Solution Flow System for Hydrothermal-Electrochemical Synthesis: New Opportunities for Multilayered Oxide Films
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ABSTRACT A solution flow system for hydrothermal-electrochemical synthesis has been constructed in our laboratory. This equipment can operate at 20'-200'C, under the pressure of 1-50 atm., at flow rate of 1-50 cm 3/min. Applicability of the flow system for low-temperature, hydrothermalelectrochemical synthesis of single-layer and multilayered thin films has been demonstrated using the BaTiO 3 -SrTiO3 system as an example. Single phase thin films as well as double layers have been deposited at 150'C, current density of 1 mA/cm 2, and flow rates of 1-50 cm 3 /min. The flow rate is an important parameter allowing additional control of the films' morphology by affecting the growth rate. The multilayers can be prepared in only one experiment by simply changing the flowing solution. Processing using the solution flow cell may serve as an inexpensive and environmentally friendly way of fabricating any multilayered thin films, including magneto-optic films. INTRODUCTION Processing of thin films has been dominated by chemical vapor deposition [1], physical vapor deposition [1], and sol gel techniques [2,3]. However, it is possible to fabricate thin films directly from solution, without subsequent heat treatments, by hydrothermal and/or electrochemical methods [4]. These techniques give similar results as any other process using fluids (such as
vapor, gas, plasma) and/or beam/vacuum processing. The total energy consumption among all these processing routes should be the lowest in aqueous solution systems because a much larger excess of energy is necessary to create melts, vapor, gas or plasma than to form aqueous solutions at the same temperatures [4,5]. Solution processing is located in the temperaturepressure range characteristic for conditions of living on earth. The other processing routes which are connected with increasing temperature and/or increasing (or decreasing) pressure, are environmentally stressed, energy consuming, and expensive [5]. Surprisingly, researchers investigating hydrothermal and hydrothermal-electrochemical synthesis of thin films have been focused only on film preparation in closed autoclaves and/or beakers. However, fabrication of thin films in a continuous process under solution flow is essential for possible future application of this technique for integration with advanced device technology. Advantages of thin film fabrication in a solution flow system are: possibility of the recycled (i.e. closed) flow, possibility of fabrication of multilayered materials in only one run, and additional control (improvement) of the film microstructure by changing conditions of the solution flow [6]. The best suited equipment for this purpose seems to be a solution flow cell for hydrothermal-electrochemical synthesis. Such equipment has been recently constructed in our laboratory. Generally speaking, its design is similar to other flow-cells used in geological studies [7] or in materials engineering (ferrite plating) [8]. We have demonstrated applicability of our solution flow system for synthesis of multilay
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