Sol–Gel Deposition of Chromium Doped Aluminium Oxide Films (Ruby) for Surface Temperature Sensor Application

One promising approach for the measurement of surface temperatures is the use of thermographic phosphor coatings. Here the deposition of chromium doped (0–7%) aluminum oxide films (ruby) with respect to this application is investigated. The optical properties of the grown films are presented in a further study. The sol–gel technique is used for film deposition. The method is rather simple and omits an expensive setup, so cheap coatings are easily prepared. Aluminum tri-isopropoxide (ATI) is used as an aluminum source and chromium acetylacetonate (Cr(acac) 3 ) is used for chromium doping. The deposition process is analyzed by thermogravimetric measurements (TG/DTA). The film morphologies are characterized by scanning electron microscopy (SEM), the chemical compositions are analyzed by energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) is used for the phase analysis. It is shown that amorphous aluminum oxide is prepared during sol–gel deposition at 563 K. It becomes well ordered (α-Al 2 O 3:Cr 3+ ) after annealing at 1403 K in an O 2 -atmosphere. The chromium content is homogeneous and identical to the stoichiometry of the aluminum and chromium sources. By using a multicycle deposition procedure well adhering films of around 3 µm thickness are grown.