Nanotubes take tiny temperatures

By Chhavi Sachdev, Technology Research News

How do you measure the temperature of something that's smaller than the tip of a thermometer? Researchers in Japan have designed a thermometer that is not only smaller than the eye can see, but is also able to measure temperatures hot enough to soften steel.

The researchers found that a carbon nanotube filled with the liquid metal gallium can act as a thermometer, said Yoshio Bando, a director at the Advanced Materials Laboratory of the National Institute for Materials Science (NIMS) in Japan. Carbon nanotubes are rolled-up sheets of carbon atoms that are a natural component of soot.

The nanotube thermometer is about 75 nanometers in diameter and 8,000 nanometers long, which is about one and a half times as long as a red blood cell. A nanometer is a millionth of a millimeter. The nanothermometer works in a way similar to the familiar clinical version that contains liquid mercury in a glass tube.

The tiny thermometer, however, measures temperatures that run much hotter -- between 50 and 500 degrees Celsius, said Bando. The wide range is possible because gallium remains a liquid over a broad range of temperatures. Gallium stays liquid for the longest range of any metal, from 30 to 2,403 degrees Celsius, said Bando.

Existing micron-sized cryogenic thermometers already measure the bottom temperature range from 0 to 80 degrees Kelvin, or -269 to -193 degrees Celsius.

In the researchers' thermometer, the gallium expands as the temperature increases, which makes its level rise within the carbon nanotube. The level can be seen through the carbon nanotube using a scanning electron microscope (SEM), which magnifies objects as much as 200,000 times. The microscope bounces a beam of electrons across a specimen; the SEM picture shows how those electrons are reflected.

"The height change can be read by comparing heights at different temperatures, and some materials beside the nanothermometer can be taken as references, such as the support grid mesh" that holds the device, said Bando.

The nanotube also expands slightly during the process. The researchers take that into account when reading the temperature. "We know the expansion coefficient of gallium [and] the relation between temperature and height," Bando said.

Gallium is particularly appropriate as a measuring liquid because it does not stick to the nanotube walls; its surface, or meniscus, is neither concave nor convex, but remains flat, which makes it easier to get accurate readings.

The nanothermometer could be used to to measure temperature shifts within very small spaces, said Bando. "This thermometer can be used in in-situ observation of chemical reaction in a micro-region," he said. It can be used, for example, to measure laser effects, he said.

Another potential use is to estimate the thermal effect of an electron beam when a material is observed in a transmission electron microscope (TEM) or a scanning electron microscope, he said.

The researchers plan next to develop a nanothermometer that measures a wider temperature range -- from 50 to 1,000 or 1,500 degrees Celsius, Bando said.

The work is interesting for two reasons, said Deepak Srivastava, a senior scientist at NASA's Ames Research Center. "First... the thermal expansion behavior of a nanoscale quantity of liquid gallium remains the same as a macroscopic quantity," he said. This may mean there are more properties that remain the same for large and microscopic samples.

Second, it increases by an order of magnitude the temperature range that can be measured over the standard microscopic cryogenic approaches, Srivastava said.

The problem with using the thermometer would be in reading it, said Srivastava. "If [the researchers] or anyone else can figure out alternative ways to measure gallium level within a nanotube, we may have [a] real nanoscale analytical tool in our hands," he said.

Bando's research colleague was Yihua Gao. They published the research in the February 6, 2002 issue of Nature. The research was funded by the Japanese government.

Timeline:   unknown
Funding:  Government
TRN Categories:  Nanotechnology; Materials Science and Engineering.
Story Type:   News
Related Elements:  Technical paper, "Carbon Nanothermometer Containing Gallium," Nature, February 6, 2002.


February 13, 2002

Page One

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Cooperative robots share the load

Nanotubes take tiny temperatures

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