Pyrolytic Graphite Crystals Research Articles

Anisotropy of the Electron Momentum Distribution in Pyrolytic Graphite Studied Using a WKa1 Spectrometer

Abstract A new spectrometer construction based on a high-voltage W-anode x-ray tube has been used to measure the directional Compton profiles of a pyrolytic graphite crystal. Owing to the focusing geometry, the monochromatic WKα1 t beam is very well collimated, which improves the momentum resolution compared with conventional241 Am spectrometers, operated almost at the same energy. The experimental anisotropy in the basal plane and in the direction of the c-axis is found to be very close to that of a recent calculation, based on the pseudopotential local-density-functional model.

A nanometer-scale galvanic cell

An electrochemicaI cell having a largest total dimension of 70 nm was assembled on the surface of a highly oriented pyrolytic graphite crystal by electrochemically depositing copper and silver pillars (diameter ≃150-200 A) in close proximity to one another using the scanning tunneling microscope (STM). When the surface containing both metals is immersed in a dilute copper plating solution, copper structures spontaneously decrease in size and, concurrently, silver structures increase in size. The direction and magnitude of these changes are consistent with a coupled electrochemical process in which copper anodically dissolves from the copper structures and plates onto the silver structures

Characterization of pyrolysed silacyclobutasilazane

Silacyclobutasilazane (SCBZ) is a candidate organosilicon polymer suited to many hightemperature applications. Pyrolysis of SCBZ occurs over two distinct temperature ranges 400–800 and 1400–1800 °C. X-ray diffraction analysis showed that amorphous SCBZ transforms to crystalline SiC above 1400 °C. Thin foils of the pyrolysed (1800 °C) SCBZ were prepared and examined using the analytical electron microscope. The material was found to contain ∼ 200 nmβ-SiC particles scattered through a matrix consisting of ∼ 50 nm radiating clusters of pyrolytic graphite crystals and a highly carbonaceous amorphous background. It is suggested that during 1400–1800 °C pyrolysis two major reactions occurred. Initially, SCBZ through nitrogen loss, decomposed to crystalline SiC and amorphous carbon. Subsequent graphitization then produced the radiating crystal clusters.

X-ray reflectivities, at low and high order of reflection, of flat highly oriented pyrolytic graphite crystals

Integrated reflectivities of flat highly oriented pyrolytic graphite crystals have been measured at various energies (5.9 ≤ E ≤ 15.10 keV) and for various orders of reflection n (1 ≤ n ≤ 5). For low n values, the reflectivity is severely affected by secondary extinction; our results show that the theory of Zachariasen represents the experiment reasonably well, a conclusion reached by other authors for the case of n = 1 reflections. For high n values, the reflectivity is more sensitive to the structure factor's temperature correction, a quantity not precisely known. Our measurements are in favour of the larger temperature correction within the range reported in the literature.

Neutron transmission measurements of poly and pyrolytic graphite crystals

The total neutron cross-section measurements of polycrystalline graphite have been carried out in a neutron wavelength from 0.04 to 0.78 nm. This work also presents the neutron transmission measurements of pyrolytic graphite (PG) crystal in a neutron wavelength band from 0.03 to 0.50 nm, at different orientations of the PG crystal with regard to the beam direction. The measurements were performed using three time-of-flight (TOF) spectrometers installed in front of three of the ET-RR-1 reactor horizontal channels. The average value of the coherent scattering amplitude for polycrystalline graphite was calculated and found to be b coh = (6.61 ± 0.07) fm. The behaviour of neutron transmission through the PG crystal, while oriented at different angles with regard to the beam direction, shows dips at neutron wavelengths corresponding to the reflections from (hkl) planes of hexagonal graphite structure. The positions of the observed dips are found to be in good agreement with the calculated ones. It was also found that a 40 mm thick PG crystal is quite enough to reduce the second-order contamination of the neutron beam from 2.81 to 0.04, assuming that the incident neutrons have a Maxwell distribution with neutron gas temperature 330 K.

Spectrometer with high energy resolution using Larmor precession

An inelastic neutron scattering spectrometer with energy resolution down to 20 μeV is described. A few, fixed, energies of the neutrons scattered by the sample are analysed by a large array of pyrolytic graphite crystals, used in backscattering. The incident beam is polarised and the energy is labeled by means of Larmor-precession modulation.

X-ray study of the structure of K1-xRbxC8.

