Kunmo Chu

Design of multi-functional dual hole patterned carbon nanotube composites with superhydrophobicity and durability

AbstractMost current research on nanocomposites has focused on their bulk attributes, i.e., electrical, microwave, thermal, and mechanical properties. In practical applications, surface properties such as robustness against environmental contamination are critical design considerations if intrinsic properties are to be maintained. The aim of this research is to combine the bulk properties of nanocomposites with the superhydrophobic surface properties provided by imprinting techniques to create a single multi-functional system with enhanced bulk properties. We report the development of a highly conductive superhydrophobic nanotube composite, which is directly superimposed with a durable dual hole pattern through imprinting techniques. The dual hole pattern avoids the use of high slenderness ratio structures resulting in a surface which is robust against physical damage. Its stable superhydrophobic properties were characterized both theoretically and experimentally. By incorporating high aspect ratio carbon nanotubes (CNTs), the dual patterned composites can also be effectively used for anti-icing and deicing applications where their superhydrophobic surface suppresses ice formation and their quick electric heating response at low voltage eliminates remaining frost. In addition, superior electromagnetic interference (EMI) shielding effectiveness (SE) was attained, with one of the highest values ever reported in the literature.

Fabrication of a Hybrid Carbon-Based Composite for Flexible Heating Element With a Zero Temperature Coefficient of Resistance

We report on a hybrid carbon-based composite for zero temperature coefficient of resistance (TCR) heating element, forming edge-island type composite comprised of carbon nanotubes (CNTs) and carbon blacks (CBs) with a polydimethylsiloxane. The island-shaped CB composite is placed between a copper connection electrode and the CNT composite, acting as both a buffer and distribution layer against current flow for a zero TCR. The degree of control over this zero TCR was characterized by examining the thickness ratio between the CNT and CB layers. The optimized edge-island composite structure showed a constant normalized resistance (with <;3% deviation) to 200 °C with a rapid heating property. This edge-island composite structure could be widely used for heat-related or sensor applications, solving safety, and accuracy issues.

Resistance Complemented Carbon-Nanotube Composite for Laser Printer Fusers Element

Resistance dependence on temperature and mechanical deformation (compression and tension) issues have to be solved, in order to utilize carbon nanotubes (CNT) polymer composite for practical heat-related applications. We report a resistance compensated laser printer fuser element using CNT composite that shows rapid heating properties. During electric heating (200 °C), highly conductive CNT/polydimethylsiloxane (PDMS) composites show a negative temperature coefficient (NTC) of resistivity resulting in the decrease of resistance (-14%), due to interconnection resistance between the CNTs. We redeem the NTC property through the use of the positive pressure coefficient effect of CNT/PDMS. In a paper-feeding test using a laser printer fuser system, CNT/PDMS film that was properly compressed by pressing roller (120 N/cm2) and heating roller showed a constant normalized resistance until 200 °C. The proposed laser printer fuser element could be used as a basis for heating unit applications.

Effects of Ag addition and Ag3Sn formation on the mechanical reliability of Ni/Sn solder joints

Abstract Formation of intermetallic compounds (IMCs) in solder joints is closely associated with the mechanical reliability of the system. Though internal voids formed in Ni/Sn solder joints are known to be related to the formation of Ni 3 Sn 4 IMC, a detailed study on the mechanical reliability has not yet been reported. In this study, the mechanical reliability of Ni/Sn joints was investigated using two different soldering systems: Ni/Ag-Ag/Sn/Ni bilayers and Ni/Sn/Ag-Ag/Sn/Ni sandwich structures. The failure mode was found to be closely related to the formation and growth of an Ag 3 Sn phase. Filling of the voids with Ag 3 Sn IMC resulted in maximum shear strength, with a failure locus through Ni 3 Sn 4 and Ag 3 Sn. However, formation of a large amount of Ag 3 Sn decreased the shear strength once again.

Enhancement of optical performance of the light emitting diode packages with advanced thermal design of die-attaching layers

To guarantee long lifetime of light emitting diodes (LEDs), thermal reliability of LED packages should be secured with suitable die-attaching materials. Die-attaching materials are important for the interconnection between optical devices and substrates, as they not only provide electrical transmission but also the emission of thermal budget generated under the operation of the devices. In this study, the joints with different die-attaching materials were closely investigated in conjunction with the optical performance of LED packages. Thermal reliability of the joints was quantitatively analyzed by estimating the thermal resistances of the metallic and intermetallic layers incorporating the joints. The experimental results revealed that insertion of nano-Ag paste between eutectic Au-Sn and Ag finish significantly improved heat emission by effectively suppressing thermal resistance of eutectic Au-Sn layer. However, to ensure long-term reliability, complete removal of numerous nano-voids among Ag nanoparticles should be accomplished to prevent accumulation of thermal budget.

Carbon Nanotube Nanocomposite Having Segregated Network Structure for Wearable Thermotherapy Application

Multiwalled nanotube (MWNT)/silicone composites having segregated MWNT network and micro-void structure were developed for wearable thermotherapy application. The nano-composites could be quickly fabricated from instant evaporation of aqueous medium during spray coating process. Fast electric heating behavior (4.8 °C/s) was demonstrated, in comparison with that (1.4 °C/s) of conventional silicone rubber composites having no void. Long-term stability was also verified with thermal aging and thermal cycling tests up to 100 h.

Effect of multiple flip-chip assembly on the mechanical reliability of eutectic Au–Sn solder joint

Eutectic Au–Sn solder has widely been used for high temperature bonding since it enables fluxless soldering and provides mechanically stable solder joint. However, effect of multiple bonding process on the mechanical reliability has not been studied in detail. In this study, microstructure evolution of Au–Sn solder joint and its effect on mechanical reliability were systematically investigated. During the multiple reflow process, the eutectic phase gradually transformed into ζ phase, which resulted in loss of solderable area. The phenomenon turned out to be responsible for degradation of bonding strength of the solder joint.