Coefficient Of Thermal Expansion Mismatch Research Articles

Toughness and R-curve behaviour of laminated Si3N4/SiCw ceramics

Laminated Si3N4/SiCw ceramics were successfully prepared by tape casting and hot-pressing. Its mechanical properties were measured and the impact resistance was discussed. The toughness of the laminated Si3N4/SiCw ceramics was 13.5 MPa m1/2, which was almost 1.6 times that of Si3N4/SiCw composite ceramics, namely 8.5 MPa m1/2. Moreover, the indentation strength of laminated Si3N4/SiCw ceramics was not sensitive to increasing indentation loads and exhibited a rising R-curve behaviour, indicating that the laminated Si3N4/SiCw ceramics had excellent impact resistance. The improved toughness and impact resistance of laminated Si3N4/SiCw ceramics was attributed to the residual stress caused by a thermal expansion coefficient mismatch between the different layers, resulting in crack deflection and bridging of SiC whiskers in the interface layer, thus consuming a large amount of fracture work.

Investigation on Die Crack in a Stacked Die Package Using Finite Element Analysis

Die crack is one of the problems in stacked die semiconductor packages. As silicon dies become thinner in such packages due to miniaturization requirement, the tendency to have die crack increases. This study presents the investigation done on a die crack issue in a stacked die package using finite element analysis (FEA). The die stress induced during the package assembly processes from die attach to package strip reflow was analyzed and compared with the actual die crack failure in terms of the location of maximum die stress at unit level as well as strip level. Stresses in the die due to coefficient of thermal expansion (CTE) mismatch of the package component materials and mechanical bending of the package in strip format were taken into consideration. Comparison of the die stress with actual die crack pointed to strip bending as the cause of the problem and not CTE mismatch. It was found that the die crack was not due to the thermal processes involved during package assembly. This study showed that analyzing die stress using FEA could help identify the root cause of a die crack problem during the stacked die package assembly and manufacturing as crack occurs at locations of maximum stress. The die crack mechanism can also be understood through FEA simulation and such understanding is very important in coming up with robust solution.

Strengthening and toughening epoxy polymer at cryogenic temperature using cupric oxide nanorods

At cryogenic temperatures thermoset polymers suffer from low fracture toughness. Herein, we present a strengthening and toughening technique for cryogenic applications using cupric oxide (CuO) nanorods. These nanorods are synthesized via a hydrothermal reaction to give an average length of 1.64 μm and a diameter of 270 nm. They exhibit a low coefficient of thermal expansion (CTE) at cryogenic temperatures, due to the magnetostriction effect. Epoxy nanocomposites containing up to 8 wt% of CuO nanorods were investigated. Both the tensile strength and the fracture energy were found to increase with CuO nanorods content up to 4 wt%, showing improvements of 18% and 133% respectively at room temperature, and of 21% and 261% at −196 °C. Current theoretical models were shown to account for the experimentally measured values of both strength and toughness at room temperature, but to significantly underestimate the observed values at −196 °C. However, modifications of these models to include thermal residual stresses generated by the CTE mismatch are proposed that lead to good agreement with the measured values. The technique to strengthen and toughen epoxy using CuO nanorods and the modified theories provide potential for designing and optimising multi-scale fibre composites for cryogenic applications such as cryogenic fuel tanks.

Influence of nitrogen gas over microstructural, vibrational and mechanical properties of CVD Titanium nitride (TiN) thin film coating

