Iridium Single Crystals Research Articles

Some Features of Cleavage Cracks in Rocks and Metals

The cracking of some rocks, namely granite, serpentinite and sandstone under tensile stress is examined in details. Brazilian testing or diametral compression, three points bending and explosion testing are used as the loading schemes in air at room temperature. Morphology of cracks in the model rocks are compared with cracks in silicon crystals, as the standard of a brittle crack, with cleavage cracks in iridium single crystals and with cracks in gallium-covered aluminum single crystals. The comparison of cracks between themselves has shown that there is additional channel for stress accommodation in the model rocks. This channel does not lead to transformation of a rock into a macroscopically ductile material, but it causes the arrest of the dangerous crack in it under tensile stress. Its influence causes transition from the brittle crack to the pore-like crack on the microscopic scale. The most probable mechanism of this transition is the dislocation emission from crack, which becomes possible in such a natural covalent solid as a rock due to Rehbinder's effect.

(Invited) Unveiling Changes in Surface Chemistry of Iridium Single Crystals and Metal Oxide Supported IrOx Nanoparticles in Oxygen Evolution Reaction Conditions

Gaining fundamental insights into the formation and the stability of iridium (Ir) surface oxides is pivotal to efficient and sustainable oxygen evolution reaction (OER) electrocatalysis, a key reaction in the field of power-to-fuels and power-to-chemicals. However, proton-exchange membrane water electrolysers (PEMWE) systems currently suffer from their high investment and operational costs, which are partly due to the use of micrometer-sized iridium oxide (IrOx) particles to electrocatalyze the OER. A passage to IrOx nanoparticles would be highly desirable but these nanomaterials face time-dependent changes in structure and chemical composition in OER conditions. To shed fundamental light into the mechanisms at stake, we used electrochemical techniques in combination to X-ray photoelectron spectroscopy (XPS), and inductively coupled mass plasma spectrometry (ICP-MS). Experiments on Ir(111), Ir(210) and nanostructured Ir(210) surfaces showed that the oxy-hydroxides layers forming in the pre-OER region feature mixed Ir oxidation states (presence of Ir(0), Ir(+III) and Ir(+IV) species), and that the fraction of each oxidation state depends on the crystallographic orientation. In the OER region, Ir(+III) species progressively dissolve leading to an enrichment of the surface and near-surface regions of the single crystals into Ir(+IV) species, and resulting in a decrease of their intrinsic activity towards the OER. The results indicate a convergence towards a more stable but less active surface state, which does not depend neither on the initial arrangement of surface atoms (crystallographic orientation, proportion of high- and low-coordinated atoms) nor on their oxidation state (initial state vs. electrochemically-activated Ir(hkl) surfaces) [1]. These findings were confirmed on more technologically relevant materials such as IrOx nanoparticles supported onto antimony-doped (ATO), niobium-doped (NTO) or tantalum-doped (TaTO) tin oxide aerogels [2]. Moreover, the combination of identical-location transmission electron microscopy (IL-TEM) and in situ ICP-MS helped in identifying which potential ranges are critical to the stability of IrOx nanocatalysts and their supports, and provided practical guidelines for the development of more active and more stable PEMWE anodes [3]. Ackowledgements This work was supported by the French National Research Agency in the frame of the MOISE project (grant number ANR-17-CE05-0033).

Molecular dynamics simulation and micropillar compression of deformation behavior in iridium single crystals

The compression behaviors of iridium single crystals with different crystalline orientations were investigated by micropillar compression tests and molecular dynamics (MD) simulations. The results indicated that the deformation process of iridium single crystals with [100] and [110] orientations was presented as the stacking faults expansion and the formation of Lomer–Cottrell locks. And the occurrence of Lomer–Cottrell locks was considered as the interaction of stacking faults on {111} planes by MD simulations. The evolution of crystal structure in compression indicated that the Lomer–Cottrell locks might contribute to the large plastic deformation of iridium single crystals. Moreover, the deformation features in MD simulations showed that the elastic modulus (E) and yield stress (σs) of iridium single crystals were significantly influenced by the temperature. The elastic modulus and yield stress gradually decreased with an increased temperature for all orientations. Meanwhile, the single crystal with a closely spaced lattice structure exhibited superior mechanical properties at a same temperature.

Single-crystal growth of iridium with [100] and [110] orientations by electron beam zone melting

Single crystal of iridium with two different orientations, [100] and [110] has been successfully prepared by electron-beam floating zone melting (EBFZM). The experimental results showed that the iridium single crystal with [100] orientation could be directly prepared without seeding at a low growth rate below 3 mm/min, and the single crystal with [110] orientation could be only obtained by seeding. The competitive growth of the bicrystal demonstrated that the [100] orientation of the iridium single crystal can overwhelm that of the [110] orientation at a growth rate of 2 mm/min. Moreover, the interface growth undercooling including curvature undercooling and kinetic undercooling has been calculated for iridium single crystal with these two orientations. The calculated data and analysis agree well with the experimental results.

