Effects of reactants proportions on features of in-situ magnesiothermic self-propagating high temperature synthesized boron carbide powder

The effects of reactant proportions were investigated on features (phase composition, micromorphology and crystal development) of B₄C (boron carbide) powder synthesized by in-situ magnesiothermic SHS (self-propagating high temperature synthesis) method, which was based on the perspective of thermodynamic design. The results showed that the reactant proportions were the fundamental reasons affecting the phase composition of the products during the reaction process. Mg addition could decrease the Mg₃B₂O₆/MgO mass ratio of the SHS products. Moreover, C addition would increase the C/B mass ratio of the leached products. When the C molar ratio was 0.2, free boron appeared in the leached products, and the composition of boron carbide was B₁₃C₂. When the C molar ratio ≥0.6, the composition of boron carbide changed into B₄C, while free carbon began to appear. The lattice parameters a and c of boron carbide crystal dropped with the increase of the C/B mass ratio from 0.1 to 0.3. Meanwhile, carbon atoms gradually entered the boron carbide unit cell. When the C/B mass ratio exceeds 0.3, the lattice parameters tended to be constants of a = 5.608 Å, c = 12.039 Å, while free carbon began to appear in the leached products.     

Synthesis and Raman characterization of boron-doped single-walled carbon nanotubes

A systematic study was carried out to dope single-walled carbon nanotube (SWNT) bundles with varying amounts of boron using the pulsed laser vaporization technique. Targets containing boron concentrations ranging from 0.5 to 10 at.% boron were prepared by mixing elemental boron with carbon paste and the Co/Ni catalysts. The laser-generated products that were obtained from these targets were characterized by high resolution transmission electron microscopy, electron energy loss spectroscopy (EELS), thermoelectric power (TEP) measurements, and Raman scattering experiments. Electron microscopy and Raman studies revealed that the presence of various levels of boron concentration in the target strongly affected the products that were prepared. SWNTs were found in the products prepared from targets containing up through 3 at.% boron, and high resolution EELS estimated that less than 0.05-0.1 at.% boron is present in the SWNT lattice. The absence of SWNT bundles in the products derived from targets containing more than 3 at.% boron implies that the presence of excess boron in the carbon plume severely inhibits the carbon nanotube growth. The overall effect of the boron incorporation primarily leads to: (i) a systematic increase in intensity of the disorder-induced band (D-band) upon boron doping, with increasing D-band intensity observed for higher doping levels, (ii) a systematic downshift in the G′-band frequency due the relatively weaker C-B bond, and (iii) a non-linear variation in the RBM and G′-band intensities which is attributed to shifts in resonance conditions in the doped tubes. Resonant Raman spectroscopy thus provides large changes in the intensity of prominent features even when the dopant concentration is below the detectable limit of EELS (0.05-0.1 at.%). Thermoelectric power data also provide complementary evidence for the presence of a small boron concentration in the SWNT lattice which transforms the SWNTs into a permanently p-type material.     

SHS-produced boron carbide: Some special features of crystal structure

Boron carbides with carbon contents (??) of 7?C24 at. % were prepared by SHS method and their lattice parameters were determined by high-precision XRD analysis. An unusually wide spread in the literature data on lattice parameters of boron carbide as a function of ?? was associated with the process of crystal ordering caused by gradual replacement of boron by carbon atoms. For SHS-produced boron carbide, the above spread turned out minimal. Lattice parameter c was found to attain the unusually high values of 12.20?C12.31 s was observed at ?? = 13.2%. The widths of diffraction lines from boron carbide were found to depend on ?? and attain their maximum values at ?? = 13.2% when the lattice is most disordered. Our results can make a basis for elaborating the means for regulating the structure/properties of boron carbide.     

Structure and mechanical properties of boron-rich boron carbides

A series of boron-rich boron carbides are investigated to explore the effect of boron/carbon ratios on the microstructure and mechanical properties of the complex material. It has been found that excess boron substitution gives rise to expanded lattice constants, orientational asymmetry of the chain structure and distortion of the icosahedra of rhombohedra B₄C units in comparison with conventional carbon-rich boron carbides. Microstructure characterization reveals a high density of stacking faults and growth twins in boron-rich boron carbides. Nanoindentation measurements demonstrate that the hardness and modulus of boron-rich boron carbides decrease with the increase of boron content while the B_10.2C sample with the highest boron substitution shows relatively high hardness and modulus. This study suggests that the structure of single-phase covalent materials could be tailored by self-alloying for improved mechanical properties.     

