Effects of oxidation curing and sintering temperature on the microstructure formation and heat transfer performance of freestanding polymer-derived SiC films for high-power LEDs

Effects of oxidation cross-linking and sintering temperature on the microstructure evolution, thermal conductivity and electrical resistivity of continuous freestanding polymer-derived SiC films were investigated. The as-received films consisting of β-SiC nanocrystals embedded in amorphous SiO_xC_y and free carbon nanosheets were fabricated via melt spinning of polycarbosilane (PCS) precursors and cured for 3 h/10 h followed by pyrolysis from 900 °C to 1200 °C. Results reveal that nanoscale structure (β-SiC/SiO_xC_y/C_free) provides an ingenious strategy for constructing highly thermal conductive, highly insulating and highly flexible complexes. In particular, the 3 h-cured films sintered at 1200 °C with satisfying thermal conductivity (46.8 W m^?1 K^?1) and electrical resistivity (2.1 × 10⁸ Ω m) are suitable for the realization of high-performance substrates. A remarkable synergistic effect (lattice vibration of β-SiC nanocrystals and close-packed SiO_xC_y, free-electron heat conduction of β-SiC and free carbon, and supporting role of oxygen vacancy) contributing to thermal conductivity improvement is proposed based on the analysis of microstructure, intrinsic properties and simulations. Eventually, the SiC films without additional dielectric layers are directly silk-screen printed with high-temperature silver paste and used as heat dissipation substrates for high-power LED devices via chip-on-board (COB) package. The final devices can emit bright light with low-junction temperature (52.6 °C) and good flexibility owing to the mono-layer SiC substrate with low thermal resistance and desirable mechanical properties. This work offers an effective approach to design and fabricate flexible heat dissipation ceramic substrates for thermal management in advanced electronic packaging fields.     

Surface Preparation of Aluminum Nitride for Metallization

Currently, the surface preparation of aluminum nitride (AlN) substrates prior to metallization includes an aqueous cleaning step. Surface reactions that occur in this step cause performance and reliability issues with AlN substrates to be used in microelectronic packaging. There is a lack of published data on the reactivity of AlN substrates with common solvents. This study investigated the effects of different solvents on the surface corrosion of AlN substrates. The variables studied were pH, aqueous vs. organic solutions, prior surface condition, and time (up to 3.6 Ms or 42 days). The solvents tested were hydrochloric acid (HCl) with pH values ranging from 2 to 5, sulfuric acid (H₂SO₄) with pH values ranging from 2 to 5, sodium hydroxide (NaOH) with pH values ranging from 8 to 12, 1 M citric acid, oleic acid, Micro-90, methanol, ethanol, isopropanol, acetone, and deionized water. Three types of surface reaction behavior were observed in this study. The substrates either showed no reaction (HCl pH = 2, methanol, ethanol, isopropanol, acetone, citric acid, and oleic acid), slight corrosion without spalling (Micro-90, HCl pH = 3, H₂SO₄pH = 3), or they were severely corroded and spalled (HCl pH = 5, H₂SO₄pH = 5, all NaOH solutions, and deionized water).     

Polishing of polycrystalline diamond by hot nickel surface

A microwave plasma technique has been employed to deposit polycrystalline diamond film over a molybdenum substrate button using a gas mixture of hydrogen and methane at a substrate temperature of 851°C. A CVD diamond coated molybdenum substrate button was mounted with a load against hot nickel plate and rotated for 3.45 h in a hydrogen ambient. Hot tungsten filament was used as a heat source to maintain the temperature of the nickel block and CVD diamond coated molybdenum button at 848°C. This experiment has reproducibly shown the successful polishing of polycrystalline CVD diamond by hot nickel. A Tencor profilometer and scanning electron microscope have been used to evaluate the surface smoothness and morphology before and after polishing the polycrystalline diamond thin films.     

Three-dimensional modeling of the heat flow into a GaAs substrate. Influence of thermal phenomena on the RF behavior of power HBTs and technological optimization

Using three-dimensional modeling of the heat flow into the substrate, and taking into account the variation of thermal conductivity with temperature, we study the sensitivity of the thermal resistance R_TH to parameters such as the substrate thickness, its nature (GaAs, Si, AlN or diamond), the topology of the power source (length and width) or even the power density. Thus, we show that the transfer of active GaAs layers onto a host substrate with a higher thermal conductivity such as Si, AlN or diamond is of greater interest than the thinning of the substrate.We developed an accurate electrothermal model for the AlGaAs/GaAs heterojunction bipolar transistor (HBT) allowing prediction of the static and mainly dynamic (RF) behavior at a high signal operating mode. We show that if the HBT layers are transferred onto a diamond substrate, a 50% gain in RF power can be expected at 10 GHz.Finally, we present the technological solutions investigated for the transfer and the heteroassembly of HBT active layers on substrates better suited for thermal dissipation.     

Friction induced paint damage as affected by clearcoat chemistry

One form of damage to automotive painted plastic composites, in particular thermoplastic bumpers and fascias, is known to be caused from compressive shear loading placed on the surface of the composite. The loading inflicted upon the composite by a stationary force (e.g. an automobile hitting a post) or a moving object (e.g. an automobile hitting another automobile), and consequent compressive shear that is induced, can result in cohesive delamination of the substrate. The cohesive delamination in the painted composite (herein referred to as ‘gouge') is not only influenced by the cohesive strength of the substrate but also by the mechanical properties of the coating. Our studies indicate that one important attribute of the coating, namely the coating's coefficient of friction, can significantly impact the ‘gouge' resistance of the painted composite. In this work, we will describe the derivation of coating attributes as they affect ‘gouge' resistance and relate changes in the coating chemistry to useful life parameters.     

