Melting of Shocked Boron Carbide

JETP Letters - The aim of this work is to determine the melting temperature of boron carbide at high shock pressures. To this end, powder boron carbide samples have been compressed by shock waves...     

Magnetic Transformations and Polymorphic Transition of Ferromagnetic Steels under Shock-Wave Loading

Technical Physics - A method for recording magnetic transformation is presented for ferromagnetic steels under shock-wave loading. The operation of the magnetic transformation gauge is considered...     

Numerical Study of Shock-Wave Loading of the W- and WC-Based Metal Composites

Technical Physics - Shock-wave loading of metal composites (Elkonites) that represent sintered refractory materials (tungsten, tungsten carbide, or molybdenum) in combinations with low-melting...     

A Free Energy Model of Boron Carbide

The assessed phase diagram of the boron-carbon system contains a single nonstoichiometric boron-carbide phase of rhombohedral symmetry with a broad, thermodynamically improbable, low temperature composition range. We combine first principles total energy calculations with phenomenological thermodynamic modeling to propose a revised low temperature phase diagram that contains two boron-carbide phases of differing symmetries and compositions. One structure has composition B₄C and consists of B₁₁C icosahedra and C-B-C chains, with the placement of carbon on the icosahedron breaking rhombohedral symmetry. This phase is destabilized above 600 K by the configurational entropy of alternate carbon substitutions. The other structure, of ideal composition B₁₃C₂, has a broad composition range at high temperature, with rhombohedral symmetry throughout, as observed experimentally.     

Experimental and Computational Study of the Shock-Wave Loading of Optically Transparent Objects

Technical Physics - Shock-wave loading of transparent objects is studied with the aid of a theoretical method based on numerical simulation on a 3D regular mesh using an explicit solver in the...     

Probing Field Emission from Boron Carbide Nanowires

High density boron carbide nanowires are grown by an improved carbon thermal reduction technique. Transmission electron microscopy and electron energy lose spectroscopy of the sample show that the synthesized nanowires are B4 C with good crystallization. The field emission measurement for an individual boron nanowire is performed by using a Pt tip installed in the focused ion beam system. A field emission current with enhancement factor of 10^6 is observed and the evolution process during emission is also carefully studied. Furthermore, a two-step field emission with stable emission current density is found from the high-density nanowire film. Our results together suggest that boron carbide nanowires are promising candidates for electron emission nanodevices.     

Evolution of Shock Waves in Hot-Pressed Ceramics of Boron Carbide and Silicon Carbide

Technical Physics - In this paper we studied the evolution of shock compression waves in hot-pressed ceramics based on boron carbide and silicon carbide at a maximum compressive stress of 32 and 34...     

Numerical Simulation of Wave Propagation and Phase Transition of Tin under Shock-Wave Loading

We undertake a nmnerical simulation of shock experiments on tin reported in the literature, by using a multiphase equation of state （MEOS） and a multiphase Steinberg Guinan （MSG） constitutive model for tin in theβ, γ and liquid phases. In the MSG model, the Bauschinger effect is considered to better describe the unloading behavior. The phase diagram and Hugoniot of tin are calculated by MEOS, and they agree well with the experimental data. Combined with the M130S and MSG models, hydrodynamic computer simulations are successful in reproducing the measured velocity profile of the shock wave experiment. Moreover, by analyzing the mass fraction contour as well as stress and temperature profiles of each phase for tin, we further discuss the complex behavior of tin under shock-wave loading.     

Re-entrant-Groove-Assisted VLS Growth of Boron Carbide Five-Fold Twinned Nanowires

We report a preferential growth of boron carbide nanowires with a five-fold twinned internal structure. The nanowires are found to grow catalytically via iron boron nanoparticles, but unusually the catalytic particle is in contact with the low-energy surfaces of boron carbide with V-shaped contact lines. We propose that this catalytical growth may be caused by preferential nucleation at the re-entrant grooves due to the twinning planes, followed by rapid spreading of atomic steps. This is consistent with the observed temperature dependence of the five-fold twinned nanowire growth.     

Optical Emission Diagnosis of Boron Carbide Synthesized Using Natural Carbon Precursors

Optics and Spectroscopy - Boron carbide (B4C) is a semiconducting material that finds a wide range of industrial and optoelectronic applications because of its unique structural and physical...     

Densification mechanism of polyacrylonitrile-based carbon fiber during heat treatment

Polyacrylonitrile (PAN)-based carbon fibers were heat treated at various temperatures for varying durations to simulate the graphitization process in the manufacture of C/C composites. Densification of the resulting fibers was confirmed by density measurement. The composition and structure of the fibers were investigated by means of elemental analysis, X-ray diffraction and Raman spectroscopy. For specified isothermal heat treatment time, the structural parameters depended strongly on heat treatment temperature. The nitrogen content decreased with increased heat treatment temperature and extended time at constant temperature. Nitrogen loss was complete at temperatures above 1900 °C. The graphite crystallite size increased rapidly with increasing heat treatment temperature, and slowly with extended isothermal heat treatment time. At 2100 °C a more ordered graphitic structure appeared. Denitrogenation induced “puffing”, which made the fibers expand. Decrease in density in the heat treatment temperature range 1500-1900 °C originated from the abrupt evolution of nitrogen, and above 1900 °C the graphitization transition induced steadily increasing density. Densification of the carbon fibers was determined both by the rate of denitrogenation and the rearrangement of carbon atoms.     

