Engineering topics
Zhang, Zhi
Publications and source records attributed to Zhang, Zhi.
A semi-empirical NO x model for LES in pulverized coal air-staged combustion
Numerical simulation of pulverized coal combustion with the large eddy simulation (LES) method can provides important instructions on the design optimization of boilers. Because of its huge computational cost, a simple but reliable NO x prediction model is required in LES. Through the air-staged combustion experiments of six kinds of coals which conducted in an electric heated down-fired furnace (DFF), the relationship between NO x reduction and CO+H 2 generation in the fuel-rich zone was identified. Based on this observation, a semi-empirical modeling strategy was proposed: instead of CHi which is difficult to calculate, the concentration of CO+H 2 is used to quantify NO homogeneous reduction. An integrated NO x prediction model was proposed and implemented into LES simulation. Here, the results proved the new model can accurately predict different NO x evolution characteristics under various conditions. Furthermore, the comparison indicated the LES method performs better on NO x prediction than the RANS method, especially in the area where the turbulent fluctuation is relatively stronger.
Carrier transport through the ultrathin silicon-oxide layer in tunnel oxide passivated contact (TOPCon) c-Si solar cells
The carrier transport through the silicon-oxide (SiO x ) layer in tunnel oxide passivated contact (TOPCon) c-Si solar cells has been studied experimentally and by simulation. The current intensity versus voltage (J-V) characteristics of GaIn/n-c-Si/SiO x /n + -poly-Si/Al structures shows a linear Ohmic characteristic, while a non-Ohmic behavior is observed in the samples without the n + -poly-Si contact layer. Conductive Atomic Force Microscopy (c-AFM) images reveal some current spikes on the surface of the samples, which could be related to the transport through pinholes. The simulation results show that 1) a rectification characteristic is obtained when only the tunneling mechanism is included, 2) both the reverse saturation current and the forward current increase when a small amount of transport through pinholes is introduced, and 3) finally a linear Ohmic behavior is observed when the pinhole transport component reaches a certain level. Furthermore, the simulation for whole TOPCon solar cells provides some useful results. For very thin SiO x (< 1.2 nm), the tunneling provides sufficient high tunneling probability and high efficiency TOPCon solar cells can be obtained without transport through pinholes if the passivation is ensured; while for a relatively thick SiO x (> 1.2 nm) without the transport through pinholes, the TOPCon solar cell shows a poor fill factor (FF) with a high series resistance (Rs) because the tunneling does not provide a sufficient high transport channel for carrier transport, and the introduction of a small number of transports through pinholes improves the FF and reduces the Rs, hence improves the PCE. However, a high possibility for carrier going through pinholes reduces all of the performance parameters and degrades PCE for all the cases simulated. Therefore, an optimized pinhole density and size distribution is critical engineering for solar cell performance optimization. However, the establishment of an optimized method to precisely control the pinhole formation and characterization is still on the way.
Increased expression of microRNA-31-5p inhibits cell proliferation, migration, and invasion via regulating Sp1 transcription factor in HepG2 hepatocellular carcinoma cell line
Accumulating evidence has suggested that microRNA-31-5p (miR-31-5p) is dysfunctional in hepatocellular carcinoma (HCC). However, the molecular mechanism of HCC remains unclear. In this study, we investigated the role of miR-31-5p in tumor formation and development of HCC. The expression of miR-31-5p was detected in HCC tissues, corresponding adjacent tissues, normal liver tissues, and HCC cell lines. miR-31-5p mimics and an inhibitor were transfected into HepG2 cells to assess the effects of miR-31-5p on cell proliferation, apoptosis, cell cycle, migration, and invasion assays. Western blotting was used to detect the expression of Sp1 transcription factor (SP1), cyclin D1, and survivin in transfected HCC cells and control cells. The expression of miR-31-5p was significantly decreased in HCC cells and HCC tissues. Overexpression of miR-31-5p inhibited HCC cell growth, migration, and invasion. Overexpression of miR-31-5p reduced the expression of SP1 and cyclin D1, and knockdown of SP1 decreased cyclin D1 expression. The dual luciferase assay showed that miR-31-5p directly targeted SP1 in HepG2. Together, the results suggested that miR-31-5p acted as a tumor suppressor to regulate SP1, and that miR-31-5p could be used as a therapeutic target for the treatment of HCC. - Highlights: • The expression of miR-31-5p was significantly up-regulated in HCC tissues. • Increased expression of miR-31-5p inhibits cell proliferation, migration, and invasion. • Sp1 transcription factor is the target gene for miR-31-5p.
