Synthesis and thermal analysis of aluminium nitride filled epoxy composites and its effective application as thermal interface material for LED applications

The improvement of heat conduction in any electronic devices has become a predominant issue in which effective heat dissipation is crucial to enhance the performance of packaged devices. This paper elucidates the application of thermally conductive particles filled composites as thermal interface material for LEDs. Present work aims on reducing the junction temperature and thermal resistance of the device under test with heavily filled ceramic-epoxy composite as the interface material between the device and metal substrate. Silane treated aluminium nitride (AlN) powder was studied for its feasibility as the filler material. The thermal conductivity values obtained by hot disc method (ISO/DIS 22007-2.2) were 0.66, 0.54 and 0.44 W/mK for 60, 50 and 40 wt% AlN filled epoxy composites respectively which were described well by thermal transient measurement of LEDs. The junction temperature and total thermal resistance of the thermal set up was reduced significantly with increased filler loading. The least junction to ambient thermal resistance (R_thJ-A) was achieved for 60 wt% followed by 50 and 40 wt% AlN filled TIM with the values of 24.8, 31.98 and 34.64 K/W respectively. Characteristics of the AlN filled composites for LED applications are discussed extensively in terms of thermogravimetric and thermo-mechanical analysis.     

Role of SiC substrate polarity on the growth and properties of bulk AlN single crystals

Two symmetrically nonequivalent silicon carbide (SiC) substrate orientations, (0001) Si-terminated and \((000\overline{1} )\) C-terminated, were used in the physical vapour transport growth of bulk aluminium nitride (AlN) single crystals. The crystals grown on Si-faces always exhibit an Al-polar growth surface. AlN growth on \((000\overline{1} )\) C-terminated surfaces of the SiC substrates was performed to obtain N-polar growth surfaces. An abrupt interface was observed between the AlN crystal and the C-face substrate which is in contrast to the growth on Si-faces where hexagonally shaped SiC hillocks are formed. The growth on C-faces is usually dominated by multi-site nucleation. Applying similar supersaturation conditions that led to step-flow growth on Si-faces to the C-faces resulted in a spiral growth mode, even on highly off-oriented substrates. The obtained broad X-ray diffraction rocking curves of such samples (full-width at half-maximum ≈380 arcsec) indicate the presence of more misfit dislocations and significant misfit stress. In addition, polarity inversion is observed in C-face grown crystals. Though the structural properties of the crystals grown on C-face are inferior to that of the crystals grown on Si-face, the incorporation of unintentional Si impurity was found to be lower (<2 wt%).     

Thermo-optical,DSC and microfractographic investigation of PE-PE mixtures

The nucleation effect of high-density polyethylene (HDPE) to low-density polyethylene (LDPE) and the structure of the resulting directional LDPE morphology was studied using thermo-optics, differential scanning calorimetry (DSC), X-ray diffraction and microfractography. Thermo-optics and DSC showed a higher nucleation activity of HDPE compared with the heterogeneities within the LDPE melt. The parallel and more perfect arrangement of the chain segments in transcrystalline LDPE resulted in a higher melting temperature, a difference in unit-cell spacing and a less ductile behaviour during fracture than with spherulitic LDPE.     

Enhancing the antifouling property of poly(vinylidene fluoride)/SiO2 hybrid membrane through TIPS method

PVDF/SiO₂ hybrid membranes with outstanding antifouling property were prepared from PVDF/glycerol triacetate system via thermally induced phase separation method, and characterized by scanning electron microscope, energy dispersive X-ray spectrometer analyses, differential scanning calorimeter, and wide angle X-ray diffraction. Their properties such as permeability, porosity, pore size distribution, and mechanical performance were also determined. The results show that SiO₂ nanoparticles modified by 3-aminopropyltriethoxysilane can be uniformly dispersed in membranes due to improved compatibility between PVDF solution and nanoparticles. The addition of SiO₂ particles to PVDF/glycerol triacetate mixture has a strong effect on crystallinity of the resulting hybrid membrane, which does not affect the type of PVDF crystal structure. Water flux recovery ratio is significantly increased from 11.7 % for pure PVDF membrane to 93.8 % for PVDF/SiO₂ hybrid membrane with addition of 8 wt% modified SiO₂. This remarkable promotion is related to the implantation of SiO₂ nanoparticles into the inner surface of membrane, which effectively restrains the adsorption of bovine serum albumin on the pore walls and improves antifouling property of the final membranes. Additionally, pure water flux of the hybrid membrane is increased by 276 %, i.e., from 85 to 320 L m^?2 h^?1, tensile strength is increased by 26.5 %, and elongation at break is increased by 85.4 % compared with that of pure membrane.     

