Photocurable High-Energy Polymer-Based Materials for 3D Printing.

Digital light processing (DLP) or stereolithography is the most promising method of additive manufacturing (3D printing) of products based on high-energy materials due to, first of all, the absence of a high-temperature impact on the material. This paper presents research results of an ultraviolet (UV)-cured urethane methacrylate polymer containing 70 wt.% of high-energy solid powder based on ammonium salts, which is intended for digital light processing. Polymerization of the initial slurry is studied herein. It is shown that the addition of coarse powder transparency for the UV radiation to resin increases its curing depth. The thickness of the layer, which can polymerize, varies from 600 µm to 2 mm when the light power density ranges from 20 to 400 mJ/cm2, respectively. In DLP-based 3D printing, the obtained material density is 92% of the full density, while the compressive strength is 29 ± 3 MPa, and the ultimate tensile strength is 13 ± 1.3 MPa. The thermogravimetric analysis shows the decrease in the thermal decomposition temperature of UV-cured resin with high-energy additives compared to the thermal decomposition temperatures of the initial components separately. Thermal decomposition is accompanied by intensive heat generation. The burning rate of obtained samples grows from 0.74 to 3.68 mm/s, respectively, at the pressure growth from 0.1 to 4 MPa. Based on the results, it can be concluded that DLP-based 3D printing with the proposed UV photocurable resin is rather promising for the fabrication of multicomponent high-energy systems and complex profile parts produced therefrom.

Nanocomposites Based on Polyethylene and Nickel Ferrite: Preparation, Characterization, and Properties.

Composite materials based on NiFe2O4 nanoparticles and polyethylene matrix have been synthesized by thermal decomposition to expand the application area of high-pressure polyethylene by filling it with nanoscale particles. The synthesized compositions were obtained in the form of a dark gray powder and compressed for further study According to TEM, the average particle size in composites was 2, 3, and 4 nm in samples with a filling of 10%, 20% and 30%. The concentration dependences of the specific electrical resistivity ρV, dielectric permittivity ε, saturation magnetization MS and the parameters of reflection and attenuation of microwave power of the obtained composites were investigated. The threshold for percolation in such materials is found to be within a concentration range of 20…30%. The electronic and atomic structure of composites was studied by methods of Mössbauer spectroscopy, X-ray diffraction and X-ray absorption spectroscopy. The closest atomic environment of nickel and iron in nanoparticles is close to that of crystalline NiFe2O4. The dependence of the nanoparticles size as well as the dependence of the number of tetrahedral or octahedral iron positions in nickel ferrite nanoparticles to their content in polyethylene matrix is established. It is shown that composite materials based on NiFe2O4 nanoparticles and polyethylene matrix can be used as components of electromagnetic compatibility systems.

Advanced Fitting Method for the Kinetic Analysis of Thermogravimetric Data.

The article considered the solution of the inverse problem of chemical kinetics of the analysis of experimental data of a thermogravimetric experiment at a constant sample heating rate. The fitting method for identifying the parameters of a kinetic triplet using the integral method for a model of a solid-state reaction based on the modified Arrhenius equation is described. The effectiveness of the proposed approach was confirmed by solving test cases for low, medium, and high rates of material conversion. Unlike other methods, setting the parameters of the reaction mechanism is not required, as they are determined by the solution. Solutions for real data of TGA studies with high and low sample heating rates were compared with the results obtained by other authors and experimental data. A description of the full cycle of calculations used to identify kinetic parameters from thermogravimetric experimental data is given, from the derivation of calculated relationships to the implementation of a short (three to five formulas) program code for MS Excel spreadsheets. The presented code is easy to verify and reproduce and can be modified to solve various problems.

"Dead" Exciton Layer and Exciton Anisotropy of Bulk MoS2 Extracted from Optical Measurements.