Numerical and experimental study of electron beam floating zone melting of Iridium single crystal

Both numerical and experimental approaches were conducted to investigate the electron beam floating zone melting (EBFZM) of iridium crystal. A finite element model was established and the temperature fields under different processing parameters were calculated and discussed. The heating power, the rod diameter and the movement of heating source significantly influence the temperature distribution in the iridium rod. Once the heating starts, the temperature quickly increases and gradually reaches the steady state. The melting zone enlarges with the increase of heating power, and the critical power for obtaining a complete melting zone is 5.5kW∼6.0kW in 20mm-diameter Iridium rod. With the upward movement of heating source, the melting zone obviously enlarges and the lower solid/liquid (S/L) interface becomes more planar. Then how to get a complete melting zone with a suitable volume and the control of a planar S/L interface were discussed. An optimized processing window was proposed and applied to the EBFZM of iridium crystal. An iridium rod 20mm in diameter was successfully produced and the microstructural morphologies indicated that the initial polycrystalline microstructure will evolve into single crystal.

Room Temperature Carbonylation of Iron–Phthalocyanines Adsorbed on a Single Crystal Metal Surface: An in Situ SFG Investigation at Near-Ambient Pressure

We report the experimental spectroscopic evidence for the room temperature carbonylation, at equilibrium with the carbon monoxide gas phase, of iron phthalocyanines adsorbed on the (111) termination of the iridium single crystal. The adsorption process occurs at CO pressures above the mbar yield. Interestingly, heme and heme-like catalytic, carrier, and enzymatic biomolecules interact with the gaseous reactants at partial pressures that are similar to the values that we observed on this model system, a 2D layer of single-atom metallorganic catalysts preassembled under ultrahigh vacuum conditions. A simple Langmuir description of the adsorption mechanism yields a CO-Fe binding energy of 0.1–0.3 eV, depending on the assumptions about the pre-exponential factor. The internal vibrational structure of the adsorbed iron phthalocyanines is also investigated.

CO Adsorption on Reconstructed Ir(100) Surfaces from UHV to mbar Pressure: A LEED, TPD, and PM-IRAS Study.

Clean and stable surface modifications of an iridium (100) single crystal, i.e., the (1 × 1) phase, the (5 × 1) reconstruction, and the oxygen-terminated (2 × 1)-O surface, were prepared and characterized by low energy electron diffraction (LEED), temperature-programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS) and polarization modulation IRAS (PM-IRAS). The adsorption of CO in UHV and at elevated (mbar) pressure/temperature was followed both ex situ and in situ on all three surface modifications, with a focus on mbar pressures of CO. The Ir(1 × 1) surface exhibited c(4 × 2)/c(2 × 2) and c(6 × 2) CO structures under low pressure conditions, and remained stable up to 100 mbar and 700 K. For the (2 × 1)-O reconstruction CO adsorption induced a structural change from (2 × 1)-O to (1 × 1), as confirmed by LEED, TPD, and IR. For Ir (2 × 1)-O TPD indicated that CO reacted with surface oxygen forming CO2. The (5 × 1) reconstruction featured a reversible and dynamic behavior upon CO adsorption, with a local lifting of the reconstruction to (1 × 1). After CO desorption, the (5 × 1) structure was restored. All three reconstructions exhibited CO adsorption with on-top geometry, as evidenced by IR. With increasing CO exposure the resonances shifted to higher wavenumber, due to adsorbate–adsorbate and adsorbate–substrate interactions. The largest wavenumber shift (from 2057 to 2100 cm–1) was observed for Ir(5 × 1) upon CO dosing from 1 L to 100 mbar.

Fundamental quantum noise mapping with tunnelling microscopes tested at surface structures of subatomic lateral size

We present a measurement scheme that enables quantitative detection of the shot noise in a scanning tunnelling microscope while scanning the sample. As test objects we study defect structures produced on an iridium single crystal at low temperatures. The defect structures appear in the constant current images as protrusions with curvature radii well below the atomic diameter. The measured power spectral density of the noise is very near to the quantum limit with Fano factor F = 1. While the constant current images show detailed structures expected for tunnelling involving d-atomic orbitals of Ir, we find the current noise to be without pronounced spatial variation as expected for shot noise arising from statistically independent events.

