Temperature oscillation calorimetry

The modulating method is based on the measurement of the temperature oscillations of a sample heated by oscillating heat power. Under isoperibol conditions, this method is called AC calorimetry [213-215]. The first AC calorimetry experiments were performed in 1962 by... 

Temperature Oscillation Calorimetry A more elegant way to estimate online the overall heat transfer coefficient without any additional measurement was developed by Carloff [ 11] by the technique known as temperature oscillation calorimetry, TOC. In this approach, the unknown product UA is computed from the analysis of the sine-shaped oscillations, which are superposed on either the reactor temperature or jacket temperature. The objective is to decouple the slow dynamic of the chemical heat production from the fast dynamic variable heat transfer during the reaction. The oscillations can be achieved either by adding a calibration heater to the system or by adding a sine signal to the set point of either T or Ty Figure 7.2 shows the evolution of the reactor and jacket temperatures in a reaction calorimeter where a sine wave temperature modulation was superimposed on the reactor jacket temperature. 

FIGURE 7.3 Evolution of the oscillating reactor and jacket temperatures, electric heater voltage and calculated overall heat transfer coefficient. With permission of VCH Verlagsgesellchaft from Carloff R, Pross A, Reichert K. Temperature oscillation calorimetry in stirred-tank reactors with variable heat-transfer. Chem Eng Technol 1994 17 406 13. 

Tietze A, Ludke 1, Reichert K. Temperature oscillation calorimetry in stirred tank reactors. Chem Eng Sci 1996 51 3131-3137. 

For a.c. calorimetry an oscillating heating power Pac=Po (1 + cos oat) is supplied to a sample (in a sample holder), usually loosely coupled to a heat bath at a given temperature Tq, and from the amplitude of the resulting temperature oscillation (see Fig. 4) the specific heat capacity of the sample can be derived. From Fig. 4, it should also be clear that for a first-order transition one can not determine the latent heat A//l in this way. 

The calorimetric thermometer measures temperature changes within the calorimeter bucket. It must be able to provide excellent resolution and repeatability. High single-point accuracy is not required since it is the change in temperature that is important in fuel calorimetry. Mercurial thermometers, platinum resistance thermometers, quartz oscillators, and thermistor systems have all been successfully used as calorimelric thermometers. 

A convenient method for determining transition times and transition temperatures of polymeric materials is dynamic mechanical analysis. One type of instrument which is particularly suitable for polymeric solids is the freely oscillating torsion pendulum (TP). Advantages of the TP include its simplicity, sensitivity, relatively low frequency ( 1 Hz) which permits direct correlation of transition temperatures with static nonmechanical methods (e.g., dilatometry and calorimetry), and its high resolution of transitions A major disadvantage of the conventional TP is that test temperatures are limited by the inability of materials to support their own weight near load-limiting transition temperatures. 

The preparation of ZnO/ PS nanocomposites preceded as follows [112] First, 110 mg bare ZnO or 110 mg PMMA-grafted ZnO were added into a three-necked bottle. Then, 10 mL styrene was added into the reactor. The mixture was stirred with the aid of ultrasonic oscillation until a uniform dispersion of the ZnO particles in styrene was attained. Afterwards, 36 mg azobisisobutyronitrile (AIBN) was added into the reactor as initiator. The subsequent polymerization was conducted at 85°C for 2.5 h. Then, the obtained composites were dried under vacuum for 24 h. The differential scanning calorimetry (DSC) heating curves of neat PS, PS/ZnO (bare), and PS/ZnO (PMMA grafted) are shown in Fig. 10. DSC traces in Fig. 10a show that neat PS has a lower glass transition temperature (Tg = 87.7°C) than PS/ZnO (bare, 7 g = 97.9°C) and PS/ZnO (PMMA grafted, Tg = 95.3°C). This behavior can be explained by the restricting effect of the nanoparticles in polymer. ZnO... 

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