Concentration Dependence Of Thermal Conductivity Research Articles

A new model to describe composition and temperature dependence of thermal conductivity for solution phases in binary alloys

Modelling temperature- and composition-dependent thermal conductivity in alloys is challengeable and is seldom studied systematically. In the present work, a new model is developed to describe the temperature and concentration dependence of thermal conductivity for binary alloys. In this new model, firstly thermal conductivity of pure metals was modelled as the function of temperature for each phase and each magnetic state by the corresponding physically sound model. Secondly, in order to describe the composition and temperature dependence of thermal conductivity for solid phases, the combination of the theories of Nordheim and Mott for electric conductivity of alloys with the Wiedemann-Franz law was performed. Thirdly, the reliability of the new model was verified by presently measured thermal conductivities for pure Co, Ni and Co-Ni alloys at 300, 600, 900 and 1100 K as well as for binary Al-Zn, Mg-Zn and U-Zr systems using the data taken from the literature. The calculated thermal conductivities can well reproduce the measured ones in one-phase regions of a series of Co-Ni alloys. The thermal conductivity in a two-phase region of the Co-Ni system is reasonably predicted as well. It is demonstrated that the new model can be utilized to evaluate the thermal conductivity over the whole investigated composition and temperature ranges for the first time and is expected to be extended to ternary and multicomponent systems by CALPHAD method, which contributes significantly to the development of computational design of materials.

Specifics of Heat Transfer in AlxGa1 – xN/GaN Heterostructures on Sapphire

The thermal conductivity of AlxGa1 – xN/GaN heterostructures (0.05 ≤ x ≤ 1) fabricated on sapphire by molecular beam epitaxy is measured. The thermal conductivities of thin AlxGa1 – xN and GaN films at room temperature are found. We analyze the concentration dependence of thermal conductivity using a virtual crystal model for thermal conductivity. A numerical model with a localized heat source is built to model heat transfer in the considered structure, and the layer thicknesses optimal for achieving a high thermal conductivity of the structure of interest are identified.

Temperatural conductivity of diluted water solutions graphen and nanostructures of graphen nanoparticles

The purpose of this work was to study the specific thermal conductivity of aqueous graphene dispersions and the diluted aqueous solution of nanostructures based on graphene and Au nanoparticles, as well as to determine the temperature and concentration dependences of the specific thermal conductivity of these aqueous dispersions.
 The objects of study were aqueous dispersions of graphene and nanostructures based on graphene and Au nanoparticles. Graphene has characteristic dimensions of the order of 150 - 200 nm in the plane. The Au nanoparticles also have an average size of about 50 nm and a star-like shape. In dry nanocomposites, graphene is oriented parallel to the substrate plane, and nanostars are evenly distributed on the sample surface.
 The specific volumetric thermal conductivity values of aqueous graphene dispersions and aqueous solutions of graphene-based nanoparticles and Au nanoparticles were obtained in the temperature range from 30оC to 60оC. A slight increase in the specific thermal conductivity was found with increasing temperature. The absolute values a/v of aqueous graphene dispersions are 1.6 times higher than in three-component systems. The concentration dependences of the thermal conductivity of the two systems studied are linear. It is determined that the values of the specific thermal conductivity of dry graphene nanofillers are 1,62 times higher than the thermal conductivity of a mixture of graphene and Au nanoparticles.

Особенности теплопереноса в гетероструктурах Al-=SUB=-x-=/SUB=-Ga-=SUB=-1-x-=/SUB=-N/GaN на сапфире

Thermal conductivity of thin layers of AlxGa1-xN/GaN (0.05≤ x≤1) heterostructures on sapphire grown with molecular beam epitaxy has been measured. Thermal conductivity values of AlxGa1-xN и GaN thin films at room temperature have been obtained. Concentration dependence of thermal conductivity has been analyzed with the virtual crystal thermal conductivity model. Thermal transport in the structure considering local heating has been simulated numerically in order to find optimal structure layers` thicknesses allowing high thermal conductivity value.

Investigations on Thermal Conductivity, Heat Transfer Coefficient and Viscosity of Graphite Nanoparticle Dispersed Nanofluid

The thermal conductivity (k), heat transfer coefficient (HTC) and viscosity of the ethylene glycol/water dispersed with graphite nanoparticles was studied under heating and cooling conditions for temperatures ranging from 0 °C to 75 °C using tubular heat exchanger system built in house. Flow rate was varied from 5 l/h to 25 l/h giving Reynolds number (Re) ranging from 50 to 750. The HTC increased with the increase of Re as well as temperature. The thermal conductivity of the nanofluid was determined at constant low Re (200) for all concentrations and temperatures used in the experiment. The concentration dependence of thermal conductivity was found to be much different than that predicted by models suggested in literature. The viscosity was measured for different concentrations of nanoparticles (0 to 0.8%) as well as temperatures 275 K to 340 K. The temperature dependence of viscosity was found to follow Arrhenius type equation η = η o exp(E/kbT) with very little change in the activation energy from that of the base fluid. On the other hand the pre-exponential constant increased six folds with the increase in nanoparticle concentration. An empirical relation derived from Brownian motion was found between the product of viscosity and thermal conductivity (Ω = kr · η r ) with respect to concentration of the nanoparticles which was found to be true for whole range of temperature and compositions studied.

