Atomic scale investigation on the distribution of boron in medium carbon steels by atom probe tomography and EELS

The medium carbon (0.5 wt% C) steels containing various boron contents were studied to observe the distribution of boron using atom probe tomography and electron energy loss spectroscopy. APT revealed the segregation of boron atoms at retained austenite for 100 ppm boron added steels and the trapped carbon atoms at micro-twins for 50 ppm boron treated steels. Moreover, it was also found that boron was randomly distributed for 20 ppm boron added steels regardless of the interactions between carbon and boron.     

Quantitative analysis of carbon content in cementite in steel by atom probe tomography

Atom probe performance in the quantitative analysis of carbon atoms in steel was investigated through analysis of stoichiometric spherical cementite (Fe₃C) in steel. The carbon concentration was estimated by determining the mean carbon number of molecular ions having a mass-to-charge ratio of 24. The apparent carbon concentration of cementite increased as the specimen temperature decreased, and it was several at% higher than the stoichiometric value (25 at%) under the preferable condition of low specimen temperature. On the other hand, the apparent carbon concentration was not changed by pulse fraction. These results indicate that the large deviation from the stoichiometric value did not arise from the preferential retention and evaporation between carbon and iron. The other mechanisms explaining the phenomenon have been discussed.     

Three-dimensional atom mapping of boron in implanted silicon

The redistribution of boron in highly implanted 〈1 0 0〉 silicon (10 keV; 5×10¹⁵ at/cm²) annealed at 600 °C for 1 h was studied using both laser-assisted wide-angle atom probe (LaWaTAP) and secondary ion mass spectrometry (SIMS). As expected, the concentration was found to increase steeply to 10²¹ boron atoms/cm³ at a distance close to 35 nm and to decrease slowly to 10¹⁹/cm³, a value close to the boron level of the silicon substrate. For depth under 75 nm, the implantation profile of boron as given by LaWaTAP was found very close to that given by SIMS investigations without any calibration of the LaWaTAP data. For larger depth, the LaWaTAP profile is observed above that of SIMS. Detection limits of LaWaTAP for low dopant concentrations are discussed. The contribution of the background noise in the spectrum and sampling errors are considered. Fine-scale fluctuations not detected in SIMS profile and related to clustering were evidenced in LaWaTAP maps and profiles. Numerous boron clusters lying on {0 0 1} planes parallel to the implanted surface, a few nanometer in size, were identified and interpreted as boron interstitial clusters (BICs), in agreement with Cristiano et al. observations. They contained between 50 and 300 atoms (Si and B). This is much higher than that generally assumed in particular in ab-initio modelling where a few atoms BICs are considered. These clusters contained 7 at% of boron in average.     

Quantitative atom probe analysis of carbides

Compared to atom probe analysis of metallic materials, the analysis of carbide phases results in an enhanced formation of molecular ions and multiple events. In addition, many multiple events appear to consist of two or more ions originating from adjacent sites in the material. Due to limitations of the ion detectors measurements generally underestimate the carbon concentration. Analyses using laser-pulsed atom probe tomography have been performed on SiC, WC, Ti(C,N) and Ti₂AlC grains in different materials as well as on large M₂₃C₆ precipitates in steel. Using standard evaluation methods, the obtained carbon concentration was 6-24% lower than expected from the known stoichiometry. The results improved remarkably by using only the ¹³C isotope, and calculating the concentration of ¹²C from the natural isotope abundance. This confirms that the main reason for obtaining a too low carbon concentration is the dead time of the detector, mainly affecting carbon since it is more frequently evaporated as multiple ions. In the case of Ti(C,N) and Ti₂AlC an additional difficulty arises from the overlap between C₂⁺, C₄²⁺ and Ti²⁺ at the mass-to-charge 24 Da.     