Factors influencing methane emission from peat soils: Comparison of tropical and temperate wetlands

Methane (CH₄) emissions from peat soils in tropical and temperate wetlands were compared. Annual CH₄ emission rates in Ozegahara, the largest wetland on Honshu main island, Japan, were higher than in drained forest wetland areas examined in Indonesia. Methane emissions from the lowland paddy fields examined in Indonesia were higher than those of peaty paddy fields in Japan. There was generally a positive correlation (r² = 0.09; P < 0.1) between CH₄ emissions and CH₄ production activities in wetland soils. In Ozegahara, there was a positive relationship (r² = 0.80; P < 0.01) between CH₄ production activities and soil pH, but there was no such relationship in Indonesia. The range of soil pH in Ozegahara was 5–7, while pH values in the Indonesian sites were lower than 5. There was a positive response of CH₄ emission with respect to groundwater level in all of these areas. In Indonesia, land-use change from swamp and drained forest to cassava or coconut field lowered groundwater levels and decreased CH₄ emission, while change to lowland paddy raised the groundwater level and increased CH₄ emission. Addition of acetate generally inhibited CH₄ production during the early period (until 2 weeks) of incubation, then enhanced it afterward in both Ozegahara and Indonesian wetland soils. Addition of hydrogen mostly enhanced CH₄ production. From the results of this study, CH₄ fluxes from peat soil to the atmosphere were positively correlated with CH₄ production activities, and CH₄ production activity in peat soil was regulated by soil pH, while land-use change from wetland to upland crop lowered groundwater level and thus reduced CH₄ production and enhanced CH₄ oxidation.     

A vacuum system for the production of clean carbon films

A compact, oil-free vacuum system has been assembled for the production of thin carbon films. Carbon rods are held in a simply constructed and easily cleaned rod holder. Using an unlubricated mechanical roughing pump and one sorption pump, the system is evacuated to the low millitorr range while completely avoiding oil vapors and contaminants that might derive from pump oil. Films produced in such systems are hydrophilic, and analytical electron microscopy measurements have shown that these films are adequately smooth and free of contaminants of both high and low atomic number.     

Bioactivity of tempe by inhibiting adhesion of ETEC to intestinal cells,as influenced by fermentation substrates and starter pure cultures

Soya bean tempe is known for its bioactivity in reducing the severity of diarrhoea in piglets. This bioactivity is caused by an inhibition of the adhesion of enterotoxigenic Escherichia coli (ETEC) to intestinal cells. In this paper, we assessed the bioactive effect of soya tempe on a range of ETEC target strains, as well as the effect of a range of cereal and leguminous substrates and starter pure cultures.     

Chemical analysis of NiAlFe thin films sputter deposited onto sapphire substrates

NiAlFe thin films were prepared onto sapphire single crystals by physical vapour deposition (PVD) and these were analysed by X-ray photoelectron spectroscopy (XPS) in combination with argon ion etching to determine the composition depth profile and interfacial characteristics of the samples. Non-linear least square fitting (NLLSF) analysis of the data was required due to the conflict of several peaks of interest. XPS depth profiles show that, for non-annealed NiAlFe–Al₂O₃, the interface is sharp and oxygen diffusion occurs at different annealing temperatures. Ni remains chemically unaffected by the presence of oxygen while the formation of aluminium oxide compounds occur. Two iron species are present in the film thickness where the low binding energy component is attributed to Fe–Fe or Fe–Al interactions and the higher one to the NiAlFe compound. The reduction-dissolution of the sapphire substrate leads to depletion of oxygen in the sapphire surface layer and the formation of alumina at the NiAlFe–Al₂O₃ interface. Within the film, aluminium and nickel are present as an intermetallic compound. Annealing of the samples induces surface oxidation and the subsequent formation of an Al₂O₃ layer. This type of interphase morphology should lead to optimal fibre/matrix (F–M) adhesion, and therefore optimal load transfer between the matrix and reinforcement.     

Bioactive HA/TiO2 coating on magnesium alloy for biomedical applications

Magnesium alloys are new class of biodegradable alloys having many favourable properties to overcome the limitations of currently used biomedical alloys. Recently, several coatings have been developed to overcome their higher degradation rate. In this regard, a new attempt has been made to develop Hydroxyapatite and Hydroxyapatite/TiO₂ coatings on magnesium alloys to increase the biocompatibility and reduce the corrosion rate. The coated surfaces were characterized by Fourier-Transform Infrared Spectroscopy (FT-IR), X-ray Diffraction Analysis (XRD), Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray (EDX) Spectroscopy, Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). Contact angle measurements proved higher hydrophilic nature of HA/TiO₂ coating compared to HA coating. In-vitro studies showed that HA–TiO₂ coated alloy exhibited higher osteoinduction compared to HA coated alloy. Hydrogen evolution studies and corrosion studies confirmed greater reduction in degradation rate of HA/TiO₂ coated alloy. Vickers microhardness test also showed enhancement in mechanical strength of the composite coated alloy compared to HA coated alloy. Three point bend test depicted better adherence of the HA/TiO₂ coating compared to HA coating on the substrate. Cell culture studies proved higher cell attachment and proliferation on composite coated alloy by controlling the release of magnesium ions into the surrounding body tissue.