Effect of Ce addition on thermal conductivity of Mg–2Zn–1Mn alloy

The concentration dependence of thermal conductivity of as-cast and as-extruded Mg–2wt.%Zn–1wt.%Mn–[x]wt.%Ce (x=0, 0.2 and 0.6) alloys was investigated using flash method at room temperature, respectively. According to the present experimental results, the thermal conductivity of as-cast ZM21-0.2Ce alloy was higher than that of as-cast ZM21 and ZM21-0.6Ce alloys, while thermal conductivity of as-extruded alloys increased unconventionally with the Ce content increasing. The obtained highest thermal conductivity of as-extruded ZM21-0.6Ce alloy at room temperature was 133.29W/(m·K). Meanwhile, as-extruded ZM21-xCe alloys exhibited higher thermal conductivity than their as-cast counterparts because of the precipitation of Mn particles and some intermetallic compounds containing Ce. Furthermore, the texture had an important effect on the thermal conductivity of as-extruded ZM21-xCe alloys.

Strong enhancement in thermal conductivity of ethylene glycol-based nanofluids by amorphous and crystalline Al2O3 nanoparticles

In the present work, the temperature and concentration dependence of thermal conductivity (TC) enhancement in ethylene glycol (EG)-based amorphous and crystalline Al2O3 nanofluids have been investigated at temperatures ranging from 0 to 100 °C. In our prior study, nanometer-sized particles of amorphous-, γ-, and α-Al2O3 were prepared via a simple sol-gel process with annealing at different temperatures and characterized by various techniques. Building upon the earlier study, we probe here the crystallinity, microstructure, and morphology of the obtained α-Al2O3 nanoparticles (NPs) by using X-ray powder diffraction with Rietveld full-profile refinement, scanning electron microscopy, and high-resolution transmission electron microscopy, respectively. In this study, we achieved a 74% enhancement in TC at higher temperature (100 °C) of base fluid EG by incorporating 1.0 vol. % of amorphous-Al2O3, whereas 52% and 37% enhancement is accomplished by adding γ- and α-Al2O3 NPs, respectively. The amorphous phase of NPs appears to have good TC enhancement in nanofluids as compared to crystalline Al2O3. In a nutshell, these results are demonstrating the potential consequences of Al2O3 NPs for applications of next-generation efficient energy transfer in nanofluids.

Anomalous concentration dependences of thermal conductivity of ceramics in TiN-AlN and ZrC-ZrB2 systems with the structure of “eutectics of rough conglomerate”

The physical nature of the experimentally found anomalous type of the concentration dependence of thermal conductivity of binary ceramic composites is interpreted. A model of the structure and procedure of calculating (predicting) the thermal conductivity of composites of the group of “eutectic models” is proposed.

Effect of Surfactants and Manufacturing Methods on the Electrical and Thermal Conductivity of Carbon Nanotube/Silicone Composites

The effect of ionic surfactants and manufacturing methods on the separation and distribution of multi-wall carbon nanotubes (CNTs) in a silicone matrix are investigated. The CNTs are dispersed in an aqueous solution of the anionic surfactant dodecylbenzene sulfonic acid (DBSA), the cationic surfactant cetyltrimethylammonium bromide (CTAB), and in a DBSA/CTAB surfactant mixture. Four types of CNT-based composites of various concentrations from 0 to 6 vol.% are prepared by simple mechanical mixing and sonication. The morphology, electrical and thermal conductivity of the CNT-based composites are analyzed. The incorporation of both neat and modified CNTs leads to an increase in electrical and thermal conductivity. The dependence of DC conductivity versus CNT concentration shows percolation behaviour with a percolation threshold of about 2 vol.% in composites with neat CNT. The modification of CNTs by DBSA increases the percolation threshold to 4 vol.% due to the isolation/separation of individual CNTs. This, in turn, results in a significant decrease in the complex permittivity of CNT–DBSA-based composites. In contrast to the percolation behaviour of DC conductivity, the concentration dependence of thermal conductivity exhibits a linear dependence, the thermal conductivity of composites with modified CNTs being lower than that of composites with neat CNTs. All these results provide evidence that the modification of CNTs by DBSA followed by sonication allows one to produce composites with high homogeneity.