The mixed layered compounds ${\mathrm{K}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Rb}}_{\mathrm{x}}$${\mathrm{C}}_{8}$ represent quasi-two-dimensional alloys interleaved by graphite hexagonal planes. We report extensive x-ray scattering investigations of the various structural aspects of these compounds. These include the variation of the layer separation with the alloy composition, the in-plane order and disorder of the alkali-metal ions in the graphite host, the stacking sequence of the alkali-metal layers, and the change of the carbon-carbon bond length as a function of the composition. We find that the interlayer separation exhibits a slight deviation from linearity independent of the graphite material used (pyrolytic graphite or single crystals), with a maximum deviation centered at x=(2/3. In the plane, the mixed alkali-metal layers form a commensurate (2\ifmmode\times\else\texttimes\fi{}2)R0\ifmmode^\circ\else\textdegree\fi{} superstructure with complete site disorder among the K and Rb ions. Extensive low-temperature scans have not revealed any tendency of the mixed layer to either super order or phase separate. The stacking sequence is independent of the alloy composition and is identical to the stacking sequence in ${\mathrm{KC}}_{8}$ as well as in ${\mathrm{RbC}}_{8}$. The carbon-carbon bond length appears not to change with the alloy composition, indicating that the charge transfer between the alkali-metal s band and the graphite \ensuremath{\pi} band remains constant with composition and is independent of the layer separation.

The intercalation of micron-size graphite powder with anhydrous ferric chloride

The experimental techniques for intercalating highly oriented pyrolytic graphite crystals and large size graphite flakes with metal halides are well known and easily carried out. However, as the size of the graphite flakes decrease below 5 microns the conditions for intercalation become more difficult and the results less predictable. Experimental results show that graphite flakes in the 2 micron range can be intercalated with FeCl 3 under the appropriate conditions of time and chlorine back pressure. The stage formation was confirmed by X-ray diffraction and supported by elemental analysis. A reaction mechanism is proposed and discussed.

Temperature dependence of positron annihilation characteristics on the surfaces of graphite powders

Positron lifetime measurements have been made on graphite powders, grafoils, and pyrolytic graphite crystals with different surface areas in the temperature range between 25° and 600 °C. Three positron lifetimes were found in these systems: a short-lived component (∼0.2 ns) due to positrons in the bulk; a component (∼0.45 ns) due to surface-trapped positrons; and a long-lived component (∼2 ns) ofo-Ps in the voids or the interfacial spaces of powders. Both bulk and surface positron lifetimes increase as a function of temperature. Correlations between the intensity of surface-trapped positrons and the surface area and between Ps formation and the surface area of graphite are found. The Ps formation probability increases as a function of temperature. A thermal desorption model interprets the emission process of Ps atoms from the surface of graphite to the vacuum and gives an activation energy of 0.23±0.02 eV.

Mossbauer diffraction on a single rotating crystal

The problem of coherent scattering of Mossbauer radiation in the case of a single rotating crystal is considered. It is shown that the resulting additional Doppler shift leads to the frequency dependence of the purely elastic intensity Iel. Due to the small value of the Mossbauer line width the intensity Iel is sensitive to small deviations of the reciprocal lattice vector from the rotating axis. In order to check the theoretical considerations, the Mossbauer diffraction experiment was carried out on a single pyrolytic graphite crystal. The authors analysed the influence of the mosaic structure of the sample as well as the deflection by the atomic planes from the crystal surface on the obtained results.

POLARIZED NEUTRON SPECTROMETER DEVELOPMENT AND EXPERIMENTS AT BROOKHAVEN

Recent work at Brookhaven has focussed on the development of polarized neutron spectrometers and reflects the resurgence of interest in polarization analysis. Both Heusler crystals and FeGe multilayers used in conjunction with pyrolytic graphite crystals have been tested as polarizing monochromators on a triple-axis instrument. The goal is to first obtain polarized beams of adequate intensity and then to evaluate various instrumental innovations which affect control of spectrometer resolution by utilizing the properties of polarized beams. Progress in these regards is reported. A recent study of the magnetic excitations in amorphous ferromagnets employing the instrumentation for polarization analysis developed thus far is also discussed.

A diffraction pattern caused by thermal diffuse scattering of X-rays in a pyrolytic graphite crystal

A defect line across the 002 diffuse spots scattered from a pyrolytic graphite plate has, for the first time, been clearly observed in X-ray films taken with unfiltered radiation from a copper target. The line was caused by the 002 Bragg reflection of the thermal diffuse scattering.

The small-angle neutron scattering spectrometer at the University of Missouri research reactor

A small-angle neutron scattering (SANS) spectrometer has been constructed at the University of Missouri Research Reactor Facility (MURR). The design of the MURR–SANS is unusual in that the size of the reactor containment building constrains the flight path to be vertical. This is achieved by Bragg scattering upward through 90° from a set of slightly misaligned pyrolytic graphite crystals to provide a neutron beam at 4.75 Å with a wavelength spread of approximately 4.1%. The beam incident on the sample is defined by two matched variable apertures located either 3.0 or 4.5 m apart. The evacuated scattered flight path is designed with removable extensions to match the primary flight path in length. The instrument has an automatic sample handling capability provided by its own dedicated PDP 11/03 computer. The detector is a large assembly of commercially available linear 3He detectors as an economic alternative to a crossed-wire two-dimensional multi-detector. An array of 43 position-sensitive proportional counters, 24 in (609.6 mm) long and 0.5 in (12.7 mm) in diameter, using charge division gives a spatial resolution of 5 x 12.7 mm. The area-averaged detector efficiency is about 84% at a wavelength of 4.75 Å. The range of scattering vectors that can be measured is 0.005 < Q< 0.15 Å−1. The instrument is well suited to a wide variety of experiments on specimens having characteristic dimensions between 20 and 500 Å. MURR–SANS is designed as a user-oriented facility which provides both reasonable resolution and intensity on sample at a modest cost, and forms part of a neutron scattering center.

