measured at a resolution of 0.2nm at room temperature. The specific surface area of Gd2O3

powder was measured by a nitrogen adsorption apparatus usingthe Brunauer–Emmett–Teller (BET) method (Belsorp 28SA, BelJapan Inc., Osaka, Japan).

III. Results and Discussion

The velocity fluctuations are the main factors that enhance theheat transfer coefficient, which leads to an increase in the evap-oration rate. Increasing the instantaneous velocity leads to anincrease in the heat transfer coefficient. Furthermore, the pres-ence of an oscillation flow of dry and hot gas of air due to PC ledto an enhancement of the evaporation rate. Dubey et al.17 con-ducted an experiment wherein the sinusoidal acoustic field in-fluenced the instantaneous droplet velocity, which is animportant variable for determination of droplet evaporation.

PC also caused fluctuations in temperature. The temperaturemeasurement at the outlet of the PC tail showed that the tem-perature fluctuated between 3001 and 3501C, as is shown inFig. 2. The steady-state simulation results of the temperaturedistribution of SP and PC are depicted in Fig. 3. In SP, a par-abolic temperature profile in the radial direction was present,with the minimum temperature along the furnace axis. On the

Fig. 1. Experimental setup of pulse combustion (PC) integrated with spray pyrolysis. Inset: Operating principle of the PC system.

3780 Journal of the American Ceramic Society—Widiyastuti et al. Vol. 90, No. 12

other hand, hot gas flow released by PC led to a temperaturedistribution in the furnace almost similar to that at the furnaceinlet. This indicated that energy transfer was enhanced by thePC system.

(1) Thermal Analysis of Precursors

Figure 4(a) shows the TG and DTG results corresponding to thepercentage weight loss of zinc acetate dihydrate due to heattreatment. The TG curve shows that the dehydration leading tothe formation of anhydrous zinc acetate occurs around 931C. A16.4% weight loss was observed, which agreed well with thedehydration calculation based on stoichiometry. On increasingthe temperature up to 2511C, a 46.5% weight loss occurred cor-responding to ZnO formation according to stoichiometric cal-culation. Increasing the temperature further up to 2901Cresulted in a 13.9% weight loss. This weight loss was due toadditional sublimation of the decomposed precursor

(ZnO4(CH3CO2)6), as measured previously by Ghule et al.12

Therefore, the total weight loss due to dehydration and thermaldecomposition processes of zinc acetate dihydrate up to 2901Cwas 76.8%.

Figure 4(b) shows the TG-DTG analysis of a gadolinium ni-trate precursor. The dehydration of the precursor began at 981C,indicated by the first peak of the DTG curve. The subsequentthree peaks observed indicated phase transitions to form Gd2O3

up to a constant weight at around 7001C. The total weight lossdue to Gd2O3 formation was 59.8%, which agreed well with thepredictions based on the dehydration process (23.9%) and thedecomposition reaction (35.9%).

TG-DTG analysis showed that the zinc acetate precursor toform ZnO had a lower decomposition temperature (at 2901C)than that of the average temperature of PC (around 3001–3501C). On the other hand, the gadolinium nitrate precursorto form Gd2O3 had a higher decomposition temperature (at7001C) than that of the average temperature of PC.

(2) ZnO Particle Preparation and their Characteristics

Conventional SP with an isothermal furnace wall temperature of5001C (SP500) generated particles of submicrometer size. Theparticles consisted of poly-crystals, which aggregated and couldnot be separated from the original drying particle. On the otherhand, PCSP without and with a heated furnace of 3001, 4001,and 6001C (PC, PCSP300, PCSP400, and PCSP600) generatednanoparticles. Figure 5 shows FE-SEM images of SP500 andPCSP600 to compare the effect of PC on the generated particles.PC plays a role in producing ZnO nanoparticles. XRD patternsare also depicted in Fig. 5. The patterns show that ZnO particleshad a zincite structure, which agrees well with the reference(JCPDS 36-1451), and that the particles synthesized by PC havea larger crystal size than those synthesized from SP. The crys-talline size was determined using the well-established Scherrer’smethod by measuring the full-width at half-maximum of thediffraction peak. Based on [101] crystal orientation at2y5 36.2511 as the highest peak of the XRD pattern, the cal-culated crystal sizes were 14.0, 24.8, 25.2, 26.4, and 32.8 nm forSP500, PC, PCSP300, PCSP400, and PCSP600, respectively.

Fig. 3. Temperature profile in the spray pyrolysis system with an isothermal furnace wall temperature of 5001C and in the pulse combustion region aspredicted by computational fluid dynamics.

Fig. 2. Temperature fluctuation of combustion flow generated in apulse combustion system at the outlet of the combustor tail.

December 2007 A Pulse Combustion-Spray Pyrolysis Process 3781

The crystallinity of the particles generated by PC integrated by aheated furnace (PCSP) increased with increasing furnace tem-perature. The energy of PC alone was sufficient to form ZnOnanoparticles, and additional heat from heated furnace (PCSP)was used for crystal growth.

