Showing papers by "Zhong Chen published in 2008"

Scratch resistance of brittle thin films on compliant substrates

Abstract: There has been intensive interest in studying the behavior of hard and brittle thin films on compliant substrates under scratch action. The examples include sol–gel protective coatings on plastic optical lenses, safe windows, and flexible electronic devices and displays. Hard ceramic coatings have been widely used to prolong the life of cutting tools and biomedical implants. In this work, the scratch resistance of sol–gel coatings with different amount of colloidal silica on polycarbonate substrates was tested by the pencil scratch test following the ISO 15184 standard. The scratch failure was found to be tensile trailing cracking in the coating and substrate gouging. The indentation hardness, elasticity modulus and fracture toughness of the coatings were determined and correlated to the observed pencil scratch hardness. Based on the analysis, the main factors to improve the scratch resistance are the elasticity modulus, thickness and fracture toughness of the coatings. General consideration for the improvement of scratch resistance of hard coatings on compliant substrates was also discussed.

Pulse electroplating of copper film: a study of process and microstructure.

Abstract: Copper films with high density of twin boundaries are known for high mechanical strength with little tradeoff in electrical conductivity. To achieve such a high density, twin lamellae and spacing will be on the nanoscale. In the current study, 10 microm copper films were prepared by pulse electrodeposition with different applied pulse peak current densities and pulse on-times. It was found that the deposits microstructure was dependent on the parameters of pulse plating. Higher energy pulses caused stronger self-annealing effect on grain recrystallization and growth, thus leading to enhanced fiber textures, while lower energy pulses gave rise to more random microstructure in the deposits and rougher surface topography. However in the extremes of pulse currents we applied, the twin densities were not as high as those resulted from the medium or relatively high pulse currents. The highest amount of nanoscale twinning was found to form from a proper degree of self-annealing induced grain structure evolution. The driving force behind the self-annealing is discussed.

Compressed sensing MRI based on nonsubsampled contourlet transform

Abstract: How to reduce acquisition time is very important in magnetic resonance imaging (MRI). Compressed sensing MRI emerges recently to suppress the aliasing when undersampling k-space data is employed. However, typical sparse transform for compressed sensing MRI ever used is wavelet, which only captures limited directional information with decay rate M1. In this paper, we introduce contourlet into compressed sensing to obtain a sparse expansion for smooth contours with decay rate C(logM)3M2 and employ nonsubsampled contourlet to increase the redundancy of basis for magnetic resonance images. We propose compressed sensing MRI based on nonsubsampled contourlet transform (NSCT). Experimental results demonstrate that NSCT outperforms wavelet on suppressing the aliasing and improves the visual appearance of magnetic resonance images.

Effect of chain length on low temperature gold-gold bonding by self-assembled monolayers

Abstract: The tensile strength of thermocompression gold joints formed with prior surface coatings of alkanethiol self-assembled monolayers (SAMs) depends on the chain length (n) of the SAM. Enhancement of bond strength is most significant at n=6 while no improvement can be achieved using octadecanethiol (n=18). These contrasting behaviors can be interpreted as a consequence of two dominant roles of alkanethiols that govern the bonding phenomenon, namely, the passivation of gold surfaces and the ease of mechanical and/or thermal displacement.

Low Temperature Copper-Copper Thermocompression Bonding

Abstract: Successful direct copper thermocompression bonding was demonstrated at room temperature under ambient environment, yielding shear strength of 21 MPa. Studies on the effect of bonding temperature on the copper joint shear strength revealed a unique phenomenon at the low temperature regime (~80degC -140degC) whereby bond integrity degrades with increasing temperature. Beyond 140degC, direct relationship between temperature and joint shear strength was observed. A hypothesis on the bonding mechanism between copper surfaces is proposed to explain the anomalous bonding behaviour with temperature.

Improvement in the contrast of CEST MRI via intermolecular double quantum coherences.

