Guang-Ping Zheng

Direct measurement of giant electrocaloric effect in BaTiO3 multilayer thick film structure beyond theoretical prediction

The electrocaloric (EC) effect of BaTiO3 multilayer thick film structure was investigated by direct measurement and theoretical calculation. The samples were prepared by the tape-casting method, which had 180 dielectric layers with an average thickness of 1.4 μm. The thermodynamic calculation based on the polarization-temperature curves predicted a peak heat adsorption of 0.32 J/g at 80 °C under 176 kV/cm electric field. The direct measurement via differential scanning calorimeter showed a much higher EC effect of 0.91 J/g at 80 °C under same electric field. The difference could result from the different trends of changes of electric polarization and lattice elastic energy under ultrahigh electric field.

Strengthening of Graphene Aerogels with Tunable Density and High Adsorption Capacity towards Pb2

Graphene aerogels (GAs) with high mechanical strength, tunable density and volume have been prepared only via soaking graphene hydrogels (GHs) in ammonia solution. The density and volume of the obtained GAs are controlled by adjusting the concentration of ammonia solution. Although volume of the GAs decreases with increasing the concentration of ammonia solution, its specific surface area maintains at about 350 m2 g−1, and the inner structure changes to radial after ammonia solution treatment. Thus, GAs are particularly suitable for the adsorption and energy storage applications owing to their high specific surface area and unique porous structure. The adsorption capacity of GAs for Pb2+ from aqueous solution maintains at about 80 mg g−1, which could reach as high as 5000 g m−3 per unit volume and they can be separated easily from water after adsorption.

Kinetic electrocaloric effect and giant net cooling of lead-free ferroelectric refrigerants

The electrocaloric effect of BaTiO3 multilayer thick film structure was investigated by direct measurement using differential scanning calorimeter. The samples show a giant electrocaloric effect of 0.89 J/g under E=176 kV/cm, which also depends on the varying rate of applied field, following a general power-law relation. Based on the large net-cooling (0.37 J/g) resulting from the difference in the varying rates of rising and falling fields, the kinetic electrocaloric effect provides a solution for the design of refrigeration cycle in ferroelectric microrefrigerator.

First-principles prediction and experimental verification of glass-forming ability in Zr-Cu binary metallic glasses.

In the field of metallic materials with amorphous structures, it is vitally important to understand the glass formation and to predict glass-forming ability (GFA) in terms of constituent elements and alloy compositions. In this study, an expression has been formulated from first-principles calculations to predict the trend of GFA by hybridizing both internal energies and atomic-scale defect structures. The prediction of GFA from compositions has been verified successfully by available experimental data in the model Zr-Cu alloy system. The physical scenario revealed here has extensive implications for the design of bulk metallic glasses with superior GFA.

The electrocaloric effect around the orthorhombic- tetragonal first-order phase transition in BaTiO3

This paper demonstrates the electrocaloric effect (ECE) around BaTiO3s orthorhombic-tetragonal first-order phase transition. By manipulating a field-induced transition of a metastable phase in the thermal hysteresis zone, a huge exothermic or endothermic peak appears after first applying or removing electric fields because of the energy change of lattice structure. A large ECE of ΔT/E = 1.4K·m/MV, equaling to latent heat, is achieved under 10kV/cm at 10°C. The entropy change for polarization ordering alone induces an ECE two orders of magnitude lower under the same condition. It confirms the dominant factor to ECE of the energy flow due to the structural phase transition.

Unification of the negative electrocaloric effect in Bi1/2Na1/2TiO3-BaTiO3 solid solutions by Ba1/2Sr1/2TiO3 doping

The microscopic mechanisms of the negative electrocaloric effect (ECE) of the single-phase (1−x)(0.94Bi1/2Na1/2TiO3-0.06BaTiO3)-xBa1/2Sr1/2TiO3 (BNT-BT-BST) perovskite solid solutions fabricated via the sol-gel technique are explored in this study. Dielectric and mechanical relaxation analyses are employed to investigate the ferroelectric and structural transitions of the samples. The electrocaloric properties of the samples were measured by thermodynamics Maxwell relations. The difference between the depolarization temperature (Td) and the maximum dielectric constant temperature (Tm) was found to decrease with increasing BST content. Doping with BST stabilized the ferroelectric phase along with unifying the EC temperature changes (ΔT) to only negative values. The origin of the uniform negative ECE of BNT-BT-BST is discussed.

