Bi-2212 Whiskers Research Articles

A modified airtight two-crucible method for growth of Bi-2212 whiskers from glassypellets

Bi2Sr2CaCu2Ox whiskers have been grown by a modified airtight two-crucible method, employingradial and vertical thermal gradients resulting from the relative position of thecrucibles. A heating profile with a constant heating rate applied from 842 to846 °C allows formation of whiskers up to 2 cm in length for a certain level of Al impurity in the glass.A cation-rich atmosphere is found to be a key intrinsic parameter. Our data confirm theimportance of the thermal gradients, but this parameter is a somewhat extrinsic one and froma practical point of view should be correlated to other intrinsic parameters that influencegrowth atmosphere (i.e. to total and partial pressures). As-grown whiskers have compositionBi:Sr:Ca:Cu = (2.9–3.5):2:(1.7–2.05):(2.4–3.0) (normalized toSr2), zero-resistancecritical temperature Tc (R = 0) = 76–98 K and maximum transport critical current density aboveJc = 104 A cm−2 at 77 K in self-field.

Area control of Bi-2212 whiskers' formation

On the synthesis of Bi-based high- T c superconducting whiskers, a scheme is proposed to control the area where the whiskers grow on a precursor. In the area-controlled fabrication technique, it is important that the partial melting temperature in some region of the precursor is intentionally varied. In the present work, Bi 2O 3 powder was partly deposited on a pellet of Bi-2212 to decease the partial melting temperature, and then the pellet was annealed for the growth of whiskers. Consequently, the area-controlled growth of the Bi-2212 whiskers was successfully achieved. The superconducting properties of the whiskers were investigated by the resistance measurement.

Synthesis and properties of high quality Bi-2212 whiskers

The whiskers synthesized by employing a new and simple method show the residual resistivity of 16 (±10) μ Ω cm at 0 K limit in transport and exhibit the second peak effect in magnetization measurement.

Growth mechanism of Bi2212 whisker from bulk precursor

Abstract The Bi2212 whisker growth from the silver-added bulk precursor was compared with those from aluminum-added precursor. Especially, microstructural observation and analysis were performed near the surface of the bulk precursors around the root of the whisker using a scanning electron microscope with an electron probe microanalyzer. From the aluminum-added bulk precursor, Bi2212 + Ca–Cu–O + Al 2 O 3 , Bi2212 + Bi–Sr–Ca–Al–O + Al 2 O 3 , and Bi2212 + Bi–Sr–Ca–Al–O were observed, suggesting that its growth mechanism is the same as that from aluminum-added glass plate. Bi2212 whiskers grew from not only an Al-added bulk precursor but an Ag-added bulk precursor. Near the Ag-added precursor surface, Bi2212 + CuO, Bi2212 + Ca–Cu–O, and Bi2212 + CuO + Ag regions were observed. Ag was extremely segregated just below the surface. A microcrucible in the Ag-added precursor may be Ag or CuO.

Junction parameter control of Bi2Sr2CaCu2O8+δ stacked junctions by annealing

The control of the critical current density (Jc) and the junction resistance (RN) along the c axis of intrinsic Josephson junctions (IJJs) on a high-Tc superconductor is very important for applying the IJJs to electronic devices. For controlling these junction parameters, we have clarified the relationship of Jc, RN, and the carrier density in Bi2Sr2CaCu2O8+δ (Bi-2212) whiskers by changing the carrier density with an annealing process. As a result, the Jc decreased, and the RN increased systematically when the carrier density decreased. The values of Jc and RN could be controlled by a change in the carrier density.

Growth, contacting and ageing of superconducting Bi-2212 whiskers

We report the growth of highly oriented microscopic whisker-like crystals in the Bi–Sr–Ca–Cu–O system by means of glassy precursors. The dependence of the growth on the stoichiometric composition and on the temperature and duration of the annealing process has been studied. Chemical impurities have been investigated from the point of view of the morphology, the elemental composition and the crystal structure, identifying the presence of CuAl2O4, Sr1.2Bi0.8O3 and Al. Electrical contacts have been fabricated by means of thermal evaporation and diffusion. Their sizes have been carefully measured, achieving contact resistivity in the range of 0.2–3.7 × 10−6 Ω cm2. A very slow degrading of the contacts and the crystals on a time scale of a few years has been detected. This is associated with a decrease in Tc, which is probably due to oxygen release from the material.

Addition of Ag, Pb, K, or Li to Bi 2Sr 2CaCu 2O y whisker

In order to examine the growth mechanism of a Bi 2Sr 2 CaCu 2O y (Bi2212) superconducting whisker, such elements as Ag, Pb, K, and Li were added to the Bi2212 powder in the molar ratio of 0.2 together with changes in sintering temperature. Potassium addition and sintering temperature at 865 °C achieved the fastest growth rate of the whisker. By varying the added elements, the diameter of whiskers is controllable. An Arrhenius plot of the growth rate of Bi2212 whiskers drew a straight line, indicating that its growth mechanism is correlated with the vapor–liquid–solid growth mechanism.

