Experimental evidence of considerable stability increase in superconducting windings with extremely high specific heat substances

The influence of rare-earth based intermetallic compounds with very high heat capacities on the transient stability of NbTi superconducting windings was experimentally investigated. The intermetallic compounds CeCu₆ and HoCu₂ were introduced as filling powders into an epoxy resin based composite in a wet winding process. Doping of about 6 vol.% of such compounds increases minimum quench energies several times even for short (of order of 1 ms) electromagnetic disturbances.The prospects and most promising applications are briefly discussed.     

Preparation and mobile ion transport studies of Ta and Nb doped Li6Zr2O7 Li-fast ion conductors

Pure and doped Li_6−x(Zr_2−xM_x)O₇, M = Nb and Ta; x = 0, 0.15 compounds have been prepared by the urea combustion method followed by annealing at 950 °C for 8 h. The samples are characterized by X-ray diffraction and impedance spectroscopy. Ionic conductivities, σ_ionic, were determined in the temperature range of 60-360 °C by impedance spectroscopy. We observe that the Ta doped Li₆Zr₂O₇ has a measurable σ_ionic at ∼160 °C, and at 300 °C exhibits a conductivity value of 1 × 10⁻³ S/cm. The temperature dependence of the conductivity in the range 100-360 °C obeys an Arrhenius relation, yielding an activation energy of E_a = 0.95 eV (for M = Ta and x = 0.15).The bond valence approach has been used to visualise Li⁺ ion migration pathways and the conductivity mechanism in these compounds. The lowest energy pathway is found to extend along the [0 1 2] direction. The Bond valence analysis also indicates a significantly anisotropic Li-ion conductivity in compounds with Li₆Zr₂O₇ type structure, predicting activation energies of 1.1 and 0.9 eV for the low energy pathway in undoped and doped Li₆Zr₂O₇.     

An aqueous sol-gel synthesis of chromium(III) doped mesoporous titanium dioxide for visible light photocatalysis

Nanoparticles of titanium dioxide doped with Cr³⁺ ions have been prepared through an aqueous sol-gel method. The mesoporous nature of both pure and Cr³⁺ doped TiO₂ powders, with specific surface area of 7.4 and 6.6 m² g⁻¹, respectively, is maintained even at calcination temperature of 800 °C. The transformation of TiO₂ from the anatase to rutile phase is suppressed up to 800 °C by Cr³⁺ ion doping. Even though surface area values are decreased, the doped materials show improved photocatalytic activity, which may be due to increased crystallinity of the anatase phase without the formation of rutile. Doped materials have a red-shift in the band gap energy and hence, photoactivity under visible light. The rate of photodegradation of methylene blue dye for both pure and doped TiO₂ under visible light has been monitored in this study. The 0.25 mol% Cr(III) doped photocatalyst, calcined at 800 °C, shows the highest photocatalytic activity under visible light with a rate constant of ∼15.8 × 10⁻³ min⁻¹, which is nearly three times higher than that of commercially available Degussa P25 titania (5.8 × 10⁻³ min⁻¹).     

Effect of yttrium doping on the dielectric properties of CaCu3Ti4O12 thin film produced by chemical solution deposition

Pure and yttrium substituted CaCu₃Ti_4 − xY_xO_{12 − x / 2} (x = 0, 0.02, 0.1) thin films were prepared on boron doped silica substrate employing chemical solution deposition, spin coating and rapid thermal annealing. The phase and microstructure of the sintered films were examined using X-ray diffraction and scanning electron microscopy. Dielectric properties of the films were measured at room temperature using electrochemical impedance spectroscopy. Highly ordered polycrystalline CCTO thin film with bimodal grain size distribution was achieved at a sintering temperature of 800 °C. Yttrium doping was found to have beneficial effects on the dielectric properties of CCTO thin film. Dielectric parameters obtained for a CaCu₃Ti_4 − xY_xO_{12 − x / 2} (x = 0.02) film at 1 KHz were k ∼ 2700 and tan δ ∼ 0.07.     

