调幅分解及形核对相分离作用机理研究

以SCN-30wt%H₂O, SCN-50 wt%H₂O和SCN-80 wt%H₂O三组透明体系, 在恒温场下实现了形核和调幅分解两种过程; 在此基础上, 施加温度梯度, 研究了第二相液滴的迁移运动规律. 结果表明, 相分离在临界成分体系以调幅分解方式进行, 在另外两种体系中以形核长大方式进行; 调幅分解与形核过程相比, 反应进行得更快, 液滴长大到同一尺寸所需时间仅为形核所需时间的1/3—1/2. 且临界成分体系有更大的不混溶间隙, 所以第二相液滴具有更多迁移时间, 揭示了偏晶体系相分离过程中在临界成分处易获得壳-核组织的内在机理. 在单向温度场中, 测量了不同半径的液滴迁移速率, 并且与理论Marangoni迁移速率值作比较, 发现液滴迁移速率和Marangoni理论迁移速率符合较好. 说明了在较好地抑制自然对流条件下Marangoni迁移对于相分离过程起主要作用.     

快速凝固Fe62.1Sn27.9Si10合金的分层组织和偏晶胞形成机理

采用自由落体和单辊急冷技术研究了三元Fe_62.1Sn_27.9Si₁₀偏晶合金的相分离和组织形成规律,理论分析了两种快速凝固条件下合金的传热特性.自由落体条件下,由于Marangoni迁移和表面偏析势的作用,液滴凝固组织主要形成富Sn相包裹富Fe相的两层壳核结构.随着液滴直径减小,冷却速率和温度梯度增大,促进偏晶胞快速生长.在单辊急冷条件下,随着辊速的增大,冷却速率从1.1×10⁷增大至6.5×10⁷ K/s,合金熔体内部的液相流动和相分离受到抑制,凝固组织发生"九层结构→两层结构→无分层结构"的转变.同时,凝固过程中FeSn+L₂→FeSn₂包晶反应受到抑制,形成与自由落体条件下不同的相组成.EDS分析显示,αFe相在快速凝固过程中发生显著溶质截留效应. 关键词： Fe-Sn-Si偏晶合金 相分离 快速凝固 溶质截留     

三元Co-Cu-Pb偏晶合金的快速凝固组织形成规律研究

在自由落体条件下实现了三元Co-Cu-Pb合金的液相分离与快速凝固. 实验发现,随液滴直径减小,Co₅₁Cu₄₇Pb₂合金液滴发生由枝晶→两层壳核→枝晶组织的转变,Co₄₇Cu₄₄Pb₉合金液滴的组织形态由壳核组织演化为均匀组织. 两种合金的快速凝固组织均由α(Co),(Cu)和(Pb)固溶体三相组成,α(Co)和(Cu)相主要以枝晶方式生长,(Pb)相分布在(Cu)枝晶间. 关键词： 液相分离 偏晶合金 快速凝固 自由落体     

液态三元Fe-Sn-Si/Ge偏晶合金相分离过程的实验和模拟研究

本文采用自由落体实验技术和格子玻尔兹曼计算方法研究了低重力条件下液态Fe-Sn-Si/Ge合金的相分离过程. 实验发现, 二种合金液滴在自由下落过程中均发生显著的液相分离, 形成了壳核和弥散组织. 当Fe-Sn-Si合金中的Si元素被等量的Ge元素替换后, 壳核组织中富Fe区和富Sn区的分布次序会发生反转. 计算表明, 在液相分离过程中冷却速率、Marangoni对流和表面偏析对壳核构型的选择和弥散组织的形成起决定性作用.     

无容器条件下Cu-Pb偏晶的快速生长

在落管无容器条件下实现了Cu-10%Pb亚偏晶和Cu_374%Pb偏晶的快速生长. 发现随着液滴 过冷度的增大, 亚偏晶中初生(Cu)相的生长形态经历“粗大枝晶→碎断枝晶→等轴晶”的转 变. 偏晶的组织形态从多个偏晶胞组织演化为单个偏晶胞组织. 理论计算表明，直径在1000 —60 μm之间的亚偏晶和偏晶合金液滴, 最大过冷度分别为269 K (02 TL_L )和245 K (02 TM_M). 亚偏晶合金中初生(Cu)枝晶的最大生长速度为24 m/s , 关键词： 无容器处理 深过冷 晶体生长 相分离     

