重力及微重力条件下Pd_(40)Ni_(40)P_(20)合金凝固组织中溶质原子的分布特征

研究了Ｐｄ４０Ｎｉ４０Ｐ２０合金在重力（１ｇ）及微重力（μｇ）条件下凝固组织中溶质原子的分布．与重力条件相比，微重力条件下凝固的样品组织中，初生相的Ｐ含量较低，而Ｐｄ的含量较高；共晶区域内的平均Ｐ含量较高而平均Ｐｄ含量较低．深入分析初生树枝晶一次枝晶间距Ｌ与冷却率υ以及固液界面前沿液相中溶质综合输运系数Ｄ的相互关系，发现地面条件下的固液界面前沿液相中溶质综合输运系数Ｄ１ｇ比空间的Ｄμｇ大１７倍．这是造成显微组织中溶质分布差异的主要原因     

重力对GaSb熔滴和液/固界面交互作用的影响

利用我国返回式实验卫星在空间微重力条件下进行了GaSb熔滴与GaP,BN和GaAs等材料的润湿性能的研究,分析了熔滴与基片之间的界面相互作用,并与重力场下实验结果进行了对比.实验表明,重力因素对润湿性存在影响,空间微重力条件下测得的GaSb熔滴与GaP及BN基片的接触角比地面测量的大.对凝固后的熔滴与基片之间的组织分析显示,重力场下,液固界面的相互作用较强,存在较宽的过渡区,这与地面浮力对流有利于物质输运密切相关.实验结果还表明,空间微重力环境下熔体凝固的组织比重力场下要均匀. 关键词： 微重力 润湿性 液/固界面     

二元合金微观偏析的相场法数值模拟

使用耦合溶质场的相场模拟研究了Ni-Cu二元合金枝晶生长过程中固相溶质扩散系数D_s 对枝晶形貌和微观偏析等的影响.计算结果表明，随着D_s的减少，固液界 面前沿溶质扩散层越薄，枝晶生长越有利，枝晶尖端生长的速度越大，侧向分支越发达；D< sub>s越小，固相中溶质成分的波动越强烈，随着D_s的增大，固相中溶质 成分的波动相应减小；溶质微观偏析程度随D_s的增大而减小. 关键词： 相场方法 微观偏析 固相扩散系数 数值模拟     

Zr65Cu17.5Ni10Al7.5合金定向凝固组织特征及其与重力场取向的关系

研究了Zr₆₅Cu_17.5Ni₁₀Al_7.5合金定向凝固过程中组织形态的演变特征，探索了不同的凝固方向与重力场方向间的相互取向关系对凝固组织形态的影响.存在着一个临界抽拉速度V_fc,当抽拉速度大于该临界值时组织方向性变差.该合金本身的物理性质决定了它具有一个较小的临界值(V_fc=5mm/min).不同的凝固方向与重力场的相对取向关系改变了液-固界面前沿的浮力对流状态.当凝固方 关键词：     

相场法模拟多元合金过冷熔体中的枝晶生长

在二元合金相场模型研究的基础上，进行扩展获得了多元合金相场模型.以Al-Si-Mg三元合金为例，采用该相场模型实现了逼真地模拟多元合金凝固过程的等轴枝晶生长，得到了二次或更高次晶臂生长等复杂的枝晶形貌.随着第三组元Mg含量的减少，枝晶的二次枝晶越发达，枝晶中溶质的偏析越严重，枝晶尖端的生长速率和半径越大，与丁二腈-丙酮体系中枝晶尖端生长速率、半径随溶质浓度变化关系的理论计算和实验结果相符合.另外，枝晶初生晶臂中心的溶质浓度最低，在被二次晶臂包围的界面区域的溶质浓度最高；固液界面区域具有较大的浓度梯度，其中枝晶尖端前沿的梯度最大. 关键词： 相场法 多元合金 凝固过程 枝晶生长     

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.     

