C. P. Wong

A precise numerical prediction of effective dielectric constant for polymer-ceramic composite based on effective-medium theory

Nanostructure polymer-ceramic composite with high dielectric constant (/spl epsiv//sub /spl tau///spl sim/90) has been developed for embedded capacitor application. This polymer-ceramic system consists of lead magnesium niobate-lead titanate (PMN-PT) ceramic particle and modified high-dielectric constant low-viscosity epoxy resin. In order to obtain precise prediction of effective dielectric constant of this composite, an empirical prediction model based on self-consistent theory is proposed. The electrical polarization mechanism and interaction between epoxy resin and ceramic filler has been studied. This model can establish the relevant constitutional parameters of polymer-ceramic composite materials such as particle shape, composition, and connectivity that determine the dielectric properties of the composite. This model is simpler, uses fewer parameters and its prediction compares better with experiment (error <10%). The precision and simplicity of the model can be exploited for predictions of the properties and design of nanostructure ferroelectric polymer-ceramic composites. The effective-medium theory (EMT) has been proved a good tool to predict effective properties of nanocomposites.

Synthesis and dielectric properties of novel high-K polymer composites containing in-situ formed silver nanoparticles for embedded capacitor applications

Dielectric properties of in-situ formed silver (Ag) incorporated carbon black (CB)/polymer composites were studied. In-situ formed Ag nanoparticles in the Ag/CB/epoxy composites increased the dielectric constant (K) value and decreased the dissipation factor (Df). The remarkably increased dielectric constant of the nanocomposite is due to the piling of charges at the extended interface of the interfacial polarization-based composites. The reduced dielectric loss might be due to the Coulomb blockade effect of the contained Ag nanoparticles, the well-known quantum effect of metal nanoparticles. The size, size distribution and loading level of metal nanoparticles in the nanocomposite were found to have significant influences on the dielectric properties of the nanocomposite system.

Fast Preparation of Printable Highly Conductive Polymer Nanocomposites by Thermal Decomposition of Silver Carboxylate and Sintering of Silver Nanoparticles

We show the fast preparation of printable highly conductive polymer nanocomposites for future low-cost electronics. Highly conductive polymer nanocomposites, consisting of an epoxy resin, silver flakes, and incorporated silver nanoparticles, have been prepared by fast sintering between silver flakes and the incorporated silver nanoparticles. The fast sintering is attributed to: 1) the thermal decomposition of silver carboxylate-which is present on the surface of the incorporated silver flakes-to form in situ highly reactive silver nanoparticles; 2) the surface activation of the incorporated silver nanoparticles by the removal of surface residues. As a result, polymer nanocomposites prepared at 230 °C for 5 min, at 260 °C for 10 min, and using a typical lead-free solder reflow process show electrical resistivities of 8.1×10(-5), 6.0×10(-6), and 6.3×10(-5) Ω cm, respectively. The correlation between the rheological properties of the adhesive paste and the noncontact printing process has been discussed. With the optimal rheological properties, the formulated highly viscous pastes (221 mPa s at 2500 s(-1)) can be non-contact-printed into dot arrays with a radius of 130 μm. The noncontact printable polymer nanocomposites with superior electrical conductivity and fast processing are promising for the future of printed electronics.

Preparation of highly conductive polymer nanocomposites by low temperature sintering of silver nanoparticles

Highly conductive polymer nanocomposites with very low resistivity (4.8 × 10−5 Ω cm) were prepared by thermal sintering of silver nanoparticles with silver flakes dispersed in a polymer matrix at 180 °C. By comparative studies of thermal behavior of Ag nanoparticles, the critical processing temperature required to obtain very low resistivity of polymer nanocomposites has been identified for Ag nanoparticles with different surface properties. The results indicate that the decomposition temperature of surface residues on Ag nanoparticles plays a key role in the sintering of Ag nanoparticles and thus the electrical resistivity of the polymer nanocomposites. Electrical measurements of the polymer nanocomposites showed that morphological changes induced by sintering of Ag nanoparticle with Ag flakes considerably contribute to the reduction of the contact resistance between conductive fillers, increasing the nanocomposite conductivity.

Silver/polymer nanocomposite as a high-kpolymer matrix for dielectric composites with improved dielectric performance

A silver (Ag)-polymer nanocomposite has been developed by in-situ formation of metal nanoparticles within the polymer matrix and utilized as a high-dielectric constant (k) polymer matrix to enhance the dielectric properties of high-k composite materials. By using an in-situ photochemical reduction method, uniformly dispersed Ag nanoparticles of size of around 10 nm were generated in polymer matrices. Self-passivated aluminium (Al) particles were incorporated into this Ag-epoxy matrix and the dielectric properties of the as-prepared composite materials were investigated. The composites showed more than 50% increase in k values as compared with an Al/neat epoxy composite with the same filler loading of Al. The dielectric loss tangent of the Al/Ag-epoxy composites was below 0.1, which meets the requirement for embedded decoupling capacitors. These results suggest that the Ag-epoxy high-kpolymer matrix effectively enhances the dielectric constant while maintaining the low dielectric loss of the high-k composites. In addition, detailed dielectric property measurements revealed that the dielectric properties and their frequency dispersion as well as the breakdown behaviors of the Al/Ag-epoxy composites were related to the incorporation and concentration of Ag nanoparticles in the high-kpolymer matrix.