Synthesis and characterization of ferromagnetic cobalt-doped tin dioxide thin films

Co-doped SnO₂ thin films are grown on sapphire (0001) substrates at 600 °C by the technique of dual-beam pulsed laser deposition. The prepared films show preferred orientation in the [100] direction of the rutile structure of SnO₂. Nonequilibrium film growth process results in doping Co into SnO₂ much above the thermal equilibrium limit. A Film with 3% of Co is ferromagnetic at room temperature with a remanent magnetization of ∼ 26% and a coercivity of ∼ 9.0 mT. As Co doping content x increases, the optical band gap absorption edge (E₀) of the Co-doped SnO₂ thin films initially shows a redshift at low x up to x = 0.12 and then increases at the higher x, which are attributed to the sp-d exchange interactions and alloying effects, respectively.     

Buckling of filament-wound composite cylinders subjected to hydrostatic pressure for underwater vehicle applications

The buckling and failure characteristics of moderately thick-walled filament-wound carbon–epoxy composite cylinders under external hydrostatic pressure were investigated through finite element analysis and testing for underwater vehicle applications. The winding angles were [±30/90]_FW, [±45/90]_FW and [±60/90]_FW. ACOS, an in-house finite element program, successfully predicted the buckling pressure of filament-wound composite cylinders with 2 ∼ 23% deviation from the test results. The analysis and test results showed that the cylinders do not recover the initial buckling pressure after buckling and that this leads directly to the collapse. Major failure modes in the test were dominated by the helical winding angles.     

Multiphonon-based comparison of erbium-doped yttria in large, fine grain polycrystalline ceramics and precursor forms

We have studied the phonon-induced non-radiative decay in erbium doped yttria (Y₂O₃). The technique employed allows for the evaluation of potential ceramic and crystalline laser materials. The frequency of the dominant phonon that deactivates the fluorescing levels and an approximate prediction of 0 K lifetime can be determined. Results show no significant quantitative difference between very large grain polycrystalline (with grain size ∼200-500 μm) ceramic, fine grain polycrystalline (with grain size ∼0.3 μm) ceramic and the precursor powder (with ∼30 nm particle size) of Er³⁺ doped Y₂O₃, when it comes to the dominant phonon frequency and the phonon occupancy number.The results show that a correct evaluation of the final product can be made in the precursor stage of the process eliminating the need to proceed to crystalline or fully sintered ceramic form to evaluate the spectroscopic properties of the material. It should be noted that the powders must be carefully prepared and handled. Adsorbed species such as water can drastically change the effective lifetimes observed in powder samples.     

Modification of the physical properties of semiconducting MgAl2O4 by doping with a binary mixture of Co and Zn ions

The effects of doping of MgAl₂O₄ by a binary mixture of Co and Zn ions on the absorbance, electrical resistivity, capacitance, thermal conductivity, heat capacity and thermal diffusivity are reported in this paper. The materials with the nominal composition Mg_1−2x(Co,Zn)_xAl₂O₄ (x = 0.0-0.5) are synthesized by solution combustion synthesis assisted by microwave irradiation. The substituted spinels are produced with a Scherrer crystallite size of 18-23 nm, as opposed to 45 nm for undoped samples, indicated by X-ray diffraction and confirmed by transmission electron microscopy. These materials also show better thermal stability in the temperature range of 298-1773 K. Three strong absorption bands at 536, 577 and 630 nm are observed for the doped samples which are attributed to the three spin allowed (⁴A₂ (F) → ⁴T₁ (P)) electronic transitions of Co²⁺ at tetrahedral lattice sites while pure magnesium aluminate remains transparent in the whole spectral range. The semiconducting behavior of the materials is evident from the temperature dependence of the electrical resistivity. Resistivity and activation energy are higher for the substituted samples. Fitting of the resistivity data is achieved according to the hopping polaron model of solids. Both dielectric constant and loss increase on account of doping. The dielectric data are explained on the basis of space charge polarization. The thermal conductivity and diffusivity are lowered and the heat capacity is increased in the doped materials. Wiedemann-Franz's law is used to compute the electronic and lattice contributions towards the total thermal conductivity.     

