Conductivity Gradient Carbon Materials Enable Dynamic Screening of Sharp Temperature Differences in Thin Films

Carbon materials offer a diverse range of microstructures, along with outstanding electrical and thermal properties, while also being cost-effective and widely accessible.^[1] These materials can be produced through high-temperature carbothermal processes starting from biopolymers like cellulose. By using catalytically active compounds, such as iron salts, the required pyrolysis temperature can be lowered. These catalytic substances influence the carbon crystal structure during graphitization, allowing for varying degrees of structural order by adjusting the concentration of iron salts.^[2]<br/>We utilized an infusion withdrawal impregnation method to create an iron salt concentration gradient and, therefore, a monotonic increase in structural order along the sample. We verified the structural change with wide and small angle X-ray scattering and Raman spectroscopy. Consequently, we observe a similar change in the closely related electrical and thermal conduction properties, measured via four-point probe technique and lock-In thermography along the sample. Considering these insights, applying an electrical current to the material leads to a constant temperature gradient measured with an infrared camera. We achieve a maximum temperature difference of 80 K on the centimenter-scaled sample within seconds. Furthermore, finite element simulation verifies our experimental observation. We also show that the evolving thermal gradient can be transferred to another thin film material such as colloidal crystals. This opens the door towards characterization of temperature dependent structures and their kinetic evolution.^[3] Since the temperature gradient evolves instantaneously, switching between the hot and cold state is simple, leading to possible applications such as continuous flow polymerase chain reaction^[4], microfluidic^[5], and battery research.^[6]<br/><br/>^[1] Z.-Y. Wu <i>et al.</i>, <i>Sci. Adv.</i> <b>2018</b>, <i>4</i>, eaat0788; A. D. Avery, et al., <i>Nat. Energy</i> <b>2016</b>, <i>1</i>, 1–9; Y. Cohen <i>et al.</i>, <i>Nanotechnology </i><b>2022</b>, <i>33</i>, 345703.<br/>^[2] M. M. Titirici, M. Antonietti, <i>Chem. Soc. Rev</i>. <b>2010</b>, <i>39</i>, 103-116; M. Sevilla, A. B. Fuertes, <i>Carbon </i><b>2006</b>, <i>44</i>, 468-474.<br/>^[3] J. S. O. Evans, I. Radosavljeć Evans, <i>Chem. Soc. Rev.</i> <b>2004</b>, <i>33</i>, 539; D. O’Nolan <i>et al.</i>, <i>J. Appl. Crystallogr.</i> <b>2020</b>, <i>53</i>, 662–670.<br/>^[4] N. Crews <i>et al.</i>, <i>Biomed. Microdevices</i> <b>2007</b>, <i>10</i>, 187–195. Z. E. Jeroish <i>et al.</i>, <i>Biomed. Microdevices</i> <b>2021</b>, <i>24</i>.<br/>^[5] A. A. Dos-Reis-Delgado <i>et al.</i>, <i>ELECTROPHORESIS</i> <b>2022</b>, <i>44</i>, 268–297; S. M. Shameli <i>et al.</i>, <i>Anal. Chem.</i> <b>2012</b>, <i>84</i>, 2968–2973.<br/>^[6] R. Carter, <i>et al.</i>, <i>Cell Rep. Phys. Sci.</i> <b>2021</b>, <i>2</i>, 100351; M. Naylor Marlow, <i>et al.</i>, <i>Comm. Eng.</i> <b>2024</b>, <i>3</i>.