Emission Spectroscopic Boundary Layer Investigation during Ablative Material Testing in Plasmatron

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution ...

Vrije Universiteit Brussel

Studying the atmospheric corrosion of copper alloy artifacts is important to acquire a better knowledge about the condition of the object and its possible conservation and restoration. The nature of the formed product, e.g. sulfate, carbonate or chloride, depends on factors such as the amount of polluting elements or humidity but may also depend on the nature of the underlying aesthetic patina, applied by the artist. The composition of the patination solution and the method of patination will both influence the nature of this aesthetic patina, i.e. its chemical composition and morphology. However, although a lot of patination recipes exist, little is known about these patinas as far as composition, structure and ageing is concerned. Therefore, a combination of several surface analytical techniques is required for the complete characterization of patina layers. In this paper, Raman spectroscopy is used to study the characteristics of several patinas obtained on copper following different traditional recipes.

Micro Raman spectroscopy used for the study of corrosion products on copper alloys: study of the chemical composition of artificial patinas used for restoration purposes.

Detection of the interactions between low molecular weight organic compounds and metals in the form of sols on a nanoscale is analytically challenging. This study aims to provide experimental evidence using a combination of UV-Vis absorption spectrometry, surface-enhanced Raman spectrometry (SERS), and static secondary ion mass spectrometry (S-SIMS). The field of application is thermography where silver images are formed via heat-catalyzed reactions. Several organic compounds called tone modifiers and stabilizers are used in thermographic materials for the optimization of the image quality. With exploitation of the strengths of each of the above-mentioned methods, an affinity ranking of several tone modifiers and a stabilizer was established on the basis of competitive adsorption experiments using different model systems. Specifically, silver sols, SERS probes, and sputter-coated silver substrates were exposed to systems with one or two additives. The UV-Vis results provided insight on the aggregation of silver nanoparticles in a hydrosol, which was necessary for the interpretation of the SERS data. Both SERS and S-SIMS measurements led to a similar ranking of the relative affinity of the additives in two components, which was largely consistent with empirical knowledge derived from macroscopic behavior.

Study of the affinity of thermographic additives for silver by time-of-flight static secondary ion mass spectrometry and surface-enhanced Raman spectroscopy on silver nanoparticles.

The surface of a biomaterial interacts with the body fluid upon implantation in the human body. The biocompatibility of a material is strongly influenced by the adsorption of proteins onto the surface. Titanium is frequently used as a biomaterial for implants in orthopedics and cardiovascular devices. Understanding the biocompatibility is very important to improve implants. The surface chemistry of an implant material and its influence on the interaction with body fluid is crucial in that perspective. The main goal of this study was to investigate the conformation of human plasma fibrinogen (HPF) adsorbed on commercially pure titanium (CP Ti) on a molecular level by means of ex situ atomic force microscopy (AFM). With X-ray photoelectron spectroscopy combined with argon ion beam depth profiling, it was shown that the oxide layer present at the surface was mainly composed of TiO2, with a small percentage of Ti2O3. Ex situ AFM imaging showed the conformation of HPF on CP Ti. Single molecules and aggregates of fibrinogen were observed. The trinodular structure of single HPF molecules (two spherical D domains at the distal ends of the extended molecule and the central spherical E domain) adsorbed onto CP Ti was visualized. Aggregate formation through the connection of the D domains of the HPF molecules was observed on CP Ti. The alphaC domains of HPF were not visible on CP Ti. The ex situ AFM images indicated conformational changes of HPF upon adsorption onto CP Ti. The conformation of the adsorbed HPF molecules was different on mica and titanium. The difference in wettability between both substrates caused a larger spread of the protein on the CP Ti surface and thus resulted in a larger perturbation to the native structure of HPF as compared to mica.

Interaction of human plasma fibrinogen with commercially pure titanium as studied with atomic force microscopy and X-ray photoelectron spectroscopy.

The deposition of n-octylphosphonic acid on aluminum oxide was studied. The substrate was pretreated in order to achieve a root-mean-square roughness of <1 nm, a hydroxyl fraction of 30%, and a thickness of approximately 170 nm. It was proven using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) that, rather than a monolayer, an organic multilayer was formed. The growth mechanism was identified as a Stranski-Krastanov one. It was also shown that the use of AFM, probing the surface topography, is essential for a reliable quantification and interpretation of data obtained with XPS.

Study of the self-assembling of n-octylphosphonic acid layers on aluminum oxide.

We report here on magnetite- and wustite-encrusted and geometrically oriented microbial-like structures (MLS) attached to the surfaces of hematite (alpha-Fe(2)O(3)) crystals in a banded iron formation. Field emission scanning electron microscope (FE-SEM) and scanning electron microscope (SEM) imaging showed a 3-D network of MLS arranged in 1 microm x approximately 20 microm coccoidal-like chains (CLC) of various geometrical shapes: dichotomous and budding-like protrusions, parallel, intersecting, triangular, or sinusoidal. Individual spheroidal forms ( approximately 1 mum in diameter), some displaying what appears to be division, were also abundant. In addition to their size, morphology, and preferred orientations, a microbial origin of these chains and single spheroidal forms is inferred by the presence of material that resembles extracellular polymeric substances (EPS) extending from the base of the chains along the mineral surface: the attachment sites show circular dissolution pits of about 100 nm diameter. Other thin structures protruding from the CLC are reminiscent of bacterial "nanowires." We were, however, unable to find any extant cells, organic carbon, or even recover DNA from the MLS, which suggests that they, if microbial, are possibly mineralogically replaced casts or mineral encrustations of cells. It is further speculated that, given the nature of the substrate upon which the forms are attached and their preferential orientations, it seems plausible that the "original cells" may have been Fe(III)-reducing bacteria that exploited structural imperfections in the crystal lattice. Importantly, the preservation of the ancient microbial shapes in mineral casts of magnetite, wustite, or both may be an overlooked means by which cellular features in the rock record are retained.

