Jianyi Jiang

Investigations on nanocrystalline Fe78B13Si9 alloys by positron annihilation spectroscopy

Polycrystalline Fe78B13Si9 alloys with nanoscale grain sizes prepared by the crystallization method have been studied by positron lifetime and Doppler‐broadened line‐shape measurements. The results obtained are different from those on amorphous or coarse‐grained crystalline alloys with the same composition. When the grain sizes are clearly smaller than the mean positron diffusion length (L+≂100 nm), saturation trapping and annihilation of the positrons in the samples should occur at traps within the interfaces. There are two kinds of defects in the interfaces of the nanocrystalline FeBSi alloys, i.e., the free volumes (the size of which is slightly smaller than that of the amorphous counterpart) and the microvoids. The dependence of grain sizes on the type of interface defects and structure‐sensitive properties has also been studied by positron spectroscopy. The measurements and comparison of the mean positron lifetime τ and the Doppler parameter S on the nanocrystalline, amorphous, and coarse‐grained cr...

Low‐temperature x‐ray diffraction of the Y1Ba2Cu3O9−x oxide ceramic superconductor

The change of the lattice parameter with temperature (82–320 K) of the single phase Y1Ba2Cu3O9−x superconductor has been investigated by x‐ray diffractometer. The experimental result shows that there exist two abrupt changes of lattice parameter at about 250 and 125 K separately in the curve of the lattice parameter versus temperature. But no change of structure symmetry was observed as shown by x‐ray diffraction. The abrupt change of lattice parameter at about 250 and 125 K may be associated to the position adjusting of oxygen atoms in oxide ceramic.

Dependence of internal friction and young's modulus on the oxygen deficiency in the superconductor YBa2Cu3O7−δ

Abstract The dependence of the internal friction and Youngs modulus on the oxygen deficiency in the superconductor YBa 2 Cu 3 O 7−δ was studied at frequencies near 2.7 kHz. Peaks observed near 110 and 230 K decreased with decreasing oxygen stoichiometry in a sample with the orthorhombic structure, and were absent in a sample with the tetragonal structure. Peaks at about 88 and 260 K did not change with the oxygen stoichiometry, and were observed both in the sample with orthorhombic structure and in the sample with tetragonal structure. The peaks at about 88 and 110 K are characteristics of relaxation processes. It is suggested that the activated event for the 110 K peak is the jump of the hole or electron in the Cu(1)-O plane and that of the 88 K peak is that in the Cu(2)-O plane. The peak at about 260 K is indicative of a phase transition. The change in the microscopic structure leading to this peak may be associated with the oxygen deficiency ordering.

TRANSMISSION MOSSBAUER-SPECTROSCOPY AND X-RAY-DIFFRACTION STUDIES ON THE STRUCTURE OF NANOCRYSTALLINE FE-CU-SI-B ALLOYS

Nanocrystalline Fe-Cu-Si-B alloys with grain sizes of 25-90 nm were synthesized by crystallization of the amorphous alloy. Two nanocrystalline phases of alpha-Fe(Si) and Fe2B were observed in all tested samples. Transmission Mossbauer spectroscopy investigation on the structure of nanocrystalline Fe-Cu-Si-B alloys showed that Si atoms are completely and substitutionally dissolved in Fe bcc lattice and the arrangement of the Si atoms in the alpha-Fe(Si) phase shows short range order (SRO), whereas 8.5-9.7 at.% of the B atoms were found as Fe2B and the remainder were located in the interfaces. When grains grow larger, the arrangement of the Si atoms in the alpha-Fe(Si) phase changes. Interestingly, x-ray diffraction results reveal that the lattice constant of alpha-Fe(Si) phase decreases rapidly with grain coarsening. Based on the thermodynamic analysis, the changes in the lattice constant of the alpha-Fe(Si) phase and SRO of Si atoms in bcc Fe lattices were attributed to the lattice expansion as a result of the variation of vacancy concentration in alpha-Fe(Si) solid solution. Meanwhile, owing to the interface contribution, the alloy with small grain size is found to exhibit large values of half linewidth (HLW) and isomer shift (IS) in the Mossbauer spectra. The results from electrical resistivity measurements agree and confirm the strong effects of the lattice distortion and interfaces.

