High Refractive Index Materials of Iron Sulfides and Poly(ethylene oxide)
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High refractive index materials of iron sulfides and poly(ethylene oxide) Tasoula Kyprianidou-Leodidou, Hans-J¨org Althaus, Yves Wyser, Daniel Vetter, Michele B¨uchler, Walter Caseri, and Ulrich W. Suter Eidgen¨ossische Technische Hochschule, Institut f¨ur Polymere, ETH-Zentrum, CH-8092 Z¨urich, Switzerland and Eidgen¨ossische Materialpr¨ufungsund Forschungsanstalt, Abt. 136, CH-8600 D¨ubendorf, Switzerland (Received 15 December 1994; accepted 10 April 1996)
High refractive index composites of iron sulfides and poly(ethylene oxide) (PEO) have been prepared by co-precipitation from aqueous solution. Several reaction parameters were varied: inorganic reactants, reactant ratios, reaction temperatures, and reaction times. Selected samples were characterized with organic microelemental analysis, x-ray fluorescence spectroscopy, x-ray diffraction, DSC, and TEM. The nanocomposites with the highest refractive indices have been prepared using PEO, Mohr’s salt, and H2 S or NaHS. The analyses indicate that the iron sulfides in these materials consist of finely dispersed mackinawite and greigite (“amorphous” FeS) and, partially, also pyrite. The refractive indexes of the resulting composites are clearly above 2 at 632.8 and 1295 nm and can assume values between 2.5 and 2.8.
I. INTRODUCTION
Composites of polymers and inorganic substances are widely used to obtain materials with advantageous properties of both materials classes. Polymers are, e.g., readily processed while inorganic materials possess physical properties unattainable with polymers. One of these properties is an extreme refractive index. The refractive index of organic compounds, including organic polymers, is usually in a range of 1.3–1.7.1,2 In contrast, the refractive index of inorganic materials varies in a much wider range.1,3 For example, the refractive index of gold is 0.2–0.4 and that of PbS is above 4, each in a broad wavelength range. Nanocomposites with extremely high or low refractive index have been prepared recently by coprecipitation or spin-coating from aqueous solutions.4–7 The high refractive index nanocomposites consist of colloidal lead sulfide embedded in a polymer matrix. Due to the high refractive index of PbS, that of the composites is up to 3, over a broad wavelength range, and to our knowledge by far the highest reported for a polymer composite. However, the high lead content precludes the use of such materials in many applications (e.g., solar cells8,9 ), and an environmentally more benign substance than lead sulfide is required. Iron sulfides might also be suitable inorganic components for high refractive index nanocomposites. For example, we measured the refractive index of a block of a Spanish pyrite crystal and obtained values above 3.5 at 632.8 and 1295 nm (an exact value cannot be given because of possible structural imperfections in the crystal surface region and, in particular, surface impurities). However, the chemistry of iron sulfides is complex. 2198
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