Bimetallic junctions...

[Externet]

Hi.

Do bimetallic junctions with no electrolyte at all, and both at same temperature (no thermoelectrics related) produce a minute amount of voltage ?

As in explosive bonding, electroplating, sputtering or other processes, where two dissimilar metals are intimately joined.

[studiot]

I've never studied this aprticular question, but I would think the answer is yes and I would look at the difference in work functions at the surfaces of the joined metals.

 

An electron crossing this area must experience a change of energy due to the difference in work functions.

I guess this would appear as a potential as in semiconductors.

 

I think you can also get this across grain boundaries within the same metal.

[fiveworlds]

Do bimetallic junctions with no electrolyte at all, and both at same temperature (no thermoelectrics related) produce a minute amount of voltage ?

 

We use them a fuses here. They are essentially two pieces of metal that when a large enough current is applied expand at different rates to break the circuit, They tend to be a lot safer than the old burn out fuses and I have never had to replace one yet.

 

http://machinedesign.com/batteriespower-supplies/thermal-magnetic-circuit-breakers

 

I am not sure if they do produce some voltage depending on which ones you put together I suppose.

Did you see this yet?

Edited October 3, 2014 by fiveworlds

[Externet]

Thanks.

See no relation to the bloom box nor circuit breakers. A termocouple bond creeps closer, but not as in the Seebeck effect, but the Seebeck coefficient.

 

Probing a zinc/copper intimate bond does not register any potential, not even a millivolt.

[fiveworlds]

Thanks.

See no relation to the bloom box nor circuit breakers.

 

It's fairly new to me to be honest found a website, http://www.galvanizeit.org/design-and-fabrication/design-considerations/dissimilar-metals-in-contact

 

Galvanic corrosion occurs when two different metals are in contact in a corrosive environment: one of the metals experiences an accelerated corrosion rate. The contacting metals form a bimetallic couple because of their different affinities (or attraction) for electrons. These different affinities create an electrical potential between the two metals, allowing current to flow.

plus the wiki http://en.wikipedia.org/wiki/Galvanic_corrosion

Edited October 3, 2014 by fiveworlds

[studiot]

Wikipedia seems to be talking about what I meant.

 

http://en.wikipedia.org/wiki/Work_function

 

You need to look up the WF for Zinc and Copper

Edited October 3, 2014 by studiot

[Enthalpy]

These are two different constructions with similar names. Two metals can pade a contact potential, they can also expand differently - and the uses differ.

 

Yes, a contact between two metals makes a potential difference. At a uniform temperature, this can't be observed with a voltmeter because the circuit closed around one contact contains one or several contacts more that go back to the original metal or material, and the net sum is zero.

 

Though, other methods can observe this potential, for instance an electron beam. This is difficult because electron work functions depend a lot on the surface condition.

 

Semiconductors have also a work function, a contact potential and so on. Their Peltier and Seebeck elements are better than metals because the Fermi level varies more than with metals.

[Externet]

... this can't be observed with a voltmeter because the circuit closed around one contact contains one or several contacts more that go back to the original metal or material, and the net sum is zero...

That ! explains it. Thanks again, Marc.

 

In my words, the 'any' potential generation from the two metals junction is also short-circuited by their bonding; showing zero.

 

So, is there a 'permanent' electrical current in the bond ?

[Enthalpy]

The contact potential is uniform across the junction, provided the temperature is even. But if you have a temperature gradient across one single junction, even without a hole at the center that would make a closed circuit patent to our eyes and understanding, then current will flow, yes.

 

This current consumes heat at the hot junction and releases a part of the heat at the cold junction, so the current can provide an electrical power but at the cost of heat, and within Carnot's efficiency limit. Worse: Peltier and Seebeck elements have a bad efficiency, far from Carnot's limit. The standard thermal engines are way more efficient.

 

Though, Seebeck's static operation can be considered advantageous, one example (is there a second one?) being Radioisotopic Thermal Generator (Wiki) on space probes far fro the Sun, which use red hot ²³⁸Pu and semiconductor Seebeck couples to provide electricity.