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Magnetic fatigue of magnetite: effect of cyclic loading on the magnetic and microstructural behavior at elevated temperatures

磁铁矿的磁疲劳：循环载荷对高温下磁性和微观结构行为的影响

基本信息

批准号：
329643415
负责人：
Privatdozentin Dr. Agnes Kontny
金额：
--
依托单位：
Institut für Angewandte Geowissenschaften (AGW)
依托单位国家：
德国
项目类别：
Research Grants
财政年份：
2017
资助国家：
德国
起止时间：
2016-12-31 至 2021-12-31
项目状态：
已结题

来源：
https://gepris.dfg.de/gepris/projekt/329643415?language=en
关键词：
Magnetic fatigue magnetite effect cyclic

项目摘要

Magnetic properties of rocks are influenced by deformation processes within the Earth crust, ranging from slow processes in plate tectonics, through volcanic eruptions to the high dynamic seismic events. Up to now, modifications of magnetic properties are mainly investigated in relation to static deformation. However, it is generally accepted that tectonic stresses first accumulate non-uniformly, and then rapidly increase. Such a rapid stress increase may be especially dominant if the tectonic loading exhibits a cyclic or a fatigue character at elevated temperatures. This assumption raises the question whether magnetic properties of rocks are sensitive to cyclic deformation at elevated temperatures. Surprisingly, such information is unavailable in the geoscience literature. Our preliminary investigations, however, show clearly that cyclic deformation is causing a degradation of magnetic properties or a magnetic fatigue of a magnetite-bearing ore. Preliminary experiments at 500°C reveal a non-linear relationship between the increasing loading frequency and the variations in the Verwey transition temperature, magnetic remanence and coercively as well as a drastic non-linear drop of magnetic susceptibility and its anisotropy with increasing cyclic loading duration. Our project aims to test the magnetic fatigue hypothesis with a special consideration of irreversible (microcracks, vacancy formation, twinning) and reversible effects (magnetostriction, elastic strains). For this purpose, samples prepared from a magnetite ore, single crystals of magnetite and magnetite-bearing granite will be cyclically loaded under various conditions and subsequently studied by different magnetic and microstructural methods. On the basis of the obtained results, fatigue deformation maps related to the magnetic behaviour for magnetite will be constructed. We expect from this project basic knowledge on the magnetic field stress sensitivity, which is currently poorly constrained but might be useful for a sophisticated interpretation of magnetic signals from deformed rocks as they occur ubiquitous in the Earth crust.

岩石的磁性受到地壳内部变形过程的影响，从板块构造的缓慢过程，到火山喷发，再到高动态地震事件。到目前为止，磁性能的改变主要是从静态变形的角度来研究的。然而，人们普遍认为，构造应力首先不均匀积累，然后迅速增加。如果构造载荷在高温下表现出周期性或疲劳的特征，这种快速的应力增加可能特别占优势。这一假设提出了一个问题，即岩石的磁性是否对高温下的循环变形敏感。令人惊讶的是，这样的信息在地球科学文献中是找不到的。然而，我们的初步研究清楚地表明，循环变形正在导致含磁铁矿矿石的磁性退化或磁疲劳。在500℃下的初步实验表明，随着循环加载时间的延长，加载频率的增加与Verwey转变温度、剩磁和矫直力的变化以及磁化率及其各向异性的急剧非线性下降之间存在非线性关系。我们的项目旨在通过特别考虑不可逆效应(微裂纹、空位形成、孪生)和可逆效应(磁致伸缩、弹性应变)来检验磁疲劳假说。为此，从磁铁矿、磁铁矿单晶和含磁铁矿花岗岩制备的样品将在不同条件下进行循环加载，然后用不同的磁性和显微结构方法进行研究。在此基础上，建立了与磁铁矿磁性有关的疲劳变形图。我们希望从这个项目中获得关于磁场应力敏感性的基本知识，目前磁场应力敏感性很差，但可能有助于对来自变形岩石的磁信号进行复杂的解释，因为它们在地壳中无处不在。