Wide Range Control of Microstructure and Mechanical Properties of Carbon Nanotube Forests: A Comparison Between Fixed and Floating Catalyst CVD Techniques

Vertically aligned carbon nanotube (CNT) forests may be used as miniature springs, compliant thermal interfaces, and shock absorbers, and for these and other applications it is vital to understand how to engineer their mechanical properties. Herein is investigated how the diameter and packing density within CNT forests govern their deformation behavior, structural stiffness, and elastic energy absorption properties. The mechanical behavior of low‐density CNT forests grown by fixed catalyst CVD methods and high‐density CNT forests grown by a floating catalyst CVD method are studied by in situ SEM compression testing and tribometer measurements of force‐displacement relationships. Low‐density and small‐diameter CNT columns (fixed catalyst) exhibit large plastic deformation and can be pre‐deformed to act as springs within a specified elastic range, whereas high‐density and large‐diameter CNT columns (floating catalyst) exhibit significant elastic recovery after deformation. In this work the energy absorption capacity of CNT forests is tuned over three orders of magnitude and it is shown that CNT forest density can be tuned over a range of conventional foam materials, but corresponding stiffness is ～10× higher. It is proposed that the elastic behavior of CNT forests is analogous to open‐cell foams and a simple model is presented. It is also shown that this model can be useful as a first‐order design tool to establish design guidelines for the mechanical properties of CNT forests and selection of the appropriate synthesis method.     

航天飞机外贮箱热防护系统

在航天飞机发射前和发射过程中,外贮箱热防护系统维护推进剂质量,保持箱体结构温度在设计要求范围内.它由两类基本材料组成:高效、硬质、闭孔泡沫塑料绝热材料和高热容、轻质、粘稠烧蚀材料.在外贮箱不同的部位,设计不同的热防护系统,满足发射航天飞机各种轨道不同的技术要求.     

陶瓷/泡沫铝/铝合金复合装甲抗射流侵彻性能

为研究泡沫铝复合装甲抗侵彻性能,根据应力波传播特性对陶瓷/泡沫铝/铝合金复合结构进行了理论分析。从不同泡沫铝夹芯厚度、相同厚度复合装甲下不同前后板厚度及布置方式和复合装甲倾角三方面研究了该复合装甲能量吸收规律、射流头部剩余速度以及不同倾角下装甲的防护性能。结果表明,泡沫铝作为夹芯层可充分降低复合装甲背板质点速度。同一倾角θ下,随着泡沫铝厚度的增大,复合装甲背板质点速度减小。泡沫铝厚度为2.4 mm时,射流头部剩余速度最低,复合装甲能量吸收最多,抗侵彻性能最优。同一泡沫铝厚度下,随着t_1/t_2值的增大,接触式复合装甲与间隔式复合装甲的射流头部剩余速度均先降低后增加。t_1/t_2=1时,间隔式复合装甲的抗侵彻性能最优。当仅布置方式不同时,间隔式与接触式复合装甲抗射流侵彻性能的差别较小。随着倾角θ的增大,复合装甲的防护性能先增强后降低。倾角为20°时,复合装甲抗射流侵彻性能最优。     

蜂窝状ZnCo2O4的制备及对AP和CL-20热分解行为的影响

为了研究双金属氧化物在固体推进剂中的催化性能,采用溶剂热法及热退火工艺成功制备出在镍泡沫(NF)上生长的蜂窝状ZnCo_2O_4(honeycombs ZnCo_2O_4,ZnCo_2O_4(HCs)),并通过X射线粉末衍射(XRD),X射线光电子能谱仪(XPS),傅里叶红外光谱仪(FT-IR),扫描电子显微镜(SEM)及N2吸附-脱附测试对其物相组成和形貌结构进行了表征分析。采用差示扫描量热法(DSC)研究了制备的ZnCo_2O_4(HCs)对高氯酸铵(AP)和六硝基六氮杂异伍兹烷(CL-20)的催化性能。结果表明,当ZnCo_2O_4(HCs)用量为20%时,ZnCo_2O_4(HCs)/AP和ZnCo_2O_4(HCs)/CL-20最低热分解峰温,分别为575.01,521.55 K。与纯AP和CL-20相比,ZnCo_2O_4(HCs)/AP和ZnCo_2O_4(HCs)/CL-20复合物的放热分解峰温分别提前了101.87,3.73 K,热分析动力学计算表明它们的表观活化能分别降低了17.88,6.23 kJ·mol~(-1)。与文献报道的纳米微晶状(NCs)、纳米线状(NWs)及纳米球状(NSs)ZnCo_2O_4相比,蜂窝状ZnCo_2O_4呈现出良好的催化活性,这可归因于ZnCo_2O_4(HCs)的多孔结构和大比表面积,能够为热分解反应提供丰富的活性位点。     

Prestoring Lithium into Stable 3D Nickel Foam Host as Dendrite‐Free Lithium Metal Anode

