Ba Sc Research Articles

Thermodynamic modelling of continental arc-adjacent magmatism: the Loicas Trough, N. Patagonia, Argentina

Continental arcs are associated with volcanism concentrated into two main belts—the main arc and back arc, often separated by fold and thrust belts. The Loicas Trough, Argentina, is a post-orogenic extensional feature that obliquely cuts the fold and thrust belts. The trough hosts large Pliocene–Holocene volcanic centres, including Domuyo and Tromen, that lie between the main arc and back arc and thus provide a rare window into this setting. We present major and trace element data for the Loicas Trough, which we combine with geochemical modelling using the Magma Chamber Simulator (MCS) to explore the origin and evolution of the volcanism. The lavas display a wide continuous range from alkaline basalts to subalkaline rhyolites. Trace elements reveal variable extents of arc enrichment (2 < Nb/U < 28), which correlate with proximity to the trench and differentiation indices. Our results and MCS models indicate that the Loicas Trough parental magmas formed from compositionally zoned mantle. Best-fit models indicate that the differentiation occurs at middle and upper crustal levels, in sharp contrast to lower crustal hot zones beneath main arcs. Assimilation of partial crustal melts drives compositional evolution and obscures source signatures. Pure or high fraction end-member partial crustal melts are also identified at Domuyo based on their low Ba (~ 250 ppm) and moderate Sc contents (~ 8 ppm). We find evidence of similar lavas in transtensional settings adjacent to continental arcs worldwide, which do not adhere to the main versus back arc volcanism binary. We suggest the term arc-adjacent magmatism, where compositions are mainly controlled by extensive assimilation and reworking in the middle to upper crust.

Thermionic Emission Capability of the Ba–Sc–Al–O Compound Synthesized by High-Temperature Solid-State Process

The dibarium scandium aluminum oxide (Ba2ScAlO5) is an important compound for thermionic emission in high-power vacuum devices. Herein, the high-temperature solid-state synthesis mechanism is investigated extensively to develop single-phase <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -Ba2ScAlO5 and evaluate its emission capacity. Barium carbonate, scandium oxide, and aluminum hydroxide are adopted as the reactants. The synthesis temperature, the intermediate phases, and the cell parameters of the final products differ from sintering the coprecipitated precursor. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -Ba2ScAlO5 single-phase material can be obtained based on the nonstoichiometric mole ratio of BaO:Sc2O3:Al2O3 = 4.4:1.1:0.9. The X-ray Rietveld refinement reveals that the substitution of Sc for Al and the defects of the Ba, Sc, and O vacancies account for the nonstoichiometry and the shrinkage of cells. The test cathode impregnated with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -Ba2ScAlO5 doped with Ca2+ exhibits an emission capability of 6.79 A/cm2 at 900 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ }\text{C}_{\text {B}}$ </tex-math></inline-formula> in the dc mode. This work demonstrates the feasibility of obtaining <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -Ba2ScAlO5 with considerable emission through the high-temperature solid-state process.

Protonation-Induced B-Site Deficiency in Perovskite-Type Oxides: Fully Hydrated BaSc0.67O(OH)2 as a Proton Conductor

One of the concepts for the design of a highly proton-conductive perovskite-type oxide is to increase the proton concentration. In this study, we focused on Ba–Sc oxide known for its brownmillerite-type structure and great potential for a high proton concentration. To achieve the highly hydrated phase, high-pressure synthesis was conducted using hydroxide as the H2O source. Although the expectation was that perovskite-type BaScO2(OH) would simply be a hydrated phase, the formation of Sc vacancies increased resulting in the greater hydration of the Ba–Sc oxide. It was likely that the fully hydrated phase was perovskite-type BaSc0.67O(OH)2. One third of the Sc sites were vacant, and the effective charge of Sc vacancies were compensated by the excess protons. First principles calculation revealed that the protons residing in the Sc vacancies were orientated toward a Sc vacancy and compensated the B-site charge in a manner similar to that of hydrogarnet substitution. This B-site deficiency caused by the extensive hydration condition is a novel mechanism to increase the proton concentration of perovskite-type proton conductors. The electrical conductivity, 10–2 S·cm–1 at 450–500 °C, was comparable to that of acceptor-doped BaZrO3, although the conductivity discontinuously changed at dehydration temperature.

