Fracture toughness of random fibrous materials with ultrahigh porosity at elevated temperatures

The fracture toughness of three‐dimensional random fibrous (3D RF) material was investigated from room temperature to 1273 K by virtue of experimental method, theoretical model and Finite Element Method (FEM) in the through‐the‐thickness (TTT) and in‐plane (IP) directions. The experiments showed that the fracture toughness in the TTT and IP directions increases (from 0.0617 to 0.0924 Mpa·m^1/2 and from 0.2958 to 0.3982 Mpa·m^1/2 for the TTT and IP directions, respectively) as the temperature until reaching a transition temperature (1123 K and 1223 K for the TTT and IP directions, respectively), then the fracture toughness decreases from 0.0924 to 0.0393 Mpa·m^1/2 and from 0.3982 to 0.3106 Mpa·m^1/2 for the TTT and IP directions, respectively. The fracture behavior was related to the bulk microstructures, the mechanical properties of fibers and the blunting of crack tip. The crack tip blunting affected the fracture toughness at elevated temperatures which was verified using the theoretical model. A FEM model with a single edge crack where special attention was drawn to the influence of the morphological characteristic was developed to simulate the fracture behavior of 3D RF material. Numerical results from the FEM modeling along with a theoretical model with crack tip blunting mechanism incorporated agreed well with the experimental results.     

Foam breaking — key process in detoxification of kraft mill effluents by foam separation

A study was undertaken to demonstrate on pilot plant scale the performance of a turbine foam breaking system and to develop the design parameters for large scale application. Among various configurations a 3-blade vaned-disc turbine was found to be optimal for foam breaking. Major process variables controlling foam breaking were system design, tip velocity, rotation speed, and foam load. A foam breaking system with only restricted liquid draw-off performed 3 - 16 times better than a conventional flow through system. A 38 cm diameter turbine operating at 1800 τ/min (3600 cm/s tip velocity) collapsed up to 1.2 m³/min (42 ft³/min) of foam. Design equations developed for sizing of foam breakers suggest that a 61 cm (2-ft) diameter 21 kW turbine will collapse 4.7 m³/min of foam. For a 95 × 10³m³/d foam separation plant, approximately 12 foam breakers are required. Capital costs are estimated at $108,000.     

Slow crack propagation in polyethylene under fatigue at controlled stress intensity

Fatigue tests were performed on circumferentially notched bars (CNB) of high density polyethylene in order to analyse the kinetics and mechanisms of crack propagation. Tests were performed at 80 °C in order to accelerate the processes. Unlike standard fatigue procedure in which the force amplitude is constant, the original system utilised in this work was capable of imposing a constant stress intensity amplitude, K^max, during the whole propagation range. This was made possible through the real-time monitoring of crack propagation, a(N), by means of a video-controlled technique. The results of the tests, for K^max in a range from 0.2 to 0.45 MPa m^1/2, show that stress intensity is the proper variable which controls crack propagation rate since, on the overall, crack speed is constant at constant K^max and no self-acceleration is observed unlike under force control. The empirical Paris law is verified under all conditions, with a stress intensity exponent close to 4. However, it is shown that constant crack speed is obtained only for K^max<0.25 MPa m^1/2, when propagation proceeds through the continuous stretching and breaking of microfibrils in the localised craze at the tip of the crack. By contrast, at larger K^max, it is observed that crack tip successively jumps across the extended crazed zone in which very coarse fibrils were previously stretched from voids nucleated in the plane of maximum normal stress at a long distance ahead of the crack tip.     

Crack-Tip Structure in Soda–Lime–Silicate Glass

The topology of crack tips in soda–lime–silicate glass was investigated using atomic force microscopy (AFM). Studies were conducted on cracks that were first propagated in water and then subjected to stress intensity factors either at or below the crack growth threshold. Exposure to loads at the crack growth threshold resulted in long delays to restart crack growth after increasing the stress intensity factor to higher values. After breaking the fracture specimen in two, the "upper" and "lower" fracture surfaces were mapped and compared using AFM. Fracture surfaces matched to an accuracy of better than 0.5 nm normal to the fracture plane and 5 nm within the fracture plane. Displacements between the upper and lower fracture surfaces that developed after a critical holding time were independent of distance from the crack tip, and increased with holding time. Despite the surface displacement, crack tips appeared to be sharp. Results are discussed in terms of a hydronium ion–alkali ion exchange along the crack surfaces and corrosion of the glass surface near the crack tip by hydroxyl ions.     

