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Power boost

In a breakthrough with potential application for energy-saving heating and cooling devices, a team of Boston College and MIT researchers has used nanotechnology to improve the efficiency of thermoelectricity, the process by which electricity is generated through temperature differences or cooling is achieved through application of electrical power.

Since the 19th century, researchers have attempted to boost the thermoelectric effect, whereby warming one end of a semiconducting wire, for example, sends electrons to the cooler end, producing an electric current. The problem has been that the transfer of electrons has typically been accompanied by a transfer of heat, resulting not only in lost energy but also in a counterproductive decline in the temperature differential.

The research team led by physicist Zhifeng Ren of Boston College and mechanical engineer Gang Chen of MIT theorized that introducing nano-size particles into the normal lattice structure of a semiconductor might impede the conduction of heat. They experimented with a common semiconducting alloy, bismuth antimony telluride, first crushing an ingot of the material into nanodust, then “hot pressing” the dust back together. Now when they applied a current, the researchers found that irregular new “grain boundaries and defects” in the alloy’s structure hindered heat flow while allowing electrons to move as before, yielding a gain in “thermoelectric merit” of 40 percent.

Their work, described in a May 2, 2008, article in Science, “sets the stage,” say the authors, for “a new nanocomposite approach” to making “high-performance low-cost” thermoelectric materials.

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Mine the gap

Between the low frequencies of radio and the escalating frequencies of light on the electromagnetic spectrum lies a relatively unexploited area known as the “terahertz gap.” It sits between 300 GHz (just above microwaves) and 10 THz, just below infrared radiation, and it takes its identify from the fact that neither antennas nor optical devices are useful in this range. To harness terahertz radiation, Boston College assistant professor of physics Willie J. Padilla and graduate student David B. Shrekenhamer are working with researchers at the Los Alamos National Laboratory and Boston University, developing resonant metamaterials—which are man-made—and new configurations of materials (man-made and natural). The researchers report their recent progress in the April 13, 2008, online edition of the journal Nature Photonics.

Padilla and his colleagues created a composite made up of microscopic units, or cells, engineered in a shape they term a “split-ring.” The cells were constructed out of a metamaterial capable of resonating in the terahertz range coupled, in a strategic pattern, with silicon, a natural semiconductor. The researchers found that they could control the metamaterial’s terahertz resonance through stimulation of the silicon by means of photoexcitation—achieving a tuning range equal to 20 percent of the terahertz frequency.

Terahertz rays can “see” through plastics, ceramics, and cardboard but unlike X-rays are non-ionizing and therefore safe. Organic molecules respond uniquely to their frequency, and so the rays may one day be used for molecular analysis.

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A code of one’s own

In the not-too-distant future, every newborn baby will receive a DNA analysis to go along with the standard slap on the bottom and quick delivery-room physical. Steps in that direction have been taken by a team led by Boston College assistant professor of biology Gabor T. Marth, who designs software that improves the accuracy of DNA decoding methods.

Marth’s efforts—as described in two articles published in the February 2008 edition of Nature Methods—build on the advances of the international Human Genome Project, which from 1990 to 2003 mapped the human DNA molecule, and on recently developed “pyrosequencer” machines that deliver the code of an individual’s DNA. His work is aimed at improving and streamlining the sequencing process and is in line with the National Institutes of Health’s goal of making individual DNA sequencing available for about $1,000. (Marth’s computational biology lab has been granted more than $3 million from NIH since 2004.)

DNA’s double helix is composed of nucleotide base pairs that, simply put, spell out life’s instructions. The emerging generation of genome sequencing machines delivers the 3 billion DNA base pairs in a person’s chromosomes at high speed, writes Marth, by producing “relatively short-read-length sequences” that then require analysis, characterization, and screening for errors. Marth’s PyroBayes software is designed to examine the data produced by one brand of sequencer and determine which base pair results are more likely to be accurate and which are not to be trusted in downstream analysis, at a speed of 50 million base pairs in 10 minutes. Mosaik, another computer program developed in Marth’s lab, compares the resulting raw data to a previously determined human genome or to DNA of other individuals, a process Marth likens to ferreting out typos in separate editions of the same book. Marth’s EagleView program, written up August 11 in Genome Research online, prepares the data graphically for analysis. Additional software, including Marth’s GigaBayes, may determine if an individual’s DNA sequence exhibits a true genetic variation and whether that variation has health implications.

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Stephanie Schorow is a writer in the Boston area.