Increase In Nucleation Temperature Research Articles

Understanding the impact of ammonium ion substitutions on heterogeneous ice nucleation.

Understanding the mechanisms underpinning heterogeneous ice nucleation in the presence of ionic inclusions is important for fields such as cryopreservation and for improved models of climate and weather prediction. Feldspar and ammonium are both present in significant quantities in the atmosphere, and experimental evidence has shown that feldspar can nucleate ice from ammonium-containing solutions at temperatures warmer than water alone. In recent work, Whale hypothesised that this increase in nucleation temperature is due to an increase in configurational entropy when an ammonium ion is included in the ice hydrogen bond network (T. F. Whale, J. Chem. Phys., 2022, 156, 144503). In this work, we investigate the impact of the inclusion of an ammonium ion on the hydrogen bond network by direct enumeration of the number of structures found using Rick's algorithm. We also determine the energy of these systems and thus compare the effects of enthalpy and entropy to test Whale's hypothesis. We find that the inclusion of an ammonium ion increases the total number of configurations under conditions consistent with a realistic surface charge. We also find that the enthalpic contribution is dominant in determining the location of the ammonium ion within the structure, although we note that this neglects other practicalities of ice nucleation.

Effect of PVD-coated chromium on the subcooled flow boiling performance of nuclear reactor cladding materials

We elucidate the separate effect of a thin Cr coating deposited by physical vapor deposition (PVD) on the subcooled flow boiling performance of zircaloy-4. First, we run flow boiling experiments on prototypical zircaloy-4 surfaces mimicking the scratch pattern and surface roughness of nuclear reactor claddings. Then, we PVD-coat a 0.3 µm thick chromium layer on the same exact surface and repeat the same flow boiling investigations. All experiments are run using deionized water at atmospheric pressure, flowing on a 1 × 3 cm2 rectangular cross section channel at a rate of 1000 kg/m2/s and a subcooling of 10 K. We measure the average temperature of the boiling surface at increasing surface heat fluxes, covering a wide range of heat transfer regimes, from single-phase forced convection to the boiling crisis. We also record high-speed videos of the boiling process, which we postprocess to measure bubble nucleation site density, growth time, departure diameter and frequency. The surface analysis reveals that, while the chromium coating does not seem change the surface roughness and morphology, it improves surface wettability. However, it decreases the critical heat flux. The chromium coating causes an increase of nucleation temperature, bubble departure diameter and growth time, and a reduction of the nucleation site density. The concurrence of these observations indicates that a size reduction of the nucleation sites, conformally covered by the chromium coating, may be the cause of the boiling performance deterioration. We confirm this hypothesis repeating the same analysis on a FeCrAl sample prepared and tested using the same protocol as the zircaloy-4 sample, but with a different initial surface texture.

Aluminum composites reinforced by γ-graphynes: The effect of nanofillers porosity and shape on crystal growth and composite strengthening

Graphyne and graphdiyne nanofillers (nanoplatelets and nanotubes), are embedded in Aluminum matrices originating graphyne and graphdiyne/Al nanocomposites. Pristine Al and the nanocomposites are melted and then recrystallized at a fixed colling rate, 1 K/ps. Al nanocomposites with planar nanofillers show a 46–53% increase in nucleation temperature and 78–88% recrystallization yield while Al nanocomposites with tubular nanofillers present a 16–32% increase in nucleation temperature and a 65–75% recrystallization yield. Al nanocomposites with planar nanofillers present a 26–36% increase in the Young's modulus, while Al nanocomposites with tubular nanofillers show a 23–31% increase in the Young's modulus. The improved mechanical properties of graphyne and graphdiyne/Al nanocomposites depend on i) the elasticity and shape (planar vs tubular) of the nanofillers ii) the level of recrystallization of the Al matrix and iii) the interfacial adhesion in the interface between the nanofillers and the Al matrix. Graphyne pore size positively affects interfacial adhesion.

