共查询到20条相似文献,搜索用时 31 毫秒
1.
Differential scanning calorimetry, thermally stimulated depolarization currents and dielectric relaxation spectroscopy techniques, covering together a broad frequency range of 10−4 to 106 Hz, were employed to investigate the effects of in situ synthesized titania nanoparticles on thermal transitions, segmental dynamics and interfacial interactions in poly(dimethylsiloxane)/titania nanocomposites. Titania particles (TiO2, 20-40 nm in diameter) were prepared and well dispersed into the polymer network through sol-gel technique, aiming at stable and mechanically reinforced systems. The interactions between polymer and fillers were found to be strong, supressing crystallinity and affecting the temperature development of the glass transition. The segmental relaxation associated with the glass transition consists of three contributions, arising, in the order of decreasing mobility, from the bulk (unaffected) amorphous polymer fraction (α relaxation), from polymer chains restricted between condensed crystal regions (αc relaxation), and from the semi-bound polymer in an interfacial layer with strongly reduced mobility due to interactions with hydroxyls on the nanoparticle surface (α? relaxation). The thickness of the interfacial layer was estimated to be in the range of 3-5 nm. Measurements using different thermal protocols proved very effective in analyzing the origin of each relaxation and the respective effects of filler addition. 相似文献
2.
J.M. Pelletier 《Journal of Non》2008,354(31):3666-3670
The effect of an annealing at a temperature above or below the glass transition temperature in a Zr41.2Ti13.8Cu125Ni10.0Be22.5 bulk metallic glass was investigated using dynamic mechanical analysis. Structural relaxation influences both the storage modulus (elastic component) and the loss modulus (viscoelastic component). Kinetics can be captured by a stretched exponential relaxation function. Experimental results are correctly described using a physical model based on the concept of defects for the mechanical response of amorphous materials and especially for the characteristic time relative to atomic mobility. 相似文献
3.
I. Avramov 《Journal of Non》2009,355(1):23-2633
Kinetics of structural relaxation and crystallization of NaPO3 glassforming melt is studied by means of thermal analyses. It is demonstrated that activation energy depends strongly on fictive temperature Tf. The dependence of the onset temperature To of the glass transition interval on the heating rate q+ is investigated for samples that were previously cooled down at a rate q− of about 850 K/min. The dimensionless fragility F is a measure of the dependence of the activation energy for spatial rearrangement on the changes of structure. According to the present results F is large for NaPO3 (i.e. phosphates are ‘fragile’ substances). 相似文献
4.
V.I. Dimitrov 《Journal of Non》2006,352(3):216-231
This is a presentation of a rigorous theory of fluidity of liquids, glass transition and melting of solids in the frame of an asymmetric double well potential model. Potential wells are doubled time to time by the local density fluctuations caused by the thermal longitudinal waves. The average frequency of doubling of potential wells is equal to the frequency of the most energetic waves which obey a law similar to Wein’s displacement law in black body radiation. Based on the equilibrium thermodynamic theory of fluctuations and the displacement law, a law of linear pre-diffusion mean-square displacement of particles in a solid is derived: the mean-square displacement of molecules within their potential wells increases linearly with temperature. It is shown that when this is broken-down (where the mean-square displacement at a certain temperature rapidly changes its slope as a function of temperature) glass devitrifies and crystal melts, and all possible solid-liquid transitions of a substance occur at the same critical mean-square displacement: any solid (not only crystals) transforms into liquid when the mean-square displacement, as a fraction of the average intermolecular distance, acquires a certain universal critical value - the same for different substances. It is proved that molecules in a liquid perform specific Brownian motion. The average jump distance is a function of temperature and it is much smaller than the nearest intermolecular distances. At a certain temperature, shown to be the Kauzmann temperature, the average jump distance of Brownian motion becomes equal to zero: the super-cooled liquid undergoes glass transition. The transition was proven to be a phase transition of the fourth order: the free energy of the system and its first, second and third derivatives are all continuous functions, but its fourth derivative with respect to temperature is discontinuous. Molecular mobility, diffusion and viscosity are obtained as functions of temperature. 相似文献
5.
