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On line investigation of the relation between magnetic properties of nickel particles on primary particle size
J. Aerosol Sci. Vol. 28, Suppl. I, pp. S315-$316, 1997 ©1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain
Pergamon
PII:SOO21-8502(97)OO205-X
oo21-ssoz~s17.0o+0.0o
ON lANE INVESTIGATION OF THE RELATION BETWEEN M A G N E T I C PR()I)Ir, RTII~S OF N I C K E L PARTICLES ON PRIMARY P A R T I C L E SIZE 'l'h. Kaufl'cldt, A. Luczak, A. Schmidt-Ott Institut tiir Verbrennung und Gasdynamik, Universitat Duisburg l~otharstr.65, 47048 Duisburg, F.R.G, email [email protected] Magnetic particles are of great interest in industrial application as well as basic research. They are used e.g. in memories and gas sensors or as quantum dots for novel electronic devices. In the nanometer range the magnetic properties are strongly dependent on particle size (Jakusch et. al. 1992). It is known that the material properties of agglomerates are defined by the size of the smallest primary particles which are hold together mainly by van der Waals forces (Schmidt-Ott 1988). We define the size of these grains as primary particle size Dp~,r. Temperature treatment of such ~Jgglonlcratcs leads to sintcring and coalescence. This process can be detected by the recast, foment of the agglomerates size l)p after heating. Experiments with other metals like gold or silver demonstrated that at low temperatures the agglomerate rearranges into a more compact structure and that further increase of the temperature leads to a growth of the primary particle size (Schmidt-Ott 1987, Friedlander et al. 1996). This process ends, when the closest packing is established and no further change of particle size occurs after increasing the temperature. Our goal is to use coalescence to study the dependence of the magnetic moment m of nickel nanoparticles versus their primary particle size. Here we report about our first experiments to vary Dppr and measure m. The particles are produced by heating a thin nickel wire by an electric current. The vapor is cooled by an inert gas stream of ultra high purity. The temperature gradient in the vicinity of the heated wire is very high, so that in a short distance of the wire the gas is cooled down and coagulating primary particles stick together without neck formation. After a coagulation time of several seconds, monodisperse agglomerates are taken out by an DMA. These are heated in a tube furnace. Afterwards the change in agglomerate size is measured using a second DMA. Alter,latively the magnetic moment is measured with a magnetic filter (Kauft~ldt et. al. 1996) fig. 1.
Particle Generator
DMA Coagulation
Chamber r
....
~
1
I I
~KrSS IF I F ~---J-~5-L--~ I I 1
=~,
OMA 2
~_~.~_
Tube Furnace
~
.
•
Magnetic Filter
Fig. 1: The experimental set up
$315
S316
Abstracts of the 1997 European Aerosol Conference
The magnetic filter consists of several parallel nickel screens, which are orientated perpendicular to the direction of flow and the homogeneous magnetic field of the electric coil the filter is situated in. Near the fibers of the nickel screens there is a strong gradient field, which attracts magnetic particles to the fiber surface. The penetration is a function of particle size and particle magnetic moment m, which can be calculated for a calibrated filter. In fig.2 the measured Dp and the measured m are shown for agglomerates with an initial diameter of 68 nm as function of the temperature in the tube furnace. 1
.-~
4
I
0,9
~
I
0,8
m
=
60 Dp
I
0,7
50
I
0,6 E E 0,5
40 ~'
/
0,4
70
3O
/
0,3 0,2
/
0,1
2O
4/
I0
/
0
~
0
~
I..~.
