Crystal growth and critical current densities of polycrystalline MgB/sub 2/ superconductor

We have investigated crystal growth and critical current densities of MgB/sub 2/ which was synthesized using stoichiometry or excess Mg and two kinds of B grain size. Excess Mg plays a role not only to promote crystal growth but also to make crystal grains fine. During crystal growth, the critical current densities decrease, although the density remains constant, approximately. Irrespective of the starting B powder, J/sub CM/ is the highest in the sample with the smallest crystal grains and highest density of MgB/sub 2/. The effects of excess Mg creates the possibility to control crystal size of MgB/sub 2/ by adjusting the quantity of Mg.

I. Introduction

The INTEREST during the early stages in MgB₂ was its high transition temperature (39 K), chemical and structural simplicity, low anisotropy and low cost. TABLE I Specifications of All Samples

	B/Mg	B size	Temperature	Time
	(mole ratio)			(h)
S1–12	2	1	1100	12
S1–24	2	1	1100	24
S1–48	2	1	1100	48
S1–96	2	1	1100	96
S2–3	1	1	1100	3
S2–6	1	1	1100	6
S2–9	1	1	1100	9
S2–12	1	1	1100	12
S2–24	1	1	1100	24
S2–48	1	1	1100	48
S3–1	1	0.25	1100	1
S3–6	1	0.25	1100	6
S3–12	1	0.25	1100	12
S3–24	1	0.25	1100	24
S3–48	1	0.25	1100	48

A lot of researchers were attracted by these properties which are attractive for superconducting electronics. For MgB₂ thin films, the first problem was the difficulty to synthesize high quality thin films due to the high reactivity of Mg oxidation, but thin films [1], [2] with high critical current densities were successfully fabricated with rather conventional processing methods. For MgB₂ wires, the first problems were a low value of irreversibility field and weak links. Therefore, a lot of researches which improve these problems are reported [3]–[8].