偏晶合金液-液相分离的格子玻尔兹曼方法模拟

本文建立了一个模拟在弥散相液滴的扩散长大、碰撞凝并和Ostwald熟化等因素的作用下偏晶合金液-液相分离过程的二维格子玻尔兹曼方法 (lattice Boltzmann method, LBM) 模型.该模型结合了Shan-Chen的两相流模型和Qin的介观粒子相互作用势模型的优点,并在LB演化方程中引入了反映相变的源项.应用该模型模拟研究了偏晶合金液-液相分离过程中单液滴的生长、两液滴的合并和多液滴的生长规律.结果表明在两液相区中第二相单个液滴的生长是一个通过扩散从非平衡态到平衡态过渡的过程.两液滴合并 关键词： 偏晶合金 液-液相分离 格子玻尔兹曼方法     

急冷条件下Cu-Pb偏晶合金的相分离研究

研究了Cu-Pb偏晶合金的急冷快速凝固和组织形成规律，并通过将金属熔体的热传导方程和Navier-Stokes方程相耦合, 理论分析了合金熔体的冷却速率、液固相变时间等物理参量与液相分离之间的相关性. 研究结果表明，在急冷快速凝固条件下，熔体的快速冷却对偏晶合金组织形成的影响要比熔体内部液相流动的影响更为显著. 快速凝固使液相分离受到抑制，Cu-Pb偏晶合金均可获得均匀的微观组织结构. 随着冷速的增大，晶粒尺寸明显减小，凝固组织显著细化，晶体形态由粗大枝晶向均匀细小的等轴晶过渡. 提高冷却速率，缩短液固相变时间是重力场中抑制液相分离、获得均匀偏晶组织结构的重要条件. 关键词： 偏晶合金 快速凝固 液相分离 微观结构     

急冷快速凝固过程中液相流动与组织形成的相关规律

研究了Fe58wt%Sn过偏晶合金的急冷快速凝固和组织形成特征． 实验发现, FeSn过偏晶合金的急冷快速凝固组织由规则排布的纤维状β-Sn相和分布其间的α-Fe相及少量金属间化合物相组成, β-Sn相的几何排列方向与合金条带表面成0—15°的夹角．根据急冷条件下金属熔体的热传导方程和Navier-Stokes方程, 对过偏晶合金的凝固行为和组织形成过程进行了理论分析, 揭示出熔体内部的动量传输对过偏晶合金的液相分离行为具有显著的影响．两相分离发生于液池底部约200μm的急冷区内, 分离的L2液滴在辊面驱 关键词： 液态 相分离 液相流动 快速凝固 晶体生长     

气泡/液滴Marangoni迁移数值模拟研究

本文利用Level Set方法,数值模拟了微重力情况下气泡/液滴的Marangoni迁移现象,分析了Marangoni数对迁移速度的影响。数值模拟结果表明,随着Marangoni数的增大,非线性热对流效应的影响会逐渐增大,导致沿相界面温度分布趋于均匀,从而降低迁移运动的驱动力,使气泡/液滴的迁移速度随Marangoni数的增加而逐渐减小。     

电磁复合场制备匀质Zn-Bi偏晶合金的物理模拟

本文通过物理模拟的方法研究了强磁场复合交变电流作用对过偏晶合金凝固组织的影响机理,考察了电磁复合场下第二相初生液滴直径的演变规律,以及洛伦兹力作用下已分层少数相被"打碎"进入基体相的过程.结果表明:电磁体积力的增加,有利于液滴初始直径的减小;而交变电流频率的影响存在最佳值,偏离该值对初生液滴直径的抑制效果下降,这与实际试验规律一致;在电磁复合场作用下,已经分层的两液相可以重新实现均匀混合.这些效应有效缓解了偏晶合金凝固前第二相由于各种因素导致的偏聚,模拟结果直观地揭示了电磁复合场制备匀质偏晶合金的主要机理 关键词： 电磁复合场 偏晶合金 物理模拟     

