Nb Substrate Research Articles

RF critical field measurement of MgB2 thin films coated on Nb

Niobium (Nb) Superconducting RF (SRF) cavities have been used or will be used for a number of particle accelerators. The fundamental limit of the accelerating gradient has been thought to be around 50 MV/m due to its RF critical magnetic field of around 200 mT. This limit will prevent new projects requiring higher gradient and compact accelerators from considering SRF structures. There is a theory, however, that promises to overcome this limitation by coating thin (less than the penetration depth) superconductors on Nb. We initiated measurements of critical magnetic fields of Nb coated with various thin film superconductors, starting with MgB2 films deposited using reactive evaporation technique, with the goal to apply this coating to SRF cavities. This paper will present first test results of the RF critical magnetic field of a system consisting of a 10 nm B and a 100 nm MgB2 films deposited on a chemically polished 2-inch single grain Nb substrate.

The influence of film thickness on photovoltaic effect for the Fe3O4/SrTiO3:Nb heterojunctions

Fe3O4 films with the thickness ranging from 5 to 160 nm have been grown on SrTiO3 : Nb (0.05wt%) substrates by the pulsed laser deposition technique. The good quality of the Fe3O4 film was confirmed by x-ray diffraction and magnetic analyses. It is found that the interfacial barrier of the resultant junctions, determined by the photovoltaic technique, decreases as film thickness increases from ∼5 to ∼40 nm, with a relative change of ∼20%, and saturates at a value of ∼1.2 eV above the thickness of 40 nm. Variation of lattice strains in the Fe3O4 film may be the reason for the thickness dependence of the interfacial barrier.

Effect of growth temperature on ferromagnetism in Mn:TiO2 thin film grown on SrTiO3:Nb substrate

We investigate the relationship between growth temperature and ferromagnetism in Mn:TiO2 thin film grown on SrTiO3:Nb substrate by plasma-assisted molecular beam epitaxy. The samples show different surface morphology, crystal quality and magnetic behavior with different growth temperature. It is concluded that the oxygen vacancies generated in the Mn:TiO2 thin film, together with the ones diffused from SrTiO3:Nb substrate, localize at the thin film/substrate interface during high-temperature growth, accounting for the more remarkable room-temperature ferromagnetism.

Anisotropic Nd-Fe-B thick film magnets prepared on various substrates

We investigated the effect of substrates for preparing perpendicularly anisotropic Nd-Fe-B thick film magnets prepared by the high-speed PLD (Pulsed Laser Deposition) method with a substrate heating system. Although it was difficult to prepare perpendicularly anisotropic films on W, Zr and Nb substrates, usage of Ta, Mo and Ti substrates enabled us to obtain anisotropic ones. It was found that the preparation of anisotropic PLD-made Nd-Fe-B thick film magnets depends on substrates.

Effect of magnetic flux penetration on the magnetic hysteresis loops of a Pt/Co/Pt triple layer on Nb(110)

The magnetic properties of Å (20 Å Pt/15 Å Co/20 Å Pt) triple layer deposited on a thick Nb(110) substrate are measured by vibrating sample magnetometry and the polar magneto-optical Kerr effect (p-MOKE). Above the superconducting transition temperature Tc = 9.1 K of Nb an out-of-plane magnetization curve M(H) is observed by p-MOKE due to the perpendicular magnetic anisotropy of the Pt/Co/Pt triple layer. Cooling the sample below Tc gives rise to a modification of the M(H) hysteresis with a sudden change of the magnetization for magnetic fields smaller than the coercive field Hcoerc of the ferromagnetic triple layer. This is due to the penetration of magnetic vortices into the center of the weak-pinning superconducting Nb disk for fields above the lower critical field. These vortices generate an enhanced local magnetic field at the Nb surface which is larger than Hcoerc of the Pt/Co/Pt triple layer and locally switches isolated domains already for applied fields smaller than Hcoerc.

