Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1998-10-01
2001-05-22
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S290000, C359S654000, C430S290000, C430S034000
Reexamination Certificate
active
06236493
ABSTRACT:
The present invention relates to optical components with a gradient structure, especially optical components with a material gradient which is induced by nanoscale particles, and to a process for producing such optical components.
The electrophoretic deposition of particles in a suspension is based on the capacity of the particles to migrate in an electric field and, as a function of the polarity of their surface charge, to become deposited at one of the electrodes. The diffusion profile in this case is dependent, inter alia, on the size and distribution of the particles and on the viscosity of the medium.
Where the diffusive processes take place in a matrix which can be hardened subsequently, it is possible to “freeze in” the respective diffusion profile and so produce a gradient material.
In accordance with the invention it has been found that, in very general terms, the directed diffusion (migration) brought about by a potential difference of any kind, of particles, especially nanoscale particles, in a liquid, (preferably thermally and/or photochemically) curable matrix of appropriate viscosity (i.e. of a viscosity which (just) permits the diffusion of the particles) can be exploited for producing optical components with a material gradient structure. If, after the desired diffusion profile of the nanoscale particles has been established, this diffusion profile is frozen in by curing the matrix phase.
The present invention therefore provides optical components with a gradient structure, in which a material gradient (which can, for example, bring about a refractive-power gradient) is induced by nanoscale particles which are embedded in a solid matrix.
The present invention additionally provides a process for producing these optical components in which a potential difference is exploited to cause nanoscale particles dispersed in a liquid, curable matrix phase to migrate in the matrix phase, thereby leading to the formation of a material gradient, and the matrix phase is subsequently cured with retention of the material gradient.
The driving force which leads to the directed migration (diffusion) of the particles in the matrix can be generated, for example, by an electrical field (as in electrophoresis), a chemical (concentration) potential or an interface potential.
If the potential difference is to be generated by an electrical field, a possible procedure is, for example, to bring the liquid, curable matrix phase with, dispersed therein, nanoscale particles which carry a surface charge between two electrodes (anode and cathode) and to cause the nanoscale particles to migrate in the direction of the electrode having the polarity which is opposite to their surface charge. The surface charge on the nanoscale particles can be generated, for example, by establishing a pH which induces dissociation of groups on the surface of the nanoscale particles (e.g. COOH→COO
−
, metal-OH&rlarr2;metal-O
−
). This approach presupposes, of course, that the viscosity of the matrix when an electrical field is applied, permits marked diffusion of the nanoscale particles. After the desired diffusion profile has become established, it is, so to speak, frozen in by curing the matrix to form a solid structure, by means of which it is possible to produce an optical component with a material gradient structure.
A chemical concentration potential can be generated, for example, as follows. In the case of the local (e.g. thermally and/or photochemically induced) polymerization of species with carbon-carbon multiple bonds, epoxy rings, etc., polymerization leads to a depletion of functional groups in the regions in which the polymerization takes place. (The term “polymerization” as used herein is intended to include not only addition polymerization but also polyaddition and polycondensation reactions.) This leads to a diffusion of species with as yet unreacted functional groups into the (heated or illuminated) regions in which the polymerization has taken place in order to compensate the chemical potential difference. In the case of photopolymers, this effect is known as the Colburn-Haines effect. In the heated or illuminated regions, this directed diffusion with subsequent polymerization leads to an increase in the density and thus to an increase in the refractive power. In the case of organic monomers, however, this change is small, since the small change in density which is established makes only a small contribution to the molar refraction. This does not apply, however, to nanoscale particles whose surface carries reactive groups capable of polymerization. In this case, the refractive-power gradient can be increased markedly in chemical potential by diffusion of the nanoscale particles, and it is possible to obtain gradient materials if, following diffusion, the matrix phase is cured, i.e. if, for example, the entire system is subjected to a thermally and/or photochemically induced polymerization. In this case, owing to the preceding immobilization of the diffused nanoscale particles (by polymerization), the material gradient is retained. An important prerequisite with this embodiment of the process of the invention as well, of course, is that the liquid matrix phase permits adequate diffusion of the nanoscale particles whose surface has been provided with reactive (polymerizable) groups; in other words, that the viscosity of the matrix phase is not excessively high.
A further option for generating a potential difference which leads to a diffusion of nanoscale particles that have been modified (on the surface) with appropriate groups in a matrix phase of appropriate viscosity to form a material gradient is to make use of the incompatibility between the surface of the nanoscale particles and the liquid matrix phase. If the nanoscale particles, for example, carry hydrophobic groups, such as fluorinated (alkyl) groups, on their surface, and the matrix phase has a hydrophilic or less hydrophobic character, the application of the liquid, hydrophilic matrix phase with hydrophobic nanoscale particles dispersed therein to a substrate causes the hydrophobic particles to migrate to the surface of the layer which leads to the lowest system energy. In general, this layer is the interface with the air, so that the hydrophobic or hydrophobically coated particles accumulate at the surface of the coating and become less concentrated at the interface with the substrate; after the coating has been cured, this produces both good adhesion between layer and substrate, and an easy-to-clean, low-energy surface.
In order to prevent separation of (hydrophilic) matrix phase and hydrophobic nanoscale particles even prior to application to a substrate, in the case of this embodiment of the process of the invention, a possible procedure, for example, is to add to the matrix phase a compatibilizer, which is removed (by evaporation, for example) after the composition has been applied to the substrate, or is incorporated stably into the matrix phase as the latter cures.
In the text below, the materials which can be employed in the process of the invention are described in more detail.
The nanoscale particles which can be employed in the process of the invention preferably have a diameter of not more than 100 nm, especially not more than 50 nm, and with particular preference, not more than 20 nm. As far as the lower limit is concerned, there are no particular restrictions, although this lower limit is for practical reasons generally 0.5 nm, in particular 1 nm and more frequently 2 nm.
The nanoscale particles comprise, for example, oxides such as ZnO, CdO, SiO
2
, TiO
2
, ZrO
2
, CeO
2
, SnO
2
, Al
2
O
3
, In
2
O
3
, La
2
O
3
, Fe
2
O
3
, Cu
2
O, V
2
O
5
, MoO
3
or WO
3
; chalcogenides, examples being sulphides such as CdS, ZnS, PbS or Ag
2
S; selenides, such as GaSe, CdSe or ZnSe; and tellurides, such as ZnTe or CdTe; halides, such as AgCl, AgBr, AgI, CuCl, CuBr, CdI
2
or PbI
2
; carbides, such as CdC
2
or SiC; arsenides, such as AlAs, GaAs or GeAs; antimonides, such as InSb; nitrides, such as BN, AlN, Si
3
N
4
or
Krug Herbert
Oliveira Peter W.
Schmidt Helmut
Sepeur Stefan
Epps Georgia
Heller Erhman White & McAuliffe LLP
Institut fur Neue Materialien gemeinnutzige GmbH
Lester Evelyn A.
LandOfFree
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