SiO2 Protective Layer Research Articles

Structural Phase Transition of AgOx Sandwiched Between ZnS–SiO2 Protective Layers Under Thermal and Laser Pulse Annealing for Super-Resolution Near-Field Recording

The as-deposited AgOx prepared at an oxygen flow ratio of 0.5 consisted of Ag2O and AgO phases. During thermal annealing, the reduction of AgO into Ag2O, decomposition of Ag2O into Ag and O2, and out diffusion and aggregation of decomposed Ag would take place successively. The chemical decomposition of AgOx film sandwiched between two ZnS–SiO2 protective layers was confirmed to be an irreversible process. As being irradiated by a high power laser pulse similar to the recording process, a hollow Ag cylinder, or ring, serving as an aperture, was formed in the AgOx mask layer, and small Ag particles, serving as light-scattering centers, were precipitated in the center region. During the readout process, the small aperture can significantly reduce the readout laser spot size, while the strong near-field interaction between precipitated Ag particles and sub-wavelength marks can effectively enhance the readout signal. That elucidates the recording and readout mechanisms of super-resolution near-field structure disk with an AgOx mask layer.

Surface Plasmon‐coupled Polarized Emission of N‐Acetyl‐l‐Trytophanamide¶

AbstractWe report an observation of ultraviolet (UV) surface plasmon‐coupled emission (SPCE) of N‐acetyl‐l‐tryptophanamide (NATA). The sample was spin coated from poly(vinyl alcohol) (PVA) solution on 20 nmaluminium film deposited on a quartz substracte. The directional UV SPCE occurs within a well‐defined narrow angle at 52dG from the normal to the coupling hemicylinder quartz prism. The NATA directional emission is highly p polarized as expected for surface plasmon‐coupled radiation.The 10 nm protective SiO2 layer deposited on top of the aluminum film significantly neutralized the fluorophore quenching by the metal surface. SPCE of NATA demonstratees a remarkable intrinsic dispersive property–the maximum of the emission spectrum depends on the observation angle. The efficient spectral resolution of SPCE can be used in the construction of miniaturized spectrofluorometers. The observation of SPCE of tryptophan opens a new possibility for the study of many unlabeled proteins with the technique complementary to surface plasmon resonance analysis.

Laser-induced damage morphology of high-reflective optical coatings

Two different kinds of 1064nm high-reflective (HR) coatings, with and without SiO2 protective layer, were prepared by electron beam evaporation. Three-dimensional damage morphology, caused by a Nd:YAG pulsed laser, was investigated for these HR coatings. Development of laser-induced damage on HR coatings was revealed by both temperature field calculation and discrete meso-element simulation. Theoretical results met experimental very well.

The Characteristics of Reactively Sputtered AgOx Films Prepared at Different Oxygen Flow Ratios and Its Effect on Super-Resolution Near-Field Properties

The characteristics of several reactively sputtered AgOx films, prepared at different oxygen flow ratios, with and without ZnS-SiO2 protective layers have been examined. For the as-deposited AgOx films, the amount and size of Ag clusters decreased, and the constituent phase of AgOx gradually transferred from pure Ag2O, to a mixture of Ag2O and AgO, then to pure AgO, as the oxygen flow ratio was increased. After annealing, the reduction of AgO into Ag2O and decomposition of Ag2O into Ag and O2 took place, and the decomposed Ag elements would diffuse outward and precipitate small silver particles on the surface of ZnS–SiO2 protective layers. The chemical decomposition of AgOx film confined by ZnS–SiO2 protective layers was confirmed to be an irreversible process. The super-resolution near field effect becomes significant only when the super-resolution near-field structure (super-RENS) disk with an AgOx mask layer prepared at oxygen flow ratios above a threshold value, where AgOx film consists of Ag2O or AgO phase.

Surface Plasmon–coupled Polarized Emission of N-Acetyl-l-Tryptophanamide¶

We report an observation of ultraviolet (UV) surface plasmon-coupled emission (SPCE) of N-acetyl-l-tryptophanamide (NATA). The sample was spin coated from poly(vinyl alcohol) (PVA) solution on 20 nm aluminum film deposited on a quartz substrate. The directional UV SPCE occurs within a well-defined narrow angle at 52 degrees from the normal to the coupling hemicylinder quartz prism. The NATA directional emission is highly p polarized as expected for surface plasmon-coupled radiation. The 10 nm protective SiO2 layer deposited on top of the aluminum film significantly neutralized the fluorophore quenching by the metal surface. SPCE of NATA demonstrates a remarkable intrinsic dispersive property-the maximum of the emission spectrum depends on the observation angle. The efficient spectral resolution of SPCE can be used in the construction of miniaturized spectrofluorometers. The observation of SPCE of tryptophan opens a new possibility for the study of many unlabeled proteins with the technique complementary to surface plasmon resonance analysis.

