Meng Kao Yeh

Bending Property of Sandwich Beams with MWNTs/Polymer Nanocomposites as Core Materials

The bending property and failure mechanism of sandwich beams were investigated. The multi-walled carbon nanotubes (MWNTs) were used as the reinforcement in the MWNTs/polymer nanocomposites and the epoxy and phenolic resins were used as the matrix. The MWNTs/polymer nanocomposites were used as the core materials of sandwich beams, which had faces made by graphite/epoxy laminates. In experiment, the hot press method was applied to fabricate the MWNTs/polymer nanocomposites and graphite/epoxy laminates. The weight percentage of MWNTs in core materials and the fiber orientation of face laminates were varied to assess their effects on the bending properties of the sandwich beams. The failure mechanism of sandwich beams with two kinds of polymer core materials was investigated. In analysis, the finite element method was used to obtain the bending behavior of the sandwich beams. The numerical results were in good agreement with the experimental ones.

Thermal Stress and Thermal Cycling Analyses of Microgyroscope Chip Models

The thermal stress and thermal fatigue life for three different microgyroscope chip models were investigated in this paper. The deformation and stress distribution in chip, at interface between microgyroscope and chip, and in the spring of microgyroscope were obtained for three different microgyroscope chip models by the finite element method. The results show that for the simplified model, no obvious differences from linear or nonlinear analyses are obtained and the fatigue life of microgyroscope chip can be predicted with the properly simplified model. Also, the model having the same process in fabricating microgyroscope and carrier has better reliability. This paper provides an effective method for the reliability analysis of microgyroscope chip.

Glass Transition Temperature of Phenolic-Based Nanocomposites Reinforced by MWNTs and Carbon Fibers

The multi-walled carbon nanotubes (MWNTs) and carbon fibers (CFs) were added to the phenolic resin to fabricate MWNTs/phenolic, MWNTs/CFs/phenolic nanocomposites and CFs/phenolic composites by hot press method. The differential scanning calorimetry (DSC) test was performed for the above-mentioned three kinds of composites. The valley points on the slope of endothermic responses correspond to the glass transition temperatures of the composites. The MWNTs/phenolic nanocomposites had the lowest glass transition temperature among the three kinds of composites discussed, which indicated a better thermal conductivity property of MWNTs. Phenolic-based composites reinforced by different weight percentages of MWNTs and CFs were also investigated. The tensile failure morphologies of nanocomposite specimens were examined using a scanning electron microscope to evaluate the possible effects on the glass transition temperature of nanocomposites..

Dynamic Mechanical Properties of MWNTs/Phenolic Nanocomposites

Two different types of multi-walled carbon nanotube (MWNT), the dispersed and the network MWNTs, were used to reinforce the phenolic resin. The MWNTs/phenolic nanocomposites were tested by a dynamic mechanical analyzer (DMA) to characterize their dynamic mechanical properties. The results showed that increasing the MWNT content can increase the storage modulus, the loss modulus and the glassy transition temperature of the MWNTs/phenolic nanocomposites. A subambient loss transition is seen in the nanocomposites with network MWNTs which results in a better impact resistance property in the nanocomposites. The glassy transition temperature of the nanocomposites with network MWNTs is higher than that of nanocomposites with dispersed MWNTs. The MWNT additive in phenolic resin can be used to improve the dynamic mechanical properties of the MWNTs/phenolic nanocomposites. The tensile failure morphologies of MWNTs/phenolic nanocomposites were also examined using field emission scanning electron microscope (FESEM) to explain the difference between the two types of nanocomposites.

Dynamic Instability of Delaminated Composite Plates under Parametric Excitation

The dynamic instability behavior of delaminated composite plates under transverse excitations was investigated experimentally and analytically. An electromagnetic device, acting like a spring with alternating stiffness, was used to parametrically excite the delaminated composite plates transversely. An analytical method, combined with the finite element method, was used to determine the instability regions of the delaminated composite plates based on the modal parameters of the composite plate and the position, the stiffness of the electromagnetic device. The delamination size and position of composite plates were varied to assess their effects on the excitation frequencies of simple and combination resonances in instability regions. The experimental results were found to agree with the analytical ones.