Solidification mechanism transition of liquid Co–Cu–Ni ternary alloy

We report a solidification mechanism transition of liquid ternary Co₄₅Cu₄₅Ni₁₀ alloy when it solidifies at a critical undercooling of about 344 K. When undercooling at ΔT<344 K, the solidification process is characterized by primary S (Co) dendritic growth and a subsequent peritectic transition. The dendritic growth velocity of S (Co) dendrite increases with the rise of undercooling. However, once ΔT>344 K, the solidification velocity decreases with the increase of undercooling. In this case, liquid/liquid phase separation takes place prior to solidification. The minor L₂ (Cu) droplets hinder the motion of the solidification front, and a monotectic transition may occur in the major L₁ phase. These facts caused by metastable phase separation are responsible for the slow growth at high undercoolings.     

High P–T Brillouin scattering study of H2O melting to 26 GPa

The properties of solid and liquid phases of H₂O at high pressure and temperature remain an active area of research. In this study, Brillouin spectroscopy has been used to determine the temperature dependence of sound velocities in H₂O as a function of pressure up to 26 GPa through the phase field of ice VII and into the liquid to a maximum temperature of 1200 K. The Brillouin shift of the quasi-longitudinal acoustic mode moves to lower frequencies upon melting at each pressure. As a test of the method, measurements of the melting of Ar by Brillouin scattering at several pressures show a similar behavior for the acoustic mode, and measured melting points are consistent with previous results. The results of H₂O melting are consistent with previously reported melting curves below 20 GPa. The data at higher pressure indicate that ice melts at a higher temperature than a number of previous studies have indicated.     

Rapid solidification and dendrite growth of ternary Fe-Sn-Ge and Cu-Pb-Ge monotectic alloys

The phase separation and dendrite growth characteristics of ternary Fe-43.9%Sn- 10%Ge and Cu-35.5%Pb-5%Ge monotectic alloys were studied systematically by the glass fluxing method under substantial undercooling conditions. The maximum undercoolings obtained in this work are 245 and 257 K, respectively, for these two alloys. All of the solidified samples exhibit serious macrosegregation, indicating that the homogenous alloy melt is separated into two liquid phases prior to rapid solidification. The solidification structures consist of four phases including α-Fe, (Sn), FeSn and FeSn2 in Fe-43.9%Sn-10%Ge ternary alloy, whereas only (Cu) and (Pb) solid solution phases in Cu-35.5%Pb-5%Ge alloy under different undercoolings. In the process of rapid monotectic solidification, α-Fe and (Cu) phases grow in a dendritic mode, and the transition "dendrite→monotectic cell" happens when alloy undercoolings become sufficiently large. The dendrite growth velocities of α-Fe and (Cu) phases are found to increase with undercooling according to an exponential relation.     

Phase equilibria in the system LiOH–H<Subscript>2</Subscript>O at 350–425°C

The system LiOH—H₂O is characterized by immiscibility phenomena both in equilibrium with vapor and solid saturated solutions or without the solid phase. The solubility of LiOH increases as a result of immiscibility phenomena at ~355°C and further increases above this temperature up to the melting point. The immiscibility region (g–l₁–l₂) is terminated by the critical phenomena (g = l₁–l₂) at ~390°C. A separation of liquid phases is observed at elevated pressures (at ~39 MPa) and temperatures up to 425°C.     

Statistic characteristics of the gas-liquid flow in a vertical minichannel

The gas-liquid upward flow was studied in a rectangular minichannel of 1.75×3.8 mm and length of 0.7 m. The experiments were carried out within the range of the gas superficial velocity from 0.1 to 10 m/s and the liquid superficial velocity from 0.07 to 0.7 m/s for the co-current H₂O/CO₂ flow under the conditions of saturation. The method for the two-beam laser scanning of structure and determination of statistic characteristics of the two-phase flow was worked through. The slug-bubble, slug, transitional, churn, and annular flows were distinguished. The statistics characteristics of liquid and gas phases motion in a minichannel were obtained for the first time including the velocities of phase motion.     

