SPECIAL REPORT/INSTRUMENTATION: Exploring Sintering Behavior through Dilatometry

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 analysis,1 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 NETZSCH ZETA® RS 4 ball mill.

Figure 1. Schematic diagram of a pushrod dilatometer.

Length Change

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 cubic symmetry.

Figure 2. Structural data of BaTiO3.

Barium Titanate Sintering Behavior

At temperatures below 120°C, BaTiO3 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.

Figure 3. DSC results of the BaTiO3 phase transitions.

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 Si3N4 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 dilatometer.†

** ZETA RS 4

Figure 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 BaTiO3 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 of 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 of 10 µm. A summary of the detected sintering temperatures and particle sizes is presented in Table 1.

Figure 5. Density change results during the sintering of three BaTiO3 samples with different particle diameters.

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.

Figure 6. Change of surface, volume and surface-to-volume ratio with decreasing particle diameter.

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.

Figure 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 size variation.

Material Development

A direct relationship exists between a material’s particle size and properties changes. The discussed example of BaTiO3 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 at@netzsch.com; or visit www.netzsch.com.

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