Seiji Kojima

By Seiji Kojima

The boson peak in the terahertz range is the low-energy excitations in glasses and disordered crystals. It is related to the excess part of the vibrational density of states. Borate glass is one of the typical network oxide glasses with covalent bonds and belongs to the strong type of glass formers. Alkali metal ions are well-known modifiers of the borate glass network and control various properties. The alkali metal effects are reviewed on basic physical properties such as elastic constants, density, and vibration modes in relation to the variation of structural units in a modified borate glass network. The alkali effect on a boson peak is discussed on the basis of experimental results of neutron inelastic scattering, neutron diffraction, Raman scattering, and heat capacity at low temperatures. The correlation is discussed between the boson peak frequency, the peak temperature of excess heat capacity, and shear modulus. The static and dynamical correlation lengths are also discussed.

Part of the book: Characteristics and Applications of Boron

By Seiji Kojima

Materials science is the interdisciplinary field to study material properties and their functionality on the basis of physics, chemistry, metallurgy, and mineralogy. Vibrational spectroscopy such as infrared spectroscopy and Raman spectroscopy is a powerful tool to investigate characteristic atomic vibrations. Especially, in the terahertz frequency range, vibrational modes are related to collective atomic vibrations reflecting interatomic/molecular interactions, characteristic units, and medium range order. Recent progress of terahertz vibrational spectroscopy using terahertz-time-domain spectroscopy, terahertz time-domain spectroscopic ellipsometry, and far-infrared spectroscopy is reviewed in advanced materials science on glassy and crystalline pharmaceuticals, ferroelectrics, and polar metallic materials. Using the terahertz spectra, phonons, polaritons, and conduction electrons of these materials are discussed.

Part of the book: Trends in Terahertz Technology

By Seiji Kojima

The dynamical properties such as fragility, and non-Debye behavior of glass-forming materials have been studied by the frequency-dependent dynamic specific heat. Kubo’s formula on the fluctuation-dissipation theorem defines dynamic specific heat using the correlation function of enthalpy fluctuations. Dynamic specific heat is important for analyzing and understanding various relaxation processes. The dielectric relaxation is caused only by polar atomic motions, while the enthalpy relaxation is caused by total degrees of freedom of atomic motions. This chapter introduces two experimental methods to measure dynamic specific heat: (1) temperature-modulated differential scanning calorimetry (MDSC) and (2) photoacoustic spectroscopy. The experimental results of the dynamical properties of glass transitions in oxide glasses with covalent bond network structures and hydrogen-bonded glass-forming materials are reviewed.

Part of the book: Innovative Heat Exchanger Technologies, Developments and Applications