Evaluation method for heat transfer coefficient of a contact interface across a microscale thin liquid polymer film

The research presented here evaluates the heat transfer coefficient of the contact interface of a thin liquid polymer film between a pair of columnar aluminum bodies with an initial temperature difference of approximately 160 K. We measured the unsteady temperature changes inside the columns. The heat transfer test was performed with three types of liquid polymers: squalane, oleic acid, and silicone oil. The heat transfer coefficient of the polymer films as a fitting parameter was obtained by ensuring the numerically computed time evolution of the columns’ temperature corresponded with the experimentally measured data. The interfacial heat transfer coefficients of the thin polymer films (mean thickness: 60 μm) for all three polymers used were 1.75 kW/m²·K, 2.75 kW/m²·K, and 4.10 kW/m²·K. The present estimating method for determining interfacial heat transfer coefficients was suitable for a material-polymer film-material contact model. The time evolution of the temperature at the contact surfaces was also effectively evaluated using the numerical simulation.     

Ab initio calculation of the adhesion and ideal shear strength of planar diamond interfaces with different atomic structure and hydrogen coverage

We propose a method to calculate the ideal shear strength τ of two surfaces in contact by ab initio calculations. This quantity and the work of adhesion γ are the interfacial parameters usually derived from tip-based friction force measurements. We consider diamond interfaces and quantitatively evaluate the effects of surface orientation and passivation. We find that in the case of fully passivated interfaces, γ is not affected by the orientation and the alignment of the surfaces in contact. On the contrary, τ does show a dependence on the atomic-scale roughness of the interface. The surface termination has a major impact on the tribological properties of diamond. The presence of dangling bonds, even at concentrations low enough to prevent the formation of interfacial C-C bonds, causes an increase in the resistance to sliding by 2 orders of magnitude with respect to the fully hydrogenated case. We discuss our findings in relation to experimental observations.     

宁东煤灰层/熔渣界面煤焦气化反应特性原位研究

以典型宁东煤-羊场湾煤为气化原料,采用热重分析仪和高温热台显微镜原位研究了1100、1200、1300℃下煤焦颗粒及其在灰层界面和熔渣界面的气化反应。结果表明,不同气化温度下灰层界面和熔渣界面的形态变化是影响煤焦颗粒气化反应性的主要因素。气化温度为1100℃,灰层在高温下收缩并包裹在煤焦颗粒表面,阻碍气化剂与煤焦颗粒的接触,使煤焦颗粒气化反应速率降低,而熔渣界面未发生明显变化,其界面处煤焦气化反应速率不变。气化温度为1300℃,灰层界面与熔渣界面均转变为液态,在表面张力作用下煤焦颗粒破碎,有效反应面积变大,传热速率增大,进而提高了煤焦的气化反应速率。     

Measurement of the dependence of interfacial charge-transfer rate constants on the reorganization energy of redox species at n-ZnO/H2O interfaces

The interfacial energetic and kinetics behavior of n-ZnO/H2O contacts have been determined for a series of compounds, cobalt trisbipyridine (Co(bpy)3(3+/2+)), ruthenium pentaamine pyridine (Ru(NH3)5 py(3+/2+)), cobalt bis-1,4,7-trithiacyclononane (Co(TTCN)2(3+/2+)), and osmium bis-dimethyl bipyridine bis-imidazole (Os(Me2bpy)2(Im)2(3+/2+)), which have similar formal reduction potentials yet which have reorganization energies that span approximately 1 eV. Differential capacitance vs potential and current density vs potential measurements were used to measure the interfacial electron-transfer rate constants for this series of one-electron outer-sphere redox couples. Each interface displayed a first-order dependence on the concentration of redox acceptor species and a first-order dependence on the concentration of electrons in the conduction band at the semiconductor surface, in accord with expectations for the ideal model of a semiconductor/liquid contact. Rate constants varied from 1 x 10(-19) to 6 x 10(-17) cm4 s(-1). The interfacial electron-transfer rate constant decreased as the reorganization energy, lambda, of the acceptor species increased, and a plot of the logarithm of the electron-transfer rate constant vs (lambda + deltaG(o)')(2)/4lambda k(B)T (where deltaG(o)' is the driving force for interfacial charge transfer) was linear with a slope of approximately -1. The rate constant at optimal exoergicity was found to be approximately 5 x 10(-17) cm4 s(-1) for this system. These results show that interfacial electron-transfer rate constants at semiconductor electrodes are in good agreement with the predictions of a Marcus-type model of interfacial electron-transfer reactions.     

