Sapphire Surface Research Articles

Radiation-Induced Surface Wettability Enhancement of an Indium Tin Oxide and Titanium Oxide Film-Coated Sapphire

Abstract Surface wettability is an important parameter that affects nucleate boiling. Irradiation can alter the surface wettability on metal surfaces without altering the surface macrostructure. The wettability characteristics of indium tin oxide and TiO2 film-coated sapphire substrates following gamma-ray and electron beam irradiation were experimentally investigated. A sapphire plate was exposed to gamma rays and electron beams. Within the irradiation dose, no evident change in the sapphire surface color was found. The surface contact angle decreased after irradiation, and surface wettability was enhanced with more irradiation. After irradiation, the contact angle recovered with time. The related mechanism is possible due to the absorption/desorption of hydroxyl groups. Our results indicate that the irradiation method can be used in indium tin oxide film-coated sapphire experiments to study nucleate boiling.

Heat flux partitioning and macrolayer observation in pool boiling of water on a surface with artificial nucleation sites

In this study, the heat transfer mechanisms in pool saturated boiling of water on a sapphire heat transfer surface with a controlled nucleation site density (NSD) were observed using a high-speed infrared camera. Artificial nucleation sites were arranged on the wall surface by ink-jet printing of a superhydrophobic agent, and NSD was varied between 24 and 318 sites/cm2. The heat transfer characteristics of each fundamental heat transfer process and its contribution to the total wall heat transfer were evaluated through the heat flux partitioning analysis. There was an upper limit to the heat transfer enhancement by increasing the NSD. The boiling heat transfer coefficient (HTC) did not increase monotonically with increasing NSD but had a maximum value at an NSD between 89 and 130 sites/cm2. When NSD was increased excessively, the microlayer formation was inhibited by bubble–bubble interaction. The contribution of the microlayer evaporation to the wall heat transfer also decreased, resulting in a decrease in the boiling HTC. At all tested NSDs, liquid-phase heat transfer dominated the wall heat transfer, as in the case of the bare surface. The contribution of the microlayer evaporation that occupies a small area to the total wall heat transfer was less than 35%. In the high heat flux region, the macrolayer, a liquid layer with suppressed liquid motion, was successfully observed at the bottom of the coalesced bubble.

Molecular-Level Correlation between Spectral Evidence and Interfacial Bonding Formation for Epoxy Adhesives on Solid Substrates.

Interfacial bonding strength of an epoxy-based adhesive depends on the interfacial interaction between the adhesive and the substrate. Normally, the curing process at the interface accompanied by the interfacial bonding formation is different from that in the bulk, and it is still a big challenge to probe the interfacial bonding formation at a molecular level. In this study, to trace the interfacial structural evolution of a representative formula of epoxy (digylcidyl ether of biphenyl A, DGEBA) and amine hardener [1,2-bis(2-aminoethoxy)ethane, EDDA] with the sapphire and silica substrates upon curing and post-curing steps, sum frequency generation (SFG) vibrational spectroscopy is employed to detect the molecular-level interfacial structural information. For the sapphire substrate, upon curing, backbone methylene (CH2) stretching signals decrease, indicating the formation of a rigid chain network structure and thus losing the local methylene order, while vibrational signals of the sapphire surface hydroxyl (OH) groups (including hydrogen-bonded and unbonded) increase significantly, indicating the formation of a strong hydrogen-bonding and polar interaction between the epoxy adhesive and the sapphire surface. Upon post-curing, increased backbone CH2 signals and decreased sapphire OH signals suggest interfacial chemical bonding formation due to the reaction between the epoxy rings and the sapphire surface OH groups. Orientation analysis confirms the enhanced ordering of the sapphire surface OH groups upon curing and post-curing, in comparison to the uncured epoxy formula. As for the fused silica, weak vibrational signals of the methylene (CH2) and methyl (CH3) groups are observed before curing, while both of them increase slightly for the cured and post-cured epoxy formulae, suggesting relatively less hydrophilic nature of the silica surface compared to that of the sapphire surface, also evidenced by the very weak OH signals upon curing and post-curing. Further measurement on the adhesion strength matches up with the above spectroscopic experimental results, substantiating the correlation between the macroscopic bonding strength of the epoxy adhesive and the microscopic molecular-level structure.

