快速加压引起的硒熔体结晶行为

开展了在513,523,533 K温度下硒熔体的快速压致凝固实验,分析了不同保温时间即0,30,60 min对凝固晶体尺寸及形貌的影响.发现随着保温时间的延长,晶粒不断发生聚集生长,晶粒尺寸变大.通过与相同温度、压力条件下非晶硒、超细晶体硒粉等温结晶的样品对比,否定了快压凝固结构为非晶硒、非晶硒晶化为晶体硒的可能性,认为硒熔体快压凝固的结构为晶体硒,在保温过程中晶体颗粒在晶界处可以发生聚集生长.分析发现熔体快速压致凝固法不能得到非晶硒的原因在于实验条件下非晶硒为不稳定相,非晶硒的晶化温度随压力关系在2 GPa前后表现出不同的变化趋势,推测压力对过冷液态硒的微观结构有影响.     

深过冷三元Ni-Cu-Co合金的快速枝晶生长

研究了深过冷条件下三元Ni₈₀Cu₁₀Co₁₀合金的快速枝晶生长, 采用电磁悬浮无容器处理方法获得了335 K(0.2T_L)的最大过冷度. X射线衍射分析与差示扫描量热分析均表明,凝固组织为α-Ni单相固溶体. 随过冷度增大, 凝固组织显著细化, 并且当过冷度达110 K时，凝固组织的形态由粗大形枝晶转变为等轴晶. 深过冷条件下溶质截留效应增强, 使得微观偏析程度减小. 对不同过冷度下合金枝晶的生长速度进 关键词： 深过冷 枝晶生长 快速凝固 溶质截留     

液态Ti-Al合金的深过冷与快速枝晶生长

采用电磁悬浮和自由落体两种试验技术研究了液态Ti-25 wt.%Al合金的亚稳过冷能力、晶体形核机制和枝晶生长过程. 试验发现, 即使电磁悬浮无容器状态下仍难以消除润湿角θ ≥60°的异质晶核, 合金熔体过冷度可达210 K (0.11T_L). β-Ti相形核的热力学驱动力随过冷度近似以线性方式增大, 其枝晶生长速度高达11.2 m/s, 从而在慢速冷却条件下实现了快速凝固. 理论计算表明, 随着过冷度的逐步增大, β相枝晶生长从溶质扩散控制转变为热扩散控制. 当过冷度超过100 K时, 非平衡溶质截留效应可使合金熔体发生无偏析凝固. 然而, 单靠深过冷状态不足以抑制β相的后续固态相变. 对于落管中快速凝固的直径77-1048 μm合金液滴, 其冷却速率最高达1.05×10⁵ K/s, 深过冷与快速冷却的耦合作用能更有效地调控凝固组织形成过程.     

液态五元Zr57Cu20Al10Ni8Ti5合金的微观结构演变与非晶形成机制

利用电磁悬浮无容器处理技术实现了液态五元Zr₅₇Cu₂₀Al₁₀Ni₈Ti₅合金的深过冷与快速凝固,同时通过分子动力学模拟计算揭示了非晶形成的微观机制.实验发现,凝固组织具有明显的核-壳结构特征,核区为非晶相,壳区主要由ZrCu, Zr₂Cu和Zr₈Cu₅晶体相组成.非晶体积分数随合金过冷度的升高逐渐增大,当达到实验最大过冷度300 K (0.26T_L)时,非晶体积分数增至81.3%.由此导出完全非晶凝固所需临界过冷度为334 K. TEM分析显示,过冷度增大并接近临界过冷度时,合金凝固组织中晶体相主要为Zr₈Cu₅相,而ZrCu和Zr₂Cu相的生长被抑制.在达到临界过冷度后,过冷液相的凝固路径由Zr₈Cu₅结晶生长转变为非晶凝固.此外,合金的晶体壳中存在少量的晶间非晶相,而非晶核中...     

