It becomes more and more challenging to satisfy the long-term demand of transmission capacity in wireless networks if we limit our research within the frame of traditional electromagnetic wave characteristics (e.g., frequency, amplitude, phase and polarization). The potential of orbital angular momentum (OAM) for unleashing new capacity in the severely congested spectrum of commercial communication systems is generating great interest in wireless communication field. The OAM vortex wave/beam has different topological charges, which are orthogonal to each other. It provides a new way for multiplexing in wireless communications. Electromagnetic wave or synthetic beam carrying OAM has a spiral wavefront phase structure, which may provide a new degree of freedom or better orthogonality in spatial domain. In this paper, we introduce the fundamental theory of OAM. Then, OAM generation and reception methods are equally demonstrated. Furthermore, we present the latest development of OAM in wireless communication. We further discuss the controversial topic "whether OAM provides a new degree of freedom" and illustrate our views on the relationship between OAM and MIMO. Finally, we suggest some open research directions of OAM.

To optimize the operating temperature and flow velocity of cooling water in a high voltage direct current (HVDC) thyristor valve cooling system, the erosion corrosion characteristics of aluminum electrodes in deionized water at various temperatures were studied. With increasing water temperature, the corrosion current of the aluminum electrode gradually increases and the charge transfer impedance gradually decreases, thus, the corrosion of aluminum tends to become serious. The aluminum electrode in 50 °C deionized water has the most negative corrosion potential (−0.930 V), the maximum corrosion current (1.115 × 10−6 A cm−2) and the minimum charge transfer impedance (8.828 × 10−6 Ω), thus, the aluminum corrosion at this temperature is the most serious. When the temperature of deionized water increases, the thermodynamic activity of the ions and dissolved oxygen in the deionized water increases, and the mass transfer process accelerates. Therefore, the electrochemical corrosion reaction of the aluminum surface will be accelerated. The corrosion products covering the aluminum electrode surface are mainly Al(OH)3. With increasing water temperature, the number of pits and grooves formed by corrosion on the aluminum surface increased. In this paper, the molar activation energy Ea and the equilibrium constant K of the aluminum corrosion reaction with various temperatures are calculated. This clarifies the effect of temperature on the aluminum corrosion reaction, which provides a basis for protecting aluminum from corrosion. The results of this study will contribute to research that is focused on the improvement of production techniques used for HVDC thyristor valve cooling systems.

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This paper considers the corrosion behavior of aluminum in a range of low-concentration sodium bicarbonate solutions at high temperature (50 °C). Aluminum exhibits enhanced corrosion inhibition (passivation) in low concentration sodium bicarbonate solutions when compared to deionized water. It was determined that an aluminum electrode in 1×10 -3 mol L -1 sodium bicarbonate solution (pH 7.77) exhibited the lowest corrosion rate based on the lowest measured corrosion current, the most positive corrosion potential, and the maximum charge transfer impedance. The corrosion of aluminum was inhibited in sodium bicarbonate solution because HCO3anions in the solution developed an ordered charge field at the interface of the aluminum/electrolyte, which limited the diffusion of aluminum ions. This ordered charge fieldmay also enhance the formation of a protective aluminum oxide film, which can effectively inhibit the corrosion of aluminum. The scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD) confirmed that the corrosion products covering the surface of the aluminum electrode were Al(OH)3 and/or Al2O3. This paper provides a basis for resolving scaling problems associated with the grading electrode in high-voltage direct current systems caused by the corrosion of the aluminum radiator.

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