Performance of a CsBr coated Nb photocathode at room temperature

Experiments performed on Nb substrates coated with thin films of CsBr indicate a substantial enhancement of 150 to 800 times of the photoyield at 257 nm relative to the uncoated substrates. Results are presented for several power density illuminations and sample thickness. Further enhancement of photoyield was observed when the laser illumination was interrupted for a short time in samples with 5–10 nm thick CsBr coatings.

Unusual transport behavior of the SrTiO3-based homojunctions

Two homojunctions composed of La0.15Sr0.85TiO3 films and SrTiO3:Nb substrates have been fabricated in the oxygen atmospheres of 10 and 20 Pa, respectively, and their transport behaviors are studied in the temperature range from 10 to 350 K. The most remarkable observations are the temperature independence of the current-voltage relations as well as the temperature/bias independence of the capacitance in the low temperature region in the junction obtained in an oxygen pressure of 10 Pa, and the exponential growth of current with reverse voltage. The rectifying behaviors can be well described by the Shockley equation at high temperatures, and the interfacial barrier, deduced from the current-voltage characteristics, are ∼1.35 and ∼0.73 eV, decreasing with oxygen pressure. The experiment results can be understood by assuming the formation of an insulating interfacial layer in the junctions.

Growth of Pb(Ti,Zr)O 3 thin films by metal-organic molecular beam epitaxy

Single-crystal Pb(Zr x Ti 1− x )O 3 thin films have been grown on (0 0 1) SrTiO 3 and SrTiO 3:Nb substrates by molecular beam epitaxy using metal-organic source of Zr and two different sources of reactive oxygen—RF plasma and hydrogen-peroxide sources. The same growth modes and comparable structural properties were observed for the films grown with both oxygen sources, while the plasma source allowed higher growth rates. The films with x up to 0.4 were single phase, while attempts to increase x beyond gave rise to the ZrO 2 second phase. The effects of growth conditions on growth modes, Zr incorporation, and phase composition of the Pb(Zr x Ti 1− x )O 3 films are discussed. Electrical and ferroelectric properties of the Pb(Zr x Ti 1− x )O 3 films of ~100 nm in thickness grown on SrTiO 3:Nb were studied using current–voltage, capacitance–voltage, and polarization–field measurements. The single-phase films show low leakage currents and large breakdown fields, while the values of remanent polarization are low (around 5 μC/cm 2). It was found that, at high sweep fields, the contribution of the leakage current to the apparent values of remanent polarization can be large, even for the films with large electrical resistivity (∼10 8–10 9 Ω cm at an electric filed of 1 MV/cm). The measured dielectric constant ranges from 410 to 260 for Pb(Zr 0.33Ti 0.67)O 3 and from 313 to 213 for Pb(Zr 0.2Ti 0.8)O 3 in the frequency range from 100 to 1 MHz.

Controllable growth of NbTe4 micro/nanostructures on Nb substrates and their field-emission performance

Single-crystalline NbTe4 microcone, microplate, and nanorod arrays were grown on Nb substrates by a surface-assisted chemical-vapor-transport under different conditions. The microcones are composed of small and large cones with a bending top. The small cones have a top diameter of about 0.2 µm, a bottom diameter of about 1.2 µm, and a length up to approx. 7.7 µm. The large cones originate from self-assembled small cones. The microplates mostly stand on Nb foils, with a diameter of about 2–3 µm and a thickness from 50 to 250 nm. The nanorods have a rectangle section of 250 × 150 to 250 × 250 nm2 or a triangle section with a side of about 250 nm, and a length up to 4.5 µm, partly standing on the substrates. Their growth mechanisms are briefly discussed. Field-emission experiments showed that the three micro/nanostructures have a notable emission current, and their turn-on fields (defined as the electric field required to produce a current density of 10 µA cm−2) are 4.7, 2.0, and 1.9 V µm−1, respectively, suggesting their potential application in electron emission devices.