Chemical and electrochemical intercalation of lithium into boronated carbons

Boron-substituted carbons have been produced by chemical vapour deposition from acetylene and boron trichloride precursors at 1140°C. Li intercalation was investigated, chemically from the Li vapour and electrochemically in Li–carbon cells. In both cases, the amount of intercalated Li increases with the boron content of the carbon, up to a value of 13 at.% boron. Above this value, the intercalation of lithium is not so efficient. This behaviour is understood by assuming that boron, which acts as an electron acceptor and hence favours the intercalation of electron donors like lithium, can enter substitutionally into the carbon lattice up to a certain limit, which is close to 13 at.% for the materials deposited at 1140°C.     

Synthesis of Nanocrystalline Boron Carbide via a Solvothermal Reduction of CCl4 in the Presence of Amorphous Boron Powder

Nanocrystalline boron carbide (B₄C) was synthesized via a solvothermal reduction of carbon tetrachloride using metallic lithium as reductant in the presence of amorphous boron powder at 600°C in an autoclave. The X-ray diffraction pattern of the product powder was indexed to the hexagonal B₄C phase, with lattice constants a =5.606 and c =12.089 Å. The sample was also characterized by Raman spectrum, X-ray photoelectron spectroscopy and inductively coupled plasma atomic emission spectrometry. Transmission electron microscopy revealed that the B₄C nanocrystallites were slightly agglomerated, with a particle size of approximately 15–40 nm in diameter.     

Non-stoichiometry of composition and dissolution of oxygen in the cubic boron nitride crystal lattice

Powders of cubic boron nitride (cBN), synthesized from multicomponent melts, were studied with a crystallization medium containing excessive nitrogen or boron. X-ray diffraction analysis was used for precision determination of the crystal lattice constants, space factor of regular sets of points of the F43m space group, dislocation density, and stacking fault concentration. Cubic boron nitride has been found to crystallize as a variable-composition compound having vacancies in the boron sublattice. These vacancies are due to a heterovalence substitution of oxygen for nitrogen, i.e. formation of the oxygen solid solution in the cBN crystal lattice. With an increase in the number of vacancies in the boron sublattice, the cBN lattice constant decreases, and the stacking fault concentration increases.     

Reactive sintering process and thermoelectric properties of boron rich boron carbides

Dense boron rich boron carbides were reactive sintered by hot pressing at 2050 °C using elementary boroncarbon compositions with carbon contents of 9.1, 11.1, 13.3 and 18.8 at.%. The following material characteristics are presented: relative density, SEM images, EDX, X-ray diffraction and corresponding lattice parameters, Seebeck coefficient, electrical conductivity and thermoelectric power factor. Significant grain growth has been obtained with increasing boron content. A deeper understanding of the boron and carbon reaction and the overall sintering process is gained by thermal and chemical analysis in combination with X-ray diffraction. Additionally a thermal experiment with boron and carbon layers illustrates the solid state diffusion behaviour. The found results of boron carbide properties of this paper correspond with results by other authors. The aim is to correlate technological aspects of sintering procedure with material properties. This should help to improve the thermoelectric efficiency of boron carbide based materials.     

用含硼T610石墨炭源材料合成透明含硼金刚石

控制石墨炭源材料的硼含量是透明含硼金刚石合成的关键问题。文中叙述了透明含硼金刚石合成的效果，讨论了石墨炭源材料中的硼含量和合成工艺对透明含硼金刚石合成的影响，指出了提高透明含硼金刚石合成质量的方法和途径。     

Influence of boron impurities on the crystal structure of cubic boron nitride

The influence of boron impurities on the unit cell parameters of the boron nitride with a sphalerite structure is investigated. It is demonstrated that the presence of boron in amounts exceeding the stoichiometric composition leads to a distortion of the cubic lattice and reduces its symmetry. The revealed oriented distortion of the crystal lattice of the cubic boron nitride is explained by the incorporation of excess boron atoms not in a random manner but between paired corrugated hexagonal layers. Original Russian Text ? S.P. Bogdanov, 2008, published in Fizika i Khimiya Stekla.     

