INVESTIGATION OF FABRICATION OF Co-Zr BASED RARE EARTH-FREE HARD MAGNETIC ALLOYS BY MELT-SPINNING METHOD

Co-Zr based alloy has attracted much interest of potential to replace the rare earth-containing hard magnetic materials due to its high coercivity. In this study, we investigated the effects of subtituting elements of M (Ti, Si and Nb) and annealing temperature on the structure and magnetic properties of Co79-xZr18+x-yMyB3 alloy ribbons (x = 0 - 2, y = 0 - 4). The alloy ribbons with a thickness of 20 µm were prepared by melt-spinning method with a rolling speed of 40 ms-1. A part of the melt-spun ribbons was annealed at different temperatures from 550 to 800 oC for various durations from 2 to 15 minutes. Their structure and magnetic properties were investigated by X-ray diffraction (XRD) and a pulsed field magnetometer (PFM), respectively. The results of the XRD analysis showed that two soft magnetic phases, namely Co and Co23Zr6, coexist with a Co5Zr hard magnetic phase in the alloy ribbons. The fraction of these phases was changed with both the concentration of the subtituting elements and annealing process. Hard magnetic properties of the alloy ribbons can be strengthened significantly, namely a large coercivity Hc 4 kOe and maximum energy product (BH)max 3.5 MGOe were obtained with an appropriate concentration of Ti, Si or Nb and annealing process. Furthermore, the subtituting elements also affect the optimal annealing temperature for these alloys. The obtained strong hard magnetic parameters of these rare earth-free alloys are of great importance in pratical application

Joint Fractional Time Allocation and Beamforming for Downlink Multiuser MISO Systems

It is well-known that the traditional transmit beamforming at a base station (BS) to manage interference in serving multiple users is effective only when the number of users is less than the number of transmit antennas at the BS. Non-orthogonal multiple access (NOMA) can improve the throughput of users with poorer channel conditions by compromising their own privacy because other users with better channel conditions can decode the information of users in poorer channel state. NOMA still prefers that the number of users is less than the number of antennas at the BS transmitter. This paper resolves such issues by allocating separate fractional time slots for serving the users with similar channel conditions. This enables the BS to serve more users within the time unit while the privacy of each user is preserved. The fractional times and beamforming vectors are jointly optimized to maximize the system's throughput. An efficient path-following algorithm, which invokes a simple convex quadratic program at each iteration, is proposed for the solution of this challenging optimization problem. Numerical results confirm its versatility.Comment: IEEE Communications Letters (To Appear

Expert opinion-based multi objective optimization: an application in plasma coating technology

Multi-objective optimization is a very important activity which is applied in many different fields. When solving this problem, it is important to determine weights for criteria. If the weight of criteria is determined according to dry mathematical formulas, the opinion of researchers will be ruined. On the contrary, if the weight of criteria is determined according to the subjective opinion of researchers, it is also easy to make mistakes. This study applies a method of determining the weight of criteria based on experts' opinions and conditions must be also strictly satisfied, thereby both of the above limitations have been remedied. Such method is known as FUCOM (FUll COonsistency Method). An application example was carried out for multi-objective optimization in the plasma coating process. Plasma coating is a modern coating technology. This method is increasingly used in many different fields. However, determining the value of technological parameters to ensure the quality of high-quality products is a very complicated job. In order to ensure many requirements of the product, it is necessary to determine the optimal value of the technological parameters. Four criteria to evaluate a coating process include the adhesion strength of the coating, the shear strength of the coating, the tensile strength of the coating, and the porosity of the coating. The task of multi-objective optimization in this study is to determine the values of three input parameters (including: spray current intensity, powder feed flow, and spray distance) to ensure that the desired values of the four criteria are simultaneously achieved. After the weight of criteria is determined by the FUCOM method, the multi-objective optimization problem has been solved. Experiments to verify the optimal results were also conducted, thereby demonstrating the correctness of the methodology. The optimal values of the technology parameters (spray current intensity, powder feed flow, and spray distance) have been determined to be 568.69 A, 31.87 g/min, and 170.19 mm, respectivel