Qing Jiang

Experimental and modelling investigations on strain rate sensitivity of an electrodeposited 20 nm grain sized Ni

Two experimental techniques of nanoindentation and tensile testing were used at room temperature to investigate the strain rate sensitivity of an electrodeposited Ni with a mean grain size (d) of 20 nm, respectively. It was found that the nanocrystalline (nc) Ni possessed a higher strain rate sensitivity exponent (m) during nanoindentation than during tensile testing. Furthermore, a higher m was accompanied by a smaller activation volume (V ). It is believed that the higher stress concentration could activate a shorter dislocation line length (L), which should be responsible for the higher m value during the nanoindentation. Based on a model of dislocation nucleation or bowing-out mechanism, the relationship between m and d for Ni and its alloys was investigated. In the end, a simple and straightforward equation relating m to d was proposed in aid of a simple assumption associating L with d, which implied that the enhanced m in nc Ni and its alloys with d> ∼ 6 nm should be due to the reduction of the dislocation line length. (Some figures in this article are in colour only in the electronic version)

Strain rate sensitivity of a nanocrystalline Cu synthesized by electric brush plating

A method for synthesizing bulk nanocrystalline Cu by an electric brush-plating technique is reported. This brush-plated nanocrystalline Cu has a fine (26nm) and quite uniform grain structure and predominant high-angle grain boundaries. A pronounced strain rate sensitivity of the stress with an m of 0.104 and the Coble creep and a subsequent transition to the power-law creep were observed in room temperature tensile and creep tests. The dominant grain boundary deformation due to the truly nanocrystalline structure of this nanocrystalline Cu is responsible for the observed strain rate sensitivity.

Effect of grain size on corrosion behavior of electrodeposited bulk nanocrystalline Ni

Nanocrystalline (NC) and coarse-grained Ni with different grain sizes (from 16 nm to 2 μm) were fabricated by direct current electrodeposition. Effect of grain size on the electrochemical corrosion behavior of these Ni deposits in different corrosion media was characterized by using potentiodynamic polarization test, electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and immersion corrosion test. Results show that in the NaOH or NaCl solution, the NC Ni exhibits improved corrosion resistance with the decrease of grain size. But in H2SO4 solution, the higher grain boundary density accelerates corrosion due to no passive process and the corrosion resistance of NC Ni decreases with refining grain size. The distinct experimental results of NC Ni in corrosion behavior can be reasonably explained by the positive or negative effect of high-density grain boundaries in different corrosion media.

Strain rate sensitivity of face-centered-cubic nanocrystalline materials based on dislocation deformation

The relationship between strain rate sensitivity and activation volume for face-centered-cubic metals is proposed based on the bow-out model of single dislocation from its source, which gives reasonable prediction of the enhanced strain rate sensitivity that occurs in nanostructured and ultrafine grained Ni and Cu.

Influence of preparation methods on photoluminescence properties of ZnO films on quartz glass

The influence of preparation methods on the photoluminescence properties of ZnO film was studied. Two methods were applied to fabricate ZnO films in a conventional pulsed laser deposition apparatus. One is high temperature (500−700 ℃) oxidation of the metallic zinc film that is obtained by pulsed laser deposition. The other is pulse laser ablation of Zn target in oxygen atmosphere at low temperature (100−250 ℃). The photoluminescence property was detected by PL spectrum. The room temperature PL spectra of the ZnO films obtained by oxidation method show single violet luminescence emission centered at 424 nm (or 2.90 eV) without any accompanied deep-level emission and UV emission. The violet emission is attributed to interstitial zinc in the films. Nanostructure ZnO film with c-axis (002) orientation is obtained by pulsed laser deposition. The ZnO film deposited at 200 ℃ shows single strong ultraviolet emission. The excellent UV emission is attributed to the good crystalline quality of the film and low intrinsic defects at such low temperature.

Study of the electrodeposition of Al-Mn amorphous alloys from molten salts

Abstract Al–Mn amorphous plating layers were electrodeposited in AlCl 3 –NaCl–MnCl 2 . The effects of current efficiency and temperature and manganese content on the structure of plating layers are discussed. The results show that the structure of deposition is face-centred cubic aluminium solution when the manganese content is smaller than 17%, amorphous phase when the manganese content is between 25% and 45%, and a mixture of amorphous and Al 8 Mn 5 intermetallic when the manganese content is larger than 45%. The Al–Mn amorphous plating layers can be obtained when the current density is between 20 and 50xa0mA/cm 2 under temperatures between 180 and 250xa0°C. The corrosion resistance of Al–Mn amorphous plating layer is about four times that of aluminium layers.

Deformation mechanism transition caused by strain rate in a pulse electric brush-plated nanocrystalline Cu

Bulk nanocrystalline Cu was synthesized by a pulse electric brush-plating technique. A very large strength (at 2% plastic strain) increase from 644 to 1451 MPa was obtained by compression tests at room temperature and strain rates from 1×10−5 to 3×100u2002s−1. A transition in plastic deformation mechanism with strain rate from a combination of the thermally activated grain boundary sliding and the dislocation emission-absorption in grain boundaries to one dominated by the dislocation activity has been revealed by the significant changes in strain rate sensitivity and apparent activation volume with strain rate.

Re-examination of Casimir limit for phonon traveling in semiconductor nanostructures

A simple and unified model is developed to predict the effective phonon mean free path (MFP) of semiconductor nanostructures based on Casimir’s work [Physica (Amsterdam) 5, 495 (1938)]. The effective MFP of nanofilms is found to be larger than that of nanowires, where the Casimir limit for nanofilms equals twice its thickness, or two times of the limit for nanowires. The theoretical formula agrees approximately with available experimental and computer simulation results for heat conduction along semiconducting nanowires, nanofilms, and superlattices.

Mechanical behavior of an electrodeposited nanostructured Cu with a mixture of nanocrystalline grains and nanoscale growth twins in submicrometer grains

A nanostructured Cu with a mixture of nanocrystalline grains and nanoscale growth twins in submicrometer grains was synthesized by electrodeposition, and its mechanical behavior was tested on MTS-810 system at different strain rates and room temperature. It exhibited high yield stress of 506–717 MPa and good ductility of 6.2%–9.1%. Two stage strain rate sensitivity (m value), different fracture directions, and different morphologies of deformed and fracture surfaces at high strain rates and low strain rates were observed in the nanostructured Cu, which demonstrated the deformation mechanism transition from dislocation deformation at higher strain rates to both dislocation deformation dominated in larger grains and grain boundary diffusion and grain boundary sliding dominated in small grains at low strain rates.

Compressive creep behavior of an electric brush-plated nanocrystalline Cu at room temperature

Creep tests were conducted on a nanocrystalline Cu at room temperature. The results at very low strain rates (<4×10−8u2002s−1) are consistent with Coble creep. An overall view of stress-strain rate behavior of this nanocrystalline Cu indicates that as the strain rate decreases, the deformation mechanism transition from predominantly dislocation activity to diffusion Coble creep, as evidenced by the strain rate sensitivity on stress trending to m=1 and activation volume trending to υ=1.5b3. The typical strain rate sensitivity of m=0.5 for surperplasticity can hardly be obtained at such low homogenous temperature.