Design and Fabrication of Thermoelectric, Self-sensing Sandwich Composites

Abstract

Sandwich composites, owing to their weight to performance ratio and structural stability, are used in many applications, including sports equipment, automotive, and aerospace. The structural stability arises from the sandwich panel structure, which consists of three layers and two thin composites (skins) of a stiff and strong material, separated by a thick core of low-density material. These sandwich structures have optimized specific flexural stiffness, as the separation of the two skins by a low-density core increases the moment of inertia, and thus, contributes to the flexural stiffness of the panel, with only a small increase in the mass of the panel. Sandwich composites are often used in structures because of these advantages; however, they pose a high risk of damage. To prevent damage, installing sensors for structural loads is necessary. The most commonly used detection method is to insert an optical fiber into the skin part of a composite structure and measure the refraction of light passing through the optical fiber or apply a resistance that the strain gauge sensor pulls and changes. However, in these instances, problems arise; for instance, hampered mechanical properties of the skin, spatial restrictions, requirement of an additional energy source, or difficulty in ensuring continuous use. To solve this problem, a self-powered sensor and structure module are required. First, regarding the self-powered sensor, a thermoelectric, piezoelectric, or triboelectric device can be used. For structures, continuous power generation is difficult, owing to repeated bending or friction. Meanwhile, since the sandwich composite material used as a structure is applied to differentiate between the inside and the outside, a temperature gradient occurs. In this case, the thermoelectric system is advantageous because it can continuously generate power via a temperature gradient. To apply such self-sensing sandwich composites, the following research objectives are proposed: (1) develop a carbon nanotube (CNT)/polyaide6 (PA6) composite n-type thermoelectric device with a segregated network formed by glass bubbles (GBs); (2) develop a p-type thermoelectric device with drawn chopped carbon fiber (CCF)/high-density polyethylene (HDPE); (3) apply a circuit process that connects the sensor and thermoelectric sensor using the skin of the sandwich composite material; (4) recycle PET-only composite sandwiches with thermal compression for extracted PET and recompressed double-depth composite skins. To develop a thermoelectric device with a composite material, the internal structure of the composite material was designed. First, a segregated network was used to compress nanomaterials and create a path for electrons to flow in an internal structural design that is advantageous for thermoelectric devices. Second, the orientation of fibers was controlled by extruding a composite material containing short fibers and adjusting the draw ratio. Thus, a structure that is advantageous for thermoelectric devices was designed. A method to visualize the change in resistance due to piezo-resistivity upon application of pressure to the sandwich is proposed, involving the placing of a sensor on the skin of the composite material. Mechanical properties of the sensor were enhanced by functionalizing the nanomaterial of the composite material to be used as a sensor, and the resistance was measured with a wire attached to the sensor and corrected by a distance-based calculation method to visualize the pressure applied to each position. Thermoelectric elements serving as a power source were arranged at the intersections to construct the core of the composite material and the circuit for driving the sensor. The circuit was composed of a DC–DC boost converter that increases the voltage, a low-power IC chip that supplies a constant power through rectification, and an analog ampere meter (μA) that evaluates the behavior of the sensor. This dissertation presents a method for manufacturing a sensing sandwich composite by creating a structure, and specifically, for mechanically mass-producing engineering polymers and carbon nanomaterials for n-type and p-type composite materials. The pressure was visualized by using a sensor that accounts for the piezo-resistance of a functionalized nanomaterial serving as the skin of a sandwich composite. It was experimentally shown that a circuit could be driven through the connection of a manufactured thermoelectric element and a sensor.