Coefficient Of Thermal Expansion Mismatch Research Articles

Non-focusing dense plasma focus device based alternative synthesis technology for ZnO thin films

Dense plasma focus (DPF) device is conventionally operated in focus mode, to achieve pinch plasma with highest possible temperature and density to maximize the soft and hard x-rays and charged particles. In this paper, we report the first ever application of non-focus mode of DPF device, which is free of magneto-hydrodynamics (MHD) instabilities, for the deposition of zinc oxide thin films (ZnO TFs) on silicon substrates for various number (5, 10, 15 and 20) of non-focused deposition shots (NFDS). The X-ray diffraction (XRD) patterns of as-deposited ZnO TFs confirms the growth along (0 0 2) orientation only. The ZnO TFs are then annealed at 600 °C temperature for 2 h. The XRD patterns of annealed ZnO-TFs confirm the wurtzite phase of ZnO with (1 0 0), (0 0 2) and (1 0 0) planes with improved crystallinity. The up and down shifting of ZnO (0 0 2) diffraction plane indicates the presence of residual stresses which are reduced in annealed ZnO TF. The surface morphology, like shape, size and the distribution of rounded nano-particles, is strongly associated with increasing number of NFDS. Raman analysis shows the development of downshifted E2 (high) and upshifted A1 longitudinal optical (LO) modes centered at 430 cm−1 and 580 cm−1 compared to bulk ZnO (430 and 575 cm−1) indicating the presence of tensile residual stress due to mismatch of thermal expansion coefficient of ZnO TF and Si substrate and due to the presence of oxygen vacancies and Zn interstitials, respectively. The XPS analysis confirms the presence of Zn, Zn–O, C–O, and Zn–OH bonds. The energy band gap and refractive index of annealed ZnO-TF are found to be 3.30 eV and 1.88, respectively. A new method of high quality ZnO TFs synthesis using MHD instability free non-focusing mode of DPF device will open a new alternative synthesis technique.

Multilayer CdHgTe-based infrared detector: 2D/3D microtomography, synchrotron emission and finite element modelling with stress distribution at room temperature and 100 K

The mechanical behaviour of a CdHgTe-based infrared detector was evaluated after processing at several temperatures to determine the impact of thermomechanical loading on residual stress and reliability. The architecture of the detector was first entirely characterized, relying on SEM, X-ray microtomography and diffraction analysis, in order to get the nature, the morphology and the crystallographic orientation of all the constitutive layers, and in particular the indium solder bumps. The results notably showed the unexpected single crystal aspect of the indium bumps with a repeatable truncated cone geometry. To obtain the thermomechanical response of the structure after processing and in the range of operating temperatures (from 430 K to 100 K), a 3D Finite Element model was then developed. As expected, the numerical results showed a stress gradient evolution in the structure from high to low temperatures, with high loca njvvl stress around 30 MPa in the CdHgTe at 100 K, mainly due to the thermal expansion coefficient mismatch between the different layers. They highlighted the significant influence of the geometry and the single crystal nature of the bumps as well as the behaviour law of the different materials.

Reduced dislocation density and residual tension in AlN grown on SiC by metalorganic chemical vapor deposition

Crack-free AlN films with threading dislocation density (TDD) below 109 cm−2 are needed for deep-UV optoelectronics. This is typically achieved using pulsed lateral overgrowth or very thick buffer layers (>10 μm), a costly and time-consuming approach. A method for conventional metalorganic chemical vapor deposition growth of AlN/SiC films below 3 μm with greatly improved quality is presented. Focusing on substrate pretreatment before growth, we reduce average film stress from 0.9 GPa (tension) to −1.1 GPa (compression) and eliminate cracking. Next, with optimized growth conditions during initial deposition, AlN films with x-ray rocking curve widths of 123 arc-sec (0002) and 304 arc-sec (202¯1) are developed, and TDD is confirmed via plan view transmission electron microscopy (TEM) to be 2 × 108 cm−2. Film stress measurements including x-ray 2θ-ω, reciprocal space mapping, and curvature depict compressively stressed growth of AlN on 4H-SiC due to lattice mismatch. The thermal expansion coefficient mismatch between AlN and SiC is measured to be Δα=αAlN−αSiC=1.13×10−6 °C−1 and is found to be constant between room temperature and 1400 °C. TEM confirms the existence of dense misfit dislocation (MD) networks consistent with MD formation near SiC step edges and low MD density regions attributed to nearly coherent AlN growth on SiC terraces. These low-TDD, crack-free AlN/SiC buffers provide a platform for deep-UV optoelectronics and ultrawide bandgap electronics.

