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His main research interests are the development of novel synthetic methods, including those based on microfluidic reactors, for the synthesis of multifunctional nanomaterials. This instrument is specially designed to have an objective-area pressure interlock and differential pumping capabilities close to the electron source, enabling the use of gas and liquid operando holders. Retrieved There are several factors for this behavior. DeLon; Perkins, James B. National Institute for Occupational Safety and Health. Thin high quality TEM samples should have a thickness that is roughly equal to the mean free path electrons that transmit through the sample, which may only be a few tens of nanometers.
Molecular dynamics simulations were also conducted, taking into consideration facets of the nanoparticles that agreed with the experimental results [ 12 ]. In addition, MD results suggested that the growth rate of particle pairs on graphene was comparable to isolated pairs, which indicated that graphene had no significant effect on the coalescence.
We found that grain boundary energy was the main factor affecting the neck growth rate at the nanoscale.
Reproduced from reference 12 with permission from the Institute of Physics. SEM images of microstructural evolution of a graphene sheet decorated with Cu nanoparticles in full view, as a function of temperature and time a — f. Quantitative neck evolution of two nanoparticles at K shown graphically and with snapshots from a movie. Methane gas is highly volatile and when mixed with air can cause explosions at higher concentrations because it is readily flammable. Thus, the development of a reliable and cost-effective methane gas sensor is important.
Room-temperature detection of methane is challenging and has been reported by others, but those lack the temperature range capability for realistic applications [ 62 - 69 ]. The sensor was operated at room temperature and detection of 0. Both analyses indicated the rutile SnO 2 structure [ 13 , 60 ]. Inset shows the ED pattern.
Reproduced from reference 13 with permission from the Institute of Physics. The rutile structure of tin di-oxide was also resolved by HAADF imaging with atomic resolution as indicated in the inset image of figure This could traduce in approximately one-third reduction of unfueled weight of space vehicles and structures.
To achieve this, commercially available CNT-based materials must have at least two times the strength of conventional carbon fibers. CNT yarns are currently the best commercially available CNT-based materials in terms of mechanical properties; however, their tensile strength is about half of conventional carbon fibers. This has to be with the weak shear interactions between carbon shells and bundles within a yarn [ 73 - 74 ]. Therefore, current efforts focus on developing protocols to improve mechanical properties of these materials. One potential route to achieve mechanical improvement is the cross-linking method induced by electron beam irradiation.
E-beam energies greater than 80 keV are needed to displace C atoms and to induce complex kinetics and recombination of lattice defects within the hexagonal carbon network, which eventually leads to cross-linking [ 77 ]. Because CNT yarns are fibers composed of several MWCNTs, the question arises as to what extent energies in this range will still promote crosslinking effectively. The study of the electrical response of CNT yarns as a function of electron dose can be a complementary route to monitor possible cross-linking events, and is important to establishing multifunctional properties of CNT yarns.
Considerable effort has been focused in e-beam irradiation methods that lead to mechanical improvement. The electrical resistivity as a function of e-beam irradiation is studied by the two-probe method, using micromanipulators inside a SEM. The electrical resistivity as a function of e-beam irradiation is presented in figure The average values of resistivity increased with irradiation time up to 30 min and decreased with further irradiation.
Effect of e-beam irradiation on CNT yarn resistivity. The inset is an SEM image of two tungsten probe electrical setup measurement. Figure 12f is a schematic that summarizes possible crosslinking sites marked with red lines of CNT constituents within the yarn at two different scales. The images were taken in thin areas located at the edges of the yarns so one can see the different planes of CNTs oriented in a given twisted direction.
For the purpose of the following discussion, only areas of the images that are in focus are described. The area enclosed by a white circle in figure 12a shows that the CNTs of yarns are double walled, and the area enclosed by a black circle is consistent with a CNT bundle structure.
Crosslinking sites in CNTs can be monitored by these HRTEM images and typically correspond to areas were the fringes are less coherent but do not completely lose their structure to form an amorphous carbon a-C structure [ 75 ]. Cross-linking events can be observed at 10 min of irradiation fig. Several types of microstructural changes of CNTs within the yarn are evident at 20 min of irradiation fig. These includes cross-linked sites area enclosed by a black box , pristine-non-cross-linked sites yellow box , a-C structure sites red box [ 82 - 87 ], and sites with a mixture of a-C and cross-linked sites white box.
