The mobility of the amorphous phase in polyethylene as a determining factor for slow crack growth

Polyethylene (PE) pipes generally exhibit a limited lifetime, which is considerably shorter than their chemical degradation period. Slow crack growth failure occurs when pipes are used in long-distance water or gas distribution though being exposed to a pressure lower than the corresponding yield stress. This slow crack growth failure is characterized by localized craze growth and craze fibril rupture. In the literature, the lifetime of PE pipes is often considered as being determined by the density of tie chains connecting adjacent crystalline lamellae. But this consideration cannot explain the excellent durability of the recent bimodal grade PE for pipe application. We show in this paper the importance of the craze fibril length as the determining factor for the pipe lifetime. The conclusions are drawn from stress analysis. It is found that longer craze fibrils sustain lower stress and are deformed to a lesser degree. The mobility of the amorphous phase is found to control the amount of material that can be sucked in by the craze fibrils and thus the length of the craze fibrils. The mobility of the amorphous phase can be monitored by dynamic mechanical analysis measurements. Excellent agreement between the mobility thus derived and lifetimes of PE materials as derived from FNCT (full notch creep test) is given, thus providing an effective means to estimate the lifetime of PE pipes by considering well-defined physical properties.     

The structure of the glass phase of Rb1−x(ND4)xD2PO4

X-ray scattering techniques have been used to study the diffuse scattering from a single crystal of Rb_1–x(ND₄)_xD₂PO₄ withx=0.65. This system has a structural glass phase at low temperatures resulting from the competing ferroelectric interactions of RbD₂PO₄ and antiferroelectric interactions of (ND₄)D₂PO₄. The diffuse scattering shows a broad peak with a maximum occurring at a wavevector of about 0.3a ^*, and the intensity of these peaks is surprisingly different for wavevectorsq on opposite sides of the Bragg reflections. A model of the D bonding is developed which suggests that the diffuse scattering arises from the interaction between ferroelectric displacements alongc, ferroelectric displacements alongb, and transverse acoustic modes polarized alongb andc. The model accounts for the incommensurate wavevector and, qualitatively, for the intensity of the diffuse scattering around different Bragg reflections. The temperature dependence of the scattering is also measured.     

微带阵列天线的时域散射特性

本文用时域有限差分方法(FDTD)研究了微带阵列天线的时域散射场,分析了入射脉冲极化方向不同以及入射方向不同时的散射场的分布情况;讨论了时域散射场的时域波形以及频谱与微带阵列天线结构的关系;用散射场分离算法讨论了地板对于微带贴片阵列散射场的影响.研究发现,有限大地板的微带阵列天线的散射场主要是由地板的边沿电流产生的,同时微带贴片阵列的谐振频率与入射脉冲的极化方向有关,因此不同的极化方向对应于不同的散射频谱. 关键词： 时域散射 微带天线 FDTD     

Temperature dependence of the inelastic scattering in a GaAs/n-AlGaAs selectively doped heterojunction with InGaAs quantum dots

Inelastic scattering processes of two-dimensional electron gas (2DEG) have been investigated in a inverted GaAs/n-AlGaAs heterojunction with self-organized InGaAs quantum dots (QDs) embedded near the 2DEG channel where the electron population in the QDs is controllable by the gate voltage V_g. By analyzing magnetoresistance, the inelastic scattering time τ_ε have been evaluated as functions of V_g at 0.6, 0.8, 1.2, and 1.7 K. It is found that τ_ε increases with V_g below 0.8 K and decreases above 1.2 K, which suggests that the dominant scattering mechanisms below 0.8 K and above 1.2 K are different. To interpret this behavior, we have calculated the inelastic scattering time theoretically. It is found that the experimental data are well explained by a theoretical model where a 2D electron is considered to be inelastically scattered both by the other 2D electrons and by the trapped electrons in QDs. It is also found that the 2DEG–2DEG scattering is dominant at low temperature, while the 2DEG-QDs scattering becomes important as the temperature increases.     

