DE3613501A1 - Process for anti-catalytic coating of thermocouples - Google Patents

Abstract

In this process, stabilised zirconium dioxide is used for coating noble metal thermocouple probes for high-temperature measurements in chemically reacting gas streams. This material is excellent in fulfilling the different requirements. A simple application technique using a slip and the subsequent firing procedure can be carried out in such a way that the essential advantages of the thermocouple probes (low cost, simple handling and many uses) are completely maintained.

Description

The invention relates to a method for coating the surface of precious metal thermocouple probes with an anti-catalytic acting oxide ceramic, so these probes for measurements high temperatures in chemically reacting gas flows are.

For the measurement of temperature fields in gas flows with high local resolution today, alongside the newer non-contact, however, sometimes quite complex optical methods [1], still the classic thermocouple probes, some of them strong miniaturized welded joints used [2]. As beneficial the low costs, the easy handling prove as well as the various possible uses. For use in chemically reacting currents at very high temperatures - These conditions are for example in combustion systems given in the combustion of hydrocarbons - are suitable especially the melting platinum or platinum / rhodium thermocouples. The ones according to DIN 43 710 and IEC are used 584-1 available pairings with standardized voltage series. These are in Table 1 with their maximum operating temperatures and the associated thermal voltages.

For the following reasons, it is now an anti-catalytic one Coating of these precious metal probes is essential:  

Tab. 1: Standardized Pt-Pt / Rh thermocouples (T _ref = 0 ° C).

• 1.) Diffusion of hydrogen or carbon into the platinum at temperatures of T <1400 ° C can cause contamination of the alloy with a corresponding decalibration of the thermal voltage. As a result, there are greatly reduced operating times [3].
• 2.) Especially through platinum are catalytic influences exerted on the reaction kinetics of the combustion. The The catalytic effect of the platinum surface is based on this on the adsorption of hydrogen, which thereby activates and shows increased responsiveness. Ignition effects and local temperature increases near the probe influence the flow field considerably and thus lead too strong measuring errors.

The anti-catalytic layer has various requirements to fulfill:

• 1.) Inert behavior towards the thermal material
• 2.) Inert behavior towards the chemically reacting  flow
• 3.) High electrical line resistance
• 4.) Small layer thickness with gas impermeable structure with a smooth surface
• 5.) No instability of the coating material, in particular no changes to the crystal lattice large temperature fluctuations
• 6.) Application technique as simple as possible without great expenditure on equipment
• 7.) Non-toxic coating material
• 8.) Melting temperature above that of the used Pt-Pt / Rh alloy
• 9.) Low procurement costs

In the coating process that is very common today Silicon dioxide according to Kaskan [4, 2] can be the requirement points 1.), 8.) and with restriction 6.) not sufficiently fulfilled will. A newer method described by Kent [3] with Beryllium oxide is often not used because of 7.).

The invention is based on the object, one in all Satisfactory coating material to select and with a suitable application technique combine that the above Main advantages of thermocouple probes (Costs, handling, versatility) are retained.

The solution to the problem is thus inventive from that to the pure metal oxides considered so far, too  the so-called partially and fully stabilized metal oxides were examined have been.

According to the invention, zirconium dioxide has proven to be very suitable, the properties of which can be changed in a targeted manner by incorporating stabilizers (for example CaO or Y₂O₃). As can be seen from the phase diagram ZrO₂-Y₂O₃ ( Fig. 1), pure ZrO₂ cannot be used here, since at 1200 ° C a reversible conversion of the lattice structure from monoclinic to tetragonal phase takes place. At the same time, the volume shrinks by about 10%. In the partially stabilized range (4.9 mol% <X _Y₂O₃ <12.5 mol%) and in the fully stabilized range (X _Y₂O₃ <12.5 mol%), irreversible crystal lattice structures can now be formed, so that there are no volume jumps when the temperature changes occur. It is known that the cubic crystals remain stable regardless of the temperature after a single temperature increase to approximately 1600 ° C. In the partially stabilized area, the cubic lattice can compensate for the phase shift of the monoclinic-tetragonal residual phase [5]. Thus, the partially stabilized zirconium dioxide with a low stabilizer concentration, the other physical properties of which are comparable to those of pure ZrO₂, is particularly suitable for the process according to the invention.

An expedient type of coating that can be carried out without major expenditure on equipment is the subject of claim 4. In this case, powder with a small grain diameter (typically, for example, d = 1 μm) is distilled with aqua dest. as well as small addition of an organic binder, a slip is stirred, the fluidity of which influences the layer thicknesses to be achieved. By immersing the thermocouple in the slip one or more times, a uniform layer can be applied, which must then dry out. The drying temperature should not exceed 35 ° C to avoid embrittlement of the surface. The layer is burned in a hydrogen-oxygen flame. The temperature must exceed 1600 ° C. The finished layer is checked under a light microscope. In this way, layer thicknesses of approx. 10 µm can be achieved.

As an example of the suitability of the method according to the invention, temperature measurements in a laminar, stoichiometric methene-air premixing flame are shown under ambient pressure conditions ( Fig. 2). A Pt-Pt / Rh 87/13 element with a weld bead diameter of d _P = 50 µm is used. With coating, the diameter of the element is d _E = 70 µm. (The temperature information exceeding the melting temperature of the alloy can be explained by the temperature correction calculations that are generally carried out via heat balances.)

• bibliography
• [1] Penner, S. S., Wang, C. P. and Bahadori, M.Y., Laser Diagnostics Applied to Combustion Systems 20th Symp. (International) on Combustion, 1984 pp 1149-1176
• [2] Fristrom, R.M. and Westenberg, A.A. Flame structure, Mc Graw-Hill, 1985
• [3] Kent, J.H. A Noncatalytic Coating for Platinium-Rhodium Thermocouples Combustion and Flame, 14, 1970
• [4] Kaskan, W.E., The Dependence of Flame Temperature on Mass Burning Velocity 6th Symp. (International) on Combustion, 1957 pp 134-143
• [5] v. Mallinckrodt, D., Impact of contamination on sintering and stabilization from ZrO₂ (CaO) Dissertation RWTH Aachen, 1981  
• [6] Stubican, V. S., Hink, R. C. and Ray, S. P., Phase Equilibria and Ordering in the System ZrO₂-Y₂O₃ J. Am. Ceram. Soc. 61, 1978, PP 17-21

Claims (5)

1. A method for the anti-catalytic coating of precious metal thermocouple probes which are used for measuring high temperatures in chemically reacting gas streams, characterized in that zirconium dioxide stabilized with Y₂O₃ is applied to the element. 2. The method according to claim 1, characterized in that zirconium dioxide stabilized with CaO on the element is applied. 3. The method according to claims 1 and 2, characterized in that both partially stabilized and fully stabilized Zirconia can be used. 4. Process according to claims 1 to 3, characterized in that from powder with a small grain diameter and aqua dest. and an organic binder is mixed with a slip which is applied in liquid form to the element. After drying, the layer can be fired in an H₂-O₂ flame at temperatures T <1600 ° C. Typical layer thicknesses of less than 10 µm can be achieved. 5. The method according to claims 1 to 3, characterized in that complex application techniques such as vacuum evaporation technology can be used.