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The Secret of Metals

The thermal conductivity, is the property of a material that indicates its ability to conduct heat. The higher the conductivity, the higher the conduction.

Aluminium : 237 W/m.K Alloy : 120—180

Gold : 318 W/m.K

Copper : 401 W/m.K

Silver : 429 W/m.K

Diamond : 900-2320 W/m.K

There is another interesting  caracteristic: the specific heat capacity (often shortened to specific heat) is the measure of heat or thermal energy required to increase the temperature of a unit quantity of a substance by one unit. For example, at a temperature of 14.5°C, the heat required to raise the temperature of a water sample by 1 K (equivalent to 1 °C) is 4186 joules per kilogram (4.186 kJ/kg.K), which is the definition of 1 Calorie.

So the higher the heat capacity, the higher the energy it need to change the material’s temperature !

Aluminium : 0.897 J/g.K

Gold : 0.129 J/g.K

Copper : 0.385 J/g.K

Silver : 0.233 J/g.K

Diamond : 0.5091 J/g.K

and we see that silver have a lower heat capacity  and a higher thermic conductivity than copper : a material of choice to spare some energy absorbed by the Peltier!

In fact the real Secret of Metal is that their thick will play the first role in the final temperature !

Peltier can produce cold, but if you see the datasheet TM-127-1_4-8.5 specification, you can see that when the difference of temperature is high between the cold and hot face, Qc, the power of refrigerating capacity on the cold side tends to go to 0. If you put a metal with bad thermodynamic property as defined above, or with a high thick, the metal will never have the temperature of the peltier face because the power needed to cold the metal is too high. The thinier the metal, the least the power it need to be refrigerated. Here is some test with different metals :

An Aluminium plate 1 mm thick, with 2 peltier. The surface is in contact with the peltier face with some thermal grease. The temperature reached is -19.7°C.

A Copper plate, nearly 3 mm thick, which is placed in the middle of 2 peltier. The temperature reached is -20.8°C. The Copper is thermodynamically better than aluminium.

Now, the metal plate is a standard aluminium foil, 25 micrometer thick. The temperature reached is -30°C. The thickness is the key.

A copper plater nearly 0.1 mm thick. The temperature reached is -25.8°C. Even if it’s copper, the temperature is slightly higher than the aluminium foil because the contact between the peltier’s surface may be worse : thinier metal have a better surface contact than the bigger one. With low thickness, thermal grease may also be a bad choice.

If we want thinier metals, one trick is to use with extreme precaution, copper or silver foil which can be as low as 5 micrometer thick ! you will easily find them on ebay for few dollars. Now, the temperature on your waterproof (which is the aim for the metals plates) peltier is the same without metal. But these sheets of metals are so fragile that copper 250 micrometer thick will be fine (silver will cost much).

The Secret of Water

A flow of fresh water must be continuously generated to remove heat from the heatsink. Hot water produced near the copper tends to stay nearby the heatsink, whereas the fresh water stay far from it.

It take time for water to have an homogenous temperature, because  the convection is too slow with such a little difference of temperature. Stirring the water yield in an homogenous solution and thus, the dissipation of heat is better. An immersion pump can do the job easily. A water below 7°C works very fine to reach temperatures in the -20°C range.

Water freeze at 0°C. Theorically, water can be liquid when supercooled up to -42°C. But a any perturbation such like a dust will make it to instananeously freeze. Supercooling does’nt help us but there is another way to have water below 0°C.

Let’s take a container of water. Let’s put some ice. Temperature of ice can be below 0°C, and when we throw some ice on water, it crack because of the thermic shock ( the difference of temperature are too high between water and ice). Ice will continously melt, until the water is at 0°C. To freeze all the water, we need to remove a considerable amount of energy from the water. When ice melt, it need energy. This energy is taken from water, and that’s why water’s temperature decrease up to 0°C .  With water at 0°C, ice won’t melt. But if you add some salt on this water, and if you mix the solution, the ice will start to melt ! Ice are unstalle in ionic solution. Ice will be stable again when the temperature of water will be far below 0°C : with a mixture of 23% of salt and 77% of water, the ice will be stable at -21°C !

Of course this temperature is only reachable in laboratory. But you can obtain easily temperature in the -10°C range. Use some crushed ice ( the surfac contact between salt water and ice is better) and mix the solution; Use a quantity of salt as pictured in the diagram. The key of the principle is that ice is the motor of the phenomenon. If we want water below 0°C we need to remove energy from water. We can do these by melting ice, which need energy ! and to do that we need to have a salt solution so the ice remain unstable until a defined temperature.

There is another secret concerning water and peltier. Consider the cold surface of one peltier. Put the thermometer on it and wait until the equilibrium is reach for example -28°C. Add a droplet of water near the thermometer, on the peltier surface : you will notice that the temperature will go to -32°C ! Very interesting. But at this temperature, water freeze rapidly. You can do the same experience with a droplet of alcohol which is liquid at -28°C (and more) : the temperature will decrease again ! That’s a good news for our Wilson machine, the flow of  alcohol will maintain a high negative temperature on the surface.

The explanation of this is that, maybe, the frozen water or the liquid alcohol act as as barrier between the atmosphere and the cold surface. Remenber that the surrounding air can warm our cold surface. A barrier could reduce the transfert of heat from the environnement. So it may be clever to isolate the cold surface from the hot surrounding air (20°C).

A last, one another idea : water usually contain some limestone which is insoluble in hot water. It can deposit in any surface, thus altering the surface. Instead of conventionnal water, we could use distillated water, free of ion (salt isn’t a problem)