Ni-Zr非晶合金晶化的透射电子显微镜研究(Ⅰ) ——Ni67Zr33晶化过程中的亚稳相

利用透射电子显微镜对Ni₆₇Zr₃₃非晶合金晶化的研究发现了两个新的亚稳相T₀与T₁0。其中T₁相为体心正交(准四方〕晶体,点阵常数a≌b=0.89nm,c=3.14nm,空间群为Iba2或Ibam。温度升高,T₁相转变为含有大量错排的A心正交Ni₁₀Zr₂相,用1/2(a+b)位移错排模型可以圆满地解释其电子衍射图中仅h+k为奇数的衍射斑沿c^*方向拉长的现象,晶化稳定相为Ni₁₀Zr,与Ni₂₁Zr₈(Ni₅Zr₂)相。 关键词：     

高Tc超导体的反铁磁理论计算

考虑到高T_c超导材料层状桔构特点,从三维各向异性Hubbard模型出发,在大U时导出了自旋波线性近似下的有效哈密顿量。利用格林函数运动方程技术,计算了系统的子晶格磁化强度、内能、比热、平行磁化率和垂直磁化率等物理量。结果表明,层间和层内反铁磁耦合强度之比δ=J_⊥/J对这些物理量有重要影响。在低温T?2J(2+δ)/k_B下,确定了特征温度T₀=2J(2δ(2+δ))^1/2/k_B,分别给出了T?T₀和T?T₀时,这些物理量的渐近表达式。 关键词：     

R型铁氧体BaFe4-xTi2+xO11的化学组态以及磁性行为的研究

本文通过传统的固相反应法制备了R型六角铁氧体BaFe_4-xTi_2+xO₁₁ (x= 0, 0.25, 0.5, 0.75, 1), 并且对它的原子价态以及磁性行为进行了研究. X 射线光电子能谱(XPS)结果显示了随着掺杂含量的增加, 体系中Fe³⁺离子逐渐减少而Fe²⁺离子逐渐增加. 由于具有非对称结构的阻挫晶格中存在各种关联作用的竞争, 使得BaFe_4-xTi_2+xO₁₁体系表现出了复杂的磁有序行为, 在T₁～250 K和T₂～83 K两处存在磁转变. 对这一系列掺杂样品, 在相变温度T₁之上表现顺磁行为, 而在相变温度T₂前后的磁化强度都表现出低场下随磁场的增加快速增加, 高场下则线性变化且在5×10⁴ Oe时还未达到饱和的行为, 表明这一系列掺杂样品是典型的倾斜反铁磁态(canted antiferromagnetic) 或者亚铁磁态.     

邻苯二甲酸二甲酯系材料中α-弛豫的降温介电谱测量与分析

对邻苯二甲酸二甲酯(dimethyl phthalate)、邻苯二甲酸二乙酯(diethyl phthalate)、邻苯二甲酸二丁酯(dibutyl phthalate)和邻苯二甲酸二异辛酯 (dioctyl phthalate)系列材料中, α-弛豫的降温介电谱进行了测量, 得出了相应材料α-弛豫的平均弛豫时间τ_α^a 随温度T的变化关系. 通过τ_α^a 的实验结果与经验的Vogel-Fulcher-Tammann)定律τ_α^a = τ_α⁰ exp (A/(T-T₀))的拟合, 获得了上述系列材料的τ_α⁰, A和T₀. 分析发现, 随邻苯二甲酸二甲酯系列分子侧链中碳原子数目n的变化, 材料的τ_α⁰, A, T₀ 和T_g 表现出一定的规律性, 具体为随n的增加, 即分子内部自由度的增多, A和T_g 都表现出近乎相同的先减后再增的趋势, 而1/τ_α⁰ 和T₀ 则表现出基本相同的先快速减小, 然后保持基本上不变的趋势. 关键词： 玻璃化转变 介电谱 α-弛豫')" href="#">α-弛豫 邻苯二甲酸二甲酯系列材料     

α-AlPO4晶体弹性温度特性的研究

研究了用缓慢升温法生长的α-AlPO₄晶体的弹性温度特性。测量采用传输法,测温范围为-30—150℃;用最小二乘法拟合弹性-温度曲线;用电子计算机处理数据。给出了弹性常数c_ij和s_ij(i,j=1,2,…,6)的温度特性方程以及T₀=25℃时的一级、二级和三级温度系数。 关键词：     