Abstract: The tremendous potential of chemical exchange saturation transfer (CEST) agents as an emerging class of magnetic resonance imaging contrast media has been demonstrated in recent years. In a CEST experiment, a high CEST contrast is always welcome. However, when the exchange rate is low, which may happen in exchangeable solute protons of low concentration, it is usually hard to obtain an excellent CEST efficiency. Recently, we noted that the intermolecular multiple quantum coherence signal is more sensitive to the changes of the magnetization magnitude than a conventional single quantum coherence signal. Consequently, it may be easier when used in obtaining a CEST contrast. In this note, a modified COSY (two-dimensional correlated spectroscopy) revamped with an asymmetric Z-gradient echo detection (CRAZED) sequence combined with an off-resonance saturation pulse followed by a standard spin-echo imaging sequence was designed to obtain a better CEST contrast image based on the intermolecular double quantum coherence method. An analytical expression was derived from a dipolar field theory. Experiments were performed on an agar-glucose phantom, and the results demonstrate the feasibility of our method.

Scratch resistance of protective sol-gel coatings on polymeric substrates

Abstract: Sol-gel coatings on polymeric substrates have been widely used in optical lenses, automobiles, safety windows, and flexible display panels. The resistance to scratch damage is an important reliability consideration. This article begins with a review of the scratch failure modes, the related mechanisms, and the parametric models related to these failure modes. Generally failure modes can be categorized into coating cracking driven and delamination driven. By analyzing the potential failure modes, the importance of characterizing the coating fracture toughness and interfacial fracture toughness between the coating and the substrate is high-lighted. Controlled buckling test is introduced as an easy and appropriate test method that can be used for the measurement of both properties. Sol-gel coatings with different amount of colloidal silica were prepared on polycarbonate substrates. Pencil scratch test was carried out following the ISO 15184 standard. The indentation hardness, elastic modulus and fracture toughness of the coatings were also determined and correlated to the observed pencil scratch hardness. The scratch failure was found to be tensile trailing coating cracking. Analysis shows that the main factors affecting the scratch resistance are elasticity modulus, thickness, and fracture toughness of the coating. Based on the current and other reported results, ways to improve scratch resistance for brittle coatings on compliant substrates are discussed.

Ambient Copper-Copper Thermocompression Bonding using Self Assembled Monolayers

Abstract: A typical copper-copper thermocompression bonding process is carried out in an ultrahigh vacuum (UHV) or inert environment at a bonding temperature >300°C. The ultraclean environment serves a single purpose – to maintain oxide-free copper surfaces, allowing intimate physical contact between copper atoms. This study investigates the temperature dependence of direct copper bonding from room temperature to 300°C under ambient condition. An anomalous thermal dependence of bond strength occurs between 80°C to 140°C where an increase in bonding temperature within this regime is in fact, detrimental to joint strength. This is interpreted as a thermal competition between oxidation and bond formation. This study also demonstrates that by simply coating the copper surface with a self assembled monolayer of 1-undecanethiol prior to bonding, Cu joints can be successfully formed at close to ambient temperature without a vacuum, yielding joint shear strengths on the order of 70MPa. The densely packed monolayer serves to passivate the copper surface against oxidation under ambient conditions. The ultrathin organic monolayer structure, as compared to a bulk oxide layer, could be easily displaced during the mechanical deformation at the bonding interface which accompanies thermocompression. This method could be an effective simple bonding solution for three-dimensional integrated chips.

TEM Microstructural Analysis of As-bonded Copper Ball Bonds on Aluminum Metallization

Abstract: In this study, the nano-scale interfacial details of ultrasonic copper ball bonding to an aluminum metallization in the as-bonded states were investigated using high resolution scanning/transmission electron microscopy with energy dispersive spectroscopy. Our results showed that ultrasonic vibration swept aluminum oxide and copper oxide in some regions of contacting surface, where an approximate 20 nm Cu-Al intermetallics (i.e. CuAl2) formed. In the regions where oxide remained, aluminum oxide layer connected with copper oxides layer. No nano-level voids or gaps were observed at the central area of the interface, including the regions with oxide. Calculation of interfacial temperature showed that the ultrasonic vibration increased the flash temperature up to 465°C which was believed to improve the interdiffusion for the formation of Cu-Al intermetallics.

