A Precision Measurement System for Triboelectric Materials

Triboelectric charge separation occurs both in nature and everyday life whenever two materials come into contact. It is, since more than a decade, the basis of mechano-electric generators harvesting from triboelectrification, frequently called tribo-electric-(nano) generators (TENGs) [1]. However, research has shown that the underlying process of triboelectrification depends on a wide range of experimental conditions, such as contact force, contact formation and separation speed, contact geometry, as well as bulk morphology, surface morphology and (chemical) surface properties of the materials involved [2]. The measurement environment, temperature, humidity and pressure influence the outcome as well. Due to the complexity and multitude of parameters to be considered in the design and optimization of TENGs, the documentation of measurement conditions for their tests is urgently required but often incomplete or not given at all in literature. This is mainly valid for common and simple testing practices like “finger tapping tests” [3] which obviously miss a standardized form, which in turn makes it challenging to compare these TENGs with others characterized with a different test. A commonly used tool to characterize triboelectric materials as such is the so-called triboelectric series (TES), which assigns different materials a positive (electron-donating) or negative (electron-accepting) character. However, the complex interplay of material characteristics, environmental and measurement conditions has resulted in the publication of several differently ordered TES, presumably as a result of different measurement conditions [4] [5]. To improve the comparability between different TENG designs and to make triboelectric materials and surface morphologies comparable, a standardized method for the characterization of TENGs and triboelectric materials is urgently required. To address this goal, we have developed a measurement system with improved control of the most important experimental conditions. This system, schematically shown in Figure 2, is designed for contact-separation mode measurements with a precise control of contact geometry, contact force and contact formation/separation speed at ambient pressure, and with a documentation of humidity and temperature. It uses force sensors embedded in a stationary platform carrying one material of a triboelectric material pair, to measure the contact-separation force in real time. Micrometer screws are used in combination with the force sensor signals to preadjust this stage parallel to a movable stage carrying the second material. This allows for an even contact force distribution between both as essential pre-requisite for a reproducible measurement. Both material samples are mounted onto standardized contact electrodes and are held in place by vacuum suction holders. The movable stage is operated with a spindle-driven linear stage, thus bringing both materials into repetitive contact while recording the output voltage or short-circuit current obtained through triboelectrification. To demonstrate system reproducibility and reliability, several material combinations were tested and characterized with respect to different measurement conditions (Figure 6). The investigated materials include metals (e.g. copper (Cu) and gold (Au)) and polymers (e.g., polyethylene (PE), polypropylene (PP) and Kapton (KP/polyimide). This allows an insight into influences of the different parameters on the triboelectric behavior of these materials and shows the advantages of a standardized condition-based comparison between different contact-separation tests.