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Characterization of nanophase titania particles processed in different flame reactors
J. Aerosol Sci., Vol. 26. Suppl 1, pp. $559-$560, 1995
Elsevier Science Lid Printed in Great Britain 0021-8502/95 $9.50 + 0.00
Pergamon
CHARACTERIZATION OF NANOPHASE TITANIA PARTICLES PROCESSED IN DIFFERENT FLAME REACTORS Guixiang Yang and Pratim Biswas Aerosol and Air Quality Research Laboratory Department of Environmental Engineering University of Cincinnati Cincinnati, Ohio 45221-0071. Flame aerosol reactors offer a potential route for the large scale processing of nanophase materials. Titania is industrially produced in large quantities in flame reactors, but typically in the submicrometer size ranges. Depending on the time-temperature history, different phases of titania have been produced. Titania has a number of different applications: the rutile phase is used as a pigmentary material, the anatase phase is known to be an effective photocatalyst, and the nanometer sizes are efficient uv absorbers and transparent in the visible regions, thus potentially having applications as cosmetic sunscreens. At very low processing temperatures, the amorphous phase has been obtained, at moderate temperatures the anatase phase has been obtained, and at very high temperatures the rutile phase is typically obtained. The objective of this work was to produce nanophase titania particles in two different designs of flame reactors: a premixed burner and a multiport, diffusion flame burner. The goal was to identify conditions to produce amorphous and anatase phases of titania using these flame reactors, and relate final product characteristics to operating conditions. The powders were characterized by X-ray diffraction and transmission electron microscopy. In situ light scattering measurements were also made to relate the evolution of the second aerosol volume moment to processing conditions. Two types of burners were used in the study: premixed, flat flame burners (Chang and Biswas, 1992) and a multiport, diffusion flame burner (Linet al., 1992). In the first case, methane, air and titanium tetraisopropoxide (atomized in an air stream) were premixed before the flame. Equivalence ratios were used to obtain temperatures around 1150 nc, as measured by radiation ratio thermometry and thermocouples. In the second case, a concentric multiport burner was used, with the atomized titanium tetraisopropoxide in air flowing in the inner most port, surrounded by methane in the next port, and by oxygen in the outer most port. The flow of oxygen was varied (0 to 4 lpm) to obtain different temperatures ranging from 1100 to 1600 °C. Additionally, experiments were conducted with a very high equivalence ratio (and low methane flow rates) powders were collected with a very short residence time in the flame zone as compared to the previous conditions. The powders were collected thermophoretically on a cooled surface close to the exit of the burner face. The powders were characterized by x-ray diffraction, and transmission electron microscopy. In addition, the light scattering intensity was monitored at 90° as a function of axial position with an Argon ion laser (400 mW) as the incident source. The different experimental conditions along with the peak temperatures and results of the Xray diffraction analysis (ratio ofrutile to anatase) are listed in Table 1. Conditions to maximize anatase formation were obtained close to 1500 C in our system. There is a lot of interest in the processing of near pure anatase as it has applications as a photocatalytic material and as an effective uv light absorber. The trends in the temperature range of 1000 to 1250 °C are similar to that reported by Kobata et al. (1991). The reasons for the specific crystal transformations and their dependence on the temperature are currently being investigated; another important factor also being the residence time (typically very short, of the order of a few milliseconds) in the flame region. The powders were also analyzed by Transmission Electron Microscopy, and the particle sizes ranged
$559
$560
G. YANG a n d P. BlSWAS
from 25 nm to 100 nm for the different experiments. For lower inlet concentrations and shorter residence times, particles as small as 25 nm could be readily produced. Using extremely short residence times, amorphous titanium dioxide samples could also be produced (Tests 16 and 17). The in situ light scattering intensity measurements (without any collection surface) for two different temperatures are shown in the Figure below. Close to the burner exit, the light scattering intensity (as particles are small, this is proportional to the second volume moment) increases as the particle concentration increases (formation by oxidation of the precursor, and growth of particles by coagulation). After reaching the peak value, the measured light scattering intensity decreases, this is possibly due to the sintering of the coagulated particles. At higher temperatures, the sintering rates are higher, and as the particles tend to spherical shapes at a faster rate, the light scattering intensity also drops off' faster. Figure 1
Table 1 Burner Type premixed burner
multiport diffusion burner A
multipon diffusion burner B
Flowr~te (liter/rain.)