Figure 6 shows TEM images of particles produced by PC,PCSP, and SP. Using PC, disperse nanoparticles were obtained.The primary particle size generated by PC increased with in-creasing furnace wall temperature as shown in Fig. 6(a–c). Ag-glomerated, polycrystalline particles were produced by SP alone(SP500) as shown in Fig. 6(d). The insets of Fig. 6 show thecorresponding selected-area electron diffraction (SAED) pat-terns of particles. Spotty ring patterns without any additionaldiffraction spots and rings of second phases reveal that particleswere of high crystalline zincite ZnO. Clearer spotted patternsindicated that an increase in crystallinity was also found withXRD measurements. Unlike PC and PCSP particles, the ZnOparticles prepared from SP (SP500) were polycrystalline, as in-dicated by circular line diffraction patterns (Fig. 6(d) inset). Ahigh-resolution TEM (HRTEM) image of a particle generatedby PC is shown in Fig. 7(a). The particle had single directionlattice fringes with a lattice spacing of 0.28 nm, which is the [100]orientation. Conversely, conventional SP particles had multi-di-rectional lattice fringes and some defect regions. Amorphousregions were also found as shown in Fig. 7(b).

(3) Gd2O3:Eu31 Particle Preparation and their

Characteristics

The FE-SEM images in Fig. 8(a) show that sphericalGd2O3:Eu

31 particles were typically generated by SP.18 In con-trast with ZnO particles, PC produced ellipsoidal–toroidalGd2O3:Eu

31 particles as shown by the FE-SEM images inFig. 8(b). The particles were also ellipsoidal–toroidal in mor-phology as shown in Fig. 8(c) when PC integrated with SP at atemperature of 8001C was used. Figure 8(d–f) show high-mag-nification FE-SEM images of particles generated by SP (SP800),PC, and PC integrated with a heated furnace (PCSP800), re-spectively. PC affects the morphology of the Gd2O3:Eu

31 par-ticles generated, giving rise to the generation of nonsphericalparticles. The change in morphology resulted in an increasedparticle surface area. The specific surface area measured by theBET method showed that particles synthesized by SP800, PC,and PCSP800 had surface areas of 12.606, 15.660, and 17.061m2/g, respectively.

XRD analysis of Gd2O3:Eu31 particles generated with PC,

SP, and PC integrated with SP are shown in Fig. 9(a). The XRDpatterns of particles generated by PC and SP at a temperature of8001C agree with the Gd2O3 crystal reference (JCPDS 12-0797).

Fig. 4. Thermogravimetric-differential thermogravimetric analysis of precursors: (a) zinc acetate dihydrate (b) gadolinium nitrate hexahydrate.

Fig. 5. X-ray diffractrometer patterns of zinc oxide particles synthesizedby spray pyrolysis (SP) at 5001C (SP500), pulse combustion (PC), and PCintegrated with SP at 3001, 4001, and 6001C (PCSP300, PCSP400, andPCSP600). Field-emission scanning electron microscopic images showsubmicrometer particle and nanoparticles generated by conventional SP(SP500) and PC integrated with SP at 6001C (PCSP600), respectively.

Fig. 6. Transmission electronic microscopic images of particles gener-ated by (a) pulse combustion (PC), (b) PC integrated with spray pyroly-sis (SP) at 4001C (PCSP400), (c) 6001C (PCSP600), and (d) conventionalSP. The insets correspond to selected analysis electron diffraction.

3782 Journal of the American Ceramic Society—Widiyastuti et al. Vol. 90, No. 12

Amorphous phase particles were observed on particles generat-ed by SP at temperatures of 3001 and 5001C, as was expectedfrom TG-DTG analysis of the precursor. Surprisingly, the XRDpattern of particles synthesized by PC without using a heatedfurnace (PC) is in good agreement with the Gd2O3 referenceeven though they had low crystallinity. The crystallinity of theparticles synthesized by PC combined with SP increased withincreasing furnace temperature.

PL intensity emission at excitation of Gd2O3:Eu31 at 254 nm

as shown in Fig. 9(b) correlates with crystallinity and crystalsize.19 Particles synthesized by PC integrated with SP at 8001C(PCSP800) had the largest peak PL intensity, followed by par-ticles generated by SP only with a furnace wall at 8001C (SP800).PC and PC integrated with a heated furnace at 3001 and 5001C(PC, PCSP300, and PCSP500) generated particles with almostsimilar PL intensities. They showed a significant luminescencepeak of Eu31 at approximately 612 nm, indicating a red5D0-

7F2 transition at C2 sites within europium.19 Particles gen-erated by conventional SP at 3001 and 5001C had the lowest PLintensity. Presumably, PC has a significant role in increasing thePL intensity because PC increases the distribution of Eu31 ac-tivators in the Gd2O3 crystals and chemical homogeneity in-creases the efficiency of the activator Eu31 cations in the hostlattice.20 However, more energy is needed to obtain a certain

crystal size by PCSP. Overall, the results show that the PL in-tensity correlates with both the crystal size and the distributionof the activator in the host lattice. From our result, PC contrib-utes to an increase in both the activation distribution and thecrystal size.

