Printable and Flexible Heater with High Electric-to-Thermal Conversion Efficiency by Force and Temperature Field Controlling

Flexible electrothermal heaters have received increasing attention due to their potential in advanced thermal management and smart wearables. However, the preparation of a heater with high electric-to-thermal conversion efficiency (η) remains a challenge. Here, a paper-based flexible electrothermal heater with a high η level was fabricated by screen printing a water-based conductive ink, composed of graphene nanoplates (GNPs), carbon black (CB), and an Ag nanowire (AgNW), on paper. It is found that its η data can be gradually improved by controlling the force and temperature field during the processing protocol. The shearing force field during screen printing promoted the horizontal alignment of the GNPs. The vertical force field under press treatment at room temperature induced interlayer densification, reduced void spaces, and enhanced the interface contacts of CB and GNP. The temperature field facilitated the junction fusion of AgNW under hot pressing, thus enhancing the electron transport efficiency. The maximum temperature of the heater reached 242.5 °C under a relatively low input voltage of 6 V with a rapid heating rate of 55.7 °C·s^-1. Its η value finally increased to 359 °C·cm²·W^-1. Furthermore, fire warnings and information encryption and decryption were realized by integrating the flexible heater with a thermochromic ink layer, which may find application in versatile fields, such as smart wearables, emergency rescues, information security, and so on.