05 November 2004
05 November 2004
Chemists from UCLA have discovered that an ordinary camera flash instantaneously welds together nanofibres made of the synthetic fibre, polyanilin, made in either a conducting or an insulating form.
The nano-scale discovery entitled “flash-welding” could have potential large scale applications, especially in areas such as chemical sensors, separation membranes, and nano devices.
""We used an ordinary 35-millimetre camera, but you could also use a laser, or any other high-intensity light source,"" said Richard B. Kaner, UCLA professor of inorganic chemistry and materials science and engineering, and a member of the California NanoSystems Institute at UCLA.
""I was very surprised,"" Kaner said. ""My graduate student, Jiaxing Huang, decided to take some pictures of his polyaniline nanofibres one evening when he heard a distinct popping sound and smelled burning plastic. Jiaxing recalled a paper that we had discussed during a group meeting reporting that carbon nanotubes burned up in response to a camera flash. By adjusting the distance of the camera flash to his material he was able to produce smooth films with no burning, making this new discovery potentially useful.""
The camera flash induces a chemical reaction; it starts a chain reaction in which the tiny nanofibres interact and cross-link, producing heat, which leads to more spontaneous cross-linking across the entire surface of the nanofibres, welding them together, Kaner said. Unlike carbon nanotubes, which burn up, this material is thermally absorbent and can dissipate the heat well enough so that it does not burn.
""We can envision welding other materials together as well,"" Kaner added. ""One way to do this is to take two blocks of a conventional polymer and insert polyaniline nanofibres between them, then induce the cross-linking reaction to produce enough heat to weld the polymer blocks together. We can weld polyaniline to itself or to another polymer or potentially use it to join conventional polymers together.""
Because only the part exposed to light welds together, chemists can create patterns by covering sections that they do not want welded; they can control what parts weld together.
Kaner's research team searched for whether any conventional techniques have this same welding property. They found a recent commercial process called laser welding, now used in the electronics industry, in which a laser beam is used to weld together conventional polymers. ""The trouble with laser welding,"" Kaner said, ""is that lasers generally have a small cross-section and consume a lot of power. Our research has the potential of revolutionizing this process.""
Nanofibres have high surface areas and important properties, from sensing to flash welding. ""This shows why nano is important,"" Kaner said. ""Here's a good example of where nano materials possess a property that conventional materials do not have.""
Kaner and Huang were the first chemists to produce large quantities of pure polyaniline nanofibres, which can also be used for sensors - findings they published last year in collaboration with Dr. Bruce Weiller and Shabnam Virji at Aerospace Corp. The nanofibres have a much greater response in a shorter time than sensors made with conventional polyaniline.
Jiaxing Huang has started a UC Berkeley postdoctoral fellowship. The research is funded by the Microelectronics Advanced Research Corp.
Optical microscope images show that flash welding through a copper grid reproduces the grid pattern on a polyaniline nanofibre film.
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