Optical Diagnostic Techniques Research Articles

Diagnostics for advanced materials processing by plasma spraying

Abstract Advanced coatings deposited by plasma spraying are used in a large variety of industrial applications. The sprayed coatings are employed typically in industry to protect parts from severe operating conditions or to produce surfaces with specific functions. Applications are found in many industrial sectors such as aerospace, automobile, energy generation, and biomedical implants. Coatings are built by the successive deposition of molten or partially molten particles that flatten and solidify upon contact on the substrate, forming lamellae. The coating properties are intimately linked to the properties of these lamellae, which in turn depend on in-flight particle properties as well as substrate temperature during spraying. Consequently, the development of diagnostic tools for monitoring and controlling these spray parameters will help provide the necessary information to study the coating formation process, optimize the coating properties, and, eventually, control the spray process in production. In this paper, a review of some recent developments of optical diagnostic techniques applied to monitor plasma-sprayed particles is presented. In the first part of the paper, two different sensing techniques for in-flight particle measurement are described. First, time-resolved diagnostics on individual particles is described. This technique is used to study the instabilities of the particle characteristics associated with the plasma fluctuations. Secondly, a technique adapted for use in an industrial production environment for measuring the particle jet characteristics as an ensemble is presented. In the second part of the paper, the use of an optical system to study the influence of the substrate temperature on the flattening and solidification of sprayed particles impacting on a flat substrate is described. The last part of this paper describes the optimization of nanostructured coatings based on a tight control of the temperature and velocity of the plasma-sprayed particles.

The Ping-Pong Cannon: A Closer Look

This paper describes the use of laser pulse photography, optical timing, and pulsed Schlieren to look more closely at the dynamics of a popular lecture demonstration—the so-called “Ping-Pong cannon” or “vacuum bazooka.”1,2 These optical diagnostic techniques are applied to two types of cannons and lead to greater knowledge of the kinematics of the accelerating ball, along with some details of the exit mechanism and subsequent target interactions.

Molecular dissociation in N2–H2 microwave discharges

A microwave N2–H2 discharge driven by a travelling surface wave is investigated as a source of ground state N(4S) and H(1s) atoms. Experimental investigations have been carried out in a plasma source operating at 2.45 GHz at low-pressure conditions (p = 0.5–2 Torr). By means of optical emission spectroscopy and probe diagnostic techniques, the population densities of ground state atoms have been detected. The dissociation kinetics is discussed in the framework of a theoretical model based on a self-consistent treatment of the main discharge balances, wave electrodynamics and plasma–wall interactions. Electron–ion surface recombination processes involving and ions are the most important sources of N(4S) gas phase atoms for the conditions considered. The relative number of N(4S) atoms in respect to the total neutral density remains approximately constant for percentages of H2 between 10% and 50% at nearly constant electron density. The competitive interplay of two important source channels of H(1s) atoms, namely electron impact dissociation of H2 and H2 dissociation via the quenching of nitrogen and metastables, determines a smooth decrease of hydrogen dissociation when the amount of hydrogen increases up to 50% in the mixture.

Theoretical analysis of frequency and time-domain methods for optical characterization of absorbing and scattering media

Measurements of the temporal distribution of radiation transmitted and/or reflected by an absorbing and scattering medium, subject to a very short pulse of light, are likely to provide information on radiative properties of the sample. However, this procedure requires precise solutions of the transient radiative transfer problem as well as appropriate nano/pico response time from detection devices. In this work, a different approach is investigated: the transient radiative transfer problem is solved in the space–frequency domain using a discrete ordinates-finite volume method and is shown to provide accurate results. Recent research work has proposed promising optical diagnostic techniques based on a radiation beam for which intensity is modulated. As a result, a preliminary sensitivity analysis is performed to determine some optimal parameters for the design of frequency-modulation based optical diagnostic techniques. The novelty here also arises from the application of the space–frequency method to media that are not necessarily optically thick.

