LaB6 Surface Research Articles

Realization of electron transpiration cooling: LaB6 heated by a plasma glow

Electron transpiration cooling has previously been predicted to be an alternative cooling mechanism for leading edges of aerospace platforms traveling at extreme velocities, where thermoelectric materials shaped as leading edges manage heat loads. Here, thermo-electron emitting LaB6 ceramics with blunt edges were placed between two conical-shaped electrodes, and a plasma glow was ignited to heat the sample. Analysis of current–voltage (I–V) curves demonstrated that insertion of LaB6 into the plasma column resulted in an increase in the electron density of the plasma, evidencing thermally stimulated electron emission. With increasing sample temperature to 1200 K, Δne increased by ∼4 × 1010 #/cm3, the electron temperature (Te) decreased from ∼8 eV (1000 K) to 2.5 eV, and the emission begins to appear to become space charge-limited. Richardson's analysis of the temperature dependence of the electron emission in the region of ion saturation, yielded an activation energy consistent with the work function of LaB6. Modulation of the LaB6 surface temperature (ΔT = 10 K) and the plasma current (ΔI = 70 μA) under electric voltage (80 V) applied between the sample and current probe was demonstrated. The results show for the first time the feasibility of electron transpiration cooling effects under plasma conditions similar to that of aerothermal environments.

Optical Properties and Optimization of LaB6 Thin Films for Photothermal Applications

AbstractThe 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.

LaB6 surface chemistry-reinforced scaffolds for treating bone tumors and bone defects

Bone tumors may lead to osteoporosis. It is of great significance to fabricate functional scaffolds for treating bone tumor, osteoporosis and repairing bone defects. In this study, LaB6 micro-nanoparticles/poly(d,l-lactide)-modified β-tricalcium phosphate scaffolds (TCP-PDLLA-LB) were successfully prepared. The LaB6 surface chemistry-reinforced TCP-PDLLA-LB scaffolds were endowed with the following excellent properties: (1) the enhanced mechanical strength. The TCP-PDLLA-5LB scaffolds possessed higher compression resistant capacity than TCP scaffolds owing to the strong adhesive property of PDLLA that bound the LaB6 micro-nanoparticles and scaffolds tightly; (2) the excellent photothermal performance. TCP-PDLLA-LB scaffolds presented distinct photothermal property because of localized surface plasmon resonance effect of LaB6 micro-nanoparticles when exposed to the near-infrared (NIR) light (wavelength: 808nm) and it was flexibly controllable through changing the quantity of LaB6 micro-nanoparticles, power density of NIR light and environmental humidity; (3) the excellent biological properties in vitro. When culturing with saos-2 bone tumor cells, the TCP-PDLLA-5LB scaffolds effectively killed the saos-2 cells through adjusting power density of NIR light, irradiation duration time of NIR light and irradiation times of NIR light. Additionally, the TCP-PDLLA-5LB scaffolds were able to release bioactive ions which were beneficial to the in vitro osteogenesis of rabbit bone marrow stromal cells (rBMSCs) by supporting the attachment and proliferation of rBMSCs as well as promoting the expressions of osteogenic genes BMP2, RUNX2 and COL 1; and (4) the abilities of effectively ablating bone tumors and supporting bone tissue regeneration in vivo. The bone tumor tissues were significantly ablated and suppressed through hyperthermia due to the existence of LaB6 micro-nanoparticles on the scaffolds. Simultaneously, TCP-PDLLA-5LB scaffolds implanted in bone defects effectively helped new bone formation regardless of NIR light irradiation. In conclusion, LaB6 surface chemistry-reinforced scaffolds possess the dual functions of ablating bone tumor and repairing bone defects, which offers a promising therapy strategy for tumor/disease-related bone tissue engineering.

Efficient negative-ion sources for radioactive ion beam applications (abstract)

Direct extraction, negative-surface ionization, and sputter-type sources have been designed to efficiently ionize specific members of the group VII A elements (F, Cl, Br, I, and At) for use in the nuclear structure physics and nuclear astrophysics research programs at the Holifield Radioactive Ion Beam Facility (HRIBF). A negative surface ionization source that utilizes a solid, spherical geometry, LaB6 ionizer (φ≅2.7 eV) for negatively ionizing Cl, Br, l, and At. During off-line evaluation, the ionization efficiency for Cl− generation ranged between 15% and 20% and for Br− generation, between 15% and 25%. Chemically active elements, such as fluorine, are often released from refractory host materials in a variety of molecular forms. Consequently, the LaB6 surface ionization source, described above, is ineffective for simultaneously dissociating and negatively ionizing this element. To overcome this problem, a new concept source, referred to as the kinetic ejection negative ion source (KENIS), was developed. The source utilizes the Cs-sputter technique to effectively dissociate molecular carriers and ionize F at efficiencies in excess of 7%. The source has been used on-line to inject >3×107 17F−/s into the tandem accelerator and deliver ∼3×106 17F9+/s (fully stripped) to the research station for completion of important nuclear-astrophysics experiments. In this article, emphasis will be placed on the design details, materials of construction, ion optics, operational parameters, thermal transport properties, emittances, and measured ionization efficiencies for these sources.