Kinetic study of graphite oxidation along two lattice directions

A systematic kinetic study of the oxidative etching of graphite basal planes along the a and c directions at temperatures up to 950 °C was undertaken. It was found that at temperatures above 875 °C, oxidation of graphite surfaces is initiated from basal plane carbon atoms as well as from point defects. This oxidation process gives rise to monolayer-depth pits with round shapes, as observed by scanning tunneling microscopy. The distribution of the diameters of the pits formed at high temperatures was broad. At lower temperatures, however, where pit formation is initiated solely at point defects, a narrow pit diameter distribution was observed. By analyzing the etching rates as a function of oxidation temperature, kinetic parameters such as etch rates and activation energies for the oxidation reactions along the a and c crystallographic directions of the graphite lattice were obtained.     

Dynamic fracture of C/SiC composites under high strain-rate loading: microstructures and mechanisms

We investigate dynamic fracture of C/SiC composites under high strain-rate compression or tension with split Hopkinson pressure bar (SHPB) and gas gun loading. Components of the as-fabricated composites are mapped and quantified with X-ray computed tomography, including C fibers and fiber bundles, SiC matrix, and inter- and intrabundle voids. Compression loading is applied along the out-of- and in-plane directions by SHPB at strain rates of 10²–10³ s⁻¹ along with in situ X-ray phase contrast imaging. Out-of-plane direction compression and tension are examined with gas gun impact at strain rates 10⁴–10⁵ s⁻¹. For the out-of-plane loading, compression induces fracture via void collapse and shear damage banding, while delamination dominates fracture for the in-plane direction compression. With increasing strain rates, the compression failure modes transit from interbundle to intrabundle fracture of SiC, and then to fiber and bundle breaking. Tensile failure involves delamination, fiber pullout and fiber breaking. In contrary to normal solids, dynamic tensile or spall strength decreases with increasing impact velocities, owing to compression-induced predamage before subsequent tensile loading.     

Porous graphite matrix for chemical heat pumps

Expanded graphite powders were prepared by rapid heating of expandable graphite powders intercalated with sulfuric acid at different heat treatment temperatures (HTT). Porous graphite matrices with 100–400 kg m⁻³ of bulk density were fabricated by pressing expanded graphite powders in order to use as heat conductive media. They were characterized using an C/S analyzer, inductively coupled plasma spectroscopy, X-ray diffraction, scanning electron microscopy, Fourier transform infrared (FTIR), nitrogen adsorption, optical microscopy and helium pycnometer before and after heat treatment. Gas permeability and thermal conductivity were measured for porous graphite matrices with different HTT and bulk densities. Chemical analysis and FTIR showed that as the HTT of expandable graphite powders increase, the residual sulfur content decreased remarkably. Nitrogen adsorption experiments for expanded graphite powders showed that specific surface area and total pore volume increased with HTT. Helium penetration results showed that porous graphite matrices with different HTT have noticeably different open porosities which were attributed to the different degrees of expansion of graphite layers. The gas permeability of porous graphite matrices was in the range of 10⁻¹²–10⁻¹⁵ m² and exhibited higher values with low HTT. Thermal conductivity values in the axial and the radial directions were in the range of 4.1–20.0 and 4.6–42.3 W mK⁻¹, respectively. A semi-empirical model was developed that can be used to correlate with the thermal conductivity of graphite matrix on the basis of solid conductivity, bulk density and porosity.     

