Anisotropic magnetoresistance and piezoresistivity in structured Fe3O4-silver particles in PDMS elastomers at room temperature.
Anisotropic magnetoresistance and piezoresistivity in structured Fe3O4-silver particles in PDMS elastomers at room temperature.
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DOI：
10.1021/la204823k
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发表时间：
2012-04
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通讯作者：
J. L. Mietta;Mariano M Ruiz;P. Antonel;O. E. Pérez;A. Butera;G. Jorge;R. Negri
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文献类型：
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作者：
J. L. Mietta;Mariano M Ruiz;P. Antonel;O. E. Pérez;A. Butera;G. Jorge;R. Negri
制备并表征了基于在室温下表现出各向异性磁阻和压阻的弹性有机基质的磁流变弹性体（MRE）。这些材料是超顺磁性磁铁矿的分散体，形成银微粒内聚集纳米颗粒的核心，银微粒分散在弹性体聚合物（聚二甲基硅氧烷，PDMS）中，在均匀磁场存在下固化聚合物。这样，当施加场诱导形成沿场方向（平行于场）排列的银覆盖的无机材料团聚物（针）的细丝时，弹性材料变得结构化。由于磁性颗粒被银覆盖，MRE 不仅具有磁性，而且还是电导体。该结构具有弹性、磁力和电各向异性特性。例如，当弹性基质中颗粒浓度较低（5% w/w）时，平行于针测量时可以获得几欧姆的电阻，或者在垂直方向上测量几兆欧姆的电阻。采用共沉淀法合成了磁铁矿纳米粒子（Fe(3)O(4) NP），然后用Ag覆盖这些纳米粒子的团聚体。所得磁铁矿纳米颗粒的平均尺寸约为13 nm，磁铁矿银颗粒（称为Fe(3)O(4)@Ag）形成微米级聚集体（1.3 μm）。通过 XRD、TEM、SEM、EDS、漫反射、伏安法、VSM 和 SQUID 对纳米颗粒、微粒和 MRE 进行了表征。在室温下，合成的磁铁矿和 Fe(3)O(4)@Ag 颗粒处于超顺磁性状态（通过 SQUID 测定，0.01 T 时 T(B) = 205 和 179 K）。使用纹理分析装置测量 MRE 的弹性特性和杨氏模量作为取向的函数。通过 FMR 研究了 MRE 复合材料的磁各向异性。当施加压力 P 时，MRE (σ) 的电导率呈指数增加，并且变化的幅度很大程度上取决于施加 P 的方向（各向异性压阻率）。此外，在固定压力下，仅当场 H 相对于电通量 J 沿共线方向施加时，在存在外部磁场 (H) 的情况下，σ 才会呈指数增加。使用考虑晶间电子传输的模型实现了实验数据 σ 与 H 和 P 的良好拟合，其中除了渗流过程中的固有晶间电阻之外，还考虑了 H 相关势垒。 H 相关势垒随着施加的场而减小，这是由于晶粒之间的银覆盖物中自旋极化的匹配性增加。该效应是各向异性的（即磁阻效应的灵敏度取决于 H 和电流 J 之间的相对方向）。对于 Fe(3)O(4)@ Ag，当 H 和 J 与 PDMS 基质中的针平行时，对于 400 mT 的场和电阻约为 1-10 Ω 的情况，我们获得了 σ 的变化高达 50%。磁阻和磁弹性特性使这些材料在柔性电子产品、电子皮肤、各向异性压力和磁场传感器中的应用非常有趣。
Magnetorheological elastomers, MREs, based on elastic organic matrices displaying anisotropic magnetoresistance and piezoresistivity at room temperature were prepared and characterized. These materials are dispersions of superparamagnetic magnetite forming cores of aggregated nanoparticles inside silver microparticles that are dispersed in an elastomeric polymer (poly(dimethylsiloxane), PDMS), curing the polymer in the presence of a uniform magnetic field. In this way, the elastic material becomes structured as the application of the field induces the formation of filaments of silver-covered inorganic material agglomerates (needles) aligned in the direction of the field (parallel to the field). Because the magnetic particles are covered with silver, the MREs are not only magnetic but also electrical conductors. The structuration induces elastic, magnetic, and electrical anisotropic properties. For example, with a low concentration of particles in the elastic matrix (5% w/w) it is possible to obtain resistances of a few ohms when measured parallel to the needles or several megaohms in the perpendicular direction. Magnetite nanoparticles (Fe(3)O(4) NP) were synthesized by the coprecipitation method, and then agglomerations of these NPs were covered with Ag. The average size of the obtained magnetite NPs was about 13 nm, and the magnetite-silver particles, referred to as Fe(3)O(4)@Ag, form micrometric aggregates (1.3 μm). Nanoparticles, microparticles, and the MREs were characterized by XRD, TEM, SEM, EDS, diffuse reflectance, voltammetry, VSM, and SQUID. At room temperature, the synthesized magnetite and Fe(3)O(4)@Ag particles are in a superparamagnetic state (T(B) = 205 and 179 K at 0.01 T as determined by SQUID). The elastic properties and Young's modulus of the MREs were measured as a function of the orientation using a texture analysis device. The magnetic anisotropy in the MRE composite was investigated by FMR. The electrical conductivity of the MRE (σ) increases exponentially when a pressure, P, is applied, and the magnitude of the change strongly depends on what direction P is exerted (anisotropic piezoresistivity). In addition, at a fixed pressure, σ increases exponentially in the presence of an external magnetic field (H) only when the field H is applied in the collinear direction with respect to the electrical flux, J. Excellent fits of the experimental data σ versus H and P were achieved using a model that considers the intergrain electron transport where an H-dependent barrier was considered in addition to the intrinsic intergrain resistance in a percolation process. The H-dependent barrier decreases with the applied field, which is attributed to the increasing match of spin-polarization in the silver covers between grains. The effect is anisotropic (i.e., the sensitivity of the magnetoresistive effect is dependent on the relative orientation between H and the current flow J). In the case of Fe(3)O(4)@ Ag, when H and J are parallel to the needles in the PDMS matrix, we obtain changes in σ up to 50% for fields of 400 mT and with resistances on the order of 1-10 Ω. Magnetoresistive and magnetoelastic properties make these materials very interesting for applications in flexible electronics, electronic skins, anisotropic pressure, and magnetic field sensors.