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In the development of urban rail transit in China, PPP mode has drawn prominent attention from all parties in recent years. This report reviews the development of PPP and policy background in China’s urban rail transit from 2017 to 2018, summarizes the application characteristics of PPP mode, and introduces the application practice of PPP mode in some cities.

Herein, the influence of annealing current intensities on magnetic properties of Gd-based microfibers was systematically investigated. The experimental results indicated that the as-prepared microfibers have amorphous structure and excellent thermal-stability, and the Curie temperature TC of which slightly change before and after annealing. The general magnetic physical magnitudes (Ms, Mr, μ0Hc and μm) of microfibers increase firstly and then decrease with the rising of annealing current intensities, and those of the microfibers annealed at 90 mA microfibers are more obvious (Ms = 255 A m2/kg and μm = 0.38 ± 0.01). Similarly, the magnetocaloric parameters of microfibers also increase firstly and then decrease with the rising of current intensities. At 5 T of magnetic field change μ0ΔH, the maximum magnetic entropy change ΔSmmax and refrigerant capacity RC of 90 mA annealed metallic microfibers are 12.77 J/(kg·K) and 906 J/kg, respectively, and which remarkably increased in compared with the as-prepared state, respectively. From the microstructural evolution perspective, a certain intensity current annealing induces the tendency of micro-regional clusters and nanocrystals, forming atomic ordered regions, and the internal residual stress release with a reduction of magnetic anisotropy. Meanwhile, the magnetic ordering and the critical magnetic-moment rotational driving-force were changed after Fe-doping, resulting in effective improvement of refrigerant capacity. Accordingly, the current-annealed Gd-Al-Co-Fe microfibers can be adopted as low-temperature magnetic refrigeration working-medium, which also provides a theoretical foundation for magnetic refrigeration technology application.

The effects of trace Fe-doping on the magnetic and magnetocaloric performances of Gd-based metallic microfibers were systematically and detailedly investigated in this article. Experimental results indicated that the Gd-based microfibers prepared by the rotated-dipping process are uniform and continuous, with typically amorphous structure characteristics and favorable thermal stability. Curie temperature TC of Gd-based amorphous microfibers increases with the rising of Fe content, and the TC of GdAlCoFe3 microfibers is the highest at 128.7 K (relatively increased by 15.3 K than before doping). Among the general magnetic performance indicators, the saturation magnetization Ms of GdAlCoFe2 microfibers reaches a peak at 264.04 emu/g. Furthermore, the values of magnetocaloric performance indexes of Gd-based amorphous microfibers increase firstly and then decrease with the rising of Fe-doping amount. When the external magnetic field Hex = 50,000 Oe, the maximum magnetic entropy change -ΔSmmax, refrigeration capacity RC and relative cooling power RCP of GdAlCoFe1 microfibers are 10.33 J/kg·K, 748.22 J/kg and 1006.90 J/kg, which increased by 0.07 times, 0.13 times and 0.16 times, respectively than before Fe-doping (namely -ΔSmmax = 9.68 J/kg·K, RC = 663.79 J/kg, RCP = 871.73 J/kg). From the perspective of microstructural changes, Fe-doping results in the formation tendency of atomic clusters and nanocrystals in micro-regions, the orderly orientation of atoms appears, and the magnetic anisotropy increases accordingly. In addition, the 3d electron layer of the doped Fe element in Gd-Al-Co-Fe quaternary alloy system enhances the RKKY indirectly exchange coupling interaction and changes the state of magnetic entropy, which in turn leads to an increase in Curie temperature and an improvement in magnetocaloric performance. Since RC and RCP are the main indicators in engineering applications, the GdAlCoFe1 amorphous microfibers can be selected as a low-temperature magnetic refrigeration working medium to provide technical guarantee for the application of magnetic refrigeration technology.

Hermetic sealing devices are produced with Au-20Sn solder at different oxygen content levels to study the reliability of the seal joints. Cross section of Au-20Sn solder joints processed with different oxygen content were examined by SEM, which shows that the microstructure displayed an obvious deterioration with the increase of oxygen content. It was found that bonding at 16 ppm oxygen content level, an uniform bonding interface with void-free was obtained with good hermeticity. However, when the oxygen content reached 55 ppm, voids and cracks were generated along the seal joint and the helium leak rate was higher than 5 × 10−8 atm cm3/s, which is the rejected limit value of MIL-STD-883 K. According to the reliability test results, we believe that the hermeticity deterioration is attributed to voids and cracks with the extension of aging time. In addition, samples with poor hermeticity were analyzed to know the failure mechanism of the seal joints and the feasible methods were proposed for improving hermeticity.