Epitaxial growth and layer-transfer techniques for heterogeneous integration of materials for electronic and photonic devices

The demand for improved electronic and optoelectronic devices has fuelled the development of epitaxial growth techniques for single-crystalline semiconductors. However, lattice and thermal expansion coefficient mismatch problems limit the options for growth and integration of high-efficiency electronic and photonic devices on dissimilar materials. Accordingly, advanced epitaxial growth and layer lift-off techniques have been developed to address issues relating to lattice mismatch. Here, we review epitaxial growth and layer-transfer techniques for monolithic integration of dissimilar single-crystalline materials for application in advanced electronic and photonic devices. We also examine emerging epitaxial growth techniques that involve two-dimensional materials as an epitaxial release layer and explore future integrated computing systems that could harness both advanced epitaxial growth and lift-off approaches. This Review Article examines the development of epitaxial growth and layer transfer techniques for monolithic integration of dissimilar single-crystalline materials for application in advanced electronic and photonic devices.

Effect of Praseodymium in Barrier Layer on the Performance of Perovskite Cathode in SOFCs

Because of the large cathode polarization of solid oxide fuel cells (SOFCs) at intermediate operating temperatures, great efforts have been made in developing new cathode material and controlling the cathode microstructure to promote cell performance. The potential reaction and the mismatch of thermal expansion coefficient (TEC) between cathode and electrolyte requires a “barrier layer” inserted between them. Gadolinium doped ceria (GDC), owing to its good ionic conductivity and good match of TEC between cathode and electrolyte layer, is widely used as the barrier layer material. The barrier layer plays an important role in charge carrier transportation and electrochemical reaction at triple phase boundary. In this work, praseodymium and gadolinium co-doped ceria, Pr0.1Gd0.1Ce0.8O2-δ (PGCO), is used as a novel interlayer to optimize the cathode performance due to its better electronic conductivity than GDC and active Pr(III)/Pr(IV) couples in oxygen reduction reaction. Distribution of relaxation time is applied to analyze the mechanism of the enhanced performance.

(Invited) Basic Ammonothermal Growth of Bulk GaN Single Crystal Using Sodium Mineralizers

GaN is a promising semiconductor of wide band gap of 3.4 eV at room temperature and can be used as a substrate material for blue/green/UV light-emitting diode (LED) and laser diode (LD) [1]. Sapphire has been the most widely used substrate for manufacturing GaN-based LEDs. However, GaN films grown on sapphire substrates include many critical defects owing to the large lattice mismatch (~15%) and large difference of thermal expansion coefficients (~25%) between GaN and sapphire. Thus, GaN substrates are needed for fabrication of the high power LEDs and LDs. Two typical routes, ammono-basic and ammono-acidic, have been studied in the ammonothermal growth of GaN crystals. Addition of alkali (Li, Na, and K) elements to ammonia typically results in a basic ammonia solution, whereas addition of halogens (F, Cl, Br, and I) to ammonia typically results in an acidic ammonia solution. The acidic ammonothermal method has a fast crystal growth rate but requires installation of Pt liner due to the corrosive environment. Thus, the basic ammonothermal method is regarded as more appropriate for mass production of GaN crystals [2]. Only a few types of basic mineralizer including NaNH2 [3] and KNH2 [4] have been used previously for the crystal growth of GaN. The reports of comparison with other sodium-based mineralizers for ammonothermally grown GaN crystals have not to be achieved. Potassium mineralizers were not included in this study because they are more sensitive to oxygen and moisture and corrosive to autoclave compared to sodium mineralizers. In this study, we demonstrate the growth rate, crystal quality, and surface morphology of the as-grown GaN single crystals according to sodium amide, sodium azide, and sodium metal as mineralizer. Also, we describe the morphology and properties of approximately 2 inch GaN crystals grown by ammonothermal method using sodium metal mineralizer. Reference [1] S. Nakamura, G. Fosol, The Blue Laser Diode, Springer, Berlin, 1997, chap. 1. [2] R. Dwilinski, R. Doradzinski, J. Garczynski, L. P. Sierzputowski, A. Puchalski, Y. Kanbara, K. Yagi, H. Minakuchi, H. Hayashi, J. Cryst. Growth 311 (2009) 3015. [3] T. Hashimoto, F. Wu, J. S. Speck, S. Nakamura, J. Cryst. Growth 310 (2008) 3907. [4] R. Dwilinski, R. Doradzinski, J. Garczynski, L. P. Sierzputowski, A. Puchalski, Y. Kanbara, K. Yagi, H. Minakuchi, H. Hayashi, J. Cryst. Growth 310 (2008) 3911.

