PRODUCT PROFILE: High-Resolution Laser Dilatometers

A new laser dilatometer offers resolution in the picometer range, providing extremely high accuracy for sensitive thermal analysis applications.

Dilatometers based on the push-rod principle of measurement-such as the Linseis L75 developed by Max Linseis, Ph.D., in the early 1950s-have been successfully used in the ceramic and other heat-related industries since 1957. With these instruments, the sample (typically 20 mm long and 7 mm in diameter) is placed in a measuring tube, and the coefficient of thermal expansion (CTE) signal is transmitted through a push rod to a linear variable differential transformer (LVDT) sensor. This sensor theoretically has an indefinite resolution; however, when used in conjunction with the other system components and associated electronics, a maximum resolution of 10 nanometers (nm) can typically be obtained.

The push rod, which transmits the expansion signal from the sample to the sensor, covers a temperature gradient of up to 1600°C from end to end; some models can even go as high as 2400°C. With such high temperature differentials on such short sample dimensions, excellent thermal conditioning of the push rod/measuring system is required to ensure accurate, reproducible results. If the push rod experiences a different thermal history from run to run, the results will not be reproducible. Other test conditions-such as the room temperature, start temperature, heat-up speed, sample atmosphere, etc.-must also remain identical from run to run to guarantee the best results.

These problems can be overcome by using a dual push-rod dilatometer, which analyzes the sample against a known reference. The measurement is always done simultaneously, and any potential errors can be eliminated by subtracting the measured results of the sample from the results of the reference. This enables the measurement of extremely small expansions-such as the expansion of quartz (e.g., cordierite), which is on the order of 10^E-8. However, for many emerging high-tech applications, even higher resolutions and accuracies are required. Additionally, the calibrations required in dual push-rod models can be difficult and time-consuming, making the technique impractical for many industrial applications.

Recently, a new dilatometer has been developed to overcome these challenges. Called the Laser Dilatometer Pico - Series 0.3 nm, the new technology can help manufacturers of advanced ceramics and glass ensure the highest possible levels of quality control with a minimal amount of time and hassle.

Optical Analysis

The new dilatometer is based on the Michelson Interferometer principle, which was first developed by A.A. Michelson in 1881. The Michelson interferometer is an optical device that splits an incident beam of light into two parts. The optical path length for one of the beams is changed by means of a movable mirror. The beams are then recombined, and the resulting beam intensity variations are measured by a detector.¹ In the new dilatometer, a modern, high-accuracy integrated dual-beam interferometer with the latest electronics are used to measure the resulting interferences and provide precise CTE data (see Figure 1).

As the name of the instrument indicates, its resolution is in the picometer range rather than being limited to nanometers (0.3 nm = 300 picometers). As a result, resolutions can be obtained that are up to a factor of 33.33 higher than the resolutions provided by conventional push-rod dilatometers. This is beneficial in applications where extremely small expansions take place that are difficult to measure with conventional push-rod instruments.

Because the new dilatometer is based on the interference measurement technique and uses modern computer calibrations, it also provides much higher accuracies. With conventional push-rod dilatometers, absolute accuracies of 1% are normal, with the best accuracies up to 100 nm. The new method allows accuracies of around 0.25% up to 30 nm. Additionally, unlike conventional systems that require infinite adjustments and/or calibration with a reference sample to ensure repeatable analyses, the laser dilatometer relies on an absolute measurement principle. As a result, the accuracy and repeatability of the measurements are not affected by external test conditions.

New Opportunities

In high-tech applications, quality control is paramount. With this highly precise measurement technique, ceramic and glass manufacturers can analyze even products with extremely small CTEs-such as cordierite components used in catalytic converters, astrophysical glass lenses used in telescopes (e.g., the Hubble telescope), or even nanostructures-with absolute accuracy and repeatability.

For more information about the new laser dilatometer, contact Linseis Inc., 20 Washington Rd., P.O. Box 666, Princeton Junction, NJ 08550; (609) 799-6282; fax (609) 799-7739; e-mail info@linseis.com ; or visit http://www.linseis.com .