Judith Monnier

Raman Studies of Corrosion Layers Formed on Archaeological Irons in Various Media

The description and identification of corrosion products formed on archaeological iron artefacts need various approaches at different observation scales. Among analytical techniques available to document phase structure at the microscopic range, Raman spectroscopy offers sensitivity and discrimination between iron corrosion products with an easy implementation. Results obtained for iron artefacts corrosion in soils and atmosphere are presented. Corrosion forms observed for anoxic and aerated soils on one hand and indoor atmosphere on the other are documented. Beyond the identification and organisation of corrosion products through hyperspectral imaging, Raman micro-spectroscopy could also provide quantitative phase proportions which will be needed in the proposition of reactivity diagnosis indicators.

An innovative approach to develop highly performant chalcogenide glasses and glass-ceramics transparent in the infrared range

An innovative way to produce chalcogenide glasses and glass-ceramics for infrared devices is reported. This new method of synthesis at low temperature combining ball-milling and sintering by SPS (Spark Plasma Sintering) is a technological breakthrough to produce efficient infrared chalcogenide glasses and glass-ceramics. This technique will offer the possibility to strongly decrease the cost of infrared devices and to produce new chalcogenide glasses. It will also permit to increase the potential of some glass compositions by allowing their shaping at desired dimensions.

Corrosion of iron from heritage buildings: proposal for degradation indexes based on rust layer composition and electrochemical reactivity

Abstract In the present work, the authors tried to establish degradation indices for heritage ferrous artefacts, especially those used in ancient buildings and submitted to indoor atmospheric corrosion. The authors focused on the site of the Amiens Cathedral in the north of France. Samples coming from this reference site were carefully characterised in order to identify the different phases constituting the corrosion scale. The scale consists in a matrix of iron oxyhydroxide goethite embedded with several ferrihydrite marblings. Other phases such as lepidocrocite and akaganeite are scarcely present in the external part of the corrosion scale. Moreover, electrochemical measurements on both references and ancient samples enable to define the reduction reactive phases. From the nature of these phases and their localisation, two degradation indices were defined to evaluate rust reactivity. Finally, a curve that links these two factors is proposed as a first step towards a corrosion diagnosis.

Magnetocaloric properties of La0.67Ca0.33MnO3 produced by reactive spark plasma sintering and by conventional ceramic route

The structural properties of ferromagnetic La0.67Ca0.33MnO3 perovskite ceramics, produced by reactive spark plasma sintering (R-SPS) and conventional solid state reaction (SSR) methods, have been investigated by combined x-ray diffraction (XRD), Mn K-edge x-ray absorption near-edge structure (XANES) and magnetometry. All the samples are single phases and crystallize in the orthorhombic structure with a Pbnm space group. A slight unit cell dilatation is observed in the R-SPS sample compared to the SSR sample. The qualitative and quantitative analyses of the XANES spectra of both samples allow us to attribute this discrepancy to a manganese electronic state change in relation to the used experimental reaction conditions: the Mn3+/Mn4+ atomic ratio is found to be equal to 0.74/0.26 and 0.66/0.34, for the R-SPS and SSR samples, respectively, the former being significantly different from that expected for stoichiometric manganite. R-SPS material processing conditions affect also the microstructure of the considered manganite. The produced ceramic is dense and fine grained, with an average grain size of about 100 nm. Consequently, the measured values of the Curie temperature, TC, and of the saturation magnetization, Ms, at low temperature decrease significantly in the former compared to the latter. The maximum of the magnetic entropy change, , is lower in the R-SPS sample than in the SSR sample, but the thermal variation of −ΔSM is broader, resulting in a higher relative cooling power (RCP) in the former.

High thermoelectric performance in Sn-substituted α-As2Te3

Lead chalcogenides PbX (X = Te, Se, S) have been the materials of choice for thermoelectric power generation at mid-range temperatures (500–700 K). Here, we report on a new family of chalcogenides α-As2Te3 that exhibit similar thermoelectric performances near 500 K. Sn doping in p-type polycrystalline α-As2−xSnxTe3 (x ≤ 0.075) provides an efficient control parameter to tune the carrier concentration leading to thermopower values that exceed 300 μV K−1 at 500 K. Combined with the structural complexity of the monoclinic lattice that results in extremely low thermal conductivity (0.55 W m−1 K−1 at 523 K), a peak ZT value of 0.8 is achieved at 523 K for x = 0.05. A single-parabolic band model is found to capture well the variations in the transport properties with the Sn concentration and suggests that higher ZT values could be achieved through band structure engineering. These results surpass those obtained in the sister compounds β-As2−xSnxTe3 and further show that α-As2Te3 based materials are competitive with other chalcogenides for thermoelectric applications at intermediate temperatures.

