Modelling the Melting of Gallium Clusters: A Path to Understanding Molecular Solids

<p>Gallium is a molecular solid with many unique properties. Comprised of Ga2 dimers but exhibiting metal-like electronic characteristics, gallium may be deemed a molecular metal. The role of this dual covalent-metallic nature may explain gallium’s fascinating thermodynamic behaviour. While bulk gallium melts at 303 K, clusters with only 10’s of atoms melt at temperatures between 500 and 800 K, according to experiment. The measured specific heat curves exhibit a strong size-sensitivity, where the addition of a single atom can alter the melting temperature by up to 100 K. This research addresses the relationship of electronic structure to the melting behaviour in small gallium clusters through a parallel tempering implementation of first-principles molecular dynamics simulations. These simulations cover 11 cluster sizes and two charge states, including neutral clusters sized 7-12 atoms and cationic clusters sized 32-35 atoms. The modelling of small clusters sets a lower size limit for melting and illustrates that greater-than-bulk melting is not universal for small gallium clusters. The larger cluster sizes allow for a direct comparison to experimental data. Each simulation reveals that the clusters have a non-covalent nature more similar to the metallic surface structure of bulk gallium than its covalently bonded interior. The dramatic lowering of melting temperatures and cluster stabilities with single atom additions supports the conclusion that the difference in the nature of bonding between bulk and clusters accounts for the melting temperature discrepancy. Finally, in order to gain additional insight into the nature of bonding in molecular solids, the cohesive energies of the solid halogens are calculated by the method of increments. These calculations investigate the relative N-body correlation energy contributions to the total cohesive energy for solid Cl2, Br2 and I2.</p>

Modelling the Melting of Gallium Clusters: A Path to Understanding Molecular Solids

<p>Gallium is a molecular solid with many unique properties. Comprised of Ga2 dimers but exhibiting metal-like electronic characteristics, gallium may be deemed a molecular metal. The role of this dual covalent-metallic nature may explain gallium’s fascinating thermodynamic behaviour. While bulk gallium melts at 303 K, clusters with only 10’s of atoms melt at temperatures between 500 and 800 K, according to experiment. The measured specific heat curves exhibit a strong size-sensitivity, where the addition of a single atom can alter the melting temperature by up to 100 K. This research addresses the relationship of electronic structure to the melting behaviour in small gallium clusters through a parallel tempering implementation of first-principles molecular dynamics simulations. These simulations cover 11 cluster sizes and two charge states, including neutral clusters sized 7-12 atoms and cationic clusters sized 32-35 atoms. The modelling of small clusters sets a lower size limit for melting and illustrates that greater-than-bulk melting is not universal for small gallium clusters. The larger cluster sizes allow for a direct comparison to experimental data. Each simulation reveals that the clusters have a non-covalent nature more similar to the metallic surface structure of bulk gallium than its covalently bonded interior. The dramatic lowering of melting temperatures and cluster stabilities with single atom additions supports the conclusion that the difference in the nature of bonding between bulk and clusters accounts for the melting temperature discrepancy. Finally, in order to gain additional insight into the nature of bonding in molecular solids, the cohesive energies of the solid halogens are calculated by the method of increments. These calculations investigate the relative N-body correlation energy contributions to the total cohesive energy for solid Cl2, Br2 and I2.</p>

Enhanced Specific Heat of Molten Salt Nano-eutectic via Nanostructural Change

In this study, the specific heat of molten salt nano-eutectic (Li2CO3-K2CO3 doped with SiO2 nanoparticles) was theoretically and computationally investigated. According to the proposed theory in the literature [1], the effective specific heat of a nano-eutectic can be significantly enhanced by the formation of needle-like nanostructures by salt eutectic. To investigate the effect of the formed nanostructure, its specific heat was theoretically calculated by the model used by Wang and other researchers [2-4]. The mass fraction of the formed nanostructure was estimated by MATLAB using the reported material characterization studies [1, 5, 6]. The theoretical prediction made a good agreement with the measured specific heat values from the literature with an error less than 3 %. Additional verification of the proposed model was performed by a molecular dynamics simulation study. The simulated specific heat of pure molten salt eutectic made a good agreement with the literature value (1.6 kJ/kg°C with an error less than 1.7 %). The simulated specific heat of nano-eutectic was 2.017 kJ/kg°C. The error between the theoretical prediction and the simulation is only 3.4 % and the value made a good agreement with the experiment (1.9 % max. error). The result shows the enhanced specific heat of a nano-eutectic can be explained by the contribution of the formed nanostructures.

