Yu Matsuda

Brillouin Scattering Study of Liquid Glass Transition in Lithium Borate Glass

Elastic properties of different compositions of lithium borate glasses xLi2O(1-x)B2O3 (x=0.06–0.28) were investigated in the temperature range from 25 to 600 °C by Brillouin scattering spectroscopy. The composition dependence of the lithium borate glass system shows a stiffening tendency and a higher onset temperature of softening with increasing mole fraction of Li2O. We successfully observed the change in longitudinal elastic modulus in the vicinity of the glass transition temperature (Tg). The elastic moduli show a marked temperature dependence above the glass transition temperature Tg, while they show little change below Tg. These results strongly relate to the structural conversion of cross-linking borate units from triangular BO3 to tetrahedral BO4 units by the addition of Li2O. The increase in lithium oxide enhances the glass network integrity in the composition range of this study. A significant decrease in the elastic modulus above Tg can be attributed to the association of boroxol ring deformation and a change in the coordination number of boron atoms. These elastic properties were discussed in comparison with lithium silicate glass.

Glass Transitions and Elastic Properties of Lithium Borate Glasses over a Wide Composition Range Studied by Micro-Brillouin Scattering

The elastic properties of lithium borate glasses, denoted by the composition formula xLi2O(100 - x)B2O3 where x is the molar concentration in mol %, have been investigated over a wide composition range 6 ≤x ≤52 mol % by micro-Brillouin scattering. From the observed values of the longitudinal and transverse sound velocities, the elastic constants such as longitudinal modulus, shear modulus, Youngs modulus, and Poissons ratio have been determined. The elastic constants have shown a strong composition dependence due to the variation of intermediate structural units. The temperature dependences of the longitudinal sound velocity and absorption coefficient of 44Li2O56B2O3 clearly show the anomalies at the glass transition and crystallization temperatures.

Elastic Properties of Potassium Borate Glass in a Wide Composition Range Studied by Brillouin Scattering

The elastic properties of potassium borate glass, xK2O·(100 - x)B2O3, where x is the molar composition of K2O in mol %, have been investigated by Brillouin scattering spectroscopy over a wide composition range of 2 ≤x ≤42 mol %. From the observed values of longitudinal sound velocity, the elastic constant has been determined and compared with that of lithium borate glass. The sound velocity increases with increasing x below x = 30; however, for a further increase in x, it decreases owing to the softening caused by the formation and increase in the number of nonbridging oxygen atoms. The absorption coefficient also increases markedly above x = 30 owing to the scattering of acoustic waves by nonbridging oxygen atoms. The temperature dependences of both sound velocity and absorption coefficient of 14K2O·86B2O3 show a markedly change at approximately the glass transition temperature Tg of 395 °C.

Dynamic Specific Heat, Glass Transition, and Non-Debye Nature of Thermal Relaxation in Lithium Borate Glasses

The physical properties of lithium borate glasses, x Li 2 O·(100- x )B 2 O 3 , vary anomalously with the lithium oxide content, known as “borate anomaly”. In this study, the compositional variations of the dynamic glass transition temperature T g ω and the non-Debye nature (or stretched exponentiality β KWW ) in the vicinity of T g have been investigated over a wide composition range. Since T g ω depends on the frequency of the temperature modulation, a more precise composition dependence can be discussed. The trend of T g ω shows a single broad maximum at about x = 26–28. Moreover, the composition dependence of the non-Debye behavior has been presented. It shows a broad maximum at about x = 40 mol %. It is found that β KWW is systematically controlled using the fraction of each intermediate structural unit, particularly the tetrahedron composed of the 4-coordinated boron atom. Both the composition dependences of T g ω and β KWW can be explained using the structure-superposition model.

