Effect of mixing temperature on microstructure of an Al-Si alloy prepared by controlled diffusion solidification

The effects of mixing temperature,i.e.,the temperatures of two precursor melts(pure Al and Al-12Si),on the temperature and solute fields of resultant mixture,the nucleation and growth,and the size and morphology of primary grains during controlled diffusion solidification(CDS) of Al-8Si alloy were investigated by using simulation and calculation.The results indicate that a lower mixing temperature is helpful for achieving more supercooled microscale Al-rich pockets in the mixture,and increasing the width and supercooling degree of supercooling zone in the Al-rich pockets,and thus,the nucleation rate.The nuclei grow up in nondendritic mode,resulting in spheroidal,at least,nondendritic grains.In a successful CDS,the superheat degrees of the two precursor melts should be limited within several degrees,and it is not necessary to extra stipulate the superheat degree of target alloy melt(Al-8Si) when the requirement about Gibbs energies of the three melts is matched.Subsequent observation on casting microstructures shows that the employed simulation and calculation processes are reasonable and the achieved results are reliable.

Numerical Diffusion Control of a Space-Time Discontinuous Galerkin Method

Variations on space-time Discontinuous Galerkin(STDG)discretization associated to Runge-Kutta schemes are developed.These new schemes while keeping the original scheme order can improve accuracy and stability.Numerical analysis is made on academic test cases and efficiency of these schemes are shown on propagating pressure waves.

Effect of mixing rate and temperature on primary Si phase of hypereutectic Al-20Si alloy during control ed diffusion solidification(CDS) process

Controlled Diffusion Solidification(CDS) is a promising process relied on mixing two liquid alloys of precisely controlled chemistry and temperature in order to produce a predetermined alloy composition. In this study, the CDS was employed to prepare hypereutectic Al-20%Si(mass fraction) alloy using Al-30%Si and pure Al of different temperatures. The mixing rate was controlled using three small crucibles with a hole of different diameters in their bottom. The effect of mixing rate and temperature on the microstructure of the primary Si-phase during the mixing of molten Al and Al-30%Si was studied. The results showed that when the diameter of the small crucible bottom hole is 16 mm, a higher mass mixing rate 0.217 kg·s-1 would results in a lower stream velocity 0.414 m·s-1. Conversely a lower mass mixing rate 0.114 kg·s-1(the diameter of the small crucible bottom hole is 8 mm) would result in a higher fluid stream velocity 0.879 m·s-1. A lower mass mixing rate would be better to refine the primary Si than a higher mass mixing rate. Meanwhile, the morphology and distribution of primary Si could also be improved. Especially, when Al-30%Si alloy at 820 °C was mixed with pure Al at 670 °C in the case of a mass mixing rate of 0.114 kg·s-1 and a pouring temperature of 680 °C, the average size of the primary Si phase would be only 18.2 μm. Its morphology would mostly be octahedral and the primary Si would distribute uniformly in the matrix microstructure. The lower mass mixing rate(0.114 kg·s-1) will enhance the broken tendency of Al-30%Si steam and the mixing agitation of resultant melt, so the primary Si phase can be better refined.

KINETICS OF DIFFUSION-CONTROLLED DRUG MATRICES RELEASE SYSTEM

A refined double integral method has been applied to the problem encountered in the diffu-sion release of a drug from a polymeric matrix.The finite external mass transfer resistance has alsobeen incorporated into the present analysis.Experimental release rates of 5-fluorouracil fromethylene-vinyl alcohol copolymer into distilled water were measured in order to test the applicabilityof these models.All system parameters were measured by experiments and models were built up.Themodels presented here are much more accurate than Higuchi model and much easier than the exactsolutions.In view of accuracy,the models are almost the same as the exact solutions.The experi-mental release data agree Well with the computed

Casting of aluminum-copper based alloy by controled difusion solidiication

A quaternary alloy(Al-5.0Cu-0.35Mn-0.25Ti, wt.%), having a similar chemical component with ZL205A, was prepared using a controlled diffusion solidification(CDS) process and a conventional casting process. The effect of the casting process on microstructure and hardness was investigated. The grain morphology and casting defects of the alloy prepared via the conventional casting and CDS were observed and compared at various pouring temperatures. Results show that the CDS process can alleviate the hot tearing defects and reduce the density of porosity, while getting rid of the riser that is generally used for feeding during conventional casting. Structure observations show that the grain morphology of the conventional cast alloy is mainly dendritic, and the grain size decreases when the pouring temperature is decreased, while the CDS cast alloy consists of a large number of spherical grains, which can decrease the thermal cracking tendency and segregation defect, and enhance the hardness of the alloy.

