Studies of the thermal expansion of solids in the temperature range 1–100 K are reviewed. Effects due to thermal vibrations of the lattice, electrons, magnetic interactions and defects are fully discussed. The survey of experimental data is intended to be comprehensive. From the large volume of theoretical work in recent years the authors have aimed to select for discussion the most significant contributions.

Thermal expansion of solids at low temperatures

A series of glasses in the TiO2-SiO2 system was prepared by the flame hydrolysis boule process. Clear glasses containing as much as 16.5 wt% TiO2 were obtained. More TiO2 caused opacity due to phase separation and anatase/rutile crystallization during glass boule formation. Glasses in the 12 to ∼17 wt% TiO2 range were metastable and showed structural rearrangements on heat treatment at temperatures as low as 750CEC (∼200° below the annealing point). These changes were accompanied by large changes in thermal expansion. Thermal treatment can be designed to produce almost any desired expansion between α-200+700=−5 × 10-7/°C and +10 × 10-7/°C. Zero expansion between 0 and 550°C was obtained. Evidence that these changes are due to phase separation and anatase formation is presented. Viscosity data in the glass transition range, refractive index, and density are also presented.

Binary Titania-Silica Glasses Containing 10 to 20 Wt% TiO2

Since the time of Galilei 400 years ago the progress of optical systems was restricted due to the lack of optical glass types with different dispersion properties and due to poor material quality. With the work of Otto Schott, which started 125 years ago, glass became a tailorable, highly reproducible and homogeneous material, thus enabling systematic design of optical systems. The demand for new glass types is still going on as well as the requirement for ever tighter tolerances and their proofs. New measurement methods provide deeper insight in the material properties. Developments in processing allow new optical elements to be designed, further advancing technology. This also holds for zero-expansion glass ceramics, another key enabling material for optical systems. This publication highlights some milestones in the history of optical glass and glass ceramics, comments on present day glass development as well as new optical elements and measurement methods and provides some new information on the materials’ properties.

Optical glass and glass ceramic historical aspects and recent developments: a Schott view

A combination of conventional and synchrotron-based techniques has been used to characterize the size-dependent structural and thermal properties of Ge nanocrystals (NCs) embedded in a silica $(a{\text{-SiO}}_{2})$ matrix. Ge NC size distributions with four different diameters ranging from 4.0 to 9.0 nm were produced by ion implantation and thermal annealing as characterized with small-angle x-ray scattering and transmission electron microscopy. The NCs were well represented by the superposition of bulklike crystalline and amorphous environments, suggesting the formation of an amorphous layer separating the crystalline NC core and the $a{\text{-SiO}}_{2}$ matrix. The amorphous fraction was quantified with x-ray-absorption near-edge spectroscopy and increased as the NC diameter decreased, consistent with the increase in surface-to-volume ratio. The structural parameters of the first three nearest-neighbor shells were determined with extended x-ray-absorption fine-structure (EXAFS) spectroscopy and evolved linearly with inverse NC diameter. Specifically, increases in total disorder, interatomic distance, and the asymmetry in the distribution of distances were observed as the NC size decreased, demonstrating that finite-size effects govern the structural properties of embedded Ge NCs. Temperature-dependent EXAFS measurements in the range of 15--300 K were employed to probe the mean vibrational frequency and the variation of the interatomic distance distribution (mean value, variance, and asymmetry) with temperature for all NC distributions. A clear trend of increased stiffness (higher vibrational frequency) and decreased thermal expansion with decreasing NC size was evident, confirming the close relationship between the variation of structural and thermal/vibrational properties with size for embedded Ge NCs. The increase in surface-to-volume ratio and the presence of an amorphous Ge layer separating the matrix and crystalline NC core are identified as the main factors responsible for the observed behavior, with the surrounding $a{\text{-SiO}}_{2}$ matrix also contributing to a lesser extent. Such results are compared to previous reports and discussed in terms of the influence of the surface-to-volume ratio in objects of nanometer dimensions.

Size-dependent characterization of embedded Ge nanocrystals: Structural and thermal properties

Numerical Data on Thermal Linear Expansion of Nonmetallic Solids

Recent critical applications for ultralow expansion materials have required thermal expansion measurements with very high precision and accuracy. A Fizeau interferometer employing a helium-neon laser has given a precision of +/-0.1 x 10(-6) cm/cm. A second, more rapid method consists of a rod type vitreous silica dilatometer. A stirred water bath is used for specimen temperature control and a high output, linear variable differential transformer serves as the extensometer. Precision of this method is +/-0.5 x 10(-6) cm/cm. Calibration procedures and results for several low expansion materials are discussed.

W A Plummer

Papers

Precision thermal expansion measurements on low expansion optical materials.