Application of secondary neutral mass spectrometry in the investigation of doped perovskites

Electromagnetic properties of doped perovskites depend sensitively on the doping level. Both the superconducting transition temperature of Bi₂Sr₂Ca(Pr)Cu₂O_8+δ compounds and the magnetic and electronic transport properties of La(Sr)Co(Fe)O₃ perovskites change dramatically with the doping level. Apart from doping, oxygen deficiency is influenced by the details of preparation processes such as calcination and sintering. Simultaneous determination of constituents is of crucial importance from sample characterization point of view. Quantitative analysis of perovskite oxides can be performed by Secondary Neutral Mass Spectrometry (SNMS) which is a suitable technique to measure the chemical composition of almost any sample because the flux of atoms sputtered from the sample is representative of the stoichiometry of the top-most layers. The composition and oxygen content of Bi₂Sr₂Ca_0.86Pr_0.14Cu₂O_8.4 and La_1−xSr_xCo_0.975Fe_0.025O_3−δ, where 0 ≤ x ≤ 0.25, were determined by SNMS. The results show that the method is equally applicable for insulating and conducting compounds. The observed electromagnetic properties reflect well the compositions obtained experimentally.     

Phase evolution and magnetic property of Bi1 − xDyxFeO3 ceramics

Rare-earth ion Dy was substituted at the bismuth site in BiFeO₃ to produce Bi_1 − xDy_xFeO₃ (x = 0, 0.1, 0.2 and 0.3) polycrystalline ceramics. A two-stage solid-state reaction method was adopted in synthesizing the materials. The effects of varying the Dy doping concentration on the crystalline structures, morphologies and magnetic properties of the final products have been investigated. It is found that Dy doping resulted in a sequence of structural phase transitions and led to small grain sizes in the materials. As a result, the magnetic property of the doped samples was much enhanced. Therefore, it seems a promising way to improve the weak ferromagnetic property of BiFeO₃ based materials by Dy doping.     

Synthesis, structural and magnetic properties of La1−xCdxFeO3 (0.0 ≤ x ≤ 0.3) orthoferrites

Nanocrystalline La_1−xCd_xFeO₃ (0.0 ≤ x ≤ 0.3) solid solutions have been synthesized by a single-step solution combustion method at a relatively low temperature of 400 °C. The combustion-synthesized solid solutions were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and magnetic measurements. The crystal structure examined by XRD indicates that the samples were single-phase, and crystallize in an orthorhombic (space group, Pbnm no. 62) structure. The parent and doped compounds showed canted antiferromagnetic behavior associated with an increase in magnetic moment with Cd doping. The changes in magnetic properties of the materials are correlated to the changes in structural features resulting from the Rietveld structural refinement of the materials.     

Effects of CuO doping on the electrical properties of 0.98K0.5Na0.5NbO3-0.02BiScO3 lead-free piezoelectric ceramics

CuO-doped 0.98K_0.5Na_0.5NbO₃-0.02BiScO₃ (0.98KNN-0.02BS-xCu) lead-free piezoelectric ceramics have been fabricated by ordinary sintering technique. The effects of CuO doping on the dielectric, piezoelectric, and ferroelectric properties of the ceramics were mainly investigated. X-ray diffraction reveals that the samples at doping levels of x ≤ 0.01 possess a pure tetragonal perovskite structure. The specimen doped with 1 mol% CuO exhibits enhanced electrical properties (d₃₃ ~ 207 pC/N, k_p ~ 0.421, and k_t = 0.424) and relatively high mechanical quality factor (Q_m = 288). These results indicate that the 0.98KNN-0.02BS-0.01Cu ceramic is a promising candidate for lead-free piezoelectric ceramics for applications such as piezoelectric actuators, harmonic oscillator and so on.     

