Calculation of Hamaker Constants in Nonaqueous Fluid Media
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Mat. Res. Soc. Symp. Proc. Vol. 625 © 2000 Materials Research Society
Most calculations of Hamaker constants have focused on aqueous media. Much of the reason for attention being paid to water is its environmentally benign nature. Yet, the majority of thick film pastes used in the electronics industry are in nonaqueous solvents, and the availability of information for these systems are not common in the literature. It is the drive of this work to fill the gap in the literature of the functions required to calculate Hamaker constants in nonaqueous media. In doing so, the role of dispersants in these media to control rheological response will be improved, and the possibility of tailoring solid and liquid to provide esired rheological properties can be explored. EXPERIMENTAL Rheological measurements were performed with a Bohlin CS-10 rheometer'. Silver slurries 2 were prepared at 40 volume % in methanol and isopropanol. Pluronic F68 3 dispersant was added at 0.4 weight % to powder to evaluate the control of interparticle force on rheological response. This dispersant is a triblock copolymer of polyethylene oxide stabilizing blocks bonded to an adsorbing block of polypropylene oxide. Experiments were performed using a 60 second preshear at 400 rpm followed by a hold time of 30 seconds. Tests were run from low shear rate to high and back to zero. The parameters for the Ninham-Parsegian expression for liquids can be found in a variety of literature sources. Water has been most extensively characterized, and the spectral parameters for water are given in [5]. Six infra-red and five ultraviolet damped oscillators with a single Debye microwave relaxation represent the dielectric behavior very well. However, this level of detail is hard to compile for most nonaqueous solvents. The static dielectric constant and index of refraction at the sodium D line are commonly tabulated [6], and the infra-red adsorption spectra for many liquids are relatively easy to find [7]. The microwave parameters for several liquids are referenced in the compilation of Buckley and Maryott [8]. Determination of the ultraviolet characteristics for several solvents has proven to be more difficult. This is a serious complication as the ultraviolet terms are the most critical for calculating the Hamaker constant. In the absence of a full spectra, UV relaxation is commonly represented by a single oscillator of magnitude equal to n2 - 1. The UV adsorption edge was chosen as the critical frequency of this oscillator and damping terms were omitted [6]. Further searches are being performed to determine the UV spectra for these nonaqueous fluids and refine the parameters for Hamaker constant calculation. From the collected data, the oscillator strengths were determined by the changes in the value of the real dielectric constant between spectral regions. The microwave strength can be determined directly from [8]. Groups of peaks located very closely together in the infra-red spectrum are represented by a single oscillator and damping terms were omitted.
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