This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)], was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

A Benchmark Study on the Thermal Conductivity of Nanofluids

Hybrid organic–inorganic perovskites (HOIPs) can have a diverse range of compositions including halides, azides, formates, dicyanamides, cyanides and dicyanometallates. These materials have several common features, including their classical ABX3 perovskite architecture and the presence of organic amine cations that occupy the A-sites. Current research in HOIPs tends to focus on metal halide HOIPs, which show promise for use in solar cells and optoelectronic devices; however, the other subclasses also exhibit a diverse range of physical properties. In this Review, we summarize the chemical variability and structural diversity of all known HOIP subclasses. We also present a comprehensive account of their intriguing physical properties, including photovoltaic and optoelectronic properties, dielectricity, magnetism, ferroelectricity, ferroelasticity and multiferroicity. Moreover, we discuss the current challenges and future opportunities in this exciting field. Hybrid organic–inorganic perovskites (HOIPs) comprise a diverse range of chemical compositions from halides and azides to formates, dicyanamides, cyanides and dicyanometallates. In this Review, advances in the synthesis, structures and properties of all HOIP subclasses are summarized and their future opportunities are discussed.

/pdf/chemically-diverse-and-multifunctional-hybrid-organic-270ock4gva.pdf

Chemically diverse and multifunctional hybrid organic–inorganic perovskites

A review of recent advances in thermophysical properties at the nanoscale: From solid state to colloids

Effects of temperature and nanoparticles concentration on rheological behavior of Fe3O4–Ag/EG hybrid nanofluid: An experimental study

The inclusion compound [(CH3)2NH2]2[KCo(CN)6] exhibits a marked temperature-dependent dielectric constant and can be considered as a model of tunable and switchable dielectric materials. Crystal structure and solid-state NMR studies reveal a switchable property between low and high dielectric states around 245 K. This originates from an order–disorder phase transition of the system, changing the dynamics of the polar dimethylammonium (DMA) cation. Furthermore, the tuning of the dielectric constant at temperatures below the phase transition point is related to increasing angular pretransitional fluctuations of the dipole moment of DMA.

Tunable and switchable dielectric constant in an amphidynamic crystal.

Significant hidden temperature gradients in thermogravimetric tests

Dielectric analysis and differential scanning calorimetry were used to study the cure reaction of an epoxy resin (the diglycidyl ether of bisphenol A (DGEBA) (n = 0)) and two curing agents, ie 1,2-diamine cyclohexane (1,2-DCH) and m-xylylenediamine (m-XDA). Various dielectric properties, such as the relaxed permittivity, unrelaxed permittivity and dipole strength, were determined as a function of the curing time, temperature and conversion. The dielectric curing properties observed were related to geometric and energetic aspects of the samples. Analysis of the results shows that the relaxed permittivity and dipole strength are higher for the system DGEBA (n = 0)/1,2-DCH than for the system DGEBA (n = 0)/m-XDA. Copyright © 2005 Society of Chemical Industry

Determination of dielectric and calorimetric properties in the cure reaction of two thermosets by dielectric analysis and differential scanning calorimetry

A cured thermoset composed of diglycidyl ether of bisphenol A and m-xylylene diamine as the cure agent was studied with different thermal analysis techniques, including differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and dielectric analysis (DEA). DSC was used to measure the glass-transition temperature and to check the absence of the heat of reaction. DMA and DEA were used to show the existence of two transitions in the temperature range of −100 to 240°C. The transition at a low temperature corresponded to the β transition. The second one, at a higher temperature, was associated with an α transition. The β transition followed Arrhenius behavior, whereas the α transition followed Vogel behavior. For an analysis of the α transition, different equations, such as the Havriliak–Negami, Vogel, and Williams–Landel–Ferry equations, were used. Important differences related to the fitting parameters were found that depended on the type of equation and the operation mode used. For this reason, a new method for calculating the α-transition temperature was examined. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2027–2037, 2005

Characterization of a thermoset by thermal analysis techniques: Criterion to assign the value of the α-transition temperature by dielectric analysis

Dielectric analysis (DEA) is a very sensitive technique, which allows for detection of small structural changes at the low scale. An advantage of DEA, with respect to other modulated techniques, is the possibility of using a wider frequency range. Molecular relaxations of the order of only a few nanometers are not observed by any other thermoanalytic method. Nevertheless, these small relaxations involve dipole changes that can be observed by DEA. Thus, this technique is used here, in combination with temperature-modulated differential scanning calorimetry (TMDSC) to obtain insightful information about the thermal transitions of poly-l-lactic acid (PLLA), one of the stereo-isomers of polylactide. Its complex thermal behavior is the subject of ongoing debate, with several overlapping crystallization and melting processes. The combined use of TMDSC and DEA provides a better insight of three important transitions of this polymer: the alpha relaxation, the enthalpic relaxation, and the cold crystallization. The dependences of the enthalpy relaxation on the dynamic glass transition relaxation and on the glass transition as a thermal event are evaluated. On the other hand, it will be shown how the cold crystallization can be identified by TMDSC, and DEA helps us understand the effect of crystallization on the dipole movements. The shape of the dielectric permittivity curve at low frequencies is compared to that of the reversing heat capacity to check whether both signals are sensitive or not to the same events. It is also verified how the experimental results of alpha relaxation of PLLA follow an Arrhenius or a Vogel trend.

Thermal characterization of poly- l -lactide by dielectric analysis and modulated DSC

Magnetorheological fluids (MRF) are usually evaluated in shear mode on a plane that is perpendicular to a magnetic field. The measurements thus obtained are perpendicular to the magnetic field. However, to better choose and design possible applications, a complete three-dimensional characterization would be needed. In the present work the rheological behavior of a MRF is evaluated in the three directions of the space while subjected to a uniform magnetic field and different mechanical stresses or deformations. The anisotropy developed in the material as a consequence of the application of a unidimensional magnetic field originates that different compression and shear moduli are obtained depending of the directions of the stresses with respect to the magnetic field. The experimental setups used here were carefully chosen to perform oscillatory measurements in different directions with respect to the applied magnetic field.

Carlos Gracia-Fernández

Papers

Significant hidden temperature gradients in thermogravimetric tests

Determination of dielectric and calorimetric properties in the cure reaction of two thermosets by dielectric analysis and differential scanning calorimetry

Characterization of a thermoset by thermal analysis techniques: Criterion to assign the value of the α-transition temperature by dielectric analysis

Thermal characterization of poly- l -lactide by dielectric analysis and modulated DSC

Measurement of single three-dimensional moduli to evaluate the effect of a uniform magnetic field on magnetorheological fluids