Composite material engineering has become an important technique to control the physical properties of materials from the macro- to the mesoscale , , , . In the static response or in the long wavelength limit for elastic  or electromagnetic waves , the Lamé constants or the dielectric function of composites can be calculated using effective medium approximations (EMAs).
In this article, we are interested in the effective dielectric function of a composite defined as a homogeneous matrix or host with a dispersive dielectric function ϵh, and inclusions that occupy a volume fraction fi with a dielectric function ϵi and their shape is defined by the depolarisation factor Li. For simplicity, the frequency dependence of the dielectric functions is assumed. The problem of EMA is to find an effective dielectric function  of the form with the constraint where fh is the volume fraction occupied by the host and F is a function of the parameters of the system. Examples of EMA include the Maxwell-Garnett approximation, the Bruggeman approximation  that is symmetric with respect to the exchange of host and inclusions, thus making it more appropriate for random porous media . EMAs are not unique and there are at least a dozen different models or recipes described in the literature to obtain the effective dielectric function. In an extensive review, Prasad and Prasad  compared these models for the same composite material showing different results depending on the model, even for dilute systems. In the static limit, where there is no dissipation, any effective medium model is bounded above and below by the Hashim-Strickman variational  bounds that impose a maximum and minimum value for the effective dielectric function. The variations in the effective models have also been explored in the context of the Casimir effect where Lifshitz model imposes the condition that the effective dielectric function has to satisfy Kramers-Kronig relations .
Experimentally, Grundquist and Hunderi ,  measured the optical response of Ag-SiO2 cermet films as well as films made by depositing Au particles on substrate. In both cases, classical models such as Maxwell-Garnett or Bruggeman do not fit the experimental data and a modification to the Maxwell-Garnett model was proposed. However, there are cases where the theoretical description of the optical properties using EMA and experiments match, such as in porous Si whose measured optical response, is described accurately by the Bruggeman model  or the calculation of the extinction coefficient of nested nanoparticles can be described by Maxwell-Garnett model .
Near field heat transfer can be tuned using composite materials. The simplest composite materials are periodic layered media that are described accurately by EMA  and the near field radiative heat transfer . For more complex structures, Ben Abdallah  considered the near field radiative heat transfer between two SiC slabs separated by vacuum, each one with a periodic array of cylindrical pores. As the volume fraction occupied by the pores changes, the near field heat transfer increases due to different mechanisms related to the allowed modes and additional surface waves present in the system. Another uniaxial system, also described by an effective dielectric function, are arrays of silicon nanowires also showing an increase in the heat transfer after a critical volume fraction of nanowires is reached . Another system where the percolation transition in the near field heat transfer was calculated  between a composite sphere made of a host of polystyrene with metallic inclusions and a SiC half-space. The changes in the radiative heat transfer close to the insulator-metal transition are shown and are described by two EMA models, Bruggeman and Lagarkov-Sarychev. In a plane-plane radiative heat transfer calculation, the EMA was used to describe the insulating-conducting transition of a thin Au film as a function of its thickness .
In this article, we study the near field radiative heat transfer between a SiC slab and a composite slab made of an homogeneous host and spherical inclusions made of Au nanoparticles. We calculate the spectral heat function and the total radiative heat transfer for this system using different EMAs, showing the validity of the different approaches.
2.1 Effective Dielectric Functions
In this article, we consider, without loss of generality, composites made of an homogeneous matrix with spherical inclusions made of Au nanoparticles of diameter 10 nm, in a host material of SiC as shown in Figure 1a.
The approximations we consider in this article for the effective dielectric function are the Maxwell-Garnett approximation Bruggemans and Looyenga-Lifshitz ,  that are obtained from the following equations
In the Mawell-Garnett approximation the term on the right-hand side of (1) is proportional to the polarisability of the inclusion. Bruggeman’s approximation yields a second-order equation for for a fixed value of the filling fraction f. The physically meaningful root is such that Finally, Looyengas-Lifshitz approximations belong to the class of general power law models where the exponent α depends on the shape of the inclusions. When α=1, the inclusions have no depolarisation and we obtain the arithmetic mean of the dielectric functions .
ϵexp(ω) being the complex experimental dielectric function, ωp=1.36×1016 rad/s, the plasma frequency, γ=4.05×1013 rad/s, is the bulk damping, vf=1.39×106 m/s the Fermi velocity of Au and γ′=vF/D, where D is the diameter of the Au nanosphere.
