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This study investigated the discharge voltage characteristics of an argon-fed lanthanum hexaboride heaterless hollow cathode to assess the influence of flow rate, discharge current, background pressure, and applied magnetic field strength. Decreasing the flow rate from 15 to 3 sccm led to a considerable increase in discharge voltage and peak-to-peak oscillation, particularly for flow rates below 5 sccm. Subsequently, variation in discharge current was tested at 4–7 A; this test revealed that the discharge voltage decreases from 53 to 48 V as the discharge current increases, while the peak-to-peak oscillation increases by approximately 2 V with the rise in discharge current. At high background pressures (8.1 × 10−4 Torr), the discharge voltage decreased by 15 V, and the peak-to-peak oscillation was maintained at 5 V. Furthermore, the spectral analysis of the discharge voltage indicated the occurrence of high-energy oscillations at 10–500 kHz owing to ionization instability. The discharge voltage decreased when the strength of an externally applied axial magnetic field increased from 0 to 118 G. Such a result can be attributed to increased ionization (caused by the applied magnetic field) in the emitter and cathode-keeper region, thereby decreasing sheath potential on the emitter surface. • Discharge voltage is used as an important parameter for analyzing the cathode performance. • When the gas flow rate decrease, the discharge voltage fluctuation increases. • High background pressure leads to the disappearance of voltage oscillation amplitude variation. • Applying a magnetic field causes a decrease in discharge voltage, and the oscillation of the discharge voltage is suppressed. • The magnetic field reduces discharge voltage by increasing the anode's surface area available for current reception. [ABSTRACT FROM AUTHOR]

This study measured the two-dimensional orifice plate surface temperature of a 20-A-class LaB6 hollow cathode using a near-infrared two-color radiation thermometer. The thermometer technique resolved the orifice plate temperature with an accuracy of approximately 166 ℃. With this technique, the surface temperature of the orifice plate was found nearly uniform, with no observed temperature gradient on the orifice plate. Consequently, the operational characteristics of the hollow cathode are discussed in relation to the average surface temperature. Data analysis revealed that the orifice plate temperature was not sensitive to the mass flow rate; conversely, it was highly sensitive to the discharge current. Moreover, for the same input power, the orifice plate temperature was higher in the high-current, low-voltage mode also known as the spot mode indicating efficient emitter heating. The measurement system developed in this study provides a more precise evaluation of the orifice plate temperature and allows for estimating the internal conditions of a hollow cathode, thus facilitating more appropriate design evaluations under various operating conditions. [ABSTRACT FROM AUTHOR]

A simple approach dealing with thermal oxidation at atmospheric pressure was used to synthesize copper oxide nanowires (CuO-NWs) on copper substrate. The vertically oriented CuO-NWs were used for further subjected to deposition of ultrathin layer (∼20 nm) of lanthanum hexaboride (LaB 6) using the electron beam evaporation technique. The observation of the characteristic vibrational mode of LaB6 in the Raman spectrum of LaB6-coated CuO-NWs (LaB6@CuO-NWs) revealed the existence of LaB6 in the deposited sample. Furthermore, confirmation of LaB6 coating on CuO-NWs was corroborated by X-ray photoelectron spectroscopy (XPS) analysis. The field electron emission (FEE) characteristics of pristine CuO-NWs and LaB6@CuO-NWs were investigated in a planer diode geometry at a base pressure of ∼ 1 × 1 0 − 8 mbar. The values of turn-on and threshold fields (defined at emission current density of 1 and 10 μ A/cm2, respectively) were found to be 5.4 V/ μ m and 1.6 V/ μ m (for pristine CuO-NWs emitter) and 6.75, and 2.85 V/ μ m (for LaB6@CuO-NWs emitter). Interestingly, a noticeably large emission current density of ∼ 1 6 7 5 μ A/cm2 has been extracted from the LaB6@CuO-NWs emitter at a relatively lower applied field of 5.8 V/ μ m. The Fowler–Nordheim (FN) plots of both the emitters showed nonlinear behavior, indicative of their semiconductive nature. Furthermore, both the emitters showed relatively stable emission behavior tested at a pre-set value of 1 μ A more than 3 h. The enhanced FEE behavior of LaB6@CuO-NWs emitter is attributed to the modulation of electronic properties due to LaB6-CuO interface-induced effects. Although the promising FEE behavior of LaB6@CuO-NWs emitter warrants its potential for practical applications, further improvement can be achieved via fabricating an array emitter due to CuO NWS and optimizing the thickness of LaB6 coating. [ABSTRACT FROM AUTHOR]

