Yttrium Hexaboride Research Articles

Superconducting YB6 nanowires

We utilize a high-pressure solid-state technique to prepare yttrium hexaboride (YB6) nanowires at very low temperatures of 200–240 °C. The obtained YB6 nanowires are single-crystals grown along [001] direction confirmed by HRTEM and SAED images. From the growth morphology at different time stages and EDS evidence, we proposed that YB6 nanowires are nucleated and grown from the submicron particle clusters. The temperature-dependent magnetization results indicate that YB6 nanostructures undergo a superconducting transition with Tc = 7.8 K, which is higher than that of reported YB6 bulk single-crystals. Furthermore, the YB6 nanostructures reveal a peak effect in superconducting state observed from the magnetic hysteresis loops, approving that YB6 belongs to a type-II superconductor. The lower and upper critical magnetic fields, Hc1 and Hc2, are determined to be 251 Oe and 1104 Oe, respectively.

A new approach to synthesize nano-yttrium boride particle through metallothermic reduction process

In the present study, a novel attempt is made to synthesize yttrium boride (YB4) nano-sized powders through metallothermic reduction method. The starting materials used were yttria (Y2O3) and boron oxide (B2O3) as reactants, and calcium (Ca) as reductant. The reaction was carried out at 9500C under argon atmosphere followed by acid washing. The product was subjected to X-ray fluorescence (XRF) which indicated its elemental constituents and purity of the prepared nanopowders. X-ray diffraction (XRD) studies revealed the formation of YB4 and YB6 phases as well as their respective crystal structures. Thermal analysis was done to calculate the weight loss and phase stability at different temperatures. It showed complete crystallization of the yttrium boride around 8000C. Field emission scanning electron microscopy (FE-SEM) images showed the agglomerated particle morphology. Energy dispersive spectroscopy (EDS) indicated the presence of Y and B elements. Transmission electron microscopy (TEM) images revealed the particle size in the order of 40 nm to 60 nm. Selected area electron diffraction (SAED) pattern were in consensus with XRD results ensuring the formation of nano-sized yttrium boride. The overall results confirmed that yttrium boride can be synthesized by the low-temperature metallothermic reduction process.

11B and 89Y solid state MAS NMR spectroscopic investigations of the layered borides YTB4 (T = Mo, W, Re).

The YCrB4 type borides YTB4 (T = Mo, W, Re) were synthesized from the elements by arc-melting and subsequent annealing. The structures were refined from single crystal X-ray diffractometer data: Pbam, a = 602.84(8), b = 1164.5(1), c = 361.20(4) pm, wR2 = 0.0404, 624 F2 values, 26 variables for YMoB4, a = 603.00(7), b = 1165.1(1), c = 360.63(6) pm, wR2 = 0.0487, 474 F2 values, 26 variables for YWB4, and a = 596.67(6), b = 1154.4(1), c = 360.21(4) pm, wR2 = 0.0465, 544 F2 values, 26 variables for YReB4. The boron atoms build up planar networks which are a tessellation of slightly distorted pentagons and heptagons. Adjacent networks coordinate the transition metal and yttrium atoms in the form of pentagonal and heptagonal prisms, respectively. The four crystallographically distinct boron sites are differentiated by high-resolution 11B solid state isotropic magnetic shifts and nuclear electric quadrupolar coupling constants. Partial site assignments are possible based on comparisons with electric field gradient calculations using the WIEN2k code. 89Y MAS NMR spectroscopic studies suggest substantially weaker Knight shift contributions to the resonance frequencies when compared to other intermetallic yttrium compounds, including other ternary yttrium boride compounds measured previously.

