Constructal blade shape in nanofluids

Blade configuration of nanofluids has been proven to perform much better than dispersed configuration for some heat conduction systems. The analytical analysis and numerical calculation are made for the cylinder--shaped and regular-rectangular-prism--shaped building blocks of the blade-configured heat conduction systems (using nanofluids as the heat conduction media) to find the optimal cross-sectional shape for the nanoparticle blade under the same composing materials, composition ratio, volumetric heat generation rate, and total building block volume. The regular-triangular-prism--shaped blade has been proven to perform better than all the other three kinds of blades, namely, the regular-rectangular-prism--shaped blade, the regular-hexagonal-prism--shaped blade, and the cylinder--shaped blade. Thus, the regular-triangular-prism--shaped blade is selected as the optimally shaped blade for the two kinds of building blocks that are considered in this study. It is also proven that the constructal cylinder--regular-triangular-prism building block performs better than the constructal regular-rectangular-prism--regular-triangular-prism building block

Numerical analysis of laminar forced convection with temperature-dependent thermal conductivity of nanofluids and thermal dispersion

Nanofluids are promising heat transfer fluids due to their high thermal conductivity. In order to utilize nanofluids in practical applications, accurate prediction of forced convection heat transfer of nanofluids is necessary. In the first part of the present study, we consider the application of some classical correlations of forced convection heat transfer developed for the flow of pure fluids to the case of nanofluids by the use of nanofluid thermophysical properties. The results are compared with experimental data available in the literature, and it is shown that this approach underestimates the heat transfer enhancement. Furthermore, predictions of a recent correlation based on a thermal dispersion model are also examined, and good agreement with the experimental data is observed. The thermal dispersion model is further investigated through a single-phase, temperature-dependent thermal conductivity approach. Numerical analysis of hydrodynamically fully developed laminar forced convection of Al2O3(20 nm)/water nanofluid inside a circular tube under constant wall temperature and constant wall heat flux boundary conditions has been carried out. Results of the numerical solution are compared with the experimental data available in the literature. The results show that the single-phase assumption with temperature-dependent thermal conductivity and thermal dispersion is an accurate way of heat transfer enhancement analysis of nanofluids in convective heat transfer. (C) 2011 Elsevier Masson SAS. All rights reserved

HEAT TRANSFER ENHANCEMENT IN LAMINAR CONVECTIVE HEAT TRANSFER WITH NANOFLUIDS

In order to utilize nanofluids in practical applications, accurate prediction of forced convection heat transfer of nanofluids is necessary. In the first part of the present study, we consider the application of some classical correlations of forced convection heat transfer developed for the flow of pure fluids to the case of nanofluids by the use of nanofluid thermophysical properties. The results are compared with experimental data available in the literature, and it is shown that this approach underestimates the heat transfer enhancement. Furthermore, predictions of a recent correlation based on a thermal dispersion model are also examined, and good agreement with the experimental data is observed. The thermal dispersion model is further investigated through a single-phase, temperature-dependent thermal conductivity approach. Numerical analysis of hydrodynamically fully developed laminar forced convection of Al2O3(20 nm)/water nanofluid inside a circular tube under constant wall temperature and constant wall heat flux boundary conditions has been carried out. Results of the numerical solution are compared with the experimental data available in the literature. The results show that the single-phase assumption with temperature-dependent thermal conductivity and thermal dispersion is an accurate way of heat transfer enhancement analysis of nanofluids in convective heat transfer

In situ Measurements of Irradiation-Induced Creep of Nanocrystalline Copper at Elevated Temperatures

We have measured irradiation-induced creep on nanocrystalline copper micropillars at elevated temperatures. The micropillars, which were approximate to 1 mu m in diameter and approximate to 2 mu m in height, were fabricated from magnetron-sputtered nanocrystalline copper films. The micropillars were compressed during 2.0 MeV Ar+ bombardment and the deformation measured in situ by laser interferometry. The creep rate was measured over the stress range 10-120 MPa at approximate to 200 degrees C. The results show linear relationships of creep rate with both applied stress and displacement rate, yielding a creep compliance of 0.07 dpa(-1) GPa(-1) (dpa:displacement per atom). The findings are in good agreement with the previous results obtained using a bulge test on free-standing thin film specimens

In situ creep measurements on micropillar samples during heavy ion irradiation

We report on the development of an in situ micropillar compression apparatus capable of measuring creep under heavy ion beam irradiation. The apparatus has a force resolution of 1 mu N and a displacement resolution of 1 nm. The experimental setup consists of a nanopositioner, a laser displacement sensor, and a microfabricated doubly clamped silicon-beam transducer. The system was tested by measuring the creep rate of amorphous Cu56Ti38Ag6 micropillars as a function of applied stress during room temperature irradiation with 2.1 MeV Ne+. Measured values of the irradiation induced fluidity are in the range 0.5-3 dpa(-1) GPa(-1), and in good agreement with values obtained by stress relaxation experiments on other metallic glasses, and with predictions of molecular dynamics simulations. The in situ apparatus provides a practical approach for accelerated evaluation of irradiation induced creep in promising nuclear materials

