# Development and Characterization of a DC-Driven Thermal Oscillator Using Acrylate-Based Composites
## 1 Introduction  
Studies on conductive polymer composites (CPCs) have received significant attention because of these composites' potential applications in various industries [1; 2; 3; 4; 5]. In general, CPCs comprise single or multiple conductive fillers (e.g., graphene [6], carbon nanotubes [7], metal powders [8], silver nanoparticles [9], and separated conductive polymers [10]) dispersed in a polymer matrix. The positive temperature coefficient (PTC) effect, which is a phenomenon found in many composite conductive polymers, was first observed in the carbon black-filled low-density polyethylene composite by [PERSON] [11] in 1945. The resistivity of materials with PTC effects exhibits order-of-magnitude variations over a temperature change of a few degrees. Many theories that describe the microscopic behaviors giving rise to the PTC effect have been reported. One of the theories considered that the PTC effect is caused by the fact that the polymer matrix expands more than the conductive fillers as the temperature increases, and, subsequently, the compressive force on conductive fillers decreases. As the temperature increases to a certain point, a very sharp increase in the resistivity of the polymer is observed [12]. Additionally, [PERSON] et al. speculated that the distribution of inter-grain gaps among graphite particles is comparatively uniform when the temperature is relatively low. As the temperature increases, the inter-grain gaps become random, which results in a sharp increase in resistivity and gives rise to the PTC effect [13]. In addition, the mechanism of the PTC effect could be attributed to the thermal expansion of the crystalline polymer during melting, which leads to the breakdown of the conductive network of fillers [14]. The above-mentioned theories have addressed various viewpoints regarding the mechanisms of the PTC effect, but the actual nature of the PTC effect has not yet been determined [15; 16]. Nevertheless, it is generally accepted that the thermal expansion of the polymer matrix near the melting temperature gives rise to the rapid resistivity increase when the temperature increases. Studies on PTC materials have attracted attention during the past decades because PTC materials can be easily employed as temperature-sensing elements. A self-adjusting heating device fabricated by connecting different types of PTC materials in series was proposed [17]. Additionally, [PERSON] et al. presented reusable current limiters realized by using materials with the PTC effect. The proposed devices limit electric current by increasing their resistance when the current exceeds the designated values [18]. Printable thermal sensors based on acrylate polymers and graphite were reported. The sensors exhibit large resistance changes near body temperature under physiological conditions with high repeatability and short response times [19]. In addition, gas sensors were realized by employing conductive carbon-black particles, of which the surface was successfully modified by grafting PE-b-PEO using a two-step method. The electric resistance of the composite increased by several orders of magnitude in several solvent vapor samples [20]. Furthermore, PTC material was used as the additive on the electrodes of a lithium-ion battery for cutting off charge transportation paths in the vicinity of internal short sites before the local temperature of the battery reaches the onset of thermal runaway [21]. These aforementioned studies demonstrated various applications of PTC materials. However, to the best of our knowledge, the oscillating behaviors induced by the temperature-sensitive resistance variation of PTC materials have not yet been reported. In this work, we present a PTC-based electrothermal oscillator by employing an ultra-sensitive temperature sensor for self-switching a microheater on and off with a DC power supply. The proposed device consists of a miniaturized ultra-sensitive temperature sensor and a microheater.