Aerial additive manufacturing with multiple autonomous robots.

Additive manufacturing methods1-4 using static and mobile robots are being developed for both on-site construction5-8 and off-site prefabrication9,10. Here we introduce a method of additive manufacturing, referred to as aerial additive manufacturing (Aerial-AM), that utilizes a team of aerial robots inspired by natural builders11 such as wasps who use collective building methods12,13. We present a scalable multi-robot three-dimensional (3D) printing and path-planning framework that enables robot tasks and population size to be adapted to variations in print geometry throughout a building mission. The multi-robot manufacturing framework allows for autonomous three-dimensional printing under human supervision, real-time assessment of printed geometry and robot behavioural adaptation. To validate autonomous Aerial-AM based on the framework, we develop BuilDrones for depositing materials during flight and ScanDrones for measuring the print quality, and integrate a generic real-time model-predictive-control scheme with the Aerial-AM robots. In addition, we integrate a dynamically self-aligning delta manipulator with the BuilDrone to further improve the manufacturing accuracy to five millimetres for printing geometry with precise trajectory requirements, and develop four cementitious-polymeric composite mixtures suitable for continuous material deposition. We demonstrate proof-of-concept prints including a cylinder 2.05 metres high consisting of 72 layers of a rapid-curing insulation foam material and a cylinder 0.18 metres high consisting of 28 layers of structural pseudoplastic cementitious material, a light-trail virtual print of a dome-like geometry, and multi-robot simulations. Aerial-AM allows manufacturing in-flight and offers future possibilities for building in unbounded, at-height or hard-to-access locations.

Location of Artinite (Mg2CO3(OH)2·3H2O) within the MgO-CO2-H2O system using ab initio thermodynamics.

The MgO-CO2-H2O system have a variety of important industrial applications including in catalysis, immobilisation of radionuclides and heavy metals, construction, and mineralisation and permanent storage of anthropogenic CO2. Here, we develop a computational approach to generate phase stability plots for the MgO-CO2-H2O system that do not rely on traditional experimental corrections for the solid phases. We compare the predictions made by several dispersion-corrected density-functional theory schemes, and we include the temperature-dependent Gibbs free energy through the quasi-harmonic approximation. We locate the Artinite phase (Mg2CO3(OH)2·3H2O) within the MgO-CO2-H2O phase stability plot, and we demonstrate that this widely-overlooked hydrated and carbonated phase is metastable and can be stabilised by inhibiting the formation of fully-carbonated stable phases. Similar considerations may apply more broadly to other lesser known phases. These findings provide new insight to explain conflicting results from experimental studies, and demonstrate how this phase can potentially be stabilised by optimising the synthesis conditions.

Nanostructured Pd hydride microelectrodes: in situ monitoring of pH variations in a porous medium.

In this study we report the exceptional potentiometric properties of pH microprobes made with nanostructured palladium hydride microelectrodes and demonstrate their application by monitoring pH variations resulting from a reaction confined in a porous medium. Their potentiometric response was found to be reproducible and stable over several hours but primarily Nernstian over a remarkably wide pH range, including alkaline conditions up to pH 14. Continuous operation was demonstrated by reloading hydrogen at regular intervals to maintain the correct hydride composition thereby alleviating the need for calibration. These properties were validated by detecting pH transients during the carbonation of Ca(OH)2 within a fibrous mesh. Experimental pHs recorded in situ were in excellent agreement with theoretical calculations for the CO2 partial pressures considered. Results also showed that the electrodes were sufficiently sensitive to differentiate between the formation of vaterite and calcite, two polymorphs of CaCO3. These nanostructured microelectrodes are uniquely suited to the determination of pH in highly alkaline solutions, particularly those arising from interfacial reactions at solid and porous surfaces and are thus highly appropriate as pH sensing tips in scanning electrochemical microscopy.

Scanning electrochemical microscopy: using the potentiometric mode of SECM to study the mixed potential arising from two independent redox processes.