Nonequilibrium energy dissipation at the interface of sliding model hydroxylated alpha-alumina surfaces

Nonequilibrium molecular dynamics simulations were performed to study the dynamics of energy transfer at the interface of a small nanoscale hydroxylated alpha-alumina surface sliding across a much larger surface of the same material. Sliding velocities of 0.05, 0.5, 5, and 50 ms and loads of 0, 0.0625, 5, 15, 25, and 100 nN were considered. Nonequilibrium energy distributions were found at the interface for each of these conditions. The velocity distribution P(v) for the atoms in a sublayer of the smaller surface oscillates during the sliding, reflecting the periodicity of the interfacial intermolecular potential. When averaged over the sliding, this P(v) for each of the sublayers is bimodal with Boltzmann and non-Boltzmann components. The non-Boltzmann component, with temperatures in excess of 1000 K and as high as 2500 K, is most important for the interfacial H-atom sublayer and becomes less important in moving to a sublayer further from the interface. Similarly, the temperature of the Boltzmann component decreases for sublayers further from the interface and approaches the 300 K temperature of the boundary. The temperature of the Boltzmann component decreases, but the importance of the non-Boltzmann component increases, as the sliding velocity is decreased. The temperature of the non-Boltzmann component is relatively insensitive to the sliding velocity. Friction forces are determined by calculating the energy dissipation during the sliding, and different regimes are found for variation in the friction force versus sliding velocity v(s) and applied load. For v(s) of 0.05, 0.5, and 5 ms, the friction force is inversely proportional to v(s) reflecting the increased time for energy dissipation as v(s) is decreased.     

Fast electrochemical triple-interface processes at boron-doped diamond electrodes

Two important mechanisms for electron transfer processes at boron-doped diamond electrodes involving the oxidation of tetramethylphenylenediamine (TMPD) dissolved in aqueous solution and the oxidation of tetrahexylphenylenediamine (THPD) deposited in the form of microdroplets and immersed into aqueous eletrolyte solution are reported. For TMPD, the first oxidation step in aqueous solution follows the equation: Remarkably slow heterogeneous kinetics at a H-plasma-treated boron-doped diamond electrode are observed, consistent with a process following a pathway more complex than outer-sphere electron transfer. At the same boron-doped diamond electrode surface a deposit of THPD undergoes facile oxidation following the equation: This oxidation and re-reduction of the deposited liquid material occurs at the triple interface organic droplet|diamond|aqueous electrolyte and is therefore an example of a facile high-current-density process at boron-doped diamond electrodes due to good electrical contact between the deposit and the diamond surface. Received: 3 February 2000 / Accepted: 18 February 2000     

Fusion-like behaviour of wood pyrolysis

The results of an experimental study of the melting rates of rods of ice, paraffin and Rilsan (polyamide 11) pressed against a spinning hot disk are reported. The observations are similar to those made with wood rods, the “fusion temperature” of which has been found close to 739 K. These results are in agreement with a theoretical study of the competition between the kinetics of thermal wood decomposition and heat transfer resistances inside and outside the wood. A second conclusion is that because of the heat fluxes that can be transferred in most of the experimental devices, it seems almost impossible to raise wood to temperatures greater than about 800 K; the direct experimental determination of wood pyrolysis reaction rate constants at temperatures above 800 K then appears impossible.     

Atomic-scale friction on diamond: a comparison of different sliding directions on (001) and (111) surfaces using MD and AFM