Naryn-Gol Creek Sapphire Placer Deposit, Buryatia, Russia

A new gem corundum occurrence has been discovered in the Naryn-Gol Creek placer of the Dzhida volcanic field (Russia). In this placer deposit, sapphire associates with large crystals of garnet, spinel, augite, olivine, enstatite, ilmenite, Ti-magnetite, and alkali feldspar. Such a combination of minerals is typical for the placer deposits associated with alkali basalts widely distributed in Southeastern Asia and Australia. We have also found sapphire crystals in phonotephrites of the nearby Cenozoic alkali-basalt paleovolcano Barun Khobol Pravyi, and in basalt sample and trachybasalt from the valley flood basalts. The chemical composition of sapphire is generally typical for ‘basalt’ corundum: it is rich in Fe, and depleted in Ti and Cr. The δ18O SMOW values of corundum and related megacrysts range from 4.6 to 6.8 ‰, thus corresponding to the isotopic signature of igneous rocks. Etched and corroded surfaces of sapphire and other megacrysts indicate that they are in non-equilibrium with their host alkali basalts. Volatile components, CO2 in particular, played a significant role during sapphire formation as gas inclusions reveal.

The Intrinsic Thermodynamic Difficulty and a Step-Guided Mechanism for the Epitaxial Growth of Uniform Multilayer MoS2 with Controllable Thickness.

Multilayer MoS2 shows superior performance over the monolayer MoS2 for electronic devices while the growth of multilayer MoS2 with controllable and uniform thickness is still very challenging. It is revealed by calculations that monolayer MoS2 domains are thermodynamically much more favorable than multilayer ones on epitaxial substrates due to the competition between surface interactions and edge formation, leading accordingly to a layer-by-layer growth pattern and non-continuously distributed multilayer domains with uncontrollable thickness uniformity. The thermodynamics model also suggests that multilayer MoS2 domains with aligned edges can significantly reduce their free energy and represent a local minimum with very prominent energy advantage on a potential energy surface. However, the nucleation probability of multilayer MoS2 domains with aligned edges is, if not impossible, extremely rare on flat substrates. Herein, a step-guided mechanism for the growth of uniform multilayer MoS2 on an epitaxial substrate is theoretically proposed. The steps with proper height on sapphire surface are able to guide the simultaneous nucleation of multilayer MoS2 with aligned edges and uniform thickness, and promote the continuous growth of multilayer MoS2 films. The proposed mechanism can be reasonably extended to grow multilayer 2D materials with uniform thickness on epitaxial substrates.

Enhancement of the light extraction characteristics and wide-angle emissive behavior of deep-ultraviolet flip-chip light-emitting diodes by using optimized optical films.

We propose the use of optical films to enhance the light extraction efficiency (LEE) and wide-angle emission of traditional packaged deep-ultraviolet light-emitting diodes (DUV-LEDs). Total internal reflection occurs easily in DUV-LEDs because they contain sapphire, which has a high refractive index. DUV-LEDs also contain an aluminum nitride (AlN) ceramic substrate, which has high light absorption in the ultraviolet band. Photons are absorbed by the sapphire and AlN ceramic substrate, which reduces the LEE of DUV-LEDs. By adding a brightness enhancement film (BEF) on the sapphire surface and a high-reflection film (HRF) on the surface of the AlN ceramic substrate, the LEE of DUV-LEDs can be increased. Moreover, we designed a single-layer metal reflective film (SMRF) on the upper surface of the quartz glass in order to achieve wide-angle emission. Experimental results indicated that compared with traditional packaged DUV-LEDs, the light output power and external quantum efficiency of DUV-LEDs with a plated BEF, HRF, and SMRF increased by 18.3% and 18.2%, respectively. Moreover, an emission angle of 160° was achieved. In a reliability test, DUV-LEDs maintained more than 95% of the initial forward voltage and light output power after 1000 h of operation at 25°C, which indicated that the addition of an optical film can improve the light efficiency and long-term reliability of DUV-LEDs.