This study demonstrates how the potentiometric mode of the scanning electrochemical microscope (SECM) can be used to sensitively probe and alter the mixed potential due to two independent redox processes provided that the transport of one of the species involved is controlled by diffusion. This is illustrated with the discharge of hydrogen from nanostructured Pd hydride films deposited on the SECM tip. In deareated buffered solutions the open circuit potential of the PdH in equilibrium between its ß and α phases (OCP(ß→α)) does not depend on the tip-substrate distance while in aerated conditions it is found to be controlled by hindered diffusion of oxygen. Chronopotentiometric and amperometric measurements at several tip-substrate distances reveal how the flux of oxygen toward the Pd hydride film determines its potential. Linear sweep voltammetry shows that the polarization resistance increases when the tip approaches an inert substrate. The SECM methodology also demonstrates how dissolved oxygen affects the rate of hydrogen extraction from the Pd lattice. Over a wide potential window, the highly reactive nanostructure promotes the reduction of oxygen which rapidly discharges hydrogen from the PdH. The flux of oxygen toward the tip can be adjusted via hindered diffusion. Approaching the substrate decreases the flux of oxygen, lengthens the hydrogen discharge, and shifts OCP(ß→α) negatively. The results are consistent with a mixed potential due to the rate of oxygen reduction balancing that of the hydride oxidation. The methodology is generic and applicable to other mixed potential processes in corrosion or catalysis.

Nanophase-photocatalysis: loading, storing, and release of H2O2 using graphitic carbon nitride.

A blue light mediated photochemical process using solid graphitic carbon nitride (g-C3N4) in ambient air/isopropanol vapour is suggested to be linked to "nanophase" water inclusions and is shown to produce approx. 50 µmol H2O2 per gram of g-C3N4, which can be stored in the solid g-C3N4 for later release for applications, for example, in disinfection or anti-bacterial surfaces.

Sensing of Damage and Repair of Cement Mortar Using Electromechanical Impedance.

Lead zirconium titanate (PZT) has recently emerged as a low-cost material for non-destructive monitoring for civil structures. Despite the numerous studies employing PZT transducers for structural health monitoring, no studies have assessed the effects of both damage and repair on the electromechanical impedance response in cementitious materials. To this end, this study was conducted to assess the effects of the damage and repair of mortar samples on the electromechanical response of a surface-mounted PZT transducer. When damage was introduced to the specimen in stages, the resonance frequencies of the admittance signature were shifted to lower frequencies as the damage increased, and an increase in the peak amplitude was detected, indicating an increase in the damping and a reduction in the material stiffness properties. Also, increasing the damage in the material has been shown to decrease the sensitivity of the PZT to further damage. During the repair process, a noticeable difference between the after-damage and the after-repair admittance signatures was noted. The root-mean-square deviation (RMSD) showed a decreasing trend during the repair process, when compared to the before repair RMSD response which indicated a partial recovery for the material properties by decreasing the damping property in the material.

Effectiveness of Nanolime as a Stone Consolidant: A 4-Year Study of Six Common UK Limestones.

Protecting stone buildings from weathering and decay is a major challenge in the conservation of built heritage. Most of the stone consolidants currently available are well suited to silicate stones, but are less compatible with limestone. In this paper we present for the first time the results over a 4-year period of various consolidation treatments carried out using nanolime on 6 of the most representative and significant stones used in historic buildings in the United Kingdom. Tests investigated the influence of stone type, environmental conditions and pre-treatments on the effectiveness of the consolidation treatment. A comprehensive and rigorous testing programme was carried out to evaluate the short (12 weeks) and longer-term (4 years) effects. Stone samples were characterised before and after treatment using light and electron microscopy, sorptivity tests and a novel methodology employing drilling resistance to interrogate the near surface effects. Results show that for some of the stones, such as Clunch and Bath Stone, the positive effect of the treatment with nanolime is noticeable after 4 years since application. However, results for other stones such as Portland and magnesian limestone showed that the initial beneficial effect of the treatment is reduced after 4 years. Nanolime treatment of Ham Stone produced an unnoticeable effect on the continuous natural reduction of the drilling resistance of the specimen over time. The results presented are of immense value to conservators as they provide essential guidance on the most appropriate repair approach. Impact to the conservation industry will be to avoid the use of nanolime on stones where there is no perceivable benefit, reducing the risk of adverse effects, including potential damage to buildings. In additional costs will be saved which might otherwise have been spent on ineffective treatments.

