Changes In Carbon Films Research Articles

Structure Changes in Carbon Films Prepared by Electron-Beam-Assisted Deposition

Carbon films 50–180 nm thick on nickel substrates are fabricated by the ion sputtering of graphite and the deposition of heavy hydrocarbons from the gas phase with simultaneous electron irradiation. Irradiation results in the formation of bonds in carbon films due to the sp and sp3 hybridization of orbitals (sp and sp3 bonds), mainly, sp3 bonds. A fraction of these bonds does not change with growth in the electron energy; it increases three-fold with a reduction in the temperature and an increase in the electron current density. Electron irradiation enhances the film microhardness which exceeds 12 GPa. The films, prepared by heavy hydrocarbon deposition, contain CHn bonds and a small fraction of sp3 bonds. The maximum value of the microhardness of the hydrocarbon films is no more than 4.5 GPa. The analysis of the proposed model of the kinetics of forming different allotropic phases in a carbon film to be deposited shows that a temperature reduction changes the specific volume of an atom in the lattice, while under conditions of simultaneous electron irradiation, it appreciably increases the content of the phase with sp3 bonds. The effect of spi-bond breakage during electron-beam-assisted deposition weakly depends on the electron energy. The weak excitations of electrons of carbon atoms can also result in the formation of sp3 bonds and increases their concentration with growth in the electron current density.

Observation of structural changes in carbon films under external influences

Carbon films obtained by the plasma chemical deposition method from the multicomponent stream of ionized particles on the various substrates are investigated. Structural formations with parallel lines were observed under the microscope on the film surface. Apparently, charge accumulation takes place along these lines. When the film locally is heated then micro tube-like structure peculiarities are formed along these lines. Observed peculiarities are periodic in nature with well-established formation phases. Possibly, the charge accumulation lines coincide with the micro tube-like formations as our observations revealed. It is important to stress that the parameters of micro tube-like formations, i.e., the period of sinus-like formations and their linear dimensions are dependent on the technological parameters. Metal–carbon film-Si sandwich-like system's volt–ampere characteristics show irreversible breakdown due to structural changes taking place in system and being observed microscopically. Obviously, this is due to applied strong electric fields (∼10 5–10 6 V/cm). Thus, origination of micro- and nano-formations both on the surface and in the bulk of carbon films under strictly controlled conditions is possible to obtain from multicomponent ion streams.

Structural changes in carbon films derived from Kapton observed by the positron annihilation lifetime technique

Structural changes in carbon films derived from Kapton observed by the positron annihilation lifetime technique

Magnetoresistance and microtexture of carbon films prepared from polyimide film LARC-TPI

Structural changes in carbon films prepared from a commercially available polyimide, LARC-TPI, of 50 μm thickness, with heat-treatment temperatures between 2000° and 2800°C were investigated by measuring magnetoresistance at liquid nitrogen temperature and by scannign electron microscopy. All the specimens exhibited granular microtextures, with mean grain diameters spanning 10–12 nm, similar to those of bulk glasslike carbons. The films heat treated between 2000° and 2600°C showed a small negative magnetoresistance characteristic of bulk glasslike carbon. The carbon films heat treated at 2750° and 2800°C gave a composite magnetoresistance attributed to granular microtexture (negative, isotropic) with graphite layer skins (positive, anisotropic). The fractured cross-sections of these films indicated layered skins about 100 nm thick. These graphitizable skins may be attributed to constraints that influenced the orientation of the molecules near the surfaces of the film when they were formed.

Structural and textural changes from polyimide Kapton to graphite: Part II. Magnetoresistance and x-ray diffraction

Structural and textural changes in carbon films prepared from the polyimide Kapton with heat-treatment temperatures up to 3000 °C were followed by measuring the magnetoresistance at 77 K, the ratio of electrical resistivity at room temperature to that at 77 K, and x-ray powder patterns. The maximum magnetoresistance (Δρ/ρ)max as a measure of graphitization changed its sign from negative to positive around 2500 °C and then increased its value abruptly. The resistivity ratio ρR.T./ρ77 K increased above 2500 °C treatment. The values of (Δρ/ρ)max and ρR.T./ρ77 K after 3000 °C treatment reached 275% and 0.93, respectively. The average interlayer spacing d002 after the 3000 °C treatment was 0.3354 nm, the same as natural graphite. The changes in structure and texture in the film were consistent with the model proposed in the previous paper from observation by transmission electron microscopy.