Browsing by Author "Chung, Wei-An"

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Fabrication of TiO2-coated nanostructured Ni foams for improved mechanical properties
(2020-02-15)
Hung, Pei-Sung
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Chou, Yu-Szu
;
Wang, Guang-Ren
;
Chung, Wei-An
;
Wu, Pu-Wei
We demonstrate a composite approach to improve the mechanical strength of nanostructured Ni foams by producing a conformal coating of TiO2 on the skeletons of the Ni foam. The nanostructured Ni foam is fabricated by a template approach in which colloidal crystals of polystyrene microspheres in 720 nm diameter are constructed by a vertical electrophoresis process, followed by electroplating of Ni to fill the interstitial voids among the closely-packed microspheres and removal of the colloidal template afterward. The formation of TiO2 overcoat is achieved by optimization of sol-gel synthesis to prepare desirable TiO2 sols and their subsequent electrophoresis toward the nanostructured Ni foam in a square wave pulsing mode. The thickness of the TiO2 deposit is proportional to the duration of pulsing electrophoresis. From nano-indentation, the TiO2 -coated nanostructured Ni foams exhibit notable improvements in hardness (53-68%) and reduced elastic modulus (26-37%) over those of pristine nanostructured Ni foams. This strategy of implementing a thin ceramic overcoat provides an effective mean to improve the mechanical properties of nanostructured metals.
High throughput fabrication of large-area colloidal crystals via a two-stage electrophoretic deposition method
(2019-09-10)
Hung, Pei-Sung
;
Liao, Chen-Hong
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Chou, Yu-Szu
;
Wang, Guang-Ren
;
Wang, Chuan-Jyun
An effective and high-throughput method is demonstrated to produce a colloidal crystal with impressive crystallinity. The fabrication procedure involves a two-stage electrophoresis process in which a vertical electrophoretic deposition of polystyrene microspheres (495 nm) is conducted on an ITO substrate to form a colloidal crystal in size of 4 x 4 cm(2), followed by a horizontal electrophoresis to improve the packing crystallinity. Relevant parameters associated with the horizontal electrophoresis step including voltage polarity, bipolar voltage offset, and operation frequency are investigated. Their effects over the crystallinity of the colloidal crystals are verified quantitatively and qualitatively by structural observation via scanning electron microscope and optical reflectance spectra. We determine that the imposition of an alternately-changing bias of 80/0/-120/0 V at 10 Hz (25 ms in each bias) enables the production of colloidal crystals with the largest grain size (106 mm) and the smallest grain boundary (170 nm), as compared to samples prepared from a single vertical electrophoresis step. We attribute the improved crystallinity of the colloidal crystal to the bipolar bias that engenders localized movements of the microspheres, rendering them to pack more closely with reduced defects. (C) 2019 Elsevier Ltd. All rights reserved.
Polydopamine and Its Composite Film as an Adhesion Layer for Cu Electroless Deposition on SiO2
(2020-03-09)
Chou, Shih-Cheng
;
Chung, Wei-An
;
Fan, Tzu-Ling
;
Dordi, Yezdi
;
Koike, Junichi
We explore dopamine (DA) and its mixtures with polyethylene glycol (PEG) or polyethylenimine (PEI) as an adhesion layer for bonding between Cu and SiO2. The DA is oxidized to form polydopamine (PDA) which deposits as aggregates on SiO2 surface with notable surface roughness. After mixing with PEG or PEI, the morphology of PDA aggregates is altered considerably. Electroless Cu deposition in a mild alkaline bath is employed to deposit Cu atop the adhesion layer. The Cu films reveal an fcc lattice with (111) preferred orientation and their thickness was around 650 nm. From measurements of four-point probe, breaking strength, and tape-peeling tests, the PDA/PEG mixture reveals impressive performance serving as a strong adhesive for robust Cu bonding. We attribute the unique adhesive ability of PDA/PEG to the hydrogen bonds established between the catechol and amine groups of DA with PEG that renders desirable film formation on the SiO2 surface for optimized interaction between Cu and SiO2. (C) 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited.
Synthesis of polystyrene@polypyrrole-COOH@Ag (core@shell@shell) microspheres for potential application in anisotropic conductive paste
(2020-03-15)
Wang, Chuan-Jyun
;
Hung, Pei-Sung
;
Chou, Shih-Cheng
;
Chung, Wei-An
;
Wu, Pu-Wei
We demonstrate a fabrication scheme for the synthesis of conductive core-shell (core@shell) nanoparticles consisting of polystyrene (PS) microspheres in size of 613 nm with consecutive coatings of functionalized polypyrrole (PPy-COOH) and Ag. The PPy-COOH precursors undergo polymerization on PS microspheres forming PS@PPy-COOH microspheres, followed by sensitization and Ag electroless deposition to produce PS@PPy-COOH@Ag microspheres in size of 717 nm. The PPy-COOH serves as the adhesive for intimate bonding of PS and Ag. Images from scanning and transmission electron microscopes confirm a Ag coating of 41-50 nm which is consisted of clustered nanoparticles forming a mono-layered film. From X-ray diffraction pattern, the Ag adopts a polycrystalline fcc structure with grain size of 12 nm. Results from thermal gravimetric analysis indicate a Ag loading of 54 wt%. By four-point-probe, the bulk conductivity for PS@PPy-COOH@Ag is 10,170 S m(-1). These microspheres show promising potential for anisotropic conductive paste. (C) 2019 Elsevier B.V. All rights reserved.