Zhan-Guo Liu

The influences of particle number on hot spots in strongly coupled metal nanoparticles chain.

In understanding of the hot spot phenomenon in single-molecule surface enhanced Raman scattering (SM-SERS), the electromagnetic field within the gaps of dimers (i.e., two particle systems) has attracted much interest as it provides significant field amplification over single isolated nanoparticles. In addition to the existing understanding of the dimer systems, we show in this paper that field enhancement within the gaps of a particle chain could maximize at a particle number N>2, due to the near-field coupled plasmon resonance of the chain. This particle number effect was theoretically observed for the gold (Au) nanoparticles chain but not for the silver (Ag) chain. We attribute the reason to the different behaviors of the dissipative damping of gold and silver in the visible wavelength range. The reported effect can be utilized to design effective gold substrate for SM-SERS applications.

Near-field laser parallel nanofabrication of arbitrary-shaped patterns

The authors present a simple and efficient technique for laser writing of arbitrary nanopatterns across a large surface area without using projection masks. It is based on the unique near-field focusing effect of a self-assembled particle array on the surface interacting with an angular incident laser beam. The spot resolution can be down to 80nm. More than 6×106 nanolines and c-shaped uniform patterns were fabricated simultaneously over an area of 5×5mm2 by a few laser shots.

Laser micro/nano fabrication in glass with tunable-focus particle lens array

Based on medium-tuned optical field enhancement effect around a self-assembled particle-lens array (PLA) irradiated with a femtosecond (fs) laser source, we demonstrated that high-precision periodical array of micro/nano-structures can be readily fabricated on glass surface or inside glass in large areas in parallel without any cracks or debris. The technique has potential for rapid fabrication of three-dimensional structures in multiple layers inside glass.

Order–disorder transition and thermal conductivity of (Yb x Nd1− x )2Zr2O7 solid solutions

X-ray diffraction, transmission electron microscopy and a laser-flash method were used to investigate the order–disorder transition and thermal conductivity of (Yb x Nd1− x )2Zr2O7 (0 ≤ x ≤ 1.00) solid solutions. The structures were found to be pyrochlore-type for 0 ≤ x ≤ 0.25, defect fluorite for 0.45 ≤ x ≤ 1.00 and a mixture of these at 0.30 ≤ x ≤ 0.40. The thermal conductivities of (Yb x Nd1− x )2Zr2O7 first gradually decrease with increasing temperature, and then increase slightly above 800°C due to the increased radiation contribution. YbNdZr2O7 has the lowest thermal conductivity due to the reduced cation mean free path at the compositional combination of equal molar Yb3+ and Nd3+ cations.

Large-area laser nano-texturing with user-defined patterns

Writing nano-sized features less than the diffraction limit of the lasers efficiently over a large area requires special technology development. This paper reports the use of a self-assembled particle lens array with near-field enhancement effect to write millions of nano-sized user-defined features, e.g. English letters, lines, curves, simultaneously by angular beam scanning. About a 5 mm × 5 mm area can be written with a single shot of a laser beam or few scans for up to 100 million identical features of nano or sub-micro scales. With the help of certain environmental conditions, such as the use of a suitable chemical solution in conjunction with the particle lens array, the characteristic of the features produced can be further controlled, including the generation of reversed (e.g. pits become hills and grooves become walls) features of laser-written patterns. The technical challenges, experimental findings and theoretical analysis/simulation are presented.

Crack-free surface sealing of plasma sprayed ceramic coatings using an excimer laser

Yttria stabilized zirconia coatings are typically used in the aerospace industry as high-temperature thermal barriers. These coatings are normally applied by plasma thermal spray, which has an inherent problem of producing coatings containing a substantial amount ofopen or closed porosity. Surface sealing of plasma sprayed ceramic coatings with CO 2 and Nd:YAG lasers is always associated with the problem of cracking on melted layers. Although some attempts such as pre-heating have been used to overcome the problem, formation of cracking is still not prevented, especially in zirconia-based ceramic coatings. The present work investigates an alternative method of surface sealing of plasma sprayed 8 wt.% Y 2 O 3 -ZrO 2 coatings using an excimer laser. The results show that smooth, crack-free and crater-free sealing can be obtained. Effects of laser operating parameters on the sealing quality and involved mechanism are also discussed.

