An Investigation of the Influence of Grain Boundaries on the Electrical Behavior of Polycrystalline ZrO2-CaO
Date of Award
Dissertation - Restricted
Doctor of Philosophy (PhD)
Electrical and Computer Engineering
The dielectric behavior of polycrystalIine zirconium oxide with calcium concentrations ranging from 12 to 19 mole per cent (m/o) were investigated as a function of frequency, from 100 Hz to 500 Hz, temperature, from 300 degree c to 900 degree c, and partial pressure of oxygen, from 10-5 atm. to 1 atm. Specimens having various grain sizes, electrode materials, and degrees of sintering were also considered in order to determine the influence of grain boundaries on the electrical conduction. The a.c. dielectric data were analyzed with the aid of impedance plotting technique "formula". This allowed an equivalent circuit representation of the specimen. The a.c. impedance of Zr02-Ca0 exhibited a general three loop behavior. At high temperatures "formula" space charge polarization due to ionized oxygen vacancies being partially blocked at the specimen electrodes seems responsible for a major portion of the dielectric behavior. At intermediate temperatures "formula" the grain boundaries contribute an important resistance and capacitance to the specimen equivalent circuit. The effective resistivity of the boundaries was found to be much higher than the bulk resistivity while the total resistance of the boundary regions was comparable to the bulk resistance. These high resistivity boundaries were rationalized in terms of space charge layers resulting from a low effective mobile carrier concentrations at the boundaries. At low temperatures "formula" the electrical behavior appears to be influenced by the high frequency limiting capacitance of the bulk material.