Delin Yang

Experimental study on the oxidation of MgB2 in air at high temperature

Oxidation of MgB2 under air conditions of room temperature to 1000 °C was studied by thermogravimetry, x-ray diffraction and scanning electron microscopy equipped with energy dispersion spectrum. The experiment shows that the obvious oxidation process begins at about 400 °C and becomes very strong when the temperature reaches beyond 700 °C. X-ray diffraction shows that after oxidation for a long time at high temperature, most of the magnesium in MgB2 becomes MgO. The analyses of the microstructure and components show that the oxidation of MgB2 is a very complex process accompanying the evaporation of boron and magnesium and the formation of MgO. Long MgO whiskers with diameters in the nanometre scale also appear, mostly on the grain surface, in the oxidation process due to the reaction of evaporated magnesium with oxygen. The oxidation activation energy of MgB2 is estimated by using the Freeman–Carroll method based on the Arrhenius equation. It is shown that in the range of strong oxidation temperature the activation energy is about 70–90 kJ mol−1.

Oxygen desorption activation energy of YBa2Cu3O7-x obtained by thermogravimetry with different heating rates

Abstract The oxygen desorption activation energy of a YBa 2 Cu 3 O 7− x specimen was determined by thermogravimetry (TG) and differential thermogravimetry (DTG) curves with different heating rates. It shows that there are clear evidences that the behavior of TG, DTG, and desorption activation energy have some relations with the oxygen stoichiometry of the specimen when its temperature changes from 500 to 800 °C. In the orthorhombic phase, the desorption activation energy has an obvious increase with the increase of temperature, while it varies smoothly in the tetragonal phase as the temperature increases.

Investigation on Oxygen Resistance Properties of RBaCo 2 O 5+δ Ceramics: Investigation on Oxygen Resistance Properties of RBaCo 2 O 5+δ Ceramics

用固相反应法制备了RBaCo 2 O 5+ δ (R=Y、Dy、Gd、Pr、Nd、Sm和Eu)系列陶瓷. 用标准四探针法测量了它们从室温到600℃之间的电阻率变化. 在温度较低时, 它们的电阻率都随着温度的升高而减小, 显示为半导体特征. 当电阻率在某一温度达到最大值后, 电阻率开始随着温度升高而缓慢增加, 显示为半金属特征. 进一步研究了RBaCo 2 O 5+ δ 系列陶瓷在高温恒温时的电阻率随着环境气氛的变化情况. 结果表明RBaCo 2 O 5+ δ 陶瓷是一类潜在的氧阻传感器材料, 并且它们的响应速率从快到慢顺序是YBaCo 2 O 5+ δ > DyBaCo 2 O 5+ δ > GdBaCo 2 O 5+ δ > PrBaCo 2 O 5+ δ > NdBaCo 2 O 5+ δ > SmBaCo 2 O 5+ δ > EuBaCo 2 O 5+ δ . 以YBaCo 2 O 5+ δ 陶瓷为例, 在700℃恒温时, 当从氧气氛切换到氮气氛时, 由于晶格中氧的脱附导致电阻率先是快速上升, 然后缓慢上升, 并在90 s内达到最大平衡值. 反之, 当从氮气氛切换为氧气氛时, 由于氧的吸附, 电阻率迅速降低, 约30 s内达到最小平衡值.用固相反应法制备了RBaCo 2 O 5+ δ (R=Y、Dy、Gd、Pr、Nd、Sm和Eu)系列陶瓷. 用标准四探针法测量了它们从室温到600℃之间的电阻率变化. 在温度较低时, 它们的电阻率都随着温度的升高而减小, 显示为半导体特征. 当电阻率在某一温度达到最大值后, 电阻率开始随着温度升高而缓慢增加, 显示为半金属特征. 进一步研究了RBaCo 2 O 5+ δ 系列陶瓷在高温恒温时的电阻率随着环境气氛的变化情况. 结果表明RBaCo 2 O 5+ δ 陶瓷是一类潜在的氧阻传感器材料, 并且它们的响应速率从快到慢顺序是YBaCo 2 O 5+ δ > DyBaCo 2 O 5+ δ > GdBaCo 2 O 5+ δ > PrBaCo 2 O 5+ δ > NdBaCo 2 O 5+ δ > SmBaCo 2 O 5+ δ > EuBaCo 2 O 5+ δ . 以YBaCo 2 O 5+ δ 陶瓷为例, 在700℃恒温时, 当从氧气氛切换到氮气氛时, 由于晶格中氧的脱附导致电阻率先是快速上升, 然后缓慢上升, 并在90 s内达到最大平衡值. 反之, 当从氮气氛切换为氧气氛时, 由于氧的吸附, 电阻率迅速降低, 约30 s内达到最小平衡值.RBaCo2O5+δ (R=Y, Dy, Gd, Pr, Nd, Sm, and Eu) ceramics were synthesized by the solid-state reaction method. Their resistivities depending on temperature were investigated by the standard four-probe method from room temperature to 600°C. At low temperature, all the resistivities decrease with the rising temperature, and exhibit semiconducting behavior. After the resistivities reach minimum values at certain temperatures, they increase slowly with temperature, and exhibit semimetal conducting behavior. Especially, the resistivity variations of RBaCo2O5+δ samples with the change of atmosphere at constant temperatures are studied. The results show that RBaCo2O5+δ could be potential oxygen resistance sensor materials, and that the responding rates of RBaCo2O5+δ are YBaCo2O5+δ > DyBaCo2O5+δ > GdBaCo2O5+δ > PrBaCo2O5+δ > NdBaCo2O5+δ > SmBaCo2O5+δ > EuBaCo2O5+δ. For YBaCo2O5+δ, the resistivity rises quickly due to the oxygen desorption when the atmosphere is switched from oxygen to nitrogen at 700°C, and reaches its maximum equilibrium value within 90 s. In addition, the resistivity drops drastically due to the oxygen adsorption when the nitrogen atmosphere is switched to oxygen, and reaches its minimum equilibrium value within 30 s.