A wireless, passive carbon nanotube-based gas sensor

A gas sensor, comprised of a gas-responsive multiwall carbon nanotube (MWNT)-silicon dioxide (SiO₂) composite layer deposited on a planar inductor-capacitor resonant circuit is presented here for the monitoring of carbon dioxide (CO₂), oxygen (O ₂), and ammonia (NH₃). The absorption of different gases in the MWNT-SiO₂ layer changes the permittivity and conductivity of the material and consequently alters the resonant frequency of the sensor. By tracking the frequency spectrum of the sensor with a loop antenna, humidity, temperature, as well as CO₂ , O₂ and NH₃ concentrations can be determined, enabling applications such as remotely monitoring conditions inside opaque, sealed containers. Experimental results show the sensor response to CO₂ and O₂ is both linear and reversible. Both irreversible and reversible responses are observed in response to NH₃, indicating both physisorption and chemisorption of NH₃ by the carbon nanotubes. A sensor array, comprised of an uncoated, SiO₂ coated, and MWNT-SiO₂ coated sensor, enables CO₂ measurement to be automatically calibrated for operation in a variable humidity and temperature environment     

Pyroelectric PVDF sensor modeling of the temporal voltage response to arbitrarily modulated radiation

Our design of transducer arrays for custom pyroelectric sensors is mainly devoted to IR laser beam characterization and control. It benefits from some of the properties of PVDF film such as low cost, low weight, mechanical flexibility, chemical stability (inert), and compatibility of thick film interconnection technologies on metallized films. By using the temporal characteristics of the source intensity and starting from a standard equivalent one-dimensional model of a multilayer thick-film transducer in the frequency domain, we developed a computer model of the PVDF sensor that determines the temporal response to arbitrarily modulated radiation. The validation of the model accuracy has been carried out with a simulation procedure performed on a PVDF sensor designed for accurate beam alignment of low power laser beams. In this case, an iterative algorithm also was developed to estimate some thermal and physical properties of the front absorbing and the metallization layers that are generally barely known. We present a fitting procedure to determine these properties by using the temporal pyroelectric response to a square wave modulated laser diode that provides a reliable reference signal.     

Two-dimensional ultrasound receive array using an angle-tuned Fabry-Perot polymer film sensor for transducer field characterization and transmission ultrasound imaging

A 2-D optical ultrasound receive array has been investigated. The transduction mechanism is based upon the detection of acoustically induced changes in the optical thickness of a thin polymer film acting as a Fabry-Perot sensing interferometer (FPI). By illuminating the sensor with a large-area laser beam and mechanically scanning a photodiode across the reflected output beam, while using a novel angle-tuned phase bias control system to optimally set the FPI working point, a notional 2-D ultrasound array was synthesized. To demonstrate the concept, 1-D and 2-D ultrasound field distributions produced by planar 3.5-MHz and focused 5-MHz PZT ultrasound transducers were mapped. The system was also evaluated by performing transmission ultrasound imaging of a spatially calibrated target. The "array" aperture, defined by the dimensions of the incident optical field, was elliptical, of dimensions 16 x 12 mm and spatially sampled in steps of 0.1 mm or 0.2 mm. Element sizes, defined by the photodiode aperture, of 0.8, 0.4, and 0.2 mm were variously used for these experiments. Two types of sensor were evaluated. One was a discrete 75-microm-thick polyethylene terephthalate FPI bonded to a polymer backing stub which had a wideband peak noise-equivalent pressure of 6.5 kPa and an acoustic bandwidth 12 MHz. The other was a 40-microm Parylene film FPI which was directly vacuum-deposited onto a glass backing stub and had an NEP of 8 kPa and an acoustic bandwidth of 17.5 MHz. It is considered that this approach offers an alternative to piezoelectric ultrasound arrays for transducer field characterization, transmission medical and industrial ultrasound imaging, biomedical photoacoustic imaging, and ultrasonic nondestructive testing.     

EXPERIMENTAL TECHNIQUES TO CUT AND WELD COPPER BY LASER - A REVIEW

This paper reports on the cutting and welding of copper sheets using a CO₂ laser. For the cutting process, the experimental data from tests on 0.2 to 4.0 mm thick copper sheets with a 2 kW CO₂ laser are described. The behavior of the critical cutting speed V, cutting widths a, the product V × b and the melted volume versus the thickness (b) for a 2 kW CO₂ laser using different focusing lenses has been investigated, The cutting speeds have been found to be significant and micrographic examinations have shown that the laser cutting quality is good. The laser welding of copper sheets is possible by overlapping layers of cupric oxide, CuO, with a small quantity of cuprous oxide, Cu₂O grown under laser beam irradiation. This experimental approach, similar to the one used for the cutting process, allows one to increase the copper surface absorption of the laser radiation. The weld tests done in this way have shown a bad quality of the butt joint; in fact, a number of inclusions in the melted zone and growth of the copper grains surrounding the weld have always been observed. Another experimental technique has also been tested using a better coating to increase surface absorptive power without producing weld defects. The first results are interesting, though the research is still in progress. Finally, by using e pulsed Nd-YAG laser and the same coated samples - cited above - some good welds have been obtained.     

