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Modeling of nanostructural design of ultrafine mullite powder particles obtained by ultrasonic spray pyrolysis
NanoSvuctured
Pergamon
PII SO9659773(99)00132-4
Materials,
Vol. 12, pp. 349-352, 1999 Elsevier Science Ltd 0 1999 Acta Metallunzica Inc. Printed in the USA. All rightskxved 0965-9773/99/$-see front matter
MODELING OF NANOSTRUCTURAL DESIGN OF ULTRAFINE MULLITE POWDER PARTICLES OBTAINED BY ULTRASONIC SPRAY PYROLYSIS ‘V.Jokanovic, 2Dj.Janackovic,3P.Spasic, 4D.Uskokovic, for Technology of Nuclear and Other Mineral Raw Materials, Franchet D*Eperey 86,llOOO Belgrade, Yugoslavia ‘Faculty for Technology and Metallurgy, Belgrade 3Military Medical Academy, Belgrade 41nstitute for Technical Sciences, Serbian Academy of Sciences and Arts, Belgrade ‘Institute
Abstract Ii this paper, a model of substructure des&n of mullite powderpartl’cles obtained by spray pyrolysis in the field of ultrasound excitation is described. Based on this model, the size of subparticles (nanopartices) constituting the substructure of so obtainedpowder is described and compared to experimentally determined values. High agreement between theoretical and experimental values confinned the value of the theoretical model and its wide applicability in estimation of particle substructure for muifite powder obtained under the conditions of the periodica physicaI field activity. 01999 Acta Metallurgica Inc. INTRODUCTION The process of spray pyrolysis performed under influence of the ultrasound force field is a typical process of the material synthesis in periodical physical field. The general model of synthesis, defined and described in our previous works (l-lo), indicated the possibility to determine average size of the powder particles and their size distribution spectrum. Apart from that, by further development of the model (which treats the aerosol drop as macroelement of the system exposed to influence of the periodical physical field, transferred onto drop in the moment of its separation from the liquid meniscus), it becomes possible to derive further substructuring of the system on the level of its basic constituting elements- subdrops. After solidification of the aerosol drops, substructure of nanostructural dimensions is obtained. In this paper, the coding mechanism determining substructure of the powder particles obtained by the ultrasonic spray pyrolysis is presented via the mechanism of harmonisation of the force physical field of the ultrasonic generator and induced oscillating field of the aerosol drop itself. EXPERIMENTAL For the mullite
(M3) synthesis, starting 0.025 M aqueous solution was prepared 349
350
FOURTH INTERNATIONAL
CONFERENCE
ON NANOSTRUCTURED
MATERIALS
in the following way: TEOS previously dissolved in ethanol was added in aqueous solution of Al(N0,),.9H,O (t=353 K, pH=l) with HNOJ addition (as a catalyst for TEOS hydrolysis), in the ratio TEOS:ethanol:water=l:2:17. In this way, the TEOS was completely hydrolyzed forming silicic acid. The solution atomization was carried out with an ultrasonic GAFVSOL-RBI9001 atomizer at a resonant frequency of 2.5 MHz and a capacity of 400 cm3/h, generating aerosol droplets of a mean diameter of 2 pm. Aerosol droplets were then inserted into an air stream at a flow rate of 30-40 dm3/h, in a one meter long furnace, at 900°C. The residence time of droplets/particles inside the furnace was 95 s assuming that the air flow rate and droplet velocities are equal. The heating rate of droplets/particles was WC/s. Transmission electron microscopy (TEM) was done by a JEOL-JEM-1OOcx instrument, with sample preparation by sealing with epoxy resin and by mechanical sample thinning. RESULTS
AND DISCUSSION
Theoretical model After separation of the aerosol drop from the surface of the liquid meniscus, the drop continues to be the carrier of excitation transferred onto it in the moment of its separation, behaving as induced mechanical oscillator whose characteristic frequency is determined by the drop geometry and frequency of excitation by degenerated frequency of the ultrasound oscillator, as already explained. Under influence of the excitation field, centrally symmetric standing wave is formed whose propagation through the aerosol drop causes formation of a great number of nanooscillators-nanodrops, which in the end of the solidification process cause formation of the system subparticles and complete define its substructure. In order to determine diameter of these substructure nanoelements one should start from the wave equation that defines propagation of disturbance through the given medium, as follows:
d2P
-=c
dt*
213--
r* dr (
r 289-
)
dr 1
Ul
