Boron Carbide Grains Research Articles

Combustion synthesized boron carbide powders by different organic carbon sources and microwave absorbing properties

AbstractBoron carbide powders were synthesized by combustion synthesis using organics chitosan, dextrin, cellulose, starch, and polyvinyl chloride (PVC) as carbon sources. The microwave absorbing properties of boron carbide powders synthesized by different carbon sources were investigated. The results showed that the powders synthesized using organic compounds containing Cl and N atoms as carbon sources have better absorbing properties. The peak value of reflection loss of boron carbide powder prepared with chitosan as carbon source is −46.41 dB at 2.1 mm thickness, with an effective absorption range of 9.21–12.30 GHz and an effective absorption bandwidth of 3.09 GHz. The small and uniform boron carbide grains and residual carbon with multiple morphologies enhance the interface polarization and electronic polarization in the material, which leads to multiple reflections and losses of the microwave. Additionally, the lattice defects and surface holes caused by Cl, N, and other atoms in chitosan and PVC increase the energy storage sites of the material, which makes the materials obtain excellent microwave absorbing properties. Boron carbides fabricated from an organic carbon source provide a potential for the preparation of high‐temperature resistant absorber.

Gamma-ray, Neutron and Beta Radiation Shielding Properties of Bi2O3-Li2O-MnO2-B2O3 Glasses

Proper shielding from harmful ionizing radiations is a mandatory requirement in nuclear facilities owing to the harmful effects on radiation workers and general public in case of radiation exposure. The present work is aimed at the investigation of the shielding properties of the prepared glass system of chemical composition (83.5+x)Bi2O3-(0.6+x)Li2O-0.1MnO2-(15.8-2x)B2O3 (with x = 0, 0.7 and 1.4 wt.% coded as L5, L10 and L15 respectively), with respect to the Gamma-ray, Beta and fast neutron radiation. The Gamma-ray shielding property of the glass system was also compared with the most commonly used Barite concrete and commercially available shielding glass ‘RS 253 G18’. Also, the neutron shielding properties was compared with common neutron shielding materials including Graphite, Boron Carbide and Polyethylene grains. An attempt was made in this study to select a better shielding material than the compared standard shielding materials. It is inferred that L15 glass sample is the best shield material for Gamma-rays, fast neutrons and also, for Beta radiation in the high energy region due to Bremsstrahlung, while in low energy region glass, L5 is better for Beta shielding.

High-temperature strength of boron carbide with Pt grain-boundary framework in situ synthesized during spark plasma sintering

Grain boundaries, twins, and defects are considered to influence the thermomechanical behavior of any covalent ceramic, as a result, monolithic B4C samples show different curve shapes of bending strength vs temperature and the present theoretical models fail to fit them over the entire temperature range. To overcome these issues, we fabricated a novel high-density boron carbide and evaluated its high-temperature bending strength. The as-obtained ceramic is composed of boron carbide grains and a fine grain-boundary metal Pt framework. The material shows a decreased strength, which is due to a non-linear increase in the volume expansion coefficient of the B4C. Recovery in strength above 1000 °C is due to the presence of twins, their growth and rearrangements. We consider twins rearrangements are the pieces of evidence for a novel ‘micro’ mechanism of high-temperature stress accommodation for the boron carbide bulks.

Effects of boron content on the microstructures and mechanical properties of reactive hot-pressed BxC-TiB2-SiC composites

Abstract BxC-TiB2-SiC ceramic composites were fabricated via reactive hot pressing using TiC, B, and Si as the raw materials. The phase transition process was studied by heating powder mixtures to different temperatures in combination with X-ray diffraction analysis. The stoichiometric ratio between B and C in boron carbide is variable. A series of powder mixtures containing excess boron (0 wt%, 10 wt%, 20 wt%, or 30 wt% B) were sintered, and the microstructures and mechanical properties of the composites were investigated. The results showed that the B6.1C-TiB2-SiC composite prepared from the starting powders with 30 wt% excess boron had the best comprehensive mechanical properties, with a relative density, hardness, bending strength, and fracture toughness of 98.32%, 33.2 GPa, 840 MPa, and 5.22 MPa m1/2, respectively. Excessive boron substitution may cause lattice distortion in boron carbide, and the boron carbide grains in this state may form a large number of twins under the compressive stress generated by the TiB2 grains, which will affect the properties of the composites.

Effect of impurities introduced by ball milling on hot pressed boron carbide

Ball milling is commonly used refining process to improve sintering properties of boron carbide. However, this process often leads to the introduction of trace metal impurities into raw materials. It was found in this study that, with prolonged holding time and increased sintering temperature at 2000 °C, these metal impurities led to the recrystallization of boron carbide grains, which significantly affected mechanical properties of sintered boron carbide. Results showed that relative density of sample increased from 98.4%–98.9%, when sintering temperature was raised from 2000 °C to 2050 °C and holding time was extended from 30 min to 60 min. However, under these conditions, flexural strength of boron carbide decreased sharply from 403 MPa to 98 MPa and fracture toughness decreased from 4.4 MPa m1/2 to 2.3 MPa m1/2.

