Preferred orientation of crystallite in carbons heat-treated under high pressure

A soft carbon and a hard carbon which were prepared from Polyvinylchloride and phenolformaldehyde resin, respectively, both by carbonizing to about 700°C, were obtained as sintered cakes by heat-treatment up to about 1800°C under the quasihydrostatic pressure of 5 kbar. Preferred orientation of crystallites relative to the compressing direction in these cakes was determined by X-ray difraction technique by using the (004) or (002) diffraction line. The soft carbon showed remarkable preferred orientation of crystallites, but the degree of orientation was dependent only a little on heat-treatment temperature (HTT). At 1200°C under 5 kbar, the hard carbon gave a sintered cake which had no appreciably preferred orientation. The degree of preferred orientation of crystallites in the cake of the hard carbon greatly increased with the increase in HTT. The difference in the dependence of preferred orientation of crystallites in the soft and hard carbons on HTT was interpreted by referring to the texture of the original carbons.     

Microstructure features of polycrystalline diamond synthesized directly from graphite under static high pressure

Recently, ultra-hard polycrystalline diamond was synthesized from graphite by direct conversion under static high pressure. This paper describes the microstructure features of thus formed polycrystalline diamond. Transmission electron microscopy and electron diffraction have revealed that the polycrystalline diamond has a mixed texture of a homogeneous fine structure and a lamellar structure. The former structure consists of fine-grained diamond particles of several tens of nanometers across, which are randomly oriented. The latter structure has bending diamond layers, which may reflect deformed shapes of locally layered graphite of starting material. The experimental results suggest that diamond particles in the homogeneous fine structure are transformed from graphite in the diffusion process, while diamond layers in the lamellar structure are formed in the martensitic process from graphite via the hexagonal diamond phase. It is also noted that significant grain growth occurred at a high temperature of 2700°C, and the lamellar structure was segmentalized to form new grain boundaries.     

Morphology change in carbons prepared from pitch-polyvinylchloride mixtures under pressure

Mixtures of fractionated pitches with different molecular weights and polyvinylchloride (PVC) were carbonized at 650°C under a pressure of 30 MPa. The morphology of the carbons obtained was studied under a scanning electron microscope and the optical texture under a polarized light microscope. Gradual changes in morphology are observed, from lump-type to coralloidal through botryoidal with gradual diminution in the size of the primary particles. The content of PVC and also the number-averaged molecular weight were found to govern the morpohology of the resultant carbon.     

Effects of crystallinity of starting carbons on diamond formation in presence of nickel under high pressure and high temperature condition

The processes of graphitization and diamond formation of several carbons in the presence of nickel were investigated under 8 GPa at temperatures up to 1800° C. Diamond was formed easily from graphitized pitch coke which had a well-developed graphitic structure and in less amount from glassy carbon preheated at about 3000° C which was partly graphitized. On the other hand, pitch coke and glassy carbon, preheated at about 2000° C and not graphitized, did not transform to diamond but remained graphitized even in the diamond stable region. Diamond from graphitized pitch coke and glassy carbon preheated at about 3000° C grew to form by direct bonding.     

Texture and graphitizability of carbons made from pitch and phenolic resin mixtures

Benzene-soluble pitch and two types of phenolic resin, resol- and novolac-type, were mixed by using pyridine. After removing pyridine, the mixtures were carbonized at 600° C and then heat-treated at 3000° C. By solvent-mixing, homogeneous optical textures within the carbons were found to be obtained, though mechanical-mixing led to heterogeneity. Changes in optical texture from coarse mosaic to isotropic through fine mosaic were observed in the carbons in the range of resin content between 40 and 60% for the pitch-resol-type-resin system, and between 30 and 40% for the pitch-novolac-type-resin system. The transitions from well graphitized to poorly graphitized carbons were observed on the 3000° C-treated samples in the same range of resin content. In the carbon fibre-carbon composites prepared by using these pitch-phenolic-resin mixtures as matrix, fine mosaic texture in the matrix appeared with a lower content of resin.     

Diamond formation from glassy carbon under high pressure and temperature conditions

The process of the formation of diamond from the glassy carbon with its characteristic bond nature was investigated in the diamond stable region at high pressures (up to 10 GPa) and temperatures (up to 3000° C), without any intentional addition of metals as solvent. The process of diamond formation was found to obey Ostwalds's step rule as follows: amorphous glassy carbon crystallized to form fully well-crystallized graphite prior to diamond formation and then the graphite crystals were converted to diamond by further heat treatment at pressures above 9 GPa. The many trigons formed are considered to be essentially a record of growth failure in the growth period. As a result of heat treatment for a longer time and/or at a higher temperature close to the diamond—graphite stability boundary, the diamond tended to grow with the (111)-face composed of the thin growth layers.     

Behaviour of B-N-Si under high pressure and high temperature

The phase relations of the B-N-Si system have been studied using a quenching method up to 10GPa and 2000 °C using a high-pressure apparatus of the octahedral anvil type. Pressure-temperature conditions for obtaining z-BN (diamond analogue of boron nitride) were delineated for turbostratic BN (t-BN), t-BN/amorphous Si₃N₄ and t-BN/-Si₃N₄. These conditions shift toward higher regimes of temperature as amorphous Si₃N₄ or -Si₃N₄ is incorporated into t-BN. Spontaneous sintering occurringin situ at high pressure yields z-BN-based composite compacts.     

Investigation of the process of diamond formation from SiC under high pressure and high temperature

SiC powder or graphite in contact with cobalt, nickel or a Ni₇₀Mn₂₅Co₅ alloy was treated at high pressure and high temperature in stable region of diamond. It was found that Ni₇₀Mn₂₅Co₅ alloy is more effective in the process of diamond formation from SiC than the others, but the difference was not apparent when graphite was used instead of SiC. Using the Ni₇₀Mn₂₅Co₅ alloy, diamond formed rapidly with the decomposition of SiC at a pressure of 5.4–6.0 GPa and temperature 1350–1570°C, and the growth tended to stagnate after 6 min, when SiC was completely exhausted. X-ray diffraction showed that the relative intensity of the diffraction lines of diamond and graphite was nearly constant in the samples synthesized under the same conditions for 2, 4 and 6 min. The results suggest that diamond and graphite may be formed directly and respectively from separated carbon atoms in a short time.     

IR determination of the orientation of molecules in polycrystalline monolayers

A theoretical treatment of the absorption of linearly polarized IR radiation by organic monolayers is given. The present approach applies to polycrystalline monolayers and enables the determination of the microcrystalline orientation and the molecular orientation inside the microcrystals.This model is quite general and can be used to calculate the optical absorption of polycrystalline thin films, irrespective of the symmetry of the microcrystals.     

Electrospun nanofiber belts made from high performance copolyimide

Electrospun nanofibers based on copolyimides were made, aiming at finding a promising method for improving the mechanical properties of electrospun polyimide nanofibers. The copolyimide had a backbone consisting of 3,3',4,4'-biphenyl-tetracarboxylic dianhydride (BPDA), biphenylamide (BPA) and 4,4'-oxydianiline (ODA) residues. The structure and composition of the copolyimide was controlled by the ratio of rigid BPA and flexible ODA moieties. The electrospun copolyimide nanofibers were collected in the form of a belt using a rotating disc with a rim of 8?mm width. Scanning electron microscopy (SEM), infrared (IR) spectroscopy, x-ray scattering and tensile testing, dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA) were used to characterize the nanofiber belts. The nanofibers had a diameter range from 80 to 300?nm and were well aligned in the belts. The thermal stability of the nanofiber belts was over 460?°C. The tensile test showed that the copolyimide nanofiber belts had much better mechanical properties than either of the flexible and rigid homo-polyimide (homo-PI) nanofiber belts. The tensile strength, modulus and elongation to break of the copolyimide nanofiber belt with BPA/ODA ratio of 40/60 are respectively 1.1 ± 0.1?GPa, 6.2 ± 0.7?GPa and 20.8 ± 1.2%, compared to 459 ± 36?MPa, 2.1 ± 0.3?GPa and 41.3 ± 2.2% for BPDA/ODA homo-PI as well as 384 ± 18?MPa, 11.5 ± 0.6?GPa and 3.9 ± 0.1% for BPDA/BPA homo-PI. The important feature is that the electrospun polymer nanofibers can be made very strong by using copolyimides as spinning materials.     