Self-heating thermionic hollow cathodes are essential components in modern plasma thrusters. To fully understand their operation, three interdependent physical domains must be considered: plasma discharge physics, thermal response of the cathode structure, and chemical evolution of plasma exposed surfaces. In this work, we develop the first self-consistently coupled plasma–thermal–chemical simulation platform for hollow cathode operation using lanthanum hexaboride (LaB 6) and Xe and study its performance against our experimentally determined temperature measurements. Results show that the customary assumptions of single-step resonant neutralization and full energy accommodation in ion-surface collisions fail to reproduce our empirical observations. We propose a two-step neutralization mechanism that consists of resonant neutralization to the first excited state of xenon followed by Auger de-excitation to the ground state, along with system specific accommodation factors. In this way, the agreement between the results of the simulations and experiments was achieved. These fundamental processes could govern neutralization in other cathode technologies where low work function emitters are employed and should therefore be accounted for in physical models. In addition, the new simulation platform allows us to better estimate the equilibrium work function of LaB 6 hollow cathode emitters. In the cathode studied here, we found that the effective work function is 2.25 eV, which is significantly lower than previous estimates, and leads to better than expected cathode material performance with important implications for space missions. [ABSTRACT FROM AUTHOR]

The development of high-entropy borides with combined structural and functional performance holds untold scientific and technological potential, yet relevant studies have been rarely reported. In this work, we report nanocrystalline (La0.25Ce0.25Nd0.25Eu0.25)B6 high-entropy rare-earth hexaboride (HEReB6-1) ceramics fabricated through the high-pressure sintering of self-synthesized nanopowders for the first time. The as-fabricated samples exhibited a highly dense (96.3%) nanocrystalline (94 nm) microstructure with major (001) fiber textures and good grain boundaries without any impurities, resulting in a remarkable mechanical, electrical, and thermionic emission performance. The results showed that the samples possessed outstanding comprehensive mechanical properties and a high electrical resistivity from room temperature to high temperatures; these were greater than the average values of corresponding binary rare-earth hexaborides, such as a Vickers hardness of 23.4 ± 0.6 GPa and a fracture toughness of 3.0 ± 0.4 MPa•m1/2 at room temperature. More importantly, they showed high emission current densities at elevated temperatures, which were higher than the average values of the corresponding binary rare-earth hexaborides. For instance, the maximum emission current density reached 48.3 A•cm−2 at 1873 K. Such superior performance makes the nanocrystalline HEReB6-1 ceramics highly suitable for potential applications in thermionic emission cathodes. [ABSTRACT FROM AUTHOR]

A 100-W class low power arcjet thruster using a hollow cathode was proposed, with the goal of reducing cathode erosion. The hollow cathode's emitter was lanthanum hexaboride expecting its excellent thermionic emission performance. The experiment used argon as a propellant and compared two different emitter lengths. The estimated emitter temperature was almost the same regardless of the emitter length, however, due to of the efficient heat exchange with the longer arc plasma region, the longer emitter provided better performance with higher discharge voltage. Additionally, the hollow cathode was compared to the conventional rod cathode, and the hollow cathode was found to have some advantages. The specific impulse of the hollow cathodes was higher than that of the rod cathode at low flow rate. The results showed that the hollow cathode is best for high specific power and high specific impulse operation. The unstable transition between low- and high-voltage modes has been observed in the rod cathode, but not in the hollow cathode configurations. Helium was put to the test as a propellant and compared to argon for simulate the future use of hydrogen as a propellant. The thrust efficiency was lower due to the high discharge voltage and significant heat loss, but the specific impulse was higher as expected. No severe recession or erosion of the emitter was observed after the test campaign, but some surface color change was seen which may affect in the long-term operation. • Arcjet using LaB 6 hollow cathode was proposed to improve cathode erosion. • Long emitter provides high efficiency due to long arc length. • High Isp at low flow rate and stable discharge compared with rod cathode. • Operable with He with higher specific impulse but lower efficiency. • No sever erosion of emitter but surface color changes were observed. [ABSTRACT FROM AUTHOR]

The processes involved in obtaining lanthanum hexaboride by boron thermal reduction using heat-treatment in a vacuum furnace and spark plasma sintering were investigated. Single-phase powder of lanthanum hexaboride was obtained at 1800 – 1900°C with excess boron content in the batch mix equal to 20 wt%. Spark plasma sintering allows the synthesis temperature of the single-phase lanthanum hexaboride powder to be lowered to 1700 and 1600°C with excess boron content 10 and 20 wt.%, respectively, in the batch. [ABSTRACT FROM AUTHOR]