High Density Boron Carbide Ceramics

Porous preforms of B4C + C and B4C + C + Si materials were impregnated by liquid (molten) silicon to obtain high-density product with a relative density of 99.0 % and porosity of 0.9%. Silicon impregnation was carried out by saturating the samples through the pores of the SiC + C sacrificial preforms. This method allows to reduce both the dissolution of boron carbide grains in the silicon solution and the formation of the B12(C, Si, B)3 phase. This reduces the fragility of the product and thereby improves the mechanical characteristics of the B4C ceramics.

The effect of Si on the microstructure and mechanical properties of spark plasma sintered boron carbide

Fully dense boron carbide discs were achieved by spark plasma sintering boron carbide powders with 10wt% silicon. The silicon did not diffuse into boron carbide grains to produce a solid solution of Si-doped boron carbide; instead the silicon reacted with impurities in the starting powder to form β-SiC and borosilicate glass. The resultant new phases facilitated densification of the multiphase ceramic through liquid phase-assisted sintering. The resultant material exhibits improved hardness (34.3GPa Vikers hardness under 1kg load) with toughness comparable to both Si-free and commercially available boron carbide.

Damage evolution of hot-pressed boron carbide under confined dynamic compression

The dynamic response of hot-pressed boron carbide was studied under uniaxial and confined loading conditions using a modified compression Kolsky bar setup at strain rates of 102 – 103 s–1. The progression of damage in the prismatic specimens was captured using a high-speed camera. This experimental approach was reproduced in a quasi-static regime (10–4 – 10–3 s–1) to study the rate dependance of the strengths and damage modes. The results showed that the compressive strength of boron carbide was both stress state and strain rate dependent. A real-time visualization showed a change in the crack path under confined compression loading. Our observations revealed the formation of column-like fragments and slabs in the uniaxial and confined compression, respectively. Collected fragments of the specimens were used to investigate failure mechanisms by SEM and TEM. Both observations showed transgranular fracture. It was found that microcracks originate at large carbon inclusions and have tensile character. TEM examinations also point to the absence of stress-induced damage at grain boundaries and inside boron carbide grains. The formation of microcracks was responsible for experimentally observed inelastic response under dynamic loading conditions.

Coarsening of boron carbide grains during the infiltration of porous boron carbide preforms by molten silicon

This work investigates the coarsening of boron carbide grains during the infiltration of porous boron carbide preforms by molten silicon with respect to fabrication of reaction-bonded boron carbide ceramics. Experimental results reveal that the shape of boron carbide grains evolve from the irregular shape to faceted shape due to dissolution-precipitation during infiltration. For infiltration temperatures below 1750°C, the boron carbide grains are irregular and exhibit an unimodal size distribution, which can be ascribed to the normal grain growth. The growth of the irregular grains follow a cubic law of diffusion control. In contrast, for infiltration temperatures above 1750°C, the boron carbide grains become faceted and exhibit a bimodal size distribution, indicative of the typical abnormal grain growth. The abnormal growth of faceted grains is proposed to be controlled by coalescence-enhanced two-dimensional nucleation.

Microstructural Characterization of a Commercial Hot‐Pressed Boron Carbide Armor Plate

Detailed microstructural characterization was carried out on a commercial‐grade hot‐pressed boron carbide armor plate. The boron carbide grains have close to B4C stoichiometry, and most of them have no planar defects. The most prominent second phase is intergranular graphite inclusions that are surrounded by multiple boron carbide grains. Submicrometer intragranular and intergranular BN and AlN precipitates were also observed. In addition, fine dispersions of AlN nanoprecipitates were observed in some but not all grains. No intergranular films were found. These microstructural characteristics are compared with the lab‐consolidated boron carbide and their effects on the mechanical properties of boron carbide are addressed.

Influence of spark plasma sintering parameters on densification and mechanical properties of boron carbide

The densification behavior of boron carbide without sintering additives is reported for temperatures in the range of 1100°C to 1800°C by spark plasma sintering (SPS) technique together with the sintering parameters (Holding Time and Pulsed DC). The influence of porosity on mechanical properties (hardness, fracture toughness and elastic modulus) of boron carbide prepared by SPS is measured. Pulsed DC current is found to play a dominant role in the densification of boron carbide and in achieving near theoretical density at lower sintering temperature compared to conventional sintering techniques. Hardness, fracture toughness and elastic modulus of fully dense B4C are measured as 37.2GPa, 2.8MPa.m1/2 and 570GPa respectively. Microstructural analysis indicates the presence of deformation twins in boron carbide grains.

