By incorporating various techniques, a broad spectrum of analytical possibilities emerges for characterizing the thermal behavior of materials and composites.
Thermal analysis methods are standardized
techniques for the investigation of physical and chemical property changes of a
variety of materials as a function of temperature and/or time. Applications
include organic samples like fats, oils and proteins; pharmaceutical active ingredients
and additives; synthetic materials like plastics, elastomers and reactive
systems; and inorganic materials like glasses, metals and ceramics. In addition
to standards for terminology in the field of thermal
there are probably a few hundred national and
international standards describing all of the important relationships and
influencing factors in the field of thermal analysis, ranging from the design
and operating modes of the measurement instruments to the measuring principles
and detailed descriptions of temperature programs and evaluation routines.
By incorporating other techniques, such as those for determining the
thermal diffusivity, thermal conductivity or even the crosslinking behavior of
reactive systems, a broad spectrum of analytical possibilities emerges for
characterizing the thermal behavior of materials and composites. A special role
is played here by evolved gas analysis (EGA) coupling methods, where
instruments for thermal analysis are coupled to spectroscopic (Fourier
transform infrared spectroscopy, or FTIR) or spectrometric (mass spectrometry,
or MS) identification methods. This makes it possible to not only characterize
and quantify the thermal effects, but to also identify the substances released.
When we break down the variety of analytical possibilities into the essentials,
however, they can be classified into three measurement categories: length
change (dilatometry), mass change (thermogravimetry) and caloric changes
(differential scanning calorimetry, DSC). Although the combination of
thermoanaly-tical methods can be even more helpful for the investigation of
material properties, the main focus in this work is on the method of
dilatometry. We will study barium titanate’s (BaTiO3
sintering behavior by using powders of various particle sizes.* The dimension
of the powder particles could be reduced in two orders of magnitude, starting
with 10 µm to 1.1 and 0.11 µm.
* Particle size changes were made with a
RS 4 ball mill.
1. Schematic diagram of a pushrod dilatometer.
The preferred means of investigating thermal
dimensional changes in test specimens is with a pushrod dilatometer. These
instruments are easy to handle and allow tracking of the length change in solid
specimens over a broad temperature range from approximately -180 to 2800°C.
With special sample holders, it is also possible to investigate pasty samples,
individual fibers or thin layers.
Test samples, typically cylindrical or block-shaped, are positioned in
the dilatometer’s horizontally oriented sample holder in such a way that one
end of the sample is in contact with an immobile barrier and the other end is
in contact with a mobile rod (see Figure 1). The forces acting on the sample
via the sample holder and pushrod are minimized so they do not influence the
length change behavior of the sample. A typical value for the contact pressure
is in the range of a few centinewtons (cN).
The far end of the pushrod (away from the sample) transfers the length
change of the sample to a measuring system that normally consists of a
so-called linear variable differential transformer (LVDT). In order to subject
the sample to a controlled temperature program, a furnace mounted on a
horizontal rail is positioned so that the sample is situated at the center of
the furnace in a homogeneous temperature zone.
Measurement software allows for the programming of complex temperature
programs with several heating and cooling segments, or even with isothermal
phases. The thermocouple on the sample serves not only to determine the exact
sample temperature, but also to control the furnace temperature by means of a
two-stage regulation process. This guarantees that the programmed temperature
profile also has an effect on the sample. Variations of the dilatometer
described above can allow for extending the temperature range, using pure inert
gas atmospheres at the sample, varying the sample geometry or simultaneously
investigating two different samples.
Measurements with the above described setup yield information on the
temperature-dependent length changes of the investigated samples. The linear
expansion of a material with temperature is known as the coefficient of thermal
expansion (CTE). Assuming that the sample is prepared and pretreated in such a
way that a homogeneous sample is obtained, and that the sample features
isotropic expansion behavior, it is possible to calculate the volume expansion
coefficient. This is especially true for samples with crystal structures of
2. Structural data of BaTiO3.
Barium Titanate Sintering Behavior
At temperatures below 120°C,
exhibits a tetragonally distorted
modification of the perovskite structure. A shift of the titanium ion from the
center of the oxygen octahedron results in a dipole moment of the unit cell and
the potential for polarizability. Barium titanate is a ferroelectric material
with pronounced hysteresis, which is why it is used as a PTC resistor
(thermistor). The structural phase transitions2
described in Figure 2 can be detected with the help of DSC, as shown in Figure
3. DSC results of the BaTiO3 phase
Samples were prepared in order to explore the influence of the
ceramic base material’s particle size on the sintering process, and
specifically on the sintering temperature of BaTiO3
The samples were calcined and milled in aqueous suspensions in a bead mill**
with an SiC coating and a Si3
agitator shaft to fractions with different particle sizes. The medians of the
particle size distributions were between 10 and 0.11 µm. From these
suspensions, pellets were made after drying and were then analyzed with a
** ZETA RS 4
NETZSCH DIL 402 C
4. Results of the BaTiO3 samples’ sintering
behavior with different particle sizes.
Figure 4 shows the results of the
investigation of the sintering behavior of three batches of
with particle diameters of 10, 1.1 and
0.11 µm (x50,3
). The beginning of the sintering process
is plotted as an extrapolated onset from the measuring data.
Table 1. Sintering results of BaTiO3 samples with different particle sizes.
The results for the sample with a median x50,3
0.11 µm show the beginning of sintering at 1108°C, which is 100 K below
the value for the sample with a median x50,3
10 µm. A summary of the detected sintering temperatures and particle sizes is
presented in Table 1.
5. Density change results during the sintering of three
BaTiO3 samples with different particle
Although identical pressing conditions were
used for the preparation of the test specimens for all samples, the green
density of the pellets of the samples with the smallest particle size was
considerably lower than that of the samples with larger particles. This lower
green density is compensated for during sintering with a considerably higher
sintering step. If one records not the length change but rather the density
change via temperature (see Figure 5), it becomes apparent that a very similar
final density is obtained for all samples after sintering.
6. Change of surface, volume and surface-to-volume ratio with decreasing
Taking into account that
very small particle sizes are necessary to have significant influence on the
melting temperature, 3
it must be noted that the sintering
behavior already changes if the particle sizes are in the micron range. Figure
6 shows a constant correlation of the surface-to-volume ratio with a reduction
of the particle size.
7. Change of the number of contacts between particles within a constant volume.
However, this would not sufficiently explain the significant
increase in sintering activity for small particles. It must be considered that
the number of contacts between the particles is decisive for the beginning of
the sintering process. As shown in Figure 7, the number of contacts between the
particles increases faster than the surface-to-volume ratio, which helps
explain the significant influence on sintering activity for the small particle
A direct relationship exists between a
material’s particle size and properties changes. The discussed example of
demonstrates the influence of
variations in particle size on the sintering behavior. It is not necessary to
thereby advance as far as the nanoscale particle size range. Often, significant
differences in the thermal behavior of materials can be detected even if the
average particle sizes are only in the micron or sub-micron range and
comparatively only small absolute differences are exhibited.
It is therefore all the more important to take
the particle size parameter into consideration during customized material
development. Changes in surface-to-volume ratio and the number of contacts are
decisive for many property changes, including sintering activity.
For additional information regarding dilatometry,
contact NETZSCH-Gerätebau GmbH, Wittelsbacherstrasse 42, 95100 Selb, Germany;
(49) 9287-8810; fax (49) 9287-881505; e-mail email@example.com; or visit www.netzsch.com.