In this experimental investigation, the influence of different N2 gas flow rates on different properties (e.g. morphological, mechanical, etc.) of chemical vapor deposited (CVD) Titanium nitride (TiN) coatings has been discussed. The TiN coatings had been grown on Si (100) substrate at elevated temperature (1000 °C) using Titanium dioxide (TiO2) powder. SEM images reveal a dense uniform microstructure with an irregular surface pattern. The surface roughness of the coatings was found to be increased from 12.42 to 28.56 nm with an increase in flow rate. XRD results indicate a B1 NaCl crystal structure of the film with reduced crystallite size with the increasing N2 flow rate. Through the corrosion test, it has been observed that due to the variation of N2 flow rate the corrosion resistance of the films decreases with increasing N2 flow rate. The mismatch of thermal expansion co-efficient in between Si substrate and TiN thin film reduces with higher N2 flow rate. The acoustic and optic phonon mode of TiN coatings have been shifted to higher intensities with higher N2 flow rate. The mechanical properties of the film reveal that the maximum value of hardness (H) and Young's modulus (E) are 30.14 and 471.85 GPa respectively.

Microstructural evolution and strengthening mechanisms in Al7075/ graphene nano-plates/ carbon nano-tubes composite processed through accumulative roll bonding

Abstract In the present research, Al7075 matrix composites containing carbon nano-tubes (CNTs) and graphene nano-plates (GNPs) processed through accumulative roll bonding (ARB). Accumulative roll bonding carried out up to 3 passes at 400 °C with 50% thickness reduction per pass. The effect of CNTs and GNPs on the microstructural evolution and strengthening mechanisms were investigated. Microstructural evolutions in processed composites were characterized by using field emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD). The grain orientation spread (GOS) analysis shows that the volume fraction of recrystallization in deformed grains depends on the level of particle stimulated nucleation (PSN) in the interface between matrix and reinforcement. The occurrence of PSN is affected by the type of reinforcement and the number of ARB passes. The level of PSN depends directly on the extent of the plastic deformation zone (PDZ) in the vicinity of the matrix/reinforcements. In the hybrid composite, the PSN mechanism was restrained due to Zener pinning of grain boundaries. Texture evolution is mainly carried out through β fiber for the composites subjected to the ARB process. In all ARBed samples, the main texture components include one or two of S, Brass, and Copper. The presence of CNTs and GNPs in the composite led to a change in both intensity and type of the main texture components formed during the ARB. In the composite containing GNPs, with increasing of ARB passes the volume fraction of PSN decreased remarkably due to strain hardening and consequently recrystallization was limited. It was found that the most significant strengthening mechanism which has the key role in the increase yield strength is the load transfer mechanism. The role of coefficient of thermal expansion mismatch and Orowan mechanisms in the increase yield strength and hardness of processed composites is less and the effect of the Hall–Petch mechanism is negligible.

Preparation and high‐temperature oxidation resistance of multilayer MoSi2/MoB coating by spent MoSi2‐based materials

AbstractSpent MoSi2 and MoB were used as raw materials to prepare multilayer MoSi2/MoB coating on molybdenum by the two‐step method of slurry deposition and spark plasma sintering. The results showed dense MoSi2/MoB coating after sintering while penetrated cracks appeared in MoSi2 coating due to coefficient of thermal expansion mismatch between the Mo substrate and coating. After the sintering of MoSi2/MoB coatings, MoB and Mo2B diffusion layers were formed between MoB transition layer and Mo substrate without defects, exhibiting good metallurgical bonding. The high‐temperature oxidation behavior of coatings (1500°C) was also explored. After oxidation of 50 h at 1500°C, lowest mass gain (0.035 mg/cm2) was obtained for MoSi2/MoB coating, and the oxide scale was dense and complete without voids, making the oxygen diffusion at elevated temperature inhibited. Compared with MoSi2 coating under the same oxidation conditions, relatively thinner silica oxide scale was acquired by MoSi2/MoB coating because of the reduction of cracks, and the multilayer coating exhibits better anti‐oxidation properties at high temperature.

