Key Engineering Materials Vol. 519

Abstract: To prepare the phase change materials for efficient low-temperature latent heat storage, microcapsules with commercial solid paraffin were synthesized by using in-situ polymerization process. Liquid paraffin was used first as core materials since it facilitates the direct observation of containment by the shell of melamine-formaldehyde resin (MF resin), and examines easily the optimal synthesizing condition. The effects of emulsion processes, pre-polymerization conditions, shell material content and reaction time on the quality of microcapsules were investigated. The experimental investigations show that the optimal process of synthesizing liquid paraffin microcapsules is adoptable to that of solid paraffin microcapsules. The as-prepared solid paraffin microcapsules show high quality. The microcapsules surface was smooth and dense, and be free of any adhesion. The measurements show that in the microcapsules solid paraffin was well encapsulated by MF resin. The microcapsule size was almost within the range of 6~33 μm and most intense distribution at 23 μm. DSC measurements gave two endothermic peaks with initial phase change temperature at 35°C and 50°C respectively, and the total enthalpy was above 134 J/g.

Abstract: The phase change materials (PCMs) can absorb, store or release large latent heat over a defined temperature range while the materials change phase or state, so they can be potentially used in thermal energy storage. In this paper, a series of microencapsulated phase change materials (MicroPCMs) with n-octadecane and n-dodecanol as core were successfully fabricated respectively, where the styrene-based copolymer, acrylic based copolymer, melamine-formaldehyde resin and polyurea were selected as shell materials. The morphology of these MicroPCMs was observed by scanning electron microscopy (SEM), and the core-shell structure and the shell thickness of microcapsules were also characterized by SEM. In addition, the phase change properties of MicroPCMs were investigated using differential scanning calorimetry (DSC) analysis. Furthermore, thermal-regulated calcium alginate fiber was produced by adding MicroPCMs in wet-spinning process; and the effects of various types of MicroPCMs on fiber was discussed.

Abstract: The electro-caloric energy conversions in single-crystal and polycrystalline barium titanate samples are investigated using differential scanning calorimetry (DSC). The DSC measurement reveals a refrigeration effect of 0.1 J/g under an electric field of 15 kV/cm at T=17.5 °C, which is related with the ferroelectric-to-ferroelectric transition in barium titanate. The thermodynamics calculation based on Maxwell’s relations leads to similar conclusion. Such EC effect shows relaxation with the heat release process more significant than the heat adsorption process when the applied field is changed, which is different with those occur near the ferroelectric-to-paraelectric transition temperatures. Furthermore, the electro-caloric energy conversion relation or the scaling relation between the maximum refrigeration effect ΔH and the applied field E, is found to follow a power law ΔH_max~E^b with b=1.72, which is significantly larger than those for EC effects related with the ferroelectric-to-paraelectric transitions.

Abstract: A kind of comb-shaped poly(oxyethylene) with nine oxyethylene units in ether side chains has been synthesized via chemical modification on ethylene oxide-epichlorohydrin copolymer [P(EO/EH)s] with a kind of alcohol ether carboxylate. The prepared polyether was characterized by FTIR spectroscopy. A series of polymer electrolytes were prepared by dissolving LiN(SO₂CF₃)₂ at different ratios in the polymer matrix. It is found that the ionic conductivity of the comb-shaped poly(oxyethylene) based polymer electrolytes could reach 4.3×10^-5 S/cm at 30°C and 6.2×10-4 S/cm at 80°C, which increase by an order of magnitude compared with that of poly(ethylene oxide) (PEO) based ones at corresponding temperatures. Both the amorphous feature of the polymer electrolytes above the ambient temperature and the highly mobile ether side chains of the comb-shaped polyether contribute to the increase of ionic conductivity.

