Boride Carbide Research Articles

Influence of silicon-containing compounds on the heat resistance of composites based on titanium boride

The reaction formation of composite materials and coatings based on titanium boride, silicon dioxide, and silicon carbide upon heat treatment of mixtures composed of initial components in air has been investigated. It is shown that the glass melt formed in the course of the chemical reaction encapsulates titanium boride and silicon carbide particles, thus imparting the high-temperature strength to the composite material. The influence of the composition, temperature, and heating conditions on the kinetics of oxidation of samples in the form of cast pieces and graphite with coatings during their heat treatment at temperatures of 1000–1300°C is studied using thermogravimetric, differential thermal, and X-ray powder diffraction analyses. The electrical resistivity of the synthesized samples is determined and its dependence on temperature is established. The compositions of coatings that ensure the effective protection of graphite from oxidation in air at high temperatures are proposed from analyzing the results of the performed investigation.

Infinite and Finite Boron Carbon Branched Chains: The Crystal Structures of New Ternary Boride Carbides RE10B7C10 and RE4B3C4*

AbstractNew ternary rare earth metal boride carbides RE10B7C10 (RE = Gd, Tb, Dy Ho, Er) and RE4B3C4 (RE = Tb, Dy, Ho, Er, Tm, Yb, Lu) were prepared from the elements by arc‐melting followed by annealing in silica tubes at 1000°C for one month. The crystal structures were investigated by means of single‐crystal X‐ray diffraction data. In the structures of both compounds the boron and carbon atoms form two different types of nonmetal arrangements: one‐dimensional (BC)∞ branched chains and finite CBC units. The branched chains and finite units are differently oriented in both structures. The new ternary rare earth metal boride carbides RE4B3C4 (R = Tb, Dy, Ho, Er, Tm, Yb, Lu) are isotypic to Gd4B3C4, the first characterized member of the rare earth metal borocarbide series in which both one dimentional (BC)∞ branched chains and finite CBC units coexist. Tb10B7C10 is the second characterized member of this family. The compounds RE10B7C10 (RE = Gd, Tb, Dy Ho, Er) are isotypic.

Pressureless Sintering of Zirconium Diboride: Particle Size and Additive Effects

Zirconium diboride (ZrB2) was densified by pressureless sintering using <4‐wt% boron carbide and/or carbon as sintering aids. As‐received ZrB2with an average particle size of ∼2 μm could be sintered to ∼100% density at 1900°C using a combination of boron carbide and carbon to react with and remove the surface oxide impurities. Even though particle size reduction increased the oxygen content of the powders from ∼0.9 wt% for the as‐received powder to ∼2.0 wt%, the reduction in particle size enhanced the sinterability of the powder. Attrition‐milled ZrB2with an average particle size of <0.5 μm was sintered to nearly full density at 1850°C using either boron carbide or a combination of boride carbide and carbon. Regardless of the starting particle size, densification of ZrB2was not possible without the removal of oxygen‐based impurities on the particle surfaces by a chemical reaction.

New examples of ternary rare-earth metal boride carbides containing finite boron–carbon chains: The crystal and electronic structure of RE15B 6C 20 ( RE=Pr, Nd)