Temperature dependent lattice expansions of epitaxial GaN-on-Si heterostructures characterized by in- and ex-situ X-ray diffraction

In-situ X-ray diffraction (XRD), with the sample temperature stepwise increased up to 900 °C, together with ex-situ high-resolution XRD (HRXRD), has been used to study the lattice expansion along the surface normal direction of GaN-on-Si heterostructures grown by metalorganic chemical vapor deposition (MOCVD). The thermal stabilities induced by step-graded (SG) AlxGa1−xN/AlN and multiple low-temperature (MLT) AlN buffer layers have been addressed for the heterostructures grown on 700 and 1500 µm thick Si (111) wafers, respectively. It reveals that the thermal expansion of the GaN epilayer is slightly larger than that of the Si substrate and the nitride growth tends to decrease the thermal expansion of the Si (111) wafer at lower temperatures. Onset of drop in the lattice expansions has been observed when the temperature is increased to a transition point, Ttr, and the Ttr of the MLT-AlN buffered GaN on the 1500 µm thick Si is ~500 °C, which is apparently lower than that of the SG-AlxGa1−xN/AlN buffered GaN on the 700 µm thick Si. These observations have been interpreted and attributed to convolutions among the residual lattice strains, their relaxations, and the thermal expansion coefficient mismatch induced wafer curvatures (opposite to that during MOCVD growth) at elevated temperatures.

The split-up of G band and 2D band in temperature-dependent Raman spectra of suspended graphene

Dedicated Raman spectroscopy investigations in the temperature range from 78 K to 298 K were performed for the graphene suspended above round holes on SiO2/Si substrate (the suspended graphene, SUG), in comparison with the surrounding graphene adhered to solid SiO2/Si substrate (the substrate supported graphene, SSG). The results confirmed that one-layer SUG (1L-SUG) was subjected to strains originated from the surrounding 1L-SSG due to the different area and shape around the hole along different directions. In further, the split-up of G band and 2D band were observed at low temperature because of the enhanced anisotropic strains in 1L-SUG due to the mismatch of thermal expansion coefficient (TEC) between the 1L-SSG and SiO2/Si substrates. The temperature-dependent (T-dependent) frequency shifts for the G band and 2D band in 1L-SUG and 1L-SSG were consistent with previous Raman investigations. Therefore, the micro-hole boundary effect in suspended graphene play a significant role in determination of the properties for graphene materials and graphene-based flexible devices.

Strain-induced variation of bandgap in (111) In2O3 epitaxial films grown on c-sapphire substrates by a pulsed laser deposition technique

The structural and electronic properties of In2O3 epitaxial films grown on c-sapphire substrates by a pulsed laser deposition technique are studied as functions of various growth conditions. Our study shows that growth temperature, growth time, oxygen partial pressure, and post-growth cooling conditions play important roles in governing the magnitude and character of the overall change in unit cell volume (volume strain) of the lattice. It is found that biaxial strain, which is developed due to lattice and thermal expansion coefficient mismatches between the layer and the substrate, leads to overall compression of the lattice. Interestingly, a hydrostatic tensile strain is found to coexist with biaxial strain in samples, which, following growth, are cooled to room temperature at a rate much slower than the natural rate of cooling. It has been observed that the overall strain can be varied from compressive to tensile by changing the growth parameters. The study further shows a systematic variation of the bandgap (from 3.8 to 3.1 eV) with a change in the unit cell volume of these layers, demonstrating the strong influence of strain on the band structural properties of the material.

Tailoring of Thermo-Mechanical Properties of Hybrid Composite-Metal Bonded Joints.

Metallic substrates and polymer adhesive in composite-metal joints have a relatively large coefficient of thermal expansion (CTE) mismatch, which is a barrier in the growing market of electric vehicles and their battery structures. It is reported that adding carbon nanotubes (CNTs) to the adhesive reduces the CTE of the CNT-enhanced polymer adhesive multi-material system, and therefore when used in adhesively bonded joints it would, theoretically, result in low CTE mismatch in the joint system. The current article presents the influence of two specific mass ratios of CNTs on the CTE of the enhanced polymer. It was observed that the addition of 1.0 wt% and 2.68 wt% of multi-walled CNTs (MWCNTs) decreased the CTE of the polymer adhesive from °C (pristine level) to °C and °C, respectively, by 22% and 41% reductions.