A Novel Approach to Join Large Coefficient of Thermal Expansion (CTE) Mismatched Thermoelectric (TE) Materials for High Temperature Applications

Radioisotope thermoelectric generators (RTGs) have been used to enable deep space exploration where solar power is scarce. Various NASA missions such as Voyager 1 and Voyager 2 have used RTGs and have traveled over 11 billion miles from Earth. Heritage RTGs have reliably provided power throughout the lifetime of the various missions but thus far have been relatively inefficient and convert less than 10% of heat to electricity. A method to achieve higher thermal to electrical conversion efficiencies is via the segmentation of advanced thermoelectric (TE) materials. High thermoelectric efficiencies can be achieved by constructing a TE device that includes combining different TE materials optimized at different temperatures and joining them together. Though advantageous to achieve high performance, segmentation can be challenging due to differences in material properties such as the coefficient of thermal expansion (CTE). Thermal stresses induced by large CTE mismatch of TE materials during the joining process can cause cracking at the bonded interface, these stresses need to be minimized in order to reduce the probability of cracking at the joint interface. Additionally, the bonds formed between materials need to be both chemically and mechanically stable and require high thermal conductivity along with low electrical contact resistance. In this study two thermoelectric materials with dissimilar CTE at room temperature were joined by using a graded-CTE bonding approach. The thermal stress states of the two thermoelectric materials were modified to avoid cracking at the bonding interface. Optical and scanning electron microscopy (SEM) was used to characterize the cross-section of the bonded thermoelectric materials. Preliminary results of a stress model shows a reduction of stress at the bonded interface and is in agreement with experimental results.

Sn-3.0Ag-0.5Cu/Sn-58Bi composite solder joint assembled using a low-temperature reflow process for PoP technology

Package-on-Package (PoP) is a popular technology for fabricating chipsets of accelerated processing units. However, the coefficient of thermal expansion mismatch between Si chips and polymer substrates induces thermal warpage during the reflow process. As such, the reflow temperature and reliability of solder joints are critical aspects of PoP. Although Sn58Bi is a good candidate for low-temperature processes, its brittleness causes other reliability issues. In this study, an in-situ observation was performed on composite solders (CSs) made of an Sn-3.0Ag-0.5Cu (SAC305) solder and an Sn58Bi solder that were mixed during reflow at temperatures of 170 °C (CS-170), 180 °C (CS-180), and 190 °C (CS-190). The volumes of the mixed-solder regions were in the order of CS-190 > CS-180 > CS-170. A calculated Sn58Bi-SAC305 isoplethal section of the Sn-Ag-Cu-Bi quaternary system was employed to elucidate the melting behavior of CS-190. CS-190 with Sn cyclic twin boundary and Bi phase preferred orientation of [0001] was demonstrated by electron backscatter diffraction. Moreover, Cu/CS-190/Cu joints with a shear strength of 46 MPa were formed. CS-190 can reduce the brittleness of SnBi solder owing to the reduced solid-solution hardening with the decrease in the Bi content. These factors can improve the reliability of PoP technology.

