Structure and thermal stability of microencapsulated phase-change materials

A series of microcapsules containing n-octadecane with a urea-melamine-formaldehyde copolymer shell were synthesized by in-situ polymerization. The surface morphology, diameter, melting and crystallization properties, and thermal stability of the microcapsules were investigated by using FTIR, SEM, DSC, TGA and DTA. The diameters of the microcapsules are in the range of 0.2–5.6 m. The n-octadecane contents in the microcapsules are in the range of 65–78wt%. The mole ratio of urea-melamine has been found to have no effect on the melting temperature of the microcapsules. Two crystallization peaks on the DSC cooling curve have been observed. The thermal damage mechanisms are the liquefied n-octadecane leaking from the microcapsule and breakage of the shell due to the mismatch of thermal expansion of the core and shell materials at high temperatures. The thermal stability of materials can be enhanced up to 10 °C by the copolymerization of urea, melamine and formaldehyde in a mole ratio 0.2:0.8:3. The thermal stability of 160 °C heat-treated microcapsules containing 8.8% cyclohexane can be further enhanced up to approximately 37 °C.     

Exergoeconomic analysis of photovoltaic thermal systems based on phase change materials and natural zeolites for thermal management

Journal of Thermal Analysis and Calorimetry - Conventional photovoltaic thermal (PVT) systems provide unstable thermal energy, which changes throughout the day. In PVT systems, phase change...     

混合高级醇固-液相变储热材料的研究

相变储热材料研究与开发是国内外的一个新兴领域[1-6]。为了解决一些仪器的内部散热问题,本文研究了混合高级醇固-液相变储热材料的凝固和熔化特性、相变焓值、固液相体积以及石墨粉对材料的导热性能。1实验部分1·1试剂与仪器CDR-1型差动热分析仪-配微机处理系统,上海天平仪器     

The PVT and thermal properties of HF

The PVT and thermal properties of pure HF have been revised. The available data,which cover two distinct regions, one at low and one at high pressure, have been organized in a consistent set of data; volume properties and enthalpy have been plotted in the temperature range between 0°C and 300°C, for pressures up to 300 atm.Some experimental work has also been done to test the volume properties of HF in the presence of an inert gas at moderate temperatures and pressures. At these conditions the association factor, which for pure HF is a function of temperature and molar volume, appears to be strongly affected by the presence of inerts.     

Synthesis and characterization of graphene oxide-based nanofluids and study of their thermal conductivity

Journal of Thermal Analysis and Calorimetry - In the present research work, we investigated the effect of loading mass concentrations (0.05%, 0.15%, and 0.25 mass %) and temperature...     

The effect of the geometry and content of the modified carbon nanotubes on the thermal properties of the composite phase-change materials

Journal of Thermal Analysis and Calorimetry - In this paper, ultrasonic dispersion and surface chemical modification were used to prepare the modified multi-walled carbon nanotubes (MWNTs)/paraffin...     

Theoretical study of electronic properties of organic photovoltaic materials

It has been proved that fullerene derivatives, in which an oligophenylenevinylene (OPV) group is attached to C(60), present an interesting photophysical phenomenon and can be incorporated into photovoltaic cells. In these systems, the OPV acts as electron donor upon excitation, and then fullerene absorbs photoexcited electrons. These new organic semiconductor materials offer the prospect of lower manufacturing costs and they present several advantages: easy fabrication, large area, flexible and light weight devices when compared with inorganic counter parts. In the present theoretical study, oligomeric chains of p-phenylenevinylene (n-PPV, n = 3-8 units) and C(60)-OPV hybrids have been studied by density functional theory (DFT). Electronic properties such as electronic absorption and emission spectra were calculated in order to determinate how the increment of spectroscopic units affects their electronic behavior. These properties were carried out with time dependent-density functional theory (TD-DFT) and ZINDO semiempirical method. The theoretical calculations of the structural properties of n-PPV and fullerene-OPV hybrids were obtained using PBE1PBE/6-31G and ONIOM two-layered version, respectively. All calculations were done with Gaussian 03W program package.     

Applications of nanofluids in condensing and evaporating systems

Nanofluids can be utilized as efficient heat transfer fluids in many thermal energy systems to improve the system’s thermal efficiency. This survey reviews and summarizes the experimental and numerical studies performed to determine the effect of nanofluids on the performance of condensing and evaporating systems. Advantages and disadvantages of nanofluid implementation in condensing and evaporating systems are evaluated and summarized. Moreover, some suggestions and recommendations are presented for future studies. This review shows that the nanoparticle deposition and nanoparticle suspension are two important factors affecting the thermal system’s efficiency. These factors should be considered when using different nanofluids in condensing and evaporating systems.     

