Heat flow measured curve

Thermogravimetry can be used to measure the amount of water [232] or other molecule adsorbed on a zeolite. DSC can be uhlized to study the thermal effects during adsorption and desorphon of water [233] because the peak area under the heat flow time curve is related to the sorption heat. 

It can be seen in Fig. 5.14 that the heat flow measured with respect to the total weight of monomer does not depend on stirring speed. Moreover, the good reproducibility of the reaction calorimeter is confirmed. By integration of the thermal curves, the enthalpy of polymerization can be calculated as -56.9 2.2 kj mol and is in good agreement with the literature s value of -57.8 kj mol" obtained in conventional solvent [32]. The error of 2.2 kJ mol is a standard deviation calculated from a series of seven measurements [33]. 

Using differential scanning calorimetry (DSC), one can measure the heat flow rate curve of polymer solid changing with the temperatures, as demonstrated in Fig. 6.15a. Heating (cooling) rates are constant. 

The main property that is measured by DSC is heat flow, the flow of energy into or out of the sample as a function of temperature or time, and usually shown in units of mW on the y-axis. Since a mW is a mj/s this is literally the flow of energy in unit time. The actual value of heat flow measured depends upon the effect of the reference and is not absolute. What matters is that a stable instrumental response or baseline is produced against which any changes can be measured. The starting point of the curve on the y-axis may be chosen as one of the starting parameters, and it should be set at or close to zero. 

Figure 10 Time evolution of heat capacity during quasi-isothermal crystallization of PHBat 296K, Ai=OAK, fp = 100s (curve (a)). Curves (b)and (c) correspond to solid and liquid heat capacities available from the ATHAS-DB, respectively. Curve (d) was estimated from a two-phase model and curve (e) from a three-phase model by using /soiid(Tg) from ACp (see Reference 85 for details). The subscript solid denotes the solid fraction of the polymer consisting of crystalline and glassy fractions. The subscript CRF denotes the crystalline fraction alone. The squares represent measurements at modulation periods ranging from 240 to 1200 s. Curve (f) shows the exothermal effect in the total heat flow, and curves (g) and (h) show the expected values from model calculations (see text and Reference 85). Data from PetIdnElmer Pyris I DSC. Reproduced with permission from Schick, C. Wurm, A. Mohammed, A. Thermochim. ActaZOOS, 396,119-132. ...

Where large samples of reactant are used and/or where C02 withdrawal is not rapid or complete, the rates of calcite decomposition can be controlled by the rate of heat transfer [748] or C02 removal [749], Draper [748] has shown that the shapes of a—time curves can be altered by varying the reactant geometry and supply of heat to the reactant mass. Under the conditions used, heat flow, rather than product escape, was identified as rate-limiting. Using large ( 100 g) samples, Hills [749] concluded that the reaction rate was controlled by both the diffusion of heat to the interface and C02 from it. The proposed models were consistent with independently measured values of the transport parameters [750—752] whether these results are transfenable to small samples is questionable. 

The determination of these curves requires not only the measurement of small amounts of heat in a microcalorimeter, but also the simultaneous determination of the corresponding quantity of adsorbed gas. Volumetric measurements are to be preferred to gravimetric measurements for these determinations because it would be very difficult indeed to ensure a good, and reproducible, thermal contact between a sample of adsorbent, hanging from a balance beam, and the inner cell of a heat-flow calorimeter. 

In the various sections of this article, it has been attempted to show that heat-flow calorimetry does not present some of the theoretical or practical limitations which restrain the use of other calorimetric techniques in adsorption or heterogeneous catalysis studies. Provided that some relatively simple calibration tests and preliminary experiments, which have been described, are carefully made, the heat evolved during fast or slow adsorptions or surface interactions may be measured with precision in heat-flow calorimeters which are, moreover, particularly suitable for investigating surface phenomena on solids with a poor heat conductivity, as most industrial catalysts indeed are. The excellent stability of the zero reading, the high sensitivity level, and the remarkable fidelity which characterize many heat-flow microcalorimeters, and especially the Calvet microcalorimeters, permit, in most cases, the correct determination of the Q-0 curve—the energy spectrum of the adsorbent surface with respect to... 

