M.Sc Thesis


M.Sc StudentYulia Raichman
SubjectUnderstanding the Nature of Inter-Nanoparticle Bonds in
Agglomerates
DepartmentDepartment of Chemical Engineering
Supervisor Professor Tsur Yoed


Abstract

The interest in nanoparticles research has grown tremendously during the last decade. This interest stems from the different and often superior properties of nanoparticles, as compared to micro-particles. Agglomeration is very common phenomenon when dealing with nanoparticles, which have high surface area and energy. In order to produce more thermodynamically stable structures, nanoparticles combine to form larger masses which are called agglomerates. However, thus they change the very character of nano-scale material. Therefore it is extremely important to investigate stabilizing techniques that result in reducing the strength of inter-nanoparticle bonds in agglomerates. Different measuring techniques are applied in order to characterize the strength of agglomerates. However, those measurement techniques can only deal with relatively large particles, in the micro range, and do not have the resolution to measure directly the inter-particle forces in the nano regime. Therefore, new tools and techniques for such investigations are needed. To study the inter-particle forces locally, one should apply a tool with spatial resolution of less than a nanometer, and with appropriate force resolution, of the order of mN. Modern Nanoindenters can provide such a tool.

In this research we studied the agglomerate strength through breakage of inter-particle bonds by a nanoindenter tip. Two silver powders were taken as a model material. We have managed to deposit and fix small agglomerates of these powders from colloidal dispersions onto hard substrates using an adhesion promoter. In the case of small agglomerates that consist of few nanoparticles, finding a proper description of the mechanical behavior becomes a problem of statistical nature. The behavior of different agglomerates within the same sample is connected with their geometry, including the number and orientation of the various stronger inter-particle bonds, the sizes and the shapes of the nanoparticles. We have found that the strength of agglomerates can be correlated with the shape of the nanoindentation load-displacement curves. We suggest using the averaged slope of the loading curve as a quantitative parameter characterizing the strength of agglomeration. The statistical distribution of this parameter can be used for characterizing the agglomeration phenomena in a given nanopowder.

At this point we can say that the method was developed successfully. It can be applied for instance for studying the strength of other powders like oxides and the influence of ambient conditions.