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排序方式: 共有1500条查询结果,搜索用时 125 毫秒
171.
172.
Z. Yu U. Yahsi J. D. McGervey A. M. Jamieson R. Simha 《Journal of Polymer Science.Polymer Physics》1994,32(16):2637-2644
Positron annihilation lifetime measurements are reported for four monodisperse polystyrenes with molar mass M = 4,000, 9,200, 25,000, and 400,000. The temperature dependences of orthopositronium (o-Ps) lifetime (τ3) and intensity (I3) were measured from 5°C to Tg + 30°C for each sample. From these data, the free volume hole size, 〈vf(τ3)〉, and fractional free volume hps=CI3〈vf(τ3)〉 were calculated. The temperature dependences of τ3, 〈vf(τ3)〉 and hps show a discrete change in slope at an effective glass transition temperature, Tg,ps, which is measurably below the conventional bulk Tg. This suggests that τ3 is sensitive to large holes which retain their liquid-like mobility in the glassy state. Good agreement was found for T > hg,ps between hps and the theoretical free volume fraction hth deduced from experimental P-V-T data for polystyrene using the statistical mechanical theory of Simha and Somcynsky. Below Tg,ps, deviations between hps and hth are observed, hps falling increasingly below hth as temperature decreases. Whereas hps and hth depend strongly on M in the melt, each essentially independent of M in the glass. A free volume quantity, computed from the bulk volume, which is in good numerical agreement with the Simha-Somcynsky h-function in the melt, gives improved agreement with hps in the glassy state. © 1994 John Wiley & Sons, Inc. 相似文献
173.
C M Srivastava 《Pramana》1985,25(5):617-633
Based on the assumption that in the groundbcs state the net gain in energy is equivalent to the repulsive electron-ion and electron-electron Darwin interactions, an expression
forT
c
has been obtained which depends on only a few atomic parameters. The theory provides a criterion for the occurrence of superconductivity
and yields satisfactory values ofT
c
for metals and alloys, and ternary chalcogenides and borides. It explains the difference inT
c
in the crystalline and amorphous states as well as the pressure dependence ofT
c
. The possibility of occurrence of high temperature superconductivity has been explored. 相似文献
174.
The microstructures of slowly solidified Cu-Ti-Zr-Ni bulk amorphous alloy were identified by X-ray diffractometry (XRD) and transmission electron microscope (TEM). XRD and TEM examinations show that the deep eutectic structures of the tested alloy consist of CuTi2-Cu10Zr7、Cu3Ti-CuZr、Cu3Ti-Cu10Zr7-CuZr low-order eutectics. Moreover, short-range ordering clusters in the melt with configuration similar to that of Cu10Zr7 compound may contribute to the glass forming ability of bulk amorphous Cu-Ti-Zr-Ni alloy. 相似文献
175.
研究了500和600℃真空退火后的纳米晶Fe38.4Co40Si9B9Nb2.6Cu合金初始磁导率随温度的变化规律,发现较高温度(600℃)退火的FeCo基纳米晶合金,在非晶相居里温度以上较宽温度范围内磁导率没有明显的衰减,这是在双相纳米晶合金中观察到的一种新现象,其磁特性不同于Fe基纳米晶合金.为了探明这种现象的起源,估算了与剩余非晶相同成分的非晶合金的居里温度及纳米晶粒间发生交换耦合作用的参数
关键词:
交换耦合作用
非晶相居里温度
交换耦合穿透深度 相似文献
176.
感应脉冲加速器的磁芯通常为铁氧体或非晶材料,而感应腔磁芯在工作脉冲下的磁性能是决定感应加速脉冲波形好坏的重要因素。搭建了低压多脉冲实验平台对铁氧体和非晶小磁环分别进行MHz重复频率的多脉冲励磁,对励磁线圈上的电压电流波形进行监测,绘制了多脉冲励磁下磁环的磁化曲线,并结合含磁芯线圈动态电感量的递推公式计算出磁环在多脉冲励磁过程中磁导率的变化曲线;在高压三脉冲实验平台上对铁氧体磁芯和非晶磁芯实验感应腔进行了高压三脉冲实验,得到的磁芯多脉冲磁化规律与低压实验的结果一致。最后对两种磁环在多脉冲励磁下的磁性能差异进行了对比分析。 相似文献
177.
178.
Jansen M Schön JC van Wüllen L 《Angewandte Chemie (International ed. in English)》2006,45(26):4244-4263
Si(3)B(3)N(7) is the parent compound of a new class of amorphous ceramics containing silicon, boron, nitrogen, and carbon that display a unique spectrum of properties. It consists of a random network in which the constituent elements are linked by predominantly covalent bonds. Similarly to quartz glass, the composition of amorphous Si(3)B(3)N(7) is virtually stoichiometric. As all three of its constituent elements can serve as the objects of various structural probes, Si(3)B(3)N(7) was selected as the basis of a systematic structural investigation, in which methods for the structure determination of solids without translational symmetry could be validated and improved. However, as the complete amorphous structure cannot be deduced from experimental data, these results must be complemented by computer simulations. Thus, five classes of structure models were generated and compared to experimental results. Only the models generated by following the actual synthesis route as closely as possible agreed well with the experimental data. 相似文献
179.
S.J. Collocott 《Journal of magnetism and magnetic materials》2010,322(16):2281-2286
Bulk amorphous ferromagnet alloys of composition Nd60Fe30Al10, Nd60Fe20Co10Al10 and Pr58Fe24Al18 have been prepared by argon arc melting and quenching into a copper mould. General insight into the magnetic behaviour of the alloys was gained from measurement of the major hysteresis loop at room temperature, and from zero-field cooled and field-cooled magnetisation measurements in the range 10-400 K. Measurements of the coercivity were made from 10 to 400 K, and for all alloys, the coercivity is seen to increase steeply with decreasing temperature to a peak at a temperature in the range 25-50 K, before decreasing. For all alloys, the temperature dependence of the coercivity between 50 and 400 K is well explained by the strong pinning model of domain walls of Gaunt [Philos. Mag. B 48 (1983) 261]. Quantities deduced from the Gaunt model, along with other relevant magnetic parameters, are used to estimate values for the exchange and anisotropy constants. 相似文献
180.
C. Monthus T. Garel 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,53(1):39-45
The directed polymer in a 1+3 dimensional
random medium is known to present a disorder-induced phase
transition. For a polymer of length L, the high temperature
phase is characterized by a diffusive behavior for the end-point
displacement R2 ∼L and by free-energy fluctuations of order
ΔF(L) ∼O(1). The low-temperature phase is characterized by
an anomalous wandering exponent R2/L ∼Lω and
by free-energy fluctuations of order ΔF(L) ∼Lω
where ω∼0.18. In this paper, we first study the scaling
behavior of various properties to localize the critical temperature
Tc. Our results concerning R2/L and ΔF(L) point towards
0.76 < Tc ≤T2=0.79, so our conclusion is that Tc is equal
or very close to the upper bound T2 derived by Derrida and
coworkers (T2 corresponds to the temperature above which the ratio
remains finite as L ↦
∞). We then present histograms for the free-energy, energy and
entropy over disorder samples. For T ≫Tc, the free-energy
distribution is found to be Gaussian. For T ≪Tc, the free-energy
distribution coincides with the ground state energy distribution, in
agreement with the zero-temperature fixed point picture. Moreover the
entropy fluctuations are of order ΔS ∼L1/2 and follow a
Gaussian distribution, in agreement with the droplet predictions,
where the free-energy term ΔF ∼Lω is a near
cancellation of energy and entropy contributions of order L1/2. 相似文献