A.R. Chambers

Preliminary results of an atomic oxygen spaceflight experiment

An atomic oxygen (AO) experiment designed to measure the AO environment and the erosion rate of selected materials is currently flying on a microsatellite (STRV-la). The experiment comprised four bare silver resistance sensors and eight silver sensors with thin film overlays of polyethylene, PTFE, carbon and silica. Preliminary analysis of the results from the bare silver films shows good agreement between the measured AO environment with that of the theoretical ESABASE environmental estimates. The calculated erosion rate for PTFE is also in good agreement with that obtained by exposing an identical sensor in a laboratory simulation facility. As predicted, after 250 days into the flight, there is no change in the status of polyethylene, carbon and silica overlay sensors.

Reactions of spacecraft materials with fast atomic oxygen

Abstract Silver films with thicknesses in the range 600–3200 A were exposed to high velocity atomic oxygen in a laboratory facility at flux levels simulating those experienced by surfaces in the low Earth orbit. Reaction rates were calculated from the resistance changes as the conducting silver was converted to its non-conducting oxides. A two-stage reaction process was observed, consisting of a rapid linear rate to a depth of approximately 340 A followed by a slower, diffusion limited parabolic rate. The reaction rates of polyethylene, PTFE, carbon and silica were established by exposing sensors comprising thin film overlays of the test material on a 300 A silver substrate to atomic oxygen. The reaction rate of PTFE was found to be higher than found in orbit, whereas the rates for polyethylene, silica and carbon were similar to reported values.

Satellite and rocket-borne atomic oxygen sensor techniques

Neutral atomic oxygen (AO)—the dominant atmospheric species at typical low Earth orbit altitudes—is responsible for the erosion, or other degradation, of many satellite materials. Therefore, AO has become an important consideration for spacecraft designers and manufacturers. The study of AO is also of interest to atmospheric physicists because it is involved in many of the chemical reactions occurring naturally in the mesosphere and lower thermosphere. Both these groups rely on atmospheric models for computer-based simulation and prediction of atomic oxygen concentrations. Such models require, or are enhanced by, empirical input data—that is, actual measurements of AO number densities. A review is presented of the different measurement techniques that, to date, have been used on satellites and sounding rockets to perform AO studies. Rather than reporting results from every sensor application, this article takes a more general view of the experimental methods, using example devices to highlight their advan...

Deformation and strain measurement techniques for the inspection of damage in works of art

Abstract The engineering techniques used for inspecting structural damage are not widely known in the conservation sector. Techniques are available based on deformation or strain measurement that have the ability to provide quantitative data. This paper reviews currently available techniques, covering point-strain measurements using resistance strain gauges and fibre-optic sensors, as well as full-field optical measurement approaches such as holography, electronic speckle pattern interferometry, photoelastic stress analysis and photogrammetry. The underlying technology of each of the techniques is described for the non-specialist. The relevance of each technique is established from a conservation perspective through accounts of usage. The application of the techniques to a wide range of artwork, including panel paintings, statues, murals and mosaics is described and the results critically reviewed. The paper also provides an insight into possible future applications of the techniques and identifies areas for further investigation.

Initial results from ground-based testing of an atomic oxygen sensor designed for use in earth orbit

Many sensors have been applied to the problem of measuring neutral atomic oxygen fluxes in low Earth orbit. The techniques used to date tend to suffer from several key disadvantages, variously: large mass and power budgets, large size, high cost, the ability to make only one measurement and poor time resolution. In this article preliminary results from ground-based testing of a novel atomic oxygen sensor based on a semiconducting metal oxide are reported. Such sensors are simple and relatively cheap while also requiring small power and mass budgets and, most importantly, are reusable. The sensors have been used in laboratory experiments to investigate the axial variation of atomic oxygen flux in a pulsed laser atomic oxygen source; the results compare well with readings taken with a carbon-coated quartz crystal microbalance. A small instrument based on these sensors has been designed and built for application on the UK’s STRV-1c microsatellite.