Recording and readout mechanisms of super-resolution near-field structure disk with a silver oxide mask layer

The chemical decomposition of AgOx sandwiched between two ZnS–SiO2 protective layers was an irreversible process. We confirmed that a hollow Ag cylinder, or ring, serving as an aperture, was formed and small Ag particles were precipitated in the center region during the recording process. The small aperture can significantly reduce the laser spot size during the readout process and the strong near-field interaction between precipitated Ag particles and subwavelength marks can effectively enhance the readout signal. That clarifies both the super-resolution effect and the near-field interaction in the super-resolution near-field structure disk.

High-temperature zinc doping of silicon from zinc-containing SiO2 films

Experimental evidence is presented that high-temperature annealing of Zn-containing SiO2 films on Si gives rise to phase separation, accompanied by the formation of Zn-enriched bands. Appreciable amounts of zinc (sufficient to change the conductivity type of Si and to raise its resistivity) diffuse to only small depths (≤10 μm) and at relatively short diffusion times. Long-term high-temperature annealing reduces the Zn concentration in the film. This process can be inhibited by applying a protective SiO2 layer.

The Synthesis of SiO2@CdS Nanocomposites Using Single‐Molecule Precursors.

AbstractFor Abstract see ChemInform Abstract in Full Text.

The Synthesis of SiO2@CdS Nanocomposites Using Single-Molecule Precursors

A chemical method for preparing cadmium sulfide nanocrystals over silica surfaces is reported. This is a one-step synthetic method involving the chemical degradation of alkyldithiocarbamato complexes of cadmium in the presence of submicrometric SiO2 particles. The X-ray powder diffraction patterns of the SiO2@CdS nanocomposites show diffraction peaks consistent with the presence of hexagonal CdS. Visible spectroscopy and scanning and transmission electron microscopy indicate that an inorganic composite formed by islands of CdS nanocrystals at the SiO2 surfaces. The SiO2@CdS nanocomposites can, in turn, be capped with a protective layer of SiO2 using the Stober method.

Improvement in oxidation resistance of carbon by formation of a protective SiO2 layer on the surface

Solid carbon was coated with silicon dioxide (SiO2) to improve the oxidation resistance at elevated temperatures. SiO2 was deposited on the surface of carbon via gas phase transport. Coating was carried out by exposing the carbon next to a bed of SiC powder in a flowing H2–H2O gas at 1400°C for 1 h. The formation of SiO2 layer was strongly dependent on the water vapor pressure (PH2O) in the gas stream. When the PH2O was around 1.3×10−2 atm, a dense and uniform SiO2 layer was formed on the carbon surface. The layer was crystalline (cristobalite) and slightly cracked apparently due to the thermal expansion mismatch. The oxidation resistance of the carbon was improved markedly by the coating layer. When oxidized in air under the same conditions, the weight loss of the coated specimen was reduced by more than a factor of 5 compared to the uncoated specimen. This improvement in oxidation resistance was attributed to the retardation of oxygen transport through the coating layer.

Editorial

SiNx IBAD coatings were applied to low-alloy steel (0.4 μm thick) and to high-alloy steels AISI 310 and 321 (0.8 μm) at 90 keV energy nitrogen ions. The samples were subjected to a high temperature corrosion test for 2000 h; low-alloy steel in air and highalloy steels in sulphidizing-oxidizing atmosphere at temperatures 500–700°C. In both gaseous environments the application of SiNx increased the corrosion resistance of the tested steels. The protective function of SiNx coating consists of its barrier effect and in some cases it supports the formation of a compact protective SiO2 layer.

Kerr effect enhancement by photon tunneling and possible application to a new scanning probe magnetic microscope

Magneto-optical effects are calculated for the film stack consisting of hemisphere glass/magnetic film (10 nm)/air gap (d nm)/glass plate. Polarized light (wave length=800 nm) is irradiated through the hemisphere glass in the total internal reflection configuration. A typical amorphous rare earth-transition metal alloy is used for the magnetic layer. We find a large monotonic change in the figure of merit (product of the reflected amplitude of light and the Kerr rotation angle) as a function of the air gap, ranging from 1 to 800 nm. Similar results are obtained for a magnetic film with a 10 nm SiO2 protective layer and for a 1-nm-thin magnetic film. This phenomenon is mostly caused by a change in the reflectivity at magnetic film/air interface due to photon tunneling. The difference in the figure of merit between perpendicular and longitudinal magnetization is about 0.6°. These results imply that it might be possible to obtain an image of perpendicular magnetic moment with photon scanning tunnel microscopy (STM). This method can be combined simultaneously with a conventional atomic force microscope or STM.