Spectral properties of backward stimulated scattering in liquid carbon disulfide

The spectral structure of backward stimulated scattering from a 10 cm-long CS₂-liquid cell is investigated by using Q-switched 10-ns and 532-nm laser pulses with different spectral linewidths. Under a narrow spectral line (∼0.1 cm⁻¹) pump condition, very strong sharp lines near the pump wavelength (λ ₀) position and the first-order stimulated Raman scattering (λ _s1) position can be observed. However, under a wide line (≈1 cm⁻¹) pump condition, only a strong and superbroadening spectral band can be observed mainly in the red-shift side of the pump wavelength. The different spectral features under these two conditions can be explained by a competition between stimulated Brillouin, Raman, and Rayleigh-Kerr scattering. Under both pump conditions, the broadening spectral distributions are not consistent with the predictions given by stimulated Rayleigh-wing scattering theories, but can be interpreted well utilizing the theoretical model of stimulated Rayleigh-Kerr scattering. Zh. éksp. Teor. Fiz. 112, 1563–1573 (November 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Neutron diffraction study of the plastic phases of polycrystalline SF6 and CBr4

An investigation of the plastic phases of polycrystalline specimens of sulphur hexafluoride (SF₆) and carbon tetrabromide (CBr₄) by neutron elastic scattering and neutron diffraction experiments is reported. A theory of neutron diffraction in plastic crystals, which treats the Bragg scattering and the diffuse scattering from a unified point of view, is developed and applied in the interpretation of the neutron results. The Debye-Scherrer peaks are analysed, both by a cumulant expansion technique and a cubic harmonic analysis, to determine the crystal structures of the plastic phase which are found to be body-centred cubic (space group Im3_m) for SF₆ and face-centred cubic (space group Fm3m) for CBr₄. The bond-orientation distribution function, f(Ω), has maxima in the <100> directions for SF₆ and in the <110> directions for CBr₄. Since, in both cases, f(Ω) is appreciably different from zero for all orientations, it is apparent that significant thermal reorientation takes place in both these crystals. The translational and librational displacements in CBr₄ are exceptionally large and give rise to extensive diffuse scattering which is analysed on the basis of a simple Einstein model. The model predicts that the centre-of-mass thermal vibration and the orientational disorder give approximately equal contributions to the total diffuse scattering. The calculated scattering is in good agreement with experiment for all wave vector transfers outside the range 2 to 3 Å^-1. Inside this range discrepancies occur which are interpreted as evidence for the existence of orientational short-range order in CBr₄.     

Bistatic scattering from forest components. Part I: coherent polarimetric modelling and analysis of simulated results

This article deals with the study of bistatic scattering by forested areas using a coherent scattering model. The contribution of the scatterers is considered (trunks, branches), as well as their interaction with the ground. This code has been extended from monostatic to bistatic configurations and we present all the tests we have performed with this tool in order to validate it as far as possible. The transmitter is at a fixed location and the receiver moves either in the incidence plane (only θ _s varies) or in all positions (θ _s and φ _s both vary). Our objective is to determine the main trends characterizing bistatic scattering by a forest. We study the variation of the scattering coefficient, when evolving from the backscattering to the specular configuration (only θ _s varies). Both radiometric and polarimetric aspects are analysed, first when considering a simple forest of trunks and then a forest with trunks and branches. Finally, we analyse the impact of both θ _s and φ _s for two cases: a vertical dielectric cylinder in free space and a set of vertical dielectric cylinders, arranged in different ways. Here again, the radiometric and polarimetric aspects are investigated to check the electromagnetic behaviour.     

Picosecond stimulated Raman scattering of BaWO4 crystal

In single-pass configuration, a picosecond pulse laser was used to investigate the stimulated Raman scattering in different BaWO₄ crystals. Using the thresholds for the first Stokes line along the a- and c-axis, the Raman gain coefficients of the BaWO₄ crystals were calculated along their respective direction. The higher Stokes (fourth) and anti-Stokes (third) lines were observed. Compared with other well-known Raman crystals, such as KGd(WO₄)₂ and Ba(NO₃)₂, BaWO₄ has favorable properties for scattering the pump radiation with picosecond pulse duration. The Raman gain coefficients of BaWO₄ are different, but do not vary for different transmission directions, which means that this Raman material can be selected more freely.     