Revised Method for the Prediction of Lattice Constants in Cubic and Pseudocubic Perovskites

The correlation between ionic radii and the lattice constants of cubic/pseudocubic perovskites is re-examined and a new empirical formula for the relationship is derived. The average absolute error in the predicted lattice constants of perovskites thus derived is expected to be about 0.60%. Discrepancies between measured lattice constants and those calculated from idealized structural models are shown to be due to stretching or compression of bond lengths and are strongly correlated to both tolerance factor and cation bond valence sums. The formula derived may be useful in the engineering of substrates or buffer layers for heteroepitaxial films, in which a lattice match is essential.     

Formation and reactivity of borides,carbides and silicides. II. Production and sintering of boron carbide

The principal methods of boron carbide production are compared and their thermodynamics are examined. Mechanism of carbide formation and sintering of the products are discussed and investigated further. High-purity stoicheiometric B₄C of submicron size was produced on a semi-technical scale by reduction of diboron trioxide with carbon and magnesium. Rates of sintering were determined from changes in surface areas and average crystallite and aggreagte sizes. Sintering of boron carbide was enhanced by increased temperature and time of calcination. Addition of chromium accelerated sintering at temperatures above 1600° and especially at 1800°. For more extensive sintering, submicron powders from the magnesium reduction process were more suitable than the coarser samples given by the electro-thermal carbon reduction; the latter required ballmilling to provide suitable grain size composition for effective hot pressing.     

Carbon Doping in Boron Suboxide: Structure,Energetics, and Elastic Properties

The structural, electronic, and elastic properties of pristine and carbon‐doped boron suboxide (B₆O) are calculated using density functional theory. The results indicate that it is energetically preferable for a single carbon atom to substitute into an oxygen site rather than a boron site. The lattice parameters and cell volume increase to relieve the residual stress created by the carbon substitution. The interstitial position is not favorable for a single atom substitution. However, if two carbon atoms substitute for two neighboring oxygen atoms, then it becomes energetically favorable to dope an interstitial oxygen, boron, or carbon atom along the C–C chain. If the interstitial dopant is either boron or carbon, a local B₄C‐like structure with either a C–B–C or C–C–C chain is created within the boron suboxide unit cell. The resulting structure shows improvements in the bulk modulus at the expense of the shear and Young's moduli. The moduli further improve if an additional carbon is substituted within a polar or equatorial site of the neighboring B₁₂ icosahedron. Based on these calculations, we conclude that carbon doping can either harden or soften B₆O depending on the manner in which the substitutions are populated. Furthermore, as B₆O samples are often oxygen deficient, C doping can occupy such sites and improve the elastic properties.     

Titanium Diboride–Tungsten Diboride Solid Solutions Formed by Induction-Field-Activated Combustion Synthesis

Solid solutions of titanium diboride–tungsten diboride (TiB₂–WB₂) were synthesized by induction-field-activated combustion synthesis (IFACS) using elemental reactants. In sharp contrast to conventional methods, solid solutions could be formed by the IFACS method within a very short time, ∼2 min. Solutions with compositions ranging from 40–60 mol% WB₂ were synthesized with a stoichiometric ratio (Ti + W)/B =½; however, samples with excess boron were also made to counter the loss of boron by evaporation. The dependence of the lattice constants of the resulting solid solutions on composition was determined. The "a" parameter decreased only slightly with an increase in the WB₂ content, whereas the "c" parameter exhibited a significant decrease over the range 40–60 mol% WB₂. Solid-solution powders formed by the IFACS method were subsequently sintered in a spark plasma sintering (SPS) apparatus. After 10 min at 1800°C, the samples densified to relative density 86%. XRD analysis showed the presence of only the solid-solution phase.     

