Xu Dong Sun

Significant improvement of critical current density in MgB2 doped with ferromagnetic Fe3O4 nanoparticles

Ferromagnetic Fe3O4-doped MgB2 bulks were first fabricated in this work by the hot pressing method. It was found that Fe3O4 does not react with Mg or B during the fabrication process. Peak Jc values of the 5 wt% Fe3O4-doped MgB2 are higher than 106 A cm−2 in the temperature range 5–30 K. Especially at 30 K, the peak Jc is 1.02 × 106 A cm−2 for the 5 wt% Fe3O4-doped MgB2, the highest values at 30 K found in the literature, and about seven times that of the 5 wt% SiC-doped MgB2 sample. The drop in Jc with increasing field for the Fe3O4-doped MgB2 is significantly slower than that of the SiC-doped MgB2 at 30 K. These results indicate that the Fe3O4-doped MgB2 is a potential superconductor to be used at temperatures greater than 25 K which is a critical temperature for large-scale practical applications.

Crystal Structure Stabilization of Gadolinium Aluminum Garnet (Gd3Al5O12) and Photoluminescence Properties

To suppress the thermal decomposition and to stabilize the crystal structure of Gd3Al5O12 (GdAG) garnet, doping GdAG with smaller Ln3+ (Ln=Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y, respectively) to form (Gd,Ln)AG solid solutions was proposed in work. Carbonate precursors of (Gd,Ln)AG with an approximate composition of (NH4)x(Gd,Ln)3Al5(OH)y(CO3)z•nH2O were synthesized via coprecipitation from a mixed solution of ammonium aluminum sulfate and rare earth nitrate, using ammonium hydrogen carbonate as the precipitant. The precursors and the calcination derived oxides were characterized using FT-IR spectroscopy, DTA/TG, XRD, BET and FE-SEM. The results showed that smaller Ln3+ doping can indeed stabilize GdAG against its thermal decomposition to a mixture of GdAlO3 (GdAP) and Al2O3 phases at elevated temperatures and at the same time effectively lowers the temperature for garnet crystallization. The carbonate precursors are loosely agglomerated and the resultant (Gd,Ln)AG powders show good dispersion and a fairly uniform particle morphology. The (Gd,Ln)AG solid solutions exhibit decreasing lattice parameters along with decreasing radius of the dopant ions at the same dopant content of 50 at%. Photoluminescence properties of some of the garnet solid solutions are also studied. The materials developed herein may potentially be used for photoluminescent and scintillation applications.

Phase evolution and microstructure of high Jc SiC doped MgB2 fabricated by hot pressing

We report the phase evolution, microstructure, and critical current density (Jc) of the SiC doped MgB2 superconductors. In our study, all samples were fabricated by hot pressing with a heating rate of 200??C?min?1. The results show that the reaction of 2Mg+SiC = Mg2Si+C can occur at about 500??C, about 50??C lower than the formation temperature of MgB2. On the other hand, according to the experimental results and thermodynamic calculations, boron (B) does not react with SiC to form B4C and Si, neither does MgB2 react with SiC to form Mg2Si and B4C. The MgB2 phase was formed via both a solid?solid reaction (during the heating process between about 500 and 650??C) and a liquid?solid reaction (>650??C) after the melting of Mg, and the two reactions resulted in differences in the grain size of the MgB2. From scanning electron microscopy, the Mg2Si particles are homogeneously distributed within the MgB2 matrix, with particle sizes ranging from 35 to 230?nm. From the perspective of superconductivity, the C substitution results in strong electron scattering centers in the MgB2 structure. It reduces the electron mean free path and thus may significantly enhance the magnetic Jc. The peak Jc of the 5?wt% SiC doped MgB2 reaches above 106?A?cm?2 at 5?K, and decreases slowly with increasing field, remaining high, above 105?A?cm?2, at 7?T.

Fabrication and Properties of Hydroxyapatite-Ti Network Composites

Hydroxyapatite (HA) powder was synthesized by a sol-gel method with Ca(OH)2 and H3PO4 as reactants. The HA granules were then coated with TiH2 powder using a mechanical mixing method. The HA-TiH2 material system produced HA-Ti composites after hot-pressing at 1050°C. The HA-Ti composites are mainly composed of HA and Ti, with small amounts of Ca2P2O7 and Ca3(PO4)2 phases. Fracture toughness and bending strength are 2.4 MPa·m1/2 and 54.3 MPa, respectively for the HA-20vol%Ti composite, higher than those of the pure HA ceramic. The improvement in properties is because of the unique 3D network structure of Ti, which is an ideal reinforcement structure for the weak and brittle HA. According to ISO/TR 7405-1984, hemolysis test was performed to evaluate the blood compatibility of the material. The results show that the hemolysis rate of the HA-20vol%Ti composite is 0.56%. Relative growth rates (RGR) of L-929 cells soaked after 6 days in the HA-20vol%Ti group, pure Ti group, black group and pure Pb group were 132%, 100%, 90% and 6% respectively, while the level of cytotoxicity was grade 0 in HA-Ti composite group. These results imply that the HA-20vol%Ti composite has good biocompatibility and bioactivity.

