Microstructure, Mechanical and Tribological Properties of Ag/Bi2Sr2CaCu2O x Self-lubricating Composites

Ag/Bi₂Sr₂CaCu₂O _x self-lubricating composites were successfully fabricated by a facile powder metallurgy method. The structure and morphology of the as-synthesized composites and the worn surface after tribometer testing are characterized by using X-ray diffraction and scanning electron microscopy together with energy dispersive spectrometry. The results indicated that self-lubricating composites are composed of superconductor phase and Ag phase. Moreover, the effects of Ag on mechanical and tribological properties of the novel composites were investigated. The friction test results showed that the friction coefficient of the pure Bi2212 against stainless steel is about 0.40 at ambient temperature and abruptly decreases to about 0.17 when the temperature is cooled to 77 K. The friction coefficients of the composites from room temperature to high temperature were lower and more stable than those of pure Bi₂Sr₂CaCu₂O _x . When the content of Ag is 10 wt.%, the Ag/Bi₂Sr₂CaCu₂O _x composites exhibited excellent tribological performance, the improved tribological properties are attributed to the formation of soft metallic Ag films at the contacted zone of the composites.     

Needle-like microstructure in Bi2Sr2CaCu2O8+x

We have obtained superconducting Bi₂Sr₂CaCu₂O_8+x ceramic compounds with a grain structure that consists of needle-like particles of about 1.5 mm in length and 100 μm in diameter, which seem to be formed by flake-like small grains. The Bi(2212) compound has been prepared following the polymer solution synthesis method. The thermal treatment used is a simplified version of the melt-textured growth process (MTG), with the sample placed in a zone of a simple tube furnace where the temperature gradient was about 15°C/cm. We have performed scanning electron microscopy (SEM) observations on the samples, before and after thermal treatment. In addition, we have measured the ac susceptibility of both samples to confirm their superconducting nature and critical temperatures of about 91 K.     

A study of the stability of superconducting crystals Bi3Sr3CaCu2.5O x

It is valuable to study the stability of highT _c oxide superconductors for their application. A series of experiments was carried out to study the stability of superconducting crystal Bi₃Sr₃CaCu_2.5O _x in water, and its reactions with several kinds of acid and alkali solutions. The results indicated that the stability of Bi₃Sr₃CaCu_2.5O _x single crystals in water is better than that of the YBa₂Cu₃O₇ superconductor. The BSCCO single crystals are easy to dissolve in hydrochloric acid and nitric acid, but difficult to dissolve in weak base.     

Pinning potential transport measurements of Bi2Sr2CaCu2O x thin films

Flux creep characteristics ofc-axis-oriented Bi₂Sr₂CaCu₂O _x thin films grown by the method of electron beam evaporation of stoichiometric target and subsequent annealing have been investigated in the absence of external magnetic field. A decrease of the pinning potential with decreasing temperature is obtained fromI–V curves. This result is explained by the presence of the spatially nonlinear pinning potential.     

Comparative Studies of the Superconducting Properties of Bi2Sr2CaCu2O8+δ and Bi1.7Pb0.3Sr2CaCu2O8+δ/Ag Random Composites

We report the results of the study of the resistive transition and the screening effect on similar random composites made of a silver matrix containing sintered superconducting grains of bismuth phase with two different stoichiometries. We show that most of the observed behaviors are compatible with a phenomenological approach in which the intergrain junctions behave as weak links whose the state depends strongly on temperature and field. We find that these links a much weaker in one of the series of samples.     

Superconductivity in epitaxial Bi2Sr2CuO6/Bi2Sr2CaCu2O8 superlattices: The superconducting behavior of ultrathin cuprate slabs

Utilizing atomic layer-by-layer molecular beam epitaxy (ALL-MBE), we have synthesized a series of high-quality superlattices in which ultrathin slabs (one-half unit cell thick) of the high-T _c superconductor Bi₂Sr₂CaCu₂O₈ alternate with up to five such layers of the low-T _c Bi₂Sr₂Cu₁O₆ phase. In all these superlattices we foundT _c to be essentially equal to that of the high-T _c Bi₂Sr₂CaCu₂O₈ phase itself, which indicates that this cuprate is a 2D superconductor insofar as the interslab coupling plays at best a secondary role. Furthermore, it is demonstrated thatT _c need not be reduced at heterostructure interfaces.     

Superconductivity of barium-doped Bi2Sr2CaCu2O y

Sintered ceramic samples of Bi₂Sr_2?x Ba _x CaCu₂O_y with nominal barium fraction 0≤x≤0.3 have been prepared by the solid-state reaction method. WDS studies verified that barium enters the superconducting phase. For slowly cooled samples, the midpointT _c of the superconducting transition is significantly increased by barium doping, whereas for quenched samplesT _c is little affected. The increase ofT _c with increasing barium fraction is consistent with a decrease in the hole concentration in the superconducting layers.     

