Fengyuan Lu

Flexible pillared graphene-paper electrodes for high-performance electrochemical supercapacitors.

Flexible graphene paper (GP) pillared by carbon black (CB) nanoparticles using a simple vacuum filtration method is developed as a high-performance electrode material for supercapacitors. Through the introduction of CB nanoparticles as spacers, the self-restacking of graphene sheets during the filtration process is mitigated to a great extent. The pillared GP-based supercapacitors exhibit excellent electrochemical performances and cyclic stabilities compared with GP without the addition of CB nanoparticles. At a scan rate of 10 mV s(-1) , the specific capacitance of the pillared GP is 138 F g(-1) and 83.2 F g(-1) with negligible 3.85% and 4.35% capacitance degradation after 2000 cycles in aqueous and organic electrolytes, respectively. At an extremely fast scan rate of 500 mV s (-1) , the specific capacitance can reach 80 F g(-1) in aqueous electrolyte. No binder is needed for assembling the supercapacitor cells and the pillared GP itself may serve as a current collector due to its intrinsic high electrical conductivity. The pillared GP has great potential in the development of promising flexible and ultralight-weight supercapacitors for electrochemical energy storage.

Synthesis of ZnO quantum dot/graphene nanocomposites by atomic layer deposition with high lithium storage capacity

Zinc oxide, as an inexpensive anode material, has attracted less attention than other metal oxides due to its poor cycling stability. A rational design of ZnO nanostructures with well-controlled particle sizes and microstructures is essential in order to improve their stability and performance as electrodes for lithium ion batteries (LIBs). Here, we demonstrate a simple approach via atomic layer deposition (ALD) to synthesize ZnO quantum dots (QDs) on graphene layers, in which the size of the ZnO QDs can be controlled from 2 to 7 nm by ALD cycles. A strong relationship between size and electrochemical performance is observed, in which smaller sized QDs on graphene display enhanced electrochemical performance. A high reversible specific capacity of 960 mA h g−1 is achieved at a current density of 100 mA g−1 for 2 nm ZnO QDs, approaching to the theoretical value of ZnO as the LIB anode. The greatly enhanced cycling stability and rate performance of the ALD ZnO QD/graphene composite electrode can be attributed to the well-maintained structural integrity without pulverization upon electrochemical charge/discharge for ZnO QDs with the grain size below a critical value.

Enhanced radiation resistance of nanocrystalline pyrochlore Gd2(Ti0.65Zr0.35)2O7

The radiation response of nanostructured materials is of great interest because of the potential of nanoscale materials design for mitigating radiation damage. We report a greatly enhanced resistance to radiation-induced amorphization in nanocrystalline Gd2(Ti0.65Zr0.35)2O7 at a particle size less than 20 nm while larger crystals with the size >100 nm are radiation sensitive. The grain size of pyrochlore, Gd2(Ti0.65Zr0.35)2O7, can be controlled by mechanical milling and subsequent thermal treatment (from 800 to 1500 °C), offering the possibility of designing pyrochlore materials at the nanoscale with enhanced performance for specific radiation environments.

Advanced phase change composite by thermally annealed defect-free graphene for thermal energy storage

Organic phase change materials (PCMs) have been utilized as latent heat energy storage and release media for effective thermal management. A major challenge exists for organic PCMs in which their low thermal conductivity leads to a slow transient temperature response and reduced heat transfer efficiency. In this work, 2D thermally annealed defect-free graphene sheets (GSs) can be obtained upon high temperature annealing in removing defects and oxygen functional groups. As a result of greatly reduced phonon scattering centers for thermal transport, the incorporation of ultralight weight and defect free graphene applied as nanoscale additives into a phase change composite (PCC) drastically improve thermal conductivity and meanwhile minimize the reduction of heat of fusion. A high thermal conductivity of the defect-free graphene-PCC can be achieved up to 3.55 W/(m K) at a 10 wt % graphene loading. This represents an enhancement of over 600% as compared to pristine graphene-PCC without annealing at a comparable loading, and a 16-fold enhancement than the pure PCM (1-octadecanol). The defect-free graphene-PCC displays rapid temperature response and superior heat transfer capability as compared to the pristine graphene-PCC or pure PCM, enabling transformational thermal energy storage and management.

Ion beam-induced amorphous-to-tetragonal phase transformation and grain growth of nanocrystalline zirconia.

Nanocrystalline zirconia has recently attracted extensive research interest due to its unique mechanical, thermal and electrical properties as compared with bulk zirconia counterparts, and it is of particular importance for controlling the phase stability of different polymorphs (amorphous, cubic, tetragonal and monoclinic phases) in different size regimes. In this work, we performed ion beam bombardments on bilayers (amorphous and cubic) of nano-zirconia using 1 MeV Kr2+ irradiation. Transmission electron microscopy (TEM) analysis reveals that amorphous zirconia transforms to a tetragonal structure under irradiation at room temperature, suggesting that the tetragonal phase is more energetically favorable under these conditions. The final grain size of the tetragonal zirconia can be controlled by irradiation conditions. A slower kinetics in the grain growth from cubic nanocrystalline zirconia was found as compared with that for the tetragonal grains recrystallized from the amorphous layer. The radiation-induced nanograins of tetragonal ZrO2 are stable at ambient conditions and maintain their physical integrity over a long period of time after irradiation. These results demonstrated that ion beam methods provide the means to control the phase stability and structure of zirconia polymorphs.