Two-dimensional condensation of water and alcohols on NaF

On the surface of NaF the adsorption isotherms of H₂O, D₂O, CH₃OH, C₃H₃OH and 1-C₃H₇OH as well as the infrared spectra of H₃O, D₂O, dilute HDO, CH₃OH and CH₃OD were measured. The adsorption temperatures of H₃O (253–308 K) were within the phase transition region where two phases of low and high density coexist, while those of CH₃OH, C₂H₅OH and 1-C₃H₃OH were yet within a super-critical region. The entropy of the 2D condensed H₂O on NaF was found to be 14.0 cal K^?1 mol^?1, which suggests that the condensed phase of water on NaF is liquid-like. The OD stretching band of dilute HDO in the 2D condensed water gives a maximum adsorption at ca. 2530 cm^?1 with a half width of ca. 150 cm^?1, being in good agreement with that in liquid water. Comparison of the integrated absorbance of the D₂O bending mode with that of the OD stretching mode suggests that the cluster size of the 2D condensed water on NaF decreases with increasing temperature. The 2D critical temperature and the occupied areas of these adsorbates enable us to conclude that the compatibility of the molecular size with the surface lattice is not important in the occurrence of the 2D condensation of the hydrogen-bonding molecules on NaF and that adsorbed molecules are randomly oriented on the surface to the extent similar to that in 3D liquid state.     

Direct synthesis of beta gallium oxide nanowires, nanobelts, nanosheets and nanograsses by microwave plasma

In this study, beta-gallium oxide (β-Ga₂O₃) nanowires, nanobelts, nanosheets, and nanograsses were synthesized through microwave plasma of liquid phase gallium containing H₂O in Ar atmosphere using silicon as the substrate. The nanowires with diameters of about 20-30 nm were several tens of microns long and the nanobelts with thickness of about 20-30 nm were tens to hundreds of microns long. The morphology and structure of products were analyzed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and X-ray diffraction (XRD). These results showed that multiple nucleation and growth of β-Ga₂O₃ nanostructures could easily occur directly out of liquid gallium exposed to appropriate H₂O and Ar in the gas phase. The growth process of β-Ga₂O₃ nanostructures may be dominated by VS (vapor-solid) mechanism.     

Thermal stability and photoluminescence property of hexagonal MoO3·0.55H2O microrods

The metastable phase of well-faceted, hexagonal, prism-like molybdenum oxide hydrate (MoO₃·0.55H₂O) was successfully synthesized by evaporating molybdic acid solution prepared through cation membrane electrolysis of Na₂MoO₄·2H₂O aqueous solution. The obtained crystals were characterized by X-ray diffraction (XRD), thermogravimetric (TG), scanning electronic microscopy (SEM) and photoluminescence (PL) spectrophotometry. The as-prepared MoO₃·0.55H₂O rods were of 2–4 μm in width and 5–12 μm in length. The MoO₃·0.55H₂O microrods displayed photoluminescence properties at room temperature and were transformed into stable orthorhombic α-MoO₃ after air annealing at 380 °C. Moreover, the influence of temperature factor on the phase transformation process, morphology and photoluminescence properties of MoO₃·0.55H₂O was investigated in detail.     

Influence of nitrogen in aluminum droplet combustion

The influence of nitrogen on the aluminum droplet combustion under forced convection conditions has been studied. An aerodynamic levitation technique of millimetric size liquid droplets heated with a CO₂ laser has been adopted to characterize the combustion of aluminum droplets and, in particular, to observe the surface phenomena. The determination of the burning rate and of the droplet temperature in several atmospheres (H₂O/O₂, H₂O/Ar, H₂O/N₂, and air) has shown that they depend only on the nature and concentration of the oxidizers (O₂ and H₂O); a comparison of experiments in nitrogen and in argon containing mixtures demonstrated that N₂ did not influence the gas phase combustion. However, for nitrogen containing atmospheres we observed the formation of solid aluminum nitride (AlN) at the droplet surface after a latency time depending on the nitrogen pressure. AlN first interacts with the oxide cap producing an aluminum oxynitride, then completely covers the droplet, and finally prevents combustion. The existence of a latency time varying with the nitrogen pressure suggests that the AlN formation is controlled by heterogeneous kinetics. The phenomenon of oxide cap regression during combustion was also observed in all gases, and it is attributed to a chemical decomposition process of alumina by aluminum forming gaseous Al_xO_y species. Therefore, nitrogen effects are significant at the droplet surface rather than in the gas phase, and it is suggested that N₂ is probably one of the main species causing the manifestation of unsteady processes during aluminum droplet burning.