Energy Resolution of STJ X-Ray Detectors with Killed Electrode. Simple Model

Superconducting tunnel junctions X-ray detectors Ti/Nb/Al,AlO _x /Al/Nb(2)/NbN with killed Ti/Nb electrode was studied as a function of bias voltage, energy of the absorbed quantum, and thickness of the Nb(2) layer. The data was compared with a simple diffusion model including the losses of excess quasiparticles due to self-recombination. It was shown that increasing of the electrode thickness reduces the self-recombination contribution and improves the linearity of the detector response.     

Bias Voltage Dependence of Quasiparticle Recombination in STJ Detectors with Killed Electrode

Superconducting tunnel junction X-rays detectors Ti/Nb/Al/AlO _x /Al/Nb/NbN with the Ti/Nb/Al/ killed electrode has been studied under irradiation by X-rays photons of different energies produced by the fluorescence method. The nonlinearity of the STJ-detector response versus photon energy has been studied as a function of the bias voltage. The minimum of the nonlinearity is observed when the tunneling probability P ₁ has maximum and the detector signals have shortest rise times. The experimental data were analyzed on the basis of the diffusion model taking into account the quasiparticle self-recombination. The simple approximate expression for the self-recombination contributions to STJ-detector signal was obtained. The nonlinearity of the response depends on the ratios of the recombination constant to the diffusion coefficient R/D and the diffusion length to the initial radius of the quasiparticle distribution Λ/a ₀ and is inverse proportional to the thickness of the electrode.      

High resolution detection of x-rays with a Nb-based STJ detector

A high energy-resolution of 88 eV has been achieved for 5.9-keV x-rays with a large area (178×178 µm²) Nb/Al-AlOx/Al/Nb superconducting tunnel junction (STJ) detector, which is stable at room temperature and robust to thermal cycles. The energy resolution is higher than those of semiconductor detectors. The resolution and the short shaping-time-constant, 2 µs, of the main amplifier used to obtain the energy resolution indicate that STJ detectors can be developed as nuclear radiation detectors with high energy resolution even for high rate radiations. Besides, a theoretical limit of energy resolution due to the statistical fluctuation of signal charges is discussed.     

Nonequilibrium phenomena in nonsymmetrical superconducting double-barrier structure Nb/NbOx/Al/AlOx/Nb

Tunneling structure Nb/NbO_x/Al/AlO_x/Nb with thin barrier in the Nb/NbO_x/Al junction and 4–6 nm thick Al interlayer was prepared and experimentally studied. Pair breaking at the tunneling processes in one junction produces nonequilibrium state of quasiparticle density in Al interlayer and consequently nonequilibrium tunneling processes in the whole structure. All joined effects, namely gap voltage suppression, presence of negative differential resistance at gap voltage and effects induced by applying of microwave radiation are similar to those observed on stacked Nb/Al/AlO_x/Nb tunnel junction current-voltage characteristics. Results may be used for the interpretation of behavior of high current density SNS or SIS type multilayers of low and high temperature superconductor structures.     

Anomaly in Fabrication Processes for Large-Scale Array Detectors of Superconducting Tunnel Junctions

The STJ array detectors with an effective detection area of 4 mm², which consist of 100 Nb/Al-AlOx/Al/Nb junctions with a size of 200×200 μm, have been fabricated. In order to improve the reproducibility of the STJ array fabrication, we investigated a correlation between the junction surface structures and the leakage currents. It has been found that the junctions near the fringe of the array detectors have a step of about 5 nm at the middle of the array detector, of which leakage currents are considerably larger than 1 μA. The step structure was formed after the etching of the bottom Nb layer for complete separation of Nb/Al/AlOx/Al/Nb/Si. In case of the sputtered Nb/Al/Nb/Si multilayers without 1 nm-thick tunneling barrier, no stepped surface was observed even after the bottom Nb layer etching. Therefore, it is apparent that the 5 nm step structure is a cause of the large leakage currents. We solved the step-fringe problem by a kind of extra patterning along the fringe of the array or lift-off patterning of the Nb/Al multilayers. It is concluded that the number of the junctions with the step structure depends on a slight difference in film deposition or etching conditions.      