Excitons (electron-hole pairs bound by the Coulomb potential) play an important role in optical and electronic properties of layered materials. They can be used to modulate light with high frequencies due to the optical Pauli blocking. The properties of excitons in 2D materials are extremely anisotropic. However, due to nanometre sizes of excitons and their short life times, reliable tools to study this anisotropy are lacking. Here, we show how direct optical reflection measurements can be used to evaluate anisotropy of excitons in transition metal dichalcogenides MoS2. Using focused beam spectroscopic ellipsometry, we have measured the polarized optical reflection of bulk MoS2 for two crystal orientations: c-axis being perpendicular to the surface from which reflection is measured and c-axis being parallel to the surface from which reflection is measured. We found that for the parallel configuration the optical reflection near excitonic transitions is strongly affected by the presence of the exciton "dead" layer such that the excitonic reflection peaks become the excitonic dips due to light interference. At the same time, the optical reflection for the perpendicular orientation is not significantly altered by the exciton "dead" layer due to large anisotropy of exciton properties. Performing simultaneous Fresnel fitting for both geometries, we were able to evaluate exciton anisotropy in layered materials from simple optical measurements.

Structure, Properties and Phase Composition of Composite Materials Based on the System NiTi-TiB2.

This article considers issues pertinent to the research of the phase composition, structure and mechanical properties of materials obtained from powders of composite (Ni-Ti)-TiB2, which have prospective applications in aerospace and automotive industry and engine construction. The starting powder materials (Ni-Ti)-TiB2 were obtained by self-propagating high-temperature synthesis (SHS). Research samples were produced using high-temperature vacuum sintering. It was shown that the use of such materials increases the wettability of the particles and allows the production of composites, the density of which is 95% of the theoretical one. Average particle size was 1.54 µm, average microhardness was 8 GPa, which is an order of magnitude higher than the average microhardness of pure nickel-based and titanium-based alloys, and the ultimate strength values were comparable to those of tungsten-based heavy alloys.

Radio-Absorbing Materials Based on Polymer Composites and Their Application to Solving the Problems of Electromagnetic Compatibility.

Recently, designers of electronic equipment have paid special attention to the issue of electromagnetic compatibility (EMC) of devices with their own components and assemblies. This is due to the high sensitivity of semiconductor microcircuits to electromagnetic interference. This interference can be caused either by natural phenomena, such as lightning strikes, or by technical processes, such as transients in circuits during fast periodic or random switching. Either way, interference implies a sudden change in voltage or current in a circuit, which is undesirable, whether it propagates along a cable or is transmitted as an electromagnetic wave. The purpose of this article is to review the works devoted to the development, creation, and investigation of modern polymeric nanocomposite materials used for shielding electromagnetic radiation and their effective application for solving problems of electromagnetic compatibility. Additionally, the approach to design EMI shielding complex media with predetermined parameters based on investigation of various properties of possible components is shown. In the review, all polymer composites are classified according to the type of filler. The issues of the interaction of a polymer with conductive fillers, the influence of the concentration of fillers and their location inside the matrix, and the structure of the nanocomposite on the mechanisms of electromagnetic interaction are considered. Particular attention is paid to a new generation of nanocomposite materials with widely adjustable electrical and magnetic properties. A wide class of modern filled polymeric materials with dielectric and magneto-dielectric losses is considered. These materials make it possible to create effective absorbers of electromagnetic waves that provide a low level of reflection coefficient in the microwave range. The model mechanisms for shielding electromagnetic radiation are considered in the paper. A detailed review of the electro-physical properties of polymer nanocomposites is provided. Multilayer electrodynamic media containing combinations of layers of filled polymer composite materials with nanoparticles of different compositions and manufactured using a single technology will make it possible to create electrodynamic media and coatings with the required electro-physical characteristics of absorption, transmission, and reflection. Within the framework of the two-layer coating model, the difference in the effects of the interaction of electromagnetic radiation with conductive layers located on a dielectric and metal substrate is demonstrated. It is shown that in order to achieve optimal (maximum) values of reflection and absorption of electromagnetic radiation in the appropriate frequency range, it is necessary to fit the appropriate layer thicknesses, specific conductivity, and permittivity. Such approach allows designers to create new shielding materials that can effectively vary the shielding, absorbing, and matching characteristics of coatings over a wide frequency band. In general, it can be said that the development of innovative polymer composite materials for shielding electronic devices from electromagnetic interference and excessive electromagnetic background is still an important task. Its solution will ensure the safe and uninterrupted operation of modern digital electronics and can be used for other applications.

Mechanisms of Liquid-Phase Exfoliation for the Production of Graphene.

