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Insulating diamond coatings on Tungsten electrodes

: Matthée, T.; Schäfer, L.; Schmidt, A.; Klages, C.-P.


Diamond and Related Materials 6 (1997), No.2-4, pp.293-297
ISSN: 0925-9635
Diamond <1996, Tours>
Journal Article, Conference Paper
Fraunhofer IST ()
diamond layer; hostile environment; Level Sensor; CVD; diamond; coating; electrical property; impedance spectroscopy

Insulating diamond layers of 5–20 μm thickness are deposited by hot filament-chemical vapor deposition (HF-CVD) on tungsten electrodes. These electrodes of about 150 mm length and 2 mm diameter are used for level sensors in hostile environments like water at 350°C. The increase of the resistivity due to N-doping is compared to resistivity of undoped layers. Studies of the homogeneity and quality of the layers are performed by scanning electron microscopy (SEM) and micro-Raman spectroscopy.
The electrical characterization is performed by impedance spectroscopy at room temperature and at elevated temperatures up to 83°C in aqueous NaCl solution (3%). The reduction of pores and pin holes is important in arriving at high values of electrical impedance (|Z| > 1 MΩ, 30 Hz), which is essential for a level sensor working in poorly conductive liquids (σwater ≈ 0.1 μS cm−1). Several cleaning procedures are necessary to reduce the pore density from about 1 per 20 mm2 to less than 1 per 1000 mm2. Highest resistivity values of N-doped diamond layers, averaged over the electrode length and film thickness, amounted to 5 × 1012Ω· cm.