Dynamic mechanical behavior of poly[ethylene terephthalate] from 4.2 to 300°K

The dynamic shear behavior of poly (ethylene terephthalate) (PET) has been investigated in the 4.2—300°K temperature region using a freely oscillating torsion pendulum at frequencies of 0.4 to 2 cps. Uniaxial stretching (draw ratio 3:1) of an amorphous specimen resulted in the appearance of a definite maximum in both the damping, δ, and the loss modulus, G', at 30°K (0.7 cps). Filled unstretched PET samples exhibited small loss maxima in the 50—90°K region. For biaxially stretched heat-set specimens the splitting of the β-damping peak (220°K, 1 cps) is found to depend on the orientations of molecular chain axes with respect to the torsion axis.     

Thermoluminescence X-irradiated NaCl:Sr crystals

Strontium-doped thermally pretreated NaCl crystals, X-irradiated at room temperature, were heated to 500°K for glow - curve measurements. The recorded glow curves show two definite peaks at 340 and 460°K. Study of the growth of these peaks with X-irradiation time indicates that the 460°K peak may be related to impurity - vacancy dipole in association with a negative ion vacancy situated in the dislocation region of the crystal.     

银、银-铝合金及其内氧化银-铝合金的内耗峯

用频率约为1周/秒的扭摆,测量了99.99％和99.95％两种纯银以及99.99％纯银分别加入0.01％、0.02％、0.05％、0.15％、0.5％铝和99.95％纯银加入0.5％铝的六种合金的内耗。并且,还比较了三种合金的多晶和其中一种合金的单晶在内氧化前和后,以及经过不同时间的内氧化处理后的内耗变化。实验的结果表明:(1)99.99％纯银试样在空气中测量,升温过程中在130℃左右出现一个内耗峯,从高温作降温测量这个峯不再明显地出现。(2)99.95％纯银试样和99.99％纯银加有少量铝的三种合金试样,在空气中作升温测量,都出现两个明显的内耗峯。低温峯出现在130℃到260℃的范围内,高温峯出现在380℃到430℃的范围内。这两种内耗峯的高度和巅值温度都随杂质含量的多少而改变。从高温作降温测量,高温峯可以重复出现,而低温峯就不再出现。加入合金元素一方面会引起一个新的高温峯,另一方面它又起着抑制低温峯出现的作用。当铝含量超过0.05％(即0.2原子％)时,就只出现一个高温峯。实验的结果指出,低温峯和氧在银中存在时的状态有关,可能是由于晶界上Ag₂O的变化导致晶界结构状态和空位的平衡浓度的改变而引起的。高温峯可能是由于富集在晶界或亚晶界处的合金元素,在一定温度的交变应力下向晶粒内扩散和反扩散所引起的。(3)经过内氧化处理后的合金试样,其内耗巅值是随处理时间的增加而减小。这种内耗的变化也是由于晶界处的铝和氧原子的平衡浓度和界面结构状态受到改变引起的。     

Deformation and annealing behavior. I. Polyethylene terephthalate films

The morphology and deformation behavior of uniaxially and biaxially oriented thin polyethylene terephthalate (PET) films was examined by bright field, diffraction and dark field electron microscopy. A large, 250-500 Å, nodule structure was observed under a variety of conditions, in addition to the smaller 75–100 Å nodules that have been reported previously. Dark field micrographs indicate that oriented PET, heat set at 180° to 240° C, is composed of small (140 Å) isometric crystallites, with a tendency to orient the 100 plane in the plane of the film. It is suggested that the nodular structure and small isometric crystallites may contribute to PET ductility at low temperatures.     

Low-temperature thermoluminescence study of GaSe:Mn layered single crystals

Mangan doped GaSe single crystals have been studied by thermoluminescence measurements performed with various heating rates between 0.4 and 1.0 K/s in the temperature range of 10?300 K. Thermoluminescence spectra exhibited four distinguishable peaks having maximum temperatures at 47, 102, 139 and 191 K revealing the existence of trapping levels in the crystals. Curve fitting and initial rise methods were applied to observed peaks to determine the activation energies of four trapping levels. Capture cross-sections of each level were also evaluated using the obtained energy values. Moreover, heating rate dependencies of the obtained peaks were investigated. It was shown that increase in the heating rate resulted in the decrease in thermoluminescence intensity and shift of the peak maximum temperatures to higher values. Discrete, single trap behaviour was established for acceptor level related with the peak at 191 K by analysing the sequentially obtained peaks with different stopping temperatures between 15 and 65 K.     

