Thermodynamic Properties of Amorphous Solids —Glass Formation and Glass Transition— (Overview)

Considerations for the selection of co-formers in the preparation of co-amorphous formulations

Co-amorphous drug delivery systems are evolving as a credible alternative to amorphous solid dispersions technology. In Co-amorphous systems (CAMs), a drug is stabilized in amorphous form using small molecular weight compounds called as co-formers. A wide variety of small molecular weight co-formers have been leveraged in the preparation of CAMs. The stability and supersaturation potential of prepared co-amorphous phases largely depend on the type of co-former employed in the CAMs. However, the rationality behind the co-former selection in co-amorphous systems is poorly understood and scarcely compiled in the literature. There are various facets to the rational selection of co-former for CAMs. In this context, the present review compiles various factors affecting the co-former selection. The factors have been broadly classified under Thermodynamic, Kinetic and Pharmacokinetic-Pharmacologically relevant parameters. In particular, the importance of Glass transition, Miscibility, Liquid-Liquid phase separation (LLPS), Crystallization inhibition has been deliberated in detail.

Amorphization of Ceramic Materials by Ion-Beam-Irradiation: Parallels to Glass Formation

Ion-beam-induced amorphization of a wide variety of ceramic materials has been investigated using in situ TEM at the HVEM-Tandem Facility at Argonne National Laboratory with 1.5 MeV Kr+ or Xe+ ions at temperatures between 20 to 1000 K. The critical amorphization temperatures and the activation energies associated with the expitaxial recovery of displacement cascades during irradiation have been determined from the temperature dependence of the critical amorphization dose. The results for phases in the A12 O3-MgO-SiO2 system suggested a parallel in the kinetics between ion-beam-induced amorphization and glass formation. Based on a cascade quenching model, a semiempirical parameter, S, which can easily be calculated from both structural and chemical parameters of a material, has been developed to predict the susceptibility of ceramics to amorphization. The critical amorphization temperature, above which irradiationinduced amorphization cannot be completed, is closely related to the glass transition temperature. The ratio between glass transition and melting temperatures can also be used to predict the susceptibility of a ceramic material to amorphization, equivalent to the Debye temperature criterion.

Metastability and Properties of Metallic Bulk Glass Forming Alloys

Glasses are generally produced from the highly undercooled liquid state by rapid quenching methods or quasi-statically at slow cooling by the effective control of potent heterogeneous nucleation sites. For metallic systems the latter method recently has led to the development of bulk metallic glass with a complex multicomponent chemistry and advanced engineering properties. With these alloys crystallization can be avoided over a broad temperature / time window. As such, the relevant thermodynamic properties of the metastable glassy and undercooled liquid states can be directly measured below and above the glass transition temperature, respectively. The obtained data give new insight into the nature of the glass transition suggesting that it is not a phase transition in the classical sense but kinetic freezing triggered by an underlying entropie instability to avoid crystallization. Further measurements of the mechanical and wear properties point to the unique engineering properties of these highly disordered materials for technological applications.

Processing of bulk metallic glass

Bulk metallic glass (BMG) formers are multicomponent alloys that vitrify with remarkable ease during solidification. Technological interest in these materials has been generated by their unique properties, which often surpass those of conventional structural materials. The metastable nature of BMGs, however, has imposed a barrier to broad commercial adoption, particularly where the processing requirements of these alloys conflict with conventional metal processing methods. Research on the crystallization of BMG formers has uncovered novel thermoplastic forming (TPF)-based processing opportunities. Unique among metal processing methods, TPF utilizes the dramatic softening exhibited by a BMG as it approaches its glass-transition temperature and decouples the rapid cooling required to form a glass from the forming step. This article reviews crystallization processes in BMG former and summarizes and compares TPF-based processing methods. Finally, an assessment of scientific and technological advancements required for broader commercial utilization of BMGs will be made.

Heterogeneous dynamics on the microsecond scale in simulated Ni0.5 Zr0.5 metallic melts far below the glass temperature

From molecular dynamics simulations for up to 2.5 micros, results are reported on structural dynamics in glass-forming metallic Ni(0.5) Zr(0.5) melts at 700 and 785 K. After elimination of high-frequency fluctuations, cascades of atoms and avalanches of atom chains, local in space and time, are identified as complex processes that govern the heterogeneous low-frequency dynamics in sufficiently aged structures near the glass temperature.

Characteristics and significance of the amorphous state in pharmaceutical systems

The amorphous state is critical in determining the solid-state physical and chemical properties of many pharmaceutical dosage forms. This review describes the characteristics of the amorphous state and some of the most common methods that can be used to measure them. Examples of pharmaceutical situations where the presence of the amorphous state plays an important role are presented. The application of our current knowledge to pharmaceutical formulation problems is illustrated, and some strategies for working with amorphous character in pharmaceutical systems are provided.