Yoonchul Sohn

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.

Anti-frost coatings containing carbon nanotube composite with reliable thermal cyclic property

One of the most important applications for superhydrophobic coatings is anti-frosting for safety and energy conservation. Safety concerns are especially critical in cold-climate regions where the daily temperature fluctuation is large. However, superhydrophobic coatings have not been studied in terms of their thermomechanical reliability. In this study, wetting characteristics and stress relaxation behavior were quantitatively investigated with multi-walled carbon nanotube (MWNT)–silicone composite films under thermal cycling conditions. It is concluded that an open structure with numerous nanopores among the fillers, constituting air pockets described as the “Cassie structure,” is of great importance not only for developing a films superhydrophobic nature but also for accommodation of thermal stress that results from a difference in coefficient of thermal expansion between the coating and the substrate. The amount of stress relaxation for a 30 vol% MWNT–silicone composite film with open structure reaches ∼80% of the value for its counterpart with a closed structure and no pores. A superhydrophobic MWNT–silicone composite film that can endure over 4000 thermal cycles (−30 °C to room temperature) is fabricated by controlling the composition and microstructure of the composite. In addition, the importance of the size and shape of the nanofillers in delaying nucleation and growth of frost on superhydrophobic coatings is also discussed.

Metallic conduction induced by direct anion site doping in layered SnSe2.

The emergence of metallic conduction in layered dichalcogenide semiconductor materials by chemical doping is one of key issues for two-dimensional (2D) materials engineering. At present, doping methods for layered dichalcogenide materials have been limited to an ion intercalation between layer units or electrostatic carrier doping by electrical bias owing to the absence of appropriate substitutional dopant for increasing the carrier concentration. Here, we report the occurrence of metallic conduction in the layered dichalcogenide of SnSe2 by the direct Se-site doping with Cl as a shallow electron donor. The total carrier concentration up to ~1020 cm−3 is achieved by Cl substitutional doping, resulting in the improved conductivity value of ~170 S·cm−1 from ~1.7 S·cm−1 for non-doped SnSe2. When the carrier concentration exceeds ~1019 cm−3, the conduction mechanism is changed from hopping to degenerate conduction, exhibiting metal-insulator transition behavior. Detailed band structure calculation reveals that the hybridized s-p orbital from Sn 5s and Se 4p states is responsible for the degenerate metallic conduction in electron-doped SnSe2.

Wafer-level low temperature bonding with Au-In system

Wafer bonding at low temperature is an essential process for next generation MEMS & Sensor packaging. Optoelectronic devices, such as image sensor module and laser diode integrated circuit, need low bonding temperature, high re-melting temperature, high thermal conductivity, and stress-relaxed structure in many cases. Eutectic Au-In system was developed as a replacement of previous Au-Sn system for specific systems require bonding temperature lower than 200degC. Bonding temperature of developed Au-In system was set at 180degC, which was 100degC lower than that of Au-Sn system. Though polymer materials has been used for low temperature bonding, out-gassing and volume shrinkage during the bonding process often degraded bonding quality and accurate alignment between the wafers. Clean packaging with accurate alignment was achieved with eutectic Au-In bonding which also possessed high re-melting temperature over 450degC. Majority of the deposited metallizations to construct the system was converted to intermetallic compounds (AuIn & AuIn2) after bonding reaction. Peak temperature and duration time were varied to investigate optimum condition of wafer-level bonding and diced separate dies are used for X-ray inspection, microstructural observation of the cross-section, and shear test. The results showed that bonding parameters critically affected mechanical reliability of the bonded joint. Failure through the solder layer (unreacted pure In) resulted in higher shear strength, while clear separation between the wafer and under bump metallization (UBM) revealed low bond strength. Re-melting temperature of Au-In system was measured using TMA and the result showed that it was closely related with melting phenomena of pre-formed intermetallic compounds such as AuIn and gamma phases. The wafer-level bonding with Au-In system showed good feasibility for MEMS & sensor packagings that require low temperature bonding with high quality.

Effect of Intermetallics Spalling on the Mechanical Behavior of Electroless Ni(P)/Pb-Free Solder Interconnection

A systematic investigation of mechanical testing was conducted to correlate the brittle fracture observed in Ni(P) metallization with Ni-Sn intermetallic spalling in Pb-free solder joints. To produce lap shear test samples, two FR4 PCBs finished with Ni(P)/Au were reflowed using two Pbfree solders, Sn-3.5Ag and Sn-3.0Ag-0.5Cu (in wt.%). Brittle fracture was found in the solder joints made of Sn-3.5Ag, while only ductile fracture was observed in the Cu-containing solder (Sn-3.0Ag-0.5Cu). For Sn-3.0Ag-0.5Cu joints, (Ni,Cu)3Sn4 and/or (Cu,Ni)6Sn5 IMCs were formed at the interface between the solder and Ni(P) film. For Sn-3.5Ag, Ni3Sn4 was formed and spalled off the Ni(P) film, causing brittle fracture in solder pads where Ni3Sn4 had spalled. From the analysis of fracture surfaces, it was found that the brittle fracture occurs through the Ni3SnP layer The growth of the Ni3SnP layer appears to be responsible for Ni3Sn4 spalling and thereby for the brittle fracture of Ni(P)/solder interconnection. To prevent IMC spalling from the Ni(P) metallization, a thin intermediate layer of Sn or Cu was deposited by electrolytic or electroless plating method on the Ni(P) film. During the reflow reaction of Sn-3.5Ag solder paste, the intermediate layers effectively suppressed Ni3Sn4 spalling during the reflow reaction at 250°C for 30 min, while Ni3Sn4 easily spalled off the Ni(P) film in a few minutes in the control samples without an intermediate layer.

