N-Type
Current In Current Out
Hot Face
Cold Face
Figure 4-5: Peltier Junction as Heat Pump
Running an electrical current through this device will drive phonons (thermal energy) into the upper conductor, which grows hot. The lower conductors lose energy, and become cold. The same principle works in reverse: heating the top conductor (or cooling the bottom one) will create an electrical voltage across the device.
A mature technology since the 1960s, semiconductor Peltier junctions work very well, and if their current draw is not
limited with a resistor, one face can easily burn up even as the other is collecting ice from the atmosphere. This inexpensive technology is often used to cool circuit boards -- especially the imaging elements of CCD cameras, although you do need a heat sink or thermal shunt to carry the heat away. Many "green"
refrigerators today employ this technology, although with present materials they are less efficient than mechanical models, and can only drop the temprature to about 40 degrees below ambient (and raise it to 40o above at the heat sink), so their use is generally restricted to portable refrigerators which can be plugged into the electrical system of a boat, car, camper or hotel room. For these uses they are advantageous because they're slim, cheap, lightweight, and solid-state.
There is a theoretical limit to the efficiency of any heat pump, regardless of design. Even with perfect materials
behaving perfectly, the maximum possible efficiency is 100% when the hot and cold faces are at the same temperature, and around 50% when the cold face is at half the absolute temperature of the hot one. The rest of the energy is lost as waste heat. And the materials in today's Peltier junctions are very far from perfect, thanks to a tradeoff between electrical resistivity and heat conduction. Natural materials offer no good solution; the best efficiencies achieved with them so far are around 10%. An optimal thermoelectric material would be simultaneously a strong electrical conductor -- perhaps even a superconductor -- and a smotheringly good thermal insulator.
Fortunately, an approximation of this ideal material actually exists, at least in the laboratory. In 2001,
scientists at the Research Triangle Institute (RTI) in North Carolina used atomically precise, layered semiconductors -- similar to quantum wells -- to create a Peltier junction that operates at 2.5 times the efficiency and 23,000 times the speed of all previous models. The electrically semiconductive
materials were unusually good radiators -- and unusually poor conductors -- of heat. The fabrication process was lengthy and expensive, but also similar in many ways to the market-proven production of nanolayered read/write heads for hard disk drives.
And the researchers are predicting another performance doubling (or more-than-doubling) in the near future. Says RTI's Rama Venkatasubriaman, "We've made a pretty good P-type material. We still have to improve the N-type, and the device design, and a lot of manufacturing issues. This is where the big payoff will be." So in coming years, we'll almost certainly see commercial products based on improved forms of this technology.
In this discussion, it's hard not to notice the clustering of important research on America's east coast. Could it be the proximity to European sites such as Delft? "Perhaps," Marcus allows, "or perhaps it's the 'east coast' personality or that Stanford is more engineering oriented, or that the weather in California is too nice for hard work. The fact is that there are isolated pockets of excellence on the west coast. Santa Barbara is a great school for this kind of stuff, for instance.
The move east is kind of new. Recall that 4 years ago, McEuen and I were out west. There's also Roukes at Caltech, who is excellent. So it may just be that the density of schools is higher in the east."
Regardless of where they're made, the most immediate
application for improved Peltier junctions will be in ultraquiet refrigerators and air conditioners. Since the technology is solid-state, it can be fashioned into a variety of shapes and probably even made flexible. Peltier "cool suits" may someday harness solar power to pump heat away from the wearer's skin (and blast it at unwary passersby). There may even be portable folding ice rinks, in the same way that today there are
inflatable swimming pools. Just remember not to touch the heat sink, which could easily double as a barbecue grill!
Flipped over, these super-Peltier junctions could also serve as thermoelectric generators, capable of capturing waste heat from many devices or processes and converting it back into electricity. This is already done onboard deep-space probes
Hacking Matter Thermodynamics and the Limits of the Possible
such as Galileo (visiting Jupiter) and Cassini (visiting
Saturn), which harvest the heat from radioactive plutonium in a device known as a Radioisotope-Thermal Generator or RTG.
Despite public hysteria, these devices are in fact so rugged that they've not only survived the explosion of launch vehicles, but have been recovered from the ocean floor afterwards, and re-flown on other missions. Closer to home, super-Peltier
junctions could recapture the heat of car engines, or even the heat of quantum dot devices shedding large numbers of electrons.
Squeezably Soft
Still another way for quantum dots to harvest energy is through the piezoelectric effect. This occurs when pressure on a material creates slight dipoles within it, by deforming
neutral molecules or particles so that they become positively charged on one side and negatively charged on the other, which in turn creates an electrical voltage across the material. This is the operating principle behind an old-fashioned phonograph:
the needle, moving over the surface of a vinyl record,
encounters ridges and grooves which deform its Rochelle-salt tip, creating voltage patterns which are amplified and converted into sound. Interestingly, the production of sound can also rely on the piezoelectric effect, or rather on its reverse: a voltage applied across certain materials will cause them to deform, and an oscillating voltage will make them vibrate.
These vibrations are then acoustically focused and amplified, in the same way that guitar-string vibrations are amplified by a hollow box of wood. This is how many stereo speakers work.
Zinc oxide, the most piezoelectric material presently known, is widely used in electronic transducers, or solid-state devices for converting electricity into mechanical force. It has other strange properties as well -- it's one of the whitest pigments available (although titanium oxide is whiter). When doped with aluminum, it's also a semiconductor, and when doped with barium or chromium oxide it has the most non-Ohmic electrical properties of any known material. Ohm's law, which holds true for virtually every substance on Earth, states that electrical voltage is equal to current times resistance, or V=IR. In these zinc oxides,
though, the equation becomes Vn=IR, where n can be upwards of 100. You get huge amounts of voltage for free, without paying the usual penalties in high current and resistive heat losses.
This bizarre effect is used in electronic components called
varistors, which are commonly used as surge arrestors in noisy electronic circuits. Feed in a huge voltage spike and the
current varies only slightly, preventing damage to any delicate electronics downstream.
If it were more common and more controllable, this effect could serve as the basis for entirely new components and entirely new types of electrical circuitry. And since it is associated with piezoelectricity, any progress in this field would also be progress in the development of new piezoelectric devices.
"Look around," Marcus says, "at the spectrum of materials that exist in the world, with all their various thermal and electrical properties. These are all made from a handful of
naturally occurring substances. By constructing materials out of manmade objects instead, we can introduce a new subtlety. The first step is putting one material next to another -- it's
amazing what you can accomplish that way. Imagine what you can do when the fundamental building blocks are designed to assign a personality to macroscopic materials. That spectrum of
possibilities gets a lot broader.
Hacking Matter Magnetism and Mechanics