Morphological effects on mechanical properties of polystyrene-polyvinylchloride blends

The morphological effects on mechanical properties of polystyrene/polyvinylchloride (PS/PVC) polymer blends were investigated through dynamic mechanical analyzer. Study reveals that the peaks of Tan δ curves of pure PVC and pure PS samples fall at temperatures 58.9 ± 0.2°C and 113.1 ± 0.1°C, respectively. Tan δ curves of 30, 50 and 70-wt% of PVC blends show two peaks indicating the immiscibility of PS/PVC blend. It has been observed that peak falling at lower temperature side shifts towards the higher temperature with the increase of PS concentration and the other one which falls at higher temperature side shifts towards lower temperature side with the increase of PVC concentration in PS/PVC blends. The variation in mechanical performance is attributed to the polymer domain interactions resulting from the different morphologies of various blend compositions.     

Temperature‐dependent Raman scattering studies of polycrystalline BiFeO3 bulk ceramics

This work reports the temperature‐dependent Raman scattering study of mutiferroic BiFeO₃ (BFO) bulk ceramics in a wide temperature range of 93–843 K. The polycrystalline samples are sintered at four different temperatures and characterized by X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), vibrating sample magnetometry, differential scanning calorimetry (DSC), and optical microscopy. The microstructure shows remarkable changes in terms of grain size and domain pattern as the sintering temperature increases. The DSC curves show prominent exothermic peaks at 645 K, the antiferromagnetic–paramagnetic phase transition temperature. The Raman spectra of all the four specimens reveal strong anomalies in the vicinity of the Neel temperature, which can be attributed to the multiferroic nature of BFO. The Raman scattering studies also reveal considerable spectral changes at a temperature range of 140–200 K in all the specimens, which can be inferred to a further spin–reorientation transition exhibited in BFO at a cryogenic temperature. Copyright © 2008 John Wiley & Sons, Ltd.     

Structure and properties of polybutylene crystallized from the glassy state. I. X-ray scattering,DSC, and torsion pendulum

Isotactic polybutene (PB) can be quenched into a completely glassy state by quenching molten films into a solid-liquid mixture of isopentane, Freon, or ethanol. The crystallization of PB from the glass form was studied by x-ray scattering, differential scanning calorimetry (DSC), and dynamic mechanical spectroscopy (torsion pendulum). As for crystallization from the melt, PB crystallizes from the glass into a tetragonal crystal structure (Form II) at ca. 0°C, depending on sample thickness, and then transforms to the twinned hexagonal structure (Form I) upon aging at room temperature. In the presence of isopentane, PB crystallizes partially from the glass into the untwinned hexagonal (Form 1′) structure at ca. -70°C; the rest of the sample starts to transform to tetragonal structure at ca. -30°C and nearly completes crystallization at ca. 0°C. The exact temperatures of both transformations depend on the amount of isopentane present and sample thickness. Upon aging at room temperature the tetragonal structure converts to the twinned hexagonal structure even faster than in the absence of isopentane. Dynamic mechanical experiments show the presence of two relaxation-like peaks for the ultraquenched samples: T_r (L) = -27°C and T_r (U) = -15°C. X-ray diffraction, DSC, and torsion pendulum experiments show that PB crystallizes from the glass at T_r (U).     

Mechanical properties of epoxy resins and glass/epoxy composites at cryogenic temperatures

The mechanical properties of four modified-epoxy resin systems were investigated. Tensile properties (including strength, modulus, and elongation), notch toughness, impact strength, the coefficient of linear thermal contraction, and thermal-shock resistance were determined for cast-resin specimens at 75, ?320, and ?423°F. Inter-laminar shear strengths (Naval Ordnance Laboratory horizontal shear and short-span shear) were obtained for glass-filament-wound-composite specimens at the three temperatures.

The two systems with the best cryogenic properties were a hybridepoxy/polyurethane (resin 4A) and a highly modified bisphenol-A epoxy (resin 2). In choosing the optimum system, the same tests (except for notch toughness, impact strength, and short-span shear) were performed on filament-wound-composite specimens at the three temperatures. In these tests the performance of resin 4A was better than, or at least comparable to, that of resin 2.     

