Laminar Metal Foam: A Soft and Highly Thermally Conductive Thermal Interface Material with a Reliable Joint for Semiconductor Packaging.

Future electronic packaging technology requires semiconductor chips having a larger size and higher power for advanced applications, e.g., new energy conversion systems, electric vehicles, and data center servers, yet traditional thermal interface materials (TIMs) with a high thermal conductivity are generally stiff materials with weak joints, which cause the accumulated thermal stress to concentrate at the chip corners, leading to cracking and popcorn problems. To address such a critical challenge, herein for the first time we report a low-cost and high-performance porous copper (Cu)-indium (In) laminar structure as TIM, which can provide a superior thermal conductivity (50 W m-1 K-1) comparable to indium, yet the Young's modulus (1.0 GPa) is an order of magnitude lower than indium, which is a state-of-the-art value. Additionally, the In-based intermetallic compound (IMC) joints enable more robust mechanical interconnection above the melting point of pure indium, providing better high-temperature performance. The discontinuous IMCs spread the global interfacial thermal stress into numerous isolated local areas, ensuring a reliable joint to resist thermal-mechanical fatigue. In the silicon-TIM-copper package testing vehicles with a large die size (1 × 1 square inch), this structure shows excellent thermal management ability and superior reliability, compared with classical indium and classic commercial silver pastes.

Exploration of the form factors of turbulence kinetic energy transfer for shear exfoliation of graphene.

Mass production of defect-free and large-lateral-size 2D materials via cost-effective methods is very important. Recently, shear exfoliation has shown great promise for large-scale production due to its simple operation, environmental-benignity and wide adaptability. However, a long-standing challenge is that with the production of more nanosheets, a ceiling yield and shattered products are encountered, which significantly limits their wider application. The method and efficiency of energy transfer in fluid is undoubtedly the key point in determining exfoliation efficiency, yet its in-depth mechanism has not yet been described. Thus, a thorough investigation of turbulence energy transfer is critically necessary. Herein, we identify two main factors that critically determine the exfoliation yield and provide a statistical analysis of the relationship between these factors and the exfoliation yield. In the initial shearing process, the coexistence of the 2D nanosheets and raw particles is the dominant factor; as time passes, the dimensional change of raw materials gradually has a greater influence on the energy transfer. These factors together lead to attenuated efficiency and a power function relationship between yield and exfoliation time. This investigation gives a statistical explanation of shear exfoliation technology for 2D material preparation and provides valuable insights for mechanical exfoliating high-quality 2D materials.

Facile Synthesis of Ant-Nest-Like Porous Duplex Copper as Deeply Cycling Host for Lithium Metal Anodes.

Suppressing the dendrite formation and managing the volume change of lithium (Li) metal anode have been global challenges in the lithium batteries community. Herein, a duplex copper (Cu) foil with an ant-nest-like network and a dense substrate is reported for an ultrastable Li metal anode. The duplex Cu is fabricated by sulfurization of thick Cu foil with a subsequent skeleton self-welding procedure. Uniform Li deposition is achieved by the 3D interconnected architecture and lithiophilic surface of self-welded Cu skeleton. The sufficient space in the porous layer enables a large areal capacity for Li and significantly improves the electrode-electrolyte interface. Simulations reveal that the structure allows proper electric field penetration into the connected tunnels. The assembled Li anodes exhibit high coulombic efficiency (97.3% over 300 cycles) and long lifespan (>880 h) at a current density of 1 mA cm-2 with a capacity of 1 mAh cm-2 . Stable and deep cycling can be maintained up to 50 times at a high capacity of 10 mAh cm-2 .

Simultaneously Homogenized Electric Field and Ionic Flux for Reversible Ultrahigh-Areal-Capacity Li Deposition.

High areal capacity and stable Coulombic efficiency (CE) in lithium metal anodes (LMAs) play pivotal roles in developing high-energy-density rechargeable batteries. However, few reported LMAs delivering high and stable CE (>50 cycles) under ultrahigh areal capacity (>10 mA h cm-2). We demonstrated that the simultaneous homogenization of electric field and Li ion flux by using self-supported and surface-oxidized 3D hollow porous copper fibers (3D-HPCFs) can greatly increase both the areal capacity and reversibility of Li deposition. Li can be easily confined inside the hollow porous fibers and within the interspaces among fibers without uncontrollable Li dendrites. The 3D-HPCF-based anode can be deeply cycled at high capacity of 15 mA h cm-2 with average CE of 98.87% for 53 cycles, enabling a practical cell to realize high capacity retentions at a surplus Li of 10%. This work provides a novel Li deposition-regulation technology in LMAs targeting for next generation high-energy-density batteries.

