Hyperelastic,Robust, Fire-Safe Multifunctional MXene Aerogels with Unprecedented Electromagnetic Interference Shielding Efficiency

MXene aerogels have shown great potential for many important functional applications, in particular electromagnetic interference (EMI) shielding. However, it has been a grand challenge to create mechanically hyperelastic, air-stable, and durable MXene aerogels for enabling effective EMI protection at low concentrations due to the difficulties in achieving tailorable porous structures, excellent mechanical elasticity, and desired antioxidation capabilities of MXene in air. Here, a facile strategy for fabricating MXene composite aerogels by co-assembling MXene and cellulose nanofibers during freeze-drying followed by surface encapsulation with fire-retardant thermoplastic polyurethane (TPU) is reported. Because of the maximum utilization of pore structures of MXene, and conductive loss enhanced by multiple internal reflections, as-prepared aerogel with 3.14 wt% of MXene exhibits an exceptionally high EMI shielding effectiveness of 93.5 dB, and an ultra-high MXene utilization efficiency of 2977.71 dB g g⁻¹, tripling the values in previous works. Owing to the presence of multiple hydrogen bonding and the TPU elastomer, the aerogel exhibits a hyperelastic feature with additional strength, excellent stability, superior durability, and high fire safety. This study provides a facile strategy for creating multifunctional aerogels with great potential for applications in EMI protection, wearable devices, thermal management, pressure sensing, and intelligent fire monitoring.     

Effects of A-site ionic size on the phase transition behavior of lead-free niobate ceramics

Understanding the phase structure evolution is important for developing high performance lead-free piezoelectric materials. In this work, the effects of A-site ionic size of monovalent ions on the phase transition behaviors for the lead-free niobate ceramics ANbO₃ (A = Li, Na, Ag, and K) are investigated using XRD analysis and dielectric measurement. The iso-valent ionic doping restrains the relaxation behavior that usually appears in the hetero-valent ionic-doped niobate ceramics. The A-site average ionic size of R_A and its ionic radius differences of ΔR_A are found to be crucial influence factors on the phase transition behaviors of the ANbO₃ ceramics. Small Li⁺ doping stabilize tetragonal phase of the ANbO₃ ceramics with R_A > 1.47 Å, but stabilize rhombohedral phase of the ones with R_A < 1.47 Å. On the other hand, The ANbO₃ ceramics without Li⁺ doping prefer to orthorhombic phase due to indistinctive ionic size differences (ΔR_A < 0.25 Å). Our results suggest that a certain phase and phase transition boundary could be designed by appropriate ionic doping for developing the niobate-based lead free piezoelectric ceramics.     

Microstructural characterization and high cycle fatigue behavior of investment cast A357 aluminum alloy

In order to observe the influence of strontium (Sr) modification and hot isostatic pressing (HIP) on an aluminum–silicon cast alloy A357 (AlSi7Mg0.6), the microstructure and the high cycle fatigue behavior of three batches of materials produced by investment casting (IC) were studied. The parts were produced by an advanced IC proprietary process. The main process innovation is to increase the solidification and cooling rate by immersing the mold in cool liquid. Its advantage is to produce finer microstructures. Microstructural characterization showed a dendrite arm spacing (DAS) refinement of 40% when compared with the same part produced by conventional investment casting. Fatigue tests were conducted on hourglass specimens heat treated to T6, under a stress ratio of R = 0.1 and a frequency of 25 Hz. One batch of material was unmodified but two batches were modified with 0.007% and 0.013% Sr addition, from which one batch was submitted to HIP after casting. Results reported in S–N diagrams show that the addition of Sr and the HIP process improve the 10⁶ cycles fatigue strength by 9% and 34% respectively. Scanning electron microscopy (SEM) observation of the fracture surfaces showed a variety of crack initiation mechanisms. In the unmodified alloy, decohesion between the coarse Si particles and the aluminum matrix was mostly observed. On the other hand, in the modified but non HIP-ed alloy, cracks initiated from pores. When the same alloy was subjected to HIP, a competition between crystallographic crack initiations (at persistent slip bands) and decohesion/failure of intermetallic phases was observed. When compared to fatigue strength reported for components produced by permanent mold casting, the studied material are more resistant to fatigue even in the unmodified and non HIP-ed states.     

