Optical Film for LED with Triangular-Pyramidal Array Using Size-Reducible Embossing Method

This study presents a modified hot-embossing process to fabricate micro-triangular-pyramidal array (MTPA). First, a tungsten (W) steel mold (as the first mold) is manufactured by precision machining including optical projection grinding, lapping, and polishing processes. The dimension of a triangular pyramid with acute angle of 85° on the W-steel mold is about 300 μm in width and 139 μm in height. The pitch between two triangular-pyramidal tips is about 170 μm. Then, only the portion of the tip area of the triangular-pyramidal patterns is transferred on bulk metallic glass (BMG, Mg₅₈Cu₃₁Y₁₁) using this modified multi-step hot-embossing method to reduce the pattern size. With a position-adjustable mechanism, size-reduced concaved-shaped MTPA can be selectively formed, used as the secondary mold. In this way, not only can the size of triangular-pyramidal patterns on W-steel mold be reduced down on BMG, but also the tool arc between each triangular-pyramid on W-steel mold caused by machine tool can be eliminated. This is based on the fact that amorphous glass alloys contain no dislocation that can be responsible for yielding in crystalline materials. Thus, BMG is expected to be strong and hard enough to be used as a mold material. Then the secondary mold is used to emboss convex-shaped MTPA on PolymethylMethacrylate (PMMA) optical film. Experiments with different embossing times and embossing pressures are conducted and discussed. Large-sized triangular-pyramidal array on the W-steel mold has been successfully and selectively miniaturized on BMG, and then transferred on PMMA. Finally, this optical film of PMMA with MTPA is packaged on light-emitting diode (LED) to improve its lighting uniformity and luminance. In comparison with commercial 3M^™ optical film (3M^™ Vikuiti^™ TBEF2-T-65i), the film with MTPA shows a good optical performance.     

Experimental and Morphological Investigations Into Electrical Discharge Surface Grinding (EDSG) of 6061Al/Al2O3p 10% Composite by Composite Tool Electrode

In this study, a special experimental setup of EDSG using EDM and surface grinding machine has been developed in the laboratory to investigate the effect of seven input parameters namely tool polarity, peak current, pulse on-time, pulse off-time, rotational speed, abrasive particle size, and abrasive particle concentration on material removal rate (MRR) as performance measure of the process. The novelty of the present research work is that successful efforts have been made to machine the 6061Al/Al₂O_3p 10% metal matrix composites (MMC) by composite tool itself. The copper-based composite tool electrodes were fabricated by powder metallurgy route with different sizes of abrasives of silicon carbide, while 6061Al/Al₂O_3p 10% MMC were fabricated through stir-casting process. The research outcome will identify the important parameters and their effect on MRR of 6061Al/Al₂O_3p 10% composite in EDSG. The experimental results reveal that tool polarity, peak current, and rotational speed are the most influential parameters that affect MRR in EDSG process. The micro-structural and morphological analysis of machined surfaces has also been carried out to analyze the surface topography. It has been concluded that the abrasive particles substantially improves the MRR after removing the resolidified layer from the machined surface.     

Synthesis and Multiferroic Properties of BFO Ceramics by Melt-Phase Sintering

Multiferroic ceramics (Bi_1.1FeO₃) were synthesized by the conventional powder metallurgy route by adopting the melt-phase sintering followed by rapid thermal quenching technique. Effect of sintering temperature on physical, structural, microstructural, electric, and magnetic properties was studied. X-ray diffraction and scanning electron microscopic studies showed that calcination and sintering promoted the desired perovskite (BiFeO₃) phase and density of the ceramics. Sintering temperature improved the bulk density of the samples as a result of this leakage current density decreased and electric polarization improved. Sample sintered at 850 °C showed bulk density up to 81%. Electric measurements showed spontaneous polarization, remnant polarization, and coercive field of 14.44 μC/cm², 5.47 μC/cm², and 25.50 kV/cm, respectively. Linear behavior of magnetization as a function of applied magnetic field confirms the antiferromagnetic nature of the BiFeO₃ compound at room temperature.     