Microwave-assisted simultaneous reduction and titanate treatment of graphene oxide

Simultaneous reduction and functionalization of graphene oxide was conducted with the assistance of microwave irradiation. A titanate coupling agent was utilized for the functionalization of graphene. TEM, XPS and FTIR were employed to characterize the changes in graphene morphology and chemistry. The results suggested that the titanate coupling agent was bonded covalently on the graphene surfaces. The surface of graphene remained hydrophilic after the reduction of graphene oxide, while the electrical conductivity of graphene was partially restored. Titanate functionalized graphene was mixed with the waterborne polyurethane (PU) to prepare nanocomposites. The graphene/PU nanocomposites exhibited a conductive percolation threshold of 0.1 wt%.

Enhanced ferroelectric and pyroelectric properties of poly(vinylidene fluoride) with addition of graphene oxides

Poly(vinylidene fluoride)/graphene oxide (PVDF/GO) nanocomposites are synthesized and their structural, ferroelectric, and pyroelectric properties are investigated. The dielectric spectrum analysis and P-E loop tests indicate that the nanocomposites exhibit enhanced ferroelectric and pyroelectric properties compared with those of poly(vinylidene fluoride) samples. The isothermal crystallization kinetics of PVDF/GO nanocomposites quantitatively determined by differential scanning calorimetry demonstrates that GOs facilitate the crystallization of the PVDF. Dynamic mechanical analyses on the PVDF/GO reveal that the amorphous and crystalline phases of PVDF are modified by the addition of GO sheets. The GO-enhanced formation of crystalline β phase in PVDF could result from the strong interaction between the –C = O groups in GO and the –CF2 groups in PVDF, and the GO-induced ordering of the microstructures of amorphous and crystalline phases. The results suggest that PVDF/GO nanocomposites could be promising d...

The microstructural, mechanical and electro-mechanical properties of graphene aerogel-PVDF nanoporous composites

Graphene aerogel-poly (vinylidene fluoride) (GA-PVDF) nanoporous composites with different concentrations of PVDF are fabricated. Scanning electron microscopy reveals that PVDF films with a typical thickness below 100 nm are coated at the graphene sheets in the nanoporous composites. The GA-PVDF composites show excellent compressibility, ductility and mechanical strength, as well as better sensitivity of stress-dependent electrical resistance compared with those of GAs. The improved mechanical and electro-mechanical behaviours of nanoporous composites are ascribed to the PVDF which possesses piezoelectricity. The structural properties of the graphene-PVDF nanosized hybrid scaffolds are analyzed by dynamical mechanical relaxation. The results demonstrate that the nanoporous composites could be used as high-performance sensors, actuators and kinetic energy harvesters.

Fabrication and characterization of CoxGd1-x nanowire arrays and core-shell-like CoxGd1-x/CoO nanowire arrays

Amorphous CoxGd1-x nanowire arrays and core-shell-like CoxGd1-x/CoO nanowire arrays with composite amorphous and crystalline structures, where 0.75 < x < 0.99, are fabricated successfully by employing electrochemical deposition with the assistance of AAO templates. Based on the experiments, dissolving of the organic salt of Gd in dimethylsulfoxide and the deposition potentials are the critical factors for formation of amorphous structures and different morphologies. Investigation of the magnetic properties shows that CoxGd1-x nanowire arrays have superparamagnetic characteristics, and core-shell-like CoxGd1-x/CoO nanowire arrays possess better soft ferromagnetic properties. Moreover, these magnetic behaviors are attributed to spin disorder of amorphous structures and shape anisotropy, which arise from the large aspect ratio of one-dimensional nanoscale materials.