The ternary rare-earth metal boride carbides RE 15B 6C 20 ( RE=Pr, Nd) were synthesized by co-melting the elements. They exist above 1270 K. Their crystal structures were determined from single-crystal X-ray diffraction data. Both crystallize in the space group P1¯, Z=1, a=8.3431(8) Å, b=9.2492(9) Å, c=8.3581(8) Å, α=84.72(1)°, β=89.68(1)°, γ =84.23(1)° ( R1=0.041 (w R2=0.10) for 3291 reflections with I o>2 σ( I o)) for Pr 15B 6C 20, and a=8.284(1) Å, b=9.228(1) Å, c=8.309(1) Å, α=84.74(1)°, β=89.68(1)°, γ=84.17(2)° ( R1=0.033 (w R2=0.049) for 2970 reflections with I o>2 σ( I o)) for Nd 15B 6C 20. Their structure consists of a three-dimensional framework of rare-earth metal atoms resulting from the stacking of slightly corrugated and distorted square nets, leading to cavities filled with unprecedented B 2C 4 finite chains, disordered C 3 entities and isolated carbon atoms, respectively. Structural and theoretical analyses suggest the ionic formulation ( RE 3+) 15([B 2C 4] 6−) 3([C 3] 4−) 2(C 4−) 2·11ē. Accordingly, density functional theory calculations indicate that the compounds are metallic. Both structural arguments as well as energy calculations on different boron vs. carbon distributions in the B 2C 4 chains support the presence of a CBCCBC unit. Pr 15B 6C 18 exhibits antiferromagnetic order at T N=7.9 K, followed by a meta-magnetic transition above a critical external field B>0.03 T. On the other hand, Nd 15B 6C 18 is a ferromagnet below T C≈40 K.

Ternary rare earth metal boride carbides containing two-dimensional boron–carbon network: The crystal and electronic structure of R2B 4C ( R=Tb, Dy, Ho, Er)

The ternary rare earth boride carbides R 2B 4C ( R=Tb, Dy, Ho, Er) have been synthesized by reacting the elements at temperatures between 1800 and 2000K. The crystal structure of Dy 2B 4C has been determined from single-crystal X-ray diffraction data. It crystallizes in a new structure type in the orthorhombic space group Immm ( a=3.2772(6) Å, b=6.567(2) Å, c=7.542(1) Å, Z=2, R1=0.035 (w R 2=0.10) for 224 reflections with I o>2 σ( I o)). Boron atoms form infinite chains of fused B 6 rings in [100] joined with carbon atoms into planar, two-dimensional networks which alternate with planar sheets of rare earth metal atoms. The electronic structure of Dy 2B 4C was also analyzed using the tight-binding extended Hückel method.

Structural arrangements of the ternary metal boride carbide compounds MB 2C 4 ( M=Mg, Ca, La and Ce) from first-principles theory

The structural arrangements of the ternary metal borocarbides MB 2C 4 ( M=Mg, Ca; La and Ce) are investigated using density-functional theory (DFT) calculations within the generalized gradient approximation (GGA). Results indicate that these compounds adopt a layered structure consisting of graphite-like B 2C 4 layers alternating with metal sheets. Within the hexagonal layers, the coloring with the –C–C–C–B–C–B– sequence is energetically more stable than that with the –C–C–C–C–B–B– one. The electronic structures of these compounds, mainly determined by the B 2C 4 sheets, can be rationalized with the simple valence electron distribution M 2+[B 2C 4] 2− xe −, with the metals essentially acting as two-electron donors with respect to the boron–carbon network, the other x electrons remaining in the relatively narrow d and/or f bands of the metals. Accordingly, MB 2C 4 are narrow band-gap semiconductors (Δ E≈0.2–0.4 eV) with M=Mg and Ca. On the other hand, with M=La and Ce, the compounds are conducting with a relatively high density of states at the Fermi level predominantly metal in character with substantial B/C π* antibonding state admixture.

