Mechanical and dielectric properties of porous and wave-transparent Si3N4–Si3N4 composite ceramics fabricated by 3D printing combined with chemical vapor infiltration

Porous Si3N4–Si3N4 composite ceramics were fabricated by 3D printing combined with low-pressure chemical vapor infiltration(CVI).This technique could effectively improve the designability of porous Si3N4 ceramics and optimize the mechanical and dielectric properties.The effects of process parameters including the deposition time and heat treatment on the microstructure and properties of porous Si3N4–Si3N4 composite ceramics were studied.The study highlights following:When CVI processing time was increased from 0 to 12 h,the porosity decreased from68.65%to 26.07%and the density increased from 0.99 to 2.02 g/cm3.At the same time,the dielectric constant gradually increased from 1.72 to 3.60;however,the dielectric loss always remained less than0.01,indicating the excellent electromagnetic(EM)wave-transparent performance of porous Si3N4–Si3N4 composite ceramics.The maximum flexural strength of 47±2 MPa was achieved when the deposition time attained 6 h.After heat treatment,the porosity increased from 26.07%to 36.02%and the dielectric constant got a slight increase from 3.60 to 3.70 with the dielectric loss still maintaining lower than 0.01.It has been demonstrated that the porous Si3N4–Si3N4 composite ceramics are a promising structural and EM wave-transparent material suitable for high temperature service.

A robust online path planning approach in cluttered environments for micro rotorcraft drones

We present in this paper a robust online path planning method, which allows a micro rotorcraff drone to fly safely in GPS-denied and obstacle-strewn environments with limited onboard computational power. The approach is based on an effi- ciently managed grid map and a closed-form solution to the two point boundary value problem （TPBVP）. The grid map assists trajectory evaluation whereas the solution to the TPBVP generates smooth trajectories. Finally, a top-level trajectory switching algorithm is utilized to minimize the computational cost. Advantages of the proposed approach include its conservation of com- putational resource, robustness of trajectory generation and agility of reaction to unknown environment. The result has been realized on actual drones platforms and successfully demonstrated in real flight tests. The video of flight tests can be found at： http：//uav.ece.nus.edu.sg/robust-online-path-planning-Lai2015.html.

Phase Evolution and Thermal Expansion Behavior of aγ′Precipitated Ni-Based Superalloy by Synchrotron X-Ray Diffraction

The phase evolution and thermal expansion behavior in superalloy during heating play an essential role in controlling the size and distribution of precipitates,as well as optimizing thermomechanical properties.Synchrotron X-ray diffraction is able to go through the interior of sample and can be carried out with in situ environment,and thus,it can obtain more statistics information in real time comparing with traditional methods,such as electron and optical microscopies.In this study,in situ heating synchrotron X-ray diffraction was carried out to study the phase evolution in a typicalγ′phase precipitation strengthened Ni-based superalloy,Waspaloy,from 29 to 1050°C.Theγ′,γ,M_(23)C_(6)and M C phases,including their lattice parameters,misfits,dissolution behavior and thermal expansion coefficients,were mainly investigated.Theγ′phase and M_(23)C_(6)carbides appeared obvious dissolution during heating and re-precipitated when the temperature dropped to room temperature.Combining with the microscopy results,we can indicate that the dissolution of M_(23)C_(6)leads to the growth of grain andγ′phase cannot be completely dissolved for the short holding time above the solution temperature.Besides,the coefficients of thermal expansions of all the phases are calculated and fitted as polynomials.