Frontiers in Thermionic Cathode Research

The high electron emission resulting from low-work-function scandate-added dispenser cathodes has generated a great deal of interest and research since the late 1970s. Despite the reported high current density at low temperatures, scandate cathodes have not yet seen wide-scale industrial adoption due to poor emission uniformity, inadequate reproducibility, and short lifetimes. This lack of industrial adoption stemming from the above issues is likely due to insufficient fundamental understanding of the role that scandium plays in the emission process. Recent work by five research teams under the U.S. Defense Advanced Research Projects Agency Innovative Vacuum Electronics Science and Technology Program aims at advancing this fundamental understanding and the cathode manufacturing processing. Emission microscopy of model surfaces of adsorbed Ba–Sc–O on single crystal tungsten and detailed characterization techniques, such as Wulff shape analysis, are being used to understand the morphology, composition and phase of each of the species that comprise the cathode emitting surfaces. The structure of tungsten grains at the cathode surface of activated and unactivated scandate cathodes and the resulting emission performance are observed. 3-D printing is being investigated as an advanced manufacturing method to meet the demanding requirements of modern vacuum electron devices. Finally, density functional theory is being employed to study the work function of perovskite oxides as a novel class of alternative materials to tungsten dispenser cathodes.

Robustness Investigation on Nanosized-Scandia-Doped Dispenser Cathodes

In order for practical applications in vacuum electron devices, the robustness of scandia-doped dispenser (SDD) cathode is a major concern. Emission properties after air exposure and gas poisoning have, therefore, been studied. A full restoring of emission has been proved after the cathodes were exposed to air and reactivated for a time much shorter than initial activation. Poisoning experiments demonstrated that the poisoning pressure thresholds of residual gases for the SDD cathode are about half to one order of magnitude higher at equivalent temperatures but with higher emission level in comparison with that of an excellent M cathode. The cathode is easier to recover after poisoning and can operate stably in a nonideal environment. These features are thought to be related to the surface Ba–Sc–O structure which has closely chemical association, better stability for air exposure, relatively high tolerance to gas poisoning, and the semiconductor emission characteristic.

A Study of Scandia-Doped-Impregnated Cathode Fabricated by Spray Drying Method

Scandia-doped-impregnated (SDI) cathodes were fabricated with scandia-doped tungsten powders using spray drying (SD) combined with two-step reduction in hydrogen atmosphere. The temperature-programmed reduction results show that the addition of Sc2O3 leads to a decrease of the reduction temperature of scandia-doped tungsten oxide powders. Using the obtained scandia-doped tungsten powders with homogenous distribution of nanosized scandia, a submicrometer porous matrix cathode composed of tungsten grains with active substances distributing uniformly around these grains has been obtained. For cathodes with the above matrices and impregnated with 411 or 612 Ba, Ca aluminates (named SDI/SD-411 or SDI/SD-612 cathodes, respectively) and space charge limited current densities of $\sim 72.5$ A/cm $^{2}$ have been reliably reached at 900 $^{\circ }\text{C}_{b}$ . After proper activation, the cathode surface is covered by a Ba–Sc–O active substance layer with preferable atomic ratios, leading to its good emission property. The average evaporation rate of $8.45\times 10^{-9}$ gcm $^{-2}~\text{s}^{-1}$ at 1150 $^{\circ }\text{C}_{b}$ for SDI/SD-612 cathode is the lowest one among Ba–W, W/Re/Os–M, SG-411 (the scandia-doped cathode fabricated by sol–gel method with impregnating 411 aluminates), and SDI/SD-612 cathodes.

Synthesis of Eu&lt;sup&gt;2+&lt;/sup&gt;-activated Rb–Ba–Sc–Si–O glass phosphors using melt synthesis technique

Synthesis of Eu&lt;sup&gt;2+&lt;/sup&gt;-activated Rb–Ba–Sc–Si–O glass phosphors using melt synthesis technique

Effect of scandia doping method on the emission uniformity of scandate cathode with Sc2O3–W matrix

Scandia doped tungsten matrix dispenser cathodes were manufactured using scandia doped tungsten powder prepared by mechanical mixing, liquid–solid doping and a spray drying method. It is found the macrostructure of the cathode depended on the powder preparation method. The cathode prepared using the powder prepared by spray drying method had a homogenous and porous matrix characterized with grains with a diameter of less than 1μm and with many nanoparticles distributing uniformly around these grains. The cathode with submicron structure and uniform distribution of scandia exhibited good emission uniformity. The emission uniformity ΔJ/J of the cathode prepared by spray drying method was 0.17, about 6 times lower than that of the cathode prepared by mechanical mixing method. The calculation results showed that the nanoparticles led to electric field enhancement. A Ba–Sc–O multilayer on the cathode surface and nanoparticles distributing mainly on W grains contributed to the emission property of the cathode.