Ligand exchanges and selective catalytic hydrogenation in molecular single crystals

Chemical reactions inside single crystals are likely to be highly selective, but examples of single crystal to single crystal (SC-SC) transformations are uncommon, because crystallinity is difficult to retain following the rearrangement of atoms in the solid state. The most widely studied SC-SC transformations involve solvent exchange reactions in porous coordination polymers or metal-organic frameworks, which take advantage of the robust polymeric networks of the hosts. Examples of reactions occurring within molecular organic crystals generally involve photo-induced reactions, such as the coupling of alkenes or alkynes within the crystal. For nonporous molecular inorganic or organometallic crystals, single-crystal transformations involving the formation or cleavage of metal-ligand bonds are rare; known examples usually involve ligand loss from the single crystal and reversible religation, a process sometimes accompanied by decay of the single crystal to a microcrystalline powder. Here we report a series of SC-SC transformations that involve the interchange of multiple small gaseous ligands (N(2), CO, NH(3), C(2)H(4), H(2) and O(2)) at an iridium centre in molecular single crystals of a pincer Ir(I) complex. The single crystal remains intact during these ligand-exchange reactions, which occur within the crystal and do not require prior ligand extrusion. We reveal a selective catalytic transformation within a nonporous molecular crystal: pincer iridium single crystals ligated with nitrogen, ethylene or hydrogen show selective hydrogenation of ethylene relative to propylene (25:1) when surface sites are passified by CO.

CARBON MONOXIDE OXIDATION ON IRIDIUM(111) SURFACES UNDER COLORED NOISE

We study the effect of external noise on the catalytic oxidation of CO on an Iridium(111) single crystal under ultrahigh vacuum conditions. This reaction can be considered as a model of catalysis used in the industry. In the absence of noise, the reaction exhibits one or two stable stationary states, depending on the control parameters such as temperature and partial pressures. When noise is added, for instance, by randomly varying the quality of the influx mixture, the system exhibits stochastic reaction rate and switching. In this work, we present the theoretical predictions for the bistable situation using a number of colored-noise approximations that consider the intensity and correlation of the external fluctuations. As we will show, only some of these approximations are suitable for bistable systems.

STOCHASTIC MODEL CALCULATION FOR THE CARBON MONOXIDE OXIDATION ON IRIDIUM(111) SURFACES

We study the effect of external noise on the catalytic oxidation of CO on an Iridium(111) single crystal under ultrahigh vacuum conditions. This reaction can be considered as a model of catalysis used in industry. In the absence of noise the reaction exhibits one or two stable stationary states, depending on control parameters such as temperature and partial pressures. When noise is added, for instance, by randomly varying the quality of the influx mixture, the system exhibits stochastic reaction rate and switching. In this work, we present two approaches: one for the monostable regime, and another for the bistable situation that relies on a white noise approximation. Both approaches rest on the assumption that spatial patterns of coverage on the Iridium plate can be neglected on a first approximation. Using mathematical models, it is possible to reconstruct stationary probability distribution functions that match experimental observations and provide support for the existence of a thermodynamic potential.

Effect of the magnetism of impurities on their diffusion in metals: Bulk diffusion of iron, cobalt, and rhodium in iridium single crystals

The coefficients and parameters of the temperature dependences of the coefficients of bulk diffusion of Fe, Co, Rh, and Au atomic probes (APs) in iridium single crystals (mono-Ir) have been determined from the diffusion profiles obtained using secondary-ion mass spectrometry of the diffusion zones. The enthalpies of activation of diffusion of Fe, Co, and Rh APs are considerably lower than the enthalpy of activation of selfdiffusion in mono-Ir. This is caused by the negative contributions of the intraatomic exchange energy and energy of relaxation of the environment of the d transition APs to the enthalpy of interaction of magnetically active APs with the vacancies in the iridium lattice. The interaction energy of partners in such complexes and the relationships between the magnetic moments of d transition APs in complexes with vacancies have been estimated. The Rh APs in complexes with vacancies in iridium possess stable magnetic moments.

Deformation tracks distribution in iridium single crystals under tension

The deformation tracks distribution in a single crystal of the high melting FCC-metal iridium, which exhibits the cleavage after considerable elongation, is considered. The octahedral slip is the sole deformation mechanism in iridium single crystal at room temperature, and, therefore, its mechanical behavior is similar to the behavior of a FCC-metal. However, in contrast to other FCC-metals, the resource of plasticity of the iridium single crystalline samples is exhausted at the initial/early stages of plastic deformation, when the octahedral slip bands are homogeneously distributed on the working surface and the necking is absent in vicinity of the dangerous crack.

