Molecular dynamics simulation of film size and vacancy defect effects on the thermal conductivities of argon thin films

It is well known that there is a size effect for the thermal conductivity of thin films and that vacancy defects in film reduce the film's thermal conduction. In this paper, the film size and vacancy defect effects on the thermal conductivities of argon thin films were studied by molecular dynamics simulations. The results show the existence of phonon boundary scattering. The results also confirm that the theoretical model based on the Boltzmann equation can accurately model the thermal conduction of thin argon films. Both the theoretical and MD results illustrate that, although, both the defect and the thickness of the thin film deduce the thermal conductivity, their physical mechanisms differ.     

Effects of the nitrogen doping configuration and site on the thermal conductivity of defective armchair graphene nanoribbons

The influence of the nitrogen (N) doping configuration on the thermal conductivity (TC) of armchair graphene nanoribbons (AGNRs) of size 15.7 nm × 7.26 nm was investigated using classical molecular dynamics (MD) simulations with the optimized Tersoff potential at room temperature. The effect of changing the N-doping site in defects on the TC of AGNRs was also investigated in detail. The variations with N concentration of the TCs of AGNRs presenting graphitic N (quarternary N), pyridinic N, and pyrrolic N doping configurations were studied. Results of MD simulations showed that, among these three doping configurations, pyridinic N was associated with the highest TC, and pyrrolic N with the lowest TC. The highest TC values were obtained when the N dopant atoms were located at the edges and at defects in the AGNR. The presence of both pyrrolic N and Stone–Wales type 1 (SW-1) defects led to a higher TC than the presence of both pyrrolic N and SW-2 defects. Phonon–defect scattering was found to be influenced by changes in C–C bond orientation. SW-1 defects were found to exert a greater influence on the TC than graphitic N doping. Furthermore, the influence on the TC of the N-doping site location in SW-1 defects was examined. Doping the central sites of SW-1 defects was found to yield higher TC values than doping the edge sites of defects. Graphitic-N doping of the more central sites in a SW-1 defect led to a higher TC than the random graphitic-N doping of sites in a SW-1 defect.     

Thermal conductivity of ferromagnetic metallic La0.95Ag0.05MnO3 manganites: role of carrier,spin waves and lattice–impurity scattering

The lattice contribution to the thermal conductivity (κ_ph) of La_0.95Ag_0.05MnO₃ manganites is theoretically analysed within the framework of Kubo model. The theory is formulated when thermal conduction is limited by the scattering of phonons from defects, grain boundaries, charge carriers, spin waves and phonons. The lattice thermal conductivity dominates in Ag-doped manganites and is artefact of strong phonon–impurity and phonon–phonon scattering mechanism in the ferromagnetic metallic state. The electronic contribution to the thermal conductivity (κ_e) is estimated following the Wiedemann–Franz law. Another important contribution in the metallic phase should come from spin waves (κ_m). It is noticed that κ_m increases with a T² dependence on the temperature. The behaviour of the thermal conductivity in manganites is determined by competition among the several operating scattering mechanisms for the heat carriers and a balance between electron, magnon and phonon contributions.     

Molecular dynamics simulations of heat conduction in multi-walled carbon nanotubes

Heat conduction in multi-walled carbon nanotubes (MWNTs) was studied using non-equilibrium molecular dynamics simulations. This research focuses on the effects of the multi-wall structure of the MWNTs on the heat conduction. The results show that the thermal conductivity of a MWNT is almost the same as that of the corresponding single-walled carbon nanotubes (SWNTs) rather than much smaller as has been suggested. Thus, the multi-wall structure does not significantly affect the thermal conduction in the MWNTs. Analysis of the temperature profiles and the phonon density of states confirms that there is almost no heat transport between the MWNT layers and that each layer conducts heat nearly independently along parallel channels. This is physically reasonable since the weak inter-wall interactions and large interfacial thermal resistances make the MWNT layers behave like parallel thermal circuits.     