With further irradiation it can be noticed that both a-C and cross-linked sites grows; however, overall the crystallinity fringes structure of CNTs is preserved in the sense that it is not totally lost. In order to understand these microstructural changes and how to correlate them with the corresponding resistivity results, a brief explanation on e-beam irradiation on CNT defect formation is needed. E-beam electrons can displace C atoms located at the hexagonal lattice network of CNTs only when critical minimum energy is used, known as the displacement threshold.
However displacement threshold depends on the local arrangement of carbon atoms relative to the electron beam and type of CNT. This has to be with the direction of momentum transfer to C atoms distributed in the hexagonal lattice. For instance, displacement threshold energies of 82 keV have been reported for small CNTs oriented perpendicular to the e-beam, and up to keV for relative bulky CNTs oriented tangential to the e-beam [ 77 ].
Once C atoms are displaced, lattice defect formation in the form of interstitials and vacancies will take place. Based on quantum mechanics calculations, defects on the form of divacancies, interstitials, and Frenkel pair interstitial-vacancy pair defects were shown to crosslink graphitic layers [ 88 ]. However, at the same time e-beam irradiation can lead to unwanted loss of lattice coherence, a processes known as amorphization.
This has to be with the kinetics of defects production rate, dynamics on specific sites of the C lattice, and agglomeration of point defects that leads to larger defects. Dynamics of defects depends on temperature. E-beam irradiation at room temperature as in this work leads to the formation of vacancies and interstitials for energies above the threshold energy , which remain relatively localized immobile at specific lattice sites.
If they do not recombine to form cross-linking sites, high concentration and agglomeration of defects can occur as e-beam time increases, which eventually cause the lattice to lose its crystallinity at those sites.
This model is consistent with the amorphous regions encountered in figure 12c—12e. In terms of electrical results, the data can be explained in terms of a competitive process between cross-linking and amorphization. Cross-linking sites reduce the resistivity by reducing the CNT-to-CNT distance, while amorphization increases resistivity. Note that the reduction of resistivity by cross-linking events is not due to conduction of electrons through sp 3 C-C bonds. The increase in resistivity with10 min of e-beam irradiation can be explained by the formation of defects in the lattice that have not produced enough cross-linking sites to enhance conductivity.
At irradiation times of 20—30 min, although cross-linking events are increasing, the resistivity is dominated by amorphization events. The reduction of resistivity at 60 min can only be explained by a significant increase in cross-linking population dominating the overall electrical conduction of electrons in the yarn. This is consistent with the microstructural data presented in figure Note that there are micron-size as well as submicron-size grains within the microstructure.
For simplicity regions of type 1 will be defined as Ge rich regions, and regions of type 2 will be referred to as Si rich. The local variations in Si and Ge content are caused by segregation during non-equilibrium solidification. The rest will be referred to as the matrix. Loads of 0.
Note that the SEM image of figure 13f was taken with the sample oriented at an angle of 54 degrees relative to the electron beam and normal to the ion beam for the milling process. The inset of figure 13f is the surface view of the NI SEM image also taken at 54 degrees , where the radial cracks can be noticed. As a final step, a protective material is deposited at the top of the NI area followed by the formation of two laterals and one front trench for cross section serial sectioning, as indicated in figure 13f.
Reproduced from reference 18 with permission from Elsevier. Quantitative results for H and E for all phases can be found in figure 14a. For this analysis, a load of 0. Average H and E values in GPa of Figure 14b gives k c for different phases, formed with data taken at a 0. The complete set of data is reported elsewhere [ 18 ]. Again, note that the SEM images are oriented at an angle of 54 degrees relative to the electron beam and normal to the ion beam see fig.
Figure 15c presents a low-magnification bright-field BF TEM image of the deformation zone enclosed by a black box. The corresponding dark-field DF TEM image of the region enclosed in figure 15c is presented in figure 15d, which shows a crack white arrow , high density of dislocations around the nanoindentation area as well as the by the area enclosed with the white circle , and shear faults lines solid white arrows ; the inset is a schematic of shear lines.
A dotted white arrow points to a crack. The inset is a schematic of radial crack formation induced by nanoindentation. Higher population of nanosizes and less or no micron sized WSi 2 grains are needed. A higher population of nanosized WSi 2 will not only improve mechanical robustness, but can also improve ZT as well.