- Structural, magnetic, and thermionic emission properties of multi-functional La1-xCaxB6 hexaboride
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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.
- Bao, Lihong,Qi, Xiaoping,Bao, Tana,Tegus
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- 11B NUCLEAR QUADRUPOLE INTERACTION IN METAL HEXABORIDES (MB6).
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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.
- Aono,Kawai
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- Thermoelectric properties of some metal borides
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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.
- Takeda, Masatoshi,Fukuda, Tadahiro,Domingo, Ferrer,Miura, Takahiro
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- On the existence of solid solutions based on magnesium diboride
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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.
- Sevast'yanova,Kazin,Kravchenko,Kuz'michev,Ponomarev,Burdina,Bulychev
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- Calcium tetraboride - Does it exist? Synthesis and properties of a carbon-doped calcium tetraboride that is isotypic with the known rare earth tetraborides
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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.
- Schmitt, Ruth,Blaschkowski, Bjoern,Eichele, Klaus,Meyer, H.-Juergen
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- Improvement of thermoelectric properties of alkaline-earth hexaborides
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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
- Takeda, Masatoshi,Terui, Manabu,Takahashi, Norihito,Ueda, Noriyoshi
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- Reversible hydrogen storage in the lithium borohydride-calcium hydride coupled system
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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.
- Pinkerton,Meyer
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- Thermal decomposition behavior of calcium borohydride Ca(BH4)2
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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.%.
- Kim, Jae-Hun,Jin, Seon-Ah,Shim, Jae-Hyeok,Cho, Young Whan
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- Ca(BH4)2-LiBH4-MgH2: A novel ternary hydrogen storage system with superior long-term cycling performance
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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.
- Gao, Mingxia,Gu, Jian,Pan, Hongge,Wang, Yiliu,Liu, Yongfeng,Liang, Chu,Guo, Zhengxiao
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- Raman scattering study of CaB6 and YbB6
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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
- Ogita, Norio,Nagai, Shinji,Okamoto, Naoki,Iga, Fumitoshi,Kunii, Satoru,Akamtsu, Toshiro,Akimitsu, Jun,Udagawa, Masayuki
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- Improved dehydrogenation properties of Ca(BH4) 2-LiNH2 combined system
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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.
- Chu, Hailiang,Xiong, Zhitao,Wu, Guotao,Guo, Jianping,He, Teng,Chen, Ping
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- Probing the structure, stability and hydrogen storage properties of calcium dodecahydro-closo-dodecaborate
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Calcium borohydride can reversibly store up to 9.6 wt% hydrogen; however, the material displays poor cyclability, generally associated with the formation of stable intermediate species. In an effort to understand the role of such intermediates on the hydrogen storage properties of Ca(BH4)2, calcium dodecahydro-closo-dodecaborate was isolated and characterized by diffraction and spectroscopic techniques. The crystal structure of CaB12H12 was determined from powder XRD data and confirmed by DFT and neutron vibrational spectroscopy studies. Attempts to dehydrogenate/hydrogenate mixtures of CaB12H12 and CaH2 were made under conditions known to favor partial reversibility in calcium borohydride. However, up to 670 K no notable formation of Ca(BH4)2 (during hydrogenation) or CaB6 (during dehydrogenation) occurred. It was demonstrated that the stability of CaB12H12 can be significantly altered using CaH2 as a destabilizing agent to favor the hydrogen release.
- Stavila, Vitalie,Her, Jae-Hyuk,Zhou, Wei,Hwang, Son-Jong,Kim, Chul,Ottley, Leigh Anna M.,Udovic, Terrence J.
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- Origin of ferromagnetism in polycrystalline Ca1+δB 6 (-0.05<δ<0.05) ceramics
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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
- Cao, Minghe,Jiang, Jun,Liu, Hanxing,Yuan, Jun
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- Electronic transport in Eu1-xCaxB6
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We have measured the electrical resistivity, the magnetoresistance, the Hall effect, and the magnetization in varying temperature ranges between 0.3 and 300 K on single crystals of EuB6, CaB6, and Eu0.8Ca0.2B6. The ferromagnetic phase transition of EuB6, marked by a sharp peak in the temperature dependence of the electrical resistivity ρ(T) just below 16 K, is shown to be accompanied by a considerable increase of the effective charge carrier concentration neff. The overall features of the transport properties of Eu0.8Ca0.2B6 are similar to those of EuB6. A phase transition at 5.3 K has been established. However, the increase of neff across this phase transition by two orders of magnitude is much more pronounced than in pure EuB6. 2000 The American Physical Society.