切向流动作用下SCN-3wt% H2O枝晶定向生长过程研究

实时观察了切向流动作用下SCN-3wt% H₂O合金的定向凝固过程,研究了该合金枝晶生长方向、尖端半径、枝晶一次臂间距以及二次臂生长速度的变化规律.实验观察到能够代表枝晶尖端溶质边界层的“亮带”,该亮带在流动作用下的非对称性证实了切向流动能够改变枝晶前沿溶质的对称分布,使溶质边界层厚度沿背流侧向迎流侧逐渐减薄,枝晶生长发生迎流偏转,且偏转角度随抽拉速度的提高而减小.同时,流场与浓度场的耦合促进枝晶间的优胜劣汰,枝晶一次臂间距显著增大.固液界面处的强迫流动还能够引发相邻枝晶间环流,加速迎流处二次臂生长而抑制背流处生长. 关键词： 定向凝固 枝晶生长 切向流动     

元胞自动机方法模拟枝晶生长

为了能够准确反映具有明显界面的枝晶生长过程，利用元胞自动机方法模拟了凝固过程中固液两相具有不同热物理性质的枝晶生长.在模拟过程中发现，凝固过程中溶质易于富集在枝晶臂之间，同时随着凝固时间的延长，界面前沿的溶质浓度梯度和温度梯度逐渐下降.利用模拟所得的枝晶尖端半径与理论计算的相比较，发现随着枝晶尖端速度的增加，枝晶尖端半径逐渐下降，模拟与理论结果符合较好，偏差小于1μm. 关键词： 枝晶生长 凝固 偏析 模拟     

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     

二元合金高速定向凝固过程的相场法数值模拟

利用相场法对二元合金高速定向凝固过程进行了模拟，为简化计算，采用了温度冻结近似.模拟了定向凝固时平界面失稳、界面形态演化、溶质浓度分布以及高速绝对稳定性条件下胞晶向平面晶的转变等，得到了不同抽拉速度下胞晶间距、胞晶尖端温度以及有效溶质分配系数等参数，显示了高速生长条件下的溶质截留效应. 关键词： 定向凝固 相场法 高速绝对稳定性 溶质截留     

Hyperfine field distribution in amorphous Fe40Ni40B20

From the Mössbauer investigation of amorphous Fe₄₀Ni₄₀B₂₀ and Fe₈₀B₂₀ alloys it was found that the substitution of Fe by Ni only shifts but does not influence the shape of the iron hyperfine field distribution contrary to that of crystalline f.c.c. Ni-Fe alloys suggesting a rather localized type of behaviour. The distributions of the linear combinations of isomer shift and quadrupole splitting are affected by this substitution.     

Stress-induced magnetic easy axis in amorphous Fe40Ni40P14B6

In a ribbon of amorphous Fe₄₀Ni₄₀P₁₄B₆ (Metglas 2826) the iron spins tend to be parallel to the plane of the ribbon, but the distribution of spin directions within the plane is nearly random. When a uniaxial tensile stress is applied to the ribbon the spins become almost completely aligned parallel to the applied stress. The technique of Mössbauer polarimetry was used to detect and measure this effect.     

The adsorption and reaction of CO and O2 on PdAu alloy wires

Temperature programmed desorption (TPD) of CO and O₂ on PdAu alloy wires has been studied. The heat of adsorption, sticking coefficient and maximum coverage of CO were recorded for Pd, 83 Pd 17 Au, 60 Pd 40 Au. For Pd and Pd-rich alloys the heat of adsorption remained fairly constant but the maximum coverage fell markedly from 0.42 for Pd to less than 0.05 for bulk palladium atom fraction X_B^pd ? 0.83. The heat of adsorption, sticking coefficient and maximum coverage of O₂ were investigated for pure Pd. A very limited adsorption was recorded on 83 Pd 17 Au and none on the more Au-rich alloys. The adsorption data are used to discuss the CO + O₂ reaction. Activation energy and frequency factor are estimated on Pd, for the TPD conditions used here. Earlier rate constants (0.2 Torr, 150°C) for CO + O₂ on PdAu as a function of Au content correlates with the maximum coverage of chemisorbed CO, which in turn is correlated with the probability of finding a Pd_9±1 ensemble in the surface. Modern results on the d-band structure of the PdAu alloys suggest that the Pd₉ ensemble, i.e. a surface Pd atom without an Au atom in its coordination shell, would tend to optimise both the donor and acceptor actions of the Pd atoms involved in chemisorbing CO.     