Incommensurate fluctuations in Bi2Sr2CaCu2O8

A special neutron scattering technique has been used to discover an incommensurate fluctuation in Bi₂Sr₂CaCu₂O₈ that appears below T_c. The fluctuation is identified as a dynamic charge density wave since its scattering intensity appears to increase with increasing momentum transfer. The fluctuation is found at a wavevector near 2k_F and could be associated with a dynamic stripe phase.     

Fabrication process of intrinsic Josephson junction stacks in Bi2Sr2CaCu2O8+x crystals by double-sided patterning process using dilute hydrochloric acid

We have developed a new double-sided patterning (DSP) process that employs dilute acid (pH = 1.65) to fabricate Bi₂Sr₂CaCu₂O_8+x (Bi-2212) stacks of intrinsic Josephson junctions. These stacks, which were fabricated from a single crystal of Bi-2212, were surrounded by an acid-treated product. The critical aspect of this process is that the Bi-2212 surrounding the photoresist pattern was converted into a transparent material, BiOCl, which was connected with the Bi-2212 crystal. Consequently, this process provides a simple way to achieve DSP of the surfaces of Bi-2212 crystals. This new DSP process realizes a remarkably improved reproducibility in fabricating Bi-2212 stacks that exhibit good current–voltage characteristics with a large hysteresis and multiple branches at T = 77 K.     

Magnetic relaxation and critical current in Bi2Sr2CaCu2O8+x single crystals

The anisotropy of critical current densityJ _c in Bi₂Ba₂CaCu₂O_8+x single crystals has been investigated as a function both of the temperature and of the applied magnetic field. An anisotropic behavior ofJ _c has been found. The decay of the remanent magnetization has been studied for fields applied both parallel and perpendicular to thec axis. A logarithmic behavior was found. A pinning energyU ₀ of about 0.01 eV, independent of the direction of the applied field, was obtained.     

Superconductivity in Na-doped Bi2Sr2CaCu2O y system

Samples with the nominal composition Bi₂Sr₂Ca_1−x Na _x Cu₂O _y (x=0, 0·1, 0·2 and 0·3) were prepared by solid-state reaction of the individual compounds. X-ray diffraction patterns indicate that the samples have a majority 2212 phase with 2223 also being present. From the DC four-probe resistance data, we have observed that the furnace-cooled samples show metallic behaviour while the quenched samples show superconductivity up to 97 K.     

Far-infrared spectra of Bi2Sr2CaCu2O8 single crystal

The far-infrared spectra of a Bi₂Sr₂CaCu₂O₈ single crystal at 2.7 and 298 K is reported for the spectral range of 30–350 cm^?1. Several characteristic bands of Cooper pairs are found at 2.7 K in the far infrared region, and accompanied by low-energy side bands.     

In-Plane and Out-of-Plane Magnetoresistance in Bi2Sr2CaCu2O8+x with Various Carrier Densities

The in-plane and out-of-plane magnetoresistance of Bi ₂ Sr ₂ CaCu ₂ O _8+x single crystals was investigated in magnetic fields of 12 T, oriented parallel to the c axes. Crystals with different carrier concentrations, corresponding to the underdoped, optimally doped, and overdoped region of the phase diagram of high-temperature superconductors, were prepared by variation of the oxygen content. Above the critical temperature, the magnetoresistance is positive for in-plane and negative for out-of-plane currents in all samples. With recent theories for superconducting order-parameter fluctuations, the in-plane magnetoresistance can be described well for all samples. Only the out-of-plane magnetoresistance of the underdoped crystal shows distinct deviations from the theoretical predictions and appears to be influenced by the pseudo-gap.     

HREM surface profile images of Bi2Sr2CaCu2O8

Surface profile images of Bi₂Sr₂CaCu₂O₈ have been obtained using high-resolution electron microscopy. The cleaved (001) surface of the crystals terminates with a single Bi-O atomic layer. The modulated structure developed in this surface atomic layer was observed directly. The (hk0) surfaces were found to decompose in air into an amorphous coating layer. This coating layer was unlikely recrystallized into the original structure under electron beam irradiation. The amorphous layer on the (hk0) surface formed in pure Ar atmosphere was relatively thin and could be recrystallized into some secondary phases in which a Bi loss was observed. The original (001) surface might also be covered by an amorphous-like layer. This disordered layer could be recrystallized under electron beam irradiation into BiSr₂Ca₂Cu₃O₉, BiSr₂CuO₅, etc. which intergrow with the parent crystal perfectly on the (001) planes.