Preparation and Improved Grain Conductivity of ( Sm1 − x Y x ) 2Zr2O7 Ceramics

(Sm 1-x Y x ) 2 Zr 2 O 7 (0 ≤ x ≤ 0.5) ceramics were prepared by a solid-state reaction process at 1973 K for 10 h in air. (Sm 1-x Y x ) 2 Zr 2 O 7 (0 ≤ x ≤ 0.3) ceramics have a single phase of the pyrochlore-type structure, whereas (Sm 1-x Y x ) 2 Zr 2 O 7 (x = 0.4,0.5) exhibit a defective fluorite-type structure. The full width at half-maxima in the Raman spectra increase with increasing Y content, which indicates that the degree of structural disorder increases as the yttrium content increases. The electrical conductivity of (Sm 1-x Y x ) 2 Zr 2 O 7 ceramics was investigated by electrochemical impedance spectroscopy over a frequency range of 1 Hz to 20 MHz in the temperature range of 623-1173 K in air and hydrogen atmospheres. The electrical conductivity of (Sm 1-x Y x ) 2 Zr 2 O 7 obeys the Arrhenius-type behavior and increases with increasing temperature. The grain conductivity and total conductivity of pyrochlore-type materials are clearly higher than those of defective fluorite-type materials in the intermediate temperature range of 623-1073 K. Both activation energy and pre-exponential factor for grain conductivity increase with increasing Y content. The electrical conductivity measured in the reducing atmosphere of hydrogen shows no significant changes, which proves that the conduction is purely ionic with negligible electronic contribution in the series.

Improvement of Corrosion Performance of HVOF MMC Coatings by Laser Surface Treatment

Abstract In order to improve corrosion performance of T800 (Co–Mo–Cr–Si) and T800 based WC HVOF sprayed coatings, laser surface modification was carried out using a high power diode laser to eliminate/reduce the microstructral defects of the coatings by precise control of treatment depth with or without melting. Characterisation of the laser modified surfaces was conducted, in terms of microstructural morphology and phase analysis. Corrosion behaviours of the coatings before and after laser treatment were evaluated using electrochemical impedance spectroscopy and immersion tests. The results indicated that the corrosion resistance of the HVOF coatings can be improved as a result of laser densification of the coatings by elimination of discrete splat structure and porosity. However, the improvement of resistance to microgalvanic corrosion between the WC and Co matrix after laser treatment can be limited depending on the extent of melting occurred to the WC within the coatings.

Coprecipitation synthesis and sintering property of (YbxSm1-x)2Zr2O7 ceramic powders

Abstract Abstract (YbxSm1-x)2Zr2O7 (0<x<1·0) ceramic powders were synthesised with chemical coprecipitation and calcination method. Thermal decomposition behaviour of precipitates was studied by differential scanning calorimetry-thermogravimetry. The powders were characterised by X-ray diffractometry, scanning electron microscopy and transmission electron microscopy with energy dispersive spectroscopy. The synthesised powders have a particle size of about 100 nm, and exhibit to a certain extent agglomeration. The sintering behaviour of (YbxSm1-x)2Zr2O7 powders was studied by pressureless sintering method at 1550-1700°C for 10 h in air. The relative densities of (YbxSm1-x)2Zr2O7 ceramics increase with increasing sintering temperature, and reach above 95% when sintered at 1700°C for 10 h in air. Sm2Zr2O7 and (Yb0·1Sm0·9)2Zr2O7 ceramics have a pyrochlore structure; however, (YbxSm1-x)2Zr2O7 (0·3<x<1·0) ceramics exhibit a defective fluorite type structure.

Influences of lattice vibration and electron transition on thermal emissivity of Nd3+ doped LaMgAl11O19 hexaaluminates for metallic thermal protection system

Normal spectral emissivity of La1−xNdxMgAl11O19 (x = 0.05, 0.10, 0.15, 0.25) ceramics has a maximum value at the Nd3+ content of x = 0.10 in the wavelength range of 3–6 μm. The difference in emissivity at short wavelengths of 3–6 μm is tightly linked to the absorption transitions from 4I9/2 to 4F3/2, 4I15/2, and 4I13/2 and the intensities of radiation transitions from 4F3/2 to 4I11/2 and 4I13/2 of Nd3+ in LaMgAl11O19 ceramics. However, in the wavelength range of 6–14 μm, the emissivity is quite similar for all the samples, which can be ascribed to the stretching vibration of Al-O tetrahedron and Al-O octahedron.