Atomic force microscopy (AFM) experiments and molecular dynamics (MD) simulations were conducted to examine single-asperity friction as a function of load, surface orientation, and sliding direction on individual crystalline grains of diamond in the wearless regime. Experimental and simulation conditions were designed to correspond as closely as state-of-the-art techniques allow. Both hydrogen-terminated diamond (111)(1 x 1)-H and the dimer row-reconstructed diamond (001)(2 x 1)-H surfaces were examined. The MD simulations used H-terminated diamond tips with both flat- and curved-end geometries, and the AFM experiments used two spherical, hydrogenated amorphous carbon tips. The AFM measurements showed higher adhesion and friction forces for (001) vs (111) surfaces. However, the increased friction forces can be entirely attributed to increased contact area induced by higher adhesion. Thus, no difference in the intrinsic resistance to friction (i.e., in the interfacial shear strength) is observed. Similarly, the MD results show no significant difference in friction between the two diamond surfaces, except for the specific case of sliding at high pressures along the dimer row direction on the (001) surface. The origin of this effect is discussed. The experimentally observed dependence of friction on load fits closely with the continuum Maugis-Dugdale model for contact area, consistent with the occurrence of single-asperity interfacial friction (friction proportional to contact area with a constant shear strength). In contrast, the simulations showed a nearly linear dependence of the friction on load. This difference may arise from the limits of applicability of continuum mechanics at small scales, because the contact areas in the MD simulations are significantly smaller than the AFM experiments. Regardless of scale, both the AFM and MD results show that nanoscale tribological behavior deviates dramatically from the established macroscopic behavior of diamond, which is highly dependent on orientation.     

交联SU-8光刻胶与Ni基底结合性的分子动力学模拟

应用分子动力学模拟软件Materials Studio构建SU-8光刻胶与Ni基底的界面结构,研究后烘温度对界面结合性的影响.结合工艺中所采用的后烘温度,模拟计算了338～368K时Ni基底上SU-8胶的交联反应,在经过反复的能量最小化和分子动力学模拟后,对最终得到的平衡结构进行了界面结合能的计算.计算结果表明界面结合能随着后烘温度的升高而增大,在368K时结合能达到最大值,说明此时界面结合最好.对分子体系进行了能量分析,结果表明界面分子间的范德华力作用能是影响界面结合的主要因素.对体系界面原子间进行了径向分布函数分析,发现范德华力作用范围内(0.31～0.60nm)出现两组Ni—O的强峰,也证实了上述结论。     

Investigation of Electrochemical Properties of Metalloporphyrin Species at the Liquid/Liquid Interface by Switching Substitutes on the Porphyrin Ring

In order to investigate the electrochemical properties of porphyrin complexes species in biological systems, metalloporphyrin with different substitutes was applied to observe the process of heterogeneous electron transfer (ET) at the interface between two immiscible electrolyte solutions (ITIES) by scanning electrochemical microscopy (SECM). Experimental results demonstrated that the process of electron transfer was affected dramatically by the presence of different substitutes. Our results also show that the rate constant follows Bulter? Volmer kinetics where the rate increases with increasing force at the low driving force, and Marcus inverted region kinetics at the high driving force where the rate decreases.     

非平衡热力学在界面传递过程中的应用

21世纪以来，随着各学科之间的交叉渗透，化学工程的研究对象越来越复杂，界面传递对多相化工过程的影响越来越显示出它的重要性，传统的传递模型已经很难对界面复杂动态的传递行为进行定量描述．本文应用线性非平衡态热力学理论，对传统物质传递模型进行描述，分析界面传递过程速率，强化的三个因素：界面传质系数K、传递截面积A、界面化学位梯度△u．以含钾化合物作为模型体系，基于非平衡热力学原理建立了描述和预测固一液界面处介质传递速率的模型，并建立了描述其溶解速率的通用模型和测定钾离子动力学数据的实验方法，通过分析探讨了多相过程速率强化的途径．     

Comparative study of electron transfer reactions at the ionic liquid/water and organic/water interfaces

The rates of electron transfer (ET) reactions at the water/ionic liquid (IL) interface have been measured for the first time using scanning electrochemical microscopy. The standard bimolecular rate constant of the interfacial ET between ferrocene dissolved in 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and aqueous ferricyanide (0.4 M-1 cm s-1) was found to be approximately 30 times higher than the corresponding rate constant measured at the water/1,2-dichloroethane interface. The driving force dependence of the ET rate was investigated over a wide range of the interfacial potential drop values (>200 mV). The observed Butler-Volmer-type dependence is discussed in terms of the interfacial model. The ET was also probed at the interface between aqueous solution and the mixture of the IL and 1,2-dichloroethane. The mole fractions in this mixture were varied systematically to investigate the transition from the water/organic to the water/IL interface. The observed decrease in the rate constant with increasing mole fraction of 1,2-dichloroethane is in contrast with the previously reported direct correlation between the electrochemical rate constant and the diffusion coefficient of redox species in solution.     