Structure and electrical transport properties of pure and Li2O-doped CuO/MgO solid solution

The different electrical properties, σ, ?′, tan δ and E_σ of pure and Li₂O-doped CuO/MgO solid solution were investigated. The mole fraction of CuO (MF) was varied between 0.048 and 0.2. Pure and doped samples were subjected to heat treatments at 673 and 1073 K. The results revealed that the amount of CuO dissolved in MgO lattice increases progressively by increasing the MF as evidenced from the progressive decrease in the intensity of all diffraction lines of undissolved CuO phase. The dissolution process of copper ions in MgO lattice was accompanied by progressive increase in its lattice parameter. This process being conducted at 1073 K was accompanied by a significant progressive increase in the values of σ, ?′ and tan δ with subsequent decrease in the value of E_σ. The increase in the MF value of CuO from 0.048 to 0.2 led to a significant increase in the value of σ_DC, measured at room temperature, from 6.33 × 10⁻¹² to 9.9 × 10⁻¹⁰ Ω⁻¹ cm⁻¹ and E_σ decreases from 0.76 to 0.58 eV.Li₂O doping of the investigated system followed by calcination at 1073 K resulted in a measurable increase in values of σ, ?′ and tan δ with subsequent decrease in E_σ. These results were discussed in the light of the possible effective increase in the charge carriers concentration (Cu²⁺ions dissolved in MgO lattice) and also to an effective increase in mobility of these charge carriers by Li₂O doping.     

Deep level transient spectroscopy on charge traps in high-k ZrO2

The deep trap properties of high-dielectric-constant (k) ZrO₂ thin films were examined by deep level transient spectroscopy (DLTS). The hole traps of a ZrO₂ dielectric deposited by sputtering were investigated in a MOS structure over the temperature range, 375 K-525 K. The potential depth, cross section and concentration of hole traps were estimated to be ∼ 2.5 eV, ∼ 1.8 × 10^− 16 cm² and ∼ 1.0 × 10¹⁶ cm^− 3, respectively. DLTS of ZrO₂ dielectrics can be used to examine the threshold voltage shift (?V_th) during the operation of SONOS-type flash memory devices, which employ high-k materials.     

Electrical properties of MgAl2-2xZrxMxO4 (M = Co, Ni and x = 0.00-0.20) synthesized by coprecipitation technique using urea

This paper presents the results of a study concerning the structural and electrical properties of MgAl_2-2xZr_xM_xO₄ (x = 0.00-0.20 and M = Co²⁺ and Ni²⁺) prepared by a coprecipitation technique using urea as a precipitating agent. The X-ray diffraction data for the pure and its doped samples are consistent with the single-phase spinel and their crystallite sizes are in the range 7-20 ± 4 nm. The DC resistivity increases from 3.09 × 10⁹ Ω cm to 6.73 × 10⁹ and 8.06 × 10⁹ Ω cm whereas dielectric constant decreases from 5.80 to 5.11 and 4.95 on doping with Zr-Co and Zr-Ni, respectively. The electrical resistivity variations with increase in the dopant contents indicate two types of conduction mechanisms in operation. Several parameters such as, hopping energy (W), metal-semiconductor transition temperature (T_MS) and Debye temperature (θ_D) have also been determined. The increase in DC resistivity and decrease in dielectric constant suggest that the synthesized materials can be considered for application as an insulating and structural material in fusion reactors.     

