Universal Control on Pyroresistive Behavior of Flexible Self‐Regulating Heating Devices

Abstract

Smart heating devices with reliable self-regulating performances and high efficiency, combined with additional properties like mechanical flexibility, are of particular interest in healthcare, soft robotics, and smart buildings. Unfortunately, the development of smart heaters necessitates managing normally conflicting requirements such as good self-regulating capabilities and efficient Joule heating performances. Here, a simple and universal materials design strategy based on a series connection of different conductive polymer composites (CPC) is shown to provide unique control over the pyroresistive properties. Hooke's and Kirchhoff 's laws of electrical circuits can simply predict the overall pyroresistive behavior of devices connected in series and/ or parallel configurations, hence providing design guidelines. An efficient and mechanically flexible Joule heating device is hence designed and created. The heater is characterized by a zero temperature coefficient of resistance below the self-regulating temperature, immediately followed by a large and sharp positive temperature coefficient (PTC) behavior with a PTC intensity of around 10 6 . Flexibility and toughness is provided by the selected elastomeric thermoplastic polyurethane (TPU) matrix as well as the device design. The universality of the approach is demonstrated by using different polymer matrices and conductive fillers for which repeatable results are consistently obtained.the merits of plastics and conductive fillers. [1] Their capability of detecting and responding to external stimuli has offered a range of promising applications by performing both sensing (damage, [2] strain, [3] humidity, [4] vapor, [5] degradation, [6] and current-limiting), [7] and actuating (artificial muscles, [8] and electroactive shape memory) [9] functions. In particular, CPCs are practical choices for pyroresistive applications like self-regulating heaters, temperature sensors, and safe battery switches, due to their capability of changing electrical resistivity upon heating. [10] Since their introduction in the late 1970s, self-regulating heating devices have been widely adopted in applications like domestic heaters and deicing units. These smart heaters can operate at a nearly constant temperature over a broad range of voltage and dissipative conditions by utilizing the positive temperature coefficient (PTC) effect. The PTC effect, where the electrical resistivity increases with increasing operating temperature, is the main property that enables self-regulating heating. [11] The PTC phenomenon is generally explained by the mismatch of thermal expansion between the polymer matrix and the filler. [12] When the temperature increases and exceeds a certain threshold, the continuous network formed by the conductive fillers is disrupted. Due to expansion of the polymer matrix, the electrical conductivity and the current passing through the sample are reduced, regulating the temperature within a desired range. [13] Great efforts have been made to design PTC materials that p...

Pyroresistivity in conductive polymer composites: a perspective on recent advances and new applications

“…(a) Resistance increases dramatically in PDMS/CB composites, allowing less current to flow; (b) secondary thermoplastic elastomeric (TPE) phase filled HDPE/GNP composites with the temperature stabilized at the self‐regulating temperature around 120 °C, and enhanced flexibility; (c) and (d) a simple and versatile sandwich‐based approach that tackles typical design compromises in smart self‐regulating heating devices by connecting different CPCs in series with distinct pyroresistive behaviours. The Ag‐coated glass sphere filled TPU composite acts as the switching unit and TPU/CNT composites at both ends behave as the heating units, leading to heating devices with significantly improved flexibility with preserved heating properties . (Reproduced with permission.…”

“…Applications of pyroresistive CPCs, ranging from temperature sensors, safe batteries, resettable fuses, to self-regulating heating devices. [18][19][20][21][22][23][24][25][26] (Reproduced with permissions.) good repeatability; (ii) suitable PTC intensity; (iii) PTC switching temperature; and (iv) elimination of an NTC effect.…”

“…The Ag-coated glass sphere filled TPU composite acts as the switching unit and TPU/CNT composites at both ends behave as the heating units, leading to heating devices with significantly improved flexibility with preserved heating properties. 19,20,63 (Reproduced with permission.) heating properties, is by adding a secondary thermoplastic elastomeric (TPE) phase to an HDPE/graphene nanoplatelet (GNP) composite.…”

“…63 Our group recently presented a versatile sandwich-based design approach by connecting different CPCs with distinct pyroresistive behaviours in series, achieving for the first time a high PTC intensity combined with excellent Joule heating as well as mechanical flexibility (Figs 4(c) and 4(d)). 20 Here, the Ag-coated glass sphere filled TPU composite acted as the switching unit, while a TPU/CNT composite at both ends of the sandwich device acted as the heating units. The use in this case of TPU as the polymer matrix, combined with the series configuration and the fillers selected, significantly improved the flexibility of such a heating device, as shown in Fig.…”

“…Applications of pyroresistive CPCs, ranging from temperature sensors, safe batteries, resettable fuses, to self‐regulating heating devices . (Reproduced with permissions.…”