Fabrication and characterization of rare earth hexaboride single-crystal materials

Oriented single crystals of LaB₆, CeB₆ and PrB₆ have been prepared by the arc float-zone refining technique, which is described. A critique of the various possible preparation techniques is given. Wet chemical methods were used to determine bulk stoichiometries of samples prepared from starting materials of various R/B compositions (where R = rare earth). It was found that LaB_6,09, a desired composition, could be grown in single-crystal form from LaB_6.2 starting material, the excess boron compensating for preferential boron vaporization. Starting materials with B/La < 6.2 produce a B/La gradient in two-pass zone-refined specimens. The level of metallic impurities in arc zone-refined LaB₆ is lower than that reported elsewhere for laser zone-refined LaB₆. Residual oxygen present in the zone-refined specimens was found to produce second-phase inclusions, which were studied in detail. The effect of contamination of LaB₆ surfaces by evaporated refractory metals has also been investigated.     

Characterization of mechanically alloyed and pressureless sintered Al-7 wt% Si-2 wt% LaB6-2 wt% (MoSi2, WSi2) hybrid composites

The purpose of this research is to investigate the dual effect of silicide (MoSi₂ or WSi₂) and LaB₆ reinforcing particles on the microstructural and mechanical properties of Al-7 wt% Si (Al7Si) matrix. Hypoeutectic Al7Si blends prepared from elemental Al and Si powders were mechanically alloyed (MA’d) for 12 h in a planetary ball mill (at 300 rpm). Afterwards, 2 wt% silicide reinforcements (MoSi₂ or WSi₂) with various particle size distributions (micron, bimodal, submicron) were separately added into these MA’d Al7Si powders together with 2 wt% of LaB₆ particles. Powders having compositions of Al7Si, Al7Si-2 wt% LaB₆, Al7Si-2 wt% LaB₆-2 wt% MoSi₂ and Al7Si-2 wt% LaB₆-2 wt% WSi₂ were milled for 30 min using a high-energy ball mill (at 1200 rpm) in order to obtain homogeneous distribution throughout the microstructure. Compositional, microstructural and mechanical characterization studies were performed on the sintered samples. The results showed that high-energy ball milling ensured the homogeneous distribution of micron-sized MoSi₂ and WSi₂ particles within the matrix rather than those of bimodal and submicron-sized ones. Micron-sized MoSi₂ and WSi₂ reinforced hybrid composites displayed dramatically higher hardness and wear resistance than those of composites reinforced with different size of MoSi₂ and WSi₂ particles. The striking point of the study was the remarkably higher hardness and wear resistance properties of the hybrid composites compared to those of un-reinforced and only LaB₆-reinforced ones. As a conclusion, hybrid composites extremely displayed promising mechanical properties.     

Microstructure and electrical properties of the SnO2 glass composite containing Cu particles precipitated by reducing copper oxides – Effect of particle size of reducing agent

An attempt was made to improve the electrical properties of SnO₂ glass composites by dispersing metal particles having low resistivity and positive temperature coefficient of resistance (TCR) in the glass matrix. Cu metal particles were precipitated by reducing Cu₂O by adding LaB₆O as a reducing agent. The effects of LaB₆O content and particle size on the microstructure and electrical properties of the SnO₂ glass composites were monitored. When coarse LaB₆O particles were used, the amount of the precipitated metal particles was large because SnO₂ was also reduced as well as Cu₂O during firing. However, in this case, the glass composite showed a porous microstructure including large pores because of the simultaneous evaporation of SnO formed as an intermediate product by reduction of SnO₂. On the other hand, the glass composite prepared using fine LaB₆O particles showed a dense microstructure uniformly dispersed with small pores. The porosity of the glass composite decreased by increasing the LaB₆O content at first and then increased by further addition of LaB₆O. The minimum of the porosity occurred at 2 wt % and 3 wt % LaB₆O for the samples containing coarsest (4.81 m) and finest (0.15 m) LaB₆O, respectively. Electrical conductivity () and TCR of the glass composites containing LaB₆O were higher and closer to zero, respectively, than those of a LaB₆O free sample. The samples containing 2–3 wt % LaB₆O showed 5–7 times higher and 50–70% smaller TCR in comparison with the sample without LaB₆O. However, at high LaB₆O content above 3–4 wt %, decreased and TCR moved in the negative direction with increase in LaB₆O content. Especially, when coarse LaB₆O was used, the declines and TCR at high LaB₆O contents were remarkable. This was due to the decrease in the continuity of conductive paths related to the increase in number of the large pores caused by the evaporation of SnO.     

