Visual measurement of the microscopic temperature of 3D porous graphene
Visual system for microscopic temperature measurement of porous graphene based on CLCMs. The inset shows the distribution of CLCMs on the porous graphene.
Since graphene was first exfoliated from graphite, it has been extensively used in diverse applications, including energy storage, single-molecule gas sensors, and photovoltaic cells owing to its unique and superior electrical, thermal, mechanical, optical, and magnetic properties. Three-dimensional (3D) porous graphene is a new type of carbon nano-material composing of two-dimensional (2D) graphene on a macroscopic scale. It not only inherits the excellent properties of graphene, but also has high specific surface area, high porosity, excellent compressibility, and an interconnected conductive network owing to its special 3D micro-nano structure. These properties make it attractive for applications such as flexible electronic equipment, thermal engineering, and catalysis loading.
With the miniaturization and integration of devices, it is important to investigate the microscopic thermal properties to improve their performance. Conventional temperature measurement methods, such as thermocouples, thermistors, and infrared thermometers, cannot accurately distinguish the microscopic temperature distribution with a high spatial resolution and high temperature sensitivity. Different new approaches have been proposed to develop ultra-small thermal sensors for microscopic temperature measurement. These approaches include temperature sensors based on carbon nanotubes, optical fiber based, atomic force microscope, and magnetic nanoparticles. Although these methods provide a high spatial resolution, they are expensive, complicated, and not intuitive enough, making them unsuitable for practical applications, particularly for 3D porous graphene. Accurately measuring the microscopic temperature of graphene is still a challenge.
The research group from Nanjing University of Posts and Telecommunications and Nanjing University proposed a simple method to visually measure the microscopic temperature of 3D porous graphene. The results have been published in Chinese Optics Letters, Vol. 18, Issue 3, 2020 (Haoyan Jiang, Yaoyi Tang, Xiaohan Zeng, Ruiwen Xiao, Peng Lü, Lei Wang, Yanqing Lu. Visual measurement of the microscopic temperature of porous graphene based on cholesteric liquid crystal microcapsules[J]. Chinese Optics Letters, 2020, 18(3): 031201).
They used cholesteric liquid crystal microcapsules (CLCMs) as temperature sensors. Based on a CLCM (∼20 μm in size), they determined the temperature variation in a small area with an accuracy of 0.1 °C. By analyzing the color changes between two CLCMs, they demonstrated the temperature changes dynamically in a region with a diameter of approximately 110 μm. Furthermore, by comparing the color evolution among the three CLCMs, they visualized the anisotropic thermal properties in the micro-zone. This convenient and low-cost temperature measurement method is expected to further improve graphene-based devices.
“CLCMs here are used as temperature microsensors which can realize non-contact detection. We can obtain temperature by the color change of images. Not only the temperature distribution at the microscopic scale is measured in the thermal equilibrium, but also the thermodynamic properties can be detected in real time through quantitative visualization.” says the corresponding author Lei Wang, “Meanwhile, the 3D porous graphene itself exhibits a black color as it absorbs all the visible light, making it beneficial in detecting the light which is reflected by the CLCMs.”
The present work has verified the feasibility of 3D porous graphene microscopic temperature visualization based on CLCMs. In the future, the temperature sensitivity and sensing range of the CLCMs will be further optimized and the local thermal characteristics of graphene will be customized to measure. The temperature characteristics of various devices based on graphene will be studied, so as to optimize the design, achieve the high-efficient thermal management, and improve the overall performance of graphene devices. The authors believe that the proposed solution also paves way for the microscopic temperature measurement of other 2D and 3D materials and devices.
三维多孔石墨烯微观温度的可视化测量
基于CLCMs的多孔石墨烯微观温度测量可视化系统。插图：CLCMs在多孔石墨烯上的分布。
近年来对于石墨烯性能的研究引起了极大关注。其中，三维多孔石墨烯不仅继承了二维石墨烯高电导率、高热导率和强化学稳定性的优点，而且由于其结构特点，具有高比表面积、高孔隙率和极好的弹性，在柔性电子、热能管理和催化加载等领域应用广泛。
面对器件小型化、集成化和性能不断提升的要求，对其微观温度的测量是一个基础且迫切的问题。传统的温度探测器，如热电偶式、热电阻式、红外测温仪等，均存在测量精度上的不足。而新型基于碳纳米管、原子力显微镜、感温光纤和磁纳米颗粒等的微观测温系统普遍成本昂贵、结构复杂，实际应用受限。精确测量石墨烯的微观温度仍面临很多挑战。
近日，来自南京邮电大学和南京大学的研究人员提出了一种简单的可视化测量三维多孔石墨烯微观温度的方法。相关研究结果发表于Chinese Optics Letters 2020年第18卷第3期（Haoyan Jiang, Yaoyi Tang, Xiaohan Zeng, Ruiwen Xiao, Peng Lü, Lei Wang, Yanqing Lu. Visual measurement of the microscopic temperature of porous graphene based on cholesteric liquid crystal microcapsules[J]. Chinese Optics Letters, 2020, 18(3): 031201）。
研究人员利用一种对温度极灵敏的胆甾相液晶胶囊作为微观温度探测器，基于一颗微胶囊的颜色变化，可以得到20 μm区域内的温度，精度达0.1 °C；通过分析两颗微胶囊，得到了在大约110 μm范围内温度的动态变化；进一步对比三个液晶微胶囊的颜色演化，可以得到三维多孔石墨烯各项异性的微区热学特性。
该研究团队的王磊副教授表示：“胆甾相液晶胶囊作为一种微型热探测器，可以无接触探测其反射的波长。通过颜色的变化即可量化感知温度，不仅可以测量热平衡时物体微观的温度分布，而且还可以实时探测热动态过程。同时，三维多孔石墨烯本身会由于吸收可见光而呈现黑色，这非常有利于图像化测量液晶胶囊反射的各种颜色的光。”
目前的工作验证了基于胆甾相液晶胶囊实现三维多孔石墨烯微区域温度可视化测量的可行性。今后将进一步优化液晶胶囊的温度灵敏度和感温范围，定制化测量石墨烯的局域热学特性，对各种基于石墨烯器件的温度特性进行研究，从而优化设计、实现对石墨烯器件的高效热管理、提升器件整体性能。该方法也可用于其他二维、三维材料及器件的微观温度测量。