Phase-Change Optical Disk Having a Nitride Interface Layer

A thin nitride layer formed at the interface of a Ge–Sb–Te recording layer and a ZnS–SiO2 protective layer successfully suppresses the phenomenon that reflectivity or signal amplitude becomes markedly small due to repeated overwrites. Based on secondary ion mass spectrometry (SIMS) observations, the 5-nm-thick interface layer was found to restrain sulfur atoms in the ZnS–SiO2 layer from diffusing into the Ge–Sb–Te layer and from changing the optical characteristics of the layer. Among several nitride materials, germanium nitride (Ge–N) sputtered film is found to have the most suitable properties as an interface layer: high barrier effect and good adhesiveness with Ge–Sb–Te and ZnS–SiO2 layers. The optical disk having the Ge–N interface layer achieves more than 5×105 cycles of overwrites with almost no changes in signal amplitude, reflectivity and jitter based on DVD-RAM specifications. The disk shows no degradation such as cracking, peeling, and corrosion after exposure to accelerated environmental conditions of 90°C and 80% RH for 200 h.

Abnormal Oxidation Behavior of Si<sub>3</sub>N<sub>4</sub>-SiC-Y<sub>2</sub>O<sub>3</sub> Composite at about 1000°C

A composite with the composition of 58 mass% Si3N4-32 mass% SiC-10 mass% Y2O3 was oxidized at 900-1400°C in dry and moist air atmospheres. The weight gain rate at 1400°C in dry air was as low as 0.5mg/cm2 for 100h, revealing that the composite is highly oxidation-resistant. The weight gain in moist air was 3-7 times larger than that in dry air at all the temperatures tested. The weight gain at 1000°C in moist air was almost the same as that at 1400°C in moist air. The mechanism of the abnormal oxidation at 1000°C is not the same but similar to that reported by Lange et al. At 1000°C, a protective SiO2 layer which retards the diffusion of oxygen into the composite was not formed and the SiC fine particles in the grain boundary region of the composite were oxidized. It was assumed that the volume expansion of about 30% accompanying the conversion of SiC into SiO2 generated cracks in the grain boundary region through which oxygen can penetrate toward the unoxidized composite. Four Si3N4-SiC composites with different compositions also showed large weight gain at 1000°C in moist air. These results indicate that special care should be taken in handling Si3N4-SiC composite in moist atmospheres at high temperatures.

Microstructural changes in GeSbTe film during repetitious overwriting in phase-change optical recording

Microstructural changes that occur in a GeSbTe film during repetitious overwriting in phase-change optical recording were investigated. The GeSbTe active layer was melted by a focused laser diode (LD) beam during each overwriting process over amorphous mark formation. The repetitious solidification and liquefaction process in such a short time as 50–200 ns resulted in microstructural changes in the active layer: a segregation, sink, and void formations. The sink was formed in the low-density active layer due to the shrinking of the volume during the resolidification process. Sink formation could be suppressed when a high-density active layer, having more than 80% of the bulk density, was used. Such a high-density GeSbTe film, however, resulted in a void formation of the size of 0.1 μm. The voids were thought to be nucleated by residual vacancies and Ar precipitation, since the active layer contained a high concentration of Ar impurities, due to the atomic peening effect. The subsequent void coalescence and migration processes across the beam scanning direction could result in the formation of thermally discontinuous grooves at the edges of the written marks. The voids could also migrate along the LD beam scanning direction, accompanied by a material flow of the active layer in the opposite direction. These phenomena were also found to depend on the material used to fabricate the protective layers which sandwiched the active layer. A TaOx protective layer enhanced the void migration across the track, resulting in the removal of voids from the center of the track. Use of the ZnS:SiO2 compound protective layer confined voids to the center of the track. The ZnS:SiO2 protective layer also promoted the formation of thermally discontinuous grooves at the edges of amorphous marks. The material flow along the track resulted in a thicker active layer at the start of the consecutive LD irradiation, and also in a high void density region at the final edge of the irradiation having a length on the order of 10 μm. This tendency was found for both the ZnS:SiO2 and TaOx sandwiching media.

