A thermoanalytical, X-ray diffraction and petrographic approach to the forensic assessment of fire affected concrete in the United Arab Emirates

For most fires, Forensic investigation takes place well after building materials have cooled and knowledge of the structural damage due to heat exposure can reveal the temperature reached during an incident. Recently, there have been significant changes in the characteristics of cementitious materials used in the United Arab Emirates. Few studies focus on the application of thermo-gravimetric and petrographic techniques on newly developed structures and this work aims to address this deficiency by utilising a series of parametric laboratory-based tests to assess the effects of heat on hardened concrete. Specimens were made with a design mix used for low-rise residential homes and storage facilities. The key constituents were: Portland cement (PC), crushed gabbro stone and dune sand with water/cement ratios of 0.4-0.5. Cement substitutes included slag (GGBS), and silica fume (SF) at replacement percentages of up to 50% and 4%, respectively. The concrete cubes were exposed to heat inside an electric furnace with pre-determined temperature regimes of 150°C, 300°C, 600°C and 900°C. Petrographic examination was utilised to compare the discolouration of the cooled concrete. Data derived from thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) are reported in order to assess the usefulness of these techniques in fire scene investigation to differentiate between these temperature regimes.. The results from the TGA indicate that the majority of the percentage weight loss for all the mixtures occurred in the range 650-700°C, which corresponds to the decarbonation of calcium carbonate, mainly from the aggregates. The endothermic DSC peak at 70-120°C relates to the loss of evaporable water. Since both of these reactions are irreversible, this information can help fire investigators estimate the temperature history of concrete after exposure to fire. On the other hand, the portlandite in the cement matrix dehydroxylates at 450-550°C but then reforms as the concrete cools. The onset temperature for the dehydroxylation of the reformed mineral is always lower than in virgin samples and its enthalpy furthermore depends strongly on the thermal history of the portlandite. Thus, this feature can be used to establish the temperature to which the material was exposed to during a fire incident

The effects of electrochemical hydrogenation on coal structure: chemical and macromolecular changes

A set of five coals have been electrochemically hydrogenated and changes to the chemical structure were monitored by elemental analysis and NMR. It was thought that the elemental analysis gave spurious results due to possible hydrogen trapping. The NMR clearly showed a decrease in aromatic material and the creation of aliphatics. Changes to the macromolecular structure following hydrogenation were monitored by measuring changes in: the amount of pyridine soluble material; the amount of volatile material; decrease in glass transition temperatures; and in solvent swelling. The increase in the amount of aliphatic material correlated well with the change in the amount of soluble material and decrease in glass transition temperature. A less perfect correlation was noted between the amount of aliphatic material and the amount of volatile material and no correlation was noted between increase in aliphatics and solvent swelling

Changes in hydrogen storage properties of binary mixtures of intermetallic compounds submitted to mechanical milling

Electrochemical loading of hydrogen in single and binary mixtures of the intermetallic compounds, TiNi, TiFe, Zr30Ni70 and Zr70Ni30 have been investigated after energetic mechanical milling. Enhanced hydrogen storage of the 50/50 (by weight) TiNi/ZrNi binary mixtures are only found after high energy milling (450 rpm for 12 h). The high energy milled 50/50 and 30/70 TiFe/Zr30Ni70 binary mixtures exhibited good electrochemical loading (98 mA h g−1) at 25 °C but this capacity was decreased by 12% on increasing the temperature to 55 °C. On the other hand, the 70/30 TiFe/Zr30Ni70 binary mixtures showed a 65% increase in the hydrogen capacity with temperature over the same range. High energy milling of the TiFe with carbon materials again led to a substantial increase in the storage capacity but here, this was attributed to a partial reduction of the surface oxides by the graphite/activated carbon. Thermal analysis of the samples using high pressure differential scanning calorimetry on the milled Zr70Ni30 sample showed a rapid deterioration of the hydrogen absorption/desorption features with thermal cycling over the range 100–310 °C due to sintering of the samples

Electrochemical behaviour of aqueous SO2 at polycrystalline gold electrodes in acidic media. A voltammetric and in-situ vibrational study. Part II. Oxidation of SO2 on bare and sulphur-modified electrodes

The electrochemical oxidation of SO2 on polycrystalline gold electrodes has been studied by means of cyclic voltammetry and in situ vibrational techniques. On bare gold electrodes, SO2 is irreversibly oxidised on forward scans at 0.6 V/RHE, featuring a diffusion-limited peak. Oxidation is inhibited by the formation of chemisorbed oxygen. A SO2 anodic current rise occurs on the reverse scan in parallel with the reduction of the metal oxide layers. As shown by FT-IR, oxidation proceeds to yield a mixture of soluble S(VI) species as stable reaction products. From vibrational spectra and results from the irreversible adsorption method, it follows that no strongly adsorbed S-O-like residues are present onto the gold surface in the region 0.3-0.5 V/RHE. On sulphur-modified electrodes improved electrocatalysis is manifested by the shift of the diffusion-limited peak to lower potentials. The best performance is observed at a sulphur coverage of 0.5. At higher coverage, sulphur adlayers impart lower catalytic efficiency and eventually show strong poisoning properties. This behaviour is exhibited by sulphur adlayers generated either in situ by SO2 reduction or ex situ by sulphide adsorption/oxidation in acidic or alkaline media

Reflectance and SERS from an ordered array of gold nanorods

The optical properties of arrays of Au nanorods were studied by specular reflectance spectroscopy. The spectra were dominated by the surface plasmon modes of the Au nanoarrays superimposed on the effects of interference through the films. The longitudinal plasmon resonance moved to longer wavelength as the aspect ratio of the nanorods increased. The reflectance spectra were modelled by applying the Maxwell-Garnett approximation to a uniaxial thin film (composite Au/alumina) and this yielded a good match to the experimental data. SERS spectra on the Au nanorod arrays were recorded at different externally applied potentials and significant differences with respect to an electrochemically roughened Au electrode were revealed. These have been attributed to the nature of the composite nanoarrays, both their nanostructuring into rods and the regular arrangement of these rods

