The fundamental principle behind thermogravimetric analysis (TGA): As a sample undergoes thermal treatment, its mass may change due to physical or chemical processes. These changes are recorded as a function of temperature or time. The thermogravimetric analysis (TGA) apparatus typically consists of a balance, a furnace and a temperature controller. The sample is placed in a small crucible that is then heated according to a pre-defined temperature program.

Key components in thermogravimetric analysis (TGA) design:

1. High-Precision Balance (Thermobalance): The heart of the system, designed to handle high temperatures and provide accurate measurements of small mass changes. Modern instruments often use top-loading or hang-down designs.

2. Furnace: Designed to reach high temperatures (typically up to 1000°C, with some reaching 1500°C or higher) and provide precise heating rates.

3. Temperature Controller & Sensors: A thermocouple is placed near the crucible to ensure accurate sample temperature monitoring.

4. Gas Delivery System: Controls the type and flow rate of gases (e.g., Ar, He, CO2, N2 etc.) in the furnace to allow inert or oxidizing environments.

5. Data Acquisition System: Software that records and processes the weight and temperature data to produce the final TGA curve (thermogram).

Three different designs of thermobalances are shown schematically in Figure 1. Nowadays, mainly compensation balances are used. With this type of balance, the position of the sample in the furnace remains exactly the same even when the mass changes. At the same time, however, one can distinguish between simple moving coil measuring systems and more sophisticated weighing cells (e.g. parallel guided). In the horizontal arrangement, simple moving coil systems have the disadvantage that samples that move horizontally during heating (e.g. during melting) generate an apparent change in mass. Use of a parallel-guided system overcomes this problem.

Figure 1. Thermobalance designs showing the top loading, hang down and horizontal arrangements.

We load the sample from the bottom (hang down), top, or side (Figure 2-4). A thermocouple near the pan monitors the sample temperature. A protective tube isolates both heating elements and cooling coils from the sample pan. A dynamic purge gas passes over the sample. The bottom-loading thermogravimetric analysis (TGA) supports the sample pan via a hang-down hook below the balance, and purge gas enters from a capillary tube from the side and exits on the opposite side of the pan (Figure 3). The top-loading thermogravimetric analysis (TGA) supports the sample pan and a thermocouple above the balance via a stem support rod. Purge gas typically enters from below the pan and exits from the top (Figure 2). In a side loading configuration, the sample support and furnace were orientated horizontally and purge gas flows over the surface of the pan (Figure 4).

Figure 2. Top Loading

Figure 3. Hang down (bottom loading)

Figure 4. horizontal arrangement (side loading)

A symmetrical electromagnetic balance has a counter pan as a counter-weight with small dynamic range. The counter pan serves to counteract the weight of the sample pan and the hang downs or rods so that the sample mass changes are within the dynamic range of the balance (without the counterweight, a greater electromagnetic force is required to correct the effect of the hang downs, sample pan, and sample crucible, which reduce balance sensitivity). Counter pans do not counteract buoyancy effects due to differences in gas flow rate and turbulency between the sample and counter pan. However, symmetrical TGAs are designed to counteract buoyancy effects. These TGAs use a high sensitivity symmetrical electromagnetic-optical balance, and instead of having the counter pan simply act as a counterweight, the counter pan is in its own furnace with a symmetrical gas flow arrangement (Figure 5). Two identical furnace chambers ensure that there is a symmetrical gas flow on both samples.

Figure 5. Counter pan in its own furnace with a symmetrical gas flow arrangement

Constructional measures have to be incorporated between the balance and the furnace to protect the balance against the effects of heat radiation and the ingress of corrosive decomposition products. In most cases the balance housing is purged with a protective gas. In some thermobalances, an ‘external’ furnace is used whereby the furnace is not in contact with the atmosphere used in the experiment and this can be useful if measurements are made in pure hydrogen atmospheres. Depending on the resolution they provide, balances are classed as semimicro- (10ug), micro- (1ug) or ultramicro- (0.1ug) balances. Besides resolution, the (continuously measurable) maximum capacity of the balance is also an important factor. This is particularly the case when measuring inhomogeneous materials where a few milligrams are often hardly representative and a larger sample mass is desirable.

A thermogravimetric analysis (TGA) instrument consists of a thermobalance, which can records weight with sensitivity around one microgram and a capacity of about a few hundred milligrams and a furnace operated in a temperature range of 50°C–800°C with a heating rate up to 100°C min−1. The thermal stability of a material can be studied in an inert or oxidative atmosphere. The thermogravimetric curves elucidate the decomposition mechanism. There are lots of factors that affect the thermogravimetric curves. The primary factors, which have a major effect, are the heating rate and size of the sample. As the heating rate and sample size increase, the decomposition temperature of the sample also increases. The particle size of the sample, the way in which it is packed, the crucible shape, and the gas flow rate can also affect the kinetics of the reaction. Thus, while comparing the thermal stability of two materials, the identical condition with respect to the above-mentioned variables must be maintained.

The adequacy of the kinetic data derived from a TGA apparatus are related to either sample or instrument conditions, which change the controlling mass and heat transfer modes. The sample conditions involve sample form, sample size, and the sample holder (pan or crucible). Sample particle granularity controls the internal mass transfer resistance, while sample mass controls the external mass transfer resistance. The instrument conditions are temperature, the rate of temperature change, the gas atmosphere and its flow rate, and the pressure.

A thermocouple near the pan measures the furnace temperature and not the actual sample temperature. Thus, both the gas composition and flow rate influence what the temperature gauge reports. Some researchers studied the temperature gradient between the thermocouple and the sample by developing a model in an infrared furnace. The model assumed convection between the thermocouple and the sample holder was negligible and the gas volume produced during the degradation was inconsequential compared to the inlet gas flow. Further, it ignored radiation and included only conduction and convection from the gas stream.

Another important factor that influences the true sample temperature in thermogravimetry is the effect of radiation on the sample temperature. Above 500℃, radiation becomes the dominant mechanism for heat transfer, especially when using an open-cup type sample pan; for example, a black sample can be at a temperature higher than a white sample at the same conditions.

Buoyancy is another phenomenon that decreases the accuracy either during heating or when switching between nitrogen and air, for example. As the temperature ramps, the gas density decreases, which translates to an apparent mass gain. This apparent mass gain depends on the volume of the pan and the density. Performing an experiment with an inert sample (empty pan) and subtracting this blank experiment can compensate for buoyancy and baseline issues. Due to the change in density of a gas as the temperature changes, buoyancy corrections must be made in TGA measurements. Without corrections every sample will appear to show a mass increase during a heating experiment. TGA measurements are usually corrected for the effect of buoyancy by performing a blank measurement. A blank experiment uses the same temperature program and crucible as the experiment but without a sample. The resulting blank curve (also called a baseline) is then subtracted from the sample measurement curve. In some instruments, a ‘standard’ baseline is automatically subtracted from all measurements. Buoyancy correction is essential for tests such as ash content where the residue at the end of the test needs to be determined accurately, and where very small weight losses are expected.