Structural relaxation in the CoP-CoNiP system upon low-temperature annealing

The atomic structure of alloys in the CoP-CoNiP system in the initial state and its behavior upon low-temperature annealing is investigated. It is shown that structural relaxation starts at temperatures of 150–200°C and results in local atomic ordering at the network boundaries. Crystals 2–5 nm in size start to undergo nucleation at the boundaries of structural heterogeneities when heated further to 250–300°C. The nanocrystal structure corresponds to the metastable phase delta-Co (ICSD 42684) and the unknown phase Co_{1 ? x} P _x . The estimated diffusion coefficient for CoP alloy is 10^?14 m² s^?1, according to the experimental data.     

Annealing studies of irradiation effects in vitreous silica

Abstract

This paper describes a series of annealing experiments devoted to the study of the thermal behaviour of B₂ and E' centres produced by heavy-ion irradiations in v-SiO₂. Thermal bleaching of the B₂ component is shown to be complete and irreversible at relatively low temperatures (?500°C). On the other hand, bleaching of E' in the same range is reversible and the corresponding absorption band can be entirely recovered in subsequent electron irradiations, suggesting the existence of a stable precursor defect for this centre. By alternating isochronal annealing cycles with electron re-irradiations, the annealing of such defects was investigated up to 1000°C, taking care to compare the behaviour of the heavy-ion damaged material with that of undamaged material. The results seem to suggest a structural difference between pre-existing E' precursors of normal SiO₂ and those produced in excess by displacing radiation, the latter being characterized by a distinct process of destruction with an activation energy Q ?0.6 eV.     

Increased anomalous low-temperature specific heat of vitreous silica due to helium or neon solutes

The incorporation of helium or neon atoms in vitreous silica generates a temperature-dependent excess specific heat, proportional to the amount of dissolved atoms. Between 0.2 and 1 K this excess is similar to the anomalous specific heat of pure vitrious silica.     

Low temperature thermal properties and intrinsic defects in vitreous silica

Measurements of the low temperature specific heatC _p (0.04KT2.4K) and thermal conductivityk (0.5KT20K) of high purity vitreous silica (Suprasil W) in the as-received and electron-irradiated states are reported. In the temperature range where the localized excitations inherent in glasses dominateC _p (T0.5K) no change is observed upon electron irradiation of up to 10¹⁹e^– (total dose). An anomalyC around 1.8K is observed inC _p which is reduced by 40% upon irradiation. However, ask is not affected by electron irradiation, localized excitations as origin forC can be ruled out. The density of paramagnetic centers in the most heavily irradiated sample is 510¹⁷cm^–3 as obtained from the ESR signal. It has been suggested that these centers have diamagnetic precursors in the unirradiated glass. Our measurements indicate that those defects are not associated with the localized excitations.     

Excess specific heat of vitreous silica

We have studied the low-temperature specific heat of vitreous SiO₂ by using the dislocation model. We have found that the contribution of the localized mode, which is caused by a vibrating dislocation, may explain qualitatively the excess specific heat of the material.     

Effect of crosslinking on the low-temperature relaxation behavior of polyethylene

The dynamic mechanical properties of polyethylene crosslinked by chemical methods and by electron irradiation were determined from 77°K to 250°K on an acoustic spectrometer at a frequency of approximately 50 Hz. Parallel stress-strain, swelling, and x-ray scattering experiments were also carried out to determine the degree of crosslinking and crystallinity. It is found that the effect of chemical crosslinking is to suppress the loss intensity of the γ relaxation in polyethylene while preserving the symmetrical shape of the loss curve without changing the temperature position. The effect of radiation crosslinking, however, also changes the symmetrical nature of the curves. Possible mechanisms of these observed changes are discussed.     

Role of vitreous matrix on the optical activity of Ge-doped silica

We report an experimental study on the relationship between the optical activity of Ge-oxygen deficient centers and dynamic properties and conformational heterogeneity of vitreous matrix in silica. We focus our attention on the absorption band at ∼5.2 eV (B_2β) and on the two related emissions at ∼4.2 eV (α_E) and at ∼3.1 eV (β). From the temperature dependence of B_2β band we estimate a mean energy value of 26 meV for local vibrational modes coupled to the electronic transition, suggesting that the chromophore and its surrounding have access to low frequency dynamics. From the thermal behavior of the two emissions we distinguish the two competitive relaxation processes from the first singlet excited state S₁: the radiative one, giving rise the α_E band, and the thermally activated intersystem-crossing process between S₁ and the triplet state T₁, originating the β band. The intersystem-crossing rate increases on increasing the temperature, determining an opposite thermal behavior of the intensity of the two emissions. However, this temperature dependence cannot be rationalized by a simple Arrhenius law and the α_E decay kinetics at high temperatures do not follow a single exponential law, suggesting a complex landscape of configurational energies of the process.     

