Sol-Gel Route to Oxygen Ion Conductors in Thin Film Form
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desired compositions Bi 2 Cu 0 .1V 0.9 0 5 .35 and Bi 2 Nb 03 V0 .7O5 .5 . The resulting solutions (0.15 M Bi3) were clear yellow (Nb-doped) or clear green (Cu-doped). These solutions are stable under nitrogen at room temperature for weeks. For bulk materials, precursor solutions were hydrolyzed with water (W=moles H20/moles alkoxide groups = 3) to a translucent gel. Gels were dried at 120 0C, then heated for 8h in air. Temperatures used were 350, 450, 550, 650, and 750 'C. Powder X-ray diffraction patterns were collected on a Siemens D5000 diffractometer using Cu Kox radiation. Films were deposited by spin-coating on quartz, silver, and platinum substrates. Quartz slides were etched in aqueous HF, then rinsed in water and baked at 600 'C before use. Precursor solutions were made as described above (0.15M Bi 3÷) and diluted by 50% with anhydrous methanol, then hydrolyzed with water (W=l). After each film layer was deposited, samples were heated on a hot plate (approx. 350 °C) for 5 minutes, then at 550 'C for 5-10 minutes before the next layer of film was added. In most cases 10-20 layers were deposited at approximately 300 nm/layer. A final anneal step involving heating at 600 'C for one hour was carried out in most cases, though higher temperatures and longer times were tested. Films were inspected for phase purity by grazing incidence X-ray diffraction (GIXRD). Film structure was investigated using environmental scanning electron microscopy (ESEM). To see the effects of assorted process parameters, W was varied in a number of experiments. Also, several additives, used in equal moles to the moles of alkoxide groups, were tested to see their effects on film quality. RESULTS AND DISCUSSION The sol-gel method has Fig. 1: XRD of Bi 2 Cu 0 .1 V0 .905. 35 (Bi 2Nb 0 .3 V 0 .7 0 5 ,5 similar.) Curves are 350, 450, 550, 650'C bottom to top. I been used previously to make the Aurivillius phase 2500o Bi 4 Ti 3 O 1 2 [16-21]. Reports x
2000
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the use of bismuth
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acetate have all cited its insolubility except
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acid, or as a co-solute with titanium 2methoxyethoxide, where its solubility result from may of the bismuth acetate with the alkoxide. the bismuth precursor prepared as described above is soluble directly in 2MOE.
The bulk-powder syntheses yielded firm translucent gels. Simultaneous DTA/TGA of the dried gels shows decomposition and loss of organic components at 350-375 'C. Powder XRD of both fired materials Fig. 2: GIXRD of Bi 2 Cu 0 . 1 V 0 . 9 0 5 . 3 5 Film showed formation of the at structure desired 2500 450'C, with no impurity (Figure 1) peaks. increased Crystallinity 200 with firing temperature, with no ingrowth of The foreign phases. ii150 indexing was consistent with that reported for the U y phase BiMeVOx 1000 I materials.[13, 14, 22] Films were spinonto quartz, coated and silver 500 •platinum, Grazing substrates. incidence XRD showed formation of the desired y _. 50 ph
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