Thermal scanning rates

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. 

Through variation of thermal scan rate or a frequency domain application of DSC, it is possible to obtain kinetics information. 

In a testing context, it refers to the first detection of exothermic-activity on the thermogram. The differential scanning calorimeter (DSC) has a scan rate of I0°C/min, whereas the accelerating rate calorimeter (ARC) has a sensitivity of 0.02°C/min. Consequently, the temperature at which thermal activity is detected by the DSC can be as much as 50°C different from ARC data. 

Differential Scanning Calorimeter (DSC) thermograms were obtained on a Perkin Elmer DSC-2 run at 10°C per minutes. Dynamic Mechanical Thermal Analysis (DMTA) spectra were obtained on a Polymer Labs DMTA at a frequency of 1Hz with a temperature range from -150°C to +150°C at a scan rate of 5°C per minute. 

In these experiments, power absorbed from the WGM-exciting diode laser, operating at higher power, heats the sphere. The laser is scanned slowly in frequency across a resonance. The scan rate is slow enough that the assumption of internal thermal equilibrium in the microsphere holds, except perhaps during the fast throughput power jumps (see Figs. 5.7 and 5.8). In equilibrium, the power absorbed depends... 

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. 

In practice, the true heating rates (dT/dt)ca and (dT/dt)cb are assumed to be equal to the programmed scan rate j3, and the true heat flow rate difference (heat flow rate difference, Ao, which reflects the intrinsic thermal asymmetry of the differential measuring system ... 

It is also possible to use microcalorimetry to obtain useful information about the kinetic processes of the instability (i.e., aggregation, proteolysis) when thermal irreversibility prevails. Scan rates will often distort the onset behavior of the melting transition that can necessarily impose a shift in the Tm, as discussed further in the following text. The scan rate dependence of the Tm may then be used to determine the activation energy of the instability, provided an Arrhenius kinetic model describes the behavior. 

Figure 13.15 A plot of In (v/Tm2) vs. 1/Tm for the thermal denaturation of thermolysin using DSC scan rates of 1.9,1.0, 0.5, and 0.2 °K/min. The plot assumes Arrhenius behavior holds. The slope of the plot is equal to -EalR. Using R = 1.9872 cal/deg-mol, one can calculate the activation energy to be 63.3 kcal/mol. (Data plotted were obtained from Reference 138.)...

Figure 3.14 shows a DSC trace obtained at the scan rate of 4°C/min from the mixtures milled for 5, 10, and 40 h. A very small thermal flow effect of either exothermic or endothermic nature is seen around 140°C at the DSC traces of mixtures milled for 5 and 10 h only. At temperatures >180°C, mixtures milled for 5 and 10 h exhibit three strong endothermic effects centered at around 271, 315 and 452°C but the one milled for 40 h shows only two endo effects at around 292 and 452°C. 

Under the DSC conditions (N, scanning rate = 10°C/min), it is apparent that the decomposition processes are occurring at a much faster rate at or near the temperature at which cure is taking place in all the pure dlcyanate samples. Both BADCy and THIOCy showed small exotherms (onset at 277°C and 226°C and peak at 308°C and 289°C, respectively). Their major decompositions began about 251°C and 246°C, respectively, as observed by TGA. On the contrary, all the 1 1 BCB/dlcyanate blends displayed the expected thermal transitions. Besides initial Tg s (20-28°C) and Tm s (171-183°C), all samples showed small exotherms in their DSC scans with maxima at 147-151°C. This is attributable to the thermally-induced crystallization in the mixtures, which also led to some initial phase separation. The polymerization exotherms are consistent with the typical temperature ranges for the known benzocyclobutene-based systems (onset 229-233°C max. 259-266°C). 

Thermal analysis, moisture uptake and dynamic mechanical analysis was also accomplished on cured specimens. Thermal analysis parameters used to study cured specimens are the same as those described earlier to test resins. The moisture uptake in cured specimens was monitored by immersing dogbone shaped specimens in 71 C distilled water until no further weight gain is observed. A dynamic mechanical scan of a torsion bar of cured resin was obtained using the Rheometrics spectrometer with a temperature scan rate of 2°C/minute in nitrogen at a frequency of 1.6Hz. The following sections describe the results obtained from tests run on the two different BCB resin systems. Unless otherwise noted all tests have been run as specified above. 

