December 2000

Optical anisotropy of silica gels doped with magnetic nanoparticles

D. Jamon, J. J. Rousseau- DIOM, Université Jean Monnet, Saint-Etienne
H. Roux- DPM, UMR 5586, Université Claude Bernard, Lyon
V. Cabuil- LI2C, UMR 7612, Université P. et M. Curie, Paris

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

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

9 samples gelled under magnetic field (from 0.75 to  10 kOe) with nanoparticles concentration varying from  0.07 to  0,.33% and 
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

temmagnet.gif (15754 octets)


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

magnet.gif (14263 octets)



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.

ferro.gif (8519 octets)

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. 


ferro2.gif (11858 octets)

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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