Glow characterization in direct current plasma polymerization of trimethylsilane

AbstractPlasma polymerization of trimethylsilane (TMS) was carried out in a direct current (dc) glow discharge and was investigated by optical photography and plasma diagnostic techniques including optical emission spectroscopy and residual gas analysis. The nature of the glow formed in TMS discharge, which deposited plasma polymers, was significantly different from that of a simple gas such as Ar. In an Ar discharge, the dominant glow was the well known negative glow, which developed at a distinctive distance from the cathode, whereas the cathode surface remained in the dark space. In strong contrast to this situation, in TMS dc discharge the dominant or primary glow was the cathode glow, which appeared at the cathode surface. At a similar location where the Ar negative glow appeared, a very feeble glow as a secondary glow was also observed in TMS glow discharge. The deposition results and plasma diagnosis data evidently indicated that in TMS glow discharge, the cathode glow resulted from the low‐energy electron‐impact dissociation of TMS molecules that creates polymerizable species, but the negative glow was related to nonpolymerizable species such as hydrogen atoms and molecules. In this article, the cathode glow formed in glow discharges of organic compounds was designated as the dissociation glow according to its underlying plasma reactions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1042–1052, 2004

Determination of titanium temperature and density in a magnetron vapor sputtering device assisted by two microwave coaxial excitation systems

We present an optical absorption diagnostic technique devoted to the simultaneous determination of titanium density and temperature during sputtering of Ti. These measurements were performed in a type of ionized physical vapor deposition reactor, consisting of a magnetron sputtering device assisted by two microwave systems for the ionization of the sputtered vapor of the magnetron. Our goal is to optimize the ionization in this reactor in order to improve the deposition process (film quality, recovery of the layers, etc.) compared to standard magnetron sputtering systems. In order to determine both titanium neutral and ion densities, we have used a titanium hollow cathode vapor lamp powered with pulsed power supply. Measurements were carried out at different positions in the reactor at different pressures (1–15 Pa). We have studied the effect of magnetron current from 100 mA to 2 A and of microwave power from 100 W to 1 kW. At lower pressures, we have shown that the titanium is not thermalized close to the magnetron, whereas it is thermalized at 10 Pa at all positions. The neutral titanium density is typically between 1010 and 3×1011 cm−3, and the ion density is ∼109 cm−3. The effect of microwave power is the decrease of neutral titanium density and the increase of its temperature. At a position located 1 cm after the crossing of the microwave plasma area, we showed that the illumination of the microwave plasma increases the degree of ionization of Ti from 2% to 10%.

New ultrahigh vacuum setup and advanced diagnostic techniques for studying a-Si:H film growth by radical beams

ABSTRACTA new ultrahigh vacuum setup is presented which is designed for studying the surface science aspects of a-Si:H film growth using various advanced optical diagnostic techniques. The setup is equipped with plasma and radical sources which produce well-defined radicals beams such that the a-Si:H deposition process can be mimicked. In this paper the initial experiments with respect to deposition of a-Si:H using a hot wire source and etching of a-Si:H by atomic hydrogen are presented. These processes are monitored by real time spectroscopic ellipsometry and the etch yield of Si by atomic hydrogen is quantified to be 0.005±0.002 Si atoms per incoming H atom.

Multi-point time-series observation of optical emissions for flame-front motionanalysis

A simple optical diagnostic technique to analyse flame-front movement isproposed. Time-series, multi-point optical emission measurements using anaberration-free Cassegrain probe coupled with multiple fibres were performedsuccessfully in turbulent premixed flames. The spatial resolution of theCassegrain optics was examined using a CCD camera in free space and also inturbulent flame conditions. Spatial intensity distributions of OH∗, CH∗ and C2∗chemiluminescence measured in a two-dimensional laminar flame were comparedwith the numerical calculation of one-dimensional profiles of these species,demonstrating the ability of the present optical technique to detect a flame front.The probability density of the instantaneous speed and orientation angle offlame-front displacement were determined locally through a geometric motionanalysis of simultaneous, time-evolutional OH* signals at three points. Flow fieldsmeasured by PIV were compared with the flame-front velocities to examinedifferences between gas flow and flame-front dynamics. Errors due to flamecurvature, thermal density change and inclined propagation were alsoestimated.

Optical detection and eradication of dysplastic Barrett's esophagus.

