General Idea

Basic research of the company's founder and resulting patents have allowed nanofluor Ltd. to produce very small and formerly unavailable metal fluoride particles in stable forms. Fluorolytic sol-gel synthesis, the prerequisite for this innovative process, has been developed since 2003 and yields a direct synthetic access to homodisperse lower nanometer metal fluoride particles.

Especially for sols (nanoparticles that are homogenously dispersed in a solvent) there is a great potential for utilization in the areas of optics, photovoltaic, ceramic, and dental applications, as well as catalysis and anticorrosive layers.

Introduction to the specific science

Metal fluorides show a number of properties that distinguish them significantly from other materials, especially the widespread applied metal oxides. Special properties like refraction index, UV and IR permeability and dielectric function permit their use in optics, laser technology and ophthalmology with superior effects than for all currently known alternative materials.

Metal fluorides are equally of interest for surface treatment because of their influence on fluxes and surface tension (hydrophobicity) and their anticorrosive, antibacterial and antifungal properties. On the basis of our fluorolytic sol-gel synthesis clear sols as well as solid nano metal fluorides (xerogels) can be produced.

Standard synthesis uses metal alkoxides, but carboxylates may also be used. Solutions of these precursors in organic solvents (usually alcohols) are realized with water-free hydrogen fluoride. Properly performed synthesis directly results into the formation of transparent metal fluoride sols with particle diameters of 3 to 30 nm, depending on the specific metal (e.g. MgF₂ 3 bis 9, AlF₃ 10 bis 15 nm).

M(OR)_n  +  nHF  →  MF_n  +  nROH    [1]

The resulting metal fluoride sols are neutral and non-corrosive (HF-free!). The particularity of these sol-based particles lies in their potential for easy to perform high quality coating: surfaces are coated with dense, transparent and mechanically stable layers that still contain the individual nano particles but show the collective characteristics of the specific material.

Potential areas of application

1. Metal Fluoride-Sols: Coatings for Optics and Photovoltaics

Light is refracted when it passes from one medium to another. The higher the difference in refraction index the higher the proportion of reflected light. 100 % non-reflexive coatings in a single coating step are currently impossible to produce.

Good anti-reflective treatments of optical systems are currently achieved with multiple alternating coatings using high and low refraction materials. Theoretically however, a glass surface could be rendered completely non-reflexive with only one coating layer if the layer's refraction index (n) is exactly 1,23.

The result would be total transparency (no reflection) and thus optimal luminous efficiency. Table 1 lists the materials with the lowest known refraction indexes.

SiO2	-1,46	CaF2	-1,40	LiF2	-1,39
MgF2	-1,38	AlF3	-1,35	Na3AlF6	-1,33

Specific introduction of porosity allows to lower the refraction index of a coating even further but this leads to a decrease in the layers's mechanical stability. The lower the refraction index of a material the lower the necessary porosity and the higher the mechanical stability of a porous layer.

Figure 1

a) Theoretical reflection depending on a coatings's refraction index

b) Transparent MgF₂-Sol (particle diameter 4 to 9 nm)

c) Hard, transparent MgF₂-Coating on borosilicate glass

Figure 1a displays calculated reflection dependency at various light wave lengths for a transparent coating material with three different refraction indexes. Already a material with n=1,30 can reduce reflection to less than 0.5%. We actually achieve this value with only one coating using metal fluoride sols. Figure 1b shows a clear MgF₂-sol that was used in coating a plane glass surface. The coating is shown in Figure 1c, next to an uncoated area (narrow right part). Further research efforts will use chemical modification to lower the refraction index of a MgF₂ coating to 1.23.

2. Inorganic-Organic Hybrid-Materials

When fluorolytic sol-gel synthesis is driven understoichiometrically regarding HF non-reacted, remaining OR-groups can be used for specific functionalization of nano particles surfaces (equ.2).

M(OR)_n  +  (n-m)HF  +  mHOX  →  MF_n-m(OX)_m  +  mROH    [2]

Organically functionalized nano metal fluorides are produced, dispersible in diverse organic systems. Figure 2 shows polymerized methacrylates, containing up to 40% of modified MgF₂. These polymers are transparent, their glass transition temperature is increased by 25º and their mechanical hardness improved by a factor of 2.5.

Figure 2 - Polymerized HEMA (MgF₂-content from left to right: 0%, 5%, 20%)

3. Ceramics

Nano materials are expected to exhibit significantly higher reactivity and thus completely different sinter behavior. In cooperation with the Fraunhofer-Institute for Ceramic Technologies and Systems (IKTS) potential uses of nano metal fluorides in improved qualities of ceramics have been studied. Nano-MgF₂ additives of only 0.1% results in lowering the final sinter temperature for corundum ceramics by up to 100º, leading not only to a reduction of energy but also to a significant improvement of grain structures and densities (Fig. 3).

Corundum ceramics obtained with nano-MgF₂ as sintering agent show optical transparency almost equal to that of glass (Fig. 2c) and an improved Vickers hardness of about 3100 compared to 2100 for conventional ceramics.

This innovation is object of ongoing research and it opens the doors to new dimensions in the production and use of high performance ceramics, as in lighting technologies, hard tools, ceramic joint replacements.

4. Catalysis

Several fluoride compounds belong the strongest Lewis acids because of the strong electron withdrawing character of fluoride. Even though the known crystalline modifications of AlF₃ are relatively weak Lewis acids, nano-AlF₃ – acquired in the aforementioned way – exhibits Lewis acid properties comparable to those of SbF₅, one of the strongest known Lewis acid. For the first time, an extremely strong sold Lewis acid is available, and unlike all other strong Lewis acids it is resistant to hydrolysis. Fluorolytic sol-gel synthesis will allow to produce solid Lewis acids from many other metals, leading to new possibilities with solid catalysts for various reactions in organic chemistry.

Completely new (Brønsted-Lewis) biacid solid matter systems can be achieved by hydrolyzing OR-groups from intermediate metal alkoxide fluorides and transforming them into OH-(Brønsted-) groups. These new biacid heterogeneous catalysts can be tuned very finely regarding their Lewis- to Brønsted-functionalities and result in improved reactivity and selectivity in various reactions when compared to the best homogeneous catalysts.