Effects of system dynamics and applied force on adhesion measurement in colloidal probe technique

Dynamic pull-in/pull-off forces were quantitatively measured using an AFM colloidal probe technique. Two spherical colloids made of silicon dioxide (SiO₂) and gold (Au) that were attached to an AFM cantilever were approached to and retracted from a silicon wafer specimen, where the speed of tip approaching/retracting (i.e., vertical dynamics) and specimen sliding (i.e., horizontal dynamics) was controlled. First, when the vertical dynamics of colloidal tip was applied on the stationary silicon wafer specimen, it was observed that the slower tip approaching showed higher pull-in force, while the pull-off force was dependent on both the applied force and the retracting speed. For the two colloidal tips, it was found that the higher applied force and the faster tip retraction led to the higher pull-off force. Next, under the constant speed of tip approach and retraction, horizontal dynamics was applied to the silicon wafer specimen. It was observed that the horizontal motion of the specimen made the pull-off force lower, which could be attributed to the breakage of adhesive asperity junctions at the interface. The pull-off force was further decreased at faster horizontal motion of the specimen due to the longer sliding distance. Therefore, from the systematic experiments of dynamic adhesion measurement, it could be known that if a micro/nano-system is under dynamic surface interaction, its adhesive force cannot be fully described by a conventional quasi-static adhesion model but it should include the effects of applied system dynamics in both normal and tangential directions.     

Improved toughness and electromagnetic shielding-effectiveness for graphite-doped SiC ceramics with a net-like structure

The graphite-doped SiC ceramics with net-like structure was fabricated via tape casting and pressureless sintering. The ceramics exhibited a step-like fracture mode, which could be attributed to the net-like structure composed of long columnar SiC grains, layered graphite, and the three-modal pore distribution. The formation of warped epitaxial graphene and large size graphite could be attributed to the pyrolysis of organics in the tape casting system. In the net-like structure, the SiC grains provide the high strength, whereas the layered graphite and three-modal pores were used to deflect the cracks and release the stress at the tip, following the crack-tip-shielding mechanism. The sample with a net-like structure exhibited a combination of a variety of extrinsic toughening mechanisms, such as crack deflection, crack bridging, crack branching and delamination, pull-out, and rupture of layered graphite, which led to improved fracture toughness of 7?MPa?m^1/2, flexural strength of 400?MPa, and (work of fracture) WOF of 3.3?kJ?m^?2. When increasing the graphite content, the electrical conductivity of the graphite-doped SiC ceramics significantly increased from 7.15?×?10^?4 to 216 S/m. The high shielding effectiveness of 34.1?dB was due to the multi-absorption on the various surfaces during the multi-reflection by the net-like structure.     

Following Single Molecules by Force Spectroscopy

Dynamic force spectroscopy of single molecules, in which an adhesion bond is driven away from equilibrium by a spring pulled with velocity V, is described by a model that predicts the distribution of rupture forces (mean and variance), all amenable to experimental tests. The distribution has a pronounced asymmetry, which has recently been observed experimentally. The mean rupture force follows a (lnV)^2/3 dependence on the pulling velocity and differs from earlier predictions. Interestingly, at low pulling velocities a rebinding process is observed whose signature is an intermittent behavior of the spring force that delays the rupture. Based on the rupture mechanism, we propose a new “pick-up-and-put-down” method to manipulate individual molecules with scanning probes. We demonstrate that the number of molecules picked up by the tip and deposited at a different location can be controlled by adjusting the pulling velocity of the tip and the distance of closest approach of the tip to the surface.     

Molecular dynamics of cleavage and flake formation during the interaction of a graphite surface with a rigid nanoasperity

Computer experiments concerning interactions between a graphite surface and the rigid pyramidal nanoasperity of a friction force microscope tip when it is brought close to and retracted from the graphitic sample are presented. Covalent atomic bonds in graphene layers are described using a Brenner potential and tip-carbon forces are derived from the Lennard-Jones potential. For interlayer interactions a registry-dependent potential with local normals is used. The behavior of the system is investigated under conditions of different magnitudes of tip-sample interaction and indentation rates. Strong forces between the nanoasperity and carbon atoms facilitate the cleavage of the graphite surface. Exfoliation, i.e. total removal of the upper graphitic layer, is observed when a highly adhesive tip is moved relative to the surface at low rates, while high rates cause the formation of a small flake attached to the tip. The results obtained may be valuable for enhancing our understanding of the superlubricity of graphite.     

