Mechanical behavior of poly(ethyleoe terephthalate) at cryogenic temperatures

The mechanical behavior of poly(ethylene terephthalate) (PET) was studied at temperatures from 4 to 300°K with a free oscillating torsion pendulum and an Instron tester.

Torsion pendulum data show the presence of relaxation maxima below 80°K. Noncrystalline specimens show a small peak at 48°K (δ) and a shoulder at 8-12° K (?). A peak at 20° K is observed in crystalline samples, both oriented and unoriented. In addition drawn specimens show a pronounced peak at 48° K and the assymmetric maximum in the 100-240° K region is split into two peaks. The logarithmic decrement in the β-loss region decreases with increasing crystallinity and orientation, while the cryogenic loss peaks increase in intensity.

Tensile tests with biaxially oriented and heat-set films show a twofold increase in elastic modulus and a tenfold increase in toughness at cryogenic temperatures when compared with “amorphous” PET.     

Electro-optic behavior of lithium niobate at cryogenic temperatures

The unclamped relative permittivity, , and the Pockels coefficient, , of congruent lithium niobate at a frequency f = 5760 Hz have been determined at low temperatures (7 K < T < 300 K). A He cryostat setup mounted to one arm of an electronically phase-stabilized Michelson interferometer was utilized for the measurement of . A continuous decrease in both parameters was observed as T → 0 K with limiting values of and , respectively.     

The electrical resistivity of amorphous alloys at low temperatures

Measurements of the temperature- and magnetic field dependence of the electrical resistance of some metallic glasses are presented. The data obtained for Cu₅₇Zr₄₃, Cu₄₀Zr₆₀ and Pd₃₀Zr₇₀ demonstrate that deviations from the high temperature behaviour extrapolated to low temperatures are caused by superconducting effects. The paraconductivity which is strongly enhanced in amorphous alloys is shown to agree quite well with theoretical models. The normal state resistance does not saturate down to temperatures of about 2 K. It still exhibits a negative temperature coefficient of the resistivity.     

The hyperfine magnetic field in Fe-Hf amorphous alloys at low temperatures

The time-differential perturbed angular correlation (TDPAC) technique was applied to study the hyperfine magnetic field in amorphous Fe-Hf alloys with different Hf contents at low temperatures. The Hf content was changed from 20 to 40 at% andthe TDPAC measurements were carried out in the temperature range from room temperature down to 18 K. Very strong perturbations were observed for the specimen of low Hf content at low temperatures due to a strong hyperfine magnetic field. The characteristic feature observed by the present study is that the temperature dependence of the perturbation for the Fe-30 at% Hf source is quite different from that for other sources, indicating a maximum around 80 K. Discussions have been given on the explanation of the experimental results in the light of magnetic properties of these alloys.     

Charge and structures of dust particles in a gas discharge at cryogenic temperatures

A dusty plasma in a dc gas discharge is considered at low (cryogenic) temperatures of the gas. The formation of dusty plasma structures consisting of monodisperse poly(styrene) particles (d = 5.44 μm) in a dc glow discharge is experimentally investigated at cryogenic temperatures in the range from 4.2 to 77 K, and the results obtained are presented. The ion velocity distribution function and the charging of dust particles at cryogenic temperatures are calculated using the molecular dynamics method. The primary attention is focused on the correct inclusion of ion-atom collisions in the analysis. This is essential to the understanding of the main mechanisms of the experimentally observed increase in the density of dust particles with decreasing temperature of the gas in the discharge.     

Mechanical properties of Co-Si-B amorphous alloys

The ternary amorphous systems Co_xSi₅B_95?x with 7070Si _y B_30?y with 5<y<18 were studied for their mechanical properties at room temperature. Structure sensitive parameters as density, Young's modulus, micro-hardness and crystallization temperature were investigated as a function of Co and Si contents. The value of density increases with higher Co content but not linearly as for Co-B. Young's modulus, micro-hardness and crystallization temperature decrease with increasing Co concentration. The packing fractionη was calculated using 12-coordinated Goldschmidt atomic radii. It is shown that changes in the proportions of metalloids contents in the alloys have more significant influence on the atomic structure and therefore on the mechanical properties than changes of Co content. The maximum tensile elastic strain for the Co-Si-B system was estimated. Influence of magnetic moment on Young's modulus is discussed.     

