Jian Wang

An in-depth exploration and development of multifunctional stealth materials with multispectral compatibility holds significant importance in addressing the increasingly deteriorating electromagnetic environment and the rapid advancement of detection systems. This study proposes an alternative fabric design that achieves precise multiscale interface construction through the ingenious combination of polymer matrices in different forms coupled with the precise customization and arrangement of functional particles. Due to the synergistic effects between dielectric/magnetic loss mechanisms and infrared shielding characteristics, the fabricated metafabric exhibits ultrahigh electromagnetic wave absorption performance and superior infrared stealth capability. The radar wave effective absorption area covers over 95.6% of the X-band spectrum and delivers a minimum reflection loss of up to -56.6 dB (at 10.11 GHz). Additionally, it provides a highly effective camouflage against infrared radiation across diverse operational environments. Furthermore, finite element simulations were employed to model the radar cross section in the far field under electromagnetic fields, investigating the actual response to electromagnetic wave signals. This study achieves highly efficient broadband stealth functionality, establishing a multifrequency compatible stealth material model based on multiscale interface design. It facilitates a deep understanding of detailed dielectric and magnetic coupling mechanisms, thereby advancing the development of theoretical frameworks and methodologies for optimizing camouflage functionality design.

Microchannel boiling heat transfer offers a promising solution to address extreme high heat flux in miniaturized and high-density integrated electronic devices. In this work, we performed numerical studies to elucidate the complex flow boiling process in a square microchannel using the open-source platform OpenFOAM. The governing equations of the two-phase flow was solved based on the finite volume framework, and the surface tension together with phase change models were incorporated to study the wettability and conjugate heat transfer in the microchannel. First, the stability of the liquid film shows that smaller contact angles lead to thicker and stable liquid films near the solid wall, while larger contact angles result in thinner and fragile films. Second, the heat transfer, represented by the dimensionless parameter Nu, shows that smaller contact angles result in higher and more uniform distribution of Nu. The velocity gradient causes the Nu to be higher for the sidewalls than for the bottom wall. The contact angle also affects the peak value of Nu. Small contact angles result in large solid wall temperature gradients and more efficient heat transfer. Finally, the simulation results show that smaller thermal conductivity of the solid results in larger temperature gradient in the flow direction, implying that the conjugate heat transfer is less efficient. Overall, the present numerical findings provide useful guidance for efficient thermal management in electronic devices.

The opposing effects of interface effect on phonon heat conduction and microwave absorption hinder the integration of thermal conduction with microwave absorption. Reconciling this contradiction by rationally optimizing the material structure remains a challenge. Herein, Fe ₂ N@CNTs with heterogeneous interfaces and multi‐dimensional encapsulated structures are fabricated through in situ chemical vapor deposition (CVD) to realize excellent thermal conductivity and microwave absorption. When the mass fraction of active material is 40 wt.%, the thermal conductivity ( λ _// ) of Fe ₂ N@CNTs composite films is 6.69 W m ⁻¹ K ⁻¹ . The minimum reflection loss (RL _min ) is up to −54.55 dB at a thickness of 3.43 mm, and the effective absorption bandwidth (EAB) is 5.52 GHz at 1.82 mm. The well‐designed Fe ₂ N@CNTs is an ideal model for gaining insight into electron transportation and phonon multi‐dimensional heat transport. Heterogeneous interface improves the dielectric loss and facilitates microwave and phonon absorption and conversion. The multi‐dimensional encapsulation structure increases the electron transport path and phonon heat transport paths making axial and radial co‐transfer possible. The multi‐dimensional encapsulation structure demonstrates extremely high microwave absorption and thermal management capabilities, which open up new possibilities for multifunctional applications in host materials with internal hollows.

Exploring interfacial thermal transport of a heterojunction interface is crucial to achieving advanced thermal management for gallium nitride-based high electron mobility transistor devices. The current research primarily focuses on material enhancements and microstructure design at the interfaces of epitaxial layers, buffer layers, and substrates, such as the GaN/SiC interface and GaN/AlN interface. Yet, the influence of different concentrations of Al/Ga atoms and interface roughness on the interfacial thermal conductance (ITC) of AlGaN/GaN interface, the closest interface to the hot spot, is still poorly understood. Herein, we focus on the rough AlGaN/GaN interface and evaluate the changes in ITC under different Al–Ga atomic concentrations and interface roughness using atomistic simulations. When the interface is completely smooth and AlGaN and GaN are arranged according to common polarization characteristic structures, the ITC gradually increases as the proportion of Al atoms decreases. When the proportion of Al atoms is reduced to 20%–30%, the impact of the interface structure on heat transfer is almost negligible. For interface models with different roughness levels, as the interface roughness increases, the ITC drops from 735.09 MW m−2 K−1 (smooth interface) to 469.47 MW m−2 K−1 by 36.13%. The decrease in ITC is attributed to phonon localization induced by rough interfaces. The phonon modes at the interface are significantly different from those in bulk materials. The degree of phonon localization is most pronounced in the frequency range that contributes significantly to heat flux. This work provides valuable physical insights into understanding the thermal transfer behaviors across the rough AlGaN/GaN interfaces.

