BingBo Wei

Viscoelastic properties of silica nanoparticle monolayers at the air-water interface

We have investigated the rheological behaviour of silica nanoparticle layers at the air-water interface. Both compressed and deposited layers have been studied in Langmuir troughs and with a bicone rheometer. The compressed layers are more homogeneous and rigid, and the elastic response to continuous, step and oscillatory compression are similar, provided the compression is fast enough and relaxation is prevented. The deposited layers are less rigid and more viscoelastic. Their shear moduli deduced from the oscillatory uniaxial compression are much smaller than those deduced from pure shear deformation suggesting that the effective shear rate is smaller than expected in the compression measurements.

Influence of the contact angle of silica nanoparticles at the air–water interface on the mechanical properties of the layers composed of these particles

We have studied the properties (surface pressure, compression and shear moduli, texture) of silica nanoparticle layers at the air–water interface. Particle hydrophobicity or, equivalently, the contact angle between particles, air and water, is the main factor that influences surface organization and surface elastic moduli. The surface layers are denser for particles of higher hydrophobicity. The compression and shear moduli, as well as the yield and melt strains, present a maximum for contact angles around 90°. The dependence of the mechanical properties on particle hydrophobicity is closely related to the foamability and stability of the foams made from dispersions.

Measurement and calculation of surface tension for undercooled liquid nickel and its alloy

The surface tensions of metastable undercooled liquid nickel and its alloy are experimentally measured and theoretically calculated by electromagnetic levitation oscillating droplet method and molecular dynamics method, respectively. The experimental undercoolings for liquid Ni and Ni90.1Si9.9 alloy are 201 and 206 K, whereas the calculated undercoolings are up to 426 and 323 K. The measured surface tension displays the same undercooling dependence as the molecular dynamics calculation. The surface tension increases linearly with the increase in undercooling and no break occurs at the melting temperature. It is found that the correlation of surface tension with temperature predicted by molecular dynamics calculation agrees with the experimental results for both pure Ni and its alloy.

Remarkable solute trapping within rapidly growing dendrites

Solute microsegregation always takes place during dendritic crystal growth. Although this may be reduced with the increase of crystal growth velocity, the realization of segregationless dendritic growth is quite difficult. Here the authors present the results of remarkable solute trapping within the rapidly growing dendrites of highly undercooled liquid Ni–5wt%Si alloy. The dendrites grow at a velocity of 15m∕s at the maximum experimental undercooling of 304K. Such a high growth velocity results in the pronounced solute trapping and almost segregationless solidification. Furthermore, a model is proposed to describe the correlation between dendritic growth velocity and undercooling. It agrees well with the experimental results in the whole undercooling regime and provides a reasonable prediction for the dendritic growth trend under extremely great undercooling conditions.

Thermophysical properties of a highly superheated and undercooled Ni–Si alloy melt

The surface tension of superheated and undercooled liquid Ni–5 wt % Si alloy was measured by an electromagnetic oscillating drop method over a wide temperature range from 1417 to 1994 K. The maximum undercooling of 206 K (0.13 TL) was achieved. The surface tension of liquid Ni–5 wt % Si alloy is 1.697 N m−1 at the liquidus temperature 1623 K, and its temperature coefficient is −3.97×10−4 N m−1 K−1. On the basis of the experimental data of surface tension, the other thermophysical properties such as the viscosity, the solute diffusion coefficient, and the density of liquid Ni–5 wt % Si alloy were also derived.

Surface tension of superheated and undercooled liquid Co–Si alloy

The surface tension of superheated and undercooled liquid Co 25wt.% Si alloy was measured by an electromagnetic oscillating drop method. The experimental temperature regime was from 1384 to 2339K and a maximum undercooling of 223K (0.14TL) was achieved. The surface tension of liquid Co 25wt.% Si alloy is 1.604Nm−1 at the liquidus temperature of 1607K, and its temperature coefficient is −4.0×10−4Nm−1K−1. On the basis of previous research results of pure Co and Si, an expression is developed to predict the surface tension of binary Co–Si alloy system. The other thermophysical properties, such as the viscosity, the solute diffusion coefficient, and the density of liquid Co 25wt.% Si alloy are also derived by the relevant theoretical models.

Containerless rapid solidification of Ag28.1Cu41.4Ge30.5 ternary alloy

Abstract Droplets of Ag28.1Cu41.4Ge30.5 ternary alloy with different sizes are solidified during containerless processing in a 3 m drop tube. The experimental result shows that the microstructural evolution depends mainly on droplet size. Generally, droplet undercooling is inverse proportional to its diameter. When the droplet diameter is large, the (Ge) primary phase and intermetallic compound η phase show granular and strip shapes, respectively. In the medium-sized droplets, the (Ge) phase grows in a dendritic mode whereas η phase has little change. Meanwhile, both the grain size and interphase spacing also decrease with the reduction of droplet size. Moreover, it is found that η phase can nucleate directly from alloy melt and does not depend on primary (Ge) phase during the formation of binary eutectic structure. If the droplet diameter is smaller than 250 μm, a spherical anomalous eutectic grain structure forms, which consists of three very fine phases and is surrounded by coarse (Ge) and η phases. There is no evident feature of macrosegregation of semiconducting (Ge) phase and it distributes quite homogeneously, which is ascribed to the reduced gravity condition and rapid cooling in the drop tube.

PERITECTIC SOLIDIFICATION UNDER HIGH UNDERCOOLING CONDITIONS

The solidification characteristics of highly undercooled Cu-7.77% Co peritectic alloy has been examined by glass fluxing technique. The obtained undercoolings vary from 93 to 203 K(0.14 TL). It is found that the α(Co) phase always nucleates and grows preferentially, which is followed by peritectic transformation. This means that the peritectic phase cannot form directly, even though the alloy melt is undercooled to a temperature far below its peritectic point. The maximum recalescence temperature measured experimentally decreases as undercooling increases, which is lower than the thermodynamic calculation result owing to the actual non-adia-batic nature of recalescence process. The dendritic fragmentation of primary α (Co) phase induced by high undercooling is found to enhance the completion of peritectic transformation. In addition, the LKT/BCT dendrite growth model is modified in order to make it appllcable to those binary alloy systems with seriously curved liquidus and solidus lines. The dendrite growth velocities of primary α (Co) phase are subsequently calculated as a function of undercooling on the basis of this model.

Rapid monotectic solidification during free fall in a drop tube

Droplets of Ni-31.4%Pb monotectic alloy with different sizes are rapidly solidified during free fall in a drop tube. The theoretical calculations indicate that the undercooling was achieved before solidification exponentially depends on droplet diameter. The maximum undercooling of 241 K (0.15Tm) is obtained in the experiments. With the increase of undercooling, the volume fraction of monotectic cells increases, and the L2(Pb) grains are refined. Calculations of the nucleation rates of L2(Pb) and α-Ni phases indicate that L2(Pb) phase acts as the leading nucleation phase during the monotectic transformation.

Rapid dendritic growth within an undercooled Ni–Cu–Fe–Sn–Ge quinary alloy

A liquid quinary alloy with composition Ni–5%Cu–5%Fe–5%Sn–5%Ge has been prepared from a containerless state by undercooling. Dendritic growth of α-Ni phase took place with a velocity of 28 m s−1 at the maximum degree of undercooling, which was as high as 405 K (0.24T L). All of the four solute elements Cu, Fe, Sn and Ge exhibited a significant solute trapping effect during the rapid dendrite growth. Segregation-less solidification is consequently realized when the degree of undercooling is sufficiently large. The lattice constant of α-Ni solid solution phase is found to increase with the amount of multicomponent solute trapping.