Effects of thermal treatments on the hydrophobicity and anticorrosion properties of as-grown graphene coatings.

Graphene grown on metal substrates has been reported to provide efficient and robust hydrophobicity during water vapor condensation on metal surfaces. However, due to the intrinsic negative coefficient of thermal expansion (CTE) of graphene, the potential thermal stress in real application environments can cause CTE mismatch and then damage the protective graphene coatings, leading to loss of surface hydrophobicity and anticorrosion properties. In this study, the effect of thermal treatments on anticorrosion properties and subsequent hydrophobicity of the graphene surface has been investigated. The as-grown graphene on nickel (Ni–Gr) is explored in terms of survival under severe thermal cycling (up to 14.62 °C s−1) and effectively maintains its surface properties. As a comparison, the as-grown graphene on copper (Cu–Gr) easily peeled off from the metal surface due to the thermal stress and intercalation of oxides. The thermal treatment at 200 °C under ambient atmosphere can elevate the corrosion rate 2.2 times and 29 times on the Ni–Gr and Cu–Gr surfaces compared to situations without thermal treatments, respectively. This study shows that the Ni–Gr surface is significantly more robust than the Cu–Gr surface as a sustainable hydrophobic surface in a complicated thermal environment.

Analysis of Frequency Drift of Silicon MEMS Resonator with Temperature.

High-quality-factor Micro-Electro-Mechanical System (MEMS) resonators have been widely used in sensors and actuators to obtain great mechanical sensitivity. The frequency drift of resonator with temperature is a problem encountered practically. The paper focuses on the resonator frequency distribution law in the temperature range of—40 to 60 °C. The four-layer models were established to analyze thermal stress caused by temperature due to the mismatch of thermal expansion coefficients. The temperature variation leads to the transformation of stress, which leads to the shift of resonance frequency. The paper analyzes the influence of hard and soft adhesive package on the temperature coefficient of frequency. The resonant accelerometer was employed for the frequency measurements in the paper. In experiments, three types of adhesive dispensing patterns were implemented. The results are consistent with the simulation well. The optimal packaging method achieves −24.1 ppm/°C to −30.2 ppm/°C temperature coefficient of the resonator in the whole temperature range, close to the intrinsic property of silicon (−31 ppm).

Characterisation and properties of low-conductivity microporous magnesia based aggregates with in-situ intergranular spinel phases

Development of microporous magnesia based aggregates serving as working-line refractories have great significance in reducing energy loss and saving resource. Microporous magnesia-based aggregates were fabricated at 1780 °C by in-situ decomposition of magnesite with addition of nano-sized Al2O3. Intergranular MgAl2O4 phases formed in situ decreased the closed-pore size, thermal conductivity and improved the ceramic bonding and thermal shock resistance. Furthermore, the results suggested that pore size distribution was the dominate factor affecting thermal conductivity. Thermal contact resistance owing to networks of intergranular spinel in magnesia could improve thermal insulation performance effectively. The mismatch of thermal expansion coefficient between spinel and magnesia and the micro-scale closed pores enhanced thermal shock resistance by accommodating thermal stress and suppressing crack propagation. Microporous magnesia-based aggregates with 3 wt% nano-sized Al2O3 presented a mean pore size of 3.42 μm, thermal conductivity of 5.76 W m−1 k−1 (800 °C), a cold compressive strength of ~285 MPa, and a residual strength retention rate of 65.0% after thermal shock cycles. The low-conductivity microporous magnesia-based aggregates with excellent thermal shock resistance show promise for future application in working-lining lightweight refractories.