Preparation of W–Sc2O3 targets and scandate cathodes with film prepared by pulsed laser deposition

Sub-micrometer Sc2O3–W powder with a narrow particle size distribution has been obtained by a sol–gel method combined with two-step hydrogen reduction process. Based on the obtained powder, the W–Sc2O3 targets have been sintered via spark plasma sintering (SPS) at 1300°C. The W–Sc2O3 targets have the average grain size of about 1μm. Both the sintering temperature and holding time are much lower than those of the targets prepared with micrometer sized powders. The obtained W–Sc2O3 targets have a high comparative density of 96.4% and rockwell hardness of 86.4 HRC. Using the target, the scandate cathode deposited with a film containing 5% Sc2O3 and 95% W has been obtained by pulsed laser deposition (PLD) method. This cathode has good emission property, i.e., the highest thermionic emission current density reaches 43.09A/cm2 of Jdiv at 900°Cb after being activated for 8h, which is much higher than that of scandate cathode without film. Scandium (Sc) supplied by the film on the surface during the activation forms a Ba–Sc–O active layer, which helps to the emission.

Preparation and characterisation of scandia doped dispenser cathodes

Scandia doped dispenser cathodes were prepared by powder metallurgy method using mixed powder of scandia doped tungsten, barium–calcium aluminates obtained by sol–gel method. The scanning electron microscope observation results indicate that the cathode has submicron quasi–spherical tungsten grains structure with homogeneous distribution of Sc2O3 and barium–calcium aluminates which dispersed over and among W grains. The cathode exhibits good emission property, i.e. the current density of this cathode can reach more than 40A/cm² at 850°Cb. The cathode has low work function of 1.43 eV. Active substance of Sc, Ba and O diffuse from the inner part of cathode to the surface to form a Ba–Sc–O multilayer having certain ratios of Ba:Sc:O which contains much more Ba than normal dispenser cathode at the surface of Sc2O3–W matrix cathode during the activation period. This active–multi–layer is considered to be responsible for copious emission capability.

Characterization of scandia doped pressed cathode fabricated by spray drying method

Scandia doped pressed cathode was prepared by a new method of spray drying combined with two-step hydrogen reduction process. The Sc 2O 3 and barium–calcium aluminate co-doped powders have sub-micrometer size in the range of 0.1–1 μm and scandium oxide and barium–calcium aluminate are distributed evenly in the powders. The cathodes sintered by powder metallurgy at 1600 °C b have a smooth surface and sub-micrometer grain structure with homogeneous distribution of scandium, barium, calcium and aluminum which are dispersed over and among the tungsten grains. This cathode has good emission, e.g., the current density of this cathode reaches 31.50 A/cm 2 at 850 °C b. After proper activation, the cathode surface is covered by a Ba–Sc–O active substances layer with a preferable atomic ratio, leading to its good emission property. The evaporation activation energy of SDP cathode with 4.58 eV is the highest among the Ba–W, M-type and SDP cathodes, and the average evaporation velocity v t of SDP cathode with 1.28 × 10 −8 g cm −2 s −1 at 1150 °C b is the lowest one.

Emission mechanism of high current density scandia-doped dispenser cathodes

The emission mechanism of scandia-doped dispenser (SDD) cathodes is characterized based on systematic investigation of microstructure, surface behavior, evaporation, and emission. High current density SDD cathodes have been developed in recent years to meet the requirements of advanced vacuum electron devices. Space charge limited current densities of over 100 A/cm2 have been reliably reached at a temperature of 950 °Cb. SDD cathodes are composed of a tungsten matrix with submicron structure and uniformly dispersed materials containing Sc, Ba, Ca, Al, and O as nanosized particulates in the matrix. The authors conclude that the copious electron emission from this cathode type results from a layer of Ba–Sc–O on W formed after activation on top of the matrix. This layer is about 100 nm thick and maintains a stable composition over its lifespan. Its emission can be explained in terms of a semiconductor layer with characteristics that differ from a metal-like emission model. The Ba–Sc–O layer is formed by codiffusion of Sc, Ba, and O (generated inside the W-matrix) to the surface of the W-matrix during activation. Furthermore, this layer guarantees a low evaporation rate of emission-relevant materials and thus enables a longer lifespan.