Characterization and properties of AA6061-based fiber metal laminates with different aluminum-surface pretreatments

In this study, an Al/CFRP/Al laminates was prepared with three types of aluminum alloy AA6061 surface treatments (sanding, anodizing, and surface modification) to improve the metal–composite interface (MCI) strengths. The results showed that the shear strengths of Al/CFRP/Al laminates modified by silane coupling agent and pretreated by phosphoric acid anodizing were similar, approximately 50 MPa, which were significantly higher than those subjected to sandpaper-sanding pretreatment (38 MPa). Results of scanning electron microscopy (SEM) revealed that the nanoscale porous anodized film produced on the AA6061 surface after anodizing pretreatment enhanced the wettability of CFRP on AA6061 sheets, leading to strong interfacial bonding. XPS analysis showed that a new chemical bond, AlOEP, appeared at the interface between CFRP and anodized AA6061, which clearly indicated that the anodized layer could enhance the mechanical anchoring effect and promote its chemical reaction. And the chemical bonds of AlOSi and SiC were found at the interface between CFRP and silane-modified AA6061, thereby proving the existence of a silane flexible layer, which enabled the adjustment of the thermal expansion coefficient mismatch between AA6061 and CFRP and the improvement in the stability of the mechanical properties of Al/CFRP/Al laminates.

Dynamic stress corrosion cracking in silicon crystal

We investigated, experimentally, the stress corrosion cracking (SCC) phenomenon in dynamic cracks propagating on two low energy cleavage systems (LECSs) of silicon crystal, (111)[ $$11\,\bar{{2}}$$ ] and (110)[ $$1\,\bar{{1}}\,0$$ ], under air and under Ar at atmospheric pressure. We used our high resolution Coefficients of Thermal Expansion Mismatch (CTEM) method to initiate and propagate the cracks. An important variable in this investigation was the gradient of the energy release rate (ERR) flow to the crack front for unit length of crack advance, $$dG_{0}/da$$ , denoted $$\varTheta $$ , in units of $$\hbox {J}/\hbox {m}^{2}/\hbox {mm}$$ . The CTEM method is capable of manipulating the value of $$\varTheta $$ . When loaded by low ERR gradient, e.g., when $$\varTheta <0.5\hbox { J}/\hbox {m}^{2}/\hbox {mm}$$ , in air, a complex and diverse SCC behavior was revealed; the cleavage energy strongly depends on $$\varTheta $$ , the environment and crystallographic structure. For higher $$\varTheta $$ , the cleavage energy is higher than that at vacuum and remains constant during crack propagation. We further show that at $$\varTheta \,0.5\hbox { J}/\hbox {m}^{2}/\hbox {mm}$$ , the SCC mechanisms vanish for both LECSs, and the cracks initiate and propagate at cleavage energy higher than that in vacuum, or the Griffith barrier of $$2\gamma _{\mathrm{s}}$$ , twice the free surface energy of the cleavage plane. This investigation suggests that the large scatter existing in the literature for the experimental cleavage energies of the current LECSs of silicon crystal and the still existing debate regarding the susceptibility of silicon crystal to SCC, is caused by the different value of $$\varTheta $$ in the various past experiments. We further suggest that $$\varTheta $$ should be stated when discussing the quasi static and dynamic cleavage energy of brittle crystals.