Effects of two-step heat treatment on crystallization behavior, densification and microwave dielectric properties of MgO-Al2O3-SiO2-TiO2-Sb2O3 glass-ceramics

Glass-ceramics with a mass fraction composition (wt%) of 19MgO-23Al2O3-53SiO2-4TiO2-3Sb2O3 (M-A-S-T-S) were prepared by two-step (nucleation and crystallization) heat treatment of the parent glass at selected temperatures for 2h and/or 20h, respectively. A microwave dielectric characterization was performed in order to evaluate the suitability of M-A-S-T-S glass-ceramics as a lower permittivity (εr<4) dielectric substrate. X-ray diffraction analysis revealed that the addition of TiO2 and Sb2O3 into MgO-Al2O3-SiO2 glass could induce the coexistence of μ-cordietire and α-cordietire as the main phases, a small fraction of SiO2, MgSiO3 and Sb6O11 additionally crystallized as the second phases in this glass-ceramic system. Meanwhile, the prolonged heat treatment time~20h at selected nucleation and crystallization temperatures would cause the appearance of a small amount of MgTiO3 phase. The longer annealing time could not promote the densification and final dielectric properties, while a slightly increase in nucleation temperature~880°C of a short-time sintering (~2h) could reduce the dielectric loss. Wherein, after the two-step heat treatment (880°C/2h+930°C/2h), a better microwave dielectric properties of εr~3.65, Q×f~7900GHz (at 13.293GHz) and τf~−7.92ppm/°C can be obtained in this M-A-S-T-S glass-ceramic sample.

Nucleation and growth of ZnO films on Si substrates by LP-MOCVD

ZnO was grown on Si (111) by low-pressure metal–organic chemical vapor deposition. The nucleation and evolution of ZnO thin films were investigated by varying growth temperature and growth time during the nucleation process. It is found that nucleation temperature and time play a key role on the nucleation process. With the increase of nucleation temperature, the size of ZnO nuclei and the spacing between nucleation sites increases. And as the nucleation time increases, the size of ZnO nucleation islands (NIs) increases and the density decreases due to the coalescence of ZnO nuclei. The density and size of initial ZnO NIs have intensely affected the growth model and quality of ZnO epitaxial layers. In addition, the crystalline quality of ZnO thin films was improved by decreasing nucleation temperature and increasing nucleation time.

Unusual Crystallization Behaviors of Anatase Nanocrystallites from a Molecularly Thin Titania Nanosheet and Its Stacked Forms: Increase in Nucleation Temperature and Oriented Growth

Crystallization behaviors of anatase nanocrystallites from an ultrathin two-dimensional reactant composed of exfoliated titania nanosheets have been studied by monitoring the heating process of their well-organized films, with which the film thickness can be controlled from a molecularly thin monolayer to a stacked multilayer structure with a stepwise increment of approximately 1 nm. The heated products were identified by means of total reflection fluorescence X-ray absorption near-edge structure analysis and in-plane X-ray diffraction measurements using a synchrotron radiation source. The films composed of five or more layers of stacked nanosheets were transformed into anatase at 400-500 degrees C, which is a normal crystallization temperature of anatase from bulk reactants. As the film became thinner by decreasing the number of nanosheet layers to five or less, the crystallization temperature was found to increase and finally reached 800 degrees C for the monolayer film. Interestingly, preferential growth of anatase along the c-axis was strongly promoted for these ultrathin films. These unusual behaviors may be understood in terms of crystallization from the two-dimensional system of scarcely distributed reactants. The titania nanosheet crystallite is much thinner than the unit cell dimensions of anatase, and therefore, extensive atomic diffusion is required for the transformation particularly for the ultrathin films with a critical number (2-3) of stacked nanosheet layers. There is some structural similarity between anatase and titania nanosheet, which may account for the oriented growth of anatase nanocrystallites.