B. Reznik

Micro- and nanostructure of the carbon matrix of infiltrated carbon fiber felts

The structural properties of carbon/carbon-composites fabricated by chemical vapor infiltration (CVI) were studied by polarized light microscopy (PLM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) on a micrometer and nanometer scale. The types of carbon bonds were estimated by electron-energy-loss spectroscopy (EELS). Using a methane/hydrogen gas mixture at a temperature of 1100°C two different methane partial pressures were applied. The carbon fibers are surrounded by ring-shaped layers with different optical reflectance. The SEM analyses of fracture surfaces revealed differences in the micromechanical behavior depending on the matrix morphology. Particular emphasis was put on the distinction of individual forms of pyrolytic carbons with similar optical behavior, which reveals significant structural differences in detailed SEM and TEM analyses.

Influence of pressure, temperature and surface area/volume ratio on the texture of pyrolytic carbon deposited from methane

Abstract The kinetics of carbon deposition from methane were studied over broad ranges of pressures, temperatures and reciprocal surface area/volume ratios. Based on these results, it was possible to distinguish between a growth and a nucleation mechanism of carbon deposition and to select conditions for the preparation of well-defined samples for texture analysis by transmission electron microscopy and selected area electron diffraction. Maximal texture degrees were obtained at medium or high values of the above parameters, but never at low values, at which carbon formation is based on the growth mechanism and dominated by small linear hydrocarbons. High-textured carbon resulting from the growth mechanism is concluded to be formed from a gas phase with an optimum ratio of aromatic to small linear hydrocarbons, which supports the earlier proposed particle-filler model of carbon formation. High-textured carbon may also be formed from a gas phase dominated by polycyclic aromatic hydrocarbons (nucleation mechanism) provided that the residence time is sufficiently long that fully condensed, planar polycyclic aromatic hydrocarbons can be formed in the gas phase.

Texture and nanostructure of pyrocarbon layers deposited on planar substrates in a hot-wall reactor

Pyrocarbon layers were deposited from methane on planar substrates (pyrolytic boron nitride) at a temperature of 1100 °C and residence times of 0.1, 0.5 and 2.5 s. The depositions were performed in a hot-wall reactor with the substrates oriented parallel to the gas flow. Transmission electron microscopy was applied to study the texture and the structure of the carbon layers on a micrometer and nanometer scale. The texture is influenced by the residence time. An alteration from medium- to high-textured carbon is observed from short to long residence times. The nanostructure of high- and medium-textured pyrocarbon is characterized by domains whose sizes do not generally differ.

Microstructure and mechanical properties of carbon-carbon composites with multilayered pyrocarbon matrix

The effect of matrix microstructure on the mechanical properties of carbon fiber felts infiltrated by isothermal chemical vapor infiltration (CVI) has been studied by optical microscopy, scanning electron microscopy and three-point bending tests. The nonbrittle fracture behavior of the investigated composites is related to multiple crack deflections caused by the interfacial sliding between pyrocarbon layers with a varying texture degree and the delamination microcracking within the highly textured pyrocarbon layer. An increase of the flexural strength is observed by the composite having a multilayered pyrocarbon matrix.

Microscopic study of failure mechanisms in infiltrated carbon fiber felts

Abstract Failure mechanisms in infiltrated carbon fiber felts have been studied by optical light microscopy, transmission electron microscopy and scanning electron microscopy combined with mechanical testing experiments. A model is presented which describes crack generation and propagation at layer–layer and fiber–matrix interfaces as well as within matrix carbon layers with different textures. Intensive cracking occurs within high- and less frequently in medium- and low-textured pyrolytic carbon layers. In particular, fracture does not occur directly at the fiber–matrix interface but within the low-textured matrix layer deposited on the fiber. Crack deflection in interface regions between layers with different textures, crack deflection along boundaries of columnar grains in high -textured layers and at interfaces between polyhedral nanoparticles, and finally crack bridging within high -textured lamellae are cooperative failure mechanisms contributing to the toughness enhancement.

