With the rapid development of electronic information technology, electromagnetic pollution and interference have become more and more serious. It is particularly important to develop microwave absorption materials with comprehensive excellent performance of "wide, thin, light and strong". Graphene materials have the advantages of light weight, high conductivity, large specific surface area and strong dielectric loss, however, the impedance matching performance is poor and the loss mechanism is single. Interestingly, the impedance mismatch can be effectively improved by doping heterogeneous elements or designing morphology and structure. Herein, based on electromagnetic wave absorption theory, this paper describes the research progress of different dimensions of graphene-based absorbing materials, and discusses the properties and microwave absorbing mechanism of different graphene-based absorbing materials in detail. The shortcomings of current research work in the field of graphene absorbing materials are also discussed. Finally, the future research direction and development prospect of graphene-based absorbing materials are prospected.
Aiming at the matter of unqualified ultrasonic inspection results of the forged GH907 alloy ring used for aero engine turbine casing, the influence of the microstructure on the ultrasonic attenuation of the forged alloy ring is quantitatively assessed by means of ultrasonic testing and microstructure observation, and the cause of the bottom wave loss observed in ultrasonic testing is elucidated. The results show that the shape of the large attenuated area in the ultrasonic bottom wave amplitude image was consistent with that of the black grain area observed in the macrostructure of GH907 alloy, and the average grain size of the black grain area was larger than that of the non-black grain area, and there were a large number of ε phase with the morphology of Widmannsttten structure. The precipitation of a large number of ε phase can increase ultrasonic attenuation by nearly 40%. It is believed that the forging process should be optimized from the following three aspects: grain refinement, controlling the uniformity of grain size and inhibiting excessive precipitation of ε phase, so that to improve the qualification rate of the forged rings.
316LN austenitic stainless steel (316LN steel) is widely used as structural components in nuclear industries for their excellent corrosion resistance and high temperature mechanical properties. With the development of next-generation nuclear reactors, it is imperative to improve the high temperature creep properties of 316LN steel to warrant the corresponding components being subjected to higher temperatures. Alloying with rare earth (RE) elements, such as Cerium (Ce), are considered as a promising approach to enhance creep properties of the austenitic stainless steels. However, the effect of Ce on the microstructure evolution and creep performance of 316LN steel have not been reported. In this study, the effect of Ce on creep behavior and microstructure of 316LN steel in the temperature range 600oC to 700oC and stresses range150 MPa to 200 MPa was investigated by electronic creep tester, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the addition of 0.032%Ce could significantly improve the creep rupture life of 316LN steel. For instance, at 700oC/150 MPa, the creep life of 316LN steel with 0.032%Ce prominently increased from 313 h to 556 h. The creep stress exponent, activation energy and threshold stress of 316LN steel without Ce were found to be 7.64, 415.3 kJ/mol and 61.7 MPa, respectively, whereas those of 316LN steel with 0.032%Ce addition were corresponding to 9.07, 454.8 kJ/mol and 76.6 MPa. The creep mechanism of those two 316LN steels were also controlled by dislocation climbing. The addition of 0.032%Ce has not changed the creep mechanism of 316LN steel but clearly raised the creep activation energy and threshold stress. Furthermore, microstructural analysis demonstrates that the addition of 0.032%Ce may obviously promote the precipitation of Laves phase within the matrix. These intragranular Laves phases could inhibit the movement of dislocations during creep deformation, notably enhancing the creep resistance of the matrix. Therefore, the addition of 0.032%Ce remarkably improved the creep properties of 316LN steel by intragranular Laves precipitation strengthening.
In order to improve the interlaminar toughness of carbon fiber/epoxy (CF/EP) composites, polyethersulfone (PES) porous fibers were prepared by wet spinning, and four PES porous fiber veils (PESV) with different areal densities were prepared by wet-laid method, which were subsequently used for interlaminar toughening of CF/EP composites by vacuum-assisted resin infusion molding (VARI). The dissolution behavior of PES porous fibers in epoxy resin, the mode I (GIC) and mode II (GIIC) interlaminar fracture toughness, the interlaminar shear strength and flexural properties of the composites were systematically examined, and the interlaminar fracture microscopic morphology of the composites was also characterized. The results showed that the PES porous fibers were completely dissolved in the epoxy resin at the curing temperature of 180oC; The GIC and GIIC of CF/EP composites were best at the areal density of 31.6 g/m2, which increased by 54.4% and 62.2%, respectively, which may be ascribed to that the phase separation of PES porous fibers dissolved in epoxy resin resulted in a two-phase PES/epoxy structure, thereby, improved the interlayer toughness; The interlaminar shear strength, flexural strength and flexural modulus of CF/EP composites also increased by 2.9%, 4.0% and 7.7%, respectively, for the areal density of 21.9 g/m2.