200
~
~
0
400 T [oc]
600
800
Fig.2: Mobility diameter and magnetic moment versus sintering temperature Below 300°C the decreasing mobility diameter indicates rearrangement of the primary particles to a closer packing (Schmidt-Ott 1987), while m remains unmeasurable small. The onset of magnetic behavior around 400°C is accompanied by rapid shrinkage of Dp and attributed to coalescence, forming magnetic domains large enough to exhibit ferromagnetism. Above 500°C m and Dp are constant, indicating that the closest packing is reached and there is no further change of the magnetic domains. Further experiments with varying the initial agglomerate will enable to detect the critical primary particle size for the change in particle magnetism with respect to particle material. Acknowledgment This work was pertbrmed as a part of the Special Research Program (SFB 209) at the university of Duisburg, sponsored by the German National Science Foundation (DFG). References Jakusch H., Veitch R., PARTEC 1992 Reprints, Nuernberg, 1992 Schmidt-Ott A., Appl. Phys. Lett. 52, 1988 Schmidt-Ott A., J. Aerosol Sci., 1988 Friedlander S.K., J. Aerosol Sci., 1996 Kauffeldt Th., Kleinwechter H., Schmidt-Ott A., Chem. Eng. Comn., 1996
Pergamon
PII:SOO21-8502(97)OO205-X
oo21-ssoz~s17.0o+0.0o
ON lANE INVESTIGATION OF THE RELATION BETWEEN M A G N E T I C PR()I)Ir, RTII~S OF N I C K E L PARTICLES ON PRIMARY P A R T I C L E SIZE 'l'h. Kaufl'cldt, A. Luczak, A. Schmidt-Ott Institut tiir Verbrennung und Gasdynamik, Universitat Duisburg l~otharstr.65, 47048 Duisburg, F.R.G, email [email protected] Magnetic particles are of great interest in industrial application as well as basic research. They are used e.g. in memories and gas sensors or as quantum dots for novel electronic devices. In the nanometer range the magnetic properties are strongly dependent on particle size (Jakusch et. al. 1992). It is known that the material properties of agglomerates are defined by the size of the smallest primary particles which are hold together mainly by van der Waals forces (Schmidt-Ott 1988). We define the size of these grains as primary particle size Dp~,r. Temperature treatment of such ~Jgglonlcratcs leads to sintcring and coalescence. This process can be detected by the recast, foment of the agglomerates size l)p after heating. Experiments with other metals like gold or silver demonstrated that at low temperatures the agglomerate rearranges into a more compact structure and that further increase of the temperature leads to a growth of the primary particle size (Schmidt-Ott 1987, Friedlander et al. 1996). This process ends, when the closest packing is established and no further change of particle size occurs after increasing the temperature. Our goal is to use coalescence to study the dependence of the magnetic moment m of nickel nanoparticles versus their primary particle size. Here we report about our first experiments to vary Dppr and measure m. The particles are produced by heating a thin nickel wire by an electric current. The vapor is cooled by an inert gas stream of ultra high purity. The temperature gradient in the vicinity of the heated wire is very high, so that in a short distance of the wire the gas is cooled down and coagulating primary particles stick together without neck formation. After a coagulation time of several seconds, monodisperse agglomerates are taken out by an DMA. These are heated in a tube furnace. Afterwards the change in agglomerate size is measured using a second DMA. Alter,latively the magnetic moment is measured with a magnetic filter (Kauft~ldt et. al. 1996) fig. 1.
Particle Generator
DMA Coagulation
Chamber r
....
~
1
I I
~KrSS IF I F ~---J-~5-L--~ I I 1
=~,
OMA 2
~_~.~_
Tube Furnace
~
.
•
Magnetic Filter
Fig. 1: The experimental set up
$315
S316
Abstracts of the 1997 European Aerosol Conference
The magnetic filter consists of several parallel nickel screens, which are orientated perpendicular to the direction of flow and the homogeneous magnetic field of the electric coil the filter is situated in. Near the fibers of the nickel screens there is a strong gradient field, which attracts magnetic particles to the fiber surface. The penetration is a function of particle size and particle magnetic moment m, which can be calculated for a calibrated filter. In fig.2 the measured Dp and the measured m are shown for agglomerates with an initial diameter of 68 nm as function of the temperature in the tube furnace. 1
.-~
4
I
0,9
~
I
0,8
m
=
60 Dp
I
0,7
50
I
0,6 E E 0,5
40 ~'
/
0,4
70
3O
/
0,3 0,2
/
0,1
2O
4/
I0
/
0
~
0
~
I..~.
200
~
~
0
400 T [oc]
600
800
Fig.2: Mobility diameter and magnetic moment versus sintering temperature Below 300°C the decreasing mobility diameter indicates rearrangement of the primary particles to a closer packing (Schmidt-Ott 1987), while m remains unmeasurable small. The onset of magnetic behavior around 400°C is accompanied by rapid shrinkage of Dp and attributed to coalescence, forming magnetic domains large enough to exhibit ferromagnetism. Above 500°C m and Dp are constant, indicating that the closest packing is reached and there is no further change of the magnetic domains. Further experiments with varying the initial agglomerate will enable to detect the critical primary particle size for the change in particle magnetism with respect to particle material. Acknowledgment This work was pertbrmed as a part of the Special Research Program (SFB 209) at the university of Duisburg, sponsored by the German National Science Foundation (DFG). References Jakusch H., Veitch R., PARTEC 1992 Reprints, Nuernberg, 1992 Schmidt-Ott A., Appl. Phys. Lett. 52, 1988 Schmidt-Ott A., J. Aerosol Sci., 1988 Friedlander S.K., J. Aerosol Sci., 1996 Kauffeldt Th., Kleinwechter H., Schmidt-Ott A., Chem. Eng. Comn., 1996