While traditional noble metal (Ag, Au, and Cu) nanoparticles are well known for their plasmonic properties, they typically only absorb in the ultraviolet and visible regions. The study of metal hexaborides, lanthanum hexaboride (LaB₆) in particular, expands the available absorbance range of these metals well into the near-infrared. As a result, LaB₆ has become a material of interest for its energy and heat absorption properties, most notably to those trying to absorb solar heat. Given the growing popularity of LaB₆, this review focuses on the advances made in the past decade with respect to controlling the plasmonic properties of LaB₆ nanoparticles. This review discusses the fundamental structure of LaB₆ and explains how decreasing the nanoparticle size changes the atomic vibrations on the surface and thus the plasmonic absorbance band. We explain how doping LaB₆ nanoparticles with lanthanide metals (Y, Sm, and Eu) red-shifts the absorbance band and describe research focusing on the correlation between size dependent and morphological effects on the surface plasmon resonance. This work also describes successes that have been made in dispersing LaB₆ nanoparticles for various optical applications, highlighting the most difficult challenges encountered in this field of study.

A novel seamless approach combining first‐principles calculations and electromagnetic device simulations is demonstrated to assess an appropriate compound material, together with an optimized device geometry, for high‐temperature infrared transducers. The electronic structures and dielectric properties of three distinct classes of materials with metallic bonds, ionic bonds, and covalent bonds are theoretically examined, ranging from elemental metals to nitrides, carbides, and borides. Among the representative candidates, CeB6 is identified as the optimal selection since the ab initio theory results show that it exhibits a low‐loss plasmonic response in the near‐infrared region (below 1.7 eV), providing a promising platform for infrared surface plasmon photonics. As a proof‐of‐concept, a metal‐insulator‐metal stripe array is simultaneously designed to examine the potential device applications for photothermal transducers. With this approach, and as a refractory conductor, CeB6 is predicted to be a promising candidate for versatile thermoplasmonic applications. It is believed that our one‐stop material‐device optimization scheme can be readily extended to high‐throughput combinatorial investigations to search for novel compound plasmonic materials and devices in a wider range of existing materials. [ABSTRACT FROM AUTHOR]

A lanthanum hexaboride (LaB6)-coated D-shape fibre was successfully employed in an erbium-doped fibre laser cavity as a saturable absorber (SA) for pulse generation via the Q-switching mechanism. The Q-switched laser was obtained with an operation wavelength of 1561.4 nm within a pump power range between 82.0 and 97.6 mW. The maximum pulse rate of 57.41 kHz, minimum pulse duration of 4.49 µs and maximum pulse energy of 73.94 nJ were obtained at a 97.6 mW pump. Our results indicate that LaB6 is a promising SA material used for generating a stable and low-cost pulsed laser. [ABSTRACT FROM AUTHOR]

A eutectic alloy in the LaB6–VB2 system was obtained by the cold crucible method. The ratio of components in the initial powder mixture was 40:60 mol%. Two-phase eutectic regions in the structure of the alloy are observed and are an LaB6 matrix filled with cylindrical VB2 fibers up to 2 μm in diameter or extended VB2 crystals with a complex cross-section and an equivalent diameter of up to 10 – 15 μm. The Vickers hardness and the fracture toughness were determined. The relationships between these characteristics and the load on the indenter, the direction of load application, and the type of eutectic structure were established. The maximum value of crack resistance (8.6 MPa·m1/2) and high hardness (20.0 GPa) were recorded for an alloy with a rod eutectic structure when a 2-N indenter load was applied parallel to the crystallographic c axis of VB2. The greatest hardening effect at loads of both 2 N and 30 N, expressed in a combination of high hardness (20.5 GPa at 2 N and 20.0 GPa at 30 N) and crack resistance (6.5 MPa·m1/2 at 2 N and 4.9 MPa·m1/2 at 30 N), was demonstrated by an alloy with a coarse conglomerate eutectic structure. [ABSTRACT FROM AUTHOR]

Lanthanum hexaboride (LaB₆) has become a material of intense interest in recent years due to its low work function, thermal stability and intriguing optical properties. LaB₆ is also a semiconductor plasmonic material with the ability to support strong plasmon modes. Some of these modes uniquely stretch into the infrared, allowing the material to absorb around 1000 nm, which is of great interest to the window industry. It is well known that the plasmon of LaB₆ can be tuned by controlling particle size and shape. In this work, we explore the options available to further tune the optical properties by describing how metal vacancies and Eu doping concentrations are additional knobs for tuning the absorbance from the near-IR to far-IR in La1-xEuxB₆ (x = 0, 0.2, 0.5, 0.8, and 1.0). We also report that there is a direct correlation between Eu concentration and metal vacancies within the Eu1-xLaxB₆.

