Sang H. Choi

Microwave power for smart material actuators

The concept of microwave-driven smart material actuators was envisioned and developed as the best option to alleviate the complexity and weight associated with a hard-wire-networked power and control system for smart actuator arrays. The patch rectenna array was initially designed for high current output, but has undergone further development for high voltage output devices used in shape control applications. Test results show that more than 200 V of output were obtained from a 6 × 6 array at a far-field exposure (1.8 m away) with an X-band input power of 18 W. The 6 × 6 array patch rectenna was designed to theoretically generate voltages up to 540 V, but practically it has generated voltages in the range between 200 and 300 V. Testing was also performed with a thin layer composite unimorph ferroelectric driver and sensor and electro-active paper as smart actuators attached to the 6 × 6 array. Flexible dipole rectenna arrays built on thin-film-based flexible membranes are most applicable for NASAs various missions, such as microwave-driven shape controls for aircraft morphing and large, ultra-lightweight space structures. An array of dipole rectennas was designed for high voltage output by densely populating Schottky barrier diodes to drive piezoelectric or electrostrictive actuators. The dipole rectenna array will eventually be integrated with a power allocation and distribution logic circuit and microbatteries for storage of excessive power. The roadmap for the development of wireless power drivers based on the rectenna array for shape control requires the development of new membrane materials with proper dielectric constants that are suitable for dipole rectenna arrays.

Transpiration Cooling Experiment for Scramjet Engine Combustion Chamber by High Heat Fluxes

A transpiration cooling experiment using an optical heating method that provided a heat flux as high as 234 W/cm 2 on the surface of specimen for a scramjet engine was performed. In this experiment, 19-mm-diam sintered, powdered, stainless-steel tubes and a 25-mm square cross-sectional tube of perforated Inconel tube were used to investigate transpiration cooling effectiveness. The cooling effectiveness by transpiration for each specimen was measured and analyzed. As a result, the transpiration cooling mechanism appeared to be a promising approach to remove a large amount of heat from the engine wall. A preliminary analysis of the transpiration cooling mechanism and a scaling conversion study that translates the results from helium tests into the case when a hydrogen medium is used are included.

Microwave power transmission using a flexible rectenna for microwave-powered aerial vehicles

In exploiting the unique capabilities of smart actuators for applications in vehicle systems, even in unmanned or micro aerial vehicles, the power issues for smart actuators and devices have not been well addressed. This is due to the fact that the driving power for smart materials has not reached the level of the power specifications for conventional devices and systems. To answer the power issue, we have developed a wireless power transmission technology using a flexible rectenna system and implemented it for a microwave-powered aerial vehicle (MPAV) system. For this application, two flexible dipole rectennas were designed, manufactured and characterized over a frequency range of 9–12 GHz. These flexible dipole rectennas were attached and tested on the complex structure of small MPAVs. The maximum converted power output of a flexible dipole rectenna array was about 300 mA at 80 VDC. The power output from this rectenna was sufficient to run the propellers of the MPAV. Each electrically driven propeller requires approximately 2 W for operation.

Cobalt oxide hollow nanoparticles derived by bio-templating

We present here the first fabrication of hollow cobalt oxide nanoparticles produced by a protein-regulated site-specific reconstitution process in aqueous solution and describe the metal growth mechanism in the ferritin interior.

Power budget analysis for high altitude airships

The High Altitude Airship (HAA) has various potential applications and mission scenarios that require onboard energy harvesting and power distribution systems. The energy source considered for the HAAs power budget is solar photon energy that allows the use of either photovoltaic (PV) cells or advanced thermoelectric (ATE) converters. Both PV cells and an ATE system utilizing high performance thermoelectric materials were briefly compared to identify the advantages of ATE for HAA applications in this study. The ATE can generate a higher quantity of harvested energy than PV cells by utilizing the cascaded efficiency of a three-staged ATE in a tandem mode configuration. Assuming that each stage of ATE material has the figure of merit of 5, the cascaded efficiency of a three-staged ATE system approaches the overall conversion efficiency greater than 60%. Based on this estimated efficiency, the configuration of a HAA and the power utility modules are defined.