Lattice instability and enhancement of superconductivity in YB6

The superconducting and normal state characteristics of yttrium hexaboride (YB$_6$) have been investigated for the single crystals with a transition temperatures $T_c$ ranging between 6 K and 7.6 K. The extracted set of microscopic parameters [the coherence length $\xi$(0) $\sim$ 320$\div$340 ${\AA}$, the penetration depth $\lambda$(0) $\sim$ 1100$\div$1600 ${\AA}$ and the mean free path of charge carriers $l$ = 31$\div$58 ${\AA}$, the Ginzburg-Landau-Maki parameters $\kappa$$_{1,2}$(0) $\sim$ 3.3$\div$4.8 and the superconducting gap $\Delta$(0) $\sim$ 10.3$\div$14.8 K] confirms the type II superconductivity in "dirty limit" ($\xi$$\gg$ $l$) with a medium to strong electron-phonon interaction (the electron-phonon interaction constant $\lambda_{e-ph}$ = 0.93$\div$0.96) and $s$-type pairing of charge carriers in this compound [2$\Delta$(0)$/k_BT_c$ $\approx$ 4]. The comparative analysis of charge transport (resistivity, Hall and Seebeck coefficients) and thermodynamic (heat capacity, magnetization) properties in the normal state in YB$_6$ allowed to detect a transition into the cage-glass state at $T^*$ $\sim$ 50 K with a static disorder in the arrangement of the Y$^{3+}$ ions. We argue that the significant $T_c$ variations in the YB$_6$ single crystals are determined by two main factors: (i) the superconductivity enhancement is related with the increase of the number of isolated vacancies, both at yttrium and boron sites, which leads to the development of an instability in the hexaboride lattice; (ii) the $T_c$ depression is additionally stimulated by the spin polarization of conduction electrons emerged and enhanced by the magnetic field in the vicinity of defect complexes in the YB$_6$ matrix.

Al insertion and additive effects on the thermoelectric properties of yttrium boride

The aluminoboride YxAlyB14 (x ∼ 0.57, 0.41 ≤ y ≤ 0.63) has been found to show striking p-n control of the thermoelectric properties through variations of the y occupancy of the Al site. The effect of Al was investigated in further extremes. Polycrystalline samples of Al-free YxB14(x ∼ 0.55; “YB25”) were successfully synthesized in sufficient amounts for bulk spark plasma sintering (SPS) samples and their thermoelectric properties were investigated. Y0.56Al0.57B14 was also prepared in comparison, and further Al was added to the samples through SPS treatment. We observed that Y0.55B14 exhibits large positive Seebeck coefficients, ∼1000 μV K−1, around room temperature and the absolute value of the Seebeck coefficient largely decreases with increase of temperature while that of Y0.56Al0.57B14 is proportional to T−1/2, indicating a strong effect of Al on the electronic structure around the Fermi level. Y0.55B14 was found to be strongly disordered with a relatively low thermal conductivity and short localization length of 0.65 Å which is close to that previously determined for the disordered and thermally glass-like compound YB66. Occupancy of Al could not be increased further for the Al-rich sample, although Al was discovered to act as a sintering aid to enhance density and ZT could be significantly improved by 50%.

Surface superconducting states in a yttrium hexaboride single crystal in a tilted magneticfield

We present the results of an experimental study of the nucleation of superconductivity at the surface of asingle-crystal YB6 in a tilted dc magnetic field. The developed experimental approach allowed us to measureHc3 at each side of the sample as a function of the angle between the dcmagnetic field and the surface. Experiment showed that the ratioHc3/Hc2≈1.28 when the dc field became perpendicular to the surface while the expected value of this ratiois 1. This sharp distinction with theory cannot be ascribed to the surface roughness.

Strong coupling features in the point-contact spectra of the YB6 superconductor

Direct experimental studies via the point-contact spectroscopy of the superconducting energy gap and the strong coupling features of the yttrium hexaboride are presented. The phonon structures in the point contact spectra in the supeconducting and normal state are compared. As a result the low energy Einstein-like phonon mode at ∼ 7.5 meV associated with the yttrium ion vibrations seems to be the main mediator of strongly coupled Cooper pairs in this system.

Synthesis and Characterization of YB66 Nanowires

The objective of the current research is to synthesize yttrium boride (YB66) nanowires. Catalyst assisted growth of the nanowires under the vapor-liquid-solid (VLS) synthesis method was performed in a quartz tube furnace. The pyrolysis of diborane (B2H6) gas over yttrium oxide (Y2O3) powders with a thermally coated layer of nickel catalyst was carried out. The reaction conditions were at 925o C and a pressure of 390 mTorr with varying reaction times. SEM analysis has shown the growth of nanowires with diameters around 400 nm. A catalyst particle was also seen at the tip of the nanowires, confirming growth by the VLS mechanism. Other analysis techniques that were used include Raman spectroscopy and TEM analysis. The Raman spectra of the nanowires were in good agreement with a Raman spectrum obtained on a YB66 single crystal. However, no other evidence was obtained that the nanowires contained yttrium or that the nanowires consisted of YB66.