A low cost microsatellite instrument for the in situ measurement of orbital atomic oxygen effects

A small, low-cost sensor for measuring orbital atomic oxygen fluxes and their corrosive effect on spacecraft materials has been developed. Its operation is based on the reaction between hyperthermal atomic oxygen and a thin silver film. The electrical conductivity of the silver oxides formed is negligible allowing the reaction rate to be monitored through resistance measurements of the film. Atomic oxygen effects on spacecraft coatings can be investigated by placing a sample above the silver film. The construction of the sensor emphasizes simple techniques and components to improve reliability and reduce costs, while its small size and mass will allow it to be used on a wide range of spacecraft, increasing flight opportunities. An instrument comprising 12 of the sensors has been tested in orbit on the Space Technology Research Vehicle (STRV-1a) microsatellite mission. Results from this experiment agree well with those from ground-based simulation facilities. The successful outcome of this demonstration fl...

Surface oxygen and chemical specificity at copper and caesium surfaces

The chemical specificity of surface oxygen at copper and caesium surfaces is shown to determine the reaction pathways for oxidation of methanol, carbon dioxide and ethene. Both photoelectron spectroscopy and temperature-programmed desorption establish that two reaction pathways are possible in methanol oxidation to give either formaldehyde or formate. Both preadsorbed oxygen, at low coverages, and coadsorbed oxygen activate CO2 adsorption on copper to give a surface carbonate at 300 K, whereas ethene carbon–carbon bond cleavage and carbonate formation occur at caesium surfaces at 80 K. The relevance of the data to studies by scanning tunnelling microscopy is discussed.

Moisture uptake in monolithic and composite materials: edge correction for rectanguloid samples

Experiments on moisture uptake of monolithic and composite materials are generally performed by immersing rectanguloid (square plate) samples in water. An edge correction factor is derived which, in a mathematically simple way, takes water uptake through all 6 faces (2 broad and 4 smaller faces) into account. Analysis shows this edge correction factor to be very accurate (deviations typically less than 2%). New expressions for moisture uptake in composites with unidirectionally aligned fibres are derived, by incorporating this edge correction factor as well as proper boundary conditions which depend on volume fraction of fibres. Experimental data on moisture uptake in these types of composite samples is successfully analysed using these expressions.

Thin-film semiconductor sensors for hyperthermal oxygen atoms

In Earth orbit, atomic oxygen (AO) erodes and degrades many spacecraft materials. Several sensors have been employed to make in situ measurements of these oxygen atom fluxes, yet none has proved suitable for general application to microsatellites. We have investigated the potential of thin-film zinc oxide (ZnO) AO sensors. It is known that single crystal and thin-film semiconductor sensors may be used to measure fluxes of low-energy (thermal) oxygen atoms. This work extends the principle to the measurement of hyperthermal (high energy) oxygen atoms in a ground-based simulation facility. It is found that, upon exposure to AO, the conductance of the ZnO sensors decreases. Moreover, the rate of sensor conductance change is proportional to the flux of oxygen atoms. Two sensors, which were manufactured simultaneously, demonstrated very similar responses when exposed concurrently to hyperthermal AO. Further experiments showed that the sensors were not affected by fluxes of molecular oxygen, but were influenced by ultraviolet (UV) radiation. A ZnO film covered with silica — to prevent the action of AO — was used to examine the influence of UV, which was shown to cause a small, permanent change of the conductance of the semiconductor.

Applications of polymer optical fibre grating sensors to condition monitoring of textiles

Fibre Bragg gratings (FBGs) in polymer optical fibres (POFs) have been used to measure the strain in a woven textile. FBGs in both POFs and silica optical fibres were attached to a woven textile specimen, and their performance characterised. It was demonstrated that the POF FBGs provide improved strain transfer coefficients and reduce local structural reinforcement compared to silica FBGs and therefore make a more suitable proposition for textile monitoring.