where cpis rate potential, c is rate of the wave propagation through the drop, r is drop radius and t is time. Solution of the given equation is of the following form:
v,= f, (ct - r ) + f2 (ct + r) 3
121
r r where f, and fi are random functions. From monochromatic spherical wave (the condition evidently satisfied in our case, since each drop corresponds to one excitation frequency), it is possible to obtain the final solution of equation [2], as follows:
FOURTH INTERNATIONAL CONFERENCE
ON NANOSTRUCTURED
MATERIALS
351
131
where f is drop excitation frequency, A is wave amplitude and k=w/c is wave number (w is circular frequency). On condition that dcp/dr = 0 and r=R, the following equation is obtained: tgkR = kR
141
Graphic solution of equation [4] has the following form:
where N and N’ are the number constants, taking the following values: N=4.49; 7.6; 10.8; 14.0; 17.2; 24.0;. . .; and N’=0.715; 1.21; 1.72; 2.23; 2.74; 3.82;... Based on obtained values of the subdrop frequency, the radii of subparticles / nanoparticles were determined according to the following equation:
Fl where d, is particle diameter, c is concentration density and M, is the solution molecular weight. M 100
of the spraying solution,
p is mullite
3exp
80 * 0 ,'
60 40 20 0
l-
Fig.2 TEM micrograph of mullite powder. Fig.1. Theoretically estimated and experimantaly determined subparticle size. Analysis of the nanostructural design of selected mullite powder particles was carried out with particles close in size to the average particle size. Theoretically determined values of frequency and size of the aerosol subdrops, determined according to equations [S] for average size of aerosol drops (l-5), and size of respective
352
FOURTH INTERNATIONAL
CONFERENCE
ON NANOSTRUCTURED
MATERIALS
subparticles - nanoparticles constituting substructure of the given powder, determined according to equation [6] are shown in Fig.1. On assumption that size of spherical subdrops (subparticles) is uniform, theoretically determined value (M3r) was found to be 79.3 nm (subdrop size) and 19.6 nm (subparticle size) for M3 powder. If replacing theoretically by experimentally obtained average size and calculated anticipated subparticle size from them, the value of 23.79 nm (M3eXpT) is obtained for mullite powder. Analysis of experimentally determined size of powder subparticles (Msexp) (by analysing the image obtained by the transmission electron microscopy) showed that their average values of mullite powder were in the range of about 20 nm (Fig.2). Following to subsequent crystallisation and sintering of obtained mullite powders, the nanocrystals whose diameter remained unchanged (x 20 nm) were connected into cylinders, about 100 nm long, formed by linking of particles along certain direction (powder particle diameter) . CONCLUSION In this paper, results of the general theoretical model enabling one to determine size of particles - subparticles obtained by harmonisation of given periodical field and internal physical field induced in the system exposed to such excitation, are presented. Verification of this model was performed with mullite powder, obtained by spraying in ultrasonic field. Results obtained by verification of agreement between theoretically estimated and experimentally determined size of particles and subparticles and their population balances confirmed that these values could be theoretically predicted. This fact is of particular importance because it indicates the possibility of the structural coding even at the finest, nanolevel, with high degree of certainty. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Jokanovic, V., JanackoviC, Dj., SpasiC, A.M., Uskokovic, D., Mater.Trans., JIM, 1996, 37, 627. Jokanovic, V., Janackovic, Dj., Uskokovic, D.,, Key Engin. Mater.,132-136, (1997), 213. JanaCkoviC, Dj., JokanoviC, V., Kostic-GvozdenoviC, Lj., Zivkovic, Lj., Uskokovic, D., J.Mater.Res., 1996,ll, 1706. JanaCkoviC, Dj., JokanoviC, V., Kostic-Gvozdenovic, Lj., Uskokovic, D., Mater.Sci. Forum., 1996,2&215. JanaCkoviC, Dj., Jokanovic, V., KostiC-Gvozdenovic, Lj., Zec, S., Uskokovic, D., J.Mater.Sci., 1997,32,163. Janackovic, Dj., Jokanovic, V., KostiOGvozdenoviC, Lj., UskokoviC, D., Nanostruct.Mater., 1998,lO, (accepted for publication). NedeljkoviC,J., Saponjic, Z., RakoEeviC, Z., Jokanovic, V., Uskokovic, D., Nanostruct.Mater., 1997,!& 125. Saponjic, Z.V., RakoEeviC, Z., Dimitrijevib, N.M., Nedeljkovic, J.M., Jokanovic, V., Uskokovic, D.P., Nanostruct.Mater., 1998, lo, (accepted.for publication). Jokanovic, V., JanaCkoviC, Dj., CurBC, R., ZivanoviC, P., Uskokovic, D., Mater.Sci.Forum, 1998,282-283,65. Jokanovic, V., Janackovic, Dj., Uskokovic, D., 1998, (to be published).
