R.L. Martens

Effects of laser energy and wavelength on the analysis of LiFePO4 using laser assisted atom probe tomography

The effects of laser wavelength (355 nm and 532 nm) and laser pulse energy on the quantitative analysis of LiFePO₄ by atom probe tomography are considered. A systematic investigation of ultraviolet (UV, 355 nm) and green (532 nm) laser assisted field evaporation has revealed distinctly different behaviors. With the use of a UV laser, the major issue was identified as the preferential loss of oxygen (up to 10 at%) while other elements (Li, Fe and P) were observed to be close to nominal ratios. Lowering the laser energy per pulse to 1 pJ/pulse from 50 pJ/pulse increased the observed oxygen concentration to nearer its correct stoichiometry, which was also well correlated with systematically higher concentrations of (16)O₂(+) ions. Green laser assisted field evaporation led to the selective loss of Li (~33% deficiency) and a relatively minor O deficiency. The loss of Li is likely a result of selective dc evaporation of Li between or after laser pulses. Comparison of the UV and green laser data suggests that the green wavelength energy was absorbed less efficiently than the UV wavelength because of differences in absorption at 355 and 532 nm for LiFePO₄. Plotting of multihit events on Saxey plots also revealed a strong neutral O2 loss from molecular dissociation, but quantification of this loss was insufficient to account for the observed oxygen deficiency.

Fabrication of specimens of metamorphic magnetite crystals for field ion microscopy and atom probe microanalysis

Field ion specimens have been successfully fabricated from samples of metamorphic magnetite crystals (Fe3O4) extracted from a polymetamorphosed, granulite-facies marble with the use of a focused ion beam. These magnetite crystals contain nanometer-scale, disk-shaped inclusions making this magnetite particularly attractive for investigating the capabilities of atom probe field ion microscopy (APFIM) for geological materials. Field ion microscope images of these magnetite crystals were obtained in which the observed size and morphology of the precipitates agree with previous results. Samples were analyzed in the energy compensated optical position-sensitive atom probe. Mass spectra were obtained in which peaks for singly ionized 16O, 56Fe and 56FeO and doubly ionized 54Fe, 56Fe and 57Fe peaks were fully resolved. Manganese and aluminum were observed in a limited analysis of a precipitate in an energy compensated position sensitive atom probe.

Grain Boundary Specific Segregation in Nanocrystalline Fe(Cr)

A cross-correlative precession electron diffraction – atom probe tomography investigation of Cr segregation in a Fe(Cr) nanocrystalline alloy was undertaken. Solute segregation was found to be dependent on grain boundary type. The results of which were compared to a hybrid Molecular Dynamics and Monte Carlo simulation that predicted the segregation for special character, low angle, and high angle grain boundaries, as well as the angle of inclination of the grain boundary. It was found that the highest segregation concentration was for the high angle grain boundaries and is explained in terms of clustering driven by the onset of phase separation. For special character boundaries, the highest Gibbsain interfacial excess was predicted at the incoherent ∑3 followed by ∑9 and ∑11 boundaries with negligible segregation to the twin and ∑5 boundaries. In addition, the low angle grain boundaries predicted negligible segregation. All of these trends matched well with the experiment. This solute-boundary segregation dependency for the special character grain boundaries is explained in terms of excess volume and the energetic distribution of the solute in the boundary.

Experimental Evaluation of Conditions Affecting Specimen Survivability in Atom Probe Tomography

It is a miraculous quirk of nature that any material can survive application of the extreme electric fields necessary to extract atoms from a specimen surface one-at-a-time without bulk rupture of the material itself. The field of atom probe tomography (APT) relies upon this natural quirk and continues to find an ever widening variety of materials can be successfully investigated [1], yet the issue of premature specimen failure remains critical for continued field growth and adoption [2].

