http://dspace.cityu.edu.hk:80/handle/2031/3769 2013-04-30T10:19:31Z 2013-04-30T10:19:31Z Tsang, Mei Ka (曾美嘉) http://dspace.cityu.edu.hk:80/handle/2031/6843 2013-04-03T05:15:37Z 2012-01-01T00:00:00Z

Title: Super-hard coatings with materials based on ternary AlMgB matrices Authors: Tsang, Mei Ka (曾美嘉) Abstract: Mechanical engineering, automotive, aeronautic and mining industries, as well as military and space industry require hard and wear resistance materials. However military and space industry often also needs light-weight materials in combination with the above wearable and hardness properties. Diamond is the hardest material known but some properties of the second hardest material, cubic boron nitride (cBN), surpass those of diamond. Although diamond is superior to cBN in hardness, its chemical stability is lower than that of cBN. Diamond is dissolved in molten steel and ferrous materials, whereas cBN is inert. The two parameters as well as friction coefficients are the reasons for using diamond and cBN as materials of choice in commercial applications. Since diamond is unsuitable for machining steels and ferrous materials and cBN coatings have not been mastered on commercial levels, new superhard coatings are needed. The single elements and binary composite films have been investigated thoroughly, but less effort has been given to ternary and quaternary composites that might reach the hardness of superhard materials. In consistence with the properties for advanced technological applications novel superhard materials in forms of films have been synthesized and studied. In this project, new materials based on aluminum, magnesium and boron were prepared and their mechanical properties were investigated in correlation with their structural composition. It is noted that composite films mentioned are related to the nano-scale since there are nanocrystallites inside the film with amorphous structure, so it means nanocomposite films. Composite films of aluminum magnesium boride (AlMgB) with ultra-hard properties could be suitable alternatives to existing lower performance materials that are currently used in mechanical applications such as cutting tools, or even they could be used as more advanced materials in military and space applications. With suitable structures and chemical compositions, these novel materials based on ternary compounds of Al, Mg and B are remarkable light and counted in the group of superhard materials. Possibly by adding optimum amount and some additives (TiB2) into the AlMgB composite, the mechanical properties of the composite films are enhanced. The hardness can be effectively increased to 45 - 50 GPa, approaching the hardness of the cBN. However, the addition of AlN and TiC would reduce film hardness. In addition, the AlMgB films are highly resistive to abrasion and have low coefficients of sliding friction. Therefore, the AlMgB materials approach the mechanical properties of cBN and diamond. The ternary AlMgB thin films were coated on silicon substrates by a sputter deposition technique using three unbalanced rectangular magnetrons (one AlMg and two boron targets). The magnetrons were installed on a deposition chamber in a closed magnetic field configuration. The deposition parameters and power supplied to individual targets are adjusted to optimize the structure and chemical composition of the film and achieve the extreme level of mechanical properties of the AlMgB films. The surface morphology and roughness were studied by atomic force microscopy and scanning electron microscopy, structural properties were analysed using x-ray diffraction (XRD) and fourier transform infrared (FTIR) spectroscopy, while compositional analysis and chemical states were investigated by energy dispersive x-ray (EDX) spectroscopy and x-ray photoelectron spectroscopy (XPS), respectively. Hardness and elastic modulus were evaluated using a nanoindentation technique. Analysis shows that the root mean square (rms) roughness varies between 1.0 and 18 nm when the AlMg target at power density alters from 0.2 to 1.0 W/cm2 and the boron target is maintained at 2 W/cm2. The chemical compositional analysis presents that aluminum, magnesium and boron are the dominant elements in the film. The coating also comprises some trace elements of oxygen, carbon and argon. The chemical composition is changed when different power densities to the AlMg targets are applied. By changing the power densities, the metal rich film Al1.38Mg0.64B with only about 33 at.% boron content was prepared. The films demonstrate the hardness value of about 30 GPa. The obtained hardness is already impressive when it is compared to the hard hydrogenated diamond-like carbon (DLC) films synthesized by plasma enhanced chemical vapor deposition (CVD) showing hardness of 24 – 28 GPa. On the other hand, boron rich films exhibit similar hardness (~30 GPa). The XRD pattern suggests that the prepared AlMgB films confine nanocrystals in amorphous matrices. The nanocrystals are identified to be B12 icosahedra forming network in amorphous AlMgB matrices that considerably contributes to the hardness enhancement of the films.

2012-01-01T00:00:00Z Foo, Yishu (胡亦抒) http://dspace.cityu.edu.hk:80/handle/2031/6842 2013-04-03T05:05:44Z 2011-01-01T00:00:00Z

Title: Numerical simulation and characterization of photonic/plasmonic devices Authors: Foo, Yishu (胡亦抒) Abstract: Nano-scale optical materials are often difficult to fabricate for experimental characterization and too complex for analytical analysis. Numerical simulation is a more cost effective way to characterize nano-scale optical materials due to increasingly cost effective computers and the availability of commercial electromagnetic-simulation software. One of the electromagnetic simulation methods that become increasingly favorable due to the ability to simulate a wide variety of problems is the Finite-Difference Time-Domain (FDTD) method. However, FDTD is not an unconditionally stable numerical simulation method. To facilitate the use of FDTD to characterize nano-scale optical materials, knowing the conditions for a numerically stable simulation is necessary. The main focus of this project is to quantify the ability of FDTD to characterize thin films using simulations which mimics ellipsometry measurements using a chosen commercial FDTD software. Simulating polarization states of light reflected by thin films is chosen due to the availability of analytical solutions to the problem. In the second part, planar chiral metamaterials (PCM), a material with periodic structure and unit cell in the shape of(left-, or right- hand) gammadions which are shown to support surface plasmon resonance with the ability to excite circularly polarized light into super-chiral light is characterized. These structures are of interest as recently, PCM have been shown to enhance the detection of biomolecules using Circular Dichroism by up to six orders of magnitudes as compared to conventional chirality detection schemes.