The oxyfluoride glass ceramics are important up-conversion luminescent materials. Er3+/Yb3+-codoped transparent oxyfluoride glasses were prepared by melt-quenching and subsequently heat-treated at different times and temperatures, and the crystallization process of fluoride nanocrystals from the glass was investigated. X-ray diffraction (XRD) and fluorescence spectra investigations reveal that fluoride nanocrystals are distributed homogeneously among the glassy matrix for the sample doped with Er3+/Yb3+. The crystallization process indicates that heat-treatment temperature influences the size of fluoride nanocrystals, while heat-treatment time influences their concentration. Moreover, the red emission intensities increase due to the incorporation of Er3+/Yb3+ into the fluoride nanocrystals under different heat-treatment methods, which are studied by fluorescence spectra. 相似文献
6.
Glasses of the system: (70−x) TeO2 + 15B2O3 + 15P2O5 + xLi2O, where x = 5, 10, 15, 20, 25 and 30 mol% were prepared by melt quench technique. Dependencies of their glass transition temperatures (Tg) and infrared (IR) absorption spectra on composition were investigated. It is found that the gradual replacement of oxides, TeO2 by Li2O, decreases the glass transition temperature and increases the fragility of the glasses. Also, IR spectra revealed broad weak and strong absorption bands in the investigated range of wave numbers from 4000 to 400 cm−1. These bands were assigned to their corresponding bond modes of vibration with relation to the glass structure. 相似文献
7.
A series of calcium aluminosilicate liquids have been experimentally heat-treated at high but variable states of undercooling, from just above the glass transition to the vicinity of the solidus. The mineralogy and chemistry of crystalline phases which appear in these experiments have been quantified using a combination of Transmission Electron Microscopy and Raman spectroscopy. The results show that mineral compositions are highly variable as a function of temperature, but that changes are governed by the contrasting and strongly temperature-dependent mobilities of network-modifying and network-forming cations. Whereas equilibrium crystals form near the liquidus, disordered and non-stoichiometric phases precipitate near the glass transition. Despite this apparently complex situation, the relative importance of thermodynamic and kinetic factors is found to be a single function of T − Tg (where T is temperature and Tg the glass transition temperature), regardless of the silicate composition. The existence of this mastercurve may be used to control the composition of novel composite materials such as glass ceramics. 相似文献
8.
Yanmin Yang Baojiu Chen Cheng Wang Qingyu Meng Xiaoxia Zhao Weihua Di Xiaojun Wang Lihong Cheng Yong Peng 《Journal of Non》2008,354(31):3747-3751
Erbium-doped glasses with composition xGeO2-(80 − x)TeO2-10ZnO-10BaO were prepared by melt-quenching technique. The phonon sideband spectra and the optical absorption band edges for the host matrix were confirmed by means of the spectral measurements. Standard Judd-Ofelt calculations have been completed to these glasses. The dependence of up-conversion and infrared emission under 980 nm excitation on the glass composition was studied. The quantum efficiencies for the 4I13/2 → 4I15/2 transition of trivalent erbium in the glasses were estimated. 相似文献
9.
The influences of different alkali and alkali-earth oxide substitutions on the properties of lithium-iron-phosphate (LIP) glasses have been studied. Na2O, K2O, MgO, CaO and BaO were used to substitute Li2O to prepare LIP glasses with molar compositions of (20 − x)Li2O − xR2O(RO) − 30Fe2O3 − 50P2O5 (x = 2.4, 4, 5.6 and 7.2). The glass transition temperature (Tg) was determined by the differential thermal analysis technique. The density and chemical durability of the prepared glasses were measured based on the Archimedes principle and the weight losses after the glasses were boiled in water. The results show that Tg decreases with the initial substitutions, whereas the density and chemical durability increase. The diminution of the aggregation effect of Li+ ions on the glass structure due to the decrease in Li+ concentration, the larger molecule weights of the substitutes, the mixed-alkali and depressing effects as well the slower mobility of substitute ions mainly contribute to the initial changes in Tg, density and chemical durability of the LIP glasses, respectively. Further increasing the amounts of substitutes brings about increasing diminution of the aggregation effect of Li+ ions and breakage of the glass network on the one hand and increasing amounts of substitutes with larger molecule weights and ion radii on the other hand. Both aspects influence the glass properties oppositely and consequently non-monotonic variations in the properties of LIP glasses with the substitutions are observed. 相似文献
10.
