Scanning Calorimetry Technique Extended to Nanoliter-Scale Samples
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tron beam gives a rate of ~2 × 10 8 positrons per second. The investigators see possible applications in positron-annihilation and Doppler-broadening spectroscopy, as well as positronium spectroscopy. TIM PALUCKA
Scanning Calorimetry Technique Extended to Nanoliter-Scale Samples Conventional differential scanning calorimetry (DSC), a technique for measuring heat exchange during chemical reactions or phase transformations, is limited to milliliter-sized, macroscopic samples. However, a team of researchers in the Materials Department at the University of Illinois at UrbanaChampaign fabricated a calorimeter for use with solid and liquid samples possessing volumes of the order of a few nanoliters. The calorimeter consists of a 0.3-µm-thick silicon nitride membrane on a silicon frame. A small “box” (volume ~35 nL) is on one side of the membrane, and a nickel heating line is on the other.
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In the October 23 issue of Applied Physics Letters, L.H. Allen, E.A. Olson, S. Lai, J.T. Warren, and co-workers report the test outcome of their “biobox” nanocalorimeter, which was fabricated at the Cornell Nanofabrication Facility. For both indium and water-droplet specimens, they demonstrated excellent agreement between both measured melting point and heat of vaporization values and the accepted values. Calorimeter performance was evaluated in scanning and heat-conductive modes for liquid and solid samples. In the scanning mode (100–150 K/s), the melting point of a 52 nL indium specimen and the heat of vaporization of water droplets (2–100 mL) were determined by heatcapacity measurements. The results demonstrate that, in this mode, the system operates with a temperature sensitivity of ±0.1 K and power sensitivity of ±7 µW. In the heat-conductive mode, which is especially useful for processes that occur near room temperature, a ~60 nL water droplet was placed in the calorimeter, and the system temperature was monitored as the
droplet evaporated. The experimental heat of vaporization fell within 25% of the predicted value, and the system’s temperature and power sensitivity were shown to be ±13 mK and ±3 µW, respectively. According to Allen, with the extension of calorimetry to nanoliter-scale materials, scientists may be able to gain insight into the thermodynamics and kinetics of biological processes and nanoscale materials, areas in which recent interest has soared. Fields in which nanocalorimetry may find application include the biological sciences in studies of basic cell processes and the microelectronics industry in studies involving individual flipchip solder balls. “Amazing things happen when the size of the matter is on the nanometer scale— for example, the melting point dramatically decreases,” said Allen. Using the new nanocalorimetry device, Zhang, Kwan, Wisleder, and co-workers report in the October 15 issue of Physical Review B that when the size of indium particles is of the order of a few thousand atoms, they melt at room temperature. “Even more amaz-
MRS BULLETIN/NOVEMBER 2000
RESEARCH/RESEARCHERS
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