Dysplastic Barrett's esophagus is a condition that offers multiple diagnostic and therapeutic challenges. The diagnosis of dysplasia within Barrett's esophagus currently relies on periodic endoscopic surveillance with multiple biopsies, a methodology limited by random sampling error, inconsistent histopathologic interpretation and delay in diagnosis. Optical spectroscopic and imaging techniques have the potential to identify dysplastic or early neoplastic lesions in real-time. These diagnostic modalities are needed to enhance the endoscopic surveillance of Barrett's esophagus in the future as well as help to define lesions for endoscopic therapy. Esophagectomy has been the standard of care for Barrett's esophagus with high-grade dysplasia although it is a procedure associated with significant morbidity and mortality. Minimally invasive endoscopic ablative therapies are attractive and less morbid alternatives to esophagectomy, with promising results obtained from the use of light-activated drugs (i.e., photodynamic therapy). The combination of novel optical diagnostic techniques and therapies will provide the endoscopist with much needed tools that can considerably enhance the management of patients with Barrett's esophagus. This article reviews the current status and future prospects of optical-based modalities for diagnosis and therapy of dysplastic Barrett's esophagus.

Optical engineering: Diagnostics for industrial applications

Optical engineering uses research and development of laser technology, modern photonic detection/imaging systems and optical metrology for engineering applications. It has produced a wide range of processes and techniques from high-power laser material processing to high-sensitivity metrology and has applications in every industrial sector. Modern optical diagnostic techniques are providing new experimental and in situ data, which hitherto were considered to be unobtainable. Engineers are analysing these data in order to provide immediate design improvements in the performance of components. In addition, they use the data to refine theoretical/computer models of engineering processes, which in turn provide more accurate performance prediction. This paper introduces technology now available to the optical engineer and describes how it is being used to provide optical diagnostic techniques for both solid and fluid mechanics applications in industry. The gas industry has to deal with gas provision safely and efficiently from ‘drill bit to burner tip’ and has benefited significantly from optical engineering. Examples of optical diagnostic techniques and applications, which are used to improve this process, are described.

Laser Doppler imaging of burn scars: a comparison of wavelength and scanning methods

Laser Doppler perfusion imaging (LDI) is a useful tool for the early clinical assessment of burn depth and prognostic evaluation of injuries that may require skin grafting. We have evaluated two commercially available laser Doppler imagers for the perfusion measurement of normal and burn scar tissue. Methods: A single wavelength (635 nm), step-wise scanning LDI and a dual wavelength (633 and 780 nm), continuous scanning LDI were used. Twenty patients with hypertrophic burn scars (time since injury: 1 month–8 years) were recruited and the color and elevation of the scar was clinically assessed using a modified Vancouver Burn Scar Scale. Perfusion of each scar region was measured using both imagers. A symmetric contralateral region of unburned skin was also imaged to record baseline perfusion. Results: Comparisons of wavelength and scanning technique were made using perfusion values obtained from 22 burn scars. Highly significant positive correlation was observed in all comparisons. In addition, output from both instruments was strongly and significantly correlated with the clinical grading of the scar. Significance: Both LDI scanners perform similar perfusion measurements. The results also indicate that red and near-infrared (NIR) wavelength photons provide similar blood flow information. The faster, continuous scanning method provides a clinical advantage without a significant loss of blood flow information. However, a critical evaluation of both instruments suggests that caution must be exercised when using these optical diagnostic techniques and that some knowledge of light–tissue interaction is required for the proper analysis and interpretation of clinical data.

A high-sensitivity in situ optical diagnostic technique for laser cleaning of transparent substrates

A differential optical transmission technique has been used to monitor in situ the efficiency of laser cleaning for the removal of sub-micrometer-sized particles on substrates transparent at the monitoring wavelength. This technique has been applied to the removal of sub-micrometer polystyrene particles on polyimide substrates using laser pulses of 30 ps duration at 292 nm while probing the material transmission at 633 nm. The sensitivity achieved -1/104 for the transmission changes induced upon single-pulse laser exposure – allows us to monitor the removal of just a few sub-micron-sized particles from the probed region inside the irradiated area.

