CAS OpenIR  > 研究所(批量导入)
基于反应-扩散的材料形貌调控研究
汪涵
学位类型博士
导师韩永生
2015-10
学位授予单位中国科学院研究生院
学位授予地点北京
学位专业化学工程
关键词形貌调控 动力学 反应 扩散
其他摘要

材料的形貌对其性能有重要的影响,对材料形貌的有效调控能极大地推动材料的工业应用,发展可控的材料形貌调控方法,有赖于对材料结晶过程中诸多动力学和热力学过程的充分认识。所以发展调控材料形貌的新方法,不仅对材料学的发展有重要影响,也对人们深入认识“结晶”这一古老的学科有重要意义。虽说人们发明了溶胶凝胶法、水热法、模板法等调控材料形貌的方法,但是这些方法大多缺乏对材料生长过程机理的认识,不易进行推广应用。近年来人们发现,在不同的动力学条件下,材料的生长会沿不同的生长路径进行,最终导致材料的形貌表现出不同于热力学条件控制下的形貌。人们根据这一现象发明出调控材料形貌的动力学调控方法,动力学调控方法这几年得到迅速发展,受到越来越多的重视。然而,人们对材料生长过程中的动力学过程认识还不充分,当前动力学方法的调控手段主要通过调节颗粒生长速率的快慢这一较为一般的动力学过程,以达到对材料形貌调控的目的。通过仔细分析,本文发现材料生长过程中,其生长界面存在着不同于宏观结构的介尺度结构,如表面轮廓的曲率、界面反应物的浓场分布等,这些介尺度结构直接决定着材料的形貌演化。反应物向材料表面传递的速率和相互之间反应的速率决定了材料生长中其表界面的介尺度结构。所以说,材料生长过程中所涉及的反应过程和扩散过程,是材料生长中最具代表性的两个动力学过程,它们之间的相互协调是导致材料形貌多样化的主要原因。传统的温度、浓度、pH值等参数的调控,无法实现传递和反应速率的独立控制,因而难以找到结构演化的规律。所以本文提出基于反应-扩散的调控材料形貌的新方法,实现通过调节反应和扩散这两个动力学因素以达到调控材料形貌的目的。本文的主要发现如下:(1)反应-扩散定性调控对碳酸钙颗粒形貌的影响。通过对碳酸钙沉淀反应中的反应速率和扩散速率的定性调控,本文发现:当反应速率较慢而扩散速率较快时,颗粒的生长在反应受限的条件下进行,可得到球状颗粒;当反应速率较快而扩散速率较慢时,颗粒的生长在扩散受限的条件下进行,可得到的碳酸钙呈枝状晶结构;当反应速率和扩散速率达到相当的状态时,本文得到了雪花状碳酸钙颗粒,这种形貌的碳酸钙颗粒是主枝和侧枝的生长处于动态平衡状态下得到的,是颗粒生长的表界面处介尺度结构动态演化的体现。通过蒙特卡洛模拟验证了雪花状碳酸钙是反应和扩散这两个动力学因素相互协调的结果,证实了反应-扩散的协调作用在颗粒形貌演化中的重要性。(2)反应-扩散定量调控对银颗粒形貌的影响。通过对银颗粒生长过程中银离子的还原反应速率和体系中材料单元扩散速率的定量调控,本文发现反应-扩散对材料生长过程的影响主要体现在两个方面:一个是对颗粒生长方式的影响,一个是对颗粒生长方向的影响。反应-扩散通过对这两个方面的影响,起到对材料形貌的调控作用。对生长方式的影响:当银离子的扩散速率或反应速率较低时,银的晶核会通过相互碰撞聚集在一起,使银颗粒的生长以聚集生长的方式进行,形成片状和纺锤棒颗粒;当Ag+的扩散速率或反应速率较高时,Ag+会通过较快的扩散速率沉积到银的晶核表面,使银颗粒的生长以传统的逐层生长的方式进行,形成多面体的小晶粒。对生长方向的影响主要针对的是以聚集生长方式生长的颗粒,扩散速率指构筑单元的扩散速率,反应速率指构筑单元生成反应的反应速率,研究发现:当构筑单元的扩散速率或反应速率较高时银颗粒倾向于沿着各向同性的方向生长,形成长径比较低的片状颗粒;当构筑单元的扩散速率或反应速率较低时,银颗粒的生长方向表现出各向异性,沿长轴方向的生长较快,形成长径比高的纺锤棒颗粒。(3)反应-扩散对材料形貌调控作用及机制的模拟研究。分别建立了基于反应-扩散的材料生长模型,对雪花状碳酸钙和银颗粒的生长进行了模拟,验证了反应-扩散对它们形貌的调控作用,发现了雪花状颗粒和高长径比的纺锤棒颗粒都是生长方向表现出各向异性的结果。而且通过计算雪花状碳酸钙颗粒和不同长径比片状银颗粒周围的浓度分布,发现了它们周围的浓度分布也表现为各向异性,而且浓度分布的各向异性和它们生长方向的各向异性一致,在颗粒生长速率较快的方向上,浓度梯度较高。所以本文得出结论,颗粒周围的浓度梯度是造成其各向异性生长的根本原因,反应-扩散对材料生长方向的调控,是通过对材料周围的浓度梯度的影响而实现的,这种浓度梯度是材料表界面的动态结构的体现。这样,本文将反应-扩散对碳酸钙和银颗粒形貌的调控作用统一到了一个共同的机制上,证明了反应-扩散调控方法对材料形貌的调控作用具有一定的普适性。