Optical properties of lightly absorbing LaB6 nanoparticles (NPs) derived from mechanical pulverization have been studied using the effective medium theory (EMT). EMT calculations are found to agree with observed optical profiles remarkably if both Drude and bound-electron terms of dielectric functions are modified for NPs from bulk values. In LaB6 NPs, the imaginary part of dielectric functions increases and the energy of initial rise in joint density of states redshifts as compared to bulk values. Origin of these behaviors has been examined in terms of lattice defects, in addition to the conventional intrinsic size effects. Lattice defects in LaB6 NPs have been identified as modest lattice expansion using x-ray diffraction and transmission electron microscopy. The observed lattice expansion is found to decrease the band gap by decreasing the anti-bonding energy of La-5d/B-2p hybridized orbitals by first-principles calculations using sX-LDA. It is concluded that, unlike conventional belief, dielectric functions can be modified from bulk values in NPs whose average particle size amply exceeds the electron mean free path. [ABSTRACT FROM AUTHOR]

The growth of highly crystalline LaB6 films with an excellent optical response and low loss is reported, which will be useful for high‐performance photothermal device applications when combined with their inherent refractory properties. Optimum growth parameters for realizing uniaxial and coherent LaB6 thin films, exhibiting an excellent plasmonic response for near‐ to mid‐infrared device applications, are established. Numerical electromagnetic simulations of the epitaxial LaB6 nanostructures revealed that the electromagnetic field at the LaB6 surface can be as high as that of the Au nanostructures. Furthermore, the LaB6 nanostructures show resonance in the visible (red) to mid‐infrared region comparable to those of Au with the added advantage of improved temperature stability that can withstand harsh photothermal device operations. [ABSTRACT FROM AUTHOR]

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To establish the mechanism of high emission efficiency of lanthanum hexaboride, which is one of the most widely used electron emitters, a comparative analysis of the state of La and B atoms in La–B binary film systems, on the one hand, and in individual La and B films, on the other hand, is performed when studying the state of carbon-monoxide molecules adsorbed on their surface as test particles sensitive to the details of the electronic and atomic structure of the adsorbent. It is shown that the formation of the La–B dipole layer on the surface is the main reason for the high emission efficiency of lanthanum hexaboride. Subnanometer La–B film systems can be promising for the use as electron emitters with a high degree of spatial localization. [ABSTRACT FROM AUTHOR]

For thermionic emission rates exceeding the incident plasma electron flux, recent theory proposes an inverse sheath regime, with promising properties for future application in fusion edge plasmas. With the aim of inducing thermionic emission in fusion-relevant plasma conditions, several lanthanum hexaboride probes were heated in the linear plasma generator Magnum-PSI.During exposures at low plasma power and additional pulsed laser heating, the probe’s floating potential was reduced by up to 12%, providing a possible indication of thermionic emission. However, these observations coincided with rapid erosion of probe material, attributed to enhanced lanthanum self-sputtering. During follow-up experiments with helium plasmas at electron temperatures around 1eV, the lanthanum ion impact energy and sputtering yield were reduced, and rapid erosion was avoided, thus confirming the thesis of self-sputtering.A parameter scan of plasma power resulted in LaB 6 surface temperatures up to 2450°C, exceeding the theoretical inverse sheath threshold temperature by over 300°C. However, the probe’s floating potential did not deviate from reference measurements using a probe with high electronic work function, indicating absence of strong thermionic emission. This apparent discrepancy is attributed to the effects of probe surface modifications as observed during these experiments: impurity deposition, erosion and cavity formation. These modifications possibly affected the LaB 6 electronic work function, thereby keeping the inverse sheath threshold out of reach.In conclusion, although LaB 6 has one of the lowest work functions available, the inverse sheath threshold conditions could not be reached with the present setup in Magnum-PSI. Surface modifications thus do form a limiting factor for the application of LaB 6 in fusion-relevant plasma conditions. Moreover, the window of stable operation for LaB 6 in dense hydrogen plasmas is limited below ∼1.5eV, and does not overlap with the conditions expected in the edge region of future fusion devices like ITER.

We have demonstrated performance-improved vertical diamond Schottky barrier diode (SBD) by introducing a lanthanum hexaboride (LaB6) interfacial layer between Zr and diamond. The composition of LaB6 was characterized using X-ray photoelectron spectroscopy. The fabricated diamond SBD with LaB6 delivers a rectification ratio of 1010, a Schottky barrier height (SBH) of 1.53 eV, a saturation current density of 1.82 × 10−19A/cm2, and a shunt resistance of 2.67 × 1012 Ω. These results indicate that with the inclusion of LaB6, diamond SBD exhibits improved SBH and reverse breakdown capability. Additionally, the trap-limited space-charge-limited current (SCLC) remains the determinant transport mechanism for diamond SBD with LaB6. Meanwhile, the interface states density is significantly reduced due to the effect of LaB6. These results suggest the great potential of vertical Zr/LaB6/p-diamond SBD as future high-performance power rectifiers. [ABSTRACT FROM AUTHOR]

