Papers by Keyword: Nucleation

Abstract: Development of low-sulfur and Pb-free free cutting steel is significant because the use of S or Pb free-cutting steel can result in serious pollution. The hypoeutectoid graphitized free cutting steel was proposed. The graphitizaion is difficult due to slow graphitization process, so it is curcial to accelerate the graphitization process. The results show that, through appropriate composition design and technology, the C atom can quickly transform to grahite within 4 hours. The microstructure of the graphitized free cutting steel consists of ferrite, graphite and carbide. Moreover, the grain size of grapite is fine and the distribution is uniform, and the carbide mainly locate the ferrtie grain boundary or in the ferrite. In addition, the nuleation of graphite not only can take place on the second-pahse particles, but also spontaneous nuleation and growth.Key words: free cutting steel, hypoeutectoed steel, graphite, carbide, nucleation

Abstract: The paper presents the concept of cryogenic growth kinetics during separation of CO₂ from natural gas using Avrami nucleation model. The interface frost layer on the glass packing of cryogenic bed is assumed as germ nuclei. The bed porosity is considered time dependent. The expression for time varying bed porosity is derived based on Avrami model. The experimentation was conducted to validate the model and the resulting simulation studies show good resemblance with experimental results.

Abstract: It is significant theoretically to study the nucleation laws of the phase transformation products of the supercooled austenite. The different iron and steel materials were used to study the nucleation of pearlite, bainite and martensite by QUANTA-400 environmental scanning electron microscope and JEM-2100 transmission electron microscope. The results show that, from the high-temperature zone to the low-temperature zone, the nucleation of the transformation products of the supercooled austenite is a gradually evolutionary process from two-phase nucleation to single-phase nucleation. Pearlite nucleates in the austenitic grain boundary, bainite nucleates preferentially in the grain boundary and sometimes in the grain interior and martensite nucleates preferentially in the interface and generally in the grain interior. The preferential nucleation sites are the interface, and with the decrease of the transformation temperature, they gradually shift to the defects in the grain interiors, which accord with the general rules of the phase transformation nucleation.

Abstract: Gold based bulk metallic glass was synthesized at with different quenching rates. Au₅₄Ag₅Pd₂Cu_24.3Si_14.7 alloy ingots were cast into wedge shape and cone shape samples. The critical casting thickness of this alloy composition was found to be 4 mm when cast in copper mold. Completely amorphous structure as found in sections thinner than 4 mm when considering a infinite copper plate geometry. In locations with slower cooling rates, there are small fractions of transformed nanocrystals. The transmission electron microscope (TEM) study of Au₅₄Ag₅Pd₂Cu_24.3Si_14.7 solidified microstructure could offer snap shots of nucleation and growth of crystals from nanometer to micrometer length-scale as evolved with different cooling rates. The phase evolution was recorded by transmission electron microscope study, electron diffraction and metallography using scanning electron microscope. The specimens comprised of both fully amorphous solid and nanocomposites which were the combination of nano-crystals of controlled sizes in amorphous matrix. Vickers hardness values increased to 350-450 Hv, when compared to sub 100-150 Hv level of conventional 18-k gold alloys as cast. The study offers a revolutionary way to create ultrahigh strength bulk metals that comprise of large percentage of nano-crystals in amorphous matrix.

Abstract: The compositions and crystallization process at low temperature of palm-based biodiesel (PME) are investigated. In this work, we show that PME is mainly composed of fatty acid methyl esters of 14-24 even-numbered C atoms: C_14:0-C_24:0, C_16:1-C_22:1, C_18:2, and C_18:3. Palm-based biodiesel crystallization comprises three steps, viz., forming supersaturated solution, nucleation and ester crystal growth; the driving force for saturated fatty acid methyl esters nucleation is the degree of supersaturation. The rate equation of nucleation is put forward. The objective of this research is to provide theoretical support for hindering the ester crystal nucleation and growth, and improving the cold flow properties of PME.

Abstract: The objective of this paper is to identify the predominant crystallographic relations between deformed state and recrystallized grains during the early stages of recrystallization of fcc metals with medium and low stacking fault energy. The experimental investigations, based on SEM/EBSD measurements, have focused on the transformations which occur in plane strain compressed single crystals with stable orientations. After annealing the disorientation across the recrystallization front 'defines' the final rotation by angles in the ranges of 25-35^o and 45-55^o around axes mostly grouped near the <122>, <012>, <112> and <111> directions located around the normals of all four {111} slip planes.

Abstract: Nucleation and condensation of mercury vapor has been investigated in various divergent angle and operating condition. Divergent angle has a great effect on droplet size at the end of nozzle. Influence of operating condition such as pressure and temperature on the size of droplet has been investigated. A one-dimensional mathematical model based on classical nucleation and growth has been developed to calculate the nucleation and condensation of mercury vapor. A mercury vapour turbine has been used in conjunction with a steam turbine for generating electricity. The mercury cycle offers an efficiency increase compared to a steam-only cycle because energy can be injected into the Rankine Cycle at higher temperature. The target of modeling is predicting the droplet size of mercury nanoparticles during rapid expansion. The results are verified by accurate experimental data available in the literature. The governing equations were solved using Runge-Kutta third-order numerical method in MATLAB software.

Abstract: Dynamic characteristics of micro bubble are the key for its applications. A thermal bubble is produced on the wall of microchannel with flowing fluid. Generation of bubble, including nucleation and growth, is observed and measured with different volume flow rates. The results show that nucleation becomes more difficult and instable with volume flow rate increasing. Bubble growth could be divided into three stages, while the second stage becomes shorter and even disappears as volume flow rate increasing. When volume flow rate increases suddenly, bubble shrinks, and it is more remarkable for larger volume flow rate. Furthermore it needs more time for bubble to reach and keep the minimum volume.

Abstract: SiC crystal is a wide band gap material of high hardness and chemical inertness. Graphene is nowadays a ubiquitous 2D material that would revolutionize many applications. Combining the characteristics of SiC and graphene higher performance and efficiency are expected, e.g. for high frequency electronic devices. The obtaining of graphene directly on SiC substrates by a single step thermal decomposition process is promising, but optimal standardized conditions are not established. We present the use of chemical-mechanical polishing (CMP) as a pre-graphene growth SiC conditioning to enable deep comprehension of the mechanisms of SiC decomposition and control towards selective formation of graphene.

Abstract: Experimental results show that the samples gained in bath added Rare earth nitrate (REN), relative to the sample got in bath without REN, improve the anti-corrosion power of the coating because of increasing of covering rate of formless crystal Zn2Fe(PO4)2·4H2O (marked P) crystals and the ratio of P/(P+H) (H is the mark of Zn3(PO4)2 crystal) in the coating, combination of which with components parsing by EDS indicates that the sequence of contribution elements P and Zn to erosion resistance of coatings is P>Zn. And the correlative mechanism was discussed, which has it clear that RE is materially a catalyst holding excellent ability of carrying oxygen and cathode depolarization, its concentration gets so constant in certain range that it is much steadier and more efficient than the usual consumptive oxidants like nitrates. In a word, REN plays the role of surface regulator, accelerant and densification agent, which speeds up the phosphating, and bids it effective to enhance the anti-corrosion power of the coating. The addition of REN, not only promote the phosphating film formation and substantial reduction or exemption of nitrite. So, REN is green phosphating accelerator of live up to one's name.