Hejun Li’s research while affiliated with Northwestern Polytechnical University and other places

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Publications (616)


An innovative fabrication method of heat sink achieving by selective-orienting co-continuous short fibers network composites
  • Article

April 2025

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32 Reads

Composites Part B Engineering

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Lehua Qi

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Hejun Li

For short fiber-reinforced composite heat dissipation components, designing the local fiber orientation and spatial structure according to heat dissipation principles is crucial for enhancing reinforcement efficiency and improving the performance of heat dissipation devices. In this work, based on the realizing of three-dimensional continuous networks of pyrolytic carbon-short carbon fiber (PyC-Csf) with controllable orientations, we propose an innovative local-global designed and integrated preparation strategy for high-thermal conductive magnesium (Mg) composites heat sink. The tunable local thermal properties of the composites are achieved by adjusting the orientations of the short fibers, and the heat sink is near-net formed by the liquid-solid extrusion following vacuum infiltration technique. The fins and substrate of the heat sink exhibit a thermal conductivity of 101.6 W/m·K and 134.6 W/m·K, which increased by 111.5 % and 180.4 % compared to the Mg matrix, respectively. The TC enhancement is mainly caused by the quasi-alignment fibers in the substrate and planar isotropic orientation in the fins of the heat sink. This work describes a scalable fabrication method for developing metal matrix composite components with thermal conductive selective-orienting co-continuous short fibers network. It highlights the potential of Csf/Mg composites for thermal management and provides an important step toward realizing their actual real-world applications.


Fig. 1 Schematic and morphology of MC-assisted AWBps-based aerogels. a Schematic of the freeze-drying preparation process for MCassisted AWBps-based aerogels. b SEM and EDS elemental mapping of representative AWBPs. c TEM image of AWBps. d HRTEM of AgBr x Cl 1-x in c. e, g, h Magnified HRTEM images of the sample shown in d (regions g, h), revealing the lattice defects, and discontinuous lattice fringe. f, i The corresponding strain fields ϵ yy were determined by the GPA method using the TEM image g, h, respectively
Fig. 2 Diagram of external force on the forming of aerogel foam/film and the related images of "network" in the diagram. a Schematic of capillary forces and large nanoparticles inducing cracks or shrinkage in the pores of the gels. b Enlarged SEM image of the nacre-like/bidirectional structure shown in Fig. S9, revealing the connection structure of layers. c SEM images of AWBps/AgNWs/MC aerogel foam. d Enlarged SEM image of c, showing the connecting filament
Fig. 3 EMI shielding behaviors of AWBps aerogel films. a Digital photo of aerogel film. b Electrical conductivity of the aerogel films of top and bottom surfaces with various mass ratios of AWBps. c EMI SE of 2 wt% AWBps aerogel film at a thickness of 0.12 mm when EM wave incidence from the top or bottom surface. d Schematic diagram of FEA simulation. e FEA simulated the electric field intensity of aerogel film and the distribution of its cut-line, particularly showing its skin depth at 0.008 mm in the z-position. f EMI SE of 5 wt% AWBps aerogel film at different thicknesses. g Total average EMI SE (SE T ) and its absorption (SE A ) and reflection (SE R ) mechanism with various mass ratios of AWBps. EMI SE versus thickness of h different nanomaterials and i waste materials (the best results for the frequency bands given in the literature). A detailed description of each data point is presented in Tables S3 and S4
Fig. 4 Microwave absorption behaviors of AWBps/AgNWs aerogel foams. a Digital photo of aerogel foam. SEM images of b upper part of foam, c lower part of foam, d top view, and e bottom view. f 2D RL plots of 1 wt% AWBps, 1.6 wt% AWBps-0.2 wt% AgNWs and 1.3 wt% AWBps-0.4 wt% AgNWs aerogel foam, containing top incidence and bottom incidence modes. g Electrical conductivity and h dielectric tangent loss of the aerogel foams of top and bottom surfaces with various mass ratios of AWBps-AgNWs
Fig. 6 EM functions of recycled AWBps aerogels. a Recyclability of AWBps aerogel. b Electrical conductivity, c SE/d, and d RL of AWBps aerogel film and/or foam before and after recasting
Electromagnetic Functions Modulation of Recycled By-Products by Heterodimensional Structure
  • Article
  • Full-text available

February 2025

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8 Reads

Nano-Micro Letters

One of the significant technological challenges in safeguarding electronic devices pertains to the modulation of electromagnetic (EM) wave jamming and the recycling of defensive shields. The synergistic effect of heterodimensional materials can effectively enable the manipulation of EM waves by altering the nanostructure. Here we propose a novel approach for upcycling by-products of silver nanowires that can fabricate shape-tunable aerogels which enable the modulation of its interaction with microwaves by heterodimensional structure of by-products. By-product heterodimensionality was used to design EM-wave-jamming-dissipation structures and therefore two typical tunable aerogel forms were studied. The first tunable form was aerogel film, which shielded EM interference (EMI shielding effectiveness (EMI SE) > 89 dB) and the second tunable form was foam, which performed dual EM functions (SE > 30 dB& reflective loss (RL) < -35 dB, effective absorption bandwidth (EAB) > 6.7 GHz). We show that secondary recycled aerogels retain nearly all of their EM protection properties, making this type of closed-loop cycle an appealing option. Our findings pave the way for the development of adaptive EM functions with nanoscale regulation in a green and closed-loop cycle, and they shed light on the fundamental understanding of microwave interactions with heterodimensional structures.