Water dissociation on TiO2 surfaces has been known for decades and holds great potential in various applications, many of which require a proper understanding of thermal transport across the TiO2–H2O interface. Molecular dynamics (MD) simulations play an important role in characterizing complex systems’ interfacial thermal transport properties. Nevertheless, due to the imprecision of empirical force field potentials, the interfacial thermal transport mechanism involving water dissociation remains to be determined. To cope with this, a deep potential (DP) model is formulated through the utilization of ab initio datasets. This model successfully simulates interfacial thermal transport accompanied by water dissociation on the TiO2 surfaces. The trained DP achieves a total energy accuracy of ∼238.8 meV and a force accuracy of ∼197.05 meV/Å. The DPMD simulations show that water dissociation induces the formation of hydrogen bonding networks and molecular bridges. Structural modifications further affect interfacial thermal transport. The interfacial thermal conductance estimated by DP is ∼8.54 × 109 W/m2 K, smaller than ∼13.17 × 109 W/m2 K by empirical potentials. The vibrational density of states (VDOS) quantifies the differences between the DP model and empirical potentials. Notably, the VDOS disparity between the adsorbed hydrogen atoms and normal hydrogen atoms demonstrates the influence of water dissociation on heat transfer processes. This work aims to understand the effect of water dissociation on thermal transport at the TiO2–H2O interface. The findings will provide valuable guidance for the thermal management of photocatalytic devices.

As the power density of GaN HEMT devices increases, its internal operating temperature increases sharply, which would affect the electrical characteristics of devices and greatly limit the service life. The operating temperature has become an important technical index to evaluate the thermal design level of GaN HEMT devices. In this work, a high-speed transient thermal simulation method for GaN HEMT devices is proposed to enable the finite element thermal analysis with high efficiency and accuracy. Based on the law of energy conservation, the functional relationship between the trough temperature and time of the transient operation is built. Then the transient trough temperature is calculated using the limit solution algorithm when the device reaches a stable state, and the initial temperature of the model is set by loading a constant temperature heat source. The finite element analysis software FloTHERM is used to calculate a small number of pulse stress cycles and then quickly obtain the transient temperature field of the dynamic equilibrium state of GaN HEMT device. It is found that the fitting degree is very high compared with the traditional simulation method. This method effectively obtains the transient temperature distribution of GaN HEMT device at any time after dynamic equilibrium, but also avoids the calculation process over ten million cycles before reaching dynamic temperature equilibrium and greatly reduces the simulation time and computational cost.

A patent-pending new solder paste material has been developed for assembly of SiP with outstanding slump resistance without compromising the paste volume printed, and exhibit acceptable solder joint service temperature. In the fine pitch solder paste using medium temperature solder powder (MTSP), low melting solder powder (LMSP) was introduced. At printing, the powder mixture behaved as regular fine-pitch solder paste. Upon heating, the LMSP melted and formed "Powder Cluster" with surrounding MTSP. The Powder Cluster resisted slump, thus avoided solder bridging, consequently enabled high yield of assembly. For both 58Bi42Sn and 52In48Sn LMSP systems used together with SAC305 in solder paste, the desired LMSP content was found to be no less than 10% w/w of solder paste for great slump resistance at 100°C, 150°C, and 200°C, and no more than 15% w/w of solder paste for 1 ^st sign of melting of reflowed solder to be no lower than around 179°C. The slump resistance achieved at 100°C which was lower than the melting temperature of both 58Bi42Sn and 52In48Sn was attributed to solid-state diffusion bonding mechanism. At LMSP content > 10%, slump increased with increasing BiSn LMSP content due to metal volume factor, due to density being higher for 58Bi42Sn than SAC. This was not true for InSn system. 52In48Sn was slightly more effective in suppressing slump than 58Bi42Sn, attributable to the lower melting temperature of 52In48Sn. Higher print thickness and higher heating temperature resulted in a higher slump rate, indicating choice of LMSP and MTSP would affect the potential of slump resistance. Print pattern was found to affect slump resistance through "Paste Crowdedness" factor, which can be used as a tool for assessing the potential of slump during stencil design phase. The overall metal load tested was found to be acceptable at 82% w/w and can be further optimized for better print and non-slump performance.