Measurement of Infrared Spectra of Dense Ceramics by Specular Reflectance Spectroscopy

It is shown that high-quality IR specular reflectance spectra can be obtained from polished surfaces of dense ceramics. The reflectance spectra can be deconvoluted by the Kramers-Kronig transformation to yield absorption spectra or the real and imaginary parts of the dielectric function. IR line shapes and peak wavenumbers are comparable to single-crystal data providing that the particle size is small compared with the IR wavelengths of interest. For noncubic structures, the spectra are a superposition of polarization components. Modes separated by less than a bandwidth are merged into a single band, thus limiting the resolution of the measurement procedure. Grain size effects were modeled using the spectra of single-crystal and polycrystalline quartz as an example.     

氧化铝陶瓷激光泵浦腔的研制

采用纯度较高的白粘土、锻烧高岭土及工业氧化铝为主要原料成功地研制了氧化铝陶瓷激光泵浦腔体.采用KZJ-30型电动抗折仪、DSDB-1型数显白度计、日立340型分光光度计及XRD,SEM等现代测试技术测定了样品的理化性能和微观结构.结果表明:制备的样品的反射率高达99%以上,抗折强度达50～100MPa.     

Application of Thermoluminescence and Reflectance Methods to Study of Lattice Defects in Alumina Ceramics

Alumina ceramics irradiated with gamma rays emit light of varying intensity as they are warmed. The resulting glow curve may be resolved into discrete peaks, each of which corresponds to a separate electron trapping level. The energy depth of each trap may be estimated from the glow-peak temperature and the temperature at which the light intensity reaches half its maximum value. Resolved glow curves are presented for a series of alumina ceramics including specimens having less than 100 p.p.m. of impurities as well as others contaminated with low concentrations of Si, Ti, Fe, Mg, and Ca oxides. Curves are also given for high-purity aluminas which were heated in oxygen and in hydrogen after firing. Where possible, glow peaks are identified with the causative impurity ion or annealing treatment, and estimated trap depths are tabulated. Because of its inverse relation to optical absorption, reflectance is also sensitive to changes in the number and type of lattice defects. Annealing in oxygen and introduction of Ca²⁺ ions produce a significant lowering of the reflectance of irradiated alumina ceramics, whereas exposure to hydrogen has the opposite effect.     

Reactive Laser Ablation Synthesis of Nanosize Alumina Powder

An aluminum (Al) target was laser ablated in an oxygen (O₂) atmosphere, producing nanosize alumina (Al₂O₃) powder. The powder surface area decreased (and the particle size increased) with both increasing oxygen pressure and laser fluence. All powders produced had surface areas between 135 and 250 m²/g, corresponding to primary particle sizes ranging from 7 to 3 nm in radius. Phase evolution with temperature was studied via X-ray diffraction. These powders showed a direct transformation from γ- to α-alumina at approximately 1200°C, bypassing other transition alumina phases, while still maintaining small particle size ( 30 nm). Despite the nanosize particles, green densities equal to 54% of the skeletal density (i.e., true density of the solid phase) were obtained by uniaxial pressing at 40 MPa.     

Constrained Sintering of Alumina Stripes on Rigid Substrates: Effect of Substrate Roughness and Coating

Micromolding in capillaries has been used to fabricate alumina stripes on smooth (R_a = 0.5 nm) and rough substrates (R_a = 900 nm). Different lateral (10–500 μm) and vertical stripe dimensions (8–27 μm) were used to study the influence of substrate roughness (smooth and rough sapphire) and substrate material (platinum coated and plain sapphire) on sintering behavior. Alumina stripes experience edge delamination during sintering on a rigid substrate independent of substrate roughness. However, enhanced substrate roughness reduced delamination length by half and lowered lateral strains by up to 0.10. Grooves in the rough substrate were found to be responsible for this feature as they act as crack propagation barriers and generate local density minima. Accompanying discrete element simulations revealed a localized triaxial stress state at the grooves of a sinusoidal‐shaped substrate interface as the main cause of the density minima. Platinum interlayers also resulted in reduced delamination by 40% in some stripe geometries while density was enhanced by 4% and lateral strains doubled in some geometries. Creep of the metal layer during sintering is thought to be the reason for this seemingly contradictory behavior.     