In situ chemical reduction and functionalization of graphene oxide for electrically conductive phenol formaldehyde composites

We report an efficient one-step approach to reduce and functionalize graphene oxide (GO) during the in situ polymerization of phenol and formaldehyde. The hydrophilic and electrically insulating GO is converted to hydrophobic and electrically conductive graphene with phenol as the main reducing agent. Simultaneously, functionalization of GO is realized by the nucleophilic substitution reaction of the epoxide groups of GO with the hydroxyl groups of phenol in an alkali condition. Different from the insulating GO and phenol formaldehyde resin (PF) components, PF composites are electrically conductive due to the incidental reduction of GO during the in situ polymerization. The electrical conductivity of PF composite with 0.85 vol.% of GO is 0.20 S/m, nearly nine orders of magnitude higher than that of neat PF. Moreover, the efficient reduction and functionalization of GO endows the PF composites with high thermal stability and flexural properties. A striking increase in decomposition temperature is achieved with 2.3 vol.% of GO. The flexural strength and modulus of the PF composite with 1.7 vol.% GO are increased by 316.8% and 56.7%, respectively.     

Main-chain poly(phosphoester)s: History,syntheses, degradation,bio-and flame-retardant applications

Nature on planet earth is dominated by poly(phosphoester)s (PPEs). They structure and determine life in the form of deoxy- and ribonucleic acid (DNA & RNA), and, as pyrophosphates, they store chemical energy in organisms. Polymer chemistry, however, is dominated by the non-degradable polyolefins and degradable polycarboxylic esters (PCEs) produced on a large scale today. Recent work has illustrated the potential of PPEs for future applications beyond flame-retardancy, the main application of PPEs today, and provided a coherent vision to implement this classic biopolymer in modern applications that demand biocompatibility and degradability as well as the possibility to adjust the properties to individual needs. This comprehensive review summarizes synthetic protocols to PPEs, their applications in biomedicine, e.g., as biodegradable drug carrier or in tissue engineering, and their flame retardant properties. We highlight recent developments that may make phosphorus-based polymers attractive materials for various future applications.     

喷涂工艺对超音速火焰喷涂Al-Cu-Fe-Si准晶合金涂层性能的影响

目的使用活性燃烧高速燃气喷涂(AC-HVAF)方法制备高质量的Al-Cu-Fe-Si准晶涂层,研究喷涂工艺对涂层性能的影响。方法采用气雾化Al-Cu-Fe-Si准晶合金粉末,利用AK02T型AC-HVAF喷涂系统制备Al-Cu-Fe-Si准晶涂层材料。通过X射线衍射及扫描电镜观察分析准晶合金粉末和涂层的组织与结构,通过电化学工作站、显微硬度计和接触角测试仪等手段分析准晶合金涂层的耐蚀性、显微硬度及抗粘性能。结果对气雾化准晶Al-Cu-Fe-Si合金粉末的研究发现,冷却速率显著影响准晶合金粉末的组织,在冷却速率较快的粉末中形成胞状晶组织,准晶I相含量较高。对准晶合金涂层进行热处理,高温退火显著提高了涂层的硬度,950℃退火12 h后,硬度值达到(724±153)HV0.1。分别对准晶合金涂层和基体45~#钢的接触角进行测量,准晶合金涂层的接触角最大为95°,而45~#钢的仅为79°。通过电化学工作站测试比较涂层的耐蚀性,发现在3.5%(质量分数)的Na Cl溶液中,喷涂在45~#钢和5052铝合金基体上的涂层腐蚀电流密度J_(corr)分别为6.8×10~(-6),2.0×10~(-7)A/cm~2。结论不同粒径的气雾化准晶合金粉末的相组成不同,选择合适的粒径是保证铝基准晶合金涂层质量的前提。对涂层进行合适的热处理可以有效地提高涂层的显微硬度,铝基准晶合金涂层的接触角较45~#钢的高,提高了基体的抗粘性。不同基体上制备的准晶合金涂层的耐蚀性有很大差异,5052铝合金基体上的准晶涂层耐蚀性优于喷涂在45~#钢基体上的涂层。     

High electromagnetic wave absorbing performance of activated hollow carbon fibers decorated with CNTs and Ni nanoparticles

