Alexander G. Anisimov

Ti3SiC2-Cu composites by mechanical milling and spark plasma sintering: Possible microstructure formation scenarios

We present several possible microstructure development scenarios in Ti3SiC2-Cu composites during mechanical milling and Spark Plasma Sintering (SPS). We have studied the effect of in situ consolidation during milling of Ti3SiC2 and Cu powders and melting of the Cu matrix during the SPS on the hardness and electrical conductivity of the sintered materials. Under low-energy milling, (3–5) vol.%Ti3SiC2-Cu composite particles of platelet morphology formed, which could be easily SPS-ed to 92–95% relative density. Under high-energy milling, millimeter-scale (3–5) vol.%Ti3SiC2-Cu granules formed as a result of in situ consolidation and presented a challenge to be sintered into a bulk fully dense sample; the corresponding SPS-ed compacts demonstrated a finer-grained Cu matrix and more significant levels of hardening compared to composites of the same composition processed by low-energy milling. The 3 vol.% Ti3SiC2-Cu in situ consolidated and Spark Plasma Sintered granules showed an extremely high hardness of 227 HV. High electrical conductivity of the Ti3SiC2-Cu composites sintered from the granules was an indication of efficient sintering of the granules to each other. Partial melting of the Cu matrix, if induced during the SPS, compromised the phase stability and uniformity of the microstructure of the Ti3SiC2-Cu composites and thus it is not to be suggested as a pathway to enhanced densification in this system.

Microhardness of ceramics produced from different alumina nanopowders by different techniques

Fine-grained ceramics (with a grain size on the order of a micron) have been produced by the spark plasma sintering (SPS) of various alumina nanopowders. We have compared the microhardness of ceramic samples prepared from 11 alumina nanopowders and that of composites based on such powders. The ceramics have been prepared by both SPS and a conventional technique (sequential pressing and sintering). We examine the effect of the phase composition and average particle size of the starting nanopowder on the microhardness of the ceramics.

Crystallization of Ti33Cu67 metallic glass under high-current density electrical pulses

AbstractWe have studied the phase and structure evolution of the Ti33Cu67 amorphous alloy subjected to electrical pulses of high current density. By varying the pulse parameters, different stages of crystallization could be observed in the samples. Partial polymorphic nanocrystallization resulting in the formation of 5- to 8-nm crystallites of the TiCu2 intermetallic in the residual amorphous matrix occurred when the maximum current density reached 9.7·108 A m-2 and the pulse duration was 140 μs, though the calculated temperature increase due to Joule heating was not enough to reach the crystallization temperature of the alloy. Samples subjected to higher current densities and higher values of the evolved Joule heat per unit mass fully crystallized and contained the Ti2Cu3 and TiCu3 phases. A common feature of the crystallized ribbons was their non-uniform microstructure with regions that experienced local melting and rapid solidification. PACS: 81; 81.05.Bx; 81.05.Kf.

New materials and technologies for pulsed power research and applications

Composites produced by explosive welding (bimetallic and multilayer materials) and composites with disordered structures produced by explosive compaction of powders were studied from the viewpoint of using them in railgun electrodes. Data on ablation are presented as a function of the electric current parameters. Results of metallographic and X-ray analysis of the post-shot electrode surfaces are reported. It is shown that using composite materials, it is possible to decrease the erosion of electrodes and increase the critical current density above which the erosion processes tend to increase considerably. The use of the method of explosive compaction of powders to produce high-temperature insulators and the barrels of railgun accelerators of projectiles is discussed.

Inter-particle interactions in partially densified compacts of electrically conductive materials during spark plasma sintering

Spark Plasma Sintering (SPS) is normally carried out to obtain consolidated materials of low residual porosity. SPS can also be used for the preparation of partially densified (porous) materials. Partial densification is achieved during SPS by using relatively low sintering temperatures or pressureless conditions. For a pressureless assembly, short punches are used; in addition, sintering without the upper punch is possible. It was shown that the number of contacts between the nickel and synthetic diamond particles was higher in the compacts partially sintered by SPS than in those cold-pressed and vacuum-annealed at the same temperature. During pressureless SPS, conditions for non-uniform current distribution can be realized causing non-uniformities in the microstructure of the compacts. It was shown that SPS of metals without the upper punch produces porous gradient structures. Electric current passing through the porous compacts and interfaces between the compacts and the punches/foils under pressureless conditions induces specific local effects evidenced by the morphology evolution of the particles and microstructure of the sintered material in those areas of the compacts.