Geometrical factors affecting the CO2 laser chemical vapour deposition of silicon oxide films in parallel configuration

Laser-induced chemical vapour deposition of SiO₂ films in a parallel configuration is a powerful technique for the growth of coatings in some special applications where other conventional low-temperature techniques cannot be applied. A CO₂ laser is more attractive than other lasers for industrial applications since it is less expensive and already widely used in the industry. Growing SiO₂ films is carried out by irradiating a gas mixture composed by SiH₄, N₂O and Ar with a continuous wave CO₂ laser. Energy absorption by the mixture causes a temperature increase in the gas phase which leads to the deposition process. Here we present a study of two important geometrical factors in our experimental set-up: the total flow rate of the reactant gases, and the distance between the laser beam and the substrate surface. Variations in gas flow cause changes in the absorption coefficient of the gas mixture and thus in the gas temperature, which mainly affects the growth rate. The beam-substrate distance influences the gas temperature owing to heat exchange between the gas and the substrate and to the collision rate of the chemical species in their diffusion path towards the substrate surface. Therefore, both the growth rate and the film properties change with this parameter.     

Hardfacing characteristics of S42000 stainless steel powder with added silicon nitride using a CO2 laser

This study examined hardfaced cladding of S42000 stainless steel powder with added silicon nitride Si₃N₄ on a medium carbon steel using a 1200-W CO₂ laser. Experimental results indicated that a well-proportioned cladding layer was obtained with an overlap of 50% at a traverse speed of 5 mm/s and a powder feed rate of 2 g/min. The degree of dilution, depth/width (D/W) ratio and microhardness of the cladding layer was increased by adding up to 5 wt.% silicon nitride. Retained austenite and wear decreased with increasing amounts of silicon nitride addition. The silicon nitride decomposed in the cladding layer during the laser treatment.     

CO2 Laser Cutting of Calcareous Stones

Portugal is one of the major European producers of natural stones. In the last decade, transformation of stones has been privileged in most of the companies and the quantity of finished product for exportation increased with a major added value. New technologies and processes have been investigated. For example, CO₂ laser has been used for cutting, marking, and drilling. The major advantage of this tool is its flexibility, and thus, it improved the working environment significantly. This article presents a report on the use of CO₂ lasers in the cutting process of marbles and limestones. The cut quality was evaluated by adjusting the laser output power and assist gas type and pressure. The CO₂ laser can be used as a feasible tool for cutting ornamental stones. Due to the economic reasons, it is specially adequate for cutting nonlinear shapes where conventional cutting tools, such as the diamond wires and saws, have limitations on both the shape and the dimensions to be cut.     

Dot Matrix Hardening of Steels Using a Fiber Optic Coupled Pulsed Nd:YAG Laser

This paper describes a novel process called “Dot Matrix Hardening” as applied to Ol, D2 and AISI 4340 steels. This process uses a pulsed laser (particularly an Nd:YAG laser) to create a uniform distribution of transformation-hardened spots to cover only a certain percentage of the desired surface. Due to significantly reduced energy input, wear resistance can be imparted to thin and intricate parts without distortion. In addition, with the use of a coupled fiber optic beam delivery system, this process provides greater flexibility compared to conventional CO₂ laser hardening for a number of applications. The use of an Nd:YAG laser also eliminates the need of absorptive coating required for hardening with a CO₂ laser. With optimized processing parameters, a relatively uniform hardened layer is obtained within the hardened spot, with a thickness of about 60 um and hardness values around 800 HV₁₀₀. The sliding wear test results show that the wear resistance of Ol samples with only 20-40% area coverage of laser-hardened spots is similar to the 100% covered laser dot hardened sample as well as the furnace hardened (Re 60) sample.     

Prospects for laser wakefield accelerators and colliders using CO2 laser drivers

Energy directly acquired by an electron from the laser electromagnetic field is quadratically proportional to the laser wavelength. Exploiting this feature, the emerging terawatt picosecond (TWps) CO₂ lasers, having an order of magnitude longer wavelength than the well-known table-top terawatt (T³) picosecond solid state lasers, offer new opportunities for strong-field physics research. Laser accelerators serve as an example where application of the new class of lasers will result in enhancement in gas ionization, plasma wave excitation, and relativistic self-focusing. Ponderomotively strong CO₂ laser permits a 100 times reduction in the plasma density without impeding the acceleration. The improved performance of the low-pressure laser wakefield accelerators (LWFA) is potentially due to higher electric charge per accelerated bunch and better monochromaticity. The multi-kilowatt average power, high repetition rate capability of the TWps-CO₂ laser technology opens new opportunities in development of compact, 1 m long, GeV accelerators and < 1 km long high-luminosity multi-stage LWFA colliders of the TeV scale. The first TWps-CO₂ laser is under construction at the BNL Accelerator Test Facility (ATF).     

Effect of surface laser treatment on the microstructure and wear behaviour of grey iron

The effect of the laser surface treatment on the microstructure and wear behaviour of grey iron was studied. Experiments were performed using a continuous CO₂ laser with a square 10 × 10 mm beam and uniform power density for 2.5 and 5 kW power output and different scan rates. The achieved microstructure and hardness of the different zones of the treated material and its wear behaviour were analyzed. By choosing the adequate working conditions, a wide range of microstructures can be obtained on the material surface layer. The wear test results showed that surface laser hardening treatment causes an improvement of the material wear behaviour.