Dynamic Properties of MWNTS/Epoxy Nanocomposite Beams

The dynamic properties of multi-walled carbon nanotubes (MWNTS)/epoxy nanocomposite beams were investigated experimentally and numerically. The MWNTs/epoxy nanocomposite beams were fabricated by hot press method. In experiment, the dynamic properties of the nanocomposite beams, such as natural frequency, and damping ratio, were obtained. A shaker was used to provide the vibration source at the fixed base of the specimens. The vibration signals of the nanocomposite beams were detected by a laser sensor, and the frequency responses were obtained by a computer-aided signal analyzer. The half power method was used to find the damping ratios of the nanocomposite beams for each mode. In analysis, the mechanical properties of MWNTs/epoxy nanocomposites were obtained and used in the free vibration analysis by the finite element method. The natural frequencies and mode shapes of the nanocomposite beams were calculated numerically. The effect of the weight percentage of MWNTs on the dynamic properties of the nanocomposite beams was investigated. The numerical results were found to be in good agreement with the experimental ones.

Force Measurement by AFM Cantilever with Different Coating Layers

Atomic force microscopy (AFM) is widely used in many fields, because of its outstanding force measurement ability in nano scale. Some coating layers are used to enhance the signal intensity, but these coating layers affect the spring constant of AFM cantilever and the accuracy of force measurement. In this paper, the spring constants of rectangular cantilever with different coating thickness were quantitatively measured and discussed. The finite element method was used to analyze the nonlinear force-displacement behavior from which the cantilever’s normal and torsional spring constants could be determined. The experimental data and the numerical results were also compared with the results from other methods. By considering the influence of coating layers and real cantilever geometries, the more accurate force measurements by AFM cantilever can be obtained.

Reliability Analysis in Flip Chip Package under Thermal Cycling

The material properties of underfill and substrate in flip chip package have temperature-dependent and moisture-sensitive characteristics. During the solder reflow process, the CTE mismatch in the package causes thermal stresses, which may reduce the reliability of the flip chip package. The package reliability can be improved by varying the die thickness, the fillet angle and the thickness of the underfill and by changing the underfill material. In this paper, the temperature- dependent properties of the underfill were established first. The flip chip reliability was then analyzed by finite element code ANSYS. Both underfill A and underfill B were used in the analysis. The results show that better reliability of the flip chip package was obtained for underfill A, for larger fillet angle of the underfill, for thinner die in the package, and for larger Youngs modulus of underfill with linear elastic assumption. Also a hygrothermal preconditioning before thermal cycling reduces the reliability of the flip chip package.

Delamination Growth in Ball Grid Array Electronic Package

The delamination problem in plastic ball grid array electronic package was investigated analytically and experimentally. The ANSYS code was used in the analysis to find the deformation and the stress distribution in electronic package due to the thermal mismatch between different materials at reflow temperature with saturated moisture pressure at the delaminated region. The stress intensity factor and the strain energy release rate at the tip of interfacial delamination were calculated using the fracture mechanics approach. The results show that the delaminaton occurs at the corner of the die pad and at the middle region of the die attach/die pad interface. The delaminaton grew from the middle region of the die attach/die pad interface, along the epoxy molding compoend/die pad interface, the epoxy molding compound/substrate interface to the exterior surface. Possible growth directions of the interfacial delamination in plastic ball grid array electronic package were identified and observed in the experimental results. In addition, the reflow temperature affected the stress distribution and the strain energy release rate at the tip of interfacial delaminaton.

Stress Reduction of Solar Cell with Nanostructures on Silicon Chip Surface

Silicon chip has been widely used in solar cell recently. The thinning of silicon chip, easily inducing surface defects, becomes necessary to produce solar cells more efficiently. The surface defects resulting in stress concentration on the silicon chip surface would be the source of chip failure. In this study, the finite element analysis was used to investigate the stress distribution near the surface crack of a solar cell on which the nanostructures were introduced to alleviate the induced stress. For the solar cell model, positive silver and negative aluminum electrodes were added on the top and bottom sides of silicon chip. The solar cell under four-point bending was simulated in analysis with and without nanostructures. The results show that the stresses reduce more than 50 % for the solar cell model with nanostructures. When the crack depth is deeper enough, the stress at crack tip is higher than that at junction near the electrode and the crack leads to the failure of solar cell. The effect of different section length of nanostructures on the stress distribution caused by the surface crack was also discussed.