Synthesis, Crystal Structure, and Properties of Two Modifications of MgB12C2

Single crystals of two modifications of the new magnesium boride carbide MgB(12)C(2) were synthesized from the elements in a metallic melt by using tantalum ampoules. Crystals were characterized by single-crystal X-ray diffraction and electron microprobe analysis (energy-dispersive (EDX) and wavelength-dispersive (WDX) X-ray spectroscopy). Orthorhombic MgB(12)C(2) is formed in a Cu/Mg melt at 1873 K. The crystal structure of o-MgB(12)C(2) (Imma, Z=4, a=5.6133(10), b=9.828(2), c=7.9329(15) A, 574 reflections, 42 variables, R(1)(F)=0.0208, wR(2)(I)=0.0540) consists of a hexagonal primitive array of B(12) icosahedra with Mg atoms and C(2) units in trigonal-prismatic voids. Each icosahedron has six exohedral B--B and six B--C bonds. Carbon is tetrahedrally coordinated by three boron atoms and one carbon atom with a remarkably long C--C distance of 1.727 A. Monoclinic MgB(12)C(2) is formed in an Al/Mg melt at 1573 K. The structure of m-MgB(12)C(2) (C2/c, Z=4, a=7.2736(11), b=8.7768(13), c=7.2817(11) A, beta=105.33(3) degrees , 1585 reflections, 71 variables, R(1)(F)=0.0228, wR(2)(I)=0.0610) may be described as a distorted cubic close arrangement of B(12) icosahedra. Tetrahedral voids are filled by C atoms and octahedral voids are occupied by Mg atoms. The icosahedra are interconnected by four exohedral B--B bonds to linear chains and by eight interstitial C atoms to form a three-dimensional covalent network. Both compounds fulfill the electron-counting rules of Wade and Longuet-Higgins.

Novel Processing of Porous Titanium Composite for Producing Open Cell Structure

Processing technique to produce open-cell porous titanium composite was developed. One of the outstanding benefits of porous titanium composite is both physical and mechanical properties can be controlled widely by changing the metal/ceramic fraction and cell structures. In this work, porous titanium composite was fabricated by a chemical reaction between titanium powder and boron carbide (B4C) powder. The reactions between titanium and B4C generates a large amount of latent heat and, therefore, it was a combustion and self-propagating mode. Precursors were made by compacting the starting powder blend (Ti and B4C), and heated in an induction furnace to induce the reaction. The reaction was strongly exothermic and, therefore, the precursor was sintered by its latent heat when the Ti/B4C blending ratio was appropriate. The reaction products were titanium boride (TiB and/or TiB2) and titanium carbide (TiC). By controlling the Ti/B4C blending ratio, it was possible to control the volume fraction of reaction products in titanium matrix. The combustion synthesized titanium composite was porous and its cell structure was strongly affected by the processing condition of the precursor (porosity and Ti/B4C blending ratio). High porosity with open pores was obtained with small Ti/B4C ratios and high porosity of the precursor, while the cell structure was closed and spherical with high Ti/B4C ratio. The cell-wall size was varied from several tens of microns to about 500 microns by changing the combustion temperature.

Portraits of some representatives of metal boride carbide and boride silicide compounds

Different ternary alkaline-earth and rare-earth metal boron carbide and silicide compounds are examined using the solid-state language of Zintl–Klemm concept, band structures, and density of states, in order to show that the topology of the non-metal sub-lattice is highly dependent on the electron count. It is also shown that the chemistry of rare-earth metal–boron–silicon does not parallel that of rare-earth metal–boron–carbon. B–C bonds are easily formed in the latter, leading to a large variety of different structural arrangements, whereas Si–B bonds are hardly observed in the former, except in insertion compounds.

Li2B12C2 and LiB13C2: Colorless Boron‐Rich Boride Carbides of Lithium

Wade was right: Li2B12C2 and LiB13C2, the first boron-rich boride carbides of lithium, are (nearly) colorless and stoichiometrically composed. Their crystal structures are characterized by B12 icosahedra and C2 and CBC units, respectively, forming a three-dimensional framework with Li cations in the voids. The charge distribution can be described by Wade's rules: (Li+)2B122−C2 and (Li+)B122−(CBC+). The synthesis succeeded in molten metals at high temperatures.

La3Cl3BC — Structure, Bonding and Electrical Conductivity.

AbstractFor Abstract see ChemInform Abstract in Full Text.