Thermal conductivity of silicon and carbon hybrid monolayers: a molecular dynamics study

Thermal conductivities of graphene-like silicon and carbon hybrid nanostructures with silicon atom percentages varying from 0?% (graphene) to 100?% (silicene) are investigated using the reserve non-equilibrium molecular dynamic (RNEMD) method and Tersoff bond order potentials. The thermal conductivity of graphene is dramatically reduced with increasing silicon concentration, and the reduction appears to be related more to the topological structures formed than the amount of doped silicon atoms present. The reduction is collectively contributed to by reduced phonon group velocities (v), phonon free paths (l ( ∞ )), and the specific heat capacity (c) of the material. For systems with high symmetry, thermal conductivity is mainly influenced by v and c. For systems with low symmetry, thermal conductivity is dominated by l ( ∞ ); such materials are also more direction-dependent on thermal flux than highly symmetric materials.     

Molecular dynamics study of the thermal conductivity of nanoscale argon films

Non-equilibrium molecular dynamics (MD) simulations were used to study the thermal conductivity of thin argon films. The MD simulations show that the argon film's thermal conductivity is affected by the thickness up to thickness of about 100 nm, which agrees with theoretical estimates. The results show that the MD method is very effective for modeling nanoscale thermal conduction. Besides pure argon films, the effect of vacancies on the argon film's thermal conductivity was also studied. The vacancies greatly reduce the thermal conductivity as a function of the vacancy concentration but not as a function of the vacancy distribution when the film's temperature is constant.     

Heat transfer simulation using energy conservative dissipative particle dynamics

Dissipative particle dynamics with energy conservation (eDPD) was used to investigate conduction heat transfer in two dimensions under steady-state condition. Various types of boundary condition were implemented to the conduction domain. Besides, 2D conduction with internal heat generation was studied and the heat generation term was used to measure the thermal conductivity and diffusivity of the eDPD system. The boundary conditions used include both the Neumann and Dirichlet boundary conditions. The Neumann boundary condition was applied via adiabatic surfaces and surfaces exposed to convection heat transfer. The DPD simulations were compared to analytical solutions and finite-difference techniques. It was found that DPD appropriately predicts the temperature distribution in the conduction regime. Details of boundary condition implementation and thermal diffusivity measurement are also described in this paper.     

十种园林植物叶片导热系数及对流换热系数测定

为研究植物层传热特性,选取校园内十种常见园林植物测定其叶片导热系数、叶片与周边空气对流换热系数,拟合导热系数与叶片含水量的近似关系式,对比实验测定对流换热系数与通过经验公式理论计算所得对流换热系数,比较与叶片接触前后空气的相对湿度。结果表明,叶片存在降温增湿作用,在5～25 ℃下叶片导热系数随温度变化较小;叶温20 ℃时,叶片导热系数随叶片含水量降低而减小;实验测试对流换热系数与理论计算结果吻合度较高。     

High Thermoelectric Performance in p‐type Polycrystalline Cd‐doped SnSe Achieved by a Combination of Cation Vacancies and Localized Lattice Engineering

Herein, a high figure of merit (ZT) of ≈1.7 at 823 K is reported in p‐type polycrystalline Cd‐doped SnSe by combining cation vacancies and localized‐lattice engineering. It is observed that the introduction of Cd atoms in SnSe lattice induce Sn vacancies, which act as p‐type dopants. A combination of facile solvothermal synthesis and fast spark plasma sintering technique boosts the Sn vacancy to a high level of ≈2.9%, which results in an optimum hole concentration of ≈2.6 × 10¹⁹ cm^?3 and an improved power factor of ≈6.9 µW cm^?1 K^?2. Simultaneously, a low thermal conductivity of ≈0.33 W m^?1 K^?1 is achieved by effective phonon scattering at localized crystal imperfections, as observed by detailed structural characterizations. Density functional theory calculations reveal that the role of Cd atoms in the SnSe lattice is to reduce the formation energy of Sn vacancies, which in turn lower the Fermi level down into the valence bands, generating holes. This work explores the fundamental Cd‐doping mechanisms at the nanoscale in a SnSe matrix and demonstrates vacancy and localized‐lattice engineering as an effective approach to boosting thermoelectric performance. The work provides an avenue in achieving high‐performance thermoelectric properties of materials.     