Boris Peter Stoicheff

Boris Peter Stoicheff

Novel Ba–Sc–Si-oxide and oxynitride phosphors for white LED

Alkaline earth silicates, which comprise a host material doped with rare-earth minerals, show excellent luminescence properties with various crystal structures and high stability. From results of this study, we report luminescence properties of Ba 9Sc 2Si 6O 24:Eu 2+ and Ba 9Sc 2+ δ Si 6O 24−3 δ N 3 δ :Eu 2+ as a novel alkaline earth silicate and silicon oxynitride phosphors for white LEDs. Using a conventional solid-state reaction, Ba 9Sc 2Si 6O 24:Eu 2+ samples were synthesized and Ba 9Sc 2+ δ Si 6O 24−3 δ N 3 δ :Eu 2+ samples were obtained by nitrization of Ba 9Sc 2+ δ Si 6O 24:Eu 2+. The samples can be excited by blue light, exhibiting green (Ba 9Sc 2Si 6O 24:Eu 2+) and yellow (Ba 9Sc 2+ δ Si 6O 24−3 δ N 3 δ :Eu 2+) efficiently, which are emissions for use in white LEDs essentially.

THE ORIGIN AND ZONING OF HYPOGENE AND SUPERGENE Fe-Mn-Mg-Sc-U-REE PHOSPHATE MINERALIZATION FROM THE NEWLY DISCOVERED TRUTZHOFMUHLE APLITE, HAGENDORF PEGMATITE PROVINCE, GERMANY

An aplite containing Fe–Mn–Mg–Sc–U–REE phosphates, some Cu–Pb–Zn sulfides, barite, and U–Nb–Ta–Ti–Fe–Mn oxides was recently discovered near Trutzhofmuhle (THM) at the western border of the Hagendorf Pegmatite Province, Germany. We describe the sequence of phosphate crystallization in six stages of mineralization (I to VI) covering the time span from the Late Carboniferous through the Recent supergene alteration, and six sequences (1a/1b to 5) reflecting the reaction of phosphate-bearing solutions with the gneissic country-rocks (exo-aplitic) and with intra-aplitic rock-forming minerals that formed during crystallization. Age dating was carried out on columbite-(Fe) and torbernite using laser-ablation techniques. Precipitation of columbite-(Fe) and early magmatic phosphates (Mn-rich apatite, monazite) in the THM aplite is correlated with a thermal event around 302 Ma postdating the intrusion of the post-kinematic Flossenburg granite. The sequences 1a and 1b, containing the lazulite solid-solution series, gordonite and childrenite–eosphorite series, reflect late magmatic and early hydrothermal exo-aplitic processes. The late magmatic and early hydrothermal stages of the intra-aplitic sequences 2 to 5 are characterized by triplite, wolfeite, triploidite, an unnamed K–Ba–Sc–Zr phosphate, an unnamed Zr–Sc phosphate–silicate, phosphoferrite, Mn-rich vivianite, and lermontovite – vyacheslavite (?). Complexing agents such as fluorine and phosphate control the formation of Sc phosphates and silicates. In contrast with the neighboring Hagendorf pegmatite, the magmatic and hydrothermal phosphate mineralization of the THM aplite does not contain any Li-bearing phosphates and is very low in F. Rockbridgeite, whitmoreite, “ferrolaueite”, Al-bearing rockbridgeite, mitridatite, “metamitridatite”, kolbeckite and strunzite appear during late hydrothermal processes and weathering. Kolbeckite formed at the transition from hypogene to supergene processes. Its morphology varies from a rather simple combination of faces (platy kolbeckite I) under hydrothermal conditions to complex mineral aggregates (stubby kolbeckite II) produced under weathering conditions. The latest supergene alteration consists of wavellite, beraunite, cacoxenite, strengite, P- and Mn-bearing “limonite”, autunite, Sc-bearing vochtenite, Sc-bearing churchite-(Y) and diadochite. The latter phosphates with predominantly Fe, Al and U in close association with kaolinite are the representatives of supergene alteration, which is related in time and space to the Miocene peneplanation between 4.8 and 6.9 Ma. The boron- and phosphate-bearing THM aplite is not directly linked to any of the granitic plutons nearby, and is not easily classified within the scheme of rare-element pegmatites.