Phase relations in the LaB<Subscript>6</Subscript>-W<Subscript>2</Subscript>B<Subscript>5</Subscript> system

The LaB₆-W₂B₅ join in the ternary system La-B-W is shown to have a eutectic phase diagram with t _e= 2220°C and a eutectic composition of 30 mol % LaB₆ + 70 mol % W₂B₅. Data are presented on LaB₆-containing systems potentially attractive for designing mixed-phase ceramics.     

Zn-assisted synthesis of high-performance adsorbent for methylene blue

The rapid removal of organic dyes from water under low temperature environment offers significant advantages when water purification is required in cold regions. In this work, porous lanthanum hexaboride (LaB₆) nanoparticles were fabricated via a facile low-temperature molten metal (LTMM) method in Zn media under argon atmosphere. As is evidenced by the characterization results, porous LaB₆ can be prepared at 400?°C. Elevated reaction temperature led to the growth of nanoparticles, serious aggregation and the residue of Zn element in the final product. Notably, LaB₆ nanoparticles prepared at 600?°C by such a LTMM method presented ultrafast adsorption, with a maximum MB sorption capacity of 606.2?mg·g^?1, which was competitive with most of reported sorbents. Owing to the photothermal conversion property, LaB₆ powder not only accelerated the melting of iced MB solution, but also achieved the efficient adsorption of MB in cold weather. LaB₆ nanoparticles could be used as an attractive sorbent for dye removal from contaminated water, because of its facile synthesis process, excellent sorption performance and photothermal conversion property.     

A new route for the synthesis of submicron-sized LaB6

Submicron crystalline LaB₆ has been successfully synthesized by a solid-state reaction of La₂O₃ with NaBH₄ at 1200 °C. The effects of reaction temperature on the crystal structure, grain size and morphology were investigated by X-ray diffraction, scanning electron microscope and transmission electron microscope. It is found that when the reaction temperature is in the range of 1000–1100 °C, there are ultrafine nanoparticles and nanocrystals that coexist. When the reaction temperature elevated to 1200 °C, the grain morphology transformed from ultrafine nanoparticle to submicron crystals completely. High resolution transmission electron microscope images fully confirm the formation of LaB₆ cubic structure.     

Field ion microscopic observations of LaB6 on tungsten

The importance of field ion microscopy as a unique surface microscopic technique has been pointed out with particular reference to the lanthanum hexaboride (LaB₆)-deposited refractory metal cathodes. In the core, field ion microscopic observations of LaB₆ deposited tungsten are described. The observations are discussed in relation to the field electron emission microscopy of LaB₆/W system. The paper ends with a few comments on the scope of further study of this or a similar system using the field ion microscopy and the atom probe field ion microscope.     

Preparation and characterization of lanthanum carbide encapsulated carbon nanocapsule/lanthanum hexaboride nanocomposites

LaC₂@CNC/LaB₆ nanocomposites were prepared using a spark plasma sintering system, and their mechanical properties and the intensity of characteristic La X-rays from the composites were characterized for application as X-ray target material. Using LaB₆ as a binder, we succeeded in producing LaC₂@CNC/LaB₆ nanocomposites with a bulk density (2.91 g/cm³) and specific electric resistance (3.12 × 10^− 4 Ω cm) through solidification at 2123 K. The fracture bending strength of LaC₂@CNC/LaB₆ nanocomposites (224 MPa) was 1.5 times larger than that observed for graphite/LaB₆ composites. The most emitted characteristic La Lα X-ray from LaC₂@CNC/LaB₆ nanocomposites was 7743 counts/mm²/s in comparison with 6372 counts/mm²/s for graphite/LaB₆ composites.     

Oxidation behaviours of carbon/carbon composite with multi-coatings of LaB6-Si/polycarbosilane/SiO2

The influence of multi-coatings of LaB₆-Si/polycarbosilane/SiO₂ on the oxidation behaviour of carbon/carbon composite materials was investigated in the temperature range from 500 to 1400 °C. The additives of LaB₆-Si offered lower oxidation rates and accelerated increases in oxidation rates at temperatures below 900 °C. The coating of polycarbosilane (PCS) improved the compatibility of the coating on the carbon/carbon composite and lowered the oxidation rates of the LaB₆-Si coated composite below the transition temperature. With the SiO₂ coating, the cracks of the LaB₆-Si/PCS coating was sealed and a good oxidation resistance of the LaB₆-Si/PCS/SiO₂ coated composite was found at temperatures up to 1300 °C.     

