Yuanhua Lin

Interface effect on thermal conductivity of carbon nanotube composites

A simple formula for the thermal conductivity enhancement in carbon nanotube composites is presented by incorporating the interface thermal resistance with an effective medium approach. This model well describes the thermal conductivity enhancement observed recently in nanotube suspensions. In particular, this simple formula predicts that a large interface thermal resistance across the nanotube-matrix interface causes a significant degradation in the thermal conductivity enhancement, even for the case with ultrahigh intrinsic thermal conductivity and aspect ratio of the carbon nanotubes embedded.

A simple model for thermal conductivity of carbon nanotube-based composites

A quite simple formula for the thermal conductivity enhancement in carbon nanotube composites is presented based on a conventional model. This simple formula predicts much higher thermal conductivity enhancement even in the dilute case of the carbon nanotubes, due to ultrahigh thermal conductivity and aspect ratio of the carbon nanotubes. By applying this model to nanotube suspensions recently reported in the literature, we show that the conventional model is still valid for the nanotube-based composites.

Ultrahigh Energy Density of Polymer Nanocomposites Containing BaTiO3@TiO2 Nanofibers by Atomic‐Scale Interface Engineering

Atomic-scale interface engineering in BaTiO3@TO2 nanofibers (TiO2 nano-fibers embedded with BaTiO3 nano-particles) leads to concurrent enhancement of electric displacement and breakdown strength in poly(vinylidene fluoride) (PVDF)-based nanocomposites. An ultrahigh energy density of ≈20 J cm(-3) is achieved with only 3 vol% nanofibers, which is by far the highest discharged energy density of PVDF-based nanocomposites.

BiCuSeO oxyselenides: new promising thermoelectric materials

BiCuSeO oxyselenides have recently acquired ever-increasing attention and have been extensively studied as very promising thermoelectric materials. The ZT of the BiCuSeO system was significantly increased from 0.5 to 1.4 in the past three years, which indicates that BiCuSeO oxyselenides are robust candidates for energy conversion applications. In this review, we first discuss and summarize the crystal structures, microstructures, electronic structures and physical/chemical properties of BiCuSeO oxyselenides. Then, the approaches that successfully enhanced the thermoelectric performances in the BiCuSeO system are outlined, which include increasing carrier concentration, optimizing Cu vacancies, a simple and facile ball milling method, multifunctional Pb doping, band gap tuning, and increasing carrier mobility through texturing. Theoretical calculations to predict a maximum ZT in the BiCuSeO system are also described. Finally, a discussion of future possible strategies is proposed to aim at further enhancing the thermoelectric figure of merit of these materials.

Enhancement of ferromagnetic properties in BiFeO3 polycrystalline ceramic by La doping

The authors present the structure transformation and magnetic properties of Bi1−xLaxFeO3 (x=0.0–0.15) ceramics prepared by a conventional solid-state reaction processing. Magnetic measurements reveal that remnant magnetization of 15% La-doped BiFeO3 has enhanced about 20 times as compared to pure BiFeO3. It is the structural phase transition (R3c–C222) near x=0.15 that destructs the spin cycloid, and thus enhances the ferromagnetic properties significantly. In these Bi1−xLaxFeO3 ceramic samples, besides the known antiferromagnetic Neel temperature TN1∼615K, another Neel temperature TN2∼260K can be observed due to the trace impurity phase of Bi2Fe4O9 in these ceramic samples.

