Identify for Fast and Easy Classification of a Zero-Expansion Material

Michael Thelen and Dr. Gabriele Kaiser, Scientific & Technical Communication

One important aspect of quality assurance is the determination of specific material properties. This is done in order to guarantee consistency with regard to product quality. Material batches which do not correspond to the desired specifications can thus be identified and filtered out.

With the introduction of measuring methods and the Identify thermoanalytical database, the NETZSCH Proteus® software now features two tools allowing for fast and easy evaluation of routine measurements. Measurement methods ensure that the same measurement parameters are always being employed. In such a method, also evaluation steps can be integrated, which can always be carried out, automatically and in an identical manner, following a measurement. This way, one can quickly see whether the respective quality characteristics have been fulfilled [1]. In order to also be able to assess a measurement without prior evaluation, Identify offers the possibility of comparing a measuring curve with the references stored in the database. In the case of DIL or TMA curves, this curve comparison is independent of the evaluation steps carried out. Through previously set tolerances, Identify is even able to assess a sample with regard to its quality criteria. Thanks to the possibility of expanding the database with one’s own measurements, the database can be adjusted to individual requirements at any time [2, 3].

A characteristic value to be determined within the scope of quality control can be, for example, the mean coefficient of thermal expansion, m.CTE. It describes the length change of a material during temperature change within a certain temperature interval. The mean linear coefficient of thermal expansion can be determined with the help of a dilatometer or a thermomechanical analyzer (TMA).

Zero Expansion = Mean Coefficient of Thermal Expansion of 0 K-1

ZERODUR® is a lithium alumosilicate glass ceramic by Schott AG with an extremely low coefficient of thermal expansion. The low thermal expansion property is realized by virtue of the fact that, during heating, the substances added to the glass melt form seed crystals on which tiny crystals grow; these crystals contract when they are heated [4]. ZERODUR® is used in various high-tech applications such as high-precision measurement technology, astronomy (e.g., for telescope mirrors) or LCD lithography [5]. ZERODUR® DK1 (expansion class 1), which was employed for these measurements, has an m.CTE of 0 K-1 in the temperature interval between 0°C and 50°C. The specified tolerance is 0.0500·10-6 K-1. For the measurement of such small changes in length, a dilatometer with high sensitivity is required. The NETZSCH DIL 402 Expedis Supreme, which features a particularly high resolution, was therefore the ideal instrument for this task.

Identify – No Prior Evaluation Needed for Classification

For the dilatometer experiments, cylindrical samples with a length of 25 mm and a diameter of 6 mm were prepared. The measurements were carried out under a helium atmosphere (gas flow: 50 ml/min) in the temperature range from -100°C to 100°C at a heating rate of 2 K/min with probe contact pressure of 250 mN using a fused silica sample holder.

In the first step, a reference sample of ZERODUR® DK1 was measured and the mean coefficient of thermal expansion was determined in the interval from 0°C to 50°C ). Figure 1 shows the evaluation of the reference measurement. Based on this measurement, a method was then created which included the applied evaluation steps (method including evaluation). A method such as this – including evaluation – is capable of such tasks as automatic determination of the coefficient of thermal expansion and presentation of the graph in the predefined scaling following the end of the respective measurement without the operator’s intervention.

The Identify database was simultaneously expanded by the measurement of the reference sample. In order to use the entry for quality control, it must be assigned to a defined class. Such a class is comprised of all of the measurements assigned to it and can be annotated with tolerances.

All in all, different batches of ZERODUR® DK1 were investigated by means of both the previously described method including evaluation and a pure measurement method (with the same measurement parameters). The results of two measurements A and B, obtained with the method including evaluation, in the temperature range between 0°C and 50°C are depicted in figure 2. It can be seen at first glance that sample A (blue) with an m.CTE of 0.0569·10-6 K-1 is beyond the tolerance of 0.0500·10-6 K-1.

 

 

Fig 1. Evaluated measurement of the ZERODUR® DK1 reference sampleFig 1. Evaluated measurement of the ZERODUR® DK1 reference sample
Fig. 2. Measurement of two ZERODUR® DK1 samples (A and B) from different batches with corresponding evaluationFig. 2. Measurement of two ZERODUR® DK1 samples (A and B) from different batches with corresponding evaluation

Sample B (red), in contrast, has an m.CTE of 0.0078·10-6 K-1 and does fulfill the requirements.

Quality assessment of the samples investigated with the pure measurement method was carried out with the help of Identify. Manual evaluation was therefore not necessary. Figure 3 presents the result of the database comparison for a measurement which is beyond the permissible tolerance. This is clearly expressed by the automatic diagram label, “QC: FAIL!”. The blue curve here represents the measurement of the sample; the red curve, that of the stored reference. If the requirements are fulfilled, on the other hand, the label reads “QC: PASS!” (see figure 4). Also here, the blue curve corresponds to the sample measurement; the red curve to the reference.

Fig. 3. Measurement of a sample from a batch that does not fulfill quality requirementsFig. 3. Measurement of a sample from a batch that does not fulfill quality requirements
Fig. 4. Measurement of a sample that fulfills quality criteriaFig. 4. Measurement of a sample that fulfills quality criteria

Summary

The NETZSCH Proteus® software – along with the Identify thermoanalytical database and the methods – features two modules which are of great help in dayto- day routine work. The use of measuring methods ensures that measurement conditions are always identical, while Identify supports the operator in classifying the results. This allows even newcomers to thermal analysis to immediately carry out sample measurements and assess them in term of whether they fulfill quality criteria.

Literature

[1] E. Füglein, Application Note 083
[2] A. Schindler, „Automatic Evaluation and Identification of DSC Curves“, Plastics Engineering 2014
[3] A. Schindler, C. Strasser, S. Schmölzer, M. Bodek, R. Seniuta und X. Wang, „Database-Supported Thermal Analysis Involving Automatic Evaluation, Identification and Classification of Measurement Curves“, Journal of Thermal Analysis and Calorimetry, 2015
[4] https://en.wikipedia.org/wiki/Zerodur
[5] http://www.us.schott.com