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Generation of ultrafine gold aerosols
J. Aerosol Sci., Vol. 17, No. 3, pp. 477-480, 1986.
0021-8502/86 $3.00+0.00 Pergamon Journals Ltd.
Printed in Great Britain.
GENERATION OF ULTRAFINE G O L D AEROSOLS C. ROTH Gesellschaft ftir Strahlen- und Umweltforschung m.b.H., Abteilung for Biophysikalische Strahlenforschung. PaulEhrlich-StraBe 20, 6000 Frankfurt am Main, F.R.G.
Inhalation risk of ultrafine particles has been underestimated for a long time because of the small mass of these particles. But with increasing industrialization the atmospheric aerosol is burdened with an increasing quantity of these small particles originating from burning processes (e.g. diesel exhaust), so that the interest in them has grown. Total deposition of ultrafine particles in the human respiratory tract was measured using a model aerosol of silver particles. Regional deposition data can be obtained by determining the clearance of particles from the human lung. These studies require an aerosol which can be labelled with a radio-isotope of short half-life. For this reason the known method of generating an ultrafine aerosol by homogeneous condensation of a supersaturated vapour is extended to the production of gold aerosols. In the tube furnace described by Scheibel (1983), silver wool positioned in a tube boat in the center of the furnace is vaporized into a regulated N2-gas stream. In the colder parts of the tube (condensation chimney) an ultrafine aerosol is formed. Concentration and size of the aerosol particles are a function of the vapour pressure of the material at a given temperature and of the temperature gradient along the condensation chimney. Because of the lower vapour pressure of gold in comparison to silver, gold has to be vaporized at higher temperatures. But at about 1400°C the evaporation of the wall material starts even after preheating of the tubes, so that the aerosol particles are no longer composed of the pure vaporized material. For this reason the condensation chimney had to be constructed out of different heat insulating and cooling elements in order to change the temperature gradient along the chimney. The final construction was determined experimentally with respect to a small standard deviation and a large modal diameter of the produced particle size distribution. The design of the tube furnace with the condensation chimney is shown in Fig. 1. In Fig. 2 the temperature profile generated in the furnace and chimney is plotted for a N2-gas flow rate of 3.6 1min- 1. With this chimney and a limited range of gas flow rates it is possible to produce gold particles in a size range between 2 and 50 nm with a furnace temperature below 1400°C. The dependence of the number concentration and of the particle size (modal diameter of the size distribution) of the produced aerosol on flow rate and temperature is shown in Figs 3-6. The number concentration of the aerosol is measured with a continuous condensation nucleus counter combined with controlled dilution systems. The calibration of the counter
.l..
It
................
L;'"
.... '
3 5 0 \ -~ Metallic tube
!
..
r--
Ceromic tube |
l
/
Tube furnoce
~Ceromic
... q L- , r . . . . . . . . . . . . . .N=~. vessel
/L
Fig. 1. Tube furnace and condensation chimney. 477
"'[.~J
478
C. ROTH Length of tube furnace
I [~N2
[\
/2
"~
zoo I \
Speci( construction
"'-* \,Without
heat
~4~...~...~q
\ insu at on
I.
\
-IO -5 5 I0 15 20 25 30 30 40 45 Distance to furnace center OM,(cm)
Fig. 2. Temperature profile of the apparatus.
jf.f'-'-
~F [0 e
@
8 o
I0 7
/
u
~
IO6
z
Z
105 I100
HSO
/ I 1200
J 1250
* ~300
i 1~50
Temperature, T (°C)
Fig. 3. Dependence of number concentration, CN, of the gold aerosol on temperature T ( Q = 3.61 min-1).
.z
/
108
/
~0~
/
Jc#
10 5 z
i
rO 4 0
i
2 3 Ftow rate, O(Irnin -r)
Fig. 4. Dependence of number concentratxon,
cN ,
of the gold aerosol on flow rate (T = 1320°C).
Generation of ultrafine gold aerosols
479
4O
E
<'b
-~ 3O
..y,J"/f
~ 2O
8 tO
/ o
i 1150
i
1200
i
1250
i
1300
1350
Temperoture, T (°C) Fig. 5.
Dependence of the modal diameter, a, of the gold aerosol size distributions on temperature, T (Q = 3.61 r a i n - 1).
