An official website of the United States government
Abstract Optoelectronics are crucial for developing energy‐efficient chip technology, with phase‐change materials (PCMs) emerging as promising candidates for reconfigurable components in photonic integrated circuits, such as nonvolatile phase shifters. Antimony sulfide (Sb2S3) stands out due to its low optical loss and considerable phase‐shifting properties, along with the non‐volatility of both phases. This study demonstrates that the crystallization kinetics of Sb2S3can be switched from growth‐driven to nucleation‐driven by altering the sample dimension from bulk to film. This tuning of the crystallization process is critical for optical switching applications requiring control over partial crystallization. Calorimetric measurements with heating rates spanning over six orders of magnitude, reveal that, unlike conventional PCMs that crystallize below the glass transition, Sb2S3exhibits a measurable glass transition prior to crystallization from the undercooled liquid (UCL) phase. The investigation of isothermal crystallization kinetics provides insights into nucleation rates and crystal growth velocities while confirming the shift to nucleation‐driven behavior at reduced film thicknesses—an essential aspect for effective device engineering. A fundamental difference in chemical bonding mechanisms was identified between Sb2S3, which exhibits covalent bonding in both material phases, and other PCMs, such as GeTe and Ge2Sb2Te5, which demonstrate pronounced bonding alterations upon crystallization.
more »
« less
Modeling nonisothermal crystallization in a BaO∙2SiO 2 glass
Abstract The accuracy of a differential thermal analysis (DTA) technique for predicting the temperature range of significant nucleation is examined in a BaO∙2SiO2glass by iterative numerical calculations. The numerical model takes account of time‐dependent nucleation, finite particle size, size‐dependent crystal growth rates, and surface crystallization. The calculations were made using the classical and, for the first time, the diffuse interface theories of nucleation. The results of the calculations are in agreement with experimental measurements, demonstrating the validity of the DTA technique. They show that this is independent of the DTA scan rate used and that surface crystallization has a negligible effect for the glass particle sizes studied. A breakdown of the Stokes‐Einstein relation between viscosity and the diffusion coefficient is demonstrated for low temperatures, near the maximum nucleation rate. However, it is shown that accurate values for the diffusion coefficient can be obtained from the induction time for nucleation and the growth velocity in this temperature range.
more »
« less
- Award ID(s):
- 1720296
- PAR ID:
- 10458302
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of the American Ceramic Society
- Volume:
- 103
- Issue:
- 4
- ISSN:
- 0002-7820
- Page Range / eLocation ID:
- p. 2471-2482
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The preparation of 0.58 Li2S + 0.315 SiS2+ 0.105 LiPO3glass, and the impacts of polysulfide and P1Pdefect structure impurities on the glass transition temperature (Tg), crystallization temperature (Tc), working range (ΔT≡ Tc‐ Tg), fragility index, and the Raman spectra were evaluated using statistical analysis. In this study, 33 samples of this glass composition were synthesized through melt‐quenching. Thermal analysis was conducted to determine the glass transition temperature, crystallization temperature, working range, and fragility index through differential scanning calorimetry. The quantity of the impurities described above was determined through Raman spectroscopy peak analysis. Elemental sulfur was doped into a glass to quantify the wt% sulfur content in the glasses. Linear regression analysis was conducted to determine the impact of polysulfide impurities and P1Pdefect impurities on the thermal properties. Polysulfide impurities were found to decrease theTgat rate of nearly 12°C per 1 wt% increase in sulfur concentration. The sulfur concentration does not have a statistically significant impact on the other properties (α = 0.05). The P1Pdefect structure appears to decrease the resistance to crystallization of the glass by measurably decreasing the working range of the glasses, but further study is necessary to fully quantify and determine this.more » « less
-
null (Ed.)Abstract Numerical cloud models require estimates of the vapor growth rate for ice crystals. Current bulk and bin microphysical parameterizations generally assume that vapor growth is diffusion limited, though some parameterizations include the influence of surface attachment kinetics through a constant deposition coefficient. A parameterization for variable deposition coefficients is provided herein. The parameterization is an explicit function of the ambient ice supersaturation and temperature, and an implicit function of crystal dimensions and pressure. The parameterization is valid for variable surface types including growth by dislocations and growth by step nucleation. Deposition coefficients are predicted for the two primary growth directions of crystals, allowing for the evolution of the primary habits. Comparisons with benchmark calculations of instantaneous mass growth indicate that the parameterization is accurate to within a relative error of 1%. Parcel model simulations using Lagrangian microphysics as a benchmark indicate that the bulk parameterization captures the evolution of mass mixing ratio and fall speed with typical relative errors of less than 10%, whereas the average axis lengths can have errors of up to 20%. The bin model produces greater accuracy with relative errors often less that 10%. The deposition coefficient parameterization can be used in any bulk and bin scheme, with low error, if an equivalent volume spherical radius is provided.more » « less
-
Surface diffusion has been measured in the glass of an organic semiconductor, MTDATA, using the method of surface grating decay. The decay rate was measured as a function of temperature and grating wavelength, and the results indicate that the decay mechanism is viscous flow at high temperatures and surface diffusion at low temperatures. Surface diffusion in MTDATA is enhanced by 4 orders of magnitude relative to bulk diffusion when compared at the glass transition temperature T g . The result on MTDATA has been analyzed along with the results on other molecular glasses without extensive hydrogen bonds. In total, these systems cover a wide range of molecular geometries from rod-like to quasi-spherical to discotic and their surface diffusion coefficients vary by 9 orders of magnitude. We find that the variation is well explained by the existence of a steep surface mobility gradient and the anchoring of surface molecules at different depths. Quantitative analysis of these results supports a recently proposed double-exponential form for the mobility gradient: log D( T, z) = log D v ( T) + [log D 0 − log D v ( T)]exp(− z/ξ), where D( T, z) is the depth-dependent diffusion coefficient, D v ( T) is the bulk diffusion coefficient, D 0 ≈ 10 −8 m 2 /s, and ξ ≈ 1.5 nm. Assuming representative bulk diffusion coefficients for these fragile glass formers, the model reproduces the presently known surface diffusion rates within 0.6 decade. Our result provides a general way to predict the surface diffusion rates in molecular glasses.more » « less
-
Abstract Controlling crystallization kinetics is key to overcome the temperature–time dilemma in phase change materials employed for data storage. While the amorphous phase must be preserved for more than 10 years at slightly above room temperature to ensure data integrity, it has to crystallize on a timescale of several nanoseconds following a moderate temperature increase to near 2/3Tmto compete with other memory devices such as dynamic random access memory (DRAM). Here, a calorimetric demonstration that this striking variation in kinetics involves crystallization occurring either from the glassy or from the undercooled liquid state is provided. Measurements of crystallization kinetics of Ge2Sb2Te5with heating rates spanning over six orders of magnitude reveal a fourfold decrease in Kissinger activation energy for crystallization upon the glass transition. This enables rapid crystallization above the glass transition temperatureTg. Moreover, highly unusual for glass‐forming systems, crystallization at conventional heating rates is observed more than 50 °C belowTg, where the atomic mobility should be vanishingly small.more » « less