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Functionally Graded Oxide Scale on (Hf,Zr,Ti)B2 Coating with Exceptional Ablation Resistance Induced by Unique Ti Dissolving

January 2025

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47 Reads

Multicomponent Ti‐containing ultra‐high temperature ceramics (UHTCs) have emerged as more promising ablation‐resistant materials than typical UHTCs for applications above 2000 °C. However, the underlying mechanism of Ti improving the ablation performance is still obscure. Here, (Hf,Zr,Ti)B2 coatings are fabricated by supersonic atmospheric plasma spraying, and the effects of Ti content on the ablation performance under an oxyacetylene flame are investigated. The (Hf0.45Zr0.45Ti0.10)B2 coating shows superior ablation resistance and cycling reliability at ≈2200°C. A functionally graded oxide scale comprising an outer dense layer and an underlying fine granular layer formed. The former is a better oxygen barrier owing to fewer cracks and the latter has high strain tolerance due to finer grain size. The uniform dissolving of ≈4 mol% Ti in the inner layer results in grain refinement via sluggish diffusion and thus stress release. For the outer layer, Ti segregation at the nanoscale leads to a metastable cubic (Hf,Zr,Ti)O2 and local severe lattice distortion, inhibiting the propagation of cracks. Ti ions’ unique dissolving in the oxide scale enables a strong oxygen diffusion barrier with high strain tolerance, which is responsible for superior performance. This study provides new insights into the ablation behavior of Ti‐containing multicomponent UHTCs.




Comparative study on the ablation resistance and mechanical properties of 2.5D C/C composites reinforced with CFMP and CFPAN

December 2024

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20 Reads

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1 Citation

Journal of Materials Science

In this contribution, we propose a new approach to enhance the ablation properties of carbon/carbon (C/C) composites. 2.5D CFMP/C composites were obtained by mesophase pitch-based carbon fiber (CFMP) as the reinforcing carbon fibers, which facilitates the high aerodynamic heat flow near the stagnation to be rapidly guided to the low heat flow region at the rear of the structure. The results show that the surface temperature of 2.5D CFMP/C composites is 100 °C lower than that of 2.5D CFPAN/C composites prepared by traditional PAN-based carbon fiber (CFPAN), and the mass ablation rate and linear ablation rate of 2.5D CFMP/C composites decreased from 3.307 mg/s to 1.412 mg/s and from 3.067 µm/s to 0.501 µm/s, respectively. In addition, the mechanical properties of the 2.5D CFMP/C composites are outstanding, which will lay the foundation for their stability in ablative environments. Our work will bring new blessings for the further development of C/C composites in the aerospace field.






Citations (41)


... Physical vapor deposition (PVD), including techniques like sputtering and evaporation, provides higher precision and better control over thin-film thickness compared to thick-film methods, but it may struggle with uniformity over large areas and requires higher vacuum conditions, increasing operational costs [51,52]. Chemical vapor deposition (CVD) offers improved scalability and the ability to coat complex geometries, yet it can be hindered by high material usage, potential contamination, and limited control at the atomic level [53,54]. In contrast, ALD surpasses these methods by offering atomic-level precision and unparalleled control over the thickness, composition, and uniformity of thin films. ...

Reference:

Molecular Layer Doping ZnO Films as a Novel Approach to Resistive Oxygen Sensors
Research progress in chemical vapor deposition for high-temperature anti-oxidation/ablation coatings on thermal structural composites
  • Citing Article
  • November 2024

Composites Part B Engineering

... Factors such as particle size, porosity, and particle morphology can be optimized to further improve the thermal insulation properties of these materials. He et al. developed mullite fiber insulating tiles coated with MoSi2borosilicate (MFIT@MoSi2), which demonstrated excellent thermal protection [62]. Even at a heat flux of 450 kW·m −2 , the surface temperature reached 1043.1 °C, while the cold surface remained close to room temperature. ...

Mechanical and Electromagnetic Interference Shielding Properties of In-situ Grown Si3N4nw Synergistic Defective-Graphene Reinforced Alumina Ceramics
  • Citing Article
  • November 2024

Composites Part B Engineering

... [1][2][3] While high frequency communication technology greatly facilitates efficient information transmission, the densely distributed electromagnetic radiation it generates seriously interferes with the normal operation of equipment and poses risks to human health. [4][5][6][7] Meanwhile, stealth technology has become a crucial factor in enhancing the survivability and combat effectiveness of weapons devices in the military field. [8,9] Therefore, the design and fabrication of new EMW absorption materials with low density, small filling ratio, and strong wave absorbing loss are of great significance in solving various electromagnetic pollution and improving the stealth performance of weapons. ...