Preparation and Mechanical Properties of a Novel Biomedical Magnesium-Based Scaffold

Magnesium has been recently recognized as a biodegradable metal for bone substitute applications. In the present work, a novel magnesium based scaffold with a specific two-layer structure was prepared for powder metallurgical process. The outer layer of the scaffold shows an interconnected porous structure, so that the fresh fluid can be easily sent into the material, allowing the ingrowth of new bone tissue. The inner compact structure reinforced by the salt particles can increase the strength of the material. Structural characterizations and mechanical tests of the materials demonstrate that the structural and mechanical properties of the magnesium-based scaffold with an appropriate salt content prepared by the current method are quite comparable to those of cancellous bone. Therefore, the magnesium-based scaffold with such a two-layer structure has the potential to serve as degradable implants for bone substitute applications.

Luminescence Behaviors of the New Upconversion Phosphors of Yb/Ho Co-Doped (Gd1-xLux)3Al5O12 (x=0.1-0.5) Garnet Solid Solutions

The metastable garnet lattice of Gd3Al5O12 (GdAG) was effectively stabilized via doping with significantly smaller Lu3+, and based on which (Gd,Lu)AG:Yb/Ho was developed in this work as a new type of upconversion phosphor. The phosphor particles calcined from the precursors synthesized via carbonate precipitation were observed to have good dispersion and fairly uniform morphologies. Optical spectroscopy found that the [(Gd1-xLux)0.948Yb0.05Ho0.002]3Al5O12 (x=0.1-0.5) garnet powders exhibit a green emission centered at ~543 nm (the 5F4,5S25I8 transition of Ho3+) and a red emission centered at ~668 nm (the 5F55I8 transition of Ho3+) under laser excitation at 978 nm. The upconversion emission intensity was found to decrease with increasing Lu3+ doping. Meanwhile, the dependence of up-conversion emission intensity on the pumping power was measured and the up-conversion mechanism was discussed in detail. The Yb/Ho codoped (Gd,Lu)AG garnet system developed herein may potentially be used as a new type of luminescent material.

Effect of SnO2 Particle Size on Properties of Ag-SnO2 Electrical Contact Materials Prepared by the Reductive Precipitation Method

Performances of Ag-SnO2 electrical contact materials can be strongly affected by the microstructure. In this work, Ag-SnO2 composite powders were synthesized by chemical reductive precipitation method. During the precipitation process, Ag particle was deposited onto the surface of SnO2 particle with the assistance of citric acid. The microstructure and properties were analyzed for the prepared Ag-SnO2 electrical contact materials. Our research reveals that the particle size of SnO2 has significant influence on the morphology of the Ag-SnO2 composite powders, and therefore on the microstructure and physical properties of the electrical contact materials. With the decrease of particle size of SnO2, hardness of the Ag-SnO2 electrical contact materials increases, while electrical conductivity decreases.

Fabrication of Transparent YAG Ceramics from Co-Precipitation Synthesized Nanopowders

YAG nanopowders were synthesized by a co-precipitation method using ammonium hydrocarbonate and ammonia water as the precipitants respectively. The influences of precipitants on chemical compositions, phase transformation and sinterability of the prepared powders, and transmittance of the vacuum-sintered YAG ceramics were studied. The sinterability of powders synthesized using ammonium hydrocarbonate as precipitant is better than that with ammonia water. Pure YAG phase can be obtained by calcining the hydrate precursor at 1200°C, while some impurity phases exist when calcining the carbonate precursor at the same temperature. Transparent YAG ceramics were fabricated by vacuum sintering at 1700°C for 5 h using the YAG nanopowders, and their in-line transmittance is about 60% in the visible light range.

Preparation of rod-like SnO2 powder and its application in Ag-SnO2 electrical contact materials

Abstract In this study, rod-like SnO2 particles with a diameter from 1 to 4 μm and the length from 5 to 50 μm were successfully prepared by chemical precipitation method. Then, SnO2 particles were mixed with silver powder by wet ball milling. After hot pressing, hot-extrusion and drawing, Ag-SnO2 electrical contact material was fabricated. The microstructure, mechanical properties and material transfer of Ag-SnO2 contact material were studied. The results showed that this kind of Ag-SnO2 electrical contact material has good performance, that is, homogeneous microstructure, high hardness HV117·55, low electrical resistivity (2·227 μΩ × cm) and low material transfer.

Ultra-Large Unilamellar Nanosheets Efficiently Exfoliated from (Y0.95Eu0.05)2(OH)5NO3·nH2O and their Self-Assembly into Oriented Oxide Film with Enhanced Photoluminescence Properties

Two dimensional nanophosphors of high quality play an important role in the miniaturization and intelligentization of opto-electronic components. In this present work, ultra-large (30μm) single crystals of (Y0.95Eu0.05)2(OH)5NO3·nH2O layered rare earth hydroxide (LRH) with a hexagonal shape have been synthisized via autoclaving the rare-earth nitrate/NH4OH reaction system in the presence of ammonium nitrate (NH4NO3). The nitrate ions, existing in the interlayer gallery of layered rare earth hydroxide, exhibit facile exchanges with oleate anions by hydrothermal anion exchange. Furthermore, the interlayer distance can thus be expanded from ~0.9 nm for the pristine LRHs to ~3.60 nm for the intercalated ones, which are then efficiently delaminated into unilamellar nanosheets with a lateral size of 10μm and a thickness of ~1.50 nm. The obtained nanosheets are single crystaline. Highly [11 oriented, dense (Y0.95Eu0.05)2O3 phosphor films with excellent optical transparency and a greatly enhanced luminescence intensity have been constructed via self-assembly of ultra-large unilamellar LRH nanosheets, followed by proper annealing.