可控团聚状态的氧化物超细粉末制备技术

有关制备技术和工艺条件对超细粉末团聚状态的影响是陶瓷学家们目前面临的一个具有现实意义的重要课题.本文就液相法制备氧化物超细粉末过程中控制团聚状态研究中的某些问题进行评述和讨论.在液相法制备超细粉末过程中,粒子的生成条件对团聚体的形成起着决定性作用.通过控制沉淀反应过程中胶粒的聚集特征就可以控制粉末的团聚状态,以制取优质陶瓷粉末.     

湿化学法制备ZrO2（Y2O3）超细粉末过程中团聚状态的控制

借助Raman光谱、IR光谱、差热分析、TEM、SEM及电动力学等测试手段,研究了液相沉淀法制备ZrO_2(Y_2O_3)超细粉末过程中团聚状态的控制。结果表明:沉淀过程中引入适宜的表面活性剂,可增强静电和位阻效应,得到均匀分散的胶粒悬浮液;经水洗后的湿凝胶,采用表面活性剂处理,可改变胶粒界面结构,避免干燥和煅烧过程中形成硬团聚体。此外,还研究了粉末团聚状态对其烧结体性能的影响。     

干燥及煅烧过程中引入表面活性剂控制ZrO2超细粉末团聚的研究

一、引言液相沉淀法制备氧化物超细粉末过程中,团聚体起源于沉淀反应过程.沉淀反应生成的聚集体内含有丰富的自由水,吸附水,结构配位水和非架桥羟基.在后续的热处理过程中,首先失去的是渗透在胶粒与胶粒之间的毛细管内的自由水.在水的表面张力的作用下,胶粒之间距离缩短,并相互接触,形成聚集.随着温度的升高,吸附水,结构配位水被排除.这些水分子具有比普通自由水高几倍至几十倍的表面张力,因而引起胶粒     

制备工艺对ZrO_2(Y_2O_3)-Al_2O_3超细粉末团聚状态的影响

本文采用由摇实密度求出团聚体密度f和改进的团聚参数AF~*(50),用以表征团聚状态,分析和讨论了工艺条件对团聚状态的影响以及粉末的团聚状态和烧结性能的相互关系,并结合烧结试样的显微结构对粉末的性能做出了评价。实验表明,f和AF~*(50)能简捷地反映粉末的团聚状态。由悬浮着超细α-Al_2O_3粉末的锆、钇混合盐溶液进行包裹沉淀,可以获得组分均匀、团聚体密度和团聚参数低的超细ZrO_2(Y_2O_3)-Al_2O_3粉末。     

超声强化溶胶-凝胶反应制备氧化锆纳米粉体

本研究探讨了不同声场条件对氧氯化锆水解反应生成氢氧化锆凝胶以及所得的氧化锆纳米粉体结构和形貌的影响。发现合适条件的超声场所产生的空化作用可以促进胶体成核速率,降低胶粒的比表面能,抑制胶粒聚集,并破坏团聚体,对氧化锆纳米粉体的制备起到强化促进作用,所得粉体颗粒较小且分布窄。相反不合适条件的声场可能会迫使胶粒聚集,促进团聚的形成,所得粉体的粒径较大且不均匀。     

湿化学法制备ZrO2超细粉末的干燥过程中控制粉末团聚的方法

本文讨论了在湿化学法制备ZrO2超细粉末的干燥过程中粉末团聚状态的控制方法，添加少量的表面活性剂以及采用快速的微波干燥。可有效控制粉末团聚，过程简单，易于操作，大大缩短了干燥时间，降低了成本。     

共沉淀中反应物浓度、反应过程pH值对纳米粉末性能的影响

通过对共沉淀过程中,不同反应物浓度、不同pH值条件下所制氢氧化锆以及氧化锆纳米粉末性能的研究,分析讨论了反应物浓度、反应过程pH值对肢体团聚状态以及粉料烧结性能的影响,获得了共沉淀反应的优化条件:氧氯化锆和氯化钇的浓度为0.8～1.0mol/L,而反应过程的PH值应在11左右。     

Y-TZP/Al_2O_3超细粉末的研究

用共沉淀法制备了3mol％Y_2O_3-ZrO_2-20Wt％Al_2O_3超细粉末。利用热分析、X射线、粒度分析仪、透射电镜和扫描电镜等研究了粉末的性能。实验结果表明:制备的粉末组分均匀,颗粒粒度为10nm,粒度分布窄,团聚体尺寸小,烧结活性高。粉末的相组成为t-ZrO_2。烧结体显微结构致密细晶,具有优异的力学性能。     

醇水共沉淀法制备氧化锆超细粉末及团聚控制

以氯氧化锆和氨水为原料，在醇水溶液中制得氧化锆前驱体，经过陈化和低温处理后干燥，前驱体在600℃下煅烧后球磨得到氧化锆超细粉末。采用TG—DSC，XRD，TEM分别对制备的粉末进行了分析。研究表明在共沉淀过程中引入无水乙醇，采用低温处理工艺，能减少粉末的团聚，避免硬团聚的形成。运用该方法可以制备出粒度为20～30nm、少团聚的超细氧化锆粉末。     