Response of the Killed Electrode in STJ X-ray Detectors

The signals of STJ detectors Ti/Nb/Al,AlO _x /Al/Nb/NbN with killed Ti/Nb electrode were studied as a function of the bias voltage, the energy of the absorbed quanta and the thickness of the electrodes. The nonlinearity of the energy calibration for the killed electrode signal had a positive curvature due to the quasiparticles self-recombination losses and 2Δ-phonon exchange. Suppression of residual signals of the killed electrode was achieved by increasing the thickness of this electrode.     

STJ X-ray detectors with titanium sublayer

Superconducting tunnel junctions (STJs) with the structure Ti/Nb/Al, AlO_x/Al/Nb/NbN and corresponding layer thickness 30/100/8/13/150/30 nm were investigated as X-ray detectors at T=1.35 K. STJs with one active electrode in which the response of the other one is suppressed due to trapping layer on the surface opposite to the tunnel barrier have a number of potential advantages. The best line width (FWHM) is 78 eV for 6400 μm² junction. Contribution of the electronic noise is about 50 eV. The intrinsic detector line width is less than 60 eV. The collected charge from inactive electrode is more than 8 times less than that from the active one. Titanium proved to be an appropriate material for a sublayer and a trap.     

Nb/Al/AlOx/Nb superconducting tunnel junctions as x-ray detectors

Nb/Al/AlOx/Nb junctions have been shown to be effective x-ray detectors that are robust to thermal cycling. We compare results from two junctions, one with a fine-grained base-layer electrode and counterelectrode and the other with an epitaxial base-layer electrode and a fine-grained counterelectrode. For a 6 keV x-ray, at 0.4 K, the epitaxial sample had a FWHM resolution of 157 eV and the fine-grained junction had a FWHM resolution of 300 eV. The differences between the junctions will be discussed.     

Hard X-Ray Microscope With Submicrometer Spatial Resolution

A high-resolution hard x-ray microscope is described. This system is capable of detecting line features as small as 0.6 µm in width, and resolving line pairs 1.2-µm wide and 1.2-µm apart. Three types of two-dimensional image detectors are discussed and compared for use with hard x rays in high resolution. Principles of x-ray image magnification are discussed based on x-ray optics and diffraction physics. Examples of applications are shown in microradiography with fiber reinforced composite materials (SiC in Ti₃Al Nb) and in diffraction imaging (topography) with device patterns on a silicon single crystal. High-resolution tomography has now become a reality.     

Photon energy dependence of spatial non-uniformity in super-conducting tunnel junction detectors between 200 ev and 10 keV

It has been demonstrated by Low-Temperature Scanning Synchrotron Microscope that the spatial profiles of Nb/Al/AlO_x/Al/Nb detectors are abnormally large in an energy range between 2 and 10 keV, and thus the detectors produce no clear total absorption peaks. Below 2 keV, on the other hand, full illumination experiments have afforded the evidence that the spatial uniformity is significantly improved, for example, an 100 μm square junction has an intrinsic energy resolution of 5.4 eV at 200 eV. These results reconfirm that carefully designed junction detectors with a reasonable size are ideal for energy-dispersive spectroscopy in a soft X-ray range.     

Quasiparticle Freeze-Out in Superconducting Tunnel Junction X-ray Detectors with Killed Base Electrode

The current–voltage characteristics of superconducting tunnel junction (STJ) X-ray detectors were measured in the temperature range from 4.2 to 0.1 K. The freeze-out of the thermal tunneling current was compared between an STJ detector with a traditional Nb/Al/Al \(_{2}\) O \(_{3}\) /Al/Nb layer structure and a Ti/Nb/Al/Al \(_{2}\) O \(_{3}\) /Al/Nb/NbN detector whose low-gap Ti film kills the X-ray response of the base electrode. The current decrease and the linear low-temperature I(V) characteristics for the detector with the killed electrode can be qualitatively explained by tunneling current contributions from the subgap states of the Ti film. The data are analyzed on the basis of the proximity theory in the dirty limit.     