A stable LaB6 nanoneedle field-emission electron source for atomic resolution imaging with a transmission electron microscope

Field-induced electron emission offers the best performance point electron source for electron microscopes. Herewith we report a new cold field-emission point electron source utilizing a nanoneedle made of lanthanum hexaboride (LaB₆) and its implementation in a spherical aberration-corrected transmission electron microscope (TEM). A sub-ångstrom resolution of 0.96 Å has been obtained with the TEM operated at an acceleration voltage of 60 kV and a smaller energy spread was also observed compared with the contemporary W(310) electron source. In particular, the LaB₆ nanoneedle cold field-emission electron source required no thermal flashing for continuous operation of extended hours (>100 hours) while exhibiting and sustaining a high stability in emission current (<1%). We attribute the excellent performance of the LaB₆ nanoneedle electron source to (i) a low work function of LaB₆; (ii) excellent alignment of the nanoneedle emitter by the electron and ion dual-beam processing with nanoscale precision and (iii) robustness of the nanometric structure that suppressed mechanical vibrations of the LaB₆ emitter even in a high electric field. This new LaB₆ nanoneedle cold field-emission electron source enables stable high-resolution imaging in TEM and it will also benefit cryogenic electron microscopy, scanning electron microscopy, electron beam lithography, and other electron beam technologies.     

Surface science studies of metal hexaborides

Abstract

Over 30 years of surface science research on metal hexaborides are reviewed. Of this class of compounds, lanthanum hexaboride has been the subject of the majority of the studies because of its outstanding properties as a thermionic emitter. The use of LaB₆ cathodes as an electron source stems from the unusually low work function of ～2.5 eV for the (100) surface combined with a low evaporation rate at high temperatures. Of particular interest has been the determination of the surface geometric and electronic structure responsible for the low work function and how the work function is affected by various adsorbates. The low-index faces of single crystals of LaB₆ and other hexaborides have been studied with a variety of ultrahigh vacuum surface science methods to gain a better understanding of the structure and properties of the clean surfaces as well as their interactions with gases such as O₂, H₂O and CO.     

Thermal conductivity of LaB<Subscript>6</Subscript> and SmB<Subscript>6</Subscript> in the range 6–300 K

The thermal conductivity of LaB₆ and SmB₆ crystals has been measured from 6 to 300 K. The electronic and lattice contributions to the thermal conductivity of SmB₆ and the temperature-dependent phonon mean free path in this hexaboride have been evaluated. The results indicate strong phonon scattering by structural defects, presumably those typical of hexaborides.     

LaB6 and TiB2 electrodes for the alkali metal thermoelectric converter

LaB₆ and TiB₂ electrodes for the alkali metal thermoelectric converter (AMTEC) were prepared from the respective powders by a screen-printing method and their electrode properties were investigated. Optimum values were obtained for particle size, thickness and porosity of electrode. When the vacuum level of the low-temperature side of the AMTEC increased above 10 Pa, the power density decreased remarkably. These results can be interpreted as different electrode processes in the AMTEC: (1) a charge transfer process, (2) surface diffusion on the electrode, (3) desorption from electrode particles, and (4) vapour-phase diffusion in the electrode pore. The maximum power density was 0.54 W cm^–2 (LaB₆) and 0.24 W cm^–2 (TiB₂ at 800 °C.     

Effect of annealing of LaB films investigated by Auger electron spectroscopy

LaB_{x = 1.9} films (with about eight (film A) or sixteen (film B) lanthanum monolayers) were deposited onto a tantalum substrate (at room temperature) by evaporation from an LaB₆ source and were annealed at gradually increasing temperatures. Auger electron spectroscopy (AES) was used to observe the variation in composition at the film surface. From the measurements it follows that, during annealing, changes in composition with temperature are very rapid and irreversible. These changes are associated with two process. The first is the diffusion of boron atoms towards the film surface (for film B) and towards the film interior (for films A and B) which occurs in the temperature range 640–740 K for film A and 550–850 K for film B. The second process is the rapid agglomeration of lanthanum atoms at the film surface in the temperature range 740–780 K for film A and 850–920 K for film B. The thickness of the islands of agglomerated lanthanum atoms was estimated on the basis of AES data and on the assumption that during the annealing phases of lanthanum boride (LaB₆ or LaB₄) and of lanthanum itself are formed.     

Phase relations in the LaB<Subscript>6</Subscript>-MoB<Subscript>2</Subscript> system

LaB₆-MoB_2.2 alloys have been prepared at 2600°C in high-purity helium in a vacuum induction furnace. As found by microstructural analysis and x-ray diffraction, the components of this system form a eutectic. The microstructure of the eutectic has been studied. MoB_2.2 is shown to undergo a transformation during cooling, with the precipitation of submicron-sized Mo₂B₅ and α-MoB particles. For this reason, its microhardness increases with an increase in the melting temperature of the alloys because the diffusion rate rises with temperature.     