Liquid- phase exfoliation (LPE) is the principal method of producing two-dimensional (2D) materials such as graphene in large quantities with a good balance between quality and cost and is now widely adopted by both the academic and industrial sectors. The fragmentation and exfoliation mechanisms involved have usually been simply attributed to the force induced by ultrasound and the interaction with the solvent molecules. Nonetheless, little is known about how they actually occur, i.e., how thick and large graphite crystals can be exfoliated into thin and small graphene flakes. Here, we demonstrate that during ultrasonic LPE the transition from graphite flakes to graphene takes place in three distinct stages. First, sonication leads to the rupture of large flakes and the formation of kink band striations on the flake surfaces, primarily along zigzag directions. Second, cracks form along these striations, and together with intercalation of solvent, lead to the unzipping and peeling off of thin graphite strips that in the final stage are exfoliated into graphene. The findings will be of great value in the quest to optimize the lateral dimensions, thickness, and yield of graphene and other 2D materials in large-scale LPE for various applications.

Heat Treatment of Corrosion Resistant Steel for Water Propellers Fabricated by Direct Laser Deposition.

The urgency of heat treatment of samples of maraging steel obtained by direct laser deposition from steel powder 06Cr15Ni4CuMo is considered. The structural features and properties of 06Cr15Ni4CuMo steel samples after direct laser deposition and heat treatment are studied. The work is devoted to research into the influence of thermal processing on the formation of structure and the mechanical properties of deposit samples. Features of formation of microstructural components by means of optical microscopy are investigated. Tests for tension and impact toughness are conducted. As a result, it was established that the material obtained by the direct laser deposition method in its initial state significantly exceeds the strength characteristics of heat treatment castings of similar chemical composition, but is inferior to it in terms of impact toughness and relative elongation. The increase in relative elongation and impact toughness up to the level of cast material in the deposit samples is achieved at the subsequent heat treatment, which leads to the formation of the structure of tempered martensite and reduction in its content at two-stage tempering in the structure of the metal. The strength of the material is also reduced to the level of cast metal.

Planetary Milling and Self-Propagating High-Temperature Synthesis of Al-TiB2 Composites.

The paper explores the influence of planetary milling on the temperature and velocity of Al-Ti-B powder mixture combustion and also on the structure and phase composition of the reaction products. It is found that the time increase of planetary milling modifies the structure of the powder particles, improves the density of compacted specimens, and increases the temperature and velocity of their combustion. These time dependences are extreme, with maximum values during 180 s planetary milling. Experiments show that the reaction products consist of an aluminum matrix with uniformly distributed particles of titanium diboride of not over 1 µm in size. The average particle size changes with the increase in the time of the planetary milling of the initial powder mixture.

Design of van der Waals interfaces for broad-spectrum optoelectronics.

Van der Waals (vdW) interfaces based on 2D materials are promising for optoelectronics, as interlayer transitions between different compounds allow tailoring of the spectral response over a broad range. However, issues such as lattice mismatch or a small misalignment of the constituent layers can drastically suppress electron-photon coupling for these interlayer transitions. Here, we engineered type-II interfaces by assembling atomically thin crystals that have the bottom of the conduction band and the top of the valence band at the Γ point, and thus avoid any momentum mismatch. We found that these van der Waals interfaces exhibit radiative optical transitions irrespective of the lattice constant, the rotational and/or translational alignment of the two layers or whether the constituent materials are direct or indirect gap semiconductors. Being robust and of general validity, our results broaden the scope of future optoelectronics device applications based on two-dimensional materials.

Ultra-thin van der Waals crystals as semiconductor quantum wells.

Control over the quantization of electrons in quantum wells is at the heart of the functioning of modern advanced electronics; high electron mobility transistors, semiconductor and Capasso terahertz lasers, and many others. However, this avenue has not been explored in the case of 2D materials. Here we apply this concept to van der Waals heterostructures using the thickness of exfoliated crystals to control the quantum well dimensions in few-layer semiconductor InSe. This approach realizes precise control over the energy of the subbands and their uniformity guarantees extremely high quality electronic transport in these systems. Using tunnelling and light emitting devices, we reveal the full subband structure by studying resonance features in the tunnelling current, photoabsorption and light emission spectra. In the future, these systems could enable development of elementary blocks for atomically thin infrared and THz light sources based on intersubband optical transitions in few-layer van der Waals materials.

