High dielectric permittivity in glass-ceramic metal nanocomposites
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Strands of nanometer-sized silver particles have been grown at the glass-crystal interface of a suitable glass-ceramic system. The samples exhibit high values of dielectric constant of the order of 500 at temperatures in the range of 80 to 200 K. This is believed to arise due to the Gorkov—Eliashberg anomaly. Above 200 K, a dielectric dispersion is observed, which is explained on the basis of a diagonal-layer heterogeneous conductor model.
Materials having high dielectric permittivity are technologically important.1 Ferroelectric materials of a wide variety have attracted considerable attention.2"4 Anomalously large dielectric permittivity values have been reported in some sol-gel-derived oxide glass systems over a narrow temperature range.5 Barrier layer capacitors have been developed, which consist of semiconducting barium titanate grains separated from each other by thin insulating layers.6 Dispersed metal particles in an insulating matrix have been thought of as a possible route for making high dielectric constant materials. However, such composites have dielectric losses too high to be of practical use. We had earlier developed a technique for making nanometer-sized silver aggregates within a glassceramic.7 By using this method we have now been able to prepare a composite glass-ceramic and nanoparticles of silver which show high values of dielectric permittivity, but have a loss factor much less than unity. The details are briefly reported in this paper. The glass used in the present investigation has the composition 55 SiO 2 , 12 ZnO, 32.2 Li 2 O, and 0.8 P 2 O 5 (in mole %). The glass is prepared by melting a mixture of the required amounts of reagent grade chemicals in alumina crucibles in an electrically heated furnace at around 1600 K. Glass plates, of dimensions 3 cm X 2 cm X 1 cm approximately, are cast by pouring the melt onto a brass plate. The glass sample is heated at a temperature of 838 K for 30 min to precipitate crystals of zinc orthosilicate and lithium phosphate, respectively, having dimensions of the order of a few microns. Glass-ceramic samples of approximate dimensions 4 mm X 2 mm X 1 mm are polished to an optical finish by grinding in a silicon carbide grit of different mesh sizes and finally in a 0.05 /Am AI2O3 powder. The polished samples are subjected to a lithium =F^ silver exchange reaction by immersing them in a molten bath of silver nitrate contained in a pyrex crucible, at a temperature of 583 K, for a period of 6 h. After washing these samples in distilled water, they are reduced in hydrogen at a temperature around 873 K for 1206
J. Mater. Res., Vol. 8, No. 6, Jun 1993
a duration of approximately 30 min. As shown earlier the glass-crystal interfaces act as the heterogeneous nucleation sites for metallic silver to grow and form nanoparticles.7 The microstructure of samples is studied in a JEM 20/0 CX electron microscope operated at 100 KV. The method of sample preparation has been described elsewhere.8 For electrical measurements two opposite faces are first of all polished to r
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