Wireless power transmission for medical applications

We studied the wireless power transmission capabilities of microwave through human skin-tissue. Microwave transmission through simulated human skins was tested with rectenna array as a power receiver located under the simulated human skin tissue. Most of transplanted medical devices and sensors require power to operate autonomously but currently by imbedded battery. Wireless power transmission alleviates the needs of imbedded power source and hard-wire power network. We used human skin-like materials, such as various polyurethanes and pork skin, under X-band microwave exposure. Transmission rate through various polyurethanes under the threshold limit value (TLV) and dielectric constant was measured in this experiment. It is also critical to measure specific absorption rates (SAR) of polyurethanes and transmission rates through polyurethanes as well as pork skin. This paper presents power transmission rates under varying thickness of polyurethanes, and effectiveness and efficiency of rectennas under the TLV of microwave power. In addition, we will discuss milimeter wave thermograph and hazards the absorption characteristics of human skin under 8-13 GHz using the results of polyurethanes and pork skin.

Design for megavolt inverse-pinch plasma switch

To meet the requirements for the output switch of an ultra-high-power (>30 GW) pulser, an upgrading design for an inverse-pinch plasma switch (INPIS) is considered. The hold-off voltage of 1 MV is met by adopting multistage rim-fire electrodes and using SF/sub 6/ as the dielectric gas of the switch. The inductance and capacitance of the switch, which are restricted by the short rise-time (<O.1 /spl mu/s) requirement, were met by adjusting the dimensions of the coaxial electodes of the switch. The input/output transmission lines attached to the switch will be immersed in oil to meet the high voltage insulation and impedence-matching requirements. Since the forwarding current is relatively low with respect to the switch capability, the lifetime of the switch is expected to exceed the requirement of 10/sup 4/ shots. The choice of the insulator and electrode materials is also discussed. Fabrication of the switch can be accomplished by well-established shop practices, and no special methods are required for its construction.

Ultrasonication of Bismuth Telluride Nanocrystals Fabricated by Solvothermal Method

The objective of this study is to evaluate the effect of ultrasonication on bismuth telluride nanocrystals prepared by solvothermal method. In this study, a low dimensional nanocrystal of bismuth telluride (Bi2Te3) was synthesized by a solvothermal process in an autoclave at 180°C and 200 psi. During the solvothermal reaction, organic surfactants effectively prevented unwanted aggregation of nanocrystals in a selected solvent while controlling the shape of the nanocrystal. The atomic ratio of bismuth and tellurium was determined by energy dispersive spectroscopy (EDS). The cavitational energy created by the ultrasonic probe was varied by the ultrasonication process time, while power amplitude remained constant. The nanocrystal size and its size distribution were measured by field emission scanning electron microscopy (FESEM) and a dynamic light scattering system. When the ultrasonication time increased, the average size of bismuth telluride nanocrystal gradually increased due to the direct collision of nanocrystals. The polydispersity of the nanocrystals showed a minimum when the ultrasonication was applied for 5 min.

Power Technology for Application-Specific Scenarios of High Altitude Airships

The High Altitude Airship (HAA) has various potential applications and mission scenarios that require onboard energy harvesting and power distribution systems. The energy source considered for HAAs is solar photon energy that allows the use of either photovoltaic (PV) cells or advanced thermoelectric (ATE) converters. Both PV cells and an ATE system were briefly compared to identify the advantages of ATE for HAA applications in this study. Utilizing the estimated high efficiency of a three-staged ATE in a tandem mode, the ATE generates a higher quantity of harvested energy than PV cells for mission scenarios. When the ATE performance figure of merit of 5 is considered, the cascaded efficiency of a three-staged ATE system approaches an overall efficiency greater than 60%. Based on this estimated efficiency, the configuration of a HAA and the power utility modules are defined.

Miniaturization of optical spectroscopes into Fresnel microspectrometers

Miniaturized optical instruments have become very important in industry as smart phones and tablet PCs increase in popularity. A chronology of spectrometer development shows that a simple numerical point of view affords important insights. A tiny spectrometer, which is smaller than a few millimeters size, cannot easily rely on the conventional Fraunhofer diffraction due to its optical criterion limit. As an alternate solution to build smaller spectrometers, a Fresnel spectrometer chip with a gradient line grating is attractive. The fabricated Fresnel spectrometers have optical path volumes of about 1 mm3 and spectral resolutions of 10 to 23 nm.