Turbostratic Layer Research Articles

Interpretation of the pair function for laminar amorphous materials in the case of coals. II. Structure of coals

From studies on six petrographic components of Polish coals by the radial distribution function method and the interatomic distances calculated from them the proportions of atoms with sp2 and sp3 hybridization and the extent of units with sp3-type structure were determined. From these data and the results of the pair-function analysis performed in paper I [Grigoriew (1988). J. Appl. Cryst. 21, 97–101], the model interpretation of the structure of these materials was carried out by matching experimental and model pair functions P(r). The model was defined as an average structural unit which scatters X-rays coherently. It appears that such units in these materials are larger than previously expected and for vitrinites and fuzinites they are in the form of turbostratic layers. The process of formation of stacks of layers was observed in components with the lowest degree of ordering.

Analyse des bandes de diffraction affectées par une fonction défaut instrumentale

A turbostratic layer lattice is characterized by asymmetric X-ray diffraction bands the shape of which is a function of the average diameter L of the two-dimensional scattering domains. The mathematical relationship between the band shape and L is known. The larger the domains, the narrower and more asymmetric is the band. The band width is also influenced by the instrumental broadening. Consequently the apparent diameter L, is smaller than L. The proposed graphical method allows L to be obtained from L, and from the characteristic parameters of the apparatus. The experimental results can be fitted from the convolution of the theoretical function corresponding to L with the experimentally measured instrumental broadening function.

Measurement of disorder in turbostratic carbons by small-angle X-ray scattering

The variation of the apparent porosity and density of a turbostratic carbon has been measured in the temperature range 1000 to 2800°C from absolute intensity small-angle scattering curves. There is agreement with bulk density values at 2800°C, but a difference develops at progressively lower temperatures. This indicates the presence of disordered regions with relatively low electron density, surrounding the turbostratic layer planes and cross-linking neighbouring crystallites. The average density of the porous/non-crystalline phase has been evaluated and is closely identified with an atomic arrangement which gives rise to diffuse scattering in high-angle X-ray patterns. The values of the weight fraction of this component, derived from both diffraction domains, are in agreement. A gradual reduction of disorder takes place with increase in heat treatment, so that a strictly porous carbon is formed by 2800°C. This variation follows the loss of impurity nitrogen atoms, most probably located in distorted or buckled layers and at layer plane edges as an integral part of the disordered phase. For the turbostratic carbon fibres used in these experiments, the mean sizes of the inhomogeneities in electron density, located between more crystalline regions and expressed by radius of gyration, mean chord intercept length and integral parameters, follow a linear relationship with Lc, the crystallite size along the c axis.

Clinical chemistry news

By using four-layered graphene/gallium nitride (GaN) Schottky diodes with an undoped GaN spacer, we demonstrate highly effective gating of graphene at low bias rendering this type of structure very promising for potential applications. An observed Raman G band position shift larger than 8.5 cm−1 corresponds to an increase in carrier concentration of about 1.2·1013 cm−2. The presence of a distinct G band splitting together with a narrow symmetric 2D band indicates turbostratic layer stacking and suggests the presence of a high potential gradient near the Schottky junction even at zero bias. An analysis based on electroreflectance measurements and a modified Richardson equation confirmed that graphene on n-GaN separated by an undoped GaN spacer behaves like a capacitor at reverse bias. At least 60% of G subband position shifts occur at forward bias, which is related to a rapid reduction of electric field near the Schottky junction. Our studies demonstrate the usefulness of few layer turbostratic graphene deposited on GaN for tracing electron–phonon coupling in graphene. Multilayer graphene also provides uniform and stable electric contacts. Moreover, the observed bias sensitive G band splitting can be used as an indicator of charge transfer in sensor applications in the low bias regime.

Clinical chemistry news

By using four-layered graphene/gallium nitride (GaN) Schottky diodes with an undoped GaN spacer, we demonstrate highly effective gating of graphene at low bias rendering this type of structure very promising for potential applications. An observed Raman G band position shift larger than 8.5 cm−1 corresponds to an increase in carrier concentration of about 1.2·1013 cm−2. The presence of a distinct G band splitting together with a narrow symmetric 2D band indicates turbostratic layer stacking and suggests the presence of a high potential gradient near the Schottky junction even at zero bias. An analysis based on electroreflectance measurements and a modified Richardson equation confirmed that graphene on n-GaN separated by an undoped GaN spacer behaves like a capacitor at reverse bias. At least 60% of G subband position shifts occur at forward bias, which is related to a rapid reduction of electric field near the Schottky junction. Our studies demonstrate the usefulness of few layer turbostratic graphene deposited on GaN for tracing electron–phonon coupling in graphene. Multilayer graphene also provides uniform and stable electric contacts. Moreover, the observed bias sensitive G band splitting can be used as an indicator of charge transfer in sensor applications in the low bias regime.

Membranes de carbone et de graphite et leurs proprietes

A thixotropic gel is formed from water suspensions of graphitic oxide, sodium graphitates or ammonium graphitates. Such gel deposited on a polished surface when all excess salt or acid is eliminated gives after drying a remarkably strong membrane of graphitic oxide or graphitate. Since it was known already that graphitic oxide can form sheets of carbon after reduction by hydriodic acid or hydrazine, it was tried first to reduce the membranes by hydrazine and by hydriodic acid in liquid or gaseous phase. These reduced carbon membranes however are extremely brittle. It was found that membranes of graphitic oxide heated at 500°C in a current of hydrogen produce also membranes essentially of carbon. A study was made of the evolution of such membranes by reheating to various temperatures between 500°C and 1000°C. The characteristics of these membranes such as density, resistivity, magnetoresistance, tensile strength along the plane, crystallographic parameters (interlayer spacing, degree of graphitization and orientation of the graphitic layers) were determined. Membranes of various thicknesses have been prepared. The thinnest permitting a study of the i.r. spectrum were deposited on a grid. They are somewhat translucent to the visible light. The evolution of the structure of the membranes is most remarkable. While for all graphitizable carbons, graphitization begins at about 1600°C and ends at about 3000°C, for such membranes an interlayer spacing of 3.39 Å is found at 1000°C with the (11) band not modulated. One finds thus a turbostratic carbon with an average spacing near that of the graphite. After heating to 2000°C the interlayer spacing decreases to 3.356 A, the order parameter P 1 (= g 2) becoming very high (0.8). One concludes that the absence of interstitial carbon atoms (nonexistent in the original graphite) leads to a spacing for turbostratic layers more nearly equal to that of the perfect graphite and to a graphitic ordering of the layers occurring easily and becoming almost complete at temperatures below 2000°C.

X-ray Study of Preferred Orientation in Pyrolytic Boron Nitride

THE preparation of boron nitride by vapour phase deposition on a hot target has been reported by the British Ceramic Research Association1 and more recently by Belforti, Blum and Bovarnick2. In the latter publication, the deposits are described as having turbostratic layer disorder and a high degree of preferred orientation of the crystallites. In support of these claims the authors show two X-ray diffraction patterns obtained with a pyrolytic sample and a randomly oriented powder sample. The turbostratic layer disorder is evident from the two-dimensional lattice type reflexion (10) in the pyrolytic pattern and from the expansion of the unit cell height which is indicated by the shift of the (00l) reflexions to lower scattering angles. The relative intensities of the two patterns suggest the presence of some preferred orientation in pyrolytic boron nitride, but do not support the authors' contention that the basal planes of the crystallites are “essentially parallel to the deposition surface”.