Weak Electron Phonon Coupling and Deep Level Impurity for High Thermoelectric Performance Pb1−xGaxTe

High ZT of 1.34 at 766 K and a record high average ZT above 1 in the temperature range of 300‐864 K are attained in n‐type PbTe by engineering the temperature‐dependent carrier concentration and weakening electron–phonon coupling upon Ga doping. The experimental studies and first principles band structure calculations show that doping with Ga introduces a shallow level impurity contributing extrinsic carriers and imparts a deeper impurity level that ionizes at higher temperatures. This adjusts the carrier concentration closer to the temperature‐dependent optimum and thus maximizes the power factor in a wide temperature range. The maximum power factor of 35 µW cm⁻¹ K⁻² is achieved for the Pb_0.98Ga_0.02Te compound, and is maintained over 20 µWcm⁻¹ K⁻² from 300 to 767 K. Band structure calculations and X‐ray photoelectron spectroscopy corroborate the amphoteric role of Ga in PbTe as the origin of shallow and deep levels. Additionally, Ga doping weakens the electron–phonon coupling, leading to high carrier mobilities in excess of 1200 cm² V⁻¹ s⁻¹. Enhanced point defect phonon scattering yields a reduced lattice thermal conductivity. This work provides a new avenue, beyond the conventional shallow level doping, for further improving the average ZT in thermoelectric materials.     

Assessment of Microbial Fouling in an Ocean Thermal Energy Conversion Experiment

A project to investigate biofouling, under conditions relevant to ocean thermal energy conversion heat exchangers, was conducted during July through September 1977 at a site about 13 km north of St. Croix (U.S. Virgin Islands). Seawater was drawn from a depth of 20 m, within the surface mixed layer, through aluminum pipes (2.6 m long, 2.5-cm internal diameter) at flow velocities of about 0.9 and 1.8 m/s. The temperature of the seawater entering the mock heat exchanger units was between 27.8 and 28.6°C. After about 10 weeks of exposure to seawater, when their thermal conductivity was reported to be significantly impaired, the pipes were assayed for the accumulation of biological material on their inner surfaces. The extent of biofouling was very low and independent of flow velocity. Bacterial populations, determined from plate counts, were about 10⁷ cells per cm². The ranges of mean areal densities for other biological components were: organic carbon, 18 to 27 μg/cm²; organic nitrogen, 1.5 to 3.0 μg/cm²; adenosine 5′-triphosphate, 4 to 28 ng/cm²; carbohydrate (as glucose in the phenol assay), 3.8 to 7.0 μg/cm²; chlorophyll a, 0.2 to 0.8 ng/cm². It was estimated from the adenosine 5′-triphosphate and nitrogen contents that the layer of live bacteria present after 10 weeks was only of the order of 1μm thick. The C/N ratio of the biological material suggested the presence of extracellular polysaccharidic material. Such compounds, because of their water-retaining capacities, could account for the related increase in thermal resistance associated with the pipes. This possibility merits further investigation, but the current results emphasize the minor degree of biofouling which is likely to be permissible in ocean thermal energy conversion heat exchangers.     

Effect of heteroatom-doped carbon quantum dots on the red emission of metal-conjugated phthalocyanines through hybridization

Quantum dots (QDs) are significant fluorescent materials for energy transfer studies with phthalocyanines (Pcs) and phthalocyanine (Pc)-like biomolecules (such as chlorophylls). Carbon-based QDs, especially, have been used in numerous studies concerning energy transfer with chlorophylls, but the numbers of studies concerning energy transfer between phthalocyanines and carbon-based QDs are limited. In this study, peripherally, hydroxythioethyl terminal group substituted metal-free phthalocyanine (H₂Pc) and zinc phthalocyanine (ZnPc) were noncovalently (electrostatic and/or π–π interaction) attached to carbon QDs containing boron and nitrogen to form QD-Pc nanoconjugates. The QD-Pc conjugates were characterized using different spectroscopic techniques (Fourier transform infrared spectroscopy and transmission electron microscopy). The absorption and fluorescence properties of QD-Pc structures in solution were studied. It was found that the quantum yields of the QDs slightly decreased from 30% to 25% upon doping the QDs with heteroatoms B and N. Förster resonance energy transfer efficiency was calculated as 33% for BCN-QD/ZnPc. For the other conjugates, almost no energy transfer from QDs to Pc cores was observed. It was shown that the energy transfer between QDs to Pc cores was completely different from the energy transfer between QDs and photosynthetic pigments, and therefore we concluded that heteroatom doping in the QD structure and the existence of zinc metal in the phthalocyanine structure is obligatory for an efficient energy transfer.     