Spreading of a droplet impacting on a smooth flat surface: How liquid viscosity influences the maximum spreading time and spreading ratio

Existing energy balance models, which estimate maximum droplet spreading, insufficiently capture the droplet spreading from low to high Weber and Reynolds numbers and contact angles. This is mainly due to the simplified definition of the viscous dissipation term and incomplete modeling of the maximum spreading time. In this particular research, droplet impact onto a smooth sapphire surface is studied for seven glycerol concentrations between 0% and 100%, and 294 data points are acquired using high-speed photography. Fluid properties, such as density, surface tension, and viscosity, are also measured. For the first time according to the authors' knowledge, we incorporate the fluid viscosity in the modeling of the maximum spreading time based on the recorded data. We also estimate the characteristic velocity of the viscous dissipation term in the energy balance equation. These viscosity-based characteristic scales help to formulate a more comprehensive maximum droplet spreading model. Thanks to this improvement, our model successfully fits the data available in the literature for various fluids and surfaces compared to the existing models.

Processes of molecular adsorption and ordering enhanced by mechanical stimuli under high contact pressure

Adsorbed molecular films, referred to as boundary films in tribology, are widely used in various industrial products as a keyway for surface functionalisation, such as lubricity, wettability, and adhesion. Because boundary films are thin nanometre-scale molecular layers and can easily be removed, their formation process cannot be elucidated in detail. In this study, to analyse the growth dynamics of boundary films, the film thickness and molecular orientation of the boundary film of a fatty acid used as an additive in rolling contact as mechanical stimuli were measured in situ. The measurements were performed on simple test lubricants, which were composed of n-hexadecane and stearic acid, at rolling tribological condition between steel and glass (or sapphire) surfaces by ultrathin film interferometry combined with sum-frequency generation spectroscopy according to a unique protocol. The results quantitatively demonstrate shear-induced boundary film formation. The insight gained from these results is anticipated to enable the formulation of high-performance lubricant additives to further reduce friction loss and high-performance glues that can be freely designed for removability.

Effects of Different Dispersants on Chemical Reaction and Material Removal in Ultrasonic Assisted Chemical Mechanical Polishing of Sapphire

Effects of different dispersing reagents on ultrasonic assisted chemical mechanical polishing (UV-CMP) of sapphire were investigated in this study. Their influences on chemical reaction and mechanical action between silica particles and sapphire surface were explored by X-ray photoelectron spectroscopy, scanning electron microscope, zeta potential and particle size analyses. The results show that ultrasonic and polyethylene glycol can synergistically promote the chemical reaction and sapphire removal rate. However, sodium polyacrylate and sodium hexametaphosphate will inhibit the chemical reaction. For different concentrations of polyethylene glycol, they affect the chemical reaction and mechanical removal due to the particle aggregation or dispersion in sapphire UV-CMP. When the content is 0.2%, the synergistic effects of chemical and mechanical action between abrasives and sapphire surface are optimal. The sapphire removal rate reaches 48.5 nm min−1 and the polished surface roughness is 0.16 nm.