Ir coating prepared on Nb substrate by double glow plasma

Abstract Compact and adhesion Ir coating was obtained on the surface of the Nb substrate by double glow plasma method which had a deposition chamber and two cathodes. Argon gas was used as the working gas. The bias voltage of Ir target and Nb substrate were −950 V and −300 V, respectively. The chamber pressure was 35 Pa. Microstructure and morphology of the Ir coating was examined by the X-ray diffraction and scanning electron microscopy, respectively. The Ir coating had a poly-crystalline structure with preferential growth orientation of (2 2 0) crystal plane. The deposition velocity was ∼5.7 μm/h. The good adhesion resulted from the buffer layer between the Nb substrate and the Ir coating. The Nb concentration distributed gradiently along the depth of the buffer layer. Because the target and the substrate were the cathode electrodes, the Nb and Ir were continuously deposited and sputtered out on the Nb substrate surface. For the higher bias voltage of the target, sputtered Nb atoms were suppressed by intense Ir atoms and formed a mixed boundary layer on the Nb substrate surface. With the Ir and Nb deposition, the Nb element in the as-deposited film gradually decreased and led the Nb atoms content decrease in the boundary layer.

Spin–orbit coupling in low‐dimensional gold

AbstractWe present experimental and theoretical evidence of the role played by the spin–orbit coupling in the electronic structure of a pseudomorphic Au monolayer on Nb(001) substrate. The bands found with the help of the angle‐resolved ultraviolet photoelectron spectroscopy (ARUPS) are compared with those obtained from ab initio self‐consistent calculations by the VASP and WIEN2k codes. The slab calculations are performed including geometric relaxation and using both the generalized‐gradient (GGA) and local‐density (LDA) approximations for the exchange–correlation energy. The dispersions and energy positions of the calculated bands agree with the experimentally determined band structure only if the LDA is used and the spin–orbit coupling is included. Therefore, both the structure relaxation and spin–orbit coupling are essential in understanding the electronic structure of the Au/Nb(001) system.magnified image(© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal stability of iridium coating prepared by MOCVD

Abstract A multilayer Ir coating with equiaxed grains was prepared using metalorganic chemical vapor deposition on Nb substrates by thermal decomposition of Ir(acac) 3 precursor. Thermal stability of the coating was investigated at 1200 °C and 1300 °C in Ar. The Ir coating exhibited different thermal stabilities depending on heating temperatures. After heat treatment, the prepared nanometer and sub-micrometer grains grow obviously and grain size increased with rising heating temperature. Pores diffused outward and their amount reduced. After heat treatment, the coating gradually transformed into a compact layer. Nb diffused into Ir coating during heat treatment, and solid solutions were formed at 1300 °C.

積層クラッドを用いたMoおよびNbに対するアルミナイド被膜の作製

In order to improve the oxidation resistances of Mo and Nb alloys, aluminide coatings were developed by lamination cladding. Mo, Nb, and Al sheets with a purity of 99.99 wt% and dimensions of 15 mm×10 mm×0.5 mm were prepared by cutting, followed by the development of a specimen piece by holding the Al sheet between two sheets of Mo or Nb. The specimen wrapped with a Ta foil to prevent oxidation was heated at temperatures of the order of the melting point of Al in a pure Ar atmosphere. On the basis of the elemental analysis by characteristic X-rays, the laminated layers formed on the Mo substrates were found to be composed of MoAl5, MoAl4, and Mo3Al8. It was also found that the main phase constituting the laminated layers changed from MoAl5 to Mo3Al8 on increasing the temperature from 973 K to 1473 K. In contrast, NbAl3 was singly formed on the Nb substrate, irrespective of the heating temperatures. However, since NbAl3 grains have a spherical shape, the NbAl3 layer was not formed on the Nb substrate.

Preparation and properties of GaFeO3 thin films grown at various oxygen pressures by pulsed laser deposition

GaFeO3 thin films were prepared on (111)-oriented SrTiO3:Nb substrates under various oxygen pressures from 0.1 to 10 Pa at 700 °C by using a pulsed laser deposition method. Effects of the oxygen pressures on the crystallinity, dielectric and ferroelectric properties of the thin films were investigated at room temperature. The results show that the GaFeO3 thin films exhibit ferroelectricity and the properties of the thin films are influenced markedly by the deposition oxygen pressures.

