Electrical properties of opal-CT |
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Authors: | Liberto de Pablo-Galán Mari´a Lourdes Chávez-Garci´a Alfonso Huanosta Tera |
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Institution: | (1) Instituto de Geologia, Aptdo. Postal 70296, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México D.F., Mexico E-mail: liberto@servidor.dgsca.unam.mx, MX;(2) Facultad de Qui′mica, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México D.F., Mexico, MX;(3) Instituto de Investigación de Materiales, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México D.F., Mexico, MX |
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Abstract: | The electrical properties of opal-CT are validated at temperatures from 600 to 840 °C and frequencies from 5 Hz to 10 MHz.
The opals are hydrothermal, containing less than 11270 ppm total of Al, Fe, Ca, Na, and K, and from 1.17 to 17.63 wt% H2O interstitial and structural. Opal-CT shows fine crystallites, measuring from 19.4 to 22.7 μm, of an ordered tridymite-M
stratification with high-cristobalite, embedded in a non-crystalline matrix. When heated to 600 °C, the non-crystalline phase
devitrifies to the same stacked high-cristobalite-tridymite-M crystals. Opals containing less than 2070 ppm of cationic impurities
are characterized by one single high-frequency complex impedance arc corresponding to the bulk polarization of the crystalline
phase, of capacitances between 25 and 30×10−12 F and resistances from 132 to 890 ohm. Opals having over 6300 ppm of cationic impurities show two superimposed high- and
low-frequency complex impedance arcs. The high-frequency arc corresponds to the bulk polarization of the crystalline phase,
of capacitance between 8 and 15.7×10−12 F and resistance from 14 to 236 ohm, less than the capacitance of 0.25 to 0.53×10−9 F and resistance from 26 to 360 ohm of the non-crystalline minor intergranular material represented by the low-frequency
impedance arc. The electric module shows one single vertex, ascribed to the bulk polarization of the crystalline phase. The
conductivities are from 10−7 to 10−4 ohm−1cm−1, in the range of poor ionic conductors, essentially constant below 1.8 kHz, rapidly increasing at higher frequencies, due
to ionic and electronic charge carriers. The activation energy changes between 0.905 and 1.003 eV for the conduction mechanism
in the crystalline phase and from 0.924 to 1.087 eV in the non-crystaline phase. X-ray diffraction and impedance spectroscopy
confirm that opal-CT is a crystalline stacked sequence of tridymite-M and cristobalite-high, in a non-crystalline matrix.
Received October 20, 1995/Revised, accepted June 15, 1996 |
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