Institute of Experimental Physics SAS, Watsonova 47, 04001 Košice, Slovakia.
Faculty of Electrical Engineering and Informatics, Technical University of Košice, Letná 9, 04200 Košice, Slovakia.
J Chem Phys. 2023 May 28;158(20). doi: 10.1063/5.0151811.
A low-frequency dielectric response of a ferrofluid based on transformer oil and MnZn ferrite nanoparticles is investigated in a gradient magnetic field. Four ferrofluid samples of various nanoparticle concentrations were introduced into planar micro-capacitors located over a magnetized tip. The dielectric spectra were measured in the frequency range from 0.1 Hz to 200 kHz and in the local magnetic field up to 100 mT. The spectra exhibit a dielectric relaxation ascribed to nanoparticle interfacial polarization. The low-frequency spectrum of each ferrofluid decreases upon application of the magnetic field up to 20 mT. The decrease in dielectric permittivity is caused by a magnetic force acting on larger nanoparticles in the gradient magnetic field. It is assumed that the interfaces of the concentrated nanoparticles in the gradient field do not contribute to the effective dielectric response. This reduces the effective relaxation time and shifts the relaxation toward higher frequencies. The dielectric spectra are well described by a relaxation fit function consisting of one Havriliak-Negami and a conductivity term. The fitting confirms that the only effect of the gradient magnetic field on the dielectric spectra is the shift of the dielectric relaxation and the decrease of the amplitude in the imaginary permittivity. This behavior is evident from a master plot, where all dielectric relaxations are superimposed on a single line. The knowledge of the presented behavior of the ferrofluid may be valuable when applying a ferrofluid to sharply magnetized parts of various electrical equipment (wires, tips, screws, nails, edges) as a liquid dielectric medium.
基于变压器油和 MnZn 铁氧体纳米粒子的铁磁流体在梯度磁场中的低频介电响应进行了研究。将四种不同纳米粒子浓度的铁磁流体样品引入位于磁化尖端上方的平面微电容器中。在 0.1 Hz 至 200 kHz 的频率范围内和高达 100 mT 的局部磁场下测量介电谱。光谱表现出介电弛豫归因于纳米粒子界面极化。施加磁场后，每种铁磁流体的低频光谱都会减小，直至 20 mT。介电常数的降低是由于梯度磁场中较大的纳米粒子所受的磁场力所致。假设梯度场中浓缩纳米粒子的界面不会对有效介电响应做出贡献。这会减少有效弛豫时间并将弛豫移向更高的频率。介电谱可以通过由一个 Havriliak-Negami 和一个电导率项组成的弛豫拟合函数来很好地描述。拟合结果证实，梯度磁场对介电谱的唯一影响是介电弛豫的位移和虚介电常数幅度的减小。从主图中可以明显看出这种行为，所有介电弛豫都叠加在单个线上。当将铁磁流体应用于各种电气设备（电线、尖端、螺丝、钉子、边缘）的急剧磁化部分作为液体介电介质时，了解铁磁流体的这种行为可能很有价值。