Porous Y<Subscript>2</Subscript>O<Subscript>3</Subscript> microcubes: synthesis and characterization

In this work, a facile route using a simple solvothermal reaction and sequential heat treatment process to prepare porous Y₂O₃ microcubes is presented. The as-synthesized products were characterized by X-ray powder diffraction (XRD), scanning electronic microscope (SEM), energy dispersive spectrometer (EDS), thermogravimetric analysis (TG), and differential thermal analysis (DTA). The thermal decomposition process of the Y₂O₃ precursor was investigated. SEM results demonstrated that the as-prepared porous Y₂O₃ microcubes were with an average width of about 20 μm and thickness of about 8 μm. It was found that the morphology of the Y₂O₃ precursor could be readily tuned by varying the molar ratio of S₂O₈²⁻ to Y³⁺. Y₂O₃:Eu³⁺ (6.6%) microcubes were also prepared and their photoluminescence properties were investigated.     

Column Formation in Suspension Plasma-Sprayed Coatings and Resultant Thermal Properties

The suspension plasma spray (SPS) process was used to produce coatings from yttria-stabilized zirconia (YSZ) powders with median diameters of 15 μm and 80 nm. The powder-ethanol suspensions made with 15-μm diameter YSZ particles formed coatings with microstructures typical of the air plasma spray (APS) process, while suspensions made with 80-nm diameter YSZ powder yielded a coarse columnar microstructure not observed in APS coatings. To explain the formation mechanisms of these different microstructures, a hypothesis is presented which relates the dependence of YSZ droplet flight paths on droplet diameter to variations in deposition behavior. The thermal conductivity (k _th) of columnar SPS coatings was measured as a function of temperature in the as-sprayed condition and after a 50 h, 1200 °C heat treatment. Coatings produced from suspensions containing 80 nm YSZ particles at powder concentrations of 2, 8, and 11 wt.% exhibited significantly different k _th values. These differences are connected to microstructural variations between the SPS coatings produced by the three suspension formulations. Heat treatment increased the k _th of the coatings generated from suspensions containing 2 and 11 wt.% of 80 nm YSZ powder, but this k _th increase was less than has been observed in APS coatings.     

Highly ordered self-organized TiO2 nanotube arrays prepared by a multi-step anodic oxidation process

Highly ordered TiO₂ nanotube arrays were prepared using a self-templating multi-step anodic oxidation process in a fluoride-containing electrolyte. The microstructures, chemical compositions, and phases of the self-organized TiO₂ nanotube arrays were analyzed by FESEM, XPS, and XRD, respectively. Hexagonal packing density in TiO₂ nanotube arrays significantly improved after the the multi-step anodic oxidation. The area densities of the hexagonal TiO₂ nanotube arrays increased approximately 3 times from the first to second step in the anodic oxidation steps process (4.9 μm⁻² to 16.4 μm⁻²), but there was no difference between the second and third step (16.4 μm⁻² to 16.0 μm⁻²). The as-anodized TiO₂ nanotube array had an amorphous structure and it transformed to an anatase phase during the annealing process at 450 °C for 1 h. The as-anodized TiO₂ nanotube arrays adsorbed the fluoride, hydrocarbon groups (CH), hydroxyl groups (OH, C-OH), and carboxyl groups (O = C-OH) on their surfaces.     

A Taguchi and experimental investigation into the optimal processing conditions for the abrasive jet polishing of SKD61 mold steel

This study introduces an abrasive jet polishing (AJP) technique in which the pneumatic air stream carries not only abrasive particles, but also an additive of either pure water or pure water with a specified quantity of machining oil. Taguchi design experiments are performed to identify the optimal AJP parameters when applied to the polishing of electrical discharge machined SKD61 mold steel specimens. A series of experimental trials are then conducted using the optimal AJP parameters to investigate the respective effects of the additive type and the abrasive particle material and diameter in achieving a mirror-like finish of the polished surface. The Taguchi trials indicate that when polishing is performed using pure water as an additive, the optimal processing parameters are as follows: an abrasive material to additive ratio of 1:2, an impact angle of 30°, a gas pressure of 4 kg/cm², a nozzle-to-workpiece height of 10 mm, a platform rotational velocity of 200 rpm, and a platform travel speed of 150 mm/s. Applying these processing parameters, it is found that the optimal polishing effect is attained using #8000SiC abrasive particles and a 1:1 mixture of water-solvent machining oil and pure water. The experimental results show that under these conditions, the average roughness of the electrical discharge machined SKD61 surface is reduced from an original value of R_a=1.03 μm (R_max: 7.74 μm) to a final value of R_a=0.13 μm (R_max: 0.90 μm), corresponding to a surface roughness improvement of approximately 87%.     

