ABSTRACT
Non-equilibrium molecular dynamics simulations of liquid flow through the surface were performed to investigate the flow resistance and thermal resistance under conditions of different solid-liquid interactions and surface temperatures. A novel phenomenon was observed in the simulation, namely the rise of surface temperature increases the flow resistance when solid-liquid interaction is weak, but decreases the flow resistance when solid-liquid interaction is strong. A higher density of the boundary layer brings a larger friction force to increase the flow resistance. For heat transfer, it is innovative to calculate the heat conduction and convection of the boundary region discretely. The results showed that the heat transfer performance of the interface is not positively correlated with the boundary liquid density, and the structure of the boundary liquid is also crucial. We believe that this research can improve the existing theory of flow heat transfer and provide a more effective method for analyzing the flow heat transfer of the solid-liquid interface.
ABSTRACT
Nanofluids for heat transfer application suffer from inevitable pump power consumption and adhesion effect with interface during flow. The hydrophobic treatment for heat transfer surface may be one of the most prospective strategies to achieve heat transfer enhancement and flow resistance reduction. However, the limitations of hydrophobic treatment technique and process make it difficult to fabricate desirable large size and high curvature hydrophobic surface. Herein, a facile displacement reaction method is applied to prepare the lath-like silver crystals and micro-nano gaps in the inner surface of copper tube with assistance of benzoic acid dispersant. The result shows that the convective heat transfer coefficient increases to 18.1% and the Darcy friction factor decreases to 4.9% at the volume concentration of 2.0% when SiO2/DI-water (deionized water) nanofluids flow through the hydrophobic surface. The hydrophobic surface-assisted strategy may provide an effective scheme for wide applications of nanofluids in heat exchange equipment.
ABSTRACT
In this paper, a 2D periodic structure made of tungsten (W) grating patch on a thin Al2O3 spacer and an opaque W film is proposed, as a broadband selective thermal emitter. We numerically investigated the spectral emissivity of the structure from the deep by finite-difference time-domain method. Geometric parameters effects on the emissivity are discussed and the mechanisms of magnetic polaritons in the structure are further analyzed in detail. In addition, by adding another metal-dielectric stack upon the pre-existing grating structure, the emissivity of the composite structure can be further enhanced, and the normal emissivity exceeds 0.95 in the wavelength range of 0.65-1.95 µm, some even close to unity. Furthermore, the composite structures are found to exhibit insensitivity to polar angle and polarization.
ABSTRACT
Second-harmonic generation (SHG) is one of the outstanding properties for practical applications. However, the great majority of molecular ferroelectric materials have very low nonlinear optical coefficients, attenuating their attractive performance. Here we synthesized (4-amino-2-bromopyridinium)(4-amino-2-bromopyridine)tetrafluoroborate (1), whose second-order nonlinear optical coefficient reaches up to 2.56 pm V(-1), 2.67 times of that of KDP, and (4-amino-2-bromopyridinium)tetrafluoroborate (2), possessing a more incredible large second-order nonlinear optical coefficient as high as 10.24 pm V(-1), 10.67 times that of KDP. The compound 1 undergoes two reversible phase transitions at around T1 = 244.1 K and T2 = 154.6 K, caused by dramatic changes of the protonated cations and order-disorder of anions, which was disclosed by differential scanning calorimetry, heat capacity, dielectric anomalies, SHG, and single-crystal X-ray diffraction analysis. The pyroelectric measurements reveal that compound 1 is a Rochelle salt type ferroelectric, which has a large spontaneous polarization of about 3 µC/cm(2).
ABSTRACT
Diisopropylammonium bromide (DIPAB) has attracted great attention as a molecular ferroelectric with large spontaneous polarization and high Curie temperature. It is hard to grow the ferroelectric phase DIPAB because of the appearance of the non-ferroelectric phase DIPAB at room temperature. Here, a ferroelectric thin film of DIPAB was successfully fabricated on a Si substrate using a spin coating method from aqueous solution via 12-crown-4 addition at room temperature. The ferroelectric DIPAB film with a thickness of hundreds of nanometers is distributed discontinuously on the substrate in narrow strips. The direction of polarization is along the narrow strip. Piezoresponse force microscopy (PFM) shows that the ferroelectric films have two kinds of domain structures: noncharged antiparallel stripe domains and charged head-to-head (H-H)/tail-to-tail (T-T) type domains. 12-crown-4 has been proved to play important roles in forming the H-H/T-T type domains. The Chynoweth method shows that the DIPAB films synthesized in this way show a better pyroelectric effect than DIPAB crystals.