Yields of H+ and He+ at the energy for elastic scattering from H2+3He+ incident on Ag and Pd

Yields, Y, of H⁺ and He⁺ emerging specularly with the energy for single binary elastic collisions have been measured from polycrystalline Ag and Pd surfaces bombarded with mixed monoenergetic (300 < E₀ < 2600 eV) beams of H₂⁺³He⁺ impinging at an angle of 45° from the surface normal. The surfaces were exposed to H₂⁺ at a dynamic pressure of 8 × 10^?3 Pa (6 × 10^?5 Torr) during the measurements. The He⁺ yields from Pd are slightly larger than from Ag, and the H⁺ yields from Pd are 10 to 40 times as large as those from Ag. These results suggest that differences between Pd and Ag in the amount of hydrogen adsorbed and in the character of the hydrogen-metal bond may be responsible for the yield differences through shadowing by, and possibly the sputtering of, adsorbed hydrogen. The Y versus E₀ curves for all four systems have qualitatively the same singly peaked shape which implies that reactive and noble gas ions undergo similar neutralization processes during elastic surface collisions. The ratios of the yields from Ag and Pd do not exhibit the theoretically expected exponential dependence on collision time over the entire range studied, but at the lowest energies the ratios lead to estimates of the difference of neutralization constants which do agree with theory. The potential utility of the large difference in proton yields from Ag and Pd for studying the Ag-Pd alloy system is noted.     

C12(d,p)C13和Ca40(d,p)Ca41基态反应质子极化的研究

本实验测量了C¹²(d,p)C¹³和Ca⁴⁰(d,p)Ca⁴¹两个基态反应质子群在六个角度上的极化值。在C¹²的反应中,小角区的结果和前人的工作相近,和半经典符号规则j_n=l_n±1/2,P=(±)一致。θ_L=115°的数据是前人没有测量过的,我们得到P=0.529±0.068,在Ca⁴⁰的反应中,小角区数据的符号也和半经典符号规则一致,和Немeц及Boschitz的实验结果相同,而和Hird,Takeda及Bercaw的实验结果相反,看来这是由于入射能量不一样所致。因为在这些实验中,三个较高入射能量和三个较低入射能量的结果,都分别有一致的符号,把已发表的极化实验数据进行比较,可以看出,半经典符号规则还是有一定的参考价值,或者,可能找出一个修改后的符号规律,以供核能谱学应用。在有些情况下,我们看到,随着氘核能量的增加,极化角分布的图形似乎有向小角区移动的趋势,这可能是直接反应的一个特性。关于截面和极化角分布间的位置对应关系,我们认为,截面角分布的极小,除了可对应于极化的变号以外,还可能对应于极化绝对值的极大,截面角分布的极大也可能对应于极化的变号,对于这些现象,我们用粗糙的扭曲波理论进行了讨论。     

Preparation and characterization of novel Pd/SiO2 and Ca-Pd/SiO2 egg-shell catalysts with porous hollow silica

Novel egg-shell structured monometallic Pd/SiO₂ and bimetallic Ca-Pd/SiO₂ catalysts were prepared by an impregnation method using porous hollow silica (PHS) as the support and PdCl₂ and Ca(NO₃)₂·4H₂O as the precursors. It was found from transmission electron microscope (TEM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) that Pd was loaded on PHS with a particle size of 5-12 nm in Pd/SiO₂ samples and the Pd particle size in Ca-Pd/SiO₂ was smaller than that in Pd/SiO₂ since Ca could prevent Pd particles from aggregating. X-ray photoelectron spectroscopy (XPS) analyses exhibited that Pd 3d_5/2 binding energies of Pd/SiO₂ and Ca-Pd/SiO₂ were 0.2 and 0.9 eV lower than that of bulk Pd, respectively, as a result of the shift of the electron cloud from Pd to oxygen in Pd/SiO₂ and to both oxygen and Ca in Ca-Pd/SiO₂. The activity of Ca-Pd/SiO₂ egg-shell catalyst for CO hydrogenation and the selectivity to methanol, with a value of 36.50 mmol_CO mol⁻¹_Pd s⁻¹ and 100%, respectively, were much higher than those of the catalysts prepared with traditional silica gel as the support, owing to the porous core-shell structure of the PHS support.     