1200 V碳化硅功率MOSFET低温特性的实验表征及分析

碳化硅功率MOSFET是宽禁带功率半导体器件的典型代表,具有优异的电气性能。基于低温环境下的应用需求,研究了1200 V碳化硅功率MOSFET在77.7 K至300 K温区的静/动态特性,定性分析了温度对碳化硅功率MOSFET性能的影响。实验结果显示,温度从300 K降低至77.7 K时,阈值电压上升177.24%,漏-源极击穿电压降低32.99%,栅极泄漏电流降低82.51%,导通电阻升高1142.28%,零栅压漏电流降低89.84%(300 K至125 K)。双脉冲测试显示,开通时间增大8.59%,关断时间降低16.86%,开关损耗增加48%。分析发现,碳化硅功率MOSFET较高的界面态密度和较差的沟道迁移率,是导致其在低温下性能劣化的主要原因。     

Photostimulated thermoluminescence of x-irradiated NaCl

The thermoluminescence and photostimulated thermoluminescence of X-ray coloured NaCl crystals has been studied, together with the thermal annealing of F, F′ and M centres. Three glow peaks centred at 315, 341 and 348°K are obtained in the temperature range 300–400°K. The first peak (315°K) is ascribed to electron trapping by Cu²⁺ centres formed by X-irradiation. The other peaks (314 and 348°K) are related to the thermal annealing of M and F centres, respectively.     

两步法制备空间取向高度一致的ZnO纳米棒阵列

采用两步法,即先用磁控溅射在Si(100)表面生长一层ZnO籽晶层、再利用液相法制备空间取向高度一致的ZnO纳米棒阵列.用扫描电子显微镜、X射线衍射、高分辨透射电子显微镜和选区电子衍射对样品形貌和结构特征进行了表征.结果表明,ZnO纳米棒具有垂直于衬底沿c轴择优生长和空间取向高度一致的特性和比较大的长径比,X射线衍射的(XRD)(0002)峰半高宽只有0.06°,选区电子衍射也显示了优异的单晶特性.光致发光谱表明ZnO纳米棒具有非常强的紫外本征发光和非常弱的杂质或缺陷发光特性. 关键词： ZnO纳米棒阵列 ZnO籽晶层 两步法 液相生长     

Die magnetische Struktur und das magnetische Verhalten von ErFe2: Eine Neutronenbeugungsuntersuchung

The magnetic structure of ErFe₂ has been determined from neutron diffraction measurements on a powderd sample in the temperature region between 4.2°K and 700°K. ErFe₂ exhibits ferrimagnetic ordering below 600°K with the Er- and Fe-ions being antiparallel oriented. The separate sublattice magnetisations of the Er- and Fe-sublattices have been determined. The saturation moments at 4.2°K have been found to beμ(Fe)=1.97μ _B andμ(Er)=8.47μ _B. The coupling between the two sublattices is by an antiparallel spin-spin interaction of the Er and Fe ions. The temperature dependence of the sublattice magnetization of the Er-ions is explained by a separation into two parts: an induced magnetization at high temperatures and a spontaneous magnetization at lower temperatures.     

The decay of low-frequency elastic oscillations in a Ba2NaNb5O15 crystal

The internal friction and shear modulus of a barium sodium niobate crystal were studied using the torsion pendulum technique at a frequency of ～25 Hz in the temperature range from 80 to 800 K. An internal friction relaxation peak of the domain nature was observed at 228 K. A relaxation process responsible for this internal friction peak is explained through compensation of the electric charge (induced by a piezoelectric effect at the 180° domain walls) by charged point defects diffusing toward the domain walls.     

快冷Fe71Al29合金中的两个内耗峰

用一个计算机控制的倒扭摆研究了快冷Fe71Al29合金中的两个内耗峰.在快冷的Fe71Al29样品中分别在180℃(P1)，340 ℃(P2)和510℃(P3)出现了内耗峰，而在慢冷的Fe71Al29样品中只发现了P3峰.快冷样品中的P1和P2峰在从650℃冷却下来的测量过程中或在350℃经过较长时间的时效后消失，其峰高随时效时间的增加 而下降，直至消失.P1和P2峰都有弛豫特征，它们的激活能分别为：H1=1．03±0．08eV(P1峰)；H2=1．64±0．05eV(P2峰).P1峰被认为是无序合金中Al原子在四面体点阵内的最近邻运动所引起，P2峰则是无序合金中Al原子在四面体点阵内的次近邻运动所引起. 关键词： Fe-Al合金 内耗     

The thermoluminescence (TL), phosphorescence and cryoluminescence of n-type hexagonal (6H) SiC crystals