Study of the Interaction of Potassium Atoms with the Sapphire Surface with the Use of an Ultrathin Spectroscopic Cell

Study of the Interaction of Potassium Atoms with the Sapphire Surface with the Use of an Ultrathin Spectroscopic Cell

Wet-etching mechanism of a semi-sphere pattern on sapphire substrate

A semi-spherical pattern can be converted to a triangular pyramid comprising three r-planes using chemical wet etching with a mixture of sulfuric and phosphoric acids. The apex Al atom on the triangular pyramid with numerous dangling bonds can be subject to etching with prolonged exposure. After removing the apex Al atoms, Al atoms underneath the r-plane can be readily etched and low incline-angle, side-facet planes are developed. The concentrations of SO42- and PO43- ions in the mixed etching solution greatly affect the etching rate on the patterns on the sapphire surface. The SO42- and PO43- etchant ions in the etching solution will react with the etching site (surface Al atoms) and form Al2(SO4)3 and AlPO4. The molar ratios of the reactants (surface Al etching sites and acid molecules) are different. Given the size of the SO42- radicals (the average bond length of the sulfate ion is 1.49 Å), the other surface Al atoms next to the Al atoms engaging with the SO42- radicals cannot be close to any free SO42- radicals in the etching solution. This effect is called the “screening effect” or “spatial barrier effect.” For forming AlPO4, one PO43- ion can bond with one etching Al site without being limited by the spatial screening effect. There is no spatial screening effect for the phosphoric acid etching of a sapphire surface. Thus, phosphoric acid would have a higher etching rate of a sapphire surface than sulfuric acid. In this study, the bottom of the triangular pyramid is not an equilateral triangle. Instead, the sides of the bottom triangular of the triangular pyramid are curved. The availability of etching Al sites on the etching side-facet surface and ridges of the triangular pyramid can explain the curved sides of the bottom triangle of the triangular pyramid.

Cathodoluminescent Analysis of Sapphire Surface Etching Processes in a Medium-Energy Electron Beam.

Sapphire crystals are widely used in optics and optoelectronics. In this regard, it is important to study the stability of crystals under external influence and the possibility of modifying their surfaces by external influence. This work presents the results of studying the processes of the action of an electron beam with an average energy of 70 keV or less under vacuum conditions on the surfaces of sapphire substrates of various orientations. The effect of etching a sapphire surface by an electron beam in vacuum at room temperature was discovered. The highest etching rate was observed for A-plane sapphire (the average pit etching rate was 10−6 µm3/s). It was shown that the rate of etching of a sapphire surface increased many times over when gold is deposited. An in situ method for studying the process of etching a sapphire surface using cathodoluminescence analysis was considered. Possible mechanisms of sapphire etching by a beam of bombarding electrons were considered. The results obtained could be important in solving the problem of the stability of sapphire windows used in various conditions, including outer space. In addition, the proposed method of metal-stimulated etching of a sapphire surface can be widely used in patterned sapphire substrate (PSS) technology and further forming low-dislocation light-emitting structures on them.

Influence of Periodic Non-Uniformities of Well-Structured Sapphire Surface by LIPSS on the Alignment of Nematic Liquid Crystal.

In this study, we report on the alignment properties of nematic liquid crystals on various transparent structured sapphire layers formed by laser-induced periodic surface structures (LIPSS). One-dimensional LIPSS (1D-LSFL) are generated by infrared femtosecond laser pulses along parallel lines covering an area of 5 × 5 mm2, with a line spacing that is varied between 7 and 17 µm. These periodic structures, employed as alignment layers, have a spatial periodicity of about 980 nm, a modulation depth of about 100 nm, and exhibit a high quality due to being characterized by a high degree of homogeneity and parallelism of the structured features. It is found that such alignment layers of the sapphire surface lead to a decreasing azimuthal anchoring energy, when the width of the unstructured gap is increased. Modifying the sapphire surface by an ITO-coating with further deposition of a polyimide film increases the azimuthal anchoring energy by a factor of about four up to Wφ ~ 4.25 × 10−6 J/m2, when the minimum width of the unstructured gap is 7 µm. Comprehensive measurements and comparisons of the azimuthal anchoring energy as well as the pretilt angle for the 1D-LSFL, unstructured gaps, and entire areas depending on the width of unstructured gaps are presented and discussed.