Transport characteristics of ZrO2 dispersed mixed (BaCl2)1−x-(KCl)x solid electrolytes

Composite solid electrolytes in the system [(BaCl₂)_1−x:(KCl)_x]_1−y:(ZrO₂)_y were prepared following the conventional ceramic powder processing route. In the mixed matrix system prepared by melt quench technique, a nominal increase in conductivity (σ) was found in (BaCl₂)_0.9:(KCl)_0.1. On ZrO₂ particle dispersion in this mixed matrix, the maximum conductivity (∼90 times that of base matrix value) was found to occur with 50 m/o of ZrO₂. Conductivity increases monotonically over the temperature range from 100 to 300 °C studied and attains the value of 10 × 10⁻⁶ S cm⁻¹ at 300 °C. The mobility (μ) of the charge carriers at room temperature was found to be 18.5 × 10⁻² cm² V⁻¹ s⁻¹ and the increase in μ with temperature was not very significant. The transference ionic number determination showed that the electrical conductivity of the electrolyte is predominantly due to ions. This study indicates that the conductivity is governed by mobile ion concentration.     

Utilization of large heat capacity substances for improving the stability of superconducting magnets

Several years ago at Kurchatov Institute the R&D program on the new type of superconducting magnets (SM) doped with large heat capacity substances (LHCS) in order to improve their stability was started. We began from an “external” doping by the LHCS powder mixed with epoxy resin using the wet-winding process. Later on at Bochvar Institute the methods to introduce LHCS inside superconducting wires (both NbTi and Nb₃Sn based) were developed. The comparative tests of LHCS doped wires and model windings with LHCS have shown positive results regarding a considerable increase of critical energies, improving of thermomagnetic stability and training behavior. The state-of-the-art of these methods is reviewed and their perspectives are discussed.     

Structure, hardness and thermal stability of TiAlBN coatings grown by alternating deposition of TiAlN and BN

Hardness and thermal stability of TiAlBN coatings prepared by alternating deposition of TiAlN and BN with high rotation speed of substrate holder (∼7 rpm) have been investigated. The TiAlN and BN were deposited by reactive sputtering of Ti_0.5Al_0.5 and BN targets with N₂ and Ar reactive gases, respectively. Despite alternating deposition, the TiAlBN coating did not show layered structure due to high rotation speed of substrate holder. By TEM analysis, the TiAlBN coating was observed to be nanocomposite with grain less than 10 nm in size. Compared to TiAlN coating, the TiAlBN nanocomposite coating showed superior hardness. Furthermore, the hardness of the coating increased after the heat-treatment in N₂ atmosphere up to 800 °C. By comparison with TiAlN/BN nanoscale multilayered coating prepared by the same deposition method except the lower rotation speed of substrate holder (∼1 rpm), the hardness enhancement after annealing, ‘self-hardening’, of the TiAlBN nanocomposite coating is believed to be due to the sharpness in chemical composition at the interface between TiAlN and BN phase.     

Phosphorous and boron doping of nc-Si:H thin films deposited on plastic substrates at 150 °C by Hot-Wire Chemical Vapor Deposition

Gas-phase phosphorous and boron doping of hydrogenated nanocrystalline thin films deposited by HWCVD at a substrate temperature of 150 °C on flexible-plastic (polyethylene naphthalate, polyimide) and rigid-glass substrates is reported. The influence of the substrate, hydrogen dilution, dopant concentration and film thickness on the structural and electrical properties of the films was investigated. The dark conductivity of B- and P-doped films (σ_d = 2.8 S/cm and 4.7 S/cm, respectively) deposited on plastic was found to be somewhat higher than that found in similar films deposited on glass. n- and p-type films with thickness below ∼ 50 nm have values of crystalline fraction, activation energy and dark conductivity typical of doped hydrogenated amorphous silicon. This effect is observed both on glass and on plastic substrates.     

Electrical characterisation of HfYO MIM-structures deposited by ALD

By an ALD process with the solid precursors HfCl₄ and (CpCH₃)₃Y and the oxidant water Yttrium doped HfO₂ was deposited on TiN layer on highly doped silicon. The films were analysed by ellipsometry, XRR, RBS and XRD. For the electrical characterisation, capacitance and I-V measurement on MIM structure were used. By doping the HfO₂ with 6.2 at.% Yttrium and annealing the film at 500 °C in N₂ the k-value increased by 60% for a 9.5 nm thick film, the leakage current also increased. The deposited amorphous film crystallises at 450 °C into the cubic phase.