N. Reinhardt, A. Venier, M. Comesana-Hermo, E. Camposilvan, J. Alberici
Keywords: zirconia nanoparticle, high refractive index, transmittance
Summary:Zirconia nanoparticles are considered as potential refractive index enhancers in different nanocomposite materials. These are required in sectors such as optics, electronics and energy, lighting (HB-LED, OLED) and cosmetics. Mathym has developed a new process for the versatile synthesis of high refractive index nanozirconia (patent pending). This process differs from known zirconia syntheses in that it is carried out by a simple protocol without the need for intermediate washing or purification steps. The synthesis is done at a pH below 7 in the presence of linear amino acids, which serve as the structure directing agents. Depending on the nature of the used amino acids, particles of different morphology and phase structure have been obtained and characterized by TEM and X-ray diffraction. In addition, doping with yttrium, cerium or gadolinium can be used to influence size, morphology and phase structure (Figure 1). Nanoparticles obtained by the process include the following grades: • spherical nanoparticles with 3 nm diameter, monoclinic phase • spherical nanoparticles with 5 nm diameter, tetragonal phase • snowball-like particles with 20 nm diameter, monoclinic + tetragonal mixed phase • anisotropic, needle-like nanoparticles of 50 nm length, monoclinic phase One additional advantage of the process is that the addition of water or solvent is not necessary when hydrated precursor salts are used. In this case, the synthesis can be conducted at an exceptionally low pressure in the range of 0.1 to 0.6 MPa, being the pressure sufficient to obtain particles with excellent crystallinity and high refractive index. Aqueous sols containing spherical particles of 5 nm diameter were examined to evaluate their optical transmittance. The aqueous particle dispersions obtained retain a high total forward transmittance over 10 mm optical length of up to 20 % at 400 nm and more than 95 % at 800 nm for a solid content of 40 wt% (Figure 2). At a concentration of 65 wt% of solid content, the viscosity below 4000 mPa.s ensures good manufacturability. For specific applications, sols can be concentrated up to 80 wt%. Refractive index of the aqueous dispersions of 5 nm sized particles was measured and displayed as a function of vol% (Figure 3). Refractive indices of the sols are ranging from 1.32 to 1.50 for concentrations of 5 to 25 vol% nanozirconia, respectively. The refractive index of the nanoparticles was estimated according to mathematical models taking into account the dispersion medium and the presence of capping agent. In this way, the resulting refractive index was found to be in a range between 2.10 and 2.15 depending on the nanozirconia grade, therefore close to the theoretical values. Suitable surface modification allows the transfer of non-agglomerated nanozirconia in organic solvents and dispersion in different host matrixes. Subsequently, the particles obtained with this process are of particular interest for use in the formulation of LED encapsulants, displays, coatings or molded materials.