六方氮化硼薄膜对石墨烯-碳化硅结构近场热辐射的增强

六方氮化硼(hBN)具有跟石墨烯类似的层状结构和晶格参数,研究发现hBN薄膜具有良好的热传导、电绝缘、光学和力学等性能.本文从理论上研究了 hBN薄膜对石墨烯-碳化硅(G/S)结构的近场热辐射的影响.研究发现在红外频段,hBN薄膜在低频率区和高频率区会增强G/S结构的近场热辐射,经计算在G/S结构中加入厚度为10 nm...     

Crystalline damage growth during martensitic phase transformations

A thermomechanical model is developed within a large deformation setting in order to simulate the interactions between martensitic phase transformations and crystalline damage growth at the austenitic grain level. Subgrain information is included in the model via the crystallographic theory of martensitic transformations. The damage and transformation characteristics are dependent of the specific martensitic transformation systems activated during a loading process, which makes the model strongly anisotropic. The state of transformation for the individual transformation systems is represented by the corresponding volume fractions. The state of damage in the austenite and in the martensitic transformation systems is reflected by the corresponding damaged volume fractions. The thermodynamical forces energetically conjugated to the rate of volume fraction and the rate of damaged volume fraction are the driving forces for transformation and crystalline damage, respectively. The expressions for these driving forces follow after constructing the specific form of the Helmholtz energy for a phase-changing, damaging material. The model is used to analyze several three-dimensional boundary value problems that are representative of microstructures appearing in multiphase carbon steels containing retained austenite. The analyses show that the incorporation of damage in the model effectively limits the elastic stresses developing in the martensitic product phase, where the maximum value of the stress strongly depends on the toughness of the martensite. Furthermore, in an aggregate of randomly oriented grains of retained austenite embedded in a ferritic matrix the generation of crystalline damage delays the phase transformation process, and may arrest it if the martensitic product phase is sufficiently brittle. The response characteristics computed with the phase-changing damage model are confirmed by experimental results.     

Nanotube growth during annealing of mechanically milled Boron

Boron powder, finely ground in a tungsten carbide ball mill in an ammonia atmosphere, has been annealed at 1200 °C in flowing nitrogen to produce small quantities of cylindrical BN nanotubes, both as isolated individuals and grouped into ropes. Thick-walled conical BN tubes are abundant in specimens annealed for longer times, and their growth was catalysed once WC debris was converted into W metal particles. Some catalytic effect of small W nanoparticles could be necessary for nanotube formation, though no tip particles have been imaged here. Given the low temperature of mechanical milling and annealing, BN growth must involve surface diffusion and solid-state reconfiguration. It could be possible to engineer desirable physical and chemical properties by exploiting the variation in cylindrical versus conical BN structures as a function of annealing time. Received: 19 December 2001 / Accepted: 3 April 2002 / Published online: 19 July 2002 RID="*" ID="*"Corresponding author. Fax: +61-2/6125-8253, E-mail: john.fitzgerald@anu.edu.au     

Densification study of ITO films during high temperature annealing by GISAXS

Three thermal routes were treated on the sol-gel ITO films, i.e. conventional thermal annealing (CTA), rapid thermal annealing (RTA) and thermal cycle annealing (TCA). The near surface and internal structures of films were characterized by grazing incidence small angle X-ray scattering (GISAXS) technique. It is found that slit-like pores show fractal structures laterally and the near surface is sparser with bigger pores. Ordered pore structure normal to the film appears when films are annealed at high heating rate. The shrinkage of pores is mainly owing to structural relaxation and diffusion during the superheating process. However, the supercooling process has no significant effect on the structures. Furthermore, CTA samples have the greatest porosity and surface roughness due to the prevailing crystallization as well as the coarsening procedure. However small pores inside the films are eliminated at low temperature.     

Effect of Shock-Wave Dusting and Ways to Suppress It

Journal of Experimental and Theoretical Physics - The ejection of particles from the free surface of metal liners accelerated to velocities of 4–5 km/s using explosive energy has been...     

Shock-Wave Pulse Compression and Stretching of Dodecane and Mineral Oils

The behavior of dodecane, vacuum, and transformer oils under shock-wave pulse compression and stretching are studied experimentally. The wave profiles are registered using a VISAR laser interferometer. The shock adiabats, the dependence of the sound velocity on the pressure, and the maximum negative pressures developed in the studied liquids are determined. It is shown that the negative pressure value does not depend on the deformation rate in the case of oils and is a strong function of the compression pulse amplitude in the case of dodecane.     

Densification mechanisms during reactive spark plasma sintering of Titanium diboride and Zirconium diboride

Abstract

In this study, dense fine-grained ZrB₂ and TiB₂ were fabricated using reactive spark plasma sintering (RSPS) of ball-milled Zr/B and Ti/B mixtures. Systematic investigations were carried out to understand the mechanisms of reactive sintering. Two densification mechanisms were found to be operating during RSPS. The first stage of densification was due to self-propagating high temperature synthesis reaction leading to formation of ZrB₂ and TiB₂ compacts having relative density of ~48 and ~65%, respectively. The second stage of densification occurred at temperatures more than 1100 °C and resulted in final relative density of more than 98%. Electron backscatter diffraction and electron microscopy studies on interrupted RSPS samples as well as dense samples showed deformed grains and presence of slip steps while grain orientation spread map and pole figure analysis confirmed plastic flow. Plastic flow-aided pore closure is shown as major mechanism during reactive sintering.