To develop high-performance microwave absorption materials with the features of lightweight, thin thickness, broad bandwidth, and strong absorption, an ultralight Ni3S2@N, S-codoped graphene aerogel with a density of 13.5 mg/cm3 has been fabricated by the use of metal-organic frameworks (MOFs) to directly initiate the gelation of graphene oxide strategy. In such a strategy, dual-functional 1D Ni-MOF nanorods not only act as the gelation agent but also afford the doping elements (N and S) originated from the organic species and the precursor for metal sulfide. Due to the synergistic effects of good impedance matching and multiple losses, the optimal reflection loss (RL) of as-prepared Ni3S2@N, S-codoped graphene aerogel reaches −46.9 dB at 17.1 GHz with only 2.0 mm and ultralow filling content (1.75 wt%). The maximum effective absorption bandwidth (EAB) reaches 6.3 GHz (11.7–18.0 GHz) at 2.38 mm, covering the whole Ku band. Moreover, the value of EAB with the RL less than −30 dB can be tuned to 12.2 GHz (5.8–18 GHz) at the absorber thickness ranging from 1.9 to 5.0 mm. This work provides insight for rational design and fabrication of multicomponent-containing graphene aerogels, showing the potential application in lightweight and high-performance microwave absorption.

During the procedure of in situ combustion (ISC) for heavy oil, the rate of heat release at the low temperature oxidation (LTO) stage directly affects the delay time and the effect of ignition. This paper aimed at various clay minerals effects on heavy oil oxidation before ignition, at different reservoir temperatures and air/oil ratios, using a small batch reactor (SBR). Characterizations of the pressure, the reaction temperature, the post-test gases, the viscosity, and the asphalt content of heavy oil before and after oxidation were carried out. The results indicated that the catalysis of clay minerals was enhanced with the increase of temperature and air/oil ratio. At a rather lower temperature (90 °C), all three types of clay minerals did not obviously affect the temperature increment or rising rate at LTO while they had a little effect on viscosity, but had great influence on asphalt content, which increased greatly after the reaction. At a rather higher temperature (150 °C), clay minerals had great influence on the heat release. Montmorillonite had the best catalytic effect, which was 4.28 times higher than the blank sample in temperature rising rate under a higher air/oil condition, while the viscosity of crude oil reduced, and the concentration of asphalt slightly increased after reaction. The results of this study provide new insight that the clay minerals in the reservoir or injecting a certain amount of clay minerals into the reservoir before ignition can accelerate the reaction rate and heat release rate, thus improving the ignition effect.

The machining of micro/nano periodic surface structures using a femtosecond laser has been an academic frontier and hotspot in recent years. With an ultrahigh laser fluence and an ultrashort pulse duration, femtosecond laser machining shows unique advantages in material processing. It can process almost any material and can greatly improve the processing accuracy with a minimum machining size and heat-affected zone. Meanwhile, it can fabricate a variety of micro/nano periodic surface structures and then change a material’s surface performance dramatically, such as the material’s wetting performance, corrosive properties, friction properties, and optical properties, demonstrating great application potential in defense, medical, high-end manufacturing, and many other fields. In recent years, the research is gradually deepening from the basic theory to optimization design, intelligent control, and application technology. Nowadays, while focusing on metal structure materials, especially on stainless steel, research institutions in the field of micro and nano manufacturing have conducted systematic and in-depth experimental research using different experimental environments and laser-processing parameters. They have prepared various surface structures with different morphologies and periods with sound performance, and are one step closer to many civilian engineering applications. This paper reviews the study of micro/nano periodic surface structures and the performance of stainless steel machined using a femtosecond laser, obtains the general evolution law of surface structure and performance with the femtosecond laser parameters, points out several key technical challenges for future study, and provides a useful reference for the engineering research and application of femtosecond laser micro/nano processing technology.

Using a pseudo-random sequence to encode the transmitted waveform can significantly improve the working efficiency and depth of detection of electromagnetic exploration. The selection of parameters of pseudo-random sequence plays an important role in correlation identification and noise suppression. A discrete cycle correlation identification method for extracting the earth impulse response is proposed. It can suppress the distortion in the early stage of the excitation field and the glitches of the cross correlation function by traditional method. This effectively improves the accuracy of correlation identification. The influence of the order and the cycles of m-series pseudo-random coding on its autocorrelation properties is studied. The numerical results show that, with the increase of the order of m-sequence, the maximum out-of-phase periodic autocorrelation function decreases rapidly. Therefore, it is very beneficial to achieve synchronization. The limited-cycle m-sequences have good autocorrelation properties. As the period of the m-sequence increases and the width of the symbol decreases, the overall autocorrelation becomes closer to the impact function. The discussion of the influence of symbol width and period of m-sequence on its frequency bandwidth and power spectral density shows that the narrower the symbol width, the wider its occupied band. The longer the period, the smaller the power spectral line spacing. The abilities of m-sequence to suppress DC (Direct-current) interference, Schumann frequency noise, and sine-wave noise are analyzed. Numerical results show that the m-sequence has excellent ability to suppress DC interference and Schumann frequency noise. However, for high-order harmonic noise, the correlation identification error appears severe oscillation in the middle and late stages of the impulse response. It indicates that the ability of m-sequence to suppress high-frequency sinusoidal noise is deteriorated. In practical applications, the parameters of the transmitted waveform should be reasonably selected in combination with factors including transmitter performance, hardware noise, and ambient noise level to achieve the best identification effect.

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