Silicon surface structure-controlled oleophobicity.

Superoleophobic surfaces display contact angles >150 degrees with liquids that have lower surface energies than does water. The design of superoleophobic surfaces requires an understanding of the effect of the geometrical shape of etched silicon surfaces on the contact angle and hysteresis observed when different liquids are brought into contact with these surfaces. This study used liquid-based metal-assisted etching and various silane treatments to create superoleophobic surfaces on a Si(111) surface. Etch conditions such as the etch time and etch solution concentration played critical roles in establishing the oleophobicity of Si(111). When compared to Youngs contact angle, the apparent contact angle showed a transition from a Cassie to a Wenzel state for low-surface-energy liquids as different silane treatments were applied to the silicon surface. These results demonstrated the relationship between the re-entrant angle of etched surface structures and the contact angle transition between Cassie and Wenzel behavior on etched Si(111) surfaces.

Guided Three-Dimensional Catalyst Folding during Metal-Assisted Chemical Etching of Silicon

In recent years metal-assisted chemical etching (MaCE) of silicon, in which etching is confined to a small region surrounding metal catalyst templates, has emerged as a promising low cost alternative to commonly used three-dimensional (3D) fabrication techniques. We report a new methodology for controllable folding of 2D metal catalyst films into 3D structures using MaCE. This method takes advantage of selective patterning of the catalyst layer into regions with mismatched characteristic dimensions, resulting in uneven etching rates along the notched boundary lines that produce hinged 2D templates for 3D folding. We explore the dynamics of the folding process of the hinged templates, demonstrating that the folding action combines rotational and translational motion of the catalyst template, which yields topologically complex 3D nanostructures with intimately integrated metal and silicon features.

Characterization of a no-flow underfill encapsulant during the solder reflow process

A challenge in flip-chip technology development is to improve the thermo-mechanical reliability of the flip-chip assembly. To increase reliability, an underfill encapsulant is applied to the gap between IC chip and substrate to provide thermal-mechanical protection as well as environmental protection to the assembly. Two processes for applying the underfill encapsulant to the gap between IC chip and substrate can be described as the fast-flow method and the no-flow (reflowable underfill) method. The fast-flow method is currently the most widely used method. The no-flow method is a new innovative method that provides cost savings. In order to develop novel underfill encapsulants for the no-flow process, a better understanding of the underfill properties during the solder reflow is needed. This paper studies two aspects of the No-Flow underfill: fluxing activity and viscosity during reflow. These two aspects are important for proper interconnect formation. Solder wetting studies were conducted by applying the no-flow underfill on top of solder beads on substrates of different metallizations. The samples were then placed in a 7-zone reflow oven on different eutectic type heating cycles. Cross sections of the samples were taken and the angle the solder makes with the substrate was determined. The viscosity of the underfill during reflow is important to allow proper solder interconnects. To acquire the viscosity of the underfill just before, during, and shortly after the solder reflow temperature, a no-flow underfill encapsulant developed at the Georgia Institute of Technology was studied. Samples of this underfill were placed in a 5-zone reflow oven on a standard eutectic cycle and taken out at different points. The samples were then analyzed by differential scanning calorimetry (DSC) to find the % conversion (amount of cure) of the underfill material. These % conversions were then used to find the complex viscosity at different points in the reflow process. In this paper, we present the experimental procedures and results of the No-Flow underfills fluxing abilities and viscosity during reflow heating conditions.

Carbon fiber paper supported hybrid nanonet/nanoflower nickel oxide electrodes for high-performance pseudo-capacitors

A composite electrode consisting of hybrid nanonet/nanoflower NiO deposited on carbon fiber paper scaffolds demonstrates a much-improved areal capacitance (0.93 F cm−2) while maintaining high rate capability and excellent cycling life. These performance characteristics are attributed to the unique electrode architecture and the nanostructures of NiO. While the nanonet NiO with a high surface area greatly facilitates the redox reactions for charge storage, the porous nanoflowers further extend the active sites for the redox reactions, leading to fast Faradic reactions for efficient energy storage.

1D Ni–Co oxide and sulfide nanoarray/carbon aerogel hybrid nanostructures for asymmetric supercapacitors with high energy density and excellent cycling stability

The fabrication of supercapacitor electrodes with high energy density and excellent cycling stability is still a great challenge. A carbon aerogel, possessing a hierarchical porous structure, high specific surface area and electrical conductivity, is an ideal backbone to support transition metal oxides and bring hope to prepare electrodes with high energy density and excellent cycling stability. Therefore, NiCo2S4 nanotube array/carbon aerogel and NiCo2O4 nanoneedle array/carbon aerogel hybrid supercapacitor electrode materials were synthesized by assembling Ni-Co precursor needle arrays on the surface of the channel walls of hierarchical porous carbon aerogels derived from chitosan in this study. The 1D nanostructures grow on the channel surface of the carbon aerogel vertically and tightly, contributing to the enhanced electrochemical performance with ultrahigh energy density. The energy density of NiCo2S4 nanotube array/carbon aerogel and NiCo2O4 nanoneedle array/carbon aerogel hybrid asymmetric supercapacitors can reach up to 55.3 Wh kg-1 and 47.5 Wh kg-1 at a power density of 400 W kg-1, respectively. These asymmetric devices also displayed excellent cycling stability with a capacitance retention of about 96.6% and 92% over 5000 cycles.