Sensing behavior of silica-coated Au nanoparticles towards nitrobenzene

In the present work, we report silica-stabilized gold nanoparticles (SiO₂/Au NPs) as a wide-range sensitive sensing material towards nitrobenzene (NB). Surface hydroxyl groups of silica selectively form Meisenheimer complex with electron-deficient aromatic ring of NB and facilitate its immobilization and subsequent catalytic reduction by Au cores. Silica-coated Au NPs were synthesized and characterized for their chemical, morphological, structural, and optical properties. SiO₂/Au NPs-modified electrodes were characterized with impedometric and cyclic voltammetric electrochemical techniques. SiO₂/Au NPs are found to have a higher optical detection window of range, 0.1 M to 1 μM and a lower electrochemical detection window of range, 10⁻⁴ to 2.5 × 10⁻² mM with a detection limit of 12.3 ppb. A significant enhancement in cathodic peak current, C ₁, and sensitivity (102 μA/mM) was observed with modified electrode relative to bare and silica-modified electrodes. The I _P was found to be linearly co-related to NB concentration (R ² = 0.985). The interference of cationic and anionic species on sensor sensitivity was also studied. Selectivity in the present sensing system may be further improved by modifying silica with specific functional moieties.     

Dense Si3N4 coatings with high friction coefficient deposited by high-velocity pulsed plasma spraying

A novel electromagnetically accelerated plasma spraying technique was applied to mixtures of αSi₃N₄, and alumina, yttria, and silica additives to deposit thin coatings (50–100 μm) onto mirror-polished stainless steel surfaces. The dense coatings consisted of crystalline αSi₃N₄ with minor amounts of β'SiAlON, traces of βSi₃N₄ and Y₃Al₅O₁₂ as well as a quinary Si−Al−N−O−Y glass. The adhesion strengths depended on the powder particle size showing values of>77 MPa for coarse powders (median grain size 25 μm) and>67 MPa for fine powders (median grain size 8 μm). The average indentation hardnesses were 450 HV_0.025 (coarse powder) and 620 HV_0.025 (fine powder); the sliding wear resistances were comparable to those of sintered Si₃N₄ used as counterbody in a pin-on-disc friction test. The friction coefficient showed surprisingly large values (1.0–1.1 in water and 1.3–1.4 in air), suggesting application of such coatings as tribological high-friction surfaces.     

Preparation and Magnetic Properties of MnFe2O4 Octahedral Microcrystals

MnFe₂O₄ octahedra have been prepared by reaction of Mn²⁺ ions and Fe³⁺ in alkaline condition via heat treatment of the coprecipitation product. The as-prepared powders were characterized in detail by conventional techniques such as powder x-ray diffraction; field emission electron microscopy and transmission electron microscopy. Vibrating sample magnetometer was used to determine the magnetic properties at room temperature. The results show that the MnFe₂O₄ octahedra were single crystals with cubic jacobsite structure and a size distribution from 0.8 to 1.0 μm. The octahedra obtained at 1100 and 1200 °C exhibited a ferromagnetic behavior with the coercive force (H _c) value of 49.03 and 39.23 Oe, saturation magnetization (M _s) value of 42.93 and 47.98 emu/g and remanent magnetization (M _r) value of 2.16 and 2.55 emu/g, respectively. It is indicated that the heat treatment temperature has a significant effect on the formation of the jacobsite structure. Furthermore, a possible mechanism was also proposed to account for the growth of these products.     

