Aaltodoc Repository :: Browsing by Department "Southeast University, Nanjing"

Browsing by Department "Southeast University, Nanjing"

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Characteristics of natural convection in n-eicosane in a square cavity with discrete heat source
(Elsevier BV, 2021-10) Yang, Shimi; Huang, Bingkun; Wang, Jun; Lund, Peter D.; Southeast University, Nanjing; New Energy Technologies; Department of Applied Physics
The natural convection of phase change material (PCM) in a two-dimensional square cavity is numerically analyzed. The cavity consists of heat surface with a constant total heated area and adiabatic wall. The Grashof and Prandtl numbers for the PCM (n-eicosane with the melting temperature, Tmelt=36°C) in basic LHS system are 9 × 10^5 and 62.7, respectively, at 350.15K. The mass, momentum and energy balance equations of the system were considered. Three basic heating surface strategies were considered (discrete heat sources): single, side-side and side-bottom heating surface. The results show that the transient Nusselt number, mean kinetic energy at the surface and energy storage rate of the fluid are effectively enhanced by proper arrangement of the discrete heat source location and heating from the bottom half of the left and right sides requires the least time for 300 kJ energy storage. The results indicate that optimally placed discrete heat sources in PCM could be a promising alternative for high-efficientthermal energy storage.
Enhanced Ferromagnetism and Tunable Magnetism in Fe3GeTe2Monolayer by Strain Engineering
(AMERICAN CHEMICAL SOCIETY, 2020-06-10) Hu, Xiaohui; Zhao, Yinghe; Shen, Xiaodong; Krasheninnikov, Arkady V.; Chen, Zhongfang; Sun, Litao; Nanjing Tech University; University of Puerto Rico; Department of Applied Physics; Southeast University, Nanjing
Recent discovery of intrinsic ferromagnetism in Fe3GeTe2 (FGT) monolayer [ Deng, Y.; et al. Nature 2018, 563, 94-99; Fei, Z.; et al. Nat. Mater. 2018, 17, 778-782 ] not only extended the family of two-dimensional (2D) magnetic materials but also stimulated further interest in the possibility to tune their magnetic properties without changing the chemical composition or introducing defects. By means of density functional theory computations, we explore strain effects on the magnetic properties of the FGT monolayer. We demonstrate that the ferromagnetism can be largely enhanced by the tensile strain in the FGT monolayer due to the competitive effects of direct exchange and superexchange interaction. The average magnetic moments of Fe atoms increase monotonically with an increase in biaxial strain from -5 to 5% in FGT monolayer. The intriguing variation of magnetic moments with strain in the FGT monolayer is related to the charge transfer induced by the changes in the bond lengths. Given the successful fabrication of the FGT monolayer, the strain-tunable ferromagnetism in the FGT monolayer can stimulate the experimental effort in this field. This work also suggests an effective route to control the magnetic properties of the FGT monolayer. The pronounced magnetic response toward the biaxial strain can be used to design the magnetomechanical coupling spintronics devices based on FGT.
Enhancing Ferromagnetism and Tuning Electronic Properties of CrI3 Monolayers by Adsorption of Transition-Metal Atoms
(AMERICAN CHEMICAL SOCIETY, 2021-05-12) Yang, Qiang; Hu, Xiaohui; Shen, Xiaodong; Krasheninnikov, Arkady V.; Chen, Zhongfang; Sun, Litao; Nanjing Tech University; Department of Applied Physics; University of Puerto Rico; Southeast University, Nanjing
Among first experimentally discovered two-dimensional (2D) ferromagnetic materials, chromium triiodide (CrI3) monolayers have attracted particular attention due to their potential applications in electronics and spintronics. However, the Curie temperature Tc of the CrI3 monolayer is below room temperature, which greatly limits practical development of the devices. Herein, using density functional theory calculation, we explore how the electronic and magnetic properties of CrI3 monolayers change upon adsorption of 3d transition-metal (TM) atoms (from Sc to Zn). Our results indicate that the electronic properties of the TM-CrI3 system can be tuned from semiconductor to metal/half-metal/spin gapless semiconductor depending on the choice of the adsorbed TM atoms. Moreover, the adsorption can improve the ferromagnetic stability of CrI3 monolayers by increasing both magnetic moments and Tc. Notably, Tc of CrI3 with Sc and V adatoms can be increased by nearly a factor of 3. We suggest postsynthesis doping of 2D CrI3by deposition of TM atoms as a new route toward potential applications of TM-CrI3 systems in nanoelectronic and spintronic devices.