In situ ellipsometry studies of electrodeposited cadmium telluride films on cadmium mercury telluride

The electrodeposition of CdTe on CdxHg1-xTe is carried out from aqueous solutions at 55 degrees C and the film growth is monitored using in situ ellipsometry. The measurements reveal that, at a growth potential of -0.55 V versus SCE, a thin (120 AA) Te layer is initially formed on the surface followed by the growth of the CdTe film (1.8 mu m) which appears to be of uniform composition for most of its thickness. Further analysis of the film using the techniques of Raman spectroscopy, differential scanning calorimetry (DSC) and energy dispersive X-ray analysis (EDAX) confirm the excess tellurium in the electrodeposited film. The EDAX measurements after electrodeposition also reveal the presence of 2-8% Hg in the film depending on the depth of analysis. The presence of Hg in the film can only be explained by the diffusion of Hg from the substrate through the electrodeposited layer

The effect of novel processing on hydrogen uptake in feti- and magnesium-based alloys

This paper discusses the production and initial evaluation of hydrogen storage alloys produced by physical vapour deposition (PVD) and mechanical alloying (MA). PVD is usually associated with the production of thin films and coatings. However, DERA Farnborough have developed a high rate vapour condensation process to produce bulk deposits, in some cases up to 44 mm thick. Vapour condensation using electron beam evaporation produces the ultimate in cooling rates with extended solid solubility and refinement of microstructure, which produce enhanced physical and mechanical properties. MA is a complimentary technique for processing hydrogen storage materials which has been developed within DERA over the past 3 years. These techniques have been applied to Mg and FeTi alloy systems and it is shown that both methods greatly enhance the amount of hydrogen uptake and the ease of activation

Electrochemical behaviour of aqueous SO2 at polycrystalline gold electrodes in acidic media: a voltammetric and in situ vibrational study: Part 1. Reduction of SO2: deposition of monomeric and polymeric sulphur

The electro-reduction of SO2 has been monitored by using cyclic voltammetry, FT-IR spectroscopy and SER spectroscopy. Prior to the bulk reduction, SO2 is reduced to yield a monomeric sulphur adlayer at a maximum coverage of about 0.25. The sulphur adlayer undergoes a reversible redox surface process at E<0.0 V (RHE), which implies a change in the frequency of the Au---S stretching mode from 270 to 300/310 cm−1. In the potential region encompassing the bulk reduction voltammetric peak, infrared spectra display a band at 2585 cm−1 attributable to a S---H vibration from a soluble species. Accordingly, H2S or H2Sx were proposed as tentative bulk reduction products. In positive sweeps a broad anodic wave develops between 0.2 and 0.6 V that leaves polymeric sulphur species adsorbed at multilayer level, with a S---S stretching mode at 460 cm−1 and a S---S---S bending vibration at 218 cm−1. Multilayer sulphur can be removed reductively under a sharp cathodic peak. According to literature of the S(-II)/Au system, removal proceeds to yield soluble S(-II) species, via intermediate polysulphides

In situ ellipsometry studies of electrodeposited cadmium telluride films on cadmium mercury telluride

The electrodeposition of CdTe on CdxHg1-xTe is carried out from aqueous solutions at 55 degrees C and the film growth is monitored using in situ ellipsometry. The measurements reveal that, at a growth potential of -0.55 V versus SCE, a thin (120 AA) Te layer is initially formed on the surface followed by the growth of the CdTe film (1.8 mu m) which appears to be of uniform composition for most of its thickness. Further analysis of the film using the techniques of Raman spectroscopy, differential scanning calorimetry (DSC) and energy dispersive X-ray analysis (EDAX) confirm the excess tellurium in the electrodeposited film. The EDAX measurements after electrodeposition also reveal the presence of 2-8% Hg in the film depending on the depth of analysis. The presence of Hg in the film can only be explained by the diffusion of Hg from the substrate through the electrodeposited layer

Enhancement of the second harmonic signal from Hg1−xCdxTe (MCT) in the presence of an anodic oxide film

Second harmonic generation (SHG) is now widely regarded as a valuable tool for investigating electrode surfaces. Typically, most studies have been limited to substrates which lack bulk symmetry and monitoring events such as sub-monolayer formation and surface reconstruction. Here, the development of a model that can be used to quantitatively describe the enhanced SH signal observed in the presence of an anodic oxide film on a non-centrosymmetric substrate, Hg1xCdxTe (MCT), is described. The aim is to further expand the utility of SHG for probing different electrode systems. The growth of the high quality oxide films was first followed by in-situ ellipsometry. For thin films (<100 nm) grown at a constant current density of 150 A cm2, an effectively uniform oxide layer is found with a refractive index n of ~2.15 0.05 and exhibiting no absorption of the incident radiation at 632.8 nm (1.96 eV). In the presence of such an oxide film of 58 nm thickness, the second harmonic (SH) signal intensity measured in reflection is found to be significantly enhanced in both the PIN-POUT and PIN-SOUT polarization configurations. To quantify the changes observed, each layer in the model is assigned its own symmetry and optical constants (at the fundamental, and harmonic (= 2) frequencies and a defined thickness. Modeling of the SH rotational anisotropy experiments carried out at different angles of incidence indicated that most of this increase could be accounted for by multiple reflections of the fundamental wave = 1064 nm (1.17 eV) in the composite ambient/oxide/MCT layer, with little contribution from charge accumulation at the buried MCT/oxide interface for this oxide thickness