The bulk material of the hyper-eutectic Al-30Si alloy was prepared via selective laser melting (SLM) technique, aiming to solve the problem of high brittleness, easy cracking, and difficulty in precise forming caused by the coarsening of primary Si in the alloy due to the coarsening of primary Si particles in the alloy during ordinary making process. Then the microstructure, mechanical properties, and thermal properties of the SLM alloy after stress-relief annealing were studied. The results showed that the room temperature tensile strength of the SLM Al-30Si alloy after annealing was 254 ± 3 MPa, which was 53.5% higher than that of the cast alloy. The hardness was 176.89 ± 8.5 HV and the specific stiffness was 35.18 m2/s2. In terms of thermal properties, the thermal expansion coefficient of the SLM Al-30Si alloy is 13.8 to 16.3 × 10-6/oC in the temperature range of -100~200oC, and the average thermal conductivity is 70.52 W·m-1·K-1. The study found that the rapid cooling characteristic of SLM could refine the primary Si particles, making the formed Al-30Si alloy have good comprehensive properties. The high specific stiffness and low thermal expansion coefficient are expected to maintain high dimensional stability for optical components.
In order to promote the practical application of lead-free piezoelectric ceramics, in this paper, a kind of sodium potassium niobate-sodium bismuth zirconate (1-x)K0.48Na0.52Nb0.96Sb0.04O3-x(Bi0.5Na0.5)ZrO3 lead-free piezoelectric ceramics were prepared, and their crystallographic structure and performance were assessed. Results show that their relative dielectric constant and the resonant frequency temperature are stable (< 10‰) with the variation of temperature; the piezoelectric ceramics are typical perovskite structure, and the most compact ceramic sample is obtained with x=0.04, which endows RD=97.43%, d33 = 463 pC/N, kp = 0.55 and Qm = 37; the piezoelectric ceramics consists of tripartite-tetragonal (R-T) two-phases and the existence of nanodomain structure may be the cause for the excellent piezoelectric properties of ceramic materials.
Developing advanced composites with high electromagnetic shielding effectiveness is of great significance to ensure safety and reliability of electronic devices in complex electromagnetic environments. We proposed a novel strategy to fabricate carbon fiber-based hybrid composites by in-situ growing zinc oxide (ZnO) nanowires onto carbon fiber (CF) and subsequently depositing carbon nanotubes (CNT), and infiltrating epoxy (EP) into laminated layers through vacuum-assisted resin transfer molding technique. Microstructures, electrical conductivity, and electromagnetic shielding performance of the acquired CNT-ZnO-CF/EP hybrid composites were investigated in detail. The hybrid composites of 2 mm thickness exhibited excellent electromagnetic shielding performance, and their total electromagnetic shielding effectiveness was up to 50 dB in the 8.2~12.4 GHz band, increasing by 51.52% in comparison to the CF/EP composites. Such high performance is mainly attributed to the high dielectric loss of ZnO nanowires, high electrical conductivity of continuous CNT films, and multiple reflection/absorption losses between laminated structures and multi-component interfaces. This work paves a way for development of advanced composites with high-efficiency electromagnetic shielding and structure/function integration.
The discharge of industrial and daily life waste water, as well as the oil leakage not only cause serious environmental disaster but also the waste of resources. Superhydrophobic sponge is a kind of ideal materials of separating and recovering oil/water mixture. Herein, benzoxazine based on renewable cardanol and dodecamine (Cd-D) was synthesized, then ZnO micro-/nano-structure and Cd-D were successively coated on the surface of melamine sponge (MS) by a simple hydrothermal process and dip-coating method to prepare superhydrophobic MS (PCd-D/ZnO/MS). A water contact angle (WCA) of 153.6° could be obtained when the concentration of Zn(NO3)2 was 0.03 mol/L and the mole ratio of Zn(NO3)2 and HMTA was 1:2 prepared at 95oC for 4 h. PCd-D/ZnO/MS exhibited a high oil absorption capacity (48.19~113.44 g/g) for organic solvents and oils with a rapid absorption rate. In addition, polybenzoxazine and ZnO (or MS) could form multiple interactions (including hydrogen bond, coordination bond, chemical bond, etc.), which made the coating be firmly adhered to the MS skeleton and thus endowed PCd-D/ZnO/MS excellent reusability. PCd-D/ZnO/MS could maintain superhydrophobicity and 96.6% of initial adsorption capacity after 30 cycles, the WCA of the modified MS could reach 147.3° with an absorption retention of 92.6% after 100 cycles. With the help of vacuum pump, PCd-D/ZnO/MS could be used for continuous oil/water separation with a separation efficiency of above 90%. Moreover, PCd-D/ZnO/MS showed excellent resistance to strong acid, alkali and salt, and then still displayed superhydrophobicity even being immersed in NaOH solution for 30 days. In summary, PCd-D/ZnO/MS not only possessed a simple, safe and economical preparation processe but also exhibited excellent oil-water separation ability, reusability and resistance to acid, alkali and salt, which made PCd-D/ZnO/MS a promising oil-water separation material.