Directing lateral growth of lithium dendrites in micro-compartmented anode arrays for safe lithium metal batteries.

Uncontrolled growth of lithium dendrites during cycling has remained a challenging issue for lithium metal batteries. Thus far, various approaches have been proposed to delay or suppress dendrite growth, yet little attention has been paid to the solutions that can make batteries keep working when lithium dendrites are already extensively present. Here we develop an industry-adoptable technology to laterally direct the growth of lithium dendrites, where all dendrites are retained inside the compartmented copper current collector in a given limited cycling capacity. This featured electrode layout renders superior cycling stability (e.g., smoothly running for over 150 cycles at 0.5 mA cm-2). Numerical simulations indicate that reduced dendritic stress and damage to the separator are achieved when the battery is abusively running over the ceiling capacity to generate protrusions. This study may contribute to a deeper comprehension of metal dendrites and provide a significant step towards ultimate safe batteries.

Fractal dendrite-based electrically conductive composites for laser-scribed flexible circuits.

Fractal metallic dendrites have been drawing more attentions recently, yet they have rarely been explored in electronic printing or packaging applications because of the great challenges in large-scale synthesis and limited understanding in such applications. Here we demonstrate a controllable synthesis of fractal Ag micro-dendrites at the hundred-gram scale. When used as the fillers for isotropically electrically conductive composites (ECCs), the unique three-dimensional fractal geometrical configuration and low-temperature sintering characteristic render the Ag micro dendrites with an ultra-low electrical percolation threshold of 0.97 vol% (8 wt%). The ultra-low percolation threshold and self-limited fusing ability may address some critical challenges in current interconnect technology for microelectronics. For example, only half of the laser-scribe energy is needed to pattern fine circuit lines printed using the present ECCs, showing great potential for wiring ultrathin circuits for high performance flexible electronics.

Conformal Pad-Printing Electrically Conductive Composites onto Thermoplastic Hemispheres: Toward Sustainable Fabrication of 3-Cents Volumetric Electrically Small Antennas.

Electrically small antennas (ESAs) are becoming one of the key components in the compact wireless devices for telecommunications, defence, and aerospace systems, especially for the spherical one whose geometric layout is more closely approaching Chu's limit, thus yielding significant bandwidth improvements relative to the linear and planar counterparts. Yet broad applications of the volumetric ESAs are still hindered since the low cost fabrication has remained a tremendous challenge. Here we report a state-of-the-art technology to transfer electrically conductive composites (ECCs) from a planar mould to a volumetric thermoplastic substrate by using pad-printing technology without pattern distortion, benefit from the excellent properties of the ECCs as well as the printing-calibration method that we developed. The antenna samples prepared in this way meet the stringent requirement of an ESA (ka is as low as 0.32 and the antenna efficiency is as high as 57%), suggesting that volumetric electronic components i.e. the antennas can be produced in such a simple, green, and cost-effective way. This work can be of interest for the development of studies on green and high performance wireless communication devices.

Towards practical application of paper based printed circuits: capillarity effectively enhances conductivity of the thermoplastic electrically conductive adhesives.

Direct printing nanoparticle-based conductive inks onto paper substrates has encountered difficulties e.g. the nanoparticles are prone to penetrate into the pores of the paper and become partially segmented, and the necessary low-temperature-sintering process is harmful to the dimension-stability of paper. Here we prototyped the paper-based circuit substrate in combination with printed thermoplastic electrically conductive adhesives (ECA), which takes the advantage of the capillarity of paper and thus both the conductivity and mechanical robustness of the printed circuits were drastically improved without sintering process. For instance, the electrical resistivity of the ECA specimen on a pulp paper (6 × 10(-5)Ω · cm, with 50 wt% loading of Ag) was only 14% of that on PET film than that on PET film. This improvement has been found directly related to the sizing degree of paper, in agreement with the effective medium approximation simulation results in this work. The thermoplastic nature also enables excellent mechanical strength of the printed ECA to resist repeated folding. Considering the generality of the process and the wide acceptance of ECA technique in the modern electronic packages, this method may find vast applications in e.g. circuit boards, capacitive touch pads, and radio frequency identification antennas, which have been prototyped in the manuscript.