MgB2 Superconducting Wire Geometry for Isotropic Critical Current
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Solution Overview
Problem
Existing MgB2 superconducting wires, particularly those produced by the mechanical milling method, often exhibit anisotropic critical current density and are difficult to shape into symmetrical round or angular forms, which are desirable for applications like MRI apparatuses.
Innovation Solution
The development of a MgB2 wire with a round or angular cross-sectional shape, achieved through high-energy mixing of magnesium and boron powders using a planetary ball mill apparatus, followed by processing methods like drawing and cassette rolling, which help in achieving a high critical current density without anisotropy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If heat treatment is performed at high temperature (800-900°C) to sinter MgB2 powder and achieve good powder bonding, then powder bonding is improved, but flux pinning deteriorates due to grain growth and reduced crystallinity
Solution Approach 1:
The patent changes the temperature parameter from conventional high-temperature sintering (800-900°C) to low-temperature heat treatment (400-600°C). This parameter change allows achieving good powder bonding while maintaining high crystallinity and flux pinning properties, as evidenced by sharp XRD peaks and high critical current density values.
2Reliability
If heat treatment is performed at low temperature (700°C or below) to maintain flux pinning, then flux pinning is improved, but powder bonding deteriorates due to insufficient sintering
Solution Approach 1:
The patent introduces a novel temperature range (400-600°C) that reconciles the contradiction between flux pinning and powder bonding. At this optimized temperature range, both sharp XRD peaks indicating good crystallinity and strong powder bonding are achieved simultaneously, eliminating the need to choose between the two competing requirements.
3Reliability
If mechanical milling is performed to increase reactivity of powders, then reactivity is improved, but anisotropy of critical current density increases and symmetrical shape formation becomes difficult
Solution Approach 1:
The patent performs mechanical milling as a preliminary action before wire drawing to increase powder reactivity and ensure complete reaction during subsequent heat treatment. By pre-milling the powders, the reaction proceeds completely at lower temperatures, and the wire drawing process then establishes the symmetrical shape, decoupling the reactivity enhancement from shape formation.
4Strength
If in situ method is used to synthesize MgB2 from magnesium and boron, then powder bonding is improved, but filling rate deteriorates due to volume contraction reaction creating voids
Solution Approach 1:
The patent uses a pre-formed MgB2 powder approach instead of in situ synthesis, avoiding the volume contraction reaction that creates voids. The pre-formed powder is then heat-treated at low temperature (400-600°C) to achieve bonding without significant volume change, thereby maintaining high filling rate while achieving good powder bonding.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The resulting MgB2 wire exhibits a high critical current density, reduced anisotropy, and improved symmetry, making it suitable for applications requiring high magnetic field homogeneity, such as MRI apparatuses, while also reducing material costs and operational complexity.
Implementation Method 1
high-energy mixing of magnesium and boron powders using a planetary ball mill apparatus
Implementation Method 2
a diameter reduction is performed by a method of drawing
Implementation Method 3
a heat treatment for synthesizing MgB2 from magnesium and boron
Implementation Method 4
after the diameter reduction, a heat treatment is performed in order to sinter MgB2 powder
Implementation Method 5
the critical current density of a superconductor is determined by flux pinning. Lines of magnetic flux are quantized and infiltrate into a (type-II) superconductor, and the Lorentz force acts on the lines of magnetic flux during energization
Data Source
Figure 1~2
Figure 3
Figure 4(a)~4(c)
AI summary
The present invention addresses the problem of providing a wire material capable of ensuring high critical current density, regardless of the cross-sectional shape thereof. This superconducting wire material is equipped with an MgB2 filament, the number density of cavities having a major axis of 10 µm or higher in a longitudinal cross-section of the superconducting wire material is in the range of 5 - 500 mm-2, and the average value of the angle formed between the major axis of the cavities and the axis of the wire material is 60 degrees or more.