An improved method for angular-resolved characterization of the optical anisotropy of pyrolytic carbon.

A description is given of an experimental technique that improves the accuracy of the measurement of light extinctions by polarized light microscopy from deposits of pyrolytic carbon. The measurements were performed using a specially developed digital photometric image‐analysis procedure providing high spatial and angular resolution of light extinctions over the reflecting optical domains with a high dynamic range of grey levels. The digital image acquisition and data processing are illustrated using circular‐shaped pyrolytic carbon matrices of infiltrated carbon fibre felts and planar layers of pyrolytic graphite. The calculated value of the extinction angle for single crystalline graphite is discussed with respect to the experimental values for pyrolytic carbon with different degrees of optical anisotropy. Practical requirements for the accuracy of the method are discussed.

Cross-section preparation for transmission electron microscopy of phases and interfaces in C/BN heterostructures

A technique is described for the preparation of transmission electron microscopy cross-sectional samples of pyrolytical carbon layers deposited on polycrystalline boron nitride substrates. To solve the problem of different abrasion rates of C and BN a filler material, Si wafers, has been bonded to both sides of the pre-thinned BN substrate. Correspondence between color and thickness of Si wafers facilitates controlled sample thickness reduction during dimpling. The samples prepared by this technique even without ion milling are thin enough for HRTEM studies.

Shock‐induced deformation phenomena in magnetite and their consequences on magnetic properties

This study investigates the effects of shock waves on magnetic and microstructural behavior of multidomain magnetite from a magnetite-bearing ore, experimentally shocked to pressures of 5, 10, 20, and 30 GPa. Changes in apparent crystallite size and lattice parameter were determined by X-ray diffraction, and grain fragmentation and defect accumulation were studied by scanning and transmission electron microscopy. Magnetic properties were characterized by low-temperature saturation isothermal remanent magnetization (SIRM), susceptibility measurements around the Verwey transition as well as by hysteresis parameters at room temperature. It is established that the shock-induced refinement of magnetic domains from MD to SD-PSD range is a result of cooperative processes including brittle fragmentation of magnetite grains, plastic deformation with shear bands and twins as well as structural disordering in form of molten grains and amorphous nanoclusters. Up to 10 GPa, a decrease of coherent crystallite size, lattice parameter, saturation magnetization (Ms), and magnetic susceptibility and an increase in coercivity, SIRM, and width of Verwey transition are mostly associated with brittle grain fragmentation. Starting from 20 GPa, a slight recovery is documented in all magnetic and nonmagnetic parameters. In particular, the recovery in SIRM is correlated with an increase of the lattice constant. The recovery effect is associated with the increasing influence of shock heating/annealing at high shock pressures. The strong decrease of Ms at 30 GPa is interpreted as a result of strong lattice damage and distortion. Our results unravel the microstructural mechanisms behind the loss of magnetization and the modification of magnetic properties of magnetite and contribute to our understanding of shock-induced magnetic phenomena in impacted rocks on earth and in meteorites.

Lingunite-a high-pressure plagioclase polymorph at mineral interfaces in doleritic rock of the Lockne impact structure (Sweden)

Lingunite nanocrystals and amorphous plagioclase (maskelynite) are identified at the contacts between augite and labradorite wedge-shaped interfaces in the doleritic rocks of the Lockne impact structure in Sweden. The occurrence of lingunite suggests that the local pressure was above 19 GPa and the local temperature overwhelmed 1000 °C. These values are up to 10 times higher than previous values estimated numerically for bulk pressure and temperature. High shock-induced temperatures are manifested by maskelynite injections into microfractures in augite located next to the wedges. We discuss a possible model of shock heterogeneity at mineral interfaces, which may lead to longer duration of the same shock pressure and a concentration of high temperature thus triggering the kinetics of labradorite transformation into lingunite and maskelynite.