Vertical Thermobalance

The TG thermobalance structure is illustrated in Figure 10.20 it includes a microbalance, furnace, temperature programmer and computer. The key component is the microbalance, which measures the mass change. A typical microbalance is able to measure mass change of 1 ptg with maximum mass of 100 mg. A commonly used microbalance is the null-point type. The null-point microbalance can maintain the sample in a vertical position when its mass changes. Figure 10.21 shows the structure of the commonly used null-point type microbalance, the Cahn microbalance. 

X-Ray Diffraction (XRD) patterns were recorded on a Philips HTK-KC diffractometer with a CuKa X-ray source, linked to Philips 386 computer. Thermo-Gravimetry (TG), was recorded on a Setaram TG-DTA 92 thermobalance. Diffuse Reflectance Spectra (DRS) were recorded on a Cary-5 spectrofotometer with a BaS04 integration-sphere in the UV-VIS-NIR region. Mossbauer spectra are recorded on a vertical constant acceleration drive in transmission geometry with a 28mCi Co(Rh) source. Isomer shift data are expressed relative to metallic Fe at 293 K which has an isomer shift of8 = -0.0888 mm/s relative to natural a-Fe. 

Thermobalances are available in two different designs, i. e. a horizontal construction (Fig. 5-1) and vertical construction. For the vertical construction there are two different versions the top-loading balance, which has the oven above the balance (Fig. 5-2), and the thermobalance with the sample tray hanging on a thin hangdown wire into the oven below the balance (Fig. 2-4). The suppliers claim some advantages for the horizontal arrangement such as complete lack of buoyancy, the avoidance of condensation in the balance cabinet, and the reduction of sensitivity to vibration. However the last two points cannot be confirmed by the author s experience. Despite an efficacious gas flow, a diffusion of condensates was observed opposite to the flow direction of the gas. The sensitivity to vibrations seems to be comparable with the vertical design. Of the vertically constructed balances, the top loader is more convenient to manipulate. 

Some suppliers recommend thermobalances in a vertical design which are equipped with two separate ovens operating with identical heating and gas flow rates for sample and counterweight. This construction completely compensates any buoyancy effects and guarantees a very high resolution of approximate 0.1 (Fig. 5-3). 

All thermobalances effectively utilize baffling to reduce the thermal convection currents that would otherwise disturb the measurement of mass and heat the balance compartment. Optimization of the atmospheric flow pattern and the baffling are critical to reduction of thermal noise and drift in the measurement of the mass. Many models will impose a water-cooled plate between the balance compartment and the furnace. When long-term stability is required, it is advisable to consider actually thermostatting the balance compartment. Since heat normally rises, vertical configurations are most susceptible. The horizontal arrangement leads to less interference from the flow patterns arising from control of the atmosphere. 

---