Abstract
Producing monomeric phenols from lignin biopolymer depolymerization in a detachable and efficient manner comes under the spotlight on the fullest utilization of sustainable lignocellulosic ...biomass. Here, we report a low-loaded and highly dispersed Ru anchored on a chitosan-derived
N
-doped carbon catalyst (RuN/ZnO/C), which exhibits outstanding performance in the reductive catalytic fractionation of lignocellulose. Nearly theoretical maximum yields of phenolic monomers from lignin are achieved, corresponding to TON as 431 mol
phenols
mol
Ru
−1
, 20 times higher than that from commercial Ru/C catalyst; high selectivity toward propyl end-chained guaiacol and syringol allow them to be readily purified. The RCF leave high retention of (hemi)cellulose amenable to enzymatic hydrolysis due to the successful breakdown of biomass recalcitrance. The RuN/ZnO/C catalyst shows good stability in recycling experiments as well as after a harsh hydrothermal treatment, benefiting from the coordination of Ru species with
N
atoms. Characterizations of the RuN/ZnO/C imply a transformation from Ru single atoms to nanoclusters under current reaction conditions. Time-course experiment, as well as reactivity screening of a series of lignin model compounds, offer insight into the mechanism of current RCF over RuN/ZnO/C. This work opens a new opportunity for achieving the valuable aromatic products from lignin and promoting the industrial economic feasibility of lignocellulosic biomass.
This letter proposes a novel direct bonded copper (DBC) layout for mitigating the current imbalance among the paralleled SiC MOSFET dies in multichip power modules. Compared to the traditional ...layout, the proposed DBC layout significantly reduces the circuit mismatch and current coupling effect, which consequently improves the current sharing among the paralleled SiC MOSFET dies in power module. Mathematic analysis and circuit model of the DBC layout are presented to elaborate the superior features of the proposed DBC layout. Simulation and experimental results further verify the theoretical analysis and current balancing performance of the proposed DBC layout.
Laser ignition (LI) allows for precise manipulation of ignition timing and location and is promising for green combustion of automobile and rocket engines and aero-turbines under lean-fuel conditions ...with improved emission efficiency; however, achieving completely effective and reliable ignition is still a challenge. Here, we report the realization of igniting a lean methane/air mixture with a 100% success rate by an ultrashort femtosecond laser, which has long been regarded as an unsuitable fuel ignition source. We demonstrate that the minimum ignition energy can decrease to the sub-mJ level depending on the laser filamentation formation, and reveal that the resultant early OH radical yield significantly increases as the laser energy reaches the ignition threshold, showing a clear boundary for misfire and fire cases. Potential mechanisms for robust ultrashort LI are the filamentation-induced heating effect followed by exothermal chemical reactions, in combination with the line ignition effect along the filament. Our results pave the way toward robust and efficient ignition of lean-fuel engines by ultrashort-pulsed lasers.
Silicon carbide (SiC) and gallium nitride metal-oxide-semiconductor field-effect transistors (MOSFETs) are capable of processing high power at high switching frequencies with less switching losses ...and conduction losses. The gate driver circuit power consumption is directly proportional to the switching frequency. The power taken from the gate supply is dissipated in the gate resistance of the conventional gate driver (CGD) circuit. Instead of dissipating all the gate driver energy, some energy can be recovered or recycled by utilizing the principle of resonance. This reduces the net power being taken from the gate supply. This paper presents a new resonant gate driver (RGD) circuit which consumes less power compared to the CGD circuit at high switching frequencies. The proposed gate driver is designed for SiC MOSFETs. It can be modified appropriately to suit for insulated-gate bipolar transistors and other MOSFETs also. The performance of the proposed circuit is simulated in LTSpice environment, and an experimental prototype of the proposed circuit is developed to validate its performance. The proposed RGD circuit has achieved nearly 50% reduction in gate driver power consumption compared to the CGD circuit.
We present an upper bound of cumulative return in financial trading time series to formulate the most possible profit of many trades. The bound can be used to formally analyze the cumulative return ...varied by the number of trades, the mean return, and transaction cost rate. We also prove and show the validation of the upper bound, and verify the trend of cumulative return is consistent with that of the proposed bound via simulation experiments. Introducing a set of stochastic assessment methodology based on bootstrap into the organization of experimental data, we illustrate the influence on cumulative return from the relationship between the mean of return and transaction cost rate, technical trading rules, and stock indexes. To the best of our knowledge, this is the first to present and prove a bound of cumulative return of a stock trading series in theory. Both theoretical analyses and simulation experiments show the presented bound is a good mathematical tool to evaluate the trading risks and chances using given trading rules in stock trading markets.
Auxiliary-source bond wires and connections are widely used in power modules with paralleled mosfets or insulated gate bipolar transistor (IGBTs). This paper investigates the operation mechanism of ...the auxiliary-source connections in multichip power modules. It reveals that the auxiliary-source connections cannot fully decouple the power loop and the gate loop such as the Kelvin-source connection, owing to their involvement in the loop of the power source current. Three effects of the auxiliary-source connections are then analyzed, which are 1) the common source stray inductance reduction, 2) the transient drain-source current imbalance mitigation, and 3) the influence on the steady-state current distribution. Finally, simulations and experimental results validate the theoretical analysis.
