Isoelectric points particle materials

One solution-based approach that works for gold catalysts, in that it produces highly active catalysts, is the deposition-precipitation (DP) method [8]. The DP method entails adjusting the pH, temperature, and gold concentration of an HAUCI4 solution to form a gold hydroxide species which is then deposited onto the support material [8]. This catalyst precursor is washed, dried, and annealed to form small (<5nm) catalyst particles [9]. The DP method has a number of limitations for example, DP cannot produce Au particles with diameters less than 5 nm on support materials with low-isoelectric points (lEPs) like SiOz and WO3 [5,10,11]. 

The pH of a slurry has a profound influence on its colloidal stability and CMP performance. Strong correlations have been established between the particle isoelectric point (lEP) and the optimal pH for slurry stability. The general rule is that the slurry is more stable at a pH that is away from the lEP, so the zeta potential of the particles is greater than 20 mV. The focus of this section is on the influence of pH on the slurry performances such as material removal rate and defectivity. In order to examine the impact of slurry pH on these two important performance features, we first take a closer look at the interaction between abrasive particles and the surface to be polished. There is a vast amount of literature on the interaction between abrasive particles and silicon dioxide surface [26]. The discussion below will focus on the interaction between ceria abrasive particles and the silicon dioxide surface to be polished. The basic principles and conclusions can be easily extended to other pairs of abrasive particles and surfaces. 

Abrasive particles are a key component in CMP slurry. The most commonly used abrasive particles include silica, alumina, ceria, zirconia, titania, and diamond. Table 21.1 listed a set of information on each type of abrasive particles such as density, microhardness, and isoelectric points (lEP). It is important to point out that the specific values for these properties depend highly on the preparation techniques and the specific states of the samples. The values listed in the table represent an average of the most commonly reported data. For example, the isoelectric point for silica is a function of the number of hydroxyl groups, type and level of adsorbed species, metal impurity in the solid matrix, and the treatment history of the materials [1]. There are three major types of silica according to their preparation methods fumed, colloidal, and precipitated. The common sources for obtaining these abrasive particles are listed in Table 21.2. As examples, some of the more specific information on... 

The isoelectric points (lEP) of the individual materials and the zero point charge (ZPC) of the mixtures as defined by Parks [12], were determined by electrophoretic migration, measuring the zeta potentials as a function of the solution pH, as in previous studies [13] using a Zeta-Meter Inc. Instrument model 3.0+ Experiments were determined with 30 mg of approximately 2 pm diameter particles, suspended in 300 ml of 10 M KCl, adjusting the pH value with 0.2 M KOH and HCl solutions. Each curve obtained was recorded at least twice to ensure reproducibility. 

At some distance from the particle surface (usually identified as the beginning of the diffuse layer in Fig. 3), a hydrodynamic shear plane exists that is characterized by the potential. The magnitude of is directly related to dispersion stability [71]. For oxides, hydroxides, and related materials, is strongly influenced by solution pH and electrolyte concentration and may be modified by surface-active species, such as oxyanions and polyelectrolytes. The key parameter characterizing a powder surface is the isoelectric point pHiep. Under pristine conditions (i.e., no surface contamination), pHiep defines the solution pH at which C = 0 and the particles exhibit a net surface charge of zero. 

Zeta potential is the potential of the surface at the plane of shear between the particle and the surrounding medium as the particle and medium move with respect to each other. In the presence of an applied electric field, the charged surface (and the attached material) tends to move in the appropriate direction, while the counterions in the mobile part of the double-layer would have a net migration in the opposite direction. On the other hand, an electric field would be created if the charged surface and the diffuse part of the double-layer were made to move relative to each other. The plane of shear is beyond the Stem plane, and the zeta potential facilitates easy quantification of the surface charge. The pH at which the calculated zeta potential value is zero is known as the isoelectric point (lEP). 

Inorganic material Chemical formula Particles dimension [pm] Isoelectric point Field of application... 

The pH at which the zeta-potential is zero is called isoelectric point (lEP). Because of specific ion adsorption, the lEP may deviate from the PZC, which is a purely material property. For this reason, the lEP is less a parameter of the particle phase but rather a characteristic parameter of the suspension. In a couple of papers, Kosmulski reviewed experimentally determined PZC and lEP-values for numerous materials in varying environments (Kosmulski 2002, 2004, 2006, 2009). 

A wide variety of materials have been implemented as abrasive particles in CMP processes. They include alumina, silica, ceria, zirconia, titania, and diamond. The effectiveness and suitability of these particles in CMP with particular applications are greatly influenced by their bulk properties (density, hardness, particle size, crystallinity etc.) and the surface properties (surface area, isoelectric electric point (lEP), OH content, etc.). This section will focus on the evaluation of alumina, silica, diamond, and ceria as the major abrasives used for the CMP of metals. 

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