40 <~
E o x3 o £
o ~ o ~ O
°°4~
20
0
i
i
i
i
i
I
2
3
4
5
Flow rate, 0 (train -~)
Fig. 6.
Dependence of the modal diameter, d, of the gold aerosol size distributions on N2-ttow rate,
Q(T=
1320°C).
and the dilution system are described elsewhere (Dreiling et al., 1985). The error of the number concentration measurement caused by diffusional losses of particles in the connecting tubes and by the accuracy of the different flow rate measurements is less than 10 %. The particle size distribution is measured with a differential mobility analyzer and an electrometer. The particle diameter is evaluated out of the mobility analyzer voltage but corrected according to the input aerosol size distribution, the charge distribution of the particles and the finite width of the analyzer slit (Roth et al., 1986). It was found that the modal diameter of the particles can be more easily varied by temperature than by flow rate variation, in contrast to the number concentration, which should be altered by different flow rates.
480
c. ROTH
The geometric standard deviation of the size distribution is nearly independent of both parameters and has a value of cr = 1.25. To obtain a high surface-to-volume ratio, necessary for vaporization, a gold foil of 15 #m thickness is used. The concentration of the aerosol formed by evaporation of the tube material is in all cases at least three orders of magnitude smaller than the concentration of the gold aerosol and is measured by exchanging the used tubes. Additionally, the gold particles were deposited on grids by means of an electrostatic aerosol sampler and checked electronmicroscopically. The photographed particles were 5-35 nm in size and had a spherical shape. The investigations demonstrate that the concentration of the produced aerosol is sufficiently high for inhalation and clearance experiments with radioactively-labelled ultrafine particles to be performed. REFERENCES Dreiling, V., Hailer, P., Helsper,C., Kaminski,U., Plomp, A., Raes,F., Roth, C., Schier,J. and Schuermann,G. (1985) 13th Conference of the Gaef, 25-27 Sept., Garmisch-Partenkirchen. Roth, C., Berlauer, U. and Schiller Ch. F., (1986). Scheibel, H. G. and Porstend6rfer, J. (1983) J. Aerosol Sci. 14, 113.
0021-8502/86 $3.00+0.00 Pergamon Journals Ltd.
Printed in Great Britain.
GENERATION OF ULTRAFINE G O L D AEROSOLS C. ROTH Gesellschaft ftir Strahlen- und Umweltforschung m.b.H., Abteilung for Biophysikalische Strahlenforschung. PaulEhrlich-StraBe 20, 6000 Frankfurt am Main, F.R.G.
Inhalation risk of ultrafine particles has been underestimated for a long time because of the small mass of these particles. But with increasing industrialization the atmospheric aerosol is burdened with an increasing quantity of these small particles originating from burning processes (e.g. diesel exhaust), so that the interest in them has grown. Total deposition of ultrafine particles in the human respiratory tract was measured using a model aerosol of silver particles. Regional deposition data can be obtained by determining the clearance of particles from the human lung. These studies require an aerosol which can be labelled with a radio-isotope of short half-life. For this reason the known method of generating an ultrafine aerosol by homogeneous condensation of a supersaturated vapour is extended to the production of gold aerosols. In the tube furnace described by Scheibel (1983), silver wool positioned in a tube boat in the center of the furnace is vaporized into a regulated N2-gas stream. In the colder parts of the tube (condensation chimney) an ultrafine aerosol is formed. Concentration and size of the aerosol particles are a function of the vapour pressure of the material at a given temperature and of the temperature gradient along the condensation chimney. Because of the lower vapour pressure of gold in comparison to silver, gold has to be vaporized at higher temperatures. But at about 1400°C the evaporation of the wall material starts even after preheating of the tubes, so that the aerosol particles are no longer composed of the pure vaporized material. For this reason the condensation chimney had to be constructed out of different heat insulating and cooling elements in order to change the temperature gradient along the chimney. The final construction was determined experimentally with respect to a small standard deviation and a large modal diameter of the produced particle size distribution. The design of the tube furnace with the condensation chimney is shown in Fig. 1. In Fig. 2 the temperature profile generated in the furnace and chimney is plotted for a N2-gas flow rate of 3.6 1min- 1. With this chimney and a limited range of gas flow rates it is possible to produce gold particles in a size range between 2 and 50 nm with a furnace temperature below 1400°C. The dependence of the number concentration and of the particle size (modal diameter of the size distribution) of the produced aerosol on flow rate and temperature is shown in Figs 3-6. The number concentration of the aerosol is measured with a continuous condensation nucleus counter combined with controlled dilution systems. The calibration of the counter
.l..
