High-density Amorphous State Research Articles

Amorphous-amorphous transitions in silica glass. II. Irreversible transitions and densification limit

In part I, we showed that the thermomechanical anomalies of silica glass are due to the reversible structural transitions that affect the intermediate-range order and that are brought about by atomic displacement modes similar to those underlying the $\ensuremath{\alpha}$-to-$\ensuremath{\beta}$ phase transformations in cristobalite silica. In this paper, we show that polyamorphic transitions can be irreversible, in particular under large compressive stresses, and provided adequate thermal activation for the necessary bond exchanges to take place. Under these conditions a stable high-density form of amorphous silica develops, which is characterized by larger ring sizes but unchanged near-range structural order. In this high-density amorphous state another anomalous feature of silica glass, i.e., negative thermal expansion, is accentuated. Pressure and temperature effects on the permanent densification of silica, as well as the nature of the newly discovered amorphous phases and its implications on the physical properties of amorphous silica are discussed.

Kinetics of the high- to low-density amorphous water transition

In situ neutron diffraction experiments have been carried out to study the kineticsof the transformation of high-density amorphous (HDA) water into itslow-density amorphous state at temperatures 87 K ≤ T ≤ 110 K. It isfound that three different stages are comprised in this transformation, namely anannealing process of the high-density matrix followed by a first-order-like transition into alow-density state, which can be further annealed at higher temperatures T ≤ 127 K.The annealing kinetics of the HDA state follows the logarithm of time as found inother systems showing polyamorphism. According to the theory of transformationby nucleation and growth the apparent first-order transition follows anAvrami–Kolmogorov behaviour. An energy barrier ΔE ≈ 33 k Jmol−1is estimated from the temperature dependence of this transition.

Pressure-induced amorphization of ZrTiCuNiBe bulk glass-forming alloy

Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 alloy can be cooled under high pressure (HP) to a bulky glassy state at very low cooling rates. The structure and properties of the bulk metallic glass (BMG) are investigated by differential scanning calorimetry, x-ray diffraction, ultrasonic study, and density measurements. The effects of pressure on the glass formation are discussed from the points of view of nucleation kinetics and thermodynamics. The BMG obtained under HP is in a high-density amorphous state that is different in structure and property from low-density amorphous phase quenched in water. Solidification under HP is a promising way not only for synthesizing BMGs with more densely packed structures and different properties, but also for understanding the glass formation mechanism.