Measurements of universal and non-universal percolation exponents in macroscopically similar systems

Chiteme, C.; McLachlan, D.S.

doi:10.1016/s0921-4526(99)00670-5

Cited by 38 publications

(33 citation statements)

References 9 publications

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“…The universality of percolation theory suggests that the dielectric constant should exhibit the same power-law dependence on the volume fraction as the conductivity below f c , i.e., s = s¢ » 1, but this is not always observed in practical continuum systems. [27] As shown in Figure 3, the experimental values of the effective dielectric constant are in good agreement with Equation 2, with f c = 0.17 ± 0.01 and s = 0.89 ± 0.05. This exponent is consistent with the universal one.…”

Section: Methodssupporting

confidence: 74%

Novel Ferroelectric Polymer Composites with High Dielectric Constants

Dang

Lin

Chen³

2003

Advanced Materials

683

438

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Fe [Co¯Fe¯], if one considers the moment of the cobalt ion being reduced due to the stabilization of the lowest Kramers doublet. [16] The slow rise of the virgin magnetization is consistent with a pinning-type magnet with no reversible region. [2] However, the S-type increase with field of the initial (virgin) magnetization after ZFC is not that expected for a random distribution of super-paramagnets, and this anomaly may be a consequence of the presence of a mean dipolar field acting on the particles, which depends on the direction and magnitude of the magnetization of surrounding particles. [18] We, therefore, think that the large coercive field reflects the ªintrinsic anisotropy of the particles enhanced by the inter-particle dipolar fieldsº. To test this hypothesis, we examine a sample (7.5 % CoFe 2 O 4 ±SiO 2 , annealed at 800 C) consisting of 3.2 nm particles with varying separation between particles. These particles are super-paramagnetic at room temperature and the ZFC±FC magnetization measurements in 5 Oe indicate a blocking temperature of 55 K (see supplementary material S2, available from the author). The super-paramagnetism is exemplified by the absence of remanance magnetization and coercivity above 55 K, while a wide hysteresis loop with a remanance magnetization of 50 % of that at saturation is observed and the coercivity is 13.5 kOe at 2 K (see supplementary material S3, available from the author). We also note that the remanance in this case is as expected for the model of Stoner for a random oriented polycrystalline magnetic sample. [19] Further work is in progress on samples prepared at different dilution of spinel and different annealing temperatures between 800 C and 1100 C in order to elucidate the effective dipolar field as a function of size and inter-particle distances. We can only note presently that the size of the particle, its blocking temperature and coercive field decrease on lowering both the concentration and sintering temperature. The lack of evidence for the presence of c-Fe 2 O 3 in the composite does not rule out the model of Skomski and Coey for the enhancement of magnetic hardness in composites consisting of hard and soft magnetic materials. This remains to be verified. [20] We have observed an unusually high magnetic hardness, characterized by a 20 kOe coercive field at 2 K, for CoFe 2 O 4 particles of a size of 12 nm in amorphous SiO 2 prepared by the sol±gel method and annealed at 1000 C. The coercivity reflects the intrinsic anisotropy enhanced by dipolar fields. Its high value for smaller particles tends to confirm this hypothesis. ExperimentalThe sample was prepared by the sol±gel method as follows: Co(NO 3 ) 2´6 H 2 O (2.18 g) and Fe(NO 3 ) 3´9 H 2 O (6.06 g) were dissolved in 4.5 g of water acidified with nitric acid (0.03 M) as a catalyst. To this solution tetraethoxysilane (TEOS, 10.6 g), dissolved in 7 g of methanol, and formamide (2.25 g), as a modifier, were added. After gelation (approximately 2 h at 40 C) and ageing (24 h), the sample was dried at 40 C f...

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Section: Methodssupporting

confidence: 74%

Novel Ferroelectric Polymer Composites with High Dielectric Constants

Dang

Lin

Chen³

2003

Advanced Materials

683

438

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“…[10,28] For the 9 vol pct NiNs + 3 vol pct NCCF composition, b is close to the expected value, but for the 15 vol pct NiNs, we find a nonuniversal value of b = 0.36. Though our value for the second critical exponent, a, is not close to the expected value of 0.87, [30] we note that it illustrates the sensitivity of the equation, as previously stated. The values found for Q c are much closer to the theoretical values.…”

Section: Resultscontrasting

confidence: 49%

Multiscale Model for the Extreme Piezoresistivity in Silicone/Nickel Nanostrand Nanocomposites

Johnson

Gardner

Seegmiller

et al. 2011

Metall Mater Trans A

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Extreme piezoresistivity was discovered in a silicone/nickel nanostrand (silicone/NiNs) nanocomposite. A novel technique was developed to study the charge transport phenomena responsible for the piezoresistive mechanism in the silicone/NiNs system using conductive nanoindentation. A quantum mechanical tunneling (QMT)/percolation model was developed, which bridges the gap between quantum effects at the nanoscopic scale and bulk material response at the macroscopic scale. The predictions of this model are compared to experimental measurements.

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“…The critical exponent in the insulation region, p=0.40, is smaller than the universal ones. The non-universal percolation exponent was reported by Chiteme et al [9]. The spatial distribution and interaction of GnPs in polymer matrices, and the nature of the interparticle contacts, might be major factors in determining the critical exponents.…”

Section: Dielectric Properties Of Compositesmentioning

confidence: 79%

Effect of Graphene Nanoplates on Phase Structure and Electrical Properties of Epoxy/Polyetherimide Composite

Pan¹,

Ma²,

Wan³

et al. 2016

Proceedings of the 4th 2016 International Conference on Material Science and Engineering (ICMSE 2016)

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Epoxy resin/polyetherimide/graphene nanoplates (EP/PEI/GnPs) composites were prepared by solution blending, and the effects of GnPs on the phase structure and electrical properties of the systems were investigated by scanning electron microscopy (SEM), optical microscopy (OM), digital ultrahigh resistance and microcurrent measuring meter and precision impedance analyzer. The results show that the phase separation morphology of the composites change from sea-island structure to bicontinuous structure and phase inversion structure with the increase of GnPs content. The composites have an electrical percolation threshold of 0.85 wt%. A high dielectric constant of more than 310 at 1 kHz could be obtained for the composite with a filler content of 1.75 wt%.

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Measurements of universal and non-universal percolation exponents in macroscopically similar systems

Cited by 38 publications

References 9 publications

Novel Ferroelectric Polymer Composites with High Dielectric Constants

Novel Ferroelectric Polymer Composites with High Dielectric Constants

Multiscale Model for the Extreme Piezoresistivity in Silicone/Nickel Nanostrand Nanocomposites

Effect of Graphene Nanoplates on Phase Structure and Electrical Properties of Epoxy/Polyetherimide Composite

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