原位电阻测试分析Mg(BH_(4))_(2)制备MgB_(2)的成相过程

In situ resistance analysis of MgB_(2)formation process from Mg(BH_(4))_(2)

Mg(BH_(4))_(2)作为优质的储氢材料,在约300℃开始分解释放H_(2),并最终生成MgB_(2.)由于Mg(BH_(4))_(2)的释氢反应可以在较低的温度下获得MgB_(2),使其成为了制备MgB_(2)超导材料的一种有效途径.本文采用了原位电阻法,通过测量Mg(BH_(4))_(2)分解过程中电阻温度曲线,详细地研究了Mg(BH_(4))_(2)分解生成MgB_(2)的相变过程.同时,利用电阻温度的微分曲线,确定了在分解过程中不同产物的成相温度(TPF).其中,MgB_(2)的成相温度可以低至410℃.通过与粉末烧结法制备MgB_(2)块材的成相温度对比,估算出反应前Mg的颗粒尺寸最低可达3.4 nm.此外,样品的XRD分析给出了生成的MgB_(2)晶粒在10—18 nm之间,在SEM图像中也同样观察到了MgB_(2)纳米纤维结构.这表明,Mg(BH_(4))_(2)分解生成的Mg与B形成了接近原子级的混合,从而使MgB_(2)可以在更低的成相温度(410℃)、更短的反应时间内成相.该方法为MgB_(2)在超导应用的制备提供了新的思路,有利于实现MgB_(2)的工业化生产.

Mg(BH_(4))_(2)was previously studied as a promising hydrogen storage material,because of its high gravimetric storage capacities for hydrogen and suitable thermodynamic properties.Mg(BH_(4))_(2)began to decompose at about 300℃,and formed MgB_(2)at the end of hydrogen desorption process with the weight content of 14.9%of hydrogen lost.Aside from the prominent hydrogen storage property,the decomposition process from Mg(BH_(4))_(2)to MgB_(2)can be a potential method for fabricating superconducting MgB_(2)at a low sintering temperature.In this paper,MgB_(2)bulk was prepared by an in-situ reaction,using the Mg(BH_(4))_(2)pressed block as a precursor.The resistance change of the sample was monitored during the Mg(BH_(4))_(2)decomposition process and the resistance-temperature(R-T)curve of this process was recorded.Phase of MgH_(2),Mg and B were formed as the block slowly release its hydrogen before MgB_(2)occurred.According to the R-T curve,the phase formation of MgB_(2)started in a relatively low temperature of 410℃.Because MgB_(2)was critically formed by Mg and B derived from Mg(BH_(4))_(2),we can compare our formation temperature with previous study on MgB_(2)prepared by Mg and B in different particle size.The fitting result indicated that the particle size of Mg and B harvest from Mg(BH_(4))_(2)decomposition was only 3.4 nm on average.The nearly atomic level mixture of Mg and B resulted in a high chemical reactivity,which was the main reason for low sintering temperature.X-ray diffraction results showed that the purity of MgB_(2)was 95.2%,and the size of MgB_(2)grains was 10–18 nm.SEM images showed that the MgB_(2)bulk had a porous structure and poor connectivity,which was caused by large amount the hydrogen release during the decomposition.MgB_(2)nanofibers can also be observed inside the bulk.In the superconductivity test,the superconducting transition temperature of the bulk was 35 K.After all,such in situ method to fabricate MgB_(2)showed a great advantage in some aspects,as its low-cost precursors,low sintering temperature,small grain-size and high superconducting transition temperature in the formed MgB_(2),which have the potential in industrial scale fabrication of MgB_(2)bulks and wires.