Study of Resbond® Ceramic Binders Used for High Temperature Non‐Contact Thermometry

Fluorescence is a good non-contact thermometry technique in hostile environments such as those found at high temperatures. Phosphors are typically rare earth-doped ceramics that emit light when excited. The intensity, rise time, decay time, and wavelength shift of this emitted light can be temperature dependent. When thermographic phosphors are applied to a surface, with an excitation source and a method to characterize the emission provided; it is possible to determine the surface temperature. One of the simpler methods to apply these coating is through the use of temperature sensitive paints (TSPs). These TSPs are created by mixing phosphor with a binder material to form a sprayable coating that can be easily and economically applied to a large area. Ideally, these phosphor paints need to survive at the limit of the existing decay time data, or 1700 C. The survivability of phosphor paint depends on the physical characteristics of the binder. The goal of this research is to discover binders that will allow phosphor paints to survive at high temperatures. Suitable binders will allow the construction of non-contact measurement devices useful in environments that are not suited for more common thermocouple or infrared devices. For a phosphor paint to be useful at a selected temperature, it must fluoresce when excited and have a measurable decay time. In this study, the Cotronics Resbond 791, 792, and 793 ceramic binders were evaluated to determine their suitability to serve as binders for a Y2O3:Eu phosphor powder. Post-thermal cycling spectral analysis was used to quantify wavelength and intensity changes in emission from ultraviolet excitation. Several of the paints utilizing their binders were able to survive at 1500 °C. INTRODUCTION Phosphors are fine powders that are doped with trace elements that give off visible light when suitably excited. Many of these phosphors have a ceramic base and can survive and function at high temperatures such as those present during combustion. When thee phosphor is applied as a thin coating, it quickly equilibrates to the ambient environment and can be used to measure the surface temperature. The basic physics of thermographic phosphors is well established, and researchers at Oak Ridge National Laboratory (ORNL) have demonstrated several useful applications [ref. 1-12]. The thermometry method relies on measuring the rate of decay of the fluorescence yield as a function of temperature. Having calibrated the phosphor over the desired temperature range, a small surface deposit is excited with a pulsed laser. The resulting fluorescent decay (typically in less than 1 ms) time is measured to calculate the temperature of the substrate. In many instances, (e.g., in a continuous steel galvanneal process) a simple puff of powder onto the surface provides an adequate fluorescent signal [ref. 1-2]. Often temperature measurements are made using thermocouples or optical pyrometry. However, in situations where rapid motion or reciprocating equipment is present at high temperatures, it is best to use other techniques. For many phosphors, the prompt fluorescence decay time (τ) varies as a function of temperature and is defined by: I = I0 exp t τ ⎧ ⎨ ⎩ ⎫ ⎬ ⎭ , (1) where: I = Fluorescence light intensity (arbitrary units), I0 = Initial fluorescence light intensity (arbitrary units), t = Time since cessation of excitation source (s), and τ = Prompt fluorescence decay time (s) [ref. 1-2]. The time needed to reduce the light intensity to e (36.8%) of its original value is defined as the prompt fluorescence decay time. An example of this quantity for several thermographic phosphors is shown in Figure 1. Notice the fluorescence decay time decreases by four orders of magnitude when the temperature increases from 600 to 1100 °C. Figure 1. Prompt fluorescence decay time for a selection of thermographic phosphors. 