Investigation on the relationship between apparent viscosity of Fe3O4@SiO2 abrasive-based magneto-rheological fluid and material removal rate of sapphire in magneto-rheological polishing

The Fe3O4@SiO2 core-shell abrasive-based magneto-rheological (MR) fluid shows promising prospect for magneto-rheological polishing (MRP) of sapphire, so it is essential to investigate the influence of its rheological properties on polishing performance of sapphire. This paper proposed an original self-assembled apparatus to measure normal force, shear force and interface area between MR ribbon and sapphire wafers during polishing, then evaluated systematically the separation rate and dynamic viscosity of Fe3O4/SiO2 abrasive-based MR fluid under various working parameters. Furthermore, the correlation between apparent dynamic viscosity of MR ribbon and the material removal rate (MRR) of sapphire was investigated. The results showed that both normal force and shear force generated on sapphire surface increase with abrasives concentration and shear velocity, while decrease with excitation distance and working distance. Both apparent dynamic viscosity and separation rate of MR ribbon decrease with working distance. The dynamic viscosity of MR ribbon increases significantly with magnetic field strength, and the larger the fraction of abrasive volume, the higher its increasing slope. The MRR of sapphire wafer increases with dynamic viscosity of MR ribbon, which shows a notably linear correlation, while the Ra value of wafer surface decreases with dynamic viscosity of MR ribbon. The largest MRR (0.323 mg/h) and the lowest Ra (2.21 nm) were achieved when the dynamic viscosity of MR ribbon was 15.33 Pa·s.

Effects of quantities and pole-arrangements of magnets on the magneto-rheological polishing (MRP) performance of sapphire hemisphere

To improve the polishing effectiveness of sapphire hemisphere, the effects of quantities and pole-arrangements of magnets in magnetic field generator on the magneto-rheological polishing (MRP) performance of sapphire hemisphere were investigated through simulation and experiment. After analyzing the distribution of magnetic field on sapphire hemisphere surface and its change with the relative motion between sapphire surface and magnetic field generator during MRP, we developed a material removal rate (MRR) model based on the Preston equation, and the material removal distribution and average removal depth of polished sapphire surface could be predicted. Then according to the calculated material removal distribution, a surface roughness model of polished sapphire was established. Both simulation and experiment results showed that the MRR of polished sapphire increases with the quantity of magnets, while the surface roughness Ra of polished sapphire decreases with the quantity of magnets. As for the effect of pole-arrangement, the cross-stacking arrangement produced a surface of Ra 2.97 nm with MRR of 0.496 μm/h, and the cross-involute arrangement achieved a smoother surface of Ra 0.79 nm with MRR of 0.435 μm/h in MRP. These results showed that the cross-involute and cross-stacking pole-arrangements are favorable for the MRP of sapphire hemisphere. The good agreement between experimental and theoretical results verified the reliability of the established MRR and Ra models.

Biomimetic sapphire windows enabled by inside-out femtosecond laser deep-scribing

Femtosecond laser machining of biomimetic micro/nanostructures with high aspect ratio (larger than 10) on ultrahard materials, such as sapphire, is a challenging task, because the uncontrollable surface damage usually results in poor surface structures, especially for deep scribing. Here, we report an inside-out femtosecond laser deep scribing technology in combination with etching process for fabricating bio-inspired micro/nanostructures with high-aspect-ratio on sapphire. To effectively avoid the uncontrollable damage at the solid/air interface, a sacrificial layer of silicon oxide was employed for surface protection. High-quality microstructures with an aspect ratio as high as 80:1 have been fabricated on sapphire surface. As a proof-of-concept application, we produced a moth-eye inspired antireflective window with sub-wavelength pyramid arrays on sapphire surface, by which broadband (3–5 μm) and high transmittance (98% at 4 μm, the best results reported so far) have been achieved. The sacrificial layer assisted inside-out femtosecond laser deep scribing technology is effective and universal, holding great promise for producing micro/nanostructured optical devices.