Growth of carbon nanotube field emitters on single strand carbon fiber: a linear electron source

The multi-stage effect has been revisited through growing carbon nanotube field emitters on single strand carbon fiber with a thickness of 11 µm. A prepared linear electron source exhibits a turn-on field as low as 0.4 V µm(-1) and an extremely high field enhancement factor of 19,300, when compared with those results from reference nanotube emitters grown on flat silicone wafer; 3.0 V µm(-1) and 2500, respectively. In addition, we introduce a novel method to grow nanotubes uniformly around the circumference of carbon fibers by using direct resistive heating on the continuously feeding carbon threads. These results open up not only a new path for synthesizing nanocomposites, but also offer an excellent linear electron source for special applications such as backlight units for liquid crystal displays and multi-array x-ray sources.

Temperature dependence of contact resistance at metal/MWNT interface

Although contact resistance of carbon nanotube (CNT) is one of the most important factors for practical application of electronic devices, a study regarding temperature dependence on contact resistance of CNTs with metal electrodes has not been found. Here, we report an investigation of contact resistance at multiwalled nanotube (MWNT)/Ag interface as a function of temperature, using MWNT/polydimethylsiloxane (PDMS) composite. Electrical resistance of MWNT/PDMS composite revealed negative temperature coefficient (NTC). Excluding the contact resistance with Ag electrode, the NTC effect became less pronounced, showing lower intrinsic resistivity with the activation energy of 0.019 eV. Activation energy of the contact resistance of MWNT/Ag interface was determined to be 0.04 eV, two times larger than that of MWNT-MWNT network. The increase in the thermal fluctuation assisted electron tunneling is attributed to conductivity enhancement at both MWNT/MWNT and MWNT/Ag interfaces with increasing temperature.

Advanced catalyst design induced enhancement of multi-walled nanotube debundling and electrical conductivity of multi-walled nanotube/silicone composites

Multi-walled nanotube (MWNT)/silicone composites were fabricated with two different kinds of MWNT bundles grown by catalysts with different morphology. The order of agglomeration of MWNTs turned out to be closely related to the shape of the catalyst particles. Though the same composition of precursors was used, catalyst particles made from gelation of the precursors followed by flame synthesis (FS) consisted of chunk-type particles, while those from spraying of the precursor solution followed by thermal decomposition (STD) were fabricated with the shape of thin sheets. After CVD growth, the MWNT bundles were entangled to form large masses for FS-catalysts but they maintained rod-like morphology for STD-catalysts. Individual bundles of the STD-MWNTs also contained a smaller population of MWNTs with more room inside, which finally resulted in highly conductive MWNT/silicone composite due to effective dispersion of the MWNTs. In this study, for the first time, direct correlation between morphology of MWNT catalysts and electrical conductivity of MWNT/polymer composites was experimentally demonstrated and a high electrical conductivity of 1407 S m−1 was acquired using a mass production compatible three roll milling process.

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.

Microstructure of Ag-Sn Bonding for MEMS Packaging

Different metallization systems and bonding designs of Ag-Sn bonding were investigated to achieve good bonding. The bonding strength was evaluated by shear force. The microstructure of bonding interface was inspected by scanning electronic microscopy and ED AX. Shear force test was performed for as-bonded dice. The test results indicate differences among different metallization systems. The bonding pair with Ti/Au as the UBM has a quite low shear strength because of the bad adhesion on the silicon substrate. The bonding pair of Ti/Ni/Sn/Au and Ti/Ni/Au/Ag obviously has higher shear strength than that of Ti/Ni/Sn/Au and Ti/Ni/Au/Ag/Au. The former is 55.17 MPa on average while the later is 36.05 MPa. The shear strength of the pair of Ti/Ni/Sn/Au and Ti/Ni/Au/Ag is similar to that of Ti/Ni/Sn/Au and Ti/Ag which has the shear strength of 55.32 MPa on average. The Ni and Au in the Ag-Sn bonding system have significant effect on the microstructure of the bonding interface. The diffusion of Au into Sn is quicker than both Ag and Ni. The diffusion between Au and Sn would induce the obstacle of the inter-diffusion between Sn and Ag. Ni will also diffuse quickly into Sn and form Ni3Sn4. The existence of Ni in Sn will also influence the diffusion of Ag into Sn and make the bad wettability during bonding. After several metallization systems have been investigated, finally a uniform bonding layer has been achieved by excluding Ni and Au in the bonding system. The bonding interface is Ag3Sn layer dispersed with some pure Ag.