Solid-Gas Phase Photo-Catalytic Behaviour of Rutile and TiOn (1 < n < 2) Sub-Oxide Phases for Self-Cleaning Applications.

The solid-gas phase photo-catalytic activities of rutile TiO2 and TiOn (1 < n < 2) sub-oxide phases have been evaluated. Varying concentrations of Ti3+ defects were introduced into the rutile polymorph of titanium dioxide through carbo-thermal reduction at temperatures ranging from 350 °C to 1300 °C. The resulting sub-oxides formed were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, impedance spectroscopy and UV-visible diffuse reflectance spectroscopy. The presence of Ti3+ in rutile exposed to high reduction temperatures was confirmed by X-ray diffraction. In addition, a Ti3+-Ti4+ system was demonstrated to enhance the photo-catalytic properties of rutile for the degradation of the air pollutants NO2 and CO2 under UV irradiation of wavelengths (λ) 376⁻387 nm and 381⁻392 nm. The optimum reduction temperature for photo-catalytic activity was within the range 350⁻400 °C and attributed to improved charge-separation. The materials that were subject to carbo-thermal reduction at temperatures of 350 °C and 400 °C exhibited electrical conductivities over one hundred times higher compared to the non-reduced rutile. The results highlight that sub-oxide phases form an important alternative approach to doping with other elements to improve the photo-catalytic performance of TiO2. Such materials are important for applications such as self-cleaning where particles can be incorporated into surface coatings.

Photosynthetic water oxidation: the role of tyrosine radicals.

This mini-review outlines the involvement of the tyrosine electron carriers, Y(D) and Y(Z), in the mechanism of electron transfer from water to P680. We discuss our data and put forward our ideas on the role of Y(D) and Y(Z).

Ammonia displaces methanol bound to the water oxidizing complex of photosystem II in the S2 state.

Ammonia and methanol both bind to the water oxidising complex of photosystem II during its turnover, possibly at sites where water binds during the normal water oxidation process. We have investigated the interaction between these two water analogues at the S2 state of the water oxidising cycle using electron magnetic resonance techniques. We find evidence that ammonia displaces methanol from its binding site.

Study of solid/gas phase photocatalytic reactions by electron ionization mass spectrometry.

This paper describes a novel methodology for the real-time study of solid-gas phase photocatalytic reactions in situ. A novel reaction chamber has been designed and developed to facilitate the investigation of photoactive materials under different gas compositions. UV irradiation in the wavelength of ranges 376-387 and 381-392 nm was provided using specially designed high efficiency light emitting diode arrays. The experiments used air containing 190 ppm NO2 in a moist environment with a relative humidity of 0.1%. Photocatalytic samples consisting of pressed pellets of rutile and anatase crystalline forms of TiO2 were monitored over a period of 150 min. An ultra-high vacuum right angled bleed valve allowed a controlled flow of gas from the main reaction chamber at atmospheric pressure to a residual gas analyser operating at a vacuum of 10(-5) mbar. The apparatus and methodology have been demonstrated to provide high sensitivity (ppb). The rate of degradation of NO2 attributed to reaction at the TiO2 surface was sensitive to both crystal structures (anatase or rutile) and wavelength of irradiation.

Ocean warming, more than acidification, reduces shell strength in a commercial shellfish species during food limitation.

