Methane is a significant contributor to global warming so reducing methane emissions, particularly from oil and gas operations, is among the most cost effective, impactful actions governments can take to achieve climate goals. Preventing methane leakage impacts economic productivity and worker safety too. Large-site leak detection requires reliable cost-effective distributed sensors.
Methane leakage is also an issue for several other industries. However, hard wiring is not practical or cost effective and battery power is unacceptable due to the need for regular changes requiring engineers working in hazardous areas at great expense. The sustainability challenge of additional travel associated with device maintenance and disposal of used batteries in the millions is also environmentally unacceptable. Worker safety monitoring with lower-cost portable methane detectors requires bulky, rechargeable battery-powered devices that the industry is seeking to avoid for operational and environmental reasons. Various low-cost sensor technologies have been applied to methane sensing (catalytic, optical - non-dispersive infrared (NDIR), semiconducting metal oxide and electrochemical) with catalytic/pellistor sensors formerly being dominant but in recent years replaced by NDIR sensors overcoming issues of accuracy, susceptibility to poisoning, short lifetimes, power consumption, recalibration and requirement for oxygen presence. It also has the advantage of being a fail-to-safe technology.
In this work, we present an optical NDIR gas sensor that uses a fast-response semiconductor light source/detector optopair operating at <1 mW power consumption, compatible with powering from photovoltaic based energy harvesting. This is a step change from current state-of-the-art gas sensor technologies and orders of magnitude lower than filament/thermopile based detectors. Fabrication of the sensor is discussed, including; semiconductor mid-IR optopair fabrication, mid-IR optical interference filter deposition and injection molded 2-mirror parabolic reflector optical system preparation. Sensor response to methane is discussed and light harvesting operation is demonstrated, enabling compatibility with wireless distributed methane sensor networks.
Optical, mechanical and erosion protective characteristics of boron and gallium phosphide have been evaluated as single films and within anti-reflection multilayers. These coatings are shown to combine broad-band infra-red transmission with environmental durability, specifically in relation to abrasion resistance and elevated temperature performance up to 500 degree(s)C. Rain erosion protection of all common IR optical materials is demonstrated from single water jet impact and whirling arm tests. Protective characteristics in relation to solid particle impact are described. Productionizing of phosphide coating processes is well advanced in relation to control, scaling and handling of hazardous feedstocks.
Pilkington Optronics (Barr & Stroud), in conjunction with the Defence Research Agency (Malvern, UK), has an ongoing development program for ultradurable coatings. Such coatings enhance environmental durability of infrared (IR) transmissive windows and domes for severe environments, such as those encountered in airborne systems. In particular, these coatings are required to provide protection against high velocity rain and sand impact. This program has to date produced one of the most effective sand and rain protective coatings, based on phosphide materials, specifically boron and gallium phosphide. The phosphide coatings are incorporated into anti-reflective (AR) multilayers, providing high transmission over the required IR waveband. Such AR coatings have been shown to be very effective in protecting windows and dome materials from rain and sand/dust impact damage. Results of single and multiple water jet impact tests, whirling arm rain erosion and sand erosion are presented. Current performance of AR coatings incorporating BP or GaP is presented. Combining BP and GaP in a composite structure, thereby maximizing IR transmission while maintaining sand and rain erosion protection, is described.
Boron phosphide (BP) has outstanding mechanical, optical and thermal properties. It has been prepared as a single crystal and as a coating by various techniques. BP coatings have been shown to combine good broad-band transmission with exceptional durability and erosion protection of all common infrared optical materials. Their protective properties on various substrates have been assessed in detail in whirling arm rain erosion tests and by water jet impact measurements, and the thickness dependences determined. Comparisons have been drawn between BP and other durable coatings. The productionising of BP coating processes is well advanced after overcoming a number of problems, in particular the exceptional hazards presented by the feedstocks.
Pilkington Optronics (Barr and Stroud Ltd) has an ongoing development and pre-production activity for ultra-durable coatings. Such coatings provide enhancement of environmental durability of IR transmissive windows and domes on airborne platforms. This activity places particular emphasis on providing protection against rain and solid particle impact at airborne velocities. This program has produced a very effective rain and san d erosion protective, anti-reflective multilayer, based on boron phosphide overcoated with diamond like carbon (DLC/BP). This coating has been demonstrated on a range of IR materials: germanium, FLIR ZnS, TUFTRAN, silicon and gallium arsenide. This paper describes a pre- production program to coat the external, face of the AV-8B and GR-7 navigational FLIR germanium windows. Results are presented of optical and environmental performance. Moreover, results of extended flight trials are presented which utilize optical scatter, transmission reduction and coating obscuration to compare durability performance of DLC/BP as compared with DLC. These tests show a factor of at least ten increase in window lifetime through use of boron phosphide.
This paper describes optical, durablility and environmental performance of a germanium carbide based durable antireflection coating. The coating has been demonstrated on germanium and zinc selenide infra-red material however is applicable to other materials such as zinc sulphide. The material is deposited using a novel reactive closed field magnetron sputtering technique, offering significant advantages over conventional evaporation processes for germanium carbide such as plasma enhanced chemical vapour deposition. The sputtering process is "cold", making it suitable for use on a wide range of substrates. Moreover, the drum format provide more efficient loading for high throughput production. The use of the closed field and unbalanced magnetrons creates a magnetic confinement that extends the electron mean free path leading to high ion current densities. The combination of high current densities with ion energies in the range ~30eV creates optimum thin film growth conditions. As a result the films are dense, spectrally stable, supersmooth and low stress. Films incorporate low hydrogen content resulting in minimal C-H absorption bands within critical infra-red passbands such as 3 to 5um and 8 to 12um. Tuning of germanium carbide (Ge(1-x)Cx) film refractive index from pure germanium (refractive index 4) to pure germanium carbide (refractive index 1.8) will be demonstrated. Use of film grading to achieve single and dual band anti-reflection performance will be shown. Environmental and durability levels are shown to be suitable for use in harsh external environments.
"Closed field" magnetron (CFM) sputtering offers a flexible and high throughput deposition process for optical coatings and thin films required in display technologies. CFM sputtering uses two or more different metal targets to deposit multilayers comprising a wide range of dielectrics, metals and conductive oxides. Moreover, CFM provides a room temperature deposition process with high ion current density, low bias voltage and reactive oxidation in the entire volume around the rotating substrate drum carrier, thereby producing films over a large surface area at high deposition rate with excellent and reproducible optical properties. Machines based on the Closed Field are scaleable to meet a range of batch and in-line size requirements. Typically, thin film thickness control to < ±1% is accomplished simply using time, although optical monitoring can be used for more demanding applications. Fine layer thickness control and deposition of graded index layers is also assisted with a specially designed rotating shutter mechanism. This paper presents data on optical properties for CFM deposited optical coatings, including anti-reflection, IR blocker and colour control and thermal control filters, graded coatings, narrowband filters as well as conductive transparent oxides such as indium tin oxide. Benefits of the CFM sputter process are described.
Optical, mechanical, and erosion-protective characteristics of boron and gallium phosphide are evaluated as single films and within antireflection multilayers. These coatings are shown to combine broadband infrared transmission with environmental durability, specifically in relation to abrasion resistance and elevated temperature performance up to 500°C. Rain erosion protection of all common IR optical materials is demonstrated from single water jet impact and whirling arm tests. Protective characteristics in relation to solid particle impact are described.
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