0 400 800 1,200 1,600 Temperature (°C) Fl uo re sc en ce D ec ay T im e (μ s) This paper will give an overview into research to find binder and phosphor combinations that can emit fluorescence and remain mechanically viable at temperatures as large as 1600 °C. Emphasis will be placed on developing procedures and techniques for the application and the pre-treatment of candidate binder and phosphor combinations. HISTORY In May 2001, a research program was initiated to evaluate TSPs that could be used in high temperature thermometry applications. This three-year program was funded by the NASA John H. Glenn Research Center in Cleveland, Ohio. A research team lead by ORNL was assembled to locate binder and phosphor combinations that can emit fluorescence and maintain structural integrity at high temperatures. The ultimate goal for this research was to locate TSPs that could be used to measure the heat flux and temperature present inside a common turbine or rocket engine. The phosphors in question are typically rare earth compounds that emit copious fluorescence and have grain sizes of less than 10 μm. Binders used for this purpose must: 1) Be easy to apply with an airbrush, 2) Set to a temperature resistant inorganic finish, and 3) Have minimal reaction to the phosphor material. During the first year of the program, research was completed to determine if heat flux could be measured using a phosphor coating. In the second year, research was completed to find phosphor and binder combinations that can emit light and remain mechanically viable at high temperatures. Results from these measurements can be found in Table I. Each paint combination was exposed to the high temperature listed in Table I for one hour and then slowly cooled. The heating rate was kept small in order to minimize effects due to the difference in expansion coefficient between the paint and the ceramic substrate. Table I. Zyp Coatings-based thermally sensitive paint survivability results. Paint Phosphor Emission At Given Cycling Temperature? Binder Fraction (Vol. %) Phosphor Emission (nm) 1200 °C 1300 °C 1400 °C 1500 °C 1600 °C Comments HPC 20% Y2O3:Eu 611 Yes Yes Paint mostly gone after heating to 1500 °C HPC 10% Y2O3:Eu 611 Yes Yes Paint mostly gone after heating to 1500 °C 75% HPC 25% LK 20% Y2O3:Eu 611 No All of paint gone after heating to 1500 °C. 75% HPC 25% LK 10% Y2O3:Eu 611 Yes Most of coating gone after heating to 1500 °C. 50% HPC 50% LK 20% Y2O3:Eu 611 Yes Yes Paint mostly gone after heating to 1500 °C 50% HPC 50% LK 10% Y2O3:Eu 611 Yes No No 611 nm peak after heating to 1500 °C. ZAP 50% Y2O3:Eu 611 Yes Yes Yes Yes Paint still intact with diminished fluorescence after heating to 1500 °C. ZAP 30% YAG:Dy 585 Yes Yes Yes Yes Yes Fluorescence still present after heating to 1600 °C. ZAP 30% YAG:Tm 420 480 Yes Coating intact but poor fluorescence after heating to 1500 °C. ZAP 30% YAG:Eu 595 611 Yes Coating intact but poor fluorescence after heating to 1500 °C. Three binders, HPC, LK, and ZAP, from Zyp Coatings, Incorporated of Oak Ridge, Tennessee were used in this research [ref. 9-10]. Each of these three binders was designed for use in high temperature applications and are available in the commercial market. A selection of rare earth compounds are used for the phosphor powders in this research: yttrium oxide doped with europium (Y2O3:Eu), and YAG doped with dysprosium (YAG:Dy), terbium (YAG:Tb), and europium (YAG:Eu). A selected binder and phosphor powder are mixed together to create a strong and durable thermally sensitive paint (TSP). Detailed information on this research can be found in references 9 and 10. Results shown in Table I show that most of the HP/LK bound samples survived heating to 1400 °C. Most of the paints were removed from the surface of the ceramic at 1500 °C. This data also shows that paint consisting of a 100% ZAP binder and 30% YAG:Dy powder by volume is intact and emits fluorescence after heating to 1500°C. This paint surface did look bumpy or mottled after heating. The other ZAP paints were also intact, but with reduced fluorescence. In fact, the ZAP binder and 30% by volume YAG:Dy TSP was also found to emit fluorescence after heating to 1600 °C. BINDERS During the third and final year of the NASA-Glenn research, several Resbond ceramic binders manufactured by Coltronics Corporation of Brooklyn, New York were evaluated using a powdered Y2O3:Eu phosphor. The main fluorescence emission from Y2O3:Eu occurs at a wavelength of 611 nm. Selected material and preparation properties for the tested Resbond binders are given in Table II. The Cotronics series of ceramic binders includes six different formulations. Of these, only the four varieties listed in Table II were tested. During initial research, Cotronics Resbond 795 did not adhere to the surface. Since the other two binders have similar properties, it was considered more productive to focus on the three remaining binders. Table II. Properties for selected Cotronics Resbond ceramic binders. Resbond Composition 791 Silicate Glass 792 Silicate Glass 793 Silica Oxide 795 Alumina Oxide Applications Adhesives Coatings Adhesives Coatings Electronics Bonds Fibrous Materials High-Purity Binder pH Mildly Basic Mildly Basic Mildly Basic Mildly Acidic Maximum Temperature (°C) 165