It
................
L;'"
.... '
3 5 0 \ -~ Metallic tube
!
..
r--
Ceromic tube |
l
/
Tube furnoce
~Ceromic
... q L- , r . . . . . . . . . . . . . .N=~. vessel
/L
Fig. 1. Tube furnace and condensation chimney. 477
"'[.~J
478
C. ROTH Length of tube furnace
I [~N2
[\
/2
"~
zoo I \
Speci( construction
"'-* \,Without
heat
~4~...~...~q
\ insu at on
I.
\
-IO -5 5 I0 15 20 25 30 30 40 45 Distance to furnace center OM,(cm)
Fig. 2. Temperature profile of the apparatus.
jf.f'-'-
~F [0 e
@
8 o
I0 7
/
u
~
IO6
z
Z
105 I100
HSO
/ I 1200
J 1250
* ~300
i 1~50
Temperature, T (°C)
Fig. 3. Dependence of number concentration, CN, of the gold aerosol on temperature T ( Q = 3.61 min-1).
.z
/
108
/
~0~
/
Jc#
10 5 z
i
rO 4 0
i
2 3 Ftow rate, O(Irnin -r)
Fig. 4. Dependence of number concentratxon,
cN ,
of the gold aerosol on flow rate (T = 1320°C).
Generation of ultrafine gold aerosols
479
4O
E
<'b
-~ 3O
..y,J"/f
~ 2O
8 tO
/ o
i 1150
i
1200
i
1250
i
1300
1350
Temperoture, T (°C) Fig. 5.
Dependence of the modal diameter, a, of the gold aerosol size distributions on temperature, T (Q = 3.61 r a i n - 1).
40 <~
E o x3 o £
o ~ o ~ O
°°4~
20
0
i
i
i
i
i
I
2
3
4
5
Flow rate, 0 (train -~)
Fig. 6.
Dependence of the modal diameter, d, of the gold aerosol size distributions on N2-ttow rate,
Q(T=
1320°C).
and the dilution system are described elsewhere (Dreiling et al., 1985). The error of the number concentration measurement caused by diffusional losses of particles in the connecting tubes and by the accuracy of the different flow rate measurements is less than 10 %. The particle size distribution is measured with a differential mobility analyzer and an electrometer. The particle diameter is evaluated out of the mobility analyzer voltage but corrected according to the input aerosol size distribution, the charge distribution of the particles and the finite width of the analyzer slit (Roth et al., 1986). It was found that the modal diameter of the particles can be more easily varied by temperature than by flow rate variation, in contrast to the number concentration, which should be altered by different flow rates.
480
c. ROTH
The geometric standard deviation of the size distribution is nearly independent of both parameters and has a value of cr = 1.25. To obtain a high surface-to-volume ratio, necessary for vaporization, a gold foil of 15 #m thickness is used. The concentration of the aerosol formed by evaporation of the tube material is in all cases at least three orders of magnitude smaller than the concentration of the gold aerosol and is measured by exchanging the used tubes. Additionally, the gold particles were deposited on grids by means of an electrostatic aerosol sampler and checked electronmicroscopically. The photographed particles were 5-35 nm in size and had a spherical shape. The investigations demonstrate that the concentration of the produced aerosol is sufficiently high for inhalation and clearance experiments with radioactively-labelled ultrafine particles to be performed. REFERENCES Dreiling, V., Hailer, P., Helsper,C., Kaminski,U., Plomp, A., Raes,F., Roth, C., Schier,J. and Schuermann,G. (1985) 13th Conference of the Gaef, 25-27 Sept., Garmisch-Partenkirchen. Roth, C., Berlauer, U. and Schiller Ch. F., (1986). Scheibel, H. G. and Porstend6rfer, J. (1983) J. Aerosol Sci. 14, 113.