Exergo-environmental cost optimization of a combined cooling, heating and power system using the emergy concept and equivalent emissions as ecological boundary
(PERGAMON-ELSEVIER SCIENCE LTD, 2021-10-15) Chen, Yuzhu; Xu, Jinzhao; Wang, Jun; Lund, Peter D.; Southeast University, Nanjing; New Energy Technologies; Department of Applied Physics
District energy systems, in particular combined cooling, heating and power (CCHP) systems, can provide cost-effective energy products, reduce fossil fuel consumption and emissions. A gas-fired CCHP system is considered here employing an internal combustion engine, whose exhaust gas is split between an organic Rankine cycle unit and absorption heat pump. A simulation model of the system is constructed and validated. An emergy based exergo-environmental cost method is proposed here to optimize the exhaust gas allocation ratio based on specific working conditions considering the equivalent emissions of the whole life-cycle chain from an ecological view. The emergy consumption in each process stage is evaluated accompanied with a sensitivity analysis. The results show that the minimize specific system cost is 310050 seJ/J when 63% of the exhaust gas flows to the ORC-unit. The ICE and AHP are responsible for >98% of the emergy consumption, dominated by the operation of the system. The sensitivity analysis shows that heating is the most sensitive of all products against key parameter variations, while electricity is the least. Increasing the service period and the operating hours, the economic performance could be improved, while the transformity of emission enlarges the cost.
Improving the accuracy of predicting the performance of solar collectors through clustering analysis with artificial neural network models
(Elsevier, 2022-11) Du, Bin; Lund, Peter D.; Wang, Jun; Southeast University, Nanjing; Department of Applied Physics
Accurate prediction of collector performance is important for optimal planning of solar thermal systems. Here, a novel prediction method combining clustering of data with artificial neural network (ANN) model is presented. A novel all-glass straight-through tube solar collector is employed as reference solar technology. In the present approach, experimental collector performance data was first collected during different weather conditions (sunny, cloudy, rainy days) subject to a clustering analysis to screen out outlier samples. The data was then used to train and verify the neural network models. For the ANN, the Back Propagation (BP) and Convolutional Neural Network (CNN) models were used. For predicting the performance (thermal efficiency) of the solar collector, solar radiation intensity, ambient temperature, wind speed, fluid flow rate, and fluid inlet temperature were used as the input parameters in the model. The prediction accuracy of the neural network models after full-data-screening were superior to that of the pre-screening and partial-screening models. The CNN model provided somewhat better efficiency predictions than the BP model. The R2, RMSE and MAE of the CNN model prediction in sunny conditions with full-screening was 0.9693, 0.0039 and 0.0030, respectively. The average MAPE of the BP and CNN models for all three weather types dropped by 30.7% and 13.8%, respectively, when applying data pre-screening and partial-screening only. The accuracy of the ANN collector prediction model can thus be improved through data clustering, which provides an effective method for performance prediction of solar collectors.
Phase structure-dependent low temperature ionic conductivity of Sm2O3
(American Institute of Physics, 2022-09-05) Ma, Longqing; Hu, Enyi; Yousaf, Muhammad; Lu, Yaokai; Wang, Jun; Wang, Faze; Lund, Peter; Southeast University, Nanjing; New Energy Technologies; Department of Applied Physics
Samarium oxide (SMO), a rare-earth oxide, has gathered great interest from researchers because of its variable valences and promising ionic conductivity. Herein, SMOs with cubic and monoclinic phases were synthesized. The changes in the crystal structure of SMOs with sintering temperature were analyzed. The cell based on cubic phase SMO achieves an excellent maximum power density of 0.876 W cm-2 along with a high ionic conductivity at 550 °C, indicating an enhanced ionic conductivity compared with monoclinic phase SMO. Further analysis of x-ray diffraction and x-ray photoelectron spectra results confirmed that there were more oxygen vacancies formed in cubic phase SMO than monoclinic phase SMO, thereby offering more active sites for fast ion transport. Furthermore, both cubic phase and monoclinic phase SMOs are dominated by proton conduction, while cubic phase SMO is further coupled with oxygen ion conduction, which leads to higher ionic conductivity of cubic phase SMO. In this study, the ionic conductivitiesof SMOs with different crystal structures are compared and reasons for the differences are disclosed, which provides guidance for further applications of SMO.
Random field-based time-dependent reliability analyses of a PSC box-girder bridge
(Springer International Publishing AG, 2019-10-01) Chen, Zheheng; Guo, Tong; Liu, Shaobo; Lin, Weiwei; Hohai University; Southeast University, Nanjing; Nanjing Institute of Technology; Department of Civil Engineering
The parameters affecting structural reliability are usually regarded as independent random variables in the current reliability analyses of bridge structures while the randomness of these structural parameters in spatial distribution is neglected. To overcome this disadvantage, the random field model should be used to describe their probability distribution. In this paper, a structural reliability analysis method considering the effect of a random field is proposed, and its validity is verified by three numerical case studies. A prestressed concrete (PSC) box-girder bridge with a main span of 150 m is selected for demonstration, and the importance sampling (IS) method is applied to estimate its failure probability, in which the influences of shrinkage and creep, stress relaxation, and shear lag on the time-dependent performance of the structure are taken into account. In addition, the random fields of the structure are discretized by using the local average method (LAM). Finally, the effects of random field parameters, such as the number of discrete elements, correlation model, and correlation length, on the reliability of the box-girder bridge are discussed.