软聚合物胶粒在3种反离子作用下的团聚融合行为

选用一种丙烯酸酯共聚物为研究对象,采用广角光散射仪的动态光散射和静态光散射方法研究其胶粒在反离子(H⁺、Na⁺和Mg²⁺)作用下的团聚和融合行为。研究发现:在3种反离子作用下,在DLCA(扩散控制团聚)条件下,团聚体分形维数D_f的值均在1. 80左右,说明胶粒在DLCA条件下没有发生融合;而在RLCA(反应控制团聚)条件下,各反离子在3种不同浓度下诱导所得的3组团聚体的平均旋转半径对无因次时间曲线、3组平均水力学半径对无因次时间曲线均在团聚实验刚开始时发生重叠,但随着团聚过程的进行,曲线不再重叠,说明胶粒在RLCA条件下发生了融合;然后在胶体中加入占固体总质量0. 70%的表面活性剂SDS(十二烷基硫酸钠),在3种反离子的作用下得到的胶粒的双颗粒形成速率常数k_1,1均随时间的增加而减小,更进一步验证此时胶粒发生了融合。不管是否加入SDS,pH相同时,3种离子对胶粒诱导失稳的作用大小为:Na⁺最稳定,H⁺次之,Mg²⁺     

PASS去除微污染原水胶体颗粒的研究

本文采用硫酸铝、PAC和PASS对珠江水系原水进行了混凝处理的对比研究，利用准弹性激光散射技术对混凝药剂及混凝前后水中胶体颗粒（小于0．45μm）的大小与分布进行了测定研究，对几种混凝剂的药剂费用进行了经济分析。     

Effect of Ultrasonication on the Microstructure and Tensile Elongation of Zirconia-Dispersed Alumina Ceramics Prepared by Colloidal Processing

To obtain dense, fine-grained ceramics, fine particles and advanced powder processing, such as colloidal processing, are needed. Al₂O₃ and ZrO₂ particles are dispersed in colloidal suspensions by electrosteric repulsion because of polyelectrolyte absorbed on their surfaces. However, additional redispersion treatment such as ultrasonication is required to obtain dispersed suspensions because fine particles tend to agglomerate. The results demonstrate that ultrasonication is effective in improving particle dispersion in suspensions and producing a homogeneous fine microstructure of sintered materials. Superplastic tensile ductility is improved by ultrasonication in preparing suspensions because of the dense and homogeneous fine microstructure.     

Precipitation of Amorphous Silica from a High-Temperature Hydrothermal Solution

The polymerization of monomeric silica in a hydrothermal solution with the formation of colloidal silica particles is investigated experimentally. The coagulation and precipitation of colloidal silica in an aqueous solution by metal cations is studied. The physicochemical characteristics of the silica precipitated are determined. The high-temperature calcination of the amorphous material precipitated leads to the formation of calcium, magnesium, and cobalt silicates.     

Physicochemical Processes during Precipitation of Silica from Geothermal Brine

Physicochemical properties of the silica–geothermal brine colloidal system are investigated. The reaction order and the rate constants for silica polymerization are determined, the radius of colloidal particles is measured, and their charge is estimated. The mechanism of coagulation and precipitation of colloidal silica by metal cations is experimentally studied. On the basis of the results obtained, a basic process to recover silica from the geothermal heat-transfer medium is designed.     

CONTRIBUTIONS TO THE THEORY OF GLASS FORMATION AND THE GLASSY STATE*

An attempt has been made to assimilate the most important results of recent glass research into a theory of glass formation and the glassy state and to discuss it in relation to present knowledge of physics, chemistry, and colloidal chemistry. The formation of glass from the liquid state, in contrast to crystallization, is a continuous path from the liquid, through the viscous (supercooled) to the brittle state. The transformation point (Tjp) represents the boundary temperature between viscous and brittle glass, at which boundary the property-temperature coefficients more or less suddenly change. Property values are influenced by thermal history which can not be explained by mechanical strains. Therefore, while a melt is cooled, an inner change of state, dynamic aggregation, is assumed which takes more and more time as the transformation point is approached and is completely checked at lower temperatures. The question of the thermodynamic stability of the glassy state thus becomes a problem. The difference between “unchecked” (lasting) and “checked” displacements of state is explained with respect to a series of properties. From measurements of the viscosity and of the electric conductivity at temperatures from 1300 down to 300°C, it is shown that the liquid or viscous state follows a simple hyperbolic law, according to which these properties can only continue down to a lower limiting temperature (7V) above the absolute zcro point. The brittle state begins at the transformation point and follows a new law, knowledge of which is made more difficult by sooner or later “checking” the displacements of state. From the previous two different laws, it follows that brittle glass is something other than a supercooled liquid. The transformation point can not be thermodynamically explained. Molecular kinetic considerations make it appear to be that temperature at which densest packing with a cessation of free molecular kinetic motion of the particles is reached. Changes in the brittle state are explained from an atom-kinetic point of view (shrinking of particle volumes) provided no changes in the state of aggregation occur. Dynamic aggregation takes place by the formation of primary and secondary particles. The structure of secondary particles is compared to a roll of coins and this picture is used to explain negative expansion under pressure, peculiarities in expansion curves, and the beginning of brittleness. According to colloidal chemical considerations, the glassy or viscous states represent a solution of secondary particles as the disperse phase in a dispersing medium of primary particles and individual molecules, and the transformation point represents the boundary temperature between viscous and brittle glass paralleled by the gelatinizing temperature characteristic of the change from a sol to a gel. Support for the colloidal-chemical conception is obtained from observations on rapidly cooled glass melts and from reaction kinetic. Changes of property in the brittle and viscous states are proportional to the logarithm of the time, z, a law which is also found in elastic after-working, in isothermal dissociation of carbonates, and in some other instances. This law can he deduced from v. Smoluchowski's theory of coagulation. The disappearance of, double refraction of chilled glasses due to annealing (cooling) can also be linked to the log Z law.     