Enhanced bioactivity of polyvinylidene chloride films using argon ion bombardment for guided bone regeneration

Polyvinylidene chloride (PVDC) is a long chain carbon synthetic polymer. The objective of this study was to improve the bioactivity of PVDC films through surface modification using argon (Ar) ion bombardment to create Ar-modified PVDC films (Ar-PVDC) to address the clinical problems of guided bone regeneration (GBR), which is technique-sensitive, and low bone regenerative ability. First, the effects of Ar ion bombardment, a low temperature plasma etching technique widely used in industry, on PVDC film wettability, surface chemistry, and morphology were confirmed. Next, fibroblast-like and osteoblast-like cell attachment and proliferation on Ar-PVDC were assessed. As a preclinical in vivo study, Ar-PVDC was used to cover a critical-sized bone defect on rat calvaria and osteoconductivity was evaluated by micro-computed tomography analysis and histological examinations. We found that the contact angle of PVDC film decreased by 50° because of the production of –OH groups on the PVDC film surface, though surface morphological was unchanged at 30 min after Ar ion bombardment. We demonstrated that cell attachment increased by about 40 % and proliferation by more than 140 % because of increased wettability, and 2.4 times greater bone regeneration was observed at week 3 with Ar-PVDC compared with untreated PVDC films. These results suggest that Ar ion bombardment modification of PVDC surfaces improves osteoconductivity, indicating its potential to increase bone deposition during GBR.     

Synthesis,characterization, photocatalytic and reusability studies of capped ZnS nanoparticles

This paper presents results of a study on the structural and morphological properties of 2-mercaptoethanol (2-ME) capped ZnS nanoparticles (NPs). The photocatalytic and reusability study of the synthesized NPs to degrade dyes was also done. ZnS semiconductor NPs were synthesized via chemical precipitation route using 2-ME as a stabilizing agent. The as-prepared NPs were characterized by X-ray diffraction (XRD) technique to confirm the nanometer sized particle formation. Morphological features of capped ZnS NPs were determined by transmission electron microscopy (TEM). Dynamic light scattering (DLS) technique was used to determine the hydrodynamic size of capped ZnS NPs. UV-Vis studies were done to determine the absorption edge and bandgap of the capped ZnS NPs. Fourier transform infrared spectroscopy (FT-IR) studies were done to confirm the presence of 2-ME on the surface of NPs. Photocatalytic studies of the as-prepared ZnS NPs were done by taking Ponceau S and crystal violet dyes as model pollutants. Their comparative degradation behaviour has been discussed. Reusability study of ZnS NPs was done to ensure its applicability as recycled catalyst in photocatalysis. The result showed photocatalytic enhancement of reused catalyst. Possible reason has been discussed in this work.     

Toward p-type conduction in Cs-doped ZnO: an eco-friendly synthesis method

An alcohol-free, eco-friendly technique was adapted for the synthesis of undoped ZnO and Cs-(cesium) doped ZnO nanoparticles (NPs). The effect of annealing and dopant concentration on its structural and optical properties was investigated. X-ray diffraction results confirmed the formation of polycrystalline hexagonal wurtzite structure and enhanced crystallinity was observed for 1 mol%: Cs-doped ZnO NPs. Scanning electron microscopy results revealed triangular-shaped NPs and increase in the crystallite size is noticed with increase in dopant concentration. UV–visible results showed shift in the band edge toward higher wave length side with increasing Cs concentration. Reduction in bandgap was observed for Cs-doped ZnO NPs, due to quantum confinement effect. Transmittance value increased to 86 % with the inclusion of Cs in ZnO lattice. Room temperature photoluminescence analysis of Cs-doped ZnO NPs reveals bandedge emission along with 450 nm emission due to Zn vacancy and Zn interstitial defects. Electrical measurements confirmed the realization of p-type conductivity in Cs-doped ZnO NPs with a carrier concentration of 1.3 × 10¹⁸/cm³.     

β-Radiation-induced decrease of adhesion in AlN/Si structure

We have studied the effect of low-intensity (I ～ 1.2 × 10⁵ cm^?2 s^?1) β radiation on the exfoliation of thin amorphous AlN films (with thicknesses on the order of 100 nm) from (100)-oriented silicon substrate as a result of scratching with a Berkovich pyramid at a linearly increasing load. It is established that AlN film beta-irradiated to a fluence of f = 2.16 × 10¹⁰ cm^?2 exfoliates at about 10% lower load, while the lateral force acting upon the indenter decreases by about 40%. The obtained results can be used for improving the bonding technology and must be taken into account in assessment of the reliability of thin-film structures subjected to (intentional or accidental) electron irradiation.     

