I am doing research in gas sensing devices and application. What would the difference be if we use gold as the material for the bottom side heater pattern instead of platinum?What effects should be considered for doing that?
If so, what thickness should be the optimum thickness of gold for reaching at least 300 deg C. ?
Metallic heater are a key topic in the technology of heated gas sensors. Platinum has become a standard because of the following reasons:
- It resistivity has a relatively high and constant temperature coefficient of temperature of around 3000ppm/degree. It therefore can serve the dual purpose of a heater and temperature sensing element
- Pt surfaces can oxidise. Pt-heater meanders can therefore form bridging oxygen bonds to substrate materials such as ceramics or SiO2. Consequently, Pt films feature a relatively good adherence to substrates.
Au is disadvantageous in both respects. In particular, it cannot oxidise. As a consequence, it has a very poor adherence to most substrates.
The normal trick of making noble metals adhere to a substrate is to deposit some kind of oxide-forming non-noble metal first (Ti, Ta, W,...) and then to deposit the noble metal on top. The method is successful, as these refractory metals form bridging oxygen bonds to the substrate and metal-metal bonds to the top noble metal layer. This works well at room temperature and little above. Adherent layers fail at high temperature because the oxidisable metals diffuse along grain boundaries in the noble metal layer to get oxidised at the free surface. In this way the integrity of the noble metal layer is destroyed and the noble metal heater will fail after some time.
In short: Pt is superior with regard to Au because it is a little less noble than Au.!
Metallic heater are a key topic in the technology of heated gas sensors. Platinum has become a standard because of the following reasons:
- It resistivity has a relatively high and constant temperature coefficient of temperature of around 3000ppm/degree. It therefore can serve the dual purpose of a heater and temperature sensing element
- Pt surfaces can oxidise. Pt-heater meanders can therefore form bridging oxygen bonds to substrate materials such as ceramics or SiO2. Consequently, Pt films feature a relatively good adherence to substrates.
Au is disadvantageous in both respects. In particular, it cannot oxidise. As a consequence, it has a very poor adherence to most substrates.
The normal trick of making noble metals adhere to a substrate is to deposit some kind of oxide-forming non-noble metal first (Ti, Ta, W,...) and then to deposit the noble metal on top. The method is successful, as these refractory metals form bridging oxygen bonds to the substrate and metal-metal bonds to the top noble metal layer. This works well at room temperature and little above. Adherent layers fail at high temperature because the oxidisable metals diffuse along grain boundaries in the noble metal layer to get oxidised at the free surface. In this way the integrity of the noble metal layer is destroyed and the noble metal heater will fail after some time.
In short: Pt is superior with regard to Au because it is a little less noble than Au.!
You can determine the changes in input power requirements as you change film thickness from equations for the resistivity and Ohm's law. See any of the myriad of materials science and engineering textbooks on electrical properties of materials to learn how to convert resistivity to resistance. Do not forget to add in the temperature dependence of Pt resistivity.
As for the power needed to reach 300 oC, that will depend on your heat transfer requirements for the system at hand. To a first approximation, do an energy balance without convection. Then add convection affects.
Gold will work - its dependence is a bit less linear compared to Pt and the resistance is lower, but generally, no major difference!
At that temperature gold will work, however beyond the 500C gold tends to agglomerate and evolves into separate, disconnected islands of material (not electrically conductive anymore) over ceramic substrates such as alumina, silicon dioxide, silicon nitrate (I assume you have one as a substrate), if you potentially use the metal heater/electrode beyond 500C, I would use Pt.
As for thickness, starting from 10 nm to 300 nm would be fine.
Engin
Due to many reasons Pt is better material for heaters working at elevated temperatures. If you are working with gas sensors and you will change substrate temperature, sometimes it is necessary to know its exact value. In this case you can utilize the known (previously measured) TCR (temperature coefficient of resistance) for Pt. It is rather constant as a function of temperature, after some initial ageing. This is widely known property of Pt.
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