Asymptotic dynamics of three-dimensional bipolar ultrashort electromagnetic pulses in an array of semiconductor carbon nanotubes.

We study the propagation of three-dimensional bipolar ultrashort electromagnetic pulses in an array of semiconductor carbon nanotubes at times much longer than the pulse duration, yet still shorter than the relaxation time in the system. The interaction of the electromagnetic field with the electronic subsystem of the medium is described by means of Maxwell's equations, taking into account the field inhomogeneity along the nanotube axis beyond the approximation of slowly varying amplitudes and phases. A model is proposed for the analysis of the dynamics of an electromagnetic pulse in the form of an effective equation for the vector potential of the field. Our numerical analysis demonstrates the possibility of a satisfactory description of the evolution of the pulse field at large times by means of a three-dimensional generalization of the sine-Gordon and double sine-Gordon equations.

Cell sorting actuated by a microfluidic inertial vortex.

The sorting of specific cell populations is an established tool in biological research, with new applications demanding greater cell throughput, sterility and elimination of cross-contamination. Here we report 'vortex-actuated cell sorting' (VACS), a new technique that deflects cells individually, via the generation of a transient microfluidic vortex by a thermal vapour bubble: a novel mechanism, which is able to sort cells based on fluorescently-labelled molecular markers. Using in silico simulation and experiments on beads, an immortal cell line and human peripheral blood mononuclear cells (PBMCs), we demonstrate high-purity and high-recovery sorting with input rates up to 104 cells per s and switching speeds comparable to existing techniques (>40 kHz). A tiny footprint (1 × 0.25 mm) affords miniaturization and the potential to achieve multiplexing: a crucial step in increasing processing rate. Simple construction using biocompatible materials potentially minimizes cost of fabrication and permits single-use sterile cartridges. We believe VACS potentially enables parallel sorting at throughputs relevant to cell therapy, liquid biopsy and phenotypic screening.

Structure and Properties of ZrO2⁻20%Al2O3 Ceramic Composites Obtained Using Additive Technologies.

This investigation focused on obtaining samples from ceramic composite materials, based on the ZrO2⁻20%Al2O3 system, using the additive layer-by-layer fusion technology for thermoplastic systems. The structure and phase composition of the initial powders were studied, experimental samples were produced, and the structure and properties of the experimental samples that were obtained using additive technologies were analysed. The measured static bending strength of the samples was 450 ± 70 MPa, microhardness was 14 GPa, and the elasticity modulus was 280 ± 25 GPa. The strength of these samples are slightly inferior to that of similar materials, obtained using Ceramic Injection Molding technology because our samples were characterised by the residual porosity of about 15%.

Combustion Synthesis of Composition Ferroalloys.

The main objective of this paper is to present results of the research in the development of a specialized self-propagating high-temperature synthesis (SHS) technology for ferroalloy composites, as applied to steelmaking. The problem of creating such a production cycle has been solved by developing a new approach to the practical implementation of self-propagating high-temperature synthesis, as applied to metallurgy. The metallurgical variation of SHS is based on the use of different metallurgic alloys (including waste in the form of dust from ferroalloy production) as basic raw materials in the new process. Here, the process of synthesis by combustion is realized through exothermic exchange reactions. The process produces a composite, based on inorganic compositions with a bond of iron and/or alloy based on iron. It has been shown that in terms of the aggregate state of initial reagents, metallurgical SHS processes are either gasless or gas-absorbing. Combustion regimes significantly differ when realized in practice. To organize the metallurgical SHS process in weakly exothermic systems, different variations of the thermal trimming principle are used. In the present study, self-propagating high-temperature synthesis of ferrovanadium nitride, which is widely used in steel alloying, was investigated. It has been shown that the phase composition of the initial alloy has a profound impact on the regular patterns in ferrovanadium combustion in nitrogen and on the mechanism itself. During the nitriding of σ-(Fe-V), process activation is taking place. The activation is due to the transformation of the intermetallide into an α-solid solution, when the temperature of phase transition is reached (~1200 °C). The composite structure of the products of ferrovanadium is nitriding by the fusion of particles-droplets composed of molten Fe and solid VN.