Microbial dissolution of hematite and associated cellular fossilization by reduced iron phases: a study of ancient microbe-mineral surface interactions.

Electrochemical (EC) quartz crystal microbalance with dissipation monitoring (ECQCM-D) is a new and powerful technique for the in situ study of adsorption phenomena, e.g., as a function of the potential of the substrate. When titanium (Ti) is employed as the substrate, its oxidation behavior needs to be taken into account. Ti is always covered with a native oxide layer that can grow by, e.g., thermal oxidation or under anodic polarization. For biomolecular adsorption studies on oxidized Ti under applied potential, a stable oxide layer is desired in order to be able to distinguish the adsorption phenomena and the oxide growth. Therefore, the oxidation of thermally evaporated Ti films was investigated in phosphate-buffered saline by means of ECQCM-D, using a specially designed EC flow cell. Upon stepping the potential applied to Ti up to 2.6 V vs standard hydrogen electrode (SHE), a fast increase of the mass was observed initially for each potential step, evolving slowly to an asymptotic mass change after several hours. The oxide layer thickness increased as a quasi-linear function of the oxidation potential for potentials up to 1.8 V vs SHE. The growth rate of the oxide was around 2.5-3 nm/V. No changes in the dissipation shift were observed for potentials up to 1.8 V vs SHE. The composition of the oxide layer was analyzed by X-ray photoelectron spectroscopy (XPS). It was mainly composed of TiO(2), with a small percentage of suboxides (TiO and Ti(2)O(3)) primarily at the inner metal/oxide interface. The amount of TiO(2) increased, and that of TiO and Ti(2)O(3) decreased, with increasing oxidation potential. For each oxidation potential, the calculated thickness obtained from ECQCM-D correlated well with the thickness obtained by XPS depth profiling. A procedure to prepare Ti samples with a stable oxide layer was successfully established for investigations on the influence of an electric field on the adsorption of biomolecules. As such, the effect of an applied potential on the adsorption behavior of lysozyme on oxidized Ti was investigated. It was observed that the adsorption of lysozyme on oxidized Ti was not influenced by the applied potential.

In situ control of the oxide layer on thermally evaporated titanium and lysozyme adsorption by means of electrochemical quartz crystal microbalance with dissipation.

The interest in self-assembling monolayer deposition on various oxide substrate surfaces is steeply increasing in the last decades. Although many studies are being performed, literature does not come with a general insight in the adsorption of these layers on oxide surfaces. Also for the deposition of phosphonic acids on aluminum oxides, there is no global consensus. In this paper, we present an original in situ analysis in order to eludicate the real layer formation mechanism. First of all, the state of the phosphonic acid molecules was determined using DOSY NMR, making sure that no structures other than free molecules were present at the concentration used. With in situ atomic force microscopy and in situ visual ellipsometry, multilayers of phosphonic acids, showing 3D island growth, were determined. It was shown that using the variation of the in situ obtained roughness and bearing ratio, together with the equivalent thickness modeled by ellipsometry, the growth of the layers occurs in situ in three different stages. They consist of increasing number of islands growth, followed by filling up the gaps between islands. At last, within the adsorption time frame measured, the islands grow further in dimensions but not in numbers. This closely corresponds with the behavior of the octylphosphonic acid films analyzed by ex situ techniques.

In situ study of the deposition of (ultra)thin organic phosphonic acid layers on the oxide of aluminum.

The early stages of nanocrystal nucleation and growth are still an active field of research and remain unrevealed. In this work, by the combination of aberration-corrected transmission electron microscopy (TEM) and electrochemical characterization of the electrodeposition of different metals, we provide a complete reformulation of the Volmer-Weber 3D island growth mechanism, which has always been accepted to explain the early stages of metal electrodeposition and thin-film growth on low-energy substrates. We have developed a Generalized Electrochemical Aggregative Growth Mechanism which mimics the atomistic processes during the early stages of thin-film growth, by incorporating nanoclusters as building blocks. We discuss the influence of new processes such as nanocluster self-limiting growth, surface diffusion, aggregation, and coalescence on the growth mechanism and morphology of the resulting nanostructures. Self-limiting growth mechanisms hinder nanocluster growth and favor coalescence driven growth. The size of the primary nanoclusters is independent of the applied potential and deposition time. The balance between nucleation, nanocluster surface diffusion, and coalescence depends on the material and the overpotential, and influences strongly the morphology of the deposits. A small extent of coalescence leads to ultraporous dendritic structures, large surface coverage, and small particle size. Contrarily, full recrystallization leads to larger hemispherical monocrystalline islands and smaller particle density. The mechanism we propose represents a scientific breakthrough from the fundamental point of view and indicates that achieving the right balance between nucleation, self-limiting growth, cluster surface diffusion, and coalescence is essential and opens new, exciting possibilities to build up enhanced supported nanostructures using nanoclusters as building blocks.

Annick Hubin

Research Group Electrochemical and Surface Engineering

Vrije Universiteit Brussel

Emission Spectroscopic Boundary Layer Investigation during Ablative Material Testing in Plasmatron

Annick Hubin

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Research Group Electrochemical and Surface Engineering

Vrije Universiteit Brussel

Emission Spectroscopic Boundary Layer Investigation during Ablative Material Testing in Plasmatron

Research Group Electrochemical and Surface Engineering