12007-99-7Relevant articles and documents
Structural, magnetic, and thermionic emission properties of multi-functional La1-xCaxB6 hexaboride
Bao, Lihong,Qi, Xiaoping,Bao, Tana,Tegus
, p. 332 - 338 (2018)
Herein, we report the synthesis of nanocrystalline La1-xCaxB6 (0 ≤ x ≤ 1) hexaboride powders by solid-state reaction and their subsequent consolidation via spark plasma sintering. The structural, magnetic and thermionic emission properties of La1-xCaxB6 hexaboride are investigated. All of the synthesized nanocrystalline hexaboride powders are single phase with the CsCl-type structure and no ferromagnetic impurity phases have been detected from X-ray diffraction. Magnetic measurements show that weak ferromagnetism at room temperature is found in nanocrystalline La1-xCaxB6 hexaboride powders, and the magnetism was attributed to the presence of the intrinsic defects, based on the data of the HRTEM. Thermionic emission measurements indicate that the maximum emission intensity for bulk La0.4Ca0.6B6 at 1873 K reached 20.02 A/cm2, which is more than three times higher as compared to bulk CaB6 (～6.04 A/cm2). When the La doping was increased to 40 at%, the work function of CaB6 decreased from 2.95 to 2.76 eV, indicating an improvement in the thermionic emission performance. Therefore, the quasibinary La1-xCaxB6 hexaboride may have an application as a promising cathode.
11B NUCLEAR QUADRUPOLE INTERACTION IN METAL HEXABORIDES (MB6).
Aono,Kawai
, p. 797 - 802 (1979)
The objective of this work is to discuss the bonding nature of the hexaborides on the basis of the experimental results on the **1**1B nuclear electric quadrupole interaction.
Thermoelectric properties of some metal borides
Takeda, Masatoshi,Fukuda, Tadahiro,Domingo, Ferrer,Miura, Takahiro
, p. 471 - 475 (2004)
Polycrystalline AlMgB14 and some hexaborides (CaB6, SrB6, YbB6, SmB6, and CeB6) were synthesized to examine their thermoelectric properties. Single phase of orthorhombic AlMgB14, which contains B12 icosahedral clusters as building blocks, was obtained at sintering temperatures between 1573 and 1823K. Seebeck coefficient (α) and electrical conductivity (σ) of the phase were about 500μV/K and 10-11/Ωm at room temperature, respectively. These values are comparable to those of metal-doped β-rhombohedral boron. On the other hand, metal hexaborides with divalent cation possessed large negative α ranging from -100 to -270μV/K at 1073K. Calculated power factors of CaB6 and SrB 6 exceeded 10-3W/K2m within the entire range of temperature measured. As a result, they can be thought as promising candidates for n-type thermoelectric material.
On the existence of solid solutions based on magnesium diboride
Sevast'yanova,Kazin,Kravchenko,Kuz'michev,Ponomarev,Burdina,Bulychev
, p. 1674 - 1680 (2003)
Dense samples of pure magnesium boride and of the compositions Mg (1-x) AxB2 (A = Na, K, Ca, Sr, Ba, Sn, Ti; 0. 05 2 samples vary only slightly as compared to those of the pure MgB2 irrespective of the sample preparation procedure, high temperature-high pressure treatment conditions, and the amount of dopant, thus indicating the absence of extended solid solution regions. The superconducting transition temperatures of all the samples did not exceed the value characteristic of MgB2 (39±1 K). The results obtained for the dense MgB2 samples using the Andreev reflection and tunneling spectroscopies confirm the two-gap nature of superconductivity in magnesium diboride and point to analogy between the superconductivity mechanisms in this compound and in cuprates.
Calcium tetraboride - Does it exist? Synthesis and properties of a carbon-doped calcium tetraboride that is isotypic with the known rare earth tetraborides
Schmitt, Ruth,Blaschkowski, Bjoern,Eichele, Klaus,Meyer, H.-Juergen
, p. 3067 - 3073 (2006)
Crystalline samples of carbon-doped CaB4 were synthesized by solid-state reactions in sealed niobium ampules from the elements Ca, B, and C. The structure was determined by single-crystal X-ray diffraction (P4/mbm, Z = 4, a = 7.0989(7) A, c = 4.1353(5) A, R1 = 0.026, and wR2 = 0.058) revealing an atom arrangement containing a three-dimensional boron network built up from B6 octahedra and B2 dumbbells which is well-known from the structures of rare earth tetraborides. Crystals of CaB4-xCx are black with a metallic luster and behave stable against mineral acids. Band structure calculations indicate that CaB4 is a stable semiconducting compound with a narrow band gap and that carbon should not necessarily be required for the stability of this compound. The presence of carbon in the crystalline samples of CaB4-xCx was indicated by electron energy loss spectroscopy, but the carbon content in the samples was estimated to be less than 5% according to inductively coupled plasma-atomic emission spectrometry measurements. The distribution of boron and carbon atoms in the structure was investigated by means of 11B and 13C solid-state magic angle spinning NMR. Measurements of the magnetic susceptibility indicate a temperature-independent paramagnetism down to 20 K.
