In Building Blocks of the Universe, Isaac Asimov listed several metals on a scale of 0 to 100, depending on their conductivity of electricity–that is, how little heat they developed when an electric current was passed through them. Silver got 100, copper 95, gold 67, aluminum 58, sodium (not known as an electrical conductor, I know) 35, and iron 16.
What I’d like to know is: How do other metals–especially mercury, the liquid metal commonly used in electric switches–rate? And what about carbon, not a metal but used by Edison in his first light bulbs?
What if we boiled carbon, in a vacuum furnace, along with an element that carbon doesn’t normally bond with, such as mercury, and let it cool? Sounds like a room temp conductor to me. You heard it here first! (what is the BP of carbon?)
Anything that’s 6000 F (3588 kelvins) is going to be glowing white. The filament of a 100 W incandescent bulb is somewhere around 2000 K. So it’s going to be much, much brighter than one of those. But not quite as bright as the sun, which is 5800 K.
Heat up carbon in air and it will react with the oxygen. To melt it, you need to do it in a vacuum or perhaps in a non-reactive atmosphere (argon or other noble gas).
The gas inside (which is converted to plasma as soon as the power is turned on). In fact, it gets so hot that the light emitted is mostly ultraviolet, which then needs to be converted to visible light via fluorescent materials on the surface.
The temperature of the gas (Hg vapor and argon) in a cool white fluorescent tube isn’t all that high. The color temperature is 5000 K, but that’s just a comparison to what wavelengths an ideal black body would radiate at that temperature. The gas in a fluorescent light is ionized, but if it were at 5000 K, the glass would eventually melt, since that’s 8450 F. In reality, the glass tube will be at a temperature similar to the gas inside.
I resisted posting for a while, but here’s what I have observed/learned.
Pure metals are rarely used in any engineering or real world building blocks. There are many more factors at play while selecting a metal for temperature resistance or electrical resistance etc. Here are some interesting observations :
1> Mercury as a coolant : Earlier air cooled airplane exhaust valves had liquid mercury cooling system. It worked great (as far as cooling is concerned) except its hard to seal the mercury and had to be abandoned.
3> Heating Elements : Although individually Iron, Aluminum or Chromium may not be great for heating elements. Together (and in the right formulation) they make Kanthal - a great heating element. If you are making an electric furnace this is a great candidate.
4> Nozzles (Burners) - Here again there are a lot of things at play. Its not the melting point of metals that come into play but high temperature fatigue, cost, machine-ability etc. Commonly Incoloy is used which is what you will find in the burners of many jet engines. For gasturbines Incoloy is used with very precision driven holes that have steam flowing through them to keep the metal cool. Steam is a great heat conductor.
One of the severe applications is in a Steel Converter - that is the purification of molten steel after it comes out of the blast furnace. This was traditionally called the bessemer’s converter and revolutionized the steel era. In this process, there’s a huge pot of molten steel - Into this pot is inserted a nozzle (sorta like a pipe) through which pure oxygen is blown. A great fireworks display happens …The hero here is the nozzle. It is made of copper cooled with water. (Newer designs have tungsten nozzles but are more expensive to make)
Ceramics are great for these applications too - but ceramics are hard (expensive) to machine or make to a certain spec. (there are improvements going on though)
5> Low temperature : Many believe that the titanic sank because the steel cracked like brittle glass when it hit the iceberg. There are many more instances of this with steel and low temperature. (Sortof like the first terminator movie when the liquid nitrogen truck spills over). Nickel again in these cases is the hero - nickel added to steel helps it keep its ductility in low temperature.
Even lower temperature : The hero is aluminum. For cryogenic applications, liquid nitrogen, LNG, liquid ethylene etc - its typically stored in aluminum vessels.
I could go and on and on - but the point I am driving is that there are many criteria that determines the selection of a material (or metal) for an application. Having a high melting point does not necessarily make a metal a good selection for a high temperature application.