Thermodielectric Analyzer

FIGURE 4.42 Inside view of the specimen holder for TDA. 1, electric conductors to the dielectric sensor 2, ceramic specimen holder for a TDA 3, nickel made concentric cylindrical electrodes 4, void where is located a platinum-rhodium thermocouple 5, hollow place where is located the samples under test in the form of powders (about 1 g of sample). 

The Physical Chemistry of Materials Energy and Environmental Applications 

IZrl is the complex impedance module of the equivalent circuit formed by the reference capacitor tilled with A1203 

The plotter, records the signal of a chromel-alumel thermocouple located inside the heating furnace near to the sample holder in the x-axis the temperature of the furnace is scanned from 27°C to 1000°C at a rate of 16°C/min. In the y-axis, the rectified output voltage, V0 (0 V V0 10 V) is plotted, in order to obtain the thermodielectric profile [15,88], 

Equation 4.34 satisfactorily describes the low-temperature effect (T 250°C) in the thermodielectric profile, since the resistance of the sample (at T 250°C) is very high and, therefore, the second term of the impedance module, [1 /R]2, can be neglected [15,88], 

---

A thermodielectric profile can be obtained with a thermodielectric analyzer [15,108-110], This device registers the relationship between the linearly scanned temperature (T) (30°C rectified output voltage (0 V dielectric sensor), which differentially compares the impedance of the test sample, in the form of a powder (particle size 0.1-0.2mm) and a reference (calcined A1203 powder) (see Figures 4.41 and 4.42 for further information). 

The effect has the form of a peak because at the origin of the peak, the increase in temperature is linked with the increasing mobility of water molecules adsorbed in the cavities and the channels of the zeolite. This event is followed by an increment of the permittivity of the zeolite sample, and this increase of the sample permittivity is detected by the thermodielectric analyzer as an increase in V0 (see Equation 4.34). 

Besides, the interaction of divalent cations with the zeolite framework and water is stronger on account of the higher charge and lower cationic radius exhibited by Ca2+ (Ca2+ 0.99 A) in contrast with Na+ and K+ (Na+ 0.95 A and K+ 1.33 A). This effect induces a lower mobility of divalent cations, on account of the fact that divalent cations are more intimately linked with the zeolite framework. Hence, the lower cationic mobility is an additional reason for the decrement in the permittivity of tested samples, and this effect is also detected by the thermodielectric analyzer as a decrease in V0 [110,119],... 

---