High strength Mg–Zn–Y alloys reinforced synergistically by Nano-SiCp and long period stacking ordered structure

In present study, high strength nano-SiCp/Mg98Zn0.8Y1.2 composites with low SiCp content reinforced synergistically by nano-SiCp and long period stacking order (LPSO) phase have been successfully fabricated. The magnesium matrix composites (MMCs) were designed with four nano-SiCp contents (0.5, 1.0, 1.5 and 2.0 vol%). The microstructures of MMCs were significantly refined by the stimulating dynamic recrystallization effect of nano-SiCp. TEM observations indicated that numerous nano-SiCps were dispersed inside α-Mg and LPSO grains, showing a relationship with dislocations, while some were distributed along grain boundaries. The microhardness and strength of MMC increased as the SiCp content increased from 0 vol% to 1.0 vol%, but they decreased when the SiCp content increased from 1.0 vol% to 2.0 vol%. The plasticity of the MMC decreased as the SiCp content increased. The 1.0 vol% nano-SiCp MMC extruded at 330 °C exhibited excellent mechanical properties with a tensile yield strength (TYS) of 441 MPa, an ultimate tensile strength (UTS) of 464 MPa and a plastic elongation (PE) of 3.2%. Besides traditional enhanced factors, including the coefficients of thermal expansion mismatch, Orowan strengthening and grain refinement, novel synergistically strengthening of nano-SiCp and LPSO structure was the other important strengthening mechanism. During MMCs deformation, the LPSO strengthening phases were strengthened by nano-SiCps which would work as obstacles by hindering the motion of LPSO structures.

Self-Assembled Triple-Conducting Nanocomposite as a Superior Protonic Ceramic Fuel Cell Cathode

Summary Here, we report an oxygen ion-proton-electron-conducting nanocomposite, BaCo0.7(Ce0.8Y0.2)0.3O3-δ (BCCY), derived from a self-assembly process, as a high-performance protonic ceramic fuel cell (PCFC) or mixed O2−/H+ dual-ion conducting fuel cell (dual-ion FC) cathode. Self-assembly during high-temperature calcinations results in the formation of a nanocomposite consisting of a mixed H+/e− conducting BaCexYyCozO3-δ (P-BCCY) phase and mixed O2−/e− conducting BaCoxCeyYzO3-δ (M-BCCY) and BaCoO3-δ (BC) phases. The interplay between these phases promotes the oxygen reduction reaction (ORR) kinetics of this composite cathode and improves its thermo-mechanical compatibility by tempering the mismatch in thermal expansion coefficient (TEC). When tested as the cathode in anode-supported dual-ion FCs and PCFCs, peak power densities (PPDs) of 985 and 464 mW cm−2, respectively, are achieved at 650°C while maintaining a robust operational stability of 812 h at 550°C. This material is ideally suited for high-performance cathodes for PCFCs and dual-ion FCs, greatly accelerating the commercialization of this technology.

Unveiling the impact of the effective particles distribution on strengthening mechanisms: A multiscale characterization of Mg+Y2O3 nanocomposites

Most models used to account for the hardening of nanocomposites only consider a global volume fraction of particles which is a simplified indicator that overlooks the particles size and spatial distribution. The current study aims at quantifying the effect of the real experimental particles spatial and size distribution on the strengthening of a magnesium based nanocomposites reinforced with Y2O3 particles processed by Friction Stir Processing (FSP). X-ray tomographic 3-D images allowed to identify the best FSP parameters for the optimum nanocomposite. A detailed analysis indicates that the observed hardening is mainly due to Orowan strengthening and the generation of geometrically necessary dislocations (GND) due to thermal expansion coefficients (CTE) mismatch between magnesium and Y2O3 particles. A multiscale characterization coupling 3D X-ray laboratory, synchrotron nanoholotomography and transmission electron microscopy (TEM) has been used to investigate particles size and spatial distribution over four orders of magnitude in length scales. Two dedicated micromechanical models for the two strengthening mechanisms are applied on the experimental particle fields taking into account the real particles size and spatial distribution, and compared to classical models based on average data. This required to develop a micromechanical model for CTE mismatch hardening contribution. This analysis reveals that the contribution from CTE mismatch is decreased by a factor two when taking into account the real distribution of particles instead of an average volume fraction.