La3Cl3BC – Structure, Bonding and Electrical Conductivity

A new rare earth carbide boride halide, La3Cl3BC, has been prepared by heating a mixture of stoichiometric quantities of LaCl3, La, B and C at 1050 °C for 10 days. La3Cl3BC (La3Br3BC type) crystallizes in the monoclinic system with space group P21/m (No. 11), a = 8.2040(16), b = 3.8824(8), c=11.328(2)Å , β =100.82(3)°. In the structure, monocapped trigonal prisms containing B-C units are condensed into chains along the b direction, and the chains are further linked by Cl atoms in the a and c directions. The condensation results in a polymeric anion 1 ∞[BC] with a spine of B atoms in a trigonal prismatic coordination by La, and the C atoms attached in a square pyramidal coordination. The B-B and B-C distances are 2.16 and 1.63 Å , respectively. La3Cl3BC is metallic. The EH calculation shows that the distribution of valence electrons can be formulated as (La3+)3(Cl−)3(BC)5− · e−.

Characteristic Features of Boron Carbide Synthesis for Iron Triad Borides

We have studied the characteristic features of synthesis of iron, cobalt, and nickel borides by boron carbide reduction of oxides. We have shown that the reaction of boron carbide with iron triad metal oxides occurs through a stages involving formation of the metals, lower boride phases, carbides and borates of the corresponding metals. A characteristic feature of such a reaction is the higher reactivity of boron carbide compared with carbon in the initial stages, leading to the appearance of B2O3 and C which react at higher temperatures to form boron carbide. The method of boron carbide reduction of oxides allows us to obtain rather pure higher borides of iron, cobalt, and nickel.

Twinning and intergrowth of rare earth boride carbides.

Twins and intergrown crystals of tetragonal rare earth boride carbides, especially those with the La(5)B(2)C(6) structure type, have been investigated by high-resolution electron microscopy and X-ray diffraction. The structure of the twin interface has been determined. It provides an explanation for coherently intergrown domains of different structure. The Sc(3)C(4) structure type is remarkable because it is frequently intergrown with La(5)B(2)C(6)-type phases. It provides, for instance, a model for the intergrowth of other types, e.g. Gd(4)B(3)C(4) and Gd(5)B(2)C(5). The presence of metal-atom square nets in different orientations in the structures accounts for a number of intergrowth phenomena. The possibilities and limitations of X-ray structure determinations are discussed with respect to actual examples.

Structure refinement of the layered composite crystal Sc2B1.1C3.2 in a five-dimensional formalism.

The crystal structure of a layered compound Sc(2)B(1.1)C(3.2), scandium boride carbide (M(r) = 140.43), has been re-refined as a commensurate composite crystal using 1795 single-crystal X-ray diffraction intensities with I > 2 sigma(I) collected by Shi, Leithe-Jasper, Bourgeois, Bando & Tanaka [(1999), J. Solid State Chem. 148, 442--449]. The crystal is composed of two layered subsystem structures, i.e. Sc--C--Sc sandwiches and graphite-like layers of the composition B(1/3)C(2/3). The structure refinement was performed in a five-dimensional formalism based on the trigonal superspace group P3m1(p00)(0p0)0m0. The unit cell and other crystal data are a = b = 3.387 (1), c = 6.703 (2) A, V = 66.59 (1) A(3), sigma(1) = (9/7 0 0), sigma(2) = (0 9/7 0), Z = 1, D(x) = 3.501 Mg m(-3). Two different three-dimensional sections through the superspace were analyzed, corresponding to two different superstructure models, one with P3m1 and the other with P3m1. A random distribution of B and C was assumed in the graphite-like layer and 41 structural parameters were introduced. R(F)/wR(F) were 0.0533/0.0482 and 0.0524/0.0476, respectively, for the first and second models. Although the difference between these R(F) or wR(F) values was too fine to exclude one of the models definitely, the advantages of using a superspace group were obvious. It not only brought about better convergence of refinement cycles by virtue of fewer parameters, but also gave an insight into the problem of symmetry of the superstructure.