Steady-state analysis of self-heated thermistors using finite elements

The purpose of this work is to validate, using numerical, finite element methods, the thermal assumptions made in the analytical analysis of a coupled thermistor probe-tissue model upon which a thermal conductivity measurement scheme has been based. Analytic, closed form temperature profiles generated by the self-heated thermistors can be found if three simplifying assumptions are made: the thermistor is spherical; heat is generated in all regions of the bead; and heat is generated uniformly in the bead. This analytic solution is used to derive a linear relationship between tissue thermal conductivity and the ratio of thermistor temperature rise over electrical power required to maintain that temperature rise. This derived, linear relationship is used to determine thermal conductivity from the observed experimental data. However, in reality, the thermistor bead is a prolate spheroid surrounded by a passive shell, and the heating pattern in the bead is highly nonuniform. In the physical system, the exact relationship between the tissue thermal conductivity and parameters measured by the thermistor is not known. The finite element method was used to calculate the steady-state temperature profiles generated by thermistor beads with realistic geometry and heating patterns. The results of the finite element analysis show that the empirical, linear relationship remains valid when all three simplified assumptions are significantly relaxed.     

Heat and mass transfer scale-up issues during freeze-drying,I: Atypical radiation and the edge vial effect

The aim of this study is to determine whether radiation heat transfer is responsible for the position dependence of heat transfer known as the edge vial effect. Freeze drying was performed on a laboratory-scale freeze dryer using pure water with vials that were fully stoppered but had precision cut metal tubes inserted in them to ensure uniformity in resistance to vapor flow. Sublimation rates were determined gravimetrically. Vials were sputter-coated with gold and placed at selected positions on the shelf. Average sublimation rates were determined for vials located at the front, side, and center of an array of vials. Sublimation rates were also determined with and without the use of aluminum foil as a radiation shield. The effect of the guardrail material and its contribution to the edge vial effect by conduction heat transfer was studied by replacing the stainless steel band with a low-thermal conductivity material (styrofoam). The emissivities (ε) of relevant surfaces were measured using an infrared thermometer. Sublimation rate experiments were also conducted with vials suspended off the shelf to study the role of convection heat transfer. It was found that sublimation rates were significantly higher for vials located in the front compared to vials in the center. Additional radiation shields in the form of aluminum foil on the inside door resulted in a decrease in sublimation rates for the front vials and to a lesser extent, the center vials. There was a significant decrease in sublimation rate for goldcoated vials (ε≈0.4) placed at the front of an array when compared to that of clear vials (ε≈0.9). In the case of experiments with vials suspended off the shelf, the heat transfer coefficient was found to be independent of chamber pressure, indicating that pure convection plays no significant role in heat transfer. Higher sublimation rates were observed when the steel band was used instead of Styrofoam while the highest sublimation rates were obtained in the absence of the guardrail, indicating that the metal band can act as a thermal shield but also transmits some heat from the shelf via conduction and radiation. Atypical radiation heat transfer is responsible for higher sublimation rates for vials located at the front and side of an array. However, the guardrail contributes a little to heat transfer by conduction.     

Atomic Structure Modification for Electrochemical Nitrogen Reduction to Ammonia

The electrochemical nitrogen reduction reaction (NRR), as an environmentally friendly method to convert nitrogen to ammonia at ambient temperature and pressure, has attracted the attention of numerous researchers. However, when compared with industrial production, electrochemical NRR often suffers from unsatisfactory yields and poor Faraday efficiency (FE). Recently, various structure engineering strategies have aimed to introduce extra active sites or enhance intrinsic activity to optimize the activation and hydrogenation of N₂. In this review, recent progress in atomic structure modification is summarized and discussed to design high‐efficiency NRR catalysts, with a focus on defect engineering (heteroatom doping and atom vacancy), surface orientation and amorphization, as well as heterostructure engineering. In addition, existing challenges and future development directions are proposed to obtain more credible NRR catalysts with high catalytic performance and selectivity.     

Heat transfer in dust structures in an RF discharge plasma

Results are presented from experimental studies of heat transfer in liquid-like plasma-dust structures. The experiments were performed with aluminum oxide grains ～3–5 μm in size in an RF discharge plasma. The heat capacity of the dust grains in plasma is measured. The thermal conductivity and thermal diffusivity of liquid-like plasma-dust structures are deduced under the assumption that the observed temperature gradients and the propagation of a thermal perturbation in a dusty plasma are related to heat conduction within the dust component. The measured temperature dependences of the thermal conductivity and thermal diffusivity are in qualitative agreement with the results of numerical simulations performed in the model of a simple single-atom liquid. It is shown that quantitative discrepancy between the experimental and numerical results is related to the energy loss of dust grains in their collisions with the neutral particles of the ambient gas.     