Fabrication of Metal-Supported Proton-Conducting Electrolysers with Thin Film Sr- and Ce-Doped BZY Electrolyte

In this work, we present fabrication and initial testing of metal-supported proton ceramic electrolyser (MS-PCEC) single cells with a thin electrolyte. Plansee ITM porous metal is used as metal support. Spray-coating is used to deposit a conducting layer of La1-xSrxTi1-yNiyO3-δ (LSTN) which is used for low temperature (600-800 °C) deposition of a gas-tight, thin-film layer (~2 μm) of electrolyte by pulsed laser deposition (PLD). Post-annealing of the electrolyte is evaluated to improve its performance. In order to have a smaller thermal expansion coefficient (TEC) mismatch towards LSTN (~10-11 × 10-6 K-1), we developed a new Sr- and Ce-doped proton-conducting electrolyte, yielding a larger TEC (~10-11 × 10-6 K-1) compared to BZY (~8 × 10-6 K-1). The cell with Pt as air electrode shows acceptable open circuit voltage and low overpotentials at 600 °C, demonstrating the potential of developing the first working MS-PCEC.

Tensile properties and strengthening effects of Al 3003 alloy weldment reinforced with TiO2 nanoparticles

This paper examines the significance of strengthening mechanisms in the fabricated Al 3003 nanocomposite weldment using accumulative roll bonded (ARB) Al 1100 – TiO2 filler rod. The wt. % of titanium oxide (TiO2) content is varied as 0, 0.75, 1.5, 2.25 and 3 to the total weight of the weld zone. The weld possesses an ultimate tensile strength of 249 ± 04 MPa in 3% reinforced whereas 137 ± 04 MPa in unreinforced condition. The results obtained from the tensile test were cross checked with the characterization studies such as X ray line profile analysis (XRD), Field emission scanning electron microscopic analysis (FESEM), Transmission electron microscopic analysis (TEM), Electron backscattered diffraction analysis (EBSD) and Energy dispersive x-ray spectroscopic analysis (EDS). The tensile test results reveal that Al – 3 wt% TiO2 nanocomposite weld exhibits higher strength which is not only due to drop in grain size but also due to mechanisms such as grain refinement strengthening, Orowan strengthening and thermal expansion coefficient mismatch strengthening and occurred incommensurate with the progressive addition of TiO2 nanoparticles in the weldment. However, it has been found that the mechanism due to Orowan strengthening has played a significant and greater role for the overall strength of the weldment.

Line Width and Capping Layer Effects on Electromigration Failure of Plasma Etched Copper Lines

Reliability of molybdenum and titanium tungsten capped copper lines of various widths prepared from the plasma-based etch process has been studied using the constant current electromigration test method. The addition of the capping layer reduced the lifetime through complicated mechanisms of interface reaction, surface oxidation, copper diffusion to the capping layer, and thermal expansion coefficient mismatch, which are augmented by the Joule heating induced temperature raise. The molybdenum capped copper line has a shorter lifetime than the titanium tungsten capped copper line has. The capping layer material is critical to the lifetime of the copper line.

Thermal damage of conventional hard chromium coatings on 416 stainless steel

Hard chromium coatings have a wide range of application due to their resistance to corrosion and wear. In the present work, chromium was commercially electroplated on 416 stainless steel. Thermal damage of the chromium coatings was investigated as a function of heating temperature and cooling rate. Specimens were heated to various temperatures and then cooled down either by air cooling or water quenching. An optical microscope was employed to assess the damage due to heating. No thermally-induced cracks were observed for specimens heated up to 600 °C. At 800 °C and higher, thermally-induced cracks were observed and increased with increasing temperature. Cracks generated during heating were likely due to thermal expansion coefficient mismatch between the coating and the substrate. Heating was found to significantly alter the residual stresses state in the coating. Residual stress increased when heated to 600 °C and dropped at 1000 °C. Spherical indentations were employed to evaluation the effects of heating on the mechanical damage of the coatings. Ring, cone, surface radial and radial bend cracks were observed on as-deposited and heated specimens up to 600 °C. Specimen heated up to 800 °C showed partially developed ring and radial bend cracks, while specimen heated up to 1000 °C displayed no indentation related cracks.