Ocean surface pH levels are predicted to fall by 0.3-0.4 pH units by the end of the century and are likely to coincide with an increase in sea surface temperature of 2-4 °C. The combined effect of ocean acidification and warming on the functional properties of bivalve shells is largely unknown and of growing concern as the shell provides protection from mechanical and environmental challenges. We examined the effects of near-future pH (ambient pH -0.4 pH units) and warming (ambient temperature +4 °C) on the shells of the commercially important bivalve, Mytilus edulis when fed for a limited period (4-6 h day(-1)). After six months exposure, warming, but not acidification, significantly reduced shell strength determined as reductions in the maximum load endured by the shells. However, acidification resulted in a reduction in shell flex before failure. Reductions in shell strength with warming could not be explained by alterations in morphology, or shell composition but were accompanied by reductions in shell surface area, and by a fall in whole-body condition index. It appears that warming has an indirect effect on shell strength by re-allocating energy from shell formation to support temperature-related increases in maintenance costs, especially as food supply was limited and the mussels were probably relying on internal energy reserves. The maintenance of shell strength despite seawater acidification suggests that biomineralisation processes are unaffected by the associated changes in CaCO3 saturation levels. We conclude that under near-future climate change conditions, ocean warming will pose a greater risk to shell integrity in M. edulis than ocean acidification when food availability is limited.

Characterisation and use of biomass fly ash in cement-based materials.

This paper presents results about the characterisation of the biomass fly ashes sourced from a thermal power plant and from a co-generation power plant located in Portugal, and the study of new cement formulations incorporated with the biomass fly ashes. The study includes a comparative analysis of the phase formation, setting and mechanical behaviour of the new cement-fly ash formulations based on these biomass fly ashes. Techniques such as X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), thermal gravimetric and differential thermal analysis (TG/DTA), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and environmental scanning electron spectroscopy (ESEM) were used to determine the structure and composition of the formulations. Fly ash F1 from the thermal power plant contained levels of SiO(2), Al(2)O(3) and Fe(2)O(3) indicating the possibility of exhibiting pozzolanic properties. Fly ash F2 from the co-generation plant contained a higher quantity of CaO ( approximately 25%). The fly ashes are similar to class C fly ashes according to EN 450 on the basis of chemical composition. The hydration rate and phase formation are greatly dependant on the samples' alkali content and water to binder (w/b) ratio. In cement based mortar with 10% fly ash the basic strength was maintained, however, when 20% fly ash was added the mechanical strength was around 75% of the reference cement mortar. The fly ashes contained significant levels of chloride and sulphate and it is suggested that the performance of fly ash-cement binders could be improved by the removal or control of these chemical species.

ESEEM studies of substrate water and small alcohol binding to the oxygen-evolving complex of photosystem II during functional turnover.

We report the first examination of exchangeable proton and MeOH interactions with the Mn catalytic cluster in photosystem II, under functional flash turnover conditions, using 2H ESEEM spectroscopy on the S2 and S0 multiline states. Deuterium-labeled water (D2O) and methyl d3-labeled methanol (DMeOH) are employed. It was discovered that a hyperfine resolved multiline S0 signal could be seen in the presence of D2O, the hyperfine structure of which depended on the presence or absence of methanol (MeOH). In the presence of DMeOH, significant dipolar coupling of the three methyl deuterons to the multiline centers in the S2 and S0 states was seen (S2, 0.65, 0.39(2) MHz; and S0, 0.60, 0.37(2) MHz). These are consistent with direct binding of the methoxy fragment to Mn. Assuming terminal Mn-OMe ligation, the couplings indicated a spin projection coefficient (rho) magnitude of approximately 2 for the ligating Mn in both the S2 and S0 states, with inferred Mn-O distances of approximately 1.9-2.0 A. In the presence of D2O, four classes of exchangeable deuterons were identified by ESEEM in S2 and S0. Three of these classes (1, 2, and 4) exhibited populations and coupling strengths that were essentially constant under various conditions of sample preparation, illumination turnover, and small alcohol addition. Class 3 could be modeled with constant coupling but a highly variable deuteron population (n3 approximately 0-10) depending in part on the preparation used. For all classes, the coupling parameters were very similar in S2 and S0. The favored interpretation is that the two strongest coupling classes (1 and 2) represent close binding of one water molecule to a single Mn which has an oxidation state of II in S0 and III in S2, and rho approximately 2 in both cases. This water is not displaced by MeOH, but either the water or MeOH is singly deprotonated upon MeOH binding. Class 4 represents approximately 2 water molecules which are not closely bound to Mn (Mn-deuteron distances of approximately 3.7-4.7 A). Class 3 probably represents protein matrix protons within approximately 4 A of the Mn in the cluster, which can be variably exchanged in different preparations.