The properties of the nonequilibrium LaB6 surface resulting from field evaporation

The nonequilibrium surface of single-crystal lanthanum hexaboride needles and its modifications are studied with a time-of-flight atomic probe. The surface is obtained by room-temperature field evaporation. The mass spectra of field evaporation shed light on the surface composition at the needle tip immediately after tip etching, corrosion in residual gases, intense cleaning by field evaporation, and the relaxation of the nonequilibrium surface by heating to 1250 K. Conditions for the breakdown of an oxide film on the tip surface and for obtaining the mass spectra of field evaporation for stoichiometric or lanthanum-enriched pure LaB6 single crystals are discussed.

Interaction of O2 with LaB6(0 0 1) surfaces as studied with MIES and UPS

Clean and O2 adsorbed LaB6(001) surfaces were investigated by metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS). Two structures appear close to the Fermi level EF and 3 eV below it in the MIE spectrum of the clean LaB6(001) surface (work function; 2.3 eV), and can be assigned to the La(5d) and dangling bond of B(2p), respectively. The peak near EF originates mainly from auto-detachment process between He*(1s2s) and La(5d). The results suggest that the clean LaB6(001) surface is terminated with La. Particular emphasis was paid to changes of these spectral features as a function of oxygen exposure. Both structures disappear at 1.5 L. It is concluded that O2 is dissociated at the La-terminated surfaces. Two oxygen-induced peaks appear in the MIES and UPS(HeII) spectra at 5.8 and 7.5 eV binding energy. It is concluded that O2 is dissociated at the La-terminated surfaces. Their origin is discussed on the basis of two mechanisms for the interaction of O2 with LaB6 surfaces.

A simple, high efficiency, negative surface ionization source

A spherical-geometry, self-extraction negative ion source has been designed and fabricated. The source utilizes direct surface ionization to form negative ion beams resulting from interactions between high electron affinity gaseous elemental or molecular materials and a negatively biased, spherical-sector LaB6 surface ionizer maintained at ∼1300 K. The design features of this source and principles upon which the source is based are discussed in this report.

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 LaB6,09, a desired composition, could be grown in single-crystal form from LaB6.2 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.

Large-area lanthanum hexaboride electron emitter

A large-area cathode assembly which is capable of continuous, high-current electron emission is described. The cathode utilizes an indirectly heated lanthanum hexaboride (LaB6) disk as the thermionic electron emitter. The LaB6 cathode emits over 600 A of electrons at an average of 20 A/cm2 continuously with no observable lifetime limits to date after about 400 h of operation in a plasma discharge. Proper clasping of the LaB6 disk is required to avoid impurity production from chemical reactions with the holder and to provide adequate support if the disk fractures during rapid thermal cycling. Modification of the LaB6 surface composition due to preferential sputtering of boron by hydrogen and argon ions in the plasma discharge has been observed. The surface appearance is consistent with the formation of LaB4 as a result of boron depletion. The electron emission capability of the cathode is not significantly altered by the surface change. This surface modification by preferential sputtering is not observed in hollow cathodes where the ion energy from the cathode sheath voltage is typically less than 50 V. The electron emission by the cathode has not been affected by exposure to both air and water during operation. Utilizing thick disks of this intermediate temperature cathode material results in reliable, high-current, long-lifetime electron emitter assemblies.

C2− Ion as Impurity from Halogen Ion Source and a Possibility of Its Application to Reaction Kinetics Studies

Abstract Negative ions from a hot LaB6 surface were studied with a quadrupole mass filter. C2− ion was formed via a charge exchange process. About 10−11 A of mass-selected C2− ion current was obtained above 40 eV of ion energies. A possibility of employing this to study a reaction kinetics is discussed.

Lanthanum Hexaboride Electron Emitter

The emitting properties of hot-pressed high-density lanthanum hexaboride rod cathodes have been measured. A current density of 100 A/cm2 is obtained at a true temperature of 1680°C. Lifetime tests have shown a useful life in excess of 200 h with an emission density of more than 50 A/cm2. The initial buildup of emission from the cathode has been found to be a gradual process and the final emission figures are not obtained until approximately 60 min have elapsed. Subsequently however, the full emission is obtained immediately on heating the cathode to operating temperature. The surface structure on the cathode has been examined with a scanning electron microscope both during the initial buildup of emission and during the operating life of the cathode. It is found that surface evaporation reveals sharp crystalline boundaries at a stage which coincides with the full emission density from the cathode. On heating the cathode to operating temperature without drawing emission, the crystalline nature of the material is revealed by the appearance of faceted hillocks on the surface. This structure is etched away when emission is drawn. There is no evidence to support the theory that lanthanum evaporates from selected emission sites. The whole LaB6 surface evaporates during operation and a certain amount of embedded material is revealed. This material is believed to contain tungsten and it accumulates gradually on the surface as operation continues.