Quantitative Analysis of Nitrogen by Atom Probe Tomography Using Stoichiometric γ′-Fe4N Consisting of 15N Isotope

The nitrogen deficiency in steels measured by atom probe tomography (APT) is considered to arise from the obscurement of singly charged dimer nitrogen ions (N2+) by the iron-dominant peak (56Fe2+) at 28 Da. To verify this by quantifying the amount of N2+ ions, γ′-Fe4N consisting of the 15N isotope was prepared on iron substrates by plasma nitriding using a nitrogen isotopic gas (15N2). Although considerable amounts of 15N2+ were observed at 30 Da without overlap with any iron peak, the observed nitrogen concentrations of γ′-Fe4N were clearly lower than the stoichiometric composition (19–20 at%), using both pulsed voltage and pulsed laser atom probes. The origin of the missing nitrogen, excluding nitrogen obscured by other ion species, was predicted to be the occurrence of neutral nitrogen or nitrogen gas molecules in field evaporation. The generation rate of iron nitride ions (FeN2+) for 15N was significantly lower than that for 14N in γ′-Fe4N, which affected the amount of the missing nitrogen. The isotope effect suggests that the isotopic ratio cannot always be determined from only one ion species among the multiple species observed in the APT analysis. We discuss the mechanism of the isotope effect in FeN2+ formation by field evaporation.

Reflections on the Spatial Performance of Atom Probe Tomography in the Analysis of Atomic Neighborhoods

Atom probe tomography (APT) is often introduced as providing “atomic-scale” mapping of the composition of materials and as such is often exploited to analyze atomic neighborhoods within a material. Yet quantifying the actual spatial performance of the technique in a general case remains challenging, as it depends on the material system being investigated as well as on the specimen's geometry. Here, by using comparisons with field-ion microscopy experiments, field-ion imaging and field evaporation simulations, we provide the basis for a critical reflection on the spatial performance of APT in the analysis of pure metals, low alloyed systems and concentrated solid solutions (i.e., akin to high-entropy alloys). The spatial resolution imposes strong limitations on the possible interpretation of measured atomic neighborhoods, and directional neighborhood analyses restricted to the depth are expected to be more robust. We hope this work gets the community to reflect on its practices, in the same way, it got us to reflect on our work.

Laser-Assisted Field Evaporation of (R = Gd, Sm) High-Temperature Superconducting Coated Conductors

In the field of high-temperature superconductors, atom probe tomography is a relatively new instrument, with the ability to provide a new perspective on the 3D nanoscale microstructure. However, field evaporation of nonmetallic materials is fraught with unique challenges that matter little in the world of metallic evaporation. In this study, we review the laser absorption, correlated evaporation, molecular dissociation, and the crystallographic effects on the field evaporation of 800-m ${\rm RB}{\rm a}_ 2{\rm C}{\rm u}_ 3{\rm O}_{ 7-{\rm \delta }}$ (R = Gd, Sm) coated conductor tapes deposited by Reactive Co-Evaporation Cyclic Deposition and Reaction (RCE-CDR). Ultraviolet 355 nm laser pulsing was found to have a substantial beneficial effect on minimizing the fracture probability compared with 532 nm illumination, especially when evaporating insulating oxide precipitates. This, in turn, allows for the 3D compositional analysis of defects such as flux pinning centers introduced by precipitation and doping. As a result, evidence for the precipitation of nanoscale ${\rm G}{\rm d}_ 2{\rm C}{\rm u}_ 2{\rm O}_ 5$ is discussed. The effect of crystallographic orientation is studied, where [001] aligned evaporation is found to develop compositional aberrations.

Do field observations agree with satellite-based evaporation products in the Miombo Woodland of Southern Africa?