Eliminating graphene wrinkles by strain engineering

Graphene layers grown on metallic substrates by chemical vapor deposition generally exhibit wrinkles due to mismatches in thermal expansion coefficients of the graphene and the substrates, which affect remarkably the tribological, mechanical and electrical properties of graphene-based devices. Here we report an isotropic-stretching method to eliminate these wrinkles. The morphology evolution of the wrinkles during the stretching process is studied using an atomic force microscope, and a simple method for calculating the critical strain required to eliminate the wrinkles is suggested. Friction and scratching tests demonstrate that for graphene layers the friction and anti-wear performances of regions with eliminated pre-existing wrinkles are as good as the wrinkle-free regions. Our method is expected to be applicable to other layered materials grown on deformable surfaces.

High Temperature Elastic Modulus of Proton Conducting Ceramics Y-Doped Ba(Zr,Ce)O3

Proton conducting ceramics (PCC) cells are promising energy conversion devices that enable high efficiency energy conversion at lower temperature range, solving the challenge of conventional solid oxide cells (SOCs) due to the high operating temperature. Electrochemical performance and chemical stability of PCC electrolyte has been investigated in recent studies, suggesting that rare-earth doped Ba(Zr,Ce)O3 perovskite-type ceramics are optimal materials exhibiting high proton conductivity and chemical stability during operations. On the contrary, mechanical stability of these PCC electrolyte materials has not been evaluated despite the fact that the mechanical properties are critically important for achieving long-term stable operation as fuel cells or electrolyser cells. For the development of conventional SOCs, mechanical stability during high temperature operation was one of the most significant challenges to deal with, which was attained as a result of detailed studies on in-situ elastic properties of composing materials such as oxygen ion conducting electrolytes and residual stresses. Similarly, for PCC cells, mechanical properties of cells and composing materials have been of significant interest in order to achieve mechanically stable long-term operation, even though PCC cells operate at lower temperature than SOCs. Furthermore, the metal-supported (MS) structure which provides superior mechanical robustness compared to anode-supported (AS) structure is expected to be applied effectively to PCC cells, which are called proton conducting ceramics – metal-supported cells (PCC-MSCs), leading to greater necessity of the mechanical evaluation of the cells and composing materials.Electrolyte is the most crucial component in an electrochemical cell and must be mechanically stable because ion transport and gas tightness made by electrolyte determines electrical performance. However, there has been important concern that larger thermal stresses might be introduced in PCC cells compared to SOCs, resulting from the thermal expansion coefficient (TEC) mismatch between the electrode and electrolyte and from the chemical expansion by the hydration that occurs in a certain temperature range. The PCC electrolyte is highly in need of investigation on in-situ mechanical properties, especially on elastic properties. In this study, elastic properties of electrochemically promising PCC, Y-doped Ba(Zr,Ce)O3 perovskite-type ceramics, were investigated under high temperature conditions. Elastic moduli such as Young’s modulus and Poisson’s ratio were measured by the method that we previously developed for elastic investigation in high temperature conditions using ultrasonic waves. This method enables highly accurate and repetitive examination of elastic properties at high temperatures in materials with poor sinterability including PCC by measuring ultrasonic sound velocities in pellets typically fabricated for electrochemical tests.Pellets of BaZr1-xYxO3-δ (BZY) with different concentrations of doped yttrium, BaZr0.9Y0.1O3-δ (BZY10), BaZr0.85Y0.15O3-δ (BZY15), and BaZr0.8Y0.2O3-δ (BZY20), were fabricated. Additionally, pellets of BaZryCe1-yY0.1O3-δ (BZCY) with different ratio of Ce to Zr, BaZr0.7Ce0.2Y0.1O3-δ (BZCY721) and BaZr0.8Ce0.1Y0.1O3-δ (BZCY811) were fabricated. Powders of PCCs above were consolidated to be thick rounded shape and sintered in air. Each prepared sample was set in an electric furnace in laboratory air atmosphere and sound velocities were measured with the sample slowly heated up to 700 °C and subsequently cooled down to room temperature to calculate elastic moduli at each measuring point. In the first series of heating and cooling measurements for as-sintered samples, hysteresis on elastic moduli in intermediate temperature range was observed. We repeatedly conducted a series of heating and cooling measurements several times, and then the hysteresis was not observed any further. Fig.1 shows final state Young’s modulus of BZCY721, BZCY811, and BZY10 (BZCY901) without hysteresis.Elastic moduli at room temperature have not changed through multiple heating and cooling measurements, and crystal structures and lattice parameters were also confirmed to remain constant by x-ray diffraction (XRD) analysis. The hysteresis found in a specific temperature range suggests that elastic moduli were influenced presumably by a change in defect structure of PCC caused by hydration or defect association of oxygen vacancies and dopants. At room temperature, Young’s modulus decreased with the increment of Ce concentration by 16 % from BZY10 to BZCY721. When materials have the same crystalline structure, Young’s modulus generally decreases as mean atomic volume of the base crystal increases. Because BaCeO3 has larger mean atomic volume than BaZrO3, this observation is qualitatively reasonable. However, in high temperatures, the difference became significant only for BZCY721, Young’s modulus decreased by 30 % from that at room temperature in BZCY721. These results suggest that Ce substitution causes different high temperature dependences. Figure 1