Improving the dielectric constants and breakdown strength of polymer composites: effects of the shape of the BaTiO3 nanoinclusions, surface modification and polymer matrix

Flexible polymer composite films are prepared by a solution cast method with polar polyvinylidene fluoride (PVDF) or non-polar epoxy as the polymer matrix. BaTiO3 nanoparticles and BaTiO3 nanofibers with large aspect ratio are used as dielectric fillers after surface modification by polydopamine. The effects of filler shape, surface modification and polarity of polymer matrix on the microstructure, dielectric constants and breakdown strength of polymer composites are investigated in detail. Surface modification by polydopamine improves the compatibility between BaTiO3 and polymer as well as passivating the surfaces of BaTiO3. At the same volume fraction, composites filled with BaTiO3 nanofibers exhibit greater dielectric constants than the composites filled with BaTiO3 nanoparticles. The polydopamine layers on BaTiO3 nanofibers give rise to stronger interfaces between the fillers and polymer matrices. Improved breakdown strengths are achieved in both composites. This work may provide a general strategy for flexible polymer nanocomposites with greatly enhanced dielectric constants and breakdown strength.

Remarkable Enhancement in Thermoelectric Performance of BiCuSeO by Cu Deficiencies

A significant enhancement of thermoelectric performance in layered oxyselenides BiCuSeO was achieved. The electrical conductivity and Seebeck coefficient of BiCu(1-x)SeO (x = 0-0.1) indicate that the carriers were introduced in the (Cu(2)Se(2))(2-) layer by Cu deficiencies. The maximum of electrical conductivity is 3 × 10(3) S m(-1) for Bicu(0.975)Seo at 650 °C, much larger than 470 S m(-1) for pristine BiCuSeO. Featured with very low thermal conductivity (∼0.5 W m(-1) K(-1)) and a large Seebeck coefficient (+273 μV K(-1)), ZT at 650 °C is significantly increased from 0.50 for pristine BiCuSeO to 0.81 for BiCu(0.975)SeO by introducing Cu deficiencies, which makes it a promising candidate for medium temperature thermoelectric applications.

Enhanced dielectric and ferroelectric properties induced by dopamine-modified BaTiO3 nanofibers in flexible poly(vinylidene fluoride-trifluoroethylene) nanocomposites

BaTiO3 nanofibers with a large aspect ratio prepared via electrospinning and modified by dopamine were used as dielectric fillers in poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE)-based nanocomposites. Highly flexible polymer nanocomposite films were fabricated via a simple solution-cast method. Enhanced dielectric permittivities were obtained at a low volume fraction of BaTiO3 nanofibers. The breakdown strength of the polymer nanocomposites was also improved, which is favorable for enhanced ferroelectric properties in the nanocomposites. Pr ∼9.1 μC cm−2 was achieved in the nanocomposites with 10.8 vol% BaTiO3 nanofibers. The improved breakdown strength and enhanced ferroelectric properties are attributed to the combined effect of the surface modification by dopamine, the large aspect ratio of the BaTiO3 nanofibers and the improved crystallinity of the polymer nanocomposites induced by the BaTiO3 nanofibers.

Giant Energy Density and Improved Discharge Efficiency of Solution-Processed Polymer Nanocomposites for Dielectric Energy Storage

Large-aspect-ratio composite nanofibers with interior hierarchical interfaces are employed to break the adverse coupling of electric displacement and breakdown strength in flexible poly(vinylidene fluoride-hexafluoropropylene) nanocomposite films, a small loading of 3 vol% BaTiO3@TiO2 nanofibers gives rise to the highestenergy density (≈31.2 J cm(-3)) ever achieved in polymer nanocomposites dielectrics.

Anomalous luminescence in Sr4Al14O25:Eu, Dy phosphors

Sr4Al14O25:Eu, Dy material with an extraordinarily long afterglow was synthesized via a traditional ceramic processing. Such a long afterglow observed can last over 20 h at recognizable intensity level (0.32 mcd/m2), which is attributed to energy exchange between the traps and emission states resulting from Dy and Eu doping. When the Eu2+ concentration doped in the host is less than 1.2 mol %, the emission spectra of Sr4Al14O25:Eu, Dy show two main peaks at 407 and 494 nm, ascribed to two types of Eu2+ centers existing in the Sr4Al14O25 host. The emission peak at 407 nm disappears slowly with increasing concentration of Eu2+ due to the energy exchange between two types of Eu2+ centers.