Direct measurements of irradiation-induced creep in micropillars of amorphous Cu56Ti38Ag6, Zr52Ni48, Si, and SiO2

We report in situ measurements of irradiation-induced creep on amorphous (a-) Cu56Ti38Ag6, Zr52Ni48, Si, and SiO2. Micropillars 1 mu m in diameter and 2 mu m in height were irradiated with similar to 2MeV heavy ions during uniaxial compression at room temperature. The creep measurements were performed using a custom mechanical testing apparatus utilizing a nanopositioner, a silicon beam transducer, and an interferometric laser displacement sensor. We observed Newtonian flow in all tested materials. For a-Cu56Ti38Ag6, a-Zr52Ni48, a-Si, and Kr+ irradiated a-SiO2 irradiation-induced fluidities were found to be nearly the same, approximate to 3 GPa(-1) dpa(-1), whereas for Ne+ irradiated a-SiO2 the fluidity was much higher, 83 GPa(-1) dpa(-1). A fluidity of 3 GPa(-1) dpa(-1) can be explained by point-defect mediated plastic flow induced by nuclear collisions. The fluidity of a-SiO2 can also be explained by this model when nuclear stopping dominates the energy loss, but when the electronic stopping exceeds 1 keV/nm, stress relaxation in thermal spikes also contributes to the fluidity. (C) 2015 AIP Publishing LLC

Effect of irradiation damage on the shear strength of Cu-Nb interfaces

The influence of irradiation-induced damage on the interfacial shear strength of Cu-Nb interfaces was characterized via compression of nanolaminate pillars performed in situ in a transmission electron microscope. Chemical mixing and interfacial roughening during MeV Kr ion irradiation leads to increased interfacial shear strength by as much as 60%, from 0.6 GPa for the as-deposited material to 0.95 GPa for samples irradiated at liquid nitrogen temperature. The increase in interfacial shear strength was most pronounced at low temperatures (similar to-196 degrees C), but it is still significant at similar to 300 degrees C. This observation was correlated with increased chemical mixing at lower temperatures, as determined from compositional profiles characterized by energy-dispersive spectroscopy. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Effect of Sm on thermal and mechanical properties of Cu-Zr-Al bulk metallic glasses

The effect of rare-earth (Sm) microalloying on the thermal stability and phase selection along with the effect of nanocrystallization on the mechanical properties of amorphous melt-spun ribbons of Zr50Cu40Al10, Zr49Cu39.2Al9.8Sm2 and Zr48Cu38.4Al9.6Sm4 alloys were investigated using differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), Vickers and nanoindentation hardness tests and micropillar compression analysis. XRD and TEM analyses showed that all samples were fully amorphous in as-spun state; however, crystallization sequences for the Sm-free and the Sm micro-alloyed samples were different during devitrification. Combined study of XRD, DSC and TEM on melt-spun ribbons show that Zr48Cu38.4Al9.6Sm4 have nanocrystallization of Cu2Sm phase with an average diameter of 10 nm, which was absent in Zr50Cu40Al10, prior to crystallization of Cu10Zr7 phase. The nanoindentation and micropillar compression tests revealed Cu2Sm nanocrystals embedded in Zr48Cu38.4Al9.6Sm4 alloy improves strength and hardness. On the other hand, presence of these nanocrystals deteriorate shear band stability and thus result in a catastrophic brittle fracture through a single shear band burst

High strength metallic wood from nanostructured nickel inverse opal materials

This paper describes a nickel-based cellular material, which has the strength of titanium and the density of water. The material’s strength arises from size-dependent strengthening of load-bearing nickel struts whose diameter is as small as 17 nm and whose 8 GPa yield strength exceeds that of bulk nickel by up to 4X. The mechanical properties of this material can be controlled by varying the nanometer-scale geometry, with strength varying over the range 90–880 MPa, modulus varying over the range 14–116 GPa, and density varying over the range 880–14500 kg/m3. We refer to this material as a “metallic wood,” because it has the high mechanical strength and chemical stability of metal, as well as a density close to that of natural materials such as wood

Grain boundary doping strengthens nanocrystalline copper alloys

Nanoindentation hardness measurements were performed on nanocrystalline (nc-) Cu alloys to test recent molecular dynamics predictions that (i) solute segregation to grain boundaries can lead to significant strengthening and (ii) solutes with large size mismatch with Cu are most effective. Results show that the hardness of nc-Cu90Nb10 is greater than 5 GPa, more than double that of pure nc-Cu, whereas similar additions of Fe solute have nearly no effect. These results are in good agreement with simulations. (c) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved