Electrometer for Triboelectric Evaluation of Materials

Some materials should be distinguishable by their triboelectric responses.

An electrometer developed for measuring the triboelectric responses of soils on Mars is also potentially useful on Earth for identifying some materials via their triboelectric responses. In operation, an array of triboelectric sensors is rubbed against the material of interest for a predetermined distance, then withdrawn from the material of interest at a predetermined speed. During this operation, the electrometer circuitry measures the time-varying sensor output voltages, which are proportional to the electric charges induced on the sensors by the rubbing. The resulting voltage-vs.-time data constitute the desired triboelectric-response data. If materials that one seeks to identify or to distinguish from each other exhibit unique, known triboelectric responses that have been catalogued, then a sample of unknown material can be identified by seeking a match between its triboelectric response and one of the catalogued responses.

Figure 1 is a simplified schematic depiction of one of the triboelectric sensors connected to a charge-sensing circuit (charge-to-voltage converter). This converter is a conventional electrometer circuit that includes a capacitor (C₁) and an operational amplifier in a follower configuration. The triboelectric sensor is an electrode covered with a layer of insulating material. The outer surface of the insulating material is what is rubbed against the material of interest.

The capacitance between the outer surface of the insulator and the electrode is C₂. The voltage associated with the triboelectric charge that accu-mulates on the rubbed insulator surface is coupled, via this capacitance, to the C₁/amplifier node. Initially during rubbing, the switch shown in Figure 1 is kept closed to prevent C₁ from charging, thereby constraining the electrometer input and output voltages to be zero. At a designated point during rubbing, the switch is opened, allowing C₁ to charge and the electrometer output voltage to depart from zero.

The triboelectric response — that is, the electrometer output voltage as a function of time — depends on several factors, including the following:

• The triboelectrically induced charge (Qi) present at the instant of liftoff following rubbing;
• C₁, C₂, and amplifier-circuit resistances;
• The speed of withdrawal;
• The time-varying capacitance C₃ between the rubbed insulator and sample surfaces; and
• The electrical resistivities of the insulator and sample materials.

Figure 2 shows triboelectric responses from an experiment in which five sensors made with five different insulating materials were rubbed against wool felt.

Electrometers based on this concept might be useful in industrial settings for inspecting and identifying incoming materials; for example, to identify fabrics coming into a clothing factory. Another potential use could be distinguishing between contaminated and uncontaminated soils. Another application might be evaluating moisture contents of soils; this should be possible because the rate of decay of triboelectric charge depends on humidity. 

This work was done by Martin Buehler and Raymond Gompf of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Intellectual Property group
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240

Refer to NPO-20684, volume and number of this NASA Tech Briefs issue, and the page number.