Linear and nonlinear low-frequency electrodynamics of surface superconducting states in an yttrium hexaboride single crystal

We report on the low-frequency dynamics of surface superconducting states in yttrium hexaboride single crystal. Tunneling and dc magnetization experiments demonstrate that ${\text{YB}}_{6}$ is a classical type-II BCS superconductor with a slightly suppressed order parameter near the surface. The ac response at frequencies 10--1500 Hz has been measured for fields ${H}_{0}$ parallel to the crystal surface. We found that for ${H}_{0}>{H}_{c2}$ the dc magnetic moment completely disappeared, while the ac response exhibited the presence of surface superconductivity. Increasing ${H}_{0}$ above ${H}_{c2}$ revealed the loss maximum in a field between ${H}_{c2}$ and ${H}_{c3}$ with a magnitude considerably larger than in mixed or normal states. In some dc fields the frequency dependence of the real part of the susceptibility is reminiscent of a spin-glass system. At the same time the imaginary component exhibits frequency dispersion which is not a typical feature of the classical spin-glass response. Analysis of the experimental data with the Kramers--Kronig relations showed the possible existence of the loss peak at very low frequencies. It was found that the amplitude of the third harmonic was not a cubic function of the ac amplitude (as it follows from the perturbation theory) even at very low excitation levels. It was shown that the critical state model cannot adequately describe the existing experimental data. The proposed model of a slow relaxing of the order parameter allows one to understand the origin of partial shielding and losses for ${H}_{0}>{H}_{c2}$.

Superconducting energy gap of YB6 studied by point-contact spectroscopy

Yttrium hexaboride has the second highest critical temperature, Tc∼8K, among all borides. The presented paper deals with the experimental study of its superconducting energy gap established by the method of the point-contact spectroscopy. The temperature dependence of the energy gap and the strength of the superconducting coupling is presented.

Electronic and optical properties of ZrB12 and YB6. Discussion on electron‐phonon coupling

AbstractWe report the optical properties of high‐quality single crystals of low temperature superconductors zirconiumdodecaboride ZrB12 (Tc = 5.95 K) and yttrium hexaboride YB6 (Tc = 7.15 K) in the range 6 meV ‐ 4.6 eV at room temperature. The experimental optical conductivity was extracted from the analysis of the reflectivity in the infrared range and ellipsometry measurement of the dielectric function in the visible range. The electronic band structure of these compounds was calculated by the self‐consistent full‐potential LMTO method and used to compute the interband part of the optical conductivity and the plasma frequency Ωp . A good agreement was observed between the interband part of the experimental optical conductivities and the band structure calculations. Different methods combining optical spectroscopy, resistivity, specific heat measurements and results of band structure calculations are used to determine the electron‐phonon coupling constant. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Semiconducting barium titanate ceramics prepared by using yttrium hexaboride as sintering aid

Nb-doped BaTiO 3 semiconducting ceramics have been prepared at 1150 °C by employing YB 6 as the sintering aid. The low-temperature sintered samples exhibit a positive temperature coefficient of resistivity (PTCR) effect. The room-temperature resistivity and the magnitude of PTCR effect depend on the amount of YB 6 added, the Nb 2O 5 content and the sintering conditions. At 1 mol% of YB 6, the sample has a low room-temperature resistivity and a resistivity jump of about 10 4. Transmission electron microscope studies revealed that the intergranular region between the crystalline BaTiO 3 grains has an amorphous structure. Y, Ba and Ti are present in the intergranular region but no Y is detectable in the crystalline grains. A liquid phase is believed to be present during sintering and it helps to facilitate the densification process and to enhance the PTCR effect.

A New Yttrium Boride: YB25

A new yttrium boride has been found to exist between YB12and YB50. A single phase was established at a nominal composition of [B]/[Y]=25.5–26.0. X ray powder diffraction and electron diffraction analyses showed that the new YB25phase has a monoclinic system witha=0.82842(3) nm,b=1.03203(3) nm,c=0.58570(2) nm,β=90.402(3)°, and space groupI121(No. 5),I1m1 (No. 8), orI12/m1 (No. 12). Rare earth elements from Gd to Ho can also form compounds isostructural to YB25.