!Test
NO I CFh 1 [ 1.00 t 2 0.3 3 0.3 4 0.6 5 0.6 6 0.6 7 0.6 8 0.9 9 0.9 10 0.9 11 1.20 12 1.20 13 1.20 [ 14 1.2 15 1.58 16 0.16
Resifl time
Air 8.3
02 3.0
3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 2.6 4.0... 7.0
0.0 1.2 00 1.2 2.0 3.0 1.5 2.5 3.5 2.0 3.0 4.0 2.0 3.2 2.5
Reac
25.
t Temp C 1150
7.0 3.0
1100 1300
lO.
13oo
12. 11. 9.0 20. 19.
141o 1410 1430 1510 1510
2!.
17.
lh'O 1520
20. 17. 22. 15. 1.0
1540 1550 1,580 1570 350
ms
Flame Temp C
,
1150
Powder Phase ,
C,ompsition
O / / , \ J ......... II. 't..... rill.......... !I "/" ~1~" x
10~0.~tile/anatase
1150 15~*'rutile/anatase 1600 20% rutile/anatase 1170 23% rutile/anatase 1620 48% rutile/anatase 1620 I 50% rutile./anatase 1620 30% ~tile./anatase 1650 100% anatase 1650 100% anatase 1650 100% anatase 1740 100% anatase 1740 100% anatase 1740 100.%anatase 1770 100% anatase 1780 100% anatase 900 100% amorphous
I
0.35
7.0
2.5
0.9
400 r 1080
mixture anatase/rutile/amor.
~X
......... •
k
l .: ira ! t
'.... . . . . tk'-- e \
//
"'111 \
\
Ii J~ 1,"
-@"" N ~ e ~ 1 ~ " ~ ....U.-" F l ~ e t ~ l ~ ' ~
I
= 1490K = 12"/0 K
I I
Q 0
I
1 17
:"
I 2
r 3
....
I 4
I 5
I 6
Distance from burner outlet (crn)
In summary, by altering conditions in the burner, different phases of titania could be produced. Conditions to maximize anatase titania formation were obtained, and time-temperature histories could be controlled to even obtain the amorphous phase. As flame reactors can be designed for larger scale production, this is potentially an effective route for processing of nanophase anatase and nanophase anatase-amorphous mixtures of titania. References Chang, H. and Biswas, P. (1992) L Colloid and Interface Sci., 153(1), 157-166. Kobata A., Kusakabe K. And Morooka S. (1991) A.I.Ch.E. L, 37, 347-359. Lin, W.Y., Sethi V. and Biswas P. (1992) Aerosol Sci. Technol., 17, 119-133.
Elsevier Science Lid Printed in Great Britain 0021-8502/95 $9.50 + 0.00
Pergamon
CHARACTERIZATION OF NANOPHASE TITANIA PARTICLES PROCESSED IN DIFFERENT FLAME REACTORS Guixiang Yang and Pratim Biswas Aerosol and Air Quality Research Laboratory Department of Environmental Engineering University of Cincinnati Cincinnati, Ohio 45221-0071. Flame aerosol reactors offer a potential route for the large scale processing of nanophase materials. Titania is industrially produced in large quantities in flame reactors, but typically in the submicrometer size ranges. Depending on the time-temperature history, different phases of titania have been produced. Titania has a number of different applications: the rutile phase is used as a pigmentary material, the anatase phase is known to be an effective photocatalyst, and the nanometer sizes are efficient uv absorbers and transparent in the visible regions, thus potentially having applications as cosmetic sunscreens. At very low processing temperatures, the amorphous phase has been obtained, at moderate temperatures the anatase phase has been obtained, and at very high temperatures the rutile phase is typically obtained. The objective of this work was to produce nanophase titania particles in two different designs of flame reactors: a premixed burner and a multiport, diffusion flame burner. The goal was to identify conditions to produce amorphous and anatase phases of titania using these flame reactors, and relate final product characteristics to operating conditions. The powders were characterized by X-ray diffraction and transmission electron microscopy. In situ light scattering measurements were also made to relate the evolution of the second aerosol volume moment to processing conditions. Two types of burners were used in the study: premixed, flat flame burners (Chang and Biswas, 1992) and a multiport, diffusion flame burner (Linet al., 1992). In the first case, methane, air and titanium tetraisopropoxide (atomized in an air stream) were premixed before the flame. Equivalence ratios were used to obtain temperatures around 1150 nc, as measured by radiation ratio thermometry and thermocouples. In the second case, a concentric multiport burner was used, with the atomized titanium tetraisopropoxide in air flowing in the inner most port, surrounded by methane in the next port, and by oxygen in the outer most port. The flow of oxygen was varied (0 to 4 lpm) to obtain different temperatures ranging from 1100 to 1600 °C. Additionally, experiments were conducted with a very high equivalence