Thermodynamic properties of small amorphous and crystalline Silica particles at low temperatures

We have measured the low-temperature (K) specific heat and heat release of small amorphous and crystalline SiO₂ particles embedded in Teflon and of Vycor. The temperature and time dependence of these properties have been interpreted in terms of the tunneling model. We found that the particle size influences the density of states of tunneling systems of the composite. The smaller the size of the particles the larger is the density of states of tunneling systems P₀. Quartz grains with dimensions in the micrometer range show similar glass-like properties as vitreous silica. In comparison with bulk vitreous silica, Vycor shows a much larger P₀ in agreement with the behavior we found for small SiO₂ particles. We discuss the implication of our results on the origin of the universal low-temperature properties of glasses. Received 9 April 1998     

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.     

Magnetization of amorphous alloys — Deviations from spin wave theory at low temperatures

Spin resonance measurements have been used to obtain the temperature dependence of the magnetization in (Fe_xNi_1?x)₇₅P₁₆B₆A?₃ alloys for 4 ? T ? 300K. With x = 0.5, spin wave theory is adequate to account for the observations. For x = 0.4 and 0.3 marked deviations from $\text{T}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ behavior are noted below ～ 70K and we propose a simple model to account for these deviations.     

Surface and bulk crystallization of amorphous alloys Ni-Si-B probed by electrical resistivity

Electrical resistivity measurements are tested as a sensitive probe of the crystallization processes in amorphous metallic alloys of Ni₇₈Si₇B₁₅, rendering the determination of nucleation rates at the surface and in bulk. It is shown that the increase in the electrical resistivity just below the crystallization temperature is mainly due to nucleation phenomena. Moreover, the Avrami coefficient, calculated from resistivity data, provides information about types of crystallization mechanisms, dimensionality of the crystallite growth and the sequence of crystallization stages.     

Enhanced ductile behavior of tensile-elongated individual double-walled and triple-walled carbon nanotubes at high temperatures

We report exceptional ductile behavior in individual double-walled and triple-walled carbon nanotubes at temperatures above 2000 degrees C, with tensile elongation of 190% and diameter reduction of 90%, during in situ tensile-loading experiments conducted inside a high-resolution transmission electron microscope. Concurrent atomic-scale microstructure observations reveal that the superelongation is attributed to a high temperature creep deformation mechanism mediated by atom or vacancy diffusion, dislocation climb, and kink motion at high temperatures. The superelongation in double-walled and triple-walled carbon nanotubes, the creep deformation mechanism, and dislocation climb in carbon nanotubes are reported here for the first time.     

Crystallization and thermal fatigue of ZrTiCuNiBe bulk amorphous alloys heated by laser pulses

The crystallization and thermal fatigue behavior of Zr41Ti14Cu12.5Ni10Be22.5 bulk amorphous alloys heated by laser pulses was investigated. The results showed that crystallization occurred even below the crystallization temperature Tx in a low cycle time. Cracks appearred before or after crystallization of the heated area in the sample. The crystallization degree increased with the heating temperature and cycles. An increase in the fatigue cracks number was also observed. The plastic deformation in the heat...     