微重力条件下Ni-Cu合金的快速枝晶生长研究

采用落管方法实现了Ni-50%Cu过冷熔体在微重力和无容器条件下的快速枝晶生长.对微重力条件下的晶体形核和快速生长进行了研究，发现随着过冷度的增大，晶体生长形态由粗大枝晶向规划均匀的等轴晶转变.实验中最大冷却速率达到8×103K/s，获得了218K(014TL)的最大过冷度.理论分析表明，过冷熔体中优先发生异质形核，形核率可达1012m-3s-1以上；Ni-50%Cu过冷熔体中的枝晶生长随过冷度的增大发生由溶质扩散控制向热扩散控制的生长动力学机理转变.在68K过冷度条件下，生长界面前沿的偏析程序最大. 关键词： 落管 微重力 深过冷 枝晶 熔体     

聚苯硫醚熔体的压致凝固行为

采用施加压力的方法将聚苯硫醚熔体凝固,凝固后获得的聚苯硫醚样品经过降温和卸压后在常温常压下回收. X射线衍射和差示扫描量热分析表明:约20 ms时间的快速压缩过程可以抑制熔体结晶,制备出非晶态聚苯硫醚块材,样品的表面及中心都是非晶态.非晶态聚苯硫醚的玻璃化转变温度和晶化温度分别为318和362 K.常压下的退火实验表明,非晶态聚苯硫醚在425 K等温结晶的产物为正交相晶型.压致凝固法中熔体的凝固不是靠温度变化,而是靠压力变化,样品表面和内部处在一致的温度下同时受压凝固,避免了热传导对非晶尺寸的影响,因此非常有利于获得结构均匀的大尺寸非晶态材料.     

Cu3Au熔体快速冷却过程微观结构演化的分子动力学模拟

利用分子动力学模拟技术研究了金属间化合物Cu3Au熔体的双体分布函数g(r)在快速凝固条件下随温度的变化情况，结果表明，Cu3Au降温至700K时第二峰已发生劈裂，液态金属中已经产生了非晶态；用键对分析技术详细考察了Cu3Au中微观组团随温度的演化特点，液体中的键对数及多面体数与温度的关系都表明，Cu3Au在向非晶转变的过程中，的确发生了微观结构组态的变化，其中以液体中的缺陷多面体随温度变化最为剧烈。非晶不是过冷液态的“冻结”。     

深过冷Cu60Sn30Pb10偏晶合金的快速凝固

实现了大体积Cu₆₀Sn₃₀Pb₁₀偏晶合金的深过冷与快速凝固. 实验获得的最大过冷度为173 K(0.17T_L). 凝固组织发生了明显的宏观偏析,XRD分析表明,试样上部是由固溶体(Sn),(Pb)相和金属间化合物ε(Cu₃Sn)相组成的三相区,下部为富(Pb)相区. 在小过冷条件下,三相区中ε(Cu₃Sn)相的凝固组织为粗大的枝晶,随着过冷度的增大,ε(Cu₃Sn)相细化成层片状组织,且层片间距随过冷度的增大而减小,而(Sn),(Pb)两相始终以离异共晶的方式存在. 富(Pb)相区中分布有少量的ε(Cu₃Sn)枝晶,枝晶长度随过冷度的增大而增大,且在大过冷条件下发生碎断. (Sn)相在ε(Cu₃Sn)相表面形核、长大,其形态类似于包晶凝固组织. 关键词： 深过冷 快速凝固 偏晶合金 层片组织     

Al-Mg合金熔体快速凝固过程中微观结构演化机理的模拟研究

采用分子动力学方法对Al₅₀Mg₅₀合金熔体的快速凝固过程进行了模拟,并采用双体分布函数、键型指数法和原子团类型指数法等方法,从微观结的不同层面对Al-Mg合金熔体快速凝固过程中微观结构的演化机理进行了深入的分析研究.结果表明：本模拟所获得的Faber-Ziman偏结构因子与实验结果符合较好.Al₅₀Mg₅₀合金熔体具有遗传性,在快速凝固过程形成了非晶态结构,其中二十面体短程序结构对非晶态结构的形成起决定性作用.基于原 关键词： Al-Mg合金熔体 快速凝固过程 分子动力学模拟 微观结构     

快速凝固Ti-Cu-Fe合金的相组成与组织演变规律

采用落管无容器处理技术实现了Ti_61.2Cu_32.5Fe_6.3三元包共晶合金在自由落体条件下的快速凝固,获得了直径为80—1120μm液滴的凝固组织.实验中获得的过冷度范围为34—293 K,最大过冷度达0.23T_L.研究发现,在自由落体条件下,由于受到无容器、微重力、超高真空等因素的影响,合金熔体的凝固组织中包含Cu_0.8Fe_0.2Ti相、CuTi₂相和CuT₃相,显著偏离了平衡状态.Cu_0.8Fe_0.2Ti为初生相,同时又与CuTi₂相形成两相共晶;CuTi₃相则呈现枝晶形貌,并发生了明显的溶质截留效应.随着过冷度的增大,共晶组织由层片共晶向不规则共晶转变,形貌由长条状共晶团变为椭球状共晶团,最终变为球状共晶胞;Cu_0.8Fe_0.2Ti相枝晶形貌由粗大枝晶变为碎断枝晶,进一步变成不规则的粒状晶粒;CuTi₃相枝晶则由碎块状转变为完整枝晶.     