Temperature and concentration dependences of thermal conductivity of solid solutions of gadolinium and dysprosium sulfides

Thermal conductivity of a number of solid solutions of gadolinium and dysprosium sulfides has been studied experimentally within the temperature range 80-400 K. The work offers the data on thermal conductivity coefficient and lattice thermal conductivity of the studied samples. It was found that replacement of gadolinium ions by dysprosium ions leads to significant decrease of the samples’ thermal conductivity and changes its temperature dependence character due to the resonance scattering of phonons by paramagnetic ions of dysprosium. Influence of this mechanism of phonon scattering conditions the area of anomalous change observed on the concentration dependence of thermal conductivity coefficient.

Thermal conductivity of single crystals of Ca1 − x Er x F2 + x and Ca1 − x Tm x F2 + x solid solutions

The thermal conductivity of single crystals of Ca1 − xErxF2 + x (x = 0.01, 0.05, 0.07, and 0.10) and Ca1 − xTmxF2 + x (x = 0.02, 0.04, and 0.06) solid solutions is studied in the temperature ranges 50–300 and 298–673 K. With increasing content of rare-earth elements, the behavior of thermal conductivity in these solid solutions changes from the characteristic of defect single crystals to glasslike. The concentration dependences of thermal conductivity for the two systems differ insignificantly.

Influence of the concentration of carbon nanotubes (CNT) on the thermophysical properties of epoxy/CNT nanocomposites at low temperatures

The influence of the content of carbon nanofillers (multi-and single-wall nanotubes) on the thermophysical properties of epoxy nanocomposites was investigated on the temperature range from −150 to 150°C. A “plateau” was found to exist in the concentration dependence of thermal conductivity on the concentration interval from 0.1 to 1.0 wt.% carbon nanotubes (CNTs). The thermal conductivity of the CNT composites exceeded that of pure epoxy resin by about 40%. A further increase in CNT content de creased the conductivity, owing to increasing interfaces between the two phases and the additional thermal resistance caused by phonon scattering on them. It is found that the temperature interval of transition of the composite from a glassy to a viscoelastic state greatly depends on the filler type and concentration. There exists a critical concentration at which a drop in the glass-transition temperature by 30% can be observed. The reason is the undercure of binder as a result of interaction between CNTs and epoxy macromolecules, which reduces the cross-linking density of structure of the polymer.

Concentration Dependence of Thermal Conductivity in Superionic Conductors

By making use of the lattice gas model with a hopping term and Kubo’s formula, we have studied the mobile ion concentration dependence of the thermal conductivity in order to consider the effects of ion-ion correlation in superionic conductors. Theoretically, it is intriguing to investigate the concentration dependence of the thermal conductivity including the formulation in the framework of the lattice gas model. The thermal conductivity has symmetry with respect to ρ =0 .5, and at ρ =0a ndρ = 1 it is zero, as expected physically. The thermal conductivity first increases with the increasing concentration and has a maximum value at ρ =0 .5. It is understood that the thermal conductivity first increases with the concentration due to the increase of the repulsive interaction among the mobile ions and then takes a maximum value. Then, beyond this maximum, the thermal conductivity decreases with the concentration due to the decreasing vacancy of the ions. At ρ =0 .5, mobile ions and vacancies occupy the lattice sites alternatively.

Thermal conductivity of pure water and aqueous SrBr2 solutions at high temperatures and high pressures

Accurate thermal conductivity measurements were made on pure water and H 2 O + SrBr 2 solutions at pressures up to 100 MPa over the temperature range 293-473 K with a parallel-plate apparatus. The concentrations studied were between 2.5 and 20 mass%. The estimated accuracy of the method is about 1.6%. The pressure, temperature, and concentration dependences of the thermal conductivity have been studied. The thermal conductivity shows an almost linear dependence on pressure and concentration. Along each isobar, at a given concentration, the λ s o l -T curves show a thermal conductivity maximum at temperatures between 405 K and 420 K depending on pressure and concentration. The measured values of thermal conductivity at atmospheric pressure were compared with values reported by other authors and values calculated with the use of various correlation equations and prediction techniques. The experimental and calculated values from IAPWS formulation of thermal conductivity for pure water show excellent agreement within their experimental uncertainties (AAD = 0.38%). The experimental results have been correlated in terms of the polynomials of x, T, P for practical use. The concentration dependence of the relative thermal conductivity λ s o l /λ H 2 O was studied. The values of apparent and partial molar thermal conductivity were derived from the measured values of thermal conductivity for solutions λ s o l and pure water λ H 2 O . Derived apparent molar thermal conductivities were extrapolated to zero concentration (x → 0) to yield partial molar thermal conductivity values Φ ∞ λ of electrolyte (SrBr 2 ) at infinite dilution.