The structure of graphite bombarded with light, gaseous ions

Natural and highly-oriented pyrolytic graphite crystals have been injected with helium, deuterium and carbon ions at energies up to 100 keV and doses up to 10 18 ions/cm 2. This results in twinning in thick samples, the density of twinning increasing with energy and dose, and varying with target temperature. Twinning is shown to provide an important plastic deformation mode and a model of twinning related to the depth variation of atomic displacements is presented: a method of calculating the strains involved is discussed. It is suggested that surface distortions observed in other graphites originate in the same way. Measurements show that some gas is retained in the samples, but its direct role is uncertain. Very thin (thickness ≲ 0.1 μm) crystals blister under gas-ion bombardment and are demonstrated to retain sufficient gas to produce the blisters observed. It is shown that the blisters can grow laterally by cleavage in graphite, rather than by increasing their curvature and bursting as in metals. The origin of these blisters is obscure.

Dead-time distortions and their elimination in reverse fourier time-of-flight spectrometry of slow neutrons

In order to disclose the so far little-known nature and practical implications of possible dead-time phenomena in neutron spectra measured with sequentially operating reverse Fourier time-of-flight spectrometers, an analysis of non-homogeneous, especially periodic, Poisson processes deformed by constant dead time is carried out. Calculations based on this treatment show that, unless the neutron beam is modulated with frequencies restricted to integer multiples of the inverse dead time, notable spectrum distortions may occur even at counting rates not higher than a few hundred neutrons/s. Such deformations depend strongly on the shape of the true time-of-flight pattern: e.g. monochromatic peaks become asymmetric and give rise to “ghost” peaks or jumps shifted towards longer flight times by a distance equal to the dead time, whereas continuous spectra, besides producing similar but broader “ghost” effects, will be distorted throughtout with the result that their tails appear more or less depressed. Of the possible methods of preventing spectrum distortions, a simple but effective technique is discussed in which the dead time is deliberately extended so as to always make it equal to the nearest integer multiple of the modulation period. The usefulness of this procedure, as well as the dead-time phenomena theoretically predicted for monochromatic peaks, are well confirmed by two examples of real reverse Fourier time-of-flight experiments on pyrolytic graphite crystals.

X-ray polarization with a pyrolytic graphite crystal

X-ray polarization with a pyrolytic graphite crystal

Is pyrolytic graphite an ideal mosaic crystal?

Absolute reflectivities, width of rocking curves and effective absorption coefficients for many pyrolytic graphite crystals at different wavelengths were measured. The results can be described within a few percent by the formulae for the ideal mosaic crystal with absorption. Effective absorption is caused by unavoidable parasitic reflections. The measured effective absorption coefficient is the same for different specimens; it does not depend on the mosaic width. By means of this effective absorption, the peak reflectivity decreases with increasing mosaic width.

The interaction of krypton and an exfoliated graphite at 77.4 K

An exfoliated graphite sample was prepared having a significant increase in basal plane area over the original highly ordered pyrolytic graphite crystal. The boundary planes of the expanded sample appeared intact and were highly reflective. The krypton adsorption at 77.4 K was at first linear with P P o , then reached a plateau over the range from 0.45 to 0.65, and then increased uniformly to P P o of 0.99, where p is pressure and p 0 is the sublimation vapor pressure of solid krypton. Each desorption branch initiated from several levels of adsorption decreased only slightly to the lowest pressures measured ( P P o ∼ 0.01 ). A model is proposed in which the adsorption process is concomitant with a contraction between adjoining graphite lamellae.

Vertically bent pyrolytic graphite crystals applied to triple-axis neutron spectrometry

This paper demonstrates the great value of verticallt bent pyrolytic graphic crystals used as monochromator or analyzer in triple-axis neutron spectrometers. A simple model is presented for estimating the flux gain to be achieved by bending the monochromator or analyzer, and the effects this bending has on instrumental resolution are also considered. Experiments with three monochromators bent to 50, 35, and 21 inch radii over a wide range of neutron energy (5 to 40 meV) gave a maximum flux gain factor of 3.5 as predicted by the model, with no measurable change in resolution. In addition, if vertical resolution is not crucial, a further intensity gain can be achieved by using a bent analyzer.

Determination of the Optical Properties of Absorbing Uniaxial Crystals from Reflectance at Oblique Incidence

Equations are presented for determining the optical properties of an absorbing uniaxial crystal by measuring the reflectance from a basal plane as a function of the angle of incidence and the angle of polarization. The technique is applied to a pyrolytic graphite crystal to determine the optical properties at 632.8 nm. The results are calculated numerically by use of a combination of the Taylor-series and gradient methods. The best-fit values found for the optical constants of pyrolytic graphite at 632.8 nm are n0 = 2.55, ne = 1.78, κ0 = 0.66, and κe = 0.00.