Wetting of TiC by Al-Cu alloys and interfacial characterization

The wetting behavior and the interfacial reactions that occurred between molten Al-Cu alloys (1, 4, 8, 20, 33, and 100 wt% Cu) and solid TiC substrates were studied by the sessile drop technique in the temperature range of 800-1130 degrees C. The effect of wetting behavior on the interfacial reaction layer was studied. All the Al-Cu alloys react with TiC at the interface forming an extensive reaction layer. The interface thickness varied with the samples, and depends on the temperature, chemical composition of the alloy and the time of the test. Wetting increases with increasing concentration of copper in the Al-Cu alloy at 800 and 900 degrees C. In contrast, at higher temperature such as 1000 degrees C wetting decreases with increasing copper content. The spreading kinetics and the work of adhesion were evaluated. The high values of activation energies indicated that spreading is not a simple viscosity controlled phenomenon but is a chemical reaction process. The spreading of the aluminum drop is observed to occur according to the formation of Al4C3, CuAl2O4, CuAl2, TiCux mainly, leading to a decreases in the contact angle. As the contact angle decreases the work of adhesion increases with increasing temperature. Al-Cu/TiC assemblies showed cohesive fracture corresponding to a strong interface. However, using pure Cu the adhesion work is poor, and the percentage of cohesion work is also too low (27-34%).     

Control of substrate temperature during diamond deposition

A method is reported for controlling substrate temperature during chemical vapor deposition of diamond in high heat flux environments such as thermal plasmas or flames. The method is based on flowing a variable-composition argon-helium mixture through channels between the substrate and a water-cooled base. A fiberoptic pyrometer temperature reading from the back of the substrate provides feedback for active substrate temperature control. Experiments in an atmospheric-pressure RF reactor under diamond growth conditions indicated that substrate temperature could be sensitively varied by over 600 K by varying the composition of the argon-helium mixture. The effect is explained in terms of the variable thermal contact resistance at the interface between the substrate and the base.     

A theoretical study of the structure of the liquid Ga-diamond (111) interface

We present the results of a computer simulation study of the structure of the interface between liquid Ga and the (111) face of diamond, with which we reinterpret the findings from an x-ray reflectivity study of that interface [W. J. Huisman, J. F. Peters, M. J. Zwanenburg, S. A. de Vries, T. E. Derry, D. Abernathy, and J. F. van der Veen, Nature (London) 390, 379 (1997); Surf. Sci. 402-404, 866 (1998)]. That experimental study has been interpreted to show that the contact of Ga with the (111) face of diamond induces the formation of Ga(2) molecules for several layers into the bulk liquid, with the axes of the Ga(2) molecules in successive layers oriented perpendicular to the diamond surface. No driving force for the proposed formation of Ga(2) molecules is identified. The simulations reported in this paper are based on a model that permits chemical binding of Ga, as a dimer, to the C=C double bonds in the reconstructed (111) face of diamond, thereby identifying the driving force for dimerization. We show that an isolated pi complex with the Ga(2) axis perpendicular to the C=C double bond is stable. We then modify the pseudopotential-based self-consistent Monte Carlo simulation scheme for describing inhomogeneous liquid metals, using the calculated potential-energy surface of Ga(2)(C=C) in the region close to the diamond surface. In this model only the Ga adjacent to the diamond is composed of dimers. The interfacial density distribution obtained from the simulations predicts an x-ray reflectivity that is in good agreement with that observed.     

Effect of heating rate and sample size on heat transfer through the sample in DSC

As it is very often the first step of the process, heat transfer plays an important role, especially for systems heated in scanning mode. The problem is studied in the simple case when no reaction occurs. In the present case, heat is transferred through the cross-section of a cylinder heated on faces with a constant heating rate. The gradients of temperature through the sample when they are established are proportional to the heating rate on faces and to the square of the diameter of the cylinder. The shape of the gradients of temperature are obtained at different times when these gradients are not established and when they are constant. The heat flux emitted through the face-calorimeter interface are also determined. The value of heat flux is certainly the best parameter for testing the validity of the model.     