Amorphization of nanocrystalline monoclinic ZrO2 by swift heavy ion irradiation

Bulk ZrO(2) polymorphs generally have an extremely high amorphization tolerance upon low energy ion and swift heavy ion irradiation in which ballistic interaction and ionization radiation dominate the ion-solid interaction, respectively. However, under very high-energy irradiation by 1.33 GeV U-238, nanocrystalline (40-50 nm) monoclinic ZrO(2) can be amorphized. A computational simulation based on a thermal spike model reveals that the strong ionizing radiation from swift heavy ions with a very high electronic energy loss of 52.2 keV nm(-1) can induce transient zones with temperatures well above the ZrO(2) melting point. The extreme electronic energy loss, coupled with the high energy state of the nanostructured materials and a high thermal confinement due to the less effective heat transport within the transient hot zone, may eventually be responsible for the ionizing radiation-induced amorphization without transforming to the tetragonal polymorph. The amorphization of nanocrystalline zirconia was also confirmed by 1.69 GeV Au ion irradiation with the electronic energy loss of 40 keV nm(-1). These results suggest that highly radiation tolerant materials in bulk forms, such as ZrO(2), may be radiation sensitive with the reduced length scale down to the nano-metered regime upon irradiation above a threshold value of electronic energy loss.

Er-Implanted Porous Silicon: a Novel Material for Si-Based Infrared LEDs

We have demonstrated a strong, room-temperature, 1.54 μm emission from erbium-implanted at 190 keV into red-emitting porous silicon. Luminescence data showed that the intensity of infrared (IR) emission from Er implanted porous Si annealed at ≤ 650°C, was a few orders of magnitude stronger than Er implanted quartz produced under identical conditions, and was almost comparable to IR emission from In 0.53 Ga 0 . 47 As material which is used for commercial IR light-emitting diodes (LEDs). The strong IR emission (much higher than Er in quartz) and the weak temperature dependency of Er in porous Si, which is similar to Er 3+ in wide-bandgap semiconductors, suggests that Er is not in SiO 2 or Si with bulk properties but, may be confined in Si light-emitting nanostructures. Porous Si is a good substrate for rare earth elements because: 1) a high concentration of optically active Er 3+ can be obtained by implanting at about 200 keV, 2) porous Si and bulk Si are transparent to 1.54 μm emission therefore, device fabrication is simplified, and 3) although the external quantum efficiency of visible light from porous Si is compromised because of self-absorption, it can be used to pump Er 3+ .

Displacive radiation-induced structural contraction in nanocrystalline ZrN

Nanocrystalline ZrN thin films with 5 nm grain size, prepared by ion beam assisted deposition, maintained their isometric structure upon intensive displacive and ionizing irradiations, indicating an extremely high stability similar to bulk ZrN. However, a unique structural contraction up to 1.42% in lattice parameter occurred only in nano-sized ZrN upon displacive irradiations. A significant nitrogen loss occurred with reduced N:Zr atomic ratio to 0.88, probably due to the production of displaced nitrogen atoms and fast diffusion along grain boundaries in nanocrystalline ZrN matrix. The accumulation of nitrogen vacancies and related strain relaxation may be responsible for the structural contraction.

Facile low temperature solid state synthesis of iodoapatite by high-energy ball milling

The apatite structure-type has been proposed as a potential waste form for the immobilization of long-lived fission products, such as I-129; however, it is difficult to synthesize iodoapatite without significant iodine loss due to its high volatility. In this study, we report a facile low temperature (∼50 °C) solid-state method for successfully synthesizing lead-vanadate iodoapatite by high-energy ball milling (HEBM) of constituent compounds: PbI2, PbO and V2O5. As-milled iodoapatite is in the form of an amorphous matrix embedded with nanocrystals and can be readily crystallized by subsequent thermal annealing at a low temperature of 200 °C with minimal iodine loss. Rietveld refinement of the X-ray diffraction patterns indicates that the thermally-annealed iodoapatite is iodine deficient with an iodine concentration of ∼4.2 at%. Thermal gravimetric analysis (TGA) indicates that low temperature annealing greatly improves the thermal stability and iodine confinement. This novel approach, using HEBM and thermal annealing, is a very promising method for synthesizing advanced materials that can confine highly volatile radionuclides, such as I-129, which pose significant challenges for the successful disposal of high-level nuclear waste.

Tailoring the radiation tolerance of vanadate–phosphate fluorapatites by chemical composition control

The apatite-type structure of AI4AII6(BO4)6(OH, F, Cl)2 (AI, AII = Ca, Na, rare earths, fission product elements such as I and Tc, and/or actinides; B = Si, P, V, or Cr) offers unique structural advantages as an advanced nuclear waste form because a wide variety of actinides and fission products can be incorporated into the structure through coupled cation and anion substitutions. However, apatite undergoes a radiation-induced crystalline-to-amorphous transition, and previously, the effect of composition on the radiation-induced transformation has not been well understood. In this study, we demonstrate that vanadate–phosphate fluorapatites radiation tolerance can be controlled by varying the composition. Enhanced radiation tolerance is achieved by replacing vanadium with phosphorus at the B-site or by replacing Pb with Ca at the A-site. Correlations among chemical composition, radiation performance and electronic to nuclear stopping power ratio were demonstrated and suggest that the ionization process resulting from electronic energy loss may enhance annealing of defects upon radiation damage.