Enhanced Optical Transmittance by Reduced Reflectance of Curved Polymer Surfaces

Subwavelength nanostructure arrays on surfaces improve their optical transmittance by reducing the reflection of light over a wide range of wavelengths and angles of incidence. A method to imprint a sub‐100 nm nanostructure array on a large surface (Ø 20 mm) made from thermoplastic materials is reported. Transmittance through the flat polymer is improved by ≈6.5%, reaching values of up to 97.5%, after imprinting. The optical properties of the nanostructured samples are highly reproducible. After eight repeated imprinting operations with the same stamp, the transmittance of the nanostructured surface is decreased by less than 0.2%. Moreover, the nanostructures can also be imprinted on curved polymethylmethacrylate surfaces, achieving a maximum transmittance of 97%. This method to prepare large‐scale antireflective nanostructures on flat and flexible curved polymer surfaces is of interest for the production of antireflective screens, optical devices, and biomedical devices such as contact lenses and intraocular lenses.

     

Laser Machining of Structural Alumina: Influence of Moving Laser Beam on the Evolution of Surface Topography

Laser machining technique has emerged as an innovative tool to effectively machine the structural ceramics, which previously was nearly impossible using various conventional machining techniques. However, obtaining a desired surface finish via laser machining is still a critical issue. As many physical phenomena act simultaneously during laser machining, it is very difficult to understand their influence in real time and predict the surface topography. To address this issue, a multiphysics‐based finite‐element modeling approach was implemented to understand the influence of moving laser beam (with lateral overlap) on the generation of corresponding surface topography/profile/roughness during laser machining of structural alumina. A computation model that coupled heat transfer and computational fluid dynamics was designed to understand the combined influence of Marangoni convection, recoil pressure, cooling rates, and surface tension on the evolving surface topography during laser machining of structural alumina under various machining conditions. Both computational and experimental results evidently showed the systematic increase in surface roughness parameters with the increase in lateral overlap (0, 17, 33, 50, 67, and 83%). The results of the computational model are also validated by experimental observations with reasonably close agreement (±3.5%).     

Evaluating the Performance of Detergent Powder Formulations Containing Nano Alumina Using Diffuse Reflectance Infrared Spectroscopy and Pattern Recognition Techniques

An analytical method has been introduced based on diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) coupled with pattern recognition techniques to determine the efficiency of various detergent powder formulations containing nano alumina. In order to reach this aim, different multivariate classification methods such as principal component analysis, hierarchical cluster analysis and linear discriminate analysis (LDA) were utilized for diffuse reflectance spectra to evaluate the classification approach. The best model was predicted by LDA, with a correct classification rate (%CC) of 93.55 %. Furthermore, sensitivity and specificity for the test set were 0.90 and 0.95, respectively. These results confirm the capability of DRIFTS coupled with chemometric techniques for estimating the performance of detergent powder formulations containing nano alumina.     

Processing of Bulk Alumina Ceramics Using Laser Engineered Net Shaping

Application of rapid prototyping (RP) in ceramics manufacturing is motivated by advances in engineering ceramics where attaining complex shapes using traditional processing is difficult. Laser Engineered Net Shaping (LENS^™), a commercial RP process, is used to fabricate dense, net-shaped structures of α-Al₂O₃. Shapes such as cylinder, cube, and gear have been fabricated successfully with 10–25 mm section sizes. As-processed structures show anisotropy in mechanical properties with a high compressive strength normal to the build direction and columnar grains along the build direction. Heat treatment did not alter strength and anisotropy, but increased the grain size from 6 to 200 μm and hardness from 1550 to 1700 Hv.     

