Thermal scanning experiments

Kamada, K., Matsunaga, K., Yoshino, A. and Ohta, K. (2003) Two-photon-absorption-induced accumulated thermal effect on femtosecond Z-scan experiments studied with time-resolved thermal-lens spectrometry and its simulation. J. Opt. Soc. Am. B, 20, 529-537. 

A thermal scan showed that the exotherm of the principal reaction can be significant if the system is neither controlled nor vented. From isothermal studies (i.e., experiments at constant temperature), time-to-maximum rate was determined which was comparable to that obtained from the DCS data. The larger scale data showed, not surprisingly, more rapid reactions at elevated temperatures. Thus, it was decided to use the DSC data at lower temperatures, and the larger scale test data at higher temperatures for hazard evaluation. 

Determination of the reaction rate from calorimetric measurements, using DSC technique, is very useful and was applied with success for many template polymerization systems and for blank polymerizations.Two types of calorimetric measurements were described isothermal and scanning experiments. The heat of polymerization can be measured by DSC method, measuring thermal effect of polymerization and ignoring the heat produced from decomposition of the initiator and heat of termination. In isothermal experiments sample is placed at a chosen temperature and thermogram is recorded versus time. Assuming typical relationship... 

In a pyrolytic or thermal LCVD experiment, the gas is transparent and the substrate absorbs the laser energy. This creates a so - called hot - spot on which a normal thermal CVD process occurs. Pyrolytic LCVD allows a very precise localization of the coating. In a sense, this technique may be compared to the cold - wall CVD technique in which the substrate may be heated by passing an electric current through it (resistance heating), or by induction, where the substrate itself acts as a susceptor. In these cases, the gas volume is not heated significantly (hence the name cold - wall CVD). The main difference between the cold-wall CVD and the pyrolytic laser CVD is that in the latter, the heated area can be localized and scanned very precisely. 

In case of ball-type Pcs, there are only two studies reported on the NLO and OL properties of metallo and double-decker Lu(III) and In(III) Pcs in the literature [42, 60]. The NL refraction and absorption dependence on thermal effect for 4-ns pulse duration in ball-type with four t-butylcalix[4]arene bridged ZnPc 2 in Fig. 1 in solution have been studied [60]. z-Scan experiments were performed on 2 in chloroform solution with a 10-cm-focal length lens. The results are shown in Fig. 24a. 

Collection of Fe Mossbauer spectra with adequate signal/noise ratios requires a minimum of several hours under ideal conditions, extending to several days or weeks for experiments where conditions are less than optimal. This is most relevant for in situ experiments, because it limits the number of temperatures and/or pressures at which spectra can be collected. One technique that avoids this limitation is thermal scanning (e.g. Nolle et al. 1983), although the information provided is generally limited to only the phase transformation temperature. 

A MicroCal MC-2 scanning calorimeter (MicroCal Inc., North Hampton, MA.) was used to perform the thermal denaturation experiments. Sample and reference volume used was 1.2 ml and enzyme concentrations ranged from 1 to 5 mg/ml. Baseline and shift settings were adjusted to give a flat water-water baseline. Cell feedback was fixed at a value of 32 and 16X sensitivity and 0.5oC/min scan rate were used. Following a scan, a 4X calibration pulse was used to scale the endotherm in practice, little instrumental drift was observ. ... 

Here, and k2 are the unfolding rate constants for the unfolding transition from N to I and from I to respectively. The unfolding kinetics can be quantitatively analyzed from the scanned intensities of the Coomassie blue stained tailspike bands on SDS gels. Figure 1 depicts results from a typical thermal unfolding experiment for wild type tailspike protein, which was performed in Tris buffer 8) and 2% SDS at 65 C. Kinetic analysis yields two rate constants 1.1 x 10 s and 4.0 x 10 s for the conversion from N to I and from I to A/, respectively. 

Microthermal analysis is a recently introduced thermoanalytical technique that combines the principles of scanning probe microscopy with thermal analysis via replacement of the probe tip with a thermistor. This allows samples to be spatially scanned in terms of both topography and thermal conductivity, whereby placing the probe on a specific region of a sample and heating, it is possible to perform localized thermal analysis experiments on those regions. 

If, in a Mossbauer experiment, the relative transmissions at two velocities, v+ and v (where v+ = -v ) are measured in a temperature scan characteristic thermal-scanning curves are obtained like those in Fig. 3.42, which can be very useful for the determination of magnetic-transition temperatures under pressure/... 

An exothermic transition process that shows an observable onset in a DTA or DSC scanning experiment, performed with a heating rate of 10 K/min, at least 100 K higher than the recommended manufacturing process temperature will not pose a thermal hazard under plant operating conditions. 

In contrast, one finds many DSCs which are used only for qualitative DTA work on transition temperatures. The often-posed question of the difference between DTA and DSC is therefore easily answered DTA is the general term covering all differential thermal analysis techniques, while DSC must be reserved for scanning experiments that yield calorimetric information. 

Table 5.3 lists the principal experimental methods used in dynamic mechanical testing. Of the experiments considered below, the thermal scan mode (method 1) is the technique most commonly used by thermal analysts. Here typical applications in quality control or processing look for differences in material batches, thermal history, different grades, reactivity, and other characteristics. The stepped isotherm (or step isothermal) experiment (method 2) is used mainly in studies involving detailed mechanical property determination for structural analysis, vibration damping applications, and for determining time-temperature superposition master curves. Method 3 (fast scan or single isotherm) is application specific. 

A simple test when accurate temperatures are required for measured transitions is to make measurements on the same samples at different thermal scanning rates, (e.g., 1, 2, S C/min). If all transition temperatures in heating experiments are coincident, then the sample is in thermal equilibrium in all tests. K higher transition temperatures are measured for the faster rates, however, then the sample is lagging behind the measured temperature. For cooling experiments lower values will be seen at the faster cooling rates if the sample temperature is lagging behind the oven temperature. 

Classical adiabatic calorimetry, in which precise quanta of heat are applied and the temperature rise of the sample is noted under shielded conditions, is an extremely slow and laborious technique. By referring the temperature of a sample to that of an inert sample experiencing a closely similar heating profile, equivalent information can be obtained in a rapid scanning experiment. This has many advantages in that structures in the polymer are not annealed and changed during fast thermal scans. 

Fig. 3 presents differential scanning experiments on isolated micro-somes, and azide-inhibited, cultured cells from tomatoes. The temperature induced transitions for membranes are sharper than those for cells. The thermal transitions observed in cells and membranes probably represent lipid phase transitions. The transition temperatures agree quite well with the break in the Arrhenius plot of Fig. 2B. 

The above is only an illustration of likely scenarios. However, it is often usefirl to perform a wide-range thermal scan (— 100-300°C, or the melting point, if known) at two frequencies (1 and 10 Hz for example). This allows easy distinction between Tg and via the frequency dependence of data. Generally, the experimental methods are best considered with real data and such examples are given in Section 4.5. However, a brief description of experiment types is given below. 

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