Effect of moisture content on the thermal properties of Horse-Eye Bean seeds relevant to its processing

The specific heat, thermal conductivity and thermal diffusivity of the Horse-Eye bean (Mucuna sloanei) were determined as a function of moisture content using the method reported by A.O.A.C (2000). The sample varieties used were the Big Sized and the Small Sized Horse-Eye bean. The specific heat and the thermal conductivity were measured using a Bomb Calorimeter. The thermal diffusivity was calculated from the measured specific heat, thermal conductivity and bulk density of the samples. Within the moisture range of 10.5% to 16.87% (b.b), the specific heat, thermal conductivity and thermal diffusivity varied with the moisture content. Results showed that the specific heat, thermal conductivity and thermal diffusivity of the Horse-Eye bean seeds ranged from 116.76 to 203.29 kJ/kgK; 21.07 to 32.23 W/moC; and 3.12 x 10-7 to 9.19 x 10-7 m 2 /s, for the Big Sized varieties, and 112.06 to 194.61 kJ/kgK; 19.85 to 24.08 W/moC; and 3.05 x 10-7 to 6.71 x 10-7 m 2 /s, for the Small Sized varieties as the moisture content increases from 10.5% to 16.87%. Regression analysis were also carried out on the thermal properties of the Horse-Eye bean varieties and moisture content, and there was positive relationship between the parameters. There were significant effects of moisture content (p < 0.05) on all the parameters conducted. The findings and the data generated will create an impact in the food processing industries for Horse-Eye bean.

Determination of Enthalpy of Pyrolysis from DSC and Industrial Reactor Data: Case of Tires

This study was motivated by the fact that differential scanning calorimetry (DSC)/differential thermal analysis (DTA) results in literature showed significant exothermic peaks while in overall, pyrolysis is an endothermic phenomenon. The specific heat of the decomposing tires has been determined with a new methodology: instead of assuming constant char properties throughout pyrolysis, the specific heat of evolving solids (char) was evaluated with increasing temperature and conversion. Measured specific heat values were observed to increase until pyrolysis was triggered at 250°C. Then, the specific heat of the solids decreased continuously until 400°C at which point they started to increase. This unexpected trend pointed out that the exothermic peak observed with DSC is an artefact generated by the control system of the apparatus. To overcome this limitation, the energy balance was performed over industrial data and the newly found heat capacity values. The enthalpy of pyrolysis was found to have a term dependent on the weight loss derivative, with a constant value of 410 kJ/kg tires. Two other terms for the enthalpy of pyrolysis have been identified, which were independent of weight loss. The first one is believed to correspond to the sulphur cross-link breakage at low temperature (65 kJ/kg), while the second one, at the final stage of pyrolysis, should correspond to charring reactions approaching the thermodynamic equilibrium (75 kJ/kg). Ultimately, this work proposes a new methodology to determine the enthalpy of pyrolysis with larger scale experimental data.

Specific heat and sound velocity at the relevant competing phase of high-temperature superconductors

Recent highly accurate sound velocity measurements reveal a phase transition to a competing phase in YBa2Cu3O6+δ that is not identified in available specific heat measurements. We show that this signature is consistent with the universality class of the loop current-ordered state when the free-energy reduction is similar to the superconducting condensation energy, due to the anomalous fluctuation region of such a transition. We also compare the measured specific heat with some usual types of transitions, which are observed at lower temperatures in some cuprates, and find that the upper limit of the energy reduction due to them is about 1/40th the superconducting condensation energy.

Study on the Thermo Physical Properties of Deeply Undercooled Silver Melts

Using the flux processing technique, the undercooling of pure silver melts could reach to 205K. Combining the Differential Scanning Calorimeter (DSC) technique, the specific heat of pure silver melts was measured, which showed a linear dependence on temperature in the range of the obtained undercooling from 0 K to 205K. The related thermodynamic properties of silver, such as the entropy change, the enthalpy change and the Gibbs free energy difference between the undercooled melt and the solid phase, were derived from the measured specific heat. The results showed that the model of Singh-Holz can reveal the reality of the non-equilibrium solidification more accurately than other models.