Theoretical and experimental study of highly textured GaAs on silicon using a graphene buffer layer
A novel heteroepitaxial growth technique, quasi-van der Waals epitaxy, promises the ability to deposit three-dimensional GaAs materials on silicon using two-dimensional graphene as a buffer layer by overcoming the lattice and thermal expansion mismatch. In this study, density functional theory (DFT) simulations were performed to understand the interactions at the GaAs/graphene hetero-interface as well as the growth orientations of GaAs on graphene. To develop a better understanding of the molecular beam epitaxy-grown GaAs films on graphene, samples were characterized by x-ray diffraction (..theta..-2..theta.. scan, ..omega..-scan, grazing incidence XRD and pole figure measurement) and transmission electron microscopy. The realizations of smooth GaAs films with a strong (111) oriented fiber-texture on graphene/silicon using this deposition technique are a milestone towards an eventual demonstration of the epitaxial growth of GaAs on silicon, which is necessary for integrated photonics application.
A peripheral component interconnect express-based scalable and highly integrated pulsed spectrometer for solution state dynamic nuclear polarization
High sensitivity, high data rates, fast pulses, and accurate synchronization all represent challenges for modern nuclear magnetic resonance spectrometers, which make any expansion or adaptation of these devices to new techniques and experiments difficult. Here, we present a Peripheral Component Interconnect Express (PCIe)-based highly integrated distributed digital architecture pulsed spectrometer that is implemented with electron and nucleus double resonances and is scalable specifically for broad dynamic nuclear polarization (DNP) enhancement applications, including DNP-magnetic resonance spectroscopy/imaging (DNP-MRS/MRI). The distributed modularized architecture can implement more transceiver channels flexibly to meet a variety of MRS/MRI instrumentation needs. The proposed PCIe bus with high data rates can significantly improve data transmission efficiency and communication reliability and allow precise control of pulse sequences. An external high speed double data rate memory chip is used to store acquired data and pulse sequence elements, which greatly accelerates the execution of the pulse sequence, reduces the TR (time of repetition) interval, and improves the accuracy of TR in imaging sequences. Using clock phase-shift technology, we can produce digital pulses accurately with high timing resolution of 1 ns and narrow widths of 4 ns to control the microwave pulses required by pulsed DNP and ensure overall system synchronization. The proposed spectrometer is proved to be both feasible and reliable by observation of a maximum signal enhancement factor of approximately −170 for {sup 1}H, and a high quality water image was successfully obtained by DNP-enhanced spin-echo {sup 1}H MRI at 0.35 T.
Bismuth-induced phase control of GaAs nanowires grown by molecular beam epitaxy
In this work, the crystal structure of GaAs nanowires grown by molecular beam epitaxy has been tailored only by bismuth without changing the growth temperature and V/III flux ratio. The introduction of bismuth can lead to the formation of zinc-blende GaAs nanowires, while the removal of bismuth changes the structure into a 4H polytypism before it turns back to the wurtzite phase eventually. The theoretical calculation shows that it is the steadiest for bismuth to adsorb on the GaAs(111){sub B} surface compared to the liquid gold catalyst surface and the interface between the gold catalyst droplet and the nanowire, and these adsorbed bismuth could decrease the diffusion length of adsorbed Ga and hence the supersaturation of Ga in the gold catalyst droplet.