Infiltration-growth of Superconductive REBCO Oxide and Simulation of Faceted Peritectic Crystal Growth

通过实验和分析研究了在空气或Ar-1%O_2气氛中渗透生长和单向凝固制备的超导REBCO（RE=Y,Nd）和REBCO/Ag氧化物中RE123（REBa_2Cu_3O_<7-x>）晶体的生长过程和结构，阐明了面形123晶体的生长和结构形成机理。将Ba-Cu-O（-RE,Ag）液相渗透到烧结好的多孔Y211（/Ag）或Nd422（/Ag）固体中，在包晶温度（Tp）下梯度冷却，得到无收缩、微观结构良好的样品。利用R-ΔT（过冷生长）和ΔT（新晶体形成）-Vc（更多冷却速率）关系，分析了宏观组织从柱状组织向等轴组织转变的临界条件和凝固过程中123相冻结前沿温度的变化，阐明了生长速率(R)、温度梯度(G)和试样尺寸（截面）的影响。采用二维数值方法模拟了123面晶在定向凝固过程中柱状和等轴结构等宏观组织的形成过程，并考虑了晶体取向和晶体在液体中的成核作用。利用变焦型光学显微镜高温原位观察装置，在MgO（100）上原位观察了多面RE123 （RD=Nd,Sm,Gd）晶体的成核和生长过程。测定了不同过冷条件下RE123晶体的成核和长大速率(u)和成核培养时间（t_< 0.05 >）。u近似为ΔTr函数：u=A_<gp>ΔTr^2/η(T)，其中A_<gp>为常数，ΔTr=ΔT/Tp, η(T)为粘度。成核速率（I_v）由成核晶体数目与时间的关系得到，用指数函数表示为：I_v ={A_n/η(T)}exp(-B/ΔTr^2)，其中an和B为常数。明确了u和Iv不仅依赖于ΔT，还依赖于RE123的Tp。少

Growth process and structures of RE123(REBa_2Cu_3O_<7-x>) crystals in superconductive REBCO(RE=Y,Nd) and REBCO/Ag oxides fabricated by infiltration-growth and unidirectional solidification in the atmosphere of air or Ar-1%O_2 were studied experimentally and analytically to clarify the growth and structure-formation mechanism of faceted 123 crystals. To get a sample without shrinkage and with fine microstructure by the infiltration-growth, Ba-Cu-O(-RE,Ag) liquid was infiltrated into sintered porous Y211(/Ag) or Nd422(/Ag) solid, and then cooled under the peritectic temperature(Tp) to grow 123 crystal under a temperature gradient. The critical transition conditions of macrostructures from columnar to equiaxed structures and the change in freezing front temperature of 123 phase during solidification were analyzed to clarify the effects of growth rate(R), temperature gradient(G) and sample size(cross section) by using R-ΔT(undercooling for growth) and ΔT(for formation of a new crystal)-Vc( … More cooling rate) relations.The formation processes of macrostructure such as columnar and/or equiaxed structures of faceted 123 crystals during unidirectional solidification were well simulated by two-dimensional numerical method, in which the effects of crystallographic orientations and the nucleation of crystals in the liquid were considered. Further, the nucleation and growth processes of faceted RE123 (RD=Nd,Sm,Gd) crystals were observed in-situ on MgO(100) by using a high temperature in-situ observation apparatus with zoom-type optical microscope. RE123 crystals nucleated and grew at various undercooling, and the growth rate (u) and incubation time (t_<inc>) for nucleation were measured. u was approximated by a function of ΔTr: u=A_<gp>ΔTr^2/η(T), where A_<gp> is a constant, ΔTr=ΔT/Tp and η(T) is viscosity.Nucleation rate (I_v) was obtained from the relationship between time and number of nucleated crystals, and it was approximated by an exponential function: I_V={A_n/η(T)}exp(-B/ΔTr^2), where An and B are constants. It was clarified that u and Iv depend on not only ΔT but also Tp of RE123. Less