Experimental analysis of overall thermal properties of Nd:La2CaB10O19crystals

Pure or rare-earth-ion-doped La2CaB10O19(LCB) is a promising optical material for nonlinear frequency conversion, laser and self-frequency doubling applications. In this paper, the thermophysical properties of Nd3+-doped LCB (Nd:LCB) crystals were measured and compared with those of pure and of Yb3+- and Er3+-doped LCB, as well as other borate nonlinear optical crystals. The melting points of rare-earth-doped LCB crystals are lower than that of pure LCB. Thermal expansion coefficients along thea,b,c,c*,a*, 〈110〉 and 〈110〉* directions have been measured, and the principal expansion coefficients were calculated to be 1.85 × 10−6, 4.13 × 10−6and 8.79 × 10−6 K−1. The measured specific heat of Nd:LCB is lower than that of pure LCB, resulting in a larger internal temperature gradient under irradiation by a pulsed laser beam. The measured thermal expansion anisotropy of Nd:LCB is stronger than that of pure LCB. The thermal conductivities of Nd:LCB along thecandc* directions are smaller than those along other crystallographic directions.

A Study of Porosity Influence on Thermal Diffusivity of Aluminium Foam by Experimental Analysis and Numerical Simulation

Aluminium foam is a unique material possessing very high thermal diffusivity due to high thermal conductivity of the cell walls accompanied with rather low overall thermal conductivity, controlled via porosity [1]. There is a presumption of increasing influence at thermal diffusivity of aluminium foam by decreasing porosity, following the presented results (e.g. by using the transient plane source method [2]) and relation between thermal diffusivity and density. Thermal diffusivity of aluminium foam considering various porosity and various compositions of precursors were observed. The Aluminium foam was prepared by the powder metallurgy route, also well known as the ALULIGHT process, and various densities were achieved by changing of parameters (temperature, time) of foaming. The following types of foamable precursors were used: AlMg1Si0.6, AlSi10, as blowing agent was used 0.8 wt. % of TiH2.The thermal diffusivity of particular precursors by the flash method was measured. Specific heat capacities of samples with different density were measured by a calorimeter for various temperatures. The coefficient of thermal conductivity as a function of temperature was calculated by heat transient experiment data and numerical simulation consequently as an inverse heat transfer task. The problem was solved by the finite element method using the engineering-scientific program code ANSYS. The results depend on the thermal diffusivity, on the porosity and the type of precursor. Despite that aluminium foam is considered as a type of composite, thermophysical properties could be calculated upon known volume of aluminium alloy and air in the pores However there is a presumption that this rule cannot be used in case of porous materials. Values obtained by the mentioned methodology shown a significant influence on the porosity and the thermal diffusivity of the aluminium foam.

Study of High Temperature Nanofluids Using Carbon Nanotubes (CNT) for Solar Thermal Storage Applications

The aim of this study is to investigate enhancement of thermal properties of various high temperature nanofluids using Carbon Nanotubes (CNT) for solar thermal energy storage applications. The specific heat of liquid carbonate salt eutectics that are doped with CNT was measured using Differential Scanning Calorimeter (DSC). A eutectic mixture of lithium carbonate (Li2CO3) and potassium carbonate (K2CO3) at a molar ratio of 62:38 is used as the base fluid (solvent). A surfactant (Sodium Dodecyl Sulfate or “SDS”) was used to obtain well-dispersed suspension of CNT in distilled water. This CNT suspension was added to an aqueous solution of two alkali carbonate salts in the form of a eutectic mixture. The resulting solution was evaporated on a hot-plate to obtain a dry mixture of CNT (at 1% concentration by weight) in the carbonate eutectic. The samples were synthesized for by evaporating at four different hotplate temperatures of 100 °C, 120 °C, 140 °C, and 160 °C. The results showed that specific heat capacities of carbonate eutectic-CNT nanofluids were linearly increased as the hotplate temperature was increased. At higher temperatures the water was evaporated faster — leading to less agglomeration of the nanoparticles in the nanofluids and thus resulting in higher values of the measured specific heat of the nanofluids.