F. I. Mopsik and S. Roth

Objectives

Develop sample and electrode configurations and sample holders to enable accurate determination of dielectric constant and loss of polymer materials, many of which can be obtained only in the form of thin films. Integrate output of high frequency instrumentation, precision LCR (Inductance, Capacitance, and Resistance) meters and impedance analyzers with that of the Dielectric Spectrometer used for lower frequencies so that data can be merged, assessed and analyzed.

Accomplishments

The LCR meters have been fully integrated for both collection and control into the computer system that controls and analyzes the Dielectric Spectrometer. This allows the data up to 30 MHz to be collected in an automated system and stored in a format that is compatible with the lower frequency data. The analysis programs have been updated to read and combine all the sets of the data in a consistent manner so that the data ranges between the different systems can be consistently merged with optimal matching points to remove minor differences in instrumental calibration.

The thin film sample holder has been modified and tested for use with the LCR meters so that the same film sample can be used for measurements from 10^-4 to 3x10⁷ Hz. Limits of accuracy were shown in the data above 5 MHz that are set by the LCR meter configuration which were consistent with limits stated by the manufacturer. Such uncertainties prevented determination of the sample loss at the higher frequencies. Furthermore, it was determined that these uncertainties can only fortuitously be calibrated out since irreproducible uncertainties prevented any further calibration.

The losses above 5 MHz were determined to be due to the attempt to use a four-terminal configuration, the nominal LCR meter configuration, at line lengths long compared to the wavelengths involved. An initial attempt was made to refer the measurement back to the instrument front panel terminals by using a four-terminal to coaxial adaptor and placing the sample at the end of a precision coaxial line. The LCR meter then was used to measure the values of capacitance and loss at a reference plane at the entry to the coaxial line and the measurement was propagated to the position of the sample. The results showed that the measurements were much more stable than using the full four-terminal method and had only small uncertainties that were capable of further analysis.

This work shows that using a lead- independent measurement with one sample holder will not be possible at frequencies much above 5 MHz. However, with further work, a second, coaxial holder using a similar, if not the same sample, should be usable to about 1 GHz once the impedance analyzer is brought on line. Only two sample configurations will be needed to cover a frequency range of 13 decades on a thin film.

Outputs

Publications
B.J. Factor, F. I. Mopsik and C.C. Han , "Dielectric Behavior of a Polycarbonate/Polyester Mixture upon Transesterification" Macromolecules, in press.