高溫高壓下超導體二硼化鎂的臨界電流密度與X光近緣吸收光譜之研究

Abstract

我們的工作為研究MgB2粉末(Alfa Aesar)在高溫與高壓下所形成之塊材。在壓力為20 kBar與加熱至溫度為900℃時得到的高溫高壓樣品，其密度約為2.36 g/cm3，接近MgB2的晶格密度2.63 g/cm3。我們利用超導量子干涉儀在外加磁場範圍為-50 kOe與+50 kOe下測量樣品的磁滯曲線，並求得臨界電流密度Jc，在溫度為10 K，外加磁場為1 tesla時，其值約為6×104 A/cm2。由X光近緣吸收光譜的測量，發現高溫高壓處理過的樣品，MgO減少了，而B2O3卻增加了。 我們在相同的高溫高壓條件下，摻雜鎳的磁性奈米顆粒，看看是否能影響樣品的臨界電流密度Jc。我們以酒精混合MgB2粉末與鎳的磁性奈米顆粒，並使用超音波震盪，成功地使奈米顆粒在樣品中均勻分佈。摻雜鎳的樣品經過高溫高壓後，其臨界電流密度Jc卻變的非常不好。研究其原因，發現主要的因素並非是鎳奈米顆粒的摻雜，雖然摻雜鎳似乎還是會稍微減少樣品的臨界電流密度Jc。我們發現樣品在經過酒精中震盪後，並經過高溫高壓的過程，確實使MgO增加了。

We report our work on the study of the bulk MgB2 prepared by hot-pressing magnesium diboride MgB2 powders (Alfa Aesar) . The hot-pressed specimen was obtained under the pressure of 20 kBar and heated up to 900℃. The density of this specimen was measured to be about 2.36 g/cm3 close to 2.63 g/cm3 of the MgB2 crystal . The hysteresis of the specimen in the magnetic field range of -50 kOe and +50 kOe was obtained by a superconducting quantum interference device (SQUID) . The critical current density Jc was about 6×104 A/cm2 for the temperature at 10 K with an applied magnetic field of 1 tesla . The hot-pressed MgB2 specimen was found to have about one order of magnitude higher Jc than MgB2 powder . The impurity of MgO was found less in the hot-pressed specimen than in the MgB2 powder , but the impurity of B2O3 was observed to increase from measured by X-ray Absorption Near Edge Spectrum (XANES). Nanoparticles of Ni was added into the MgB2 under the same hot-pressed condition to see if it affect Jc . The uniform distribution of nanoparticle was achieved by mixing the MgB2 powders and Ni nanoparticle in alcohol under supersonic vibration . The critical current density of Ni nanoparticle added sample became worse . The cause of it was investigated and found that the Ni nanoparticle addition was not the major cause , although it seems to decrease Jc a little . The vibration process in alcohol was found to make MgO increase after the hot-press process.