<p>Evaporation is a major constraining factor of water availability at the land surface which makes its assessment a highly significant prerequisite for application in hydrological, agricultural, climate studies and many other disciplines at various scales. However, its importance and calculation procedures have largely been crafted around and often limited to crop productivity. The overarching consequence of this is inaccurate estimates of evaporation for other land surfaces and particularly for forest systems. Due to limited field evaporation observations attention has been focused on the application of satellite-based products. However, in the case of Africa, and the Miombo ecosystem in particular, the number of flux towers is extremely limited (very few if any) which makes it extremely difficult to evaluate available satellite-based evaporation products. In this study we used the energy balance Bowen ratio approach to estimate field evaporation in a dense Miombo Woodland which we then used to evaluate four energy balance evaporation models. The models evaluated included the MOD16, SEBS, SSEBop and WaPOR. Furthermore, cluster analysis was used to assess the similarity of the models in simulating evaporation. The results show that at daily and dekadal scale the simulated evaporation by the four models significantly varied from field evaporation observations. However, less variations were observed at monthly scale.  Furthermore, all four models overestimated evaporation during the dry season (June-September) with RMSE ranges between 0.21 – 0.38 mm.day<sup>-1</sup> and 6.64 - 9.91 mm.month<sup>-1</sup>. Based on the RMSE and biases the MOD16 (RMSE = 6.64 mm.month<sup>-1</sup>; Bias = 2.04 mm.month<sup>-1</sup>), SEBS (RMSE = 8.69 mm.month<sup>-1</sup>; Bias = 5.72 mm.month<sup>-1</sup>) and WaPOR (RMSE = 7.44 mm.month<sup>-1</sup>; Bias = 6.67 mm.month<sup>-1</sup>) ranked higher than the SSEBop (RMSE = 9.91 mm.month<sup>-1</sup>; Bias = 9.84 mm.month<sup>-1</sup>) in simulating evaporation in the Miombo Woodland. Three clusters were observed with the SEBS and WaPOR grouped together indicating their close similarity in simulating evaporation in the Miombo ecosystem while the MOD16 and SSEBop were each grouped separately. Results of this study could aid the interpretation of these evaporation models in Miombo Woodland covered basins such as the Zambezi River Basin in Southern Africa. This could help in monitoring basin water availability and ecosystem reactions and feedbacks to climate change and anthropogenic impacts.</p>

Effect of ultrahigh-frequency electromagnetic field on the properties of associated liquids

The influence of the electromagnetic field on the refractive index, evaporation rate and surface tension of wa-ter, propanol-1 and pentanol-1 solutions have been studied. It was shown that the properties of these liquids depend on the field frequency and the time of exposure. The action of the field on the structure of water and alcohols is selective; changes in their properties are due to frequencies that are individual for each liquid. Both deceleration and acceleration of the alcohols evaporation occurs depending on the frequency of the elec-tromagnetic field. Evaporation of the field exposed water is slowing down at all the studied frequency range. There is an increase in the surface tension for water and pentanol, and a decrease for propanol. The properties of alcohols return to their initial values, and the properties of the water remain unchanged after the termina-tion of the field action. Thermodynamic functions of surface water and propanol-1 have been calculated on the basis of the temperature dependence of the surface tension. It has been demonstrated that the total internal energy of the surface increases for water and reduces or propanol-1. This indicates the strengthening of the structure in an aqueous solutions and a weakening of intermolecular interaction in the propanol-1 medium.

Field evaporation and atom probe tomography of pure water tips

Measuring biological samples by atom probe tomography (APT) in their natural environment, i.e. aqueous solution, would take this analytical method, which is currently well established for metals, semi-conductive materials and non-metals, to a new level. It would give information about the 3D chemical structure of biological systems, which could enable unprecedented insights into biological systems and processes, such as virus protein interactions. For this future aim, we present as a first essential step the APT analysis of pure water (Milli-Q) which is the main component of biological systems. After Cryo-preparation, nanometric water tips are field evaporated with assistance by short laser pulses. The obtained data sets of several tens of millions of atoms reveal a complex evaporation behavior. Understanding the field evaporation process of water is fundamental for the measurement of more complex biological systems. For the identification of the individual signals in the mass spectrum, DFT calculations were performed to prove the stability of the detected molecules.

Use of Amount-of-Substance Terminology and Equations in Field Desorption Theory

This note proposes that the theories of field evaporation and field desorption, as used in atom-probe microscopy and related atomic-level contexts, should be consistently formulated in terms of a set of “seven-dimensional (7-D)” formulae and equations that involve the physical quantity “amount of substance”, but make use of an atomic-level constant effectively equal to “one atom” (or, more generally, “one entity”). It is argued that the term “count” should be introduced as an alternative name (more suited to atomic-level contexts) for the quantity “amount of substance”. For field evaporation/desorption theories, relevant definitions and formulae are proposed, and compared with the “six-dimensional” system (based on the dimensionless quantity “number of atoms/entities”) sometimes used in the literature. Advantages of using a 7-D system are noted. It is argued that there is also an increasing need for a comprehensive system of official nomenclature for atomic-level constants and units, for all three of the extensive quantities “mass”, “electric charge” and “amount of substance”. It is also argued that, in the longer term, considerations of the kind being proposed here for field evaporation/desorption theories might usefully be applied more generally in atomic-level rate theory.