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Physics News Update
Number 828, June 13, 2007 by Phil Schewe and Ben Stein

Turning Heat Into Electricity Through Sound

Turning heat into electricity through sound has been demonstrated by the University of Utah group of physicist Orest Symko. The group has built devices that can create electricity from the heat that would otherwise be wasted in objects such as computer chips. The devices might potentially make extra electricity from the heat of nuclear power plant towers, or remove extra heat from military electronics.

At last week's meeting of the Acoustical Society of America in Salt Lake City, five of Symko's students demonstrated the latest versions of the devices, which they have been developing for a few years. The devices first convert heat into sound, and then sound waves into electricity. Typically, each device is a palm-sized cylinder containing a stack of materials such as plastic or metal or fiberglass.

Applying a heat source, such as a blowtorch, to one end of the stack creates a movement of air which then travels down the cylindrical tube. This warm, moving air sets up a sound wave in the tube, similar to the way in which blowing air into a flute creates sound.

The pitch, or frequency, of the sound wave depends on the dimensions of the tube; current designs blast audible sound, but smaller devices would create ultrasound. The sound wave then strikes a piezoelectric crystal, a commercially available material that converts sound into electricity when the sound waves put pressure on the crystal. Symko says a ballpark range of 10-25% of the heat gets converted into sound in typical situations.

The piezoelectric crystals then convert about 80-90% of the sound energy into electrical energy. Symko (orest@physics.utah.edu) expects the devices to be used in real-world applications within two years, and may provide a better alternative to photovoltaic solar cells in some situations. (Session 5aPA at meeting; also see University of Utah press release at http://www.unews.utah.edu/p/?r=053007-1)

Polonium Is The Only Element With Simple Cubic Crystal Structure

Polonium is the only element with a simple cubic crystal structure, and new theoretical work explains why that is. In a solid piece of polonium the atoms sit at the corners of a cubic unit cell and nowhere else (see figure).

Many other materials have more crowded structures. For instance, in a face centered cubic (fcc) structure, atoms (such as copper, gold, nickel, and iridium) sit at the corners of the cube and in the center of each face. In body centered cubic structure (bcc), atoms (such as potassium, sodium, iron, and tungsten) sit at the corners of the cube and in the very center of the cube. Only polonium has the simple cubic (SC) structure.

One reason the study of Po so difficult is that it is highly radioactive and spews forth decay products; indeed, polonium has more isotopes, 36, than any other element. Physicists at the Academy of Sciences in the Czech Republic have now produced the first detailed theoretical explanation for polonium's unique crystal structure: it is the result of the complicated interplay of relativistic effects which become important in such heavy atoms as polonium (element 84).

Specifically they have identified the so-called mass-velocity term (describing the relativistic increase in mass of electrons traveling with velocities comparable to the velocity of light) as the cause of the simple-cubic structure of polonium.

Another polonium oddity: its elastic anisotropy is greater than for any other solid. That is, it is about 10 times easier to deform a Po crystal along the direction diagonal to the consolidated cubic cells than it is to deform the crystal in a direction perpendicular to any of the cubic faces. According to Dominik Legut (legut@ipm.cz, +420-530229-461), this property results directly from the simple cubic structure of polonium.

Polonium is a hazardous element that appears in the air and soil and in such plants as tobacco, tea, and mushrooms. (Legut et al., Physical Review Letters, upcoming article; text available at Physics News Select.

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