Strength and microplasticity of biocarbons prepared by carbonization in the presence of a catalyst

The microdeformation has been investigated under uniaxial compression of beech-derived biocarbons partially graphitized during carbonization in the presence of a Ni- or Fe-containing catalyst. The strength and ultimate fracture strain have been determined at different temperatures of carbonization of the samples in the absence or in the presence of a catalyst. It has been shown using high-precision interferometry that the deformation of biocarbon samples under uniaxial loading occurs through jumps (in magnitude and rate of deformation) with axial displacements in the nanometer and micrometer ranges. The use of a catalyst leads to a decrease in the size of nanometer-scale jumps and in the number of micrometer-scale jumps. The standard deviations of the strain rate on loading steps from the smooth average dependence of the strain rate on the displacement have been calculated for micrometer-scale jumps. A similar characteristic for nanometer- scale jumps has been determined from the distortion of the shape of beats in the primary interferogram. It has been shown that the variation in the standard deviation of the strain rate with a change in the carbonization temperature is similar to the corresponding dependence of the ultimate fracture strain.     

Jumpwise deformation of polymethyl methacrylate in the microplasticity region

The deformation rate with a step of 325 nm has been measured under uniaxial compression at the initial stage of creep and shape recovery of a polymethyl methacrylate (PMMA) sample after unloading. The effect of low γ-ray doses and magnetic fields on the deformation has been studied. It has been shown that a weak pre-exposure of the PMMA sample structure to radiation and magnetic fields can cause a slight hardening in the microplasticity region. The deformation jump sizes have been determined on micro- and nanoscales. The effect of irradiation and magnetic fields manifests itself as redistributed contributions of various jumps to the deformation.     

Effect of activation on the porous structure and the strain and strength properties of beech wood biocarbon

The effect of activation on the size, specific volume, and surface area of pores in a monolithic biomorphic material obtained by carbonization of beech wood is studied. It is shown that under optimal activation mode with a steam heated to 970°C, the total pore volume and surface, determined by adsorption curves, increased by 20 and 18 times, respectively. With the use of high-precision interferometric procedure, strain curves are obtained under uniaxial compression with a stepwise loading, and the strain rate is measured with a step of moving of 325 nm for activated and nonactivated samples. Despite an increase in porosity, the strength and maximum deformation of the samples do not decrease. The behavior of the strain rate jumps is analyzed in the micro- and nanometer range. It is shown that the maximum size of the micrometer jumps (4 μm) correlates well with the average size of the possible strain area in the samples (the average distance between the pores of small size), and the minimum dimensions of the strain jumps are close to the size of mesopores. Assessment of the strain change and its rate upon activation indicates that the effect of activation on the strain and strength characteristics is defined by nanometer defects, the most likely of which are microand mesopores.     

Micro- and nanoscale instabilities of deformation in polymers

The rate and magnitude of the deformation in polymers under constant compressive stresses at room temperature have been measured. The use of laser interferometer has made it possible to perform measurements at small intervals of variations in the specimen length Δl = 0.325 μm, and the analysis of the form of beats has made it possible to estimate oscillations of the strain rate in nanoscale displacements. It has been shown that the average strain rate of polymers continuously varies and no creeping interval with a constant rate is observed. At all stages of smooth variations in the average rate, jumps of its current values corresponding to Δl from several nanometers to a hundred and more nanometers have been found. Changes in the structure with an increase in the deformation manifest themselves in an increase in the size of nanoscale jumps and in a complication of their shape.     

Nanoscale deformation irregularities of γ-irradiated poly(methyl methacrylate)

Development of deformation jumps in the creep of poly(methyl methacrylate) (PMMA) has been studied. The structural levels of deformation have been determined from the creep rate oscillation periods (deformation jumps) measured by the interferometric method. Special attention is given to a new method of data processing, which enables one to reveal previously undetectable nanoscale deformation jumps. By the example of PMMA specimens preliminarily exposed to γ radiation with doses D=55–330 kGy and unexposed specimens, the presence of nanoscale deformation jumps with the values dependent on the dose D and time of creep has been shown. The obtained results confirm the existence of 10–20-nm domains in amorphous polymers and make it possible to study the multilevel organization of the deformation process, starting from the nanoscale.     

Effect of γ-radiation on the strain characteristics of a high-filled wood-plastic composite

The strain rate and the characteristics of the jumps at micro- and nanolevels were measured by the high-precision interferometric method for a wood-plastic composite irradiated to doses of 0–100 kGy. Radiation was shown to strengthen the material and change the characteristics of strain rate and value jumps. Strain jumps and mean-square deviations of the measured strain rate from its smoothened time dependence were determined for micro- and nanosized jumps. The change of these characteristics depending on the radiation dose of specimens was traced. A relation between the characteristics of micrometer jumps and the macroscopic strain was revealed.

