In this paper, degradation mechanisms of optical materials, used in the light emitting diode (LED)-based products, are reviewed. The LED lighting is one of the fastest technology shifts in human history. Lighting accounts for almost 20% of the global electrical energy use, inferring that replacement of traditional lighting sources with LEDs with higher efficiencies will have major positive implications for the global energy consumption. Organic optical materials are key components in LEDs in the sense that they control the functionality of the device and they have decisive effects on the durability and reliability of LEDs. This paper aims at describing the influences of chemical structure and service conditions on the degradation mechanisms of organic optical materials in LEDs which lead to the lumen depreciation, discolouration, and colour shift of the LED light output. The contributions of different degradation mechanisms of optical and package materials in LED-based products to the lumen depreciation and colour shift are methodically reviewed.

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This paper aims at studying microstructure and mechanical properties of spark plasma sintered (SPSed) Stellite®-6 cobalt-based superalloy. SPS is a sintering technique, based on a relatively fast resistance heating using a pulsed current. Fast sintering process, associated with minimum grain growth, results in excellent mechanical properties. Samples were sintered at temperatures ranging from 950 to 1100 °C. Microstructure of samples were studied using scanning electron microscope (SEM), energy-dispersive X-ray spectroscope (EDS), X-Ray diffraction (XRD), and optical microscope. Hardness, impact test, as well as room and high temperature compression tests were used to evaluate the effects of sintering temperature and duration on the mechanical properties of SPSed samples. Results show that optimum mechanical properties can be obtained after sintering at 1050 °C for 10 min. The correlation between sintering parameters, microstructure, and mechanical properties are discussed.

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Isothermal ageing of Al–Mg–Si alloys, stored at room temperature for more than 5 months, is associated with an unexpected significant increase in the overall electrical resistivity. This unexpected anomalous increase is not observed in alloys with shorter storage (natural ageing) times. This phenomenon is explained with a scenario, based on the evolution of the size distribution of Guinier–Preston (GP) zones during natural ageing and during subsequent artificial ageing. The proposed scenario can explain the contribution of natural ageing atomic clusters to this anomalous increase in the electrical resistivity. A physically based combined precipitation–electrical resistivity model, with the former being based on simultaneous nucleation-growth-coarsening reactions and the latter based on the Bragg scattering of electrons from atomic clusters, has been used to explain the electrical resistivity evolution. It is shown that the proposed model is capable of reproducing the experimental data in both short natural ageing (less than 5 months) and long natural ageing (more than 5 months) regimes.@en

This paper investigates the failure of superheater tubes, made of AISI 304 stainless steel, after 10 years of service in a power plant. The failure is in the form of fish mouth rupture and is mostly observed at U-bent elbows. Microstructure of the degraded samples was analyzed using optical and scanning electron microscopes. Phases were identified with energy-dispersive x-ray spectroscopy analysis as well. Results show that grain boundaries are heavily oxidized, and this has resulted in the embrittlement of the alloy. The fractography analysis showed that cracks propagated along grain boundaries, resulting in an intercrystalline fracture mode. At the end, some tentative remedial measures are recommended to mitigate similar failure in power plants.

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This paper investigates the effects of carbon nanotubes (CNT) addition on the structure and mechanical properties of Ni ₃ Al-xB (0.0 < x < 1.5 at%) intermetallic compound. Ni ₃ Al-xB-1wt%CNT nanocomposite powders were first synthesized by mechanical alloying. Effects of CNT addition on the lattice strain and crystallite size of synthesized powders were investigated by means of X-ray diffraction (XRD) analysis. Scanning electron microscope (SEM) was used to study powder morphologies. Powders, synthesized by mechanical alloying, were then consolidated using spark plasma sintering (SPS), conducted at 950 °C under pressure 50 MPa. Microhardness and shear punch tests were employed to study the mechanical properties of sintered samples. Results show that CNT addition is accompanied by a decrease in crystallite size and a significant improvement of mechanical properties of Ni ₃ Al-xB (0.0 < x < 1.5 at%) intermetallic compounds.

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This study aims at studying the microstructure and mechanical properties of nanocrystalline Ni₃Al-xB (0.0<x<1.5 at%) alloy, made by mechanical alloying (MA) and spark plasma sintering (SPS). Effects of milling time as well as boron addition on the crystallite size, lattice strain, and powder morphologies of synthesized powders were studied using scanning electron microscope (SEM) and X-ray diffractometer (XRD). Synthesized powders were consolidated under uniaxial compression 50 MPa at 950 °C for 10 min. Shear punch and hardness tests were used to evaluate mechanical properties of consolidated specimens. Results show that Ni₃Al compound were synthesized after 30 h of milling in a planetary ball mill with the rotation speed of 350 rpm and ball-to-powder ratio 10:1. Synthesized powders have crystallite size in the range 15–20 nm depending on the milling time. Results also show that boron addition is influential the most when boron content is between 0.5–1.0 at%. Boron addition is accompanied with a significant improvement of hardness and shear strength and a drop in the ductility of Ni₃Al-xB (0.0<x<1.5 at%) alloy. The details concerning structure-properties relationship in this system are discussed in this paper.

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This work investigates the effects of grain boundary oxidation on the failure of AISI 304H stainless steel elbows in a heat exchanger in a power plant after 8 years of service at 400 °C. Optical and electron microscopes, elemental mapping, electron dispersive spectroscopy (EDS), inductively-coupled plasma atomic energy spectroscopy (ICP-AES) and hardness tester were used to understand the failure mechanism. Elemental mapping of the samples operating for 8 years showed high concentrations of oxygen and carbon at the grain boundaries. Results showed that cracks were propagated along the grain boundaries due to the brittleness of the oxide layer at the boundaries. Results confirmed the intergranular and brittle nature of the fracture. Microhardness profilometry showed that oxidized/carburized grain boundaries are significantly harder than matrix inside grains.@en

In a highly competitive and demanding microelectronics market, reliable non-destructive methods for quality control and failure analysis of electronic components are highly demanded. Any robust non-destructive method should be capable of dealing with the complexity of miniaturized assemblies such as chip-scale packages and 3D IC stacks. Scanning acoustic microscopy (SAM) is indeed one the best non-destructive tools for failure analysis purposes. It is also a useful technique for imaging the morphology, location and size distribution of defects in different microelectronics components. SAM can detect delaminations at sub-micron thicknesses. It is also one of the only available techniques capable of efficiently evaluating popcorning in PBGA's and is a also useful device to detect sub-micron air gaps. SAM can also be used to measure the thickness of an internal layer of material. Overall, SAM is an efficient tool for evaluating such a wide range of different defects in printed circuit boards, underfills, BGAs, wire bonds, discrete components, and wafers. In SAM a focused sound is directed from a transducer at a small point on a target object, as is schematically shown here. Sound, hitting a defect, inhomogeneity or a boundary inside material, is partly scatted and will be detected. The transducer transforms the reflected sound pulses into electromagnetic pulses which are displayed as pixels with defined gray values thereby creating an image. This article aims at giving an overview of scanning acoustic microscope (SAM) and explaining its operating principles and its limitations. A few examples are also given for further clarification.

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