Structure study of rare earth doped vitreous silica by molecular dynamics simulation

A series of erbium doped vitreous silica, with erbium concentration as low as 0.25 per cent up to 5 per cent, has been modeled, using molecular dynamics. It has been found that as the Er content decreases, the percentage of erbium ions in pairs remains almost constant until very low concentration, when pairing is the only clustering mechanism and those that are not in pairs are isolated.     

Envelope solitons in acoustically dispersive vitreous silica

Acoustic radiation-induced static strains, displacements, and stresses are manifested as rectified or 'dc' waveforms linked to the energy density of an acoustic wave or vibrational mode via the mode nonlinearity parameter of the material. An analytical model is developed for acoustically dispersive media that predicts the evolution of the energy density of an initial waveform into a series of energy solitons that generates a corresponding series of radiation-induced static strains (envelope solitons). The evolutionary characteristics of the envelope solitons are confirmed experimentally in Suprasil W1 vitreous silica. The value (- 11.9 ± 1.43) for the nonlinearity parameter, determined from displacement measurements of the envelope solitons via a capacitive transducer, is in good agreement with the value (- 11.6 ± 1.16) obtained independently from acoustic harmonic generation measurements. The agreement provides strong, quantitative evidence for the validity of the model.     

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.     

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.     

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.     

Observation of an anomalous density minimum in vitreous silica

Vitreous silica is known to have several anomalous variations in its physical properties including a temperature dependent density maximum. We report the results of high-resolution density relaxation experiments that conclusively show for the first time the existence of a density minimum for silica below its glass-transition temperature at approximately 950 degrees C. This unusual density-temperature relationship is shown to be consistent with the existence of a polyamorphic phase transition in silica with a negative Clapeyron slope.     

Anomalous ultrasonic attenuation in vitreous silica at low temperatures

The attenuation of longitudinal sound waves of 750 and 970 MHz has been determined in vitreous silica down to 0.4 K. The unsaturated attenuation observed at the smallest incident acoustic intensities allows a quantitative comparison with the “tunneling model” for glasses.     

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.     

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.     

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.     

掺杂硅石纳米粉末的染料随机激光发射

针对染料随机激光器特性与应用研究,讨论了无序介质中荧光粒子和额外散射粒子的发光性质、浓度变化以及颗粒尺寸与随机激光发射阈值之间的关系。采用时域有限差分法直接求解Maxwell方程组及速率方程组的方法,模拟仿真出了相应的发射谱线及浓度变化与激光发射阈值的关系曲线。所设计的样品中Rh6G-SiO2的质量分数为4%,其阈值大小为8.5 J/pulse时,额外微小散射体TiO2的加入对该介质随机激光发射产生的影响很小,可忽略不计。在此基础上,通过进一步模拟分析了处于不同条件和背景下随机激光的阈值特性。     

掺杂硅石纳米粉末的染料随机激光发射

针对染料随机激光器特性与应用研究,讨论了无序介质中荧光粒子和额外散射粒子的发光性质、浓度变化以及颗粒尺寸与随机激光发射阈值之间的关系。采用时域有限差分法直接求解Maxwell方程组及速率方程组的方法,模拟仿真出了相应的发射谱线及浓度变化与激光发射阈值的关系曲线。所设计的样品中Rh6G-SiO2的质量分数为4%,其阈值大小为8.5 J/pulse时,额外微小散射体TiO2的加入对该介质随机激光发射产生的影响很小,可忽略不计。在此基础上,通过进一步模拟分析了处于不同条件和背景下随机激光的阈值特性。     

Lattice vibrational spectra of vitreous silica densified by pressure

The results of the measurement of Raman scattering spectra and infra-red absorption spectra of densified vitreous silica are reported. Raman active-low frequency vibrations are largely changed by compression.     

掺钛空心玻璃微球制备技术

为实现惯性约束聚变靶用空心玻璃微球的均匀掺杂,采用溶胶-凝胶技术结合炉内成球法,以钛酸四丁酯为掺杂剂,对钛掺杂玻璃溶胶-钛掺杂玻璃凝胶-钛掺杂干凝胶粒子-钛掺杂空心玻璃微球（Ti-HGM）这一掺钛空心玻璃微球的制备方法进行了探索。实验结果表明：掺钛效应使空心玻璃微球壁厚均匀性、同心度有所降低,但对壁厚、直径控制基本无影响;Ti与Si原子分数比为2.23%的Ti-HGM掺杂基本均匀,近70%微球的保气半寿命在一个月以上。