超细粉体表面包覆与改性课件

1、超细粉体表面包覆与改性超细粉体表面是指表面的一个或几个原子层，有时指厚度达几微米的表面层。表面是体相结构的终止，表面向外的一侧没有近邻原子，表面原子有一部分化学键形成悬空键。超微粉体内部的三维周期势场在表面中断，表面原子的电子状态也和体内不同，然而表面不是体相结构的简单终止。由于超微粉体表面有悬空键，因而有剩余成键能力。为了使表面能降低，所有超微粉体的表面原子都会离开它们原来在体相中应占的位置而进入新的平衡位置，发生弛豫和重构。因此导致颗粒表面有很高的活性。超细粉体表面特性超细粉体表面的润湿性超细粉体的表面电性（颗粒与其他介质接触时，表面会有电荷转移，这种转移往往正负电荷数量不一致，从而产生电

2、势差。）超细粉体的表面能超细粉体表面改性表面改性是指采用物理或化学方法对固体颗粒进行表面处理，即根据应用需要有目的地改变颗粒表面物理化学性质与表面形态结构工艺。可以根据改性途径和赋予的改性产物功能分为三个方面：第一，有机改性剂在颗粒表面的覆盖，以提高无机粉体在有机基体中的分散性和界面结合强度；第二，通过化学沉积或机械力化学作用将固体小颗粒（子颗粒）或均一物质膜在较大颗粒表面（母颗粒）均匀包覆形成复合颗粒，从而赋予复合颗粒新的功能；第三，采用高能电晕放电、紫外线照射或等离子辐射等方法在颗粒表面形成不饱和程度大的电子层或化学键，从而提高颗粒表面活性以及与其他物质的界面结合程度。表面化学改性：超微粉

3、体的表面化学改性是指在原来单一组分的基元物质表面上，均匀地引入一种或者是多种其他组分的物质，以改变原来基元的基本性质的方法。常见的有：表面吸附包覆改性、化学反应包覆改性、微乳液法、胶囊化改性和化学镀法等。表面改性目的改变颗粒表面的晶体结构(表面无定形化)、溶解性能、化学吸附和反应活性(增加表面的活性点或活性基点)等。具体：偶联剂具有两性结构，其分子中的一部分基团可与粉体表面的各种化学官能团反应，形成强有力的化学键。另一部分基团则与有机高聚物发生某些反应或物理缠结，从而将两种性质差异很大的材料牢固地结合起来，使无机填料和有机高聚物之间产生具有特殊功能的“分子桥”。接枝聚合改性通过调节表面聚合改性

4、单体的配比，进行控制共聚物层及其无机粒子界面层的结构和性质。静电分散作用：通过表面改性使颗粒的荷电量增大；通过表面改性改变颗粒表面性质，消除液桥力的影响。降低纳米粉体的团聚度、提高其流动性合金粉末真空雾化设备The electrochemical properties of MmNi5-type metal hydride powders produced by gas atomization were improved by acid surface modification. FIB specimen preparation followed by FE-TEM observation w

5、as used to analyze the surface structure before/after surface modification. The original oxide layer of the surface was removed and a nickel rich layer was formed by acid treatment. AbstractThe structure of the nickel rich surface differed with the treatment conditions. For acetic acid treated powde

6、r prepared in heated solution, a nanocrystalline nickel layer was formed at the surface. In the absence of any auxiliary conductivity additives, discharge capacity of this powder was improved to 254 mAh/g versus 52 mAh/g for non-treated powder and 62 mAh/g for hydrochloric acid treated powder.Introd

7、uction(Mm: La 33%, Ce 48%, Pr 5%, Nd 14%)MmNi5-type metalhydride (MH) alloy powders are widely used for the negative electrodes of nickel/MH batteries. Much effort has been devoted to finding substitutes for Ni to improve basic electrode properties such as discharge capacity and cycle life. Modern a

8、lloy systems are primarily of the type MmNiCoMnAl.The aim of this work is to clarify the surface structures of before/after acid treated MH powders prepared by gas atomization as well as discharge capacity change by surface modification.ExperimentalThe MH alloy powder whose composition is Mm1.0Ni3.6

9、Co0.6Mn0.6Al0.2 (Mm: La 33%, Ce 48%, Pr 5%, Nd 14%)was produced by Ar gas atomization after being melted in a 2 kg capacity induction furnace under an Ar atmosphere.The atomized powder was sieved to less than 180 m and its average particle diameter was 59 m.The sieved powder was then heat treated at

10、 1073K for 36 ks under an Ar atmosphere. A SEM image of the atomized powder is shown in Fig. 1.Surface modification was terminated when the solution pH reached 6.5 for hydrochloric acid or solution temperature exceeded 368K for acetic acid.A TEM specimen was prepared with FIB for thinning the area o

11、f interest of the MH powder particle and observed with FE-TEM. The surface layer of the particle was characterized by EDS analysis and electron diffraction.ResultsThe pH value varied from 0.8 to 6.5 with incremental changes as a function of time during hydrochloric acid surface modification, and the

12、 pH value of acetic acid solution showed an almost linear increase from 2.6 to 3.1 for 0.6 ks at room temperature. After heating to 368 K, the pH rose to 4.2, as shown in Fig. 2.Table 1 shows the discharge capacity before/after hydrochloric acid and acetic acid surface treatment. Without surface tre

13、atment, the value of the standard conductivity electrode was 213 mAh/g, which is only 63% of the theoretical capacity (340 mAh/g). After acid treatment, the discharge capacity of the standard electrode increased, nearing the theoretical value for both acids.TEM micrographs of the powder surfaces are

14、 shown in Figs. 57. The insets show the results of EDS analysis of areas of interest (marked as white circles) and their electron diffraction patterns. The darkest contrast was observed at the surface of the heat treated powder, where a mischmetal oxide layer might be formed. DiscussionConsidering t

15、he electrochemical properties, the surface of the non-modified atomized powder was covered with a thin oxide layer which prevented electrical contact of the powder, so that the electrochemical activity of the electrode declined. The electrochemical properties were drastically improved by surface mod

16、ification in which the original surface layer disappeared and a Ni-rich layer was formed.ConclusionsThe effects of surface modification on surface structure as well as electrochemical properties of gas atomized MH powder were investigated using two acidic solutions, hydrochloric and acetic. For low

17、conductivity electrodes conditions, the electrochemical capacity of gas atomized powder was inferior even after hydrochloric acid treatment.After heated acetic acid treatment, however, discharge capacity was greatly improved. TEM observation revealed that the original surface oxide layer was removed by surface modification, and a Ni-rich layer was formed. T