Here, we report the synthesis of high quality monolayer graphene on the pre-treated copper (Cu) foil by chemical vapor deposition method. The pre-treatment process, which consists of pre-annealing in a hydrogen ambient, followed by diluted nitric acid etching of Cu foil, helps in removing impurities. These impurities include native copper oxide and rolling lines that act as a nucleation center for multilayer graphene. Raman mapping of our graphene grown on pre-treated Cu foil primarily consisted of ∼98% a monolayer graphene with as compared to 75 % for the graphene grown on untreated Cu foil. A high hydrogen flow rate during the pre-annealing process resulted in an increased I2D/IG ratio of graphene up to 3.55. Uniform monolayer graphene was obtained with a I2D/IG ratio and sheet resistance varying from 1.84 – 3.39 and 1110 – 1290 Ω/□, respectively.
Excellent weldability and high temperature stability make Inconel 718 (IN718) one of the most popular alloys to be produced by additive manufacturing. In this study, we investigated the effects of laser powder bed fusion (LPBF) parameters on the microstructure and relative density of IN718. The samples were fabricated with independently varied laser power (125-350 W), laser scan speed (200-2200 mm/s), and laser scan rotation (0°-90°). Archimedes' method, optical microscopy, and scanning electron microscopy were employed to assess the influence of LPBF parameters on the relative density and microstructure. Optimal processing windows were identified for a wide range of processing parameters, and relative density greater than 99.5% was achieved using volumetric energy density between 50 and 100 J/mm 3 . Microstructural features including melt pool geometry, lack of fusion defect, keyhole porosity, and sub-grain cellular microstructure were examined and quantified to correlate to LPBF parameters. A simple empirical model was postulated to relate relative sample density and LPBF volumetric energy density. Melt pool dimensions were quantitatively measured and compared to estimations based on Rosenthal solution, which yielded a good agreement with the width, but underestimated the depth, particularly at high energy input, due to lack of consideration for keyhole mode. In addition, the sub-grain cellular-dendritic microstructure in the as-built samples was observed to decrease with increasing laser scan speed. Quantification of the sub-micron cellular-dendritic microstructure yielded estimated cooling rate in the order of 10 5 -10 7 K/s.
A rectenna is adopted in a microwave-based wireless power transfer system to rectify the received RF to DC power. Each rectenna includes a DC-DC converter at the DC output port to ensure a stable power supply and a Schottky diode for the DC-combining network. The results demonstrate that the RF-DC conversion efficiencies of the rectifier circuit and 4 × 1 rectenna were 71% and 71.2% when the unit-cell RF input level was 19 dBm and 19.6 dBm. The DC output levels of a 4 × 8 rectenna with and without the DC-DC converter were 28 V and 30 V when the unit-cell RF input level was 20.2 dBm. The RF-DC conversion efficiency was measured as 52.6% and 59.9% with and without the DC-DC converter, respectively. The Schottky diode combined with the D-DC C converter at the DC output port exhibits an increase in the DC power combining level, making it suitable for an extendable array rectenna system.
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