癸基萘磺酸钠胶束中分子动态的质子核磁共振研究

在298、303和313 K下对浓度为0.82 和2.59 mmol/L的癸基萘磺酸钠的重水溶液中质子的自旋-自旋弛豫（T₂）时间进行了测量. 浓度为0.82 mmol/L 的溶液(临界胶束浓度CMC以下)中的所有质子的T₂值均随温度上升而增长，说明整个分子运动变的更自由. 然而，浓度在CMC以上(2.59 mmol/L)时，分子中一部分参与形成栅栏层的质子的T₂值却表现出相反的效应，它们的运动随温度上升而变慢，说明在高温下为防止水分子进入疏水胶束壳内，这些栅栏层的质子堆积的更紧密. 同时分子中其他质子的T₂值随温度上升而增长，表现出正常的分子运动的温度效应.     

MBBA和CC混合物的螺距测量及螺距与温度关系经验公式

我们用布喇格散射法测定了混合液晶MBBA:CC=22.7:77.3(重量比)在不同温度下的螺距。从测得的结果,结合文献中对其它材料所测的数据,我们提出一个螺距p随温度T变化的经验公式:p=p₀-b/(T-T₀)对左旋材料螺距p<0,对右旋材料p>0。除接近相变温度或旋向翻转温度附近外,公式不但适用于单体螺旋状液晶也适用于混合液晶。对一些材料,从实验数据中定出了与它们相对应的常数p₀,b和T₀。 关键词：     

纯铜[0 1 1]倾侧型非共格∑3晶界结构稳定性分子动力学模拟研究

有研究表明, 非共格∑₃晶界的行为在中低层错能面心立方金属晶界 特征分布演化中发挥着重要作用. 为了掌握不同界面匹配的非共格∑₃晶界的特性, 本文利用分子动力学(MD)模拟方法首先研究了纯铜的[0 1 1]倾侧型 非共格∑₃晶界在700–1100 K温度范围内和常压下的结构稳定性. MD模拟采用原子间相互作用长程经验多体势, 步长为5×10^-15 s. 模拟结果表明: 所研究的五个非共格∑₃晶界, 其结构稳定性存在很大差异, 其一般规律是, 与(1 1 1)/(1 1 1)共格孪晶界之间的夹角(Φ角)越小, 晶界匹配值越大, 则非共格∑₃晶界越稳定; 反之亦然. Φ角最小的 (2 5 5)/(2 1 1)非共格∑₃晶界较稳定, 在退火过程中几乎不发生变化. 随着Φ角的增大, 非共格∑₃晶界不再稳定, 这类晶界会通过Miller指数较高一侧晶体每三层原子面合并为一层原子面 (或Miller指数较低一侧晶体每一层原子面分解为三层原子面)的机理 转变为亚稳的“台阶”状晶界, 台阶面部分地处于精确的能量极低 的{111}/{111}共格孪晶界上; 当提高温度退火时, 这种“台阶”状晶界最终会全部转变成稳定平直的{111}/{111}共格孪晶界. 关键词： 纯铜 ∑₃晶界')" href="#">非共格∑₃晶界 分子动力学模拟     

High temperature strain glass in Ti–Au and Ti–Pt based shape memory alloys

Strain glass is a frozen short-range strain ordered state found in shape memory alloys recently, which exhibits novel properties around the ideal glass transition temperature T₀. However, the T₀ of current strain glass systems is still very low, limiting their potential applications and experimental studies. In this paper, we reported two new strain glass systems with relatively high T₀. In Ti₅₀Au_50-xCr_x alloys, the strain glass appears at x = 25, and exhibits a T₀ of 251 K, while in Ti₅₀Pt_50-yFey alloys, the strain glass takes place at y = 30, and shows a T₀ of 272 K. Both of them are comparable with the highest T₀ value reported so far. Moreover, the phase diagrams of main strain glass systems in Ti-based alloys were summarized. It is found that the influence of the martensitic transformation temperature of the host alloy on the T₀ of the strain glass is limited. This work may help to design new strain glass systems with higher T₀ above ambient temperature.     

不同能量的氦原子与同位素分子H2(D2，T2)碰撞分波截面的理论计算

用Tang-Toennies势模型和密耦近似方法计算了不同能量下惰性气体原子He与H₂及其同位素D₂,T₂替代碰撞体系的振转激发碰撞截面. 通过分析He-H₂(D₂,T₂)各碰撞体系分波截面的差异,总结出在H₂分子的对称同位素替代情形下He-H₂(D₂,T₂)碰撞体系分波截面随量子数和体系 关键词： 散射截面 密耦方法 同位素     

Spinodal ordering in Cu3Au

Results from, X-ray diffuse scattering and inelastic neutron scattering demonstrate the presence of long-lived fluctuations in order up to 5.0°C above the first-order phase transition, T_c = 383.2°C. The temperature for continuous ordering has been determined to be 358.2°C. The coherent phase boundary is 3–13°C below T_c.     