Synthesis of cubic boron nitride from rhombohedral form under high static pressure

The cubic, zincblende-type boron nitride (z-BN) has been synthesized from the rhombohedral form (r-BN) under high static pressures greater than 6 GPa without any planned addition of catalysts. The process of forming z-BN has been delineated from isobaric and isothermal series of data. At 6GPa, r-BN begins conversion to the graphite-type form (g-BN) upon heating to 600 °C. This conversion terminates at 1200 °C forming single-phase g-BN, which in turn transforms into z-BN at temperatures higher than 1300 °C. The appearance of z-BN occurs at lower temperatures when the pressure is raised to 7 or 8 GPa. At pressures beyond 10 GPa the wurtzite-type form (w-BN) is observed between 400 and 1200 °C, whereas z-BN is formed above 1000 °C. The boundary of pressure-temperature conditions for synthesizing z-BN from r-BN runs through 6GPa and 1300 °C, and is located near to the lowest bound hitherto known for non-catalytic z-BN synthesis from g-BN.     

Laser-assisted processing of Ni-Al-Co-Ti under high pressure

Laser-assisted processing and in-situ characterization of a Ni_0.7-Al_0.1235-Co_0.15-Ti_0.0265 alloy were carried out under a range of simultaneous hydrostatic high pressures of ～30?GPa and high temperature conditions ～2000°C using a laser-assisted heating in diamond anvil cell with synchrotron X-ray micro-diffraction. The characterization of the microstructure and X-ray diffraction analysis at ambient conditions confirmed the formation of the cuboids of ordered γ′ phase in the disordered γ matrix. The isothermal bulk modulus (B₀) and its first-order derivative (B₀’) of the alloy were determined to be B₀?=?123?±?9?GPa and B₀’?=?5.7?±?2.8. The in-situ characterization of the alloy at high temperatures under high pressures revealed that the γ′ phase transforms into the tetragonaly distorted D0₂₂-type structure. This transformation is similar to the transformation that occurs in the ordered Ni₃Al, responsible for the improved strength at high temperatures. High pressure was found to increase the onset temperature of the structural distortion. The pressure–temperature phase diagram of the Ni_0.7-Al_0.1235-Co_0.15-Ti_0.0265 up to ～30?GPa and ～2000°C was determined and is reported here.     

Transformation pathways of silica under high pressure

Network-forming oxides with rigid polyhedral building blocks often possess significant capacity for densification under pressure owing to their open structures. The high-pressure behaviour of these oxides is key to the mechanical properties of engineering materials and geological processes in the Earth's interior. Concurrent molecular-dynamics simulations and first-principles calculations reveal that this densification follows a ubiquitous two-stage mechanism. First, a compact high-symmetry anion sublattice forms, as controlled by strong repulsion between the large oxygen anions, and second, cations redistribute onto the newly created interstices. The same mechanism is observed for two different polymorphs of silica, and in the particular case of cristobalite, is corroborated by the experimental finding of a previously unidentified metastable phase. Our simulations not only clarify the nature of this phase, but also identify its occurrence as key evidence in support of this densification mechanism.     

Studies on elemental tellurium under high pressure

A non-linear behaviour in the resistance of tellurium as a function of pressure has been observed. At each pressure the resistance shows a time variation. At lower pressures, the normalized resistance increases with time, and at higher pressures, the normalized resistance decreases exponentially as a function of time. This change in behaviour with respect to time occurs in the region of the steepest descent in the resistance versus pressure plot. However, the magnitude of the change in the resistance with time is small compared to a change in the resistance with pressure. The origin of this behaviour is suggested to be linked with the generation and annealing of localized charged defects. This revised version was published online in November 2006 with corrections to the Cover Date.     

Rheology of cement paste under high pressure

The objective of this study was to investigate whether cementitious materials undergo changes during pumping processes due to pressure variation. The influence of pressure on the rheological properties of cement pastes, which assumably represented the lubricating layer that forms along the profile of concrete during pumping, was evaluated using a rotational rheometer with a high-pressure cell. Cement pastes with water-to-cement ratios ranging from 0.35 to 0.6 were tested according to a protocol designed to simulate the conditions of an actual pumping process based on field tests. The shear rates, shearing durations, and pressure levels from 0 to 30 MPa were experimentally simulated. The test results indicated that below a certain water-to-cement ratio (0.40) elevated pressures lead to changes in the rheological properties, while changes were negligible when the ratio was above this threshold. Further, at low water-to-cement ratios the thixotropy of the cement paste can reverse into rheopexy after pressurization.     

Thermal expansivity of teflon under high pressure

Results of (T/P) measurements are presented involving Teflon. The dependence of its thermal expansivity on the pressure has been determined within the stability range for various modifications of this material (P13 kbar, 0T50°C).Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 4, pp. 671–673, October, 1980.