It is well-established that stressing nanocrystalline metals can lead to grain growth which can result from tensile or compressive stress, high pressure torsion or the result of microhardness testing with an indenter [1][2][3]. The effect can be large, especially in the case of high-purity single-element nanocrystalline metals. The grain growth in the vicinity of a Vickers indenter during hardness testing of nanocrystalline copper has shown that an initial average grain size of about 20 nm doubles after 10 seconds of indenter dwell time [2]. The grain growth persists over hours of dwell time. At liquid nitrogen temperature grain growth is even more striking, with the rapid appearance of some large grains approaching micrometer size. The fast growth at the cryogenic temperature suggests that the grain growth is stress driven. It is not clear what the mechanism is for grain growth during indentation.To shed some light on these questions, we have carried out electron microscopy studies of the internal structure of nanocrystalline Cu near microhardness indentations. Foils taken from the vicinity of indents have been examined for changes in the internal structure and for chemical information using high resolution (scanning) transmission electron microscopy (HR(S)TEM) operated at 200 kV and equipped with an energy dispersive X-ray (EDX) spectrometer for chemical analyses. The nanocrystalline samples used in this study are made by inert gas condensation and in-situ compaction. In the present study, foils have been examined from indents made at room temperature with 10 sec indenter dwell time and 30 min. dwell time, and from indents made at 77 K with 1 min. dwell time and 30 min. dwell time. A large number of grains and grain boundaries have been examined which has made it possible to draw some general, qualitative conclusions about the effect of indenter dwell time and temperature of indentation on the internal structure of the nanocrystalline Cu.General observations about the effect of indenter dwell time and temperature show grains are smaller away from the indents. At both room temperature and 77 K, the grain sizes in the vicinity of the indenter become larger as the dwell times increase. At both the temperatures, the number of low angle grain boundaries is greater at the longer dwell time. Arrays of parallel dislocation lines are seen at room temperature (Fig.1). The arrays are present especially at 30 min dwell time when the grains have grown considerably; fewer such arrays are seen at 77 K. Twins have been observed with a (111) twinning plane in the <110> direction. But, less frequently, twins are also seen with a (110) twinning plane in the <111> direction (Fig. 2). The number of twins appear to increase with increasing indenter dwell time, indicating that they are deformation twins. They are more frequent at 77 K than at room temperature.The results indicate that besides observations of grain growth, the number of low angle grain boundaries increase with indenter dwell time and decreasing temperature, suggesting...
A combination of high-resolution electron microscopy and electron diffraction methods is used to obtain microstructural information of lithium-bearing glass ceramics, Li20-A1203-4SIO2 with TiO2 as a nucleating agent (system A), and of the commercial system 0.68Li203 A120 3 6.1SIO 2 0.13ZnO 0.03Na20 0.01K20 0.11TiO 2 0.077ZRO 2 (system D). The experiments reveal the presence of small amounts of y-spodumene in system A. In system D, the volume fraction of residual glass is estimated and the microstructure of various nucleating agents is elucidated.With the development of lithium-bearing glass ceramic materials for use in a diversity of commercial applications, the crystal structure and phase equilibrium studies in Li20-AI203-SiO 2 glass ceramics have become increasingly important. The superior properties of glass ceramics depend to a large extent on the crystal phases present, the microstructure, morphology and the residual glass (e.g. McMillan, 1975). Particular attention is paid to fl-and y-spodumene phases that can be formed in these low-or zero-expansion lithium-bearing glass ceramics. A combination of high-resolution electron microscopy of phasecontrast structure imaging and electron diffraction methods has been used here to examine crystallographic, microstructural and residual glass properties as well as the nature of nucleating agents of Li20-AI203-4SiO2 glass ceramic with TiO 2 added as a nucleating agent (system A) and of the commercial glass ceramic system 0.68Li203 AI203 6-ISiO 2 with nucleating agents 0.13ZnO 0.03Na20 0.01K20 0.11TiO 2 0-077ZRO 2 (system D) as such microstructural information is not revealed by conventional X-ray diffraction
Controlled reduction and oxidation reaction experiments on metal oxide catalysts, sulphides, and iron-titanium oxide minerals have been conducted in an environmental (or gas reaction) cell fitted to an AEI-EM7 high voltage electron microscope operating at 1MV. With this technique it is possible to observe dynamic heterogeneous reactions using gas pressures in the range from a few torr to an atmosphere, and temperatures from room temp, to ∼1000°C. A full dynamic description of the behaviour of the solid with respect to temperature, pressure, gaseous environments can therefore be obtained. These observations are useful in understanding mechanisms of heterogeneous catalysis, redox processes, and migration of the reduced species, which affect catalyst selectivity and activity. Examples of some in-situ reactions have been reported in the literature.
The possibility of substitutional doping in carbon nanotubes (CNT) to manipulate their electronic properties for use in molecular electronics and sensors has created considerable interest [1][2][3]. Doped CNTs offer the opportunity to understanding dopant-iduced perturbations on physical properties in one-dimensional materials, and to exploiting their unique properties in these nanotechnologies. The feasibility of tailoring structural and electronic properties of CNTs by using boron and nitrogen have been reported [1][2][3]. Boron and nitrogen dopants are of particular interest because of these expected modifications of the electronic properties of CNTs. Controlling the concentration of nitrogen atoms in the carbon lattice can be useful to tune the conducting properties of single wall carbon nanotubes (SWCNTs) by in situ doping. The substitution of boron in the carbon lattice is expected to increase the hole-type charge carrier concentration, thus influencing the conductivity.We have carried out a systematic study to dope single wall carbon nanotubes (SWCNTs) with varying amounts of boron using the pulsed laser vaporization technique. Targets containing boron concentrations, varying from 0.5 to 10 at % boron, were prepared by mixing elemental boron with carbon paste and Co-Ni catalysts. The products were examined by atomic resolution electron microscopy, nano-electron energy loss spectroscopy (nano-EELS), thermoelectric power (TEP) measurements and Raman scattering experiments. Electron microscopy and Raman studies have revealed that the concentration of boron in the targets greatly affects the type of product. SWCNTs were found in the products prepared from targets containing up to 3 at % boron. While there is no clear evidence of B in SWCNT lattice using electron spectroscopy measurements (within the detection limit of 0.05-0.1 at %B), the sign reversal in TEP (figure 1) and Raman spectra indicate changes to the electronic structure in the presence of boron. At higher boron concentrations (3.5 at % and higher), there are significant changes in the nanostructure, with the formation of a small concentration of double wall carbon nanotubes (DWCNT) (figure 2) and graphite layers. The absence of SWCNTs at these higher amounts of boron may be attributed to the poisoning of the catalyst particles by boron. With the higher B levels in the target material, B appears to play a catalytic role in the formation of DWCNTs. We have achieved the direct growth of DWCNTs without a Co-Ni catalyst, enabling us to control the nanostructural evolution in doped SWCNTs, using B as the variable.
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