Yuling Zhang

Electrorheological fluid with an extraordinarily high yield stress

Surface modified complex strontium titanate microparticles are synthesized by means of a modified sol-gel technique. A suspension composed of these particles immersed in a silicone oil exhibits excellent electrorheological properties attractive to industry and technology applications: a yield stress as high as 27 kPa in an applied electric field of 3 kV/mm, a low leakage current, wide dynamic ranges in temperature and shear rate, and a long-term stability against sedimentation. In addition to the high dielectric constant of strontium titanate, surfactant and water-free character of the particles may be responsible for the dramatic improvement of the electrorheological properties of the suspension

Subsolidus phase relations of the LaCoCu system and crystal structure of LaCoCu12

The subsolidus phase relations of the La-Co-Cu ternary system in the La-poor region on annealing at 800 °C were investigated by X-ray powder diffraction. A new ternary intermetallic compound was discovered. The crystal structure of LaCoCu,2 was refined by the Rietveld profile fitting technique.

Crystal structures and superconductivity of superconducting phases in the Tl-Ba-Ca-Cu-O system

Abstract We found a series of superconducting phases in the Tl-Ba-Ca-Cu-O system and determined the crystal structures of these superconducting phases by means of the X-ray powder diffraction method. All of these superconducting phases belong to the tetragonal symmetry. They have the same lattice constant a and different lattice constant c . These superconducting phases can be divided into two types. The space group of type I is P4/mmm. The space group of type II is 14/mmm. The chemical formulae of type I and type II can be generalized as TlBa 2 Ca n −1 Cu n O 2 n +2.5 ( n =2, 3, 4) and Tl 2 Ba 2 Ca n −1 Cu n O 2 n +4 ( n =1, 2, 3), respectively. T c increases as n increases. On the condition that n is the same, type II has a higher T c than type I. The relations among the crystal structures of the superconducting phases are discussed. We proposed a method which can deduce the crystal structure of the superconducting phase from the d 1 -value of the first diffraction line (00 l ) or lattice parameter c and which is confirmed by experimental data.

Magnetoresistance behaviour of compounds

The magnetoresistance behaviour of has been systematically investigated between 77 and 300 K; it depends strongly upon the content in the sample. For and , the samples exhibit antiferromagnetic and semiconducting characters, and the resistance obeys Motts law: . For , the sample is ferromagnetic and metallic in the temperature range below . A negative magnetoresistance as large as 50% was observed in a bulk sample with in a field of 1 T. In the two phase ranges and , antiferromagnetic domains and ferromagnetic domains are assumed to distribute randomly in one crystallographic lattice, and the effective-medium theory provides a satisfactory interpretation for the resistance behaviour of the samples.

CRYSTAL STRUCTURE AND SUPERCONDUCTIVITY OF Bi2Sr2CaCu2O8 COMPOUND

The crystal structure of the ideal compositional compound Bi2Sr2CaCu2O8 with superconducting transition temperature Tc(0)=81.4K has been determined by X-ray powder diffraction method. The basic structure of this compound has the tetragonal symmetry with the lattice constants a=3.825A, c=30.82A. The most possible space group is . Each unit cell contains two chemical formulae. The arrangements of atoms in the unit cell are as follows: 2Ca cations occupy 2(a) equivalent point position 4Sr, 4Bi and 4Cu cations occupy the same equivalent point position 4(e) with respective parameters z=0.1, 0.3, 0.45. 16O anions respectively occupy 8(g) (z=0.45) and two 4(e) equivalent point positions (z=0.2 and z=0.385). The crystal structure of Bi2Sr2CaCu2O8 can be derived from Aurivillius type structure. The other possible superconducting phases in the Bi-Sr-Ca-Cu-O system can be considered as the structure having different stacking on the basis of Aurivillius phase.

NEW SUPERCONDUCTING SYSTEM SR-CA-BI-CU-O SYSTEM

Abstract In Sr-Ca-Bi-Cu-O system, we have found superconductors with Tc around LN 2 temperature. Those superconductors are multiphases, but one of superconducting samples is almost single phase. In this system, the crystal structure of the superconducting phase differs from the known structure of Ba 2 YCu 3 O 7-y .

Tunable oxygen vacancy configuration by strain engineering in perovskite ferroelectrics from first-principles study

Comprehensive first-principle calculations indicate that the oxygen vacancy (VO) configuration of a prototypical perovskite oxide PbTiO3 (PTO) can be tuned by strain engineering. Remarkably, we found that VO located along the c axis (Vc), normally resulting in a polarization-harming tail-to-tail domain configuration in strain-free PTO (Vcud), can be readily tuned to a polarization-harmless head-to-tail domain configuration (Vcsw) by applying an ab-biaxial compressive strain of 3%. Also, VO located in the ab-plane (Vab), another type of polarization-harmless vacancy, can be stabilized by applying a compressive strain of 2%.

Enthalpies of formation of rare earth-3d metal alloys and intermetallic compounds

The enthalpies of formation of rare earth-3d metal alloys and intermetallic compounds have been calculated by Miedemas semiempirical method. The calculations agree well with experimental enthalpy data and with phase diagram information. A brief comment on Miedemas model is given and the possibility of predicting the enthalpies of formation of multicomponent alloys and intermetallic compounds is discussed.

High-Tc superconductivity above 110 K in the Tl-Ba-Ca-Cu-O system

High-Tc superconductivity above 110 K has been observed in Tl-Ba-Ca-Cu oxides. The dependence of electrical resistivity and AC magnetic susceptibility on temperature show that the zero-resistance transition temperature is about 114 K and a dramatic magnetic susceptibility drop appears at 117 K. The superconductivity remains stable after several cooling and heating cycles. The observation of superconductivity in this system was carried out at least eight times.

Thermal characterization and crystal structure of an electrorheological material - Complex strontium titanate

A new type of anhydrous complex strontium titanate (STO) electrorheological fluid (ERF) material is obtained, which demonstrates a good electrorheological (ER) character even at lower applied electric field. Its shear stress is about 8 kPa at electric field of 2.4 kV/mm. The current density is only 1.6 mu A/cm(2) leaking through the fluids. Some additive in our samples prevents sedimentation. The shear stresses vary only slightly in a wide temperature range. The thermal characteristics of the solid phase STO including the heat capacity and thermal stability were investigated. The value of the activation energy of crystallization was derived to be about 100 kJ/mole. The crystal structure of Y-doped STO was refined by the Rietveld method. Y3+ substitutes for Sr2+ in the 1(a) position of space group Pm3m.