Numerical analysis of the reliability of photovoltaic modules based on the fatigue life of the copper interconnects

A typical photovoltaic (PV) module is composed of different layers bonded together, each with a different material and thus, a different coefficient of thermal expansion (CTE). While under operation, it is subjected to thermal loads due to continuous temperature variations. The CTE mismatch induces thermo-mechanical stresses, whose cyclic nature causes fatigue failure. Due to their relative small size, the copper interconnects are one of the most vulnerable constituents.In this work, the finite element (FE) analysis has been used to calculate these thermal stress/strain variations. To make the simulations more reliable, well-known material models have been adopted from the literature and used for each constituent of the PV module. Furthermore, to reduce computational time and complexity, a simplified modeling approach has been proposed in this work. In this approach, a 2-D FE global model was first used to identify the region that undergoes maximum strain. Then a 3-D FE local model was used for that particular region only. This approach helps calculate the maximum stress/strain variations. Thus, using the proposed modeling approach coupled with a fatigue criterion, one can calculate the fatigue life of the PV module, while significantly reducing the computational time. The initial conditions, in terms of residual stresses, due to the lamination process for the fabrications of the PV module are also accounted for in our approach. We applied our approach to silicon-based PV module under desert weather conditions of Doha (Qatar) and the results show a reduced life compared to the one provided by the manufacturer.

Bamboo like SiC Nanowires Grown in a Dual-Temperature Zone Reaction System Enhance the Oxidation and Thermal Shock Resistance of SiC Coatings

The life of high temperature coating mainly depends on its oxidation resistance and bonding strength with the substrate. SiCnws / SiC coatings were prepared by a self-made two temperature reaction system and chemical vapor deposition method. The oxidation resistance of the composite coating at 900 ℃ and 1500 ℃ and the thermal shock resistance at 1500 ℃ were studied. The fracture toughness and bonding strength of the composite coating were increased by 53.8% and 9.8 times, respectively, due to the stable mechanical linkage effect between the bamboo like SiC nanowires with special nodes and SiC. Due to the healing effect of glass phase SiO2, the weight loss rate of bamboo like SiCnws / SiC coating sample after oxidation at 900 ℃ for 30 h is about 5.28%, and that after isothermal oxidation at 1500 ℃ for 60 h is only 0.74%. In addition, bamboo like SiCnws can effectively alleviate the thermal expansion coefficient mismatch between SiC coating and substrate. After 25 thermal shock cycles, the final oxidation weight loss rate of composite coating is only 2.03%.