;

The shape of particles is crutial for their performance in applications. A versatile approach for obtaining shape-controlled particles relies on the understanding of the thermodynamics and kinetics involved in the crystallization of particles. Therefore, developing a versatile approach for controlling the shape of particles will greatly promote the industrial applications of particles, as well as benefit the development of crystallization science. To control the shape of particles, scientists invented sol-gel method, hydrothermal method, template method, etc. Most of these previous approaches are not based on mechanistic understanding, and difficult to be put into industrial application.Over decades, researchers discovered that, kinetic condition can alter the growth pathway of particles, leading to particles with shapes deviating from thermodynamic predictions. Based on this recognition, the kinetic approach was invented to control the shape of particles, and this approach is becoming more and more popular. Previous kinetic approaches mainly focus on adjusting the reaction rate of particle growth, which lacks more indepth understanding in the kinetic processes of particle growth. By viewing the growth of the particles, we find that, the mesoscale structures in the growing front the particles, including the contour curvature of the particles and the concentration distribution, are crutial for shape development of the particles. The transport rate of reactant towards the particle and their reaction rate influence the mesoscale structure in the growing front of the particle. Therefore, we propose a new kinetic approach, that is controlling the shape of particles via regulating the reaction and diffusion of chemicals. Our main findings in this thesis are shown in the follows:(1) Control on the shape development of CaCO3 by qualitatively manipulating the reaction-diffusion. By adjusting the reacion rate and diffusion rate of chemicals in the precipitation of CaCO3, we obtain sphere particles, dendritic structures and snow-shaped particles at different kinetic conditions. When the reaction rate is low and diffusion rate is high, the obtained particles show sphere shape, because it is a reaction-limited condition. When the reaction rate is high and diffusion rate is low, the particles show dendritic structures, because this is a diffusion-limited condition. When the interplay between reaction and diffusion comes to balance state, we obtain snow-shaped CaCO3 particles. Through Monte Carlo simulation, we confirm that the formation of snow-shaped particle is a result of compromise between reaction and diffusion. In this work, the importances of reaction and diffusion on the shape development of particles are verified. (2) Control on the shape development of silver particles by quantitatively manipulating the reaction-diffusion. Through adjusing the diffusion rate of diffusive units and reduction rate of Ag+, we obtain small crystals with smooth surface, rectangle plates and spindle rods. We find that the reaction-diffusion influences the particle growth in two aspects, which is the reason for the shape variation at different reaction-diffusion conditions. One of the two aspects is in terms of the growth pathway, which switches between classic layer-wise growth to coalescence growth; another one is in terms of the the growth direction which switches between isotropic mode and anisotropic mode. For the growth model, when the diffusion rate of Ag+ is low or reduction rate is low, the silver nuclei will preferentially aggregate through collision, favoring the coalesence growth pathway for silver growth, forming rectangle plates and spindle rod; when diffusion rate of Ag+ is high or reducion rate of Ag+ is high, the Ag+ deposits onto the surface of silver nuclei at fast pace, favoring the classic layer-wise growth for silver particle, leading to the formation of small crystals with smooth surfaces. For the growth direction, the switch happens when the particles grow through the coaleacence growth pathway. When the diffusion rate of building blocks is high or the reaction rate of building blocks formation is high, the growth direction of silver particles tends to follow the isotropic growth way, leading to the formation of rectangle plates; when the diffusion rate of building blocks is low or the reaction rate of building blocks formation is low, the growth direction of silver particles tends to follow the anisotropic growth, making the long axis of particles grow faster, forming spindle rods with higher aspect ratio.(3) Simulation studies on influences of reaction-diffusion on the shape development of particles. Growth models based on reaction-diffusion are built up to simulate the growth of snow-shape CaCO3 particles and silver particles. Through the simulation work, we confirm the effects of reaction-diffusion on the shape development of CaCO3 and silver particles. The snow shape of CaCO3 and spindle-rod shape of silver particles are both results of anisotropic growth. By disclosing the concentration distribution around the growing snow-shaped particles and silver aggregates, we find anisotropic concentration gradient around the growing particles, and the concentration gradient agrees well with the anisotropic direction of particle growth. Therefore, the concentration gradient created by diffusion-reaction is the key mechanism that determines the shape of particles. The shape development of CaCO3 and silver particles have been correlated by the mutual governing mechanism, that is the concentration gradient. Therefore, we verify that the reaction-diffusion approach promises a general protocol for shape-controlled synthesis of particles.

语种中文
文献类型学位论文
条目标识符http://ir.ipe.ac.cn/handle/122111/21355
专题研究所(批量导入)
推荐引用方式
GB/T 7714
汪涵. 基于反应-扩散的材料形貌调控研究[D]. 北京. 中国科学院研究生院,2015.
条目包含的文件
文件名称/大小 文献类型 版本类型 开放类型 使用许可
汪涵-20151201.pdf(7249KB)学位论文 开放获取CC BY-NC-SA浏览 请求全文
个性服务
推荐该条目
保存到收藏夹
查看访问统计
导出为Endnote文件
谷歌学术
谷歌学术中相似的文章
[汪涵]的文章
百度学术
百度学术中相似的文章
[汪涵]的文章
必应学术
必应学术中相似的文章
[汪涵]的文章
相关权益政策
暂无数据
收藏/分享
文件名: 汪涵-20151201.pdf
格式: Adobe PDF
所有评论 (0)
暂无评论
 

除非特别说明,本系统中所有内容都受版权保护,并保留所有权利。