Ferroelectric thin films with complex composition of PNN–PZN–PMN–PZ–PT and excess NiO

Abstract: Ferroelectric thin films of the 0.1Pb(Ni1/3Nb2/3)O3–0.35Pb(Zn1/3Nb2/3)O3–0.15Pb (Mg1/3Nb2/3)O3–0.1PbZrO3–0.3PbTiO3 (PNN–PZN–PMN–PZ–PT) complex oxide system were prepared on Pt/Ti/SiO2/Si substrates using a polymer-modified sol-gel method followed by a rapid thermal annealing (RTA) process. It was found that the addition of excess NiO is effective in stabilizing the perovskite phase while suppressing the pyrochlore phase. The crystalline structure and morphology of the films with different amounts of access NiO were studied with x-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM), respectively. The electrical properties, including dielectric, ferroelectric, and piezoelectric, showed a significant improvement with excess NiO. The film sample with 3 mol% of excess NiO exhibited optimized electrical properties. Different parameters, including tolerance factors on the basis of ionic radii, electronegativity differences between cations and anions, and oxygen bond valences, were applied to analyze the stability of the perovskite phase with different amount of excess NiO. Analysis results indicated that only the bond-valence theory could explain the effect of excess NiO on the stability of the perovskite phase under the assumption that the excess Ni2+ entered the A sites of the perovskite structure.

Investigations on the Effects of NMR Experimental Conditions in Human Urine and Serum Metabolic Profiles

Abstract: Studying on the effects of the experimental conditions to the spectra data are very important for NMR-based metobonomics since the resonance signals were sensitive to the experimental conditions. In this paper, NOEPER and NOEPER-CPMG pulse sequences are adopted to acquire the 1H NMR spectra of the human urine and serum samples respectively, and two important parameters, i.e., the experimental temperature and the saturation power of NOEPER sequence, are investigated. Resonance peaks shifting and signal intensity changing can be observed in both the urine spectra and the serum spectra when the experimental temperature was changed. For example, intensity of resonance signal of LDL, VLDL, valine and choline would increase obviously in the serum spectra with the experimental temperature increasing. Those evidences implied that it is important to acquire the 1H NMR spectra of the samples in the same temperature. In addition, water suppression profiles of the NOEPER pulse sequence are obtained with several different values of saturation power, in which signals whose chemical shifts are closed to that of water would decrease their intensity with the saturation power rising. Those results demonstrate that trade-off between the water suppression and signal decrease must be taken into consideration. As a matter of experience, lower saturation power is recommended if the water signal has been suppressed enough.

Investigation on Diurnal Variation and Gender Difference in Human Urine Using NMR-Based Metabonomics

Abstract: 1H NMR-based metabonomics has shown considerable potential as a means of rapidly detecting and characterizing a myriad of metabolic components. The 1H NMR spectra obtained for human urine samples were visually inspected and analyzed using principal component analysis (PCA). Urine samples were collected eight times daily (8:00, 10:00, 12:00, 14:00,18:00, 20:00) from healthy volunteers with a standard diet and lifestyle for 2 consecutive days. Separation of urinary data according to time of sampling collection was achieved. The fluctuations of creatinine, citrate and observed between urine samples collected during the different time of day may be related to hormone levels. In addition, gender-difference was also investigated and the PCA score plots revealed several metabolites that were considered significant for distinguishing between male and female: creatinine, glycine and citrate. These results showed the value of metabonomic methods in the investigation of physiological variation and rhythms, and provided suggestions to collect significative metabonomics data in clinical studies.

Numerical simulations of edge detection effect due to chemical shift variation in intermolecular multiple-quantum coherence MRI

Abstract: Combined with the theories of intermolecular multiple-quantum coherences (iMQCs) and distant dipolar field (DDF) effect, edge detection effect due to chemical shift variation in magnetic resonance imaging (MRI) was simulated and discussed deeply using an efficient numerical algorithm based on the nonlinear Bloch equations. Simulation results show that, different from the conventional MRI signal, chemical shift in iMQC MRI can provide new imaging information, an edge detection method to search regions containing spins with chemical shift offset. The edge detection method can present plentiful information about various kinds of edges of object regions.

