Preparation and Characterization of Fine Indium Tin Oxide Powders by a Hydrothermal Treatment and Postannealing Method
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S. Nasu Division of Materials Physics, Department of Physical Science, School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan (Received 9 September 1999; accepted 3 March 2000)
Indium tin oxide powder with an In to Sn mole ratio of 95:5 was successfully prepared from a coprecipitated In–Sn hydroxide gel by hydrothermal processing followed by calcination at relatively low temperatures (∼500 °C). Hydrothermal treatment of the In–Sn coprecipitated gel at 300 °C for 24 h led to the formation of a single phase of tin-doped indium oxyhydroxide (InOOH:Sn). Under hydrothermal conditions the oxyhydroxide phase appeared well crystallized with particles ∼80 nm in size. Calcination of the oxyhydroxide phase above 450 °C in air yielded a substitutional-vacancy-type solid solution of In2Sn1−xO5−y.
I. INTRODUCTION
Sn-doped In2O3 (or ITO) is an advanced ceramic material with many electronic and optical applications due to its high electrical conductivity (up to 104 ⍀−1 cm−1) and transparency to light.1 ITO thin films are used in transparent electrodes for display devices, transparent coatings for solar energy heat mirrors, and windows films in n-p heterojunction solar cells, etc. The thin films are commonly prepared by the magnetron sputtering, which utilizes dense tin doped indium oxide targets.2 Few works have been reported on the preparation of ITO targets.3–6 The sputtering efficiency and properties of the sputtered films are mainly dependent on the characteristics of sputtering targets.3 To optimize the efficiency, the targets should be as dense as possible, but densification of ITO is rather difficult due to rapid vaporization of In2O3 above 1200 °C.7 An oxidizing atmosphere allows high densification by sintering at 1600 °C, whereas a neutral atmosphere (Nitrogen or Argon) leads to poor densification.4 The preparation of the ITO targets is normally achieved using conventional solid-state reaction routes requiring the high-temperature (above 1200 °C) reaction of In2O3 and SnO2 under an O2 atmosphere.6
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J. Mater. Res., Vol. 15, No. 6, Jun 2000 Downloaded: 14 Mar 2015
We believe that the characteristics of the starting powder strongly influence the properties of the final ITO target. In practice, small spherical particles may improve the high density. The direct calcination of the coprecipitated In–Sn hydrated gel failed to form pure ITO powders.8 The aim of this study is to make very fine crystalline Sn-doped In2O3 powder with controlled stoichiometry at low temperatures to use as the starting material for fabricating dense ITO targets. Therefore, hydrothermal techniques were used to obtain the tindoped indium oxyhydroxide phase from the coprecipitated In–Sn amorphous gel. Various experimental conditions have been tested to optimize the crystallinity of the hydrothermally produced powders. Postannealing of this powder leads to the production of fine ITO powders. No literature data a
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