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.
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.
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.
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.
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.
Improved dehydrogenation properties of Ca(BH4) 2-LiNH2 combined system
Chu, Hailiang,Xiong, Zhitao,Wu, Guotao,Guo, Jianping,He, Teng,Chen, Ping
, p. 10585 - 10587 (2010)
Ca(BH4)2-LiNH2 combined system is shown to release hydrogen at much lower temperature compared to the pure Ca(BH 4)2. The improved dehydrogenation in this system can be ascribed to a combination reaction between [BH4] and [NH2] based on the reaction mechanism of positive H and negative H. The Royal Society of Chemistry 2010.
Origin of ferromagnetism in polycrystalline Ca1+δB 6 (-0.05<δ<0.05) ceramics
Cao, Minghe,Jiang, Jun,Liu, Hanxing,Yuan, Jun
, p. 39 - 43 (2005)
We have investigated systematically the relationship of ferromagnetic properties of polycrystalline Ca1+δB6 ceramics with Ca content, as δ ranges from -0.05 to 0.05. We demonstrated that the origin of the high-temperature ferromagnet
Flux Growth and Magnetic Properties of CaB6 Crystals
Otani, Shigeki,Mori, Takao
, p. 1791 - 1792 (2002)
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Effect of additions of Ca compounds to the filling powder on the reduction of MgO and the critical current density properties of ex situ processed MgB2 tapes
Fujii, Hiroki,Ishitoya, Akira,Itoh, Shinji,Ozawa, Kiyoshi,Kitaguchi, Hitoshi
, p. 650 - 656 (2016)
We have studied the effect of additions of Ca compounds, CaC2 and CaH2, to the filling powder on the transport critical current density (Jc) properties of ex situ processed MgB2 tapes. These additives are expected to reduce MgO present in as-received commercial MgB2 powder and improve the grain coupling of MgB2 in the tapes. Heat treatment of the hand mixed powders, MgB2 and the Ca compounds, brings about both disappearance of MgB4 and formation of CaB6 and Mg, compared with heat treated MgB2 powder. They are observed much more clearly for the mixtures with CaH2 addition. The MgO content decreases by the additions of those compounds. Neither of the a-axis and the c-axis parameters of MgB2 are changed by the additions, suggesting no reactions of substitutions of Ca for Mg and C for B in MgB2. The Jc properties of the tape samples using the hand mixed filling powder with CaC2 addition degrade, whereas those with CaH2 addition are slightly improved. When using the ball milled mixtures, slight contraction of the a-axis parameter occurs only by CaC2 addition. Although the additions cause the reactions similar to those observed for the hand mixed powders, the MgO content reduces more clearly, compared with the MgB2 content. The Jc properties of the tapes are enhanced in the high-field and low-field regions by the additions of CaC2 and CaH2, respectively. These enhancements are attributed to the carbon substitution, the improved grain coupling and the increased content of MgB2.
Synthesis of Calcium Hexaboride Powder via the Reaction of Calcium Carbonate with Boron Carbide and Carbon
Zheng, Shuqi,Min, Guanghui,Zou, Zengda,Yu, Huashun,Han, Jiande
, p. 2725 - 2727 (2001)
The synthesis of calcium hexaboride (CaB6) powder via the reaction of calcium carbonate (CaCO3) with boron carbide (B4C) and carbon has been investigated systematically in the present study. The influences of heating temperature and holding time on the reaction products have been studied using X-ray diffractometry, and the morphologies of CaB6 obtained at various temperatures and holding times have been investigated via scanning electron microscopy. The interaction in the CaCO3-B4C-carbon system by which CaB6 is formed is a solid-phase process that passes through the transition phases Ca3B2O6 and CaB2C2. The optimal conditions for CaB6 synthesis are a holding time of 2.5 h at a temperature of 1673 K, under vacuum (a pressure of 10-1 Pa). CaB6 powder has the same morphology as B4C, and the properties and the shape of CaB6 powders can be improved by choosing good-quality raw materials.
Calcium borohydride for hydrogen storage: Catalysis and reversibility
Roennebro, Ewa,Majzoub, Eric H.
, p. 12045 - 12047 (2007)
We demonstrate a new solid-state synthesis route to prepare calcium borohydride, Ca(BH4)2 by reacting a ball-milled mixture of CaB6 and CaH2 in a molar ratio of 1:2 at 700 bar of H2 pressure and 400-440°C. Moreover, doping with catalysts was found to be crucial to enhance reaction kinetics. Thermogravimetric analysis and differential scanning calorimetry revealed a reversible low-temperature to high-temperature endothermic phase transition at 140°C and another endothermic phase transition at 350-390°C associated with hydrogen release upon formation of CaB6 and CaH2, as was evident from X-ray diffraction analysis. Thus, since Ca(BH4)2 here is shown to be prepared from its anticipated decomposition products, the conclusion is that it has potential to be utilized as a reversible hydrogen storage material. The theoretical reversible capacity was 9.6 wt % hydrogen.
Ferromagnetism in lanthanum doped CaB6: Is it intrinsic?
Mori, Takao,Otani, Shigeki
, p. 287 - 290 (2002)
Magnetism of flux grown single crystals of undoped and lanthanum doped Ca1-xLaxB6 was systematically investigated, taking especial care in the preparation and quality control of crystals. Ca1-xLaxB6 (x = 0.0029, 0.0051, 0.015) crystals showed ferromagnetism at 300 K, which could be removed by treatment with hydrochloric HCl acid, indicating that the phenomena is due to iron impurities rather than being intrinsic. Chemical analysis which has been lacking in previous reports, was performed on HCl treated and untreated Ca1-xLaxB6 crystals and showed that the magnitude of observed ferromagnetism can be explained by the iron content. Crystals grown using HCl treated aluminum flux revealed that undoped CaB6, which has a higher resistivity, accumulates less iron on the surface compared to Ca1-xLaxB6 and is diamagnetic, agreeing with our previous idea that iron is electrochemically plated onto the crystal surface during the flux removal procedure. The temperature dependence of the spurious magnetism of Ca0.985La0.015B6 crystals was measured at high temperatures and exhibited anomalies around 780 K but also at 640 K. Previously, differences of the transition temperature with bulk iron metal were given as one of the reasons to support the ferromagnetism of Ca1-xLaxB6 to be an intrinsic effect, but this result indicates that this shift can be attributed to the environment of iron impurities on the crystal surface. These results strongly indicate that the ferromagnetism in lanthanum doped CaB6 is not an intrinsic phenomenon.
A novel strategy for reversible hydrogen storage in Ca(BH4)2
Yan, Yigang,Remhof, Arndt,Rentsch, Daniel,Züttel, Andreas,Giri, Santanab,Jena, Puru
, p. 11008 - 11011 (2015/07/07)
We report that decomposition pathway of Ca(BH4)2 can be efficiently controlled by reaction temperature. That is, it decomposes into CaB6 at a lower temperature range of 320 to 350 °C, but into amorphous boron at 400 to 450 °C. We identified the formation of CaB2H6 as the crucial intermediate step on the way to CaB6 that only forms at 320 to 350 °C.
