- Synthesis and characterization of ternary layered Nb2SB ceramics fabricated by spark plasma sintering
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In this paper, B-containing MAX phase of Nb2SB ceramics with high purity of 96 wt% (4 wt% NbB impurity) was successfully synthesized using the molar ratio of Nb: S: B = 2: 1.3: 1 by spark plasma sintering at 1350 °C under 30 MPa. The reaction path, microstructure, physical and mechanical properties were systematically studied. It is found that the formation of Nb2SB is from the combination of Nb, NbS2, and Nb5B6, or NbB and NbS. The full dense sample (RD 99.7%) possesses the fine grains about 6 μm in length and 3.6 μm in width. The grains of Nb2SB show a layered structure, which is same to other MAX phases. The thermal expansion coefficient is 7.1 × 10?6 K?1 in the range of 24–1100 °C. In the temperature range of 25–800 °C, the thermal diffusivity of Nb2SB ceramic increases from 5.58 mm2/s to 7.07 mm2/s. At 25 °C, the heat capacity is 0.36 J·g?1·K?1, the thermal conductivity is 13.79 W·m?1·K?1, and the electrical conductivity is 1.17 × 106 Ω?1·m?1. Additionally, the obtained Nb2SB ceramics exhibit excellent mechanical properties of Vickers hardness of 11.89 GPa (10 N load), flexural strength of 249 MPa, fracture toughness of 4.76 MPa·m1/2, and compressive strength of 1157 MPa.
- Fan, Longfeng,Feng, Qingguo,Grasso, Salvatore,Hu, Chunfeng,Qin, Yanru,Zhou, Yanchun
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- Preparation of niobium borides NbB and NbB2 by self-propagating combustion synthesis
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Preparation of niobium borides NbB and NbB2 was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts in this study. Effects of the sample green density, preheating temperature and starting stoichiometry on combustion characteristics, as well as on the composition of final products were studied. Experimental evidence indicates the self-sustained reaction zone propagating along a spiral trajectory for the reactant compacts without prior heating or preheated at 100 °C. The increase of initial sample temperature to 200 and 300 °C by prior heating brings about a planar flame-front propagating in a steady mode. As the preheating temperature or sample green density increased, the combustion temperature was found to increase and the propagation rate of combustion wave was correspondingly enhanced. According to the temperature dependence of combustion wave velocity, the activation energies associated with the Nb + B and Nb + 2B reactions were determined to be 151.8 and 132.4 kJ/mol, respectively. As indicated by the XRD analysis, the final composition of burned products was essentially governed by the starting stoichiometry of reactant compacts. Synthesized products composed of a single boride phase NbB and a small amount of unreacted Nb were obtained from the reactant compacts of Nb:B = 1:1. In addition, the SHS reaction of powder compacts with an initial composition Nb:B = 1:2 yielded niobium diboride NbB2 as the dominant phase, along with another boride phase Nb3B4 in a minor quantity.
- Yeh,Chen
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- Synthesis of niobium boride powder by solid state reaction between niobium and amorphous boron
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Niobium boride powders were synthesized by solid state reaction between niobium metal powder and amorphous boron powder. The formation of niobium borides was found to be dependent on temperature. Single phases of the stable borides, NbB and NbB2, were formed by heating mixed powders corresponding to the stoichiometric compositions at 1000°C for 60 min. A single phase of Nb3B4 was obtained at a higher temperature of 1800°C as a result of the promoted diffusion of boron atoms in niobium metal. All synthesized powders were well dispersed and had particle sizes of 5 - 10 μm.