Corrosion behavior of a positive graphite electrode in vanadium redox flow battery

The graphite plate is easily suffered from corosion because of CO₂ evolution when it acts as the positive electrode for vanadium redox flow battery. The aim is to obtain the initial potential for gas evolution on a positive graphite electrode in 2 mol dm⁻³ H₂SO₄ + 2 mol dm⁻³ VOSO₄ solution. The effects of polarization potential, operating temperature and polarization time on extent of graphite corrosion are investigated by potentiodynamic and potentiostatic techniques. The surface characteristics of graphite electrode before and after corrosion are examined by scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. The results show that the gas begins to evolve on the graphite electrode when the anodic polarization potential is higher than 1.60 V vs saturated calomel electrode at 20 °C. The CO₂ evolution on the graphite electrode can lead to intergranular corrosion of the graphite when the polarization potential reaches 1.75 V. In addition, the functional groups of COOH and CO introduced on the surface of graphite electrode during corrosion can catalyze the formation of CO₂, therefore, accelerates the corrosion rate of graphite electrode.     

Raman spectra of ground natural graphite

Structural changes in Ceylon natural graphite with grinding were studied by Raman spectroscopy along with X-ray diffraction. The natural graphite shows a single Raman band at 1580 cm^?1, but the ground graphite samples exhibit two Raman bands at 1360 and 1620 cm^?1 in addition to the 1580 cm^?1 graphite band. The 1360 cm^?1 band increases in intensity with increasing grinding time, and becomes much stronger than the 1580 cm^?1 band after 200-hr grinding. Raman results are compared with structural parameters such as effective Debye parameter and C₀ spacing obtained from X-ray diffraction measurements, and discussed in terms of structural defects introduced into the crystal lattice of natural graphite. A linear relationship was obtained for the ground graphite when the relative intensity of the 1360 cm^?1 band is plotted as a function of effective Debye parameter. The slope of the linear plot is different for the ground graphite from that for the graphitized cokes, indicating a difference in the type of structural defects involved.     

Intermolecular interactions dictating adhesion between ZnO and graphite

Zinc oxide (ZnO) was recently demonstrated to strongly adhere to both pristine graphitic surfaces and oxidized surfaces; however, the bonding interaction between the two is unknown and has yet to be investigated. In this study, atomic force microscopy (AFM) liftoff in the dry state was performed with a ZnO coated AFM tip on highly oriented pyrolytic graphite (HOPG) such that the bonding interaction can be identified and used to design high strength interfaces. It is found that the ZnO film on the AFM tip becomes negatively charged after contacting an oxygen plasma treated HOPG surface, and once charged can strongly interact with other surfaces such as pristine HOPG through induced polarization to create high adhesive energy.     

A theoretical analysis of frictional and defect characteristics of graphene probed by a capped single-walled carbon nanotube

Using molecular dynamics simulations, we show that a probing tip using a short capped single-walled carbon nanotube is able to capture the frictional characteristics and faithfully resolve the graphene lattice through the measurements of oscillatory lateral force or normal force. By averaging the oscillatory lateral force and normal force along the tip moving path, we extract the friction coefficient. It is found that the friction coefficient decreases with increasing both the initial tip–surface distance and the number of graphene layer. The underlying energy dissipation arises from the periodical acceleration–deceleration of the tip, causing the conversion of kinetic energy into thermal energy. We also study the interaction of the tip with a single-layer graphene containing a vacancy or Stone–Thrower–Wales defect, and reveal that the change in lateral and normal forces can be used to differentiate these defects. The present study demonstrates that a short single-walled capped nanotube can serve as an ideal candidate for high-resolution surface probing.     

Graphitization behaviour of chemically derived graphene sheets

Graphene sheets were prepared via chemical reduction of graphite oxides and then graphitized at 2800 °C. The structure changes from pristine graphite to graphitized graphene sheets were monitored using X-ray diffraction and Raman spectroscopy. It was found that the graphitized graphene sheets exhibited relatively low degree of graphitization and high level of structural defects. XPS spectra revealed that oxygen functionalities could be completely eliminated after graphitization. Morphology observations indicated that graphitization could induce the coalescence and connection of the crumpled graphene agglomerations into compressed grains. The connections included the joint of graphitic sheets along the c-axis with van der Waals force between graphitic sheets and the joint of sheets in the in-plane with covalent bond between carbon atoms. New structures such as the formation of loop at the tip of graphene sheets and the formation of 3D concentric graphene nanoparticles occurred in the graphitized graphene sheets, as a result of self-organization to achieve their lowest potential energy. Our findings should provide some experimental implications for understanding of graphitization behaviour and thermal stability of strictly 2D graphene monolayers.     

