Evaluation of energy band parameters of carbons

The systematic dependence of the Hall effect of carbons on the electronic band parameters has analytically been studied on the basis of Yazawa's band model which takes into account in a simple way the interlayer perturbation. Many experimental results on the magnetic-field dependence of the Hall effect of carbons can be satisfactorily explained by adjusting the band parameters. The change in the Hall voltage with the band parameters is so drastic as to provide a powerful tool for evaluating these parameters of carbons. The evaluation procedures are described in detail and some examples of the application to experimental results presented. It is concluded that as-deposited pyrolytic graphite has a high concentration of acceptors with small activation energy (~ l0 meV), which may probably originate from the structural defects. On the other hand, the activation energy of boron in graphite is found to exceed 100 meV.     

Dry granulation of organic powders—dependence of pressure 2D-distribution on different process parameters

Nowadays dry granulation of powders has become a very important research topic because it is the most economic way of granulation, making the particle handling easier, and avoiding the loss of material during particle processing, or particle transfers. This kind of process has been deeply studied but a better knowledge appears necessary to control the great number of parameters of the process. This is particularly important in cases where the nature of the powder may lead to very complex phenomena during compaction.In order to try and optimise dry granulation process for organic compounds, a roll press has been designed with a series of instruments enabling to control the compaction process. The apparatus consists of three parts: a vertical container with rotating steel blades avoiding arches into which the powder is poured, a feeder transferring the powder towards the rolls; the feeder is equipped with an horizontal helical screw in a cylindrical draft tube (10 mm in internal diameter, 500 mm long) and in the end of the feeder, a junction allows the change from the cylindrical symmetry of the feeder to the prismatic symmetry existing in the roll gap. The roll press (0-500 kN, load per unit length 0-) has been developed to record different major classical parameters: the roll speed, the roll gap, the press strength, the rotation angle, and the feeding rate (between 0 and ). In comparison with different kinds of roll press described in the literature, in this work an original instrumentation system has been developed to catch specific data. The 3D-pressure distribution profiles at the interface between powder and the roll wall and the drive torque applied to the rolls were measured. A large-sized smooth steel-made roll (240 mm diameter, 50 mm width) has been chosen to compare the results to the industrial scale.The results obtained with an organic compound exhibited the dependence—sometimes unexpected—of the rotation angle, the feeding and the rotation speed on the pressure distribution, the roll width, and the drive torque.     

Deformation of polycrystalline graphite under pressure

Linear strains at pressures up to 600 psi (4.14 MPa) have been measured in uncoated and coated graphites at room temperature. The bulk moduli found in this way are found to be very dependent on the nature of the graphite. Two empirical equations relating modulus to porosity are applied to the data, and the results are discussed in terms of the structure of graphite. The influence of neutron irradiation on bulk modulus has been studied and is interpreted in the same context. The effects of uniaxial prestressing and of increasing the temperature to 300°C have also been investigated.     

Bend strength of graphite under pressure

The 4-point bend strength and strain to fracture at room temperature have been measured in three graphites at atmospheric pressure and at a gas pressure of 600 psi (4.14 MPa). The surface fracture stress was reduced under pressure by between 2 and 7%, and the fracture strain was reduced by between 13 and 18%. The Young's modulus was also increased significantly. Whereas the latter result can be explained in qualitative terms, there does not appear to be a satisfactory explanation of the results for fracture stress and strain.     

Modeling the principle physical parameters of graphite carbon foam

Graphite carbon foam, a mesophase, pitch-based material, portrays highly ordered topology structures which exhibit superior mechanical and thermal properties. Typical graphite carbon foam with dimensions 5 cm³, can have a surface area greater than 11 m², making it an excellent candidate for heat transfer applications. Accurate three dimensional modeling of carbon foams is necessary to study and predict their properties in simulation. This paper describes a computer algorithm for modeling POCO Foam® and similar carbon foams. The algorithm, written in MATLAB, captures the principle physical parameters of the carbon foam including bubble and pore diameter ranges and overall foam void percentage while retaining the random dispersal of spherical bubbles found in manufactured foams.     

The influence of impurity-doping on temperature dependence of dislocation-ribbon width in graphite

By means of an electron microscope, the width of dislocation ribbons in a stress-annealed pyrolytic graphite and doped with iodine chloride (ICl) and bromine was investigated as a function of temperature in the range 100°K to 300°K. The impurity content amounted to 1·1 at % for ICl and 1·6 at % for bromine. Through measurements of the ribbons widths W in steps of 20°K, the average temperature coefficients $\text{1}\text{W}\text{·}\text{dW}\text{?dT}$ were found to be 5 × 10^?4, 3 × 10^?4 and 1 × 10^?4°K^?1, for the original, ICl-doped and bromine-doped material respectively; this shows that the temperature dependence of the ribbon width in graphite is appreciably decreased by the presence of halogen impurities. The phenomenon is quantitatively analyzed in terms of the chemical interaction mechanism between stacking faults and impurities. A discussion of the viscous drag function of impurities for dislocations in relation to the internal friction is included.     

On the pressure dependence and the Grüneisen parameters of the B1u-lattice vibration in high density polyethylene

Far infra-red spectroscopic measurements have been made on a high density (0.973 g cm?3) sample of linear polyethylene to determine the frequency of the zone-centre B1u translational lattice mode (vibrating in the b-axis direction) as a function of pressure up to 7 kbar. The variation of frequency over this range is found to exceed, considerably, its total variation with temperature up to the melting point. These measurements and X-ray data from the literature give a volumetric mode Grüneisen parameter γB1uV varying from 3.15 at low pressures to about 5.5 at 7 kbar. Theoretical calculations using the potential functions of Hägele et al. give, for the special (anisotropic) Grüneisen mode parameters, γB1ua = 6.1, γB1ub = 2.2, and predict a pressure variation of $\text{v}\text{?}\text{(B}{}_{\mathrm{1u}}\text{)}$ similar to but greater than that observed experimentally. The results are compared with other data in the literature.     