Intermolecular multiple-quantum coherence NMR signals modulated by double distant dipolar fields

Abstract: NNSF of China [20573084, 10605019, 10774125]; project of Science and Technology of Fujan [2006H0092]

Improvement of one-dimensional gradient shimming method in nuclear magnetic resonance experiments

Abstract: Nuclear magnetic resonance (NMR) plays an important role in the fields of physics, chemistry, biology and medicine. The homogeneity of the magnetic field is very significant for NMR experiment. Based on the principle of field mapping, gradient shimming is an efficient method to achieve a homogeneous magnetic field. Reflecting accurately the spatial magnetic field distribution produced by every shim coil, gradient shimming can optimize the homogeneity efficiently and quickly. In this paper, one-dimensional gradient shimming method was studied, and the corresponding computational process was improved. Experiments were done on the NMR spectrometer, and the results show that the improved algorithm operates more efficiently and still works in a great inhomogeneous magnetic field.

Fracture Toughness Assessment of a Solder Joint using Double Cantilever Beam Specimens

Abstract: Conventional assessment of solder joint reliability uses either ball shear test or solder ball pull test. The test results are reported in terms of materials strength in either shear or tensile mode, and the strength values are size-dependent. Therefore these test results are largely useful only for qualitative comparison and qualification of the products. In the current effort, we aim at developing an assessment scheme for solder joint interface fracture toughness. The obtained results, in terms of critical energy release rate, predict the joint failure based on the principle of fracture mechanics. The results can be used as a materials property in the reliability design of various types of solder-ball joined packages. Double cantilever beam (DCB) specimens made of 99.9 wt% copper were selected in the current work. Eutectic Sn-37Pb solder was used to join two pieces of the copper plates with controlled solder thickness. The test record showed steady propagation of the crack along the solder / copper interface, which verifies the viability of such a testing scheme. Interface fracture toughness for as-joined, extensively-reflowed and thermally aged samples has been measured. Both the reflow treatment and the thermal aging lead to degradation of solder joint fracture resistance. To understand the degradation, fractographic analysis by scanning electron microscopy (SEM) has been carried out from both top and cross-sectional views of the fractured surfaces.

Toxicity Study of Oral Vanadyl Sulfate by NMR-based Metabonomic

Abstract: Vanadium compounds have been believed to be ideal drugs for diabetes biological therapy in future, but they suffer setback for the potential toxicity now. Toxicity study is necessary for vanadyl drugs development. This paper investigated the toxicity effects of vanadyl sulfate (VOSO4) oral administration in male Wistar rats using 1H NMR-based metabonomic analysis and clinical biochemical analysis. Rat urine were collected and their 1H NMR spectra were acquired, and then subjected to multi-variable statistical analysis. Compared to control groups, urinary excretion of lactate, TMAO, creatinine, taurine and hippurate increased following VOSO4 dosing, with concomitant decrease in the level of acetate and succinate. The dosed groups can be readily discriminated from the control groups by principle component analysis. The results showed that VOSO4 can affect energy metabolism process, interrupted intestinal microfloral metabolism, and induced liver and kidney injury. NMR-based metabonomic can offer additional information to traditional clinical chemistry in the sensitivity and specificity of results obtained.

A Novel NMR Simulation Algorithm and Its Potential Application in In Vivo Research

Abstract: Nuclear magnetic resonance (NMR) provides noninvasive detection for molecular information. Recently, intermolecular multiple quantum coherences (iMQCs) was proved to be a promising NMR method to get high-resolution in vivo magnetic resonance spectroscopy (MRS) under inhomogeneous field. The simulation of NMR plays an important role in predicting the result of a complex experiment, developing new NMR experiments, etc. However, in the simulation of iMQC-MRS, there is obstacle of combining scalar couplings with the classic formalism. This paper introduces an iMQC simulation algorithm based on the combination of product operator matrix (POM) and nonlinear Bloch equation, and its applications in developing high-resolution spectroscopy with solvent suppression and region-localized in vivo molecular spectroscopy.