Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userA transmission electron microscopy study of presolar hibonite
148
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We report isotopic and microstructural data on five presolar hibonite grains identified in an acid residue of the Krymka LL3.1 ordinary chondrite. Isotopic measurements by secondary ion mass spectrometry (SIMS) verified a presolar circumstellar origin for the grains. Transmission electron microscopy (TEM) examination of the crystal structure and chemistry of the grains was enabled by in situ sectioning and lift-out with a focused-ion-beam scanning-electron microscope. Comparisons of isotopic compositions with models indicate that four of the five grains formed in low-mass stars that evolved through the red-giant/asymptotic-giant branches, whereas one grain formed in the ejecta of a Type II supernova. Selected-area electron-diffraction patterns show that all grains are single crystals of hibonite. Some grains contain stacking faults and small spreads in orientation that can be attributed to a combination of growth defects and mechanical processing by grain-grain collisions. The similar structure of the supernova grain to those from RGB/AGB stars indicates a similarity in the formation conditions. Radiation damage, if present, occurs below our detection limit. Of the five grains we studied, only one has the pure hibonite composition of CaAl12O19. All others contain minor amounts of Mg, Si, Ti, and Fe. The microstructural data are generally consistent with theoretical predictions, which constrain the circumstellar condensation temperature to a range of 1480 K to 1743 K, assuming a corresponding total gas pressure between 1 x 10-3 and 1 x 10-6 atm. The TEM data were used to develop a calibration for SIMS determination of Ti contents in oxide grains. Grains with extreme 18O depletions, indicating deep mixing has occurred in their parent AGB stars, are slightly Ti-enriched compared to grains from stars without deep mixing, most likely reflecting differences in grain condensation conditions.
rate research
Read More
For quantitative electron microscopy high precision position information is necessary so that besides an adequate resolution and sufficiently strong contrast of atoms, small width of peaks which represent atoms in structural images is needed. Size of peak is determined by point spread (PS) of instruments as well as that of atoms when point resolution reach the subangstrom scale and thus PS of instruments is comparable with that of atoms. In this article, relationship between PS with atomic numbers, sample thickness, and spherical aberration coefficients will be studied in both negative Cs imaging (NCSI) and positive Cs imaging (PCSI) modes by means of dynamical image simulation. Through comparing the peak width with different thickness and different values of spherical aberration, NCSI mode is found to be superior to PCSI considering smaller peak width in the structural image.
Alnico is a prime example of a finely tuned nanostructure whose magnetic properties are intimately connected to magnetic annealing (MA) during spinodal transformation and subsequent lower temperature annealing (draw) cycles. Using a combination of transmission electron microscopy and atom probe tomography, we show how these critical processing steps affect the local composition and nanostructure evolution with impact on magnetic properties. The nearly 2-fold increase of intrinsic coercivity ($H_text{ci}$) during the draw cycle is not adequately explained by chemical refinement of the spinodal phases. Instead, increased Fe-Co phase ($alpha_1$) isolation, development of Cu-rich spheres/rods/blades and additional $alpha_1$ rod precipitation that occurs during the MA and draw, likely play a key role in $H_text{ci}$ enhancement. Chemical ordering of the Al-Ni-phase ($alpha_2$) and formation of Ni-rich ($alpha_3$) may also contribute. Unraveling of the subtle effect of these nano-scaled features is crucial to understanding on how to improve shape anisotropy in alnico magnets.
In the quest for dynamic multimodal probing of a materials structure and functionality, it is critical to be able to quantify the chemical state on the atomic and nanoscale using element specific electronic and structurally sensitive tools such as electron energy loss spectroscopy (EELS). Ultrafast EELF, with combined energy, time, and spatial resolution in a transmission electron microscope, has recently enabled transformative studies of photo excited nanostructure evolution and mapping of evanescent electromagnetic fields. This article aims to describe the state of the art experimental techniques in this emerging field and its major uses and future applications.
It has been implicitly assumed that ices on grains in molecular clouds and proto planetary disks are formed by homogeneous layers regardless of their composition or crystallinity. To verify this assumption, we observed the H2O deposition onto refractory substrates and the crystallization of amorphous ices (H2O, CO2, and CO) using an ultra-high-vacuum transmission electron microscope. In the H2O-deposition experiments, we found that three-dimensional islands of crystalline ice (Ic) were formed at temperatures above 130 K. The crystallization experiments showed that uniform thin films of amorphous CO and H2O became three-dimensional islands of polyhedral crystals; amorphous CO2, on the other hand, became a thin film of nano crystalline CO2 covering the amorphous H2O. Our observations show that crystal morphologies strongly depend not only on the ice composition, but also on the substrate. Using experimental data concerning the crystallinity of deposited ices and the crystallization timescale of amorphous ices, we illustrated the criteria for ice crystallinity in space and outlined the macroscopic morphology of icy grains in molecular clouds as follows: amorphous H2O covered the refractory grain uniformly, CO2 nano-crystals were embedded in the amorphous H2O, and a polyhedral CO crystal was attached to the amorphous H2O. Furthermore, a change in the grain morphology in a proto-planetary disk is shown. These results have important implications for the chemical evolution of molecules, non-thermal desorption, collision of icy grains, and sintering.
Single atoms can be considered as basic objects for electron microscopy to test the microscope performance and basic concepts for modeling of image contrast. In this work high-resolution transmission electron microscopy was applied to image single platinum atoms in an aberration-corrected transmission electron microscope. The atoms are deposited on a self-assembled monolayer substrate which induces only negligible contrast. Single-atom contrast simulations were performed on the basis of Weickenmeier-Kohl and Doyle-Turner scattering factors. Experimental and simulated intensities are in full agreement on an absolute scale.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