Carbide characterization in low-temperature tempered steels

The nature of the initial carbides formed during the early stages of the tempering of steels is still a matter of debate. Conventionally, the main transition carbide is described as epsilon carbide, with a composition of approximately Fe_2.4C. However, earlier one-dimensional atom probe (1DAP) results indicated the existence of carbon-rich regions having much lower carbon contents, with maxima of around 10 at%. There was some uncertainty about the interpretation of the 1DAP results, because of possible problems with alignment of the aperture and with trajectory aberration effects. We have therefore re-visited this topic, using the three-dimensional (3D) atom probe, and studying both a model Fe–Ni–C alloy and a well-known engineering steel (AISI4340). We demonstrate that, for both materials, low-temperature (20–150 °C) aging produces carbon-rich regions with average peak carbon contents of up to 10%. We show for the first time the three-dimensional structure of these carbon-rich regions, and demonstrate that fine-scale faulting exists within them.     

APT analyses of deuterium-loaded Fe/V multi-layered films

Interaction of hydrogen with metallic multi-layered thin films remains as a hot topic in recent days. Detailed knowledge on such chemically modulated systems is required if they are desired for application in hydrogen energy system as storage media. In this study, the deuterium concentration profile of Fe/V multi-layer was investigated by atom probe tomography (APT) at 60 and 30 K. It is firstly shown that deuterium-loaded sample can easily react with oxygen at the Pd capping layer on Fe/V and therefore, it is highly desired to avoid any oxygen exposure after D₂ loading before APT analysis. The analysis temperature also has an impact on D concentration profile. The result taken at 60 K shows clear traces of surface segregation of D atoms towards analysis surface. The observed diffusion profile of D allows us to estimate an apparent diffusion coefficient D. The calculated D at 60 K is in the order of 10⁻¹⁷ cm²/s, deviating 6 orders of magnitude from an extrapolated value. This was interpreted with alloying, D-trapping at defects and effects of the large extension to which the extrapolation was done. A D concentration profile taken at 30 K shows no segregation anymore and a homogeneous distribution at c_D=0.05(2) D/Me, which is in good accordance with that measured in the corresponding pressure–composition isotherm.     

Gallium-enhanced phase contrast in atom probe tomography of nanocrystalline and amorphous Al-Mn alloys

Over a narrow range of composition, electrodeposited Al-Mn alloys transition from a nanocrystalline structure to an amorphous one, passing through an intermediate dual-phase nanocrystal/amorphous structure. Although the structural change is significant, the chemical difference between the phases is subtle. In this study, the solute distribution in these alloys is revealed by developing a method to enhance phase contrast in atom probe tomography (APT). Standard APT data analysis techniques show that Mn distributes uniformly in single phase (nanocrystalline or amorphous) specimens, and despite some slight deviations from randomness, standard methods reveal no convincing evidence of Mn segregation in dual-phase samples either. However, implanted Ga ions deposited during sample preparation by focused ion-beam milling are found to act as chemical markers that preferentially occupy the amorphous phase. This additional information permits more robust identification of the phases and measurement of their compositions. As a result, a weak partitioning tendency of Mn into the amorphous phase (about 2 at%) is discerned in these alloys.     

Study of structures at the boundary and defects in organic thin films of perchlorocoronene by high-resolution and analytical transmission electron microscopy

Both the periodic and non-periodic structures of perchlorocoronene (C₂₄Cl₁₂) crystals were characterized by high-resolution transmission electron microscopy (HRTEM), electron diffraction (ED), electron energy-loss spectroscopy (EELS), and energy-filtered transmission electron microscopy (EFTEM). The HRTEM images at the boundary of the C₂₄Cl₁₂ crystals exhibit the flexibility of defect structures, where molecules align to compensate for the discontinuity between two different domains. Emphasized by the filtered images, it was found that the non-periodic regions are created everywhere with a small electron beam irradiation (∼10⁶ electrons nm⁻²) and then spread over the entire regions to completely destroy the periodic structures after a higher electron dose (∼2×10⁶ electrons nm⁻²). The effect of the electron beam irradiation was monitored by ED, EELS, and EFTEM, where periodic structures and content elements are well preserved up to 10⁶ electrons nm⁻², but chlorine atoms decreased with a much higher electron dose. This is explained by the breakage of the C–Cl bond to detach chlorine atoms, confirmed by energy-loss near the edge structures (ELNES) of carbon π? peaks and chlorine loss at the edge of the specimen, as well as by theoretical simulation. The detachment of chlorine is localized at the peripheral edge around a hole confirmed by core-loss EFTEM imaging.     