Synthesis of LaB<Subscript>6</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocomposite powders via ball milling-assisted annealing

In this study, LaB₆–Al₂O₃ nanocomposite powders were synthesized via ball milling-assisted annealing process starting from La₂O₃–B₂O₃–Al powder blends. High-energy ball milling was conducted at various durations (0, 3, 6 and 9 h). Then, the milled powders were annealed at 1200 °C for 3 h under Ar atmosphere in order to obtain LaB₆ and Al₂O₃ phases as reaction products. X-ray diffractometry (XRD), scanning electron microscopy/energy-dispersive spectrometry (SEM/EDS) and transmission electron microscopy (TEM) techniques were utilized to carry out microstructural characterization of the powders. No reaction between the reactants was observed in the XRD patterns of the milled powders, indicating that high-energy ball milling did not trigger any chemical reactions even after milling for 9 h. LaAlO₃ and LaBO₃ phases existed in the annealed powders which were milled for 0, 3 and 6 h. LaBO₃ phase was removed after HCl leaching. 9-h milled and annealed powders did not exhibit any undesired phases such as LaAlO₃ and LaBO₃ after leaching step, and pure nanocrystalline LaB₆–Al₂O₃ composite powders were successfully obtained. TEM analyses revealed that very fine LaB₆ particles (~?100 nm) were embedded in coarse Al₂O₃ (~?500 nm) particles.     

Tensile properties of in situ synthesized (TiB + La2O3)/β-Ti composite

A novel (TiB + La₂O₃)/Ti-alloy composite was In situ synthesized through homogeneously melting in a non-consumable vacuum arc remelting furnace. Ti–35Nb–2Ta–3Zr β titanium alloy was chosen as the matrix Ti-alloy and different mass fractions of LaB₆ were chosen as additions. Microstructure observations were examined by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscopy (TEM). The phase analysis was identified by X-ray diffraction (XRD). Largest ultimate tensile strength around 580 MPa and highest elongation around 30% is obtained in 0.1% LaB₆-additioned specimen. The appearance of too many La₂O₃ particles and the reduction of oxygen in the matrix alloy also attribute much to the strength and plasticity of (TiB + La₂O₃)/Ti composites. Lower ultimate tensile strength around 526 MPa is obtained in 0.5% LaB₆-additioned specimen.     

Point-contact spectroscopy of the electron-phonon interaction in single-crystal LaB6

Point-contact spectra of single-crystal LaB₆ are obtained, yielding the energy positions of all the phonon modes up to 160 meV. A relatively strong anisotropy of the spectra is observed. It is related to the anisotropy of the phonon system. The point-contact electron-phonon interaction function and the point-contact electron-phonon interaction parameter are compared with published calculated data and a qualitative agreement is found. From the measured spectra the temperature dependence of the LaB₆ electrical resistivity and heat capacity are calculated.     

Properties of very thin La-B films deposited onto tantalum

The properties of very thin (up to about ten lanthanum monolayers) La-B films deposited onto a tantalum substrate were investigated by Auger electron spectroscopy, by measuring changes in the work function and by ion sputtering to remove successive layers of the film. The composition of the films was changed from almost pure lanthanum to stoichiometric LaB₆ and in some cases to almost pure boron.Decreases in the work function during film deposition depend on the boron content in the film and are greatest for the $\text{LaB}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$ film. We propose a qualitative explanation for this dependence. Films with boron contents up to LaB₃ are inhomogeneous in the direction perpendicular to the surface, i.e. the boron content is greatest near the substrate surface. This indicates that boron atoms are bound more strongly to the substrate atoms than to the lanthanum atoms.The work functions and compositions of the surfaces of the films were investigated during annealing in the temperature range 20–800 °C.     

Mechanical activation of crystallization of amorphous boron and synthesis of Al<Subscript>3</Subscript>Ti under cold isostatic pressing of B–Al–(LaB<Subscript>6</Subscript>–TiB<Subscript>2</Subscript>) powder

The powder mixture consisting of amorphous boron, crystalline aluminum, and a composite material LaB₆–TiB₂ is cold-isostatically-pressed under a pressure of up to 0.6 GPa and then heated to 1000°C. It is found out that almost the entire amount of amorphous boron contained in the mixture has been crystallized and the synthesis of Al₃Ti has been noted, while the crystallization of amorphous boron usually occurs at a temperature of 1500°C.