The low temperature (down to liquid helium temperature) TL, phosphorescence and cryoluminescence of n-type 6H SiC crystals is described. The crystals contained nitrogen as the major impurity at concentrations of about 10¹⁶ cm^-3. The glow curves exhibited peaks at about 25, 45, 70 and 90°K (in addition to a peak at 250°K). Thermal activation energies for the above peaks ranged between 0.02 and 0.14 eV (0.30 eV for the 250°K peak). These are much lower than energies reported earlier for nitrogen donor levels in 6H SiC. The values obtained for the 70–90°K peaks (0.08–0.14 eV) fit quite well those obtained by electrical transport measurements and Raman scattering.The crystals exhibited strong phosphorescence even at liquid helium temperature. This was shown to be only partly due to thermal release from traps, the other components being due to pair-recombination and optical release from the shallow traps by the black body radiation (BBR) from the walls of the cryostat. This BBR was found to be responsible also for the observed cryoluminescence.     

Investigation of Pendant Group and Chain Segment Motions in P(MMA/BA) Copolymers by Thermally Stimulated Depolarization Current

Poly(methyl methacrylate/butyl acrylate) [P(MMA/BA)] copolymers (M_η ～2×10⁵) with different mass percentages of MMA were synthesized by the method of solution polymerization. Thermally stimulated depolarization current (TSDC) technique was used to investigate the effect of copolymerization on pendant group and chain segment motions. Three TSDC peaks were observed over the temperature range from 310 to 400 K. The highest temperature, ρ peak originates from the detrapping of trapped charge carriers. The lower temperature, α peak corresponds to the glass transition. The activation energy of the α relaxation decreases from 1.2 eV for PMMA to 0.98 eV for MMA(75)/BA(25). In the fitting process, another peak, β′, is separated on the low temperature side. The apparent energy barrier of the β′ for PMMA is 0.80 eV. The β′ relaxation is thought to correspond to the motion of pendant groups including intra‐ and inter‐molecular interactions. All three peaks move to lower temperatures with an increase in BA component, and the activation energy for the α and β′ relaxations also decreases with the increase of BA component in copolymers, indicating that the flexible side groups of BA have an effect of plasticization on the glass transition and motion of pendent groups. The temperatures of the α and β′ peaks of P(MMA/BA) copolymers follow the Fox equation. Fitting results gives the α peak at 238 K and β′ peak at 225 K for polybutyl acrylate (PBA).     

FOX-7转晶行为的太赫兹光谱及理论计算研究

1, 1 -二氨基- 2, 2 -二硝基乙烯(FOX-7)是一款新型高能钝感炸药, 为了研究温度变化对其分子结构特性的影响, 利用太赫兹时域光谱技术对持续升温(298K→393 K)过程中FOX-7 在0.2—2.5 THz 频率范围内的吸收光谱进行了在线探测, 结果发现随着样品温度的升高, FOX-7 的吸收谱带发生变化, 于384 K 时出现一个新的吸收特征峰, 且该吸收峰的峰强逐渐升高. 基于密度泛函理论(DFT), 对样品在298 K 和393 K 温度下的晶体结构进行了小于2.5 THz 范围内吸收频谱的模拟计算, 完成了对FOX-7 两种晶型实验吸收特征峰的指认. 分析表明FOX-7 的分子结构会受温度的影响而发生改变, 互为异构晶型的基团表现出的振动模式不同, 温度384 K 时FOX-7 开始发生α→β 晶型转变, 且该晶型转变过程是可逆的, 新出现的1.12 THz 特征峰在393 K 时的振动是由—NO₂ 和—NH₂ 的摆动及各自的扭动所致.     

Lattice location of Co implanted in silicon

Thermal treatment of CaF₂ has a significant influence on the number and intensity of the peaks seen by thermo-luminescence. A combination of ion implants and anneal cycles leads to the conclusion that the 90°C glow peak is derived from a defect of a substitutional trivalent impurity (e.g. Ce⁺³) linked to an interstitial fluorine ion. Perturbations of this centre by other defects modify the centre and the glow peak temperature is raised to 110°C.

The peaks at 180, 220 and 350°C all involve intrinsic defect clusters.

The building of models for the different glow peaks was helped by a comparison of impurity and self ion implantations.     

A low-temperature high-pressure apparatus with a temperature control system

Abstract

A low-temperature high-pressure apparatus was designed using commercial cryogenic equipment. Pressures up to 1 GPa and temperatures down to 40 K can be obtained in a volume of up to 30 cm³. The apparatus is of the piston-cylinder type with a piston diameter of 45 mm, and the pressure can be varied at all temperatures, An adaptive temperature control system keeps the temperature inside the pressure cylinder constant to within ±0.1 K.