Molecular dynamics studies of the kinetics of phase changes in clusters III: structures, properties, and crystal nucleation of iron nanoparticle Fe331

Molecular dynamics (MD) computer simulations have been carried out to study the structures, properties, and crystal nucleation of iron nanoparticles with 331 Fe atoms or with diameter around 2 nm. Structure information for the nanoparticles was analyzed from the MD simulations. Three crystalline phases and one amorphous phase were obtained by cooling the nanoparticles from their molten droplets at different cooling rates or with different lengths of cooling time periods. Molten droplets froze into three different solid phases and a solid-solid transition from a disordered body-centered cubic (BCC) phase to an ordered BCC phase were observed during the slow cooling and the quenching processes. Properties of nanoparticle Fe₃₃₁, such as melting point, freezing temperature, heat capacity, heat of fusion, heat of crystallization, molar volume, thermal expansion coefficient, and diffusion coefficient, have been estimated. Nucleation rates of crystallization to two solid phases for Fe₃₃₁ at temperatures of 750, 800, and 850 K are presented. Both classical nucleation theory and diffuse interface theory are used to interpret our observed nucleation results. The interfacial free energy and the diffuse interface thickness between the liquid phase and two different solid phases are estimated from these nucleation theories.     

Dynamic response of geometrically constrained vapor bubbles

We consider a two-dimensional model of a vapor bubble between two horizontal parallel boundaries held at different temperatures. When the temperatures are constant, a steady state can be achieved such that evaporation near the contact lines at the hot bottom plate is balanced by condensation in colder areas of the interface near the top. The dynamic response of the bubble is probed by treating the case of time-dependent wall temperatures. For periodic modulations of the wall temperature the bubble oscillates about the steady state. In order to describe such time-dependent behavior we consider the limit of small capillary number, in which the effects of heat and mass transfer are significant only near the contact lines at the bottom plate and in a small region near the top. When the bottom temperature is modulated and the top temperature is held fixed, the amplitude of forced oscillations is constant for low-frequency modulations and then rapidly decays in the high-frequency regime. When the top temperature is modulated with fixed bottom temperature, the dynamic-response curve is flat in the low-frequency regime as well, but it also flattens out when the frequency is increased. This shape of the response curve is shown to be the result of the nonmonotonic behavior of the thickness of the liquid film between the bubble interface and the top plate: when the temperature is decreased, the film thickness increases rapidly, but then slowly decays to a value which is smaller than the initial thickness. The dynamic response is also studied as a function of the forcing amplitude.     

Heat capacity of fluoropolymers

The heat capacities of poly(vinyl fluoride), poly(vinylidene fluoride), and polytrifluoroethylene have been measured between 80 and 340°K. The results can be expressed as the sum of two terms: the optical contribution calculated from the vibrational band assignments in the literature and the acoustical contribution calculated according to the Tarasov continuum model. Combining our data with those for polyethylene and polytetrafluoroethylene, it is concluded that the force constant for one-dimensional intrachain interaction is approximately constant for all polymers with carbon backbones. The force constant for three-dimensional interchain interaction is one order of magnitude smaller than the one-dimensional force constant, and decreases when the hydrogen atoms in polyethylene are replaced by fluorine atoms. The thermodynamic functions for the three polymers have been evaluated. Glass transitions at 228°K and 305°K have been found in poly(vinylidene fluoride) and polytrifluoroethylene, respectively.     

Kinetic analysis of distribution of weak acid in an oil-gelatin microcapsule/water system studied by microcapillary injection and microabsorption methods.

The distribution of anthracene-9-carboxylic acid across dibutyl phthalate/gelatin-membrane/water interface of a single microcapsule was analyzed using microcapillary manipulation and microabsorption methods. The partitioning ratio and the distribution rate in the microcapsule/water system were measured for various pH values in the water phase. Results were compared with those in the dibutyl phthalate/water system in the absence of the gelatin membrane. The distribution rate could be analyzed on the basis of a first-order type reaction. The observed rate constant was linearly proportional to the inverse of the microcapsule radius, indicating that the distribution rate is limited by interfacial mass transfer. Analysis of the pH dependence of the interfacial mass transfer rate suggests that the mass transfer of the neutral species of anthracene-9-carboxylic acid (AnH) competes with the ion transfer of the dissociated species (An-) at the liquid/liquid interface in the gelatin membrane of the microcapsule.