     

Regularities of Microstrain of Ultrahigh-Molecular-Weight Polyethylene Modified with Halloysite Additives

The effect of additives of 1 and 3 wt % of halloysite on the rate and small jumps of deformation under uniaxial compression of ultrahigh-molecular-weight polyethylene was investigated. A procedure for precision interference measurement with a resolution of 325 nm for displacement and 1 kHz for frequency enabled the detection of several levels of deformation in the micro- and nanometer ranges. The addition of halloysite results in a decrease in the strain rate under the same loading conditions and a change in the characteristics of the strain jumps. Calorimetric measurements showed that melting of polyethylene with a different concentration of halloysite causes a change in the transition energy and the degree of crystallinity.     

Inhomogeneity of the strain rate of polymer materials with different supramolecular structures

The characteristics of rate oscillations in submicron deformation increments in the course of creep under compression of polymer materials of different classes, namely, amorphous poly(vinyl butyral), amorphous-crystalline poly(tetrafluoroethylene), and a composite consisting of polyimide with graphite particles, have been investigated. The strain rate has been measured using an interferometer on a deformation base of 300 nm. The periods of rate oscillations have been used to determine the deformation jumps, and the amplitude of rate oscillations has been used to determine the jump sharpness. It has been demonstrated that the radical differences in the structure of materials manifest themselves in the parameters of deformation jumps at different stages of creep. The type of jumps makes it possible to reveal the type of molecular packing in the starting polymer or the packing formed during deformation.     

Multilevel character of deformation of polymers

The inhomogeneity in the creep rate of polymers on different scales of deformation has been studied by laser interferometry. The main results have been obtained for the amorphous-crystalline polymer polytetrafluoroethylene. The deformation characteristics are the oscillation periods of the rate (jumps of deformation), oscillation amplitudes of the rate, and the scatter of these quantities. Application of computer methods for processing of the results has made it possible to determine the difference and similarity between jumpwise deformations on different structural levels, including the nanolevel. For a more distinct separation of deformation levels, the measurements have been made in a magnetic field and outside the magnetic field. Deformation jumps have been found on five levels: from 4 nm to more than 10 μm. Introduction of a sample into a magnetic field changes the characteristics of jumps; in this case, the scatter in the values of jumps always increases, whereas their average value varies differently on different scale levels. The measurement of the parameters of deformation jumps on different scales allows one to study the laws of the development of the deformation process and the evolution of structural inhomogeneities.     

Inhomogeneity of deformation of solids at the nanometer level

A new method for processing interferometrically recorded deformation data has been implemented for studying an inhomogeneity in the rate and parameters of deformation jumps at the nanostructure level, which provides detection of deformation jumps of less than 300 nm. It is shown that the lower limit for deformation jumps lies in the range 10–30 nm for aluminum and is 130 nm for amorphous polymer (poly(methyl methacrylate)). It is assumed that the sizes of jumps correspond to scales of ordered structures, as was previously established for higher level structures. The results obtained make it possible to investigate more thoroughly the multilevel character of deformation and to evaluate the sizes of the nanostructural units, their evolution during deformation and under the effect of external fields, as well as their relation to the microscopic and macroscopic inhomogeneities of deformation.     

Influence of plastic deformation on fracture of an aluminum single crystal under shock-wave loading

The plastic deformation and the onset of fracture of single-crystal metals under shock-wave loading have been studied using aluminum as an example by the molecular dynamics method. The mechanisms of plastic deformation under compression in a shock wave and under tension in rarefaction waves have been investigated. The influence of the defect structure formed in the compression wave on the spall strength and the fracture mechanism has been analyzed. The dependence of the spall strength on the strain rate has been obtained.     

Structural heterogeneity and jumplike deformation of polymers on the mesoscopic level

This paper reports on the results of research into the jumplike deformation of two polymers based on poly(oxymethylene) (POM) with structural aggregates (spherulites) of different micrometer-scale sizes at a temperature of 290 K, as well as of polyimide (PI) and a PI + graphite composite at temperatures of 290 and 690 K. The creep rate under compression is measured with a laser interferometer in 0.3-μm deformation increments. It is found that, in the course of deformation on the micrometer scale, the creep rate varies nonmonotonically. Periodic variations of the creep rate correspond to a jumplike (stepwise) behavior of the creep. It is shown that the mean jumps in the microdeformation correspond to the mean sizes of poly(oxymethylene) grains and graphite particles in polyimide. The results obtained are in agreement with previously drawn conclusions: the deformation jumps are determined by the scale of ordered microaggregates typical of the structure under investigation.     

Thermopower coefficient of pine-wood biocarbon preforms and the biocarbon-preform/copper composites

The thermopower S(T) of the composites prepared by filling empty sap channels in high-porosity biocarbon preforms of white pine wood by melted copper in vacuum and the thermopower S(T) of these preforms have been measured in the temperature range 5–300 K. The biocarbon preforms have been obtained by pyrolysis of pine wood in an argon flow at two carbonization temperatures, 1000 and 2400°C. An analysis of the experimental data has demonstrated that the thermopower of the composites is determined by the contribution related to the copper filling the channels of the biocarbon preform and exhibits a characteristic temperature dependence with a deep minimum close to 20 K. This suggests that copper in the preform channels is essentially a Kondo alloy with the iron and manganese impurities entering into it from the carbon preform in the course of infiltration of melted copper.     