Synthesis, Characterization, Structural and Theoretical Analysis of a New Rare-Earth Boride Carbide: Lu3BC3

Synthesis, Characterization, Structural and Theoretical Analysis of a New Rare-Earth Boride Carbide: Lu3BC3

CaB2C2: Reinvestigation of a Semiconducting Boride Carbide with a Layered Structure and an Interesting Boron/Carbon Ordering Scheme

Calcium diboride dicarbide, CaB(2)C(2), was synthesized as a crystalline powder and investigated by electron energy loss spectroscopy, X-ray powder diffractometry, conductivity measurements, and LMTO band structure calculations. A new structure model was derived, and the crystal structure was refined by Rietveld methods in the tetragonal space group I4/mcm (No. 140, a = 537.33(1) pm and c = 741.55(2) pm, Z = 4). The boron and carbon atoms are well ordered within layers consisting of four- and eight-membered rings. A convincing coloring scheme is proven by the detection of a superstructure reflection. An earlier assignment of the compound into the LaB(2)C(2) structure family (space group P&fourmacr;2c or P4(2)/mmc, respectively) has been shown to be incorrect. LMTO band structure calculations suggest semiconducting behavior for CaB(2)C(2), which has been confirmed by conductivity measurements.

B and B–C as interstitials in reduced rare earth halides

In this review we describe a variety of rare earth metal boride halides and boride carbide halides. In boride halides boron atoms occur either as discrete atoms octahedrally coordinated by rare earth metal atoms or as chains of interconnected B 4 rhomboids in rods of fused metal atom bisphenoids. Characteristic building units in boride carbide halides are quasi-molecular B–C, C–B–C, C–B–B–C, and C 2–B–B–C 2 entities. In these compounds B centers trigonal prisms of rare earth metal atoms, and C is located in tetragonal pyramids. Owing to the electropositive character of rare earth metals, B atoms, B 4 units, and C n B m groups tend to be anions. Band structure calculations as well as electrical and magnetic measurements have been performed. The compounds show semiconducting or metallic behavior, depending on whether excess electrons according to the Zintl–Klemm formalism are localized or delocalized. La 9Br 5(CBC) 3 is superconducting with T c∼6 K. Ce and Gd compounds are paramagnetic. In most cases anti-ferromagnetic ordering is observed at low temperature.

Zirconium boride and tantalum carbide coatings sprayed by electrothermal explosion of powders

Refractory zirconium diboride and tantalum monocarbide ceramic powders were sprayed using an electrothermal explosion caused by a high-voltage electric breakdown and large-current discharge heating. This spray technique was improved using a purpose-designed powder container, which made it possible to melt the powder completely and accelerate it to impinge on substrates. The electrical energy applied to the powder was estimated to be about twice the energy theoretically needed to melt just the powder. Although the ceramics used in this work are hard-sintered materials by nature, they could be sprayed and deposited to form coatings on metal substrates without additives and sintering agents. The coatings formed exhibited no chemical decomposition in the boride, and only small amounts of decarburization in the carbide due to its nonstoichiometry. The tantalum carbide coating mixed with iron and aluminum substrates in the range of 10 μm to several tens of micrometers.

Magnetic structure of TbNi2B2C.

Neutron-diffraction techniques have been used to study the magnetic structure of Tb${\mathrm{Ni}}_{2}$${\mathrm{B}}_{2}$C. The measurements, performed on single crystals of this compound, show that below approximately 15 K the moments order in an almost longitudinal spin wave with wave vector along ${\mathbf{a}}^{*}$. The magnitude of the wave vector is close to that obtained in the Ho, Er, and Gd compounds. This observation provides additional evidence that there are common Fermi-surface nesting features along ${\mathbf{a}}^{*}$ in the rare-earth nickel boride carbides which cause the magnetic ordering of the rare-earth moments via the Ruderman-Kittel-Kasuya-Yosida mechanism. Below approximately 8 K, the experimental results indicate the development of a small ferromagnetic component.