Strain robust spin gapless semiconductors/half-metals in transition metal embedded MoSe2monolayer
(IOP Publishing Ltd., 2020-08-26) Yang, Qiang; Kou, Liangzhi; Hu, Xiaohui; Wang, Yifeng; Lu, Chunhua; Krasheninnikov, Arkady V.; Sun, Litao; Nanjing Tech University; Queensland University of Technology; Department of Applied Physics; Southeast University, Nanjing
The realization of spin gapless semiconductor (SGS) and half-metal (HM) behavior in two-dimensional (2D) transition metal (TM) dichalcogenides is highly desirable for their applications in spintronic devices. Here, using density functional theory calculations, we demonstrate that Fe, Co, Ni substitutional impurities can not only induce magnetism in MoSe2 monolayer, but also convert the semiconducting MoSe2 to SGS/HM system. We also study the effects of mechanical strain on the electronic and magnetic properties of the doped monolayer. We show that for all TM impurities we considered, the system exhibits the robust SGS/HM behavior regardless of biaxial strain values. Moreover, it is found that the magnetic properties of TM-MoSe2 can effectively be tuned under biaxial strain by controlling the spin polarization of the 3d orbitals of Fe, Co, Ni atoms. Our findings offer a new route to designing the SGS/HM properties and modulating magnetic characteristics of the TM-MoSe2 system and may also facilitate the implementation of SGS/HM behavior and realization of spintronic devices based on other 2D materials.
Thermodynamic performance analysis and modified thermo-ecological cost optimization of a hybrid district heating system considering energy levels
(PERGAMON-ELSEVIER SCIENCE LTD, 2021-06-01) Chen, Yuzhu; Hua, Huilian; Wang, Jun; Lund, Peter D.; Southeast University, Nanjing; New Energy Technologies; Department of Applied Physics
Utilization of the renewable resources in district heating systems can reduce the use of fossil fuels, operating costs and protect the environment. In this study, an integrated hybrid system consisting of concentrating photovoltaic/thermal collectors (PV/T), geothermal (GSHP) and absorption (AHP) heat pumps is considered for district heating. The thermodynamic performance of the system at various conditions is explored through detailed simulations. A modified thermo-ecological cost (TEC) method is used to optimize the structure of the PV/T by considering contributions of different flows. The results show that a higher solar irradiance level and a higher PV coverage ratio have a positive impact on the thermal performance of the hybrid system. The TEC-based optimization shows that a 66% PV coverage ratio of PV/T yields a minimum thermo-ecological heating cost of 6.86 J/J, which is slightly lower than cost with a conventional method. Based on the sensitivity analysis, other key parameters except the operating time and the PV coverage ratio have a negative influence on the economic performance of the district heating system, because of the increasing cumulative exergy consumption of the GSHP or PV/T.
Tunable electronic properties and enhanced ferromagnetism in Cr2Ge2Te6monolayer by strain engineering
(IOP Publishing Ltd., 2021-11-26) Liu, Lifei; Hu, Xiaohui; Wang, Yifeng; Krasheninnikov, Arkady V.; Chen, Zhongfang; Sun, Litao; Nanjing Tech University; Department of Applied Physics; University of Puerto Rico; Southeast University, Nanjing
Recently, as a new representative of Heisenberg's two-dimensional (2D) ferromagnetic materials, 2D Cr2Ge2Te6 (CGT), has attracted much attention due to its intrinsic ferromagnetism. Unfortunately, the Curie temperature (T C ) of CGT monolayer is only 22 K, which greatly hampers the development of the applications based on the CGT materials. Herein, by means of density functional theory computations, we explored the electronic and magnetic properties of CGT monolayer under the applied strain. It is demonstrated that the band gap of CGT monolayer can be remarkably modulated by applying the tensile strain, which first increases and then decreases with the increase of tensile strain. In addition, the strain can increase the Curie temperature and magnetic moment, and thus largely enhance the ferromagnetism of CGT monolayer. Notably, the obvious enhancement of T C by 191% can be achieved at 10% strain. These results demonstrate that strain engineering can not only tune the electronic properties, but also provide a promising avenue to improve the ferromagnetism of CGT monolayer. The remarkable electronic and magnetic response to biaxial strain can also facilitate the development of CGT-based spin devices.

Browsing by Department "Southeast University, Nanjing"

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