Space Environment Simulation for Material Processing by Acoustic Levitation

Single-axis acoustic levitation of four polymer samples has been realized in air under the ground-based laboratory conditions for the purpose of space environment simulation of containerless processing.The levitation capabilities are investigated by numerical calculations based on a model of the boundary element method corresponding to our levitator and following Gor'kov and Barmatz's method.The calculated results,such as the resonant distance between the reflector and the vibrating source and the positions of levitated samples,agree well with experimental observation,and the effect of gravity on the time-averaged potential for levitation force is also revealed.As an application,the containerless melting and solidification of a liquid crystal,4-Pentylphenyl-4'-methybenzoate,is successfully accomplished,in which undercooling up to 16 K is obtained and the rotation and oscillation of the sample during solidification may result in fragmentation of the usual radiating surface growth morphology.

Solute Diffusion Controlled Dendritic Growth Under High Undercooling Conditions

Liquid Cu_(70)Co_(30) alloy is undercooled up to 363 K(0.22TL)by using glass-fluxing technique.The LKT/BCT dendritic growth theory is modified and made applicable to predict the kinetic characteristics of dendritic growth in those alloy systems which have extremely curved liquidus and solidus lines.Actual calculations reveal that the dendritic growth of primary α-Co phase in Cu_(70)Co_(30)alloy is constantly controlled by solute diffusion.The growth velocity of α-Co dendrite has a maximum value of 54.5mm/s at an undercooling of 333K.The calculated solubility of Cu in α-Co dendrite decreases with the increase of undercooling,which agrees well with the experimental data.Prior to peritectic transformation,the growth of α-Co dendrite is close to equilibrium solidification.

Ternary Eutectic Growth in Microgravity Environment

Rapid growth of Ag_(38.5)Cu_(33.4)Ge_(28.1) ternary eutectic alloy was accomplished in a 3m drop tube and its phase selection and growth mechanism were investigated.The experimental results revealed that the semiconductor phase(Ge)was the primary nucleating phase during solidification,which agrees with the calculated results of nucleation rate.The solid solution phase(Ag)and intermetallic compound phase η(Cu_(5)Ge_(2))grew cooperatively and lamellar structures similar to binary eutectic formed.Moreover,with the decreasing of droplet size,the growth morphology of primary(Ge)phase transformed from platelike to granular shape and a kind of anomalous ternary eutectic formed.The microgravity environment has a significant effect on the crystal growth process,which makes the(Ge)phase distribute homogeneously and the anomalous eutectic grains show good geometrical symmetry.The calculation of cooling rate versus droplet diameter showed that it was the high cooling rate and large undercooling that brought about the eutectic growth morphology transition.

Electrostatic Levitation of Plant Seeds and Flower Buds

We report the electrostatic levitation of various kinds of seeds and flower buds.Coral berry and pepper near a spherical shape show a stable levitation state.The prolate ellipsoid soybeau and flower buds are always “standing” in the free space with satisfactory levitation stabiiity.For the irregular mushroom and wheat grain,the levitation state is characterized as a “top-heavy” posture.These special stable equilibrium states are proved by the analysis of surface charge distribution.The obtained saturation polarization charge of samples presents a good accordance with experimental data.The levitation ability is weighed by thefactor m(ε_(r) + 2) / (εrD^(2)).

Crystal Growth in Al_(72.9)Ge_(27.1) Alloy Melt under Acoustic Levitation Conditions

The nonequilibrium solidification of liquid Al72.9Ge27.1 hypoeutectic alloy is accomplished by using single−axis acoustic levitation.A maximum undercooling of 112 K(0.16TL)is obtained for the alloy melt at a cooling rate of 50 K/s.The primary(Al)phase displays a morphological transition from coarse dendrite under a normal conditions to equiaxed grain under acoustic levitation.In the(Al)+(Ge)eutectic,the(Ge)phase exhibits a conspicuous branched growth morphology.Both the primary(Al)dendrites and(Al)+(Ge)eutectics are well refined and the solute content of the primary(Al)phase is extended under acoustic levitation.The calculated and experimental results indicate that the solute trapping effect becomes more intensive with the enhancement of bulk undercooling.

Hypercooling and the Specific Heat Capacity of Cu-Ni Alloy

The average heat capacities of the undercooled Cu-25%Ni,Cu-50%Ni,Cu-50%Ni and Cu-75%Ni melts were derived by using the glass fluxing technique.The undercoolings of the above alloys were 381,380,349 and 431K,respectively,which exceed the critical undercooling of the classical nucleation theory.A detailed analysis of the heat transfer condition during the solidification process was carried out,which suggested a linear relationship between the time duration of thermal arrest ta and the undercoolingΔT.The hypercooling points of the alloys,derived from the relationship between ta andΔT,were determined to be 457.7,461.1,448.4 and 528.K,respectively.