火灾烟颗粒分形模型和球形模型光散射的比较研究

对烟颗粒的光散射进行模拟计算是研究火灾烟颗粒光散射特性的重要手段,目前对于火灾烟颗粒光散射的数值计算多采用球形或椭球模型.实际上,火灾烟颗粒的形貌与球形和椭球均存在着显著差异.扫描电子显微镜图像表明,烟颗粒具有近似分形的结构.本文利用离散偶极近似方法计算了随机取向的火灾烟颗粒分形凝团以及同体积的球形颗粒的光散射Muller矩阵,并对两者的归一化Muller矩阵元素随散射角的分布进行了比较.研究表明:火灾烟颗粒分形模型和球形模型的归一化矩阵元素F₁₁(θ)/     

应变Si/(001)Si1-xGex电子迁移率

依据离化杂质散射、声学声子散射和谷间散射的散射模型,在考虑电子谷间占有率的基础上,通过求解玻尔兹曼方程计算了不同锗组分下,不同杂质浓度时应变Si/(001)Si_1-xGe_x的电子迁移率.结果表明:当锗组分达到0.2时,电子几乎全部占据Δ₂能谷;低掺杂时,锗组分为0.4的应变Si电子迁移率与体硅相比增加约64%;对于张应变Si NMOS器件,从电子迁移率角度来考虑不适合做垂直沟道.选择相应的参数,该方 关键词： 电子谷间占有率 散射模型 锗组分 电子迁移率     

激光-电子康普顿散射物理特性研究

对激光-电子康普顿散射物理特性即能量特性和微分截面角分布进行了仔细的研究.计算结果显示出光子能量和微分截面角分布的简单结构.康普顿散射X射线光源具有散射光子的能量易调节、方向性好等特点.在入射电子束能量很高时,X射线近乎单向出射.光源色散度较大,但实验上可以获得色散（带宽）小的X射线.对于各种波长的激光,在很宽的电子束能量范围（1 MeV—10 GeV）内,散射X射线光子的总截面和前向发射圆锥内（半圆锥角1/γ,其中γ=E/m₀ 关键词： 康普顿散射 能量特性 微分截面 角分布     

Low energy scattering data and phase shifts

We point out that at any given low energyσ _tot and a ratio of integrated scattering data likeF/B can, in principle, be used unambiguously to finds-andp-wave phase shifts. Thus efforts to obtain other low energy data like elastic dσ/dΩ andP/E are unnecessary. It is also indicated that the mere knowledge whetherF/B is greater than or less than unity enables us to draw important conclusions about the nature of the interaction in thep-state without performing detailed calculations. Thus a strong case is made out for obtaining much more preciseF/B data than are presently available. The discussion refers mainly to low energy ∧p scattering data.     

Dependence of the scattering length for hydrogen atoms on effective mass

The possibility that the non-adiabatic correction to the scattering length of a pair of hydrogen atoms interacting via the ground state molecular potential, X¹Σ_g ⁺, of H₂ could be made by replacing the mass of each nucleus by a different effective mass is explored. The Born-Oppenheimer potential with adiabatic, relativistic and radiative corrections is used in calculations of the scattering lengths and the mass-dependent shifts of the rotationless vibrational levels with fixed and varying effective masses. The shifts are compared with established values and it is demonstrated that the semi-classical formula for the scattering length accounts well for the effect of changing the mass. A perturbing potential that is equivalent to a change in mass is derived and it is compared to a published local non-adiabatic potential.     