Electrical and thermal conductivity of polymers filled with metal powders

The electrical and thermal conductivity of systems based on epoxy resin (ER) and poly(vinyl chloride) (PVC) filled with metal powders have been studied. Copper and nickel powders having different particle shapes were used as fillers. The composite preparation conditions allow the formation of a random distribution of metallic particles in the polymer matrix volume for the systems ER–Cu, ER–Ni, PVC–Cu and to create ordered shell structure in the PVC–Ni system. A model is proposed to describe the shell structure electric conductivity. The percolation theory equation σ∼( ϕ− ϕ c) t with t=2.4–3.2 (exceeding the universal t=1.7 value) holds true for the systems with dispersed filler random distribution, but not for the PVC–Ni system. The percolation threshold ϕ c depends on both particle shape and type of spatial distribution (random or ordered). In contrast to the electrical conductivity, the concentration dependence of thermal conductivity shows no jump in the percolation threshold region. For the description of the concentration dependence of the electrical and thermal conductivity, the key parameter is the packing factor F. F takes into account the influence of conductive phase topology and particle shape on the electrical and thermal conductivity.

Thermal conductivity and flocculated structure of suspension of carbon black (abstract)

A modified concentric cylinder viscometer was used for the simultaneous measurement of thermal conductivity and apparent viscosity. The thermal conductivity of disperse medium is 100 times higher than that of carbon black particles, suggesting the particles behave as obstacles to thermal conductance. In flow field, heat transfer is mainly caused by forced convection and the effect of particle orientation on the thermal conductivity of the system becomes large. After cessation of shear flow, the thermal conductivity decreases drastically depending on the rate of shear before the cessation of flow, and recovers gradually with elapsed time. The process of thermal conductance at rest state is by radiation. The thermal conductivity of the disperse system where the particles are dispersed uniformly agents. The thermal conductivity of the system containing a small amount of dispersing is comparatively low, but aggregation of particles results in an increase of conductance because of the path for thermal conductance. In the disperse system, aggregation of particles is depressed by the addition of dispersing agents. The thermal conductivity of the system containing a small amount of dispersing agent does not change with time. With increasing concentration of disperse particles, the thermal conductivity of the system is reduced. The concentration dependence of thermal conductivity can be expressed by Hamilton’s theory, qualitatively. However, in the higher concentration region, the experimental values of thermal conductivity are larger than the theoretical values, because highly developed flocculated structures are constructed.

Thermal conductivity of magnetite magnetic fluids

Temperature and concentration dependences of thermal conductivity of hydrocarbon-based magnetic fluids containing colloidal Fe 3O 4 particles coated with oleic acid are obtained. Measurements have been made in the volumetric Fe 3O 4 particle concentration range 0.01<Ø⪅0.2 and in the temperature range 20°C< t<80°C.

Hole-phonon scattering in zinc doped GaSb

The phonon thermal conductivity values in Zn-doped GaSb are explained for hole concentrations ranging from 2 × 10 17 to 4 × 10 19 cm -3. The concentration dependence of thermal conductivity above 3 × 10 18 cm -3 is similar to other doped materials and is explained by considering the scattering of phonons by free electrons (holes) given by Ziman and Kosarev and applying the screening effects suggested by Crossby and Grenier. The density-of-states effective mass is kept constant and the dilatation deformation potential is found to increase with hole concentration. For impurity concentrations less than 3 × 10 18 cm -3, the thermal conductivity is found to increase with hole concentration. Since this concentration region is below the metal-insulator transition, the theory of scattering of phonons by holes bound to the impurity atoms explains the conductivity results. The values of acceptor hole radius and deformation potential constants are in agreement with the experimental values.

The thermal conductivity of semiconductors of the system As2(Se1−xTex)3 depending on temperature and pressure

The thermal conductivity of the compounds As2Se3 and As2Te3 and solid solutions on their base are investigated in solid and liquid states for crystalline and glassy modifications depending on composition, temperature and hydrostatic pressure. The temperature and concentration dependences of thermal conductivity of polycrystalline samples can be explained by the influence of U-processes, N-processes, and point defects on thermal resistance of lattice. Anomaly of temperature course of the thermal conductivity in softening range and increase of the thermal conductivity at hydrostatic pressure up 3.5 kbar can be explained by the change of elastic parameters of the lattice and phonon velocity. [Russian Text Ignored].