Lithium metal is considered a “Holy Grail” of anode materials for high‐energy‐density batteries. However, both dendritic lithium deposition and infinity dimension change during long‐term cycling have extremely restricted its practical applications for energy storage devices. Here, a thermal infusion strategy for prestoring lithium into a stable nickel foam host is demonstrated and a composite anode is achieved. In comparison with the bare lithium, the composite anode exhibits stable voltage profiles (200 mV at 5.0 mA cm^?2) with a small hysteresis beyond 100 cycles in carbonate‐based electrolyte, as well as high rate capability, significantly reduced interfacial resistance, and small polarization in a full‐cell battery with Li₄Ti₅O₁₂ or LiFePO₄ as counter electrode. More importantly, in addition to the fact that lithium is successfully confined in the metallic nickel foam host, uniform lithium plating/stripping is achieved with a low dimension change (merely ≈3.1%) and effective inhibition of dendrite formation. The mechanism for uniform lithium stripping/plating behavior is explained based on a surface energy model.     

Constraining Si Particles within Graphene Foam Monolith: Interfacial Modification for High‐Performance Li+ Storage and Flexible Integrated Configuration

Pulverization of electrode materials and loss of electrical contact have been identified as the major causes for the performance deterioration of alloy anodes in Li‐ion batteries. This study presents the hierarchical arrangement of spatially confining silicon nanoparticles (Si NPs) within graphene foam (GF) for alleviating these issues. Through a freeze‐drying method, the highly oriented GF monolith is engineered to fully encapsulate the Si NPs, serving not only as a robust framework with the well‐accessible thoroughfares for electrolyte percolation but also a physical blocking layer to restrain Si from direct exposure to the electrolyte. In return, the pillar effect of Si NPs prevents the graphene sheets from restacking while preserving the highly efficient electron/Li⁺ transport channels. When evaluated as a binder‐free anode, impressive cycle performance is realized in both half‐cell and full‐cell configurations. Operando X‐ray diffraction and in‐house X‐ray photoelectron spectroscopy confirm the pivotal protection of GF to sheathe the most volume‐expanded lithiated phase (Li₁₅Si₄) at room temperature. Furthermore, a free‐standing composite film is developed through readjusting the pore size in GF/Si monolith and directly integrated with nanocellulose membrane (NCM) separator. Because of the good electrical conductivity and structural integrity of the GF monolith as well as the flexibility of the NCM separator, the as‐developed GF/Si‐NCM electrode showcases the potential use in the flexible electronic devices.     

消失模振动台的人机界面系统设计

介绍了新型雨淋加砂振动台的工作过程及电气控制设计方案,重点阐述了振动台的人机界面设计以及特点;该方案操作方便,能满足不同铸件的生产工艺要求。     

泡沫铝/改性环氧树脂复合材料压缩力学性能的试验研究

为实现功能结构一体化,提高泡沫铝复合材料的力学性能和吸能性能,制备硅橡胶改性环氧树脂的高分子材料填充泡沫铝的复合材料。静态压缩试验表明,填充改性环氧树脂的泡沫铝的平台屈服阶段明显抬升,改善泡沫铝的力学性能,提高泡沫铝的吸能能力。     

Graphene-Based Ultralight Compartmentalized Isotropic Foams with an Extremely Low Thermal Conductivity of 5.75 mW m−1 K−1

The importance of high-performance thermal insulation materials is rapidly emerging due to energy conservation and the management of temperature-sensitive device perspectives. Recent thermal insulation materials including complex structures have been developed either by reducing the structural connectivity to mitigate thermal transport through solid conduction or forming directionally aligned confined inner pores to suppress the internal gas convection. In this study, to create a highly efficient thermal insulating material that suppresses thermal transport in all directions, graphene-based anisotropic closed-cellular structures (CCS) are devised with a highly ordered assembly of hollow compartments with extremely thin walls (≈50 nm). This uniquely designed CCS made from microfluidically synthesized graphene solid bubbles exhibited a remarkably low thermal conductivity of 5.75 mW m⁻¹ K⁻¹ thanks to effective suppression of both solid conduction and gas conduction/convection. Therefore, the proposed strategy in this work offers a novel toolkit for implementing next-generation high-performance insulation materials.     

泡沫对激光制导武器系统干扰效能分析

泡沫作为一种新兴的光电无源干扰手段,对激光制导武器干扰作用明显。首先对泡沫干扰进行了介绍,然后对激光制导武器系统的组成、制导原理进行了描述,探讨了利用泡沫干扰激光制导武器系统的可行性。并根据泡沫干扰及激光制导武器系统探测目标的要求,分别计算了激光制导武器系统的遮蔽面密度和泡沫两点间面密度,通过两种面密度的比较,对泡沫的干扰效能进行了分析。本文的研究结论对泡沫干扰效能评估及泡沫干扰战术的优化具有重要的意义。