碳热还原法制备碳化硼粉末的工艺研究

采用碳热还原法制备了碳化硼粉末样品,讨论了硼碳比、煅烧合成和粉碎过程等工艺参数对其粉末性能的影响。借助XRD分析手段测试了其成分,并用化学方法分析了粉末的总碳含量,用激光粒度分布仪测试了其粒度分布。实验结果表明:以工业用硼酸和炭黑为原料,在1700～1850℃、保温0.5～1h煅烧合成能制得纯度较高的碳化硼粉末。其总碳含量为20.7%,接近理论值。中位径为32.56μm,经球磨粉碎后,其中位径可以达到2.42μm。     

Doping-induced anisotropic lattice strain in homoepitaxial heavily boron-doped diamond

As a result of the larger covalent radius of boron (r_B = 0.88 Å) when compared to that of carbon (r_C = 0.77 Å), the introduction of substitutional boron into diamond leads to an expansion δa/a of the lattice parameter. This has been found previously to follow a linear interpolation (Vegard law) as long as the boron content is lower than about 0.5 at.% in MPCVD epilayers or 1.5 at.% in HPHT bulk crystals.Above those concentrations, the expansion is less pronounced than predicted by Vegard. In order to explain this effect, we have performed ab initio calculations on C:B substitutional alloys. The results show that the presence of interstitial boron and of boron clusters is not necessary to explain the experimental data available in the literature. Moreover, quantitative estimates are proposed for the deformation potential of the valence band maximum and for the steric effect associated to boron pairing. We then apply these conclusions to discuss the different variations of δa/a vs boron contents observed by high resolution XRD experiments performed on “insulating” and metallic (and superconducting) p⁺⁺ diamond epilayers grown by MPCVD on (100)- and (111)-oriented type Ib substrates, for which boron concentration profiles have been determined by Secondary Ion Mass Spectroscopy.     

Influence of chemical composition on mechanical properties of spark plasma sintered boron carbide monoliths

Fully dense boron carbide monoliths exhibiting fine microstructure (i.e., submicrometric grain size) are sintered by Spark Plasma Sintering. Two different commercial powder batches, exhibiting different stoichiometries (i.e., B/C ratio and oxygen content) and various amounts of secondary phases (i.e., boric acid and free carbon), are used. Their chemical composition is well‐defined by coupling different methods (Transmission Electron Microscopy associated with XRD analyses, and Instrumental Gas Analysis), and are correlated with their mechanical properties, characterized from meso‐ to macro‐scopic scales by nano‐indentation and ultrasonic pulse echography. The presence of secondary phases (graphite and boric acid) is evidenced in various proportions in each powder batch. If the boric acid disappears during sintering, the graphite remains. However, for the considered amounts of graphite (lower than 1 wt%), the low variations in graphite content have no significant effect on hardness and elasticity values. At the opposite, the presence of oxygen in boron carbide lattice, leading to a boron oxycarbide phase, induces a decrease in both hardness and elasticity properties.     

Effect of isotope content on the cubic boron nitride lattice thermal conductivity

New theoretical data are presented on the lattice thermal conductivity temperature dependence of cubic boron nitride single crystals. Their thermal conductivity increases with increasing isotope purity, with the maximum thermal conductivity occurring with either isotopically pure ¹¹B or ¹⁰B. The thermal conductivity is a symmetric function of isotopic composition with a minimum at 50% ¹⁰B.     

The System Zirconium—Carbon

Phase relations in the system zirconium-carbon were established by DTA, X-ray, and metallo-graphic methods. Only one phase, a monocar-bide with the NaCl structure, was found. The low-carbon phase boundary for the monocarbide contains 38.5 at.7, carbon between 1900'and 3300OC. The carbon-rich boundary between 2850" and 330O°C is at 48.9 at.% carbon. ZrC melts congruently at 342O°C and 46 at.% carbon. The lattice parameter versus composition curve shows a maximum at 46 at.% carbon. The Zr-ZrC eutectic is at 186O°C and 1 at.% carbon; the Zr-ZrC eutectic is at 285O°C and 65 at.% carbon. Liquidus curves are inferred from the invariant points.     

Structural Parameters of Mullite Formed During Heating of Diphasic Mullite Gels

Diphasic gels of four different compositions were synthesized. Lattice parameters, crystallite size, and strain values of mullite formed at different stages of annealing of diphasic gels have been calculated by quantitative standard less analysis using X-ray diffraction. Results show that all three parameters change on progressive heating. Changes in the microstrain of mullite show a reciprocal relationship with crystallite size. Silica-rich gel shows that the relative changes in the calculated values of three structural parameters of mullite formed during its formation process are because of an increase in its crystallinity rather than changes in composition. However, gels of composition in the solid solution range show that the relationship between lattice constants and chemical composition of mullite formed at high temperature agrees with Cameron data.