铜基大块非晶合金添加微量元素对腐蚀行为的影响机理研究

通过分子动力学方法模拟了Cu-Al合金液相,然后模拟降温过程得到Cu-Al非晶合金.通过计算机编程建立了Cu-Al-M非晶基体、Cu-Al-M非晶表面及吸附O原子Cu-Al-M非晶表面原子结构模型.利用实空间连分数方法,研究了添加微量合金元素Zr,Nb,Ta,V,Y,Sc对Cu基大块非晶合金的腐蚀行为的影响机理.研究发现合金元素Zr,Nb,Ta,V,Sc不向清洁Cu基非晶表面偏聚,但除Y外向有氧吸附的表面偏聚,说明有氧吸附后Cu基非晶表面偏聚发生逆转.键级积分计算表明Zr,Nb,Ta,V,Y,Sc元素均增大与氧之间的结合力,易形成氧化膜,提高Cu基大块非晶的耐蚀性.稀土Y提高Cu基大块非晶的耐蚀性可能是由于它向合金与氧化膜界面偏聚并提高了合金与氧化膜的结合力.     

Magnetic behavior of small amorphous particles produced by spark erosion

A model which admits a distribution in the Curie temperature values is proposed to explain an apparently conflicting low- and high-field magnetization behavior, in the critical transition region, in small amorphous particles produced by spark erosion.     

块体非晶合金的黏度与玻璃形成能力的关系

基于经典结晶理论讨论了非晶合金的晶化动力学因素和晶化热力学因素对玻璃形成能力（GFA）的影响.分析表明,合金的等温转变（TTT）曲线“鼻尖”温度T_n对应的黏度与晶化阻力因子成正比;重新加热时晶化开始温度T_x对应的黏度与晶化驱动力因子成反比.由此得到了新的GFA参数ω₀=（T_g-T₀）/（T_x-T₀）-（T_g-T₀）/（T_n-T₀）,其中T_g为玻璃转变温度,T₀为理想玻璃转变温度.统计结果显示,ω₀与临界冷却速率具有较高的相关性,R²高达09626.进一步分析表明：新提出的ω₀参数可以合理地解释过冷熔体的黏度、脆性、液相稳定性、热稳定性以及T_rg、ΔT_x、γ、γ_m、ΔT_rg、α、β、δ和φ等参数与GFA的关系. 关键词： 块体非晶合金 黏度 脆性 玻璃形成能力     

基于分数阶流变模型的铁基块体非晶合金黏弹性行为研究

采用分数阶黏弹单元替代经典模型中的黏壶, 结合非晶合金在外加载荷作用下的微观结构演化, 建立了以分数阶微积分表示的非晶合金黏弹性本构模型. 并根据Hertz弹性理论及分数阶黏弹性本构模型, 推导了块体非晶合金在纳米压痕球形压头下的位移与载荷及时间关系式. 基于推导的解析式, 对铁基块体非晶合金在表观弹性区的纳米压痕位移与载荷及时间曲线进行了非线性拟合分析. 相较于整数阶模型, 分数阶模型不仅具有较高的拟合精度, 其拟合参数能敏锐地反应加载速率对块体非晶合金黏弹性行为的影响, 且参数的变化规律与载荷作用下非晶合金微观结构演化呈现出较强的相关性.     

Experimental investigation of muon-catalyzed dt fusion at cryogenic temperatures

An experimental investigation of muon-catalyzed fusion (μCF) in gaseous, liquid and solid mixtures of deuterium and tritium was performed. The target conditions included the range of densities of 0.03 ≲ φ ≲ 1.5, tritium concentrations of 2% ≲ c_t ≲ 90% and temperatures of 13 ≲ T ≲ 40K. The study was based on the analysis of observed cycling rates extracted from the measured time distributions of the fusion neutrons. For the first time, the density dependence of the rates λ_(dtμ)d⁰ for the dtμ molecule formation in collisions of tμ atoms in the lower (F=0) hyperfine state with D₂ molecules was determined experimentally. Evidence for the proposed effects of below-threshold resonances on molecular formation was found. Measurements at low temperatures in mixtures of different molecular compositions revealed a clear predominance of the λ_(dtμ)d⁰ formation rate. The probability q_1s for dμ atoms to reach the ground state was determined as a function of density and tritium concentration.