1.
Objective The objective of this work was
to demonstrate induced birefringence of silica gels doped with
magnetic nanoparticles. Particles orientation which introduce the
optical anisotropy was obtained during gelation under magnetic
field. After gelation the induced birefringence is permanent and
do not changes in presence of a magnetic field.
Several optical devices based
on this principle can be easily made. We have realized a quarter
wavelength plate with D=90 +- 0.5°2.
Experimental procedures
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The silica gel is made as usual with
hydrolyzed TEOS (Tetra ethyl Ortho silicate) in
ethanol
The ferrofluid phase is constituted by Nanoparticles of g-Fe2O3
suspended in water
The equipment used to prepare the composite
is shown below |
Birefringence Dn is proportional to the
magnetic nanoparticles concentration and can be fixed at a pre-selected value
under magnetic
field (H) |
14 samples have been used for this study
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9 samples
gelled under magnetic field (from 0.75 to 10 kOe) with
nanoparticles concentration varying from 0.07 to 0,.33% and |
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5 references samples
gelled without magnetic field for the
shake of comparison |
3. Gels
structure
Observed under Electron
Transmission Microscope all samples shows a well dispersed nanocomposite without
visible aggregation and good homogeneity
4.Permanent Birefringence
Concentration effects
Gels birefringence shows a linear relationship with Fe2O3
nanoparticle concentrations and its absolute value at a given concentration is
increasing with the magnitude of the magnetic field used for the gelation as
shown below
Magnetic field effects
The figure below shows the dependence of the doped silica
gels birefringence for three nanoparticle concentrations for increased values of the
magnetic field used during gelation. In comparison pure suspension of nanoparticles have
been measured under the same conditions.
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Birefringence of gels and pure ferrofluids
under different magnetic field |
One can see that the agreement in the induced
birefringence between the pure ferrofluids and the derived gels is excellent.
5. Magneto induced Birefringence
Silica doped gels prepared with and without magnetic field
were placed under magnetic field of increasing intensity in order to evaluate the impact
on birefringence. Again a pure ferrofluid was used for the shake of comparison.
From the figure below one can see that:
1. Silica doped gels prepared in absence of a magnetic
field they don't show any birefringence initially (random distribution of nanoparticles,
absence of optical anisotropy. When these gels are placed under magnetic field no
birefringence is induced indicating that nanoparticles once fixed on a random distribution
within the silica gels will not change their orientation even under high intensity
magnetic fields.
2. Silica doped gels prepared under magnetic field show a
birefringence which is permanently recorded in the gel and doesn't changes for
the same reasons as above under magnetic
field.
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Comparison of
the magneto induced birefringence between pure ferrofluids
and doped gels made with and without magnetic field |
6. Conclusions
Birefringence
can be induced durably and totally on silica gels doped with magnetic nanoparticles.
Its value varies as a function of the
nanoparticle concentration and the magnetic field intensity applied during gels
preparation.
The technique has been used to
realize a quarter wavelength plate with D=90±0.5°
Presented at "Journées
Nanoparticules" October 19, Ecole Polytechnique, France
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