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Dilatometers are valuable tools in the investigation of ceramics, particularly when measuring the dimensional changes that occur upon sintering.
A dilatometer is a scientific instrument that measures the length change of a material as a function of change in temperature. Dilatometers are valuable tools in the investigation of ceramics, particularly when measuring the dimensional changes that occur upon sintering.
Modern dilatometers are available in both horizontal and vertical configurations, as well as newer non-contact technology that uses optical measurement. The success and accuracy of the dilatometric measurement of a ceramic material greatly depends on choosing the proper instrumental configuration. Substantial differences exist between the various configurations, each being better-suited for a particular measurement and application.
Horizontal dilatometers are popular and commonly used for the accurate evaluation of the coefficient of linear thermal expansion. The horizontal furnace arrangement ensures outstanding temperature accuracy and the absence of convection currents.
The horizontal dilatometer is usually comprised of a cantilevered horizontal tube protruding into a horizontal furnace cavity (see Figure 1). A pushrod is positioned at the center of and parallel to the long axis of the tube. The pushrod is affixed to a high-sensitivity linear displacement transducer. The sample (usually a cylindrical or rectangular slab) rests on a cradle or holder, which then rests on the bottom inside face of the furnace tube. Most frequently, the top half of the tube is cut away to facilitate sample exchange.
The end of the tube may be sealed permanently, either by fusing the tube to the base or by a mechanical connection. The sample is positioned against the sealed end of the tube and is held in place by a force maintained on the pushrod from the opposite end. The force applied through the pushrod must be sufficient to maintain complete contact between the stationary tube face, the sample and the pushrod.
For a mechanically secured tube, this contact pressure must also be sufficient to keep the mechanical connection secure. This often requires the application of a considerable force, which is inconsequential when measuring most rigid solids. However, the need to apply this contact force is a limitation when measuring ceramic samples that soften, shrink or sinter, as these will contract rather than expand when heated.
If the pushrod force is low (to prevent indentation/deformation of the sample), it may be insufficient to overcome the friction associated with large dimension changes, leading the sample to lose contact with the stationary end plate. As the sample shrinks, a gap may develop between it and the plate, which will severely compromise measurement accuracy. Data collected in such errant measurements may look believable and smooth, but will greatly underestimate the sample contraction during sintering.
Another commonly encountered issue with the horizontal dilatometer design is the sagging of the alumina pushrod and sample holder tube at high temperatures (> 1500°C), which is caused by slow creep of the cantilevered section under the weight of the measuring system and sample. While the process is very slow and has no detrimental short-term effects (no single test will be affected), the measuring system will eventually become mechanically distorted and will require replacement. Alternative tube and pushrod materials such graphite are available and do not sag, but graphite has its own set of atmospheric operational limitations, making it impractical for many ceramic studies.
For these reasons, horizontal systems are ideal for coefficient of thermal expansion measurements below 1500°C, but are not recommended for high-temperature ceramic shrinkage or sintering studies.
An alternative to the horizontal pushrod dilatometer is a vertical pushrod dilatometer. As illustrated in Figure 2, this usually consists of a tube furnace or a pot furnace in which the sample and pushrod dilatometer are inserted vertically. Gravity acts along the long axis of the tube, making this design much more resistant to temperature-induced sagging. In addition, vertical creep of the alumina tube is generally not a factor up to 1700°C.
A major advantage of vertical dilatometers is realized in ceramic applications when the sintering or softening of samples are studied. The specific issues noted for the horizontal device are addressed by design, as the sample is held against the bottom plate by gravity, ensuring reliable contact throughout the test.
Vertical designs also afford long pushrod travel, which is critical for ceramic processes that undergo large dimensional changes; 10-30% shrinkage is not unusual. Vertical dilatometers employ linear bearings to hold the pushrod in line and a static counterbalance by weights to reduce the tip pressure of the pushrod on the sample. Finally, because the sample stands up on one end, loading stability and tracking are often better in a vertical device.
By design, vertical furnace cavities are more subject to convection and, as a result, may have larger temperature gradients than horizontal models. This is mitigated by reducing sample length and through the use of multiple thermocouples for accurate temperature measurement.
For these reasons, a vertical dilatometer design is favored when studying processes that involve substantial contraction (such as ceramic sintering).
Non-Contact Optical Dilatometers
An alternative to both pushrod dilatometer designs is an optical dilatometer, which measures a sample’s length change without the need for mechanical contact with the sample. As a result, these instruments are ideally suitable for the measurement of challenging samples, such as thin materials, plastic components, or ceramic samples that may contract significantly during the sintering process.
In one non-contact optical dilatometer,* a high-performance GaN-LED emits a planar light beam on to the sample via a diffusion unit and a collimator lens (see Figure 3, p. 30). The sample shadow is registered by a high-resolution CCD sensor in the receiver via a filter and a lens system, and then evaluated by a digital edge-detection processor. This principle is also known as the “shadowed-light method.”
The sample is located in the center of the disc-shaped furnace on a platform, and no forces are exerted on the sample to make the measurement. This design makes it possible to measure thin samples, plastic and/or easily deformed materials, and even the solid-liquid-solid phase. The sample does not need to be positioned precisely on the platform; it only needs to be located in the light beam emitted by the optical measuring device. The initial length is automatically determined and saved, in order to be available later for calculation of the linear thermal expansion coefficient.
Since this optical measurement method is an absolute process, it is not necessary to correct the measured results to obtain high accuracy and resolution on ceramic samples (see Figure 4). The optical dilatometer is also free from the limitations of the traditional pushrod designs. As there is no mechanical transducer, pushrods and the associated temperature/contact issues are eliminated.
In addition, the furnace of the optical dilatometer has a disc-shaped heating element above and below the sample. This configuration provides an absolutely homogeneous temperature profile within the sample chamber. The upper disc furnace is completely retracted while open, making it simple to place the sample inside, and also supports the use of sampling automation. Heating rates up to 100K/min and rapid cooling from 1400°C to 50°C in 10 minutes enable tests to be carried out very quickly for optimized productivity.
*The TA Instruments DIL 806 optical dilatometer
When considering a dilatometer for the measurement of processes associated with ceramics, it is important to choose a configuration that provides the best measurement accuracy, temperature uniformity and ease of use. In general, vertical dilatometers are favored over horizontal configurations due to their ability to maintain contact with the ceramic sample even during regions of significant contraction.
The optical dilatometer is ideal for ceramic sintering studies. This configuration is not affected by temperature/materials limitations, measurement accuracy is not compromised during contraction of the sample, and the thermal environment is consistent and well-controlled.
For more information, contact TA Instruments-Waters, LLC at 159 Lukens Dr., New Castle, DE 19720; call (302) 427-4000; email email@example.com; or visit www.tainstruments.com.
The substantial contributions of Peter S. Gaal and Heinz Bähr are gratefully acknowledged.
To find suppliers of thermal analysis equipment, visit http://directories.ceramicindustry.com.