The impurity resistivity and resistivity minima in AlCr and AlMn alloys

The impurity resistivity of $\text{Al}$Cr between 1.5 and 50°K was determined with a characteristic temperature for the T² variation θ₁=960±40°K. The behaviour of the resistivity minimum both in $\text{Al}$Cr and $\text{Al}$Mn alloys with impurity concentration provides evidence that a T³ phonon resistivity is found also in aluminium with anomalous impurity resistivity.     

Self-organization of the structure in the Cu60Fe40 composite during deformation-thermal treatment

The deformation-thermal stability of a clusterized amorphous-crystalline structure prepared from a Cu₆₀Fe₄₀ powder mixture at a logarithmic strain e = 4.6 and subjected to isochronous (40 min) annealings at T _a = 200–800°C has been investigated. Periodic changes (ΔT = 300°C) in the order and disorder with a maximum ordering at T _a = 300 and 600°C and a maximum disordering at T _a = 400 and 700°C have been observed. The periodicity of the dominant crystallographic order with a period ΔT = 400°C in the annealing temperature has been revealed for face-centered cubic copper phase planes separated by a singular point at T = 500°C characterized by the dominant body-centered cubic iron phase ordering. It has been shown that the sawtooth shape of the size distribution of strain clusters formed within the crystal structure of deformed samples slowly changes with increasing annealing temperature from exponential (T _a = 200–700°C) to linear (T _a = 800°C). This indicates a high density of internal local distortions in structural units.     

Effect of calcinations on the structural and magnetic properties of magnesium ferrite nanoparticles prepared by sol gel method

Magnesium ferrite nanoparticles calcined at 300 °C, 350 °C, 400 °C, 450 °C were synthesized by sol-gel method. The effects of calcinations on the cation distribution, structural and magnetic properties have been investigated. X-ray diffraction (XRD) and vibrating scanning magnetometer (VSM) were used to characterize the structural and magnetic properties. X-ray diffraction analysis revealed the formation of single phase MgFe₂O₄ in all the samples. Lattice constant and crystallite size increased with calcination. X-ray diffraction data were used to estimate the average cationic distribution among A site and B site. Cationic distribution shows that there is migration of cation between tetrahedral A site and octahedral B site. Saturation magnetization increased with particle size. Coercivity decreased with calcination temperature as a result of decrease in pinning effect at the grain boundary. Curie temperature (T_C) decreased slightly due to weakening of A-B exchange interaction. Low temperature magnetic measurement revealed that blocking temperature (T_B) increased due to strong magnetic interaction.     

Study of magnetoresistance and conductance of bicrystal grain boundary in La0.67Ba0.33MnO3 thin film

La_0.67Ba_0.33MnO₃ (LBMO) thin film is deposited on a 36.7°C SrTiO₃ bicrystal substrate using laser ablation technique. A microbridge is created across bicrystal grain boundary and its characteristics are compared with a microbridge on the LBMO film having no grain boundary. Presence of grain boundary exhibits substantial magnetoresistance ratio (MRR) in the low field and low temperature region. Bicrystal grain boundary contribution in MRR disappears at temperature T>175 K. At low temperature, I-V characteristic of the microbridge across bicrystal grain boundary is nonlinear. Analysis of temperature dependence of dynamic conductance-voltage characteristics of the bicrystal grain boundary indicates that at low temperatures (T<175 K) carrier transport across the grain boundary in LBMO film is dominated by inelastic tunneling via pairs of manganese atoms and tunneling through disordered oxides. At higher temperatures (T>175 K), magnetic scattering process is dominating. Decrease of bicrystal grain boundary contribution in magnetoresistance with the increase in temperature is due to enhanced spin-flip scattering process.     

Evidence for ordered regions in poly(n-butyl)methacrylate from light-scattering studies

From light-scattering studies on polybutylmethacrylate, a polymeric glass, the variation of the velocity and attenuation of thermally excited hypersonic phonons with temperature has been measured. Measurement of the temperature dependence of the ratio of the intensity of the Rayleigh line to the Brillouin lines is interpreted as due to a configurational rearrangement within the glass above the glass transition temperature, T_g . Only light scattered from longitudinal phonons was observed. The distinct change in the temperature dependence of the velocity, attenuation and intensity ratio identified the glass transition.