THE LATTICE STRUCTURE OF NANOCRYSTALLINE FE-CU-SI-B ALLOYS

Nanocrystalline Fe-Cu-Si-B alloys with different grain sizes were synthesized by crystallization of an amorphous alloy. Two nanophases, alpha-Fe(Si) and Fe2B, were noticed in all samples. XRD results reveal that the lattice constant of the alpha-Fe(Si) phase increases; the a-axis is elongated and the c-axis is shortened in the Fe2B phase upon reducing the grain size. Based on the thermodynamic analysis, the changes in the lattice parameters were attributed to the solution of vacancies in the above two phases. Owing to the lattice distortion of the alpha-Fe(Si) and Fe2B phases, the crystallite with small size is found to exhibit a disordered character to some extent, which is manifested by large values of the half linewidth (HLW) and isomer shift (IS) of various Fe configurations in the Mossbauer parameters.

Structural analysis of nanocrystalline FeCuSiB alloys by Mössbauer spectroscopy

The distribution of metalloid atoms, Si and B, in α-Fe(Si) and Fe2B nanophases of nanostructured FeCuSiB alloys, as well as its variation with grain growth by means of Mossbauer spectroscopy are the concerns of this paper. Results show that B atoms exhibit two states in the nanocrystalline alloys studied. Si atoms are totally dissolved in α-Fe phase and show short range order. With grain size refinement the distribution of Si and B atoms was found to change, especially for short range order of Si atoms in bcc Fe lattice points.

Preparation, structure and properties of nanocrystalline FeMoSiB alloys

Abstract Using X-ray diffraction, transmission electron microscopy and transmission Mossbauer spectroscopy, the formation, structure and properties (including microhardness and electrical resistivity) of nanocrystalline FeMoSiB alloys have been investigated. By annealing at 833–913 K for 1 h, nanocrystalline materials with grain sizes of 15–70 nm can be obtained. Resistivity and microhardness measurements show that the properties of nanocrystalline FeMoSiB alloys exhibit a strong grain size effect, which is in reasonable agreement with the results of Mossbauer spectroscopy.

STUDYING IN-SITU ON THE SUPERCONDUCTING TRANSITION AND INTERNAL FRICTION OF A HIGH Tc SUPERCONDUCTOR YBa2Cu307−δ

The resistence, internal friction and modulus of a high Tc superconductor YBa2Cu3O7−δ were measured in-situ in 80–300 K. Two obvious peaks of internal friction were observed about Tc. On account of character of peaks, it is suggested that these two peaks are associated with the electronic relaxation between Cu3+ and Cu2+. The peak and anomalies of Young’s modulus observed in the range between 160 K and 280 K may be related to the adjustment of deficiency of oxygen.

A STUDY ON INTERNAL FRICTION AND YOUNG’S MODULUS OF A HIGH Tc SUPERCONDUCTOR YBa2Cu3O7−x

Young’s modulus (E) and internal friction (Q−1) measurements were performed. A Q−1 peak and a sign of anomaly of E were observed near the superconducting transition temperature. There are also two peaks and anomalies of E above Tc. The relation between these charactristic features with superconducting transition is discussed.

Effects of Fe on the superconductivity of 110 K phase superconductor Bi‐Pb‐Sr‐Ca‐Cu‐O

We have investigated the effects of Fe on the structure and properties of 110 K phase superconductor Bi‐Pb‐Sr‐Ca‐Cu‐O. The zero resistance temperature decreases and the x‐ray diffraction intensity ratio of (200) to (001_4_) peak increases, with the increasing of the Fe atom concentration. These samples are of single (2223) phase structure. It may be suggested that a 95 K superconducting phase exists. Its structure is very similar to that of the Bi2Sr2Ca2Cu3Oy phase. The Fe atoms make the 110 K phase turn into the 95 K phase.