2011-01-01T00:00:00Z Chan, Hiu Laam (陳曉嵐) http://dspace.cityu.edu.hk:80/handle/2031/6445 2012-08-15T03:13:44Z 2011-01-01T00:00:00Z

Title: Light emitting diodes based on ZnO nanowires Authors: Chan, Hiu Laam (陳曉嵐) Abstract: The fabrication and characterization of light emitting diodes (LEDs) based on n-ZnO nanowires/p-GaN thin film heterojunction were studied. Closely packed and vertically aligned n-ZnO nanowire array was first grown on Si substrates by a hydrothermal method and then optimized on the p-GaN substrates. The average diameter and length of the n-ZnO nanowires were around 95 nm and 520 nm, respectively. The perfect wurtzite structure of n-ZnO nanowires was verified by the dominant E2 high mode (439 cm-1) in the Raman spectrum of the n-ZnO nanowires. The sharp (002) diffraction peak with a small full width at half maximum (FWHM) of 0.3° in X-ray Diffraction pattern shows the good crystallinity and [001] preferred growing direction of the n-ZnO nanowires. LEDs based on n-ZnO nanowires/p-GaN thin film heterojunctions were successfully fabricated by a hydrothermal method. Arrays of n-ZnO nanowires were grown on two commercially available p-GaN substrates (S1 and S2). According to the results of Four-point Hall Measurement, the carrier density of substrate S1 (2.00 × 1017 cm-3) is double of that of S2. Substrate S2 on the other hand has higher carrier mobility (176 cm2/Vs) than S1 (33 cm2/Vs). Both assembled LEDs exhibit diode-like rectifying behavior in current-voltage (I-V) measurements. Blue emission (440 nm) is dominated in the assembled LED based on substrate S1, while weak UV (370 nm) and green (550 nm) emissions are also detected. The blue emission is visible even at a forward bias as small as 7 V. Interference fringes found in the Electroluminescence emission of the assembled LED based on substrate S2 indicate that those well-faceted n-ZnO nanowires act as waveguiding cavity in the photon emission.

2011-01-01T00:00:00Z Liu, Hao (劉浩) http://dspace.cityu.edu.hk:80/handle/2031/6052 2011-09-20T05:14:22Z 2010-01-01T00:00:00Z

Title: Electrochemical performance of Li(NMC)O2 cathode materials for Li-ion batteries Authors: Liu, Hao (劉浩) Abstract: Li-ion batteries are widely used in portable electronic industry, and have been considered for large scale applications in electric or hybrid electric vehicles. The rapid development in electronics industry calls for better batteries of higher energy storage, lower capacity decay, and lighter weight. A better understanding on materials’ electrochemistry would help us optimize the battery’s performance. This project is therefore focused on the electrochemical performance of a LiNiyMnzCo1-y-zO2 type cathode material. LiNiyMnzCo1-y-zO2 powder was successfully synthesized through co-precipitated hydroxide route, with a resultant composition of LiNi0.36Mn0.29Co0.35O2 determined from EDX. The particle morphology was examined by SEM. It was found that the particles possessed a polyhedral geometry with an average size of 1 μm. XRD analysis on lattice parameters, I003/I104 parameter and R-factor showed a well layered structure in this material with little cation mixing between Li+ and Ni2+. Both particle morphology and well indexed diffraction pattern pointed to the formation of highly crystallized and pure compound. This material’s electrochemical properties were characterized with various electrochemical measurements. Cyclic voltammetry was performed at different scan rates. The oxidation and reduction peaks were found to be at ~3.8 V and ~3.7 V, respectively, which corresponds to the redox reaction of Ni2+/Ni4+. No Mn3+ was found in this compound due to a lack of redox peak at 3.0 V; and this confirms that the major oxidation state of Mn ion is 4+. Cycling charge-discharge performance was carried at two rates: 0.5 C (80 mA/g) and 2 C (320 mA/g). The corresponding discharge capacity was 154 mAh/g and 143 mAh/g at 0.5 C and 2 C rate, respectively. By the end of the 30th cycle, the discharge capacity decays to 137 mAh/g and 122 mAh/g, respectively. The Coulombic efficiency after the first cycle was over 100%. A linear capacity fading model was adopted for this material, and the capacity decay rate was 0.52 % per cycle. Li-ion chemical diffusion coefficient was determined as a function of cell voltage using both GITT and PITT techniques, yielding DLi of ~10-10 cm2/s from GITT and ~10-11 cm2/s from PITT. This range is consistent with the DLi obtained using CV curves at different scan rates, which yielded a value of 2.4×10-11 cm2/s. The equilibrium voltage vs. amount of lithium extraction showed a change of slope at 30% lithium de-intercalation. Finally, a brief investigation on crystal structural change after 30 cycles at 0.5 C and 2 C rates was carried out using XRD, which revealed severe cation mixing and low hexagonal ordering upon cycling by analyzing the I003/I104 parameter, R-factor and the c/a ratio. The increase of (110) over (108) peaks intensity might infer an irreversible Li loss which accounts for the deterioration of the layered structure.

2010-01-01T00:00:00Z