Simulation and Experimental Study of the Warpage of Fan-Out Wafer-Level Packaging: The Effect of the Manufacturing Process and Optimal Design

In this paper, warpage of fan-out wafer-level packaging (FOWLP) throughout the manufacturing process is investigated to minimize the warpage. FOWLP technology has the advantages of low cost, small form factor, efficient electrical performance, and increased input–output counts. The main benefit of fan-out packaging is that it utilizes the redistribution layer to interconnect the die and solder joints instead of using the substrate. However, warpage control is still a critical issue in the wafer molding process. Warpage is induced by the coefficient of thermal expansion mismatch between the die and the molding compound. Throughout the manufacturing process, the warpage of each step was recorded by utilizing the continuity simulation. The debonding process, grinding process, and annealing process at room temperature were analyzed via the simulation results and the experimental results. To simulate the wafer molding process in a continuous manner, the element birth and death technique was employed using finite-element modeling. This method can be used to reactive and deactivate elements when the material is added to or removed from the system. The results of the measurement and simulation were highly similar, with a difference of less than 10%. However, the results differed considerably when the wafer thickness was small. The results revealed a gap between the experimental testing and simulation modeling. Because of the wafer stiffness, the wafer is too thin to support the gravity force in the experimental testing process. Therefore, it is important to determine the calibration factor to decrease the gap between experimental testing and simulation modeling. In addition, it is crucial to determine the key factor controlling the warpage behavior and to minimize the warpage by evaluating the die arrangement and the die size in this paper. Overall, the simulation modeling represents a feasible approach for predicting the warpage during the manufacturing process.

Effect of molecular stereoregularity on the transcrystallinization properties of poly(l-lactide)/basalt fiber composites

A comparative study on interfacial crystallization of poly(l-lactic acid) (PLLA) with different stereoregularity (PLLA2003D 96.0%, PLLA4032D 98.5%, and PLLA290 99.2%) surrounding macroscopic basalt fibers (BF) has been carried out in polymer composites. The effect of stereoregularity on the interface crystallization was systemically studied by microstructure. From polarized optical microscopy (POM) analysis, it was found that the transcrystallinity of PLLA was influenced by temperature. Moreover, stereoregularity significantly affects the crystallization kinetics. It was also found that increased molecular mobility providing faster orientation with high stereoregularity and low molecular weight. It might be the reason for the observation that the PLLA290 samples crystallized more rapidly than the PLLA4032D samples. Appropriate molecular stereoregularity exhibits a clear increase in extensional stress that is directly correlated with the crystalline orientation of the crystallization samples. The mechanism of formation of transcrystallinity underwent the process of “a mismatch in thermal expansion coefficients” with the extensional stress.

Effect of copper over-pad metallization on reliability of aluminum wire bonds

Recently, there have been moves to replace Si power devices with SiC ones, which will require new materials with higher thermal conductivity for power module assemblies owing to their higher operating temperatures. Al wire bond interconnections suffer from a mismatch in the coefficient of thermal expansion (CTE) between the SiC chip and the wire material, leading to a degradation of the power module reliability owing to the higher junction temperature (Tj) operation caused by the current density increase. Various studies have been conducted to improve the reliability of such interconnections. In this study, Al wires were bonded on a thick Cu over-pad metallization (OPM) layer that was formed on an Al pad of a SiC Schottky barrier diode, and active power cycling tests were performed while changing Tjmax to investigate the contribution of such a thick Cu-OPM layer on the reliability of Al wire bonds. Compared with the case of Al wire bonds on an Al pad, in active power cycling tests, the lifetime of Al wire bonds on a 25 μm-thick Cu-OPM layer was 1.3-times longer at ΔTj = 75 °C, and 1.5-times longer at ΔTj = 100 °C. The results of Joule heating analysis and elastic thermal-stress analysis showed that the lifetime prolongation effect of Al wire bonds on Cu-OPM was not derived from the heat dissipation effect of the thick Cu-OPM layer, but from a stress-reduction effect obtained by keeping the Al wire bonds away from the SiC diode chip, with which the CTE mismatch is large.