In this manuscript, weld pool dynamics in laser welding of various series of aluminum alloys were investigated by the in situ X-ray phase contrast imaging system. The experimental results showed that metal irradiated by laser was evaporated immediately, which generated the keyhole. Then metal surrounding the keyhole was melted gradually with the heat from keyhole. The growth rate of keyhole depth had a positive linear correlation with the total content of low boiling temperature elements (TCE), so did the keyhole depth and diameter at the stable stage. Longitudinal view area of the molten pool had a negative linear correlation with the thermal conductivity of aluminum alloy. The measured laser absorption rate had the same variation trend with the ratio of keyhole depth to diameter, and the highest absorption rate of 58% appeared in laser welding of aluminum alloy with TCE equal to 2.1%. Violent fluctuation in keyhole shape was avoided in aluminum alloy with TCE lower than 2.1%, where the surface tension and recoil pressure of metal vapor were balanced. To sum up, the effect of alloy element on weld pool dynamics in laser welding of aluminum alloys was firstly quantified in this manuscript.

A blue diode laser has a higher absorption rate than a traditional laser, while the maximum power is limited. We report the structure and laser beam profile of a 250 W high-power blue laser (445 nm) for material processing. The absorption rate of the blue laser system for the steel was 2.75 times that of a single-mode fiber laser system (1070 nm). The characteristics of the steel after laser irradiation were determined, validating the potential of this high-power blue laser for material processing, such as heat treatment and cladding. The cost of the developed laser system was lower than that of the existing one. To the best of our knowledge, this is the first blue laser with a power as high as 250 W.

Organic crystals that exhibit mechanochromic luminescence (MCL) switch their emission colors in response to mechanical stimuli, and the original emission colors usually recover upon heating the mechanically changed state. Contrary to the recent enthusiastic investigations of MCL crystals, only a limited number of organic crystals transform to another state showing a third emission color upon sequential exposure to mechanical and thermal stimuli. The development of near-infrared (NIR) emissive organic crystals is another challenge. Although several NIRemissive crystals that can respond to mechanical stimuli have been developed, further investigations are still required for the multistimuli-responsive emission involving the NIR region. In this study, we investigate the mechano-responsive properties of crystalline thienylbenzothiadiazole derivatives. The maximum emission wavelengths of these crystals are observed between blue-green to NIR (502−731 nm), which depend on the substituents R 1 and R 2 on the thiophene and benzothiadiazole rings, respectively. All derivatives shift the emission wavelength in the hypsochromic direction upon grinding. Moreover, derivatives having a carbonyl group on R 2 display further hypsochromic shifts of their emission bands by thermal annealing of the ground states. Specifically, a stepwise hypsochromic shift of the emission from the NIR region has been achieved for a derivative bearing phenyl and formyl groups (R 1 = Ph, R 2 = CHO). Powder X-ray diffraction analysis, differential scanning calorimetry, and spatially resolved fluorescence microscopy have suggested that the stepwise emission-color switching should be attributed to the crystal-size-dependent luminescence of the carbonyl-substituted derivatives. The present study can be expected to provide useful insights into developing diverse multi-stimuli-responsive luminescent organic crystals and thus accelerate the practical applications of luminescent sensor systems.

Microcrystalline silicon thin films were formed on quartz substrates by ultrahigh-frequency ͑UHF͒ plasma enhanced chemical vapor deposition from a mixture of silane (SiH 4) and hydrogen (H 2) gases at low substrate temperatures (T s). The UHF plasma was excited at a frequency of 500 MHz. The deposition rate and the crystallinity of the films were investigated as a function of H 2 dilution, total pressure, mixture ratio of SiH 4 to H 2 and T s. A crystalline fraction of 63% with a high deposition rate of 7.7 Å/s was obtained even at a T s of 100°C. At a temperature of 300°C, a crystalline fraction of approximately 86% was achieved at a deposition rate of 1.4 Å/s. Diagnostics of the UHF plasma have been carried out using a Langmuir probe, ultraviolet absorption spectroscopy, and optical emission spectroscopy. Good crystallinity was explained by the balance of the sheath voltage and atomic hydrogen densities in the UHF plasma. Namely, the UHF plasma source achieving a high density plasma with a low electron temperature enabled us to reduce the ion bombardment energy incident on the substrates while maintaining a high density of hydrogen atoms, and which improved the crystallinity at low T s .

Herein, we propose a new strategy to tune the magnitude of mechanoresponsive shift of the maximum emission wavelength (Δλem). The Δλem of thienylbenzothiadiazole crystals has been extended to 69 nm...