High sensitiv AC chip calorimetry for nanogram samples

摘要

Calorimetry is known as a very powerful tool for the characterization of a wide variety of materials and their transitions. There is an ongoing interest in improving the technique in order to achieve high sensitivity and precision. High sensitive differential scanning calorimeters are developed mainly to measure biological samples showing small effects in highly diluted systems, e.g. (1). For reduced sample masses AC-calorimetric techniques are used in this field too (2). For more than 30 years, AC-calorimetry has been known as a sensitive technique to measure thermophysical properties of small sized samples. To further increase sensitivity a differential AC-calorimeter was developed too (3). In order to allow measurements on sub micron films addenda heat capacity has to be reduced dramatically. The combination of silicon technology and calorimetry opens up new possibilities in this research area as demonstrated recently (4). The thin membrane allows a good ratio between sample and addenda heat capacity for sub micron films. Different chip calorimeters as from Xensor Integrations, NL are used for samples in the microgram range, but also specialized home made sensors for nanogram samples (5). Using chip calorimeter in high vacuum under essentially adiabatic conditions the glass transition in polymer films down to 3 nm is measured (5). This is realized using a fast scanning calorimeter at heating rates up to 1 000 000 K/s. Also differential setups are possible with this technique. Because of the essentially adiabatic conditions only measurements at rapid heating are possible. Thin film calorimeter operated under non-adiabatic conditions allow heating as well as cooling at rates up to 10.000 K/s (6) and possibly faster. Using such high rates yield often non-equilibrium states of the sample under investigation. As an example, for several fast crystallizing polymers it is possible to prevent crystallization on cooling totally and to reach the amorphous glassy state (6). But often one would prefer to measure thermal properties of small samples at or at least close to thermodynamic equilibrium. This can be achieved by a combination of chip calorimetry and AC calorimetry (4). As common in AC calorimetry a small periodic heat flow is provided and the resulting complex temperature amplitude is measured. The measurements are done at slow scanning or at constant bath temperature. The frequency chosen provides a well defined time scale of the experiment. In several cases, e.g. at glass transition, a direct comparison with results from other dynamic methods like dielectric spectroscopy is possible. Such an AC-chip calorimeter for small samples using a single commercially available sensor under non-adiabatic conditions is described in (7). The sensitivity of this system is about 10 nJ/K at room temperature. This setup would allow measuring the glass transition of polymer films down to 500 nm thickness. For measuring the glass transition of much thinner polymer films the sensitivity of the calorimeter has to be enhanced. Based on a differential AC-calorimeter we show an improved experimental setup (see figure 1) combining the advantages of the different methods already described (8). The differential AC chip calorimeter is based on a commercially available chip sensor from Xensor Integrations, NL, which was already used in (7, 9) for AC and in (6, 10) for fast scanning calorimetry. Due to the differential setup we achieve a sensitive in the pico Joule per Kelvin range allowing to measure samples below one nanogram. Consequently films down to 1 nm thickness can be measured. Because of the small total heat capacity (addenda + sample) not only a high sensitivity is achieved but AC measurements at relative high frequencies are possible too (9). The calorimeter allows heat capacity measurements in the frequency range 1 Hz to 1 kHz.