Effects of carbon content on the high temperature friction and wear of chromium carbonitride coatings

Chromium nitride-based coatings are often used in application at high temperature. They possess high wear and oxidation resistance; however, the friction coefficient is typically very high. Therefore, we doped CrN coatings by carbon with the aim to improve tribological properties at elevated temperature, particularly to lower the friction. CrCN coatings were prepared by cathode arc evaporation technology using constant N₂ flow and variable C₂H₂ flow. The coatings with a thickness of 3-4 μm were deposited on hardened steel substrates and high-temperature resistant alloy. The carbon content varied from 0 at.% (i.e. CrN) up to 31 at.%. The standard coating characterization included the nano-hardness, adhesion, chemical composition and structure (including hot X-ray diffraction). Wear testing was done using a high temperature tribometer (pin-on-disc); the maximum testing temperature was 700 °C. The coatings with carbon content 12-31 at.% showed almost identical tribological behaviour up to 700 °C.     

The influence of elastic strain on the early stages of decomposition in Cu–1.7 at% Fe

The initial stage of decomposition of homogenized Cu–1.7 at% Fe at 722 K was investigated by means of field ion microscopy (FIM), atom probe tomography (APT) and computer-assisted field ion image tomography (cFIIT). The agglomeration of atoms depending on time could be investigated and the growth of precipitates with a diameter of few nanometers was observed during ongoing nucleation.     

Behaviour of TEM metal grids during in-situ heating experiments

The stability of Ni, Cu, Mo and Au transmission electron microscope (TEM) grids coated with ultra-thin amorphous carbon (α-C) or silicon monoxide film is examined by in-situ heating up to a temperature in the range 500–850 °C in a transmission electron microscope. It is demonstrated that some grids can generate nano-particles either due to the surface diffusion of metal atoms on amorphous film or due to the metal evaporation/redeposition. The emergence of nano-particles can complicate experimental observations, particularly in in-situ heating studies of dynamic behaviours of nano-materials in TEM. The most widely used Cu grid covered with amorphous carbon is unstable, and numerous Cu nano-particles start to form once the heating temperature reaches 600 °C. In the case of Ni grid covered with α-C film, a large number of Ni nano-crystals occur immediately when the temperature approaches 600 °C, accompanied by the graphitization of amorphous carbon. In contrast, both Mo and Au grids covered with α-C film exhibit good stability at elevated temperature, for instance, up to 680 and 850 °C for Mo and Au, respectively, and any other metal nano-particles are detected. Cu grid covered Si monoxide thin film is stable up to 550 °C, but Si nano-crystals appear under intensive electron beam. The generated nano-particles are well characterized by spectroscopic techniques (EDXS/EELS) and high-resolution TEM. The mechanism of nano-particle formation is addressed based on the interactions between the metal grid and the amorphous carbon film and on the sublimation of metal.     

Two-dimensional ordering of pentafluorobenzenethiol self-assembled monolayers on Au(1 1 1) prepared by ambient-pressure vapor deposition

Formation and surface structures of pentafluorobenzenethiol (PFBT) self-assembled monolayers (SAMs) on Au(1 1 1) prepared by ambient-vapor phase deposition were examined by means of scanning tunneling microscopy (STM) as a function of deposition temperature. PFBT SAMs formed at room temperature have disordered phases with bright aggregated domains, which are very similar to benzenethiol SAMs. As deposition temperature increases to 50 °C, partially ordered domains and large aggregated domains appeared. High-resolution STM clearly revealed that PFBT SAMs formed at 75 °C were composed of long-range, two-dimensional (2D) ordered domains, which can be described as a c(2×√3) structure. The results of this study clearly demonstrate that deposition temperature is a crucial factor for obtaining PFBT SAMs on Au(1 1 1) with a high degree of structural order.     

