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Summary Therminol‐based nanofluids with graphite nanopowders (GNs) were proposed as an efficient working fluid in direct‐absorption solar collectors (DASCs), especially for elevated‐temperature applications. In this work, Therminol VP‐1/GN nanofluids (TG nanofluids) were prepared in two steps with surfactant (Triton X‐100). In the synthesis procedure, the vortex generator for coating GN with sticky surfactant (μ = 240 mPa⋅s) and the ultra‐sonicator for diluting GN/Triton X‐100 mixture with Therminol VP‐1 are used. We have demonstrated excellent dispersion stability up to 4 weeks and even after heating over 100°C without any surface modification of nanoparticles. The feasibility of using the synthesized TG nanofluid as a working fluid in DASCs was experimentally demonstrated by measuring its absorption coefficient as well as by characterizing photo‐thermal conversion performance. The absorption coefficient of the TG nanofluid at 0.004 wt.% of GNs was measured to be 1.12 cm−1, which is more than eight times enhanced from that of bare Therminol VP‐1 (ie, 0.13 cm−1) in the wavelengths from 350 to 1600 nm. Finally, the temperature increment of 43°C was achieved after t = 3600 s under the 5‐sun condition in the static photo‐thermal conversion experiment. Moreover, the low viscosity of TG nanofluids (only 2.2% higher than that of the bare Therminol VP‐1) supports the superiority of TG nanofluids to be applied as working fluid in DASCs.

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This Letter reports the formation, evolution, and prevention of thermally induced defects on germanium upon high-temperature (up to 890 °C) vacuum (2×10−6 Torr or 2.67×10−6 mbar) annealing. It is found that the shape of defects evolves depending on the annealing temperature and duration. The defect shape can be classified into three groups (pyramid, expanded pyramid, and doughnut), considering the interplay of chemical desorption and surface diffusion. The effects of annealing conditions on the density and size of defects are also investigated with scanning electron microscopy and atomic force microscopy. To prevent thermally induced defects, a simple covering method is proposed and demonstrated. In addition, formation and prevention of defects resulting from high-temperature (1150 °C) vacuum annealing are applied to silicon.