Multifunctional 3D pristine graphene with tunable electromagnetic properties via self-assembly strategy
  • Citing Article
  • September 2024

Chemical Engineering Journal

... Common carbon materials are carbon nanotubes and graphene. However, these artificially designable carbon materials have limited productivity and application prospects due to their high cost and complex preparation [19,20]. In contrast, biomass-derived carbon materials have great potential to replace traditional carbon materials due to its advantages such as easy to obtain from nature, low price and environmental friendliness [21]. ...

Construction of core-shell structured SiC nanowires@carbon nanotubes hybrid conductive network for supercapacitors and electromagnetic interference shielding
  • Citing Article
  • July 2024

Carbon

... Moreover, EMW absorbing materials are also widely used in aircraft stealth and other military fields [9][10][11][12]. Until now, a variety of EMW absorbing materials have been developed [13][14][15]. EMW absorbing materials can be classified as resistive materials, dielectric materials and magnetic materials according to the loss mechanism [16]. ...

Electromagnetic wave absorption of polymer derived ceramic composites tuned by multi-component oxide solid solution
  • Citing Article
  • June 2024

Composites Part B Engineering

... For the oxides of tantalum, the O1s pattern of stoichiometric Ta 2 O 5 is around 531.9 eV, 35,36 and the O1s pattern of unstoichiometric TaO x , produced from the ablation of TaC or oxidation of Ta, is usually located between 529.8 and 530.3 eV. [37][38][39] In this work, the peak of O1s starts from 529 to 535 eV. The width is much larger than normal O1s peak, indicating the complex chemical valence state. ...

Comparison of ablation performances and sublayer interaction of CVD-(TaC/SiC)3 alternate coatings with different crystallite morphologies
  • Citing Article
  • May 2024

Surface and Coatings Technology

... Among them, Si 3 N 4 nanowire is particularly suitable for high-temperature applications due to its high thermal shock resistance, high-temperature stability and excellent oxidation resistance [13,14,18,19]. In our previous research, we fabricated a Si3N4 nanowire film (SN) with porous architecture [20,21]. However, there were two issues that need to be addressed in the SN film. ...

Surface decoration of flexible Si3N4 nanowire membrane by hydroxyapatite micron-flake with excellent thermal insulation at 1300 °C
  • Citing Article
  • May 2024

Materials Characterization

... In addition to improving the overall electrochemical performance, the cost is reduced as pyrolyzed Advances in Engineering Technology Research BEMIC 2024 ISSN:2790-1688 Volume -12-(2024) carbon replaces a portion of the base material. The material used for the experiments by Wu et al [29] was CFs reinforced carbon matrix composites with a unique core-shell structure, with carbon fibers (CFs) as the core and chemical vapor deposition pyrolytic carbon (PCs) as the layered shell ( Fig. 4.(a)).Li + was preferentially adsorbed on the defects, edges, and microporous surfaces and then inserted into the graphite lattice ( Fig. 4.(b)).The CFs were cycled at 0.5 C The capacity retention rate was 80 % after 350 cycles, and the average Coulombic efficiency was as high as 99.7 %; the CFs electrode had a high discharge specific capacity of 235 mAh g -1 at 0.1 C, and was maintained at 166 mAh g -1 at 2 C, as shown in Fig.2.(c).The PC layer on the CFs buffered the volume expansion induced by the embedding of Li + , covered the interstitial space and the defects, reducing the specific surface area and electrolyte consumption, and realizing the high Coulombic efficiency and good cycling performance of the electrode. Composite carbon materials with a core-shell structure have been prepared using a layer of amorphous carbon material coated on the surface of a silicon-based anode material (Fig. 2.(e)). ...

Highly Conductive Carbon/Carbon Composites as Advanced Multifunctional Anode Materials for Structural Lithium‐Ion Batteries

... The layered structure includes randomly multilayered structures, gradient distribution structures, sandwich-like structures, alternating multilayered structures, and "brick and mortar" structures, etc. [65] . By varying the EM characteristics and thickness of each layer in the multilayer absorber, frequency-tunable EM absorption performance and wide EAB can be achieved [20,66] . Usually, an EM reflection layer, an EM adsorption interlayer, and a surface impedance matching layer contribute to a multilayered structure. ...

Layered composites made of polymer derived SiOC/ZrB2 reinforced by ZrO2/SiO2 fibers with simultaneous microwave absorption and thermal insulation
  • Citing Article
  • February 2024

Journal of Material Science and Technology

... Carbon based materials [1][2][3][4][5], such as C/C composites, are widely used in the nose cones, wing leading edges of space shuttles, and nozzles of rocket engines, and when high energy lasers are applied to these components causing them to ablate, it can have catastrophic consequences for the overall equipment. However, C/C composites have poor oxidation resistance and begin to oxidize above about 400 • C in the atmosphere environment [6][7][8]. Therefore, it is necessary to design and prepare coating materials with good laser damage resistance for C/C composites. At present, the main forms of protection for high energy lasers include insulation protection [9,10], ablation protection [11][12][13], reflection protection [14][15][16], and composite protection [17]. ...

Ablation behavior of ZrC-SiHfOC-MoSi2 coating for carbon/carbon composites under Ar-O2 plasma flame

Corrosion Science