Influence of scattering on the anisotropy of relaxation time in indium alloys

The low field Hall effect in indium at low temperatures is very sensitive to the kind of scatterers introduced. In the presence of small angle dislocation scattering, the Hall coefficientR ₀ is positive, while diffuse scattering such as that caused by boundary scattering leads to a negative value forR ₀. Data taken on alloys of indium with Ga, Tl, Pb, Sn, Hg, Cd and Bi will be analyzed by comparing them with the dislocation and size effect and by interpreting the differential cross sections of these scatterers in term of their effect onR ₀. The behaviour ofR ₀ of these alloys in function of temperature is also described.     

Amplitudes of two-channel scattering by a two-dimensional potential barrier with a constant height

The immersing method is used to solve the two-channel scattering problem in the case of concrete potential. In particular, we consider the particle scattering by a two-dimensional barrier which is constant in the scattering direction and is arbitrary in the transverse direction. For this case the scattering amplitudes t ₁, t ₂, r ₁, and r ₂ are determined. Expressions for the transmission and reflection amplitudes are obtained for the case of δ-potential. The behavior of the scattering amplitudes in the limit k ₂ → 0 is studied. It is also shown that the ratio of products of transmission and reflection amplitudes for two channels does not depend on the coordinate of the middle of coordinate.     

Mode dependent scattering of phonons by domain walls in ferroelectric KDP

Thermal conductivity and ballistic phonon imaging measurements in KH₂PO₄ (KDP) at low temperature (T<3K) indicate that scattering from domain walls has a large effect on phonon transport. kDP has a ferroelectric phase transition from tetragonal to orthorhombic structure atT _c =122 K. BelowT _c domains of opposite electric polarization and crystal orientation form unless the sample is colled in an electric field. Thermal conductivity measured along the [100] (tetragonal) axis drops 30% when domain walls are present, which is independent of sample size and temperature. We attribute this decrease to phonon polarization-dependent scattering at the domain boundaries. This is verified by measurements of ballistic transport, using phonon imaging techniques, which reveal the phonon polarization and mode dependence of the scattering. The scattering is successfully modelled using continuum acoustics with simple acoustic mismatch at the domainwall. The interface scattering is found to be mode dependent: Caustic structures in the phonon images due to slow transverse phonons are most affected by the domain wall scattering, which channels these phonons along parallel planes by multiple reflections without mode conversion. Mode conversion scattering, though possible for a number of phonons, has little effect on the overall phonon transmission.     

基于delta-P1近似模型的空间分辨漫反射一阶散射参量灵敏度研究

以delta-P1近似光学模型为基础, 推导了双点源近似下空间分辨漫反射一阶散射参量μ_s'灵敏度的解析式, 并进行了数值分析和比较. 研究表明, 与混合漫射近似模型和漫射近似模型相比, delta-P1 近似模型能更好地描述强散射较强吸收情况下近光源区域生物组织漫反射光子的分布, 且在有效反照率a'>0.83时, 获得最佳优化距离ρ_opt, ρ_opt 随μ_s' 的增大而减小, 且在距光源约2.7—4个输运平均自由程处μ_s'的变化对测量吸收的影响最小. 这项研究对于优化传感器几何结构以及生物组织光学参量的测量具有重要意义.     

Electric-field influence on the neutron diffuse scattering near the ferroelectric transition of Sr0.61Ba0.39Nb2O6

ABSTRACT

Uniaxial relaxor ferroelectric Sr_0.61Ba_0.39Nb₂O₆ single crystal has been investigated in the vicinity of its phase transition using neutron scattering and dielectric spectroscopy. A global-type thermal hysteresis is evidenced by both techniques in the ferroelectric phase and up to about 15 K above T_c. In addition, a part of the transverse neutron diffuse scattering in the 001 Brillouin zone, presumably related to static nanodomain structure, can be suppressed by prior poling the crystal in electric field of 3 kV/cm. The remaining part of the transverse neutron diffuse scattering and the real part of permittivity show a similar temperature dependence. The temperature position of the maximal scattering intensity T_max depends significantly on the scattering wave vector. T_max shifts monotonically to higher temperature with the increasing wave vector in all investigated cooling and heating regimes. It is concluded that the critical fluctuations have space correlations which depend on frequency and wave vector.