DEVELOPMENT OF MICRO-DIESEL INJECTOR NOZZLES VIA MEMS TECHNOLOGY AND EFFECTS ON SPRAY CHARACTERISTICS

Micro-machined diesel injector nozzles have been designed, fabricated and used with commercially produced diesel injection systems in the study of spray dynamics. Such a system, properly designed, may improve spray behavior in DI diesel engines due to improved atomization and fuel-air mixing. In this work, fourteen micro-planar orifice nozzles were fabricated using the MEMS (Micro-Electro-Mechanical-Systems) technique. Circular orifice diameters were varied from 40 to 260 microns and the number of orifices was varied from one to 169. Three plates with noncircular orifices were also fabricated to examine the effect of orifice shape on spray characteristics. These nozzles were then attached to commercial injectors and the associated injection systems were used for the spray experiments. Given these novel injection systems, jet spray characteristics of micro-planar orifice nozzles were investigated experimentally using optical diagnostic techniques in a pressurized constant volume cylindrical chamber. Local drop sizes were measured by the laser diffraction technique, and average drop sizes of the whole sprays were measured by the light extinction * Corresponding author : Address : 295 Roger Adams Laboratory, Box C-3, 600 South Mathews Avenue, Urbana, IL 61801, USA Tel: +1-217-333-5233 E-mail address: sbaik@scs.uiuc.edu 1 technique. Current test results show expected qualitative trends in spray kinematics and drop sizes, but quantitative magnitudes of the behavior are less dependent on orifice geometry than first anticipated. Droplet coalescence among adjacent sprays were apparent for the multiple orifice nozzles because all the orifices were aligned in the same direction with limited spacing. Non-planar configurations are under development and may show improved performance.

A “Cool” Thermal Tuft for Detecting Surface Flow Direction

An optical flow diagnostic technique was developed to detect surface flow direction in weakly separated flows. The technique makes use of thermochromic liquid crystals applied to a surface that is first coated with flatblack paint. A thin reflective material is affixed to the test surface. The surface is heated uniformly from above with infrared heaters. The reflective surface remains cool relative to the nearby black surface. A thermal tuft of relatively cool air (having just passed over the reflector) is advected in the direction of the surface flow thereby creating a comet tail of liquid crystal color response. The comet tail points in the direction of the surface flow The cool thermal tuft successfully provides a means for detecting surface flow direction. The advantages of this method are the following: it is sensitive enough to detect the presence of weakly separated flows; it is non-obtrusive; it is reversible so does not require clean-up between runs; and it can be applied to vertical surfaces because there is nothing to drip

Use of a blood substitute to determine instantaneous murine right ventricular thickening with optical coherence tomography.

A satisfactory imaging technique to determine regional wall thickening of the murine myocardium is not available. Although cardiovascular imaging with light offers a novel solution, application is problematic because scattering by erythrocytes causes significant optical attenuation. Optical coherence tomography (OCT) is a technique for detailed resolution imaging of highly scattering biological tissues. To reduce the high level of blood scattering, a method was devised whereby murine blood was replaced with a hemoglobin-based blood substitute. The scattering and absorption properties of in vitro preparations of whole blood and dilutions of blood with a blood substitute were determined with a spectrophotometer and an inverse-adding doubling algorithm. OCT imaging of the same dilutions demonstrated a significant reduction in scattering at a hematocrit <5%. A fiber-optic OCT imaging system was used to image the murine right midventricular free wall before and after isovolumic replacement with blood substitute. Strong light attenuation prevented full thickness imaging before replacement, whereas visualization of the full ventricular thickness was possible after replacement. Baseline and imaging hematocrits were 52.4+/-3.8% and 3.7+/-1.2%, respectively. End-systolic and end-diastolic thickness values were 0.458+/-0.051 mm and 0.352+/-0.047 mm. Percent thickening fraction was 30.8+/- 7.5%. Optical imaging of the intact beating murine right ventricle was substantially improved by isovolumic blood replacement with a hemoglobin-based blood substitute. Although the current study has been directed toward imaging the murine heart, a blood substitute may be applied to various optical diagnostic and therapeutic techniques under investigation in cardiovascular medicine.

