Homogeneous bubble nucleation limit of lead

Liquid heavy metal coolant in a fast reactor, as well as in accelerator driven systems, is exhibited to large thermal and pressure shocks which can cause cavitation in the coolant. Here we calculated the work of the critical bubble formation in the lead coolant and the nucleation rate in terms of the generalized Gibbs approach. It is demonstrated that such approach provides a more adequate description of the process of bubble nucleation as compared with the classical nucleation theory.Теплоносители на основе жидких тяжелых металлов в быстрых реакторах и реакторах, управляемых ускорителем, подвержены значительным тепловым и гидравлическим ударам, что может приводить к кавитации теплоносителя. Рассчитаны работа образования критических пузырьков в свинцовом теплоносителе и скорость их зарождения в рамках обобщенного подхода Гиббса. Показано, что такой подход обеспечивает более адекватное описание процесса зарождения пузырьков по сравнению с классической теорией нуклеации.Теплоносії на основі рідких важких металів у швидких реакторах і реакторах, керованих прискорювачем, схильні до значних теплових і гідравлічних ударів, що може призводити до кавітації теплоносія. Розраховані робота утворення критичних пухирців в свинцевому теплоносії та швидкість їх зародження в рамках узагальненого підходу Гіббса. Показано, що такий підхід забезпечує більш адекватний опис процесу зародження пухирців в порівнянні з класичною теорією нуклеації

Homogeneous bubble nucleation limit of mercury under the normal working conditions of the planned European Spallation Source

In spallation neutron sources, liquid mercury is the subject of big thermal and pressure shocks, upon adsorbing the proton beam. These changes can cause unstable bubbles in the liquid, which can damage the structural material. While there are methods to deal with the pressure shock, the local temperature shock cannot be avoided. In our paper we calculated the work of the critical cluster formation (i.e. for mercury micro-bubbles) together with the rate of their formation (nucleation rate). It is shown that the homogeneous nucleation rates are very low even after adsorbing several proton pulses, therefore the probability of temperature induced homogeneous bubble nucleation is negligible.Comment: 22 Pages, 11 figures, one of them is colour, we plan to publish it in Eur. Phys. J.

Steady-State Crystal Nucleation Rate of Polyamide 66 by Combining Atomic Force Microscopy and Fast-Scanning Chip Calorimetry

Copyright © 2020 American Chemical Society. Homogeneous crystal nucleation in polyamide 66 (PA 66) was studied by a combination of atomic-force microscopy (AFM) and fast-scanning chip calorimetry (FSC), with a specific experimental setup allowing precise correlation of AFM images and FSC heating curves. PA 66 was subjected to Tammann's two-stage crystal nuclei development method, including quenching the relaxed melt to the nuclei-formation temperature of 310 K, annealing for different times to allow crystal nucleation, supplemented by an analysis of nuclei growth to crystals by heating the sample. With the number of crystals/nuclei detected by AFM imaging and by the variation of the nucleation time, a steady-state nucleation rate of (2.3 ± 0.2) × 1018 m-3 s-1 (2.3 μm-3 s-1) at 310 K has been determined. Comparing the total enthalpy of melting of the crystals, obtained by FSC, with the number of crystals, observed by AFM, yields the enthalpy of melting/formation of one single crystal of (5.2 ± 0.5) × 10-13 J at the specific growth conditions applied. Application of this approach, that is, correlating transition enthalpies with nuclei numbers, for analysis of crystal nucleation in a wide range of temperatures between 310 and 375 K yielded a maximum homogeneous nucleation rate close to 1020 m-3 s-1 (100 μm-3 s-1) at around 350 K. The present study offers a qualitatively new approach of analyzing the kinetics of homogeneous nucleation of polymers. In addition, it suggests employing the specific AFM-FSC setup as a valuable tool for direct observation of crystal-specific enthalpies of crystallization

Crystal nucleation and growth in cross-linked poly(ε-caprolactone) (PCL)

The crystal nucleation and overall crystallization kinetics of cross-linked poly(ε-caprolactone) was studied experimentally by fast scanning calorimetry in a wide temperature range. With an increasing degree of cross-linking, both the nucleation and crystallization half-times increase. Concurrently, the glass transition range shifts to higher temperatures. In contrast, the temperatures of the maximum nucleation and the overall crystallization rates remain the same, independent of the degree of crosslinking. The cold crystallization peak temperature generally increases as a function of heating rate, reaching an asymptotic value near the temperature of the maximum growth rate. A theoretical interpretation of these results is given in terms of classical nucleation theory. In addition, it is shown that the average distance between the nearest cross-links is smaller than the estimated lamellae thickness, which indicates the inclusion of cross-links in the crystalline phase of the polymer

Growth and dissolution of crystal nuclei in poly(l-lactic acid) (PLLA) in Tammann's development method

© 2020 Elsevier Ltd By fast scanning calorimetry (FSC), the influence of the transfer of nuclei from the nucleation stage at low temperature to the growth stage at higher temperature was systematically studied. Heating rates above 1000 K s−1 are sufficient to prevent growth of crystal nuclei at the transfer from 60 °C to 125 °C in Poly(L-lactic acid) (PLLA). The critical heating rate for preventing nuclei growth is about 1000 times higher than the critical heating rate to prevent crystal growth in a nucleated sample on heating. In a second experiment, the cluster size distribution after isothermal nucleation of PLLA at 60 °C for 1000 s was estimated. In the temperature interval from 125 °C to 145 °C, the density of critical nuclei decreases from the initial value ∼4 × 1024 m−3 to zero, as nuclei with R > 2 nm, the critical radius for 145 °C, were not formed at the nucleation temperature

Updated definition of glass-ceramics

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