Temperature-Induced Explosive Behaviour and Thermo-Chemical Damage on Pyrite-Bearing Limestones: Causes and Mechanisms

In this investigation, two different varieties of ‘Prada’ limestones were studied: a dark grey texture, bearing quartz, clay minerals, organic matter and pyrites, and a light grey texture with little or no presence of such components. We have observed two effects of different intensity when heating the dark texture from 400 °C: (1) the explosion of certain samples and (2) greater thermal damage than in the light grey texture. Chemical and mineralogical composition, texture, microstructure, and physical properties (i.e. colour, open porosity, P and S-wave velocity) have been evaluated at temperatures of 105, 300, 400, and 500 °C in order to identify differences between textures. The violence of the explosive events was clear and cannot be confounded with ordinary splitting and cracking on thermally treated rocks: exploded samples underwent a total loss of integrity, displacing and overturning the surrounding samples, and embedding fragments in the walls of the furnace, whose impacts were clearly heard in the laboratory. Thermogravimetric results allowed the identification of a process of oxidation of pyrites releasing SO2 from 400 °C. This process jointly with the presence of microfissures in the dark texture, would cause a dramatic increase in pore pressure, leading to a rapid growth and coalescence of microcracks that leads to a process of catastrophic decay in rock integrity. In addition to the explosive events, average ultrasound velocities and open porosity showed a greater variation in the dark grey texture from 400 °C. That result also points towards a significant contribution of oxidation of pyrites on the thermo-chemical damage of the rock, among other factors such as the pre-existence of microfissures and the thermal expansion coefficient mismatch between minerals. Implications in underground infrastructure and mining engineering works are critical, as the explosive potential of pyrite-bearing limestones bears risk for mass fracturing and dramatic strength decay from 400 °C. Moreover, SO2 released has harmful effects on health of people and the potential to form acid compounds that corrode materials, shortening their durability and increasing maintenance costs.

Interface Control in Additive Manufacturing of Dissimilar Metals Forming Intermetallic Compounds-Fe-Ti as a Model System.

Laser metal deposition (LMD) has demonstrated its ability to produce complex parts and to adjust material composition within a single workpiece. It is also a suitable additive manufacturing (AM) technology for building up dissimilar metals directly. However, brittle intermetallic compounds (IMCs) are formed at the interface of the dissimilar metals fabricated by LMD. Such brittle phases often lead to material failure due to thermal expansion coefficient mismatch, thermal stress, etc. In this work, we studied a Fe-Ti system with two brittle phases, such as FeTi and Fe2Ti, as a model system. Fe was grown on top of Ti at various process parameters. The morphologies and microstructures were characterized by optical microscopy (OM) and scanning electron microscopy (SEM). No cracks along the interface between pure Ti and bottom of the solidified melt pool were observed in the cross-sectional images. Chemical composition in the fabricated parts was measured by Energy-dispersive X-ray spectroscopy (EDS). Electron backscatter diffraction (EBSD) was performed in addition to EDS to identify the crystalline phases. The Vickers hardness test was conducted in areas with different phases. The chemical composition in the melt pool region was found to be a determining factor for the occurrence of major cracks.

Thermodynamics Models for V-pit Nucleation and Growth in III-Nitride on Silicon

Silicon and sapphire are common substrates for AlN, InGaN, and GaN thin films in several applications such as photovoltaic and light-embedded diodes. Threading dislocations are generated at interfaces between III-nitride (III-N) layers and these substrates because of large lattice and thermal expansion coefficient (TEC) mismatches. These dislocations penetrate the top surface of III-N layers to relax the system by forming V-pit defects. This work presents a thermodynamics-based model to study V-pit formation and growth in InGaN/GaN epilayers on either silicon or sapphire substrates. The model calculates the evolution of V-pit defects in thin films through the energy balance between the strain energy in the III-N layer, dislocation deterioration energy to form new V-pits, and V-pit facet energies that result because of facet formations. The impact of different lattice and TEC mismatches as well as a novel approach, the embedded void approach, on V-pit nucleation and growth is also investigated.