A. A. Golyshev

Polymorphic transformations in nanostructured anatase (TiO2) under high-pressure shock compression

The action of dynamic pressure and temperature on polymorphic transformations in nanostructured (grain size of 8–20 nm) anatase (TiO2) is studied. The dynamic pressure of a loading pulse (10–45 GPa) is measured with a manganin gauge. The temperature of shock-compressed specimens, which is varied by varying the initial temperature and initial porosity, is found to fall into the range 500–2500 K. It is shown that as the temperature and shock compression pressure rise, the nanostructured anatase turns into a nanoanatase-nanocolumbite or columbite-rutile mixture or into almost impurity-free (pure) nanocolumbite or impurity-free microcrystalline rutile.

Phase transformations in nanostructural anatase TiO2 under shock compression conditions studied by Raman spectroscopy

Shock-wave-induced phase transformations in nanostructural titanium dioxide TiO2 (anatase) powders of two types have been studied by Raman spectroscopy. It is established that, at a shock-wave compression to 42 GPa, anatase particles can either transform into a columbite phase or exhibit amorphization.

Energy balance in high-quality cutting of steel by fiber and CO2 lasers

The energy balance of laser cutting of low-carbon and stainless steel sheets with the minimum roughness of the cut surface is experimentally studied. Experimental data obtained in wide ranges of cutting parameters are generalized with the use of dimensionless parameters (Peclet number and absorbed laser energy). It is discovered for the first time that the minimum roughness is ensured at a certain value of energy per unit volume of the melt (approximately 26 J/mm3), regardless of the gas type (oxygen or nitrogen) and laser type (fiber laser with a wavelength of 1.07 μm or CO2 laser with a wavelength of 10.6 μm).

Electrical resistivity of plastic insulation at megabar shock pressures

This paper presents the results of experimental determination of the electrical resistivity of an insulating polymer composition (Teflon film and high-vacuum leak sealant) under stepwise shock compression at pressures up to 150 GPa. The data obtained can be used in experiments to measure the electrical conductivity of materials in this range of shock pressures.

Structural and Morphological Changes Induced by Intense Shock Waves in Carbon Nanotubes

The morphological and structural characteristics of multiwalled carbon nanotubes subjected to multi shock-wave compression to pressures of about 120 GPa and temperatures of 2000 K are comprehensively analyzed experimentally and theoretically. The calculations relevant to the thermal history of shock compression followed by unloading of the multiwalled carbon nanotubes are performed. Diffractometer structural studies of the samples before and after shock loading are carried out. The morphological properties of the multiwalled carbon nanotubes are studied by scanning electron microscopy and transmission electron microscopy of the pristine and explosion-treated samples. It is shown that multiwalled carbon nanotubes subjected to a strong shock compression undergo the following changes: (a) most nanotubes transform into graphite structures with an increased spacing between graphene planes and (b) some nonvolatile nanotubes exhibit irreversible distortions in shape.

Shock-wave studies of anomalous compressibility of glassy carbon

The physico-mechanical properties of amorphous glassy carbon are investigated under shock compression up to 10 GPa. Experiments are carried out on the continuous recording of the mass velocity of compression pulses propagating in glassy carbon samples with initial densities of 1.502(5) g/cm3 and 1.55(2) g/cm3. It is shown that, in both cases, a compression wave in glassy carbon contains a leading precursor with amplitude of 0.135(5) GPa. It is established that, in the range of pressures up to 2 GPa, a shock discontinuity in glassy carbon is transformed into a broadened compression wave, and shock waves are formed in the release wave, which generally means the anomalous compressibility of the material in both the compression and release waves. It is shown that, at pressure higher than 3 GPa, anomalous behavior turns into normal behavior, accompanied by the formation of a shock compression wave. In the investigated area of pressure, possible structural changes in glassy carbon under shock compression have a reversible character. A physico-mechanical model of glassy carbon is proposed that involves the equation of state and a constitutive relation for Poisson’s ratio and allows the numerical simulation of physico-mechanical and thermophysical properties of glassy carbon of different densities in the region of its anomalous compressibility.

Jump in the electrical conductivity of shock-compressed glassy carbon

The effect of high dynamic pressures on the electrical conductivity of the amorphous conducting carbon phase (glassy carbon) has been studied. The electrical conductivity of glassy carbon samples has been measured under the condition of shock compression and subsequent release wave. The history of the shock loading of glassy carbon has been calculated with the developed semiempirical equations of state. It has been shown the electrical conductivity of glassy carbon samples in the compression phase at a pressure of 45(5) GPa decreases abruptly by two orders of magnitude. In the relief phase, partially reversible change in the electrical conductivity of an amorphous carbon sample occurs. The recorded effect has been treated as a result of a partially reversible physicochemical transformation of shock-compressed amorphous carbon.

Shock-wave-induced grain refinement and phase state modification in coarse-grained and nanocrystalline titanium

We have experimentally studied the structure and phase variations in polycrystalline titanium stimulated by stepwise shock compression at pressures up to 40 GPa with subsequent unloading. The experiments were performed on samples of commercial titanium (VT1-0 grade) in a broad range of initial grain sizes (0.2–50 μm). The phenomenon of grain refinement and phase composition variation in titanium under stepwise shock-wave action has been studied by transmission and scanning electron microscopy techniques. Specific features of these changes are discussed.

Equation of state of polytetrafluoroethylene for calculating shock compression parameters at megabar pressures

Semi-empirical equations of state (thermal and caloric) are obtained to calculate not only the kinematic parameters (shock wave velocity, particle velocity, and reverberation of waves) but also the thermodynamic parameters (temperature, pressure, and compression) of monolithic and porous polytetrafluoroethylene at high shock pressures. The equations of state are used to model wave interaction in shock-wave experiments using the developed hydrocode. The equations are verified by comparison simulation results with published results of experiments and the data of our shock compression tests of solid and porous samples of PTFE in the range of 10–170 GPa.

Structural Transformations of Amorphous Carbon (Glassy Carbon) at High Shock Pressures

Amorphous carbon (glassy carbon) samples were shock compressed up to 80 GPa and temperatures up to 1700 K for several microseconds. Glassy carbon samples before and after an explosive action are analyzed by X-ray diffraction, electron microscopy, and electron-probe microanalysis. It is shown that as a result of microsecond shock pressure exposure, glassy carbon is compacted to ρCG ≈ 2.3(5) g/cm3 and is partly transformed into a graphite-like nanomaterial with a cellular structure. At the level of crystallites, the density of glassy carbon increases via a decrease in the interplanar spacings and an increase in the crystallite thickness and width. Spheres from 20 nm to 80 μm in diameter are found to be formed during shock-wave compression of glassy carbon in a copper container and high-temperature shock heating posteffects. Spheres 20 μm in diameter consist of a copper-rich core and a carbon shell.