Temperature sensors are used across a broad spectrum of human activities such as in medicine, home appliances, meteorology, agriculture, and industrial and military contexts, resulting in temperature being by far the most commonly measured physical quantity. New concepts in temperature measurement methods and probes are needed for contemporary applications, the most important of which are nanotechnology, biotechnology, and integrated optics. Additionally, non-contact thermometry of moving or contact-sensitive objects, difficult to access pieces, or bodies in hazardous locations, is of a growing interest. Since the size of devices rapidly decreases making them more sensitive to thermal overstress, the reliable detection of hot-spots in electrical devices is becoming even more important.
For surface luminescence temperature measurements, phosphor materials in forms of films and coatings are the most promising ones. If sufficiently thin, coatings and films rapidly equilibrate with their surroundings, and local temperature can be monitored in real-time. Also, it enables thermal imaging over the complete surface of interest. Such coatings and films can be prepared from almost all luminescence thermographic materials mixed with appropriate binders by different methods: spin-, spray-, dip-coating, doctor blade method, electrophoretic, etc. Inorganic phosphor materials for use could be rare earth or/and transition metal doped oxides, silicates, titanates, phosphates, etc. The most frequently used temperature read-out scheme in current practice is founded on luminescence intensity ratio (LIR – the ratiometric intensity reading) of different emission bands in luminescent material. This method may exploit emissions from two emission centers in binary luminescence thermometric material.
Here, we aimed to develop the supersensitive luminescence thermometric binary film and coatings which utilize the ratio of two spectrally distinct emissions from two luminescence centers. Transition ion Mn4+ was one center whose emission intensity rapidly quenches with temperature, and rare earth ion Ho3+ was the one whose luminescence is insensitive to temperature changes over the temperature range up to 100°C. As powder precursors, Mg2TiO4:1%Mn4+ and Y2O3:1.5%Ho3+ were prepared by Pechini and Polymer complex solution methods, respectively. To avoid spectral overlapping, the powders were selected based on its efficient luminescence in different spectral regions and similar powder morphologies. Various ratios of starting powder precursors were studied to optimize probe.
Luminescence emissions were measured by 465nm excitation from 450W Xenon lamp on Fluorolog-3 Model FL3-221 spectrofluorometer system (Horiba Jobin-Yvon), and the LIR was calculated to obtain the calibration curve. To test the thermographic performance of the newly developed probe, an uncertainty analysis is conducted and repeatability measurements were performed. By using electrophoretic deposition of the probe, a film was formed on ITO glass, while brush coating was applied on the glass substrate with an organic (PVA) and inorganic (SHMP) binder. For such obtained probes, luminescence thermometry parameters were evaluated based on the LIR method.