Mechanical Behavior and Oxidation Properties of Molybdenum-Modified Cr3Si/Cr5Si3 Intermetallics

AbstractA series of two-phase chromium silicide base intermetallics have been investigated with respect to creep behavior and oxidation resistance. The first results of these investigations are reported. Good oxidation resistance has been demonstrated in one composition, which was due to the formation of the protective vitrious SiO2 layer, while the chromium oxide layer is very helpful at the intermediate temperature range.

Anomalous Oxidation of Intermetallics

AbstractMoSi2, (γ-NiAl and TiAl with Cr additions are of interest for high temperature applications in oxidizing environments, where an oxide layer such as SiO2 or Al2O3 should form to protect the base material. At elevated temperatures (600-1700°C), a protective SiO2 layer forms on MoSi2, while near 500°C pesting and/or accelerated oxidation could disintegrate the material to powder as Mo and Si oxidize to form a complex, thick, non-protective oxide layer. Use of γ -TiAl is limited by poor oxidation resistance, whereby layered mixed oxides of TiO2 and Al2O3 form. With the addition of Cr from 4 to 34 at%, results are varied: protective Al2O3 formation, mixed oxide formation as with TiAl or more rapid oxidation than TiAl. NiAl is currently used as a diffusion coating on Ni-based superalloys and is being considered for use as a structural material itself because of its excellent oxidation resistance, i.e. forming α-alumina above 1000°C. Recent work indicates that pure NiAl oxidized under low oxygen partial pressures in a contained atmosphere develops nonprotective oxide scales similar to accelerated oxidation of MoSi2. This study explores the parameters defining protective behavior of these intermetallics and attempts to describe and explain anomalies at low temperatures and pressures.

Low-temperature oxidation of molybdenum disilicide

Cyclic oxidation rates of 95 to 97 pct dense, powder-source molybdenum disilicide (MoSi2) in dry air, wet air, and oxygen have been measured between 400°C and 600°C. Dense MoSi2 does not disintegrate catastrophically (pest) in these atmospheres for exposure times up to 688 hours. Between 400°C and 500°C, Mo and Si oxidize simultaneously to form amorphous SiO2, monoclinic Mo9O26, and vapor-deposited MoO3 plates, and the oxidation rate of MoSi2 in air is influenced by its microstructure, composition, and surface defects. Rapid oxidation obeying a linear rate law occurs over a narrow temperature range near 500°C, where Mo vapor transport by (MoO2) n species is sufficiently rapid to produce large numbers of surface MoO3 plates but simultaneously is slow enough to allow nucleation and growth of solid Mo oxides in conjunction with SiO2. Addition of water vapor to the oxidant stream at 500°C retards nucleation and growth of solid Mo oxides by formation of MoO3·H2O (g), which has a high vapor pressure relative to those of (MoO3) n species. The transition from nonselective oxidation to high-temperature selective oxidation of Si to form a protective SiO2 layer occurs between 500°C and 550°C. Preoxidation of MoSi2 at 1200°C creates a SiO2 barrier layer which prevents further oxidation upon subsequent exposure at 500°C. The oxidation kinetics and microstructural observations support the model of MoSi2 pest in which oxidation in pores and cracks is required for disintegration. Based on these results, low-temperature oxidation phenomena are not expected to restrict the use of MoSi2 as a high-temperature material.

Microstructure of the Oxidized Surface of Rare-Earth Disilicate - Silicon Nitride Ceramics

Silicon nitride ceramics are being considered for use as structural materials in high-temperature oxidizing environments. Silicon nitride itself is extremely oxidation resistant due to the protective SiO2 layer, which results from exposure to high oxygen partial pressures at elevated temperatures. However, hightemperature exposure of silicon nitride sintered with oxide additives leads to accelerated oxidation attributable to the enhanced oxidation of Si3N4 dissolved in a silicate phase present at the surface. Since diffusion of additive and impurity cations through an amorphous grain-boundary phase from the bulk to the surface oxide scale is rate limiting, obtaining crystalline phases in equilibrium with SiO2 should eliminate the driving force for cation diffusion and improve oxidation resistance. Characterization by electron microscopy of the oxidized surfaces resulting from high-temperature oxidation is necessary to understand and solve these problems.

Comparative Oxidation Studies of Polycrystalline HP and CVD Silicon Carbides by TEM

The excellent oxidation resistance of SiC at high temperature results from formation of a protective SiO2 layer in a strongly oxidizing environment. The oxide layer is often initially amorphous, may transform to a crystalline phase for extended reaction time, especially crystobalite above 1200°C. Our objective is use of high resolution electron microscopy methods to determine the oxide layer microstructure dependence on SiC substrate type, and especially to investigate existence of an intermediate Si-O-C phase between the oxide layer and substrate.