Papers by Author: Jan Ma

Abstract: In the present work, a piezoelectric diaphragm pump was designed and investigated. The pump uses a piezoelectric diaphragm transducer as the driving component, and two check valves located at the inlet and outlet to control the flow direction. The displacement of the transducer was first measured statically and dynamically. Then the vibration of check valves and the effects of chamber depth were investigated. Finally the pump performance was characterized. Maximum flow rate of 200 mL/min and pressure head of 5 mH₂O can be achieved.

Abstract: This paper proposes an effective and systematical method to obtain reliable rate-dependent material models used in FEM simulation for polymers. Compressive stress-strain curves of two types of polymer are obtained at different strain rates. Rate-dependent elastic-plastic models are applied to describe the observed rate-dependent behaviors, whereby the input data of material model are determined from the test data obtained. Verification of the material models is proposed via comparing FEM simulation with test data of quasi-static tension tests and dynamic compression tests of different strain rates.

Abstract: Bi₂Te₃ and its solid solution remain the state-of-the-art thermoelectric materials for refrigeration applications in microelectronics industry, such as dissipating the heat generated by various devices. The fabrication method and associated processing parameters are to be optimised to get desirable composition exhibiting better electrical and thermal transport properties. Carrier concentration and mobility are found to be crucial in achieving high thermoelectric cooling efficiency and energy conversion. In this paper, we present the fabrication and analysis of thermoelectric thin films deposited by RF-magnetron sputtering from n-type Bi₂Te_2.7Se_0.3 and p-type Bi_0.5Sb_1.5Te₃ targets on a silicon substrate. X-ray diffraction, Scanning electron microscopy combined with energy dispersive spectrometry, electrical resistivity, Seebeck coefficient and thermal diffusivity measurements were used for the thermoelectric thin films characterization. We studied the effect of sputtering process parameters, on the structural, electrical and thermal transport characteristics of films. The observed results demonstrate both n-and p-type doped Bi₂Te₃ films exhibit desirable properties and could be potential candidates for thermoelectric micro-cooler applications.

Abstract: Thermoelectric is an ever evolving field that serves many critical needs (cooling and power generation) in the industry. The key objective of this work is to fabricate Bismuth Telluride (Bi₂Te₃) thin-films by varying the various process parameters using a radio-frequency (RF) magnetron sputtering disposition technique. Characterization methods such as four point probe resistivity, surface profiler, atomic force microscopy (AFM), X-ray diffraction (XRD), Seebeck coefficient and thermal diffusivity are performed on the N and P-type Bi₂Te₃ films. The samples are analysed for their electrical properties in relation to the evolved microstructures, for how the process parameters of sputtering and annealing affect these changes. The results demonstrate that N-Type film (S2) processed using sputtering parameters of 7mT, 100W, 50sccm of argon flow under room temperature for 30mins with no annealing and the P-Type film processed using sputtering parameters of 7mT, 100W, 60sccm under room temperature for 30mins with institute annealing at 200°C for 2h exhibit desirable thermoelectric properties suitable for cooling application in microelectronic and optoelectronic devices, optimizing their performance and reliability.

Abstract: The aim of this work is to fabricate and develop the innovative ceramic coatings for protection applications with both high hardness and coating adhesion particularly performing in extremely environmental conditions. In this study, nanocomposite AlCrTiSiN coatings were developed and deposited on Carbide coupons (WC with Co binder) substrate using Cathodic Arc Deposition techniques. The resultant coatings were evaluated with respect to fundamental mechanical properties such as hardness, modulus and coating adhesion. It was found that the nanomposite AlCrTiSiN coatings has a good adhesion to the substrate and retained extremely high hardness as superhard coatings with hardness values above 50 GPa.

Abstract: Recently, thermoelectric thin films have been gaining attention as potential thermoelectric generators that can be used to power external devices. Such films can recover electrical energy from waste heat and are environmentally friendly. Micro fabrication of thin films is achieved by sputtering on silicon films. In this study, the sputtering of Bismuth Telluride (N-type, P-type) films was investigated. Research has verified the efficiency of Bismuth Telluride films, but little is known about how the sputtering process affects the film's quality. Thus, the focus of this study explores how sputtering parameters of discontinuous sputtering intervals, exposure to normal atmospheric conditions and in situ annealing affect the thickness, thermoelectric properties, and microstructure of films. This will bring about a better understanding of the relationship between the sputtering process and the properties of the produced film for both N and P type materials. Recommendations based on this study can contribute to the production of more efficient thin films suitable for energy harvest application.

Abstract: Highly transparent Er:YAG ceramics with different Er concentration were fabricated by a solid-state reaction and vacuum sintering method. The optical properties, the microstructure and the upconversion luminescence of the Er:YAG ceramics were investigated. For 3 mm thick samples, the in-line transmittances of the as-fabricated Er:YAG ceramics at the wavelength of 1100 nm and 400 nm were about 84% and 82%, respectively, which was very close to the theoretical transmittance of YAG ceramics. The micrograph of the Er:YAG transparent ceramics exhibited a pore-free structure and the average grain size was about 10 μm. The grain boundary of the ceramics was clean and no secondary phase was detected. When pumped by a 980 nm LD, the strong green and red upconversion luminescences in the Er:YAG ceramics were observed. The different upconversion mechanisms depending on Er concentration in the Er:YAG ceramics were also discussed.

Abstract: High quality holmium doped Yttrium Aluminum Garnet (YAG) transparent ceramics were fabricated by a reactive sintering method under vacuum. Fully dense Ho:YAG ceramics with the average grain size of ~ 10 μm were obtained after vacuum sintering at 1780 for 8 h. The optical properties, microstructures and photoluminescence spectra of the fabricated Ho:YAG ceramics were investigated. The transmittances of Ho:YAG ceramic are higher than 82% at 400 nm and 84% at 2400 nm. The absorption coefficient was 1.32 cm^-1 for 1.5 at.% Ho:YAG at 1907 nm. And the absorption cross section of the Ho:YAG ceramic is 0.645×10^-20 cm².

Abstract: Yb³⁺ doped YAG ceramics have been successfully fabricated using a vacuum reactive sintering method. High purity Y₂O₃, Al₂O₃ and Yb₂O₃ powders were used as the starting materials. Samples of 5.0 at.% Yb³⁺ doping concentration have been obtained. The in-line transmittance of the sample reaches 84% at 1100 nm wavelength. The absorption and emission bandwidth were measured to be 19.5 nm and 11.07 nm respectively. The results show that Yb:YAG ceramics will be a promising laser material for high power and ultrafast laser applications.

Abstract: Natural hydroxyapatite has been electrophoretically deposited on medical grade 316L stainless steel. Stable suspensions were prepared by mixing 40 g/L milled natural hydroxyapatite powder in isopropyl alcohol and stabilized by polyethylenimine as dispersing agent and binder. The stability of suspensions was investigated by measuring zeta potential. It was found here that the suspension which was stabilized with 4 g/L polyethylenimine revealed a high value of zeta potential and stability. Deposition was achieved on the cathode at constant voltages of 30, 60, and 90 V for 1 to 5 minutes. After deposition, the samples were dried at room temperature for 24 hours and deposition weight, roughness, and thickness of the coatings were measured. The surface morphology of the coated samples was studied by a scanning electron microscope. The results of the electrophoretic deposition process showed that the sample coated at 60 V and 3 minutes led to an adherent, continuous, and crack-free coating. The coating efficiency and thickness increased with increasing deposition time and yielded to saturation at the constant applied voltage. Also, the current density decreased and yielded to saturation at the constant applied voltage during electrophoretic deposition.