Evidence of a third kind of Johnscher’s like universal dielectric response

Polymer/metal composites(PMC)comprising of polyvinylidene fluoride/nanocrystalline nickel with varying volume fractions of nickel(fcon)prepared under cold press show an insulator to metal transition(IMT)at percolation threshold(f_(c)=f_(con)=0:27).The two kinds of generalized Johnscher’s universal dielectric response(UDR)laws on both sides of IMT hold good,while for the percolative sample,none of the two laws hold good.Neither the concept of dipolar relaxation nor anomalous low frequency dispersion stands valid for f_(c)=0:27,while a completely different,neutral and competing electrical behavior is observed over the entire range of frequencies.The emerged third kind of Johnscher’s like UDR for fc is observed and the relaxation law has been formulated as the ratio of imaginary and real parts of dielectric constant remains constant over the entire range of frequency starting from dc to any higher frequency.The value of the constant is attributed to depend on the PMC,the dielectric constant of the polymer,the differences of conductivity and fractions of the components of the PMC and also on their connectivity arising due to the difference of their process conditions.The emerged unique dielectric relaxation consists of multiple relaxations arising due to the combination of other relaxations(arising due to the two different types of species)present in the sample,f_(con)=0:27.This novel material may be suitable for certain specific applications in electrical and electronics engineering.

Frequency-dependent scaling behavior of polyvinylidene fluoride/metal composites

The frequency-dependent percolation and scaling behavior of a variety of polymer/metal composites(PMC),based on polyvinylidene fluoride(PVDF)matrix and various types of fillers such as;metal/alloy particles of different sizes,prepared through cold/hot pressing process conditions have undergone investigation.The universal percolation behavior in the vicinity of percolation threshold((fc),i.e.,σ_(e_(ff))(ω,f_(con)≈f_(c))α/ω^(x)andε_(eff)(ω,f_(con)≈f_(c))α/ω^(-y)is well satisfied,which suggests fc to be independent of frequency,where
eff and"eff are the effective ac conductivity and effective dielectric constants of the composite andωis the frequency of applied ac signal.The obtained experimental values of the exponents are consistent with the inter-cluster polarization model(x=0:72 and y=0:28),satisfying x+y=1.The widely used percolative equations are well fitted with the experimental results of all PMC at all values of the frequency.The value of fc is found to be independent of frequency of the applied signal,suggesting the studied PMC are real percolating systems.The critical exponents(s and s0)which characterize the divergence of"ε_(ff)and
ε_(ff)in the vicinity of f_(c)are found to decrease with the increase of frequency.The rate of decrease of‘s’and‘s′’with increase of frequency is attributed to the method of preparation,size of the fillers,adhesiveness of polymer/filler and the rate of decrease of e_(ff) with frequency(due to the absence of different extents of contributions of various types of conventional polarizations).

Dielectric spectroscopy of polymer-metal composites across the percolation threshold

Polymer(polar/nonpolar)/metal composites(PMC)were prepared under different process conditions.In polar PMC,dipolar relaxation plays a predominant role below percolation threshold(f_(c))and anomalous low frequency dispersion(ALFD)becomes dominant above f_(c)while ALFD is the only likely possibility for nonpolar PMC above f_(c).The magnitude of relaxation exponents"m","p"and"n",evaluated from the experimental results using Jonscher's universal dielectric response(JUDR)laws,falls within the universal limit
[0,1]with additional feature of strong dependence on volume fraction of conductor(f_(c)on).The decrease in the relaxation exponent"m"with an increase of f_(c)on is directly linked with decrease in the number of dipoles of the polymer in the composite and is accompanied by a distribution of relaxation time due to increased heterogeneity of the system.The magnitude of the relaxation exponent"n"decreases at f_(c),due to the prevalence of Maxwell–Wagner–Sillar polarization contributed by uncorrelated electrons.