Measurement of pressure dependence on the shear viscosity of polymer melts

We built a rheometer which has a simple structure and can measure the pressure dependence of the viscosity consuming small amount of sample. Main part of the rheometer consists of two sample chambers connected through a slit channel in which two pressure transducers and a pressure adjusting valve are mounted. The double piston arrangement enables to use the materials over and over by the reciprocating flow of the polymer melts from one chamber to the other chamber. The viscosity of polymer melts can be measured at wide range of shear rate and pressure with only about 3 g sample. The pressure coefficients of various polymers are measured by the designed equipment. Measured values are compared to reported values and provide a good agreement.     

Effect of hydrogen implantation on the graphite used in high pressure diamond synthesis

The objective of the present work is to investigate the effect of hydrogen implantation on graphite in the high pressure diamond synthesis. A comparison of the graphite/diamond conversion for different fluences of hydrogen implantation revealed that the diamond nucleation and the total mass yield are always higher (up to 46%) than in experiments without implantation. The maximum nucleation for the studied cases occurred at a fluence of 1×10¹⁷ hydrogen ions/cm². This behavior is not observed when other ions, such as krypton and argon, are implanted on the graphite, or when hydrogen is present in the reaction cell but not implanted on the graphite. The results were interpreted as a consequence of the creation of additional tetrahedral sp³ bonded carbon atoms when the graphite is hydrogen implanted, which would act as effective diamond nucleation sites in the high pressure synthesis.     

A note on using expanded graphite for achieving energy- and time-efficient production of graphene nanoplatelets via liquid phase exfoliation

Although easily scalable, the production of graphene nanoplatelets (GNP) by the means of liquid-phase exfoliation of graphite flakes (GF) remains an energy- and time-intensive process. In this work, we demonstrate that significant time and energy can be saved in GNP production when employing expanded graphite (EG) in a surfactant-assisted liquid phase exfoliation process. Owing to its increased interlayer distance, the exfoliation of EG can be accomplished in a much shorter time (<30 min) compared to GF (approximately 7 h in the present case). Moreover, the energy required for the EG exfoliation is close to 80-fold lower than that for GF exfoliation. Monitoring of the mean lateral dimension, specific surface area, and graphite flake-to-GNP transition during exfoliation was performed experimentally using several analytical techniques. The EG-derived GNPs are produced much faster and require less energy for exfoliation compared to GF, thus making it a more efficient alternative technique.     

Hyperfiltration of nonelectrolytes: dependence of rejection on solubility parameters

The dependence of hyperfiltration rejection of nonelectrolyte solutes in single-solute water solutions on solubility parameters is demonstrated using hyperfiltration results reported in the literature. The hyperfiltration systems are characterized by a solubility parameter derived empirically from the rejection-solubility parameter dependence. A criterion for high rejection follows.     

Evidence of band bending and surface Fermi level pinning in graphite oxide

We present the electronic structure of graphite oxide in the vicinity of the Fermi level measured using ultraviolet photoemission and inverse photoemission spectroscopies and compare it with X-ray absorption spectra. The expected p-type behavior of graphite oxide is not observed at the surface and the presence of band bending is invoked. The observed electronic structure of graphite oxide exhibited an n-type semiconducting band structure with a band gap of 2.3 ± 0.4 eV. An oxygen related state, at 0.8 eV above Fermi level, and the suppression of the unoccupied carbon weighted states at the conduction band minimum suggests that the oxygen vacancies at the surface of graphite oxide contribute to the n-type semiconducting electronic structure of the surface.     

A physical model of the pressure dependence and biaxial mechanical properties of solid polymers

We give here a model for the pressure dependent, biaxial mechanical behavior of glassy polymers based on the thermally activated growth of deformation zones (Somigliana dislocation loops). The Coulomb criterion of plasticity, σ_c = S ? mσ_n, is found as the critical threshold needed to propagate Somigliana loops, in the same way as yield in crystals is found as the stress to move Volterra dislocation loops. While S is the shear strength, it is proposed that m follows basically from chain spacing fluctuations in the polymer glass; the temperature dependences of both parameters are derived. Application to tensile and compressive tests under a confinement pressure P is developed, with the aim to derive the pressure dependent (biaxial) strain-rate law. In particular, the pressure effect on dislocation density, that is, on plasticity defect nucleation, is shown to have a definite role in the plasticity of these solids. It introduces in the strain-rate law a normal stress dependent term (exp Dσ_n), which may have a decisive importance in a number of situations like multiaxial solicitations, solid state polymer shaping, second phase effects in polymer blends, and so on. Finally, a set of constant strain rate experiments is presented on an unsaturated polyester resin crosslinked with styrene. Measurements fit reasonably well with the predictions of the above model up to ～50 K below the glass transition, at which collective molecular motions invalidate its basic assumptions. The fit includes: (i) the Coulomb Criterion and its temperature dependence; and (ii) the dilative and shear apparent activation volumes at yield at all pressures.     

A note on the Buckingham equation

Friction losses of the laminar flow of a Bingham plastic fluid through a pipe is characterised by a friction factor, f, which depends on two dimensionless parameters, the Reynolds and the Bingham numbers. The relationship is a fourth degree equation for f, named after Buckingham (1921). In this short communication, we present a novel expression for its only physically meaningful solution, which is easier to handle than those presented previously by other authors.