Phase separation and rapid solidification of liquid Cu<Subscript>60</Subscript>Fe<Subscript>30</Subscript>Co<Subscript>10</Subscript> ternary peritectic alloy

The metastable liquid phase separation and rapid solidification of Cu₆₀Fe₃₀Co₁₀ ternary peritectic alloy were investigated by using the drop tube technique and the differential scanning calorimetry method. It was found that the critical temperature of metastable liquid phase separation in this alloy is 1623.5 K, and the two separated liquid phases solidify as Cu(Fe,Co) and Fe(Cu,Co) solid solutions, respectively. The undercooling and cooling rate of droplets processed in the drop tube increase with the decrease of their diameters. During the drop tube processing, the structural morphologies of undercooled droplets are strongly dependent on the cooling rate. With the increase of the cooling rate, Fe(Cu,Co) spheres are refined greatly and become uniformly dispersed in the Cu-rich matrix. The calculations of Marangoni migration velocity (V _M) and Stokes motion velocity (V _S) of Fe(Cu,Co) droplets indicated that Marangoni migration contributes more to the coarsening and congregation of the minor phase during free fall. At the same undercooling, the V _M/V _S ratio increases drastically as Fe(Cu,Co) droplet size decreases. On the other hand, a larger undercooling tends to increase the V _M/V _S value for Fe(Cu,Co) droplets with the same size. Supported by the National Natural Science Foundation of China (Grant Nos. 50121101 and 50395105) and the Scientific and Technological Creative Foundation of Youth in Northwestern Polytechnical University of China (Grant No. W016223)     

The formation and stability of quasicrystal in Al80Mn14Si6 under high static pressure

Abstract

The method of quenching in fusing state under high static pressure (MQFSHP) was applied for the first time to prepare the quasicrystal icosahedral phase of Al₈₀Mn₁₄Si₆ alloy. The pressure was from 2.8GPa to 3.1GPa and the cooling rate during quenching was of about 100°C/s. Some sharp electron diffraction spots showing an arrangement with a five-fold symmetry axis and noncrystalline ring have been observed in electron diffraction experiment.

The crystallization temperature of I phase obtained from high pressure(HP) is close to that of rapid cooling ribbon, but the cooling rate of the sample obtained is lower than that of rapid cooling ribbon.     

小注入下晶体管Ic-VBE特性的指数因子的研究

本文讨论了影响小注入下晶体管的l_c-V_BE特性的各种因素。作者发现发射结势垒区准费米能级降落的影响对于解释实验中发现的n^*≡d(V_BE/V_T)/dInl_c<1的情况是重要的。文中也给出了精确测量集成晶体管对的n^*值的差分方法。     

聚丁二烯中的等待时间效应

测量了高聚物材料聚丁二烯的比热,发现在温度T_g=178K出现玻璃转变且转变点附近的比热与降温过程有关。在降温过程中,若控制样品在某一温度T_wg等待时间t_w,则升温比热测量表明,T_g处的比热跃变△c_p存在明显的等待时间效应,即△c_p随t_w的增大而增大。在T_w=169K条件下,△c_p(t< 关键词：     

The D.C. voltage-dependent resistivity of CuCl powder compacts under pressure

The CuCl powder compacts are found to exhibit d.c. voltage dependent resistivity. The resistivity drops by six orders of magnitudes when the applied voltage exceeds a critical value V_c.V_c is found to decrease with the increase in pressure. The high conducting phase shows a positive temperature coefficient of resistance at 1 atm of 0.004°C^-1 which increases with pressure at a rate 0.0004°C^?1 GPa^?1. It is suggested that the pressure induced transition reported at 4 GPa could arise from the application of d.c. voltage.     