The influence of Cu addition on precipitation in Fe–Cr–Ni–Al–(Cu) model alloys

Precipitation in Fe–Cr–Ni–Al–(Cu) model alloys was investigated after ageing for 0.25, 3, 10 and 100 h at 798 K. Characterization of nanoscale precipitates was performed using three-dimensional atom probe microscopy and transmission electron microscopy. The precipitates are found to be enriched in Ni and Al (Cu) and depleted in Fe and Cr. After 0.25 h of ageing the number density of precipitates is ∼8×10²⁴ m⁻³, their volume fraction is about 15.5% and they are near-spherical with an average diameter of about 2–3 nm. During further ageing the precipitates in the both alloys grow, but the coarsening behaviour is different for both alloys. The precipitates of the Cu-free alloy grow much faster compared with the Cu-containing alloy and their density decreases. Precipitates in Cu-free alloy change to plate shaped even after 10 h of ageing, whereas those of Cu-containing alloy remain spherical up to 10 h of ageing. The influence of Cu addition on precipitation in these model alloys is discussed with respect to the different coarsening mechanisms.     

Effect of decomposition of the Cr-Fe-Co rich phase of AlCoCrCuFeNi high entropy alloy on magnetic properties

Splat-quenched, as-cast and aged (2 h at 600 °C after casting) AlCoCrCuFeNi high entropy alloys were investigated by means of transmission electron microscopy and three-dimensional atom probe (3D-AP). 3D-AP revealed anti-correlated fluctuations of the Cr and Fe-Co compositions in Cr-Fe-Co-rich regions of the as-cast alloy. The ferromagnetic behavior of AlCoCrCuFeNi high entropy alloy was correlated with the decomposition of the Cr-Fe-Co-rich regions into ferromagnetic Fe-Co-rich and antiferromagnetic Cr-rich domains, the size of which was determined by statistical analysis of 3D-AP data. The splat-quenched alloy showed a softer magnetic behavior as compared to the as-cast and aged alloys. The aged alloy possessed a higher saturation magnetization and coercivity as compared to the as-cast alloy.     

Investigation of some dielectric properties of phosphate glasses doped with iron oxides,by a microwave technique

The effects of iron ions on dielectric properties of lithium sodium phosphate glasses were studied by non-usual, fast and non-destructive microwave techniques. The dielectric constant (ε′), insertion loss (L) and microwave absorption spectra (microwave response) of the selected glass system xFe₂O₃·(1 − x)(50P₂O₅·25Li₂O·25Na₂O), being x = 0, 3, 6, … , 15 expressed in mol.%, were investigated. The dielectric constant of the samples was investigated at 9.00 GHz using the shorted-line method (SLM) giving the minimum value of ε′ = 2.10 ± 0.02 at room temperature, and increasing further with x, following a given law. It was observed a gradual increasing slope of ε′ in the temperature range of 25 ? t ? 330 °C, at the frequency of 9.00 GHz. Insertion loss (measured at 9.00 GHz) and measurements of microwave energy attenuation, at frequencies ranging from 8.00 to 12.00 GHz were also studied as a function of iron content in the glass samples.     

