Nuffer, Jürgen ; Riedel, Ralf (2011)
Reply to the “Comment on ‘Piezoresistive Effect in SiOC Ceramics for Integrated Pressure Sensors’ ”.
In: Journal of the American Ceramic Society, 94 (1)
doi: 10.1111/j.1551-2916.2010.04075.x
Artikel, Bibliographie

Kurzbeschreibung (Abstract)

As part of the authors of the original paper,1 we would like to take the opportunity to reply to the Comment.2 The basic argument in the Comment is that the experimental method of measuring the piezoresistive behavior of the SiOC specimen, as used in our study is flawed because we applied a 2-point measurement system instead of a 4-point technique to determine the electrical resistivity of our samples. Chung2 clearly describes why the latter method is considered to be superior to the analysis we used. Generally, we agree to this judgement and wish to thank the author of the Comment2 for the helpful statements. The main reason why the 2-point method was used is that our specimens are rather small in size (5 × 5 × 5 mm). The application of the 4-point method to these specimens would have needed a time-consuming experimental procedure, which in our opinion is not justified. The main purpose and message of our paper were the proof that piezoresistivity exists in SiOC. In our opinion, the 2-point test method is sufficient for this purpose, as there are several other indications showing that the analyzed change in electrical resistance with applied pressure is not seriously affected by side effects, like contact resistance as described by Chung.1 The most important indicator is that we also have investigated SiOC samples containing electrically conductive MoSi2 as filler material. In these samples, we obtained 0.5–1 Ω as the electrical resistance without mechanical load, using the identical setup and experimental procedure. This result clearly shows that the complete measurement procedure, including all contact resistances, will not exceed these values.

The 4-point method is required if it is expected that the contact resistance is in the same order as that of the piezoresistivity to be measured. However, in our case, we found differences in resistance of about 30 Ω between mechanically unloaded and fully loaded state (see Fig. 2 in the above mentioned paper). Thus, all additional electric resistances other than that related to piezoresistivity will, as stated above, reach a maximum value in the order of 1 Ω in the worst case, meaning an error of about 3–4%.

The gauge factor k=145, as reported in the original paper1 is also affected by this error and should attain lower values as stated by Chung.2 If we again assume an error of 1 Ω in our measurement, then the gauge factor is reduced to about k=130. This deviation is acceptable and does not significantly change the scientific conclusions derived from our studies, indicating that the gauge factor is by far high enough for technical exploitation of the piezoresistive effect. Therefore, the main message of our results can be summarized as that (a) the k-value is much higher than that for strain gauges, and (b) the piezoresistive effect of SiOC is considered to be strong enough to be exploited in future applications.

The most important result of our paper is the proof of a measurable piezoresistive effect in SiOC ceramics. This finding was mainly qualitative in nature, and for this purpose, our measurement technique can be accepted to be sufficient enough to prove the piezoresistive behavior of SiOC ceramics. For any future research, which elaborates more quantitative data for the piezoresistive behavior of polymer-derived ceramics, it should be considered, depending on the desired accuracy, to apply additional methods for the exact quantifcation of all unwanted additional resistances, for example, the 4-point method as suggested by Chung.2

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