A graphite furnace and modular design are enabling a new
thermal analysis system to address the high-temperature needs of the ceramic
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
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.
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 (Si3
carbon to prepare silicon carbide (SiC). A mixture of Si3
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 Si3
+ 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
+ 3 C
→ 3 SiC + 2 N2
Figure 2. An α-SiC cube undergoes a global expansion
between ambient and 1310°C.
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
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
Figure 3. Yttria shows two exothermic effects after cooling
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
The system's symmetrical balance beam enables samples of
up to 35 g to be studied without any loss of balance resolution.
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.