Influence of Grain Boundary Character and Annealing Time on Segregation in Commercially Pure Nickel

Commercially pure nickel (Ni) was thermomechanically processed to promote an increase in Σ3 special grain boundaries. Engineering the character and chemistry of Σ3 grain boundaries in polycrystalline materials can help in improving physical, chemical, and mechanical properties leading to improved performance. Type-specific grain boundaries (special and random) were characterized using electron backscatter diffraction and the segregation behavior of elements such as Si, Al, C, O, P, Cr, Mg, Mn, B, and Fe, at the atomic level, was studied as a function of grain boundary character using atom probe tomography. These results showed that the random grain boundaries were enriched with impurities to include metal oxides, while Σ3 special grain boundaries showed little to no impurities at the grain boundaries. In addition, the influence of annealing time on the concentration of segregants on random grain boundaries was analyzed and showed clear evidence of increased concentration of segregants as annealing time was increased.

Improving Data Quality in Atom Probe Tomography

Figure 1 schematically illustrates several methods to improve APT specimen yield through decreasing stress by decreasing the evaporation field required during data collection: 1) decrease ion detection rate, 2) increase specimen base temperature, 3) use laser rather than voltage pulsing, and 4) increase laser pulse energy. The current work explores the use of thin coatings to modify the thermal and/or optical properties of 302 stainless steel, SiN, or a Si/SiO2/Si/Ni test structure (Fig. 2) in order to improve yield [7]. The APT data collection parameters were nominally: specimen temperature 30 K, laser pulse energy 30 pJ at 625 kHz, and a detection rate of one ion every 333-1000 pulses.

Atom Probe Tomography Analysis of Bulk Chemistry in Mineral Standards

Atom Probe Tomography (APT) analysis of materials is an established technique for atomic level compositional analysis. Extensive research has been performed on many alloys, compounds, multilayered thin films, integrated circuits, and even polymers [1]. While some geological materials, mainly minerals and biominerals, have been previously studied [2-4], the use of APT for the resolution of bulk chemistry of minerals and its relationship to stoichiometry are insufficiently constrained. Often, the mass spectra from natural geological samples exhibit many complex ionic species, and their interpretation can lead to differing determinations of composition.

Atom Probe Tomography Studies of FePt Thin Films

FePt films deposited directly onto Si <001> wafers adopt a strong {111} fiber texture as well as the A1 disordered phase However if FePt films are deposited onto a Introduction Th ti bit f l t d it h d d t i Focus Ion Beam ‘lift-out’ of an Atom Probe Tip: T i t i th t ti f th t b d t t T i i El t Mi (TEM) i i . , MgO <001> substrate heated at 350°C or higher, the FePt film adopts the L10 ordered phase and the [001] orientation. Several CrRu seed-layers were processed before attaining the correct composition and thickness to facilitate the epitaxial growt. Fig 3(a) is an atom probe reconstruction of both the CrRu seed-layer and the FePt film. An isosurface has been added to indicated the CrRu/FePt interface. Fig 3(b) is an Fe atom map oriented to depict the Fe (001) planes. Fig 3(c) and 3(d) are spatial distribution maps (SDM) generated from a 5 nm by 5 nm cube near the (001) crystallographic pole. From these SDMs the d001 was determined to be 3.64Å. e magne c or area s orage ens y as ecrease o a s ze that is rapidly approaching the superparamagnetic barrier; the thermal stability limit where magnetization randomly fluctuates because of the small magnetic volume. The thermal stability of very small magnetic crystals or grains can be improved if the material has a large uniaxial magnetocrystalline anisotropy, Ku. The L10 phase of FePt has been identified as a candidate material for ultra-high magnetic storage media because of its high Ku [1]. When this intermetallic is sputter-deposited as a polycrystalline thin film, a metastable solid solution face-centered-cubic phase (A1) is formed [2]. By annealing at temperatures in excess of 500°C, the crystalline lattice atomistically orders into the desired hard ti L1 h Th h b li it d i t l t di h o ass s n e recons ruc on o e a om pro e a a se s, ransm ss on ec ron croscopy mag ng was performed on a Focus Ion Beam (FIB) lift-out specimen of an atom probe tip, as shown in Fig.1. The TEM was performed using a 200keV FEI Tecnai F20. The FIB milling was performed using a FEI Quanta 3D dual beam FIB. The phase identification of the films was determined by XRD using a Bruker Discovery D8 diffractometer operating at 40kV and 35 mA with Co K radiation as the source.