Keh-Chyang Leou

In the design of a rotary-type magnetic refrigerator, a high field of a coaxial magnet is desired. Typically, a high-field design can be achieved with a small duty cycle, which might not be optimized from the viewpoint of the thermal hydraulics of a magnetic refrigerator. In this work, a numerical simulation analysis of a graded coaxial magnet designed using a COMSOL program for a rotary-type active magnetic refrigeration (AMR) system was performed. The magnet structures are based on neodymium–iron–boron permanent magnets with thin gadolinium (Gd) and gadolinium-terbium alloy (Gd-Tb) plates as AMR materials. For a rotary-type magnetic cooling system, from the thermal–hydraulic point of view, the best duty cycle of a coaxial magnet should be 50% if the magnetic field can be kept constant during the period of duty cycles. However, the simulation calculation shows a serious reduction in the magnetic field strength at higher duty cycles, resulting in lower magnetic cooling efficiency. After considering the thermos-hydraulic part, the optimized duty cycle is around 30% in the case of a temperature span of 8 K between the hot and cold ends on a rotary-type magnetic cooling system. By applying graded Gd-Tb alloy along the flow direction, the performance of magnetic refrigeration improves significantly. Compared to a pure Gd AMR system, it is demonstrated that more than three times the increase in the cooling capacity can be achieved.

In a design of rotary type magnetic refrigerator, a high field of coaxial magnet is desired. Typically, a high field design can be achieved with a small duty cycle which might not be optimized from the viewpoint of the thermal hydraulics of a magnetic refrigerator. Numerical simulation analysis of a coaxial magnet-designed using COMSOL program for rotary type active magnetic refrigeration (AMR) system has been performed for magnet structures based on neodymium-iron-boron permanent magnets. The AMR material considered was thin gadolinium plates. For a rotary type magnetic cooling system, from the thermal hydraulic point of view, the best duty cycle of coaxial magnet should be 50% if the magnetic field can be kept as constant during the period of duty cycles. However, the simulation calculation shows that a serious reduction of the magnetic field strength at higher duty cycles resulting in lower magnetic cooling efficiency. After considering the thermo-hydraulic part, the optimized duty cycle is around 30% in case of a temperature span of 8 K between the hot and cold ends on a rotary type magnetic cooling system.

Silicon-vacancy (SiV) centers were produced in single crystalline diamond (SCD) and ultrananocrystalline diamond (UNCD) nanostructures via Si ion implantation or in situ Si doping. SiV-embedded UNCD (SiV-UNCD) was fabricated by both top-down and bottom-up methods. The spectral properties of the SiV centers, including the zero phonon line (ZPL) width and decay time, were investigated in the SCD and UNCD nanostructures. All the SiV-UNCD nanostructures showed bright emission regardless of the preparation method. However, the decay time of the SiV centers was affected by the synthesis procedure. A SiV decay time of τ ∼ 0.19 ns was observed for UNCD nanostructures formed by in situ doping, whereas the SiV decay time was ∼0.43 ns for SiV-UNCD clusters prepared by Si ion implantation into UNCD deposited on Ti/sapphire substrates. The ultrasonication of UNCD clusters on Ti/sapphire pyramids produced bright SiV-UNCD nanoclusters with sizes of ∼50 nm, a ZPL width of 13.5 nm, and a decay time of 0.35 ns, suggesting promising potential in bioimaging applications. SiV-containing SCD (type Ia or type IIa) showed enhanced SiV spectral properties with a ZPL width of 6.08 nm and longer decay time of 1.3 ns.

The field electron emission (FEE) properties of nitrogen-incorporated nanocrystalline diamond films were enhanced due to Li-ion implantation/annealing processes. Li-ion implantation mainly induced the formation of electron trap centers inside diamond grains, whereas post-annealing healed the defects and converted the a-C phase into nanographite, forming conduction channels for effective transport of electrons. This resulted in a high electrical conductivity of 11.0 S/cm and enhanced FEE performance with a low turn-on field of 10.6 V/μm, a high current density of 25.5 mA/cm² (at 23.2 V/μm), and a high lifetime stability of 1,090 min for nitrogen incorporated nanocrystalline diamond films.

A “patterned-seeding technique” in combination with a “nanodiamond masked reactive ion etching process” is demonstrated for fabricating vertically aligned diamond-graphite hybrid (DGH) nanorod arrays. The DGH nanorod arrays possess superior field electron emission (FEE) behavior with a low turn-on field, long lifetime stability, and large field enhancement factor. Such an enhanced FEE is attributed to the nanocomposite nature of the DGH nanorods, which contain sp²-graphitic phases in the boundaries of nano-sized diamond grains. The simplicity in the nanorod fabrication process renders the DGH nanorods of greater potential for the applications as cathodes in field emission displays and microplasma display devices.