Differential thermal analysis (DTA) data was obtained using a DuPont 1090 thermal analyzer using 0.04 SCF/h of air as purging gas and heating rates of 10°C/min. All powder diffraction measurements were obtained with a Siemens D-500 diffractometer at a scan rate of l°/min using monochromatic Cu-K radiation. The preparation of DFCC mixtures containing sepiolite hSs been described elsewhere (4). 

During the TSR process, the concentration of holes and electrons is determined by the balance between thermal emission and recapture by traps and capture by recombination centers, hi principle, integration of corresponding equations yields ric(t,T) and p t,T) for both isothermal current transients (ICTs) or during irreversible thermal scans. Obviously, the trapping parameters hsted together with the capture rates of carriers in recombination centers determine these concentrations. Measurement of the current density J = exp(/in c + yUpP) will provide trap-spectroscopic information. The experimental techniques employed in an attempt to perform trap level spectroscopy on this basis are known as Isothermal Current Transients (ICTs) [6], TSC [7]. 

It must be pointed out that, at the laser power and scanning rates used, the surface temperature of the sample did not rise more than a few degrees above ambient, as shown recently by Tsao and Ehrlich (10). Since the laser exposure time was five orders of magnitude shorter in our experiments as in Tsao s work, thermal initiation can be neglected. Further support of an exclusive photoeflect in the laser curing of these resins came from the absence of any detectable polymerization when the photoinitiator was not introduced in the formulation. 

This instrument was designed to yield information intermediate between the ARC and the DSC. A sample of 0.2-0.5 g is loaded into a tube-like container and placed into the device (larger sample sizes may be used at slower scan rates). A thermocouple is connected to the outside of the tube and the cell is fitted with a pressure transducer. A similar, empty cell in the same oven with thermocouple serves as a thermal reference. The oven is heated at a slow, linear rate (0.5 to 1 °C/min), and the pressure and differential thermal data are collected. The data are presented in a fashion similar to DSC - Heat Rate (mW) vs. Temperature (°C). The thermal data are enthalpically calibrated by means of a series of standards (cahbration at high heat rates may be non-linear). Detection of thermal events approaches the sensitivity of the ARC. 

Differential scanning calorimetry (DSC) [3] The DSC thermogram for zaleplon is shown in Figs. 8.2 and 8.3, and was obtained using Shi-madzu DSC-50 thermal analyzer at scan rate of 10 and 5 °C/min under a nitrogen atmosphere over a temperature interval of 20-300 °C. 

Differential Scanning Calorimetry (DSC) Studies. Hairless mouse abdomen stratum corneum, extracted lipids and protein residues were studied with a Perkin Elmer 4 differential scanning calorimeter (DSC) equipped with a thermal analysis data system (TADS). Scanning rates were 10°C per minute over the temperature region -10 to 237°C. Stratum corneum, extracted lipid and protein residue samples obtained from the abdomen of the hairless mice (average 10 mg/sample) were studied in the desiccated state following evaporation of any residue water or solvents by vacuum drying at 10 4 Torr. 

Fig. 6.17. Cyclic voltammograms of o-phenylenediamine (101 M) oxidation for W03 thermal-treated (350°C) anodic films (b) and smooth platinum electrode (c) first sweep (curves 1) and repeated sweep (curves 2) scan rate was 80 mV/cm2. The left picture shows a schematic representation of the morphology of thermal-treated anodic W03 film tungsten support, highly defective oxide (including the continuous donor clusters), moderately doped oxide (non-shaded region), poly-o-phenylenediamine deposits.

The thermal stability of the complexes was determined by thermo gravimetric analysis (TGA) with a Mettler TA 3000 System at a scan rate of 10°C/min under nitrogen atmosphere. The complexes were found quite stable up to 430 °C. The maximum weight loss occurred at about 450 °C. 

PC instruments are preferable to HF instruments for isothermal studies, and in studies in which the temperature scanning rate is high, because the very small sample and reference holders in the PC type have much smaller thermal inertia than the relatively large heating block in the furnace of the HF type (Hatakeyama and Quinn, 1994). 

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