Boron carbide is a strategic material, finding applications in nuclear industry, armour for personnel and vehicle safety, rocket propellant, etc. Its high hardness makes it suitable for grinding and cutting tools, ceramic bearing, wire drawing dies, etc. Boron carbide is commercially produced either by carbothermic reduction of boric acid in electric furnaces or by magnesiothermy in presence of carbon. Since many specialty applications of boron carbide require dense bodies, its densification is of great importance. Hot pressing and hot isostatic pressing are the main processes employed for densification. In the recent past, various researchers have made attempts to improve the existing methods and also invent new processes for synthesis and consolidation of boron carbide. All the techniques on synthesis and consolidation of boron carbide are discussed in detail and critically reviewed.

Synthesis and consolidation of boron carbide: a review

Transparent ceramics: Processing, materials and applications

Magnesium aluminate (MgAl2O4) spinel (MAS) is a synthetic material with cubic crystal structure and excellent chemical, thermal, dielectric, mechanical and optical properties. These properties made MAS an indispensable material for optically transparent windows, domes and armours, and for certain refractory applications. High processing cost of dense MAS ceramics is in the main responsible for its limited usage in certain important applications despite its excellent performance in them. The volume expansion (∼8%) associated with MAS phase formation from alumina and magnesia does not allow obtaining dense MAS bodies in a single-stage reaction sintering process. Therefore, dense MAS bodies are made by following a double stage firing process, which is expensive. The existing literature suggests that the processing cost of dense MAS ceramics could be reduced to a great extent by decisive selection of starting raw materials, powder processing and densification conditions, and by understanding the under...

A review on magnesium aluminate (MgAl2O4) spinel: synthesis, processing and applications

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The Science And Engineering Of Materials

Cerium modified birnessite-type manganese dioxides (Ce-MnO2) with different doping ratios were prepared by a redox reaction of KMnO4 with (NH4)2C2O4 in the presence of Ce(NO3)3. The as-synthesized Ce-MnO2 samples were characterized by XRD, SEM, TEM, BET, XPS, ICP-AES, H2-TPR, Raman and in situ DRIFTS. The results indicate that doping of cerium significantly enhance the performance of birnessite for HCHO removal at low temperature. Ce-MnO2(1:10) with the nominal Ce/Mn ratio of 1:10 exhibited the best activity and achieved complete HCHO conversion at 100 °C and better activity at room temperature than undoped birnessite. Upon the doping of cerium, the growth of MnO2 crystal was inhibited, leading to smaller particle size and higher specific surface. In addition, CeO2 nanocrystal formed even at low doping ratio (0.1:10), resulting in close contact between CeO2 and MnO2 nanocrystals. As a result, the doped material owned higher content of oxygen vacancies and surface adsorbed oxygen species, which contributed to its high activity for HCHO oxidation. This investigation provides a new point of view about how to design an inexpensive efficient catalyst for indoor air purification via introducing appropriate content of cerium.

Cerium modified birnessite-type MnO2 for gaseous formaldehyde oxidation at low temperature

Mixtures of elementary oxides, MgO-Al 2 O 3 , were used to fabricate transparent polycrystalline magnesium aluminate spinel specimens by means of the spark plasma sintering technique. A sintering aid, 1 wt% of LiF, was added to the mixed powder. The presence of the additive promotes the synthesis of spinel that starts at 900°C and is completed at 1100°C. The LiF additive wets spinel on its melting and promotes densification, which is completed at 1600°C. LiF vapor plays a cardinal role in eliminating residual carbon contamination and in the fully dense state, allows attaining a 78% level of optical transmittance. The optimal conditions for achieving adequate transparency were determined and the role of the LiF addition in the various stages of the process is discussed.

Synthesis and Densification of Transparent Magnesium Aluminate Spinel by SPS Processing

The previously reported model that accounts for the formation of the core-rim structure in reaction-bonded boron carbide composites (RBBC) is expanded and validated by additional experimental observations and by a thermodynamic analysis of the ternary B–C–Si system. The microstructure of the RBBC composites consists of boron carbide particles with a core-rim structure, β-SiC and some residual silicon. The SiC carbide particles have a polygonal shape in composites fabricated in the presence of free carbon, in contrast to the plate-like morphology when the initial boron carbide is the sole source of carbon. In the course of the infiltration process, the original B4C particles dissolve partly or fully in molten silicon, and a local equilibrium is established between boron carbide, molten silicon and SiC. Overall equilibrium in the system is achieved as a result of the precipitation of the ternary boron carbide phase B12(B,C,Si)3 at the surface of the original boron carbide particles and leads to the formation of the rim regions. This feature is well accounted for by the “stoichiometric saturation” approach, which takes into account the congruent dissolution of B4C particles. The SiC phase, which precipitates form the silicon melt adopts the β-allotropic structure and grows preferably as single plate-like particles with an {1 1 1}β habit plane. The morphology of the SiC particles is determined by the amount of carbon available for their formation.

Microstructural evolution during the infiltration of boron carbide with molten silicon

The study deals with the effect of the SPS parameters and LiF doping on the mechanical and optical properties polycrystalline magnesium aluminate spinel (PMAS) with emphasis on the grain size of the final product. Sintering at 1300°C of undoped powder yielded fully dense submicrometer (0.4–0.6 μm) samples with elevated mechanical properties (1600HV and 300MPa bending strength). Doped samples had a larger, 40 μm grain size, lower, 1450HV, hardness and 150MPa bending strength. The transmittance of the doped samples (80% at 500 nm wavelength) was higher than that of the undoped ones. Thus, the required functionality of the ceramic dictates the choice of parameters for the fabrication of dense transparent PMAS.

The Effect of Grain Size on the Mechanical and Optical Properties of Spark Plasma Sintering‐Processed Magnesium Aluminate Spinel MgAl2O4

The present paper is concerned with the static and the dynamic mechanical properties at strain-rates up to 10 3 s −1 , of ceramic composites based on porous B4C infiltrated with molten Si. The static mechanical properties of the infiltrated composites depend on the amount of the residual silicon. The Young’s modulus and the hardness increase, while the flexural strength and K1C decrease with decreasing fraction of residual silicon. The dynamic strength and the initiation fracture toughness K1D have significantly higher values than the corresponding static properties, and are insensitive to both the residual silicon fraction and the strain-rate up to u = 10 3 s −1 . © 2007 Elsevier B.V. All rights reserved.

Static and dynamic mechanical properties of infiltrated B4C–Si composites

The presence of unreacted, free silicon lowers the mechanical properties of reaction-bonded boron carbide. The fraction of free silicon can be reduced by increasing the green density of the initial boron carbide performs. The use of multimodal boron carbide mixtures allows attaining 75% green density. After reaction bonding with molten silicon, the composites consist of four phases, namely the original B 4 C particles, the B 12 (B,C,Si) 3 phase, product of the dissolution―precipitation process, β-SiC, and residual Si. The volume fraction of residual Si in the composites is in the 8―10% range. The infiltrated composites display elevated values of the mechanical properties with a high Weibull modulus.

Nahum Frage

Papers

Synthesis and Densification of Transparent Magnesium Aluminate Spinel by SPS Processing

Microstructural evolution during the infiltration of boron carbide with molten silicon

The Effect of Grain Size on the Mechanical and Optical Properties of Spark Plasma Sintering‐Processed Magnesium Aluminate Spinel MgAl2O4

Static and dynamic mechanical properties of infiltrated B4C–Si composites

The Effect of Particle Size Distribution on the Microstructure and the Mechanical Properties of Boron Carbide‐Based Reaction‐Bonded Composites