We report the synthesis of silicon-vacancy (SiV) incorporated spherical shaped ultrananocrystalline diamond (SiV-UNCD) particulates (size ~1 μm) with bright luminescence at 738 nm. For this purpose, different granular structured polycrystalline diamond films and particulates were synthesized by using three different kinds of growth plasma conditions on the three types of substrate materials in the microwave plasma enhanced CVD process. The grain size dependent photoluminescence properties of nitrogen vacancy (NV) and SiV color centers have been investigated for different granular structured diamond samples. The luminescence of NV center and the associated phonon sidebands, which are usually observed in microcrystalline diamond and nanocrystalline diamond films, were effectively suppressed in UNCD films and UNCD particulates. Micron sized SiV-UNCD particulates with bright SiV emission has been attained by transfer of SiV-UNCD clusters on soda-lime glass fibers to inverted pyramidal cavities fabricated on Si substrates by the simple crushing of UNCD/soda-lime glass fibers in deionized water and ultrasonication. Such a plasma enhanced CVD process for synthesizing SiV-UNCD particulates with suppressed NV emission is simple and robust to attain the bright SiV-UNCD particulates to employ in practical applications.

Needle-like diamond grains encased in nano-graphitic layers are an ideal granular structure of diamond films to achieve high conductivity and superior electron field emission (EFE) properties. This paper describes the plasma post-treatment (ppt) of ultrananocrystalline diamond (UNCD) films at low substrate temperature to achieve such a unique granular structure. The CH₄/N₂ plasma ppt-processed films exhibit high conductivity of σ = 1099 S/cm as well as excellent EFE properties with turn-on field of E₀ = 2.48 V/μm (J_e = 1.0 mA/cm² at 6.5 V/μm). The ppt of UNCD film is simple and robust process that is especially useful for device applications.

Here we report the development of a microwave interferometer based on a ridged microstrip transmission line structure for the monitoring of plasma density in plasma processing tools. A special ridged shaped microstrip structure with a quartz dielectric is adopted for yielding a large phase shift of the microwave, and thus a higher sensitivity of the interferometer. During operation, the plasma density sensor is installed on the chamber wall where the microstrip transmission line is immersed in plasma and a microwave is launched from one end of the line and exits through the other end. As in conventional microwave interferometers, the plasma density is determined by the phase shift of the microwave propagating through the transmission line. 3D electromagnetic numerical simulations, where plasma is treated as a dielectric medium having a plasma permittivity determined by plasma density and microwave frequency, were employed to determine the phase shift/plasma density relation of this sensor. The sensor is designed to operate at 2.4 GHz microwave frequency, with a compact size and materials that are compatible with most plasma processing tools. Measurement results show that plasma density measured by the sensor, although placed at the chamber wall, does reflect the variations of the plasma density near the chamber center. In the real-time plasma etch process, the dependence of plasma densities on source powers and pressures measured by the sensor is also consistent with the results of ion current on the wafer electrode obtained from an impedance meter.

Here we present the design of a low loss top metal silicon (Si) hybrid dielectric-loaded plasmonic waveguide (TM-SiHDLW) and a compact, high performance optical resonator by numerical simulation based on finite element method. The waveguide adopted a thick (200 nm) top metal stripe structure to yield optimal performance due to reduced Ohmic loss in conductor around the stripe edge/corner. Moreover, a relatively thick (150 nm) dielectric spacer between the Si ridge and the metal stripe was employed to achieve both long propagation length and good field confinement. The effect of a thin (10 nm) silicon nitride (SiN x) layer covering the waveguide which was added for minimizing uncertainties on optical properties of SiHDLW resulting from high density of dangling bonds on Si surface was also investigated. Simulation results show that there is no significant degradation on the performance of the TM-SiHDLW. For the proposed plasmonic waveguide, a propagation length of 0.35 mm and a mode area around 0.029 µm 2 were demonstrated. The TM-SiHDLW waveguide was then used as the basis for an optical resonator, which was designed to operate at the fundamental TE 011 mode for yielding high quality factor at a relatively small footprint size. A metal enclosure was also adopted to reduce the radiation loss, and a high quality factor of ∼1900 was obtained, more than double the results in other disk or ring resonators of comparable size. Compared to the resonators based on a rounded top metal Si hybrid dielectric-loaded plasmonic waveguide (RTM-SiHDLW) which has a much longer propagation length than the TM-SiHDLW, as reported in our previous work, the performance is essentially the same. This is simply because, for the resonators, the radiation loss is the dominate loss mechanism and the dissipation in the waveguide structure itself, thus, contribute little to the final quality factor of the plasmonic resonators.