ratio (and low methane flow rates) powders were collected with a very short residence time in the flame zone as compared to the previous conditions. The powders were collected thermophoretically on a cooled surface close to the exit of the burner face. The powders were characterized by x-ray diffraction, and transmission electron microscopy. In addition, the light scattering intensity was monitored at 90° as a function of axial position with an Argon ion laser (400 mW) as the incident source. The different experimental conditions along with the peak temperatures and results of the Xray diffraction analysis (ratio ofrutile to anatase) are listed in Table 1. Conditions to maximize anatase formation were obtained close to 1500 C in our system. There is a lot of interest in the processing of near pure anatase as it has applications as a photocatalytic material and as an effective uv light absorber. The trends in the temperature range of 1000 to 1250 °C are similar to that reported by Kobata et al. (1991). The reasons for the specific crystal transformations and their dependence on the temperature are currently being investigated; another important factor also being the residence time (typically very short, of the order of a few milliseconds) in the flame region. The powders were also analyzed by Transmission Electron Microscopy, and the particle sizes ranged
$559
$560
G. YANG a n d P. BlSWAS
from 25 nm to 100 nm for the different experiments. For lower inlet concentrations and shorter residence times, particles as small as 25 nm could be readily produced. Using extremely short residence times, amorphous titanium dioxide samples could also be produced (Tests 16 and 17). The in situ light scattering intensity measurements (without any collection surface) for two different temperatures are shown in the Figure below. Close to the burner exit, the light scattering intensity (as particles are small, this is proportional to the second volume moment) increases as the particle concentration increases (formation by oxidation of the precursor, and growth of particles by coagulation). After reaching the peak value, the measured light scattering intensity decreases, this is possibly due to the sintering of the coagulated particles. At higher temperatures, the sintering rates are higher, and as the particles tend to spherical shapes at a faster rate, the light scattering intensity also drops off' faster. Figure 1
Table 1 Burner Type premixed burner
multiport diffusion burner A
multipon diffusion burner B
Flowr~te (liter/rain.)
!Test
NO I CFh 1 [ 1.00 t 2 0.3 3 0.3 4 0.6 5 0.6 6 0.6 7 0.6 8 0.9 9 0.9 10 0.9 11 1.20 12 1.20 13 1.20 [ 14 1.2 15 1.58 16 0.16
Resifl time
Air 8.3
02 3.0
3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 2.6 4.0... 7.0
0.0 1.2 00 1.2 2.0 3.0 1.5 2.5 3.5 2.0 3.0 4.0 2.0 3.2 2.5
Reac
25.
t Temp C 1150
7.0 3.0
1100 1300
lO.
13oo
12. 11. 9.0 20. 19.
141o 1410 1430 1510 1510
2!.
17.
lh'O 1520
20. 17. 22. 15. 1.0
1540 1550 1,580 1570 350
ms
Flame Temp C
,
1150
Powder Phase ,
C,ompsition
O / / , \ J ......... II. 't..... rill.......... !I "/" ~1~" x
10~0.~tile/anatase
1150 15~*'rutile/anatase 1600 20% rutile/anatase 1170 23% rutile/anatase 1620 48% rutile/anatase 1620 I 50% rutile./anatase 1620 30% ~tile./anatase 1650 100% anatase 1650 100% anatase 1650 100% anatase 1740 100% anatase 1740 100% anatase 1740 100.%anatase 1770 100% anatase 1780 100% anatase 900 100% amorphous
I
0.35
7.0
2.5
0.9
400 r 1080
mixture anatase/rutile/amor.
~X
......... •
k
l .: ira ! t
'.... . . . . tk'-- e \
//
"'111 \
\
Ii J~ 1,"
-@"" N ~ e ~ 1 ~ " ~ ....U.-" F l ~ e t ~ l ~ ' ~
I
= 1490K = 12"/0 K
I I
Q 0
I
1 17
:"
I 2
r 3
....
I 4
I 5
I 6
Distance from burner outlet (crn)
In summary, by altering conditions in the burner, different phases of titania could be produced. Conditions to maximize anatase titania formation were obtained, and time-temperature histories could be controlled to even obtain the amorphous phase. As flame reactors can be designed for larger scale production, this is potentially an effective route for processing of nanophase anatase and nanophase anatase-amorphous mixtures of titania. References Chang, H. and Biswas, P. (1992) L Colloid and Interface Sci., 153(1), 157-166. Kobata A., Kusakabe K. And Morooka S. (1991) A.I.Ch.E. L, 37, 347-359. Lin, W.Y., Sethi V. and Biswas P. (1992) Aerosol Sci. Technol., 17, 119-133.