The Influence of the Substrate on the Deposition of Cold-Sprayed Titanium: An Experimental and Numerical Study

The deposition of cold-sprayed titanium on various substrates is studied in this work. A rather coarse powder of titanium (−70 + 45 μm) was sprayed under uniform spraying conditions using a cold spray system onto five different substrates: two aluminum-based alloys (AISI 1050-H16 and AISI 2017-T4), copper, stainless steel AISI 304L, and Ti-6Al-4V. All the spraying experiments were carried out using alternatively nitrogen (N₂) or helium (He) as the process gas. Thick coatings were formed on the various substrates, with the exception of the AISI 2017 substrate. When N₂ was used as the process gas, only a few particles remained adhering to the AISI 2017. The thick pre-existing superficial oxide layer on AISI 2017, which was detected by Electron MicroProbe Analysis (EPMA), appeared to prevent adhesion of cold-sprayed titanium particles. The interaction of the sprayed particles with the various substrates was also studied by means of numerical simulations to better understand the adhesion mechanisms. The microstructure and the characteristics of the coatings were investigated. Deposition efficiency and coating density were found both to be strongly improved by spraying helium as the process gas.     

Enhanced Characteristics of HVOF-sprayed MCrAlY Bond Coats for TBC Applications

This study is focused on the variation of the microstructures of different CoNiCrAlY bond coats sprayed by the high-velocity oxy-fuel (HVOF) process for thermal barrier coating (TBC) applications. Three different size fractions of the CoNiCrAlY bond coat powder have been considered for this investigation: AMDRY 9951 (5-37 μm), AMDRY 9954 (11-62 μm), and AMDRY 995C (45-75 μm). The influence of HVOF process parameters and process conditions have been studied in detail to achieve quality bond coats in terms of low porosity level, low oxygen content, and high surface roughness. The results have been promising and have shown that dense bond coats with low porosity can be achieved by HVOF spraying through the appropriate selection of powder size and process parameters. Importantly, HVOF bond coats appear to be competitive to VPS bond coats in terms of its oxygen content and high surface roughness.     

Influence of Plasma Intensity on Wear and Erosion Resistance of Conventional and Nanometric WC-Co Coatings Deposited by APS

The effects of plasma intensity and powder particle size on wear and erosion resistance have been evaluated for WC-12 wt.%Co coatings deposited by Air Plasma Spraying. Coatings were deposited from micrometric and nanostructured powders. SEM and XRD characterization showed the presence of WC, W₂C, W, and an amorphous Co-rich matrix. The performance of the different coatings was compared in sliding wear tests (ball-on-disk), under dry friction conditions. Wear debris and tracks were analyzed by SEM. The debris generated during the test was found to have a great influence on the sliding properties. Wear follows a “three-body abrasive mechanism” and is dominated by coating spallation because of sub-surface cracking. In order to evaluate erosion behavior, solid particle erosion tests were conducted. Eroded coatings were analyzed by SEM, and erosion mainly occurs by a “cracking and chipping mechanism.” The study shows that wear and erosion behavior is strongly affected by plasma arc intensity.     

Influence of processing parameters on the phase composition of ZrN-Si<Subscript>3</Subscript>N<Subscript>4</Subscript> synthesized from zircon

The optimum parameters were determined for synthesizing ZrN-Si₃N₄ composite powder from zircon by carbothermal reduction-nitridation (CTRN) process. The samples were prepared by mixing the carbon black of an average particle size less than 30 μm and the zircon of 40 μm with C/ZrSiO₄ mass ratios of 0.2, 0.3, 0.4, and 0.5. The prepared samples were subjected to the CTRN process at temperatures of 1673, 1723, 1753, and 1773 K for 6, 9, and 12 h. The CTRN process was conducted in an atmosphere-controlled tubular furnace in a nitrogen gas flow of 1.0 L/min. All the products were examined by X-ray powder diffraction to determine the transformation. The results showed that the proper transformation of ZrN-Si₃N₄ occurred at 1773 K for 12 h with a C/ZrSiO₄ mass ratio of 0.4.     

