Datt Gopal, Kotabage Chetan, Abhyankar A C
Department of Materials Engineering, Defence Institute of Advanced Technology, Girinagar, Pune 411025, India.
Phys Chem Chem Phys. 2017 Aug 9;19(31):20699-20712. doi: 10.1039/c7cp03953k.
The effect of cationic disorder and particle morphology on the ferromagnetic resonance (FMR) of NiCoFeO nanoparticles (NPs) and the electromagnetic shielding effectiveness of flexible composites (wherein the nanoparticles are used as fillers) has been presented. Upon annealing at 1000 °C, spherical, ∼25 nm, single crystalline (as-prepared) NPs are transformed into octahedral, ∼200 nm, polycrystalline (annealed) NPs and change the cationic distribution significantly. The effect of shape, size and cationic distribution on the resonance properties has been discussed using the randomly-oriented anisotropic-axis model. The temperature dependent evolution of FMR spectra has been found to be consistent with a Bloch spin-relaxation model. Analysis of the FMR spectra reveals that NiCoFeO nanoparticles have a large internal magnetic field along with broad FMR linewidths of ∼2-3 kOe, signifying high magnetic losses that are essential for the absorption of electromagnetic (EM) waves. Next, NiCoFeO-carbon black (NCF-CB) hybrids grafted in a PVA matrix, as flexible composite films with a thickness of ∼1.5 mm, are assessed for EM wave absorption properties in the range of 8-18 GHz. As compared to annealed-NCF-CB/PVA (21 dB, ∼99.5%), the as-prepared-NCF-CB/PVA composite film exhibits significantly large SE of 27 dB (∼99.9% attenuation of the EM wave), with a dominant contribution from absorption (SE ∼ 21 dB). The electrical conductivity, the electric modulus, and Cole-Cole plots reveal that the dielectric losses in the as-prepared-NCF-CB/PVA have significant contributions from cationic disorder and particle size, as compared to the annealed-NCF-CB/PVA composites. Cationic disorder increases the d-d electron transition probability between adjacent ionic pairs such as Co/Fe and a reduced particle size creates large interfacial polarization in the as-prepared NCF/CB hybrids. Considerably large values of the Landes g-factor, magnetic anisotropy and better impedance matching indicate a dominant magnetic loss contribution in ap-NCF (g = 4.5) as compared to an-NCF (g = 2.5) at 300 K.
研究了阳离子无序和颗粒形态对NiCoFeO纳米颗粒（NPs）铁磁共振（FMR）以及柔性复合材料（其中纳米颗粒用作填料）电磁屏蔽效能的影响。在1000℃退火后，球形、约25nm的单晶（制备态）NPs转变为八面体、约200nm的多晶（退火态）NPs，并显著改变了阳离子分布。使用随机取向的各向异性轴模型讨论了形状、尺寸和阳离子分布对共振特性的影响。发现FMR光谱随温度的演变与布洛赫自旋弛豫模型一致。对FMR光谱的分析表明，NiCoFeO纳米颗粒具有较大的内磁场以及约2 - 3kOe的宽FMR线宽，这表明其具有高磁损耗，这对于吸收电磁波至关重要。接下来，评估了接枝在PVA基质中的NiCoFeO - 炭黑（NCF - CB）杂化物作为厚度约为1.5mm的柔性复合薄膜在8 - 18GHz范围内的电磁波吸收性能。与退火态 - NCF - CB/PVA（21dB，约99.5%）相比，制备态 - NCF - CB/PVA复合薄膜表现出显著更大的屏蔽效能，为27dB（约99.9%的电磁波衰减），其中吸收起主要作用（屏蔽效能约为21dB）。电导率、电模量和科尔 - 科尔图表明，与退火态 - NCF - CB/PVA复合材料相比，制备态 - NCF - CB/PVA中的介电损耗主要来自阳离子无序和颗粒尺寸。阳离子无序增加了相邻离子对（如Co/Fe）之间的d - d电子跃迁概率，并且减小的颗粒尺寸在制备态NCF/CB杂化物中产生了大的界面极化。在300K时，Landes g因子、磁各向异性的值相当大且阻抗匹配更好，表明制备态 - NCF（g = 4.5）中的磁损耗贡献比退火态 - NCF（g = 2.5）更显著。