Surface topography and surface chemistry of radiation‐patterned P(tBuMA)—analysis by atomic force microscopy

Poly‐(tert‐butyl methacrylate) (P(tBuMA)) thin‐film surfaces were patterned by UV radiation at doses in the range 10–100 mJ cm^?2, in order to induce laterally differentiated surface chemistry with µm resolution. The most likely pathway for the radiation chemistry predicts a transition from hydrophobicity to hydrophilicity. Outcomes of analysis by atomic force microscopy under air ambient conditions were consistent with that prediction. Topographic and lateral force imaging, in combination with friction loop analysis, revealed shrinkage and increased friction arising from exposure. Force versus distance analysis revealed greater adhesion in hydrophilic regions, due to greater meniscus force acting on the tip. The thickness of adsorbed moisture, increased by a factor of 2.5 from ca 0.8 nm for the unirradiated surface, as a result of greater hydrophilicity induced by radiation. The latter observation shows that the increased friction was due principally to the greater normal force on the tip from an additional meniscus force. Copyright © 2003 Society of Chemical Industry     

Molecular weight dependence of the fracture toughness of glassy polymers arising from crack propagation through a craze

We investigate criteria for craze failure at a crack tip and the dependence of craze failure on the molecular weight of the polymer. Our micromechanics model is based on the presence of cross-tie fibrils in the craze microstructure. These cross-tie fibrils give the craze some small lateral load bearing capacity so that they can transfer stress between the main fibrils. This load transfer mechanism allows the normal stress on the fibrils directly ahead of the crack tip in the center of the craze to reach the breaking stress of the polymer chains. We solve for stress field near the crack trip and use it to relate craze failure to the external loading and microstructural quantities such as the craze widening (drawing) stress, the fibril spacing, the molecular weight, and the force to break a single polymer chain. The relationship between energy flow to the crack tip due to external loading and the work of local fracture by fibril breakdown is also obtained. Our analysis shows that the normal stress acting on the fibrils at the crack tip increases linearly as the square root of the craze thickness, assuming that the normal stress distribution is uniform and is equal to the drawing stress acting on the craze-bulk interface. The critical crack opening displacement, and hence the fracture toghness is shown to be proportional to [1–(M_e/qM_n)]², where M_e is the entanglement molecular weight, M_n is the number average molecular weight of polymer before crazing, and q is the fraction of entangled strands that do not undergo chain scission in forming the craze.     

Chemistry of graphite intercalation by nitric acid

Gas-phase intercalation of graphite by nitric acid is accompanied by evolution of a brown gas which has been identified as nitrogen dioxide by chemiluminescence measurements. The reaction is inhibited by the presence of oxygen or water vapor, but not by nitrogen. These results, and the existence of induction times of 5–20 min before intercalation begins, is interpreted as evidence that intercalation is effected by oxidation of graphite by adsorbed nitronium ions with the release of NO₂. The graphite lattice then intercalates NO₃^? ions along with neutral HNO₃ molecules.     

Determining Surface Potential of the Bitumen‐Water Interface at Nanoscale Resolution using Atomic Force Microscopy

Atomic force microscopy (AFM) was used to measure the surface forces between a silicon nitride AFM tip and a deposited layer of Athabasca bitumen; the measurements were carried out in pure water (pH 6.0–6.5) and 1 mM KCl solution (pH 9). An AFM pyramidal‐shaped tip was moved stepwise using an operator‐controlled offset (10 nm per step) and the tip‐bitumen colloidal forces were measured at each location. Surface charge densities at the bitumen‐water interface were calculated from the measured colloidal forces using a theoretical model that combined both electrostatic and van der Waals forces for a conical tip‐flat substrate system. Fitted values of the bitumen surface charge density ranged from –0.002 to –0.004 C/m² in water (pH 6.0–6.5), and –0.005 to –0.022 C/m² in KCl solution (pH 9); the variation of local charge density along the bitumen surface appeared random. Bitumen surface potentials were also calculated from the surface charge densities using the Graham equation; the values ranged from –90 to –130 mV in water, and –45 to –110 mV in KCl solution. This study suggests the presence of bitumen surface domains of different surface charge densities/surface potentials. The domains are estimated to have characteristic sizes of 20 to 40 nm or less.