Electric-Field Effects on Reactions Between Oxides
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electric field. A description of the apparatus is given elsewhere. [7] A diffusion couple was placed between two Pt electrodes under slight pressure. The Pt electrodes were supported on 1cm by 3cm sheets of A12 0 3. To ensure proper oxygen transport to the surface of the diffusion couple, the electrodes were finely scratched with an abrasive in order to roughen the surface of the Pt. The entire apparatus was then placed in a box furnace where electrical leads were attached that were brought in through the top insulation of the furnace. A second diffusion couple without the applied field often served as a standard. This diffusion couple was placed with the thin film side down on a piece of Pt foil in close proximity to the sample being reacted in the field to ensure similar reaction conditions. The reacted thin films were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. SEM and TEM samples were prepared in cross-section. The preparation of the SEM cross-section samples required the use of diamond lapping films followed by Syton© to produce a polish which allowed the thin films to be imaged. The SEM samples were then coated with approximately 0.5 nm of Pt to reduce charging and then
imaged in a Hitachi S-900 field-emission SEM operating at 5 kV in back-scattered electron mode. The TEM cross-section samples were produced by conventional dimpling techniques followed by ion milling at liquid nitrogen temperatures. After preparation the samples were imaged in a Philips CM30 operating at 300 kV. RESULTS AND DISCUSSION Feg.Q/MgO System The reaction between Fe 20 3 and MgO results in the formation of spinel, MgFe 20 4 .Figure 1 is a montage of cross-section backscattered electron images of three different Fe 2 O3/MgO diffusion couples which have been reacted for a sequence of times at a temperature of 1150'C in an applied field of 2000V/cm. The thin-film was in contact with the cathode (negative electrode). The spinel phase appears brightest in the backscattered electron images. This sequence of images illustrates the effect of the applied field on the transport of the cations in the spinel. Because of the induced as a result of the ionic current, MgO has formed on the surface of the spinel. This MgO forms Mg 2 + cations having a higher mobility in the spinel in comparison to the Fe 3+ cations. Mg 2 + ions which have been driven past the spinel layer react with oxygen at the cathode and form MgO. This MgO layer forming at the cathode occurs at the expense of MgO at the anode side. Equations 1 and 2 below describe the reactions occurring at the electrodes. cathode:Mg2 + + -O 2 (g) + 2e' = MgO
(1)
anode:MgO = ½02 (g) + 2e' + Mg2+
(2)
Similar effects are seen in the system MgAI2 0 4. When single crystals of MgA120 4 were placed in the presence of an electric field at elevated temperatures it was found that a layer of MgO was precipitated at the cathode. [6] The sequence of images shown in figure 1 also illustrates that the MgO forming on the spinel goes through a n
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