Effect of the sintering temperature on the microstructure of MgB2 bulks manufactured by in-situ technique (original) (raw)
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Effects of nano-carbon doping and sintering temperature on microstructure and properties of MgB2
Physica C: Superconductivity, 2009
We fabricated nano-carbon (NC) doped MgB 2 bulks using an in situ process in order to improve the critical current density (J c) under a high magnetic field and evaluated the correlated effects of the doped carbon content and sintering temperature on the phase formation, microstructure and critical properties. MgB 2Àx C x bulks with x = 0 and 0.05 were fabricated by pressing the powder into pellets and sintering at 800°C, 900°C, or 1000°C for 30 min. We observed that NC was an effective dopant for MgB 2 and that part of it was incorporated into the MgB 2 while the other part remained (undoped), which reduced the grain size. The actual C content was estimated to be 68-90% of the nominal content. The NC doped samples exhibited lower T c values and better J c (B) behavior than the undoped samples. The doped sample sintered at 900°C showed the highest J c value due to its high doping level, small amount of second phase, and fine grains. On the other hand, the J c was decreased at a sintering temperature of 1000°C as a result of the formation of MgB 4 phase.
Improvement in the Performance of Bulk MgB2 Material through Optimization of Sintering Process
Physics Procedia, 2015
Enhancement of the critical current density (J c) in MgB 2 bulk material is very important for engineering applications. It has been found that the optimization of the sintering temperature leads to high J c values in bulk MgB 2 material. In our recent work, the highest trapped field value was recorded in the sample sintered at 775 o C. The present investigation has focused on methods to improve further J c values of the disk-shape bulk MgB 2 superconductors by optimizing sintering conditions. MgB 2 samples were subjected to a solid state reaction at 775, 780, 785, 795, 800 and 805 o C for 3h in pure Ar atmosphere. XRD analysis showed that all the samples were single phase MgB 2. The magnetization measurements confirmed a sharp superconducting transition with onset T c at around 38.7 K. Atomic force microscopy observation indicated that the number of nano meter size grains is more in samples processed 3h at 805 o C has compared to the samples sintered at 775 o C. As a result, the highest J c value of 245 kA/cm 2 at 20 K was obtained for the sample sintered at 805 o C for 3h.
Superconductor Science and Technology, 2003
We report here on a new heat treatment to prepare both dense polycrystalline and single crystal MgB 2 high quality samples in one single process. Resistivity measurements for polycrystalline part of the sample gives a residual resistivity ratio RRR=16.6 and a very low normal state resistivity ρ ο (40 Κ) = 0.28 µΩcm. Both SEM and SQUID study on polycrystals reveal the high quality, dense character and well coupling of grain boundaries. On the other hand, the high quality single crystals have a unique shape that resembles the hexagonal crystal structure. SQUID measurements reveals very weak flux pinning character implying our single crystals to be very clean. In this study, we conclude that heat treatment is playing a major rule on the characteristics of both polycrystalline and single crystal MgB 2. Samples are thoroughly characterized by x-ray, resistivity, dc SQUID and SEM.
Preparation and characterization of MgB2 polycrystalline with different milling method
In this study, we focus on the synthesis of MgB2 polycrystalline by using three different methods. MgB2 polycrystalline was prepared by conventional and mechanical alloying method. Three variations of milling method were used, which are agate mortar for a half hour; High energy ball milling (HEBM) under air atmosphere for 2 hours; and HEBM under argon for 2 hours. MgB2 polycrystalline was prepared using magnesium and boron powder with a ratio of Mg:B =1:2. After milling, the mixed powders were poured into a stainless steel SS304 tube, compacted by the press machine and sintered at 800°C for an hour in an air atmosphere. After sintering, the sample was removed from the tubes and analyzed the phase formation, microstructure, resistivity and critical temperature (Tc) of the samples. Hexagonal phase MgB2 can be seen as a dominant phase for all samples based on a qualitative analysis of X-Ray Diffraction (XRD), but there was a small amount of Mg and MgO phase as impurities. We obtained the qualitative and quantitative analysis of sample mixed with the agate mortar was the optimal sample with less impurity phase compared to other samples. Sample by using HEBM under air has the highest Tconset of 42.3 K and Tczero of 41.9 K.