Figure 11.5 Typical curve for a continuous titration calorimetry study of an exothermic reaction, using the calorimeter of Figure 11.1 in the heat flow isothermal mode of measurement./ is the frequency of the constant energy pulses supplied to the heater C in Figure 11.1 b. Adapted from [196,197],...

If, as illustrated in figure 12.6, the isothermal starting lines of the various curves do not coincide, then A< >o, A< cai, and Aheat transfer change between runs, for example, due to a variation in the purge gas flow or the fact that it is virtually impossible to relocate the crucible containing the sample exactly in the position used for the calibrant run (normally the reference crucible remains in place throughout a series of runs). Note that a similar correction should have been used in the computation of heat flow or area quantities if, in the example of figure 12.4, the isothermal baselines of the main experiment and the zero line were not coincident. 

Many different test methods can be used to study polymers and their physical changes with temperature. These studies are called thermal analysis. Two important types of thermal analysis are called differential scanning calorimetry (DSC) and differential thermal analysis (DTA). DSC is a technique in which heat flow away from a polymer is measured as a function of temperature or time. In DTA the temperature difference between a reference and a sample is measured as a function of temperature or time. A typical DTA curve easily shows both Tg and T . 

Differential Scanning Calorimetry (DSC) This is by far the widest utilized technique to obtain the degree and reaction rate of cure as well as the specific heat of thermosetting resins. It is based on the measurement of the differential voltage (converted into heat flow) necessary to obtain the thermal equilibrium between a sample (resin) and an inert reference, both placed into a calorimeter [143,144], As a result, a thermogram, as shown in Figure 2.7, is obtained [145]. In this curve, the area under the whole curve represents the total heat of reaction, AHR, and the shadowed area represents the enthalpy at a specific time. From Equations 2.5 and 2.6, the degree and rate of cure can be calculated. The DSC can operate under isothermal or non-isothermal conditions [146]. In the former mode, two different methods can be used [1] ... 

The sample (typically 1 mg) is sealed in a glass container (capillary or ampoule) in a nitrogen or air atmosphere. The reference material is typically an empty sealed container. The sample and reference are heated in an oven at a constant temperature rate (typically 10 °C per minute) from ambient to 400 °C. Energy flow to the sample is measured as a slight deviation in its local temperature. Data are presented in terms of plot of heat flow vs. temperature. Processes (phase changes, reactions) total enthalpies are determined by integration of the heat rate curve. 

When the measured heat-flow rate (qtot) curve at 25°C is compared with the results obtained from the IR signal (see Fig. 8.5), it again becomes clear that the initial peak of the qtot curve, which is not visible in the IR signal, is not related to the chemical reaction but to the mixing. In this simple case, we have an excellent example of how the simultaneous... 

It appears, therefore, that the measurement of heat flow using direct calorimetry, is a suitable technique for biogeochemical studies in aquatic systems since under carefully controlled conditions a reproducible "fingerprint" (or power-time curve) is obtained. 

Calorimetric measurements show that the addition is exothermic [34], The heat release rate is mainly feed controlled, as the square shape of the heat flow curve demonstrates (see Figure 5.5). Whenever feed is added, the heat flow responds without delay. 

The relationship of the thermal conductivities of fabrics and volume fractions of water in the interfiber spaces was expressed by a quadratic curve when the heat flow was normal to the fabric surface and by a straight line when the flow was parallel to the warp yarns. Except for hairy wool fabrics, the thermal conductivity of various wet fabrics may be calculated from the equations of Naka and Kamata (J3). An earlier investigation used an environmentally controlled room as a periodic heat source, and observed conductivities of 1-2 x 10 l cal/cm-sec °C for cotton, linen, and wool fabrics, and changes to 2-10 x 10 when the water content of these fabrics were increased ( ). After correcting for anisotropic effects, good agreement between actual conductivity measurements of wool fabrics and those calculated from a mathematical model of a random arrangement of fibers was observed. 

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