Already made. Still subject to the same laws of thermodynamics.
http://en.wikipedia.org/wiki/Thermophotovoltaic
All machines that we have made, from heatsinks on computers to jet turbines, are all subject to the laws of thermodynamics. To oversimplify; no machine can exceed 40% efficiency. Of the energy that enters a system, only 40% of that energy can converted into a useful force (such as movement) while the other remaining total is wasted. Few machines truly even get close to 40%, the closest are the turbines used to generate power in various powerplants.
The 40% rule applies to machines, which are inefficient due to their size. Nano materials work both chemically and mechanically and don't have that problem as long as the required minimal energy is present. The chemical attributes enable them to work on the basis of chemical reactions, here's the basic example:
A + B --> C + D + energy
The product, releasing energy, is always favored (due to increase in stability, entropy of the system, other factors). So the reaction drives itself to the product side as long as there are enough reactants to meet the reaction rate. If there is an overflow of the reactants A or B or both, then the reaction drives itself to the products until balance is reestablished. This is the principle of all chemical reactions.
Nano materials behave this way, but with an added bonus of also behaving mechanically (spinning molecules, kinetic/steric molecule bumping, etc.). This means you can potentially make nano sized molecular machines like miniature electric generators. Then that generator is connected to another molecule that absorbs the impact energy via kinetic/steric bumping (since heat is really just excited molecules zooming all over the place that bump into other molecules, exchanging heat) to turn the motor. Another possibility is to forget the motor entirely and simply have a molecular material that absorbs kinetic molecular impacts to promote electrons into the system (thus creating a potential difference, and creating an electric current).
The chemical behavior is what can make this very efficient:
Energy (heat) + Molecule in conformation A <--> Molecule in Conformation B <--> Molecule in conformation A + energy (in this case, electrical)
As long as there is a specific amount of heat and a lack of electric potential in the system, a reaction like this would continue to absorb the heat energy and create electric potential. The only things that would stop it is if the heat energy were too low, or the battery this system was delivering energy to was full.
The mechanical factors of nano materials gives the possibility of such a reaction to become Irreversible (or atleast, demand a larger amount of electric energy on the right side to be reversed). Then the minimal amount of heat needed to drive the reaction will depend on the Rate (speed) at which the reaction progresses. It will get faster as heat rises.
Such a material could convert large amounts of heat into electric energy very very fast, so long as the circuit is closed. Open the circuit by turning the switch Off or something and this entire reaction will not be able to create the end product, and thus will cease.
Whoever makes and perfects something like this is going to be loaded.
