High-Temperature Thermal Characterization

A graphite furnace and modular design are enabling a new thermal analysis system to address the high-temperature needs of the ceramic industry.

The Setsys Evolution features a graphite furnace capable of controlling temperatures of up to 2400°C.

Developments in ceramic technology are providing industry with ever-increasing high-temperature solutions. This trend has placed increased demand on developers of analytical instruments to provide systems capable of the measurement of various parameters at these higher temperatures.

Temperatures above 1750°C are defined as high temperature because of the mechanical challenges that going over this temperature presents. Up to 1750°C, analytical furnaces are typically lined with alumina, which acts as a barrier to aggressive environments. Above 1750°C, alumina loses its desired mechanical properties and limits most commercially available thermal analysis systems.

However, a new system* has been developed that offers many benefits for ceramic applications. The system's most important feature is a graphite furnace that can control temperatures up to 2400°C, enabling the high-temperature analyses the industry depends on. An additional benefit is that the system features a modular design. The furnace and gas control system are common between all of the measurement techniques, allowing for the key sensor to be changed by the operator and enabling a number of different critical mechanical property measurements, including thermogravimetric analysis, thermomechanical analysis and differential thermal analysis.

*Setsys Evolution, developed by Setaram Inc., Pennsauken, N.J.

Figure 1. Synthesis of SiC.

Thermogravimetric Analysis

One of the most common thermal techniques, thermogravimetric analysis (TGA) is defined as the measurement of mass variation within control temperature environments. The effects that can be studied are wide and include decarbonation (loss of CO2), loss of water, oxidation, reduction, and adsorption and desorption studies.

The new system offers a symmetrical balance beam,** which, within the inclusion of a counter weight, enables samples of up to 35 g to be studied without any loss of balance resolution. Data obtained using samples that are heterogeneous or show very small mass loss can be greatly improved over traditional methods.

The following example illustrates the classic reaction of silicon nitride (Si3N4) with carbon to prepare silicon carbide (SiC). A mixture of Si3N4 and carbon in a 1:6 ratio is heated in a graphite crucible under helium flow, and two successive mass losses are observed (see Figure 1). Between 1100 and 1300°C, a mass loss of approximately 1% corresponds to the reaction of the reduction of the layer of silica that is at the surface of Si3N4 :

SiO2 + 3 C → SiC + 2 CO↑

And between 1300 and 1500°C, a mass loss of approximately 26% corresponds to the final reaction from silicon nitride into silicon carbide and nitrogen:

Si3N4 + 3 C → 3 SiC + 2 N2

**Setsys Microbalance.  

Figure 2. An α-SiC cube undergoes a global expansion between ambient and 1310°C.

Thermomechanical Analysis

Thermomechanical analysis (TMA), defined as the measurement of any dimensional variation under controlled conditions, is another widely used technique. Classic applications include the measurement of a material's softening point and coefficient of expansion. The most common application is the measurement of sintering behavior. The sample volume decreases as sintering occurs because the powdered sample becomes more closely associated.

The unique design of the new system also allows for vertical measurement, which enables the operator to apply zero retaining force so there is no compaction of the powdered sample prior to sintering. With horizontally designed measurement, some force is always required. This force changes the density of the sample and therefore adversely effects the detection and evaluation of dimensional changes during sintering.

In the following example, the production of dense SiC using pressureless sintering is studied. The sample is an α-SiC cube(9 mm) under an argon atmosphere and a small load of 5 g. The sample is baked at 480°C for three hours and then heated to 2200°C at 15 k/min. The curve in Figure 2 clearly shows a global expansion between ambient and 1310°C. Above 1310°C, shrinkage due to the sintering of the SiC is clearly evident and is completed at approximately 1815°C. The global shrinkage of the sample after cooling is approximately 2.7%.

Figure 3. Yttria shows two exothermic effects after cooling from 2400°C.

Differential Thermal Analysis

Differential thermal analysis (DTA) is the simultaneous measurement of the temperature difference between a sample and a reference under an identical environment. Therefore, any phase transitions that are not accompanied by a mass change or endothermic or exothermic dimensional variation (i.e., melting) can be shown.

If the system is correctly calibrated, the energy associated with this transition can also be reported. In the following example, yttria shows two exothermic effects after cooling from 2400°C (see Figure 3). The first is crystallization after melting, and the second is a phase transition. Understanding these transitions is critical for any research project, and in this case both occur above 1750°C, which is the limit of most analytical furnaces.

The system's symmetrical balance beam enables samples of up to 35 g to be studied without any loss of balance resolution.

Hot Stuff

While the characterization of ceramic sample behavior through thermal analysis has long been understood, the limitations of traditional equipment have hampered analyses beyond the 1750°C temperature range. However, recent advances in thermal analysis equipment are enabling ceramic manufacturers to gather vital information on a range of mechanical properties-from sintering and enthalpies of phase transitions to mass variations during reactions-at temperatures of up to 2400°C.

For additional information regarding the thermal characterization of high-temperature ceramics, contact Setaram Inc. at 7905 Browning Rd., Ste. 206, Pennsauken, NJ 08109; (856) 910-9990; fax (856) 910-1150; or visit www.setaram.com.


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