A powder mixture of eutectic composition in the LaB6–NbB2 system has been prepared by carboborothermic reduction of a La(OH)3 + NbO0.5–2(OH)4–1 hydroxide mixture in vacuum at an isothermal holding temperature of 1650°C. The hydroxide mixture was prepared by reverse coprecipitation in a suspension of amorphous boron and carbon from aqueous solutions of lanthanum nitrate, niobium fluoride, and niobium oxyfluoride. We have examined the effect of the boron-to-carbon ratio on the elemental and phase compositions of the boride mixture. The average particle diameter of the synthesized borides has been determined to be 200–250 nm. The powders have been sintered at 1700°C to a relative density of 87% and arc-remelted. The resultant rodlike eutectic structures had a NbB2 rod diameter near 600 nm. The structure of the solidified materials has been studied using scanning electron microscopy, X-ray microanalysis, and X-ray diffraction, and the eutectic composition has been accurately determined (58 mol % LaB6). [ABSTRACT FROM AUTHOR]

Detailed results from field emission studies of lanthanum hexaboride (LaB6) coated multiwalled carbon nanotube (MWCNT) films, pristine LaB6 films, and pristine MWCNT films are reported. The films have been synthesized by a combination of chemical and physical deposition processes. An impressive increase in field enhancement factor and temporal stability as well as a reduction in turn-on field and threshold field are observed in LaB6-coated MWCNTs compared to pristine MWCNT and pristine LaB6 films. Surface morphology of the films has been examined by scanning electron microscopy. Introduction of LaB6 nanoparticles on the outer walls of CNTs LaB6-coated MWCNTs films is confirmed by transmission electron microscopy. The presence of LaB6 was confirmed by X-ray photoelectron spectroscopy results and further validated by the Raman spectra. Raman spectroscopy also shows 67% increase in defect concentration in MWCNTs upon coating with LaB6 and an upshift in the 2D band that could be attributed to p-type doping. Ultraviolet photoelectron spectroscopy studies reveal a reduction in the work function of LaB6-coated MWCNT with respect to its pristine counterpart. The enhanced field emission properties in LaB6-coated MWCNT films are correlated with a change in microstructure and work function. [ABSTRACT FROM AUTHOR]

Polyelemental solid solutions of hexaborides in the La–Ce–Sm–B system are obtained by the solid-phase borothermal reduction of a mixture of rare-earth oxides in a 10–2 Pa vacuum at a temperature of 1450 to 1650°C. The morphology of solid solution particles is studied using scanning electron microscopy. The average particle size of the solid solutions ranged from 200 to 400 nm. It is found that the temperature of the onset of the secondary recrystallization of solid solutions lies in the range 1550–1650°С. The phase and chemical composition of the powders is determined by the methods of X-ray phase and micro X-ray spectral analyses. The size of the crystallites and parameters of the crystal lattice are determined. The dependence of the crystal lattice parameters on the concentration of the substituent atoms demonstrates a positive deviation from Vegard's law. [ABSTRACT FROM AUTHOR]

We have demonstrated the feasibility of lanthanum hexaboride powder synthesis from a chloride–oxide melt via electrochemical codeposition of its components on an inert cathode. The synthesis temperature, electrolyte composition, and current density have been optimized. The phase composition of the synthesized powder has been determined by X-ray diffraction. Using scanning electron microscopy, the powder has been shown to have cubic morphology and its particle size has been determined. The particle size distribution of the powder has been determined by particle size analysis. [ABSTRACT FROM AUTHOR]

C f /LaB 6 preform was used to prepare LaB 6 doping C/C-ZrC-SiC composites by precursor infiltration and pyrolysis method, and effects of LaB 6 on the microstructure and ablation resistance of C/C-SiC-ZrB 2 -LaB 6 composites were investigated. Results show that LaB 6 was reacted with Zr-precursor or its products to generate ZrB 2 and other compounds. Thanks to the reactive sintering effect of LaB 6 , a compact ceramic skeleton was established in the substrate, which played a vital role in densifying and resisting ablation. The oxidation of the evenly-distributed La- and Zr-compounds produced the homogeneous oxide scale. The ternary phases of La 2 Si 2 O 7 , La 0.71 Zr 0.29 O 1.65 and La 2 Zr 2 O 7 were participated in the formation of the oxide scale, evolving the layer from SiO 2 to SiO 2 -La 2 Si 2 O 7 and then to SiO 2 -La 2 Si 2 O 7 -La 0.71 Zr 0.29 O 1.65 -La 2 Zr 2 O 7 -ZrO 2 layer. After plasma ablation for 360 s, the mass and linear ablation rates were 0.3848 mg/s and 0.3694 μm/s, respectively. The seamless multi-phase layer can effectively prevent the composites from long-time ablation. [ABSTRACT FROM AUTHOR]