Effect of CaO/SiO_(2) Slag Mass Ratio on Dissolution Rate of Alumina-based Refractory Ceramics

The dissolution of alumina-based refractory ceramics in CaO-Al2O3-SiO_(2)slag melts was performed based on the in-situ observation system of an ultra-high-temperature laser confocal microscope,and the effect of the CaO/SiO_(2)slag mass ratio(C/S ratio)on the dissolution rate of alumina-based refractory ceramics was investigated.The results indicate that the dissolution rate increases with an increase of the C/S ratio and is mainly controlled by diffusion.During the early stage of dissolution,for all C/S ratios,the dissolution process conforms to the classical invariant interface approximation model.During the later stage of dissolution,when the C/S ratio is≥6,the dissolution process is significantly different from the model above because of the formation of a thick interfacial layer,which can be explained by dissolution kinetics.

基于动力学和扩散分段控制的Mg/H_2O反应模型及数值分析

为了数值研究宽广温度范围内Mg/H2O的反应特性,分别建立了考虑部分MgO在液滴表面凝聚的Mg/H2O扩散燃烧模型和基于Arrhenius公式的Mg/H2O反应动力学模型。数值研究了Mg/H2O反应速率在扩散控制和化学动力学控制下随反应条件变化的规律。研究结果表明,Mg液滴扩散燃烧时间计算结果与文献值相符;提高温度和水蒸汽浓度有利于扩散燃烧速率增大,增大压强使燃速降低。不同温度段Mg/H2O反应速率控制因素不同,低温段反应速率受动力学控制,高温段反应速率受扩散控制。

Kinetics and Mechanism of Interfacial Reaction in a SiC_f/Ti Composite

In order to evaluate the interfacial reaction, a SiCf/Ti (TA1) composite was fabricated by a vacuum hot pressing method and then heat-treated in vacuum at 800℃ for up to 100 h. The elemental distributions of C, Si and Ti at the interfacial reaction zone were investigated. It was found that the reaction zone occurs during the fabrication process and continuously grows at high temperature because the Si and C atoms diffuse from SiC fibers to the matrix and Ti atoms diffuse in the opposite direction. The growth of the reaction zone is diffusion controlled and the mechanism of the reaction can be described by a reactive diffusion model of solid-state growth of an AmBn layer between two elementary substances A and B.

Kinetics of oxidation-precipitation of cobalt(Ⅱ) from solution by ozone

In order to precipitate cobalt(Ⅱ) in cobalt chloride solution, a novel method using ozone as the precipitant for its strong oxidability was proposed. The results show that the precipitation reaction is diffusion-controlled. The main factors affecting the oxidation rate such as the stirring speed, solution temperature, ozone partial pressure, initial concentration and flow rate were investigated. The kinetics equation of each condition was established. The results indicate that the oxidation rate is independent of the initial concentration or solution temperature. The oxidation rate increases obviously with increasing the stirring speed. The linear relationship between ozone partial pressure or flow rate and oxidation rate is found.

环境感应式控制释放开关膜的研究进展

药物控制释放是目前十分活跃的研究领域,因为它具有长效、高效、靶向、低副作用等特点,因此具有智能开关的环境感应式控制释放膜成了目前膜学与医用高分子材料的研究热点。文中基于大量研究文献,主要综述了温度感应型、pH感应型以及葡萄糖浓度感应型等环境感应式控制释放开关膜的研究进展。

Diffusion-reaction compromise the polymorphs of precipitated calcium carbonate

Diffusion is seldom considered by chemists and materialists in the preparation of materials while it plays an important role in the field of chemical engineering. If we look at crystallization at the atomic level, crystal growth in a solution starts from the diffusion of ions to the growing surface followed by the incorporation of ions into its lattice. Diffusion can be a rate determining step for the growth of crystals. In this paper, we take the crystallization of calcium carbonate as an example to illustrate the microscopic processes of diffusion and reaction and their compromising influence on the morphology of the crystals produced. The diffusion effect is studied in a specially designed three-cell reactor. Experiments show that a decrease of diffusion leads to retardation of supersaturation and the formation of a continuous concen- tration gradient in the reaction cell, thus promoting the formation of cubic calcite particles. The reaction rate is regulated by temperature. Increase of reaction rate favors the formation of needle-like aragonite particles. When diffusion and reaction play joint roles in the reaction system, their compromise dominates the formation of products, leading to a mixture of cubic and needle-like particles with a controllable ratio. Since diffusion and reaction are universal factors in the preparation of materials, the finding of this paper could be helpful in the controlled synthesis of other materials.