Effect of pressure on the phase composition of Li(Na)/W/Mn/SiO2 composites and their catalytic activity for oxidative coupling of methane

The phase state of Li/W/Mn/SiO₂ and Na/W/Mn/SiO₂ composites after exposure to high pressures (2.5 GPa at 500°C) and subsequent exploitation in oxidative coupling of methane (OCM) was studied. Comparison of the catalytic activity of the composites before and after exposure to high pressures indicates that the formation of Li(Na)/W/Mn/SiO₂ composites catalytically active for OCM is significantly influenced by high pressures.     

Comparison of the performance of natural latex membranes prepared with different procedures and PTFE membrane in guided bone regeneration (GBR) in rabbits

This work assessed the performance of membranes made of natural latex extracted from Hevea brasiliensis prepared with three different methods: polymerized immediately after collection without the use of ammonia (L1); polymerized after preservation in ammonia solution (L2); and polymerized after storage in ammonia, followed by Soxhlet technique for the extraction of substances (L3). Polytetrafluoroethylene (PTFE) membrane was used as control. Two 10-mm diameter bone defects were surgically made in the calvaria of thirty adult male New Zealand rabbits. Defects (total n = 60) were treated with guided bone regeneration (GBR) using L1, L2, L3 or PTFE membranes (n = 15 for each membrane). Ten animals were euthanized after 7, 20 and 60 days postoperatively so that five samples (n = 5) of each treatment were collected at each time, and bone regeneration was assessed microscopically. The microscopic analysis revealed defects filled with blood clot and new bone formation at the margins of the defect in all 7-day samples, while 20-day defects were mainly filled with fibrous connective tissue. After 60 days defects covered with L1 membranes showed a significantly larger bone formation area in comparison to the other groups (P < 0.05, ANOVA, Tukey). Additionally, bone tissue hypersensitization for L1 and PTFE membranes was also investigated in six additional rabbits. The animals were subjected to the same surgical procedure for the confection of one 10-mm diameter bone defect that was treated with L1 (n = 3) or PTFE (n = 3). Fifty-three days later, a second surgery was performed to make a second defect, which was treated with the same type of membrane used in the first surgery. Seven days later, the animals were euthanized and samples analyzed. No differences among L1 and PTFE samples collected from sensitized and non-sensitized animals were found (P > 0.05, Kruskal–Wallis). Therefore, the results demonstrated that latex membranes presented performance comparable to PTFE membranes, and that L1 membranes induced higher bone formation. L1 and PTFE membranes produced no hypersensitization in the bone tissue.     

Porous asymmetric SiO2-g-PMMA nanoparticles produced by phase inversion

A new kind of asymmetric organic–inorganic porous structure has been proposed. Asymmetric lattices of polymer grafted silica nanoparticles were manufactured by casting and phase inversion in water. Silica nanoparticles were first functionalized with 3-(dimethylethoxysilyl)propyl-2-bromoisobutyrate, followed by grafting of poly(methylmethacrylate) (PMMA) segments, performed by atom-transfer radical polymerization. Mechanically stable self-standing films were prepared by casting a dispersion of functionalized nanoparticles in different solvents and immersion in water. The resulting asymmetrically porous morphology and nanoparticle assembly was characterized by scanning electron and atomic force microscopy. The PMMA functionalized SiO₂ hybrid material in acetone or acetone/dioxane led to the best-assembled structures. Porous asymmetric membranes were prepared by adding free PMMA and PMMA terminated with hydrophilic hydroxyl group. Nitrogen flow of 2800 L m^?2 h^?1 was measured at 1.3 bar demonstrating the porosity and potential application for membrane technology.     

Optical properties of Er3+–Ag co-doped ZnO nanocrystals prepared by combustions method

The Er³⁺–Ag co-doped ZnO nanocrystals have been synthesized by citric acid-assisted combustions method. The effect of different concentration of silver nanoparticles (NPs) on Er³⁺ doped ZnO nanocrystals and the optical behaviors are explored. The nanocrystals were characterized by X-ray diffractions, scanning electron microscopy, UV–Vis–NIR absorption spectra, X-ray photoelectron spectroscopy, and photoluminescence, respectively. The luminous intensity of Er³⁺ doped ZnO nanocrystals was significantly influenced by the concentration of silver NPs. A large enhancement in up-conversion intensity has been observed when the concentration of silver NPs was 0.10 mol%. The effect of localized surface plasmon resonance of silver NPs and the energy transfer between the silver NPs and Er³⁺ ions (²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2) are discussed as the sources of enhancement or quenching.     