In this study, silicon carbide-lanthanum hexaboride (SiC–LaB 6) and silicon carbide–lanthanum hexaboride–zirconium boride (SiC–LaB 6 –ZrB 2) ceramics were fabricated by spark plasma sintering at 1900 °C, and their ablation resistance was tested under plasma flames over 2300 °C. The results indicate that the SiC–LaB 6 –ZrB 2 ceramic exhibits better ablation resistance than the SiC–LaB 6 ceramic. After ablation under the plasma flame for 60 s, the mass and linear ablation rates of the SiC–LaB 6 ceramic were 15.83 μg/s and 1.08 μm/s, respectively, while those of SiC–LaB 6 –ZrB 2 were -8.42 μg/s and -0.27 μm/s. With the addition of ZrB 2 , SiC–LaB 6 –ZrB 2 ceramic attained a high density and fewer inner oxygen diffusion channels. Moreover, the ZrO 2 –La 2 O 3 –SiO 2 oxide scale with good self-healing ability and excellent stability was formed in the ablation centre, which can retard the further oxidation during ablation. [ABSTRACT FROM AUTHOR]

In this study, first of all, sub-micron LaB6 reinforcing particles were synthesized in the size range of 100–500 nm via mechanochemical synthesis and leaching steps. Afterwards, Al-7 wt% Si matrix alloy and 2 wt% laboratory synthesized LaB6 powders were mechanically alloyed (MA'd) at different durations to constitute Al-7 wt% Si/2 wt% LaB6. Followed by compaction under a pressure of 450 MPa in a uniaxial cold press, samples were sintered at 570 °C for 2 h in flowing Ar gas atmosphere. Microstructural characterizations of the powders and sintered samples were performed using X-Ray Diffractometry (XRD) and Scanning Electron Microscopy (SEM) techniques. Effects of both MA (0, 1, 4 and 8 h) and LaB6 reinforcement (0 and 2 wt%) on the mechanical properties of the composites were determined in terms of microhardness and dry sliding tests. Both MA process and LaB6 reinforcement were found to make positive contribution to the hardness and wear resistance values. As-blended and sintered Al-7 wt% Si matrix alloy had a hardness value of 0.43 GPa. On the other hand, 8 h MA'd and sintered Al-7 wt% Si/2 wt% LaB6 composite had the highest hardness (0.97 GPa) and the 4 h MA'd one had the lowest wear rate (3.25 × 10−6 mm3/N mm) among all the sintered samples. The hardness and wear resistance values were increased by approximately 2 times in the 4 h MA'd and sintered Al-7 wt% Si-2 wt% LaB6 composite compared to its 1 h MA'd counterpart. [ABSTRACT FROM AUTHOR]

The development of effective anti-reflective coatings of electromagnetic radiation is a necessary basis to solve a wide range of fundamental and applied problems. We studied the conditions for the preparation of lanthanum hexaboride (LaB6) films by electron beam deposition on Al2O3, AlN, ZrO2, Si, and W substrates, and CeB6 and W coatings. The phase and elemental composition, the microstructure and optical characteristics of samples obtained under various deposition conditions were studied. The relationship between the reflection spectra and the surface roughness of the samples was investigated. The region of the electromagnetic spectrum in which the obtained films can be used as anti-reflection coatings is determined. [ABSTRACT FROM AUTHOR]

The structure and phase composition of samples of a ceramic material fabricated from lanthanum hexaboride by the sintering method were investigated. It was found that the formation of a high-quality consolidated structure with closed pores is observed at sintering temperature 1850°C. It is shown that the addition of nickel intensifies the sintering process and levels the influence of the sintering temperature on the grain size and volume fraction of the pores. Nickel in this case is found in the base material of the LaB6 grains as well as in the form of finely disperse intermetallics on grain boundaries. [ABSTRACT FROM AUTHOR]

