Through passively emitting excess heat to the outer space, radiative cooling has been demonstrated as an efficient way for energy saving applications. Selective surface with unity emittance only within the atmospheric window as well as zero absorption within the solar spectrum is sought to achieve the best sub-ambient radiative cooling performance during the daytime. In this work, we proposed a bulk radiative cooler consisting of a 1-mm-thick lithium fluoride crystal coated with silver backing, which exhibits a solar absorptance of 4.7% and nearly ideal infrared selectivity with high emission exactly within the atmospheric transmission band (i.e., 8-13 m).Excellent daytime cooling performance was demonstrated in an outdoor test with stagnation temperature below the ambient temperature by 5 ℃ under solar irradiance above 900 W/m 2 and a net cooling power of about 60 W/m 2 when the cooler is in thermal equilibrium with the ambient. As a bulk material with the highest ultraviolet-visible-near infrared transmittance, lithium fluoride crystal has been widely employed as the optical windows and mirrors in various applications. The proposed simple selective infrared emitter based on lithium fluoride would open up an innovative way to radiatively cool optical systems.
Piezoelectric energy harvesting using cantilever-type structures has been extensively investigated due to its potential application in providing power supplies for wireless sensor networks, but the low output power has been a bottleneck for its further commercialization. To improve the power conversion capability, a piezoelectric beam with different electrode coverage ratios is studied theoretically and experimentally in this paper. A distributed-parameter theoretical model is established for a bimorph piezoelectric beam with the consideration of the electrode coverage area. The impact of the electrode coverage on the capacitance, the output power and the optimal load resistance are analyzed, showing that the piezoelectric beam has the best performance with an electrode coverage of 66.1%. An experimental study was then carried out to validate the theoretical results using a piezoelectric beam fabricated with segmented electrodes. The experimental results fit well with the theoretical model. A 12% improvement on the Root-Mean-Square (RMS) output power was achieved with the optimized electrode converge ratio (66.1%). This work provides a simple approach to utilizing piezoelectric beams in a more efficient way.
The polarized absorption spectra, polarized fluorescence spectra and fluorescence decay curve of Nd3+ : LiLuF4 crystals are measured at room temperature. The peak absorption cross-sections are 6.944×10−20 cm2 and 1.664 × 10−20 cm2 at around 795 nm with full-width at half-maximum of 3.4 nm and 3.2 nm for π-polarization and σ-polarization, respectively. Based on Judd–Ofelt theory, the spectral parameters of Nd3+ in LiLuF4 crystals are investigated. The emission probabilities, branching ratio and radiative lifetime for the transitions from 4F3/2 are calculated. The radiative lifetime, fluorescence lifetime and quantum efficiency are 536 µs, 489 µs and 91%, respectively. Its spectroscopic parameters are also compared with those of some other important Nd3+-doped laser crystals.
In order to reduce power consumption of sensor nodes and extend network survival time in the wireless sensor network (WSN), sensor nodes are scheduled in an active or dormant mode. A chain-type WSN is fundamentally different from other types of WSNs, in which the sensor nodes are deployed along elongated geographic areas and form a chain-type network topology structure. This paper investigates the node scheduling problem in the chain-type WSN. Firstly, a node dormant scheduling mode is analyzed theoretically from geographic coverage, and then three neighboring nodes scheduling criteria are proposed. Secondly, a hybrid coverage scheduling algorithm and dead areas are presented. Finally, node scheduling in mine tunnel WSN with uniform deployment (UD), non-uniform deployment (NUD) and optimal distribution point spacing (ODS) is simulated. The results show that the node scheduling with UD and NUD, especially NUD, can effectively extend the network survival time. Therefore, a strategy of adding a few mobile nodes which activate the network in dead areas is proposed, which can further extend the network survival time by balancing the energy consumption of nodes.
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