Thermal Expansion and Phase Stability Investigations on Cs-Substituted Nanocrystalline Calcium Hydroxyapatites

The high-temperature phase stability of Ca_10−x Cs _x (PO₄)₆(OH)₂, (x = 0–3) compositions synthesized by various wet chemical methods was investigated. The thermal expansion property of Ca₁₀(PO₄)₆(OH)₂ (abbreviated as CaHAp) and Cs-substituted CaHAp was measured by high-temperature XRD and dilatometry. The average crystallite size of the powders synthesized by wet chemical methods was found to be 10–50 nm range as shown by XRD and TEM. Up to 30 mol% Cs loading was observed to show only the apatite phase by XRD when the apatite powder was nanocrystalline in nature. However, high-temperature stability of the Cs-substituted system is limited to ≤5 mol%. Cs₃(PO₄) is observed to be separated out on heating the material above 773 K for compositions substituted with more than 5 mol% of Cs in the Ca-sublattice. The coefficient of thermal expansion measured by HTXRD is α_a = 12.42 × 10⁻⁶ K⁻¹, α_c = 14.98 × 10⁻⁶ K⁻¹; and α_a = 12.62 × 10⁻⁶ K⁻¹, α_c = 12.57 × 10⁻⁶ K⁻¹ for CaHAp and Ca_9.78Cs_0.2(PO₄)₆(OH)_1.96, respectively, in the temperature range of 298-1083 K. Bulk thermal expansion measurements are seen to be in agreement with the lattice expansion results.     

The Heusler Phase Ti25(Fe50 − x Ni x )Al25 (0 ≤ x ≤ 50); Structure and Constitution

Alloys with composition Ti₂₅(Fe_50 − x Ni _x )Al₂₅ (0 ≤ x ≤ 50) were investigated employing electron probe microanalysis (EPMA) and X-ray powder diffraction (XPD). For TiFe₂Al, in situ neutron powder diffraction (ND) was used for the inspection of phase constitution covering the temperature range from 27 °C (300 K) to 1277 °C (1550 K). Combined Rietveld refinement of ND and XPD data for TiFe₂Al revealed that Fe atoms occupy the 8c site in space group Ti with a small amount of Al sharing the 4a site, and the remaining Ti and Al atoms adopting the 4b site. This structural model was successfully applied in the refinement of all alloys Ti₂₅(Fe_50 − x Ni _x )Al₂₅ (0 ≤ x ≤ 50). Partial atom order exists on the Fe-rich side while complete order is observed for the Ni-rich side. Profiles recorded by in situ neutron powder diffraction for TiFe₂Al in the range of investigated temperatures show two phases, namely Heusler phase and MgZn₂-type Laves phase. Diffraction peaks from the Heusler phase dominate the profiles at lower temperatures but at higher temperatures the MgZn₂-type Laves phase is the main phase. No CsCl-type phase was found in the alloy in the investigated temperature range. The thermal expansion coefficient of TiFe₂Al is 1.4552 × 10⁻⁵ K⁻¹.     

Microstructure and wear studies of laser clad Al-Si/SiC(p) composite coatings

Coatings of a composite material consisting of an Al-Si matrix reinforced with SiC particles were produced by laser cladding on UNS A03560 cast Al-alloy substrates from mixtures of powders of Al-12 wt.% Si alloy and SiC. The influence of the processing parameters on the microstructure and abrasive wear resistance of the coatings was studied. For an interaction time of 0.08 s and a power density of 330 MW/m², corresponding to a specific energy of 26 MJ/m², the interaction between SiC and liquid Al is limited and the reinforcement particles remain essentially undissolved. The coating's microstructure is formed of SiC particles dispersed in a matrix consisting of primary α-Al dendrites and interdendritic α-Al + Si eutectic. For interaction times of 0.3 and 0.45 s and a power density of 193 MW/m², corresponding to specific energies of 58 and 87 MJ/m², SiC reacts with molten Al and partially dissolves. The resulting microstructure consists of undissolved SiC particles, found mainly at the bottom of the clad tracks, where the maximum temperature reached during processing is lower, and Al₄SiC₄ and Si particles dispersed in a matrix of α-Al + Si eutectic. The coatings prepared with higher specific energy (58 MJ/m²) present a hardness of 250 V and an abrasive wear rate in three-body abrasion tests with SiC as abrasive of 1.7 × 10^− 4 mm³/m, while those produced with 26 MJ/m² present a hardness of 120 V and a wear rate of 0.43 × 10^− 4 mm³/m. These results show that Al₄SiC₄ and Si increase the hardness of the material by dispersion hardening but do not contribute to its abrasive wear resistance, because they are softer than the abrasive particles, and confirm that the parameters used to prepare Al-Si-SiC composite coatings by laser cladding must be selected so that only minimal reactions occur between SiC and molten Al.     