Effect of Mechanochemical Processing on the Oxidation of 3Nb + Al Mixtures

Mechanochemical processing is shown to influence the oxidation behavior of 3Nb + Al mixtures. As determined by thermogravimetric analysis and x-ray diffraction, mechanical activation leads to the formation of solid solutions. The processing time affects the reactivity of the mixtures. The highest reactivity is offered by the mixture ball-milled for 5 min: oxidation of this mixture during heating in air from 25 to 600°C results in the formation of Nb₂O₅ and Al₃Nb.     

Microstructure of interfacial reaction zone in SCS-6 SiC fibre reinforced super α2 composites

Abstract

The microstructure of the interfacial reaction zone in SCS-6 SiC/super α2 composites heat treated at 700°C for 3000 h was investigated by means of analytical transmission electron microscopy. The very fine grained reaction layer adjacent to the carbon coating of the SiC fibre was found to consist of two sub layers, determined to be (Ti, V)C and (Ti, V,Nb)₅Si₃. The second layer is (Ti,Nb)C with large equiaxed grainsfollowed by the third layer consisting of the (Ti,Nb)₃(Al,Si)C phase. This layer is separated from the matrix by a fourth layer with the phase composition (Ti,Nb)₅(Si,Al)₃. At some interface positions, the two layers of(Ti,Nb)C and (Ti,Nb)₃(Al,Si)C are separated by an additional layer of the (Ti,Nb)₃(Si,Al) phase. The thickening of the interfacial reaction zone at 700°C is mainly due to the layers of (Ti, Nb)₃(Al,Si)C and (Ti,Nb)₃(Si,Al). The growth of these two layers is probably responsible for the degradation of the mechanical properties of the composites.     

Structural evolution during mechanical alloying and annealing of a Nb-25at%Al alloy

Mixtures of pure elemental Al and Nb powders of Nb-25at%Al composition was mechanically alloyed, and structural evolution during high energy ball milling has been examined. Al dissolved in Nb from the early stage of the ball milling, and amorphization became noticeable after longer than five hours of milling. However the dissolution of Al in Nb was not completed before the amorphization. No intermetallic phase formed during the mechanical alloying. Before complete amorphization, metastable nitride of Nb_4.62N_2.14 (i.e., -NbN) with hexagonal structure has formed in nanocrystalline size through nitrogen incorporation from ambient environment. The lattice parameter of Nb increased significantly (up to 3.3433 Å after 5 hours of milling) during the milling. Upon annealing above 950 °C, Nb₂Al became the dominant feature with the -NbN, and Nb₃Al did not form from the samples milled at ambient environment. Nb₃Al appeared only from a sample milled at Ar environment. Structural evolution during mechanical alloying of the Nb-Al system is critically dependent the upon milling environment.     

Superconducting Solid-State Particle Spectrometers for Atoms and Macromolecules of 3–20 keV

Superconducting detectors have no dead surface-layer. It has been found that even if there is a 700 nm-thick SiO₂ layer on the sensitive area, the detectors produce measurable output pulses for molecule impact. This feature is very attractive in solid-state spectroscopy of low-energy atoms or molecules for basic chemistry, nuclear physics, and life science. The superconducting tunnel junction detectors enable the measurement of the deposited energy for individual particle impacts in contrast to conventional particle detectors that rely on secondary particle emission. A study of the particle-surface interaction with atoms, proteins, and synthetic polymers has revealed that there are three regions. As the mass value increases, the pulse height reduction, or the decrease of the deposited energy, is remarkable in a mass range below 2,000, the pulse height increases in 2,000–100,000, and finally almost constant pulse height appears in 100,000–1,000,000.      