(4) Proposed Particle Formation Mechanism

Experimental results show that a material with a decompositiontemperature lower than the pulse average temperature, such asZnO, can produce nanoparticles. Presumably, the primary par-ticles of nucleated ZnO were formed just before droplets enter-ing the furnace. The initial submicrometer size of ZnO particlestends to undergo fragmentation due to velocity and pressurefluctuations generated by PC. In the case of a higher decompo-sition reaction such as Gd2O3:Eu

31, ellipsoidal–toroidal parti-cles in submicrometer size were generated. PC affects on thesurface droplet-like particles that deformed easily to an ellipsoi-dal–toroidal morphology. In this case, fluctuating velocity andpressure are not strong enough to break the molecular chain in asurface drying precursor.

There are two types of interactions between two differentphases, e.g. fluctuating flow generated by PC (gas phase) andfast precipitated droplets (solid/liquid). These gas–liquidinterface interactions can lead to the fragmentation of agglom-erate crystals in a precipitated droplet in the case of ZnOsynthesis and deformable particles acting as bubbles in thecase of Gd2O3:Eu

31 synthesis depending on their characteris-tics.21,22 Two characteristic dimensionless parameters arebond number and Weber number. The bond number representsthe ratio of the apparent gravity of an impacting particle tothe surface tension force ðBo ¼ ðDrad2

dÞ=sÞ.23Dr, a, dd, and s

are the difference in the densities of the droplet and thesurrounding fluid, the acceleration due to a change in thegas velocity, the droplet size, and the surface tension, respec-tively. The Weber number defines the square root of the ratioof the kinetic energy of the impacting particle to the surfacetension (We 5 (rgU

2dd)/(2s)).24 rg and U are the gas density

and the maximum speed in the acoustic disturbance, respective-ly. High fluctuating oscillation generated by PC yield thehigh bond and Weber numbers. They lead to droplet-like par-ticles’ deformation. However, the ability to disperse primaryparticles in an agglomerate particle formed by evaporation,precipitation, and nucleation of a precursor in a droplet is

Fig. 7. High-resolution transmission electronic microscopic images ofzinc oxide particles generated by (a) pulse combustion and (b) spraypyrolysis at 5001C (SP500).

Fig. 8. Field-emission scanning electron microscopic images of gadolinium oxide-doped with europium particles synthesized by (a) spray pyrolysis (SP)at 8001C (SP800), (b) pulse combustion (PC), and (c) PC integrated with SP at 8001C (PCSP800). (d–f) show high-magnification images.

December 2007 A Pulse Combustion-Spray Pyrolysis Process 3783

influenced by its agglomeration behavior such as primary par-ticle size and number, density, and interparticle bond strength.22

The fragmentation of dried submicrometer nanoparticle aggre-gates into dispersed nanoparticles under low pressure conditionswas also investigated by our group.25 Pressure differencesbetween liquid and solid nanoparticles inside droplets inducedby rapid drying and evaporation rates are considered to bethe main reasons for particle fragmentation or droplet distor-tion. A simple proposed mechanism of ZnO and Gd2O3:Eu

31

particle synthesis by PC is illustrated in Fig. 10. The PC is ac-tually a complex process and will become a specific topic to beinvestigated.

IV. Conclusions

A new method for synthesizing nanoparticles and submicrom-eter-aspherical particles was developed using an integratedPCSP process. Zinc acetate and gadolinium nitrate having low-er and higher decomposition temperatures, respectively, than

the average gas temperature generated by PC were used to gen-erate nano ZnO and submicrometer-aspherical Gd2O3. Thisstudy showed that the precursor decomposition temperaturehas a significant role in determining the final particle morphol-ogy. In addition, PC contributes toward increasing the charac-teristics of the materials prepared such as crystallinity,luminescence, and surface area.

Acknowledgments

The authors wish to thankM. Hazata and T. Ozaki for their assistance with theexperiments, and T. Ogi for TEM analysis. Mr. K. Kato, M. Sabi, and N. Ike fromDai-Ichi Kogyo Seiyaku Co. Ltd., Kyoto, Japan, as well as T. Kubotani and M.Wada from Pultech Corporation, Kobe, Japan, are thanked for valuable discus-sions. The Ministry of Education, Culture, Sports, Science and Technology(MEXT) of Japan and the Japan Society for the Promotion of Science (JSPS)are acknowledged for providing doctoral scholarships (W. W. and A. P.) and apostdoctoral fellowship (W. N. W.), respectively.

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