Substantial attention has been given to depolymerization of lignin into monomeric phenols in recent years because lignin is a renewable and CO2-netural aromatic resource. Recent results indicated ...that the base can shift the selectivity from C3-fragmented phenols to C2-fragmented phenols partially in transition metal-catalyzed lignin hydogenolysis, while reaction mechanisms have remained elusive. Using a series of dimeric, trimeric, and polymeric β-O-4 lignin mimics, as well as their deuterated analogues, we now report an in-depth experimental study on the mechanism of Ru/C-catalyzed hydogenolysis lignin. Experimental evidence based on substrate probes, reactivity examination of possible intermediates, and isotopic labeling experiments confirmed that the reported pathways, such as enol ether generated via α,β-dehydration reaction or Cα carbonyl compounds generated via dehydrogenation or consecutive Cβ–O and Cγ–OH bonds hydrogenolysis, are irrelevant to current reactions. For C3-fragmented phenols with Ru/C catalyst under neutral condition, we deduced that the monolignol such as coniferyl alcohol is formed primarily through a concerted hydrogenolysis process, where Cα–O and Cβ–O bonds are ruptured synchronously. For C2-fragmented phenols generated by the combination of Ru/C and Cs2CO3, the reaction should start from quinone methide specie generated from the dehydration (or demethanolization) reaction between phenolic proton and Cα–OH (or Cα–OMe). The followed deprotonation of Cγ–OH and the coordination of oxygen with Ru results in a Ru specie, which undergoes Cβ–H, Cβ–O, and Cβ–Cγ bonds cleavage to release 4-vinylphenol. In the case of Ru/C-catalyzed hydrogenolysis of an enzymatic mild acidolysis lignin (EMAL) derived from birch tree, the effects of some key parameters such as temperature, reaction time, as well as the type and dosage of base were also examined in terms of monomer yields and selectivity. We found the formation of C2-phenols is a base-dependent process, which is in line with the proposed mechanism. Under optimized conditions, a high proportion of C2-phenols (44%) could be obtained with 26.6 wt % total monomers yield.
Femtosecond filament-induced breakdown spectroscopy (FIBS) is an efficient approach in remote and in situ detection of a variety of trace elements, but it was recently discovered that the FIBS of ...water is strongly dependent on the large-bandgap semiconductor property of water, making the FIBS signals sensitive to laser ionization mechanisms. Here, we show that the sensitivity of the FIBS technique in monitoring metal elements in water can be efficiently improved by using chirped femtosecond laser pulses, but an asymmetric enhancement of the FIBS intensity is observed for the negatively and positively chirped pulses. We attribute the asymmetric enhancement to their different ionization rates of water, in which the energy of the photons participating in the ionization process in the front part of the negatively chirped pulse is higher than that in the positively chirped pulse. By optimizing the pulse chirp, we show that the limit of detection of the FIBS technique for metal elements in water, e.g., aluminum, can reach to the sub-ppm level, which is about one order of magnitude better than that by the transform-limited pulse. We further examine the FIBS spectra of several representative water samples including commercial mineral water, tap water, and lake water taken from two different environmental zones, i.e., a national park and a downtown business district (Changchun, China), from which remarkably different concentrations of Ca, Na, and K elements of these samples are obtained. Our results provide a possibility of using FIBS for direct and fast metal elemental analysis of water in different field environments.
We report on measurements of both the critical power for self-focusing of a Ti: Sapphire 800 nm femtosecond laser and the peak intensity clamped inside a single filament in an ethanol-air flame on an ...alcohol burner array. By observing the shift of focal position of femtosecond laser pulses, we determine the critical power in the flame to be 2.2 ± 0.3 GW, which is 4-5 times smaller than the usually quoted one in air. The clamped laser intensity inside the filament is measured to be roughly half of that in air. Our results provide insights into the understanding of femtosecond laser filamentation in flames for practical application of combustion diagnostics.
Soot, which is produced through the rapid growth of hydrocarbon molecules in fuel-rich flames, has been proved to be responsible for global warming and harmful to the respiratory system of human. ...However, the complex formation process of soot in flames has not been understood yet because of the lack of efficient time-resolved methods for monitoring soot and its precursor species. We present here a viable pump–probe approach to measure ultrafast processes of the soot particles in flames with femtosecond time resolution. The scattering of femtosecond ultraviolet (UV) light pulses is used to monitor soot particles along alkanol–air diffusion flames from the inception phase up to the burnout region after the formation of a femtosecond laser filament in the flames. The time evolution of the scattered UV signal reveals unexpected ultrafast swelling and shrinking of soot particles induced by the femtosecond laser filament in the different flame regions. Our in situ and time-domain measurements of soot particles with the femtosecond time resolution unveil their unique properties in the flames.
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