Preparation of flexible,hydrophobic, and oleophilic silica aerogels based on a methyltriethoxysilane precursor

We report the synthesis of flexible, hydrophobic, and oleophilic silica aerogels through a two-step acid–base sol–gel reaction followed by supercritical drying, in which methyltriethoxysilane (MTES) is used as a precursor, ethanol (EtOH) as a solvent, and hydrochloric acid (HCl) and ammonia (NH₃·H₂O) as catalysts. At the optimal molar ratio of MTES:EtOH:H₂O:HCl:NH₃·H₂O is 1:18:3.5:1.44 × 10^?4:1.2, MTES-based silica aerogels show the minimum density of 0.046 g/cm³ and the maximum compression ratio of 80 % with 15.09 kPa stress. They are superhydrophobic with a water contact angle of 157° and thermally stable up to 350 °C. We also find that they show the excellent adsorption for ethanol with a ratio of 1400 %.     

Janus nanofiber: a new strategy to achieve simultaneous enhanced magnetic-photoluminescent bifunction

Magnetic-photoluminescent bifunctional Janus nanofibers have been successfully fabricated by electrospinning technology using a homemade parallel spinneret. NaYF₄:Eu³⁺ and Fe₃O₄ nanoparticles (NPs) were respectively incorporated into polyvinyl pyrrolidone (PVP) and eleactrospun into Janus nanofibers with NaYF₄:Eu³⁺/PVP as one strand nanofiber and Fe₃O₄/PVP as another strand nanofiber. The morphologies, structures, magnetic and photoluminescent properties of the as-prepared samples were investigated in detail by X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, energy dispersive spectrometry, vibrating sample magnetometry and fluorescence spectroscopy. The results show Janus nanofibers simultaneously possess superior magnetic and luminescent properties due to their special structure, and the luminescent characteristics and saturation magnetizations of the Janus nanofibers can be tuned by adding various amounts of NaYF₄:Eu³⁺ NPs and Fe₃O₄ NPs. Compared with Fe₃O₄/NaYF₄:Eu³⁺/PVP composite nanofibers, the magnetic-photoluminescent bifunctional Janus nanofibers provide better performances due to isolating NaYF₄:Eu³⁺ NPs from Fe₃O₄ NPs. The novel magnetic-photoluminescent bifunctional Janus nanofibers have potential applications in the fields of new nano-bio-label materials, drug target delivery materials and future nanodevices owing to their excellent magnetic and luminescent performance. More importantly, the design conception and construction technology are of universal significance to fabricate other bifunctional Janus nanofibers.     

Synthesis,structural and optical characterization of ZrO2 core–ZnO@SiO2 shell nanoparticles prepared using co-precipitation method for opto-electronic applications

Nanocrystalline Zirconia (ZrO₂) and Zinc oxide (ZnO) as well as Silica (SiO₂) coated ZrO₂ core–shell structures were synthesized by both Co-precipitation and seeded polymerization technique. The phase analysis and the core–shell structure formation were confirmed by X-ray diffraction (XRD), FESEM and high resolution transmission electron microscopy (HRTEM) analysis. The existence of SiO₂ on ZrO₂@ZnO was characterized by FT-IR measurement. UV–Vis study reveals coating of ZnO over Zirconia shows red shift in the absorption spectra. Photoluminescence studies show the non-monotonous variation in luminescence behavior of these core–shell nanoparticles. This investigation explains that the interfacial effect between the core (ZrO₂) and the shell materials (ZnO and SiO₂) can be exploited to tune the optical properties of the material. This implies that we can envisage the core–shell materials as potential candidates for optical–electronic devices.     