The Effect of Small Additions of Carbon Nanotubes on the Mechanical Properties of Epoxy Polymers under Static and Dynamic Loads

The results of measurements of the mechanical characteristics of cured epoxy composites containing small and ultrasmall additions of single-walled carbon nanotubes in the concentration range from 0 to 0.133 wt % under static and dynamic loads are presented. Static measurements of strength characteristics have been carried out under standard test conditions. Measurements of the Hugoniot elastic limit and spall strength were performed under a shock wave loading of the samples at a deformation rate of (0.8–1.5) ß 10⁵ s^-1 before the fracture using explosive devices by recording and subsequent analyzing the evolution of the full wave profiles. It has been shown that agglomerates of nanotubes present in the structure of the composites after curing cause a significant scatter of the measured strength parameters, both in the static and in the dynamic test modes. However, the effects of carbon nanotube additions in the studied concentration interval on the physical and mechanical characteristics of the parameters were not revealed for both types of loading.     

Serrated creep of zinc single crystals under compression in a magnetic field

The compressive creep rate of zinc single crystals was measured for sample deformation increments of 150 nm, which permits the measurement of deformation jumps larger than 300 nm. A weak magnetic field B = 0.2 T is shown to increase the average creep rate and decrease the height and sharpness of submicron-sized deformation jumps. Preliminary holding of a sample in a magnetic field also influences the creep rate and the characteristics of deformation jumps. The data are explained in terms of a model relating the effect of a magnetic field to the destruction of barriers to dislocation motion.     

Jumplike deformation of polymer materials on the micrometer and submicrometer structural levels

Jumplike creep is considered as a reflection of the structural heterogeneity of amorphous polymers on the mesoscopic and nanoscopic levels. The D-450 epoxy resin, poly(vinyl chloride), poly(vinyl butyral), and a composite consisting of the D-450 epoxy resin and diabase microparticles are studied at a temperature of 290 K. The creep rate of the specimens under compression is measured with a laser interferometer in submicrometer-scale deformation increments. Periodic variations of the creep rate with time or under deformation correspond to a jumplike (stepwise) behavior of the creep. It is shown that diabase particles (5–10 μm in size) are responsible for the appearance of micrometer-scale jumps in the creep of the composite and that the deformation jumps on the nanometer level are comparable to the sizes of the globules. The role of the resolution of the method employed in the evaluation of the scale of deformation jumps and structural units is considered.     

Step deformation of solid amorphous polymers

The variation of step deformation kinetics in solids is studied as a function of morphological factors. Oscillations of creep rate at micrometer increments of the amount of deformation, which reflect the step nature of the process, are investigated from an interferogram. It is shown that the plasticization of polymethyl methacrylate by dibutyl phthalate blurs the steps, while their height varies insignificantly. The results are explained using the concept of the netlike structure of amorphous polymers. The data obtained confirm the universal nature of jumps as a mode of evolution of deformation in various solids. The jumps reflect the cooperative nature of motion of kinetic units, and the regular variation of the characteristics of the jumps lends support to the definition of creep as a process of structural self-organization.     

Thermal conductivity of high-porosity heavily doped biomorphic silicon carbide prepared from sapele wood biocarbon

The electrical resistivity and thermal conductivity of high-porosity (～52 vol %, channel-type pores) bio-SiC samples prepared from sapele wood biocarbon templates have been measured in the temperature range 5–300 K. An analysis has been made of the obtained results in comparison with the data for bio-SiC samples based on beech and eucalyptus, as well as for polycrystalline β-SiC. The conclusion has been drawn that the electrical resistivity and thermal conductivity of bio-SiC samples based on natural wood are typical of heavily doped polycrystalline β-SiC.     

Dislocation motion in icosahedral quasicrystals at elevated temperatures: Numerical simulation

We have simulated shear deformation of an icosahedral model quasicrystal at elevated temperatures with molecular dynamics. The generation of a dislocation loop was studied with a new visualization technique and a critical stress almost as large as the theoretical shear strength was measured. Built-in dislocations started to move at a temperature-dependent critical stress lower by one order of magnitude. While at zero temperature the dislocation propagated intermittently by large jumps, its motion became viscous as temperature increased. The dislocation cores bulged considerably owing to pinning at obstacles inherent in the structure. A calculation of the energy of a Peierls-Nabarro dislocation moving rigidly through the sample allowed us to determine the dominant obstacles. The results are considered in relation to two different models of quasicrystalline plasticity.     

Jumplike deformation of γ-irradiated polymethyl methacrylate

Nonuniformity of the microdeformation rate and the parameters of microdeformation jumps were studied in the creep regime for a polymethyl methacrylate irradiated by various dozes of the Co-60 γ radiation. The creep rate during compression of the polymethyl methacrylate was measured by an interferogram on 300-nm deformation increments. It is shown that the periods L of rate oscillations (jumps of deformation) on three scale levels are dependent on the irradiation doze and are also changed after prolonged exposure of samples in air. In the doze range 0 to 330 kGy, both a decrease and an increase in L are observed, which corresponds to the unstable kinetics of radiation chemical processes. The deformation jumps permit estimates of the radiation effect on various structural levels. It is concluded that the effect of radiation on coarser microstructural formations is the largest.