Structural and electrical properties of Pb(Zr,Ti)O3 films grown by molecular beam epitaxy

Single-crystal, single-phase Pb(ZrxTi1−x)O3 films (x=0–0.4) were grown on (001) SrTiO3 and SrTiO3:Nb substrates by molecular beam epitaxy. Layer-by-layer growth of the Pb(Zr,Ti)O3 films was achieved by using PbTiO3 buffer layers between the SrTiO3 substrates and the Pb(Zr,Ti)O3 films. The layers with low Zr content showed high crystallinity with full width at half maximum of ω-rocking curves as low as 4arcmin, whereas increase in Zr concentration led to pronounced angular broadening. The PbZr0.07Ti0.93O3 films exhibited remanent polarization as high as 83μC∕cm2, but local areas suffered from nonuniform leakage current.

Deposition of MgB<sub>2</sub> Thin Films on Nb Substrates Using an In Situ Annealing PLD Method

MgB2 thin films have been coated on Nb substrates without any buffer layers. An in situ pulsed laser deposition (PLD) method was used to prepare the coating. The interface between films and substrates has been characterized by scanning electron microscopy (SEM). Surface impedance has been measured for the MgB2 films on Nb substrates. The results were discussed with regard to the potential large scale applications in superconducting RF cavities.

Hydrothermal synthesis of anatase-type TiO 2 films on Ti and Ti–Nb substrates

The hydrothermal synthesis of anatase-type TiO 2 films on Ti and Ti–Nb substrates was examined with a focus on crystallinity and surface morphology of the films. The substrates were chemically treated with H 2O 2/HCl or H 2O 2/HNO 3 solution to prepare amorphous-like TiO 2. The hydrothermal treatment of the resultant films was performed in an autoclave with deionized water or NH 3 aqueous solution. The synthesized anatase-type TiO 2 films showed high crystallinity and rectangular morphology. The (Ti, Nb)O 2 film could be obtained by using Ti–Nb alloys as substrates.

Improvement of high temperature stability of Pd coating on Nb by intermediate layer comprising NbC and Nb 2C

Non-porous intermediate layers comprising NbC and Nb 2C were prepared between thin Pd films (100 nm) and Nb substrates, and the high temperature stability (573–873 K) of Pd coating was examined by hydrogen absorption experiments. This type of intermediate layer provided better stability of Pd coating at/below 673 K than the layers consisting solely of Nb 2C. Sharp reduction and recovery in hydrogen absorption rate were observed after heat treatments at 773 and 873 K, respectively. The mechanism underlying such complicated temperature dependence was discussed.

Infrared vacuum brazing of Ti–6Al–4V and Nb using the Ti–15Cu–15Ni foil

The microstructural evolution of infrared brazed joint is primarily depended upon redistribution of alloying elements such as Cu, Ni and Nb across the joint. For the specimen infrared brazed at 970 °C for 180 s, the joint primarily consists of coarse eutectic Ti 2Cu, Ti 2Ni intermetallics and transformed β-Ti. Increasing the brazing time to 600 s results in a fine eutectoid microstructure dominating the entire joint due to the depletion of Cu and Ni from the braze alloy into Ti–6Al–4V substrate. Ti 2Cu, Ti 2Ni and transformed β phases are almost absent from the specimen infrared brazed for 3600 s, and the β-Ti is stabilized by alloying high concentration of Nb. Based on the shear test result, all cracks propagate along the Nb substrate with typical ductile dimple rupture appearance. Consequently, Ti–6Al–4V and Nb can be vacuum brazed by Ti–15Cu–15Ni foil with good joint strength.

Optimized dielectric properties of SrTiO3:Nb∕SrTiO3 (001) films for high field effect charge densities

The authors report the growth, structural and electrical characterizations of SrTiO3 films deposited on conductive SrTiO3:Nb (001) substrates by high pressure reactive rf magnetron sputtering. Optimized deposition parameters yield smooth epitaxial layers of high crystalline perfection with a room temperature dielectric constant ∼200 (for a thickness of 1150Å). The breakdown fields in SrTiO3:Nb∕SrTiO3∕Ag capacitors are consistent with induced charge densities >1×1014cm−2 for both holes and electrons, making these films ideal for high charge density field effect devices.