Low-temperature specific heat of compacted vitreous silica

We present low-temperature specific-heat measurements of two types of irreversibly compacted vitreous silica (Suprasil W and Suprasil I, differing in their OH⁻ content). In the whole temperature range investigated (0.1 K<T<35 K), the specific heatC is reduced by up to 50% compared to that of uncompacted samples but exhibits a similar temperature dependence, with the characteristic maximum inC/T ³ shifted to higher temperatures. This coherent change ofC, which is roughly the same for both types of vitreous silica, gives strong evidence that the low-energy excitations characteristic of amorphous materials have a common structural origin. Possible relations to the microscopic changes upon compaction are discussed, giving support to the coupled-rotation model of SiO₄ tetrahedra. Phenomenologically, the change ofC upon compaction can be understood by the soft-potential model. The relationship between height and position of theC/T ³ maximum observed in normal and compacted states for both types of vitreous silica and, surprisingly, also for α-quartz hints at a correlation between the mechanisms leading to theC/T ³ maximum for amorphous and crystalline materials with similar short-range order.     

Near-surface modification of optical properties of fused silica by low-temperature hydrogenous atmospheric pressure plasma

In this Letter, we report on the near-surface modification of fused silica by applying a hydrogenous atmospheric pressure plasma jet at ambient temperature. A significant decrease in UV-transmission due to this plasma treatment was observed. By the use of secondary ion mass spectroscopy, the composition of the plasma-modified glass surface was investigated. It was found that the plasma treatment led to a reduction of a 100 nm thick SiO2 layer to SiOx of gradual depth-dependent composition. For this plasma-induced layer, depth-resolved characteristic optical parameters, such as index of refraction and dispersion, were determined. Further, a significant plasma-induced increase of the concentration of hydrogen in the bulk material was measured. The decrease in transmission is explained by the plasma-induced near-surface formation of SiOx on the one hand and the diffusion of hydrogen into the bulk material on the other hand.     

Molecular dynamics simulation of Pt on a vitreous silica surface

The molecular dynamics (MD) computer simulation technique was used to simulate the deposition and cluster growth processes of Pt on a vitreous silica surface. Using a combination of a modified Born-Mayer-Huggins potential (for the substrate) and a Lennard-Jones potential (for the adatoms), the structural features of clusters resulting from four different deposition processes were analyzed and compared to EXAFS results of a similar system. Two of the four deposition processes allowed cluster growth with little interaction with the substrate (by physical separation) and showed comparable results to the EXAFS data. In the two remaining deposition processes, cluster formation occuring with increasing interaction with the substrate resulted in smaller, less three-dimensional particles. This result is in accordance with experimental and theoretical calculations suggesting limited mobility of metal atoms to diffuse once in contract with the amorphous surface.     

Hyper-raman scattering observation of the boson peak in vitreous silica

Hyper-Raman spectroscopy is used to investigate low frequency vibrations of various silica glasses. A strong boson peak is observed. The corresponding modes are inactive in infrared and Raman spectra, and are nonacoustic in nature. The shape of this boson peak essentially matches the total density of vibrational states (DOS), with a constant coupling coefficient C. This and other indications suggest that these modes actually dominate the DOS of silica.     

Observation of the boson peak at the surface of vitreous silica

The boson peak is an excess in the phonon density of states compared to the Debye model that appears in almost all glasses. It has been repeatedly measured in the bulk by a variety of methods, but its origin is still highly debated. Here we present first experimental evidence of the boson peak on the v-SiO2 surface. The measurements were obtained by helium atom scattering. The boson peak appears as a dispersionless mode of approximately 4 meV in the recorded time-of-flight spectra. It is clearly identified as an excess contribution to the low energy Debye-like region in the surface phonon spectral density which is extracted from the time-of-flight spectra using a straightforward theoretical model.     

Structural relaxation effects on the hyperfine parameters in Fe80B20 amorphous alloys

We have studied the influence of the relaxation effects on the different kinds of short range order structures which can be distinguished inFe ₈₀ B ₂₀ amorphous alloys by Mössbauer spectroscopy. The analysis of the distribution function of the hyperfine fields gives results on the variations of the chemical and topological short-range orders in good agreement with those known in literature. Attempt to give quantitative results is made.     

非晶态石英的变电荷分子动力学模拟

基于迭代变电荷方法,用分子动力学模拟了非晶态石英的结构与振动特征.首先利用熔化-猝火方法得到了非晶石英的平衡结构.在此基础上获得了体系不同原子对之间的对关联函数、键角分布函数和振动频谱等,结果与实验数据均符合较好.变电荷方法的计算结果表明,非晶石英体系内粒子的电荷与石英晶体内粒子电荷显著不同,并且出现了较大的涨落. 关键词： 分子动力学 变电荷 非晶石英     

Brillouin scattering of vitreous silica under high pressure

The pressure dependence of Brillouin spectra of vitreous silica exhibits a maximum in the hypersonic absorption and a minimum in the sound velocity at about 2 GPa almost independent of temperature. The results are discussed on the basis of relaxing defects residing in double-well potentials with a Gaussian distribution of barrier heights and a Lorentzian distribution in the asymmetry. Our analysis suggests that both the distribution functions are strongly changed at higher pressures. It is striking that a phase boundary between α-quartz and coesite exists just at the pressure where absorption and sound velocity of vitreous silica pass their extremum. This might be a hint that a corresponding structural change also occurs in the glass.