- Matsudiara,Itoh,Naka,Hamamoto
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- A comparative study on combustion synthesis of Nb-B compounds
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A comparative study on the preparation of specific niobium borides (including Nb3B2, NbB, Nb5B6, Nb3B4, and NbB2) in the Nb-B system was experimentally conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts of their corresponding stoichiometries. Effects of the sample green density, preheating temperature, and starting stoichiometry on combustion characteristics, as well as on product composition were studied. Experimental evidence indicated that except for the samples of Nb:B = 3:2, upon ignition a planar and self-sustained combustion front was established and proceeded throughout the entire sample in a steady manner. However, combustion of the samples with Nb:B = 3:2 was characterized by a localized reaction zone propagating along a spiral trajectory, due largely to the low combustion temperatures which further resulted in a poor degree of phase conversion with a significant amount of Nb left unreacted. The incomplete reaction in the Nb:B = 3:2 powder compact produced boride compounds Nb3B4 and NbB in minor quantities. Reactant compacts of Nb:B = 1:1 and 1:2 were shown to yield practically single-phase monoboride NbB and diboride NbB2, respectively. In contrast, multiphase compounds consisting of Nb3B4, NbB, and NbB2 were synthesized from the powder compacts with starting stoichiometries Nb:B = 3:4 and 5:6. However, it was found that two boride phases Nb3B2 and Nb5B6 did not appear in the end products from any of the reactant compacts. Based upon the temperature dependence of combustion wave velocity, the activation energies associated with combustion synthesis of NbB and NbB2 were determined to be 151.8 and 132.4 kJ/mol, respectively.
- Yeh,Chen
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- Preparation of borides in Nb-B and Cr-B systems by combustion synthesis involving borothermic reduction of Nb2O5 and Cr2O3
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An experimental study on the preparation of metal borides in the Nb-B and Cr-B systems was conducted by self-propagating high-temperature synthesis (SHS) involving the reduction of Nb2O5 and Cr2O3 by amorphous boron. The starting stoichiometry of the reactant compact was shown to make a great impact on the combustion behavior and the phase composition of the final product. For the powder compacts of Nb2O5 and boron, self-sustaining combustion was performed under a molar ratio of B/Nb2O5 between 5 and 10, but complete reduction of Nb2O5 was achieved when B/Nb2O5 ≥ 8. Partial reduction of Nb2O5 caused a decrease in the combustion temperature and velocity, and was responsible for the presence of NbO2 in the final products. For the samples with stoichiometry of 6 ≤ B/Nb2O5 ≤ 8, three boride phases NbB, Nb3B4, and NbB2 were synthesized. An increase in the boron content up to B/Nb2O5 = 8.5-10 resulted in not only full reduction of Nb2O5, but also formation of single-phase NbB2. On the other hand, the SHS process involving Cr2O3 and boron was feasible for the powder compacts of 4 ≤ B/Cr2O3 ≤ 9, wherein the highest combustion temperature and the fastest reaction front were observed in the compact with B/Cr2O3 = 6. During combustion Cr2O3 was fully reduced, leading to the formation of three borides Cr5B3, CrB, and CrB2 in either monolithic or composite form. With a boron content more than the stoichiometric amount, the powder compacts of B/Cr2O3 = 4, 5, and 9 yielded single-phase Cr5B3, CrB, and CrB2, respectively.
- Yeh,Wang
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p. 366 - 371
(2010/05/01)
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- Low-temperature specific heat of the niobium borides
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The specific heat of the borides Nb3B2, NbB, and Nb3B4 has been studied at 60-300 K.Their characteristic temperatures and their fundamental thermodynamic functions under standard conditions (298.15 K) have been calculated.
- Bolgar, A. S.,Blinder, A. V.
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p. 449 - 450
(2007/10/02)
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- THERMAL EXPANSION STUDIES ON THE GROUP IV-VII TRANSITION METAL DIBORIDES.
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The thermal expansions of the group IV-VII transition metal diborides were studied with the aid of X-ray powder diffraction. The diborides were studied over the temperature range 298 - 1500 K. All the diborides except for CrB//2 display larger thermal expansion coefficients in the c direction than in the a direction. The expansion coefficients in the c direction decrease with increasing radius of the metal atom, a fact which can be correlated to an increase in metal-boron bond strength. The thermal expansion coefficient in the a direction changes very little with the size of the metal radius, owing to the fact that the bonding strength in the basal plane is determined by the strong B-B bonds within the boron layer.
- Loennberg
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p. 145 - 156
(2008/10/08)
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