Improvement of thermoelectric properties of alkaline-earth hexaborides
Takeda, Masatoshi,Terui, Manabu,Takahashi, Norihito,Ueda, Noriyoshi
, p. 2823 - 2826 (2006)
Thermoelectric (TE) and transport properties of alkaline-earth hexaborides were examined to investigate the possibility of improvement in their TE performance. As carrier concentration increased, electrical conductivity increased and the absolute value of
Reversible hydrogen storage in the lithium borohydride-calcium hydride coupled system
Pinkerton,Meyer
, p. L1-L4 (2008)
We report large reversible hydrogen storage in a new coupled system, LiBH4/CaH2, via the reaction 6LiBH4 + CaH2 ? 6LiH + CaB6 + 10H2 having a theoretical hydrogen capacity of 11.7 wt% and an estimated reaction enthalpy of ΔH = 59 kJ/mol H2. Samples that include 0.25 mol (18.2 wt%) TiCl3 reproducibly store 9.1 wt% hydrogen, corresponding to 95% of the available hydrogen. H2 is the only evolved gas detected by mass spectrometry. X-ray diffraction confirms that the sample cycles between LiBH4 and CaH2 in the hydrogenated state and LiH and CaB6 in the dehydrogenated state.
Thermal decomposition behavior of calcium borohydride Ca(BH4)2
Kim, Jae-Hun,Jin, Seon-Ah,Shim, Jae-Hyeok,Cho, Young Whan
, p. L20-L22 (2008)
The thermal decomposition behavior of adduct-free Ca(BH4)2, prepared by heating Ca(BH4)2·2THF powder under vacuum, was investigated by X-ray diffraction and thermal analyses. It has been found that Ca(BH4)2 undergoes a polymorphic transformation at 440 K and eventually decomposes in two steps between 620 and 770 K. CaH2 and an unknown intermediate compound form after the first step, but CaH2 is the only crystalline phase observed after the second step with a total weight loss of about 9.0 wt.%.
Ca(BH4)2-LiBH4-MgH2: A novel ternary hydrogen storage system with superior long-term cycling performance
Gao, Mingxia,Gu, Jian,Pan, Hongge,Wang, Yiliu,Liu, Yongfeng,Liang, Chu,Guo, Zhengxiao
, p. 12285 - 12292 (2013)
A ternary hydrogen storage system, of superior cyclic stability and high capacity, was developed from a mixture of Ca(BH4)2, LiBH4 and MgH2 in molar ratios of 1:2:2. Investigation on both non-isothermal and isothermal hydrogen desorption/absorption properties shows that the hydrogen desorption starts from 320 °C and completes at 370 °C under a heating rate of 2 °C min-1, releasing ca. 8.1 wt% H2. The finishing temperature of desorption is much lower and the capacity much higher than any of the two-hydride mixtures in the ternary system. In particular, hydrogenation of the ternary system initiates at an extremely low temperature of ca. 75 °C and the onset dehydrogenation temperature is significantly reduced by 90 °C after the initial dehydrogenation/ hydrogenation cycle, which is ascribed to the formation of an active dual-cation hydride of CaMgH3.72 for dehydrogenation in the hydrogenation process. There is ca. 7.6 wt% H2 absorbed at 350 °C and 90 bar H2 for 18 h for the system post-dehydrogenated at 370 °C for 30 min, demonstrating a reversibility of over 94%. The capacity seems to fade mainly in the initial few cycles and stabilizes after further cycling. The reversibility is as high as 97% and a dehydrogenation capacity of ca. 6.2 wt% H2 at the 10th cycle. Favourable kinetics and thermodynamics of hydrogen desorption/absorption are achieved, which are responsible for the low completion temperature and the superior cycling performance. Mechanisms of the improved dehydrogenation/hydrogenation properties including the cyclic behaviour of the system are also proposed in relation to microstructural analyses.
Raman scattering study of CaB6 and YbB6
Ogita, Norio,Nagai, Shinji,Okamoto, Naoki,Iga, Fumitoshi,Kunii, Satoru,Akamtsu, Toshiro,Akimitsu, Jun,Udagawa, Masayuki
, p. 461 - 465 (2004)
Phonon spectra of CaB6 and RB6 (R=Yb, Ce, and Pr) have been investigated by Raman scattering. We found clear spectral difference between divalent cation hexaboride and trivalent one. Eg mode shows the doublet spectra for o