Study of Densification Process and Microstructure for Highly Dense Doped MgB2 Bulk Samples
Journal of Superconductivity and Novel Magnetism, 2012
In order to give a contribution in the subject of MgB 2 processing technique to obtain low cost high density bulk samples with improved superconducting properties, studies on dilatometry and on the microstructures of in situ and ex situ MgB 2 bulk samples prepared in the presence of carbon compounds are presented in this paper. The ex situ densification was performed by isostatic pressure followed by high temperature treatment in air, which promoted high density samples compared to the common magnesium diboride in situ samples. It was observed that each carbon compound addition (hydrocarbon-C 8 H 18 , SiC and silicon oil-SiC 2 H 6 O), corresponds to one microstructure, which can be noticed even after the ex situ process for sintering the pellets. Among the compounds, the SiC showed to present the most irregular structure for the in situ samples. However, this microstructure showed to change completely and reaches higher density (90 %) very rapidly when heat treated in the ex situ process. The X ray diffraction (XRD) analyses also revealed that the presence of spurious phases-like
Journal of Alloys and Compounds
The present investigation focuses on methods to further improve the J c values of disk-shaped bulk MgB 2 superconductors by optimizing the sintering conditions. We prepared two sets of bulk MgB 2 material from commercial high-purity powders of Mg metal and amorphous B using a single-step solid-state reaction process. To optimize the sintering time, a set of samples was sintered at 775 C with sintering duration ranging between 1 and 10 h (pure Ar atmosphere). A second set of samples was produced similarly at 775, 780, 785, 795, 800 and 805 C (3 h, pure argon atmosphere). X-ray diffraction analysis showed that both sets of samples were single phase MgB 2 . Magnetization measurements confirmed a sharp superconducting transition with T c,onset z 38.2 Ke38.8 K. The critical current density (J c ) values for MgB 2 samples produced for 1 h were the highest in all processed materials, i.e., the high J c value of 270,000 A/cm 2 and 125,000 A/cm 2 (20 K, self-field and 1 T) were achieved in the sample produced at 775 C, without any additional doping. In contrast, the second series of samples clearly indicated that at 805 C (3 h) the highest J c of 245,000 A/cm 2 and 110,000 A/cm 2 (20 K, self-field and 1 T) were achieved. AFM and EBSD observations indicated that largest amount of fine grains do exist in the sample sintered at 775 C, but the narrowest distribution of grains does exist in the sample sintered at 800 C. The present results clearly demonstrate a strong relation between the microstructure and the pinning performance. The optimization of the sintering conditions is crucial to improve the performance of bulk MgB 2 samples.
Structure and Properties of MgB2: Effect of Ti-O and TiC Additions
IEEE Transactions on Applied Superconductivity, 2018
The effects of the additions of powdered titanium carbide (TiC) and polyvalent titanium oxides (Ti-O) to MgB 2 (synthesized under high pressure (2 GPa)-high temperature (800 and 1050°C) conditions) on the critical current density, j c , critical magnetic fields, B C2 and B irr , and the related transformations in the materials structures are reported in this paper. The superconducting characteristics are compared with that of MgB 2 with titanium (Ti) additions synthesized under the same conditions. The synthesis temperature, the ability of Ti-containing compounds to act as a getter, and the different diffusion rates of Mg and B into grains of these additions influence the distribution of the elements (boron, magnesium, and impurity oxygen) in the structures of MgB 2-based materials. This in turn affects the formation of pinning centers, the resulting flux pinning and eventually the superconducting properties (j c , B C2 , B irr).
Journal of the Australian Ceramic Society, 2019
Magnesium diboride, MgB 2 , is known to be an important high-temperature superconductor material. High-energy ball milling with subsequent low-temperature sintering remains an attractive solid-state method for its synthesis. One main optimization parameter of this method is to ensure a balance between cold welding and fracturing at the milling stages. Therefore, in many cases, the use of a process control agent at the milling stage is considered to be the preferred solution in the prevention of particle size enlargement. The aim of this paper is to determine the effect of dry and toluene-assisted milling on the magnetic and surface topology properties of obtained MgB 2. The calculated critical current density of 2.89 × 10 5 A/cm 2 at 10 K 0.3 T for a tolueneassisted sample, compared with the 2.37 × 10 5 A/cm 2 for a dry-milled sample as well as the relatively higher mean surface roughness, clearly indicated a notable positive contribution of surfactant use on the true alloying of MgB 2 by mechanical activation.