Measuring temperature dependence of photoelectric conversion efficiency with dye-sensitized solar cells

It’s well known that the drift velocity of electrons in conductors depends on temperature in accordance with thermodynamics, which influences also photoelectric conversion efficiency of solar cells. The article presents experimental data for studying temperature influence of photoelectric conversion efficiency with dye-sensitized solar cells (DSSCs). The measured DSSCs were built in three layers, the photoelectrode, the electrolyte, and the counter electrode, which were made in the CCT laboratory, National Taipei University of Technology, Taiwan. The photoelectrode is coated by using ? = 21 nm nano TiO₂ and dye as well as the counter electrode using ? = 5 nm nano carbon black on their individual ITO glass. The fluidic electrolyte is used in this work due to its good ionic drift property. In process, the DSSC was waterproof and immersed in the constant temperature water tank for temperature adjusting. The measured temperature range was from ca. 5 °C to 80 °C at an interval of ca. 10 °C. The results show the higher temperature, the lower photoelectric conversion efficiency of DSSCs.     

Optimum HRTEM image contrast at 20 kV and 80 kV--exemplified by graphene

The dependence of high-resolution transmission electron microscopy (HRTEM) image contrast of graphene on the adjustable parameters of an aberration-corrected microscope operated at 80 and 20 kV has been calculated and, for 80 kV, compared with measurements. We used density functional theory to determine the projected atom potential and obtained the image intensity by averaging over the energy distribution of the imaging electrons, as derived from the electron energy loss spectroscopy measurements. Optimum image contrast has been determined as a function of energy spread of the imaging electrons and chromatic aberration coefficient, showing that significant improvement of contrast can be achieved at 80 kV with the help of a monochromator, however at 20 kV only with chromatic aberration correction and bright atom contrast conditions.     

Influence of laser irradiation condition on a femtosecond laser-assisted tomographic atom probe

Influence of femtosecond laser pulse condition on the performance of an energy-compensated optical tomographic atom probe has been investigated. The unstable oscillator makes the mass peaks significantly broadened. Double 80 fs pulse train with 10 ns interval makes the mass peaks slightly shifted to the higher mass side. The mass peak shift corresponds to the fight time of ions triggered by laser pulsing. Chirping ratio for the laser pulses ranging from 80 fs to 10 ps is controlled by the pulse compressor for the fragile specimens such as oxide dispersion strengthen steel or insulator materials. A first-principle calculation for optical dielectric breakdown in diamond has been successfully demonstrated. It is shown that effective conductive increase has appeared at the laser intensity around 10¹³ W/cm².     

The effect of body temperature on the accuracy of arterial and end-tidal carbon dioxide measurement

Measurement of carbon dioxide has great clinical significance during mechanical ventilation, in the adjustment of ventilatory parameters and detection of respiratory complications. The main objective is to investigate the correlation between end-tidal carbon dioxide pressure (PetCO₂) and partial pressure of arterial carbon dioxide (PaCO₂) measured at 37 °C and corrected for body temperature in patients with thermal instability. Altogether, 110 measurements were analyzed, and the correlation was statistically more significant for corrected temperature than measured PaCO₂. The difference between corrected and uncorrected PaCO₂ varies from 3% per °C for hypothermic patients and 6.5% per °C for hiperthermic patients. The difference between PaCO₂ measured and PetCO₂ (Pa-etCO₂) resulted in an increase for all temperature degree, reaching a maximum difference of 9 torr. In contrast, Pa-etCO₂ has little variation when corrected PaCO₂ was used for calculation around −2.1 to 3.1 torr for hypo and hiperthermic patients. Thus, PetCO₂ reflects temperature corrected PaCO₂ more adequately than measured PaCO₂.     

Detecting density variations and nanovoids

A combination of simulated and experimental data has been used to investigate the size range of nanovoids that can be detected in atom probe tomography data. Simulated atom probe tomography data have revealed that nanovoids as small as 1 nm in diameter can be detected in atom probe tomography data with the use of iso-density surfaces. Iso-density surfaces may be used to quantify the size, morphology and number density of nanovoids and other variations in density in atom probe tomography data. Experimental data from an aluminum-yttrium-iron metallic glass ribbon have revealed the effectiveness of this approach. Combining iso-density surfaces with atom maps also permits the segregation of solute to the nanovoids to be investigated. Field ion microscopy and thin section atom maps have also been used to detect pores and larger voids.