In situ and real-time ellipsometry diagnostic techniques towards the monitoring of the bonding structure and growth kinetics: silicon oxide coatings

We present a modern methodology concerning the applicability of in situ and real-time optical diagnostic techniques for the study of the optical properties, bonding structure evolution during film growth, in the case of silicon oxide films deposited by e-beam evaporation. Fourier Transform IR Spectroscopic Ellipsometry (SE) was used for in situ and real-time monitoring of the film optical properties during growth. This technique enables the investigation of the films bonding structure and stoichiometry at different growth stages that were associated with the partial pressures of the evaporated species. In addition, films’ optical properties, density and surface and interface quality were studied by multi-wavelength SE in UV–vis region. This methodology reveals the potential of in situ and real-time diagnostic techniques, for the optimization of process control by the preparation of the appropriate transparent oxide coatings for various applications.

Optical Diagnostics of Plasma and Particle in an Atmospheric Pressure Dusty Plasma

The different optical diagnostic techniques for determining dusty plasma parameters are presented. A comparative analysis of different methods for determining the gas-phase temperature and the density of alkali atoms in low-temperature thermal dusty plasma are carried out. The results obtained show that conventional gas-plasma diagnostics are inapplicable to dusty plasmas. Diagnostic methods allowing for the presence of dust grains are proposed and a system for determining the parameters of dusty plasma is developed. A combination of novel methods for determining the temperature, mean size, density and spectral refractive index of particles in high-temperature flows is described. The technique is based on empirical inversion of the forward-scattering transmittance and spectral emission-absorption measure ments. The measurements were successfully inverted for Al2O3, CeO2, and ash particles in plasma flows. The particle temperature, concentration, mean size, and complex refractive index are retrieved.

High-speed visualization of flame propagation in explosions

Flow visualization data is presented to describe the structure of flames propagating in methane-air explosions in semi-confined enclosures. The role of turbulence is well established as a mechanism for increasing burning velocity by fragmenting the flame front and increasing the surface area of flames propagating in explosions. This area increase enhances the burning rate and increases the resultant explosion overpressure. In real situations, such as those found in complex process plant areas offshore, the acceleration of a flame front results from a complex interaction between the moving flame front and the local blockage caused by presence of equipment. It is clear that any localised increase in flame burn rate and overpressure would have important implications for any adjacent plant and equipment and may lead to an escalation process internal to the overall event. To obtain the information required to quantify the role of obstacles, it is necessary to apply a range of sophisticated laser-based, optical diagnostic techniques. This paper describes the application of high-speed, laser-sheet flow visualization and digital imaging to record the temporal development of the flame structure in explosions. Data is presented to describe the interaction of the propagating flame with a range of obstacles for both homogeneous and stratified mixtures. The presented image sequences show the importance of turbulent flow structures in the wake of obstacles for controlling the mixing of a stratified concentration field and the subsequent flame propagation through the wake. The data quantifies the flame speed, shape and area for a range of obstacle shapes.

A probe to estimate the local fuel concentration in spark ignition engines: design and validation study of catalytic hot wire probe

A catalytic hot wire probe (CHWP) technique has been developed to estimate the local fuel concentration near the spark plug of a spark ignition engine. Knowledge of this local concentration is highly useful in studying the combustion process. The small fuel concentration variation is measured by superimposing a catalysis effect in the thermal balance of the hot wire. Various parameters such as pressure, temperature and sampling velocity have been tested for their effect on the hot wire catalytic response. To validate this CHWP technique, local fuel concentration was also measured by two optical diagnostic techniques: planar laser-induced fluorescence (PLIF) and fuel/air ratio laser-induced exciplex fluorescence (FARLIEF). Comparison with PLIF measurements shows good agreement, and the capabilities and limitations of both techniques. With the FARLIEF/CHWP study, we can characterise the time and spatial variation of the fuel/air ratio in the vicinity of the spark plug in the gasoline direct injection engine.

Optical access and diagnostic techniques for internal combustion engine development

Optical access and diagnostic techniques for internal combustion engine development