A Novel Spray Pyrolysis System for the Deposition of High Tc Superconductors
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A NOVEL SPRAY PYROLYSIS SYSTEM FOR THE DEPOSITION OF HIGH TC SUPERCONDUCTORS TOBY J. CUMBERBATCH÷, STEPHEN DEANE*, PAUL E. BARDEN* AND R.YU* + Interdisciplinary Research Centre in Superconductivity, The Cavendish Laboratory, Madingly Road, Cambridge CB3 OHE, England. * Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, England.
ABSTRACT A spray pyrolysis system has been developed in which the use of airless atomisation and optical heating permits the deposition of thin film superconductors at growth surface temperatures exceeding 650"C. A comparison between centrifugal and hydrodynamic atomisation techniques shows that each possesses advantages and disadvantages for the deposition of thin films by this technique. The presence of a corona discharge has been found to be an essential ingredient for growth at high substrate temperatures.
INTRODUCTION The use of spray pyrolysis for the deposition of the new oxide superconductor families has been investigated by several groups [ 1-5]. There are two distinctly different approaches to the use of this technique. The first uses substrate temperatures below about 550"C to deposit a thin film of the precursor solutes which is subsequently pyrolysed at temperatures above about 800'C [1-4]. The second, less common, method is to use a temperature high enough to drive the pyrolytic decomposition reaction during the deposition phase [5]. This method may have the potential to achieve superior uniformity of film composition and density. Experience with other materials has shown that there are increasing difficulties with spray pyrolysis as the substrate temperature is raised. The growth rate and materials utilisation drop sharply with temperature as it becomes more difficult for the droplets to reach the growth surface. The authors have experience in the development of new techniques intended for the commercial deposition of cadmium sulphide at substrate temperatures of around 500"C [6]. This work describes the application of those techniques to the deposition of oxide superconductors.
SPRAY PYROLYSIS SYSTEM DESIGN AND ATOMISATION TECHNIQUES During the first part of this investigation, an air-assisted hydrodynamic atomisation system was used for aerosol generation. The results were very encouraging and suggested that our approach was ideally suited to the high temperature route for thin film deposition. However, problems with nozzle erosion and corrosion within the precursor delivery feed line made the system unserviceable. We therefore changed to a high speed centrifugal atomisation system which overcomes both these problems in addition to providing a much greater degree of control over the droplet size. However, we have found that the dynamic behaviour of the droplets generated by each system is fundamentally different. The net effect is a significant reduction in the deposition rate with a centrifugal atomiser which suggests a need for fundamental design changes. A summary of the differences between these two techniques is given in Table
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