Field evaporation of LaB[sub 6] single crystals resulting in a large variety of ionized clusters of LaB[sub n][sup m+] type, where n=1,2,...,6 and m=1, 2, 3, 4, is investigated in a collaborative experimental and theoretical study. The aim of the present work was to compare the measured appearance probability of various clusters in a flux with the calculated energetic stability of corresponding free clusters. The experiments were carried out with a time-of-flight atom probe. The appearance probability of cluster ions in the atom probe spectra was estimated by calculating its rating in many selected spectra. The methods for determining such ratings have been developed and discussed. The theoretical calculations are based on ab initio quantum chemical Hartree–Fock self-consistent field and configuration interaction methods. A double ζ basis set was used to determine the cluster geometries, energies, and relative stability. For the inner-shell electrons of lanthanum we have used relativistic compact effective potentials. The geometry optimization was carried out using the analytical gradient method. The experimentally obtained ratings of some ionic species were compared with the energetic stability of free clusters calculated as described previously. The main conclusion of the present study is that unstable clusters, e.g., LaB[sup 3+], or almost unstable LaB[sub n][sup m+] clusters were preferably observed in the ion flux. The physical explanation of this phenomenon lies in the possibility of a stable cluster to redistribute its interatomic bonds and thus to remain attached to the surface. An unstable cluster does not have this characteristic and as a result this must leave the surface. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the thermoelectric power and direct current conductivity of La[sub 2/3]TiO[sub 3-sigma]. Negativity of the thermoelectric power between 80 and 350 K; Correlation between the measured thermoelectric power and temperature; Indication that the charge carrier in the material is a small polaron; Observation of a hopping process involving small polarons.

Eutectic LaB6–ZrB2 powder was synthesized by borothermal reduction of an La(OH)3–ZrO(OH)2 mixture under vacuum with isothermal curing at 1200 – 1600°C. The mixture of hydroxides was prepared via coprecipitation from aqueous solutions of La and Zr nitrates in a suspension of amorphous boron. The effect of an excess of B on the phase and elemental compositions of the boride mixture was studied. The particle size of the obtained powders depended on the synthesis temperature. [ABSTRACT FROM AUTHOR]

Deals with an experimental study of large lanthanum hexaboride cathodes and their emission properties. Description of experiment; Results and conclusion.

Boron-rich plasmas have been obtained using a LaB6 target in a high power impulse sputtering (HiPIMS) system. The presence of 10B+, 11B+, Ar2+, Ar+, La2+, and La+ and traces of La3+, 12C+, 14N+, and 16O+ have been detected using an integrated mass and energy spectrometer. Peak currents as low as 20 A were sufficient to obtain plasma dominated by 11B+ from a 5 cm planar magnetron. The ion energy distribution function for boron exhibits an energetic tail extending over several 10 eV, while argon shows a pronounced peak at low energy (some eV). This is in agreement with models that consider sputtering (B, La) and gas supply (from background and 'recycling'). Strong voltage oscillations develop at high current, greatly affecting power dissipation and plasma properties. [ABSTRACT FROM AUTHOR]

Abstract In this study, x wt% LaB 6 (x = 0, 2, 5 and 10) particulate-reinforced eutectic Al-12.6 wt% Si powders and composites were prepared from commercial Al, Si and LaB 6 starting materials via powder metallurgy methods. Mechanical alloying (MA) was conducted on the blended powders using a high-energy ball mill (at 1060 cycles/min) for 2, 4 and 8 h. Mechanically alloyed (MA'd) powders were subjected to cold pressing (CP, under 450 MPa) and spark plasma sintering (at 450 °C for 180 s) processes. Powder particle morphologies changed from flaky to semi-equiaxial shape after MA duration of 4 h. Calculations performed through the XRD patterns of Al phase revealed that the average crystallite size increased and the average lattice strain decreased with increasing MA duration and LaB 6 content. Spark plasma sintered (SPS'd) samples were examined in terms of composition, microstructure, densification and mechanical properties (hardness, yield strength, compressive strength and wear volume loss). Studies revealed that 4 h was the optimal MA duration and 10 wt% was the optimal reinforcement content for the LaB 6 particulate-reinforcedAl-12.6 wt% Si composites with respect to their mechanical properties. Al-12.6 wt% Si-10 wt% LaB 6 sample had the highest hardness (1.72 ± 0.12 GPa), yield strength (373 MPa), compressive strength (578 MPa) and lowest wear rate (0.001 mm3/Nm) values among all SPS'd samples. Graphical abstract Unlabelled Image Hıghlıghts • Al-based MMCs were synthesized by MA followed by SPS. • Effect of MA time and LaB 6 content on Al12.6Si matrix was investigated. • Lattice strain and crystallite size values were significantly retained after SPS. • 4 h MA'd and 10 wt% LaB 6 reinforced composites had the best mechanical properties. • Wear rate of the composites decreased by increasing LaB 6 content. [ABSTRACT FROM AUTHOR]

