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Metal-free room-temperature phosphorescence (RTP) materials are of great significance for many applications; however, they usually exhibit low efficiency and weak intensity. This article reports a new strategy for the preparation of a high-efficiency and strong RTP materials from crystalline thermal-annealed carbon dots (CDs) and boric acid (BA) composite (g-t-CD@BA) through grinding-induced amorphous to crystallization transition. Amorphous thermal-annealed CDs and BA composite (t-CD@BA) is prepared following a thermal melting and super-cooling route, where the CDs are fully dispersed in molten BA liquid and uniformly frozen in an amorphous thermal annealed BA matrix after super-cooling to room temperature. Upon grinding treatment, the fracture and fragmentation caused by grinding promote the transformation of the high-energy amorphous state to the lower energy crystalline counterparts. As a result, the CDs are uniformly in situ embedded in the BA crystal matrix. This method affords maximum uniform embedding of the CDs in the BA crystals, decreases nonradiative decay, and promotes intersystem crossing by restraining the free vibration of the CDs, thus producing strong RTP materials with the highest reported phosphorescence quantum yield (48%). Remarkably, RTP from g-t-CD@BA powder is strong enough to illuminate items with a delay time exceeding 9 s.

Nanostructure-enhanced pool and flow boiling has the potential to increase the efficiency of a plethora of applications. Past studies have developed well-ordered, nonscalable structures to study the fundamental limitations of boiling such as bubble nucleation, growth, and departure, often in a serial manner without global optimization. Here, we develop a highly scalable, conformal, cost-effective, rapid, and tunable three-tier hierarchical surface deposition technique capable of holistically creating micropores, microscale dendritic clusters, and nanoparticles on arbitrary surfaces. We use this technique to investigate the pool boiling heat transfer performance with focus on the bubble departure diameter and frequency. By tuning the structure length scale, the pool boiling characteristics were optimized through a multipronged approach, including increasing nucleation site density (micropores), regulating bubble evolution behavior (dendritic structures), improving surface wickability (nanoscale particles and channels), and separating liquid and vapor pathways (micropores and micro/nanochannels). Ultrahigh critical heat fluxes (CHF) ≈400 W/cm2 were obtained, corresponding to an enhancement of ≈245% compared to smooth copper surfaces. To study in situ bubble departure and coalescence dynamics, we developed and used high-magnification in-liquid endoscopy. Our work reveals the existence of a linear relationship between the bubble departure diameter/frequency near the onset of nucleate boiling and CHF enhancement. Our study not only develops a highly scalable, conformal, and rapid micro/nanostructuring technique, it outlines design guidelines for the holistic optimization of boiling heat transfer for energy and water applications.

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