Free volume model is widely used as a tool for understanding the nature of glass transition phenomenon and the related consequences. Recently, an analytical derivation of Kissinger's equation has been proposed in literature using free volume model. In this derivation, the glass transition activation energy has been assumed constant throughout the whole glass transition temperature range. The present paper investigates the applicability of free volume model for derivation of Kissinger's relation by checking the constancy of the glass transition activation energy (Eg) throughout the glass transition temperature range. Performing series of experiments, we have legalized the possibility for usage of the Kissinger relation for determination of the glass transition activation energy if the peak value of the endo-thermal effect is taken as temperature of glass transition. 相似文献
11.
Two series (A and B series) of oxynitride glasses were prepared by melting batches at 1580 °C for 3 h under local CO reducing atmosphere in a Si-Mo-heated resistance furnace. Nominal compositions of A and B series glasses in equivalent percent (eq.%) are (28−x)Y:xMg:48Si:24Al:83O:17N and (28−x)Y:xMg:56Si:16Al:83O:17N (x = 0, 7, 14, 21), respectively. The influences of Mg/Y and Al/Si ratios on the properties such as glass transition temperature (Tg), crystallization temperature (TC), knoop hardness (H), three-point bending strength (σ) and chemical durability in 20%HF were investigated. At the same time, the relationship between these properties and the structures of the glasses were discussed. At constant ratio Si-Al-O-N, Tg decreases nonlinearly but glass leaching ratio increases linearly with increasing Mg/Y ratio. However, H and σ increase first and then decrease as the Mg/Y ratio increases. When the Y/Mg/O/N ratio is constant, Tg decreases slightly but H and σ increase slightly as the Al/Si ratio increases. 相似文献
12.
《Journal of Non》1999,243(2-3):116-122
Viscosities of GexS1−x melts (0.30⩽x⩽0.44) have been measured by penetration viscometry from 107 to 1013 Pa s. The temperature dependence of equilibrium viscosities in this range can be expressed approximately by a simple Arrhenius equation. Both the heat capacity change at the glass transition and the activation energy of viscous flow monotonously increase with germanium content as predicted by the Adam–Gibbs theory. Therefore, the connectivity of the germanium–sulfur network is reduced due to decreasing concentration of sulfur which causes increasing fragility of the undercooled liquid. The glass transition temperature exhibits a maximum near the GeS2 composition where heteropolar bonds are predominantly formed. 相似文献
13.
Viscometry, ultrasonic echography and mechanical spectroscopy were applied to explore the thermo-mechanical properties of a photo-thermo-refractive (PTR) glass. Newtonian shear viscosity and ultrasonic velocity were used to determine the Maxwell time of structural relaxation. Two fast relaxation modes are found in PTR glass below glass transition temperature, which are attributed to alkali and mixed cation mobility. Respective activation energies, Eγ = 72 ± 4 kJ mol−1 and Eβ = 117 ± 4 kJ mol−1, are considerably lower than that of viscous flow at the glass transition Eα ≈ 465 kJ mol−1 as quantified from mechanical spectroscopy. Decoupling ratios of Eγ/Eα = 0.155 and Eβ/Eα = 0.255 are similar to those of sodium trisilicate glass but at considerably longer timescales. These observations imply that β- and γ-relaxations are delayed as local structural arrangements are more complex for the multi-component PTR glass. 相似文献
14.
Saswati Ghosh 《Journal of Non》2008,354(34):4081-4088
Several compositions based on BaO-CaO-Al2O3-SiO2 (BCAS) glass system have been studied in this investigation to see their applicability as sealant for solid oxide fuel cell (SOFC). The glasses as well as the corresponding glass-ceramics have been systematically characterized by differential thermal analysis, dilatometry, X-ray diffractometry, electron microscopy and impedance analysis to examine their suitability as sealant. While the glass transition temperature (Tg) determined from DTA are within 600-665 °C, the coefficient of thermal expansion (CTE) can be tailored between 9.5 and 13.0 × 10−6 K−1. These glasses are found to be well adhered with metallic interconnects, such as commercial ferritic steel (Crofer22APU), at an optimum sealing temperature of 850 °C. The shrinkage behavior of the developed glasses in their pellet form has also been investigated. The resistivities of the glass-ceramics, as obtained from impedance analysis, are found to be within 104-106 Ω cm at 800 °C. Under sandwiched condition between two metals, some of the developed compositions are found to maintain this high resistivity even after 100 h of operation. One of the glass compositions has shown a low leak-rate of the order of ∼10−7 Pa m2 s−1. 相似文献
15.
