Sick Silicon Carbide Heating Elements
If the Silicon Carbide heating elements in your dental sintering furnace look like the ones pictured above, full stop, you have a problem.
It's all about the oxide
Similar to the MoSi2 elements we talk about so frequently, SiC elements form a protective oxide layer. If you have ever looked at a brand new SiC element, you have probably noticed an iridescent sheen – not dissimilar to the rainbow affect oil has on water – on the surface of the element. What you are seeing is the SiO2 (Silicon Oxide) that formed on the surface of the element during its manufacturing.
Like the Alumina formed by MoSi2 (Super) elements, this oxide layer protects the base material of the element, allowing the element to operate at a far higher temperature than without the oxide layer, even in challenging atmospheres (we don’t see atmospheres in dental sintering, but they are often used in industrial applications).
What has happened in the above picture is that the elements have been overheated.
You may be thinking, “wait – my furnace reports that it never goes over 1600C when I am sintering, and I know that SiC elements are capable of up to 1650C!”
Yes, that is true. Remember though, that your furnace elements are never going to run at the same temperature that your furnace is set to operate at. In an ideal situation, they are going to run approximately 50C – 100C higher than your furnace chamber due to the fact that the furnace chamber is large, porous, and – at least at the beginning of your cycle – has a bunch of cold material inside.
Think of it like this – if you put a jet engine in your living room, turning it on will make the interior of your living room nice and, um, toasty in short order.
But, if you put an incandescent light bulb in the same space, although the surface of the light bulb is too hot to touch, it will have little to no net effect on the temperature in the room. You could put 500V into that light bulb, and still nothing will happen. Well, save for your light bulb burning out.
The story of power and Mr. Watt
It all comes down to power – and the same thing that is happening to the hypothetical light bulb mentioned above is doing its magic on your heating elements . After all, heating elements are just larger, fancier light bulbs.
Depending on how your furnace manufacturer designed your furnace, they may not have enough elements in the furnace to meet the power demands of the application. The calculation is straight forward – if your furnace needs, let’s say, 20kW to sinter 5kg of Zirconia in 8 hours, and there are 4 elements, each capable – at maximum – of producing 4.5kW each for a total of 18kW, the elements will be easily over powered (read: driven to a high, catastrophic temperature) if the furnace controls are dumping in 20kW worth of power.
What actually is happening, and how to fix it
Silicon Carbide vaporizes (sublimates for you chemistry types) at 1700C. When that happens, it vaporizes out of the hottest part of the element – the core – and when it reaches the relatively cooler surface of the element, it recrystallizes, giving the bubbly appearance.
This is not good for two reasons – (a) your elements will not last as long, and all (b) SiC contains natural impurities – such as Iron – that will negatively affect your finished product.
Unfortunately, there is not much you, as the lab owner, can do on your own to address this issue as the furnace is designed to work with it’s specific control scheme and power supplies.
That said, there may be some options. Being SiC heating element experts, we can analyze your existing elements and perhaps offer a larger diameter (or longer) element that will still work with your controls but run at a lower surface temperature, reducing the effects of the higher element temperature.
Doing so is a fun exercise. So, if you are needlessly bubbly (er, just your heating elements), drop us a line and we will be glad to help.