Composition Dependence of the Boson Peak and Universality in Lithium Borate Binary Glasses: Inelastic Neutron and Raman Scattering Studies

We study the low-energy vibrational dynamics, called the boson peak (BP), of binary lithium borate glasses as a function of the composition of these glasses by inelastic neutron and Raman scattering. Firstly, we analyze the variations in the properties of the BP with the composition. The position and intensity of the BP linearly change with increasing x owing to the changes in the intermediate glass structures. Secondly, we demonstrate that all spectral shapes with different compositions can be scaled by a single master curve, and are the same for all spectra. The results suggests that there is a universal distribution of the vibrational density of states, which does not change with the composition, even though the structures of lithium borate glasses markedly change. The effect of Li 2 O doping can be understood to be a chemical structure-induced densification.

High Temperature Brillouin Scattering of Potassium Borate Glasses

High temperature elastic properties of potassium borate glasses, xK2O(100-x)B2O3 (x=4, 10, 14, 20, 28, and 34 mol %), have been investigated between 20 and 1100 °C by Brillouin scattering. Longitudinal sound velocity of the glasses changes only slightly up to the glass transition temperature (Tg), and, with further heating above Tg, it decreases markedly. This sudden decrease in sound velocity is caused by structural rearrangements that make glass networks soft above Tg. With an increase in K2O composition, the slope of sound velocity just above Tg increases. The composition dependence of the fragility of potassium borate glass is suggested on the basis of the slope. The activation energy of the relaxation process of 28K2O72B2O3 measured by Brillouin scattering is estimated to be 6.84 kcal/mol, which is comparable to the energy of formation of a boroxol ring structure in a liquid phase.

Fragility Variation of Lithium Borate Glasses Studied by Temperature‐Modulated DSC

The fragility of lithium borate glass system has been investigated by Temperature‐Modulated Differential Scanning Calorimetry (TMDSC). The frequency and temperature dependences of dynamic specific heat have been observed in the vicinity of a glass transition temperature Tg. It is shown that the value of the fragility index m can be determined from the temperature dependence of the α‐relaxation times observed by TMDSC, when the raw phase angle is properly corrected. The composition dependence of the fragility has been also discussed.

Complex Heat Capacity of Lithium Borate Glasses Studied by Modulated DSC

Complex heat capacity, Cp* = Cp′ − iCp″, of lithium borate glasses Li2O⋅(1−x)B2O3 (x = 0.00 – 0.33) has been investigated by Modulated DSC (MDSC). We have successfully observed the frequency dependent Cp* by MDSC in the frequency range 0.01 to 0.1 Hz, and the average relaxation time of glass transition has been determined as a function of temperature. Moreover, the composition dependence of the thermal properties has been investigated. The calorimetric glass transition temperatures become higher with the increase of concentration of Li2O and show the board maximum around x = 0.26–0.28. The width of glass transition region becomes narrower as Li2O increases. These results relate to the change of the fragility of the system. It has been proven that the complex heat capacity spectroscopy by MDSC is a powerful tool to investigate the glass transition phenomena.

Elastic Properties of Lithium Germanate Glasses Studied by Brillouin Scattering

To investigate the correlation between structural changes and physical properties, the elastic properties of lithium germanate glasses, xLi2O(100-x)GeO2 represented as a function of Li2O mole fraction, have been studied in the composition range 6≤x≤36 mol % by Brillouin scattering spectroscopy. Raman spectra have also been measured to clarify the composition variation of structural changes. Several elastic properties, such as sound velocity and elastic modulus, have been determined from the Brillouin shift. The longitudinal and transverse sound velocities increase up to about x=20 mol %, and above x=20 mol %, they become nearly constant. Elastic moduli, such as longitudinal modulus (L) and shear modulus (G), increase gradually up to x=22 mol % and then decrease with a further increase in Li2O content. It is considered that such behaviors are due to the change in the coordination number of germanium atoms and the formation of nonbridging oxygen. Differently from other elastic moduli, Poissons ratio (σ) shows a similar behavior to sound velocity.