Growth and electrical transport characteristics of Bi 2Sr 2Ca 1Cu 2O x and Bi 2Sr 2CuO x single-crystal whiskers using tellurium-doped precursors

Bi 2Sr 2CaCu 2O x (Bi2212) and Bi 2Sr 2CuO x (Bi2201) single-crystal whiskers were grown by annealing sintered Te-doped Bi–Sr–Ca–Cu–O precursors. The growth of Bi2212 or Bi2201 whiskers definitely depends on the Ca content of precursors; the optimal composition is Ca-rich Bi 2Sr 2Ca 2Cu 2Te 0.50O x for Bi2212 and Bi 2Sr 2CaCuTe 0.50O x for Bi2201. In the precursors Te ions exist in the phase of (SrCa) 3TeO 6. The whiskers contain no Te element. The Bi2212 whiskers have a critical temperature of about 80 K and the critical current with a multibranched structure of the current–voltage characteristics due to the intrinsic Josephson junctions along the c-axis is quite uniform, showing that the whiskers are homogeneous and of good quality. The Bi2201 whiskers show semiconducting resistance with no superconducting transition down to 4.2 K.

Carrier density control of Bi-2212 whiskers

The superconducting transition temperature ( T c), the junction resistance ( R N) and the critical current density along the c-axis ( J c) of Bi-2212 whiskers with different oxygen content have been measured. T c showed a systematical change for the c-axis length. Since the change of c-axis length corresponds to the change of oxygen content, it can be considered that this behavior explains the clear change in carrier density on the whisker from overdoped to underdoped region. J c of intrinsic Josephson junctions fabricated on the whisker exponentially decreases with decreasing the carrier density on whiskers. It indicates that the transport along the c-axis is due to a tunneling process through an insulating barrier.

Critical current in cross-whiskers Josephson junctions and mechanism of cuprate superconductivity

Contacting the c planes of two Bi2212 whiskers we made cross-whiskers junctions with intrinsic Josephson junction nature. The cross-angle dependence of the critical current has a fourfold symmetry and therefore the symmetry of the superconducting order parameter is d-wave like. However, the angular dependence is much stronger than that of the conventional d-wave. Recently, neutron scattering experiments in cuprate superconductors showed the occurrence of very large charge fluctuations with relatively low frequencies along the q x and q y axes. Taking account of this fact, we calculated the superconducting order parameter originating from the Coulomb channel and obtained the d-wave like order parameter whose structure is consistent with the present experiments.

Fabrication of Bi2212 cross-whiskers junction

Intrinsic Josephson junctions have been successfully fabricated without any micro-fabrication technique. Two Bi2212 whiskers were crossed with one another and joined by post-annealing. The inter-whisker electrical transport properties were measured by the four-probe method. The temperature dependence of resistance exhibited metallic behavior above T c. The resistance decreased to zero around 80 K, corresponding to the superconducting transition. The current–voltage characteristics at 5 K exhibited a small hysteresis and voltage jump, which can be explained by the intrinsic Josephson effect.

Studies on the growth aspects of Bi 2Sr 2CaCu 2O 8+ δ whiskers by vapour–liquid–solid mechanism

Growth aspects of superconducting Bi 2Sr 2CaCu 2O 8+ δ (Bi-2212) whiskers by the vapour–liquid–solid (VLS) mechanism are reported. Formation of a Bi-rich liquid phase at the nucleation site is found to be the driving force for the growth by the VLS mechanism. The growth proceeds by the accumulation of atomic species from the liquid and vapour phases and precipitation at the solid–liquid interface. The decrease in the degree of supersaturation of the liquid phase leads to the compositional inhomogeneity in the grown whiskers. A modified annealing process for the continuous supply of atomic species at the growth interface facilitates the growth of Bi-2212 whiskers of uniform composition. Whiskers grown by this mechanism are free from the inter-growth of either low T c (2201) or high T c (2223) phases. The resistivity measurements reveal a superconducting transition at 82 K.

Vapour–liquid–solid (VLS) growth mechanism of superconducting Bi–Sr–Ca–Cu–O whiskers

Growth of superconducting Bi-2212 whiskers by the vapour–liquid–solid (VLS) mechanism is reported for the first time. Formation of a Bi-rich liquid phase at the nucleation site is believed to cause the growth by the VLS mechanism. The growth proceeds by the precipitation of atomic species at the solid–liquid interface as long as the liquid phase is in the superstaturated state. Whiskers grown by this mechanism are free from the inter-growth of either low T c (2 2 0 1) or high T c (2 2 2 3) phases. The grown whiskers possess compositional inhomogeneity due to the decrease in degree of supersaturation of the liquid phase as the growth progresses. Continuous supply of constituent atomic species at the growth site facilitates the growth of large whiskers of uniform composition. The resistivity measurements reveal a superconducting transition at 78 K.