Synthesis of silica-based carbon dot/nanocrystal hybrids toward white LEDs

We report herein the synthesis of silica-based carbon dots (CDs)/nanocrystal (NC) hybrids with white fluorescence and their application in white LEDs. First, silane-functionalized CDs with strong fluorescence (quantum yield = 20 %) were prepared by the hydrothermal method from citric acid and (3-aminopropyl) triethoxysilane (APTES). The optical properties of the CDs were thoroughly investigated, as well as the structural properties. Then silica-based CD/CdS NC hybrids emitting bright white light under UV excitation were realized by co-hydrolyzing the blue fluorescent CDs functionalized by APTES with yellow-emitting CdS NCs modified by (3-mercaptopropyl) trimethoxysilane (MPTMS). The hybrids show many merits, like excellent fluorescence, easy access, low cost, and good compatibility with silicone, suggesting that they could be promising candidates as white phosphors for energy-efficient optoelectronic devices. To realize their potential, a white light-emitting diode (LED) was finally fabricated by the as-prepared CD/CdS NC hybrids as converters, which exhibits white light with a color coordinate of (0.27, 0.32).     

Correlation between in vivo stresses and oxidation of UHMWPE in total hip arthroplasty

The possibility of in vivo, stress-induced oxidation in orthopaedic UHMWPE has been investigated. EtO sterilised, uncrosslinked UHMWPE liners, explanted or shelf-aged, have been collected. Linear wear and wear rate were assessed and FTIR spectroscopy was employed to detect oxidation and to build up oxidation products spatial maps across the liners section. Oxidation profiles have been compared to stress distribution profiles, resulting from a FE analysis conducted on the same liners geometries and couplings. It was found that oxidised and stressed areas followed the same asymmetrical, localized distribution profile. It was therefore possible to establish a correlation between stressed areas and observed oxidation.     

Superhydrophilic titania wall coating in microchannels by in situ sol–gel modification

An in situ reaction sol–gel method to synthesize superhydrophilic titania films in silicon microchannels at room temperature is reported. Superhydrophilic surface can be realized on TiO₂ films with thickness less than 10 nm. The water flow velocity in the TiO₂-coated silicon channels reached almost 4 times of the velocity in SiO₂-coated channels. The ultra-thin superhydrophilic TiO₂ films fabricated by this method show the ability to strongly improve the capillary of microchannels without affecting the morphology of the channel walls, indicating potential applications to biomolecule analysis and surface tension driven microfluidic systems. Due to its low operating temperature, this method is also suitable for polymer microstructures such as PDMS and PMMA microfluidic chips.     

Near-white light luminescent material from Eu(III) complexes encapsulated in silica/PMMA matrices

A novel ternary molecular hybrid material has been obtained by immobilization of Eu-Salen complex into silica matrix and poly (methyl methacrylate) (PMMA) matrix. Here, the Salen-type Schiff-base ligand H₂Salen (N,N′-bis(salicylidene)ethylenediamine) which has been successfully modified by 3-(triethoxysilyl)-propyl isocyanate (TESPIC) has been used as a flexible linker and the antenna. The obtained solid hybrid material shows not only the characteristic red emission of Eu³⁺ but also the blue emission of Salen-Si host arising from the inefficient energy transfer from antenna to Eu³⁺, leading to the unexpected near-white light color of the material. For comparison, the binary hybrid without PMMA has also been prepared. Photoluminescent spectra suggest that the introduction of PMMA can enhance the emission intensity whereas the chromaticity nearly has not been changed. Moreover, thermal analysis has revealed the good thermal stability of the ternary hybrid material. X-ray diffraction patterns demonstrate the amorphous structure of both hybrid materials.     

Nanoscale structuration and optical properties of thin gold films on textured FTO

In this work, we propose a methodology to synthesize metallic nanoparticles on textured Fluorine Tin Oxide (FTO) surface by laser irradiations of deposited Au films. In particular, the breakup of the Au films into nanoparticles (NPs) is observed as a consequence of the melting and solidification processes induced by laser irradiations. The mean Au NPs size and surface density evolution are analyzed as a function of the laser fluence. Optical characterizations of the glass/FTO/Au NPs multilayer show, in the absorption spectra, plasmonic peaks due to the Au NPs and an improvement of the light absorption efficiency from the sample with larger Au NPs. The simulated trends of the ratio between the scattering and absorption cross section suggest that the absorption efficiency dominates over the scattering efficiency in the spectral range between 200 and 600 nm. The simulation shows that, by varying the NPs radius from about 18 to 24 nm, the radiation-scattered intensity remains symmetric in forward and reverse directions. These results indicate that the surface coverage size distribution of Au NPs is the key parameter to correlate the structural and optical properties of the glass/FTO/Au NPs multilayer. Furthermore, electrical characterizations highlight a reduction in the sheet resistance of the textured FTO due to the presence of the NPs. We compare these results with those obtained for the same systems when standard furnace annealing processes are used to obtain the Au NPs on the textured FTO surface.