Note on the quantum displacement of the critical point

The quantum corrections to the law of corresponding states are studied by calculating the critical pressure, temperature, and density to first order in Planck's constanth on an exactly soluble model. The ratio of the critical parameters to the corresponding classical values are found to be (p _c/p _c ⁰)^1/2=_c/_c ⁰ = T_c/T_c ⁰ = 1–0.67, with=h _c ^1/3(mkT _c)^–1/2. The critical ratio is independent ofh to first order. The results are compared with critical data for noble gases and hydrogen isotopes.     

Single-crystal elasticity of the rhodochrosite at high pressure by Brillouin scattering spectroscopy

ABSTRACT

The sound velocity properties of single-crystal rhodochrosite (MnCO₃) were determined up to 9.7?GPa at ambient temperature by Brillouin scattering spectroscopy. Six elastic constants were calculated by a genetic algorithm method using the Christoffel's equations at each pressure. The elastic constants increased linearly as a function of pressure and its pressure derivatives ?Cij/?P for C₁₁, C₃₃, C₄₄, C₁₂, C₁₃, C₁₄ were 5.86 (±0.36), 3.82 (±0.44), 2.06 (±0.39), 5.07 (±0.27), 5.34 (±0.44), 1.52 (±0.24), respectively. Based on the derived elastic constants of rhodochrosite, the aggregate adiabatic bulk and shear moduli (K_s and G) were calculated using the Voigt-Reuss-Hill averages and the linear fitting coefficients (?K_s/?P)_T and (?G/?P)_T were 5.05(±0.26) and 0.73(±0.05), respectively. The aggregate V_p of rhodochrosite increased clearly as a function of pressure and its pressure derivative ?V_p/?P was 7.99(±0.53)?×?10^?2?km/(s?GPa), while the aggregate V_s increased slowly and ?V_s/?P was only 1.19(±0.12)?×?10^?2?km/(s?GPa). The anisotropy factor for As of rhodochrosite increased from ～40% at 0.8?GPa to ～48% at 9.7?GPa, while A_p decreased from ～19% to ～16% at the corresponding pressure.     

High-pressure study of the non-Fermi liquid material U 2 Pt 2 In

The effect of hydrostatic pressure ( p ? 1.8 GPa) on the non-Fermi liquid state of U₂Pt₂In is investigated by electrical resistivity measurements in the temperature interval 0.3-300 K. The experiments were carried out on single-crystals with the current along ( I || c ) and perpendicular ( I || a ) to the tetragonal axis. The pressure effect is strongly current-direction dependent. For I || a we observe a rapid recovery of the Fermi-liquid T²-term with pressure. A comparison of the data with the magnetotransport theory of Rosch provides evidence for the location of U₂Pt₂In at an antiferromagnetic quantum critical point. For I || c the resistivity increases under pressure, indicating the enhancement of an additional scattering mechanism. Received 2 March 2001 and Received in final form 29 June 2001     

Ferroelastic phase transition and anomalous elastic and thermal properties of betaine borate (CH3)3NCH2COO·H3BO3

The temperature and pressure derivatives of the elastic constants of orthorhombic betaine borate, (CH₃)₃NCH₂COO·H₃BO₃, have been determined by measuring temperature and stress induced shifts of resonance frequencies of thick plates at ca. 15 MHz in the range between 140 and 300 K and 0 and 3 kbar. The elastic ‘shear’ resistance c₄₄ exhibits a value as low as 0.0492×10¹⁰Nm^-2at 293 K. With decreasing temperature c₄₄ approaches zero at ca. 142.5 K, indicating an acoustic soft mode behaviour connected with a ferroelastic phase transition. The softening of c₄₄ is described in a good approximation by c₄₄(T)_p=0 =alogT/T₀ with a=0.0663×10¹⁰Nm^-2 and T₀ = 139.5 K. Further, c₄₄ decreases with increasing pressure according to the linear relation c₄₄(p)_{T=293 K} = 0.0492?0.184×10^-4p (p in bar, c₄₄ in 10¹⁰ Nm^-2). All other elastic constants show a quite normal temperature and pressure dependence. At 293 K the transition is induced by a pressure of 2.65 kbar. The transition temperature T_c depends linearly on pressure according to T_c = 142.5+0.0568 p (pinbar, T_cinK). Passing through the transition no discontinuous change of the lattice constants is observed. The three principal coefficients of thermal expansion and the pressure derivatives of the dielectric constants exhibit discontinuities at the transition. The transition is of strongly second order.