Laser Electron Beam Heating

Electron beam heating has been used widely in the fields of melting, welding, sputtering, and micromanufacturing. The electrons emit from the cathode of the electron gun where the temperature is very high due to the application of the high voltage that is necessary for the emission of electrons. Thus the high vaccum must be kept in the electron gun. Actually, the electron gun inside a TEM can be conveniently used for electron beam heating and irradiating of the materials. Another efficient method is laser heating, which has several advantages: the heating source is outside the evaporation system; any materials, including metals, compounds, ceremics, etc., can be evaporated; there is no contamination from/for the heating source. Both the laser and the electron beam heatings are efficient for evaporation of materials with high melting points.

A method was developed for formation of carbon nano-tubes, carbon nanocapsules, and carbon nanoparticles in which polyyne-containing carbons were heated and irradiated by an electron beam in a TEM [146-149]. The technique was applied to carry out in-situ observation of those formation processes. Though carbon nanoparticles were formed accompanied by carbon nanotubes, carbon nano-particles and nanocapsules were formed independently of the carbon nanotubes [146]. The carbon nanotubes were preferentially inside of the polyyne-containing carbon films, while the carbon nanoparticles/nanocapsules were outside of the films. The differences on the inside and the outside were discussed to understand the formation process of the carbon nanoparticles/nanocapsules. From in-situ observation, the existence of metal particles, the high surface energy, the high wettability, and the high viscosity of the polyyne-containing carbons were assumed to be relevant to the preferential formation of the carbon nanoparticles/nanocapsules to the carbon nanotubes.

Carbon onions were produced in a TEM by electron irradiation of amorphous carbon in the presence of Pd clusters [147]. HRTEM revealed the structural changes of the onion surface, and the atom clouds were observed at the pentagonal vertices. In some onions, Pd atoms were intercalated between the graphite onion sheets, and a structural model for the intercalation was proposed. Electron beam irradiation of amorphous carbon in the presence of Pd clusters was shown to be an effective method for the formation of intercalated onions.

Metallic nanocrystals, such as cobalt or gold, were encapsulated by spherical graphitic shells under high-temperature electron irradiation [148]. The irradiation promoted a heavy contraction of carbon onions. The contraction forced the metal atoms to migrate outward through the shells, even without further irradiation. This led to a gradual but complete displacement of the encapsulated crystals. In-situ observation in a TEM allowed shrinkage of the encapsulated crystals and migration of the atoms through the shells to be monitored. The spherically curved graphene layers were permeable to metal atoms.

Copper substrates were implanted with carbon ions at temperatures ranging from 570 to 973 K [149]. Carbon onions and nanocapsules were observed together with amorphous carbon layers. Most of the nanocapsules were found to be hollow and rarely included copper nanoparticles. The encapsulating of Cu nanoparticles with graphene layers, the gradual shrinkage of the encapsulated clusters, and finally the disappearance of the clusters (leaving behind hollow nanocapsules) were observed under electron irradiation at 783 K. Statistics of cluster size as a function of ion fluence, implantation temperature, and substrate crystallinity gave insights into the nucleation processes of onions and nano-capsules. One process involved the formation of graphene layers on grain boundaries to encapsulate copper particles. Another process was the nucleation of graphene cages, probably fullerenes, due to a high concentration of carbon atoms and a high amount of radiation damage.

Nanocapsules of crystalline boron nitride with diameters ranging from 50 to 300 nm were synthesized by pulsed-laser vaporization of BN, where the laser plume was controlled by the modulated plasma jet flow field [150]. Their shapes varied from polyhedrons to cocoons and the interlayer spac-ings along the c axis were enlarged according to their size and shape (curvature). Without the synchronization of the laser pulses with the plasma modulation, sootlike BN was obtained.

Magnetic Fe3C and a-Fe nanoparticles were prepared by laser-induced pyrolysis of Fe(CO)5 and C2H4 [151]. It was found that not only was oxygen content of the a-Fe particles much higher than that of the Fe3C particles, but the oxygen was in different states for the two nanoparticles. The oxygen present on the Fe3C particles was primarily in absorbed form, compared to chemically combined oxygen as in the a-Fe particles. Iron carbonitride nanoparticles (20-80 nm in size) were synthesized by laser-induced pyroly-sis of a Fe(CO)5-NH3-C2H4 mixture [152]. Surface morphology, structural characteristics, oxidation behavior, and magnetic properties of the iron carbonitride particles were reported. The unilateral lattice expansion of the iron car-bonitride compound was interpreted in terms of the structural and chemical bonding features of the iron carbonitride compound.

a-Fe (bcc) and y-Fe (fcc) nanoparticles were prepared by laser-induced pyrolysis [153]. The structures, morphologies, and magnetic properties of the oxide layers covering the iron nanoparticles were investigated. The iron oxide layers consisted of very fine crystallites of 2-5 nm in diameter, and the layers were nonferromagnetic or superparamagnetic. The iron oxide layer formed on iron nanoparticles at room temperature was Fe3O4, rather than a mixture of Fe3O4 and y-Fe2O3.

y-Fe(N) nanoparticles (containing 5.9% N, atomic fraction) were prepared by laser induced pyrolysis [154]. The martensitic transformation temperature of the nanoparticles was much lower than that of the bulk materials with the same composition. The effect of particle size on the transformation temperature was discussed. It was suggested that the hydrostatic pressure resulting from surface tension of the nanoparticles was responsible for the decrease of the transformation temperature. In order to verify the plastic deformation induced martensitic transformation of the y-Fe(N) nanoparticles, the effects of pressurization (0.5-4.0 GPa) on the martensitic transformation and microstructures of the particles were presented.

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