A Study of Ta 2 O 5 for use as High Temperature Piezoelectric Sensors
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types (labeled slabs A and B) consist of pentagons and lozenges. Folding chain-like strings of pentagons, with lozenges between chains form the slab B which are interrupted by slab A structures which consist only of pentagons. JoosteP and Jooste and Viljoen' first reported the piezoelectric behaviour of polycrystalline orthorhombic /3-Ta2 0 5 . Jooste deposited films on a variety of substrates including Si (001), Al, W and stainless steel 304. Since the objective of the work is the manufacture of a high temperature resistant sensor, this study is focused on the deposition of Ta2 0 5 on Inconel. Ta 20 5 films formed by reactive magnetron sputtering with no intentional substrate heating are generally amorphous in the as-deposited state'. Annealing of amorphous Ta2 0 5 at temperatures above 600 'C is needed for crystallization to occur 8 and the crystallization is thermally activated; for example it takes an annealing time of 14 h at 600 TC for Ta 2 0s to be fully crystalized but only 5 minutes at 900 OCS. In the present investigation an XRD investigation of as-deposited Ta 20 5 films, annealed at different temperatures above 600TC for different periods of time, shows the formation and disappearance of certain crystal orientations. Sensor evaluation tests were done in parallel with the annealing studies to compare the crystal orientation of specimens with their sensing performance and it is shown that orthorhombic /3- Ta 20 5 shows promising piezoelectrical properties. EXPERIMENT Deposition, Annealing and Sensor Evaluation Ta 20 5 films were sputter deposited in a DC magnetron sputtering system. Details of the sputtering conditions are reported by Jooste and Viljoen'. Polished and chemically cleaned Inconel coupons (0.05x10x30 mm) were used as substrates. Substrates were not heated by an external source during film deposition and as-sputtered films were amorphous. It is interesting to note that despite the lack of order as measured by X-ray diffraction (XRD), the films exhibit (albeit weak) piezoelectric behaviour. This observation is consistent with studies by Shimizu et al.9 who used microdiffraction to obtain insight into the structure of Ta2 05 films considered 'amorphous' by conventional XRD techniques. Microcrystalline regions of fI-Ta 2 05 were found, only a few nanometer in size. Rapid thermal annealing (RTA) similar to the annealing done by Pignolet et al.10 was done on the reactively sputtered, amorphous Ta2 0 5 films. The Ta2 0 5 films were placed in a small pre-heated pulsed current furnace, for periods of one minute and removed to cool down under ambient conditions. Longer annealing studies were done at annealing periods of ten minutes and one hour. Two different temperatures were investigated; 800'C and 900'C and three different annealing periods were used for each temperature, 1 min.,10 min. and 60 min. The crystallinity of the annealed films were investigated using standard Rigaku XRD apparatus with Cu-K, radiation. In order to evaluate the strength of the piezoelectric effect in the fabricated Ta 2 0
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