Electronic structure, low-temperature transport and thermodynamic properties of polymorphic β-As2Te3

β-As2Te3 belongs to the prominent family of Bi2Te3-based materials, which show excellent thermoelectric properties near room temperature. In this study, we report a joint theoretical and experimental investigation of its electronic and thermal properties at low temperatures (5–300 K). These results are complemented by specific heat measurements (1.8–300 K) that provide further experimental evidence of the first order lattice distortion undergone by β-As2Te3 near 190 K. Data taken on cooling and heating across this transition show that the lattice distortion has little influence on the electronic properties and further evidence a weak hysteretic behavior. Although first-principles calculations predict a semiconducting ground state, these measurements show that β-As2Te3 behaves as a degenerate p-type semiconductor with a high carrier concentration of 1020 cm−3 at 300 K likely due to intrinsic defects. Calculations of the vibrational properties indicate that the extremely low lattice thermal conductivity values (0.8 W m−1 K−1 at 300 K) mainly originate from low-energy Te optical modes that limit the energy window of the acoustic branches. This limited ability to transport heat combined with a relatively large band gap suggest that high thermoelectric efficiency could be achieved in this compound when appropriately doped.

Effect of Ni, Bi and Se on the tetrahedrite formation

Materials based on Cu12Sb4S13 tetrahedrites have been seen in recent years as promising materials for thermoelectric applications. However, the effect of small amounts of additional elements (used to tune the electrical transport properties) on the formation of the phases was not investigated. In this work we present such study, by means of powder X-ray diffraction, scanning electron microscopy complemented with energy-dispersive spectroscopy, and differential scanning calorimetry, using the Taguchi method to design the experiments. The effect of Ni, Bi and Se (i) in the volume percentage of tetrahedrite, (ii) in the temperature at which the Class III (tetrahedrite + chalcostibite + antimony → skinnerite) reaction occurs and (iii) in the final melting temperature of Cu12−xNixSb4−yBiyS13−zSez materials rapidly cooled from 950 °C was investigated. Se was observed to have a strong positive influence on the formation of tetrahedrite, while Ni and Bi were seen to promote the decrease of its volume percentage. A decrease of the Class III reaction and final melting temperatures was also observed after the introduction of Se, with Ni inducing the increase of the reaction and the decrease of the melting temperatures and Bi having only minor effects. The analysis of the microstructures indicate that high Ni concentrations lead to the first solidification of the NiS phase, while in the other compositions the (tetrahedrite/skinnerite) phase or mixtures of phases is first formed.

High-temperature thermoelectric properties of the β-As2−xBixTe3 solid solution

Bi2Te3-based compounds are a well-known class of outstanding thermoelectric materials. β-As2Te3, another member of this family, exhibits promising thermoelectric properties around 400 K when appropriately doped. Herein, we investigate the high-temperature thermoelectric properties of the β-As2−xBixTe3 solid solution. Powder X-ray diffraction and scanning electron microscopy experiments showed that a solid solution only exists up to x = 0.035. We found that substituting Bi for As has a beneficial influence on the thermopower, which, combined with extremely low thermal conductivity values, results in a maximum ZT value of 0.7 at 423 K for x = 0.017 perpendicular to the pressing direction.

Reactivity studies of atmospheric corrosion of heritage iron artefacts

Abstract: The authors consider the case of iron atmospheric corrosion to illustrate the possibility of developing a conservation diagnosis for a given material in a given environment. In the particular case of iron atmospheric corrosion, samples from the site of the Amiens Cathedral in the North of France have been characterised in order to identify the different phases constituting the corrosion product layers. The layers consist of a matrix of iron oxy-hydroxide goethite embedded with several ferrihydrite marblings. Other phases such as as lepidocrocite, maghemite and akaganeite are present in minor quantities. A degradation index is first defined from the phase proportions and from the intrinsic electrochemical properties of those phases. Further, the electrochemical reactivity of scratched rust powders has been studied to define a second degradation index. From these two degradation indices a first step towards a corrosion diagnosis method is proposed.

Nanoscale investigation by TEM and STEM-EELS of the laser induced yellowing

Nd-YAG QS laser cleaning of soiled stone at 1064 nm can sometimes result in a more yellow appearance compared to other cleaning techniques. Especially in France, this yellowing effect is still considered as a major aesthetic issue by the architects and conservators. One explanation states that the yellowing is linked to the formation of iron-rich nanophase(s) through the laser beam interaction with black crusts that would re-deposit on the cleaned substrate after irradiation. To characterize these nanophases, a model crust containing hematite was elaborated and laser irradiated using a Nd-YAG QS laser. The color of the sample shifted instantaneously from red to a bright yellow and numerous particles were ablated in a visible smoke. Transmission electron microscopy (TEM) was used to examine the morphology and the crystallinity of the neo-formed compounds, both on the surface of the samples and in the ablated materials. In addition, an investigation of the chemical and structural properties of the nanophases was conducted by X-ray dispersive energy (EDX) and electron energy loss (EELS) spectroscopies. It was found that both the surface of the sample and the ablated materials are covered by crystallized nano-spheres and nano-residues, all containing iron and oxygen, sometimes along with calcium and sulfur. In particular an interfacial area containing the four elements was evidenced between some nanostructures and the substrate. Magnetite Fe3O4 was also identified at the nanoscale. This study demonstrates that the laser yellowing of a model crust is linked to the presence of iron-rich nanophases including CaxFeySzOδ nanostructures and magnetite Fe3O4 at the surface after irradiation.