Ceramic Question

[pbhuter]

I'm looking into ceramics for a high-temperature (up to 2000-deg C) high resistance application (resistance = heat). Are there ceramics that I can apply a current to (not quite sure how much power I will have available, but it should be pretty high) and have it generate enough heat to get up to 2000-deg C? I see that something like alumina loses resistivity as the temperature goes up, is that common for ceramics? Lowest temperature I need will be 650-deg C, but if I can get up to 2000, that would be better. Thanks.

 

[pbhuter]

UPDATE - let's say I have 12 kW to work with.

 

[unclesyd]

You didn't give any specific requirements for the material you require so here goes a shot in the dark.

Silicon Carbide and Molybdenum Disilicide are used for heating elements and are about the only thing readily available at a reasonable cost.

http://www.isquaredrelement.com/

 

[pbhuter]

Thanks unclesyd. Will either of these materials get up to 2000-deg C? I realize that 12 kW is not a lot of power to work with. If cost weren't a factor, what else could I use? What other parameters would you need to help me choose a material?

 

[unclesyd]

If you check the literature a the link given in my post you will see that 1700C is the upper limit, You might be able to get a little higher in some atmospheres.

Myself and other members will help if a little more information is given so others can contribute.

What shape do you need?
Working atmosphere?
Duty cycle?

Any Other information laying around.

 

[pbhuter]

unclesyd - I wasn't sure if that link was meant just for me, or if it was a "signature" - I'll check it out, though. I'm looking at a spiral heating element similar to the one on an electric stove (but bigger, although not sure exact dimensions). It needs to operate pretty much throughout the atmosphere (sea level and up to some unknown altitude). I need to operate the heating element for hours at a time and not have to replace it after each use (especially if using a more expensive ceramic). Like I said, I will be applying about 12kW to the element to get it to heat up. Does this help? I'm going to check out that link, now. Thanks again.

 

[Ceramicguy]

Think hard about the temperature you must have versus the temperature that is nice to have. Heating elements for higher temperature are much more costly than those for lower temperatures. Furthermore, there is a strong inverse relationship between process temperature and heating element lifetime.

The rest of this note assumes you are in an oxidizing atmosphere. Metallic heating elements can reach element temperatures up to 1400 C. Silicon carbide will get you to about 1625 C. The moly disilicide is good to about 1850 C. All these elements are protected by growing an oxide film at high temperatures. Changes in oxygen partial pressure can have a dramatic effect on element life. If you truly need to get to 2000 C, consider using zirconium oxide which will work well as an ionic conductor of oxygen above 600 C. This means you have to get the element to at least 600 C before you can get enough power through it to begin heating. Ultimate element temperature is on the order of 2100 C.

I have talked about element temperatures here. In a furnace environment it wouldn't be unusual to see the environment temperature a couple hundred degrees less than the element temperature.

Bruce

http://www.accuratus.com

 

[pbhuter]

Ceramicguy - Thanks for the response. I'd like a minimum temperature of 650C, and do not necessarilly need 2000C. That makes sense that temperature and lifetime are inversely proportional. How does the oxygen parial pressure effect things? I understand that the partial pressure will be higher at sea level and lower at altitude, but there may still be high pressures (compressed airflow) over the element. Thanks again.

 

[Ceramicguy]

The active corrosion occurs with oxygen pressures in the millitorr to decitorr range. Essentially, the protective SiO2 film that would normally grow is an SiO film which has a vapor pressure an order of magnitude or greater than SiO2. It simply evaporates. Since you are working at higher pressures with high gas flows, I would like to direct you to the following link wherein you may find some useful information.

http://www.ornl.gov/.../More_ObsAccelSiCReces_JACS 83-11-2000.pdf

As an aside, I have used Kanthal heating elements for many years and find them consistently high quality.

Bruce

http://www.accuratus.com

 

[pbhuter]

Ceramicsguy - that link didn't work. I see a "..." in there that may be causing the problem. Would you mind reposting? Thanks.

 

[Ceramicguy]

I uploaded the file to engineering.com . Hopefully you can access it now. It has a lot of good references for you. Sorry about the misstep.

Bruce
www.accuratus.com

 

[pbhuter]

Thanks Ceramicguy, I'll take a look at it. But overall, it's possible to get a heating element hot enough using 12 kW of power for my application? I appreciate all the help I've gotten.

 

[Ceramicguy]

Sorry, I can't answer your question. There isn't enough information regarding the heating element resistance, configuration, heat loss mechanisms, gas flow rates, etc. etc.

Bruce

http://www.accuratus.com

 

[pbhuter]

I understand. I was going with the fact that some of these elements can get hot enough for my application. I'll have to figure out calculations for if these elements will actually get hot enough with 12 kW. Thanks.

 

[cloa]

pbhuter are comfortable with all the basic electrical and thermodynamic calculations P=V^2/R etc. (can your electrical supply sufficient a lot of current and wiring handle a lot of current). I think pbhuter just wants to work out if the project is roughly doable.

 

[pbhuter]

If these elements can get up to 1850-deg C, there has to be a way to do it. I'm looking at an application where 12 kW of power is supplied to a heating element. I'm not familiar with the thermodynamics of resistors (the element is basically a resistor, right?), but I can figure that sort of thing out. Thanks for all the help.

 

[Ceramicguy]

The element is a resistor. Power dissipated in the resistor can be obtained from the voltage drop across is as cloa states above or the current through is as P=I^2*R. Either relation is dependent on element resistance. You will have to look at both and determine if your power supply has enough capacity to meet the voltage and current demands of the element. The resistance of the elements varies with temperature so you will need to look at your design at room temp. and design temp. at a minimum. I encourage you to consult with the element manufacturers for details.

Bruce

http://www.accuratus.com

 

[pbhuter]

Ceramicguy - I sent a message to Kanthal asking how much power would be required to get their element up to 1850-deg C (which is listed as the highest temperature). Hopefully they'll get back to me and let me know. Thanks again for all your help.

 

[PKerEng]

I would throw-in my two pennies, just to maintain the momentum. The forum should be promoted and expanded by any possible means. It is the way of the future.
Here come my technical comments: the problem should be more clearly defined from the physical point of view. First of all, the only solid restrains I can read is the total awailable power of 13 kW ready to be dissipated and on top of it a desire to achieve certain minimum temperature level. This is not even close to any physical description of the system if nothing is said about heat transfer conditions. Here we also have to specify the regime: is it steady-state or transient (here heat capacity issues will kick-in). Also, do not ignore the electric power transfer issues: current & voltage of the source has to match the heating element.
The bottom line is: let's try to help each other but let's strive to build-up some kind of engineering discipline while describing the problem.

Slawomir

http://www.pkerengineering.com

 

[pbhuter]

PKerEng - what would you need to help me out with this problem? It's been years since I took a thermodynamics course, and I don't remember covering electrical heating. I found some literature on the Internet that described how heating is a function of power and resistance of the material. I then found that ceramics made good heating elements, and that led me to this forum. I understand that the resistance of a ceramic element will go down as the heat increases. I also recognize that 12 kW of power probably isn't a lot. I emailed a manufacturer of elements to see how much power would be required to achieve the highest temperature, and I'll post here when they respond.