Toward a modeling study of thermal conductivity of nanofluids using LSSVM strategy

Journal of Thermal Analysis and Calorimetry - In the present study, a comprehensive model based on least square support vector machine algorithm (LSSVM) was developed to estimate thermal...     

Photogalvanic and photovoltaic effects in systems based on metal complexes of Schiff bases

The nature of the processes that occur when electrodes modified with complexes [M(Schiff)] (M = Ni, Pd, Pt; Schiff denotes four-dentate Schiff base ligands) are irradiated with visible light for the potential use of these electrodes in photoelectrochemical energy conversion devices is considered. The factors responsible for shifts in the electrode potential upon photoexcitation, i.e., the nature of the metal site, the nature of the substituents in the sensitizer, and the oxygen concentration are discussed. Tentative mechanisms of the photovoltaic effects observed for conventional and semiconductor electrodes modified with [M(Schiff)] complexes are determined.     

Use of emanation thermal analysis in the study of thermal behaviour of inorganic materials

Emanation thermal analysis [1, 2] (ETA) is based on the measurement of the inert gas release from samples previously labelled. The high detection sensitivity of radioactive nuclides used for the measurement makes it possible to use very low concentrations of the inert gases (10^–14 at %) so that no influence of the inert gases on the properties of the solids can be supposed.

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Zusammenfassung Emanations — Thermoanalyse [1, 2] (ETA) basiert auf der Messung von freigesetztem inerten Gas aus zufor markierten Proben. Die große Nachweisempfindlichkeit der bei der Messung verwendeten radioaktiven Nuklide ermöglicht die Anwendung sehr niedriger konzentrationen (10^–14 Atom%) für das inerte Gas. Somit kann angenommen werden, daß die Eigenscahften des Feststoffes durch das inerte Gas nicht beeinflußt werden.

     

Preparation and characterization of pavement materials with phase-change temperature modulation

A kind of pavement crack repairing material with temperature regulation property was successfully prepared through one-step method, in which the paraffin was incorporated into the polyurethane/epoxy resin-interpenetrating polymer networks. Differential scanning calorimeter results indicated that the phase-change latent heat of sample A was 14.4 kJ kg^?1, and the phase transition temperature was ??0.3 °C. FTIR and thermogravimetry measurements verified that the paraffin was successfully incorporated into the interpenetrating polymer network without leakage and reacted with the carrier, which exhibited high thermal stability above 300 °C. After 1 year of road test, there was no breakage for the repairing pavement with paraffin–polyurethane/epoxy resin-interpenetrating polymer networks, and there was almost no change for the accumulated attenuation of phase-change latent heat. Therefore, the materials of paraffin–polyurethane/epoxy resin-interpenetrating polymer networks have good chemical stability and thermal stability.

     

A study on the thermal conductivity of resilient materials

The floor system of apartments must have a certain level of floor impact sound performance (light-weight impact sound is 58 dB or less, heavy-weight impact sound is 50 dB or less), and comply with the building energy code to prevent energy loss in residential buildings in Korea. In this study, expanded polystyrene (EPS), expanded polypropylene (EPP), ethylene-vinyl acetate copolymer (EVA), polyethylene (PE) and urethane foam, which are widely used in Korea as resilient materials for floating floor system of apartments, were measured. Their thermal conductivity and apparent density were measured to analyze their thermal properties and their relation was analyzed by material. In case of resilient materials made of EPS and EPP, thermal conductivity tended to decrease as apparent density increased, whereas in case of resilient materials made of EVA and PE, as apparent density increased, thermal conductivity tended to increase as well.     

Abilities of porous materials for energy saving in advanced thermal systems

Journal of Thermal Analysis and Calorimetry - Using materials with porous natures to enhance the thermal efficiency of various thermal systems has attracted much attention. This is owing to the low...     