Stability of plasma-sprayed coatings tested at White Sands Solar Facility

Plasma-sprayed coatings on steel plates were tested at the White Sands Solar Facility at temperatures from 200/sup 0/C to 1000/sup 0/C. Analysis of the specimens before and after testing showed erbium dodecaboride (ErB/sub 12/), yttrium hexaboride (YB/sub 6/), titanium diboride (TiB/sub 2/), and chromium oxide (Cr/sub 2/O/sub 3/) to be stable above 600/sup 0/C. A heat balance on the water-cooled specimens of these coatings showed 73% to 97% heat recovery efficiency.

Properties of melt-grown single crystals of “ YB68”

Single crystals of yttrium boride YB n with n = 61 ± 3 were grown from the melt. Precision density and lattice parameter measurements indicate a congruent melting point at n = 61.7 and a stoichiometric composition at n = 68. Measurements of elastic constants, acoustic attenuation, electrical resistivity and optical absorption are presented. High resolution transmission electron microscopy reveals a complex crystal structure similar to that found by using X-rays. A comparison of the properties of YB n with those of β-boron show that there are many similarities.

Preparation and some properties of metalloceramic alloys of tungsten and yttrium hexaboride: G A Kudintseva et al, Neorg Mater, 6 (7), 1970, 1335–1336 ( in Russian).

Preparation and some properties of metalloceramic alloys of tungsten and yttrium hexaboride: G A Kudintseva et al, Neorg Mater, 6 (7), 1970, 1335–1336 ( in Russian).

Structure cristalline de la phase du systeme bore-thorium. Un nouveau borure a structure type YB 66

A single crystal of the δ phase has been obtained in the boron-thorium system by slow cooling of a liquid corresponding to an atomic ratio B Th = 70 . The structure has been established, from two sets of 650 independent reflexions. Absorption correction was made necessary by the thorium atoms. From least-squares refinement, Fourier syntheses and electron density maps it appears that the δ phase is isotypic with the yttrium boride YB 66. The crystal is cubic with a = 23.53 A ̊ ; the space group is Fm3 c, O h 6. There are 1584 boron atoms and 24 thorium atoms in the unit cell. 1248 boron atoms are contained in 13 icosahedron units corresponding to the formula B 12(B 12) 12. B-B bonds within icosahedra range from 1.749 to 1.911 Å. The shortest intericosahedral bond is 1.598 Å, the longest 1.837 Å. The remaining boron atoms are statistically distributed in channels resulting from packing of the B 12(B 12) 12 units and form nonicosahedral cages as in YB 66. The thorium atoms are inserted in the channels; the thorium-boron bonds range from 2.71–2.84 Å.

Cermet alloys of rhenium with yttrium hexaboride

1. Alloys of rhenium with yttrium hexaboride (1–95 wt.%) have been produced by the powder metallurgy method. It is shown that an alloy of rhenium with 1% YB6 is a solid solution of YB6 in rhenium. X-ray and metallographical analyses of the alloys with 20, 50, 70, and 95% YB6 have shown that the components of their phases were formed on the basis of Re7B3, ReB3, YB4, and YB6. 2. The electrical resistance of the alloys increases with increase in their content of YB6 to the alloy Re+ 50% YB6. In alloys with a content < 50% Re, the specific resistance falls with decrease in the latter. The lowest work function in the given system was obtained for the alloy Re+ 20% YB6. 3. The different value of the work function of the alloys Re+ 1% YB6 and W+ 1% YB6 are discussed from the point of view of the possibility of electron exchange between the absorbed yttrium atoms and the substrate, leading to the stabilization of stable electron states in the absorbent-absorbate system.

Preparation and emissive properties of some yttrium and gadolinium borides

1. The investigation has established the optimum conditions of preparation of yttrium and gadolinium tetra and hexaboride powders, as well as conditions suitable for the hot pressing of these powders. 2. A study was made of the emissive properties of yttrium and gadolinium hexaborides and tetraborides. It was shown that yttrium hexaboride is an unstable compound, which decomposes during cathode operation with the formation of yttrium tetraboride. Yttrium tetraboride and the gadolinium borides are more stable. The maximum current densities obtained for the yttrium and gadolinium borides at a temperature of 1600°C and a field intensity of 5500 V/cm were 0.6 and 1.1 A/cm2, respectively.