Heating rate dilatometry

Ceramic powders of BaCeo.9Yo.1O2.95 (BCYIO) have been prepared by the sol-gel method [115]. Barium and yttriimi acetate and cerium nitrate were used as ceramic precursors in a water solution. The reaction process studied by DTA-TG and XRD showed that calcination of the precursor powder at r>1000°C produces a single perovskite phase. The densification behavior of green compacts studied by constant heating rate dilatometry revealed that the shrinkage rate was maximal at 1430 °C. Sintered densities higher than 95% of the theoretical one were thus obtained below 1500 °C. The bulk and additional blocking effects were characterized by impedance spectroscopy in an wet atmosphere between 150 and 600 °C. Proton conduction behavior was clearly identified. 

While the common heating rate for DTA and TG investigations is 10°C/min, a more appropriate heating rate for dilatometry is 3 to 5°C/min. The specimen dimensions used in dilatometry are generally much larger than those used in DTA or TG time must be allowed for heat to propagate from the specimen surface to its interior. Temperature gradients within... 

The crystal phases in the glass-ceramics were determined by XRD analysis. All instruments were precisely and identically set to ensure a high precision to obtain the integral peak area. The microstructure of the fresh fractured cross section of the glass-ceramics was observed by SEM. The thermal expansion coefficient (TEC) was calculated from room temperature to 500 °C at a heating rate of 5°C/min in the dilatometry analyser (NETZSCH, DIL402PC). The flexural strength was determined in a 3-point bend test at a constant strain ratio of 0.5mm/min. 

Dilatometry is an older method, but it has the advantage of being adaptable to very low heating rates (1-2 K day ), which can be valuable as both Tg and T a depend upon the rate. 

Dilatometry. DTA extrapolated to zero heating rate. Rapid heating on hot stage. Table 5 Extrapolated for Linear Polyethylene... 

Dilatometry and rotating sector techniques were combined to follow the photopolymerization of acrylamide in AOT reverse micelles with AIBN as the initiator and toluene as the organic phase [51,52]. Very high polymerization rates were observed with total convCTsion to polym achieved in a few minutes and good heat transfer. A monoradical termination reaction was found (R oc [I] [M]° ), caused by a degradative chain transfer to toluene. The reaction occurs at the water-oil interface by transfer of the growing polymer radical to toluene, followed by the exo-diffusion of tiie new ben lic radical, the latter bdng too stable to reinitiate polymerization. 

Although the glass transition resembles characteristics of a second-order thermodynamic transition such as changes in the coefficient of expansion and heat capacity, the temperature of the transition is a function of the heating or cooling rate and of the rate of deformation. The methods used to determine Tg are based either on static or dynamic mechanical processes. The former uses volume effects (dilatometry) and heat capacity effects in differential scanning calorimetry (DSC), entailing conditions of very low deformation. The latter utilizes the response to imposed deformation of the system. 

Polymerization rates were determined by recording capacitance dilatometry . In these experiments S (1.90 g), AN (1.1 g), and the appropriate solvent (50 ml) were heated in the dilatometer with tBPP (0.06 g) and ZnCla (0.22 g) when required. 

At sufficient supercooling, the sphemhtic superstmcture may be described with only one growth rate, v, as shown in Fig. 3.83. The increase in crystal volume, V, is given by the second equation and allows the computation of the enthalpy evolved by multiplication with the product of density and specific heat of fusion (Ahf in 1 g ). With nucleation data, the crystallization rate of the whole sample can be computed and linked to growth rates measured by dilatometry or calorimetry. Results for the LiPOj crystallization, which is discussed in Sect. 3.1.6, are also listed in Fig. 5.83 [15]. The changes of the growth rale with temperature are given in the table. Note that the crystallization of the polymer is in this case coupled with the polymerization reaction [1], and increases with temperature. 

Figure 4.67[A] shows a typical isothermal experiment carried out with a DSC. Similar experiments could be carried out with isothermal calorimeters, dilatometry and other teehniques sensitive to crystallinity changes. After attainment of steady state at point 0, the experiment begins. At point 1, the first heat flow rate is observed, and when the heat flow rate reaches 0 again, the transition is complete. The shaded area is the time integral of the heat flow rate, and if there is only a negligible instrument lag, it represents the overall kinetics. In case of an excessive heat flow-rate amplitude, lag calibrations with sharply melting substances of similar thermal conductivity may have to be made (see Figure 4.22). Processes faster than about 1 min...

Thermomechanical analysis (TMA) is a technique that measures the deformation of a substance under non-oscillatory load or strain as a function of temperature or time. Thermo-dilatometry (see Section 2.2.4) is a technique that measures dimensional changes of a sample as a function of temperature or time. Both of these techniques can be applied using the same apparatus. The sample is heated or cooled at a certain rate, or is maintained isothermally at a fixed temperature. 

Results may be quoted in terms of seconds or minutes, depending upon the crystallization rate. Crystallization rates involving half-times in excess of a few minntes are rarely of interest to commercial enterprises. The determination of half-times in excess of an hour with a differential scanning calorimeter is not very practical due to the difficulty of measuring the very low heat transfer rates involved if such measurements are desired, dilatometry is a more practical alternative. 

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