固载桥结构对Ru(II)-GO纳米复合物光伏性能的影响

本文分别利用非共轭桥和共轭桥将三联吡啶钌配合物固载到氧化石墨烯表面,制备了复合物Ru1-GO和Ru2-GO。采用IR、UV-Vis、EDS、TEM及XRD等多种方法对复合物进行了详细表征,并利用光电流响应测试对它们的光伏性能进行了比较。结果显示,含有共轭桥的复合物Ru2-GO产生的光电流强度可以达到3.5μA/cm~2,大约是复合物Ru1-GO的3.3倍,说明固载桥结构对钌配合物共价修饰的石墨烯纳米复合物的光伏性能有着重要的影响。     

Determination of nanolayer thickness and effective thermal conductivity of nanofluids

Nanofluids having high thermal conductivity enhancement relative to conventional pure fluids are fluids engineered by suspending solid nanoparticles into base fluids. In the present study, calculating the Van der Waals interaction energy between a nanoparticle and an ordered liquid nanolayer around it, the nanolayer thickness was determined, the average velocity of the Brownian motion of nanoparticles in a fluid was estimated, and by taking into account both the aggregation of nanoparticles and the presence of a nanolayer a new thermal conductivity model for nanofluids was proposed. It has been shown that the nanolayer thickness in nanofluids is independent on the radius of nanoparticles when the radius of the nanoparticles is much greater than the nanolayer thickness and determines by the specific interaction of the given liquid and solid nanoparticle through the Hamaker constant, the surface tension and the wetting angle. It was proved that the frequency of heat exchange by fluid molecules is two orders of magnitude higher than the frequency of heat transfer by nanoparticles, so that the contribution due to the Brownian motion of nanoparticles in the thermal conductivity of nanofluids can be neglected. The predictions of the proposed model of thermal conductivity were compared with the experimental data and a good correlation was achieved.     

Exploring the potential of tetraazaacene derivatives as photovoltaic materials with enhanced photovoltaic parameters

A series of D-π-A type molecules have been designed for their potential use in organic photovoltaic devices. Photovoltaic and optoelectronic properties of newly designed molecules have been explored by comparing with a reference molecule R comprising of the central core (2,3,8,9-tetrakis(thiophen-2-ylethynyl)-5,7,10,12-tetrakis((trimethylsilyl)ethynyl)pyrazino[2,3-b]phenazine) and π-bridge (thiophene). The end groups are (2-(2-ethylidene-3-oxo-2,3-dihydro-1H-inden-1 ylidene)malononitrile), (2-ethylidenemalonitrile), (methyl 2-cyanoacrylate) and (3-methyl-5-methylene-2-thioxothiazolidin-4-one) in the newly designed molecules. Among the investigated molecules M1 and M2 exhibit a broad absorption range of 627 and 626 nm with respect to the reference. All the designed molecules exhibited a lower bandgap as compared to R which indicates a better transfer of electron density from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO). The reorganization energy values show that all designed molecules have efficient charge transport capability. This study proves that end-capped acceptor modification is an effective strategy for designing optimistic molecule for high performance future organic solar cells fabrication.     

Numerical simulation of nanofluids for improved cooling efficiency in a 3D copper microchannel heat sink (MCHS)

In this paper, laminar nanofluid flow in 3D copper microchannel heat sink (MCHS) with rectangular cross section, and a constant heat flux, has been treated numerically using the computational fluid dynamics software (FLUENT). Results for the temperature and velocity distributions in the investigated MCHS are presented. In addition, experimental and numerical values are compared in terms of friction factors, convective heat transfer coefficients, wall temperature and pressure drops, for various particle volume concentrations and Reynolds numbers. The numerical results show that enhancing the heat flux has a very weak effect on the heat transfer coefficient for pure water, but an appreciable effect for the case of a nanofluid. For all considered volume fractions, the sink friction factor decreases by increasing the Reynolds number and slightly increases by increasing the volume fractions, and, with increasing the volume fractions and the Reynolds number, the pressure drop increases.     

The effects of oil/MWCNT nanofluids and geometries on the solid oxide fuel cell cooling systems: a CFD study

Journal of Thermal Analysis and Calorimetry - In this numerical study, the variations in the surface area of the cooling channels in a solid oxide fuel cell with different cross sections...     

Effect of copper nanoparticle aggregation on the thermal conductivity of nanofluids

The thermal conductivity of water and glycerol is investigated via the transient hot wire method by adding small amounts of copper nanoparticles to solutions. At a 0.2% copper nanoparticle concentration, the thermal conductivity coefficient rises to 25% for the Cu + glycerol system, and to 35% for Cu + water system. A mechanism and mathematical model for describing the nanoparticle aggregation effect on the thermal properties of nanofluids are proposed, based on an analysis of the accumulated experimental data. It is shown that the enhancement of nanofluid thermal conductivity at low nanoparticle concentrations is directly proportional to their volume fraction and thermal conductivity coefficient, and (in accordance with the literature data) is inversely proportional to the radius and the aggregation ratio. The proposed model describes the existing experimental data quite well. The results from this work can be applied to the rapid cooling of electronic components, in the power engineering for ensuring the rapid and effective transfer of thermal energy in a nuclear reactor, and in the oil industry for thermal stimulation.