Embedding Nichrome wire in Stoneware/clay - will it crack?

[irpheus]

Hello All,

I have a question that I hope one of you here can help with ... I would really appreciate if this was so ...

As it is I'm currently underways with a project where I consider embedding a Nichrome wire into a clay circular plate. The clay plate, which is Ø150 mm and 12 mm thick, will eventually be kiln fired to become "Stoneware" or another "household clay-ceramic" - I use this word in lack of a better wording. The end result should be a clay/ceramic material that will have a use similar to that of ceramic kitchen pots or earthenware dishes/bowls. Inside the clay there will be a circular pattern of the nichrome wire from the center to the perimeter.

After the kiln firing - this is in daily use - the nichrome resistance wire will be heated >= 1000 times to about 150 degr. celsius (80% of the times) and the other 20% of the times to about 250 degrees celsius. I.e. several hundred times to 250 degrees celsius - and it should be able to do so without cracking or breaking the ceramics/stoneware - or break itself.

To ensure this I'm considering increasing the diameter around the nichrome wire inside the clay so that there's a bit of space inside the ceramics for the nichrome wire to "move" during expansion/contraction.

Would any of you here have experience with this - and can say whether it's likely to work? Or maybe have some better suggestions? I would also be most interested in a tip on how to terminate the hot Nichrome wire towards the normal wire leading away from the Nichrome in a way that lasts "forever" () ...

Thanks for any help you may be able to give

Jesper

 

[IRstuff]

I would take apart a $20 consumer hair dryer to see how that's done, which I think is typically a crimped connection. What you propose sounds reasonably, but you may want to think about embedded some supports for the nichrome, to keep it from getting bunched up in the cavity; again, see what's done in a hair dryer or possibly a toaster.

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[Compositepro]

As long as you allow the wire to expand without stressing the ceramic, there should be no problem. Thermal shock is another issue to be aware of. If you heat the ceramic too quickly, it can break due the stress caused by thermal gradients within the ceramic.
Terminate the nichrome wire by doubling it over at the end and twisting it. Then crimp or clamp onto the wire. Doubling the wire increases its mechanical strength and lowers its resistance so less heat is generated near the termination. The termination should be in a relatively cool zone.

 

[tomwalz]

Might look at how kilns, ovens, etc. are wired.

It will grow and shrink. The ceramic will crack.

Wind the wire into coils. We used a 1/4" dia. rod chucked into a small electric drill.
Wear a leather glove to guide the wire. Wind it tight then let it relax. It will expand to some odd diameter which will be about right.
Pull the coil out until the is lots of room between the individual turns so they do not touch and short out.
Consider running it in tunnels. The coils will get hot, weak and layover.



Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[irpheus]

Hi All,

- & thank you very much for your feedback and suggestions

@ IRstuff: . I'll look into a toaster and consider if supports may be needed ...

@ Compositepro: Fine idea with doubling the nichrome wire - will do that and also terminate it in cooler area. I also should be able to make the electronics ramp up reasonably slowly so as to avoid heating the ceramics too quickly ...

BTW noting your alias name I was wondering if you know of a solid, very cheap and environmentally friendly insulator material (available as a plate in appr. 5 mm thickness, or as a potting material to go on the ceramics)? I was thinking of e.g. magnesium oxide but might you know of an even better insulator material?

@tomwalz: ... Fine idea - I will do this and also make provisions for a "relaxed" course of the wire.

Your ideas & suggestions have been fine in steering me in what I consider very useful directions. Now time for investigating a bit and trying it out ..

Cheers

Jesper

 

[EdStainless]

The method that Tom suggested is how heating elements were made in our grandparents' days.
Not considered safe because you could poke into the grooves in the bottom and contact the wire.
If you select the correct wire gage and length you will get the desired resistance to power this off of your desired voltage and get the correct power draw. consider that commercial hot plates are typically between 1kW and 2kW

They also used a ceramic composition that was much more tolerant of thermal shock. I believe that mullite (3 parts Al2O3 and 2 parts SiO2) is what was common. but I don't know if this was strictly for properties or for price.
If the ceramic gets moist and you heat it it will break.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Ceramicguy]

Will the nichrome survive the kiln firing of the clay plate?

An alternative could be to machine the grooves in a piece of pyrophyllite (known in industry as Grade A Lava).

As mentioned above, not allowing the nichrome to expand independently of the ceramic will result in mechanical failure of the ceramic.

Your temperature requirements are cool from a ceramic perspective. The risks for thermal shock/stress will increase with faster temperature ramp rates and temperature differentials on mating surfaces.

If you wish to try the clay, you might be able to build a mold you can press into the clay while it is wet thus forming the element grooves. Using a clay body with a range of particle sizes and some porosity will improve thermal stress resistance.

There are a number of ceramics suitable for your application should you decide to have the part fabricated.

-Bruce

Bruce

http://accuratus.com