Pyrometry in laser surface treatment

The pyrometers specially developed for laser machining were applied to analyse temperature fields in laser cladding. It is shown that 2D temperature mapping is useful to optimise the cladding parameters: zone of powder injection in relation to laser beam, temperature gradients and their evolution versus cladding parameters. Multi-wavelength pyrometer was applied to restore the value of true temperature that is useful to control melting/solidification when complex powder blends are used and when it is necessary to minimise thermal decomposition of certain compounds. The developed multi-wavelength pyrometer and the applied notch filters are appropriate instruments to measure the evolution of surface temperature produced by Nd:YAG laser pulses of millisecond duration. The variations of several characteristics of the thermal cycle, such as the maximum peak temperature, T_max, the instant when melting starts, t_m, the melt lifetime, τ_lt, the duration of the solidification stage, τ_s, with various energy inputs (in the range 10-33 J) and pulse durations (10-20 ms), have been determined for rectangular laser pulses. By appropriate modification of the laser pulse shape (keeping the same energy input and pulse duration), it is possible to realise rather different temperature profiles to vary the melt lifetime and the instant when melting starts. In order to minimise surface temperature variation, to minimise thermal decomposition of certain melt compounds and to increase the melt lifetime, it is necessary to apply higher energy density flux at the beginning of the laser pulse. To obtain a higher peak of the surface temperature, for the given energy input and pulse duration, it is necessary to apply higher energy density flux at the pulse end. This will minimise the melt lifetime as well. In general, it is possible to impose the instant when melt cooling starts and thus to realise an intensive melt cooling during laser irradiation. The above results correspond to the action of laser pulses in the millisecond range with relatively low energy density flux (4 · 10⁸-10⁹ W m^− 2) on metallic materials, whose thickness is larger than the heat affected zone (i.e. semi-infinite body from the heat transfer point of view).     

Improvement of the sinterability of thermally-treated UO2 powder by horizontal rotary ball milling

Horizontal rotary ball milling has been demonstrated to be a useful method for reducing the particle size of ceramic powder in remote operation in shielded hot cells. Techniques, equipment and operating parameters, such as milling media, media wear and rotor speed were investigated with Al₂O₃ powder to evaluate its performance prior to contamination with nuclear fuel material. The established operating parameters were then verified with UO₂ powder, which had been produced by a thermal process to make fuel pellets. The sintering of the milled UO₂ powder showed the higher sintered densities obtainable by the milling, and the milling process seemed to be an important factor in improving the powder characteristics. This article is based on a presentation made in the “Symposium on Nuclear Materials and Fuel 2000“, held at the Korea Atomic Energy Research Institute (KAERI), Taejon, Korea, August 24–25 under the auspices of the Ministry of Science and Technology (MOST).     

Effects of Arc Spray Process Parameters on Corrosion Resistance of Ti Coatings

In order to improve the corrosion resistance of carbon steel structures in marine environment, Ti wires were sprayed to a steel substrate using arc spray technique, and orthogonal experimental design was used to investigate the effects of the fluctuation in the main parameters of spray process on the microstructure and the corrosion resistance of sprayed coatings. The results show that the corrosion resistance of sprayed coatings is very sensitive to spray process parameters, corrosion current density can decrease from 997.7 to 5.08 μA cm⁻² by optimizing process parameters. The coatings are composed of TiN and Ti₂O, and the corrosion resistance of coatings can be improved with the decrease in the contents of oxides. The spray distance should be exactly monitored and controlled in arc spray process because of its great effects on the quality of sprayed coatings.