Activated hollow carbon fibers (ACHFs) decorated with carbon nanotubes (CNTs) and nickel nanoparticles (CNTs–Ni–ACHFs) were prepared by thermal reduction and chemical vapor deposition technique. Microwave reflection loss, permittivity and permeability of CNTs–Ni–ACHFs composites as novel electromagnetic wave absorbents were studied in the frequency range of 2–18 GHz. It was demonstrated that CNTs–Ni–ACHFs absorbents possessed the best microwave absorbing performances whose minimum reflection loss was −43.457 dB at 13.10 GHz with a thickness of 2.0 mm, which is much better than those of Ni–ACHFs and ACHFs samples. The dielectric polarizations and magnetic loss derived from the effect of the porous structures, Ni nanoparticles, and defects in the CNTs–Ni–ACHFs composites are playing an important role for the excellent microwave absorbing performances.     

Evolution of structure and electrical properties with lanthanum content in [(Bi1/2Na1/2)0.95Ba0.05]1−xLaxTiO3 ceramics

Lead-free perovskite [(Bi_1/2Na_1/2)_0.95Ba_0.05]_1−xLa_xTiO₃ (x = 0.00–0.10) ceramics were fabricated via the solid state reaction method and their crystal structures and electrical properties were systematically studied. Transmission electron microscopy examination reveals a transition from a rhombohedral R3c phase with micron-sized complex domains to a mixture of rhombohedral R3c and tetragonal P4bm phases with nanodomains as La content was increased. X-ray diffraction analysis on bulk samples, however, indicates a pseudocubic structure in La-doped compositions. Electrical poling seems to transform the pseudocubic structure to a rhombohedral phase in compositions with 0.00 < x < 0.03. With La addition, the depolarization temperature (T_d) is observed to decrease and the dielectric constant (ɛ_r) within 100–350 °C becomes more temperature independent, promising for applications in high-temperature capacitors. Electric field-induced polarizations and strains display complex changes with respect to La concentration, with the highest strain of 0.35% achieved in the composition x = 0.04 under 70 kV/cm at room temperature. The piezoelectric coefficient d₃₃ initially increases with La content, reaching a maximum value of 151 pC/N in the composition x = 0.02, and then diminishes.     

Effect of thermo-mechanical process on the microstructure and secondary-deformation behavior of 22MnB5 steels

22MnB5 steel specimens were deformed at 923 K and 693 K to three strain levels to study the effect of applied strain level on the microstructure and secondary-deformation behavior. As the steel was deformed at 923 K, deformation induced ferrite transformation (DIFT) occurred even when a small strain of 0.044 was applied, and the volume fraction of deformation induced ferrite (DIF) increases with increasing applied strain level. When deformed at 693 K, deformation induced bainite transformation (DIBT) was observed when the applied strain was larger than 0.109. The incubation period for DIFT is shorter than that for DIBT, but the DIBT proceeds much faster than DIFT. Sub-size tensile specimens were cut from the hot deformed 22MnB5 steel specimens, and digital image correlation technique was employed to investigate the secondary-deformation behavior of the sub-size tensile specimens at room temperature. It is found that the appearance of DIF or DIB (deformation induced bainite) decreases the yield strength and ultimate tensile strength (UTS) but increases the elongation and strength–ductility product of the hot deformed 22MnB5 steel specimens compared with the as-quenched 22MnB5 steel specimen with full martensite.     

Synthesis of the uniform hollow spherical Sr2SiO4:Eu2+ phosphors via an h-BN protective method

The uniform hollow spherical Sr₂SiO₄:Eu²⁺ green emitting phosphors have been successfully synthesized using hollow silica spheres as templates by an h-BN protective method. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results directly confirmed the existence of the hollow spherical structure with a narrow size distribution and a shell thickness of 15–25 nm. The h-BN protective film, observed by high resolution TEM, plays an important role in the formation of the hollow spherical morphology and the improvement of photoluminescence properties. Comparing with the Sr₂SiO₄:Eu²⁺ micron-phosphor prepared by the traditional solid state reaction method, the hollow spherical phosphor with nano-sized grains exhibits stronger green emission under ultraviolet–blue light excitation. This could be attributed to the elimination of surface defects by the h-BN coating. This research gives an economic and convenient way to synthesize uniform spherical phosphors with high quantum efficiency.