Thermophysical Properties of Lignocellulose: A Cell-Scale Study Down to 41K

Thermal energy transport is of great importance in lignocellulose pyrolysis for biofuels. The thermophysical properties of lignocellulose significantly affect the overall properties of bio-composites and the related thermal transport. In this work, cell-scale lignocellulose (mono-layer plant cells) is prepared to characterize their thermal properties from room temperature down to ∼40 K. The thermal conductivities of cell-scale lignocellulose along different directions show a little anisotropy due to the cell structure anisotropy. It is found that with temperature going down, the volumetric specific heat of the lignocellulose shows a slower decreasing trend against temperature than microcrystalline cellulose, and its value is always higher than that of microcrystalline cellulose. The thermal conductivity of lignocellulose decreases with temperature from 243 K to 317 K due to increasing phonon-phonon scatterings. From 41 K to 243 K, the thermal conductivity rises with temperature and its change mainly depends on the heat capacity''s change.     

Large Thermal Conductivity Differences between the Crystalline and Vitrified States of DMSO with Applications to Cryopreservation

Thermal conductivity of dimethyl-sulfoxide (DMSO) solution is measured in this study using a transient hot wire technique, where DMSO is a key ingredient in many cryoprotective agent (CPA) cocktails. Characterization of thermal properties of cryoprotective agents is essential to the analysis of cryopreservation processes, either when evaluating experimental data or for the design of new protocols. Also presented are reference measurements of thermal conductivity for pure water ice and glycerol. The thermal conductivity measurement setup is integrated into the experimentation stage of a scanning cryomacroscope apparatus, which facilitates the correlation of measured data with visualization of physical events. Thermal conductivity measurements were conducted for a DMSO concentration range of 2M and 10M, in a temperature range of -180°C and 25°C. Vitrified samples showed decreased thermal conductivity with decreasing temperature, while crystalline samples showed increased thermal conductivity with decreasing temperature. These different behaviors result in up to a tenfold difference in thermal conductivity at -180°C. Such dramatic differences can drastically impact heat transfer during cryopreservation and their quantification is therefore critical to cryobiology.     

Measurement of the Thermal Conductivity of Unfrozen and Frozen Food Materials by a Steady State Method with Coaxial Dual-cylinder Apparatus

Coaxial dual-cylinder apparatus was used to measure the effective thermal conductivity of aqueous solutions of glucose, sucrose, gelatin and egg albumin over a temperature range from –20° to 20°C by the steady state method. The accuracy of the apparatus was confirmed by testing with water and ice. The effective thermal conductivity decreased with an increase in the total solid content in both the frozen and unfrozen states. In the unfrozen state, the effective thermal conductivity was slightly dependent on temperature. In the frozen state, however, the effective thermal conductivity was strongly dependent on temperature; lower temperatures gave higher effective thermal conductivity, reflecting the increase in the ice fration. For the unfrozen samples, the intrinsic thermal conductivity of each solid component was calculated by heat transfer models. All the models tested, series, parallel and Maxwell–Eucken, were equally applicable to describe the heat conduction in the unfrozen state. In the frozen state, however, the strong temperature dependency of the effective thermal conductivity suggests that the effect of the temperature dependency of the ice fraction should be incorporated into theoretical models.     

Heat transport mechanisms in vascular tissues: a model comparison

We have conducted a parametric comparison of three different vascular models for describing heat transport in tissue. Analytical and numerical methods were used to predict the gross temperature distribution throughout the tissue and the small-scale temperature gradients associated with thermally significant blood vessels. The models are: an array of unidirectional vessels, an array of countercurrent vessels, and a set of large vessels feeding small vessels which then drain into large vessels. We show that three continuum formulations of bioheat transfer (directed perfusion, effective conductivity, and a temperature-dependent heat sink) are limiting cases of the vascular models with respect to the thermal equilibration length of the vessels. When this length is comparable to the width of the heated region of tissue, the local temperature changes near the vessels can be comparable to the gross temperature elevation. These results are important to the use of thermal techniques used to measure the blood perfusion rate and in the treatment of cancer with local hyperthermia.