Mechanism and Simulation of Removal Rate and Surface Roughness During Optical Polishing of Glasses

Glass optics with ultra‐low roughness surfaces (<2 Å rms) are strongly desired for high‐end optical applications (e.g., lasers, spectroscopy, etc.). The complex microscopic interactions that occur between slurry particles and the glass workpiece during optical polishing ultimately determine the removal rate and resulting surface roughness of the workpiece. In this study, a comprehensive set of 100 mm diameter glass samples (fused silica, phosphate, and borosilicate) were polished using various slurry particle size distributions (PSD), slurry concentrations, and pad treatments. The removal rate and surface roughness of the glasses were characterized using white light interferometry and atomic force microscopy. The material removal mechanism for a given slurry particle is proposed to occur via nano‐plastic deformation (plastic removal) or via chemical reaction (molecular removal) depending on the slurry particle load on the glass surface. Using an expanded Hertzian contact model, called the Ensemble Hertzian Multi‐gap (EHMG) model, a platform has been developed to understand the microscopic interface interactions and to predict trends of the removal rate and surface roughness for a variety of polishing parameters. The EHMG model is based on multiple Hertzian contacts of slurry particles at the workpiece–pad interface in which the pad deflection and the effective interface gap at each pad asperity height are determined. Using this, the interface contact area and each particle's penetration, load, and contact zone are determined which are used to calculate the material removal rate and simulate the surface roughness. Each of the key polishing variables investigated is shown to affect the material removal rate, whose changes are dominated by very different microscopic interactions. Slurry PSD impacts the load per particle distribution and the fraction of particles removing material by plastic removal. The slurry concentration impacts the areal number density of particles and fraction of load on particles versus pad. The pad topography impacts the fraction of pad area making contact with the workpiece. The glass composition predominantly impacts the depth of plastic removal. Also, the results show that the dominant factor controlling surface roughness is the slurry PSD followed by the glass material's removal function and the pad topography. The model compares well with the experimental data over a variety of polishing conditions for both removal rate and roughness and can be extended to provide insights and strategies to develop practical, economic processes for obtaining ultra‐low roughness surfaces while simultaneously maintaining high material removal rates.     

Influence of Co-Doping on the Sintering Path and on the Optical Properties of a Submicronic Alumina Material

Sintering in air was performed on pure and CaO/TiO₂ co-doped slip cast α-alumina samples. The relative density/grain size relationship, referred to as sintering path, has been obtained for each composition. Fully dense samples were obtained after an additional hot isostatic pressing (HIP) step on the pure and doped materials. After grinding and polishing, the optical properties were measured on flat windows (thickness around 1 mm) in the ultraviolet, visible, close-infrared, and mid-infrared ranges. The influence of a tiny residual porosity on the optical properties, impossible to quantify using standard methods, has been investigated.     

Size Resolution of Laser Optical Particle Counters

The size-resolving capabilities of laser optical particle counters have been explored. The ideal size resolution displays a singular behavior (i.e., the leading edge of the pulse height spectrum observed when calibrating with strict monodisperse aerosol increases, whereas the trailing edge falls off infinitely). When a real test aerosol is used the singularity is suppressed; as a consequence, it has become common practice to take the observed regular pulse height spectrum as “the” resolution.

The tendency to judge the alignment of laser optical particle counters from the symmetry of the observed pulse height spectrum results in definite alignment errors, with a serious deterioration in functioning. The ambient Junge size distribution will not be distorted when sizing it by means of a laser optical particle counter. Essentially, the particle size-dependent number concentration will be underestimated up to some 15% for large particles.

When interpreting measured size distributions using a commonly assumed Gaussian resolution, differences of up to some 25% could result, particularly in the large size regime.     