Numerical Solutions for Optimal Control of Stochastic Kolmogorov Systems

This work is concerned with controlled stochastic Kolmogorov systems.Such systems have received much attention recently owing to the wide range of applications in biology and ecology.Starting with the basic premise that the underlying system has an optimal control,this paper is devoted to designing numerical methods for approximation.Different from the existing literature on numerical methods for stochastic controls,the Kolmogorov systems take values in the first quadrant.That is,each component of the state is nonnegative.The work is designing an appropriate discrete-time controlled Markov chain to be in line with(locally consistent)the controlled diffusion.The authors demonstrate that the Kushner and Dupuis Markov chain approximation method still works.Convergence of the numerical scheme is proved under suitable conditions.

Near-Optimal Controls of Differential Systems with Switching and Random Jumps Subject to Fast Switching and Wideband Noise Perturbation

This work develops near-optimal controls for systems given by differential equations with wideband noise and random switching.The random switching is modeled by a continuous-time,time-inhomogeneous Markov chain.Under broad conditions,it is shown that there is an associated limit problem,which is a switching jump diffusion.Using near-optimal controls of the limit system,we then build controls for the original systems.It is shown that such constructed controls are nearly optimal.

Modeling of Cell/Dendrite Transition During Directional Solidification of Ti-Al Alloy Using Cellular Automaton Method

Solute diffusion controlled solidification model was used to simulate the initial stage cellular to dendrite transition of Ti44Al alloys during directional solidification at different velocities. The simulation results show that during this process, a mixed structure composed of cells and dendrites was observed, where secondary dendrites are absent at facing surface with parallel closely spaced dendrites, which agrees with the previous experimental observation. The dendrite spacings are larger than cellular spacings at a given rate, and the columnar grain spacing sharply increases to a maximum as solidification advance to coexistence zone. In addition, simulation also revealed that decreasing the numbers of the seed causes the trend of unstable dendrite transition to increase. Finally, the main influence factors affecting cell/dendrite transition were analyzed, which could be the change of growth rates resulting in slight fluctuations of liquid composition occurred at growth front. The simulation results are in reasonable agreement with the results of previous theoretical models and experimental observation at low cooling rates.

Kinetic study of electrochemically produced hydrogen bubbles on Pt electrodes with tailored geometries

Understanding bubbles evolution kinetics on electrodes with varied geometries is of fundamental importance for advanced electrodes design in gas evolution reaction.In this work,the evolution kinetics of electro-generated hydrogen bubbles are recorded in situ on three(i.e.smooth,nanoporous,and nanoarray)Pt electrodes to identify the geometry dependence.The bubble radius shows a time-dependent growth kinetic,which is tightly-connected to the electrode geometry.Among the three electrodes,the smooth one shows a typical time coefficient of 0.5,in consistence with reported values;the nanoporous one shows a time coefficient of 0.47,less than the classic one(0.5);while the nanoarray one exhibits fastest bubble growth kinetics with a time coefficient higher than 0.5(0.54).Moreover,the nanoarray electrode has the smallest bubble detachment size and the largest growth coefficient(23.3)of all three electrodes.Based on the experimental results,a growth model combined direct bottom-injection with micro-convection is proposed to illustrate the surface geometry dependent coefficients,i.e.,the relationship between geometry and bubble evolution kinetics.The direct injection of generated gas molecules from the bottom of bubbles at the three phase boundaries are believed the key to tailor the bubble wetting states and thus determine the bubble evolution kinetics.

Classification of different sustainable flood retention basin types

Using a revised version of a previously published expert classification system, a database of potential Sustainable Flood Retention Basins has been developed for Scotland. The research shows that the majority of small and former （often old） drinking water reservoirs are kept full and their spillways are continuously in operation. Utilising some of the available capacity to contribute to flood control could significantly reduce the costs of complying with the European Union Flood Directive. Furthermore, the application of a previously developed classification model for Baden in Germany for the Scottish data set showed a lower diversity for basins in Scotland due to less developed infrastructure. The classification system appears to be robust and has the potential, with minor modifications, to be applied across Europe. The principle value of this approach is a clear and unambiguous categorisation, based on standard variables, which can help to promote communication and understanding between stakeholders.