Effect of Exfoliated Graphene on Thermal Conductivity Enhancements of Graphene-Ironoxide Hybrid Nanofluids: Experimental Investigation and Effective Medium Theories

Reasoning of particular mechanism of anomalous thermal transport behaviours are not identified yet for the nanofluids. In this study, iron oxide (Maghemite: MH) and graphene (Gr) flake dispersed deionized water (DW) hybrid nanofluid system were developed for the first time to evaluate the thermal conductivity (TC) enhancements along with the analysis of anomalous TC behavior implementing modified effective medium theories (EMTs). A solvo-thermal two-step method was used to develop the MH nanoparticle and exfoliated Gr flake dispersed hybrid nanofluids with different compositions. Stability of as-prepared hybrid nanofluids were monitored using Ultraviolet-Visible (UV-Vis) spectroscopy. The maximum sedimentation rate was observed ~ 8.4 % after 600 hours. The results showed an overall maximum TC enhancement of ~ 43 % at 25 °C. EMTs were modified with the consideration of flat geometry of Gr flake. It is found that, modified EMTs with the crumpled factor (due to the non-flatness or crumple of Gr flake) of ~ 0.205 the predicted effective TC enhancements are agreed with the experimental TC’s of Gr-NMP/MH-DW hybrid nanofluids samples. The estimated crumple factor value of exfoliated Gr flakes using images analysis was also found nearly similar (~ 0.232). This agreement exposed that, Gr flake’s with negligible thickness compared to its extremely wide basal plane dimensions and its non-flatness or crumpled geometry in the nanofluids have the leading impacts on the effective TC properties of the Gr flake dispersed nanofluids. This modified model opens the new doors to analyse the insight of the thermophysical properties of various types of nanofluids by introducing potential other parameters.

Plasmonics of Diffused Silver Nanoparticles in Silver/Nitride Optical Thin Films

Metal-dielectric multilayers are versatile optical devices that can be designed to combine the visible transmittance of dielectrics with the electronic properties of metals for plasmonic and meta-material applications. However, their performances are limited by an interfacial optical absorption often attributed entirely to the metal surface roughness. Here, we show that during deposition of AlN/Ag/AlN and SiNx/Ag/SiNx multilayers, significant diffusion of Ag into the top dielectric layer form Ag nanoparticles which excite localized surface plasmon resonances that are primarily responsible for the interfacial optical absorption. Based on experimental depth profiles, we model the multilayer’s silver concentration profile as two complementary error functions: one for the diffused Ag nanoparticles and one for the interface roughness. We apply the Maxwell-Garnett and Bruggeman effective medium theories to determine that diffusion characteristics dominate the experimental absorption spectra. The newfound metal nanoparticle diffusion phenomenon effectively creates a hybrid structure characteristic of both metal-dielectric multilayer and metal-dielectric composite.

Experimental study of crack density influence on the accuracy of effective medium theory

SUMMARY Effective medium theory plays an important role in the characterization of fractured reservoirs. The main goal of this paper is to assess the accuracy of three classical effective medium theories (compliance-based non-interaction approximation, Hudson theory and anisotropic self-consistent approximation) and investigate their applicable frequency range. To evaluate the crack density limitation of these models, we construct a series of physical models with crack density varying from 0 to 12 per cent. To satisfy the long-wavelength approximation of effective medium theory, we consider a crack diameter and aperture of 3 and 0.12 mm, respectively. To study their appropriate high-frequency laboratory condition, we use the ultrasonic transmission method to measure the elastic wave velocities at frequencies of 500, 250 and 100 kHz. The experimental results are then compared with the effective medium theories to discuss their accuracy. We also highlight and compare the accuracy of these theories for the inversion procedure to quantify the crack density.

Strain-accumulation mechanisms in sands under isotropic stress

Determining the pressure dependence of dynamic moduli in unconsolidated sediments is still an open problem in applied geophysics. This is because several petrophysical parameters affect the elastic response of the granular medium during compression. Effective medium theories based on the Hertz–Mindlin contact law estimate the effective moduli from petrophysical parameters. Among them, the Pride and Berryman model assumes that new contacts between grains are progressively created during compression. Furthermore, the gaps around rattlers are distributed following a power law with distance and the global strain can change either linearly or quadratically with the local strain. This identifies two types of strain accumulation. Quadratic strain accumulation is associated with grain rotation. We simplified this model by assuming a flat distribution of gaps around rattlers and we applied this simplified model to published ultrasonic measurements. By means of these measurements, we studied how the strain-accumulation mechanism affects the coordination number during isotropic compression. The coordination numbers were estimated by applying a DEM-based correction to the average-strain model. We observe that the majority of the experimental trends lay between the linear and the quadratic accumulation trends. Based on this result, we assume that the strain accumulation is a combination of the two mechanisms and we propose a formula to estimate the contribution of each mechanism. Furthermore, we note that, in the studied datasets, rotation affects larger grains (diameter approximately 500 μm) more than smaller grains (diameter approximately 100 μm). If further validated, this correlation could guide the determination of pressure trends for sands.

Constraining the detectability of water ice in debris disks

Context. Water ice is important for the evolution and preservation of life. Identifying the distribution of water ice in debris disks is therefore of great interest in the field of astrobiology. Furthermore, icy dust grains are expected to play important roles throughout the entire planet formation process. However, currently available observations only allow deriving weak conclusions about the existence of water ice in debris disks. Aims. We investigate whether it is feasible to detect water ice in typical debris disk systems. We take the following ice destruction mechanisms into account: sublimation of ice, dust production through planetesimal collisions, and photosputtering by UV-bright central stars. We consider icy dust mixture particles with various shapes consisting of amorphous ice, crystalline ice, astrosilicate, and vacuum inclusions (i.e., porous ice grains). Methods. We calculated optical properties of inhomogeneous icy dust mixtures using effective medium theories, that is, Maxwell-Garnett rules. Subsequently, we generated synthetic debris disk observables, such as spectral energy distributions and spatially resolved thermal reemission and scattered light intensity and polarization maps with our code DMS. Results. We find that the prominent ~3 and 44 μm water ice features can be potentially detected in future observations of debris disks with the James Webb Space Telescope (JWST) and the Space Infrared telescope for Cosmology and Astrophysics (SPICA). We show that the sublimation of ice, collisions between planetesimals, and photosputtering caused by UV sources clearly affect the observational appearance of debris disk systems. In addition, highly porous ice (or ice-rich aggregates) tends to produce highly polarized radiation at around 3 μm. Finally, the location of the ice survival line is determined by various dust properties such as a fractional ratio of ice versus dust, physical states of ice (amorphous or crystalline), and the porosity of icy grains.