On brittle fracture in polycrystalline iridium

The brittle intercrystalline fracture (BIF) in iridium and low plasticity of this refractory fcc-metal in polycrystalline state for wide temperature range, give occasion to supposition that a metal having fcc lattice can be brittle substance like silicon due to specific features of interatomic bonds. On the other hand, experience has shown that such behavior of fcc-metal should be impurity induced effect. High plasticity of iridium single crystals (by means of octahedral slip), which however cleave under tension, does not support idea on “classic” inherent brittleness of this metal. High sensibility of its mechanical properties to impurities (10 ppm is critical level for carbon!) has been severe restriction for experiments aimed to searching of inherent fracture mode for polycrystalline iridium. In present paper fracture behavior of high purity polycrystalline iridium and iridium, where grain boundaries (GBs) have been contaminated, are considered. Fine grained impurity-free metal exhibits 100% brittle transcrystalline fracture (BTF) and cracked GBs on the fracture surface and this is an inherent fracture mode of polycrystalline iridium. Re-crystallization in vacuum leads to appearance of BIF regions, whose total square can reach 100% of the fracture surface in coarse grain aggregates. Contaminated iridium mainly fails on grain boundaries, at that portion of BIF on the fracture surface (50 ÷ 100%) does not depend on grain size.

Intrinsic and 'Extrinsic' Mechanisms of the Volume Diffusion of Gold in Iridium Single Crystals

Intrinsic and 'Extrinsic' Mechanisms of the Volume Diffusion of Gold in Iridium Single Crystals

A New Model of Thermal Desorption of Nitrogen from a Nonhomogeneous Catalyst Surface

A new mathematical model is proposed for temperature recombination of atomic nitrogen on the (111) face of the iridium single crystal and on iridium foil, allowing for geometrical nonhomogeneity of the catalyst surface. The model describes the main qualitative features of laboratory experiments: the effect of the catalyst surface structure and the adsorbed oxygen on nitrogen thermal desorption spectra. Conditions are determined when an additional low-temperature peak appears in the N2 thermal desorption spectrum. The results are compared with calculations using traditional models of associative thermal desorption from homogeneous surfaces, including models that allow for lateral interactions in the adsorption layer. The numerical analysis is carried out using deterministic and stochastic approaches.

A monolithic superconducting micro-calorimeter for X-ray detection

A cryogenic micro-calorimeter detector using a small bulk iridium single crystal as both absorber and as transition edge thermometer has been prepared. The detector is voltage biased, and the current through it is measured with a DC-SQUID. In this mode of operation an improvement of the detector performance by the electrothermal feedback is expected. In fact, we observed a stabilization of the working temperature due to this effect. By irradiating the micro-calorimeter with an 241Am X-ray and γ-ray source an RMS energy resolution of σ=150 eV for 14 keV X-rays and σ=250 eV for 60 keV γ-rays was observed.

Strain Hardening of Fcc Metal Surfaces Induced by Microploughing

Microploughing experiments were used as a method for better understanding the ploughing mechanism in gold and iridium single crystals. The plough depths ranged from 20 nm in iridium to 1600 nm in gold. Yield stress profiles and TEM analyses indicate that both materials strain harden even when very small volumes of material are involved. Strain hardening theory, as applied to bulk material, is useful in analyzing the results.

The cause of cleavage in iridium single crystals

The cause of cleavage in iridium single crystals

The main features of plastic deformation of iridium single crystals

A technique for growth of massive iridium single crystals up to 55 mm in diameter and 5 kg in mass has been developed. The single crystals are subjected to plastic deformation. For deformation conditions to be appropriately chosen features of plastic behaviour of iridium single crystals have to be understood. However the data on iridium deformation available in the scientific literature are contradictory [1-3], thus not allowing them to be used in full to develop the process. On the other hand different authors have produced different tensile and compression strength values of iridium attributing them more often to the presence of impurities than to the crystallographic characteristics of plasticity. This letter is an attempt to link plastic behaviour anisotropy of iridium single crystals to the crystallographic characteristics of their deformation. Iridium single crystals were prepared in crucible less zone melting apparatus around oriented seed per one zone pass. The starting material for single crystal production was subjected to deep refining in the process of induction, oxidation melting and electron beam melting. As a result of these operations we managed to reduce gaseous impurities (oxygen, nitrogen, hydrogen and carbon) to 0.1 p.p.m. (wt), most metallic impurities (W, Mo, Nb, Zr, Cu, Gd, Y, AI, Ga, Ni, Pd, Zn, Mg, Ca) to 0.1 to 1 p.p.m. (wt), concentration levels of Pt, Rh, Fe down to 10p.p.m. (wt), and completely clear the iridium from oxide inclusions. The single crystals produced had a ratio P290/Pso = 6.1 to 6.7.