The effect of Sm-doping on optical properties of LaB6 nanoparticles

Chao, Luomeng,Bao, Lihong,Shi, Junjie,Wei, Wei,Tegus,Zhang, Zhidong

, p. 618 - 621 (2015)

Nanocrystalline particles of LaB6, SmB6 and Sm-doped LaB6 have been prepared by a solid-state reaction in order to investigate the optical properties of ternary rare-earth hexaborides. The sizes of prepared nanoparticles range from dozens to more than 200 nm, as confirmed by XRD, SEM and TEM examinations. The optical property concerning the absorption spectra was tested with ultraviolet-visible-near infrared (UV-vis-NIR) absorption spectrum. All samples exhibit high absorbance in NIR range and UV range. The increase of Sm-doping amount shifts the position of minimum absorptance value of LaB6 to the long-wave direction. Density functional theory (DFT) is employed to interpret the optical properties of Sm-doped LaB6, and results indicate that Sm 4f states change the DOS at near Fermi surface of LaB6 after Sm doping and the reduced number of conduction electrons results into the change of absorption spectra.

Peculiarities of electronic, phonon and magnon subsystems of lanthanum and samarium tetraborides

Novikov,Mitroshenkov,Matovnikov

, p. 906 - 911 (2015)

Abstract Experimental research was carried out to study the temperature dependences of heat capacity Cp(T = 2-300 K), lattice parameters a(T), and c(T), (5-300 K) of lanthanum and samarium tetraborides. A comparison with data obtained previously for LuB4 reveals the peculiar influence of lanthanide contraction and the rare-earths mass on the thermodynamic properties of rare earth tetraborides at low and high temperatures. Sharp anomalies were found in the heat capacity and thermal expansion for SmB4 at TN = 25.1 K, conditioned by the phase transition into antiferromagnetic state. The more poorly defined heat capacity anomaly around 7 K is referred to the quadrupole orbital fluctuation of the atomic magnetic moments for Sm3+ ions. The electronic, lattice, and magnetic contributions to the heat capacity and thermal expansion of samarium tetraboride were defined. Our approach makes it possible to adequately approximate the lattice components of heat capacity and thermal expansion by combining the Debye and Einstein contributions, which are based on the joint analysis of calorimetric and X-ray data. The influence of the frustration of the atomic magnetic moment system for Sm3+ ions on the thermodynamic characteristics of the samarium tetraboride magnetic phase transition was revealed.

A solvothermal approach for the preparation of nanostructured carbide and boride ultra-high-temperature ceramics

Kelly, James P.,Kanakala, Raghunath,Graeve, Olivia A.

, p. 3035 - 3038 (2010)

The use of a solvothermal process for the synthesis of tantalum carbide (TaC) and lanthanum hexaboride (LaB6) powders in fused-quartz test tubes is reported in order to demonstrate the synthesis of these powders using thermal and chemical ignition techniques and to prove that the process is of a self-propagating high-temperature synthesis type, obviating the need for an autoclave. X-ray powder diffraction showed phase pure powders with crystallite sizes of 25 and 80 nm, while dynamic light scattering showed average particle sizes of 97 and 130 nm, for TaC and LaB6, respectively. The data demonstrates that the powders have a very low level of agglomeration. Scanning electron microscopy shows that the TaC powders have a spherical morphology, while the LaB6 powders have a mixture of cubic and spherical morphologies.

Shiota, M.,Tsutsumi, M.

, (1980)

Low-temperature synthesis of nanosized metal borides through reaction of lithium borohydride with metal hydroxides or oxides

Pan, Wei Yuan,Bao, Qian Wen,Mao, Yang Jun,Liu, Bin Hong,Li, Zhou Peng

, p. 666 - 672 (2015)

In this study, we report a novel and facile synthesis approach of boron-rich transition metal borides such as LaB6 and TiB2 through reaction of lithium borohydride (LiBH4) with corresponding metal hydroxide or oxide at temperatures below 600 °C. A fast endothermic reaction occurred at around 350 °C in the ball milled mixture of 6LiBH4 + La(OH)3 or 12LiBH4 + La2O3, efficiently producing crystalline LaB6 nanoparticles of a size smaller than 100 nm. In comparison, the reaction of LiBH4 with TiO2 proceeded within a wide temperature range from 120 °C to 500 °C, resulting in the formation of nanocrystalline TiB2 of only a few nanometers. This synthesis method proved to be a facile and general route to fabricate nanosized transition metal borides.

