The photoreforming of lignocellulose is a novel method to produce clean and sustainable H 2 energy. However, the catalytic systems usually show low activity under ultraviolet light; thus, this reaction is very limited at present. Visible light-responsive metal-free two-dimensional graphite-phased carbon nitride (g-C 3 N 4 ) is a good candidate for photocatalytic hydrogen production, but its activity is hindered by a bulky architecture. Although reported layered g-C 3 N 4 modified with active functional groups prepared by the chemical exfoliation enhances the photocatalytic activity, it lost the intrinsic structure and thus is not conducive to understand the structure−activity relationship. Herein, we report an intrinsic monolayer g-C 3 N 4 (∼0.32 nm thickness) prepared by nitrogenprotected ball milling in water, which shows good performance of photoreforming lignocellulose to H 2 driven by visible light. The exciton binding energy of g-C 3 N 4 was estimated from the temperature-dependent photoluminescence spectra, which is a key factor for subsequent charge separation and energy transfer. It is found that monolayer g-C 3 N 4 with smaller exciton binding energy increases the free exciton concentrations and promotes the separation efficiency of charge carriers, thereby effectively improving its performance of photocatalytic reforming of lignocellulose, even the virgin lignocellulose and waste lignocellulose. This result could lead to more active catalysts to photoreform the raw biomass, making it possible to provide clean energy directly from locally unused biomass.
With the application prospect of a new generation of a liquid rocket engine and the status that system safety and reliability are weakened by the friction and wear of existing rolling bearings, a kind of compound bearing for axial balance is proposed in this paper, which is a combination of a high-temperaturesuperconducting bearing and a hydrodynamic fluid-film bearing. The most important feature of the compound bearing lies in a kind of "superconducting tilting pad" structure. The structure of the compound bearing and its detailed feasible application scheme in a rocket engine are given. The static characteristics of the compound bearing based on a decoupling analytical method are studied, such as the pressure distribution, the temperature distribution of the film, the minimum film thickness, and the bearing capacity. Theoretical results indicate that a compound of a high-temperature superconductor and a hydrodynamic fluid film is an effective approach to improve the static characteristics of bearings for a rocket engine.Index Terms-Compound bearing, high-temperature superconducting (HTS), hydrodynamic, rocket engine, static characteristics. 1051-8223
Rotating machinery has extensive industrial applications, and rolling element bearing (REB) is one of the core parts. To distinguish the incipient fault of bearing before it steps into serious failure is the main task of condition monitoring and fault diagnosis technology which could guarantee the reliability and security of rotating machinery. The early defect occurring in the REB is too weak and manifests itself in heavy surrounding noise, thus leading to the inefficiency of the fault detection techniques. Aiming at the vibration signal purification and promoting the potential of defects detection, a new method is proposed in this paper based on the combination of singular value decomposition (SVD) technique and squared envelope spectrum (SES). The kurtosis of SES (KSES) is employed to select the optimal singular component (SC) obtained by applying SVD to vibration signal, which provides the information of the REB for fault diagnosis. Moreover, the rolling bearing accelerated life test with the bearing running from normal state to failure is adopted to evaluate the performance of the SVD-KSES, and results demonstrate the proposed approach can detect the incipient faults from vibration signal in the natural degradation process.
Purpose To meet the high stiffness requirement of bearings used in high-precision spindles, this paper aims to propose a novel kind of bearing composited by hydrostatic cavities and tilting pads with preload. Design/methodology/approach Cavities are cut on the oil seal surface of a hybrid bearing, in which the tilting pads are set up. The load of the bearing is carried by the hydrostatic cavities and tilting pads. The structural features of this compound bearing and the controlling variables of the main stiffness coefficient are presented. Two basic design principles are proposed on the basis of equal machining clearance (EMC) and equal installation clearance (EIC). Findings The theoretical analysis indicates that the stiffness of compound bearings under the EMC condition increases to infinity monotonously when the preload coefficient of the tilting pad tends to 1, while the stiffness under the EIC condition has a peak value. Therefore, a synthetic design principle is proposed by synthetically using the above-mentioned two principles. The applicable range of the three principles is discussed through an example. Originality/value The study about technological combination of hydrostatic cavity and tilting pad in this paper can provide suggestions for the design of a high-stiffness bearing in a precision spindle.
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