Characterization of Electron Beam Physical Vapor-Deposited Thermal Barrier Coatings Using Diffuse Optical Reflectance

The use of diffuse optical spectral reflectance as a nondestructive tool to characterize the microstructure of electron beam physical vapor deposition (EB-PVD) thermal barrier coatings (TBCs) has been investigated and the contributions of intercolumnar gaps and intracolumnar pores distinguished. It is shown that the reflectance is controlled by the refractive index mismatch and that the optical scattering coefficient depends on the thickness of the TBC due to the porosity distribution through the thickness of the coating. The sensitivity of the reflectance to the porosity suggests that optical reflectance can be used to characterize the microstructure of EB-PVD TBC for both quality control and nondestructive evaluation purposes.     

Preparation and Photoproperties of a Transparent Alumina Film Doped with Energy-Transfer-Type Laser Dye Pair

Doping of an energy-transfer-type laser dye pair into an alumina film was tried in order to increase the efficiency of its photoproperties. A transparent alumina film was prepared from an aqeous sol solution by the sol—gel process. Changes in the fluorescence of the pyranine were monitored during the process from aqueous sol solution to the dried gel. The undesirable dye aggregation usually observed in aqueous solution was largely reduced in the film. The laser emission of rhodamine B by N₂ laser pumping can be increased by the addition of rhodamine 6G, which enhances pumping efficiency. It indicates that this doped film operates as a compact dye laser film.     

反饱和吸收光限幅效应与激光防护

采用表面模板组装途径合成了MCM-41介孔材料,研究了这类载体上键合钛酸 摘要:随着激光技术在军事、民用领域的广泛应用,激光防护技术越来越受到人们的重视。本文介 绍了激光对人眼的危害性,对目前的激光防护技术进行了总结比较,着重论述了基于反饱和吸收 (RSA)非线性光限幅原理、主要材料及其性能。     

Laser Ablation Synthesis and Optical Characterization of Silicon Carbide Nanowires

Silicon carbide (SiC) nanowires were synthesized at 900°C by the laser ablation technique. The growth morphology, microstructure, and defects in SiC nanowires were characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The Raman scattering study indicated that the Raman peaks corresponding to the TO and LO phonon modes of the SiC nanowires had larger red shifts compared to those of bulk SiC material. The red shift, broadening peak, and the asymmetry of the Raman peak could be explained by the size confinement effect in the radial and growth directions. The growth mechanism of SiC nano-wires was discussed based on the vapor–liquid–solid reaction.     

Scanning Tunneling Microscopy of Optical Fiber Corrosion: Surface Roughness Contribution to Zero-Stress Aging

Strength, fatigue resistance, and zero-stress aging behavior control the long-term mechanical reliability of optical fibers. Zero-stress aging refers to the loss of strength of high-strength glass fibers after exposure to some corrosive environments in the absence of stress. Understanding the effect of the chemical environment under zero stress on the subsequent fracture strength of optical fibers is important because optical fibers in service will probably encounter water and other chemical species while exposed to zero- or low-stress conditions. In this work, the strength of fibers aged under zero-stress conditions at 80°C in deionized water has been measured. Scanning tunneling microscopy was also used to measure the roughening of the fibers from corrosion at intervals during the aging. The product of the median inert strength of fibers aged for various times and the square root of the roughness depth of fibers was constant within experimental error. The results show that surface roughening contributes to zero-stress aging in silica fibers.     

Dependence of the Non-Linear Refractive Index on the Optical Parameters of Laser and Optical Glasses for Powerful High-Energy Pulse Radiation Amplifiers

Glass Physics and Chemistry - The influence of the refractive index, dispersion coefficient, and average dispersion on the non-linear refractive index (NRI) of athermal neodymium laser glasses for...     

A Comparison of the Kubelka-Munk Reflectance with the Exact Reflectance for Isotropic Scattering

For a medium that scatters isotropically, the exact reflectance, for a variety of distributions of incident light, is compared with the Kubelka-Munk reflectance. It is shown that, although the Kubelka-Munk reflectance is not identical to the exact reflectance forany incident light, it does have a good correlation with the reflectance produced by a single beam incident at 63d? to the surface normal.