Korsukova, M. M.,Gurin, V. N.,Lundstroem, T.,Tergenius, L.-E.

, p. 73 - 82 (1986)

Fabrication and characterization of ZrB2-based ceramic using synthesized ZrB2-LaB6 powder

Spring, Alex,Guo, Wei-Ming,Zhang, Guo-Jun,Wang, Pei-Ling,Krstic, Vladimir D.

, p. 2763 - 2765 (2008)

ZrB2-LaB6 powder was obtained by reactive synthesis using ZrO2, La2O3, B4C, and carbon powders. Then ZrB2-20 vol% SiC-10 vol% LaB6 (ZSL) ceramics were prepared from co

Low temperature synthesis of low thermionic work function (LaxBa1-x)B6

Hasan,Cuskelly,Sugo,Kisi

, p. 67 - 72 (2015)

This study presents investigations of the microstructure, morphology and emission properties of the promising thermionic material (LaxBa1-x)B6. The material was synthesised by solid-state reaction without post-synthesis purifications. Powder X-ray diffraction revealed that samples prepared at a temperature ≥1500 °C had formed a significant proportion of solid solution (above 54 mass%). Subsequent sintering at 1950 °C caused the formation of a mixture of three solid solutions with the dominant phase being (La0.31Ba0.69)B6 ～85% (by mass). The Richardson work function and emission constant for this boride mixture were found to be 1.03 eV and 8.44 × 10-6 A cm K-2 respectively.

The preparation and composition design of boron-rich lanthanum hexaboride target for sputtering

Chen, Defang,Min, Guanghui,Wu, Yan,Yu, Huashun,Zhang, Lin

, p. 380 - 386 (2015)

Lanthanum Hexaboride (LaB6) nano-film has been proved to be promising transparent thermal insulation material, while its properties are limited on purity and composition. High-purity LaB6 polycrystalline powder was prepared through boron carbide reduction method in this work. A series of techniques such as scanning electron microscopy, X-ray diffraction, laser particle analyzer and inductively coupled plasma emission spectrometer were employed to characterize LaB6 powder. As raising the content of La2O3 in reactants, more uniform, finer (2.686 μm) and purer (99.5139 wt%) LaB6 powder is prepared, with only 0.4434 wt% residual B4C. The density of targets increases with the rise of sintering temperature and the extension of sintering time, while crystallite size increases simultaneously with the extension of sintering time. The introduction of B powder in target is conductive to sintering process, increasing hardness and flexural strength of targets. X-ray photoelectron spectrometer was used to characterize the composition and microstructure of LaB6 nano-film which is tentatively considered to be composed of LaB6 nanocrystalline and amorphous microstructure of La and B atoms. The film LaB6.0627±0.02 was obtained when the ratio of B and La of sputtering target reached 12.5. The thickness and deposition rate decrease with the increase of B content in targets.

Self-catalyst growth of LaB6 nanowires and nanotubes

Xu, Junqi,Zhao, Yanming,Zou, Chunyun

, p. 138 - 142 (2006)

LaB6 nanowires have been successfully fabricated with self-catalyst method using lanthanum (La) powders and boron trichloride (BCl3) gas mixed with hydrogen and argon. Our results show that lanthanum hexaboride (LaB6) nanowires are 20-200 nm thick and several micrometers long. Transmission electron microscopy (TEM) reveals that the nanowires are highly crystalline. In our experiment LaB6 nanotubes have also been observed for the first time. Our results indicate that LaB6 nanotubes are polycrystalline with 200-400 nm thick and around several hundred nanometers to one micrometer long. A growth mechanism based on the self-catalyst process is proposed for the formation of the nanowires.

Structural, magnetic, and thermionic emission properties of multi-functional La1-xCaxB6 hexaboride

Bao, Lihong,Qi, Xiaoping,Bao, Tana,Tegus

, p. 332 - 338 (2017/10/17)

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.