Hardness of moist agglomerates in relation to interparticle friction, capillary and viscous forces

Wet agglomerates deform plastically until they break through crack propagation. On the particulate level, liquid bridges are responsible for the strength of the wet agglomerate as they hold the particles together. Recent micro-scale studies have identified the role of liquid surface tension, bridge Laplace pressure and liquid viscosity, which, in combination, explain the axial strength of pendular liquid bridges. Different situations exist depending on the degree the liquid wets the particles and on the saturation of the agglomerate mass.On the wet agglomerate level, the hardness is related to three factors: the liquid binder surface tension and viscosity and the interparticle friction. A simple model is developed in this paper, based on the powder and liquid binder properties, which shows that the forces due to interparticle friction are generally predominant in wet agglomerates made from non-spherical particles. Although mechanical interlocking is not accounted for, the model yields accurate prediction of wet agglomerate hardness independently measured on wet masses of varying composition. This theoretical hardness could prove an interesting tool for wet granulation research and technology.     

New perspectives on capturing particle agglomerates in CFD modeling of spray dryers

Abstract

Current computational fluid dynamic (CFD) models of spray dryers lack the capability to predict the structure of the agglomerates formed; loose or compact agglomerates. This is mainly due to the conventional simplistic approach in numerically “fusing” of the colliding particles forming the agglomerate. A new theoretical treatment is introduced in this work, suitable for implementation in CFD simulations, which numerically fuses the particles and yet retain information on the structure of the agglomerate. This new theoretical treatment is based on tracking the reduction of the agglomerate surface area as the agglomerate is progressively formed. Analysis revealed that the reduction in the agglomerate surface area exhibits a unified correlation with the degree of compactness of the agglomerate. Further analysis comparing this new approach to the conventional numerical fusing of the particles revealed inherent numerical discrepancies, which has not been noted in the literature before. Understanding these discrepancies will provide clarity to the interpretation of the modelling and simulation of spray drying particle agglomeration in CFD. Moreover, this work lays the groundwork for a more comprehensive CFD model for agglomeration which can be potentially utilized to predict final powder properties.     

密相流化床中介尺度流动结构的流体力学特性研究

在一套流化床冷模实验装置中对黄沙颗粒和黄沙-硅微粉 （20 μm）混合颗粒进行实验。测量固含率时间序列信号并进行统计分析,提出并建立复杂光纤脉动信号的解耦方法,实现稠密气固流中介尺度流动结构的准确识别。基于统计矩一致性原理提出气泡阈值的计算方法,通过遍历法确定气泡阈值。对气泡阈值变化规律进行分析,发现加入细颗粒有助于改善流化质量,随表观气速的增加,气泡阈值减小。对气泡、乳化和聚团三相的相分率进行统计,发现在黄沙颗粒中加入少量（5%,质量分数）细颗粒能够显著改善流化质量,细颗粒添加量过多时（10%）,对流化质量的改善将减弱。对气泡的流体力学特性进行分析,发现加入10%硅微粉后,气泡弦长增大,频率降低,速度略有降低。对颗粒聚团流体力学特性进行分析,发现随硅微粉含量增加,表观气速对聚团速度的影响减弱,聚团弦长略有减小。加入5%硅微粉后,颗粒聚团的出现频率较小且径向上分布均一。加入10%硅微粉后,聚团频率有所增大,说明加入过多硅微粉会促进聚团的形成。     

细颗粒的团聚流态化及其判定

本文在考查了不同细颗粒的流态化过程及物料的气动特性与粘附力关系的基础上，提出了当量流态化的概念；实验发现聚团密度的减小是改善细颗粒流化性能的一种有效途径；根据气动情况下不同的成团结果，把细颗粒的聚团流态化分成了三类：沟流；似A类聚团流态化；似B／D类聚团流态化；同时，结合实验结果，给出了不同细颗粒聚团流态化类型的定量判据