Haiming Lu

Ag Dendrite-Based Au/Ag Bimetallic Nanostructures with Strongly Enhanced Catalytic Activity

Dendritic Ag/Au bimetallic nanostructures have been synthesized via a galvanic replacement reaction (GRR) of Ag dendrites in a chlorauric acid (HAuCl4) solution. After short periods of time, one obtains structures with protruding flakes; these will mature into very porous structures with little Ag left over. The morphological, compositional, and crystal structural changes involved with reaction time t were analyzed by using scanning and transmission electron microscopy (SEM and TEM, respectively), energy-dispersive X-ray spectrometry (EDX), and X-ray diffraction. High-resolution TEM combined with EDX and selected area electron diffraction confirmed the replacement of Ag with Au. A proposed formation mechanism of the original Ag dendrites developing pores while growing Au flakes cover this underlying structure at longer reaction times is confirmed by exploiting surface-enhanced Raman scattering (SERS). Catalytic reduction of 4-nitrophenol (4-NP) by sodium borohydride (NaBH4) is strongly enhanced, implying promising applications in catalysis.

Size-dependent ordering and Curie temperatures of FePt nanoparticles

The analytic models for size-dependent ordering and Curie temperatures of FePt nanoparticles have been proposed in terms of the size-dependent melting temperature. It is found that the order-disorder transition temperature TO and Curie temperature TC decrease with decreasing the particle size D, and the drop becomes dramatic once the size decreases to about 3 and 6 nm below for TO and TC, respectively. Moreover, the suppression in TC(D) is nearly twice as large as that in TO(D) when D is in the range of 5–20 nm. The accuracy of the developed model is verified by the recent experimental and computer simulation results.

Thickness and grain size dependent mechanical properties of Cu films studied by nanoindentation tests

The hardness and the elastic modulus of Cu films with thickness (t) and grain size (d) have been investigated by nanoindentation tests. The d and the indentation depth increase linearly with the increase in t. The hardness rises with the decrease in t, whereas the elastic modulus is independent of t and it is about 20% less than conventional coarse-grained Cu. The enhanced hardness is attributed to the smaller d and the indentation depth. The analysis of load–displacement curves indicates that the scope of the critical shear stress for different thick Cu films ranges from 3.2 to 4.1 GPa, which is similar to the theoretical shear stress of single crystalline Cu. The present results are explained by the dislocation mediated mechanism even if d reaches about 16.4 nm for the Cu film with t = 180 nm.

Indentation size dependent plastic deformation of nanocrystalline and ultrafine grain Cu films at nanoscale

Nanoindentation creep tests were performed in the depth range from about 28 to 190 nm on nanocrystalline (NC) and ultrafine grain (UFG) Cu films. Pronounced indentation size effects on hardness, creep strain rate (e), and strain rate sensitivity (mc) are observed. Both e and mc are dependent not only on contact depth (hc) but also on grain size. The experiment results and analysis support that the creep deformation of NC and UFG Cu is dominated by grain-boundary-mediated process and diffusion along the interface of tip sample, respectively, under a critical hc and dislocation-mediated process begin to work as hc increases further.

Size-dependent Curie transition of Ni nanocrystals

The mechanical spectroscopy and magnetization measurements are performed on Ni nanocrystals from room temperature to 650 K. It is found that the peak temperatures of internal friction are in agreement with the corresponding Curie temperatures of Ni nanocrystals obtained from the magnetization-temperature curves, showing that the traditional mechanical spectroscopy can also be employed to investigate the Curie transition of ferromagnetic nanocrystals. Moreover, the analytical model for size-dependent Curie temperature is proposed in terms of a size-dependent melting temperature model. The Curie temperature drops with decreasing grain size in Ni nanocrystals, which agrees with the corresponding experimental results.

Order-disorder transition and Curie transition in Ni70Fe30 nanoalloy

This letter addresses the issue of the order-disorder and Curie transitions in Ni70Fe30 nanoalloy. The ordered phase is observed at room temperature while the disordered phase appears when the nanoalloy is heated up to 773 K. By means of mechanical spectroscopy, x-ray diffraction, and vibrating sample magnetometer measurements, the order-disorder and Curie transition temperatures of 20 nm Ni70Fe30 nanoalloy are determined to be 636 and 728 K, both lower than the corresponding values in the coarse-grained form. Moreover, the reduction in these two critical temperatures is consistent with the predictions of a thermodynamic analytical model.

Chemical ordering phase transitions in Ni–Fe nanoalloys

The chemical ordering phase transitions in Ni75Fe25 and Ni70Fe30 nanoalloys are investigated by differential scanning calorimetry (DSC), mechanical spectroscopy (MS), vibrating sample magnetometer (VSM) measurements and thermodynamical calculation. An internal friction peak occurs at 646 K in the Ni75Fe25 nanoalloy with an average grain size of 23 nm diameter during MS measurement. An exothermic peak appears during the DSC tests of nanoalloys. Associated with the results of thermodynamical prediction and VSM measurements, both the exothermic peak and the internal friction peak are convinced to be originated from chemical ordering phase transition. Compared with inefficacy of electron diffraction and x-ray diffraction, it is an effective route of employing DSC, MS, VSM and thermodynamical prediction in investigating the chemical ordering phase transitions in Ni–Fe nanoalloys.