Pergamon
PII SO9659773(99)00132-4
Materials,
Vol. 12, pp. 349-352, 1999 Elsevier Science Ltd 0 1999 Acta Metallunzica Inc. Printed in the USA. All rightskxved 0965-9773/99/$-see front matter
MODELING OF NANOSTRUCTURAL DESIGN OF ULTRAFINE MULLITE POWDER PARTICLES OBTAINED BY ULTRASONIC SPRAY PYROLYSIS ‘V.Jokanovic, 2Dj.Janackovic,3P.Spasic, 4D.Uskokovic, for Technology of Nuclear and Other Mineral Raw Materials, Franchet D*Eperey 86,llOOO Belgrade, Yugoslavia ‘Faculty for Technology and Metallurgy, Belgrade 3Military Medical Academy, Belgrade 41nstitute for Technical Sciences, Serbian Academy of Sciences and Arts, Belgrade ‘Institute
Abstract Ii this paper, a model of substructure des&n of mullite powderpartl’cles obtained by spray pyrolysis in the field of ultrasound excitation is described. Based on this model, the size of subparticles (nanopartices) constituting the substructure of so obtainedpowder is described and compared to experimentally determined values. High agreement between theoretical and experimental values confinned the value of the theoretical model and its wide applicability in estimation of particle substructure for muifite powder obtained under the conditions of the periodica physicaI field activity. 01999 Acta Metallurgica Inc. INTRODUCTION The process of spray pyrolysis performed under influence of the ultrasound force field is a typical process of the material synthesis in periodical physical field. The general model of synthesis, defined and described in our previous works (l-lo), indicated the possibility to determine average size of the powder particles and their size distribution spectrum. Apart from that, by further development of the model (which treats the aerosol drop as macroelement of the system exposed to influence of the periodical physical field, transferred onto drop in the moment of its separation from the liquid meniscus), it becomes possible to derive further substructuring of the system on the level of its basic constituting elements- subdrops. After solidification of the aerosol drops, substructure of nanostructural dimensions is obtained. In this paper, the coding mechanism determining substructure of the powder particles obtained by the ultrasonic spray pyrolysis is presented via the mechanism of harmonisation of the force physical field of the ultrasonic generator and induced oscillating field of the aerosol drop itself. EXPERIMENTAL For the mullite
(M3) synthesis, starting 0.025 M aqueous solution was prepared 349
350
FOURTH INTERNATIONAL
CONFERENCE
ON NANOSTRUCTURED
MATERIALS
in the following way: TEOS previously dissolved in ethanol was added in aqueous solution of Al(N0,),.9H,O (t=353 K, pH=l) with HNOJ addition (as a catalyst for TEOS hydrolysis), in the ratio TEOS:ethanol:water=l:2:17. In this way, the TEOS was completely hydrolyzed forming silicic acid. The solution atomization was carried out with an ultrasonic GAFVSOL-RBI9001 atomizer at a resonant frequency of 2.5 MHz and a capacity of 400 cm3/h, generating aerosol droplets of a mean diameter of 2 pm. Aerosol droplets were then inserted into an air stream at a flow rate of 30-40 dm3/h, in a one meter long furnace, at 900°C. The residence time of droplets/particles inside the furnace was 95 s assuming that the air flow rate and droplet velocities are equal. The heating rate of droplets/particles was WC/s. Transmission electron microscopy (TEM) was done by a JEOL-JEM-1OOcx instrument, with sample preparation by sealing with epoxy resin and by mechanical sample thinning. RESULTS
AND DISCUSSION
Theoretical model After separation of the aerosol drop from the surface of the liquid meniscus, the drop continues to be the carrier of excitation transferred onto it in the moment of its separation, behaving as induced mechanical oscillator whose characteristic frequency is determined by the drop geometry and frequency of excitation by degenerated frequency of the ultrasound oscillator, as already explained. Under influence of the excitation field, centrally symmetric standing wave is formed whose propagation through the aerosol drop causes formation of a great number of nanooscillators-nanodrops, which in the end of the solidification process cause formation of the system subparticles and complete define its substructure. In order to determine diameter of these substructure nanoelements one should start from the wave equation that defines propagation of disturbance through the given medium, as follows:
d2P
-=c
dt*
213--
r* dr (
r 289-
)
dr 1
Ul
where cpis rate potential, c is rate of the wave propagation through the drop, r is drop radius and t is time. Solution of the given equation is of the following form:
v,= f, (ct - r ) + f2 (ct + r) 3
121
r r where f, and fi are random functions. From monochromatic spherical wave (the condition evidently satisfied in our case, since each drop corresponds to one excitation frequency), it is possible to obtain the final solution of equation [2], as follows:
FOURTH INTERNATIONAL CONFERENCE
ON NANOSTRUCTURED
MATERIALS
351
131
where f is drop excitation frequency, A is wave amplitude and k=w/c is wave number (w is circular frequency). On condition that dcp/dr = 0 and r=R, the following equation is obtained: tgkR = kR
141
Graphic solution of equation [4] has the following form:
where N and N’ are the number constants, taking the following values: N=4.49; 7.6; 10.8; 14.0; 17.2; 24.0;. . .; and N’=0.715; 1.21; 1.72; 2.23; 2.74; 3.82;... Based on obtained values of the subdrop frequency, the radii of subparticles / nanoparticles were determined according to the following equation:
Fl where d, is particle diameter, c is concentration density and M, is the solution molecular weight. M 100
of the spraying solution,
p is mullite
3exp
80 * 0 ,'
60 40 20 0
l-
Fig.2 TEM micrograph of mullite powder. Fig.1. Theoretically estimated and experimantaly determined subparticle size. Analysis of the nanostructural design of selected mullite powder particles was carried out with particles close in size to the average particle size. Theoretically determined values of frequency and size of the aerosol subdrops, determined according to equations [S] for average size of aerosol drops (l-5), and size of respective
352
FOURTH INTERNATIONAL
CONFERENCE
ON NANOSTRUCTURED
MATERIALS
subparticles - nanoparticles constituting substructure of the given powder, determined according to equation [6] are shown in Fig.1. On assumption that size of spherical subdrops (subparticles) is uniform, theoretically determined value (M3r) was found to be 79.3 nm (subdrop size) and 19.6 nm (subparticle size) for M3 powder. If replacing theoretically by experimentally obtained average size and calculated anticipated subparticle size from them, the value of 23.79 nm (M3eXpT) is obtained for mullite powder. Analysis of experimentally determined size of powder subparticles (Msexp) (by analysing the image obtained by the transmission electron microscopy) showed that their average values of mullite powder were in the range of about 20 nm (Fig.2). Following to subsequent crystallisation and sintering of obtained mullite powders, the nanocrystals whose diameter remained unchanged (x 20 nm) were connected into cylinders, about 100 nm long, formed by linking of particles along certain direction (powder particle diameter) . CONCLUSION In this paper, results of the general theoretical model enabling one to determine size of particles - subparticles obtained by harmonisation of given periodical field and internal physical field induced in the system exposed to such excitation, are presented. Verification of this model was performed with mullite powder, obtained by spraying in ultrasonic field. Results obtained by verification of agreement between theoretically estimated and experimentally determined size of particles and subparticles and their population balances confirmed that these values could be theoretically predicted. This fact is of particular importance because it indicates the possibility of the structural coding even at the finest, nanolevel, with high degree of certainty. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Jokanovic, V., JanackoviC, Dj., SpasiC, A.M., Uskokovic, D., Mater.Trans., JIM, 1996, 37, 627. Jokanovic, V., Janackovic, Dj., Uskokovic, D.,, Key Engin. Mater.,132-136, (1997), 213. JanaCkoviC, Dj., JokanoviC, V., Kostic-GvozdenoviC, Lj., Zivkovic, Lj., Uskokovic, D., J.Mater.Res., 1996,ll, 1706. JanaCkoviC, Dj., JokanoviC, V., Kostic-Gvozdenovic, Lj., Uskokovic, D., Mater.Sci. Forum., 1996,2&215. JanaCkoviC, Dj., Jokanovic, V., KostiC-Gvozdenovic, Lj., Zec, S., Uskokovic, D., J.Mater.Sci., 1997,32,163. Janackovic, Dj., Jokanovic, V., KostiOGvozdenoviC, Lj., UskokoviC, D., Nanostruct.Mater., 1998,lO, (accepted for publication). NedeljkoviC,J., Saponjic, Z., RakoEeviC, Z., Jokanovic, V., Uskokovic, D., Nanostruct.Mater., 1997,!& 125. Saponjic, Z.V., RakoEeviC, Z., Dimitrijevib, N.M., Nedeljkovic, J.M., Jokanovic, V., Uskokovic, D.P., Nanostruct.Mater., 1998, lo, (accepted.for publication). Jokanovic, V., JanaCkoviC, Dj., CurBC, R., ZivanoviC, P., Uskokovic, D., Mater.Sci.Forum, 1998,282-283,65. Jokanovic, V., Janackovic, Dj., Uskokovic, D., 1998, (to be published).