Gi-Dong Sim

Tensile characteristics of metal nanoparticle films on flexible polymer substrates for printed electronics applications

Metal nanoparticle solutions are widely used for the fabrication of printed electronic devices. The mechanical properties of the solution-processed metal nanoparticle thin films are very important for the robust and reliable operation of printed electronic devices. In this paper, we report the tensile characteristics of silver nanoparticle (Ag NP) thin films on flexible polymer substrates by observing the microstructures and measuring the electrical resistance under tensile strain. The effects of the annealing temperatures and periods of Ag NP thin films on their failure strains are explained with a microstructural investigation. The maximum failure strain for Ag NP thin film was 6.6% after initial sintering at 150 °C for 30 min. Thermal annealing at higher temperatures for longer periods resulted in a reduction of the maximum failure strain, presumably due to higher porosity and larger pore size. We also found that solution-processed Ag NP thin films have lower failure strains than those of electron beam evaporated Ag thin films due to their highly porous film morphologies.

Improving the stretchability of as-deposited Ag coatings on poly-ethylene-terephthalate substrates through use of an acrylic primer

In this paper, we report that silver films evaporated on poly-ethylene-terephthalate (PET) substrates coated with an acrylic primer can be stretched beyond 70% without fracture. As-deposited films show a larger failure strain than annealed coatings. These observations are rationalized in light of a ductile fracture mechanism where debonding from the substrate coevolves with strain localization. The results of this study indicate that PET substrates coated with an acrylic primer layer may be suitable for stretchable electronics.

Scanning AC Nanocalorimetry Study of Zr/B Reactive Multilayers

The reaction of Zr/B multilayers with a 50u2009nm modulation period has been studied using scanning AC nanocalorimetry at a heating rate of approximately 103u2009K/s. We describe a data reduction algorithm to determine the rate of heat released from the multilayer. Two different exothermic peaks are identified in the nanocalorimetry signal: a shallow peak at low temperature (200–650u2009°C) and a sharp peak at elevated temperature (650–800u2009°C). TEM observation shows that the first peak corresponds to heterogeneous inter-diffusion and amorphization of Zr and B while the second peak is due to the crystallization of the amorphous Zr/B alloy to form ZrB2.

Tensile and fatigue behaviors of printed Ag thin films on flexible substrates

Flexible electronics using nanoparticle (NP) printing has been highlighted as a key technology enabling eco-friendly, low-cost, and large-area fabrication. For NP-based printing to be used as a successive alternative to photolithography and vacuum deposition, stretchability and long term reliability must be considered. This paper reports the stretchability and fatigue behavior of 100u2009nm thick NP-based silver thin films printed on polyethylene-terephthalate substrate and compares it to films deposited by electron-beam evaporation. NP-based films show stretchability and fatigue life comparable to evaporated films with intergranular fracture as the dominant failure mechanism.

Effects of Conductive Particles on the Electrical Stability and Reliability of Anisotropic Conductive Film Chip-on-Board Interconnections

The effects of conductive particles on the electrical stability and reliability of anisotropic conductive film (ACF) joints for chip-on-board applications were investigated. In this paper, two types of conductive particles were prepared. One was a conventional Ni/Au-coated polymer ball, and the other was a Ni/Au-coated polymer ball with Ni/Au-projections. According to the results of a nano-indentation experiment of a conductive particle, the elastic recovery of a single conductive particle decreased as the applied load and the deformation of the conductive particle increased. The evaluated conductive particles which had the same polymer core showed the similar load-deformation behaviors regardless of the existence of Ni/Au-projections. The contact resistances of ACF joints using each conductive particle as a function of bonding pressure were measured, and the results showed that the contact resistances of ACF joints with the metal-projection-type conductive particles were lower and more stable than those with the conventional conductive particles. Especially at lower bonding pressure, ACF joints with the metal-projection-type conductive particles were much more stable and had lower contact resistances than the conventional ACF joints. According to the results of thermal cycling (T/C) reliability test, the metal-projection-type conductive particles also enhanced T/C reliability of ACF joints when compared with the conventional conductive particles.

Nanotwinned metal MEMS films with unprecedented strength and stability

Sputter deposited nanotwinned metal alloy films that possess exceptional properties attractive for next generation MEMS devices. Silicon-based microelectromechanical systems (MEMS) sensors have become ubiquitous in consumer-based products, but realization of an interconnected network of MEMS devices that allows components to be remotely monitored and controlled, a concept often described as the “Internet of Things,” will require a suite of MEMS materials and properties that are not currently available. We report on the synthesis of metallic nickel-molybdenum-tungsten films with direct current sputter deposition, which results in fully dense crystallographically textured films that are filled with nanotwins. These films exhibit linear elastic mechanical behavior and tensile strengths exceeding 3 GPa, which is unprecedented for materials that are compatible with wafer-level device fabrication processes. The ultrahigh strength is attributed to a combination of solid solution strengthening and the presence of dense nanotwins. These films also have excellent thermal and mechanical stability, high density, and electrical properties that are attractive for next-generation metal MEMS applications.

Kinetic Role of Carbon in Solid-State Synthesis of Zirconium Diboride using Nanolaminates: Nanocalorimetry Experiments and First-Principles Calculations.

Reactive nanolaminates afford a promising route for the low-temperature synthesis of zirconium diboride, an ultrahigh-temperature ceramic with metallic properties. Although the addition of carbon is known to facilitate sintering of ZrB2, its effect on the kinetics of the formation reaction has not been elucidated. We have employed a combined approach of nanocalorimetry and first-principles theoretical studies to investigate the kinetic role of carbon in the synthesis of ZrB2 using B4C/Zr reactive nanolaminates. Structural characterization of the laminates by XRD and TEM reveal that the reaction proceeds via interdiffusion of the B4C and Zr layers, which produces an amorphous Zr3B4C alloy. This amorphous alloy then crystallizes to form a supersaturated ZrB2(C) compound. A kinetic analysis shows that carbon lowers the energy barriers for both interdiffusion and crystallization by more than 20%. Energetic calculations based on first-principles modeling suggest that the reduction of the diffusion barrier may be attributed to the stronger bonding between Zr and C as compared to the bonding between Zr and B.

Thermo-mechanical response of single-phase face-centered-cubic AlxCoCrFeNi high-entropy alloy microcrystals

ABSTRACT The response of [100]-oriented single-crystal face-centered-cubic Al0.1CoCrFeNi and Al0.3CoCrFeNi high-entropy alloy (HEA) microcrystals tested from 293 to 573u2009K by means of in situ micro-compression is reported. At all temperatures, plasticity is governed by dislocation slip with significant strain hardening and intermittent strain bursts observed. A model, which is in good agreement with experimental measurements, is also developed to predict the effects of Al concentration, temperature, and crystal size on the strength of HEAs. The model interestingly predicts a softening response with an increase in Al concentration when the crystal size is ≤0.1u2009µm. Finally, this model can guide the development of advanced HEAs for small-scale applications. GRAPHICAL ABSTRACT IMPACT STATEMENT In situ scanning electron microscopy (SEM) experiments are performed to quantify the thermo-mechanical response of AlxCoCrFeNi microcrystals. A physics-based model is also proposed to predict the strength of high-entropy alloys as a function of crystal size, temperature, and Al concentration.