High Hardness BaCb-(BxOy/BN) Composites with 3D Mesh-Like Fine Grain-Boundary Structure by Reactive Spark Plasma Sintering

Boron carbide B4C powders were subject to reactive spark plasma sintering (also known as field assisted sintering, pulsed current sintering or plasma assisted sintering) under nitrogen atmosphere. For an optimum hexagonal BN (h-BN) content estimated from X-ray diffraction measurements at approximately 0.4 wt%, the as-prepared BaCb-(BxOy/BN) ceramic shows values of Berkovich and Vickers hardness of 56.7 +/- 3.1 GPa and 39.3 +/- 7.6 GPa, respectively. These values are higher than for the vacuum SPS processed B4C pristine sample and the h-BN -mechanically-added samples. XRD and electronic microscopy data suggest that in the samples produced by reactive SPS in N2 atmosphere, and containing an estimated amount of 0.3-1.5% h-BN, the crystallite size of the boron carbide grains is decreasing with the increasing amount of N2, while for the newly formed lamellar h-BN the crystallite size is almost constant (approximately 30-50 nm). BN is located at the grain boundaries between the boron carbide grains and it is wrapped and intercalated by a thin layer of boron oxide. BxOy/BN forms a fine and continuous 3D mesh-like structure that is a possible reason for good mechanical properties.

Structure and phase formation in boron carbide and aluminum powder mixtures during hot pressing

The paper examines the structure and phase formation during hot pressing of boron carbide powder mixtures containing 20–50 vol.% aluminum and analyzes the microhardness and fracture toughness of the hot-pressed samples. It is established that the particle size of the starting powder has an essential effect on the structure and mechanical properties of boron carbide composites. A matrix structure consisting of aluminum carboboride and borates and inclusions of boron carbide forms during hot pressing of mixtures containing coarse B 4 C powder. The microhardness of the matrix varies from 20 to 25 GPa and that of boron carbide grains from 42 to 44 GPa. Homogeneous fine- grained structure mainly consisting of aluminum nitride and borates forms in hot-pressed mixtures containing fine boron carbide. Its microhardness is 33–37 GPa and fracture toughness 5.4–5.7 MPa ⋅ m 1/2 .

The high-strain-rate dynamic response of boron carbide-based composites: The effect of microstructure

The dynamic high-strain-rate behavior of boron carbide-based composites with similar phase composition yet different microstructural features, namely, amount of residual silicon, average grain size and morphology of the SiC particles, was investigated as a function of the planar impact strength. The composites consisted of boron carbide preforms (compacted or partly sintered powder) with or without free carbon addition, infiltrated with molten silicon. The dynamic response of the composites depends strongly on the amount of residual silicon, on the average size of the boron carbide grains, and on the morphology of the SiC particles. The preliminary sintering of the preforms exerts no effect on the dynamic properties of the composites. The deformation and failure mechanisms under dynamic loading are discussed.

Microstructure of Vacuum Plasma-Sprayed Boron Carbide

In the present study, boron carbide was deposited on Ti-6%Al-4%V alloy by vacuum plasma spraying. Chemical and phase compositions of the initial starting powder and the as-sprayed deposit were characterized using hot gas extraction, x-ray photoelectron spectroscopy, Raman spectroscopy, x-ray diffraction, and transmission electron microscopy. Mechanical properties of the deposition were assessed by microhardness and nanohardness indentation. The microstructure consisted of equiaxed boron carbide grains, microcrystalline particles, and amorphous carbon regions. The amount of boron oxide and amorphous carbon in the coating was increased compared with the initial powder. The measured microhardness was slightly higher than values reported previously (1033±200 HV). There was significant variation in measured nanohardness (−100 + 39 GPa) from point to point caused by multiple phases, splat boundaries, and porosity in the deposited structure. Carbon segregation to grain boundaries and/or splat boundaries in boron carbide was observed directly using spatially resolved electron energy loss spectroscopy method.

Pressureless sintering and related reaction phenomena of Al2O3-doped B4C

The effects of alumina on the densification of boron carbide and related reaction phenomena in alumina-doped B4C were studied. Pressureless sintering was conducted at various temperatures for 15 min in a flowing Ar atmosphere. The addition of alumina improved the densification of boron carbide. Maximum density of 96% theoretical was obtained with the 3 wt % alumina-doped B4C sintered at 2150°C. Abnormal (or exaggerated) grain growth was observed in the specimen containing more than 4 wt % alumina. In the B4C-Al2O3 reaction couples, good wetting of the liquid phase on the boron carbide grains was observed. X-ray diffraction and Auger electron spectra showed that the AlB12C2 phase was formed by the reaction between boron carbide and alumina. It is suggested that these phenomena promote the densification of boron carbide.

Cleaning of metal-contaminated graphite tiles of the TEXTOR ALT-II pump limiter blades

Graphite is the preferred material for plasma facing components in today's fusion experiments and it is a prime candidate material also for coming fusion devices. At least in the existing tokamaks it cannot be excluded that the graphite surface will become contaminated, e.g. with metals, which leads to undesired effects on the plasma. Instead of an expensive exchange for new graphite parts, an appropriate cleaning method should be applied. This paper describes comparative tests of different mechanical cleaning methods applied to metal contaminated graphite tiles of the ALT-II toroidal belt pump limiter in the TEXTOR tokamak. Scanning electron microscope techniques have been used to characterize the cleaning efficiency. Best results have been obtained by sandblasting the graphite surface with boron carbide grains, which proves to be a very effective, inexpensive and fast cleaning method. After reassembly of the tiles, in-situ tests by exposure to tokamak discharges showed undetectably low (≤10 −5) metal concentrations in the TEXTOR plasma.