Papers by Author: Feng Sun

Abstract: With global warming, more and more people begin to pay close attention to the development of new energy, especially wind power. To promote the healthy development of the wind power industry, countries are required to wind farms with a function of low voltage ride through. The research object of this paper is the capacity for 49.5MW wind farm, using German power system simulation software DIgSILENT/Power Factory to build wind farm model, proposed a new method to achieve LVRT for wind farms by short-circuit test. By comparing the simulation results with the test results, we can verify that the method can achieve the desired requirements.

Abstract: γ-aluminum oxynitride (γ-ALON) powder was prepared by carbothermal reduction method using γ-Al2O3 and activated carbon as raw materials. Effects of activated carbon content, synthesizing temperature, holding time and carbon impurity on ALON powder were studied. It was found a ‘two-step’ procedure, first sintering at about 1550°C-1650°C then at about 1750°C-1850°C, was effective to prepare high-purity ALON powder. The removal of carbon impurity was crucial for the preparation of ALON powder, which effectively improved the purity of ALON powder. The prepared powder was characterized by XRD and SEM, which indicated that pure ALON powder prepared by a ‘two-step’ procedure had well-developed crystalline and homogenous granularity. Transparent ALON ceramics were prepared by hot-press sintering at 1850~1900°C for up to 2~4h using ALON powder and some sintering additive, the transmission of sample with thickness of 1mm was 81% at infrared wave.

Abstract: Silicon nitride ceramics were prepared by gas pressure sintering (GPS) with different sintering additives, including La2O3, Sm2O3 and Al2O3. Effect of sintering additives on the phase-transformation, microstructure and mechanical properties of porous silicon nitride ceramics was investigated. The results show that the reaction of sintering additives each other and with SiO2 had key effects on the phase-transformation, grain growing and grain boundaries. With 9MPa N2 atmosphere pressure, holding 1h at 1850°C, adding 10wt% one of the La2O3, Sm2O3, Al2O3, porous silicon nitride was prepared and the relative density was 78%, 72%, 85% respectively. The flexural strength was less than 500MPa, and the fracture toughness was less than 4.8MPam1/2. Dropping compounds sintering additives, such as La2O3+Al2O3, Sm2O3+Al2O3 effectively improves the sintering and mechanical properties. The relative density was 99.2% and 98.7% with 10wt% compounds sintering additives. The grain ratio of length to diameter was up to 1:8. The flexural strength was more than 900MPa, and the fracture toughness was more than 8.9MPam1/2.

Abstract: The sections of Φ55mm silicon nitride balls, sintered by gas pressure sintering were analyzed. The results show that temperature gradient during of the sintering process from the surface to the core of large size silicon nitride balls occurred because of the lower thermal conductivity of Si3N4. With the diameter increasing, the temperature gradient was more visible. The impurity of raw materials, such as free Si, free C and other metal oxides, like the SiO2 could produce gas, such as SiO, CO and so on, during the sintering process through the thermodynamic analysis. The producing gas exhausted more difficultly with the diameter of silicon nitride balls increasing. These factors were the most important to the defects of large size silicon nitride balls during the sintering process and made cracks and crescent on the surface of balls.

Abstract: An oxidation kinetics model of hot-pressed ZrB2-SiC composites was established and then proved by experiments. The relationship between oxidation rate and temperature was attained by reaction activation energy and frequency factor at the different oxidation stages. Results showed that the relations between the oxidation gain of unit area and the oxidation time are linear, conic and parabolic, respectively, during the stage of the oxidation prophase, midterm and anaphase.

Abstract: High-purity alumina ceramics were prepared by sintering at low temperature, using rare earth oxide Y2O3 and Tm2O3 as sintering aids. Adding the mixture of Y2O3 and Tm2O3 in the alumina powders can effectively improve the sintering ability and mechanical properties. The mixture, Y2O3+Tm2O3, can restrain the grains growth, fine the grain size, make the grain uniform and finally help to form compact structure. The transgranular fracture was the main fracture mode. It was found the relative density of alumina ceramics with 0.3wt% Y2O3 and Tm2O3, sintered at 1620°C for 3hrs, can be over 99.2% and the strength, toughness and microhardness reach 533.1MPa, 4.9MPa⋅m1/2 and 17.2GPa, respectively.