- Paschen,Pushin,Schlatter,Vonlanthen,Ott,Young,Fisk
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- Ferromagnetism induced by Ca vacancy in CaB6
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CaB6 doped with a La atom, Ca1-cursive Greek chiLacursive Greek chiB6, is a ferromagnet, but the magnetic and transport properties of the nondoped mother material, CaB6, remain unclear. In order to reveal these we prepared a nondoped sample of CaB6 from CaO and B by the oxidation-reduction method. We observed that a sample synthesized at an appropriate temperature shows a diamagnetic magnetization, while a sample prepared at a higher temperature exhibits ferromagnetism which is one order of magnitude larger than that in La-doped CaB6. We have ascribed this appearance of ferromagnetism to Ca vacancies which were produced during the synthesis. When we compared experimental results of the electrical resistivity with the magnetic properties, we failed to find any correlation between them. From these observations, we wish to suggest that the Ca vacancy yields not only the carrier doping effect but also another effect such as inhomogeneity or lowering of local symmetry in the crystal, the latter being possibly more effective in terms of the appearance of the ferromagnetism.
- Moriwaka, Tomoaki,Nishioka, Takashi,Sato, Noriaki K.
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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
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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.
- Fujii, Hiroki,Ishitoya, Akira,Itoh, Shinji,Ozawa, Kiyoshi,Kitaguchi, Hitoshi
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- Low Temperature Synthesis and Characterization of Cubic CaB6 Ultrafine Powders
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Ultrafine calcium hexaboride (CaB6) powder was successfully synthesized at 500°C in an autoclave by using CaCl2 and NaBH 4 as the reactants. X-ray powder diffraction pattern and Raman spectra indicate the formation of CaB
- Shi, Liang,Gu, Yunle,Chen, Luyang,Yang, Zeheng,Ma, Jianhua,Qian, Yitai
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- Synthesis of Calcium Hexaboride Powder via the Reaction of Calcium Carbonate with Boron Carbide and Carbon
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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.
- Zheng, Shuqi,Min, Guanghui,Zou, Zengda,Yu, Huashun,Han, Jiande
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- The nature of Fe impurity phase in ferromagnetic CaB6
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Since the discovery of unexpected ferromagnetism in CaB6, the role of Fe impurity has been suspected, but its identity has never been properly determined. By an experimental study of the microscopic structure and related ferromagnetism observed
- Cao, Minghe,Jiang, Jun,Liu, Hanxing,Yuan, Jun
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- Calcium borohydride for hydrogen storage: Catalysis and reversibility
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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.
- Roennebro, Ewa,Majzoub, Eric H.
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- Self-catalyst growth of single-crystalline CaB6 nanostructures
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Large-scale calcium hexaboride (CaB6) nanostructures have been successfully fabricated with self-catalyst method using calcium (Ca) powders and boron trichloride (BCl3) gas mixed with hydrogen and argon. X-ray diffraction (XRD), scan
- Xu, Junqi,Zhao, Yanming,Zou, Chunyun,Ding, Qiwei
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- Ferromagnetism in lanthanum doped CaB6: Is it intrinsic?
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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.
- Mori, Takao,Otani, Shigeki
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- Unoccupied electronic states in CaB6 studied by density functional theory and EELS measurements
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The electronic structure of CaB6 has recently attracted great interest because of the discovery that this compound is unexpectedly ferromagnetic and exhibits a surprisingly large thermoelectric effect. We have concentrated on studying the unoccupied electronic states by investigating the boron 1s core-level spectroscopy. Comparison of our data from electron energy-loss spectroscopy with that from x-ray absorption spectroscopy using the total electron yield mode reveals evidence for sample inhomogeneity. The ab initio band structural method has been successfully applied to simulate the overall features found in the boron 1s absorption spectrum. Correlation with the results of a ground state energy-band calculation allows the interpretation of the spectral features in terms of the density of states of the unoccupied conduction band, in particular the identification of a feature associated with hybridization of Ca d orbitals with B p orbitals. This interpretation is confirmed by comparison with similar calculations for related boride systems. However, simulation spectral data cannot be reconciled with high-resolution experimental data at the absorption threshold. The possibility of either the failure of the theoretical method employed to account for many-body effects and/or the need for improved experimental measurements is discussed.