The effect of back-bombardment (BB) electrons is considered one of the main obstacles for extensive use of thermionic RF guns (T-RF). The 10 hexaboride materials named (Ca, Sr, Ba, La, Ce, Pr, Nd, Sm, Eu, and Gd) B6 are investigated in this paper to survey the effect of the cathode material on reducing BB electrons and compare them with LaB6 (which is widely used as a thermionic cathode). A numerical model was used to conduct this paper. Besides the numerical calculations, an experiment has been performed to determine the work functions of CeB6 and LaB6. The results from the numerical calculations revealed that (Ba, Ca, and Nd) B6 are less affected by BB electrons: 42%, 50%, and 59%, respectively, compared with LaB6, for low beam current applications. In contrast, for high beam current duties, (Nd, Ce, and Sm) B6 have minimum influence by BB electrons compared with other hexaborides. The study concluded that BB electrons are strongly affected by the properties of the cathode material, especially thermionic emission and material density. Moreover, the study suggests that it is worthwhile to prepare BaB6, CaB6, NdB6, and SmB6 as cathode materials, then to subject them to a real experimental test using T-RF gun to compare their performance and BB effect against LaB6. [ABSTRACT FROM AUTHOR]

Abstract: An effective way to improve sintering densification of LaB6 was proposed and confirmed experimentally. Firstly, LaB6 nanopowders with a cube-like shape of ~ 94.7 nm were fabricated by molten salt synthesis route at 800 °C for 1 h. Then, LaB6 bulk material of 98% density was prepared by hot pressing sintering of as-synthesized LaB6 nanopowders under 1800 °C/50 MPa/30 min. The acquired LaB6 bulk material had a work function of 2.87 eV and exhibited an excellent thermionic emission property. The saturation emission current density at 1500 and 1600 °C reached 37.4 and 44.3 A/cm2, respectively.Graphical Abstract: [ABSTRACT FROM AUTHOR]

In the present study, we report the morphological tuning of carbonaceous nanocrystalline lanthanum hexaboride (LaB6) using an arc plasma route. It was possible to obtain different morphologies, namely: pebbles, rods and sheets, on a nano-scale by means of plasma assisted vapour phase growth and by varying the plasma current. At elevated plasma currents, as a consequence of graphite electrodes being used in the arc geometry, the synthesized product was obtained in the form of carbonaceous LaB6 having a high degree of crystallinity. Detailed morphological analysis using electron microscopy has revealed that nanorods and nanosheets of carbonaceous LaB6 can be grown by elevating the plasma currents. The field emission (FE) current was found to be enhanced from 200 μA cm−2 to 660 μA cm−2 for the hierarchical structure of nanosheets as compared to that of nanopebbles at a much reduced turn-on potential. A stable FE current is an important feature exhibited by all these LaB6 nanostructures. [ABSTRACT FROM AUTHOR]

Tantalum carbide (TaC)-based composites containing 0.1–30 mol% lanthanum hexaboride (LaB 6 ) were prepared by hot pressing at 1900 °C for 30 min under 30 MPa pressure. Effects of LaB 6 content on densification, microstructure and mechanical properties of the consolidated TaC-LaB 6 composites were investigated. Sinter-forge was performed to test creep resistance of the TaC-LaB 6 composites at temperatures up to 1950 °C. Addition of LaB 6 increased densification of the TaC-LaB 6 composites and the compositions with ≥1 mol% LaB 6 almost reached full densities. LaB 6 reacted with TaC and the Ta 2 O 3 impurity to form minor phases of TaB 2 , B 2 O 3 and C. Average size of the TaC grains in the consolidated composites showed a decrease trend with increase in the LaB 6 content. The TaC-LaB 6 composites with 1–2 mol% LaB 6 addition had relatively good flexural strength, Vickers hardness and fracture toughness with a value of 530 MPa, 15.6 GPa and 5.29 MPa m 1/2 , respectively. Addition of LaB 6 modified interfacial bonding strength of the TaC grains, hence increase in creep resistance under sinter-forge up to 1950 °C. [ABSTRACT FROM AUTHOR]

Heater materials and geometry are analyzed to address the requirements of large lanthanum hexaboride hollow cathodes. Larger hollow cathodes are capable of increased discharge currents but require higher heater powers to ignite. Existing cathode heaters suffer from material interactions and increased failure rates at high temperatures and are currently not viable for high-power operation; graphite heaters can provide order-of-magnitude increases in heater power and operational life simultaneously. Two models, a simplified circuit model and a finite-element model, were developed to predict the heater operating temperature and resistance. The operational life of the heater was estimated using a vacuum sublimation model. A graphite heater was fabricated and tested using a large-diameter lanthanum hexaboride hollow cathode to demonstrate feasibility as a high-power heater material. The prototype has repeatedly achieved cathode ignition and was tested at up to 4.5 kW of heater power. Heater voltage and current were measured prior to ignition and used to calculate the heater resistance. The measured resistance is within 8% of the predicted value for the circuit model and within 2.5% of the finite-element prediction. At cathode ignition temperatures, the vacuum sublimation model predicts an order-of-magnitude increase in operational life as compared to existing cathode heaters; for a peak surface operating temperature of 1500 °C, the vacuum sublimation model predicts a 1%-loss life of approximately 400 kh. [ABSTRACT FROM AUTHOR]