Effect of Nb and Nb2O5 additives on mechano-thermal processing of TiAl/Al2O3 nano-composite

In this research, TiAl matrix nano-composite with Al₂O₃ reinforcement was obtained by mechanical activation of TiO₂ and Al powder mixture and its subsequent heat treatment. Effect of Nb and/or Nb₂O₅ additions on the process was investigated. Structural changes and thermal behavior of the samples were evaluated by X-ray diffraction and differential thermal analysis, respectively. Moreover, the microstructure was characterized by transmission electron microscopy. The results confirmed the partial dissolution of Nb in Al during the milling stage in the Nb-added samples. The reaction mechanism during heat treatment in the sample without any additives was a two-stage process that was quite similar to the sample with Nb addition. However, Nb₂O₅ addition led to the progress of reaction through a single stage and with a higher rate. Both additives promoted formation of the Ti₃Al phase in the final products. The results confirmed the formation of nano-sized Al₂O₃ particles in a nano-crystalline Ti–Al matrix with a mean crystallite size of 30 nm.     

Structure and superconductivity studies on ternary equiatomic silicides,MM′Si

A series of ternary equiatiomic transition metal silicidesMM′Si (M=Zr, Nb, Ta;M′=Ru, Re) and Zr_1/2 M _1/2 RuSi (M=Nb, Ta) have been synthesized and characterized. Guinier x-ray data clearly indicate that the compoundsMM′Si (M=Nb, Ta;M′=Ru, Re) crystallize with the orthorhombic TiFeSi-structure. This is contrary to one of the literature reports ascribing the hexagonal ZrRuSi structure toMReSi,M=Nb, Ta. ZrRuSi and Zr_1/2 M _1/2RuSi (M=Nb, Ta), however, crystallize with the ZrRuSi structure. All the compounds are metallic and exhibit small and negative Seebeck coefficients. Superconductivity has been discovered inMM′Si,M=Nb, Ta;M′=Ru, Re and the transition temperatures, as measured by the low frequency a.c. susceptibility technique, range from 2·7 to 5·1 K. Zr_1/2Nb_1/2RuSi is not superconducting above 2·2 K. The observed properties are explained in terms of the electronic charge transfer effects and crystal structures adopted by these metal excess ternary silicides. Dedicated to the memory of Prof. N S Satya Murthy.     

Interfacial reaction kinetics of SiC fiber-reinforced Ti3Al matrix composites

• 1.(1) SiC fibers reacted with Ti₃Al + Nb to form multilayer reaction products during consolidation and extended isothermal exposure. Complex carbides and silicides genertated from the reactions between Ti, Nb, Al and SiC appears to be the major components in the reaction zone. The C-rich layer on the surface of a SiC fiber affects the development of the reaction zone and the distribution of the reaction products.
• 2.(2) The fiber/matrix interfacial reaction is diffusion-controlled with an activation energy of 271.49 KJ/mole and 218.11 KJ/mole for the SCS-6/Ti-24A1-11Nb and Sigma/Ti-24Al-11Nb composites, respectively, at the early stage of reaction.
• 3.(3) The results of this study indicate that in the SCS-6/Ti-24Al-11Nb composite, the activation energy is higher, the growth rate of reaction zone is slower, and the consumption of C-rich layer is much slower than that in the SCS-6/Ti-6Al-4V composite.

     

Improvement of sulfide-corrosion resistance of gas-turbine blades with sufficient heat resistance by rational alloying

We present the results concerning the influence of alloying elements (Cr, Al, Ti Mo, W, Nb, and Co) on sulfide corrosion resistance in a 75% Na₂SO₄+25% NaCl melt at a temperature of 900°C for 30 h and heat resistance (τ ₂₀₀ ⁹⁰⁰ ) of nickel alloys. To provide high resistance to sulfide corrosion and longterm strength at the level of τ ₂₀₀ ⁹⁰⁰ , nickel-base alloys must contain 14–16% Cr, Ti/Al=0.8–1.5 for ∑Al+Ti=6–7.5%, Cr/Al≥6, , 3–4% Mo, 3–4% W, 4–6% Co, and 1.5–2% Nb. Frantsevych Institute for Problems in Materials Science, Ukrainian Academy of Sciences, Kiev. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 35, No. 4, pp. 79–83, July–August, 1999.