For samples annealed well above T_g, T_g was found to be about 0°C from the light-scattering studies, 12°C from differential scanning calorimetry (DSC), and 20°C from refractive index measurements. For an unannealed sample the behaviour of the above parameters with temperature was found to be different. T_g for the unannealed sample was 14°C from light-scattering, 18°C from DSC and 20°C from index of refraction measurements.     

First observation of a wetting phase transition in low-angle grain boundaries

The wetting phase transition at low-angle intercrystallite grain boundaries has been experimentally observed. In contrast to the high-angle grain boundaries with the misorientation angels θ > 15°, the low-angle grain boundaries (θ < 15°) are not continuous two-dimensional defects, but constitute a discrete wall (network) of lattice dislocations (edge and/or helical). The theory predicts that, depending on θ, either a continuous layer of the liquid phase or a wall (network) of microscopic liquid tubes on wetted dislocation nuclei is formed at completely wetted low-angle grain boundaries. It has been shown that the continuous liquid layers at low-angle grain boundaries in the Cu-Ag alloys appear at the temperature T _wminL = 970°C, which is 180°C higher than the onset temperature T _wmin = 790°C and 50°C lower than the finish temperature of the wetting phase transition at high-angle grain boundaries, T _wmax = 1020°C.     

Molecular dynamics simulation of the melting of a twist Σ=5 grain boundary

The existence of a possible grain boundary disordering transition of the melting type in a =5 (001) twist boundary of aluminium bicrystal below the melting temperature was investigated using a constant pressure molecular dynamics simulation. The calculated melting temperature T _cm of the bulk Al is about 960 K. The total internal energy, the structure factor, and the pair distribution function were calculated at different layers across the grain boundary. The mean atomic volume, the grain boundary energy, and the thermal expansion coefficients were also calculated using the same simulation method. This simulation also allows us to image the grain boundary structure at different temperatures. The equilibrium grain boundary structure at 300 K retains the periodicity of the coincident site lattice, so that the lowest energy structure corresponds to the coincident site arrangement of the two ideal crystals. With increasing temperature, the total internal energy of the atoms for both the perfect crystal and the grain boundary increases, as do the number of layers in the grain boundary. The grain boundary core exists and the perfect crystal structure still exists outside the grain boundary at 0.9375 T _cm. However, two atomic layers of the equilibrium grain boundary structure at 0.9375 T _cm lose the coincident site lattice periodicity and attain a structure with liquid-like disorder. Therefore, partial melting of the grain boundary has occurred at the temperature above 0.9375 T _cm which is in agreement with the experimental results.     

Effect of Temperature on the Composition and Properties of Poly[(alkylamino)borazine] Precursor to Boron Nitride

A novel poly[(alkylamino)borazine] (PPAB) precursor to boron nitride (BN) was synthesized and the effect of synthesis temperature (T_syn) on its composition and properties was investigated. The results showed that with elevation of temperature, the precursor turned from a colorless transparent liquid to a yellow foamed solid. The softening point (T_s) can be adjusted according to the equation T_s = 1.43T_syn – 125.7, in which the T_syn was between 130°C and 170°C. The PPAB was composed of B, C, N, and H elements and the C content in PPAB decreased gradually with an increase of T_syn. The soluble and fusible precursor, with a softening point of 95°C, provided good processability into polymer fibers. The weight loss of PPAB in N₂ can be divided into three ranges: below 240°C, 240～700°C, and 700～1200°C. The ceramic yield of PPAB was improved from 43 wt% to 55 wt%, when the T_syn was elevated from 110°C to 190°C. Furthermore, the residue from PPAB pyrolyzed under N₂ showed characteristic IR spectra of hexagonal BN.     

The glass transition temperature of amorphous poly(ethylene terephthalate) by thermally stimulated currents

The relation between the temperature T_α of the dipolar relaxation, obtained by the technique of thermally stimulated currents (TSC) and the glass transition temperature T_g has been studied in amorphous poly(ethylene terephthalate) samples. The temperature T_α depends fundamentally on the polarization temperature T_p, the polarization time t_p, and the heating rate v. For each heating rate a maximum T_α, T_M, was obtained for an optimum polarization temperature T_po. The value of T_po is 70°C, independent of the heating rate, and very close to the glass transition temperature obtained by differential scanning calorimetry (69°C). The resulting value for T_M coincides with T_po in the limits of null heating rate and null isothermal polarization time, and, consequently, T_M gives the value of the glass transition temperature for each heating rate as a function of the isothermal dipolar contribution on polarizing at the temperature T_po.