Breakthrough Solar Thermal (High Temperature Photonic Crystals)

A team of MIT researchers has developed a way of making a high-temperature version of a kind of materials called photonic crystals, using metals such as tungsten or tantalum. The new materials — which can operate at temperatures up to 1200 degrees Celsius — could find a wide variety of applications powering portable electronic devices, spacecraft to probe deep space, and new infrared light emitters that could be used as chemical detectors and sensors.

Compared to earlier attempts to make high-temperature photonic crystals, the new approach is “higher performance, simpler, robust and amenable to inexpensive large-scale production,” says Ivan Celanovic ScD ’06, senior author of a paper describing the work in the Proceedings of the National Academy of Sciences. The paper was co-authored by MIT professors John Joannopoulos and Marin Soljačić, graduate students Yi Xiang Yeng and Walker Chen, affiliate Michael Ghebrebrhan and former postdoc Peter Bermel.

These new high-temperature, two-dimensional photonic crystals can be fabricated almost entirely using standard microfabrication techniques and existing equipment for manufacturing computer chips, says Celanovic, a research engineer at MIT’s Institute for Soldier Nanotechnologies.

While there are natural photonic crystals — such as opals, whose iridescent colors result from a layered structure with a scale comparable to wavelengths of visible light — the current work involved a nanoengineered material tailored for the infrared range. All photonic crystals have a lattice of one kind of material interspersed with open spaces or a complementary material, so that they selectively allow certain wavelengths of light to pass through while others are absorbed. When used as emitters, they can selectively radiate certain wavelengths while strongly suppressing others.

Photonic crystals that can operate at very high temperatures could open up a suite of potential applications, including devices for solar-thermal conversion or solar-chemical conversion, radioisotope-powered devices, hydrocarbon-powered generators or components to wring energy from waste heat at powerplants or industrial facilities. But there have been many obstacles to creating such materials: The high temperatures can lead to evaporation, diffusion, corrosion, cracking, melting or rapid chemical reactions of the crystals’ nanostructures. To overcome these challenges, the MIT team used computationally guided design to create a structure from high-purity tungsten, using a geometry specifically designed to avoid damage when the material is heated.

NASA has taken an interest in the research because of its potential to provide long-term power for deep-space missions that cannot rely on solar power. These missions typically use radioisotope thermal generators (RTGs), which harness the power of a small amount of radioactive material. For example, the new Curiosity rover scheduled to arrive at Mars this summer uses an RTG system; it will be able to operate continuously for many years, unlike solar-powered rovers that have to hunker down for the winter when solar power is insufficient.

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Gravitational Force and Electromagnetic Base

Until a few years ago scientists believed that all forces could be categorized into five classes:

  • Gravitational force - the force of attraction between any two objects with mass.
  • Electric force - a force of attraction or repulsion between charged objects.
  • Magnetic force - a force of attraction or repulsion between ferro magnetic objects.
  • Strong force - the force holding protons and neutrons together in the nucleus.
  • Weak force - the force which causes radioactive decay.

In recent years it has been shown that the magnetic, strong, and weak forces are all variations of the electric force now called the electro-weak force. Many scientists believe that the gravitational force may also have an electromagnetic base, but no proof exists as of now.