Sc 2O 3–W matrix impregnated cathode with spherical grains

Sc 2O 3–W matrix cathodes have been prepared by using a liquid–liquid doping method combined with high-temperature sintering. The microstructure and physical behavior of active substances of scandia-doped tungsten matrix and impregnated cathode has been studied by SEM and AES methods. The results show that the matrix has a homogeneous structure composed of W grains with spherical shape and superfine Sc 2O 3 particles dispersed uniformly over and among W grains. After impregnation, this Sc-type impregnated cathode has high emission capability. Space-charge-limited current density could reach 52 A/cm 2 at 850 °C b. The high emission results from a Ba–Sc–O active layer with a thickness of about 80 nm, which is formed at the cathode surface during the activation period. Both the decrease of the thickness of active surface layer and the decrease of the content of Sc at the surface could lead to the degradation of current density during operation.

A study of scandia and rhenium doped tungsten matrix dispenser cathode

Scandia and rhenium doped tungsten powders were prepared by solid–liquid doping combined with two-step reduction method. The experimental results show that scandia was distributed evenly on the surface of tungsten particles. The addition of scandia and rhenium could decrease the particle size of doped tungsten, for example, the tungsten powders doped with Sc 2O 3 and Re had the average size of about 50 nm in diameter. By using this kind of powder, scandia and rhenium doped tungsten matrix with the sub-micrometer sized tungsten grains was obtained. This kind of matrix exhibited good anti-bombardment insensitivity at high temperature. The emission property result showed that high space charge limited current densities of more than 60 A/cm 2 at 900 °C could be obtained for this cathode. A Ba–Sc–O multilayer about 100 nm in thickness formed at the surface of cathode after activation led to the high emission property.

Surface coating influence on scandate cathode performance

It was found that the electron emission from scandate-impregnated cathodes coated with W-scandia films depends on scandium concentration and film thickness. W-scandia cathodes with low Sc-concentration films (2.8 at.% Sc) produce high electron emission when film thickness is 670 nm. On the contrary, the cathodes with high Sc-concentration films (13, 22, or 33 at.% Sc) showed a tendency that the thinner film (10 nm) provided higher electron emission than a thicker film (30–220 nm). The electron emission dependence is related to surface scandia density. It was shown that almost all of the scandia in the high Sc-concentration films is segregated to the surface from the inside. A surface covered by too much scandia can not provide high electron emission, because the Ba–Sc–O monolayer on the scandia area might not produce a low work function. However, scandia segregation in a low Sc-concentration film might be less than that in a high Sc-concentration film, so a large tungsten area might remain on the cathode surface. If so, the Ba–Sc–O monolayer can be formed on the tungsten area and produce a low work function.

Nonuniform emission distribution on a scandate impregnated cathode

The effect of barium supply into a (W–Sc 2W 3O 12)-coating film on the emission distribution of a scandate cathode has been investigated. Scandium concentration on the cathode surface was increased by activation heating. This increase was coincident with the rise in emission current by heating. However, when the film was coated on the unimpregnated tungsten pellet, the scandium concentration did not increase. These results indicate that the barium supply into the film is necessary for liberating scandium atoms from the scandium tungstate in the film and for forming a monolayer of Ba–Sc–O on the surface. We measured the heating-induced change in the scandium content of the film. The result showed that the scandium content decreased only in the part of the film that covered the impregnated areas. No decrease was observed in the film that covered tungsten metal. The liberation of scandium is considered to take place only in the film to which barium atoms are mainly supplied. This nonuniform scandium liberation in the film will cause nonuniformity in the Ba–Sc–O monolayer on the cathode surface, which in turn causes emission nonuniformity.

Work functions for models of scandate surfaces

The electronic structure, surface dipole properties, and work functions of scandate surfaces have been investigated using the fully relativistic scattered-wave cluster approach. Three different types of model surfaces are considered: (i) a monolayer of BaScO on W(100), (ii) Ba or BaO adsorbed on Sc 2O 3 + W, and (iii) BaO on Sc 2O 3 + WO 3. Changes in the work function due to Ba or BaO adsorption on the different surfaces are calculated by employing the depolarization model of interacting surface dipoles. The largest work function change and the lowest work function of 1.54 eV are obtained for Ba adsorbed on the ScO monolayer on W(100). The adsorption of Ba on Sc 2O 3 + W does not lead to a low work function, but the adsorption of BaO results in a work function of about 1.6–1.9 eV. BaO adsorbed on Sc 2O 3 + WO 3, or scandium tungstates, may also lead to low work functions.