Growth of Bi 2Sr 2CaCu 2O x superconducting whiskers from a Bi-rich melt

Bi 2Sr 2CaCu 2O x (Bi-2212) superconducting whiskers grow by annealing a melt-quenched glass precursor. We have confirmed that the Bi-2212 whiskers grow from a Bi-rich melt in this method. The bottom of the whisker is completely surrounded by the Bi-rich melt during the growth. The Bi-rich melt stays at the bottom of the whisker for a long heating duration keeping the chemical composition constant.

Preparation of long Bi-2212 whiskers and their superconducting properties

The effects of oxygen gas flow rate on the growth of Bi 2Sr 2CaCu 2O x (Bi-2212) superconducting whiskers have been studied. Increasing the gas flow rate inhibits whisker growth and reduces both maximum and average whisker lengths. Flow rate should be kept as low as possible to obtain a long whisker. Apart from the top part, chemical composition and T c are uniform in a whisker of 14 mm length. It is confirmed that the superconductivity of whiskers is not suppressed during growth of long duration.

Effects of Cooling Rate on Growth of Bi-2212 Superconducting Whiskers

Bi2Sr2CaCu2Ox (Bi-2212) superconducting whiskers have been prepared by using a melt-quenched glass precursor method and it has been reported that the glass precursor was necessary to the growth of the long whiskers. In this work to ascertain the necessity, the effects of the cooling rate of the melt on the growth of Bi-2212 whiskers were studied. Our experimental results indicated that the glass precursors were not needed for the whisker growth. In addition we report that long YBa2Cu3Ox whiskers of 1∼3 mm in length can be prepared by melting and furnace cooling.

Single Cooper-pair tunneling junctions using high-T/sub c/ superconducting materials

We introduce the single electron (Cooper-pair) tunneling junctions using c-axis Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+d/ (Bi-2212) superconducting single crystal whiskers. Focused-ion-beam (FIB) etching patterned the Bi-2212 whiskers. The fabricated small stacked junctions have in-plane area S smaller than <1 /spl mu/m/sup 2/. The junctions showed current-voltage (I-V) characteristics with the periodic structure of current peaks. The stacking layered structure of Bi-2212 works as a multi-junction array which decreases the effective capacitance, C/sub /spl Sigma//=C/sub 0//N, where C/sub 0/ is the capacitance of the junction and N is the layer number of elementary junctions. The period of current peaks of I-V curves corresponds to the charging energy of the single Cooper pair, 2Ec (=e/sup 2//C/sub /spl Sigma//).

Grain Growth, Alignment, and Properties of Bi-based Superconducting Materials.

Hot pressing effect on critical current density (Jc) is studied on Bi2Sr2CaCu2Ox (Bi-2212) superconducting materials. Hot pressing is effective to hasten the grain growth, align the grains, and improve the bulk density of the Bi-2212 materials. In consequence of hot pressing, Jc increases by more than 10 times at 77K under zero magnetic field. Hot pressing effect is enhanced by incorporation of large single crystalline Bi-2212 whiskers. Jc is 3. 1×103 A/cm2, which is about 8 times higher than that for the hot pressed Bi-2212 material without the whiskers, at 77K under zero magnetic field for the Bi-2212 whisker composite. This is due to the haste of grain growth and improvement of grain alignment around the whiskers.

C-axis IV-curves in Bi2212 whiskers with applied magnetic fields parallel to the a-axis

We have studied c-axis IV-curves of nominal Bi2212 whiskers in a magnetic field applied in the crystalline a-direction. In zero applied field we observe branches in the IV-plane which for low fields are retained. For high currents, they all merge into a linear IV dependence which extrapolates to the origin. The critical current as a function of field shows a linear decrease to a level that stays constant in higher fields.

Preparation and critical current density of Bi-based superconducting whisker composites

Bi 2Sr 2CaCu 2O x (Bi-2212) superconducting whisker composites densified and textured by hot pressing have been prepared. The composites consist of Bi-2212 whiskers and Bi-2212 powder and include the whiskers by 5.0 and 10 wt%. The ab-plane of the whiskers and the grains is aligned parallel with the pressed plane of the composites. Although zero resistance temperature ( T c,zero ) is not so different between the composites and the sample without the whiskers, critical current density ( J c ) for the 10 wt% composite is 14 times larger than that for the sample without the whiskers at 77 K under zero magnetic field. The increase in J c is caused by an improvement of the grain growth and alignment around the whiskers.