First Six Dimensional Phase Space Measurement of an Accelerator Beam.

This Letter presents the first complete six dimensional phase space measurement of a beam in an accelerator. The measurement was made on the Spallation Neutron Source Beam Test Facility. The data reveal previously unknown correlations in the six dimensional phase space distribution that are not visible in lower dimensionality measurements. The correlations are shown to be intensity dependent.

Plasmon-induced nanoscale quantised conductance filaments.

Plasmon-induced phenomena have recently attracted considerable attention. At the same time, relatively little research has been conducted on electrochemistry mediated by plasmon excitations. Here we report plasmon-induced formation of nanoscale quantized conductance filaments within metal-insulator-metal heterostructures. Plasmon-enhanced electromagnetic fields in an array of gold nanodots provide a straightforward means of forming conductive CrOx bridges across a thin native chromium oxide barrier between the nanodots and an underlying metallic Cr layer. The existence of these nanoscale conducting filaments is verified by transmission electron microscopy and contact resistance measurements. Their conductance was interrogated optically, revealing quantised relative transmission of light through the heterostructures across a wavelength range of 1-12 µm. Such plasmon-induced electrochemical processes open up new possibilities for the development of scalable devices governed by light.

Low Stress Ion Conductance Microscopy of Sub-Cellular Stiffness.

Directly examining subcellular mechanics whilst avoiding excessive strain of a live cell requires the precise control of light stress on very small areas, which is fundamentally difficult. Here we use a glass nanopipet out of contact with the plasma membrane to both exert the stress on the cell and also accurately monitor cellular compression. This allows the mapping of cell stiffness at a lateral resolution finer than 100 nm. We calculate the stress a nanopipet exerts on a cell as the sum of the intrinsic pressure between the tip face and the plasma membrane plus its direct pressure on any glycocalyx, both evaluated from the gap size in terms of the ion current decrease. A survey of cell types confirms that an intracellular pressure of approximately 120 Pa begins to detach the plasma membrane from the cytoskeleton and reveals that the first 0.66 ± 0.09 µm of compression of a neuron cell body is much softer than previous methods have been able to detect.

Graphitic platform for self-catalysed InAs nanowires growth by molecular beam epitaxy.

We report the self-catalysed growth of InAs nanowires (NWs) on graphite thin films using molecular beam epitaxy via a droplet-assisted technique. Through optimising metal droplets, we obtained vertically aligned InAs NWs with highly uniform diameter along their entire length. In comparison with conventional InAs NWs grown on Si (111), the graphite surface led to significant effects on the NWs geometry grown on it, i.e. larger diameter, shorter length with lower number density, which were ascribed to the absence of dangling bonds on the graphite surface. The axial growth rate of the NWs has a strong dependence on growth time, which increases quickly in the beginning then slows down after the NWs reach a length of approximately 0.8 µm. This is attributed to the combined axial growth contributions from the surface impingement and sidewall impingement together with the desorption of adatoms during the diffusion. The growth of InAs NWs on graphite was proposed following a vapour-solid mechanism. High-resolution transmission electron microscopy reveals that the NW has a mixture of pure zinc-blende and wurtzite insertions.

Pipette-surface interaction: current enhancement and intrinsic force.

There is an intrinsic repulsion between glass and cell surfaces that allows noninvasive scanning ion conductance microscopy (SICM) of cells and which must be overcome in order to form the gigaseals used for patch clamping investigations of ion channels. However, the interactions of surfaces in physiological solutions of electrolytes, including the presence of this repulsion, for example, do not obviously agree with the standard Derjaguin-Landau-Verwey-Overbeek (DLVO) colloid theory accurate at much lower salt concentrations. In this paper we investigate the interactions of glass nanopipettes in this high-salt regime with a variety of surfaces and propose a way to resolve DLVO theory with the results. We demonstrate the utility of this understanding to SICM by topographically mapping a live cell's cytoskeleton. We also report an interesting effect whereby the ion current though a nanopipette can increase under certain conditions upon approaching an insulating surface, rather than decreasing as would be expected. We propose that this is due to electroosmotic flow separation, a high-salt electrokinetic effect. Overall these experiments yield key insights into the fundamental interactions that take place between surfaces in strong solutions of electrolytes.