- Gao, Shang-Peng,Jiang, Jun,Cao, Minghe,Zhu, Jing,Yuan, Jun
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p. 214419-1-214419-9
(2004)
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- A novel strategy for reversible hydrogen storage in Ca(BH4)2
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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.
- Yan, Yigang,Remhof, Arndt,Rentsch, Daniel,Züttel, Andreas,Giri, Santanab,Jena, Puru
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p. 11008 - 11011
(2015/07/07)
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- Depth profile of iron in a CaB6 crystal
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Scanning electron microscopy, energy dispersive x-ray spectroscopy, and depth profiling with Auger electron spectroscopy were used to characterize surface iron present on a CaB6 single crystal. The crystal was grown by the aluminum flux method and displayed ferromagnetism at 300 K. For similarly prepared samples, treatment with hydrochloric acid eliminated the ferromagnetism. In previous work it was speculated that the iron is deposited onto the CaB6 surface during flux removal. The Auger depth profiles presented here reveals that the iron is indeed concentrated in the surface region.
- Meegoda, Chandana,Trenary, Michael,Mori, Takao,Otani, Shigeki
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- Electrical transport properties and small polarons in Eu1-xCaxB6
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Temperature- and field-dependent resistivity ρ(T,H) and Hall effect measurements have been carried out for the Eu1-xCaxB6 (x=0.2, 0.4, 0.6, and 0.9) compounds. The replacement of Eu with Ca invoked drastic changes in the ρ(T) although Ca and Eu are isoelectronic. While Eu0.8Ca0.2B6 showed a phase transition similar to that of pure EuB6, the ρ(T) of Eu1-xCaxB6 with x = 0.4 and 0.6 showed a rapid increase at low temperatures (T ≤ 10 K). The upturn of the ρ(T) was suppressed as the magnetic field increased, resulting in negative magnetoresistance (MR). For Eu0.1Ca0.9B6, the MR changed from positive at 10 K ≤ T ≤ 50 K to negative at T~2 K. It was found that the observed exotic ρ(T) is due to the change of the effective carrier density neff and Hall mobility μH determined from the Hall measurements. Analysis of the Hall mobility based on a small polaronic model showed that the carrier transport is dominated by hopping between the polaron sites in a nonadiabatic regime with four-site hopping for the Eu-rich side and three-site hopping for the Eu-poor side of the compounds. This polaronic scenario of the transport is consistent with the observed ρ(T,H), MR(T), neff(T,H), and μH(T) variation.
- Rhyee, Jong-Soo,Cho, B. K.,Ri, H.-C.
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- Raman scattering investigation of RB6 (R = Ca, La, Ce, Pr, Sm, Gd, Dy, and Yb)
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The excitation-energy, polarization, and temperature dependeces of Raman scattering spectra have been measured for the RB6 crystals (R=Ca 2+, La3+, Ce3+, Pr3+, Sm 2.6+, Gd3+, Dy3+, and Yb2+). In the divalent crystals of CaB6 and YbB6, the Eg phonon shows doublet, while the line shape is a broad single peak for the trivalent case. In the trivalent crystals, two kinds of the extra peaks have been clearly observed at around 200 cm-1 and at around 1400 cm -1 in addition to the Raman-active phonons and crystal electric-field (CEF) excitations. The peak intensities at around 200 cm -1 show the anomalous decrease with decreasing temperature, and this temperature dependence correlates with that of the mean-square displacement of the R ion. Furthermore, their energies decrease with the increase of the cage space due to B6. These peaks are originated from the second-order excitation of acoustic phonons. The broad peak at around 1400 cm-1 is assigned as the second-order excitation of the T2g phonon. For the T2g phonon, the energy difference and its line-shape change between the trivalent and divalent crystals suggests the existence of electron-phonon interaction. We have observed the CEF excitation in PrB 6 and have obtained the Lea, Leask, and Wolf parameters with x=0.94 and W=9.2 K.
- Ogita, Norio,Nagai, Shinji,Okamoto, Naoki,Udagawa, Masayuki,Iga, Fumitoshi,et al.
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- Preparation of W2B5-based Powder from Scheelite Concentrate
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W2B5 powder was prepared from scheelite concentrate by vacuum reduction with carbon and boron at 1100-1380°C for 1-2.5 h. Phase composition of the products was determined by x-ray diffraction and chemical phase analysis.
- Sokol,Budkina,Vlasova
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p. 787 - 790
(2008/10/08)
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