Thin films of AlN, AlGaN, and AlGaN-based device structures of approximately 1 cm2 formed on a sapphire substrate were successfully exfoliated from the substrate by immersion in heated (115°C) and pressurized (170 kPa) water. These thin films were crystalline grown on periodically formed AlN nanopillars. The water was permeated through intentional voids formed in the AlGaN or AlN crystalline layers by using periodic AlN nanopillars. The exfoliated AlGaN exhibited clear X-ray diffraction peaks from its (0002) plane diffraction. Transmission electron microscopy confirmed that exfoliation introduced few additional dislocations and that the device structure was maintained.
Instantaneous three-dimensional (3D) density distributions of a shock-cell structure of perfectly and imperfectly expanded supersonic microjets escaping into an ambient space are measured. For the 3D observation of supersonic microjets, non-scanning 3D computerized tomography (CT) technique using a 20-directional quantitative schlieren optical system with flashlight source is employed for simultaneous schlieren photography. The 3D density distributions data of the microjets are obtained by 3D-CT reconstruction of the projection's images using maximum likelihood-expectation maximization. Axisymmetric convergent-divergent (Laval) circular and square micro nozzles with operating nozzle pressure ratio 5.0, 4.5, 4.0, 3.67, and 3.5 have been studied. This study examines perfectly expanded, overexpanded, and underexpanded supersonic microjets issued from micro nozzles with fully expanded jet Mach numbers M j ranging from 1.47 -1.71, where the design Mach number is M d = 1.5. A complex phenomenon for free square microjets called axis switching is clearly observed with two types "upright" and "diagonal" of "cross-shaped". The initial axis-switching is 45˚ within the first shock-cell range. In addition, from the symmetry and diagonal views of square microjets for the first shock-cells, two different patterns of shock waves are viewed. The shock-cell spacing and supersonic core length for all nozzle pressure ratios are investigated and reported.
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