A molecular-beam investigation of the reaction H2 + 12O2 → H2O on Pd(111)

Using molecular-beam relaxation techniques and isotopic exchange experiments, the water-formation reaction on Pd(111) has been shown to proceed via a Langmuir-Hinshelwood mechanism. The reaction product H₂O is emitted from the surface with a cosine distribution. The rate-determining step is the formation of OH_ad in the reaction O_ad + H_ad → OH_ad. The activation energy for this step is 7 kcal/mole with a pre-exponential factor, v, of 4 × 10^?8 cm² atom^?1 sec^?1. This value for v lies well below that observed for simple second-order desorption of dissociatively adsorbed diatomic gases, but is roughly of the order of that obtained for the oxidation of CO on Pd(111). The formation of H₂O proceeds differently under conditions of excess O₂ or H₂. In an excess of H₂, the kinetics is dominated by the transport of atomic hydrogen between the bulk and the surface as was found for the H?D exchange reaction on Pd(111). In an excess of O₂, diffusion of hydrogen into the bulk is blocked by adsorbed oxygen and the hydrogen reservoir available for reaction at the surface is decreased by several orders of magnitude. This results in a drastic reduction of the reaction rate which can be reversed by increasing the partial pressure of H₂.     

Electrical properties and enhanced room temperature magnetoresistance in La0.7Ca0.2Sr0.1MnO3/Pd composites prepared by chemical plating

A series of bulk polycrystalline La_0.7Ca_0.2Sr_0.1MnO₃ (LCSMO)/Pd composites were prepared by chemical plating and structural, electrical, magnetic, and magnetoresistance (MR) properties were investigated. It is found that Pd additions are uniformly distributed on the grain boundaries of the LCSMO grains, which decrease the resistivity and the saturation magnetic moment of the matrix. An interesting phenomenon is observed that at a given field, when the plating time increases, the MR increases at low addition level (0>t (plating time)<40 min) and decreases at high addition level (t>40 min), indicating an optimal plating time of 40 min, at which the MR value is maximum. Our analysis suggests that the improvement of grain boundaries originating from Pd addition plays an important role in enhancing the MR.     

Galvanomagnetic effects in Fe40Ni40B20

Hall effect and magnetoresistance measurements on the amorphous ferromagnet Fe₄₀Ni₄₀B₂₀ are reported. Both properties are approximately independent of temperature (1.5–300 K) and related to magnetization. The extraordinary Hall coefficient is 3.5 × 10^?8 m³/As. The temperature dependence of the resistivity (1.5–30 K) is also reported.     

The decay of isoscalar giant resonances in 24Mg and 40Ca

In this paper we present data on the charged-particle decay of the isoscalar 2⁺ strength between 10 and 20 MeV excitation energy (E_x) in ²⁴Mg and ⁴⁰Ca. The isoscalar strength was excited by inelastic scattering of 120MeV α-particles at 14° and 12.5° for ²⁴Mg and ⁴⁰Ca, respectively. The charged particles originating from the decay were detected in coincidence with the α′ particles at several angles in the scattering plane. J^π assignments of the decaying states were made on the basis of the angular correlation pattern of the α₀ decay to the ground state of ²⁰Ne and ³⁶Ar, respectively, using a DWBA calculation for the m-state population of the decaying state.For ⁴⁰Ca, about 40% of the E2 EWSR is found to be located in the interval E_x = 13.5 ± 1.5 MeV, which is similar to what has been found from previous inelastic scattering experiments at E_x = 18 ± 2 MeV, but much more than such experiments located in the region E_x = 12–15 MeV. The difference for the region E_x < 16 MeV is due to the fact that from our α₀ angular correlation pattern we conclude that virtually no continuum is excited in the (α, α′) process up to E_x = 16 MeV while all previous inelastic hadron scattering experiments assumed such a continuum to be present. The E2 strength distribution for ⁴⁰Ca thus obtained is very different from what previous theoretical calculations predict. For ²⁴Mg about 30% of the E2 EWSR is present in the interval 12.5 ? E_x ? experiments. 16.5 MeV which again is about twice as much as deduced from previous inelastic scattering The observed branching ratios are compared with calculated ones assuming statistical decay. Reasonable agreement was obtained for ⁴⁰Ca, but for ²⁴Mg especially the α₀-decay branch and to a lesser extent also the p₁ one are much stronger than the statistical calculations predict, indicating that especially the α₀ decay occurs mainly in a non-statistical way.A similar conclusion can be drawn from the behaviour of the forward-backward asymmetry in the angular correlations of the decay particles as a function of the excitation energy FBA(E_x). For ⁴⁰Ca, FBA(E_x) for all decay channels increases smoothly on the average once E_x is above a well-defined threshold, which is due to the onset of knock-out processes. For ²⁴Mg, however, the FBA(E_x) for the α₀ shows a large fluctuation as a function of E_x, indicating an interference process between semi-direct decay and knock-out processes.