Re-deposition of ITER-grade Be on plasma gun facility QSPA-Be: Characterization & plasma cleaning

Studies of contaminants obtained by spraying ITER-grade Be on a quasi-stationary plasma gun facility QSPA-Be are presented. Contaminating films, consisting mainly of Be and O in approximately equal proportions, were deposited on substrates of quartz, sapphire, single crystalline silicon (SC-Si) and NaCl crystal. Characterization of the deposits was performed using SEM, XPS, EBS, AFM, SE, TEM&SAED and micro-interferometry showing polycrystalline BeO films similar as found in JET-ILW. Films on SC-Si and NaCl were used to characterize their composition and morphology. The cleaning rates in the 81.36 MHz RF discharges in He or D2 at 2 Pa were measured on the SC-Si target. The measured etching rate of the deposited films was several times higher than the rate calculated from the theoretical value of beryllium oxide sputtering yield. Test cleaning of these contaminants was carried out in a capacitively coupled RF discharge (CCRF) from the surface of sapphire and quartz plates, which were considered as a mock-ups of the protective window of the first mirror unit (FMU) designed for ITER divertor Thomson scattering diagnostic system (DTS).

Metal-stimulated decomposition of sapphire surface in flux electrons with an energy of 70 keV

Abstract: Metal-stimulated decomposition of sapphire surface at room temperature in a flux of electrons with energy of 70 keV. The stimulant used is an ensemble of crystalline islands of gold. It was first established that the presence of an ensemble of gold islands on the surface of sapphire significantly reduces substrate temperature and electron energy at which decomposition occurs surfaces. Etching pits form near gold islands and have an elongated shape with the following dimensions: length up to 1.2 μm, width up to 0.8 μm and average depth 20 nm. Maximum area the base of the islands increases from 0.6 to 1.8 μm2. Keywords: sapphire, gold, metal-stimulated decomposition, electron flux.

Определение толщины зародышевого слоя AlN, сформированного на поверхности Al-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-(0001) в процессе нитридизации, методами РФЭС и ИК-спектроскопии

The effect of different degrees of the sapphire surface nitridation process completion on the AlN buffer layer morphology has been studied. It was found that ~85% completion of the AlN crystalline phase formation promotes the growth of a two dimensional AlN buffer layer with a smooth surface morphology, regardless of the substrate temperature and ammonia flux. In contrast, during the AlN nucleation layer formation as a result of weak or excessive sapphire nitridation, a polycrystalline or three-dimensional AlN structures with a high density of inversion domains, respectively, were formed. Using independent methods of X-ray photoelectron spectroscopy and infrared spectroscopy of surface polaritons, the thickness of the AlN nucleation layer was determined at ~85% degree of the nitridation process completion, which amounted to ~1 monolayer.

Determination of the AlN nucleation layer thickness formed on the Al-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-(0001) surface during nitridation process by XPS and IR spectroscopy

The effect of different degrees of the sapphire surface nitridation process completion on the AlN buffer layer morphology has been studied. It was found that ~85% completion of the AlN crystalline phase formation promotes the growth of a two dimensional AlN buffer layer with a smooth surface morphology, regardless of the substrate temperature and ammonia flux. In contrast, during the AlN nucleation layer formation as a result of weak or excessive sapphire nitridation, a polycrystalline or three-dimensional AlN structures with a high density of inversion domains, respectively, were formed. Using independent methods of X-ray photoelectron spectroscopy and infrared spectroscopy of surface polaritons, the thickness of the AlN nucleation layer was determined at ~85% degree of the nitridation process completion, which amounted to ~1 monolayer. Keywords: ammonia molecular beam epitaxy, AlN, sapphire, reflection high-energy electron diffraction, nitridation, inversion domains, X-ray photoelectron spectroscopy, surface polaritons.