The effect of the finite size of the particle is shown in Figure 2, where we present the real part (2a) of the dielectric function with and without finite size correction, and the imaginary part also with and without correction (2b). Thus, finite size effects have to be considered in the calculation of the effective dielectric function of the composite.
The dielectric function of SiC ϵh is given by a Lorentz type oscillator:
with ϵ∞=6.7, ωL=1.827×1014 rad/s, ωT=1.495×1014 rad/s, and γ=0.9×1012 rad/s.
With the dielectric function of the host and the Au inclusions, we calculate the effective dielectric function using the three different models as a function of wavelength and for different values of the filling fraction. This is shown in Figure 3 for the real part of the effective dielectric function and in Figure 4 for the imaginary part. The filling fractions considered are f=0.01, 0.05, 0.1, 0.15. For small filling fractions, Bruggeman and Maxwell-Garnett models give the same results at large wavelengths. The largest difference occurs at around a wavelength of λ=λp=620 nm that corresponds to the plasmonic resonance of the Au nanoparticles. For λ>λp, Looyenga’s model shows the largest variation as compared with the other approximations.
3 Near Field Radiative Heat Transfer
The near field radiative heat transfer (NFHT) is calculated between a SiC half-space at temperature T2=1 K and the composite slab at a temperature T1=300 K, both slabs separated by a distance d. In the case of the composite slab, the nanospheres are randomly distributed and the effective dielectric function is isotropic. Using the known results of fluctuating electrodynamics , , the heat flux Q is calculated as
where θ(ω, T)=ħω/(exp(ħω/kBT)−1) is Planck’s distribution function, q is the parallel component to the slabs of the wave vector, and the perpendicular component is For either p or s polarised waves, the transmission coefficient is
The terms and are the Fresnel coefficients for the slabs at temperature T1 and T2, respectively. The Fabry-Perot type term in the denominator is
Setting the temperatures at T1=300 K and T2=1 K, we calculate the heat flux as a function of the separation d for the different effective medium models at various filling fractions of the nanospheres. In Figure 1b, we show the configuration of the system. In Figure 5, we present the values of the heat flux, normalised to the far field value where σ=5.670×10−8 Wm−2 K−4. As a reference, the value of Q for two homogeneous SiC slabs is also plotted. For a filling fraction of f=0.010=1%, we see that there is no significant difference between the different effective medium models, except for Looyengas that shows a smaller value of Q for all separations. As the filling fraction increases to f=0.1 and f=0.15, we observe differences on the predicted value of Q depending on the model. Looyenga’s approximation gives a lower bound for the total near field radiative heat transfer for all values of the filling fractions. At a filling fraction of f=0.33, the Bruggeman model predicts a percolation transition, that corresponds to the close packing of the spheres. At this filling fraction, we see the largest differences in the predicted values of Q.
For a system with a small filling fraction, Maxwell-Garnett does not match the experimental measurements of the optical properties for the systems studied by Grundquist and Hunderi. Neither does the Bruggeman approximation. Grundquist introduced an ad-hoc correction ,  to the Maxwell-Garnett model. The effective dielectric function in this case is
For comparison, we can write Maxwell-Garnett equation (1) as
For two different filling fractions, we show in Figure 6 the total heat flow obtained using Maxwell-Garnett (8) of the Grundquist-Hunderi (10) EMAs. At a filling fraction of f=0.1, the value of Q differs for both models for all separations considered. However, for f=0.15, the difference becomes smaller. This is because the larger the amount of Au in the sample, the effective dielectric function becomes more metallic and the dielectric function of Au begins to be more prevalent in the effective response.
In this article, we calculate the near field heat transfer between a composite medium and an homogeneous slab using different models for the effective dielectric function of the composite. The real and imaginary parts of the effective dielectric function have different values depending on the model used. This in turn yields also different values for the total heat transfer Q, even for small filling fractions. We also show that ad-hoc models fitted to experimental optical data, such as Grandquist-Hunderi approximation, differ from the values of Maxwell-Garnett model, even at small filling fractions. Furthermore, in the case of metallic inclusions, the different models differ around the plasmonic resonance of the inclusions.
The sensitivity of both the effective dielectric function and the radiative heat transfer shows that any prediction based on effective medium models gives at best an estimate of the heat transfer and the choice of the effective dielectric function can only be validated if experimental data of the optical properties are available.
Partial support from DGAPA-UNAM project IN110916, PIIF-IFUNAM projects and CONACyT Fronteras project No. 1290.
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About the article
Published Online: 2016-12-19
Published in Print: 2017-02-01