Abstract: A novel approach to the functionalization of nanometer-sized graphene was presented in this work. Covalent bonding between the filler and matrix was formed, with minimal disruption to the sp² hybridization of the pristine graphene sheet. Functionalization proceeded by covalently bonding a 4-substituted benzoic acid monomer to the surface of expanded graphene, via ‘‘direct Friedel-Crafts” acylation in mild reaction medium of polyphosphoric acid (PPA)/phosphorous pentoxide. Polyamide 6 (PA6)-functionalized graphene (FG) composites were prepared by in situ polymerization of ε-caprolactam in the presence of nanometer-sized FG. Nanocomposite fiber with 0.1 wt.% content of nanometer-sized FG was prepared with a piston spinning machine and hot-roller drawing machine. The nanometer-sized FG performed homogeneous dispersion in the polymer matrix. The mechanical properties of the PA6-FG composites fiber was enhanced by adding FG in the polymer matrix. The new functionalization method paves the way to prepare graphene-based nanocomposites fiber simply, without disrupting the primary structures of nanometer-sized graphene.

Abstract: Studies of ion motion in crystalline and glassy materials, Jonscher power law, which was frequently referred to as the so-called universal dynamic response (UDR) is an effective method, but the power law exponents obtained from curve-fitting are sensitively affected by the frequency window employed in the analysis. So how to choose frequency window and which window can truly describe the nature of the materials, are important for every researcher. In this paper, through analyzing the experimental data of frequency dependence of ac conductivity in 50Li₂O–50P₂O₅ glasses, a simple method to choose frequency window was found and further discussions indicated our choice was reasonable.

Abstract: Doped ceria-based (Gd0.2Ce0.8O1.9, GDC) solid electrolytes were prepared by Solid-phase synthesis method. The effect of doping bismuth oxide and samarium oxide on the phase and microstructure of GDC was investigated. The phase composition was analyzed by the X-ray diffraction (XRD).The single cubic fluorite structure was observed after doping these oxides. Appearance and microstructure of doped ceria-based solid electrolytes were analyzed by the scanning electron microscopy (SEM) and the transmission electron microscopy (TEM). The results showed that the doped trivalent cations had entered into the ceria structure uniformly. The density, porosity rate and water absorption of GDC were measured by Archimedes principle. It indicated that the density of doped GDC solid electrolyte increased with the rising of sintering temperature.

Abstract: The idealized microstructure of NiO/YSZ anode of solid oxide fuel cell (SOFC) is that YSZ structures the bone of the anode and NiO is segregated by YSZ to keep Ni from congregation. To obtain this microstructure, the mixture of coarse YSZ and fine YSZ was used. The effect of the particle size gradation of YSZ on the microstructure of anode was studied. The results show that the homogenous microstructure can be obtained when the mass ratio of the coarse YSZ (d50＝3.38µm) to the fine YSZ (d50＝0.4µm) is 7:3. The pre-sintering temperature has obvious effect on the porosity of the NiO/YSZ. The bending strength of NiO/YSZ and Ni/YSZ are 123MPa and 85MPa, respectively. The reduction of NiO to Ni has less effect on the bending strength for 40vol%Ni/YSZ (7/3).

Abstract: A cobalt-free perovskite oxide Ba_0.5Sr_0.5Fe_0.8Cu_0.2O_3−δ (BSFCu) is applied as the cathode material for intermediate temperature micro-tubular solid oxide fuel cells (SOFCs) with asymmetric structure anode. The NiO-YSZ hollow fiber anode as support was prepared by the phase inversion technique. The as-prepared fiber anode shows a special asymmetric structure consisting of porous sponge-like structure in the middle and finger-like porous structure on the inner and outer side. Dense thin YSZ electrolyte membrane and SDC transition layer were deposited on the NiO-YSZ hollow fiber anode by a vacuum-assisted dip-coating and co-sintering technique. Laboratory-sized micro-tubular single cells of NiO-YSZ/YSZ+SDC/BSFCu-SDC were tested at 650-750°C with humidified H2 as the fuel and static ambient air as the oxidant. The peak power densities of 437.2, 326.5 and 214.4 mWcm^-2 can be obtained at 750, 700 and 650°C, respectively. The experimental results indicate that the cobalt-free perovskite oxide BSFCu is a promising cathode material candidate for the developed intermediate temperature micro-tubular SOFC with asymmetric hollow fiber anode.