Preparation and electrical erosion resistance of TiB2/Cu nanocomposites

A procedure is described for producing nanocomposite TiB2/Cu powders containing up to 57 vol % TiB2. Using shock compression of composite powders, we have prepared electrode materials offering enhanced electrical erosion resistance at high arc discharge currents. The effect of titanium diboride nanoparticles embedded in the copper matrix on the erosion behavior of the nanocomposites is examined. The nanoparticles are shown to suppress the copper droplet entrainment during the service of the electrode. TiB2/Cu nanocomposite electrodes containing more than 10 vol % TiB2 retain their shape and dimensions in the course of electrical erosion tests and offer enhanced service life.

Analysis of physical effects on the electrode surfaces in rail launchers

The paper analyzes the possibility of some physical effects occurring on the electrode surface in plasma-armature rail launchers when the linear current density is higher than the critical values. It is shown the melt on the electrode surface is unstable. An analysis of the possible development of a number of instabilities showed that under experimental conditions, Rayleigh-Taylor and Kelvin-Helmholtz instabilities and magnetohydrodynamic (MHD) instabilities due to the interaction of the flowing current with the self-magnetic field can develop over times much smaller than the discharge time. These instabilities can lead to the entry of the electrode material into the plasma armature and limit acceleration velocities of solids. X-ray recording of the electrode surface confirmed the presence of small-scale inhomogeneity and ejection of the material from the surface. Under certain conditions, the emergence of metal conducting jets from the electrode surface was detected.

On analysis of physical processes on the electrode surface in a rail launcher

This paper analyzes some physical effects that occur on the electrode surface in plasma‐armature rail launchers when the linear current density is higher than critical value. It is shown that under typical experimental conditions, Rayleigh–Taylor and Kelvin–Helmholtz instabilities and magnetohydrodynamic instabilities, which arise from the interaction of the current with the self‐magnetic field, can develop over times much smaller than the launcher operation time and can be responsible for the entry of the electrode material into the discharge. Flash radiography of the electrode surface confirmed the presence of inhomogeneities and ejection of the material from the surface. Under certain conditions, the emergence of conducting metal jets from the electrode surface was detected.

On analysis of physical effects on the electrode surface in rail launchers

The paper analyzes the possibility of some physical effects occurring on the electrode surface in plasma-armature rail launchers when the linear current density is higher than the critical values. It is shown the melt on the electrode surface is unstable. An analysis of the possible development of a number of instabilities showed that under experimental conditions, Rayleigh-Taylor and Kelvin-Helmholtz instabilities and magnetohydrodynamic (MHD) instabilities due to the interaction of the flowing current with the self-magnetic field can develop over times much smaller than the discharge time. These instabilities can lead to the entry of the electrode material into the plasma armature and limit acceleration velocities of solids. X-ray recording of the electrode surface confirmed the presence of small-scale inhomogeneity and ejection of the material from the surface. Under certain conditions, the emergence of metal conducting jets from the electrode surface was detected.

Pressure chamber with a viewport and magnet manipulators to study biological samples

ABSTRACT A simple easy way for production in laboratory chamber is described to study biological samples subjected to gas pressure up to 5 MPa with a transparent viewport and light-emitting diode light. The main application of the chamber is to study the dynamics of plant growth under changes in external pressure. However, equipping the chamber with magnetically controlled manipulators allows the simultaneous treatment of samples with chemical agents and chemical fixation of samples for electron microscopy directly under pressure which allows one to extend the range of tasks. As an example of practical application of the chamber, the dynamics of growth of maize seedling roots under the air pressure of 2 MPa, and electron microscope photos of the cross section of maize root segments fixed directly at 2 and 4 MPa and after pressure dampening from 4 MPa to the atmosphere level are shown.