Our investigation demonstrates the role of interfaces for molecular mobility of glasses. Usually, the role of the surface layer is neglected because it contains about 10−8 of the number of molecules of the sample. However, the interface is important in several cases: For the thin films; for the processes taking part at the interface (e.g. surface crystallization) and in the cases when bulk molecules are practically immobile. We define a critical temperature Tc, below which surface molecules have more important contribution then the bulk ones. As soon as this happens in the vicinity, or rather below, the glass transition temperature, the samples behave solid like. So, the contribution of the surface mobility is not sufficient to cause changes in human life period but it is large enough to permit tiny changes in historical periods. This could be an explanation why cathedral glasses are a couple of microns thicker at the bottom side. 相似文献
16.
The effects of TM and RE elements on glass formation ability (GFA) of the Al-TM-RE systems are studied systematically. The TM elements show distinct differences: critical sizes of the Al-TM-Ce systems are in the order of Ni > Co > Fe > Cu. However, the RE elements show similar effect on glass formation. These results are discussed in terms of the GFA-related factors, i.e., reduced glass transition temperature (Trg = Tg (Tx)/TL, where Tg, Tx and TL are the glass transition temperature, onset crystallization temperature and liquidus temperature, respectively) and the mixing enthalpy ΔHAl-TM. A new parameter of TL − Tg (Tx) is potentially proposed and used to evaluate the GFA of the Al-based alloys, as well as bulk glass-formers. 相似文献
17.
Er3+ doped multicomponent fluoride based glass was prepared. These precursor fluoride glass samples were then heated using different schedules. Crystalline phase particles were successfully precipitated in the multicomponent fluoride glass samples after heat treatment. The influence of heat treatment on the spectroscopic properties of Er3+ in multicomponent fluoride based glass samples were discussed. Small changes of the Judd-Ofelt parameters Ωi (i = 2, 4, 6) were found in multicomponent fluoride glass samples before and after heat treatment compared to oxyfluoride telluride glass. Preparation conditions used to produce transparent multicomponent fluoride glass ceramics doped with rare-earth ions are discussed. 相似文献
18.
The correlations between the pressure sensitivity and the fragility/glass transition temperature have been addressed in various bulk metallic glasses in the present work. The results demonstrate that the pressure sensitivity of bulk metallic glasses is closely related to both the fragility index (m) and the glass transition temperature (Tg). The physical origin of the correlations has been discussed from their disordered structure, which is determined by the glass transition behavior and the glass transition temperature. 相似文献
19.
We report the fabrication of waveguides using the fiber on glass (FOG) method. Taking advantage of a Thermal Mechanical Analyzer (Shimadzu TMA-50), we were able to produce a new type of waveguide by coupling an erbium doped fiber core onto a planar glass substrate. Both optical fiber core and substrate were fabricated from tellurite glass. Important thermal characteristics of the substrate and fiber like the transition temperature Tg, the temperature for the crystallization onset Tx and the maximum crystallization temperature Tc were determined by Differential Thermal Analysis (DTA). The thermal expansion coefficient of the tellurite glass was determined by Thermal Mechanical Analysis (TMA). 相似文献
20.
Akira Doi 《Journal of Non》2006,352(5):469-472
In an ion-conducting glass, there may be the ionic transition which characterizes ‘melting’ of the modifying ions in a similar way as the glass transition characterizes ‘melting’ of the glass network former. The ionic transition can be observed, electrically, by the thermally stimulated depolarization current technique. It is demonstrated that the relaxation time for the ionic transition is the same (near 100 s) with that for the glass transition in so far as the heating/cooling rate lies within, say, 10−1-10−5 K/s as adopted in our routine works. 相似文献